Early Time Photoconductance in Quantum Dot Solids Probed by Ultrafast Photocurrent Spectroscopy

Abstract

Understanding and controlling carrier transport and recombination dynamics in colloidal quantum dot (QD) films is key to their application in electronic and optoelectronic devices. Towards this end, we have conducted transient photocurrent measurements (10-50 ps time resolution) to monitor charge-transport dynamics in lead selenide QD films as a function of pump fluence, temperature, electrical bias, and surface treatment [1, 2]. Room temperature dynamics reveal two distinct timescales: one sub-nanosecond and the other, tens-to-hundreds of nanoseconds. The first of these processes is assigned to relaxation of one type of carriers (presumably, electrons) into low-mobility intra-gap states, which pins the corresponding quasi-Fermi level at below band-edge energies [3]. This effect is likely responsible for a considerable photovoltage deficit typical of QD PVs. A longer-time transient photocurrent exhibits memory-less decay due to nongeminate recombination of the remaining mobile charges with the trapped carriers of the opposite sign (pre-existing and photogenerated). In addition to potentially modifying the chemical nature and/or abundance of trapping sites, application of different QD surface treatments also alters the initial (“dark”) occupancy of intra-gap states, which has a profound effect on mobile-carrier lifetimes. The peak photoconductance observed immediately after short-pulse excitation is temperature-independent suggesting a tunnelling mechanism of early time phototransport. Further, low temperature measurements reveal an important role of the excitonic fine structure and, specifically, the electron-hole exchange interaction (exchange blockade) in early time photocurrent dynamics. This effect is likely universal as it necessarily arises following photoexcitation when an electron and a hole are generated in the same QD and hence are strongly coupled by the exchange interaction, which creates a barrier to their separation between adjacent QDs. Finally, side-by-side comparison of photocurrent transients using excitation with low- and high-photon energies (1.5 vs. 3.0 eV) reveals clear signatures of carrier multiplication (CM), that is, generation of multiple excitons by single photons [2]. Based on photocurrent measurements of QD solids and optical measurements of solution based samples, we conclude that the CM efficiency is unaffected by inter-dot coupling, and therefore, the results of previous numerous spectroscopic CM studies conducted on dilute QD suspensions should, in principle, be reproducible in electronically coupled QD films used in devices. 1. Fidler, A.F., J. Gao, and V.I. Klimov, Electron-hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots. Nat. Phys, 2017. advance online publication. 2. Gao, J.B., A.F. Fidler, and V.I. Klimov, Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots. Nat. Comm. 2015. 6. 3. Nagpal, P. and V.I. Klimov, Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films. Nat. Comm. 2011. 2: p. 486.