title: Structural, electronic, and linear optical properties of organic photovoltaic PBTTT-C14 crystal.
abstract: 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.
authors: Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA. longhua.li@northwestern.edu
authors: Long-Hua
authors: LH
authors: Li
authors: Oleg Y
authors: OY
authors: Kontsevoi
authors: S H
authors: SH
authors: Rhim
authors: Arthur J
authors: AJ
authors: Freeman
mesh_terms: D011211:Electric Power Supplies; D004583:Electrons; D015394:Molecular Structure; D055620:Optical Phenomena; D011108:Polymers / Q000737:chemistry*; D011789:Quantum Theory; D013876:Thiophenes / Q000737:chemistry*
chemical_list: D011108:Polymers; D013876:Thiophenes