Final Report Summary - INNOVASOL (Innovative Materials for Future Generation Excitonic Solar Cells)
Executive summary:
INNOVASOL project aimed at developing radically new nanostructured materials for photovoltaic (PV) excitonic solar cells (XSCs) really competitive with traditional energy sources. The target was to leapfrog current limitations of third-generation PV devices through a drastic improvement of the materials used for assembling XSCs (with particular attention to light absorbers, hole transport materials and electron transport solids). Taking into account that the state of the art DSC device was 11% efficiency, INNOVASOL XSC device target was to reach 11-15% efficiency.
As a first activity, benchmarking analysis to evaluate the effect of commercially available organic dyes, titanium dioxide, conductive SnO2:F (FTO) coated glasses and electrolytes on the efficiency of laboratory "spot-cells" (being typically between 0.16 and 0.64 cm2 in size) was carried out: this activity was fundamental to measure the performances of new INNOVASOL materials in a reproducible and comparable fashion. A detailed analysis of literature data and their comparison with preliminary results of INNOVASOL project was carried out especially during the first year of the project, in order to select the most promising candidates for the core materials of the new XSC devices.
In particular, the screening involved four classes of XSC materials:
i) quantum dot light absorbers (QD),
ii) molecular relays,
iii) hole transport materials,
iv) mesoscopic electron transport materials.
In this respect, development of innovative preparation routes allowing the optimization of new core materials needed to increase the actual performances of XSC devices.
Project Context and Objectives:
INNOVASOL project aimed at the development of new nanostructured materials for photovoltaic (PV) excitonic solar cells (XSCs) really competitive with traditional energy sources. The main objective of the project was to overcome current limitations of third-generation PV devices though a drastic improvement of the materials used for assembling XSCs.
A combination of radically new materials in novel solar cell architectures was proposed in the frame of the project to overcome the current limitations of dye sensitised solar cell (DSSC) devices. The research activity involved the following tasks:
i) use of semiconductor quantum dots (QDs) as the light harvesting units with a fine tuning of the optical cross section and of the band gap (by modifying both the chemical composition (e.g. PbS, PbSe, PbSe/PbS core/shells etc) and the size of the nanoparticles);
ii) synthesis of suitable amphiphilic dye molecules (e.g. cyanines, squaraine dyes, zinc phthalocyanine and dipolar stilbene-like dyes), designed to act as molecular relays (MRs) that connect the QDs to electron conductor materials;
iii) development of novel solid-state electrolytes (and quasi-solid electrolytes) as p-type hole conductors, such as layered or porous materials suitable to host anionic redox couples (e.g. I-/I3-);
iv) development of specifically designed n-type semiconductor, e.g. the mesoscopic oxide film or the transparent conductive oxides (TCO), such as ZnO nanorods or TiO2 nanofibers and nanotubes with architecture, morphology and surface structure suitable to maximise the efficiency of the charge transfer processes at the QD/MR/SC interface and electron transport in the nanocrystalline TCO films.
Innovative materials and device architectures were designed to have low production cost, high stability to UV and high light harvesting efficiency.
QDs had already demonstrated to be very efficient in photon capture, with optical cross sections which can be far greater than the molecular dyes currently used in dye-sensitized solar cells. A revolutionary improvement is feasible by multiple exciton generation (MEG) processes, where more than one exciton is generated by a single energy photon, leading to quantum efficiency larger than 100%. Such processes were observed in colloidal silicon QDs, and in small band gap IV-VI semiconductors like PbS and PbSe. INNOVASOL project explored this radically new approach to increase the XSC performance, by designing, producing and testing suitable QDs and QD/MR composite materials.
The photon capture by the QD followed by energy transfer to the molecular relay and the subsequent electron injection from the excited relay into the charge carrier conductors were carefully tuned, to be competitive with thermal recombination, by judicious engineering the QD/conductors interface. A striking improvement was demonstrated by an unprecedented architecture of supramolecular (self-organised) hierarchical nanostructures, where the QDs are covered by self-assembled monolayers (SAM) of amphiphilic dye molecules as basic light harvesting units mimicking the photosynthetic antenna system. In this configuration, the dye molecules act as molecular relays (MRs), connecting the photon capture unit and the TCO electron conductor, i.e. the negative electrode of the PV cell. The photogenerated excitons transfer their energy to the MRs by Förster resonance energy transfer (FRET) mechanism through dipole-dipole interactions (i.e. without the need of electron transfer), then the MRs inject electrons into the oxide. INNOVASOL designed, synthesized and tested several MRs in order to select the most photostable and efficient, e.g. the ones matching their HOMO-LUMO energy differences with both the QDs band gap and the conduction band of TCO semiconductor at the best.
In order to select the most promising materials for light harvesting and electron injection, the effects of specific physical and chemical modifications (i.e. the size of the NCs, the presence of ionic liquid electrolytes within the layered or porous solids, the changes of the material properties at the different layer interfaces, etc.) were reproduced. The properties of interest could be modelled as a function of the physical and chemical nature of the systems. State-of-the art computational procedures were adopted, even combining different approaches (since the nature of the INNOVASOL XSC is intrinsically multi-phase, and required different theory levels for the different parts of the devices) in newly developed mixed computational procedures.
Beside the development of innovative materials, partners performed an accurate screening of the electronic properties of QDs already available, in order to identify the most promising light absorbers. In addition, a screening of the proposed molecular relays to explore their binding on quantum dots and thin conductive oxide surfaces and their role in the energy transfer surfaces was accomplished.
The innovative materials developed by the project partners were accurately characterized by spectroscopic and structural analysis, and modelled with suitable theoretical procedures (based on various computational codes, capable of describing each layer of the composite material with the suitable level of theory), in order to find the most promising candidates for the XSC devices developed in the project. In addition, theoretical and computational techniques have been designed and exploited in order to support synthesis and characterization of new materials.
Finally, INNOVASOL aimed at substituting the liquid electrolyte with radically new solid-state (or quasi-solid) conductors, in order to reduce potential environmental risks and ensure a much greater stability in outdoor operating conditions. The project designed and produced a very wide spectrum of radically new materials and nanomaterials with porous or layered structure, high specific surface area and complex engineered architectures, which were employed to host redox active species. During the first year of the project some key materials were identified and tested for the final XSC devices: a commercial porous silica, two families of synthetic clays, such as saponite and organo-modified talc, and a synthetic layered silicate known as Na-RUB-18.
The technological objective of INNOVASOL was the integration of innovative materials in XSC devices to be used as environmentally clean and renewable electric power source, which may pave the way for long term applications, such as automotive roof panels, transparent windows and civil/industrial buildings requiring stringent performances (long lifetime, high stability, high efficiency and high relative humidity resistance). These were very ambitious targets, which could lead to highly relevant societal impacts.
One of the most relevant technological objective was to optimize process strategies suitable to integrate radically new materials in proof-of-concept devices, which were strictly needed for testing the INNOVASOL materials and composites. This was essential for understanding that a real upgrade in the properties of the materials was being reaching, and for developing radically new paths leading to highly innovative, long term research in the field of materials for solar cell applications.
The competitive cost/efficiency ratios of INNOVASOL materials could be maintained (or even improved) developing efficient synthesis approaches and surface functionalization methods thus to propose reliable applications of XSC devices on a large scale by Solaronix partner. Hybrid approaches, liquid and vacuum based, were optimized to deposit the active materials with suitable morphologies for enhanced electro-optical properties. Both new self-assembly (e.g. layer-by-layer) techniques and low cost deposition and patterning processes (e.g. spin coating, ink jet) were used. Controlled nanostructured active layers were developed using hydrothermal and CVD deposition processes.
Project Results:
The main scientific and technological results obtained during the three-year's project are reported in this section in relation to the different type of materials/activities that the Consortium developed in the frame of the Project. It is worth noting to recall that DSC state of the art in 2009 (at the beginning of the Project) exhibited ca. 11% efficiency and that INNOVASOL XSC device target was to reach 15% efficiency, together with an enhancement of the device lifetime. These targets were reached by preparing materials used as molecular relays, n-conductors and quasi-solid electrolytes suitable for system integration. In addition, attention was paid to decrease cost production of the materials (e.g. by upscaling material preparation).
In the sections below, the main results obtained in the frame of the Project are reported following the work plan of the INNOVASOL Project (i.e. Work package organisation).
1.1.3.1 Development of computational procedures for modelling of molecular relays and nanostructured composite materials (activity related to WP1)
According to INNOVASOL project, high level theoretical modelling was joined to the experimental activity in order to: 1) help the pre-screening of materials on the basis of their predicted relevant properties; 2) orient the synthesis of new materials with tailored physico-chemical characteristics; 3) support the interpretation of the experimental data, obtained by spectroscopic, electrochemical and other techniques.
To achieve this goal, the preliminary step was the development and implementation of a multiscale computational protocol, able to integrate the different codes and programs available to the partners in a common framework. Such a development led to support and/or development of the following activities:
- Computational procedures provided by partners
1) Particle-in-a-box, developed and parameterized at TUD allowing for the description of optical properties of very large nanoclusters of different chemical composition.
2) Semiempirical approaches with different parameterizations, namely PM3, MNDO, AM1 as coded in Gaussian03 package, PM6 in MOPAC2009, MSINDO in MSINDO3.3: these codes are available at UNIPMN and UNITO, where they were locally modified.
3) Density Functional Theory (DFT) methods with various functionals and basis sets, through Gaussian03 and TURBOMOLE6 packages, available at UNIPMN and UNITO.
4) DFT methods with periodic boundary conditions, for the modelling of crystal structures, with various functionals and basis sets, coded in CRYSTAL06 package, available at UNIPMN.
- Parameterization and optimization of the procedures
1) The particle-in-a-box parameterization was performed in the first months, and the method was already applied to some of the nanoclusters of interest, to help selecting the most promising materials.
2) To ensure the transferability of results, the databases of TURBOMOLE and CRYSTAL were modified, so that the density functional definition (B3LYP) and the atomic basis sets (Pople 6-31 set plus polarization and diffusion functions, and Hay-Wadt pseudopotential with double-z basis sets) match exactly those defined in Gaussian03.
3) The present MSINDO parameterization was verified to ensure that it could be suitable to describe the systems of interest.
- Integration of the procedures in a multiscale protocol
The developed multiscale procedure is based on the combination of different levels of theory in the same calculation, following the scheme called ONIOM in Gaussian suites: this is expected to be crucial for the description of structural and optical properties of molecular relays adsorbed onto the nanoclusters, and the electron transfer between nanocrystals, relays and electron-conducting layers.
The two theory levels selected for such a combined approach are DFT with localized basis sets and hybrid functionals (as implemented in Gaussian03) and semiempirical with up-to-date parameterization (namely MSINDO and PM6 implemented in MOPAC2009).
1.1.3.2 Synthesis and characterisation of nanomaterials (activity related to WP2)
a)Quantum dot absorbers
During the project implementation, the TUD activity was focused on the colloidal synthesis of semiconductor nanocrystals, their assembly and the grafting of molecular relays to the nanocrystals obtained. Applying an aqueous approach, a facile one-step synthesis of near infrared (NIR) absorbing and emitting CdTe and alloyed CdHgTe quantum dots (QDs) was developed (Lesnyak et al., J. Mater. Chem. 2009, 19, 9147-9152). Focusing on the current demand for low-toxic nanocrystalline materials, blue emitting alloyed ZnSeTe QDs was also obtained employing a one-pot synthetic protocol (Lesnyak et al., Chem. Commun. 2010, 46, 886-888) Furthermore, chemically non toxic SnS and FeS nanoparticles were produced by means of a hot-injection synthetic technique, and the non-injection synthesis of ternary CuInSe2 and quaternary Cu2ZnSnS4 nanocrystals was also explored. These materials were designed to act as the light absorbing layer in the XSC devices instead of, or in combination with, the organic dyes currently used in the present generation of dye sensitized solar cells. However, after being tested in solid state excitonic cells at EPFL neither Cu-containing nanocrystals, nor iron or tin sulfides provided reasonable light harvesting efficacy.
