Size dependent charge separation and recombination in CsPbI3 perovskite quantum dots

CsPbI3 perovskite quantum dots (QDs) have shown great potential in light-harvesting and light-emitting applications, which often involve the transfer of charge carriers in and out of these materials. Here, we studied size-dependent charge separation (CS) and charge recombination (CR) between CsPbI3 QDs and rhodamine B (RhB) molecules, using transient absorption spectroscopy. When the average size decreases from 11.8 nm to 6.5 nm, the average intrinsic CS time constant decreases from 872 +/- 52 ps to 40.6 +/- 4.3 ps and the corresponding charge recombination time constant decreases from 3829 +/- 51 ns to 1384 +/- 54 ns. The observed trend of size-dependent CS and CR rates can be well explained by Marcus theory using the theoretically calculated CS and CR driving forces (Delta G(CS) and Delta G(CR)), molecular reorganization energy (lambda(RhB)), and electronic coupling strength between QD and RhB (H-CS and H-CR). Unlike the extensively studied more strongly quantum confined Cd chalcogenide QDs, the CsPbI3 QDs are in a weak quantum confinement regime in which size-dependent coupling strength plays a dominant role in the size-dependent charge transfer properties.