Phosphonic anchoring groups in organic dyes for solid-state solar cells

We report the synthesis and the optoelectronic characterization of three new 4H-pyran-4-ylidene and thiazole derivatives (pyt) as metal-free organic dyes for solid-state dye-sensitized solar cells (DSSCs). We investigate the performance and the long-term stability of devices employing pyt dyes functionalized with carboxylic and phosphonic acids as TiO₂ anchoring groups. In contrast to reports on liquid electrolyte DSSCs, we show that solid-state DSSCs prepared with phosphoric pyt derivatives can achieve similar power conversion efficiency to their carboxyl analogues. We make use of the Mott–Schottky analysis and equivalent circuit models to demonstrate that a phosphonic group induces a significant increase in built-in voltage at the TiO₂-hole transporter interface, which results in a higher open circuit voltage.