Efficient Singlet Fission in Perylenediimide Derivative Nanocrystals

Singlet fission (SF) is an effective multi-exciton generation process. The photoelectric conversion efficiency of photovoltaic devices can be improved by absorbing a high-energy photon and converting it into multiple excitons. Among the known SF materials, crystal materials with high SF efficiency have attracted extensive attention because of their long triplet exciton diffusion length; however, their limited absorption wavelength range and low triplet state energy limit their application. Perylenediimide (PDI) and its derivatives are SF materials with good photostability. However, most studies on their SF remain in solutions and amorphous or polycrystalline films. In this study, we fabricate solutions, polycrystalline films, and nanocrystal films of several PDI derivatives, and analyze their SF processes. The results of the steady-state spectra, time-resolved emission kinetics, and transient absorption spectra indicate that SF exists in nanocrystals; additionally, the charge-transfer (CT) state, which plays a mediating role in the SF process of materials, is found. The SF rates (kSF) of nanocrystal films are considerably higher than those of polycrystalline films, particularly, PTCDI-C5 (C5) nanocrystal film, which can reach 2.69 ns-1. The triplet yield of C5 nanocrystals can still reach 103%, even in the case of exciton annihilation. Our results reveal the SF mechanism in PDI derivative nanocrystals, which not only proves that PDI derivative crystals are prospective SF materials, but also indicates that a reasonable design of the molecular structure can help improve the SF efficiency of materials.