The effects of oxidation on lead sulfide quantum dots for photovoltaic devices

Abstract

Colloidal quantum dots (CQDs) are of great interest in application of photovoltaic (PV) devices, due to their cost-effectiveness and solution-based processability. They also have excellent size-tunable optical properties which are highly desirable for optoelectronic devices. Lead(Ⅱ) sulfide (PbS) CQDs are promising material for PV devices due to their strong quantum confinement and high dielectric constant. However, the relatively low power-conversion-efficiency and poor air-stability are remaining challenges. It is essential to understand oxygen reaction on CQD PV devices for better performance. But only few reports have focused on the effects of oxidation. In this thesis, Schottky junction PVs comprised of PbS CQDs were fabricated. Uniform PbS CQDs (average diameter ~ 3 nm) were synthesized with thermal decomposition method. The PbS quantum dot (QD) films were produced via layer-by-layer spin-coating method using 1,2-ethanedithiol. Finally, we fabricated Schottky junction PVs by metal electrodes deposition. We employed a simple pre-oxidation treatment on PbS QD films prior to deposition of metal electrodes. It significantly enhanced an open-circuit voltage up to ~28%, fill factor up to ~ 72% and power conversion efficiency up to ~65%. Also it improved the capability of PCE retention in ambient condition. We systemically analyzed oxidation effects on PbS QD PVs with X-ray photoelectron spectroscopy (XPS), UV-visible spectrophotometer, transmission electron microscopy (TEM) analyses and current-voltage measurements. We found that the improvements of device characteristics are due to the formation of oxygen compounds: PbO, PbSO3 and PbSO4. The oxidation changed the characteristics of PbS QDs. Interestingly post-oxidation treatment also increased the device performance in case of no pre-oxidation. But it involved rapid degradation of device performance, while pre-oxidized devices showed gradual degradation. Pre-oxidation treatment improved not only device perf...