Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

Nanocrystal (NC)-sensitized triplet-fusion upconversion is a rising strategy to convert long-wavelength, incoherent light into higher-energy output photons. Here, we chart the photophysics of tailor-functionalized CdSe NCs to understand energy transfer to surface anchored transmitter ligands, which can proceed via correlated exciton transfer or sequential carrier hops. Varying NC size, we observe a pronounced acceleration of energy transfer (from kquench = 0.0096 ns-1 ligand-1 to 0.064 ns-1 ligand-1) when the barrier to hole-first sequential transfer is lowered from 100 +/- 25 meV to 50 +/- 25 meV. This acceleration is 5.1x the expected effect of increased carrier wave function leakage, so we conclude that sequential transfer becomes kinetically dominant under the latter conditions. Last, transient photoluminescence shows that NC band-edge and trap states are comparably quenched by functionalization (up to similar to 98% for sequential transfer) and exhibit matched dynamics for t > 300 ns, consistent with a dynamic quasi-equilibrium where photoexcitations can ultimately be extracted even when a carrier is initially trapped.