Picosecond Dexter-Type Energy Transfer in Device-Grade InAs Quantum Dot Films

InAs quantum dots (QDs) have emerged as a promising replacement for highly toxic lead- and mercury-based QDs for infrared optoelectronic devices. In order to understand the performance of InAs QD-devices and exploit their full potential, it is essential to elucidate the mechanisms of exciton migration or energy transfer in the films of InAs QDs, which, however, have remained lacking. Here we investigate exciton transfer dynamics in device-grade InAs QD films used in infrared photodetectors using femtosecond transient absorption spectroscopy. Interdot distances were precisely controlled by using InAs QDs of different sizes capped with ligands of varying lengths. Through minimizing interdot distances with halide ligands, we observed an energy transfer time constant as short as 1.7 ps. The distance dependence of the energy transfer rates was found to follow a Dexter-like mechanism with a damping coefficient of β = 0.31 ± 0.03 Å^-1, which is a relatively small value compared to prior charge/energy transfer studies enabled by the strongly delocalized exciton wave functions of InAs QDs. These results provide hitherto lacking fundamental insights into the energy transfer/migration mechanisms inside device-grade InAs QD films, with direct relevance to optoelectronic devices ranging from photodetectors and solar cells to light-emitting diodes.