Sputtered-deposited Zn(S,O) is an attractive alternative to CdS for the window layer of Cu(In,Ga)Se 2 (CIGS) thin-film solar cells due to its higher band gap, which allows greater blue-photon collection. However, distortions to current-voltage ( J-V) curves are observed in some cases. A straightforward photodiode model with a secondary barrier at the Zn(S,O)/CIGS interface is employed to explain the physical mechanisms of the experimental J-V distortions. The primary contributor to the secondary barrier is the conduction-band offset (CBO), whose magnitude is determined by the oxygen fraction in Zn(S,O) and by the carrier density of Zn(S,O); the latter may be increased with indium-doping. Comparison of experimental and simulated J-V characteristics with variation in oxygen fraction, window carrier density, and temperature, allows a reasonably compelling test of the secondary-barrier model for this system.