Achieving longitudinal doping of specific ions by surface treatment remains a challenge for perovskite solar cells, which are often limited by dopant and solvent compatibility. Here, with the flowing environment created by CsBr colloidal nanocrystals, ion exchange is induced on the surface of the perovskite film to enable the homogeneous distribution of Cs+ and gradient distribution of Br- simultaneously at whole depth of the film. Meanwhile, assisted by long-chain organic ligands, the excess PbI2 on the surface of perovskite film is converted to a more stable quasi-2D perovskite, which realizes effective passivation of defects on the surface. As a result, the unfavorable n-type doping on the top surface is suppressed, so that the energy level alignment between perovskite and hole transport layer is optimized. On the basis of co-modification of the surface and the bulk, the PCE of champion device reaches 23.22% with enhanced VOC of 1.12 V. Device maintains 97.12% of the initial PCE in dark ambient air at 1% RH after 1056 h without encapsulation, and 91.56% of the initial PCE under light illumination of 1 sun in N2 atmosphere for more than 200 h. The approach demonstrated here provides an effective strategy for the nondestructive introduction of inorganic ions in perovskite film.
Keywords: colloidal nanocrystals; energy level match; gradient ion doping; ion exchange; perovskite solar cells; quasi-2D perovskite.
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