Deliverables
TThe BSF is a component of the solar cell structure, which suppresses losses due to carrier recombination on the cell back surface. Current multi-crystalline Si solar cell production technology includes an Al alloying operation for BSF formation, which also provides a low resistance back contact. The quality of this BSF structure does not satisfy the requirements for next generation high efficiency cells.
Previous experiments with single crystal FZ Si cells demonstrated that an improvement in the BSF could be achieved by simultaneous Al & B instead of Al only, alloying/diffusion. The application of this process to mc Si solar cell fabrication technology improves the quality of the BSF and increases the cell parameters dependant on back surface recombination (long wavelength spectral response and consequently photo generated current, open circuit voltage, fill factor, FF) and conversion efficiency Eff., as a result of the improvement of the above parameters.
The cell fabrication technology was developed to evaluate the BSF improvement on a structure similar to that of future industrially produced high efficiency cells.
Parameters of these cells were: thin solar cells (215 +/-10mm), uniformly low doped emitter with sheet resistance 60-100/square(instead of a selectively doped emitter with the analogous low doped regions), passivated front surface (by a thermally grown SiO2 layer instead of a SiN coating), vacuum deposited and plated Ti-Pd-Ag front contact (instead of fine screen printed contacts).
BSF structures were formed at a temperature of 950 oC for 1 or 2 hrs using for each experiment wafers with an Al layer covered by the spin-on Boron dopant composition and, for comparison, with an Al only layer.Simultaneously with Al and B diffusion P drive-in and front surface oxidation took place.
Several batches of mc-Si solar cells with Al only and Al & B doped backs were fabricated using the above process with 2 hrs BSF doping. The principal positive effect of additional B doping for BSF structure formation was demonstrated: increase of long wavelength response, short circuit current and open circuit voltage.
Considerable improvement in performances of mc Si solar cells with BSF formed by Al & B doping was shown: an increase in efficiency at least of 0.5% abs. up to 17.5 17.8% was demonstrated.
The reference point of the process development within the INDHI-project was a TiO2 coated homogeneous emitter of a multicrystalline silicon solar cell. An improvement of surface passivation and bulk lifetime can be achieved by the incorporation of a silicon nitride SiNx layer into the standard industrial silicon solar cell. The antireflection properties of SiNx are comparable to those of TiO2, if the index of reflection and the layer thickness are well adapted. The open question which had to be addressed is the stability of this SiNx layer. Stability with regard to illumination with UV-light which the solar cells have to endure in the final application in a module on the one hand and the influence of heat treatment during cell processing e. g. contact firing on the other hand, has to be evaluated considering bulk lifetime and surface passivation properties of the SiNx layer.
The bulk lifetime investigations were carried out on mc-Si acquired for the project from Deutsche Solar. The process sequence carried out includes a firing step for the front contact, whereas no screen-printing had been performed. After that the SiN layer as well as the previously formed phosphorous emitter were removed and silicon nitride was deposited on both wafer surfaces, optimised for surface passivation leading to surface recombination velocities of less than 10 cm/s. Thus it was possible to determine the bulk lifetime of the material. The lifetime mappings before and after bulk passivation are 118 µs and 184 µs respectively. That means despite of a high lifetime before processing the average lifetime could have been improved by about 50%.
Experiments with FZ-Si proved the stability of the surface passivation during the firing process in the IR belt furnace.
A joint solar cell manufacturing experiment was set up between ENEA, EniTecnologie and ISFH. A series of 24 wafers was prepared to be introduced into the solar cell production line of EniTecnologie. These solar cells with a SiNx layer from ISFH showed efficiencies up to 15.1 % (Voc: 614 mV, Jsc: 32.4 mA/sqcm, FF: 76.0 %, cell area > 100 sqcm).
A further experiment between UKON and ISFH showed the combined effect of an acidic surface texture and a SiNx layer. An efficiency of 15.9 % was achieved (Voc: 613 mV, Jsc: 34.1 mA/sqcm, FF: 76.0 %, cell area: 25 sqcm). The main contribution to the efficiency improvement originates from an enhanced short circuit current leading to an average increase in cell efficiency of 1 % absolute.
The UV stability of the cells manufactured together with EniTecnologie could have been demonstrated. The cells were exposed to UV-light with wavelengths above 360 nm at about 5 suns for up to 16 hours.
The transfer of the process for surface and bulk passivation was successfully established on on an industrially relevant system, the SiNA from Roth & Rau. The surface and bulk passivation properties of the SiN films from the SiNA system are as good as from the laboratory tool. Also the stability requirements are fulfilled as stated above.
In workpackage two WP2 the most promising, low cost and innovative solutions were studied, to enhance the carrier collection in the blue spectral region, and to minimise surface recombination. Selective emitters formed by screen printing and low temperature passivation schemes were setup, and an increase of one absolute point in efficiency respect to homogeneous emitter, titanium dioxide layer cells were demonstrated.
A selective emitter formation process was developed, based on the selective printing of dopant pastes.
The purpose was to establish the P-incorporation conditions by using a screen printed deposition of a doped paste, especially elaborated by one of the partner of this project, with a grid design similar to the grid used for the contacts in order to form the low resistive part of the emitter under the metallic contacts. For the low doped region of this selective emitter we used both a dopant paste and a spray-on deposition followed by firing step that will yield to form the two high and low doped regions that are suitable for selective emitter industrial silicon solar cells.
The selective emitter was optimised and tested based on the following parameters: measurement of the differential of doping by scanning 4-point probe, reflectance etc, control of the lateral diffusion of the doped paste, measurement of the gradient of doping, characterisation of the junctions properties and effects on the base material.
The selective emitter formation was achieved by screen printing; for our experiments we have employed the Soltech P101 thick film dopant paste.
The first part of the work consisted in verify the effectiveness of selective emitter formation on single crystal, Cz wafers.
First, the homogeneous diffusion from Soltech P101 was optimized, to find the best processing conditions.
Firing was carried out in a standard conveyor belt furnace, in air atmosphere, at temperatures of about 920-950 °C, for 15-20 min, giving, on homogeneous devices, 20 - 40 ohm/square sheet resistance value. The set-up of a full screen printing selective emitter process is impossible without a suitable system of re-alignment of the different patterns, metallisation and selective diffusion. This was achieved on our high resolution screen printing system, equipped with a special optically resolved recognizing system, which allows the realignment of the different patterns within 10 microns.
Selective emitters (20/100 ohm/square) have been obtained by screen printing the dopant paste through a diffusion mask with the same grid pattern of the metallization mask, followed by an optimized firing step in the belt furnace.
Due to out-diffusion induced by the screen printed doped paste, a high-low emitter is formed, allowing a significant increase in the blue spectrum region carrier collection.
To complete the selective emitter structure, different kinds of front surface treatments have been studied: lightly doped screen printed emitters, sprayed-on, thermal oxidation, etc.
Contacts have been made by screen printing (for back contact, an Al based paste was used). Firing of contacts has been carried out in air atmosphere, in an IR belt furnace (peak Temp 860 C, v=50-60 ipm), and temperature profiles have been adjusted to get the best Fill Factor.
A short circuit current gain of up to 3 mA/sqcm has been demonstrated, respect to homogeneous emitters, and open circuit increase of 15 mV.
Several batches of cells have been fabricated with efficiency of about 15%.
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