Effect of TiOx compact layer with varied components on the performance of dye-sensitized solar cells

• TiOx compact layers with varied components are deposited by sputtering deposition.
• TiOx compact layers suppressed the recombination at the FTO glass/ electrolyte interface effectively.
• 20 nm-TiOx compact layer with the lowest x value (named T1) gave the highest charge transfer or transport and reduced recombination most.
• Lower value of x in TiOx showed slightly better transmittance.
• Lower value of x in TiOx reveals higher conductivity and better charge transfer from the porous TiO2 to the substrate.

In this study, approximately 20 nm thick compact layers of TiOx with varied components are deposited by physical vapor deposition. The performance of these layers in solar cells is investigated. The TiOx compact layers consist of T1 (with Ti0, Ti2+, Ti3+, and Ti4+), T2 (with Ti3+ and Ti4+), and T3 (with Ti4+). Results show that the optimum compact layer is T1, which exhibits an approximately 61% enhancement in energy conversion efficiency compared with the bare cell. Mott–Schottky plots indicate that the carrier concentration decreases and the flatband becomes less negative with decreasing x, which consequently increases the likelihood of charge transfer from the nanoporous TiO2 to the TiOx compact layers. Furthermore, a decrease in the x value of TiOx results in lower resistance. Voltage decay and electrical impedance spectrum (EIS) show that the electron-carrier lifetime and charge recombination reduction are improved the most by T1. Consequently, TiOx with smaller x works better as a compact layer. However, a solar cell with T2 shows weak enhancement of photovoltaic performance. Cyclic voltammetry and EIS illustrate that the low recombination blocking and high resistance of T2 may be a result of its large pore size and weak adhesion to fluorine-doped tin oxide glass.

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