Mesoscopic perovskite solar cells with an admixture of nanocrystalline TiO₂ and Al₂O₃: Role of interconnectivity of TiO₂ in charge collection

Perovskite solar cells with high power conversion efficiency usually employ mesoporous TiO₂, however the role of the TiO₂ layer has not been clearly resolved. Here we prepared MAPbI₃ (MA = CH₃NH₃) perovskite solar cells with an admixture of nanocrystalline TiO₂ and Al₂O₃ to investigate the role of the mesoporous TiO₂ layer. The Al₂O₃ content was varied from 0% (pure TiO₂) to 100% (pure Al₂O₃) with nominal composition of (1 - x)TiO₂ + xAl₂O₃ (x = 0, 0.25, 0.5, 0.75 and 1). The photocurrent density and fill factor decreased as Al₂O₃ content increased, whereas the open-circuit voltage was hardly changed. Steady-state photoluminescence (PL) was less quenched as the Al₂O₃ content increased due to its non-electron-injecting characteristics, where a decrease in PL intensity with increasing TiO₂ content was correlated to an increase in photocurrent. Electron injection to TiO₂ was also evidenced by time-resolved PL and time-limited photocurrent measurements, where interconnection of TiO₂ particles played an important role in charge collection. The slight change in voltage with Al₂O₃ content was explained by balancing the Fermi position due to a trade-off between charge recombination and the Fermi level. The results observed from the admixture mesoporous layer comprising electron-injecting and electron-non-injecting oxides suggest that electron-injection characteristics play an important role in determining photovoltaic parameters.