Influence of the TiCl₄Treatment on Nanocrystalline TiO₂Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and Quantification of Recombination Losses at Short Circuit

TLDR

TiCl4 treatment is commonly used to enhance photocurrent in dye‑sensitized TiO₂ solar cells, and charge separation in these devices involves injection and dye regeneration. This study uses charge density, kinetic data, and transient optical experiments to demonstrate that TiCl4 causes an 80 mV downward shift of the TiO₂ conduction band and a 20‑fold reduction in electron/electrolyte recombination. Photocurrent transients, charge extraction, and CCTPV measurements—validated against transient absorption and modeling—show that TiCl4 has little effect on electron transport at short circuit but markedly lowers recombination. The reduced recombination and band‑edge shift raise charge‑separation efficiency and photocurrent, increase Voc by offsetting the expected loss, and confirm that transport improvements do not drive the benefit.

Abstract

Chemical bath deposition of TiO2 from TiCl4 is an often used treatment that improves the photocurrent from dye-sensitized TiO2 solar cells. In this paper, charge density and kinetic data are used to show that the main effects of this treatment are an 80 mV downward shift in the TiO2 conduction band edge potential and a 20-fold decrease in the electron/electrolyte recombination rate constant. Together, these changes increase the quantum efficiency of charge separation at the interface, thus providing the observed increase in the photocurrent. The reduction in the recombination rate constant allows a greater concentration of electrons to accumulate at Voc, thus offsetting the Voc loss otherwise expected from the conduction band edge shift. Photocurrent transients and charge extraction data are used to show that the TiCl4 treatment has little effect on the transport of electrons at short circuit. The electron/electrolyte recombination rate constant at short circuit has been measured with the CCTPV (Constant Current Transient PhotoVoltage) technique. The results further confirm that any improvements in transport could not cause the beneficial effect of the TiCl4 treatment. Verification of the CCTPV technique is undertaken by comparison to transient absorption and by a model of the technique. Charge separation in dye-sensitized cells concerns two steps, charge injection and dye regeneration. Transient optical experiments to determine which process is improved by the TiCl4 treatment are discussed.