Singlet fission in disordered acene films: from photophysics to organic photovoltaics

Singlet fission produces low-energy triplet excited states from higher energy absorbed photons through an ultrafast photophysical process wherein two triplets are produced from one initially prepared singlet. Appropriately harnessed in organic photovoltaics (OPV) solar cells, singlet fission can augment power conversion efficiencies beyond that of the thermodynamic Shockley-Quessier limit for single junction devices by limiting thermalization losses. However, a delicate balance must be found as a low-energy triplet may not be energetically competent to separate into free charges at an OPV D/A interface owing to higher-energy charge transfer states. ❧ Amorphous thin films of vapro-deposited 5,12-Diphenyltetracene (DPT) were found to exhibit no X-ray or electron diffraction, contrasting directly with the strong diffraction witnessed for tetracene. Concurrently, DPT thin film absorption and emission show little spectral change from solution measurements whereas tetracene film absorption and emission red-shifts to lower energy wavelengths due to aggregation and inter-chromophore coupling in the solid state. Ultra-fast transient absorption measurements reveal triplet production in DPT thin films over two timescales, approximately 1 ps and 100 ps, faster than inter-system crossing. Fits to the transient decay allow for extraction of DPT triplet population densities. At low excitation densities DPT triplet production approaches 140%, confirming active singlet fission in the thin film.