Enhancing the efficiency of TiO 2 –perovskite heterojunction solar cell via evaporating Cs 2 CO 3 on TiO 2

In a TiO 2 –perovskite heterojunction solar cell (TiO 2 –PHSC), besides the perovskite CH 3 NH 3 PbX 3 , TiO 2 as one side of the TiO 2 /CH 3 NH 3 PbX 3 heterojunction also plays an important role in the photovoltaic effect. In order to improve the performance of the TiO 2 –PHSC with the structure of glass/FTO/compact TiO 2 /mesoporous TiO 2 /CH 3 NH 3 PbI 3– x Cl x /poly‐TPD (poly( N , N ′‐bis(4‐butylphenyl)‐ N , N ′‐bis(phenyl)benzidine))/Au, a 2 nanometer thick Cs 2 CO 3 layer is thermally evaporated on the mesoporous TiO 2 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 TiO 2 –PHSC free of mesoporous TiO 2 , where Cs 2 CO 3 is evaporated on the compact TiO 2 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 TiO 2 –PHSC or the mesoporous‐TiO 2 ‐free TiO 2 –PHSC with a nanometer‐thick Cs 2 CO 3 layer on mesoporous TiO 2 or compact TiO 2 are discussed. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)