Vapor-assisted deposition of CsPbIBr2 films for highly efficient and stable carbon-based planar perovskite solar cells with superior Voc

CsPbIBr2 perovskite, as a promising light harvester, possesses the most balanced bandgap and stability characters among all-inorganic perovskite materials. However, the poor quality of the traditionally one-step solution-processed CsPbIBr2 film always leads to a severe recombination loss and thus a low output potential difference (Voc). Herein, we demonstrate a novel vapor-assisted deposition strategy to construct high-quality CsPbIBr2 films for the first time, in which the crystallization kinetics of the CsPbIBr2 is more easily controllable than that of the one-step spin-coated one. The PbBr2 film acting as the template in the CsPbIBr2 crystal growth is firstly prepared via an antisolvent-washing technique and CsI is then vacuum evaporated onto the PbBr2 layer. By precisely tuning the thickness of the CsI film, highly phase-pure and crystallized CsPbIBr2 crystals are successfully obtained. The optimized CsPbIBr2 film also exhibits a homogeneous morphology and full coverage over the substrate with large grain sizes up to microscale and ultrahigh light absorption capability. The corresponding carbon-based CsPbIBr2 solar cells achieve a champion power conversion efficiency of 8.76% with a superior Voc of 1.289 V. The large-area (1 cm2) devices also deliver an efficiency of 6.78% with an impressive Voc of 1.336 V. Moreover, under the protection of the highly hydrophobic and chemically stable CuPc layer and carbon counter electrode, the unencapsulated devices present excellent moisture and thermal stabilities. Our work provides a new approach for the preparation of cost-effective, highly efficient and robust CsPbIBr2 photovoltaics.