The physics and chemistry of transport in CdSe quantum dot solids

Abstract

Semiconductor quantum dots (QDs) have tunable opto-electronic properties and can be chemically synthesized and manipulated with ease, making them a promising novel material for many diverse applications. An understanding of the physics of charge transport in QDs is not only important for realizing QD based electronic devices, but it also provides crucial insight into the chemical and optical properties of QDs. This thesis highlights how photoconductivity measurements are valuable to advancing our understanding of QD physics because they are exquisitely sensitive to the optical, chemical, and electronic properties of QDs. The work presented in this thesis emphasizes how the chemistry and physics of QD films are deeply entwined. Chapter 2 demonstrates that the photoconductivity and dark conductivity of CdSe QD films are enhanced following annealing at high temperatures. Chapter 3 illustrates that the purity of the QD capping reagent (tri-n-octylphosphine) and the methods used for film preparation can each affect the observed photocurrent by two to three orders of magnitude. In Chapter 4, the methods for CdSe film preparation developed in Chapter 3 are used to make films that exhibit photoconductivity properties consistent with having a low density of trapped charges, in contrast to previous studies. Chapter 5 also uses chemistry to bring CdSe QD films into a new regime of photoconductivity physics. Post-deposition chemical treatments that increase photocurrent by up to three to four orders of magnitude are presented. The voltage dependence of the photocurrent after treatment is consistent with having achieved unity exciton separation efficiency. Furthermore, by bringing CdSe QD films into this

(cont.) new regime of higher photoconductivity physics it is found that energetics prevent the facile injection of charges from gold electrodes into CdSe QDs, but there is no barrier to charge extraction.