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Photoreduction of CO2 without photosensitizers and scavengers runs into the development bottleneck for lack of excellent photocatalysts and ambiguous reduction mechanism. Herein, an ultrathin CoO layer containing Zn‐dopants and O‐vacancies (Vo‐Zn‐CoO) is designed as an archetype to explore the influence mechanism of Zn on O‐vacancies in ultrathin nanolayer for CO2 photoreduction. DFT calculations illustrate that Zn‐dopants not only reduce formation barriers of *COOH and *CO intermediates, but also form π‐back‐bonding with *CO stimulating CH4 evolution. Finally, Vo‐Zn‐CoO layer significantly enhances CO2 photoreduction efficiency and CH4 selectivity with 26.8 µmol g−1 h−1 (63.8%) compared to 7.2 µmol g−1 h−1 (23.6%) for CoO layer with O‐vacancies. Moreover, the synergistic effect of Zn and O‐vacancies benefits the stability of O‐vacancies in photocatalysts, achieving durable photocatalytic performance of Vo‐Zn‐CoO layer. This work manifests that the strategy of metal atoms synergistic O‐vacancies is effective to optimize CO2 photocatalytic efficiency, selectivity, and stability of photocatalyst with O‐vacancies. [ABSTRACT FROM AUTHOR]

All‐inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. However, the inferior film quality and doped hole transport layer (HTL) have a strong tendency to degrade the perovskite under high temperatures or harsh operating conditions. To solve these problems, a one‐source strategy using the same polymer donor material (PDM) to simultaneously dope CsPbI2Br perovskite films via antisolvent engineering and fabricating the HTL is proposed. The doping assists perovskite film growth and forms a top–down gradient distribution, generating CsPbI2Br with enlarged grain size and reduced defect density. The PDM as the HTL suppresses the energy barrier and forms favorable electrical contacts for hole extraction, and assemble into a fingerprint‐like morphology that improves the conductivity, facilitating the creation of a dopant‐free HTL. Based on this one‐source strategy using PBDB‐T as PDM, the CsPbI2Br perovskite solar cell with a dopant‐free HTL achieves a power conversion efficiency (PCE) of 16.40%, which is one of the highest PCEs reported among all‐inorganic CsPbI2Br pero‐SCs with a dopant‐free HTL. Importantly, the devices exhibit the highest thermal stability at 85 °C and operational stability under continuous illumination even with Ag as the top electrode and present good universality. [ABSTRACT FROM AUTHOR]