Researchers in China have developed an electrical imaging technique using three-dimensional (3D) tomographic conductive atomic force microscopy (TC-AFM) to go beyond indirect characterization of perovskite film to be able to examine the effects of passivation treatments intended to improve perovskite solar cell performance.
“The key novelty is the use of three-dimensional tomographic conductive atomic force microscopy to directly visualize how electrical conductivity and carrier transport are distributed inside perovskite films, rather than being limited to surface measurements or a single averaged value for the entire film,” corresponding author of the research, Chuanxiao Xiao, told pv magazine.
“This approach allows us to map current pathways and defect regions throughout the full film thickness with nanometer resolution, which has been challenging for conventional characterization techniques,” said Xiao.
The 3D nature of TC-AFM provides a detailed electrical analysis, both laterally and vertically, offering a comprehensive view of current performance at various depths, according to the research. The approach enables visualization of the current distribution across perovskite films by sequentially measuring local electrical conductivity.
In a demonstration of the TC-AFM method, FAPbI3 perovskite solar films were characterized after being treated as follows: untreated, passivation with guanidinium iodide (GAI), surface passivation with phenylethylammonium iodide (PEAI), and combined bulk and surface passivation (GAI+PEAI).
An analysis showed that the untreated films exhibited “extensive low-conductivity regions that hindered charge transport,” whereas bulk passivation primarily “improved conductivity inside the film and along grain boundaries.” The surface passivation mainly suppressed defects near the top surface. The combined passivation treatment offered “better suppression of high-resistance regions and enhanced film conductivity compared to individual passivation treatments,” according to the study.
To confirm the passivation effect on device performance, the scientists fabricated corresponding devices. For p-i-n devices, which have the electron transport layer at the top, the power conversion efficiency increased from approximately 23.3% in the untreated device to 24.5%, 24.7%, and 25.1% following bulk, surface, and combined passivation treatments, respectively, according to the paper.
“Importantly, we show that combining bulk and surface passivation produces a much more uniform and conductive film, with high-resistance regions almost completely eliminated. When combined with ultrafast spectroscopy, these results help explain the observed improvements in device efficiency and stability,” said Xiao.
The study is detailed in “Three-dimensional mapping of electrical behavior in perovskite films using tomographic conductive atomic force microscopy,” published in Newton. The research team included members from Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS), Ningbo University, Tianjin University, PetroChina Company Limited, Hunan Normal University, Soochow University, and Ningbo New Materials Testing and Evaluation Center.
The researchers are now extending the use of the three-dimensional electrical imaging technology to other perovskite compositions, interfaces, and degradation processes, with a particular focus on reliability and long-term stability. “For example, we are studying light-induced phase segregation and degradation in wide-bandgap perovskite materials for tandem applications,” said Xiao.
More broadly, the team is developing advanced nanoscale characterization tools to directly link microscopic material behavior with real device performance in next-generation solar cells.
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