Liquid Crystallinity as a Self-Assembly Motif for High-Efficiency, Solution-Processed, Solid-State Singlet Fission Materials

Solution and solution-deposited thin films of the discotic liquid crystalline electron acceptor–donor–acceptor (A-D-A) p-type organic semiconductor FHBC(TDPP)2, synthesized by coupling thienyl substituted diketopyrrolopyrrole (TDPP) onto a fluorenyl substituted hexa-peri-hexabenzocoronene (FHBC) core, are examined by ultrafast and nanosecond transient absorption spectroscopy, and time-resolved photoluminescence studies to examine their ability to support singlet fission (SF). Grazing incidence wide-angle X-ray (GIWAX) studies indicate that as-cast thin films of FHBC(TDPP)2 are “amorphous,” while hexagonal packed discotic liquid crystalline films evolve during thermal annealing. SF in as-cast thin films is observed with an ≈150% triplet generation yield. Thermally annealing the thin films improves SF yields up to 170%. The as-cast thin films show no long-range order, indicating a new class of SF material where the requirement for local order and strong near neighbor coupling has been removed. Generation of long-lived triplets (µs) suggests that these materials may also be suitable for inclusion in organic solar cells to enhance performance.