Enhanced light-scattering properties of sputtered Ag/Al:Si bilayers deposited at a low substrate temperature-A potential back reflector for highly efficient flexible thin-film photovoltaic devices

摘要

Nanotextured Ag/Al:Si back reflectors with enhanced light-scattering efficiency for flexible thin-film solar cells were developed at a low substrate temperature (T_s) by sophisticated control of the microstructural evolution of metal films. Development was based on a structure zone model and a bilayer concept involving a high reflectivity Ag top layer deposited onto a surface-textured Al:Si bottom layer. The texture evolution with a root-mean-square (σ_(rmS)) surface roughness of 60.1 nm in the Al:Si bottom layer was successfully achieved at a low Ts of 100 °С by abnormal grain growth. The growth was induced by the low melting point of Al and the impurity drag effect on the grain boundaries caused by the Si dopant, whereas the σrms of the pure Ag film deposited at T_s=200 °C was 46.9 nm. After room temperature deposition of highly reflective Ag films onto the nanotextured Al:Si bottom layers, significant improvement in the light-scattering properties of the Ag/Al:Si bilayers was obtained compared to the Al:Si films without Ag films, for which the reflection significantly decreased due to the interband absorption and low reflectivity of Al. Under optimum conditions, the light-scattering efficiency of nanotextured Ag/Al:Si bilayers deposited at T_s=75 °С is comparable to that of reference Ag films at T_s=200 °C. When the nanotextured Ag/Al:Si bilayers deposited at T_s=75 °С as back reflectors were applied to flexible nc-Si:H thin-film solar cells, a high conversion efficiency of 8.6% was successfully achieved, and no detrimental reduction in cell performance was observed in comparison with the solar cell using the reference Ag back reflectors deposited at 200 °C.