Unreported resistance in charge transport limits the photoconversion efficiency of aqueous dye-sensitised solar cells: an electrochemical impedance spectroscopy study

In this work, a thorough electrochemical impedance spectroscopy study is performed of both liquid and polymeric aqueous dye-sensitized solar cells (a-DSSCs), which are also compared with conventional organic solvent-based devices. The main purpose is unveiling phenomena limiting the efficiency of water-based photovoltaics. Indeed, electrochemical impedance spectroscopy spectra of a-DSSCs show two peculiar (and unreported) features that are not observed in organic-based DSSCs. The higher frequency one (R45°) is likely associated with a slowdown of the diffusion kinetics of the redox mediator: it is due to the breakdown of the hydrogen-bond network of the aqueous environment, which was also supported by density functional theory calculations. The lower-frequency feature is associated with the additional amount of energy required for the breakdown at the semiconductor/FTO interface of the adducts between protons (coming from the solvent) and electrons localized in the TiO2 surface trap-states. This ‘disruption energy’ results in a resistive element (RIC) that inversely correlates with the device efficiency. Very interestingly, RIC depends on the applied potential and becomes negligible only at much more positive values than VOC. Tailored equivalent circuits implementing simultaneously R45° and RIC are currently under investigation.