Charge Carrier Hopping Dynamics in Homogeneously Broadened PbS Quantum Dot Solids

Energetic disorder in quantum dot solids adversely impacts charge carrier transport in quantum dot solar cells and electronic devices. Here, we use ultrafast transient absorption spectroscopy to show that homogeneously broadened PbS quantum dot arrays (sigma(2)(hom):sigma(2)(inh) > 19:1, sigma(inh)/k(B)T < 0.4) can be realized if quantum dot batches are sufficiently monodisperse (delta less than or similar to 3.3%). The homogeneous line width is found to be an inverse function of quantum dot size, monotonically increasing from 15 meV for the largest quantum dots (5.8 nm diameter/0.92 eV energy) to similar to 55 meV for the smallest (4.1 nm/1.3 eV energy). Furthermore, we show that intrinsic charge carrier hopping rates are faster for smaller quantum dots. This finding is the opposite of the mobility trend commonly observed in device measurements but is consistent with theoretical predictions. Fitting our data to a kinetic Monte Carlo model, we extract charge carrier hopping times ranging from 80 ps for the smallest quantum dots to over 1 ns for the largest, with the same ethanethiol ligand treatment. Additionally, we make the surprising observation that, in slightly polydisperse (delta less than or similar to 4%) quantum dot solids, structural disorder has a greater impact than energetic disorder in inhibiting charge carrier transport. These findings emphasize how small improvements in batch size dispersity can have a dramatic impact on intrinsic charge carrier hopping behavior and will stimulate further improvements in quantum dot device performance.