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ECN applied improved processing for texturing, emitter formation and BSF on material from three batches of TOPSICLE material. The best efficiency obtained is 16.7%. This cell has a single layer SiNx:H coating. With an additional MgF2 coating the efficiency could be improved with about 0.5% absolute.
Bottom-to-top approach using screen printing ECN optimized the process on Topsicle material (mainly Topsicle I material) in phases:
- Optimizing texturing and emitter formation
Texturing using in-line acidic etching was adjusted for the Topsicle I material. After texturing an emitter of 80 Ohm/sq emitter was applied. Firing optimization and using a single layer SiNx:H ARC resulted in a best cell efficiency of 16.7%. After applying an additional MgF2 layer the efficiency should increase with about 0.5%. On other material (commercially available) an independently confirmed efficiency of 17.0% was reached.
- Improved BSF formation
The optimized BSF process found in WP2 was applied on Topsicle I material. The efficiency gain using the improved BSF formation was about 0.2%. The improved BSF formation is used to obtain 17.0%. With an additional MgF2 coating the efficiency could be improved with about 0.5% absolute. The 17.0% process was applied on a larger amount mc-Si wafers and the average efficieny obtained was 16.8%. A first full size module made of these cells has an encapsulated cell efficiency of 16.8% corresponding to a power of 94.3 Wp.
The knowledge developed on this topic, will on one hand be assessed by the industrial partner SCHOTT Solar, and on the other hand contribute to the basic knowledge of the researchers.
SCHOTT Solar GmbH is a direct end-user and will exploit the results of the project. One important step achieved within the project was the development of a high-efficiency mc Si solar cell process sequence based on isotexturing and screen-printing together with the consortium partner ECN. Comprehensive assessment at SCHOTT Solar and common experiments with ECN and UKON within the project revealed that both, enhancement of efficiency and decrease of depreciation and consumables cost, help to reduce the solar cell production cost per Wp significantly. The isotexturing cell process adaptation to industrial boundary conditions was already started at SCHOTT Solar during the project and is now under development. Approval for introduction into solar cell production is expected with the next year.
The researchers will exploit the public knowledge through publications and conferences, and the specific knowledge through their knowledge position.
Several novel processes were investigated during the first part of the Topsicle project:
- Single-Step Selective Emitter
- Fine-Line Printing
- Angled Buried Contact (abc) Concept
Single-Step Selective Emitter:
University of Konstanz was working on a single step selective emitter cell using porous silicon as a diffusion barrier. Tests with short wavelength lasers were done to find optimal laser parameters necessary to remove the porous silicon (in the selected regions) prior to the diffusion. In a first experiment the cleaning step before the diffusion unfortunately removed the porous silicon. Further experiments have been carried out to optimise the porous silicon thickness and diffusion parameters to have optimal the sheet resistance in both the highly and lowly doped regions.
More research is needed (and will be carried out) before this novel process can be assessed for dissemination.
Fine-Line Printing:
With the aim of achieving both narrow and high fingers for low shadowing losses and high finger conductivity a method of fine-line printing was investigated. By printing and drying several times it was possible to achieve fingers of about 60 µm width and 60 µm height. Multiple prints in exactly the same position were demonstrated. Test wafers with up to 10 printed layers were made. Solar cells were made with 3x and 5x front side printing on both iso- and un -textured wafers. The cells show an approximately 0.5 mA/cm2 higher current compared to standard screen-printed cells. In the case of the un-textured cells, the increase in Jsc was accompanied by a decrease in fill factor (approximately 2-3% absolute). No decrease in fill factor was seen for the iso-textured cells. These results are promising, and can probably be used to disseminate outside the consortium.
Angled Buried Contact Concept:
The Angled Buried Contact cell design is a front metallisation scheme that results in negligible shading. The metal is deposited in angled grooves and is in the ideal case not visible by looking from a perpendicular view to the cell surface. The process does not require any additional process steps compared to a standard buried contact process, it is slightly simplified. The proof of concept was shown and an increase in short circuit current of 0,7mA/cm2 was demonstrated.
The LPCVD (low pressure) silicon nitride has to be replaced by PECVD (plasma enhanced) silicon nitride because a directional deposition is neccesary. Depostion pressure of PECVD system must be adjusted to reach low pinhole density in the silicon nitride layer. Overplating (and thus higher shading losses) because of silicon nitride pinholes and local shunting were the main problems of first cell runs.
Processing techniques known and proven in high efficiency processing of monocrystalline solar cells are being transferred, adapted and investigated on multicrystalline silicon.
UPM-IES worked on adaptation of techniques from mono-crystalline cell processing including both P/Al and P/B structures.
In particular, a PERC structure (passivated emitter, rear local contact) is proposed. When passivating the rear side with dry oxide a result in the range of 17% has been achieved. The analysis carried out to explain results shows that final lifetimes are modest, so that improvement of rear surface passivation does not have a real impact on cell performance, which on the other hand is based on low frontal reflectance (thanks to a DARC, even with an alkaline textured surface) and good light trapping conditions. On the other hand, another alternative is proposed, based on rear side passivation by wet oxide. Multicrystalline test samples reach final lifetimes in the range of 100 µs and 17.1% has been reached, even with a non-optimal deposited DARC.
