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(PTTh2)2, which belongs to the
dithienosilole-pyridylthiadiazole family of chromophores. In combination
with the PC70BM 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.