Passivation of Bulk and Interface Defects in Sputtered-NiO_x-Based Planar Perovskite Solar Cells: A Facile Interfacial Engineering Strategy with Alkali Metal Halide Salts

Nickel oxide (NiOx) (deposited by sputtering (sp)) is a promising hole transport layer (HTL) for inverted planar perovskite solar cells. However, poor CH3NH3PbI3 crystallization, elimination of CH3NH3+, and formation of residual PbI2 grains, induced by defects present on the surface of sp-NiOx, have limited the device efficiency. Herein, a facile approach is reported to passivate the surface defects in sp-NiOx and simultaneously induce complete perovskite crystallization (without residual PbI2 grains) via modifying the sp-NiOx/CH3NH3PbI3 interface with various alkali metal halide salts (AMHSs). Comprehensive film and device characterizations reveal the additional influence of AMHSs, especially cesium bromide (CsBr), on the structural, morphological, photophysical, and photovoltaic performance. It is found that incorporation of a CsBr interlayer significantly improves the perovskite crystallization, producing high-quality MAPbI3 films with enlarged grain sizes (without any residual PbI2 grains) contrasting without the CsBr-interlayer case. CsBr (and other AMHSs) additionally reduces the band tail states and passivates the surface defects in sp-NiOx (as revealed by X-ray photoelectron spectroscopy and photodeflection spectroscopy), thereby suppressing interfacial disorder and recombination centers and improving the overall charge collection property across the sp-NiOx/CH3NH3PbI3 interface. This leads to improvement in the device efficiency (with active area = 1 cm2) and long-term operational stability at the maximum power point under continuous illumination for 8000 s and against ambient atmosphere for ∼2200 h. On the basis of the results, a possible crystallization process is discussed which provides insights into the engineering of the sp-NiOx/CH3NH3PbI3 interface rendering improved device performance.