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We obtained 17.9% cell efficiency on thin and large mc-Si REC wafers using ECN's metal-wrap-through (MWT) concept. Optimization of several cell processing steps led to an increase of more than 2% absolute in cell efficiency. With these cells 36-cell modules were manufactured at 100% yield in our industry scale module pilot line. The highest module efficiency obtained (as independently confirmed by JRC-ESTI) was 17%. In this module the average cell efficiency was 17.8%; this shows a small difference between cell and module efficiency. Copyright © 2011 John Wiley & Sons, Ltd.

Thin films of polysilicon are an attractive material to reduce the cost of photovoltaic energy. Among the different polysilicon techniques, fine-grained polysilicon deposited directly onto a foreign substrate is an interesting option, since no extra seed layer or crystallization step is needed. However, the fabrication of efficient fine-grained polysilicon solar cells is a real challenge due to the large number of defects and the small average grain size of only 0.2 µm. This paper reports on our recent progress with fine-grained polysilicon solar cells. Using a diffuse refracting top surface and a more efficient two-step hydrogenation, we obtained an efficiency of 5.0%.

Thin-film polycrystalline-silicon solar cells might become an alternative to bulk silicon solar cells if sufficiently high efficiencies can be obtained. In this work we made pc-Si layers using aluminium-induced crystallization and thermal CVD on alumina substrates. By using plasma texturing and optimizing the cell structure, we increased the current density of our cells and achieved a cell efficiency of 8.0%. At present, our cell efficiency seems to be mainly limited by the presence in our layers of a high density of electronically active intragrain defects. Intragrain quality improvement will therefore be very important to further increase our pc-Si cell efficiency.

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc-Si layers obtained by aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc-Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc-Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc-Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin-on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a-Si layers that yield much higher Voc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc-Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc-Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.

Thin-film polycrystalline-silicon solar cells offer a promising alternative to standard silicon solar cells. Until now, however, obtained efficiencies are too low to lead to a cost reduction in photovoltaics. To obtain cells with higher efficiencies, polycrystalline-silicon layers need large grains, good passivation and an optimized cell design. In this work we compare pc-Si solar cells with an amorphous silicon–crystalline silicon heterojunction emitter to cells with a diffused phosphorus emitter. In heterojunction cells, hydrogen passivation was carried out before emitter formation, which appears to make it more effective. Open-circuit voltages ( V oc ) were much higher for cells with a heterojunction emitter, reaching values up to 520 mV. A maximum efficiency of 5.3% was obtained on the heterojunction cells, while the homojunction cells led to substantially lower efficiencies. A cell concept based on a heterojunction emitter seems therefore most favorable to lead to highly efficient thin-film pc-Si solar cells. The high V oc and efficiency values obtained in this work form an important step towards cost-effective polycrystalline-silicon solar cells.

Thin-film polycrystalline-silicon (pc-Si) solar cells with a high efficiency could lower the price of photovoltaic electricity substantially. Efficient thin-film solar cells will not only lead to a cost reduction by the use of less silicon material, but will also reduce the module fabrication costs if a monolithic module process is used that integrates cell interconnection with cell contacting. Aluminium-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening recently proved to be a simple way to obtain large-grained pc-Si thin films with excellent properties for solar cells. However, cell processes different from those for bulk-Si cells have to be implemented to fully exploit the pc-Si material quality and to obtain working solar cells. In this work, we propose a simple monolithic module process for thin-film pc-Si solar cells, in which all contacts are on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts, using pc-Si layers on ceramic substrates grown by AIC in combination with high-temperature epitaxy. Next, we made pc-Si modules using a simplified metallization scheme. The interdigitated pc-Si cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to a lower series resistance and a higher current density. The maximum obtained cell efficiency was 5.6%, which is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. First module results showed that good cell separation and isolation is crucial to obtain proper working modules. Our interdigitated cell results indicate that monolithic thin-film modules with interdigitated top contacts based on pc-Si layers made by AIC will most likely lead to high efficiencies.

We propose a novel approach to thin-film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS-HJ). High crystallographic quality mono-Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free-standing mono-Si (FMS) films were processed into solar cells by creating a-a-Si/c-Si heterojunction. In our preliminary experiments a thin-film FMS-HJ solar cell with 9.6% efficiency was realized in a 20-μμm-thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.

