Charge Transport in Colloidal Quantum Dot Solids

Open Access
Author:
Kim, Jihye
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 02, 2013
Committee Members:
  • John B Asbury, Dissertation Advisor
  • John B Asbury, Committee Chair
  • David Lawrence Allara, Committee Member
  • Tom Mallouk, Committee Member
Keywords:
  • charge transport
  • lead sulfide quantum dot
  • transient infrared absorption spectroscopy
  • quantum dot photovoltaics
Abstract:
The most abundant energy source on earth is solar energy. Solar cells, also called photovoltaic (PV) cells, convert the sunlight directly into electricity. The most efficient solar cells to date are based on silicon. However, due to the high cost per kilowatt-hour of silicon based photovoltaic (PV) devices, inexpensive semiconductor nanocrystals that exhibit quantization effects have attracted a great level of interest. Here, we focus in particular on lead sulfide (PbS) colloidal quantum dots (CQDs). PbS has a large Bohr exciton radius that allows wide band gap tunability to absorb the Sun’s broad spectrum. Solar cell technologies based on colloidal quantum dots are low in cost and energy consumption since large area solution processing at low temperature is available. Although charge transport and recombination in CQD solids has been extensively studied, reported mechanisms on transport states have been controversial. In this work, we investigate the nature of charge transport and recombination, and identify the charge carrier transport state in the most efficient PbS CQD photovoltaic devices. Our main tools are based on transient mid-infrared absorption spectroscopy techniques measured over seven orders of magnitude in time- 100 femtoseconds to microseconds. Collaborative efforts focused on electrical measurements and computational studies strongly support our findings. We conclude that it is the Stokes-shifted sub-gap state that mediates transport of photocarriers in the PbS CQD solids. Based on our findings, we suggest pathways to achieve CQD solar cells with higher power conversion efficiencies. The findings on the charge transport state provide insights to understand different features observed in the transient absorption spectra of n-type PbS CQD films. Furthermore, preliminary data on environmentally benign SnS nanocrystals which are promising photovoltaic absorber materials are introduced and discussed in this dissertation.