Two-step annealed CdS/CdSe co-sensitizers for quantum dot-sensitized solar cells

•Stepwise structure of TiO2/CdS/CdSe was annealed using a two-step procedure.•TiO2/CdS/CdSe was also conventionally annealed at one temperature (150 or 300 °C).•The two-step annealed sample led to significantly enhanced charge transport through CdS.•The two-step annealed sample also provided better light harvesting.•These two beneficial effects enhanced the cell efficiency significantly.

CdS/CdSe co-sensitizers on TiO2 films were annealed using a two-step procedure; high temperature (300 °C) annealing of TiO2/CdS quantum dots (QDs), followed by low temperature (150 °C) annealing after the deposition of CdSe QDs on the TiO2/CdS. For comparison, two types of films were prepared; CdS/CdSe-assembled TiO2 films conventionally annealed at a single temperature (150 or 300 °C) and non-annealed films. The 300 °C-annealed TiO2/CdS/CdSe showed severe coalescence of CdSe QDs, leading to the blocked pores and hindered ion transport. The QD-sensitized solar cell (QD-SSC) with the 150 °C-annealed TiO2/CdS/CdSe exhibited better overall energy conversion efficiency than that with the non-annealed TiO2/CdS/CdSe because the CdSe QDs annealed at a suitable temperature (150 °C) provided better light absorption over long wavelengths without the hindered ion transport. The QD-SSC using the two-step annealed TiO2/CdS/CdSe increased the cell efficiency further, compared to the QD-SSC with the 150 °C-annealed TiO2/CdS/CdSe. This is because the 300 °C-annealed, highly crystalline CdS in the two-step annealed TiO2/CdS/CdSe improved electron transport through CdS, leading to a significantly hindered recombination rate.