Thin-film polysilicon solar cells are a promising low-cost alternative for bulk silicon solar cells due to their reduced material thickness. Recently, we showed that the use of an amorphous silicon/polycrystalline silicon heterojunction emitter instead of a diffused homojunction emitter led to a boost in the open-circuit voltage by 90 mV. Now, we present a full evidence that shows that this improvement is related to the absence of dopant smearing along the grain boundaries. By using scanning spreading resistance microscopy, we found an enlargement of the junction area by a factor of five in case of a homojunction. The tips of the dopant spikes represent lowly doped areas with an enhanced recombination.

A considerable cost reduction in photovoltaics could be achieved if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) films. Although hydrogen passivation of pc-Si films is crucial to obtain good solar cells, the exact mechanism of hydrogen diffusion through pc-Si layers is not yet understood. In this letter, the influence of the junction and the grain size are investigated. We find that the presence of a p-n junction acts as a barrier for hydrogen diffusion in thin-film polysilicon solar cells. Therefore, pc-Si solar cells should preferably be passivated before junction formation. Furthermore, pc-Si layers with large grains retain less hydrogen after passivation than layers with small grains. This indicates that hydrogen atoms get mainly trapped at the grain boundaries.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1186-1188, This presentation will show how an improved understanding of multicrystalline wafer quality can explain the variations in cell performance across multicrystalline silicon blocks. Using infrared scanning a silicon block was sorted out with precipitates in the middle of the block. Photoluminescence combined with defect etching revealed needle-like precipitates along the grain boundaries. The precipitations lead to reduced shunt resistance. In the middle of the block numerous crystallographic defects were identified that lower the current collection and the final cell efficiency.

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could lower the price of photovoltaic electricity substantially. At the MRS conference in 2006, we presented a pc-Si solar cell with an efficiency of 5.9% that had an absorber layer made by aluminum-induced crystallization (AIC) of amorphous silicon followed by high-temperature epitaxial thickening. The efficiency of this cell was mainly limited by the current density. To obtain higher efficiencies, we therefore need to implement an effective light trapping scheme in our pc-Si solar cell process. In this work, we describe how we recently enhanced the current density and efficiency of our cells. We achieved a cell efficiency of 8.0% for pc-Si cells in substrate configuration. Our cell process is based on pc-Si layers made by AIC and thermal CVD on smoothened alumina substrates. The cells are in substrate configuration with deposited a-Si heterojunction emitters and interdigitated top contacts. The front surface of the cells is plasma textured which leads to an increase in current density. The current density is further enhanced by minimizing the back surface field thickness of the cells to reduce the light loss in this layer. Our present pc-Si solar cell efficiency together with the fast progression that we have made over the last few years indicate the large potential of pc-Si solar cells based on the AIC seed layer approach.

Thin-film polysilicon solar cells are a promising low-cost alternative for bulk silicon solar cells. Due to their reduced material thickness, these solar cells are less dependent on the silicon feedstock price. Until now these devices showed a worse performance compared to bulk Si solar cells due to the small grain size and the high recombination velocity at the grain boundaries. A better understanding of hydrogen passivation is therefore of crucial importance to improve the efficiency of polysilicon solar cells. In this work we characterized fine-grained polysilicon layers with a grain size of only 0.2 μm before and after passivation. Plasma hydrogenation led to a higher hydrogen concentration in the first micron of the layer than nitride passivation. The highest efficiency of 5.0 % was reached when nitride passivation was followed by plasma passivation.

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) layers. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening is an effective way to obtain large-grained pc-Si layers with excellent properties for solar cells. To obtain efficient solar cells, the electronic quality of the pc-Si material obtained by AIC has to be optimized and the cell design has to be adapted to the material. In this paper, we report on pc-Si solar cells made by AIC in combination with thermal CVD on ceramic alumina substrates. We made pc-Si solar cells on alumina substrates that showed Voc values up to 533 mV and efficiencies up to 5.9%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of silicon was used. We demonstrate that the quality of the pc-Si material can be improved drastically by reducing the substrate roughness using spin-on oxides. We further show that a-Si/c-Si heterojunctions lead to much higher Voc values than diffused homojunctions. A cell concept that incorporates spin-on oxides and heterojunction emitters is therefore best suited to obtain efficient pc-Si solar cells on alumina substrates.

Hydrogenation by high temperature rapid annealing of SiNx:H is found to be very effective on the defects responsible for the carrier trapping effect in multicrystalline silicon. The passivation effect is reversible and is annihilated by a long thermal annealing. As for the passivation of deep, lifetime killing defects, the efficiency of “trap” removal by the short thermal treatment depends on the density of the SiNx:H layer. This effect is, in fact, well correlated with performance improvement observed in solar cells. The parallelism between the trap and recombination center passivation effects suggests that they originate from the same defect.