Inorganic surface ligand effects on nanocrystal-based photovoltaic devices

Abstract

Colloidal quantum dots (QDs) are very small crystals on the scale of several nanometers in diameter. QDs demonstrate interesting size-related phenomena resulting from quantum confinement of electronic energy states. Many fundamental optoelectronic properties are heavily influenced by the ligand that resides at the QD surface. Because QDs have a large surface to volume ratio compared to bulk materials, macroscopic observables like charge-carrier mobility, photoluminescence, sinterability, etc. are strongly controlled by the atoms (or lack of atoms) present on the surface of the QD. By understanding which chemical species and how to control what resides at the surface (through ligand exchange, synthetic protocol, or other treatments) promotes electronic transport and passivate traps allows for realization of optoelectronic devices. While this thesis focuses on photovoltaic devices, the results herein are applicable to a broad range of applications including transistors, light emitting diodes, thermoelectrics, batteries, and other applications where tuning QD properties through surface chemistry is important. The findings of this thesis begin with development of ligand-exchanges and the study of exciton dynamics and charge transport in films of QDs. Using metal chalcogenide ligands, specifically In2Te3, promotes exciton delocalization over six nearest-neighbor QDs despite quantum confinement still present in the absorption spectra. This is the first observation that excitons can occupy an interaction space larger than the physical extent of the QD that absorbed the excitation. We then examined the charge carrier lifetime and yield-mobility product (of excitons that disassociate into free charges) with time resolved microwave conductivity measurements. Measurements indicate that using Na2Te ligands increases both lifetime and yield-mobility compared to a treatment with pyridine and CdCl2. This main result demonstrates that ligands play a crucial role in how nanocrystals sinter and the resulting bulk properties. Exploring the physics related to grain growth to induce large columnar crystal grains in sintered CdTe solar cells enabled greater than 11% power conversion efficiency with a pyridine CdCl2 treatment. Moving forward with inorganic ligation strategies could enable improvements in device efficiency. Applying new ligand exchange strategies to devices improved upon the state-of-the-art of solution processed PbS solar cells. Controlling surface passivation to improve charge transport in PbS QD solar cells leads to cells with an efficiency of 7.3%. I show how controlling what elements are present at the surface of the QDs enables tuning of the valence band energy level and work function in arrays of QDs. Using this control to create an energy cascade throughout the device active area improves carrier collection efficiency. The PbS thickness is then optimized at nearly 3x thicker than world-record devices reported in literature implying increased carrier lifetime (lower recombination) with the novel ligand treatments devised. The final portion of my thesis combines the device physics developed with CdTe and PbS cells I developed and a full optical model to calculate the necessary layer thicknesses to join cells into tandem solar cells. I test the validity of the model and develop recombination layers to experimentally realize tandem solar cells.