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CsPbI 3 all-inorganic perovskite has now drawn much attention due to its superior thermal stability compared with the organic-inorganic hybrid counterparts. Despite the great progress achieved recently in this field, CsPbI 3 still suffers from low phase stability when exposed to moisture. We found the phase transition of CsPbI 3 would be accelerated after depositing the conventional hole transport material (HTM) 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]− 9,9′-spirobifluorene (spiro-OMeTAD). This is attributed to the negative effects of the additives in spiro-OMeTAD HTM: the hydrolysis of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), the corrosivity of 4-tert-butylpyridine (TBP), and the evaporation of TBP. By slightly modifying the additive content, full coordination between LiTFSI and TBP could be achieved and the negative effects mentioned above could be mitigated. Furthermore, tris(2-(1 H-pyrazol-1-yl)− 4-tert-butylpyridine)cobalt(III)-tris(bis(trifluoromethylsulfonyl)imide) (Co(III)TFSI) was added to promote oxidation of spiro-OMeTAD HTM in inert environment. With these approaches of additive engineering, HTMs with better interface contact and charge transport capability could be obtained. The device with optimized spiro-OMeTAD HTM achieved a champion power conversion efficiency (PCE) of 10.61% compared with 6.63% of the control one. Moreover, the optimal device maintained 81% of its initial PCE after storage for 30 days, exceeding that (68%) of the control one. Our results highlight the importance of spiro-OMeTAD HTM on the stability of CsPbI 3 all-inorganic perovskite and provide a facile and feasible way to increase the stability and performance of corresponding solar cells. [Display omitted] • Spiro-OMeTAD HTMs triggered accelerated phase transition of black-phase CsPbI 3. • The negative effects of each component in HTMs were evaluated. • The stability of CsPbI 3 improved with modified spiro-OMeTAD HTM. • CsPbI 3 PSCs with optimized spiro-OMeTAD HTM achieved a champion PCE of 10.61%. [ABSTRACT FROM AUTHOR]

CsPbI 3 all-inorganic perovskite has attracted much attention due to its improved thermal stability compared with organic-inorganic hybrid counterparts. The shallow conduction band of CsPbI 3 leads to an energy band mismatch between it and the widely used electron transport material SnO 2 , causing a severe loss in the performance. As a result, most high-efficiency inorganic perovskite solar cells are based on high-temperature sintered TiO 2 with better energy band alignment. To exploit the potential of SnO 2 in inorganic perovskite solar cells, SnO 2 nanocrystals were complexed with ethoxylated polyethyleneimine (PEIE). This strategy affords continuous adjustment of SnO 2 band structure by varying PEIE content, leading to an ideal energy band alignment with CsPbI 3. The introduction of PEIE also promotes the growth of CsPbI 3 films, resulting in larger grains. Moreover, improved charge extraction is achieved after PEIE introduction. As a result, the power conversion efficiency of CsPbI 3 all-inorganic perovskite solar cells significantly increases from 11.61 % to 13.89 %. Our work provides a simple strategy to produce high-performance CsPbI 3 all-inorganic perovskite solar cells with low-temperature solution process. [Display omitted] • PEIE incorporation enables continuous adjustment of SnO 2 band structure. • Favorable energy band alignment is achieved between PEIE-SnO 2 and CsPbI 3. • PEIE improves SnO 2 charge transport and facilitates CsPbI 3 crystal growth. • PEIE-SnO 2 boosts the PCE of low-temperature processed CsPbI 3 PSC to 13.89 %. [ABSTRACT FROM AUTHOR]