Exploring the versatility of lead sulfide quantum dots in low-temperature, solution-processed solar cells

Abstract

Solution processability and optoelectronic tunability makes lead sulfide quantum dots (PbS QDs) promising candidates for low-temperature, solution-processed thin film solar cells. Central to this thesis is the crucial role of QD surface chemistry and leveraging surface modification to prepare QDs suitable for optoelectronic device applications. The work presented here explores the versatility of PbS QDs integrated into two main device architectures, where the primary role of the QD is unique in each case. In p-i-n planar perovskite solar cells, efforts to utilize PbS QDs as a hole transport material and the effects of size tuning and surface passivation with cadmium on device characteristics are discussed. A combination of QD size reduction and minimal cadmium-to-lead cation exchange is found to improve the open circuit voltage and hole extraction into the PbS QD layer. In ZnO/PbS QD heterojunction solar cells, the feasibility of preparing fully inorganic, halometallate-passivated PbS QD inks for use as the absorber layer is discussed. A modified biphasic ligand exchange strategy is presented and in order to further elucidate electronic passivation in these QD ink systems, optical properties were investigated with steady state and time-resolved photoluminescence. Significantly, PbS QDs exhibit comparable quantum yields in solution before and after ligand exchange and no significant trap state emission was observed in solution and in film. Ink devices were fabricated with one- and two-layer depositions, which significantly reduce fabrication time compared to traditional layer-by-layer deposition, and devices exhibit anomalous efficiency improvement throughout storage in air.