Multiple exciton dissociation and hot electron extraction by ultrafast interfacial electron transfer from PbS QDs

In addition to size dependent optical properties, strong quantum confinement in semiconductor nanoparticles (QDs) also affects their exciton relaxation, annihilation and dissociation dynamics, leading to their potential applications in third generation solar energy conversion devices. Multiple exciton generation and hot electron extraction have been proposed as potential approaches to increase the solar conversion efficiencies beyond the Shockley–Queisser limit in QD based solar cells. A common challenge faced by these two approaches is the need for ultrafast exciton dissociation to compete with ultrafast carrier cooling and exciton annihilation. In this review, we summarize our recent studies on multiple exciton generation and dissociation dynamics in PbS-MB+ complexes as well as strong electronic coupling and ultrafast electron transfer from PbS QDs to TiO2 nanocrystalline thin films. In PbS-MB+ complexes, we demonstrate that the multiple exciton generation efficiency in PbS QDs are unaffected by the presence of electron acceptors, and the multiple excitons can be fully dissociated by these acceptors via ultrafast electron transfer. For PbS QDs on TiO2 nanocrystalline films, we observe strong electronic coupling induced broadening of the 1S exciton band, which indicates a ∼6 fs electron transfer process according to the Newns–Anderson model of chemisorption. This transfer time is faster than the reported hot electron relaxation time (a few hundred femtosecond), which suggests the feasibility of hot electron extraction prior to their relaxation in these materials.