Ab initio study of a molecular crystal for photovoltaics: Light absorption, exciton and charge carrier transport

Using ab initio methods we examine the molecular and solid-state electronic properties of a recently synthesized small-molecule donor, p-DTS(PTTh ₂)₂, which belongs to the dithienosilole- pyridylthiadiazole family of chromophores. In combination with the PC ₇₀BM acceptor, p-DTS(PTTh₂)₂ can be used to fabricate high-efficiency bulk heterojunction organic solar cells. A precise picture of molecular structure and interchromophore packing is provided via a single-crystal X-ray diffraction study; such details cannot be easily obtained with donor materials based on conjugated polymers. In first-principles approaches we are limited to a single-crystallite scale. At this scale, according to our investigation, the principal properties responsible for the high efficiency are strong low-energy light absorption by individual molecules, large exciton diffusion length, and fast disorder-resistant hole transport along π-stacks in the crystallite. The calculated exciton diffusion length is substantially larger than the average crystallite size in previously characterized device active layers, and the calculated hole mobility is 2 orders of magnitude higher than the measured device-scale mobility, meaning that the power conversion "losses" on a single-crystallite scale are minimal.