Singlet Exciton Fission for Solar Cell Applications: Energy Aspects of Interchromophore Coupling

Abstract

Singlet exciton fission, a process that converts one singlet exciton to a pair of triplet excitons, has the potential to enhance the efficiency of both bulk heterojunction and dye-sensitized solar cells and is understood in crystals but not well understood in molecules. Previous studies have identified promising building blocks for singlet fission in molecular systems, but little work has investigated how these individual chromophores should be combined to maximize triplet yield. We consider the effects of chemically connecting two chromophores to create a coupled chromophore pair and compute how various structural choices alter the thermodynamic and kinetic parameters likely to control singlet fission yield. We use density functional theory to compute the electron transfer matrix element and the thermodynamics of fission for several promising chromophore pairs and find a trade-off between the desire to maximize this element and the desire to keep the singlet fission process exoergic. We identify promising molecular systems for singlet fission and suggest future experiments.

Quantitative Intramolecular Singlet Fission in Bipentacenes

“…31 In order for SF to take place, the energy of the singlet state must be approximately twice the energy of the triplet. Using a method similar to that of Greyson et al 29 (Supporting Information), we calculated the energy of the singlet to be roughly isothermic with two triplets for all three bipentacenes discussed. Satisfaction of the energetic requirement 2E(T 1 ) ≤ E(S 1 ), within the margin of error for density functional theory (DFT) energy calculations, suggests that these compounds are feasible candidates for iSF.…”

Microscopic theory of singlet exciton fission. I. General formulation

“…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. 9 It may thus be permissible to uniformly scale the electronic coupling matrix elements when investigating singlet fission.…”

Triplet Excitons of Carotenoids Formed by Singlet Fission in a Membrane

“…An additional benefit of singlet fission is that the process could be modulated by changes in the relative orientation or spacing of the proximal carotenoids. [1,3] Our proposal is not intended as an alternative to other mechanisms of photoprotection. [43,44] Instead, the findings here motivate further investigation of biological systems in which membraneembedded carotenoids are known to exist in close proximity with one another, such as in some types of LHCs, [21][22][23] or in primate maculae.…”