Korean researchers revealed that efficiency losses in heterojunction solar cells are due to two coexisting types of defects: dangling bonds and weak silicon-silicon bonds. Their findings explain how passivation of hydrogenated amorphous silicon helps reduce these defects and improve cell performance.
A group of scientists led by the Korea Institute of Energy Research (KIER) has conducted extensive research on defects in silicon heterojunction solar cells and 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,” said the study’s lead author Ka-Hyun Kim. pv magazine. “This approach simply simplifies defects as ‘more’ or ‘less,’ overlooking the importance of defect quality. Furthermore, most characterization methods only examine macroscopic responses, so they cannot resolve the contributions of individual defect types, making it difficult to understand how mixed defects affect material properties.”
“In our study, we show that defects in silicon heterojunction solar cells transform dynamically rather than remaining in a single static state,” he continued. “Through the decomposition of the two-phase capacitance that is transient into slow and fast components, we separate mixed defects that were previously interpreted as one. These two phases reflect different defect configurations that develop and reconfigure on different time scales depending on the deposition conditions, layer stacking and annealing.”
The research team found that passivation quality is not only determined by the defect concentration, but also by the bonding configurations of the defects. These configurations can shift to deeper or shallower levels, and such transformations directly affect the recombination behavior at the heterointerface. “The key novelty of this work is that it provides a clear framework connecting defect transformations with passivation quality. This new perspective enables more precise control of the defect state and provides practical guidance for process optimization in semiconductor heterostructures, including next-generation HJT and tandem solar cells,” Kim continued.
Image: KIER
In the newspaper “Unraveling mixed-defect transformations and passivation dynamics in silicon heterojunction solar cells”, published in Advanced functional materials, the researchers have shown for the first time that the specific types of defects responsible for efficiency loss in HJT cells exhibit a biphasic behavior, consisting of 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, mainly reducing the open-circuit voltage of the cell. They act as small “broken” bonds on the silicon surface and serve as traps for electrons. Similarly, weak Si-Si bonds can break during processes such as sputtering, etching or annealing, creating vulnerable spots at the silicon interface. When they break, they form defects that trap charges and ultimately reduce the efficiency and stability of the cell.
Prior to this work, defects in heterojunction solar cells were largely assumed to belong to one category.
In particular, using Deep-level transient spectroscopy (DLTS), the researchers found that the defects caused by indium tin oxide (ITO) sputtering continue to evolve after annealing and eventually transform into shallower energy states that affect the material’s properties. They also found that annealing performed after ITO sputtering does not eliminate the sputtering-induced deep defects.
“This finding further explains why hydrogenated amorphous silicon (a-Si:H), especially when rich in silicon-hydrogen (Si-H2) and free from parasitic epitaxial growth, improves passivation and shows recovery after sputtering damage,” the research group pointed out. “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 also enable us to realize world-class tandem solar cells using KIER’s proprietary technologies,” said co-author Hee-Eun Song.
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