Engineering of Doping and Transport for Enhanced Colloidal Quantum Dot Photovoltaics

Abstract

Colloidal Quantum Dots (CQDs) are nanoscale quantum-tuned semiconductor particles suspended in solution. When deployed as optoelectronic materials, CQDs are closely packed together into thin films enabling charge transport. This also enables the formation of semiconductor junctions and contacts with other bulk semiconductor materials and metals, respectively. However, limited attention has been given to understanding the fundamental electronic behavior of these materials as bulk-like films, gaining fine control over their semiconducting properties, and then leveraging these insights to make better semiconductor devices. In this thesis, I explore two of the most fundamental concepts to CQD semiconductor device physics: charge carrier doping density and charge carrier transport. With the aid of optoelectronic simulation, I show that these are the most important paths to pursue in order to improve the photovoltaic device performance. I develop a doping density theory that I then rigorously test for the PbS CQD materials system; this theory is also applicable to other types of CQD materials. I demonstrate doping densities on the order of 1016 cm-3 to 1018 cm-3 for both p- and n-type films. My work enables a previously unavailable p-n homojunction within one CQD materials system, and furthermore allows to grade the doping within the active absorber layer to reach power conversion efficiencies (PCEs) exceeding 7%. I then study CQD size polydispersity, and use it to investigate the details of charge transport in the rough energetic landscapes inherent to these materials. Here, I find that midgap trap elimination is the most important concept in rapidly obtaining dramatic photovoltaic performance gains. By directly measuring the diffusion length in highly coupled CQD films, and combined with optoelectronic modeling, I was able to develop a new passivation strategy achieving a record 8.5% PCE. My research serves as a roadmap for future performance improvements in CQD photovoltaics.Ph.D