Determination of Exciton Diffusion Length by Transient Photoluminescence Quenching and Its Application to Quantum Dot Films

We present a framework for analyzing transient photoluminescence interfacial quenching experiments to extract exciton diffusivity and diffusion length. Through analytical solutions and finite element simulations at the continuum level, we derive spatiotemporal exciton distributions in films with arbitrary optical thickness and under noninstantaneous photoexcitation, paying particular attention to the effects of imperfect quenching and time-dependent diffusivity (i.e., subdiffusive transport). We demonstrate the utility of our model by applying it to a colloidal quantum dot (QD) thin film interface found in a recently reported record-efficiency QD light-emitting device. We find the exciton diffusion length in these CdSe/CdS core/shell QD films to be in the range 19–24 nm, in agreement with recent measurements of similar materials. We discuss limitations of the continuum-level analysis due to the finite size of individual QDs and an apparent subpopulation of “stationary” excitons that do not diffuse.