Cu2-xS films in situ formed on brass substrates were annealed at different temperatures (100, 200, 300, and 400 degrees C) in flowing Ar gas, and introduced as the counter electrodes (CEs) for CdSe quantum dot-sensitized solar cells (QD-SSCs). The QD-SSC with the bare Cu2-xS CE exhibited higher cell performance than that with the Pt CE. The QD-SSC with the 300 degrees C-annealed Cu2-xS CE achieved the best energy conversion efficiency of 4.25%, showing a 34% increase compared to the cell performance of that with the bare Cu2-xS CE. This is because the Cu2-xS film annealed at 300 degrees C provided rapid charge transport and the increased electrochemical active sites due to the improved crystallinity and surface roughening, leading to significantly enhanced electrocatalytic activity. On the other hand, when the Cu2-xS film was annealed at 400 degrees C, secondary phases, such as CuO, Cu2O, CuSO4 and ZnO, formed., which reduced the cell performance of the QD-SSC with the 400 degrees C-annealed Cu2-xS CE due to the poor electrocatalytic activity. (C) 2013 The Electrochemical Society. All rights reserved.