Microcrystalline Silicon Germanium for Thinner Silicon-Based Tandem Solar Cells

Microcrystalline silicon germanium (μc-Si1-xGex:H) alloys have a potential advantage over microcrystalline silicon (μc-Si:H) as infrared-active absorbers in thin film Si multijunction solar cells. In principle Ge incorporation provides an enhanced infrared absorption allowing for a significant reduction of the material thickness in the devices, but it can also give rise to adverse effects on carrier transport. Here μc-Si1-xGex:H films were prepared by VHF-PECVD at 100-MHz using a SiH4-GeH4-H2 gas mixture. The Ge content in the films has been estimated from the peak position of the Si-Si optical mode in the Raman spectra. The optical properties down to the subgap region have been investigated by spectrophotometry and Photothermal Deflection Spectroscopy. The material potentialities as absorber layer in solar cells have been tested by fabricating p-i-n type devices with 0.5 μm thick μc-Si1-xGex:H i-layer with different x values. The spectral response is found to be affected by collection problems due to the reduced carrier lifetime in the defective material. Further experiments are underway to improve the quality of the μc-Si1−xGex:H films.