Low Band Gap Fused Bicyclic Polymers with Heteroatoms Se and Te: A DFT-PBC Study.

Designing low band gap conjugated polymers is critical for the development of advanced materials in organic electronics. This study focuses on DFT-PBC analysis of a series of bicyclic fused polymers containing heteroatoms Se and Te, using the hybrid functional B3PW91. The polymer geometries defined by cells containing two monomers connected in different configurations were initially optimized with an assumption that all the atoms are on the same plane due to interchain interactions in the solid state. Subsequently, the structures were optimized without these restrictions, starting from the nearly planar geometry, as small deviations were anticipated due to insufficient interchain interactions required for planar geometry. According to the band structure calculations, the band gap value for the planar structure with 4-6 connection positions, where two Se atoms occupy both heteroatom positions, was found to be 0.779 eV, which is very close to the experimentally determined value of 0.76 eV (Patra 11). The finding indicates that polymerization primarily occurs through this connection within theoretical limits. The band gap values for other structures with the same connection positions but different heteroatom pairs (Se-Te, Te-Se, and Te-Te) were also low, at 0.905, 0.745, and 0.730 eV, respectively. Conducting properties of the title polymers were assessed by comparing band gaps, bandwidth, and effective mass values. In conclusion, the planar structures of the polymers with 4-6 and 2-4 connections exhibit bandwidths comparable to those of polypyrrole and polythiophene, with their effective masses that are either improved or comparable with these benchmark materials. Furthermore, atomic and subshell compositions of the frontier orbitals were analyzed to gain insight into the variation of the band gap as a function of the heteroatom.