Study of deep-level defects in perovskites photovoltaics

• A few days ago, the team of Professor Xu Jixian from the University of Science and Technology of China, the team of Professor Wu Xiaojun, and the team of researchers Chen Qi and Lin Hongzhen from Suzhou Institute of Nanotechnology, Chinese Academy of Sciences, revealed the relationship between multiple modes of the perovskite/polymer interface through the mutual reflection of experimental tests and theoretical calculations. The interactions, and the corresponding relationship between these effects and the passivation of deep-level defects, provide important references for greatly improving device efficiency and further developing broad-spectrum passivation strategies.
  

• In recent years, metal halide perovskite solar materials have been gradually improved, and they are promising to become the next generation of thin-film photovoltaic technology. During the rapid transformation of perovskite precursor solutions into polycrystalline films, high defect concentrations (~10^15/cm3) are easily formed, among which deep-level defects for minority carriers can cause severe interfacial recombination loss. Therefore, the precise passivation of deep-level defects is one of the central research issues to push perovskite solar cells closer to the theoretical limit of Shockley–Queisser. In addition, it is very challenging to explore a passivation strategy with wide applicability for the complex situation of perovskite with various components and various types of defects. Compared with traditional small molecule passivation agents, polymer passivation is generally more thermally stable, and has higher complexity and design freedom in functional groups and molecular configurations, and has become one of the research focuses of broad-spectrum passivation. There are still many questions to be answered here. For example, is there only one mode of interaction between a functional group and the perovskite surface interface? And can these interactions accurately suppress minority carrier deep-level defects instead of shallow-level defects or many-carrier defects?
  

• In response to the above problems, the research team took polyethyleneimine (PEI) polymer and its derivatives as the model research object, and discovered the multi-mode interaction mechanism between the high-density amine group and the perovskite surface. Through surface-sensitive characterization tests such as high-sensitivity XPS, depth-resolved XPS tests, and sum-spectroscopy techniques, the in situ amine group protonation reactions at the perovskite/PEI interface were discovered and confirmed (Figures A-D). This in situ surface chemical reaction is different from the traditionally considered amine-based physical adsorption and metal chelation modes at the perovskite/PEI interface, and expands the understanding that together constitute a multimodal interaction of a single functional group with the complex surface of perovskite . Further, the research team combined methods such as deep-level defect transient spectroscopy (DLTS) and density functional theory (DFT) to carefully study the correspondence between these modes of action and different defects (Fig. Based on the effects of depth, concentration, and capture cross-section, it was found that the in-situ protonation mode can effectively passivate the minority carrier deep-level defects of perovskite, while the off-site protonation has no such effect.

  
  

  At the same time, the defect passivation effect of traditional physical adsorption mode and metal chelation mode is weak or more inclined to passivate multi-child defects. In addition, the research team compared the passivation effects and device volt ampere characteristics of PEI with different configurations such as chain, branched, and branched partial alcoholization, and further pointed out the synergistic effect between functional groups and configurations (Figure I-J). The above results show that in order to improve the applicability of passivation, in addition to the conventional idea of ​​increasing the types of functional groups, the multi-mode passivation effect of single functional groups is also worth exploring.
  

• In the trans-type perovskite device structure (p-i-n structure), this technology has achieved significant improvement in perovskite solar devices with different band gaps such as 1.65eV and 1.55eV. This applicability is conducive to its application in various stacked cells such as crystalline silicon/perovskite stacks and perovskite/perovskite stacks. The device can show no obvious decline in accelerated life tests such as maximum power point tracking for more than 1000 hours and 85-degree heating aging. Among them, a 1.55 eV solar cell achieved a photoelectric conversion efficiency of 24.3% (Figure K), one of the highest efficiencies reported for trans-perovskite devices.
  

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• Fig. (A, B) Perovskite/PEI in situ reaction diagram; (C) XPS signal of N atom; (D) surface sum frequency signal; (E, F) deep level defect transient spectrum DLTS test results; (G,H) Minority carrier defect passivation calculated by density functional theory (DFT); (I,J) Device effects of PEI with different configurations; (K) 1.55eV trans p-i-n device performance improvement
  

• Zhu Zhengjie, Mao Kaitian, doctoral students from the School of Chemistry and Materials Science, University of Science and Technology of China, and Zhang Kai, a postdoctoral fellow at the National Research Center for Physical Science at the Microscale, are the co-first authors of the paper. Professor Xu Jixian from the University of Science and Technology of China is the corresponding author. This work was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Chinese Academy of Sciences, the Energy Research Institute of Hefei Comprehensive National Science Center, and the China Fujian Energy Device Science and Technology Innovation Laboratory (21C-LAB). Professor Xu Jixian thanked the National Synchrotron Radiation Laboratory and the Tencent Foundation Science Exploration Award for their support.

  
  

  source: solarpwr.cn