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Macropore formation by photoelectrochemical (PEC) etching of N-type crystalline Si under front side illumination has been investigated. We report on two important parameters that influence the pore morphology: the applied anodic bias and the spectral distribution of the illumination source. On (100) c - Si, the pore morphology changes from a round shape at low bias (0 V/NHE) to a conical shape at high bias (2 V/NHE). In the latter case we show that, by adjusting the spectrum of the white light source with longpass filters of different cut-on wavelengths, we can control the cone angle of the pores from 50° with white light to 10° with infrared light. Optical measurements indicate that macropores formed with a narrow cone angle (20°) could be of interest for Si solar cell texturization since they result in a very low reflectivity (2.6% at 800 nm) and in an efficient light trapping.

Metal-assisted chemical etching (MAE) was used as an alternative route to texturise the front surface of multicrystalline Si solar cells. Ag nanoparticles deposited by an electroless method on the Si surface acted as catalysts for Si etching in a solution containing HF and H2O2, an oxidizing species. MAE formed a duplex structure at the surface of the Si wafer consisting in a mesoporous Si layer that after dissolution in NaOH revealed the second structure, which is a macrotexturised surface. After texturisation, the effective reflectivity (AM1.5G, 400-1000 nm) ranged between 12% and 15% depending on the duration of the dissolution treatment of the mesoporous layer. MAE texturised solar cells were compared to NaOH texturised solar cells made with similar wafers (∼25% in effective reflectivity). MAE texturisation leads to a gain in fill factor (FF) but not in short-circuit current (JSC) (from average values). The best MAE cell exhibited higher JSC (+2%) and efficiency (16.1%) compared to the best NaOH cell (15.7%). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Taking advantage of the electrochemical formation of macropores in aqueous HF under illumination, we have developed an efficient photoelectrochemical (PEC) texturization on n-type multicrystalline Si (mc-Si) wafers which leads to a decrease in the reflectivity of solar cells. An optimized mc-Si texturized surface exhibits an 8.5% effective reflectivity (λ=400-1000 nm, AM1.5 standard solar illumination).

The formation of porous Si (PSi) antireflection coating (ARC) on n + -p multicrystalline Si solar cells has been investigated in view of its implementation at the last stage of the solar cell processing. PSi layers were formed either by electrochemical (EC) etching or by chemical stain (CS) etching and led to effective reflectivity around 13% (AM1.5). Solar cells with solar energy conversion efficiency of 13.3% and 12.6% were obtained with PSi ARC formed by EC- and CS-etching, respectively, the difference originating from a 2-3% larger short-circuit current for the EC-etched cells. Additionally, the FF was higher by 1-2% (a.v.) after PSi formation, which demonstrates that both methods are non-deleterious for the front contact quality and hence suitable for a standard solar cell process.

Conditions of macropore formation on multicrystalline medium doped silicon were deduced from a systematic study of the macroporous morphology of (100) and (111) oriented monocrystalline silicon in function of the porous etching parameters and silicon doping. A new promising method to texturize the surface of multicrystalline silicon wafers has been developed which leads to a macroporous surface with a very low effective reflectivity ∼9%. Macroporous multicrystalline silicon solar cells exhibit high efficiency larger than 13.5%.