In contrast, thermodynamically stable orthorhombic CsPbI 3 (δ-CsPbI 3 , yellow phase) has a bandgap of about 2.82 eV, which is not suitable for photovoltaic application. This phase transition issue calls for the efforts on developing other all-inorganic perovskite materials. By replacing halide to increase Gold-Schmidt tolerance factor, CsPbI 2 Br with more stable lattice structure is emerging as an alternative for photovoltaics. [12][13][14][15][16][17] The challenges remain in the fabrication of high-quality CsPbI 2 Br film and thus high-efficiency solar cells.Several strategies have been attempted to improve the performance of CsPbI 2 Br perovskite solar cells (PSCs). For example, the CsPbI 2 Br quantum dots were synthesized as absorber layer to deliver a V oc of 1.32 V and a PCE of 12.02%. [18] The hotcast method was applied to increase the PCE of CsPbI 2 Br perovskite solar cells to 14.81%. [16] A significant PCE of 16.07% was achieved by improving the crystallization of CsPbI 2 Br film via isopropanol-assisted antisolvent processing. [4] More recently, the PCE of CsPbI 2 Br PSCs has been pushed to 16.58% by regulating the grain growth and perovskite surface. [19] Although the efficiency is still lower than those of hybrid organic-inorganic PSCs and even CsPbI 3 devices, the much less phase transition issue and sensitivity to moisture for CsPbI 2 Br compared with CsPbI 3 make it promising for the investigation of challenges and solutions in all-inorganic perovskite photovoltaic devices.Up to now, high-efficiency all-inorganic PSCs are usually based on doped hole transporting materials (HTMs) such as sprio-OMeTAD and PTAA with additives of lithium salts or cobalt salts and 4-tert-butyl pyridine (TBP). [20,21] It is well known that hygroscopic Li salts and TBP could adsorb moisture, leading to serious device degradation. [22,23] To address this issue, hydrophobic materials (such as polystyrene and PVDF) are introduced as interlayers to improve the moisture resistance of devices. [24,25] Poly(3-hexylthiophene) (P3HT) is another alternative additive-free HTM in view of its excellent hole mobility and low cost. [26][27][28] Unfortunately, due to severe nonradiative recombination at the perovskite/P3HT interface, the P3HTbased PSCs still exhibit low efficiency with low open-circuit voltage. [29] The highest PCE for dopant-free P3HT-based allinorganic PSCs is just 14.08% although the devices show much CsPbI 2 Br is emerging as a promising all-inorganic material for perovskite solar cells (PSCs) due to its more stable lattice structure and moisture resistance compared to CsPbI 3 , although its device performance is still much behind this counterpart. Herein, a preannealing process is developed and systematically investigated to achieve high-quality CsPbI 2 Br films by regulating the nucleation and crystallization of perovskite. The preannealing temperature and time are specifically optimized for a dopant-free poly(3hexylthiophene) (P3HT)-based device to target dopant-induced drastic performance degradation for spiro...

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