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Heterojunction solar cell with 25.18% efficiency, 85.42% fill factor

Scientists from the Chinese Academy of Sciences (CAS) have built a heterojunction solar cell with highly conductive boron-doped hydrogenated amorphous silicon (a-Si:H) thin films. They claim it reduces fill factor losses to a minimum.

“Our cells were conceived for applications in large-scale solar modules,” researcher Wenzhu Liu told pv magazine.

The scientists fabricated the cell with a 244.63 cm2 Czochralski n-c-Si wafer provided by China-based Sichuan Yongxiang Silicon Material Company. They used light soaking to improve the dark conductance of the a-Si:H films, in order to help the diffusion and hopping of weakly bound hydrogen atoms, resulting in efficient boron tetrasilicide (B−Si4) doping.

“The light-soaking devices are made of LED arrays and were developed by Chinese companies at our request,” Liu said.

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The scientists have found that boron doping, which stems from weakly bound hydrogen atoms, plays a crucial part in the formation of metastable hydrogen configurations in the a-Si:H films. Under standard illumination conditions, the solar cell achieved a power conversion efficiency of 25.18%, an open-circuit voltage of 749 mV, and a fill factor of 85.42%, which the scientists described as the highest values among two-side contacted silicon solar cells.

An encapsulated device was also able to retain 98.70% and 97.59% of the fill factor and efficiency, respectively, after 1,000 hours. The scientists claimed that this demonstrates high stability against extreme climate degradation factors.

The scientists described the process in “Light-induced activation of boron doping in hydrogenated amorphous silicon for over 25% efficiency silicon solar cells,” which was recently published in Nature Energy. “We participated in their design and now both the cells and the light-soaking devices will be available in the market,” said Liu.

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Source: pv magazine