A group of scientists led by the Korea Institute of Energy Research (KIER) and Chungbuk National University (CBNU) has conducted extensive research on defects in silicon heterojunction solar cells and has found a direct correlation between these defects and passivation quality.
“Conventional defect analysis in solar cells and semiconductor devices has largely focused on measuring defect concentration,” the research's lead author, Ka-Hyun Kim, told pv magazine. “This approach oversimplifies defects as simply ‘more' or ‘less,' overlooking the importance of defect quality. Furthermore, most characterization methods probe only macroscopic responses, so they cannot resolve the contributions of individual defect types, making it difficult to understand how mixed defects influence material properties.”
“In our study, we show that defects in silicon heterojunction solar cells transform dynamically instead of remaining in a single static state,” he went on to say. “Through decomposition of the dual-phase capacitance transient into slow and fast components, we separate mixed defects that had previously been interpreted as one. These two phases reflect distinct defect configurations that evolve on different time scales and reconfigure depending on deposition conditions, layer stacking, and annealing.”
The research team found that passivation quality is determined not only by defect concentration but also by the bonding configurations of the defects. These configurations can shift toward deeper or shallower levels, and such transformations directly influence recombination behavior at the heterointerface. “The main novelty of this work is that it provides a clear framework connecting defect transformations with passivation quality. This new perspective enables more precise defect-state control and offers practical guidance for process optimization in semiconductor heterostructures, including next-generation HJT and tandem solar cells,” Kim went on to say.
Image: KIER
In the paper “Unraveling Mixed-Defect Transformations and Passivation Dynamics in Silicon Heterojunction Solar Cells,” published in Advanced Functional Materials, the researchers demonstrated, for the first time, that the specific types of defects responsible for efficiency loss in HJT cells exhibit a dual-phase behavior, comprising a slow phase and a fast phase, corresponding to two different passivation-related defect configurations: dangling bonds (DBs) and weak silicon-silicon (Si–Si) bonds.
Dangling bonds (DBs) are known to cause recombination losses in HJT devices, primarily reducing the cell’s open-circuit voltage. They act like tiny “broken” connections on the silicon surface, serving as traps for electrons. Similarly, weak Si–Si bonds can break during processes such as sputtering, etching, or annealing, creating fragile sites at the silicon interface. When they break, they form defects that trap charges and ultimately reduce the cell’s efficiency and stability.
Prior to this work, defects in heterojunction solar cells were largely assumed to belong to a single category.
Using Deep-level transient spectroscopy (DLTS), the researchers discovered, in particular, that the defects induced by indium tin oxide (ITO) sputtering continue to evolve after annealing, ultimately transforming into shallower energy states that affect the material’s properties. They also ascertained that annealing executed after ITO sputtering does not eliminate sputtering-induced deep-level defects.
“This finding further explains why hydrogenated amorphous silicon (a-Si:H), particularly when rich in silicon-hydrogen (Si–H2) and free from parasitic epitaxial growth, enhances passivation and exhibits recovery from sputtering damage,” the research group emphasized. “Our findings demonstrate that multiple defect types coexist and evolve dynamically under deposition and thermal processes.”
“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,” said co-author Hee-Eun Song.
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