Professor xu Jianbin, CUHK and Professor Li Gang, HKUST: the synergistic effect between organic cations and halogen anions on the stability and photoelectric properties of perovskite

Lead organic-inorganic hybrid perovskite has a series of excellent and unique photoelectric properties, including high visible light absorption coefficient, long carrier diffusion length, high mobility, high tolerance of defect state, so it has been widely concerned in many fields, especially in the field of solar cells After several years of research and development, its photoelectric conversion efficiency has rapidly increased from 3.8% in 2009 to 22.7%, which is comparable to commercial inorganic solar cells Its preparation method is simple, mainly solution preparation method, and the production cost is low, so it has a potential application prospect in the field of solar cells At present, although its photoelectric conversion efficiency has reached the industrial level, its stability still needs to be further strengthened, mainly including humidity stability, thermal stability and light stability The mixed cation mixed halogen type perovskite CS xfa1-x pbiybr 3-y, one of the organic-inorganic hybrid perovskites, has been widely used in single junction and cascade solar cells due to its adjustable band gap structure and excellent stability Recently, Professor xu Jianbin of the Chinese University of Hong Kong and Professor Li Gang of the Hong Kong Polytechnic University have studied CS x FA 1-x PBI y br 3-y using a series of in situ detection methods The effect of organic cations and halogen anions on their intrinsic stability and photoelectric properties was studied It was suggested that there was a simultaneous volatilization of formamidine cations and bromine ions in the process of low-temperature heating, which could accelerate the decomposition and phase separation of perovskite This work emphasizes that the synergism between organic cation and halogen anion can optimize the stability and photoelectric properties of organic-inorganic hybrid perovskite (DOI: 10.1002 / ADMA 201801562) Introduction to Professor xu Jianbin group of the Chinese University of Hong Kong xu Jianbin research group mainly focuses on the research and development of new solar energy technologies, including perovskite based solar cells; graphene and new two-dimensional solid-state semiconductor electronic and optoelectronic materials and devices; organic solid-state semiconductor electronic and optoelectronic devices (such as device physics, Surface plasmon photonics in semiconductor electronics and optoelectronic devices Since 2013, the research group has gradually carried out research on the photoelectric properties and stability of perovskite solar cells Based on the chemical coordination engineering, the defect concentration of perovskite optical absorption layer is reduced, the crystallization process is optimized and its stability and photoelectric conversion performance are improved simultaneously Prof Xu Jianbin, Ph.D xu Jianbin, received his Bachelor of science and master's degree from Nanjing University in 1983 and 1986 At the beginning of 1988, he went to Konstanz University (Elite University) in Germany for further study and studied with Klaus Dransfeld, a famous German solid state physics and applied physicist (academician of German Academy of Sciences), mainly engaged in the research of nano science and technology, especially scanning probe microscopy and near-field microscopy and their application in energy transfer In the summer of 1993, he obtained a doctor's degree in German natural science, and then joined the Department of electronic engineering of the Chinese University of Hong Kong He also became a member of the Hong Kong Institute of Engineers (HKIE) fellow, the American Institute of electrical and Electronic Engineers (IEEE) fellow, the American Physical Society and the American Society for material research In 1995, preparation began for the establishment of the Nanotechnology Laboratory of the Chinese University of Hong Kong Since 2007, he has been the director of the materials research center of the Chinese University of Hong Kong Since 2011, he has been the general recruiter of the nano energy materials research program of the College of engineering Now he is the Secretary General of the Hong Kong materials society, the editorial board member of the microelectronics Journal and the director of the China Vacuum Society So far, more than 200 SCI articles have been published in nature photo., nature nanotechnol., NAT Commun., adv mater., j.am Chem SOC., adv energy mater., adv func Mater., acsnano and other high impact magazines, with a total number of citations of more than 8000 times Brief introduction of Professor Li Gang group of Hong Kong University of science and Technology Professor Li Gang's research group mainly focuses on optoelectronic devices based on organic semiconductors and organic-inorganic hybrid perovskite films, including film morphology characterization and control, interface engineering, device structure, device physics, flexible electronics and device printing and preparation technology Prof Li Gang profile Prof Li Gang obtained his Bachelor of Science Degree in Space Physics Department of Wuhan University in 1994 and his doctor of condensed matter physics and master of electrical engineering degree in Iowa State University in 2003 In 2007-2011, he presided over the research and development of organic polymer solar cells of start-up company Solarmer Energy Inc From 2011 to 2016, he served as an associate professor of research at UCLA, and then joined the Department of electronic and information engineering of Hong Kong Polytechnic University as an associate professor in August 2016 to prepare and host the advanced materials and electronics laboratory Professor Li Gang has published more than 100 papers (more than 40 highly cited papers) in nature materials, nature photonics, science, chemical reviews, advanced materials and other academic journals, which have been cited more than 47000 