Phase degradation of all-inorganic perovskite CsPbI 2 Br films induced by a p-type CuI granular capping layer
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Published online 2 September 2020 | https://doi.org/10.1007/s40843-020-1462-4
Phase degradation of all-inorganic perovskite CsPbI2Br films induced by a p-type CuI granular capping layer 1,2
1,2
1,3,4,5
Zhi Zhu , Wenjing Su , Jianyong Feng 1,3,4,5 1,2,3,4 Zhaosheng Li and Zhigang Zou
ABSTRACT It is necessary to evaluate the interactions between the different functional layers in optoelectronic devices to optimize device performance. Recently, the I-rich allinorganic perovskite CsPbI2Br has attracted tremendous attention for use in solar cell applications because of its suitable band gap and favorable photo and thermal stabilities. It has been reported that the undesirable phase degradation of the photoactive α phase CsPbI2Br to the non-perovskite δ phase could be triggered by high humidity. To obtain stable devices, it is thus important to protect CsPbI2Br from moisture. In this paper, CuI, a non-hygroscopic p-type hole-transporting material, is found to induce the phase degradation of α-CsPbI2Br to the δ-CsPbI2Br. The rate and extent of phase degradation of CsPbI2Br are closely associated with the heating temperature and coverage of a CuI granular capping layer. This discovery is different from the widely reported water-induced phase degradation of CsPbI2Br. Our work highlights the importance of careful selection of hole-transporting materials during the processing of I-rich all-inorganic CsPbX3 (X=Br, I) perovskites to realize high-performance optoelectronic devices. Keywords: CsPbI2Br, CuI, phase degradation, perovskite, holetransporting material
INTRODUCTION Recently, research on inorganic perovskite solar cells (PSCs) based on CsPbIxBr3–x has been progressing rapidly [1–6]. Among known light-absorbing materials, CsPbI2Br has attracted wide attention from researchers [7–9] because of its narrow band gap [10,11], high theoretical 1 2 3 4 5
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1,2
1,2
, Jincheng Li , Xiaopeng Han , Tao Yu
1,2,3,4*
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power conversion efficiency (PCE) [12], good photo and thermal stabilities, and suitable tolerance factor [13,14]. However, CsPbI2Br is sensitive to humidity. The photoactive α phase of CsPbI2Br (α-CsPbI2Br, black phase) easily converts to the non-perovskite δ phase (δ-CsPbI2Br, yellow phase) under high humidity [15–18]. It is essential to establish a suitable physical barrier between α-CsPbI2Br and moisture to improve the stability of α-CsPbI2Br-based devices in the atmosphere. Moreover, inadvertent introduction of water in other layers of PSCs can induce phase degradation of CsPbI2Br during device fabrication. Previous study revealed that CsPbI2Br PSCs using doped 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)-9,9ʹ-spirobifluorene (Spiro-OMeTAD) as a hole-transporting material (HTM) showed much lower stability than that of a pure CsPbI2Br film under the same conditions [16]. Further studies indicated that the lithium salt contained in Spiro-OMeTAD is hygroscopic, which exacerbates device instability [16,19–22]. Therefore, water should be excluded from the whole device fabrication process, including the i
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