Designing Hybrid Materials for Photon Up- and Downconversion
Sean T. Roberts, University of Texas at Austin
Singlet exciton fission is a process that occurs in select organic materials wherein a spin-singlet exciton redistributes its energy to form a triplet exciton pair. Incorporating singlet fission materials into light harvesting platforms offers potential to enhance their performance. Likewise, singlet fission’s inverse process, triplet fusion, can yield upconversion systems that produce high-energy excitons from pairs of low-energy photons for applications in sensing and catalysis. Intrinsic to the design of any singlet fission or triplet fusion-based device, however, is the exchange of energy, typically in the form of a spin-triplet exciton, between an organic material and an inorganic semiconductor. Hybrid materials consisting of semiconductor quantum dots functionalized with organic molecules are a premier platform for the study of this energy transfer process. The high surface to volume ratio of these materials effectively means they consist entirely of interfacial molecules and the energy level tunability of quantum dots allows exploration of how the redox properties of the interface impact energy transfer.
Here, we report results on both PbS and Si quantum dots interfaced with a range of acene and rylene energy acceptors. We find that by tuning the energy level alignment of PbS quantum dots to that of rylene acceptors, the transfer of charge carriers across the interface can be varied by an order of magnitude. Interestingly, electronic structure calculations suggest this rate variation stems from electrostatic effects that both alter interfacial energy level alignment and shift the average orientation of molecules tethered to PbS. For Si quantum dots, energy transfer to acene and rylene acceptors is decidedly slow, unfolding on nanosecond to microsecond timescales due to weak coupling. Nevertheless, this process is highly efficient due to a lack of competing deactivation pathways. Interestingly, we find subtle changes in the structure of the triplet exciton energy acceptor lead to large changes in the energy transfer rate. These rate changes are unexpected on the basis of electronic structure calculations performed on molecules tethered to Si(111) surfaces, suggesting more exotic surface structures may play a key role in facilitating triplet energy transfer from silicon to organic molecules.