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 Sip+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 anilayer deposited using strong hydrogen dilution we obtain 1.25±0.04 V. We speculate that interface dipoles between thep+andilayers significantly influence the built‐in potential. ©1996 American Institute of Physics.