Doped hydrogenated amorphous silicon (a-Si:H) films of only a few nanometer thin find application in a-Si:H/crystalline silicon heterojunction solar cells. Although such films may yield a field effect at the interface, their electronic passivation properties are often found to be inferior, compared to those of their intrinsic counterparts. In this article, based on H2 effusion experiments, the authors argue that this phenomenon is caused by Fermi energy dependent Si–H bond rupture in the a-Si:H films, for either type of doping. This results in the creation of Si dangling bonds, counteracting intentional doping of the a-Si:H matrix, and lowering the passivation quality.

/pdf/nature-of-doped-a-si-h-c-si-interface-recombination-51yt3xv2cd.pdf

Nature of doped a-Si:H / c-Si interface recombination

Recent advances in the production of Si nanostructures from electroless etching are reviewed, including stain etching, metal-assisted etching and chemical vapour etching. A brief review of the explosion in applications of porous silicon over the past 18 months is also given. The stain film that results from the etching of (poly- or single-)crystalline Si is composed of a porous network of nanocrystalline silicon. Few mechanistic studies of electroless etching have been performed, but the more extensively studied anodic etching of silicon in fluoride solutions provides many clues as to how porous films are formed. Intriguing recent results have shown that control over the properties of the film can be obtained by exercising control over the composition of the etchant.

Silicon nanostructures from electroless electrochemical etching

Polycrystalline silicon thin-film solar cells: Status and perspectives

When exposed to light, boron doped monocrystalline Czochralski grown silicon suffers from degradation of the minority carrier lifetime due to the formation of recombination active boron-oxygen related defects. The so called regeneration procedure is able to convert these recombination active defects into a new less recombination active state characterized by a higher minority charge carrier lifetime and stability under illumination. However, the exact working principle on microscopic scale is still unknown even though some influencing factors were identified. The role of hydrogen in the regeneration process is investigated in this work. We find that the characteristic regeneration time constant is subject to variation depending on the process parameters of a Plasma Enhanced Chemical Vapor Deposition a-SiNx:H deposition, namely the applied gas flows, as well as on the thermal history of the sample prior to applying the regeneration procedure. The positive effect of a short high temperature (800–900 °C) step leads to the idea that the presence of atomic hydrogen in the silicon bulk is crucial for the regeneration effect to occur. The different regeneration behavior of samples with variable thickness of a hydrogen diffusion barrier, namely an Al2O3 layer capped by SiNx:H, supports those results. Finally, the importance of hydrogen for regeneration is directly shown on samples having different hydrogen bulk concentrations due to direct hydrogenation in a Microwave Induced Remote Hydrogen Plasma reactor. A new model to explain the effect of the regeneration of boron-oxygen related defect centers based on the possible role of atomic hydrogen is presented.

/pdf/influence-of-hydrogen-on-the-regeneration-of-boron-oxygen-1b1r0vvfhq.pdf

Influence of hydrogen on the regeneration of boron-oxygen related defects in crystalline silicon

Semiconductor solar cells: Recent progress in terrestrial applications

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.

8% Efficient thin-film polycrystalline-silicon solar cells based on aluminum-induced crystallization and thermal CVD

Thin-film polycrystalline silicon solar cells on ceramic substrates by aluminium-induced crystallization

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.

Carrier trap passivation in multicrystalline Si solar cells by hydrogen from SiNx:H layers

Defect etching revealed a very large density (∼109cm−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 0.93eV) 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.

/pdf/electrical-activity-of-intragrain-defects-in-polycrystalline-2dr7qn4546.pdf

L. Carnel

Papers

8% Efficient thin-film polycrystalline-silicon solar cells based on aluminum-induced crystallization and thermal CVD

Thin-film polycrystalline silicon solar cells on ceramic substrates by aluminium-induced crystallization

Carrier trap passivation in multicrystalline Si solar cells by hydrogen from SiNx:H layers

Electrical activity of intragrain defects in polycrystalline silicon layers obtained by aluminum-induced crystallization and epitaxy

Fabrication and characterization of highly efficient thin-film polycrystalline-silicon solar cells based on aluminium-induced crystallization