The photovoltaic properties and electronic structures of amorphous In-Ga-Zn-O (a-IGZO)/p-type Si heterojunctions were investigated. An optimized solar cell with a conversion efficiency of 5.3% was obtained by vacuum annealing at 300 degrees C. The short circuit current was 28.1 mA.cm(-2), which approaches the theoretical limit without an electron back reflector. The open circuit voltage (V-oc) was 0.31 V, which was limited by the built-in potential of 0.39 V. This result combined with hard x-ray photoemission spectroscopy provided the electron affinity and ionization potential of a-IGZO, indicating the low V-OC was controlled by the deep conduction band bottom of a-IGZO. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3595741] All rights reserved.