Efficient Energy Transfer and Singlet Fission in Co-Deposited Thin Films of Pentacene and Anthradithiophene

Co-deposited molecular heterostructures with statistical intermixing of the constituents are attractive candidates to tune the optical and the transport properties, as well as the ability to promote photophysical processes like singlet fission. In order to comprehend and control the singlet fission mechanism in these systems, it is of utmost interest to study the underlying excited state dynamics. In this work, thin films of anthradithiophene blended with the efficient singlet fission material pentacene are investigated by means of time-resolved and temperature-dependent photoluminescence spectroscopy with a time resolution of a few picoseconds. The analysis of the photoluminescence dynamics points toward efficient funneling of excitons from anthradithiophene via isolated pentacene molecules to agglomerates of pentacene, where eventually singlet fission occurs. The efficient and largely temperature-independent quenching of the luminescence in anthradithiophene is attributed to a favorable cascade-like alignment of the energy levels, and it is hypothesized that Forster resonance energy transfer is the main driving mechanism for exciton transport to pentacene agglomerates. The system investigated here can serve as a blueprint for the design of other molecular heterostructures with spatially separated light harvesting and singlet fission regions. Co-deposition of statistically mixing organic compounds offers a high flexibility to tailor material properties such as absorbance or singlet fission, which could enhance the performance of next generation solar cells. This work demonstrates that adding small concentrations of pentacene to an absorbing layer of anthradithiophene enables efficient singlet fission, which occurs after resonant energy transfer of excitons into pentacene agglomerates.image