Ab Initio Study of a Molecular Crystal for Photovoltaics:
Light Absorption, Exciton and Charge Carrier Transport
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Abstract
Using
ab initio methods we examine the molecular and solid-state
electronic properties of a recently synthesized small-molecule donor, <i>p</i>-DTS(PTTh<sub>2</sub>)<sub>2</sub>, which belongs to the
dithienosilole-pyridylthiadiazole family of chromophores. In combination
with the PC<sub>70</sub>BM acceptor, <i>p</i>-DTS(PTTh<sub>2</sub>)<sub>2</sub> 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