-Self assembly of quantum dots
Since one of the most facile and straightforward self-assembly technique is the attachment of various components using chemical binding (Gaponik, N., J. Mater. Chem. 2010, 20, 5174-5181) the work in this task concentrated on establishing a number of linking procedures that allowed the efficient and irreversible binding of QDs to transparent conducting substrates based on ZnO and TiO2. To achieve this goal, monodisperse PbS, ZnO and TiO2 nanoparticles were synthesized and characterized using transmission electron microscopy, X-Ray diffraction and optical spectroscopy. A great deal of emphasis was placed on the role of the linker and it was shown that the linker is a major key to the understanding of the charge carrier transfer processes and has an important influence by not only transferring or blocking the electron and hole intermediates in such systems but through its own inherent and often substantial contribution to the optoelectronic response. To gain a greater understanding concerning the complex role played by the linker, a number of different molecules with different functional groups and different alkyl chain lengths were used to connect the QDs to ITO/TiO2 and ITO/ZnO substrates, the attachment of the linkers and nanocrystals being verified by spectroscopic methods, and the optical and optoelectronic response studied using cyclic voltammetry, electrochemical impedance spectroscopy and photocurrent spectroscopy (Krüger et al., J. Phys. Chem. C 2011, 115, 13047-13055). Additionally, analysis of the substrates was carried out by using UV-Vis, Fourier-Transform Infrared Spectroscopies and X-Ray diffraction to give a conception of material composition and the nature of the molecular contact characteristics. The obtained results provided important insights into the role of the linker molecules at the electrode interface in the mediation of charge transfer, which allowed for further improvement of the performance of QD sensitized solar cells and hybrid dye/QD based cells.
-Grafting of molecular relays to quantum dots
TUD group fabricated and studied several different composites of CdSe and CdSe/ZnS QDs with specially designed red-NIR squaraine dyes (VG0, VG1, VG1S6, VG2) provided by the UNITO partner (vide infra). In order to prove the concept of a molecular relay architecture and to investigate its energy transport properties, squaraine dye-QD composite films deposited onto quartz glass plates, QD-squaraine-ZrO2 and QD-squaraine-Al2O3 systems as analogues of the TiO2 based architecture were prepared and characterized by means of optical spectroscopy (absorption, steady state and time resolved photoluminescence measurements). In these composites the QDs complemented the optical properties of the dyes acting as additional light absorbers covering the UV-Vis region of the solar spectrum and thus were capable of energy transfer to the dyes. This study allowed to probe these systems and revealed a number of energy relationships between the various components. In all the systems investigated energy transfer from the donor (QD) to the acceptor (dye) was observed.
b) Design and preparation of novel molecular relays
The acitivity of UNITO unit was mainly devoted to the design, synthesis and characterization of several NIR dyes in order to select the most interesting structures to be used by the consortium partners in conjunction with the appropriate QD sensitizer. Moreover, the unit was also directly involved and responsible for the preparation and characterization of composite nanostructured materials made of MRs linked to QDs and their characterization. In particular, the following activities were carried out:
1.Design and synthesis of a variety of polymethine dye structures (mainly squaraines) with absorption spectra with maximum wavelengths ranging from 640 nm to 850 nm.
More than 30 new different structures were synthesized and fully characterized resulting in an optimal tuning of optical and electrochemical properties, number and position of anchoring groups, length of the alkyl protecting chain. The synthesis of all these new structures was settled up firstly with standard synthetic methods, but during the project new methods based on a microwave approach were exploited.
This research resulted in the overcoming of a series of excellent synthetic challenges:
i) number of reaction steps reduced (it is no more necessary the hydrolysis of emisquarate intermediate);
ii) yield of each step raised up for all dyes, reaching or overcoming in most of them the value of 75%;
iii) reaction time of each step reduced from day and hours to tenths of minute and minutes, respectively;
iv) purification performed, mostly of the time, only by simple washing and crystallization avoiding the expensive use of chromatographic techniques, giving in all cases completely reproducible dyes with high purity ;
v) synthesis of all intermediates and dyes up-scaled from milligram to multigram level.
-Identification of a new target in NIR dyes for DSSC: a low cost highly efficient symmetric sensitizers
The use of organic sensitizers, respect to standard Ru-based dyes, in DSC is mainly due to their larger molar extinction coefficients (30 000-300 000 M-1 cm-1), extended spectral sensitivity even in the NIR light portion and simple preparation and purification, offering lower cost, while exhibiting excellent characteristics. A number of Ru-free dyes have so far been reported in the literature; among them, squaraines are well known for their intense, but sharp absorption in the red/near-IR regions that can be tuned to the desired long-wavelength region, while exhibiting a considerable photo- and thermal stability. The common idea in literature until last year was to focus on asymmetrical rather than symmetrical squaraines, in order to favor a directionality of the charge-transfer band in the excited state. Thanks to the work done during this project UNITO was able to demonstrate that very simple symmetric dyes show promising photovoltaic properties, sometimes even superior to standard NIR dyes, but with undoubtedly advantages in the low cost and easiness of synthesis for symmetrical structures. In fact, symmetrical and unsymmetrical squaraines have different characteristics and properties, but they can be both excellent sensitizers for DSC, giving comparable efficiency in the cell. In fact, the number of anchoring groups, as well as the length and the nature of alkyl chain, play an important role as much on optical properties as on cell efficiency. The symmetrical dye can be obtained according to a two-step procedure using commercially available precursors. This easiness of synthesis comes in opposition to the unsymmetrical, which requires five steps combined to two chromatographic purification procedures. Worth noting that symmetrical dyes can be easily purified by crystallization, a feasible process for upscaling. A DFT optimization of molecular structures for both symmetric and non-symmetric dyes was performed to gain insight into the energy levels of the different conformations (trans-cis) and into the nature of the excited states involved in the electronic injection. In the case of the symmetric dye the cis conformation has higher polarity (4.05 Debye), thus indicating a high charge separation and an electron density anisotropy comparable to that of the asymmetrical model dye.
-Identification of a suitable chain able to interact with Quantum Dots in XSC
Another important point in the INNOVASOL project deals with the preparation and characterization of composite nanostructured materials made of MRs (squaraines) linked to QDs. These materials should be then used as 'complex sensitizers' in a XSC based on cosensitization/FRET principle. Firstly mercapto groups as possible linkers for QDs were selected and an organic colorless linker for PbS QDs synthesized and used in a solid state cell. After this first step, as far as MRs is concerned, mercapto groups were added to the end of the alkylic chains, but their use unfortunately was not successful due to the formation of unreactive intramolecular disulfide bonds. Beside this, a deep investigation on a series of symmetric squaraines bearing chains of different length was done in order to find the better FRET couples with suitable QDs. Thanks to these measurements done both in solution, in solid state and on TiO2 electrodes it was found that appropriate chain lengths (i.e. C10) may lead the formation of van del Waals molecular assemblies exploitable in FRET XSCs.
-Identification of a simple reaction able to make a stable covalent bond between squaraines (Molecular relays) and Quantum Dots
Finally in order to take advantage of the synergic interaction between MRs and QDs, a simple reaction able to create a covalent bond between QDs and a squaraine was studied. A good compromise was found introducing azido ligands at the end of the QDs synthesis and a triple bond at the end of one of the alkyl chain of a symmetric squaraine. In the case of multiple triple bonds on the organic moieties, olygomeric insoluble structures were formed during the reaction with QDs nanocrystals; however, in the presence of a single anchoring (to the QD) moiety the composite material is still soluble in alcohols and presents the absorption behavior of the two species, showing thus the possibility to be used in the solar cell as complex sensitizer.
1.1.3.3 Development of novel p-type conductors
In the frame of the Project different classes of materials were used as additives for the preparation of non-liquid or gel electrolytes for the development of XSCs. This activity was carried out mainly by UNIPMN and UNICAMP partners that prepared a wide class of materials with different chemical composition and structure. In addition, CRF also produced several types of anodic aluminas.
In particular, a wide spectrum of materials for the preparation quasi-solid electrolytes was developed. Novel syntheses of porous and lamellar solids with high specific surface area and complex architectures were attempted to tune structure, morphology (e.g. crystal shape) and textural properties (e.g. porosity and surface area). Structural (XRD, HRTEM, AFM, etc.), morphological/textural (SEM microscopy, gas physisorption) and spectroscopic (UV-Vis-NIR, IR, Raman, SS-NMR) characterization of all synthesised materials were performed. Diffraction and spectroscopic techniques used at variable temperature and under controlled atmospheres were also employed.
-Solids with non-structural porosity
Commercial F5 silica (from Azko Nobel) with high specific surface area (450 m2/g) was used as a reference solid for the preparation of non-liquid electrolytes. In addition, the surface of the silica was modified in order to increase the affinity with electrolyte species aiming at the preparation of stable electrolyte suspensions or gels. To reach this aim, the F5 silica surface was modified by introducing organic functionalities (in particular aminopropyl groups) trough grafting procedures.
Solids with completely different surface charge were prepared in order to investigate on the influence of this parameter on final performances of non-liquid electrolytes. In addition, nanosized alumina samples characterised by particle size of ca. 5-10 nm and high specific surface area (from 370 to 450 m2/g) were prepared using a modified sol-gel procedure. These materials were also functionalised to introduce on the surface the I-/I3- redox couple by post-synthesis methods by two different approaches. In the first approach, samples were treated with iodopropionic acid and then contacted with methyl imidazole to promote the nucleophilic substitution between the iodide and the imidazole group, obtaining iodide species as counterions. Finally, materials were contacted with an appropriate amount of a I2 solution in acetonitrile to obtain I-/I3- redox couple with a molar ratio of 10.
- Solids with structural porosity
Different families of mesoporous solids (MCM-41 and SBA-15 silicas) were synthesized and their surface modified to improve the affinity with the liquid electrolyte. Thermally stable mesoporous MCM-41 silica with hexagonal channel porosity (pore diameter of ca. 22 Å) with mean particle size around 200-300 nm was synthesized by supramolecular assembly according to the literature using cetyltrimethylammonium bromide (CTMAB) as structure-directing agent (Beck, et al. J. Am. Chem. Soc. 1992, 114, 834). Nanosized MCM-41 silica was also prepared according to literature data (K. Suzuki, et al. JACS 2004, 126, 462-463). The formation of a mesoporous silica phase with regular arrangement of hexagonal pores was detected by XRD diffraction studies. As indicated by TEM images, the mesoporous silica particles are characterized by a mean diameter of 20-50 nm.
In addition, SBA-15 silica, with well-ordered hexagonal array of one-dimensional channels, high pore diameter and hydrothermal stability were prepared according to methods proposed in the literature (D. Zhao et al., J. Amer. Chem. Soc. 120 (1998) 6024).
The synthesis method was also varied to allow the modification of SBA-15 morphology as recently proposed in the literature (S.-Y. Chen et al. Chem. Mater. 2004, 16, 4174-4180). This preparation is carried out by introducing a Zirconium precursor (ZrOCl2 ∗ 8H2O) directly in the synthesis gel, during the condensation of TEOS source. The presence of Zr precursor modifies the hydrophilicity/hydrophobicity of the Pluronic micelles used as structure-directing agents, whose morphology and dimensions are modified, and this have a driving effect on the final morphology of the SBA-15 particles.
Beside nanosized and mesoporous alumina, the preparation of anodic porous alumina was also carried out (CRF). Two different approaches have been investigated:
-Anodization of pure aluminum foil in order to obtain a thick (greater than 40 μm) self-standing porous alumina film to be sandwiched between the electrodes;
-Evaporation of aluminum on the glass/TCO/Pt counter electrode and subsequent anodization to obtain thin (1-20 μm) porous alumina film directly grown on the counter electrode;
Four different samples were prepared:
i) Self-standing alumina film made with phosphoric acid; thickness: 40 ÷ 50 μm; pores diameter: 200 ÷ 250 nm,
ii) self-standing alumina film made with oxalic acid; thickness: 40 ÷50 μm; pores diameter: 40 ÷ 60 nm,
iii) alumina film made with phosphoric acid grown on cathode; thickness: 40 ÷ 50 μm; pores diameter: 200 ÷ 250 nm,
iv) alumina film made with oxalic acid grown on cathode; thickness: 40 ÷ 50 μm; pores diameter: 40 ÷ 60 nm.
-Layered materials
Layered materials with neutral, positive and negative charged layers were used as additives for the preparation of non-liquid electrolytes for XSC purposes.
Both UNIPMN and UNICAMP research units were involved in this activity. More specifically, UNIPMN developed saponite and hydrotalcite materials, whereas UNICAMP prepared different kinds of organo-modified talcs and layered V2O5 and MoO3 materials.
Cationic clays: saponite
Cationic saponite clays, especially when in the form of nanosized particles, appeared particularly effective for improving the efficiency of liquid electrolytes.
During the INNOVASOL project, particular attention was given to the preparation of saponite sample with different chemical composition and particle size. The optimisation of a synthesis method allowing to reduce as much as possible the particle dimension of inorganic saponites and the preparation and functionalisation with I-/I3- redox couple of hybrid organic-inorganic saponites, aiming at improving the affinity of the solid with the liquid electrolyte, were carried out.
Inorganic saponite clays were prepared by hydrothermal synthesis by properly adapting the procedures reported in the literature (Kloprogge, et al. Clays Clay Miner. 1993, 41, 103). In the first 12M of the project it was found that the H2O/Si ratio used for the preparation of the synthesis gel influences the morphology and the aspect ratio of the produced materials passing from 200 nm particle size for solids prepared with H2O/Si ratio of 20 to ca. 50 nm for saponite obtained with H2O/Si ratio of 110.
