Molecular engineering of thieno[3,4-b]pyrazine-based materials: Toward organic photovoltaic applications: DFT and TD-DFT investigation

Herein, we theoretically design four new π-conjugated thieno[3,4-b]pyrazine derivatives for organic solar cells (OSCs), denoted 2,3-di([2,2′:5′,2″:5″,2‴-quaterthiophen]-5-yl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine, 2,3-bis(5'''-(4-fluorophenyl)-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine, 2,3-bis(5'''-(4-methoxyphenyl)-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine, and 2,3-bis(5'''-(9-methyl-9H-carbazol-2-yl)-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine. The geometric, electronic, optical, photovoltaic, and nonlinear optical (NLO) properties of the investigated chemical structure were investigated using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations at the B3LYP/6-31G(d,p) level. Structural changes on the chemical structure induce a narrow energy gap in the range (2.08 – 2.12 eV), and an enhancement of the absorption in the visible range (λmax = 682 – 688 nm). In addition, a low value of exciton binding energies is obtained (Eb = 0.28 – 0.31 eV). Analysis of the density of states (DOS), transition density matrix (TDM), electron localization function (ELF), and localized orbital locator (LOL) showed effective charge delocalization and donor-to-acceptor electron transfer. The photovoltaic performance was assessed using the open circuit voltage (Voc) obtained between Highest Occupied Molecular Orbital (HOMO) donor to Lowest Unoccupied Molecular Orbital (LUMO) acceptor levels and was found promising (Voc = 1.32 – 1.44 V). The large hyperpolarizability and high nonlinearity response of the NLO results also indicated the potential for these molecules to be used in various optoelectronic applications.