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) (C 5 PDI) in the solid state is 1.1 eV, so that achieving singlet fission (SF) in crystalline films of C 5 PDI 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 C 5 PDI single crystals and thin polycrystalline films. The X-ray single-crystal structure of C 5 PDI 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 (T 1 T 1 ) in the C 5 PDI 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 C 5 PDI polycrystalline thin film, 1 (T 1 T 1 ) formation occurs in τ = 169 ± 6 ps. The quantum yield of the 1 (T 1 T 1 ) 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.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films

“…Following 580 nm excitation, the TREPR spectrum exhibits an eae / aea spin polarization pattern (low to high magnetic field), where ( e ) and ( a ) denote emissive and enhanced absorptive signals, respectively. Simulating the spectrum gives zero-field splittings of | D | = 1350 MHz and | E | = 80 MHz, similar to those of PDI derivatives with relative sublevel populations [ p x , p y , p z ] = [0.24, 0.71, 0.04]. , Interestingly, the observed TREPR spectrum suggests triplet formation is induced via spin–orbit intersystem crossing (SO-ISC) at 85 K since it does not exhibit the unique polarization pattern typically observed for SF-born decorrelated triplets ( aee / aae when D > 0) . While it is possible that the polarization pattern is consistent with SF-born triplets that undergo selective spin relaxation through the T z sublevel, as with some tetracene derivatives, we observe no evolution of the triplet spectrum within the time window of the experiment that would suggest this spin relaxation (Figure S19).…”

“…For example, we recently reported on SF in bis(3′-aminopentyl)-PDI (C 5 -PDI) single crystals in which the X-ray single crystal structure of C 5 PDI shows that it forms π-stacked dimers, wherein the PDIs are twisted ∼51°relative to one another. 20 Formation of the correlated triplet pair state 1 (TT) in the C 5 PDI single crystals occurs in τ = 56 ± 4 ps. Although this is the fastest SF time constant reported to date for a PDI derivative, it is still limited by the endoergic nature of the SF process in C 5 PDI.…”

“…Among the many chromophores reported to undergo SF, − perylene-3,4:9,10-bis­(dicarboximide) (PDI) derivatives have emerged as robust candidates for applications because SF in the solid state occurs with reasonable efficiencies even though SF is endoergic by about 150 mV and relatively slow (τ ≳ 200 ps). ,, The physical properties of PDI can be readily changed by modifying the functional groups on the periphery of the molecule . For example, by eliminating one imide group, perylene-3,4-dicarboximide (PMI) has a higher singlet excited state energy than PDI, while retaining a 1.1 eV triplet state energy; thus making SF in PMI systems slightly exoergic.…”

“…Given that SF most often involves virtual charge transfer states and SF in PDI derivatives is endoergic, the SF rate and efficiency are bound by the requirements of electron transfer theory, , so that SF in PDI is very sensitive to the interchromophore electronic coupling. For example, we recently reported on SF in bis­(3′-aminopentyl)-PDI (C 5 -PDI) single crystals in which the X-ray single crystal structure of C 5 PDI 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 (TT) in the C 5 PDI single crystals occurs in τ = 56 ± 4 ps.…”

Influence of Excited-State Delocalization on Singlet Fission: Tuning Triplet-Pair-State Emission in Thin Films

“…To bring singlet fission toward application in solar cells, it is crucial to minimize the impact of loss channels. This can be achieved by the first step, triplet-pair formation, and the second step, which can be either triplet dissociation or charge collection from the triplet-pair state, outcompeting alternative deactivation pathways for each respective state. , One of the most common ways to manipulate the triplet-pair formation rate of solid-state singlet fission materials is a modification of the molecular arrangement, either by introducing structural changes via chemical modification of the molecules − or by a variation of the growth conditions. ,,− On the basis of these studies, it can be expected that in most cases the triplet-pair formation rate is higher in samples with better structural order − ,, and stronger intermolecular coupling. ,,− …”