Deliverables
The main objective of this project was the development of cost-effective technologies for the manufacturing of photovoltaic (PV) cells based on thin (below 200micrometer) conventionally cast multicrystalline silicon wafers and EFG sheets. The EU's target cost for PV systems is 7Euro/Wp for the short and 3Euro/Wp for the medium term to help reach the White Paper target of 3GWp capacity by 2010.
To achieve the project's objective, a consortium was formed to develop innovative cell structures and fast (>1dm{2}/3sec) low-stress manufacturing technologies suitable for processing thin wafers with high yield.
Good progress has been achieved in all work packages of the project. All deliverables and milestones that were foreseen until the MTA meeting were fulfilled:
- Isotexturing processes have been developed and successfully demonstrated for both, multicrystalline wafers and EFG sheets. A wetbench that is suited for pre-diffusion cleaning and iso-texturing has been developed, designed and is currently under construction.
- P pastes resulting in two different target emitter sheet resistance values and P surface concentrations (A: 1019 P atoms/cm{3}; B: >1020 P atoms/cm{3}) when applied in the same optimised diffusion step have been developed and investigated. Samples of a boron paste have been supplied as well.
- Diffusion processes for homogeneous shallow emitter diffusion (60ohm/sq.) have been developed, applied and optimised.
- An advanced method for the parasitic edge removal applying no stress to the thin wafers has been developed based on a novel KOH paste suited for selective Si and PSG etching. Resulting fill factors of 76% and higher for POCl(3) diffused solar cells demonstrate clearly that the parasitic junction removal is successfully performed with this contactless method.
- Ag pastes allowing contacting shallow emitters (60ohm/sq.) have been developed, supplied to the project partners and successfully applied after optimising the contact firing step.
- Al pastes resulting in lower wafer bowing (? 1mm bow on ?125x125mm{2} wafers; full Al BSF) without degrading the Al back surface field (BSF) have been developed and successfully tested.
- A transverse probe double wavelength lifetime measurement set-up has been designed, built up and successfully tested. This set-up is in principle suited to extract and monitor the minority carrier lifetime in the wafer bulk and the surface recombination velocities of the rear and front surfaces and thus will help further process optimisation.
- Necessary improvement for handling and metallisation printing of thin wafers has been identified and specified.
Alternative rear side metallisation schemes applying local rear contacts, rear surfaces passivation and reflector layers and suited isolation processes are under development
First attempts to run advanced integrated process sequences that are suited for thin mc-Si and EFG wafers have been successfully executed.
Solar cell efficiencies in excess of 16.1 % have been achieved on thin mc-Si wafers (MTA milestone: > 15.5 %)
Solar cell efficiencies of 14.5% on 200 µm thin and > 15 % on 300 µm thick 100 cm2, 3 ohm cm EFG wafers have been demonstrated (MTA milestone on thin EFG: 14.5 %).
The advantageous results will be individually exploited by the respective project partners as described in the TIP (technology implementation plan):
- Merck and DuPont will try to commercialise the respective developed pastes. Astec will try to commercialise the developed wetbench concept.
- RWE SCHOTT Solar will try to implement the developed integral processing sequences including iso-texturing and shallow emitters.
- IMEC will disseminate the developed technologies through journal articles and conference proceedings and offer to transfer the gained know-how to interested PV companies.
- DIE-UNAP will offer the developed characterisation possibilities as commercially available service for a wider public and publish the scientifically relevant progress.
- Technion will publish the fundamental scientific know-how that has been gained and apply the etching and texturing technology also to other fields in the micro-electronic sector.
One of IMECs tasks in the project is the optimisation of what is known as the classical screen printing process for thin (200 micron or less) multicrystalline silicon solar cells. This process incorporates a limited number of steps: isotropic texturisation, shallow emitter diffusion, parasitic junction removal, dielectric ARC and passivation and finally screen printed metallisation. These process steps have been optimized first on an individual level.
Afterwards each process step was looked at carefully to allow use in an integrated process sequence. Care has been taken to allow application on thin wafers: this is mostly evident on the steps related to isotropic texturisation (to avoid development of dislocations) and screen printing of the rear contact (to avoid bending of the wafers). For the latter, the applied material (Al-paste) as well as the printing parameters are important. Further work has been done to increase the blue response of the cells by optimisation of the POCl3-diffusion and the front metallisation.
Application of the process has resulted in efficiencies around 16% on 200 micron thick multicrystalline silicon solar cells (size:100 cm2).
Novel method of silicon etching in alkaline electrolytes with the application of negative high potential, (termed Negative Potential Dissolution [NPD]) can be applicable not only for polishing but rather also to texturing of multi silicon in the solar cells industry.
Targets and Objectives:
- Perform and understand electrochemical texturing of silicon single crystals.
- Evaluate the influence of the following parameters on mono-silicon texturing:
-- Depth of negative potential
-- Alkaline concentration
-- Process time
- Introduce the concept of electrochemical texturing into multi silicon
- Perform both anisotropic and isotropic texturing of multi silicon and EFG silicon
- Evaluate and study the parameters controlling both isotropic and anisotropic texturing
- Ultimately, to develop and introduce new inexpensive, ultra-rapid texturing technology which do not use either HF or nitric acid.
Achievements and Milestones:
- Anisotropic silicon texturing was obtained with all three mono-crystalline silicon. Characteristic texturing for each single crystal was obtained. For example, in <100> pyramids with a base length of more than 20 Ým could be obtained and for <110> we observed in the formation of a prism.
- Negative potentials were scanned and it was established that surface texturing starts only at potentials below -20V. As the potential scanned down to negative values of -100 V new textured surfaces are formed.
- Alkaline concentrations of 8-50% were studies as a function of the applied potential. We found that the optimal alkaline concentration was in the range of 20-24%. In this potential the current drawn from the system was the highest and thus fast removal of silicon from the surface was feasible.
- Texturing of single crystal as-cut silicon could be obtained in time scales of less than 60 seconds.
- Anisotropic texturing of both multi silicon and EFG silicon was successfully obtained with this novel electrochemical method. The obtained textured surfaces have no steps in grain boundaries.
- During the passed 18 months of FANTASI project we successfully isolated the parameters controlling the texturing of multi-silicon. We found the optimal conditions in terms of value of negative potential, alkaline concentration and process time.
- We successfully obtained isotropic texturing of IMEC multi silicon. This process is achieved at totally different conditions than anisotropic texturing is obtained. Namely, alkaline concentration was increased dramatically to 50% and the process time was shortened to 20-30 seconds with a silicon removal rate of 15 Ým/mi. Although the process is not optimizes yet we can observe that no steps are formed in-between grain boundaries during the process.
Future Technology work:
- Optimize IMEC multi silicon isotexturing with the use of NPD process.
- Study and investigate RWE EFG silicon isotexturing by NPD technology.
- Increase working area from 1 cm2 to 4 cm2.
A contactless, all-optical and non-destructive technique for simultaneous measurement of minority carrier recombination lifetime and surface recombination velocity, at low injection level, in silicon samples is presented. Being contactless and non-destructive with respect to the surface to be analysed, the method is suitable for routine lifetime characterisation in solar cell process. The technique is applicable to the measurement of bulk recombination lifetime and surface recombination velocity on thin multi-crystalline Silicon samples
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