Enhanced electrocatalytic activity of the annealed Cu_2-xS Counter electrode for quantum dot-sensitized solar cells

Cu_2-xS films in situ formed on brass substrates were annealed at different temperatures (100, 200, 300, and 400?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 Cu_2-xS CE exhibited higher cell performance than that with the Pt CE. The QD-SSC with the 300?C-annealed Cu_2-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 Cu_2-xS CE. This is because the Cu_2-xS film annealed at 300?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 Cu_2-xS film was annealed at 400?C, secondary phases, such as CuO, Cu₂O, CuSO₄ and ZnO, formed, which reduced the cell performance of the QD-SSC with the 400?C-annealed Cu_2-xS CE due to the poor electrocatalytic activity.