Aiming at verifying the possibility to prepare a nanosized saponite by further increasing the synthesis gel dilution, the synthesis procedure was conducted by using a H2O/Si ratio of 150. As in the case of the other saponites, the obtained material was then submitted to a classical ion exchange procedure in order to introduce only Na+ ions in the interlamellar space. The XRD pattern diffraction of the novel sample was compared to those of samples prepared previously by using different H2O/Si ratio.
The adopted hydrothermal synthesis procedure affords in general high yield of layered product, with essentially low amounts of amorphous by-products. This fact is demonstrated by the XRD profiles showing for all samples the presence of reflections characteristics of natural saponite (H. Suquet et al, Clays Clay Miner. 23 (1975) 1).
The dilution of the gel mixture has a clear effect on the average size of the lamellae, being larger than 200 nm for SAP20 and less than 50 nm for SAP150. The latter sample is formed of very thin particles. Differences in the XRD profiles of the three samples can be then justified on the basis of the mean dimension of the platelets. Indeed, the vanishing of the basal (001) reflection when the H2O/Si ratio of the gel is different from 20 may be due to a scarce tendency of small sheets (all with comparable stiffness) to stack orderly and to assemble along the c axis.
The research work on saponite solids was devoted to the introduction, through different approaches, of organic species aiming at increasing the affinity of the clay to the electrolytic solution and at hosting I-/I3- redox couple to favour the occurrence of Grotthus-like conduction mechanisms. Solids containing the redox couple were tested for the preparation of quasi-solid electrolytes. Nevertheless, interesting effects related to the presence of the redox couple in the interlayer space were not observed, whereas interesting results were obtained for inorganic samples exchanged with Na+ ions.
Anionic clays: Hydrotalcite
Different hydrotalcite samples were prepared aiming to intercalate I-/I3- species. Particular attention was given to the development of two hydrotalcite solids characterised by different morphology and particle size. Microsized Zn-Al-LDH were synthetized using the urea method. The solid obtained was washed 2 times and dried in oven at 80°C; the formula was [Zn0.61Al0.39(OH)2](CO3)0.195*0.5H2O. The chloride form (hereafter ZnAl-Cl) was obtained by titrating the carbonate form. The solids were analyzed by XRPD.
The ZnAl-Cl was then contacted with an aqueous solution 1 M of potassium iodide under magnetic stirring for 1 day, using CO2-free and de-ionized water and working under nitrogen atmosphere in order to introduce I- species in the interlayer space. Finally, the ZnAl-I was equilibrated with 0.1 M solutions of iodine in different organic solvents: ethanol, diethyl ether, chloroform, acetonitrile, under magnetic stirring for 3 days, to favour the formation of I3- species. The solids obtained were recovered by centrifugation and washed two times with the solvent used for the intercalation and dried in an oven at 80°C. This procedure was repeated also using 0.5 M iodine solution in the above indicated solvents.
The presence of I-, I3- and polyiodide species was found by DR-UV-Vis spectroscopy.
Nanosized hydrotalcite samples containing nitrate anions (LDH-NO3) in the interlayer region were synthesized adapting the urea method. Zn(NO3)2.6H2O and Al(NO3)3.9H2O were dissolved in a mixture of water and ethylene glycol 1:2 (v/v). Solid urea was added at this solution so that the molar ratio urea/Al was equal to 4. The clear solutions were heated, under stirring at reflux temperature for 6 hours. The gelatinous material obtained was recovered by centrifugation and washed two times with water. Finally, the material was stored as dispersion in water and ethylene glycol 1:2 (v/v). The formula of the solid obtained was: [Zn0.74Al0.26(OH)2](NO3)0.26*0.5H2O.
Nanometric LDH-NO3 was converted in the corresponding chloride form and then exchanged with I- species, thus obtaining a solid named n-LDH-I with the following chemical composition [Zn0.74Al0.26(OH)2](Cl)0.08I0.18 • 0.43H2O. The n-LDH-I was contacted with a solution 0.5 M of iodine in acetonitrile in order to generate triiodide in the interlayer region. The solid functionalized has the formula [Zn0.74Al0.26(OH)2](Cl)0.08I0.18(I2)0.11 • 0.57H2O (I2 wt%= 61%). Interesting results were obtained using hydrotalcite-based solids especially for the preparation of gel-based electrolytes.
Neutral layered oxides
Layered V2O5 and MoO3 were prepared aiming to intercalate I-/I3- redox couple. V2O5 and MoO3 are neutral solids made of thin layers, which are attractive materials due to their easy reduction. The first step in the modification was to promote a controlled reduction of V and Mo sites using polyelectrolytes, thus producing I-/I3- intercalated structures (F. J. Quites et al., J. Colloid Interface Sci., 2012, 368, 462).
The conclusion of these studies was that although the insertion of the redox pair on polyelectrolytes-modified V2O5 and MoO3 improved the general conduction properties of the materials in a 5 wt.% suspension in methoxypropionitrile, it did not improve them beyond that obtained with the use of electrolyte only. This was particularly true for V2O5-based materials. For MoO3-based materials the situation was even poorer: besides diminishing the results for conduction, in PDDA composites, a short circuit was observed. The general conclusion, related to the vanadium and molybdenum oxides, was that these materials seemed interesting for application in solar cells but the idea of turning them into anion exchangers by using polyelectrolytes did not work as expected. In fact, the long PDDA and PAH chains inside the interlayer spaces might have improved the conduction properties of V2O5 and MoO3 because they provided amine groups that could transport charge, however, adding the redox pair improved properties even better.
The key issue in the case of these materials is that their stability under light was not adequate, in fact, loss of conduction properties of ca. 96-97 % in the case of vanadium oxides and 30-60% for molybdenum oxides makes them uninteresting materials for this application.
Neutral clays: talcs
Talc is a layered hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Its elementary sheet is composed of a layer of magnesium-oxygen/hydroxyl octahedra, sandwiched between two layers of silicon-oxygen tetrahedral (R. B. Ferreira, et al., Langmuir 2008, 24, 14215).
In that composition, the material is neutral and has no counterion in the interlayer space. Because of that it can be modified with pending groups, and suitable organic functions were introduced in the interlayer space in order to have different energies of interaction with the ionic pair I3-/I-. Two materials were initially developed, the propylamine-magnesium silicate and the propylethylenediamine-magnesium silicate. The diffraction profiles are typical of phyllosilicate-type materials. In the case of the sample with monoamine modification, the XRD profile, shows the 001 and the 002 diffractions at ca. 5o and 10o 2, indicating that this material is more organized than the diamine-modified one. In these samples the amount of organics is rather varied: in the monoamine sample every Si atom bears a pending group, that is where the 100% naming comes from. In the case of the diamine, only 8.6% of the total Si atoms has one pending group attached; keeping in mind that every pending group has two N atoms then the N/Si molar ratio is 17.2. In these cases, Solaronix measured 5 wt.% solid suspensions in ethoxypropionitrile and unfortunately few samples were miscible with the ionic liquid, therefore EPFL measured only three samples.
100% aminopropyl-modified talcs are not the best samples, probably due to diffusional characteristics of these solids, whereas with diamines the situation is different. The samples prepared have approximately the same amount of pending groups/total silicon atoms and their behavior is better than the ionic liquid. In this case the diffusion problem is strongly diminished because one pending group bears two N atoms. The case of slightly larger number of pending groups is the better sample.
Organized Silicate
Organized silicate as Na-RUB-18 was also used as a means to prepare a quasi solid gel to be employed as electrolyte in a spot XSC. The samples were prepared following the procedure already reported in the literature (M. Borowski et al., J. Phys. Chem. B 1997, 101, 1292-1297) with smaller adapting modifications.
In the case of this material, the measurements at a 5 % solids suspension in methoxypropionitrile are not as good as the ones obtained for the talcs. The sample was only 104 % efficient in relation to the liquid electrolyte, with high density of current (11.68 mAcm-2) but low voltage at open circuit (0.76 V), not much better than the talcs. As a matter of fact, the results obtained in gels, measured at EPFL, were really better also because they involved a procedure that is more adequate for fast preparation of cells. These quasi-solid conditions range from viscous liquids to very thick gels, difficult to work with in preparing the cells. Despite that, the materials prepared in this work showed excellent results when in viscous liquid conditions. The concentration of 7% of sample in Z952 ionic liquid seems to be the best compromise between performance and ease of handling. All these results point unequivocally to the fact that Na-RUB-18 is as good a sample to prepare the quasi-solid electrolyte for the XSCs as any of the talcs discussed in the beginning of this report.
On the basis of results collected at EPFL and Solaronix, F5 silica, talc, saponite and Na-RUB-18 were found to be the most promising additives for XSC quasi-solid electrolytes.
1.1.3.4 Transparent conductive oxide (TCO)
-Synthesis of Nanostructured ZnO thin films for XSC devices
Over the course of the project, UCAM made advances in the synthesis and characterisation of a variety of ZnO-based nanostructured thin films for use in XSC devices. During the three-year period, UCAM was able to demonstrate complete control over the length and properties of the ZnO active layers. The method utilised a solution-based growth procedure - preparation of a suitable substrate, sputtering of a seed layer and placement into an aqueous solution of metal salts and complexing agents. Through careful control of the substrate surface, seed deposition procedure and growth conditions, it was prepared an active layer that showed superior performance compared to many of the materials reported in the literature.
The synthetic method showed a number of unique features compared to other methods available. Firstly a seed layer deposited simply from a modified metal sputtering system was used. By simply and cheaply depositing a thin layer of zinc metal down on our substrate, and through careful control of the sputtering conditions, a film ideally suited for XSC devices was formed. This seed layer fulfilled two roles - not only did it provide nucleation sites for the growth of the NWs, it also acted as a blocking layer when the substrates were eventually used in XSC devices. Through careful investigation of the deposition conditions, UCAM was able to control roughness and grain size, which gave a complete control of the NW arrays that were grown subsequently.
Concerning the growth of the NW arrays, they were able to develop aqueous solutions that ensured highly crystalline ZnO crystals grew from the seed layer surface. This was achieved through careful control of the chemical composition of the solution, through the use of additives in the solution and by control of the heating and reaction timing. In this way, the length of the wires and the crystallinity of the structures were controlled.
One important aspect of the ZnO wires is the stability of their surfaces - inefficient chemical binding of the sensitisers (mainly developed for TiO2-based devices) and the presence of surface traps can often cause poor device performances. New methods to improve the surface chemistry of the ZnO by synthesising core-shell structures were therefore developed. Two methods were used for this - a solution method and an ALD method. Both had advantages and disadvantages. Nevertheless, both methods showed that device performances could be substantially improved with the use of thin shells.
-Investigating unique device architectures
During the course of the INNOVASOL project, UCAM was able to investigate the growth and mechanical behaviour of ZnO arrays directly produced onto compliant substrates. These stretchable ZnO islands were extremely robust, being able to withstand high stretching regimes without failure of electrical compliance. This work paves the way for developing nanostructured devices that are no longer rigid and can be incorporated into advanced architectures and applications. UCAM demonstrated, for the first time, a ZnO-based diode on a stretchable polymer.
1.1.3.5 Testing INNOVASOL materials for the preparation of XSC devices
Materials prepared in the frame of the INNOVASOL project were tested (alone or in combination) for the preparation of XSC devices. Tests were carried out both by researchers of EPFL and Solaronix SA. In particular, Solaronix activities were mainly focused to the testing of novel non-liquid electrolytes prepared by adding to methoxyproprionitryle-based electrolyte solids prepared by UNIPMN and UNICAMP and to the development of novel DSSC modules with improved efficiency. On the other hand, EPFL tested all materials produced over the course of the INNOVASOL project aiming at increasing the performances of laboratory scale devices by using novel and especially designed components.
-Assembly of XSC with novel ZnO-based photoanode electrodes
ZnO nanowires (NWs), various core-shell based ZnO NWs core and doped ZnO nanowires were examined as photo-anode electrodes in a complete XSC. EPFL was responsible on the assembly of the cells and their photovoltaic characterization more than 200 cells were made in this task.
In the beginning, the suitable dye for the ZnO NWs photo-anode was found. Four organic dyes C101, B11, C205 and C218 were selected because of their high molar extinction coefficient and spectral response in the visible region and tested using liquid electrolyte. C218 dye gave the best PV results. The Z960 electrolyte was used for all cells, because this electrolyte gave the best efficiencies in XSCs and good stability using C218 sensitizer.
Therefore, the rest of the experiments were made using with the C218 dye and the Z960 electrolyte.
The best performance was achieved using long NWs. The longer ZnO NWs reached 10m length.
Using those long ZnO NWs an efficiency of 1.25% at 1.5AM (full sun conditions) was achieved. This is one of the highest efficiency in the literature for bare ZnO NWs cell.
