10001059_Triple Cation NH3+C2H4NH2+C2H4NH3+-induced Phase-stable Inorganic α-CsPbI3 Perovskite Films for Use in Solar Cells

Triple Cation NH3+C2H4NH2+C2H4NH3+-induced Phase-stable Inorganic α- CsPbI3 Perovskite Films for Use in Solar Cells Xihong Ding1, and Songyuan Dai1* 1Beijing Key Laboratory of Novel Thin-Film Solar Cells, Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing 102206, China In spite of the outstanding photoelectronic properties of the all-inorganic cesium lead iodide (CsPbI3) perovskite1, which does not contain volatile and hygroscopic organic components, only a few CsPbI3 devices have been developed mainly owing to its spontaneous phase transition from the desired black α-phase to the inactive yellow δ-phase at room temperature. Herein, it is demonstrated that employing a small quantity of triple cation NH3+C2H4NH2+C2H4NH3+ (denoted as DETA3+) without introducing the layered low- dimensional structure could effectively stabilize α-CsPbI3 for 60 d via a facile one-step deposition method without any encapsulation. Through a combination of physical and spectroscopic methods (FT-IR, XRD, UV-vis spectroscopy, SEM, PL, TRPL, XPS, Raman spectroscopy and contact angle measurement), reasons for the remarkable improvement of phase-stability for CsPbI3 are generalized into three aspects: the reduction of CsPbI3 crystalline grains, avoiding spontaneous octahedra tilting caused by ion bonds between NH3+ or RNH2+ groups of DETA3+ and I- of [PbI6]4- in CsPbI3 perovskite crystal formed on the surface and grain boundaries of the CsPbI3 and enhancement of hydrophobicity of inorganic perovskite layers induced by the oil-wet (hydrophobic) hydrocarbon chains of DETA3+ aggerated on the surfaces. Excellent photovoltaic properties of the CsPbI3·xDETAI3 perovskite films with reduced nonradiative recombination are achieved. The best solar cell with the configuration of FTO/compact-TiO2/CsPbI3·xDETAI3/HTM/Au shows a PCE of 7.89% and retains 92% of initial PCE after storage for 1008 h stored in a dark dry box without any encapsulation. In addition, we also find other polycationic organic ammonium iodine as a proof-of-concept, such as ethylenediamine iodine (EDAI2) and pyrazine iodine (PZI2), could stabilize α-CsPbI3, indicating that employing a small quantity of polycationic organic ammonium iodine is a general effective approach to solve the formidable phase-related issue encountered by CsPbI3. Based on our findings, more analogues of DETAI3 with better photoelectronic properties are expected to be found or designed by the community. Moreover, the method of this phase manipulation is also expected to be utilized in stabilizing the active phase of other mutable perovskite materials, such as FAPbI3, FASnI3, and CsSnI3, by the community in future. Fig. 1. schematic of surface passivation of DETAI3, ion bonds and octahedra tilting. References 1. Huang Y., Yin W.-J. and He Y., The Journal of Physical Chemistry C, 2017, 122, 1345-1350.