Development of hydrogenated microcrystalline silicon-germanium alloys for improving long-wavelength absorption in Si-based thin-film solar cells

Abstract

Hydrogenated microcrystalline silicon-germanium (c- Si 1-x Gex:H) alloys were developed for application in Si-based thin-film solar cells. The effects of the germane concentration (R GeH 4) and the hydrogen ratio (R H 2) on the c- Si1-x Gex:H alloys and the corresponding single-junction thin-film solar cells were studied. The behaviors of Ge incorporation in a- Si1-x Gex:H and c- Si1-x Gex:H were also compared. Similar to a- Si1-x Gex:H, the preferential Ge incorporation was observed in c- Si1-x Gex:H. Moreover, a higher RH2 significantly promoted Ge incorporation for a- Si1-x Gex:H, while the Ge content was not affected by RH2 in c- Si1-x Gex:H growth. Furthermore, to eliminate the crystallization effect, the 0.9 m thick absorbers with a similar crystalline volume fraction were applied. With the increasing RGeH4, the accompanied increase in Ge content of c- Si1-x Gex:H narrowed the bandgap and markedly enhanced the long-wavelength absorption. However, the bias-dependent EQE measurement revealed that too much Ge incorporation in absorber deteriorated carrier collection and cell performance. With the optimization of RH2 and RGeH4, the single-junction c- Si1-xGex:H cell achieved an efficiency of 5.48%, corresponding to the crystalline volume fraction of 50.5% and Ge content of 13.2 at.%. Compared to c-Si:H cell, the external quantum efficiency at 800 nm had a relative increase by 33.1%.