Poly(2,5-bis(3-tetradecylthiophen-2yl)thieno(3,2-b)thiophene) (PBTTT-C14) is an important electro-optical polymer, whose three-dimensional crystal structure is somewhat ambiguous and the fundamental electronic and linear optical properties are not well known. We carried out first-principles calculations to model the crystal structure and to study the effect of side-chains on the physical structure and electronic properties. Our calculations suggest that the patterns of side-chain has little direct effect on the valence band maximum and conduction band minimum but they do have impact on the bandgap through changing the π-π stacking distance. By examining the band structure and wave functions, we conclude that the fundamental bandgap of the PBTTT-C14 crystal is determined by the conduction band energy at the Q point. The calculations indicate that the bandgap of PBTTT-C14 crystal may be tunable by introducing different side-chains. The significant peak in the imaginary part of the dielectric function arises from transitions along the polymer backbone axis, as determined by the critical-point analysis and the large optical transition matrix elements in the direction of the backbone.
Structural, electronic, and linear optical properties of organic photovoltaic PBTTT-C14 crystal
Long-Hua Li, Oleg Y. Kontsevoi, S. H. Rhim, Arthur J. Freeman; Structural, electronic, and linear optical properties of organic photovoltaic PBTTT-C14 crystal. J. Chem. Phys. 28 April 2013; 138 (16): 164503. https://doi.org/10.1063/1.4802033
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