Figure 1. The formation of Na-related defects in CZTSe with Fermi level changes; Na migration path (b).
Figure 2. The formation energies of SnGa and CuGa in CuGaS2 as a function of Fermi level Arrows indicate the location of the adiabatic charge-transfer level (a); the dielectric of SnGa0, SnGa-and SnGa + in the sub-bandgap energy region of CuGaS2 The imaginary part of the function (b).
The components of CZTSe are abundant and nontoxic in the earth. With a small amount of sulfur substituted for selenium, the band gap can be adjusted between 1.0-1.5 eV, making it an advantageous low-cost thin film solar cell material. At present, the highest efficiency of CZTSe is only 12.6%, which is much lower than 22.6% of its sister compound CIGS.Experimental studies show that Na doping can increase the carrier (hole) concentration in CZTSe material and enhance p Type conductance, thereby increasing the battery efficiency.But the current doping effect on its mechanism is not yet clear.
Accordingly, the Chinese Academy of Sciences Institute of Solid State Physics, Hefei Institutes of Physical Research Zeng pheasants on the CZTSe materials impurities and defects in the nature of the study conducted by the research team using first-principles calculated Na-related defects in the formation of energy, charge Transfer energy level and migration path.The results show that except for NaSn in CZTSe, other Na-related defects are shallow donors or acceptors, among them, NaZn formation energy is very low and can exist in a large amount in the material, therefore, The intrinsic deep level defect SnZn competes to reduce recombination of electron-hole pairs and enhances cell efficiency. Meanwhile, NaZn has a very shallow charge-transfer level, which can contribute holes to the material and enhance the p-type conductance of the material. Na It is easy to migrate in the form of interstitial Na atom and NaCu in CZTSe material, which contributes to the production of VCu shallow acceptor.The related research results are published in Physical Chemistry Chemical Physics.
Copper-based compound CuGaS2 has a bandgap of 2.43eV at room temperature, which is close to the best intermediate bandgap of the host material and is an ideal intermediate solar cell material.In recent years, the intermediate solar cell can realize the three-photon absorption process with a theoretical limit efficiency of 46 %, And therefore has attracted wide attention by researchers.Experiments and theories have investigated CuGaS2 with various doping elements (Sn, Fe, Ti, Cr, etc.), but the results are not clear.For example, for Fe-doped CuGaS2 material , The experimental results show that the optical current and voltage decrease as the doping amount increases, so the group uses the optimized hybrid density functional theory to study the defect in Sn-doped CuGaS2 from the perspective of defect physics It is found that the SnGa in CuGaS2 is a bipolar trap and its radiation recombination is equal to the possibility of excitation, which limits the lifetime of carriers, that is, the photocurrent size. In addition, the host of SnGaGa The spontaneous formation of the host, the two charge compensation, the Fermi level pinned at EV +1.4 eV at this time, ionized SnGa + and CuGa -, 2-defect limits the range of available light.This study from the theory On the explanation The current observed phenomena in the experiment provide a new idea for the future study and understanding of the properties of the impurities in the middle zone.Related research work published in Physical Review B.
The research was supported and funded by the National Key Basic Research and Development Program (973 Program), the CSC and Hefei Supercomputing Center.
