Improvement of epitaxial crystalline silicon thin-film solar cells at Fraunhofer ISE

Due to the increased costs of silicon feedstock, the interest in silicon thin-film solar cells has been intensified in the last years. The aim of this paper is to present improvements to the epitaxial crystalline silicon thin-film solar cells concept that have been achieved at Fraunhofer ISE. Within the thin-film technologies, our approach is based on the concept of an epitaxial wafer-equivalent. It consists of a crystalline silicon thin-film on a highly doped silicon substrate, which can then be processed using a “standard” wafer cell process. Simple process optimisation has been carried out, focusing on the epitaxial back surface field, the base thickness and an enhanced doping profile. Simulation results were confirmed experimentally. A simplification of the solar cell process by growing the emitter in-situ after the base deposition is investigated. Efficiencies up to 14.9% with open circuit voltages up to 655 mV were reached with base thicknesses below 20 μm. However, to further improve the performance a front side texture has to be implemented. Initial results show that plasma etching of a thick epitaxial emitter creates shunts. Remote plasma hydrogen passivation (RPHP) produces a significant enhancement of the open circuit voltage Voc of epitaxial wafer equivalents on low cost silicon substrates. An increase of up to 12 mV is shown on ungettered and up to 16 mV on gettered highly doped multicrystalline silicon substrates. Combining all process optimisations should result in efficiencies higher than 15% on Cz and 14% on mc.