A post-deposition annealing approach for organic residue control in TiO 2 and its impact on Sb 2 Se 3 /TiO 2 device performance.

We report a systematic investigation on the influence of two-step post-deposition treatments (PDTs) on TiO 2 buffer layers deposited by ultrasonic spray pyrolysis (USP) for emerging Sb 2 Se 3 photovoltaics. Air annealing is a typical method for recrystallizing chemically deposited TiO 2 films. However, organic residues (such as carbon species) from a precursor solution based on titanium tetraisopropoxide and acetylacetone may still remain on the TiO 2 surface, therefore requiring an additional annealing step. We demonstrate that vacuum annealing can be a suitable technological approach to decrease the concentration of carbon species in TiO 2 films. Vacuum annealing was performed at temperatures at 160-450 °C prior to the 450 °C air annealing step. It was found that vacuum annealing at 160 °C followed by subsequent air annealing led to better device performance. This was explained by achieving an optimal balance between the removal of carbon content during vacuum annealing and the active recrystallization of TiO 2 during air annealing. The decrease of carbon concentration by employing the two-step approach was supported by changes in the lattice parameters of TiO 2 and proven by X-ray photoelectron spectroscopy (XPS). The given study provides experimental evidence on how nanoscale carbon species in the TiO 2 heterojunction partner layer of a Sb 2 Se 3 solar cell can affect the device's performance. By this approach, we generate complementary insights on how the quality of the main interface has an impact and can take a key role despite the optimized Sb 2 Se 3 grain structure and orientation.