Atomic layer deposition of high performance ultrathin TiO₂ blocking layers for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) often use a thin insulating or semiconducting layer (typically TiO₂) between the transparent conductive oxide and the mesoporous TiO₂ to block electron/hole recombination at the conducting oxide/electrolyte interface. The blocking layer (BL) is essential to maintain efficient charge generation under low light conditions, at which DSSCs perform well compared to common semiconductor-based photovoltaic devices. In this work, we show that atomic layer deposition (ALD) can produce ultrathin (<10 nm) BLs that significantly impede charge recombination in functional DSSCs, leading to improved photocurrents, open-circuit photovoltages, and fill factors; this results in an increase in the overall efficiency from ≈7% to ≈8.4% under AM 1.5G illumination. The 5-10 nm ALD BLs are the thinnest optimized DSSC BLs reported to date. The BL retards the open-circuit voltage decay and extends the electron lifetime from ≈0.2 s to more than 10 s at 0.3 V, confirming that the ALD films significantly impede photogenerated charge recombination. By preparing BLs through other deposition techniques, we directly demonstrate that ALD results in better performance, even with thinner films, which is ascribed to the lower pinhole density of ALD materials.