Photoemission Studies of Charge Transport, Passivation and Electronic Structure for Quantum Dot Solar Cells

UoM administered thesis: Phd

  • Authors:
  • Philippa Clark

Abstract

Colloidal quantum dot (CQD)-based solar cells are a promising technology that could provide low-cost energy in the future. In this thesis the passivation of PbS CQD surfaces, carrier dynamics in PbS CQD-based solar cell systems, and the electronic structure of a potential photoanode material (CdO) are studied using a variety of photoemission spectroscopies and complementary techniques. The surface passivation of PbS CQDs is investigated using X-ray photoelectron spectroscopy (XPS) and synchrotron radiation-excited depth profiling XPS, which gives information on the chemical composition and oxidation state of the surface. PbS CQDs were studied after treatments including Cd cation exchange, halide (Cl, Br, I) ligand exchanges, short organic ligand (EDT) exchange, and combinations of these treatments. For Cd cation exchanged PbS CQDs a thicker Cd shell results in less oxidation, and an optimum effective shell thickness of 0.1 nm provides suffcient protection against oxidation without introducing a large insulating barrier which can hinder charge transfer from the CQD. For combined ligand treatments, evidence of passivants being etched from the surface is found. This significantly reduces the air stability and device performance, which is found to be strongly connected to the number of passivants present on the PbS CQD surface. Laser-pump photoemission-probe spectroscopy is used to investigate both charge transport at the PbS CQD-ZnO photoanode interface, and band bending and carrier dynamics at the surface of PbS CQD solids. Charge injection was observed from PbS CQDs into ZnO, the dynamics of which are limited by the persistent photoconductivity of the ZnO. For PbS CQD solids passivated with EDT, MPA, PbI2, and a quasiperovskite MAI/PbI2 shell, band bending at the solid-vacuum interface is observed for the first time. Comparison of the charge carrier dynamics for different CQD solids show that the dynamics can occur on timescales between microseconds (MAI/PbI2) and seconds (MPA, PbI2). Oxygen contaminants (observed with XPS), creating deep traps, is suggested as the reason for the slower dynamics. The origin of the two-dimensional electron gas (2DEG) on the surface of CdO is also investigated with angle-resolved photoemission and core-level XPS. Surface adsorbates and interstitial hydrogen are found to donate electrons to the surface, occupying conduction band states. The effects on the occupancy of the 2DEG with removal of adsorbates and diffusion of atomic hydrogen via cracking are explored.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2019