The molecular and electronic structure of poly-[2,7-(benzo[2,1;3,4-b′]dithiophene)-alt-2,2′-(3,3′-didodecyl-5,5′-bithiophenyl)] (PBTT): A periodic DFT approach

We report the results of periodic DFT calculations on quasi-2D chains of PBTT. Geometry optimizations used GGA and hybrid functionals. Several polymer space groups were explored, P 2122 yielding the lowest unit cell energy. The band gap varies from 1.15 to 2.39 eV vs. exp 2.0 eV. The B3LYP and B3PW functionals yield the best agreement, GGA ones predicting too small band gaps. When empirical dispersion effects are added, a reorganization of the lateral dodecyl chains occurs and the conjugation of main polymer backbone is lost; however, these changes seem unphysical. Further experiments and calculations of the real 3D material are needed.