Rutile TiO2-based perovskite solar cells

Abstract

A perovskite solar cell based on rutile TiO₂ film was prepared and its photovoltaic performance was compared to an anatase TiO₂-based perovskite solar cell. Rutile TiO₂ nanoparticles with aspect ratio of 0.2 (20 nm wide and 100 nm long) were prepared by hydrolysis of TiCl₄ at ambient temperature. Anatase TiO₂ nanoparticles with diameter of about 50 nm were hydrothermally synthesized. The annealed rutile film showed porosity of 60.6%, while lower porosity of 49.1% was detected in the anatase TiO₂ film. CH₃NH₃PbI₃ perovskite was deposited on TiO₂ film using either a one-step spin coating or two-step dipping method. 2,2′,7,7′-Tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) was used as a hole transporting material. One-step deposition led to average power conversion efficiency (PCE) of 8.19% from the rutile-perovskite solar cells and 7.23% from the anatase-perovskite solar cells, while two-step deposition resulted in higher average PCE of 13.75% for the former device and 13.99% for the latter one. Regardless of the deposition methodologies, the rutile–perovskite solar cell showed generally higher J_sc and lower V_oc. Slower electron transport and longer electron lifetime were observed for the rutile-based perovskite solar cell than for the anatase-based one. Although the same perovskite material was used for both rutile and anatase TiO₂, the difference in electronic behavior indicates that photo-excited electrons are in part injected to TiO₂ and the extent of electron injection can be influenced by the crystal phase of TiO₂. Despite longer electron lifetime, the slightly lower voltage of the rutile-based device might be due to the fact that the amount of injected electrons was relatively larger for rutile than anatase, leading to a lower Fermi energy level at equilibrium between TiO₂ and perovskite. Using a 260 nm-thick rutile TiO₂ film, the highest PCE of 14.46% was achieved by depositing CH₃NH₃PbI₃ using a two-step method, in which photocurrent density of 20.02 mA cm⁻², open-circuit voltage of 1.022 V and fill factor of 0.71 were demonstrated.