Shanghai Institute Improves Conversion Efficiency of Silicon Heterojunction Solar Cells
May 23, 2022
Recently, Liu Zhengxin's team from the New Energy Technology Center of the Key Laboratory of Microsystem Technology of the Shanghai Institute of Microsystems found in the doped amorphous silicon (a-Si:H) thin film of amorphous silicon/crystalline silicon heterojunction (SHJ) solar cells. Anomalous Staebler-Wronski effect, and proved that the anomalous effect is the physical essence of using light injection to improve the photoelectric conversion efficiency of SHJ solar cells. The result was published on May 13, 2022 in Nature Energy (https://doi.org/10.1038/s41560- 022-01018-5, impact factor 60.868).
Light was first discovered in the laboratory in 1977 by American electrical engineer David L. Staebler and Penn State electrical engineer and professor emeritus Christopher R. Wronski will reduce the dark conductivity of a-Si:H thin films, this phenomenon was later named "Staebler-Wronski effect", this phenomenon caused great trouble to the reliability of amorphous silicon optoelectronic devices, and also affected amorphous silicon Development and utilization of thin film solar cells.
In the field of amorphous silicon, it is believed that the main form of H atoms in thin films is Si-H covalent bonds. In 2020, based on a large number of experimental data, Liu Wenzhu et al. found that the above structural model is not permanent. Combined with FTIR, SIMS, TA, Sinton Lifetime Tester, Keithley and DFT and other technical means, it is proved that there is a large amount of doping a-Si:H in the The bridging weak H atoms with a density as high as 1021 cm-3 or more will "poison" the doping efficiency of B and P atoms in the a-Si:H network. When light irradiation (optical injection) or an applied electric field (electrical injection) is used to give energy quanta greater than 0.88 eV, these weak H atoms gain enough energy and diffuse or jump in the lattice, thereby reactivating B, P atoms, B The dark conductivity of the doped p-type a-Si:H film increases significantly, which belongs to the obvious "abnormal Staebler-Wronski effect" (Fig. a). After the illumination was removed, the dark conductivity gradually decayed to the initial value before illumination (Fig. b). We found that the decay behavior of this dark conductivity can be described as a combination of Debye decay and Williams-Watts decay, the former representing free diffusion of H atoms and the latter representing H atoms hopping between chemical bonds (Figure c). By further comparing the performance parameters of solar cells, we found that the "abnormal Staebler-Wronski effect" can quantitatively describe SHJ solar cells using light injection to improve photoelectric conversion efficiency and dark-state decay. With the help of the light injection process of strong light irradiation of 60 times the standard sunlight, a high conversion efficiency of more than 25% was obtained on the industrially produced large-scale SHJ solar cells (Fig. d, e; third-party independent certification in Germany and China).
Further research found that the dark conductivity of P-doped n-type a-Si:H can be increased by more than 100 times under sunlight. Therefore, the "abnormal Staebler-Wronski effect" can be used to further study the physical mechanism and process technology to improve the photoelectric conversion efficiency of SHJ solar cells.
