Electroabsorption measurements and built-in potentials in amorphous silicon-germanium solar cells

We report electromodulated reflectance spectra in n-i-p solar cells with hydrogenated amorphous silicon-germanium alloy absorber layers. At lower photon energies the spectra are determined by bulk electroabsorption, and exhibit peaks near the optical gap of the absorber layers. Voltage scaling of the electroabsorption spectra indicate a built-in potential of V_bi=1.17 V in cells with absorber layer band gaps of 1.50 eV; in conjunction with earlier work, this value argues against a systematic decline in V_bi with an absorber layer band gap. At higher photon energies the spectra are due to direct electroreflectance; the voltage scaling was consistent with model predictions for the electric field at the interface of the p-type and absorber layers.