Hefei Research Institute copper-based thin film solar cell material defect research progress
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Figure 1. Changes in formation energy of Na-related defects in CZTSe with Fermi level (a); Na migration pathway (b).
Figure 2. The variation in the formation energy of SnGa and CuGa in CuGaS2 with the Fermi level. The arrows indicate the location of the adiabatic charge transfer energy level (a); the imaginary part of the dielectric function of SnGa0, SnGa-, and SnGa+ in the sub-band gap energy region of CuGaS2 (b).
The elements of CuZnSnSe (CZTSe) are abundant and non-toxic in the earth. By replacing sulfur with a small amount of sulfur, the bandgap can be adjusted between 1.0-1.5 eV, which is an advantage of low-cost thin-film solar cell materials. At present, the highest efficiency of CZTSe is only 12.6%, which is much lower than 22.6% of its sister compound copper indium gallium selenide (CIGS). Experimental studies have shown that Na doping can increase the concentration of carriers (holes) in the CZTSe material, enhance the p-type conductance, and improve the cell efficiency. However, the effect of doping on its mechanism is not yet clear.
Based on this, the research group of the Institute of Solid State Physics, Chinese Academy of Sciences, Hefei Institute of Physical Physics, Zeng Jun, conducted an in-depth study of the nature of impurities and defects in CZTSe materials. The team used first principles to calculate the formation energy, charge transfer levels, and migration paths for Na-related defects. The results of the study indicate that in addition to NaSn in CZTSe, other Na-related defects are shallow donors or acceptors. Among them, NaZn has a very low formation energy and can exist in large amounts in the material, so it competes with intrinsic deep-level defects SnZn, reduces the recombination of electron-hole pairs, and enhances the efficiency of the battery; at the same time, NaZn has a very shallow charge transfer. Energy levels can contribute holes to the material and enhance the p-type conductivity of the material; Na easily migrates in the form of interstitial Na atoms and NaCu in the CZTSe material, which contributes to the generation of shallow acceptors of VCu. Related research results are published on Physical Chemistry Chemical Physics.
The copper-based compound CuGaS2 has a bandgap of 2.43 eV at room temperature, which is close to the best intermediate band gap of the parent material, and is an ideal intermediate-band solar cell material. In recent years, three-photon absorption processes have been realized in the middle-zone solar cells, and the theoretical limit efficiency is as high as 46%. Therefore, researchers have received extensive attention. Both experiments and theories have investigated CuGaS2 in various doping elements (Sn, Fe, Ti, Cr, etc.), but the results are not clear. For example, for Fe doped CuGaS2 materials, experimental studies have found that light absorption increases with doping, but the photocurrent and voltage are decreasing. To this end, the research group used the optimized hybrid density functionals to study the defects in Sn-doped CuGaS2 from the perspective of defect physics. The study found that SnGa in CuGaS2 is a bipolar trap, and the radiation compound is equal to the possibility of excitation, thus limiting the lifetime of carriers, ie, the size of the photocurrent. In addition, the SnGa donor induces the spontaneous formation of CuGa acceptors, and both charges are compensated and the Fermi level is pinned at EV +1.4 eV. At this time, ionized SnGa+ and CuGa-, 2 - defects limit the range of available light. This study theoretically explains the phenomena observed in experiments at present, and provides new ideas for the future study and understanding of the nature of impurities in the intermediate strips. Related research work was published in Physical Review B.
The research work has received funding and support from the National Basic Research Development Program (973 Program), the China Scholarship Council and the Hefei Supercomputer Center.
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