Influence of epitaxial anatase and rutile TiO₂ thin films as electron transport layers for perovskite solar cell

The effective electron transport layer (ETL) is essential for high-power conversion efficiency (PCE) of perovskite solar cell (PSC). TiO₂ is the most widely used material for ETL owing to proper band alignment, enough optical transmittance, and high electron mobility. There are two representative thermodynamically stable crystal phases of TiO₂: anatase and rutile. However, it is still debated which phase is more effective for the ETL. To solve the concern, single-phase thin film is strongly needed.
We deposited epitaxial anatase- and rutile-TiO₂ thin films on LaAlO₃ and g-sapphire substrate, respectively, using pulsed laser deposition technique. X-ray diffraction analysis shows that both films have high crystallinity. In addition, both phases of epitaxial Nb-doped TiO₂ films are grown in order to enhance carrier injection/extraction of ETL. The variation of chemical states of elements resulting from Nb dopants was observed by X-ray photoelectron spectra. Steady-state photoluminescence (PL) spectra and time-resolved PL reveals PSC with anatase-TiO₂ has more effective carrier transporting and charge separating than with rutile-TiO₂. Finally, the photocurrent density-voltage (J-V) measurements were conducted for estimating PCE.