Photocharging and Band Gap Narrowing Effects on the Performance of Plasmonic Photoelectrodes in Dye-Sensitized Solar Cells.

Abstract

The incorporation of plasmonic nanostructures in active electrodes has become one of the most attractive ways to enhance the photoconversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). Although an enhancement of PCE because of the incorporation of plasmonic nanostructures of different sizes, either bare or coated, has been demonstrated, the fundamental mechanisms associated to such enhancement are still unclear. Besides, the photocurrent enhancement of plasmonic DSSCs is frequently associated to the strong surface plasmon resonance (SPR) absorption of metal nanoparticles. In this work, through oxygen K-edge soft X-ray absorption and emission spectroscopies of plasmonic electrodes and electrodynamical characterization of the fabricated cells, we demonstrate a band gap narrowing and photocharging effect on the plasmonic electrodes that definitely contribute to the PCE enhancement in plasmonic DSSCs. The incorporation of bare metal nanoparticles in active metal-oxide semiconductor electrodes such as TiO2 in optimum concentration causes an upward shift of its valence band edge, reducing its effective band gap energy and enhancing the short-circuit current of DSSCs. On the other hand, small perturbation-based stepped light-induced transient measurements of photovoltage and photocurrent of the operating DSSCs revealed an upward shift of quasi-Fermi level of photoelectrodes because of the photocharging effect induced by the incorporated metal nanoparticles. The upward shift of the quasi-Fermi level causes an increase in open-circuit voltage ( VOC), nullifying the effect of band gap reduction. The short-circuit photocurrent enhancement was controlled by the band gap narrowing, screening the SPR contribution. The results presented in this work not only clarify the contribution of SPR absorption in plasmonic DSSCs, but also highlight the importance of considering the corrections in the effective base voltage because of the quasi-Fermi level band shift during the estimation of the transport and recombination parameters of an assembled DSSC.