Zn-Porphyrin propped with hydantoin anchor: synthesis, photophysics and electron injection/recombination dynamics

In this work, Zn-porphyrin with a hydantoin anchor (ZnPHy) was designed and synthesized for dye-sensitized solar cell (DSC) applications. The synthesized ZnPHy was well characterized using IR, NMR and mass spectral techniques, and satisfactory results were obtained. Cyclic voltammetry, UV-visible absorption, steady-state fluorescence, time-resolved fluorescence and transient absorption spectroscopic techniques were employed to elucidate the electrochemical and photophysical properties of ZnPHy. The obtained properties revealed that the synthesized ZnPHy can be used as a photosensitizer for DSC applications. The nature of ZnPHy binding onto the TiO₂ surface was investigated using ATR-FTIR and UV-Vis absorption measurements. The amount of adsorbed ZnPHy on TiO₂ surface was reasonably fit using the Langmuir adsorption isotherm, with a binding constant value of 1.03 × 10⁵ M^-1. Time-resolved measurements were used to elucidate the rate of electron injection and the regeneration and recombination kinetics for ZnPHy/TiO₂ film. The ZnPHy showed a high electron injection rate with a ϕ_inj of 99%. Intriguingly, the rate of electron recombination is much slower than the rates reported for carboxyl-based Zn-porphyrins. Such a high electron injection and slow electron recombination rate are beneficial to produce long-lived electrical current in a photovoltaic device. Thus, the ZnPHy-sensitized TiO₂ electrode showed the best photovoltaic performance, with the short-circuit photocurrent density (J_SC), open-circuit voltage (V_OC) and fill factor (ff) of 3.49 mA cm^-2, 0.6 V and 0.52, respectively, yielding an overall conversion efficiency (η) of 1.01%. For comparison, the ZnCOOH-sensitized electrode was also fabricated under the same conditions and yielded the η value of 0.84%. Hence, the hydantoin moiety could be a potential alternative anchoring group for DSC applications.