Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Structure optimization of CH 3 NH 3 PbI 3 by higher-valence Pb in perovskite solar cells with enhanced efficiency and stability Bing Liu a,c , Rongli Cui a , Huan Huang a , Xihong Guo a , Shouwei Zuo b,c , Jinquan Dong a , Huanli Yao a , Ying Li a,c , Dangui Zhao a,c , Jiahao Wang a , Jing Zhang b , Yu Chen b , Junliang Yang b,c , Baoyun Sun a,c, ⁎ a CAS Key Lab for Biomedical Effects of Nanomaterials & Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China c University of Chinese Academy of Science, China ARTICLE INFO Keywords: Perovskite solar cells Structure optimization Higher-valence lead Stronger Pb-I bond Fullerene ABSTRACT The crystal structure of perovskite has a significant influence on the photovoltaic performance and stability of perovskite solar cells (PSCs). Pb 4+ is introduced into CH 3 NH 3 PbI 3 (MAPbI 3 )-based PSCs by participating the octahedral [PbI 6 ] 4- structure, then to induce the formation of stronger Pb-I bond and reduce [PbI 6 ] 4- octahedron distortion, which would improve structural symmetry, decrease the degree of disorder and be beneficial to the crystallization of perovskites. Synchrotron based X-ray absorption fine spectroscope (XAFS) revealed that the existence of higher valence state lead could be realized by doping Pb 4+ directly or induced by electron with- drawing group, in consequence the [PbI 6 ] 4- octahedral structure becomes more stable. Grazing incidence X-ray diffraction (GIXRD) especially demonstrates that Pb 4+ with appropriate proportion may well replace part of Pb 2+ to form an uniform phase in the primal perovskite structure to improve the crystallization on the surface and homogeneous out-of-plane (OOP) ordered crystal accumulation in the bulk, which is also important for improving the efficiency and stability of PSCs. As a result, a power conversion efficiency (PCE) exhibits a 42.1% increase with the doping of 0.03% PbF 4 and 0.075% PCBM compared with a pristine device and its stability improves markedly after 30 days of storage in ambient atmosphere. 1. Introduction Perovskite solar cells have received broad attention and research due to their rapidly improving efficiency since the first article by Kojima et al. was published in 2009 (Kojima et al., 2009). The PCE of 3.8% improves considerably to 25.2% in ten years (Kim et al., 2019), nevertheless, to improve the efficiency and stability is still a major problem. Different possibilities are available for the development of conversion efficiency such as expanding the absorption spectrum (Fu et al., 2016; Li et al., 2015; Pratheek and Predeep, 2020; Yang et al., 2015), suppressing interface and surface recombination (Guo et al., 2019; Mei et al., 2014; Zhou et al., 2014; Zou et al., 2019), and/or increasing grain size to reduce defects (Nie et al., 2015; Zhang et al., 2019; Zhu et al., 2019), as well as new types of solar cells, e.g., per- ovskite/silicon tandem (Werner et al., 2016). Besides of that, the de- velopment of stability mainly concentrates upon interface enhance- ments at this stage. However, the improvement of perovskite material itself which should be paid attention to is still limited. Further research is needed in this aspect to realize the goal of efficiency and stability. The optoelectronic properties (absorption cross section, band gap, charge carrier motility etc.) of perovskite materials hinge on its struc- ture. Any crystal distortion and phase transformation could detrimen- tally impact its optoelectronic properties which would degrade the photovoltaic performance of PSCs. The Goldschmidt tolerance factor t, which generally suggests the symmetry and structural stability of a 3D perovskite structure, can be expressed by = + + t r r r r 2( ) A X B X , where r A , r B and r X are the radii of monovalent cation, divalent metal cation and monovalent halide anion, respectively (Chen et al., 2016; Leijtens et al., 2015). The perovskite materials usually keep 3D cubic structure for 0.9 ≤ t ≤ 1, whereas an ideal 3D cubic perovskite structure has a value of t =1(Noh et al., 2013). A network of BX 6 octahedra exits in the ideal cubic perovskite structure, which may be subjected to distortion in lower symmetry structures (Smith et al., 2014; Zhou and Zhao, 2019). For this reason, raising the value of t could heighten the https://doi.org/10.1016/j.solener.2020.05.028 Received 23 March 2020; Received in revised form 1 May 2020; Accepted 9 May 2020 ⁎ Corresponding author at: CAS Key Lab for Biomedical Effects of Nanomaterials & Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. E-mail address: sunby@ihep.ac.cn (B. Sun). Solar Energy 205 (2020) 202–210 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T