Effect of different aromatic groups on photovoltaic performance of 1,1′-bis (diphenylphosphino)ferrocene functionalized Ni (II) dithiolates as sensitizers in dye sensitized solar cells

Abstract

A series of five new heteroleptic 1,1′-bis (diphenylphosphino)ferrocene (dppf) appended nickel (II) dithiolates namely, [Ni (dppf)(benzylcyanidedithiolate)] (Bz-Ni), [Ni (dppf)(2-cyanobenzylcyanidedithiolate)] (CN-Ph-Ni) and [Ni (dppf)(pyridine-2-cyanidedithiolate)] (2-Py-Ni), [Ni (dppf)(pyridine-3-cyanidedithiolate)] (3-Py-Ni), Ni (dppf)(thiophene-2-cyanidedithiolate)] (Th-Ni) have been prepared and characterized using spectroscopic techniques and in one case by single-crystal X-ray diffraction analysis for CN-Ph-Ni. The geometry around Ni (II) in CN-Ph-Ni is distorted square planar which is fulfilled by two S and two P centers of dithiolate and dppf ligand, respectively. The electronic absorption spectra for sensitizers display major bands between 400 and 550 nm that could be ascribed to an intra-molecular charge transfer (ICT) between the ferrocene and functionalized dithiolates to engender an efficient charge separated state. All these newly synthesized compounds are used as sensitizers in dye sensitized solar cells (DSSCs). The photovoltaic performances for the five featured sensitizers follow the order 3-Py-Ni>2-Py-Ni˃CN-Ph-Ni˃Bz-Ni˃Th-Ni. The assembly fabricated with 3-Py-Ni displays Jsc of 13.15 mA cm−2 and Voc of −0.633 with conversion efficiency (η) and incident photon-to-current conversion efficiency (IPCE) of 5.05% and 52%, respectively. The relatively slower charge recombination, greater dye loading capacity on TiO2 nano-particulate and relative smaller total resistance are the possible reasons behind the superior photovoltaic performance of the cell fabricated using the 3-Py-Ni as sensitizer. Also, density of states calculations have been performed which suggested that in the best performer 3-Py-Ni+(TiO2)30 cell set-up, the minimum inter-band excitation energy lies ~1.4 eV. Also, efficient electron coupling between sensitizer and (TiO2)30 results in efficient sensitization of TiO2 nanocluster.