Improved perovskite photostability via application of TiO2, ZnO and AZO thin films by pulsed laser deposition

The UV-induced degradation of perovskite solar cells has been a barrier to commercialization. Thin film encapsulation represents a promising solution for extending perovskite solar cell lifetimes. Three materials with suitable bandgaps for blocking UV light are identified: TiO2, ZnO and AZO. Herein, the optical properties of TiO2, ZnO and AZO thin films grown by pulsed laser deposition at room-temperature are optimized by varying the oxygen partial pressure between 2, 5 and 20 mTorr during deposition. UV-Vis spectroscopy reveals facile bandgap tuning via the PLD O2 pressure for maximized UV light absorption/reflection. In particular, the TiO2 bandgap is found to narrow with decreasing O2 partial pressure as a result of an increase in oxygen vacancies and Ti3+ interstitials. Conversely, the AZO bandgap widens with decreasing O2 partial pressure, as explained by the Burstein-Moss effect. Finally, ZnO films have stable optical properties at all investigated oxygen pressures. The analysis finds that ZnO, TiO2, and AZO coatings exhibit maximum percentages of absorbed/reflected UV-A light of 72.13 +/- 3.05%, 52.30 +/- 2.72%, and 52.06 +/- 2.20% at 5, 2 and 20 mTorr O2, respectively. Accelerated UVaging experiments find that the films which screen the most UV light are also most effective in delaying perovskite degradation. A comparison of TiO2, ZnO and AZO photoprotective coatings concludes that ZnO is most suitable for perovskite solar cell encapsulation since it provides a similar to 50% reduction in UV-degradation without significantly compromising visible light transmittance.