Depleted Bulk Heterojunctions in Thermally Annealed PbS Quantum Dot Solar Cells

We have studied the detailed interface structure and energy conversion behavior of TiO2/PbS heterojunction solar cells. Nanoscale structure and composition analysis have revealed that thermal annealing causes intermixing of the TiO2 and PbS phases and influences the morphologies and optical properties of the heterojunction film. This intermixing increased the junction area within the depleted bulk heterojunction (DBH) layer and promoted the carrier extraction from PbS QDs to TiO2. In addition, the thermal annealing caused interparticle necking between PbS QDs and increased the crystallinity of the PbS QD film. Compared with unannealed PbS/TiO2 heterojunction solar cells, the formation of the DBH layer and the partial sintering of PbS QDs led to a doubling of the short-circuit current (J(sc)) and an improved energy conversion efficiency, by 39%. Electric force microscopy analysis confirmed the presence of a DBH layer. The electron lifetime and fill factor (FF) of the solar cells decreased when the TiO2/PbS mixed film was thermally annealed, and this was assigned to a lower recombination resistance in the DBH layer. Post-treatment of PbS/TiO2 DBH films with ethanedithiol was found to increase the recombination resistance at PbS/TiO2 interface and to enhance the energy conversion efficiency to similar to 4%.