A P/B cell can benefit from the boron BSF and the light trapping associated to a bifacial structure. But the standard process in UPM-IES implies a great number of thermal steps (masking oxide; B diffusion; B protection; P diffusion and final passivation), which multicrystalline material cannot withstand easily (although P diffusion has gettering effect that recovers lifetime to some extent).
So, efforts were dedicated to re-design the process to reduce thermal loads. As a first step, monocrystalline wafers were used to check the potential of the approach.
Three approaches are pursued: avoidance of masking oxidations, spin-on B source and use of screenprinted emitters.
Although the results look promising, it is too early to come to final conclusions. Further research is carried out to establish the potential gain of these processes.
Using this hybrid screen-print/buried contact process, which has been specially optimised for multi-crystalline silicon material, an efficiency of 18.1% on a large area cell has been achieved. The front side is made by the conventional buried contact approach, whereas the rear side has a back surface field made by screen printing and firing aluminium paste.
The front side was mechanically texturied by a single blade resuluting in parallel v-grooves. The cell was made on Polix material from Photowatt, which had a Voc of 636 mV, a Jsc of 36.91 mA/cm2 and a FF of 77%. Fraunhofer ISE certified the efficiency. The formerly reached efficiency of 17.6% was clearly topped.
The improvement was mainly contributed by a better spectral response in the long wavelength region due to a more effective back surface field and more homogeneous material quality. Cell thickness was higher, differences could also be seen at the front surface reflectance, the texture was deeper and pointier for the 18.1% cell.
Applying a similar process to TOPSICLE material resulted in best efficiency of 15.8%. This lower efficiency was partly due to the fact that the TOPSICLE wafers were only NaOH etched rather than being textured. However, there is also evidence that the efficiency of the LGBC cells on TOPSICLE wafers was limited by relatively poor minority carrier diffusion length.
Analysis of LGBC cells made on TOPSICLE I wafers showed that the inclusion of MIRHP in the process increased the mean bulk diffusion length. However, screen-printed cells fabricated on neighbouring wafers had an even higher mean diffusion length. It was apparent that MIRHP as applied in the buried contact process is not effective in realising the optimum bulk lifetime of the TOPSICLE material.
A batch of 50 high quality wafers was used to process high efficiency buried contact cells. Textureing, BSF formation and MIRHP passivation were done by UKON, all remaining process steps by NaREC. Due to technical contraints the cells could not be finished in time.
The roadmap discusses two different cell concepts. A screen-printed cell was investigated and possible improvements that can be obtained with input from other workpackages were discussed within the roadmap.
The buried contact cell design was the second considered approach of the roadmap. A detailed loss analysis and comparison of the 17.6% and 18.1% mc-Si efficiency cells has been carried out. The efficiency potential was determined using PC1D modelling, literature information and again input from another worpackage, leading to future possibilities to reach large area mc-Si 20% solar cells.
The loss analysis of the 18.1% solar cell showed that major improvement can be done by replacing the full area aluminium back surface field by a local rear contact scheme that provides low back surface recombination velocity and high optical rear surface reflectivity. Front surface recombination velocity can be lowered by using an additional thin thermal silicon oxid layer underneath the silicon nitride antireflection coating. Fillfactor of the 18.1% cell was 77%. Using a similar plating process, we have previously achieved fill factors of up to 78.8% and a fill factor of 79.0% does not seem unrealistic.
Another high efficiency improvement that may be implemented is the use of the zero-shading loss cell design, the angled buried contact concept. By succesfully applying all improvement the large area mc-Si solar cell this roadmap is leading to has a short circuit current density of 39.6mA/cm2, an open circuit voltage of 642.9 mV and a fillfactor of 79%, resulting in an efficiency of 20,1%.
This assessment compares in detail the costs, environmental impact of equipment, consumed materials and waste treatment in several high-efficiency mc-Si cell processing sequences developed within TOPSICLE. For this SCHOTT Solar has worked out a Methodology how to collect and analyse the detailed and complex data. The focus of this assessment was to serve as a steering instrument the TOPSICLE development efforts.
Detailed economical analysis showed that a newly developed in-line process resulting in 16.5% cell efficiency and based on isotexturing and screen-printing could result in a cost reduction of about 6% compared to a reference process with 15.5% cell efficiency. For a 17% cell process the cost reduction will be 8-9%. The ABC cell concept is more complex, but will result in higher efficiencies. Based on the currently available data, it is estimated that the efficiency for the ABC cells should be about 2% absolute higher to achieve the same cost per Wp.
A comprehensive study on the developed processes was carried out with respect to the national legislation and the EC directives. For all newly developed processes a limited environmental effect is expected. All emissions will be below 10% of the limits when the exhaust of chemical and furnace processes is purified and recycled. All this can be done with state-of-the-art technologies.
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