Improvements in device performance for ZnO NW-based photovoltaic devices were observed when the NWs were coated with thin layers of semiconducting or insulating materials. The purpose of these shells is to provide a more stable binding site for dye absorption and suppress carrier recombination. Core-shell structures of ZnO-TiO2 were synthesized using ALD methods (see deliverable D3.3) allowing precise control of the shell thickness coating the NW surface.
By trying different TiO2 shell thickness such us 10nm, 20nm and 40nm it was observed that the TiO2 shell thickness of 20 nm gave the best photovoltaic performance using 1m thickness of ZnO/ TiO2 core shell film. Using this finding 10m long core ZnO NWs covered with 20nm TiO2 shell, thus creating ~10m thickness ZnO/TiO2 films, were made.
-Molecular relays - Energy transfer between Squarine dyes and CdSe QDs
EPFL was responsible of the assembly of a solar cell composed of squarine dye as acceptor and CdSe QDs as donor. This assembly shows a simple structure of FRET system inside a XSC device. The donors are CdSe QDs, which have broad absorption spectrum in the visible region. The acceptor is a molecularly engineered squaraine sensitizer (and molecular relay), the VG1-C10 prepared by UNITO, that has an additional carboxylic acid group and two long carbon chains compared to the standard squaraine dye. The presence of two carboxylic acid anchoring groups, and the hydrophobic long chains provides better dye stability and allows efficient energy transfer from the high energy QD's to squaraine sensitizer. The use of cobalt complex (Co2+/Co3+) as electrolyte in these cells permit direct contact between the QDs and the electrolyte. Moreover there is no need to change the original ligands of the QDs prior to deposition; the two C10 chains of the dye molecules and the oleic acid ligands coated the QDs provide the optimum distance for FRET, hence the preparation and the structure of the cell are simple. As a result of the energy transfer the cell power conversion efficiency was increased and its solar response was expanded from the visible to the near infra-red.
-Assembly of PbS QDs/TiO2 heterojunction solar cells
PbS QDs were used as light harvesters in conjunction with films composed of 18nm-sized TiO2 nanocrystals containing both meso-and macropores. Also, at the same time the PbS acts as a hole conductor, rendering superfluous the use of an additional p-type material for transporting positive charge carriers.
In order to increase further the photovoltaic performance of the PbS QDs solar cells, nanosheets of anatase TiO2 were tried. The TiO2 nanosheets have an exposed (001) facet, which is different than the normal anatase TiO2 nanoparticles (NPs) which have (101) dominant exposed facet. Researchers have shown that the (001) facet has a higher surface energy than the (101) facet therefore their surface is more reactive. Thin films of those nanosheets were deposited by spin coating on the FTO glass.
Two sizes of PbS QDs were tested, energy gap (Eg) of 1.38eV and Eg of 1.24eV and two sizes of TiO2 nanosheets, 30nm and 80nm, were tested as well.
The best results were achieved with 30nm size of nanosheets and PbS QDs with Eg of 1.38eV. The power conversion efficiency in this case was 4.73%, which is one of the highest reported in literature.
-Assembly of XSC cells with novel quasi-solid electrolytes
A pre-screening of materials for quasi-solid electrolytes was done at Solaronix by assembling spot cells preparing 5wt% suspension of solid in methoxyproprionitrile based electrolytes. Best results were obtained by using talc and saponite clays as additives for electrolyte.
Moreover, gels (quasi-solid electrolytes more suitable for industrial preparations) were prepared by dissolving 20% wt of powder in ionic liquid. EPFL was responsible for the cell architecture, assembling, dyes adsorbing, J-V and IPCE measurements.
The cell composed of working electrode, glass covered by FTO, and 12μm TiO2 nanocrystals layer made in EPFL. Counter electrode, TEC 15 glass covered by FTO, and Pt.
All cells were made using gel electrolytes, which were applied by a doctor blading technique.
The first step was the screening of all the materials using the conditions mentioned below:
(1) Z907 dye;
(2) 20%wt of the material was mixed in the ionic liquid electrolyte in order to get the gel;
(3) Doctor blading technique to spread the gel on the dye-sensitized TiO2 film.
34 materials were screened, and based on the photovoltaic results the best materials were chosen:
i) F5 Silica particles,
ii) Hydrotalcites,
iii) Nanosized saponites ,
iv) Layered silica NaRub-18,
v) Talcs.
In order to further optimize the cell performance few changes in the cell composition were made: organic dye Y123 was used, transparent TiO2 layer of 5.7 m thickness, different loading of the powder in the ionic liquid electrolyte.
Multiple - Exciton Generation
According to equation 1 it is possible to calculate the Light Harvesting Efficiency (LHE) of the QDs film from the absorbance spectra.
(1)
The absorbed photons-to-current efficiency (APCE) values taking into account the light- harvesting efficiency (LHE), or light actually absorbed by the monolayers of QDs. The APCE can be calculated according to equation 2.
(2)
The APCE values are over 100% for photon energies of 2.8eV - 3.1eV which are around 2 times the QDs Eg. Those APCE values, which exceed 100%, can suggest the possibility of multiple exciton generation (MEG) effect in PbS QDs solar cells.
1.1.3.6 Preparation of XSC modules
During the first period of the project, materials such as ruthenium dyes, organic dyes, titanium oxides, conductive SnO2:F (FTO) coated glasses and electrolyte compositions were investigated to build the best testing device for the project's materials. This activity was mainly carried out by Solaronix SA Company.
All efficiency data were measured at 1000 W/m2 Class A light at 25°C with black tape masked cells.
Various electrolyte compositions have been studied for their performance. The best electrolyte was still the "AN50", i.e. an acetonitrile based electrolyte with 50 mM tri-iodide, followed by "Z-946" and "R150", which are based on 3-methoxypropionitrile, giving best performances in stability tests thanks to their low volatility compared to acetonitrile.
Four types of ruthenium based dyes and one pure organic dye (squaraine SQ2) were compared in spot-cells. Following obtained results the best dye was still Ru535 bisTBA (a.k.a. N-719) and the blue-green coloured squaraine dye gave an appreciable efficiency of 3.2 % in the spot-cell.
The relative size of the light blocking mask is crucial for consistent efficiency measurements with small solar cells. To avoid large errors, the mask is always 1 mm wider than the TiO2 active area, resulting in a relative mask area of a factor of 2. The choice of the conductive glass was also critical for performance improvements, thus switching from FTO22-7 (a 7 ohm/sq 2.2 mm float glass) to Asahi U (a 1 mm thick 10 ohm/sq FTO glass) gave nearly 10 % relative increase in output.
Titania screen printable paste made using narrow titania particle size distribution presented:
-bad adhesion properties that could be attributed to the formation of a compact film on the glass substrate.
-high viscosity after few days. This high viscosity leads to a bad printability of the paste and a poor quality of the film formed.
The unfavorable adhesion properties led to the impossibility to perform a post TiCl4 treatment. The latter is essential for best cell performance.
To solve these issues, the particle size distribution has been tuned by introducing a small amount of larger particles while keeping the distribution centered around 18 nm (optimum size for DSSC).
Introducing these bigger particles allowed to control the film porosity. TiCl4 treatment was performed on the formed film without delamination. The treatment led to a dramatic increase in the cell performance.
Summing up all these new improvements, the best test cell gave 9.4 %, while keeping the process compatible with industrial manufacturing.
Once this spot cell based testing tool was setup, the various materials supplied by the projects partners were evaluated in «real» conditions. The results concerning the screening these electrolyte additives are presented in detail in Deliverables 2.4 2.6 and 4.2.
In parallel to the work on new electrolyte materials, SOLARONIX developed liquid and gel electrolyte based prototypes. These prototypes consisted in serially interconnected dye sensitized solar cells to form a module. W-type module were first investigated. W-type modules are semi-transparent bifacial devices where the counter and the working electrodes are alternately printed on the same side creating the serial connection with the other side.
As the titania and platinum counter electrodes are printed on the same glass, the platinum is usually poisoned during the staining process. This deactivation of the catalytic platinum led to poor fill factor and low final performance (around 3% using liquid electrolyte). SOLARONIX worked on improving liquid based devices before introducing the use of suspension and gel electrolyte.
To avoid platinum layer degradation a selective staining process was setup. The first approach was to protect the platinum layers using low adhesion adhesive tape. This approach allowed a dramatic increase in the final module performance but this method is time consuming and not easy to scale up. A modification of this strategy was finally used. A silicon mold was designed to allow titania selective staining. This mold was easy to put in place (less time consuming) and the platinum protection was effective. This approach is scalable to any module size.
Summing up the previously described improvement and all the enhancements developed earlier in INNOVASOL, the state of the art W-module (10 x 10 cm, 65 cm2 active area) based on liquid electrolyte and N-719 dye reached a high power conversion efficiency (PCE) of 7.86 % (Voc = 8.3 V; Isc = 106 mA) under one sun (AM1.5G). This device was measured at various sun intensities to evaluate the performance in situation matching real outdoors conditions. Under one third of sun the PCE was as high as 9.19 %.
This high efficiency is impressive especially considering the fact that the titania layer was only around 8 microns thick, semi transparent and the module made with only industrially available raw materials.
Liquid electrolyte cell stability was tested. Cells made with Z-907 dye and Z-946 electrolyte, were placed under constant illumination, one sun AM 1.5G at 55 °C during the first 4000 hours and at 65 °C for the rest of the experiment. All cells exhibited good stability with less than 2% efficiency loss after 1000 hours and around 20% after 8000 hours in these conditions.
The stability of some W-modules was tested in the same experimental conditions than the ones previously described. The module exhibited good stability over 2000h. Some defects in the sealing finally led to the module death after this time.
Further work consisted in developing thin glass based modules to build INNOVASOL prototype. The idea was to apply the process previously developed for high efficiency modules to hundred microns thick flexible glass.
A new design to fit CRF specifications was created and consisted in an interconnected module of seven individual cells on a 7 x 10 cm substrate. Several steps needed to be optimized for this thin glass like: i) FTO deposition, ii) Two glass lamination, iii) Hole drilling, iv) Titania printability and adhesion.
To conclude, INNOVASOL led to several achievements:
-Test cells went from around 7% to above 9%.
-Ionic liquid cells went from around 2.5% to above 6%.
-W modules went from around 3% to around 8% and even above 9% under 1/3 of sun.
-First thin modules at Solaronix were obtained.
-High stability cells were obtained (one year under 1 sun during 24h/day, corresponding to over 5 years of outdoors conditions).
All these results were obtained using only industrially available raw materials or techniques and summing up the various improvements developed during INNOVASOL.
Potential Impact:
INNOVASOL socio-economic potential impact - Automotive application
The INNOVASOL objective is to implement novel XSC devices to harvest the sun energy. Even if the efficiency is lower respect to crystalline Si cells, the superior performances of this kind of modules are:
-efficient temperature-independent in the range 2565 ?C (Si-based declines by approximately 20%).
-light capture less sensitive to the angle of incidence (indoor possible integration).
-more efficient than pc-Si in diffuse light or cloudy conditions.
-printable resulting in low cost and easy fabrication (below 1 EURO/W) even on flexible substrates.
The capability to produce energy in any light intensity condition is particularly important for transportation allowing the integration of these cells also on interiors surfaces with above 2m2 surface available (medium size car); this results in potential production of 250-400Wh considering average efficiency 4% and 8h exposition under diffuse light (300W/m2).
A further potential impact of this innovative energy harvesting approach on transportation is that a more constant power output respect to the other Si-PV (changing T and light intensity) results in a easier power management for the charging of batteries; this impacts positively also on battery lifetime and costs.
The application on vehicle exteriors (typically solar roof) and interiors (dashboard) of large surface solar cells built with INNOVASOL materials can bring significant social and environmental advantages in terms of:
-reduced overall fuel consumption at the European level
-reduced CO2 emissions in terms of g/km emitted aiming at complying with more severe regulations
The demonstration of the potential impact of INNOVASOL concept requires to calculate the current cost of the kWhel produced by the alternator allowing to estimate the financial and fuel savings expected and the reduction of CO2 emissions (greenhouse gases).
INNOVASOL Socio-economic impact (solar cell production)
Thanks to the novel materials developed during the INNOVASOL project, such as printable 'gellified' electrolytes, ionic liquid compositions showing excellent thermal stability, it became now possible to produce XSC modules using an all printed technology, giving for example semi-transparent modules that could find their way into the automotive application.
Taking the example of a 7 x 10 cm solar module powering a convenience light in a car, and a estimated volume of 500000 modules per year, the cost estimation shows that such a rather small production may produce customized XSC for less than 4 EUROS per module.
The cost structure shows that labor and materials are making up most of the expenditures, whereas the cost associated with the production equipment is only 15 %, as such a production line is mostly based on printing, heating and lamination techniques well known to industry.
