An Assessment of a-SiGe:H Alloys with a Band GaP of 1.5Ev as to their Suitability for Solar Cell Applications

Hydrogenated amorphous silicon germanium alloys a-Si1-xGex:H are being actively investigated for their application as a low band gap material in cascade solar cells [1,2]. To date, such alloys produce material of reasonable electronic quality only if the Ge-content is kept low (<40 at.%) such that the band gap is not decreased much below 1.5 eV. Conversion efficiencies of -5% have been obtainedwith alloys having such a band gap, and tandem cells have shown conversion efficiencieswhich are lower than those of good quality single layer a-Si:H devices. Thus, the performance of alloys is well below that necessary to achieve conversion efficiencies of >16%, which are ultimately hoped to be obtained using the cascade approach [2]. Other low band gap alloys such as a-Si1-xSnx:H have been shown to be even less suitable with regard to their electronic properties [3]. The cause of the degradation in electronic properties with increased alloying is not yet understood. Factors such as preferential attachment of H to Si rather than Ge [4] or microstructure observed in alloys have been suggested as a cause for the electronic degradation, [5,6] but no unique correlations have been established between such findings and the electronic properties.