times, and H-factor 63 He is a global highly cited scientist (Material Science (2014-2017) and Physics (2017) discipline) of Thomson Reuters from 2014 to 2017 Frontier research achievements: the influence of organic cations and halogen on the stability and photoelectric properties of mixed perovskite Professor xu Jianbin's team systematically studied the coordination of various chemical components in the perovskite precursor, and discussed the influence of coordination degree on the colloid size and perovskite crystallization process in the precursor In this work, it was found for the first time that perovskite precursor is essentially a colloidal solution, and it was pointed out for the first time that coordination plays a decisive role in the crystallization and morphology of perovskite, opening up a direction for the subsequent precursor to regulate the growth of perovskite films (j.am.chem.soc., 2015, 137, 4460-4468) In addition, they found a new and efficient synthesis formula of perovskite film, that is, hpbi 3 plus Ma solution replaced the traditional PbI 2 plus Mai solution The coordination of Pb in hpbi 3 is more complete than that of PbI 2 At the same time, excessive Ma will guide the directional crystallization of perovskite on the one hand, and passivate the grain boundary on the other hand The final unsealed perovskite film has excellent stability at 65% humidity, and there is no impurity phase in two months (NAT Commun., 2016, 7, 13503) Figure 1: crystal transformation process of perovskite: from PbI 2 to hpbi 3, and then react with excessive Ma to prepare mapbi 3 (source: Nat Commun., 2016, 7, 13503) for the preparation of organic-inorganic hybrid perovskite films based on formamidine (fabpi3-xbr x), a large amount of anti solvent is needed in the spin coating process, which brings some difficulties to the operation and reduces the repeatability of the device preparation Therefore, hpbi 2Br, an intermediate compound, was introduced to replace PbI 2, and then a halogen exchange reaction was carried out with non stoichiometric Fai to prepare high crystallinity and large grain fabbi 3-xbr x films The introduction of hpbi 2Br can effectively delay the crystallization process The thermal instability of hpbi 2Br can make the intermediate exchange halogen slowly with FAI in the heating process, avoid the formation of disordered crystal structure, and finally form perovskite film Its grain size reaches 2-3 micrometres (adv energy mater., DOI: 10.1002 / aenm 201601882) Figure 2: high quality fabbi 3-x br x perovskite films (source: adv energy mater, DOI: 10.1002/aenm.201601882) were prepared by halogen exchange reaction between intermediate hpbi 2 BR and fai Recently, Professor xu Jianbin of Chinese University of Hong Kong and Professor Li Gang of Hong Kong University of science and technology discussed the high stability of perovskite CS x FA 1-x PBI y br A series of thermal degradation mechanisms of 3-y were studied For the mixed cation mixed halogen type perovskite CS xfa1-x pbiybr 3-y, based on its adjustable band gap structure and excellent thermal stability, it can be widely used in single junction and cascade solar cells However, due to the complexity of its crystal structure and composition, the influence of various elements in perovskite on its stability and photoelectric properties is not clear The influence of heat treatment on CS xfa1-x pbiybr 3-y was analyzed by means of a series of in-situ characterization methods, and the change trend of element composition, morphology, crystal structure and photoelectric properties was detected Figure 3: in situ XRD detection of perovskite phase composition changes during heating (source: adv.mater, Doi: 10.1002/adma.201801562) in situ XRD showed that there was no impurity phase in perovskite before 160 ℃, but the main peak position of perovskite shifted to the left gradually, and the expansion of its lattice structure was due to the release of smaller br ions from the perovskite structure Due to the relatively small amount of released, it did not cause the formation of detectable impurity phase Fig 4: change process of FA and Br content during heating process detected by in situ XPS (source: adv.mater, Doi: 10.1002/adma.201801562) further in-situ XPS analysis showed that the perovskite films after heat treatment at 100 ° C were deficient in FA and Br elements, and these ion vacancies would promote the ion movement in perovskite, resulting in the decrease of stability and the significant increase of hysteresis Further quantitative analysis shows that the release of FA and BR is a synchronous process Figure 5: surface introduction of additional FA and Br to repair vacancy defects caused by heating process (source: adv.mater, Doi: 10.1002/adma.201801562) in order to solve the lack of FA and Br ions in the annealing process, the author uses surface ion treatment, i.e treating the mixed perovskite surface with a low concentration of Fabr solution, to repair these ion vacancy defects in the heating process After treatment, the hysteresis of solar cells was effectively suppressed, and the light stability was also improved This work reveals the importance of the synergistic effect of organic cations and halogen ions on the stability of perovskite, which is of great significance to further improve the photoelectric conversion efficiency and intrinsic stability This achievement was published on advanced materials by Mingzhu long, Tiankai Zhang, Mingzhen Liu, Zefeng Chen, Chen Wang, Weiguang Xie, Fangyan Xie, Jian Chen, gang Li, Today, people and scientific research are getting more and more attention in the economic life China has ushered in the "node of science and technology explosion" Behind the progress of science and technology is the work of countless scientists In the field of chemistry, in the context of the pursuit of innovation driven, international cooperation has been strengthened, the influence of Returned Scholars in the field of R & D has become increasingly prominent, and many 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