Picosecond Charge Transfer and Long Carrier Diffusion Lengths in Colloidal Quantum Dot Solids

• Published in: Nano letters Vol. 18; no. 11; pp. 7052 - 7059
• Main Authors: Proppe, Andrew H, Xu, Jixian, Sabatini, Randy P, Fan, James Z, Sun, Bin, Hoogland, Sjoerd, Kelley, Shana O, Voznyy, Oleksandr, Sargent, Edward H
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14.11.2018
• Subjects: carrier transport; quantum dot photovoltaics; Ultrafast spectroscopy; diffusion lengths; charge transfer
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/acs.nanolett.8b03020

Quantum dots (QDs) are promising candidates for solution-processed thin-film optoelectronic devices. Both the diffusion length and the mobility of photoexcited charge carriers in QD solids are critical determinants of solar cell performance; yet various techniques offer diverse values of these key parameters even in notionally similar films. Here we report diffusion lengths and interdot charge transfer rates using a 3D donor/acceptor technique that directly monitors the rate at which photoexcitations reach small-bandgap dot inclusions having a known spacing within a larger-bandgap QD matrix. Instead of relying on photoluminescence (which can be weak in strongly coupled QD solids), we use ultrafast transient absorption spectroscopy, a method where sensitivity is undiminished by exciton dissociation. We measure record diffusion lengths of ∼300 nm in metal halide exchanged PbS QD solids that have led to power conversion efficiencies of 12%, and determine 8 ps interdot hopping of carriers following photoexcitation, among the fastest rates reported for PbS QD solids. We also find that QD solids composed of smaller QDs (d = ∼3.2 nm) exhibit 5 times faster interdot charge transfer rates and 10 times lower trap state densities compared to larger (d = ∼5.5 nm) QDs.