Development of Hydrogenated Microcrystalline Silicon-Germanium Alloys for Improving Long-Wavelength Absorption in Si-Based Thin-Film Solar Cells

Abstract

Hydrogenated microcrystalline silicon-germanium ( μ c-Si1-xGex:H) alloys were developed for application in Si-based thin-film solar cells. The effects of the germane concentration (RGeH4)and the hydrogen ratio (RH2) on the μ c-Si1-xGex:H alloys and the corresponding single-junction thin-film solar cells were studied. The behaviors of Ge incorporation in a-Si1-xGex:H and μ c-Si1-xGex:H were also compared. Similar to a-Si1-xGex:H, the preferential Ge incorporation was observed in μ c-Si1-xGex:H. Moreover, a higherRH2significantly promoted Ge incorporation for a-Si1-xGex:H, while the Ge content was not affected byRH2in μ c-Si1-xGex:H growth. Furthermore, to eliminate the crystallization effect, the 0.9 μ m thick absorbers with a similar crystalline volume fraction were applied. With the increasingRGeH4, the accompanied increase in Ge content of μ c-Si1-xGex: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 ofRH2andRGeH4, the single-junction μ 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 μ c-Si:H cell, the external quantum efficiency at 800 nm had a relative increase by 33.1%.