the main concern for degrading the device stability; for example, CH 3 NH 3 PbI 3 could be decomposed into lead Iodide (PbI 2 ) and CH 3 NH 3 I, and the organic molecules could be vapored under thermal or humidity environment. [15] Therefore, addressing the long-term stability is a primary concern for the perovskite solar cells community. [16] As an alternative, all-inorganic perovskites (CsPbX 3 , X = I, Br, Cl, or their mixtures), prized for their excellent thermal stability, have received increasing attention. [17][18][19][20][21][22][23] Among them CsPbI 3-x Br x with the band gap of around 1.73 eV showed promising, by stabilizing the α-phase, control growth of the perovskite layer, and also the interface engineering, significant progresses have been achieved. For example, Luther et al. have shown CsPbI 3 quantum dot solar cells with the efficiency of 10.77% and 13.4%, subsequently. [24,25] Our group has developed a solvent controlled growth of CsPbI 3 in dry environment, and showed a 14.67% certificated efficiency with 500 h light-soaking stability. [26] Li et al. invented a novel gradient thermal annealing to control the growth of CsPbI 2 Br film (band gap = 1.92 eV), and a champion PCE of 16.07% with a V OC of 1.23 V was realized. [27] Recently, an outstanding efficiency of 17.06% with V OC of 1.1 V was realized by Zhao et al. via using HPbI 3 as a precursor combined with the PTABr surface modification. [28] Although there are significant progresses in inorganic perovskite solar cells, the PCE is still far behind the hybrid PSCs, even compared with the I-Br mixed hybrid perovskite with a similar band gap (1.75 eV). [29] It can be found that the opencircuit voltage loss (V oc loss) is still the main reason of low performance of inorganic perovskite solar cells, which strongly related to energy band matching and defects at the interface or in the bulk of perovskite. [10,14] Most recently, Yip et al. applied PN4N as cathode interlayer to reduce the work function of the SnO 2 electron transporting layer (ETL) for tuning the electron extraction property and combing with poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT) as hole transporting layer, leading to a significant enhancement in V OC of the CsPbI 3-x Br x PVSCs from 1.06 to 1.3 V [30] ; however, the V oc loss is still as high as 0.62 V .Here, we develop an inorganic shunt-blocking layer lithium fluoride (LiF) between SnO 2 and CsPbI 3-x Br x perovskites, which push forward the conduction band of the electron transport Cesium-based inorganic perovskite solar cells (PSCs) are promising due to their potential for improving device stability. However, the power conversion efficiency of the inorganic PSCs is still low compared with the hybrid PSCs due to the large open-circuit voltage (V OC ) loss possibly caused by charge recombination. The use of an insulated shunt-blocking layer lithium fluoride on electron transport layer SnO 2 for better energy level alignment with the conduction band minimum of the CsPbI...

Low bandgap lead-tin mixed perovskite solar cells have achieved high power conversion efficiency in excess of 17%. However, methylammonium (MA) cation was usually contained, and the thermal stability of MA is always a great concern. In this work, accoring to composition engineering, we tried to explore a nearly formamidinium (FA) based lowbandgap Pb-Sn mixed perovskite FAPb 0.75 Sn 0.25 I 3 as the absorber layer. Combined with interface enginering by replacing PEDOT:PSS layer with NiO x as hole transport layer, a power conversion efficiency of 17.25% has been obtained. This low-bandgap perovskite solar cell maintains about 91% of its original efficiency at 80 O C for 20 hours, and 92% of its initial performance after 46 days storage at the room temperature. The good thermal stability of nearly FA based low-bandgap perovskite could be good for delivering efficient and stable perovskite-perovskite tandem solar cells.

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