Theoretical investigation of the open circuit voltage: P3HT/9,9- bisfluorenylidene derivative devices

The calculation of the power conversion efficiency (PCE) value for a bulk heterojunction (BHJ) organic solar cell is complex due to the wide number of parameters involved in the processes. This study focuses the attention on the molecular parameters involved into the open circuit voltage and the PCE definitions and in particular on the electronic coupling and on the lowest unoccupied molecular orbital (LUMO) of the acceptor. A simplified model system composed by a polymer as donor and a novel class of molecules (9,9′-bisfluorenylidene derivatives) as acceptor has been proposed as prototype to simulate the BHJ organic solar cell interface. Several substituents on different positions are tested and the chemical nature/position of substituents have a relevant influence on the electronic coupling and energy level values. Geometrical and electronic properties are obtained using density functional theory (DFT) and time-dependent (TD)-DFT calculations, respectively. A new hypothesis suggests that the minimization of the electronic coupling between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor can enhance the PCE reducing the recombination interface processes and calculations showing the possibility to minimize this parameter and fine-tune acceptor energy level through the acceptor functionalization. An accurate balance between electronic coupling and on the LUMO of the acceptor allows to propose the more performing candidate as electron acceptor in a P3HT/99′BF derivative BHJ solar cell.