A joint research team led by Dr. Hee-Eun Song of the Photovoltaics Research Department at the Korea Institute of Energy Research (President Yi Chang-Keun, hereafter "KIER") and Prof. Ka-Hyun Kim of the Department of Physics at Chungbuk National University (President Koh Chang-Seup, hereafter "CBNU") has successfully identified, for the first time, the specific types of defects responsible for efficiency loss in silicon heterojunction (SHJ) solar cells. The findings are expected to significantly contribute to improving solar cell efficiency when combined with defect-suppression (passivation) techniques.
* Silicon Heterojunction Solar Cell (SHJ): A silicon-based solar cell structure with the highest efficiency among existing silicon technologies. Recently, it has been actively used in tandem solar cells, where different types of solar cells are stacked together to achieve even higher efficiencies.
Various defects that occur within solar cells cause losses and reduce both conversion efficiency and power output. To prevent this, passivation techniques, such as applying surface coatings, are used to control these defects. For passivation to be applied effectively, it is essential to fully understand the types and characteristics of the defects present in each solar cell.
The conventional defect-analysis method known as Deep Level Transient Spectroscopy (DLTS) applies a short voltage pulse to the solar cell, temporarily altering its electronic properties, and then analyzes the device by measuring its response as it returns to its normal state (hereafter "transient response"). However, because the time it takes for the device to relax back to equilibrium is extremely short, on the order of milliseconds, previous approaches typically measured only two points: once immediately after the voltage pulse and once when the device had fully returned to its steady state, rather than capturing the entire transient response.
This approach is suitable for analyzing devices with simple structures, but because it does not capture the full transient response, it is not appropriate for devices such as silicon heterojunction solar cells, which contain multiple, complex defects. As a result, even until recently, the defects and characteristics of silicon heterojunction solar cells could only be inferred indirectly, making it difficult to determine their true nature.
To address this limitation, the research team refined the conventional analysis method and proposed a new interpretation technique capable of examining the solar cell's entire transient response. Through this approach, they discovered that the key defect in heterojunction solar cells, previously assumed to be a single type, is in fact a superposition of two distinct defects. In other words, they have, for the first time, identified that defects in silicon heterojunction solar cells exist in two combined forms.
The two defect types identified by the research team are a slow component (deep-level defect) and a fast component (shallow-level defect). By analyzing each component separately, the team extracted various defect characteristics, such as the defect energy level, spatial location within the device, and atomic bonding configuration. This demonstrates that, for effective integration with passivation technologies, both quantitative evaluation (e.g., increases or decreases in defect density) and qualitative assessment focused on the defects' impact on device performance are critically important.
The research team also found that the two types of defects can undergo changes in atomic bonding configuration depending on the solar cell's fabrication process and device operation conditions. In particular, they experimentally demonstrated that hydrogen present within the solar cell plays a key role in driving these defect-state transformations.
Dr. Hee-Eun Song of the Photovoltaics Research Department at KIER stated, "We expect that this study will accelerate the development of high-efficiency silicon heterojunction solar cells and, furthermore, enable us to realize world-class tandem solar cells using KIER's proprietary technologies."
Professor Ka-Hyun Kim of the Department of Physics at Chungbuk National University stated, "This study provides a fundamental understanding of the relationship between defects and passivation," adding that "the developed analysis method can be extended not only to solar cells but also to a wide range of semiconductor and display applications, including sensors, LEDs, and CMOS devices."
The joint research was conducted using SHJ solar cells fabricated at the Center for Advanced Solar PV Technology (CAST) at KIER, and the DLTS measurements were analyzed at CBNU. The results of this work provide a solid foundation for high-efficiency tandem solar cells.
This research was supported by KIER's Basic Research Program and was selected as a cover article for the October 2025 issue of Advanced Functional Materials (IF 19, top 4.5% in JCR), a world-renowned journal in materials science.
