Combustion-processed NiO/ALD TiO2 bilayer as a novel low-temperature electron transporting material for efficient all-inorganic CsPbIBr2 solar cell

Low-temperature ALD TiO2 electron transporting layers (ETLs) are promising for all-inorganic perovskite solar cells (PSCs), such as the CsPbIBr2-based ones. However, the non-ideal interfacial level-alignment between ALD TiO2 and CsPbIBr2, as well as the concomitant defects in ALD TiO2 during preparation of upper CsPbIBr2 film severely limit the performance of final PSC. We report herein a new design of ETL by combining ALD TiO2 with low-temperature combustion-processed NiO. Although the underlying NiO layer has a p-type conductivity and is known as a hole transporting layer (HTL), the NiO/ALD TiO2 bilayer can serve as an ETL with fewer traps, larger conduction band minimum (CBM) offset with CsPbIBr2 film, along with the similar optical transmittance, in contrast with individual ALD TiO2 ETL. Consequently, the resulting optimized CsPbIBr2PSC yields the superior efficiency of 9.71% and photovoltage of 1.272 V, both of which exceed those of the one based on individual ALD TiO2 ETL and even so-gel TiO2 ETL. Our work verifies the great applicability of NiO/ALD TiO2 ETL for CsPbIBr2 PSC and thereby explores a promising way to develop more low-temperature ETLs by combining conventional HTLs with ALD TiO2 layers.