Nature Energy,a top international academic journal, published the latest joint achievement of researcher Zheng Kun (Institute of Solids) and professor Zhang Yongzhe (Institute of New Energy Materials) from Faculty of Materials and Manufacturing of BJUT. The publication is titled “Identification of embedded nanotwins at c-si/a-si: Hinterface limiting the performance of high-efficiency silicon heterojunction solar cells”(DOI:https://doi.org/10.1038/s41560-020-00768-4), with the first unit and sole communication unit as BJUT. According to statistics, since its founding, Nature Energy has published only 499 papers (including Articles and Reviews). Its immediate impact factor has exceeded 58.

At UN General Assembly in September 2020, China undertook solemnly to achieve the carbon discharges peak value by 2030 and realize carbon neutrality by 2060. To reach the target, PV generation based on the clean and renewable solar energy is a main technical solution. However, the relatively low photoelectric conversion efficiency makes it hard to lower the cost of PV generation and expand its application, which has long been a core problem for the PV industry. Considering the advantages of their material and performances, as well as their mature manufacturing processes, c-Si/a-Si:H heterojunction (SHJ) solar cells are now the hottest investment spot for investment and enjoy the largest production capacity among the latest generation of high-efficiency solar cells. In 2020, the globe saw a large-scale industrialization of SHJ cells. It is agreed that the interface defect and passivation mechanism of c-Si/a-Si:H is the key factor affecting the conversion efficiency of SHJ cells. Therefore, to precisely recognize and regulate c-Si/a-Si:H interface structure on atomic scale has become an urgent and serious challenge.

Since signing the strategic cooperation agreement in 2012, Faculty of Materials and Manufacturing of BJUT and Hanergy Chengdu R&D Center have made several breakthroughs through cooperation and collaborative innovation. In the research work of this paper, researchers chose mass-produced SHJ cells with world-leading performance indicators and conversion efficiency of 24.85% (M2 substrate, area 244.5cm2) as the object of research, employed the combination of observation through spherical aberration corrected transmission electron microscope and theoretical calculation and analysis to conduct ex situ/in situ observation, and established the atomic structure characteristics of interface and the law of structural evolution caused by annealing. With respect to the a-Si:H film with passivated c-Si{111} surface defect, researcher not only observed the phenomenon of conventional epitaxial growth, but also discovered for the first time the “embedded nano twin” as deep energy-level recombination center in the 2-3 nanometer epitaxial layer, which leads to a strain force field in the film and shorter life of photogenerated minority carriers and thereby limits the conversion efficiency of SHJ cells, which is a subversive and novel insight. A consensus derived from the extensive research on SHJ cells at home and abroad over the past 40-odd years is that the only deep energy-level recombination center existing at the interface is Si-dangling bond point defect. However, the latest result of the research shows that there is also a non-Si-dangling bond interface defect less prone to conventional passivation, that is, nano twin defect. Moreover, based on the law of reversing the nucleation-growth-termination and symmetry of “nano twin defect”, the research further clarified the factors inducing “nano twin defect” and the mechanism of action, providing the industry with a scientific basis of universal reference value and guiding significance. The new process of inhibiting “nano twin defect” on this basis reduces the density of states of such new defect (around 1/3 compared with the original process), laying the foundation for mass-produced SHJ cells to achieve new world record of conversion efficiency (25.11%, certified by internationally recognized third-party German research institute ISFH).