Temperature-Dependent Atomic Layer Deposition of Passivating ZnO Nanolayers for Dye-Sensitized Solar Cells

The influence of ZnO nanolayers as a passivating layer prevents electrons from recombining with the electrolyte or oxidized dye molecules at the interface by acting as a blocking layer for semiconducting materials. At 300 °C, it was observed that FTO-ZnO 500-cycle samples recorded the lowest Rq and Ra values of 1210 nm and 0.877 nm, respectively, resulting in homogeneous, crystalline, and smooth surface thin films. SEM images of FTO-ZnO 500 cycles-300 °C (150.00 KX) show a much more crystalline and homogeneous layer, while FTO-ZnO 500 cycles-100 °C (150.00 KX) show an irregular and agglomerated surface. Energy-dispersive spectroscopy also revealed that ALD successfully deposited ZnO on the FTO glass substrates, especially at 300 °C, resulting in uniform layers. In visible light wavelength (400 nm–800 nm), FTO-ZnO 500 cycles-300 °C exhibited the highest stable transmittance value of 0.78 a.u. However, it can be observed that the temperature with the slowest grain growth at 500 cycles of ZnO deposition was 200 °C, with a layer thickness of 60 nm. The device efficiency increased progressively with deposition temperature, reaching a maximum power conversion efficiency of 4.63% for ZnO films deposited at 300 °C with 500 ALD cycles. The observed enhancement is attributed to improved crystallinity, grain growth, and film uniformity at elevated deposition temperatures, which collectively enhance charge transport and reduce recombination losses. These results demonstrate that optimizing the ALD temperature is a key factor in achieving high-quality ZnO films and improved DSSC performance.