Determination of sensitizer regeneration efficiency in dye-sensitized solar cells

Regeneration of the sensitizing dye in dye-sensitized solar cells (DSCs) is frequently studied using the transient absorption (TA) technique. However, TA measurements are generally not performed using complete DSCs at the maximum power point (MPP) on the current-voltage (j-V) characteristic, and the electron concentration in the nanocrystalline TiO2 films used in these devices is often not well characterized, which may lead to results that are not relevant to actual solar cell operation. Here, dye regeneration kinetics were studied at the MPP and at open circuit (where interpretation of results is simpler) in DSCs employing a "robust" nonvolatile 3-methoxypropionitrile-based electrolyte solution. Using a combination of TA, differential incident photon-to-current efficiency measurements, and impedance spectroscopy, the dependence of electron-dye recombination rate and overall sensitizer regeneration efficiency on TiO2 electron concentration is unambiguously demonstrated. We also examine the validity of a commonly used approach for determining regeneration efficiency in which the electron-dye recombination rate constant is estimated from TA decays of cells employing a redox-inactive electrolyte solution. We find evidence that this widespread practice may be unsuitable for accurate determination of the regeneration rate constant or efficiency. We go on to show that, despite near-quantitative regeneration at short circuit or low photovoltage, power conversion efficiency is limited by inefficient regeneration in stable DSCs with practically relevant electrolyte solutions.