Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excite state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells is raises the maximum theoretical efficiency of a solar cell form the Schockly-Queisser Limit of 33% to around 45% by effectively harvesting the energy from high energy photons. SF has been reported and extensively studied in crystalline acenes, and more recently acene dimers to better understand the fundamental photophysics and materials requirements for SF. Incorporation of these SF materials in to functional solar cells, although demonstrating modest efficiency enhancements, have had limited success. In our efforts to produce higher efficiency printed organic solar cells we had the desire to incorporate solution processible SF materials in printed organic solar cells, however most of the reported SF materials are highly crystalline and either do not promote SF in the solid state or controlling crystallisation is difficult. Therefore we were interested in developing a new range of SF materials that could be solution processed, promoted SF in the solid state, and were compatible with deposition methods used in printed organic solar cells.
We reasoned that materials designed to promote intra-molecular SF [1] would allow us greater control over the SF process. In addition greater control over the pre-organisation of the chromophores would enhance SF yields. Design criteria outlined by Busby et al. [1] suggested an Acceptor-Donor-Acceptor (A-D-A) structure may promote SF, and control of the coupling between the acceptors (triplet hosts), and the donor could mdoify SF yield.
In our own work we have been examining A-D-A chromophores, such as BTR or BQR [2], as p-type organic semiconductors in organic solar cells. However, calculations indicated that the triplet energy levels in these materials were too high. Of interest to this study was that BTR and BQR have a rich phase space with high temperature nematic liquid crystalline phases. Could we use the self-organisation inherent if the BTR or BQR structure to promote self organisation in new SF A-D-A chromophores? In our preliminary studies we coupled the benzodithiophene (BDT) core in BTR or BQR with thiophene substituted diketopyrrolopyrrole (TDPP), which has a known triplet energy of around 1.0 eV. The new material BDT(TDPP)2has a singlet energy level of 1.88 eV and a measured triplet energy of 0.95 eV, which is on the borderline for the energy requirement for SF, that is E(S1) ≥ 2xE(T1).[3] We reported that BDT(TDPP)2 shows modest SF in the solid state, perhaps due to the low singlet energy level.
With BDT(TDPP)₂ there was no evidence self assembly, and the singlet energy level was low. We needed a SF host, with i) a singlet energy around 2.0 eV, ii) stronger self-association through the core, and iii) solution processability. We have already described such a material where we had coupled TDPP to a fluorenyl substituted hexabenzocornene core (FHBC) as a p-type organic semiconductor.[4] The discotic liquid crystalline FHBC(TDPP)2material forms hexagonally packed columns and has a singlet energy level of 2.00 eV, however we did not report it's triplet energy. We report here our SF studies on FHBC(TDPP)2 and demonstrate that a triplet yield of 150% in amorphous thin films, increasing to 170% in thermally annealed films.