(285e) Multiple Exciton Generation In Organic Materials Through Singlet Fission: a Theoretical Perspective
Future generations of solar cells may break theoretical efficiency limits by simultaneously harvesting high and low energy photons. This possibility has sparked interest in singlet fission, the process where one absorbed photon generates two triplet charge carriers. A detailed understanding of the mechanism is required, however, to exploit the efficiency gain promised by singlet fission. Theoretical study of singlet fission is especially challenging and requires characterization of multi-exciton (multi-electron/hole pair) states, which are inaccessible to most simulation techniques. In this talk, we detail our efforts to characterize multi-exciton states using restricted active space double spin-flip configuration interaction (RAS-2SF). Multi-exciton states cannot form directly by photon absorption, but instead form via coupling to the optically allowed excited states. RAS-2SF simulations show this coupling proceeds through pi-pi interactions caused by intermolecular motion. This analysis shows how singlet fission proceeds through molecular excited states in acene crystals, and this process is distinct from multiple exciton generation mechanisms occurring in inorganic materials.