Effect of metal ions in the electron-transfer mechanism on the photovoltaic performance of SALPHEN-based DSSC: experimental and theoretical studies

Herein, we report the synthesis and characterization of metal complexes of Fe(iii), Co(ii), and Cu(ii) with SALPHEN (N,N-bis(salicylimine)-o-phenyldiammine) and their potential application as sensitizers in dye-sensitized solar cells (DSSCs). The high thermal stability up to 550 °C observed for these compounds suggests that they may be possible candidates for the fabrication of solar cell devices without pyrolysis or thermo-oxidative degradation during the solar cell operation. The photovoltaic parameters of SALPHEN and its metal complexes were investigated experimentally, and it was observed that the metal ion coordination generally enhanced the power conversion efficiency (η) where the maximum η of 46.2% was obtained for the Co(ii) complex due to its high molar extinction coefficient and enhanced charge transfer by improved π-conjugation and electron-withdrawing capability. In the theoretical calculations, the B3LYP functional was employed to compute the ground-state geometries and the frontier molecular orbitals for all the complexes. The ground-state and excited-state energy transfer, which specifically contribute to the light harvesting properties of SALPHEN and its metal complexes, were analyzed using the functionals with corrected dispersion CAM-B3LYP/dgdzvp basis set (double zeta) and functional (B3PW91, B3P86/6-311++G(d,p) basis set (triple-zeta)). It was observed that the HOMOs of the metal complexes were localized on the d orbitals with π(SALPHEN nitrogen and O atoms) orbital character, whereas their LUMOs have π*(O) orbital character. The light-harvesting efficiency (LHE) was analyzed using the theoretically obtained band-gap values for SALPHEN and its metal complexes. In general, metal ion chelation showed increased charge transfer transitions in both the ground and excited states. Moreover, the charge density difference in each compound was observed through three-dimensional (3D) visualization of its electron density isosurface (contour 0.05 e Å), which supports understanding the mechanism involved in the energy transfer.