The conformational control of small D-A-D organic solar cells for large power conversion efficiency: A deep quantum chemistry analysis

Charge transport rate of designed molecules. [Display omitted] •Theoretical calculations with long range corrected hybrid functional.•Morphological variations at the positions of donor attachment sites.•Influence of Gibbs free energy on charge transport rates.•Detail mechanism of open circuit voltages with exciton generation.•Minimum power conversion efficiency with constant current density. Four naphthalene diimide (NDI) based small organic solar cells have been studied with for photoexcited and photovoltaic properties. Density functional theory and Tamm-Dancoff Approximation (TDA) with range-separation gap tunning have been used for ground and excited state calculations. The theoretical calculated results are highly reliable as the difference between the fundamental energy gap (Eg) and ionization gap (Ig) are nearly same. During this research study, the positions of donor units changed with symmetrical and unsymmetrical attachment sites which produced the interesting results of optoelectronic properties at both ground and excited states. The triphenyl amine (TPA) donor units at unsymmetrical units de-stabilized the HOMO and stabilized the LUMO energy levels significantly. The small vertical excitation energies of NDI-TPA molecules increased the transition coefficients from HOMO to LUMO. The emission rate of electron from S1 excited state to S0 is large for DPA-based molecules due to large vertical emission oscillator strength.