Water-Induced Fine-Structure Disorder and Its Effect on the Performance of Photoelectrodes in Dye-Sensitized Solar Cells

Water incorporation in an electrolyte solution has become a popular method for increasing the photoconversionefficiency (PCE) of dye-sensitized solar cells (DSSCs). Although thePCE enhancement in DSSCs has been associated with incorporationof a precise amount of water, the fundamental mechanisms underlyingsuch an enhancement remain unclear. Enhanced photocurrent inaqueous electrolyte DSSCs has also been linked to band-edge shifting,which leads to higher electron injection efficiencies. From X-rayabsorptionfine-structure spectroscopy (XAFS) of TiO2electrodes ofdismantled DSSCs and electrodynamical characterization of theassembled aqueous DSSCs, we show that a less static disordered(lowest Debye-Waller value) and more relaxed lattice (interatomicdistances comparable to bulk TiO2) are thefingerprints for theoptimized aqueous DSSC. On the other hand, lower efficiency of aqueous DSSCs is associated with a more static disordered (higherDebye-Waller values compared to the optimal aqueous DSSC) and less relaxed lattice (lower interatomic distances compared tobulk TiO2). For the optimum amount of water, small perturbation-based stepwise light-induced transient measurements of thephotocurrent of the operational DSSCs revealed a decrease in the overall trap density in the intragap states of the TiO2electrode.The decrease in intragap states induces a downward shift of the quasi-Fermi level, which is responsible for the short-circuitphotocurrent amplification. Thefindings of this study establish a direct link between the structural parameters such as theinteratomic distance and the Debye-Waller factor with electrodynamical and photovoltaic parameters such as the total trap densityand PCE for the electrodes used in aqueous DSSCs, paving the way for research into more stable and environmentally friendly solar cells.

Automated summary

The study reveals that a more relaxed lattice and less static disorder are the characteristics of optimized aqueous DSSCs. The decrease in intragap states leads to a downward shift of the quasi-Fermi level, amplifying the short-circuit photocurrent.