List of Websites:
The address of project website is http://www.innovasol.eu
INNOVASOL project aimed at developing radically new nanostructured materials for photovoltaic (PV) excitonic solar cells (XSCs) really competitive with traditional energy sources. The target was to leapfrog current limitations of third-generation PV devices through a drastic improvement of the materials used for assembling XSCs (with particular attention to light absorbers, hole transport materials and electron transport solids). Taking into account that the state of the art DSC device was 11% efficiency, INNOVASOL XSC device target was to reach 11-15% efficiency.
As a first activity, benchmarking analysis to evaluate the effect of commercially available organic dyes, titanium dioxide, conductive SnO2:F (FTO) coated glasses and electrolytes on the efficiency of laboratory "spot-cells" (being typically between 0.16 and 0.64 cm2 in size) was carried out: this activity was fundamental to measure the performances of new INNOVASOL materials in a reproducible and comparable fashion. A detailed analysis of literature data and their comparison with preliminary results of INNOVASOL project was carried out especially during the first year of the project, in order to select the most promising candidates for the core materials of the new XSC devices.
In particular, the screening involved four classes of XSC materials:
i) quantum dot light absorbers (QD),
ii) molecular relays,
iii) hole transport materials,
iv) mesoscopic electron transport materials.
In this respect, development of innovative preparation routes allowing the optimization of new core materials needed to increase the actual performances of XSC devices.
Project Context and Objectives:
INNOVASOL project aimed at the development of new nanostructured materials for photovoltaic (PV) excitonic solar cells (XSCs) really competitive with traditional energy sources. The main objective of the project was to overcome current limitations of third-generation PV devices though a drastic improvement of the materials used for assembling XSCs.
A combination of radically new materials in novel solar cell architectures was proposed in the frame of the project to overcome the current limitations of dye sensitised solar cell (DSSC) devices. The research activity involved the following tasks:
i) use of semiconductor quantum dots (QDs) as the light harvesting units with a fine tuning of the optical cross section and of the band gap (by modifying both the chemical composition (e.g. PbS, PbSe, PbSe/PbS core/shells etc) and the size of the nanoparticles);
ii) synthesis of suitable amphiphilic dye molecules (e.g. cyanines, squaraine dyes, zinc phthalocyanine and dipolar stilbene-like dyes), designed to act as molecular relays (MRs) that connect the QDs to electron conductor materials;
iii) development of novel solid-state electrolytes (and quasi-solid electrolytes) as p-type hole conductors, such as layered or porous materials suitable to host anionic redox couples (e.g. I-/I3-);
iv) development of specifically designed n-type semiconductor, e.g. the mesoscopic oxide film or the transparent conductive oxides (TCO), such as ZnO nanorods or TiO2 nanofibers and nanotubes with architecture, morphology and surface structure suitable to maximise the efficiency of the charge transfer processes at the QD/MR/SC interface and electron transport in the nanocrystalline TCO films.
Innovative materials and device architectures were designed to have low production cost, high stability to UV and high light harvesting efficiency.
QDs had already demonstrated to be very efficient in photon capture, with optical cross sections which can be far greater than the molecular dyes currently used in dye-sensitized solar cells. A revolutionary improvement is feasible by multiple exciton generation (MEG) processes, where more than one exciton is generated by a single energy photon, leading to quantum efficiency larger than 100%. Such processes were observed in colloidal silicon QDs, and in small band gap IV-VI semiconductors like PbS and PbSe. INNOVASOL project explored this radically new approach to increase the XSC performance, by designing, producing and testing suitable QDs and QD/MR composite materials.
The photon capture by the QD followed by energy transfer to the molecular relay and the subsequent electron injection from the excited relay into the charge carrier conductors were carefully tuned, to be competitive with thermal recombination, by judicious engineering the QD/conductors interface. A striking improvement was demonstrated by an unprecedented architecture of supramolecular (self-organised) hierarchical nanostructures, where the QDs are covered by self-assembled monolayers (SAM) of amphiphilic dye molecules as basic light harvesting units mimicking the photosynthetic antenna system. In this configuration, the dye molecules act as molecular relays (MRs), connecting the photon capture unit and the TCO electron conductor, i.e. the negative electrode of the PV cell. The photogenerated excitons transfer their energy to the MRs by Förster resonance energy transfer (FRET) mechanism through dipole-dipole interactions (i.e. without the need of electron transfer), then the MRs inject electrons into the oxide. INNOVASOL designed, synthesized and tested several MRs in order to select the most photostable and efficient, e.g. the ones matching their HOMO-LUMO energy differences with both the QDs band gap and the conduction band of TCO semiconductor at the best.
In order to select the most promising materials for light harvesting and electron injection, the effects of specific physical and chemical modifications (i.e. the size of the NCs, the presence of ionic liquid electrolytes within the layered or porous solids, the changes of the material properties at the different layer interfaces, etc.) were reproduced. The properties of interest could be modelled as a function of the physical and chemical nature of the systems. State-of-the art computational procedures were adopted, even combining different approaches (since the nature of the INNOVASOL XSC is intrinsically multi-phase, and required different theory levels for the different parts of the devices) in newly developed mixed computational procedures.
Beside the development of innovative materials, partners performed an accurate screening of the electronic properties of QDs already available, in order to identify the most promising light absorbers. In addition, a screening of the proposed molecular relays to explore their binding on quantum dots and thin conductive oxide surfaces and their role in the energy transfer surfaces was accomplished.
The innovative materials developed by the project partners were accurately characterized by spectroscopic and structural analysis, and modelled with suitable theoretical procedures (based on various computational codes, capable of describing each layer of the composite material with the suitable level of theory), in order to find the most promising candidates for the XSC devices developed in the project. In addition, theoretical and computational techniques have been designed and exploited in order to support synthesis and characterization of new materials.
Finally, INNOVASOL aimed at substituting the liquid electrolyte with radically new solid-state (or quasi-solid) conductors, in order to reduce potential environmental risks and ensure a much greater stability in outdoor operating conditions. The project designed and produced a very wide spectrum of radically new materials and nanomaterials with porous or layered structure, high specific surface area and complex engineered architectures, which were employed to host redox active species. During the first year of the project some key materials were identified and tested for the final XSC devices: a commercial porous silica, two families of synthetic clays, such as saponite and organo-modified talc, and a synthetic layered silicate known as Na-RUB-18.
The technological objective of INNOVASOL was the integration of innovative materials in XSC devices to be used as environmentally clean and renewable electric power source, which may pave the way for long term applications, such as automotive roof panels, transparent windows and civil/industrial buildings requiring stringent performances (long lifetime, high stability, high efficiency and high relative humidity resistance). These were very ambitious targets, which could lead to highly relevant societal impacts.
One of the most relevant technological objective was to optimize process strategies suitable to integrate radically new materials in proof-of-concept devices, which were strictly needed for testing the INNOVASOL materials and composites. This was essential for understanding that a real upgrade in the properties of the materials was being reaching, and for developing radically new paths leading to highly innovative, long term research in the field of materials for solar cell applications.
The competitive cost/efficiency ratios of INNOVASOL materials could be maintained (or even improved) developing efficient synthesis approaches and surface functionalization methods thus to propose reliable applications of XSC devices on a large scale by Solaronix partner. Hybrid approaches, liquid and vacuum based, were optimized to deposit the active materials with suitable morphologies for enhanced electro-optical properties. Both new self-assembly (e.g. layer-by-layer) techniques and low cost deposition and patterning processes (e.g. spin coating, ink jet) were used. Controlled nanostructured active layers were developed using hydrothermal and CVD deposition processes.
Project Results:
The main scientific and technological results obtained during the three-year's project are reported in this section in relation to the different type of materials/activities that the Consortium developed in the frame of the Project. It is worth noting to recall that DSC state of the art in 2009 (at the beginning of the Project) exhibited ca. 11% efficiency and that INNOVASOL XSC device target was to reach 15% efficiency, together with an enhancement of the device lifetime. These targets were reached by preparing materials used as molecular relays, n-conductors and quasi-solid electrolytes suitable for system integration. In addition, attention was paid to decrease cost production of the materials (e.g. by upscaling material preparation).
In the sections below, the main results obtained in the frame of the Project are reported following the work plan of the INNOVASOL Project (i.e. Work package organisation).
1.1.3.1 Development of computational procedures for modelling of molecular relays and nanostructured composite materials (activity related to WP1)
According to INNOVASOL project, high level theoretical modelling was joined to the experimental activity in order to: 1) help the pre-screening of materials on the basis of their predicted relevant properties; 2) orient the synthesis of new materials with tailored physico-chemical characteristics; 3) support the interpretation of the experimental data, obtained by spectroscopic, electrochemical and other techniques.
To achieve this goal, the preliminary step was the development and implementation of a multiscale computational protocol, able to integrate the different codes and programs available to the partners in a common framework. Such a development led to support and/or development of the following activities:
- Computational procedures provided by partners
1) Particle-in-a-box, developed and parameterized at TUD allowing for the description of optical properties of very large nanoclusters of different chemical composition.
2) Semiempirical approaches with different parameterizations, namely PM3, MNDO, AM1 as coded in Gaussian03 package, PM6 in MOPAC2009, MSINDO in MSINDO3.3: these codes are available at UNIPMN and UNITO, where they were locally modified.
3) Density Functional Theory (DFT) methods with various functionals and basis sets, through Gaussian03 and TURBOMOLE6 packages, available at UNIPMN and UNITO.
4) DFT methods with periodic boundary conditions, for the modelling of crystal structures, with various functionals and basis sets, coded in CRYSTAL06 package, available at UNIPMN.
- Parameterization and optimization of the procedures
1) The particle-in-a-box parameterization was performed in the first months, and the method was already applied to some of the nanoclusters of interest, to help selecting the most promising materials.
2) To ensure the transferability of results, the databases of TURBOMOLE and CRYSTAL were modified, so that the density functional definition (B3LYP) and the atomic basis sets (Pople 6-31 set plus polarization and diffusion functions, and Hay-Wadt pseudopotential with double-z basis sets) match exactly those defined in Gaussian03.
3) The present MSINDO parameterization was verified to ensure that it could be suitable to describe the systems of interest.
- Integration of the procedures in a multiscale protocol
The developed multiscale procedure is based on the combination of different levels of theory in the same calculation, following the scheme called ONIOM in Gaussian suites: this is expected to be crucial for the description of structural and optical properties of molecular relays adsorbed onto the nanoclusters, and the electron transfer between nanocrystals, relays and electron-conducting layers.
The two theory levels selected for such a combined approach are DFT with localized basis sets and hybrid functionals (as implemented in Gaussian03) and semiempirical with up-to-date parameterization (namely MSINDO and PM6 implemented in MOPAC2009).
1.1.3.2 Synthesis and characterisation of nanomaterials (activity related to WP2)
a)Quantum dot absorbers
During the project implementation, the TUD activity was focused on the colloidal synthesis of semiconductor nanocrystals, their assembly and the grafting of molecular relays to the nanocrystals obtained. Applying an aqueous approach, a facile one-step synthesis of near infrared (NIR) absorbing and emitting CdTe and alloyed CdHgTe quantum dots (QDs) was developed (Lesnyak et al., J. Mater. Chem. 2009, 19, 9147-9152). Focusing on the current demand for low-toxic nanocrystalline materials, blue emitting alloyed ZnSeTe QDs was also obtained employing a one-pot synthetic protocol (Lesnyak et al., Chem. Commun. 2010, 46, 886-888) Furthermore, chemically non toxic SnS and FeS nanoparticles were produced by means of a hot-injection synthetic technique, and the non-injection synthesis of ternary CuInSe2 and quaternary Cu2ZnSnS4 nanocrystals was also explored. These materials were designed to act as the light absorbing layer in the XSC devices instead of, or in combination with, the organic dyes currently used in the present generation of dye sensitized solar cells. However, after being tested in solid state excitonic cells at EPFL neither Cu-containing nanocrystals, nor iron or tin sulfides provided reasonable light harvesting efficacy.
-Self assembly of quantum dots
Since one of the most facile and straightforward self-assembly technique is the attachment of various components using chemical binding (Gaponik, N., J. Mater. Chem. 2010, 20, 5174-5181) the work in this task concentrated on establishing a number of linking procedures that allowed the efficient and irreversible binding of QDs to transparent conducting substrates based on ZnO and TiO2. To achieve this goal, monodisperse PbS, ZnO and TiO2 nanoparticles were synthesized and characterized using transmission electron microscopy, X-Ray diffraction and optical spectroscopy. A great deal of emphasis was placed on the role of the linker and it was shown that the linker is a major key to the understanding of the charge carrier transfer processes and has an important influence by not only transferring or blocking the electron and hole intermediates in such systems but through its own inherent and often substantial contribution to the optoelectronic response. To gain a greater understanding concerning the complex role played by the linker, a number of different molecules with different functional groups and different alkyl chain lengths were used to connect the QDs to ITO/TiO2 and ITO/ZnO substrates, the attachment of the linkers and nanocrystals being verified by spectroscopic methods, and the optical and optoelectronic response studied using cyclic voltammetry, electrochemical impedance spectroscopy and photocurrent spectroscopy (Krüger et al., J. Phys. Chem. C 2011, 115, 13047-13055). Additionally, analysis of the substrates was carried out by using UV-Vis, Fourier-Transform Infrared Spectroscopies and X-Ray diffraction to give a conception of material composition and the nature of the molecular contact characteristics. The obtained results provided important insights into the role of the linker molecules at the electrode interface in the mediation of charge transfer, which allowed for further improvement of the performance of QD sensitized solar cells and hybrid dye/QD based cells.
