Rational design of naphthalimide based small molecules non-fullerene acceptors for organic solar cells

To explore optical electronic and charge transfer properties, a series of D-pi-A type of molecules with central core of 9,9-dimethyl-9H-fluorene as donor linked by 6-fluoro-4-(prop-1-yn-1-yl)benzo[1,2,5]thiadiazole as pi-bridge to variable end group acceptor materials have been designed for organic solar cells (OSCs). Optoelectronic properties of designed molecules M1-M4 with similar central core and pi-bridge but with different end groups were compared with R as representative of our system with 1,8-naphthalimide as end group. These optoelectronic properties are influenced by different end groups. Lower band gaps and longer wavelength of absorption have been observed for molecules by analyzing their frontier molecular orbitals. Furthermore, the computed reorganization energies for designed molecules are also comparable to the R so these molecules can be used as electron and hole transport materials. Among designed molecules M3 has higher wavelength of absorption along with minimum band gap and suitable distribution pattern of HOMO and LUMO during transition. The results presented display that varying the end groups is a highly promising approach in order to develop a series of D-pi-A type of materials for organic photovoltaics. So, our computed results display that these designed molecules can be used as an excellent candidates for OSCs.