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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 (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.

Thin-film silicon solar cells on low-cost foreign substrates could lead to a large cost reduction of photovoltaic electricity if sufficiently high efficiencies could be obtained. A possible approach is to make polycrystalline-silicon solar cells by epitaxial thickening of large-grained thin seed layers made by aluminium-induced crystallization (AIC) of silicon. Until now however, obtained efficiencies are too low to lead to the desired cost reduction. We report on the influence of the AIC seed layer morphology (grain size and presence/absence of secondary crystallites on top of the surface) on the epitaxial growth of absorber layers and on the resulting cell parameters. To increase the grain size of the seed layers, we investigated the use of a nitric acid treatment to oxidize the Al layers prior to the amorphous silicon deposition. We compared seed layers oxidized by nitric acid treatment to seed layers oxidized by a short exposure to the ambient air. The nitric acid treatment led to a larger grain size and strongly affected the structure of the secondary crystallites on most investigated samples. Removing these crystallites before epitaxial growth clearly led to larger grains and increased the open-circuit voltages (Voc) of our solar cells. Using the nitric acid treatment, absorber layers with grain diameters up to 50 μm were made, that reached efficiencies of 4.9%, with Voc values around 500 mV. However, seed layers that were made through oxidation by air exposure and had grain diameters below 12 μm led to similar Voc values and even higher efficiencies (5.3%). We believe that regions with very small grain size on the large-grained samples are responsible for the relative poor behaviour of the large-grained solar cells. Our results show that increasing the average grain size of AIC seed layers does not automatically lead to better solar cells.

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.

Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. When hydrogen is introduced into multicrystalline Si that is often used for the fabrication of solar cells, the H atoms become trapped by carbon impurities to produce defect structures known at H2*(C). These defects act as both a source and a sink for hydrogen in H-related defect reactions. IR spectroscopy has been used to determine what H- and C-related defects are formed in multicrystalline Si when the carbon concentration is varied. A process that is used by industry to introduce hydrogen into Si solar cells is the post-deposition annealing of a hydrogen-rich SiNx layer. The H2*(C) defects provide a strategy for estimating the concentration and penetration depth of the hydrogen that is introduced by this method.

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.

Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. When hydrogen is introduced into multicrystalline Si that is often used for the fabrication of solar cells, the H atoms can become trapped by carbon impurities to produce defect structures known at H2*(C). These defects act as both a source and a sink for hydrogen in H-related defect reactions. IR spectroscopy has been used to determine what H- and C-related defects are formed in multicrystalline Si when the carbon concentration is varied. A process that is used by industry to introduce hydrogen into Si solar cells is the postdeposition annealing of a hydrogen-rich SiNx layer. The H2*(C) defects provide a strategy for estimating the concentration and penetration depth of the hydrogen that is introduced by this method.

Defect etching revealed a very large density (similar to 10(9) cm(-2)) of intragrain defects in polycrystalline silicon (pc-Si) layers obtained through aluminum-induced crystallization of amorphous Si and epitaxy. Electron-beam-induced current measurements showed a strong recombination activity at these defects. Cathodoluminescence measurements showed the presence of two deep-level radiative transitions (0.85 and 0.93 eV) with a relative intensity varying from grain to grain. These results indicate that the unexpected quasi-independence on the grain size of the open-circuit voltage of these pc-Si solar cells is due to the presence of numerous electrically active intragrain defects. (c) 2007 American Institute of Physics.

Large-grained polycrystalline silicon (poly-Si) films were prepared on foreign substrates by epitaxial thickening of seed layers. The seed layers were formed by the aluminum-induced layer exchange (ALILE) process which is based on aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). The epitaxial thickening was carried out at two different temperature regimes (low- and high-temperature approach). Using these large-grained poly-Si films first thin-film solar cells have been prepared. The best poly-Si thin-film solar cell obtained so far has reached an efficiency of 4.5% (high-temperature approach).