-Grafting of molecular relays to quantum dots
TUD group fabricated and studied several different composites of CdSe and CdSe/ZnS QDs with specially designed red-NIR squaraine dyes (VG0, VG1, VG1S6, VG2) provided by the UNITO partner (vide infra). In order to prove the concept of a molecular relay architecture and to investigate its energy transport properties, squaraine dye-QD composite films deposited onto quartz glass plates, QD-squaraine-ZrO2 and QD-squaraine-Al2O3 systems as analogues of the TiO2 based architecture were prepared and characterized by means of optical spectroscopy (absorption, steady state and time resolved photoluminescence measurements). In these composites the QDs complemented the optical properties of the dyes acting as additional light absorbers covering the UV-Vis region of the solar spectrum and thus were capable of energy transfer to the dyes. This study allowed to probe these systems and revealed a number of energy relationships between the various components. In all the systems investigated energy transfer from the donor (QD) to the acceptor (dye) was observed.
b) Design and preparation of novel molecular relays
The acitivity of UNITO unit was mainly devoted to the design, synthesis and characterization of several NIR dyes in order to select the most interesting structures to be used by the consortium partners in conjunction with the appropriate QD sensitizer. Moreover, the unit was also directly involved and responsible for the preparation and characterization of composite nanostructured materials made of MRs linked to QDs and their characterization. In particular, the following activities were carried out:
1.Design and synthesis of a variety of polymethine dye structures (mainly squaraines) with absorption spectra with maximum wavelengths ranging from 640 nm to 850 nm.
More than 30 new different structures were synthesized and fully characterized resulting in an optimal tuning of optical and electrochemical properties, number and position of anchoring groups, length of the alkyl protecting chain. The synthesis of all these new structures was settled up firstly with standard synthetic methods, but during the project new methods based on a microwave approach were exploited.
This research resulted in the overcoming of a series of excellent synthetic challenges:
i) number of reaction steps reduced (it is no more necessary the hydrolysis of emisquarate intermediate);
ii) yield of each step raised up for all dyes, reaching or overcoming in most of them the value of 75%;
iii) reaction time of each step reduced from day and hours to tenths of minute and minutes, respectively;
iv) purification performed, mostly of the time, only by simple washing and crystallization avoiding the expensive use of chromatographic techniques, giving in all cases completely reproducible dyes with high purity ;
v) synthesis of all intermediates and dyes up-scaled from milligram to multigram level.
-Identification of a new target in NIR dyes for DSSC: a low cost highly efficient symmetric sensitizers
The use of organic sensitizers, respect to standard Ru-based dyes, in DSC is mainly due to their larger molar extinction coefficients (30 000-300 000 M-1 cm-1), extended spectral sensitivity even in the NIR light portion and simple preparation and purification, offering lower cost, while exhibiting excellent characteristics. A number of Ru-free dyes have so far been reported in the literature; among them, squaraines are well known for their intense, but sharp absorption in the red/near-IR regions that can be tuned to the desired long-wavelength region, while exhibiting a considerable photo- and thermal stability. The common idea in literature until last year was to focus on asymmetrical rather than symmetrical squaraines, in order to favor a directionality of the charge-transfer band in the excited state. Thanks to the work done during this project UNITO was able to demonstrate that very simple symmetric dyes show promising photovoltaic properties, sometimes even superior to standard NIR dyes, but with undoubtedly advantages in the low cost and easiness of synthesis for symmetrical structures. In fact, symmetrical and unsymmetrical squaraines have different characteristics and properties, but they can be both excellent sensitizers for DSC, giving comparable efficiency in the cell. In fact, the number of anchoring groups, as well as the length and the nature of alkyl chain, play an important role as much on optical properties as on cell efficiency. The symmetrical dye can be obtained according to a two-step procedure using commercially available precursors. This easiness of synthesis comes in opposition to the unsymmetrical, which requires five steps combined to two chromatographic purification procedures. Worth noting that symmetrical dyes can be easily purified by crystallization, a feasible process for upscaling. A DFT optimization of molecular structures for both symmetric and non-symmetric dyes was performed to gain insight into the energy levels of the different conformations (trans-cis) and into the nature of the excited states involved in the electronic injection. In the case of the symmetric dye the cis conformation has higher polarity (4.05 Debye), thus indicating a high charge separation and an electron density anisotropy comparable to that of the asymmetrical model dye.
-Identification of a suitable chain able to interact with Quantum Dots in XSC
Another important point in the INNOVASOL project deals with the preparation and characterization of composite nanostructured materials made of MRs (squaraines) linked to QDs. These materials should be then used as 'complex sensitizers' in a XSC based on cosensitization/FRET principle. Firstly mercapto groups as possible linkers for QDs were selected and an organic colorless linker for PbS QDs synthesized and used in a solid state cell. After this first step, as far as MRs is concerned, mercapto groups were added to the end of the alkylic chains, but their use unfortunately was not successful due to the formation of unreactive intramolecular disulfide bonds. Beside this, a deep investigation on a series of symmetric squaraines bearing chains of different length was done in order to find the better FRET couples with suitable QDs. Thanks to these measurements done both in solution, in solid state and on TiO2 electrodes it was found that appropriate chain lengths (i.e. C10) may lead the formation of van del Waals molecular assemblies exploitable in FRET XSCs.
-Identification of a simple reaction able to make a stable covalent bond between squaraines (Molecular relays) and Quantum Dots
Finally in order to take advantage of the synergic interaction between MRs and QDs, a simple reaction able to create a covalent bond between QDs and a squaraine was studied. A good compromise was found introducing azido ligands at the end of the QDs synthesis and a triple bond at the end of one of the alkyl chain of a symmetric squaraine. In the case of multiple triple bonds on the organic moieties, olygomeric insoluble structures were formed during the reaction with QDs nanocrystals; however, in the presence of a single anchoring (to the QD) moiety the composite material is still soluble in alcohols and presents the absorption behavior of the two species, showing thus the possibility to be used in the solar cell as complex sensitizer.
1.1.3.3 Development of novel p-type conductors
In the frame of the Project different classes of materials were used as additives for the preparation of non-liquid or gel electrolytes for the development of XSCs. This activity was carried out mainly by UNIPMN and UNICAMP partners that prepared a wide class of materials with different chemical composition and structure. In addition, CRF also produced several types of anodic aluminas.
In particular, a wide spectrum of materials for the preparation quasi-solid electrolytes was developed. Novel syntheses of porous and lamellar solids with high specific surface area and complex architectures were attempted to tune structure, morphology (e.g. crystal shape) and textural properties (e.g. porosity and surface area). Structural (XRD, HRTEM, AFM, etc.), morphological/textural (SEM microscopy, gas physisorption) and spectroscopic (UV-Vis-NIR, IR, Raman, SS-NMR) characterization of all synthesised materials were performed. Diffraction and spectroscopic techniques used at variable temperature and under controlled atmospheres were also employed.
-Solids with non-structural porosity
Commercial F5 silica (from Azko Nobel) with high specific surface area (450 m2/g) was used as a reference solid for the preparation of non-liquid electrolytes. In addition, the surface of the silica was modified in order to increase the affinity with electrolyte species aiming at the preparation of stable electrolyte suspensions or gels. To reach this aim, the F5 silica surface was modified by introducing organic functionalities (in particular aminopropyl groups) trough grafting procedures.
Solids with completely different surface charge were prepared in order to investigate on the influence of this parameter on final performances of non-liquid electrolytes. In addition, nanosized alumina samples characterised by particle size of ca. 5-10 nm and high specific surface area (from 370 to 450 m2/g) were prepared using a modified sol-gel procedure. These materials were also functionalised to introduce on the surface the I-/I3- redox couple by post-synthesis methods by two different approaches. In the first approach, samples were treated with iodopropionic acid and then contacted with methyl imidazole to promote the nucleophilic substitution between the iodide and the imidazole group, obtaining iodide species as counterions. Finally, materials were contacted with an appropriate amount of a I2 solution in acetonitrile to obtain I-/I3- redox couple with a molar ratio of 10.
- Solids with structural porosity
Different families of mesoporous solids (MCM-41 and SBA-15 silicas) were synthesized and their surface modified to improve the affinity with the liquid electrolyte. Thermally stable mesoporous MCM-41 silica with hexagonal channel porosity (pore diameter of ca. 22 Å) with mean particle size around 200-300 nm was synthesized by supramolecular assembly according to the literature using cetyltrimethylammonium bromide (CTMAB) as structure-directing agent (Beck, et al. J. Am. Chem. Soc. 1992, 114, 834). Nanosized MCM-41 silica was also prepared according to literature data (K. Suzuki, et al. JACS 2004, 126, 462-463). The formation of a mesoporous silica phase with regular arrangement of hexagonal pores was detected by XRD diffraction studies. As indicated by TEM images, the mesoporous silica particles are characterized by a mean diameter of 20-50 nm.
In addition, SBA-15 silica, with well-ordered hexagonal array of one-dimensional channels, high pore diameter and hydrothermal stability were prepared according to methods proposed in the literature (D. Zhao et al., J. Amer. Chem. Soc. 120 (1998) 6024).
The synthesis method was also varied to allow the modification of SBA-15 morphology as recently proposed in the literature (S.-Y. Chen et al. Chem. Mater. 2004, 16, 4174-4180). This preparation is carried out by introducing a Zirconium precursor (ZrOCl2 ∗ 8H2O) directly in the synthesis gel, during the condensation of TEOS source. The presence of Zr precursor modifies the hydrophilicity/hydrophobicity of the Pluronic micelles used as structure-directing agents, whose morphology and dimensions are modified, and this have a driving effect on the final morphology of the SBA-15 particles.
Beside nanosized and mesoporous alumina, the preparation of anodic porous alumina was also carried out (CRF). Two different approaches have been investigated:
-Anodization of pure aluminum foil in order to obtain a thick (greater than 40 μm) self-standing porous alumina film to be sandwiched between the electrodes;
-Evaporation of aluminum on the glass/TCO/Pt counter electrode and subsequent anodization to obtain thin (1-20 μm) porous alumina film directly grown on the counter electrode;
Four different samples were prepared:
i) Self-standing alumina film made with phosphoric acid; thickness: 40 ÷ 50 μm; pores diameter: 200 ÷ 250 nm,
ii) self-standing alumina film made with oxalic acid; thickness: 40 ÷50 μm; pores diameter: 40 ÷ 60 nm,
iii) alumina film made with phosphoric acid grown on cathode; thickness: 40 ÷ 50 μm; pores diameter: 200 ÷ 250 nm,
iv) alumina film made with oxalic acid grown on cathode; thickness: 40 ÷ 50 μm; pores diameter: 40 ÷ 60 nm.
-Layered materials
Layered materials with neutral, positive and negative charged layers were used as additives for the preparation of non-liquid electrolytes for XSC purposes.
Both UNIPMN and UNICAMP research units were involved in this activity. More specifically, UNIPMN developed saponite and hydrotalcite materials, whereas UNICAMP prepared different kinds of organo-modified talcs and layered V2O5 and MoO3 materials.
Cationic clays: saponite
Cationic saponite clays, especially when in the form of nanosized particles, appeared particularly effective for improving the efficiency of liquid electrolytes.
During the INNOVASOL project, particular attention was given to the preparation of saponite sample with different chemical composition and particle size. The optimisation of a synthesis method allowing to reduce as much as possible the particle dimension of inorganic saponites and the preparation and functionalisation with I-/I3- redox couple of hybrid organic-inorganic saponites, aiming at improving the affinity of the solid with the liquid electrolyte, were carried out.
Inorganic saponite clays were prepared by hydrothermal synthesis by properly adapting the procedures reported in the literature (Kloprogge, et al. Clays Clay Miner. 1993, 41, 103). In the first 12M of the project it was found that the H2O/Si ratio used for the preparation of the synthesis gel influences the morphology and the aspect ratio of the produced materials passing from 200 nm particle size for solids prepared with H2O/Si ratio of 20 to ca. 50 nm for saponite obtained with H2O/Si ratio of 110.
