Solution-Processable, Solid State Donor-Acceptor Materials for Singlet Fission

The exploitation of singlet fission (SF) materials in optoelectronic devices is restricted by the limited number of SF materials available and developing new organic materials that undergo singlet fission is a significant challenge. Using a new strategy based on conjugating strong donor and acceptor building blocks, the small molecule (BDT(DPP)2) and polymer (p-BDT-DPP) systems are designed and synthesized knowing that bisthiophene-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) has a low lying triplet energy level, which is further confirmed by time-dependent density functional theory (TD-DFT) calculations. TD-DFT and natural transition orbital (NTO) analysis are conducted to gain insight into the photophysical properties and features of excited states in BDT(DPP)2, respectively. Femtosecond and nanosecond transient absorption spectroscopies are used to investigate the excited state kinetics in the synthesized compounds. Fast formation of triplet pairs in thin film of p-BDT-DPP and BDT(DPP)2 and the equilibrium formation of correlated triplet pairs and S1 from triplet�triplet annihilation in solution of BDT(DPP)2 are further evidence of SF in these compounds. The short triplet lifetime, as a result of fast biexcitonic recombination, provides additional support for triplet pair formation through singlet fission

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 acceptordonoracceptor (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