First principles studies of some polymer-PCBM complexes for PV cells

A detailed first principles theoretical study of assemblies of five polymer (as donor)-PCBM (as acceptor) assemblies is presented here, with a view to assessing the potential of these organics photovoltaics (PV) in comparison to the more efficient Si -based PV cells. The geometric structures, electronic and optical properties are calculated using density functional theory (DFT) and time dependent DFT (TDDFT). Charge transfer upon absorption of light is analyzed using natural transition orbitals (NTOs). For all the polymer-PCBM assemblies studied, structural optimization leads to the polymer wrapping around the PCBM, irrespective of its initial mode of attachment to the PCBM, leading to a considerable change in the energy level spectrum of each assembly. Thus, these systems behave as molecular tweezers, which have a wide variety of applications, such as sensors. Although each polymer-PCBM assembly shows weaker absorption than the polymer -only system, the charge transfer characteristics , either direct or indirect, are excellent for all the systems. The detailed theoretical study presented in this work, considering several different properties of these polymer-PCBM assemblies in tandem, will help in understanding the viability of polymer-PCBM assemblies for use in PV applications giving pointers for improving their efficiency and providing an impetus for further experimental work on these promising systems.