A model is presented for heterojunction solar cells, in which interface recombination is the dominant diode current mechanism, that relates the charging of interface states to the voltage dependence of the light‐generated currentjL(V). The model also shows how a diode ideality factor greater than two can occur. The calculations presented are for the small‐signal case, and do not treat the intensity dependence of the collection factor &eegr;(V)=jL(V)/jsc. The parameters chosen were such as to account for the trends seen in the CuInSe2/CdS solar cell.