Mechanism for Singlet Fission in Pentacene and Tetracene: From Single Exciton to Two Triplets

Abstract

Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment. Intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers. From this localized state, nonadiabatic coupling mediated by intermolecular motion between the optically allowed exciton and a dark multi-exciton state facilitates SF without the need for a nearby low-lying charge-transfer intermediate. An estimate of the crossing rate shows that this direct quantum mechanical process occurs in well under 1 ps in pentacene. In tetracene, the dark multi-exciton state is uphill from the lowest singlet excited state, resulting in a dynamic interplay between SF and triplet-triplet annihilation.

Singlet fission in linear chains of molecules

“…(1) is true for organic crystalline materials like pentacene and hexacene derivatives, the former being the most studied material for SF purposes. 3,9,[17][18][19][20][21][22][23][24][25] For hexacene, instead, a possible fission of the singlet in three triplets has been recently proposed, due to the extremely low energy of T1. 26 However, hexacene is very unstable 27 and, for this reason, is usually functionalized.…”

“…22 A main controversy in SF mechanism arises from the coupling between ME and S1 states: some studies suggest that the main interaction driving the ME←S1 transition is the direct coupling between these two states; 5,17,18,31 another school of thought indicates that direct coupling is not sufficient to explain the rates of SF and suggests that indirect coupling via charge transfer (CT)…”

“…17 In a successive paper, Zimmermann et al also studied pentacene and tetracene through QM/MM calculations, adopting the RAS-2SF method; 36 his results suggest that ME←S1 transition is driven by the direct coupling between them and no CT state is involved in the process. 18 Zhu's group argues that the ultrafast dynamics arising from available experiments cannot be justified by direct coupling between S1 and ME. Multi-state DFT and density matrix modeling were employed to show that indirect coupling through CT states is two orders of magnitude higher than direct coupling.…”

In situ measurement of exciton energy in hybrid singlet-fission solar cells

“…In the solid state the triplet energy has been measured indirectly using variable temperature host guest fluorescence 16 , electron backscattering from crystalline pentacene 17 , and inelastic tunnel ling spectroscopy on amorphous pentacene 18 . In solution it has been studied using broadband flash photolysis [19][20][21] , and estimated via a range of theoretical calculations [22][23][24][25][26] . However it is unclear that these energies are relevant to ionization at a polycrystalline pentacene heterojunction.…”

Pentacene/K12 solar cells formed by organic vapor phase deposition

“…21 Even triplet transfer after singlet fission has been reported for that system, see e.g., Rao et al 26 There are two key mechanisms for photocurrent generation, namely Channel I (electron transfer) and Channel II (hole transfer). 27 In a simple analysis, the Channel I mechanism (the splitting of an exciton at the heterojunction) takes place if the energy difference between the electron affinity (EA) levels of the donor (D) and acceptor (A) materials is larger than the exciton binding energy. This can be described as E EA;D − E EA;A ≥ E EX where E EX is the binding energy of an exciton in the donor material and E EA;D (E EA;A ) is the EA level in the donor (acceptor) relative to the vacuum energy level.…”