Application of Intramolecular Singlet Fission in Photovoltaics: Control over Multiexciton Generation and Triplet–Triplet Annihilation

摘要： Singlet ?ssion (SF) is the spin-allowed photophysical conversion of a high-energy singlet exciton into two independent triplets. This intriguing phenomenon was suggested in 1965 to explain delayed ?uorescence in anthracene crystals. After nearly 50 years, Hanna and Nozik realized that SF could be used to overcome the Shockley–Queisser limit imposed on photovoltaics by reducing thermally wasted excessive energy. While there has been controversy over the details of the SF mechanism, it is widely accepted that a singlet exciton [S1] evolves into a spin-entangled coupled double-triplet [1(TT)], followed by dissociation into two independent triplets [T1 + T1 or 2 × T1]. The strategy to harvest electrons generated from SF depends on the type of chromophore and the electronic state of interest. There are two types of SF chromophore depending on the number of molecules involved in the SF process: intermolecular SF (xSF), where at least two chromophores participate in SF, and intramolecular SF (iSF), where SF occurs within a single chromophore. iSF chromophores have some advantages over xSF ones, such as independence of morphology/packing structure and easily tunable coupling. Numerous multichromophore systems such as biacene and quinoidal oligomers have been synthesized and suggested as iSF materials. In terms of target electronic states, there are two electron-donor candidates after one singlet exciton undergoes SF: 1(TT) and 2 × T1. Independent triplets which are present for ~μs timescales could travel longer and have a higher chance of reaching the donor–acceptor junction than singlet exciton. However, disentanglement of a bound correlated double-triplet requires further energy, resulting in retardation in SF and loss of a non-negligible number of excitons.