Inorganic perovskite CsPbI3 has been studied as a promising alternative light‐absorbing material for photovoltaic application due to its suitable band gap for converting solar light and enhanced stability toward ambient conditions compared to the organic–inorganic halide perovskite. However, the photoactive α‐phase of CsPbI3 can only be stabilized at a high temperature and the phase transition from α‐phase to δ‐phase easily occurs at room temperature. Herein, ytterbium (III) chloride (YbCl3) as a bifunctional additive is introduced into the perovskite precursor to stabilize the black α‐phase, and high‐quality CsPbI3 films are obtained at a temperature as low as 80 °C. Yb3+ partly replaces Pb2+ to enhance the tolerance factor and favors the formation of α‐phase. The Lewis adduct complex of YbCl3·DMSO in the perovskite film can passivate the perovskite film with reduced defects and help enhance the stability of the α‐phase. The YbCl3‐modified planar‐type α‐CsPbI3 perovskite solar cells show a champion power conversion efficiency of 12.4% with an impressive stability at ambient conditions. The additive induced controlled crystallization provides a simple and promising way to improve the photovoltaic performance of inorganic perovskite solar cells.

It is extremely important to develop nontoxic solvent and additive‐processed high‐performance all‐polymer solar cells (all‐PSCs) that are suitable for printing preparation of large‐scale devices. Herein, it is demonstrates that a simple random copolymerization of two acceptor monomers (benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) and 5,6‐difluoro‐2H‐benzo[d][1,2,3]triazole (FTAZ)), alternating with Si atom‐containing benzo[1,2‐b:4,5‐b′]dithiophene donor comonomer, forms a successful approach by which to synthesize donor copolymers with excellent solubility/processability for nontoxic‐solvent‐processed all‐PSCs. The incorporation of a higher degree of BDD in the backbone lowers the frontier energy levels, as well as redshifts, with higher absorption coefficients; however, it adversely affects solubility in a 2‐methyltetrahydrofuran (MeTHF). An impressive power conversion efficiency, of about 8.0%, is achieved from PJ25 (25 mol% BDD)‐based all‐PSC when paired with N2200‐F30 acceptor random copolymer by using MeTHF as the processing solvent without any additive. Another interesting point is that the air stability of the all‐PSCs increases with increasing FTAZ content due to strong noncovalent interaction and resistance to humidity and oxidation caused by the F‐atoms in FTAZ units. Not only does this study establish a structure–property–performance relationship through a series of structural, morphological, and electrical characterization techniques, but it also provides a promising and easy way to develop nontoxic‐solvent‐processed high‐performance all‐PSCs.