Singlet fission in pentacene through multi-exciton quantum states

Abstract

Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multiexciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multiexciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here.A lthough the maximum efficiency of single-junction solar cells is limited to 31% (ref. 1), multi-exciton generation (MEG) may enable dramatically higher efficiencies. MEG produces multiple charge carrier pairs from single photons, whereas typical solar cells only produce one exciton per photon. Singlet fission, in which two triplets are created from one excited singlet state, is the molecular analogue of MEG and has the inherent advantage that because the generated charge carriers are triplets, the forbidden triplet to ground-state singlet transition would decrease carrier recombination rates, preserving higher efficiencies.Singlet fission has been observed in crystalline pentacene 2-4 and may also govern MEG in materials other than pentacene or other polyacenes. Any MEG process necessarily entails the formation of a multi-exciton (ME) quantum state following photoexcitation, so taking advantage of the fission process to design more efficient photovoltaic materials will require a detailed understanding of both the MEG mechanism and the precise nature of the participating ME states.The singlet excitons generated by photoexcitation of crystalline pentacene undergo rapid, temperature-independent fission into two triplet excitons 2-4 . The lowest optically allowed singlet excited state (S 1 ) of crystalline pentacene has an excitation energy of more than twice its triplet energy (E(S 1 ) . 2 × E(T 1 )), making unactivated singlet fission energetically possible. In contrast, this is not the case in tetracene, because E(S 1 ) is less than 2 × E(T 1 ) and consequently singlet fission is thermally activated [5][6][7] . In pentacene crystals, both magnetic field measurements 8 and long (.1 ns) photoinduced absorption lifetimes show that triplets are present after optical excitation 3,4 . The energy of this absorption matches our theoretically predicted triplet-triplet excitation (vide infra), further confirming the generation of triplets. The observed yield of triple...

Microscopic theory of singlet exciton fission. I. General formulation

“…(9), using only the DFT HOMO and LUMO from B3LYP and the dielectric constant of pentacene ǫ = 3.6, predicts a transition energy of 2.95 eV, much improved from the HOMO-LUMO CIS result (note that there is, however, no a priori reason that the dielectric constant for bulk pentacene should be physically meaningful for a single molecule). Only multi-reference perturbation theory 24 and full many-body GW/BSE calculations 52 yield quantitative accuracy, predicting 2.1 and 2.2 eV, respectively. Similarly, the electronic couplings in this basis may not be quantitatively accurate, but have already been shown in other work to provide useful qualitative insight into the efficiency of singlet fission through investigation of their relative magnitudes 26 and dependence on molecular orientation.…”

“…Perhaps most significantly, high-level quantum chemistry calculations have identified a multi-exciton state that is composed of two triplets coupled into an overall spin singlet. 24,25 The transition to this multi-exciton state is thus spin-allowed and should proceed rapidly, while its triplet-triplet character suggests that it should naturally relax to separated triplets on a longer timescale. Unfortunately, the multi-exciton nature of this state implicitly prevents its direct photoexcitation such that the state is spectroscopically "dark" and difficult to observe.…”

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.…”

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.…”