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![Homojunction perovskite solar cells: opportunities and challenges](https://image.oaes.cc/ad4c7d93-f5b4-46ed-975a-7b415aed57cf/4403.fig.4.jpg)
Figure 4. Extrinsic doping for perovskite homojunctions. (A, B) A simple heterovalent metal halide surface treatment strategy enables the self-assembly of an n/n+ perovskite homojunction, which synergistically enlarges the built-in electric field. Reproduced from