Optoelectronic Applications of Conjugated Organic Polymers: Influence of Donor/Acceptor Groups through Density Functional Studies

Abstract

Organic photovoltaics (OPVs) are fast emerging as an attractive alternative to resolve the future energy crisis by maximum harness of solar energy. The optically active layer of OPVs consists of a p-type polymer as the donor and an n-type molecule as the acceptor. The efficiency of the polymeric donor material is decided by the type of monomeric units and nature of side groups attached to the backbone of the polymeric chain. This study focuses on the polymeric structures of polythiophene and its analogues, polyfuran and polyselenophene, to explore the relationship between structure and electronic/optical properties via static and time-dependent density functional calculations. Each polymer chain is substituted with a variety of electron releasing/accepting side groups to understand their influence on the band gap and energy shifting of frontier orbitals with respect to a well-known acceptor (phenyl-C70-butyric acid methyl ester (PC70BM)). All the underlying physical processes starting from excitation of an electron from the HOMO of the donor material to its LUMO, breaking of exciton into free charge carriers, and diffusion of a free electron and hole to the donor–acceptor interface have been investigated thoroughly. The photovoltaic efficiency has been interpreted in terms of electronic (Egap) and optical (Eexe) band gap, open circuit voltage (VOC), and short circuit current density (JSC). The reported derivatives have Egap in the range 1.69–2.78 eV which enables exciton generation by photon absorption in the visible region. It has been concluded that while electron-donating groups result in maximum lowering of Egap, the CN-substituted derivatives exhibit maximum efficiency with a VOC of about 1.63–1.75 eV.