Chemical compatibility between a hole conductor and organic dye enhances the photovoltaic performance of solid-state dye-sensitized solar cells

A series of organic dyes having an unsymmetrical geometry, 3-(5'-{4-[(4-tert-butyl-phenyl)-(4-fluoro-phenyl)-amino]-phenyl}-[2,2']bithio-phenyl-5-yl)-2-cyano-acrylic acid (D-F), 3-(5'-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2']bithiophenyl-5-yl)-2-cyano-acrylic acid (D-CH3), and 3-(5'-{4-[(4-tert-butyl-phenyl)-(4-methoxy-phenyl)-amino]-phenyl}-[2,2']bithiophenyl-5-yl)-2-cyano-acrylic acid (D-OCH3), were designed and synthesized for use in solid-state dye-sensitized solar cells (sDSCs). The dye regeneration energy levels and surface properties were characterized to determine the hole transfer yield from the oxidized dye to the hole conductor (spiro-OMeTAD) by measuring the degree of pore-filling by the spiro-OMeTAD and the transient absorption spectra (TAS). An electrode sensitized with D-OCH3 exhibited the highest spiro-OMeTAD filling fraction and hole transfer quantum yield (F) to spiro-OMeTAD, resulting in an enhanced photocurrent and a power conversion efficiency of 3.56% in the sDSC, despite a lower energy driving force for hole transfer compared to those of D-F, or D-CH3. This result illustrates the importance of the chemical compatibility between the hole conductor and the dye on the surface of TiO2.