Oxygen annealing of the ZnO nanoparticle layer for the high-performance PbS colloidal quantum-dot photovoltaics

Though numerous researches regarding the influence of annealing atmospheric condition of ZnO have been carried out, the impact of annealing atmosphere on the carrier transporting properties and the performance of the ZnO-based optoelectronics has not been well-established. Here, the effects of annealing atmosphere (i.e., N2, ambient air, and O2) used to generate ZnO nanoparticle (NP) layers are elucidated. The chemical nature of ZnO layers, especially the amount of oxygen vacancies in ZnO NPs, is modulated by the annealing atmosphere. As the composition of O2 gas increases in the annealing atmosphere, a notable reduction of oxygen vacancies of ZnO NPs and electron mobility enhancement are observed, indicating that O2 gas contributes to a reduction of surface defects on ZnO NPs during the annealing process. In addition, trap-filling by reduced oxygen vacancies of air- and O2-annealed ZnO layers, induces the enhanced built-in potential in colloidal quantum-dot photovoltaic (CQDPV) devices. As expected, PbS CQDPVs with an air- and O2-annealed ZnO layer demonstrate significantly improved power conversion efficiencies than CQDPVs with an N2-annealed ZnO layer. Further analysis shows that the interfacial recombination is reduced for CQDPVs with an air- and O2-annealed ZnO layer due to the reduced trap states of ZnO NPs.