Optimization of PbS Hole Transport Layer using Hybrid Ligand Treatment

Abstract

Quantum dots have emerged as a promising low-cost solution processable solar harvesting material, with efficiencies having increased to 12% within a relatively short 10-year time span since its inception. This technology is also driven by the flexibility to fine tune the bandgap and electronic properties via surface ligand engineering. Of the different quantum dot materials, PbS quantum dots have been demonstrated to be the most promising. PbS quantum dot solar cells were responsible for the certified efficiency of 12% in 2018, and have demonstrated good air stability, indicating the opportunity for large scale commercial implementation in the future. This high efficiency was achieved by utilizing a n+-n-p structure where the n-doped and p-doped layers were based on PbS quantum dots passivated with different ligands. The p-type layer has generally suffered from low mobility. This is due to the organic ligand 1,2-ethanedithiol that is used to modify the quantum dot surface such that p-doping is achieved. This naturally limits the device thickness as the carrier diffusion length is limited by the low mobility. The work done in this thesis aims to improve the properties of the p-type layer through hybrid ligand treatments. By treating the p-type layer with two types of ligands, 3-mercaptopropionic acid and 1,2-ethanedithiol, the PbS quantum surface was passivated by a combination of the two ligands. The combination of the two ligands resulted in an overall improvement in open circuit voltage, fill factor and current density, leading to an improvement in cell efficiency from 6-8% to 9-10%, with a champion cell efficiency of 10.4%. This achievement was a result of a reduction in interdot distance, leading to an improvement in film conductivity. The hybrid ligand treatment conditions also drastically affect the results. 1,2-ethanedithiol treatment concentration particularly affected the penetration of the ligand into the active layer, which resulted in an undesirable drop in current density. The thesis concluded that the treatment with low mobility organic ligands should be done minimally to only passivate the hole transport layer without further deeper penetration into the n-type layers.