Tunable TiO<sub>2</sub>–pepsin thin film as a low-temperature electron transport layer for photoelectrochemical cells

<p>Electron transport layer (ETL) in perovskite-based solar cells (PSCs) is attractive from the perspective of photon-to-electron conversion efficiency. TiO<sub>2</sub> thin film is preferred in situations where microstructure, stability and charge transport characteristics are important. In this regard, we illustrate an approach that involves the synthesis of TiO<sub>2</sub>–pepsin thin film that can be used either as a mesoporous or compact layer depending on the device architecture of PSC devices. It was observed that pepsin acts as organic glue to TiO<sub>2</sub> nanoparticles and facilitates in controlling morphology of the thin film. The addition of pepsin did not have any adverse effect on TiO<sub>2</sub> anatase phase. Thickness and roughness of mesoporous and compact TiO<sub>2</sub> thin film suggested that controlled agglomeration can be achieved by tuning the concentration of pepsin in the precursor colloidal solution. The spin-coated thin films indicated high bulk mobility (205 cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>) and good thermal stability over a wide range temperature (25–700°C). Furthermore, in the presence of pepsin group in TiO<sub>2</sub> thin film, the inherent optical properties were retained, as confirmed by UV–vis analysis.</p>