Reports: ND649639-ND6: Charge Traps in Colloidal Quantum Dot Photovoltaic Materials Examined with 2DIR Spectroelectrochemistry

John B. Asbury, PhD, Pennsylvania State University

Funds from the Petroleum Research Fund have supported the development of a novel approach to examine defects at interfaces of nanocrystalline materials based on time-resolved infrared spectroscopy.  We solve the problem of studying low density defects by allowing charge carriers to find defects for us.  By creating an initial population of charge carriers in nanocrystalline materials using a pulsed laser, we are able to examine the trapping of the initially free charges and then follow their subsequent recombination.

The approach permits us to elucidate the interactions of molecular ligands with colloidal quantum dot surfaces through the vibrational spectra of the ligands and to simultaneously establish the corresponding charge trap energies and densities associated with the ligands.  We obtain molecular information about ligand-nanocrystal interactions giving rise to trap states by observing transient vibrational spectra of ligands that are perturbed by the localization of charges in surface trap states.  We observe transient bleach spectra corresponding to the spectra of ligands in their neutral states (negative-going narrow features) and simultaneously measure transient absorption spectra of the ligands that are perturbed by the change in charge distribution associated with the charge trapping event.  These narrow vibrational features are superimposed onto a broad electronic transition that we have shown results from excitation of trapped charges back into delocalized core states of the nanocrystals.  Thus, the spectra of the electronic transitions provide information about the charge trap depth.  The combination of information about molecular interactions at quantum dot surfaces and the corresponding electronic properties arising from those interactions provides a new tool to understand how ligand-nanocrystal interactions determine the electronic properties of nanocrystalline materials.

Having developed the technique, we began collaborating with a leader in the colloidal quantum dot solar cell field, Edward H. Sargent, and together have made significant strides toward understanding how molecular interactions of ligands with quantum dot surfaces influence the electronic properties of devices.  Our work has led to development of world-record power conversion efficiency colloidal quantum dot solar cells recently reported in Nature Materials (2011) and ACS Nano (2012).  Students supported by the project have benefited from rich interdisciplinary research opportunities afforded by the highly collaborative nature of the project.  This training will position them to address complex interrelated issues in the energy and catalysis fields in the future.