Thickness-dependent hole-blocking capability of RF-sputtered nickel oxide compact layers in dye-sensitized solar cells

Photo-generated charge carrier recombination in dye-sensitized solar cells (DSSCs) is observed to be suppressed significantly at the interface between transparent fluorine-doped tin oxide (FTO) and titanium dioxide (TiO2) by coating nickel oxide (NiO) thin film by RF sputtering. UV-Visible optical absorption spectroscopic measurements performed in the wavelength window of 300-800 nm showed similar to 60% average transmittance for NiO thin films coated for 10 min. The calculated optical bandgap value for NiO was 3.4 eV. The RF-sputtered NiO films were thoroughly characterized by X-ray photo-electron spectroscopy to examine Ni 2p(3/2) and Ni 2p(1/2) along with O 1s. The present study assessed the effect of 5, 10, and 15 min RF-sputtered NiO thin films at the interface between FTO and mesoporous TiO2. Results showed that charge transport in DSSCs is highly sensitive to NiO thickness at the interface between FTO and TiO2. It was specifically noticed that 10 min coating of NiO on FTO yielded DSSCs with photo-conversion efficiency (eta) of similar to 6.8% while DSSCs with no NiO on FTO showed only 4.9%. Further increase in NiO thickness affected the performance of DSSCs due to the significant reduction in tunneling probability from TiO2 to FTO.