Harvesting singlet fission for solar energy conversion: one versus two-electron transfer electron transfer from the quantum superposition state

Singlet fission (SF) is being explored to increase the efficiency of organic photovoltaics. A key question is how to effectively extract multiple electron-hole pairs from multiple excitons with the presence of other competing channels such as electron transfer from the singlet state. Recent experiments on the pentacene and tetracene show that a quantum superposition of the singlet (S$_{1}$) and multiexciton (ME) state is formed during SF. However, little is known about the kinetics of electron transfer from this quantum superposition. Here, we apply time-resolved photoemission spectroscopy to the tetracene/C$_{60}$ interface to probe one and two electron transfer from S$_{1}$ and ME states, respectively. Because of the relatively slow (~7 ps) SF in tetracene, both one- and two-electron transfer are allowed. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photo-excited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C$_{60}$. Kinetic analysis shows that ~60\% of the quantum superposition transfers one electron through the S$_{1}$ state to C$_{60}$ while ~40\% undergoes two-electron transfer through the ME state.