Molecular design of electron-donor materials for fullerene-based organic solar cells

A set of model symmetrical and unsymmetrically substituted squaraine and croconine dyes is designed as potential electron donor component in organic photovoltaic bulk heterojunction solar cell (BHJ) where [60]PCBM fullerene is set as acceptor. Ground-state geometries and electronic structures were investigated using density functional theory (DFT) and time-dependent (TD-DFT) density functional theory at the B3LYP/6-31+G(d,p) level. The effects of the electronrich heterocycles on these squarilium/croconium based organic dyes are studied with respect to the electronic and transport properties of the systems. The estimated HOMO-LUMO gaps of all model dyes fall in the range of the typical organic semiconductors’ gap of about 2 eV. The HOMO and LUMO energy levels of the dyes are compared with respect to the acceptor’s and the rigorous conditions for an effective charge transfer is discussed. The calculated high values of the oscillator strengths for all proposed dyes are indicative for large absorption coefficient. Based on the optimized molecular geometries, relative positions of the frontier orbitals, absorption maxima and transport properties we propose some of these dyes as suitable components for optoelectronic devices.