The performances of n‐indium‐tin‐oxide (ITO)/n‐InGaAsP/p‐GaAs solar cells in which the chemical treatment at the ITO/InGaAsP heterointerface is varied are compared: HNO3 treatment, HCl treatment, and nontreatment. The cells with HNO3 treatment show good solar‐cell performance in spite of large lattice mismatch between ITO and InGaAsP. Others do not show even rectifying behavior. The structure of the cells with HNO3 treatment is thought to be a semiconductor‐insulator‐semiconductor structure, and its current model follows the tunneling model. By Auger analysis the oxide layer, which is thought to be formed by HNO3 treatment, was ascertained. Using the heterostructure back‐surface field for the cells with HNO3 treatment, the highest efficiency attained so far is 10.9% (total area) under AM1 illumination normalized to 100 mW/cm2; the corresponding open‐circuit voltage, short‐circuit current density, and fill factor are 0.56 V, 28.8 mA/cm2, and 0.677, respectively.