Aiming at verifying the possibility to prepare a nanosized saponite by further increasing the synthesis gel dilution, the synthesis procedure was conducted by using a H2O/Si ratio of 150. As in the case of the other saponites, the obtained material was then submitted to a classical ion exchange procedure in order to introduce only Na+ ions in the interlamellar space. The XRD pattern diffraction of the novel sample was compared to those of samples prepared previously by using different H2O/Si ratio.
The adopted hydrothermal synthesis procedure affords in general high yield of layered product, with essentially low amounts of amorphous by-products. This fact is demonstrated by the XRD profiles showing for all samples the presence of reflections characteristics of natural saponite (H. Suquet et al, Clays Clay Miner. 23 (1975) 1).
The dilution of the gel mixture has a clear effect on the average size of the lamellae, being larger than 200 nm for SAP20 and less than 50 nm for SAP150. The latter sample is formed of very thin particles. Differences in the XRD profiles of the three samples can be then justified on the basis of the mean dimension of the platelets. Indeed, the vanishing of the basal (001) reflection when the H2O/Si ratio of the gel is different from 20 may be due to a scarce tendency of small sheets (all with comparable stiffness) to stack orderly and to assemble along the c axis.
The research work on saponite solids was devoted to the introduction, through different approaches, of organic species aiming at increasing the affinity of the clay to the electrolytic solution and at hosting I-/I3- redox couple to favour the occurrence of Grotthus-like conduction mechanisms. Solids containing the redox couple were tested for the preparation of quasi-solid electrolytes. Nevertheless, interesting effects related to the presence of the redox couple in the interlayer space were not observed, whereas interesting results were obtained for inorganic samples exchanged with Na+ ions.
Anionic clays: Hydrotalcite
Different hydrotalcite samples were prepared aiming to intercalate I-/I3- species. Particular attention was given to the development of two hydrotalcite solids characterised by different morphology and particle size. Microsized Zn-Al-LDH were synthetized using the urea method. The solid obtained was washed 2 times and dried in oven at 80°C; the formula was [Zn0.61Al0.39(OH)2](CO3)0.195*0.5H2O. The chloride form (hereafter ZnAl-Cl) was obtained by titrating the carbonate form. The solids were analyzed by XRPD.
The ZnAl-Cl was then contacted with an aqueous solution 1 M of potassium iodide under magnetic stirring for 1 day, using CO2-free and de-ionized water and working under nitrogen atmosphere in order to introduce I- species in the interlayer space. Finally, the ZnAl-I was equilibrated with 0.1 M solutions of iodine in different organic solvents: ethanol, diethyl ether, chloroform, acetonitrile, under magnetic stirring for 3 days, to favour the formation of I3- species. The solids obtained were recovered by centrifugation and washed two times with the solvent used for the intercalation and dried in an oven at 80°C. This procedure was repeated also using 0.5 M iodine solution in the above indicated solvents.
The presence of I-, I3- and polyiodide species was found by DR-UV-Vis spectroscopy.
Nanosized hydrotalcite samples containing nitrate anions (LDH-NO3) in the interlayer region were synthesized adapting the urea method. Zn(NO3)2.6H2O and Al(NO3)3.9H2O were dissolved in a mixture of water and ethylene glycol 1:2 (v/v). Solid urea was added at this solution so that the molar ratio urea/Al was equal to 4. The clear solutions were heated, under stirring at reflux temperature for 6 hours. The gelatinous material obtained was recovered by centrifugation and washed two times with water. Finally, the material was stored as dispersion in water and ethylene glycol 1:2 (v/v). The formula of the solid obtained was: [Zn0.74Al0.26(OH)2](NO3)0.26*0.5H2O.
Nanometric LDH-NO3 was converted in the corresponding chloride form and then exchanged with I- species, thus obtaining a solid named n-LDH-I with the following chemical composition [Zn0.74Al0.26(OH)2](Cl)0.08I0.18 • 0.43H2O. The n-LDH-I was contacted with a solution 0.5 M of iodine in acetonitrile in order to generate triiodide in the interlayer region. The solid functionalized has the formula [Zn0.74Al0.26(OH)2](Cl)0.08I0.18(I2)0.11 • 0.57H2O (I2 wt%= 61%). Interesting results were obtained using hydrotalcite-based solids especially for the preparation of gel-based electrolytes.
Neutral layered oxides
Layered V2O5 and MoO3 were prepared aiming to intercalate I-/I3- redox couple. V2O5 and MoO3 are neutral solids made of thin layers, which are attractive materials due to their easy reduction. The first step in the modification was to promote a controlled reduction of V and Mo sites using polyelectrolytes, thus producing I-/I3- intercalated structures (F. J. Quites et al., J. Colloid Interface Sci., 2012, 368, 462).
The conclusion of these studies was that although the insertion of the redox pair on polyelectrolytes-modified V2O5 and MoO3 improved the general conduction properties of the materials in a 5 wt.% suspension in methoxypropionitrile, it did not improve them beyond that obtained with the use of electrolyte only. This was particularly true for V2O5-based materials. For MoO3-based materials the situation was even poorer: besides diminishing the results for conduction, in PDDA composites, a short circuit was observed. The general conclusion, related to the vanadium and molybdenum oxides, was that these materials seemed interesting for application in solar cells but the idea of turning them into anion exchangers by using polyelectrolytes did not work as expected. In fact, the long PDDA and PAH chains inside the interlayer spaces might have improved the conduction properties of V2O5 and MoO3 because they provided amine groups that could transport charge, however, adding the redox pair improved properties even better.
The key issue in the case of these materials is that their stability under light was not adequate, in fact, loss of conduction properties of ca. 96-97 % in the case of vanadium oxides and 30-60% for molybdenum oxides makes them uninteresting materials for this application.
Neutral clays: talcs
Talc is a layered hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Its elementary sheet is composed of a layer of magnesium-oxygen/hydroxyl octahedra, sandwiched between two layers of silicon-oxygen tetrahedral (R. B. Ferreira, et al., Langmuir 2008, 24, 14215).
In that composition, the material is neutral and has no counterion in the interlayer space. Because of that it can be modified with pending groups, and suitable organic functions were introduced in the interlayer space in order to have different energies of interaction with the ionic pair I3-/I-. Two materials were initially developed, the propylamine-magnesium silicate and the propylethylenediamine-magnesium silicate. The diffraction profiles are typical of phyllosilicate-type materials. In the case of the sample with monoamine modification, the XRD profile, shows the 001 and the 002 diffractions at ca. 5o and 10o 2, indicating that this material is more organized than the diamine-modified one. In these samples the amount of organics is rather varied: in the monoamine sample every Si atom bears a pending group, that is where the 100% naming comes from. In the case of the diamine, only 8.6% of the total Si atoms has one pending group attached; keeping in mind that every pending group has two N atoms then the N/Si molar ratio is 17.2. In these cases, Solaronix measured 5 wt.% solid suspensions in ethoxypropionitrile and unfortunately few samples were miscible with the ionic liquid, therefore EPFL measured only three samples.
100% aminopropyl-modified talcs are not the best samples, probably due to diffusional characteristics of these solids, whereas with diamines the situation is different. The samples prepared have approximately the same amount of pending groups/total silicon atoms and their behavior is better than the ionic liquid. In this case the diffusion problem is strongly diminished because one pending group bears two N atoms. The case of slightly larger number of pending groups is the better sample.
Organized Silicate
Organized silicate as Na-RUB-18 was also used as a means to prepare a quasi solid gel to be employed as electrolyte in a spot XSC. The samples were prepared following the procedure already reported in the literature (M. Borowski et al., J. Phys. Chem. B 1997, 101, 1292-1297) with smaller adapting modifications.
In the case of this material, the measurements at a 5 % solids suspension in methoxypropionitrile are not as good as the ones obtained for the talcs. The sample was only 104 % efficient in relation to the liquid electrolyte, with high density of current (11.68 mAcm-2) but low voltage at open circuit (0.76 V), not much better than the talcs. As a matter of fact, the results obtained in gels, measured at EPFL, were really better also because they involved a procedure that is more adequate for fast preparation of cells. These quasi-solid conditions range from viscous liquids to very thick gels, difficult to work with in preparing the cells. Despite that, the materials prepared in this work showed excellent results when in viscous liquid conditions. The concentration of 7% of sample in Z952 ionic liquid seems to be the best compromise between performance and ease of handling. All these results point unequivocally to the fact that Na-RUB-18 is as good a sample to prepare the quasi-solid electrolyte for the XSCs as any of the talcs discussed in the beginning of this report.
On the basis of results collected at EPFL and Solaronix, F5 silica, talc, saponite and Na-RUB-18 were found to be the most promising additives for XSC quasi-solid electrolytes.
1.1.3.4 Transparent conductive oxide (TCO)
-Synthesis of Nanostructured ZnO thin films for XSC devices
Over the course of the project, UCAM made advances in the synthesis and characterisation of a variety of ZnO-based nanostructured thin films for use in XSC devices. During the three-year period, UCAM was able to demonstrate complete control over the length and properties of the ZnO active layers. The method utilised a solution-based growth procedure - preparation of a suitable substrate, sputtering of a seed layer and placement into an aqueous solution of metal salts and complexing agents. Through careful control of the substrate surface, seed deposition procedure and growth conditions, it was prepared an active layer that showed superior performance compared to many of the materials reported in the literature.
The synthetic method showed a number of unique features compared to other methods available. Firstly a seed layer deposited simply from a modified metal sputtering system was used. By simply and cheaply depositing a thin layer of zinc metal down on our substrate, and through careful control of the sputtering conditions, a film ideally suited for XSC devices was formed. This seed layer fulfilled two roles - not only did it provide nucleation sites for the growth of the NWs, it also acted as a blocking layer when the substrates were eventually used in XSC devices. Through careful investigation of the deposition conditions, UCAM was able to control roughness and grain size, which gave a complete control of the NW arrays that were grown subsequently.
Concerning the growth of the NW arrays, they were able to develop aqueous solutions that ensured highly crystalline ZnO crystals grew from the seed layer surface. This was achieved through careful control of the chemical composition of the solution, through the use of additives in the solution and by control of the heating and reaction timing. In this way, the length of the wires and the crystallinity of the structures were controlled.
One important aspect of the ZnO wires is the stability of their surfaces - inefficient chemical binding of the sensitisers (mainly developed for TiO2-based devices) and the presence of surface traps can often cause poor device performances. New methods to improve the surface chemistry of the ZnO by synthesising core-shell structures were therefore developed. Two methods were used for this - a solution method and an ALD method. Both had advantages and disadvantages. Nevertheless, both methods showed that device performances could be substantially improved with the use of thin shells.
-Investigating unique device architectures
During the course of the INNOVASOL project, UCAM was able to investigate the growth and mechanical behaviour of ZnO arrays directly produced onto compliant substrates. These stretchable ZnO islands were extremely robust, being able to withstand high stretching regimes without failure of electrical compliance. This work paves the way for developing nanostructured devices that are no longer rigid and can be incorporated into advanced architectures and applications. UCAM demonstrated, for the first time, a ZnO-based diode on a stretchable polymer.
1.1.3.5 Testing INNOVASOL materials for the preparation of XSC devices
Materials prepared in the frame of the INNOVASOL project were tested (alone or in combination) for the preparation of XSC devices. Tests were carried out both by researchers of EPFL and Solaronix SA. In particular, Solaronix activities were mainly focused to the testing of novel non-liquid electrolytes prepared by adding to methoxyproprionitryle-based electrolyte solids prepared by UNIPMN and UNICAMP and to the development of novel DSSC modules with improved efficiency. On the other hand, EPFL tested all materials produced over the course of the INNOVASOL project aiming at increasing the performances of laboratory scale devices by using novel and especially designed components.
-Assembly of XSC with novel ZnO-based photoanode electrodes
ZnO nanowires (NWs), various core-shell based ZnO NWs core and doped ZnO nanowires were examined as photo-anode electrodes in a complete XSC. EPFL was responsible on the assembly of the cells and their photovoltaic characterization more than 200 cells were made in this task.
In the beginning, the suitable dye for the ZnO NWs photo-anode was found. Four organic dyes C101, B11, C205 and C218 were selected because of their high molar extinction coefficient and spectral response in the visible region and tested using liquid electrolyte. C218 dye gave the best PV results. The Z960 electrolyte was used for all cells, because this electrolyte gave the best efficiencies in XSCs and good stability using C218 sensitizer.
Therefore, the rest of the experiments were made using with the C218 dye and the Z960 electrolyte.
The best performance was achieved using long NWs. The longer ZnO NWs reached 10m length.
Using those long ZnO NWs an efficiency of 1.25% at 1.5AM (full sun conditions) was achieved. This is one of the highest efficiency in the literature for bare ZnO NWs cell.
