Effect of Crystallinity on Endoergic Singlet Fission in Perylenediimide Single Crystals and Polycrystalline Films

Abstract

The triplet state energy of bis(3′-aminopentyl)-perylene-(3,4:9,10)-bis(dicarboximide) (C5PDI) in the solid state is 1.1 eV, so that achieving singlet fission (SF) in crystalline films of C5PDI can provide a potential means of delivering triplet excitons to silicon-based solar cells, whose band gap is also 1.1 eV, to enhance their performance by utilizing blue light in the solar spectrum. Here, we use transient absorption spectroscopy and microscopy to assess the effect of solid-state order on SF dynamics by comparing C5PDI single crystals and thin polycrystalline films. The X-ray single-crystal structure of C5PDI shows that it forms π-stacked dimers, wherein the PDIs are twisted ∼51° relative to one another. Formation of the correlated triplet pair state 1(T1T1) in the C5PDI single crystals occurs in τ = 56 ± 4 ps mediated by a mixed state having both excited singlet and charge-transfer character, while in a solvent-vapor-annealed C5PDI polycrystalline thin film, 1(T1T1) formation occurs in τ = 169 ± 6 ps. The quantum yield of the 1(T1T1) state formation in each case is nearly unified, yet the free triplet exciton quantum yield in the single crystals is 70%, while that in the annealed polycrystalline film is only 29%. Steady-state and time-resolved photoluminescence measurements indicate that the disorder in the polycrystalline film hinders free triplet excitons via long-lived excimer trap states at sites with suboptimal electronic coupling. The higher free triplet yield in the single crystal also clearly shows that the high degree of molecular order in the crystal enables competition between triplet annihilation and diffusional escape, which is critical for utilizing the triplet excitons to enhance solar cell performance.