The phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic‐CsPbI3−xBrx perovskite solar cells. In this work, stable cubic perovskite (α)‐phase CsPbI2Br is successfully achieved by Pb(Ac)2 functioning at the grain boundary under low temperature. Ac− strongly coordinates with CsPbI2Br to stabilize the α‐phase and also make the grain size smaller and film uniform by fast nucleation. PbO is formed in situ at the grain boundary by decomposing Pb(Ac)2 at high‐temperature annealing. The semiconducting PbO effectively passivates the surface states, reduces the interface recombination, and promotes the charge transport in CsPbI2Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO‐passivated CsPbI2Br solar cells, while Pb(Ac)2‐passivated device exhibits 8.7% performance and the highest stability, much better than the control device with 8.5% performance and inferior stability. This article highlights the extrinsic ionic grain boundary functionalization to achieve stable and efficient inorganic CsPbI3−xBrx materials and the devices.

Organic–inorganic hybrid lead halide perovskites have recently realized significant development. However, the toxicity of lead (Pb) and the poor stability might eventually hamper the commercialization of perovskite solar cells (PSCs). Here, we present an environment friendly and stable all inorganic rudorffite AgBiI4 as solar absorber by solution-based synthesis of thin-films. AgBiI4 films fabricated by 0.6 M solution and annealed at 150 °C show dense grains and high surface coverage. Furthermore, the AgBiI4 films exhibit greater thermal stability and photostability than CH3NH3PbI3. Ultimately, by judiciously choosing the hole transport material with appropriate energy level, the solar cell device demonstrates high carrier extraction efficiency and achieve a power conversion efficiency (PCE) of 2.1% under standard 1 sun (AM 1.5). The devices also show excellent long-term air stability and still maintain 96% of its initial PCE even after 1000h at relative humidity of 26%. This work highlights new directions for further exploration of Pb-free and stable solar cells.

PVK has been extensively tried in conventional OIH-PVK. Pure Sn, [10] Sn-Pb, [11,12] Sn-Cu-Pb, [13] and In-Pb [14] based alloys reduced ≥10% Pb content in OIH-PSCs. Regardless of the instability of Sn 2+ , high efficiency is achieved in Sn-Pb alloyed solar cells because of the energy state tuning and high quality perovskite film formation. Doping of Pb is also tried in CsPbX 3 based I-PVK. 10% Pb is reduced in CsPb 0.9 Sn 0.1 IBr 2 , which was applied in carbon counter electrode based I-PSCs. Other doping elements of Sr, [15] Bi, [16] Mn, [17,18] Ln 3+ , [19] and In 3+[20] were tried aiming to stabilize the phase or increase the performance. However, the doping amounts were usually below 5%, which still remains a large amount of Pb. Because of the special physicochemical property of Pb, large amount of new substitute might greatly change the energy band structure and induce trap states in the bandgap to deteriorate opto-electronic property. Therefore, seeking stable substitutes to achieve lead-less CsPbX 3 PSCs with high PCE is a great challenge.Aiming for high PCE, high quality large PVK crystals, and even single-crystal with a few GBs is pursued to reduce the defect states in OIH-PVK. [21][22][23][24][25] Moreover, the GB is modified to further passivate the defect states. [26,27] Recently, obtaining high quality I-PVK films with low defect states became a focus of attention, which largely boosts the PCE. [17,[28][29][30][31][32] However, the crystalline process of Pb-reduced I-PVK might be particularly different from the pure CsPbX 3 films because of the differed physicochemical property between the substitute ion and the Pb analog. [10][11][12] Therefore, formation of high quality Pb-reduced I-PVK films should be paid special attention to.In this work, stable and abundant Zn is used to reduce ≥10% Pb in CsPbI 2 Br. Due to the different physicochemical property of Zn 2+ , the crystal morphology and energy band structure are effectively tuned. 10% Pb reduction in CsPb 0.9 Zn 0.1 I 2 Br successfully diminishes GB trap states and slightly reduces the energy bandgap, greatly contributing to the enhanced performance of PSCs. Stable Zn 2+ and high quality CsPb 0.9 Zn 0.1 I 2 Br also benefit the device stability.To reduce the Pb content, PbI 2 is partly replaced by ZnI 2 in the starting material, and the molar ratio of Cs:(Pb+Zn):(Br+I) in the precursor is kept to be 1:1:3. The ZnI 2 doped precursors and the films are noted as CsPb 1−x Zn x I 2 Br (x is the molar percentage of Zn, x = 6%, 10%, and 12% in our work). The X-ray Fabrication of efficient Pb reduced inorganic CsPbI 2 Br perovskite solar cells (PSC) are an important part of environment-friendly perovskite technology. In this work, 10% Pb reduction in CsPb 0.9 Zn 0.1 I 2 Br promotes the efficiency of PSCs to 13.6% (AM1.5, 1sun), much higher than the 11.8% of the pure CsPbI 2 Br solar cell. Zn 2+ has stronger interaction with the anions to manipulate crystal growth, resulting in size-enlarged crystallite with enhanced growth orientation. Moreover, the grain ...

There exists stronger chemical interaction between Zn2+ and CH3NH3+/I−, which effectively influences the morphology of perovskite film during annealing process.

The organic-inorganic perovskite material with better energy alignment in the solar cell device will have a profound impact on the solar cell performance. It is valuable to tune the energy states by element substitution and doping in perovskites. Here, we present that Ca2+ is incorporated into CH3NH3PbI3, which up-shifts the valence band maximum and the conduction band minimum, leading to a difference between the bandgap and the Fermi level in the device. Consequently, Ca2+ incorporation results in an enhancement of the photovoltage and photocurrent, achieving a summit efficiency of 18.3% under standard 1 sun (AM 1.5). This work reveals the doped perovskite to improve the solar cell performance by tuning the energy state.