Enhancing the efficiency of TiO2–perovskite heterojunction solar cell via evaporating Cs2CO3 on TiO2

In a TiO₂–perovskite heterojunction solar cell (TiO₂–PHSC), besides the perovskite CH₃NH₃PbX₃, TiO₂ as one side of the TiO₂/CH₃NH₃PbX₃ heterojunction also plays an important role in the photovoltaic effect. In order to improve the performance of the TiO₂–PHSC with the structure of glass/FTO/compact TiO₂/mesoporous TiO₂/CH₃NH₃PbI_3–xCl_x /poly‐TPD (poly(N,N ′‐bis(4‐butylphenyl)‐N,N ′‐bis(phenyl)benzidine))/Au, a 2 nanometer thick Cs₂CO₃ layer is thermally evaporated on the mesoporous TiO₂ layer. The short‐circuit current density (J_sc) raises from 17.7 mA cm^–2 to 18.9 mA cm^–2, the open‐circuit voltage (V_oc) from 0.81 V to 0.87 V, and the fill factor (FF) from 55.2% to 67.3%; as a result, the power conservation efficiency (PCE) increases from 8.0% to 11.1% under AM 1.5G solar illumination (100 mW cm^–2). Moreover, in a TiO₂–PHSC free of mesoporous TiO₂, where Cs₂CO₃ is evaporated on the compact TiO₂ layer, the J_sc, V_oc, FF and PCE values increase from 16.0 mA cm^–2, 0.83 V, 50.8% and 6.7% to 17.9 mA cm^–2, 0.90 V, 59.3%, and 9.5%, respectively. The reasons of the PCE increase for either the first kind of TiO₂–PHSC or the mesoporous‐TiO₂‐free TiO₂–PHSC with a nanometer‐thick Cs₂CO₃ layer on mesoporous TiO₂ or compact TiO₂ are discussed. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)