fig2
![Homojunction perovskite solar cells: opportunities and challenges](https://image.oaes.cc/ad4c7d93-f5b4-46ed-975a-7b415aed57cf/4403.fig.2.jpg)
Figure 2. Current electrical doping characterization methods for perovskite thin films and homojunction structures. XSP: X-ray spectroscopy; UPS: ultraviolet photoelectron spectroscopy; KPFM: Kelvin probe force microscopy; HRTEM: high-resolution transmission electron microscopy; TRPL: time-resolved photoluminescence.