Theoretical studies on opto-electronic properties of Congo red dye based dye-sensitized solar cells

Abstract

Designing a structural moiety of metal-free organic dyes is vital for dye-sensitized solar cells' electronic and optical properties. In this study, a donor–π–acceptor based molecular design using N-methyl-4,5-diazocarbazole as the donor unit, Congo red dye as acceptor, 2-phenyl-2h-1,2,3-benzodiazphosphole as the π -spacer and cyano acrylic acid as the auxiliary acceptor unit is presented. Density functional theory studies were employed to study the effect of an electron-donating amino group and electron-withdrawing nitro group on the photo-electronic properties of the dyes. The D-A2- π -A1 strategy was adopted to extend the conjugation and delocalization of electrons, lower the HOMO-LUMO gap and improve the efficiency of the dye-sensitized solar cell. The strategy was developed to enhance the opto-electronic properties and, consequently, the photovoltaic performance. The best performance was depicted by diazo carbazole-3,3′-NO2 disubstituted Congo red dye-phosphole-cyanoacrylic acid, which possessed the lowest HOMO-LUMO gap. The presence of the nitro group on the Congo red dye increased the conjugation and intramolecular charge transfer resulting in a bathochromic shift. A similar observation was depicted by the electron-releasing amino group on donor carbazole moiety leading to a red shift.