Electroabsorption measurements and built-in potentials in amorphous silicon p-i-n solar cells

We present a technique for using modulated electroabsorption measurements to determine the built-in potential in semiconductor heterojunction devices. The technique exploits a simple relationship between the second-harmonic electroabsorption signal and the capacitance of such devices. We apply this technique to hydrogenated amorphous silicon (a-Si:H)-based solar cells incorporating microcrystalline Si p⁺ layers. For one set of cells with a conventional plasma-deposited intrinsic (i) layer we obtain a built-in potential of 0.98±0.04 V; for cells with an i layer deposited using strong hydrogen dilution we obtain 1.25±0.04 V. We speculate that interface dipoles between the p⁺ and i layers significantly influence the built-in potential.