Improvements in device performance for ZnO NW-based photovoltaic devices were observed when the NWs were coated with thin layers of semiconducting or insulating materials. The purpose of these shells is to provide a more stable binding site for dye absorption and suppress carrier recombination. Core-shell structures of ZnO-TiO2 were synthesized using ALD methods (see deliverable D3.3) allowing precise control of the shell thickness coating the NW surface.
By trying different TiO2 shell thickness such us 10nm, 20nm and 40nm it was observed that the TiO2 shell thickness of 20 nm gave the best photovoltaic performance using 1m thickness of ZnO/ TiO2 core shell film. Using this finding 10m long core ZnO NWs covered with 20nm TiO2 shell, thus creating ~10m thickness ZnO/TiO2 films, were made.
-Molecular relays - Energy transfer between Squarine dyes and CdSe QDs
EPFL was responsible of the assembly of a solar cell composed of squarine dye as acceptor and CdSe QDs as donor. This assembly shows a simple structure of FRET system inside a XSC device. The donors are CdSe QDs, which have broad absorption spectrum in the visible region. The acceptor is a molecularly engineered squaraine sensitizer (and molecular relay), the VG1-C10 prepared by UNITO, that has an additional carboxylic acid group and two long carbon chains compared to the standard squaraine dye. The presence of two carboxylic acid anchoring groups, and the hydrophobic long chains provides better dye stability and allows efficient energy transfer from the high energy QD's to squaraine sensitizer. The use of cobalt complex (Co2+/Co3+) as electrolyte in these cells permit direct contact between the QDs and the electrolyte. Moreover there is no need to change the original ligands of the QDs prior to deposition; the two C10 chains of the dye molecules and the oleic acid ligands coated the QDs provide the optimum distance for FRET, hence the preparation and the structure of the cell are simple. As a result of the energy transfer the cell power conversion efficiency was increased and its solar response was expanded from the visible to the near infra-red.
-Assembly of PbS QDs/TiO2 heterojunction solar cells
PbS QDs were used as light harvesters in conjunction with films composed of 18nm-sized TiO2 nanocrystals containing both meso-and macropores. Also, at the same time the PbS acts as a hole conductor, rendering superfluous the use of an additional p-type material for transporting positive charge carriers.
In order to increase further the photovoltaic performance of the PbS QDs solar cells, nanosheets of anatase TiO2 were tried. The TiO2 nanosheets have an exposed (001) facet, which is different than the normal anatase TiO2 nanoparticles (NPs) which have (101) dominant exposed facet. Researchers have shown that the (001) facet has a higher surface energy than the (101) facet therefore their surface is more reactive. Thin films of those nanosheets were deposited by spin coating on the FTO glass.
Two sizes of PbS QDs were tested, energy gap (Eg) of 1.38eV and Eg of 1.24eV and two sizes of TiO2 nanosheets, 30nm and 80nm, were tested as well.
The best results were achieved with 30nm size of nanosheets and PbS QDs with Eg of 1.38eV. The power conversion efficiency in this case was 4.73%, which is one of the highest reported in literature.
-Assembly of XSC cells with novel quasi-solid electrolytes
A pre-screening of materials for quasi-solid electrolytes was done at Solaronix by assembling spot cells preparing 5wt% suspension of solid in methoxyproprionitrile based electrolytes. Best results were obtained by using talc and saponite clays as additives for electrolyte.
Moreover, gels (quasi-solid electrolytes more suitable for industrial preparations) were prepared by dissolving 20% wt of powder in ionic liquid. EPFL was responsible for the cell architecture, assembling, dyes adsorbing, J-V and IPCE measurements.
The cell composed of working electrode, glass covered by FTO, and 12μm TiO2 nanocrystals layer made in EPFL. Counter electrode, TEC 15 glass covered by FTO, and Pt.
All cells were made using gel electrolytes, which were applied by a doctor blading technique.
The first step was the screening of all the materials using the conditions mentioned below:
(1) Z907 dye;
(2) 20%wt of the material was mixed in the ionic liquid electrolyte in order to get the gel;
(3) Doctor blading technique to spread the gel on the dye-sensitized TiO2 film.
34 materials were screened, and based on the photovoltaic results the best materials were chosen:
i) F5 Silica particles,
ii) Hydrotalcites,
iii) Nanosized saponites ,
iv) Layered silica NaRub-18,
v) Talcs.
In order to further optimize the cell performance few changes in the cell composition were made: organic dye Y123 was used, transparent TiO2 layer of 5.7 m thickness, different loading of the powder in the ionic liquid electrolyte.
Multiple - Exciton Generation
According to equation 1 it is possible to calculate the Light Harvesting Efficiency (LHE) of the QDs film from the absorbance spectra.
(1)
The absorbed photons-to-current efficiency (APCE) values taking into account the light- harvesting efficiency (LHE), or light actually absorbed by the monolayers of QDs. The APCE can be calculated according to equation 2.
(2)
The APCE values are over 100% for photon energies of 2.8eV - 3.1eV which are around 2 times the QDs Eg. Those APCE values, which exceed 100%, can suggest the possibility of multiple exciton generation (MEG) effect in PbS QDs solar cells.
1.1.3.6 Preparation of XSC modules
During the first period of the project, materials such as ruthenium dyes, organic dyes, titanium oxides, conductive SnO2:F (FTO) coated glasses and electrolyte compositions were investigated to build the best testing device for the project's materials. This activity was mainly carried out by Solaronix SA Company.
All efficiency data were measured at 1000 W/m2 Class A light at 25°C with black tape masked cells.
Various electrolyte compositions have been studied for their performance. The best electrolyte was still the "AN50", i.e. an acetonitrile based electrolyte with 50 mM tri-iodide, followed by "Z-946" and "R150", which are based on 3-methoxypropionitrile, giving best performances in stability tests thanks to their low volatility compared to acetonitrile.
Four types of ruthenium based dyes and one pure organic dye (squaraine SQ2) were compared in spot-cells. Following obtained results the best dye was still Ru535 bisTBA (a.k.a. N-719) and the blue-green coloured squaraine dye gave an appreciable efficiency of 3.2 % in the spot-cell.
The relative size of the light blocking mask is crucial for consistent efficiency measurements with small solar cells. To avoid large errors, the mask is always 1 mm wider than the TiO2 active area, resulting in a relative mask area of a factor of 2. The choice of the conductive glass was also critical for performance improvements, thus switching from FTO22-7 (a 7 ohm/sq 2.2 mm float glass) to Asahi U (a 1 mm thick 10 ohm/sq FTO glass) gave nearly 10 % relative increase in output.
Titania screen printable paste made using narrow titania particle size distribution presented:
-bad adhesion properties that could be attributed to the formation of a compact film on the glass substrate.
-high viscosity after few days. This high viscosity leads to a bad printability of the paste and a poor quality of the film formed.
The unfavorable adhesion properties led to the impossibility to perform a post TiCl4 treatment. The latter is essential for best cell performance.
To solve these issues, the particle size distribution has been tuned by introducing a small amount of larger particles while keeping the distribution centered around 18 nm (optimum size for DSSC).
Introducing these bigger particles allowed to control the film porosity. TiCl4 treatment was performed on the formed film without delamination. The treatment led to a dramatic increase in the cell performance.
Summing up all these new improvements, the best test cell gave 9.4 %, while keeping the process compatible with industrial manufacturing.
Once this spot cell based testing tool was setup, the various materials supplied by the projects partners were evaluated in «real» conditions. The results concerning the screening these electrolyte additives are presented in detail in Deliverables 2.4 2.6 and 4.2.
In parallel to the work on new electrolyte materials, SOLARONIX developed liquid and gel electrolyte based prototypes. These prototypes consisted in serially interconnected dye sensitized solar cells to form a module. W-type module were first investigated. W-type modules are semi-transparent bifacial devices where the counter and the working electrodes are alternately printed on the same side creating the serial connection with the other side.
As the titania and platinum counter electrodes are printed on the same glass, the platinum is usually poisoned during the staining process. This deactivation of the catalytic platinum led to poor fill factor and low final performance (around 3% using liquid electrolyte). SOLARONIX worked on improving liquid based devices before introducing the use of suspension and gel electrolyte.
To avoid platinum layer degradation a selective staining process was setup. The first approach was to protect the platinum layers using low adhesion adhesive tape. This approach allowed a dramatic increase in the final module performance but this method is time consuming and not easy to scale up. A modification of this strategy was finally used. A silicon mold was designed to allow titania selective staining. This mold was easy to put in place (less time consuming) and the platinum protection was effective. This approach is scalable to any module size.
Summing up the previously described improvement and all the enhancements developed earlier in INNOVASOL, the state of the art W-module (10 x 10 cm, 65 cm2 active area) based on liquid electrolyte and N-719 dye reached a high power conversion efficiency (PCE) of 7.86 % (Voc = 8.3 V; Isc = 106 mA) under one sun (AM1.5G). This device was measured at various sun intensities to evaluate the performance in situation matching real outdoors conditions. Under one third of sun the PCE was as high as 9.19 %.
This high efficiency is impressive especially considering the fact that the titania layer was only around 8 microns thick, semi transparent and the module made with only industrially available raw materials.
Liquid electrolyte cell stability was tested. Cells made with Z-907 dye and Z-946 electrolyte, were placed under constant illumination, one sun AM 1.5G at 55 °C during the first 4000 hours and at 65 °C for the rest of the experiment. All cells exhibited good stability with less than 2% efficiency loss after 1000 hours and around 20% after 8000 hours in these conditions.
The stability of some W-modules was tested in the same experimental conditions than the ones previously described. The module exhibited good stability over 2000h. Some defects in the sealing finally led to the module death after this time.
Further work consisted in developing thin glass based modules to build INNOVASOL prototype. The idea was to apply the process previously developed for high efficiency modules to hundred microns thick flexible glass.
A new design to fit CRF specifications was created and consisted in an interconnected module of seven individual cells on a 7 x 10 cm substrate. Several steps needed to be optimized for this thin glass like: i) FTO deposition, ii) Two glass lamination, iii) Hole drilling, iv) Titania printability and adhesion.
To conclude, INNOVASOL led to several achievements:
-Test cells went from around 7% to above 9%.
-Ionic liquid cells went from around 2.5% to above 6%.
-W modules went from around 3% to around 8% and even above 9% under 1/3 of sun.
-First thin modules at Solaronix were obtained.
-High stability cells were obtained (one year under 1 sun during 24h/day, corresponding to over 5 years of outdoors conditions).
All these results were obtained using only industrially available raw materials or techniques and summing up the various improvements developed during INNOVASOL.
Potential Impact:
INNOVASOL socio-economic potential impact - Automotive application
The INNOVASOL objective is to implement novel XSC devices to harvest the sun energy. Even if the efficiency is lower respect to crystalline Si cells, the superior performances of this kind of modules are:
-efficient temperature-independent in the range 2565 ?C (Si-based declines by approximately 20%).
-light capture less sensitive to the angle of incidence (indoor possible integration).
-more efficient than pc-Si in diffuse light or cloudy conditions.
-printable resulting in low cost and easy fabrication (below 1 EURO/W) even on flexible substrates.
The capability to produce energy in any light intensity condition is particularly important for transportation allowing the integration of these cells also on interiors surfaces with above 2m2 surface available (medium size car); this results in potential production of 250-400Wh considering average efficiency 4% and 8h exposition under diffuse light (300W/m2).
A further potential impact of this innovative energy harvesting approach on transportation is that a more constant power output respect to the other Si-PV (changing T and light intensity) results in a easier power management for the charging of batteries; this impacts positively also on battery lifetime and costs.
The application on vehicle exteriors (typically solar roof) and interiors (dashboard) of large surface solar cells built with INNOVASOL materials can bring significant social and environmental advantages in terms of:
-reduced overall fuel consumption at the European level
-reduced CO2 emissions in terms of g/km emitted aiming at complying with more severe regulations
The demonstration of the potential impact of INNOVASOL concept requires to calculate the current cost of the kWhel produced by the alternator allowing to estimate the financial and fuel savings expected and the reduction of CO2 emissions (greenhouse gases).
INNOVASOL Socio-economic impact (solar cell production)
Thanks to the novel materials developed during the INNOVASOL project, such as printable 'gellified' electrolytes, ionic liquid compositions showing excellent thermal stability, it became now possible to produce XSC modules using an all printed technology, giving for example semi-transparent modules that could find their way into the automotive application.
Taking the example of a 7 x 10 cm solar module powering a convenience light in a car, and a estimated volume of 500000 modules per year, the cost estimation shows that such a rather small production may produce customized XSC for less than 4 EUROS per module.
The cost structure shows that labor and materials are making up most of the expenditures, whereas the cost associated with the production equipment is only 15 %, as such a production line is mostly based on printing, heating and lamination techniques well known to industry.
List of Websites:
The address of project website is http://www.innovasol.eu
