Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excited state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells raises the maximum theoretical efficiency of a solar cell from 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, has 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. We aim to remove the local order constraint in high efficiency solid-state SF materials by, i) designing intra-molecular SF materials, and ii) using secondary self-association to pre-organise chromophores. Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excited state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells raises the maximum theoretical efficiency of a solar cell from the Schockly-Queisser Limit of 33% to around 45% by effectively harvesting the energy from high energy photons, with a potential 35% reduction in delivered cost per Watt of power. 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 into functional solar cells, although demonstrating modest efficiency enhancements, has 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. We aim to remove the local order constraint on high efficiency solid-state SF materials by, i) designing intra-molecular SF materials, and ii) using secondary self-association to pre-organise chromophores.

We have recently published our proof of principle sduies on new SF systems.[3] Here we report our recent studies of new solid-state singlet fission materials using liquid crystallinity to promote self-assembly and to pre-organise triplet host chromophores. Using design criteria outlined by Busby et al. [1], suggesting an Acceptor-Donor-Acceptor (A-D-A) structure may support intra-molecular SF. However, we have used the core Donor in intra-molecular SF materials to promote self association, removing a requirement for local order in the triplet host chromophores. We have used strong pi-pi interactions of substituted hexabenzocoronene (HBC) donors to promote strong self-assembly, coupled with thienyl-substituted diketopyrrolopyrrole (TDPP) as the triplet host. In addition, we needed to design our intra-molecular SF host, with i) a singlet energy around 2.0 eV if TDPP was to be our triplet host, ii) strong self-association through the core, and iii) solution processability. Thin films of the discotic liquid crystalline FHBC(TDPP)₂ material form hexagonally packed columns and has a singlet energy level of 2.00 eV [2]. SF studies on FHBC(TDPP)₂ demonstrate a triplet yield of 150% in amorphous thin films, increasing to 170% in thermally annealed films.