Theoretical And Theoretical Studies On The Structure And Properties Of Cu ₂ ZnSnS 4 Quaternary Compounds

The constituents of multi-compound semiconductor Cu2ZnSnS4 (CZTS) have various advantageious, including:low-cost, non-toxic, and earth-abundant elements. As a direct band-gap semiconductor, the absorption curve of CZTS is very well matching with solar radiation, with a large optical absorption coefficient. Besides, its crystal structure, contitutent, and properties can be easy adjust and control. So, it presents excellent optoelectric performance, and is considered as the ideal key materials for the green, low-cost, high-efficent, and stable thin film solar cells. Meanwhile, as the novel energy conversion materials, the applications of CZTS in the fields of photocatalysis and thermoelctrics have shown excellent performance.However, CTZS is made up of a lot of elements and the preparation process is easy to form a variety of miscellaneous phase; Its structure also presents diversification and these structures are easily confused with each other, so it is difficult to distinguish, which brings great inconvenience to the experimental preparation and theoretical study; At present, the photoelectric efficiency of CZTS is far lower than its theoretical value and ion doping can effectively improve its performance, however, the doping mechanism is not clear. All these problems seriously restrict the development of CZTS.While the materials simulations and calculations based on the density functional theory is an effective method to study the micro-structure and properties. not only can deeply understand the micro-structure and electronic structure of CZTS, which can verify experiment observations and predict the unexpected performance, but also can provide theoretical guidance for the preparation of experiment.In this paper, we use of Materials Studio 7.0 CASTEP material calculation software. The structural, electronic, optical, and mechanical properties of CZTS are calculated using the density functional theory within the generalized gradient approximation (GGA) method. We conducted several systematic studies about the diamond-like structure of semiconductor, four crystal structure of CZTS and Na-doping kesterite CZTS, The research results can be summarized as follows:(1) The crystal structure of diamond-like semiconductors has a lattice distortion during the evolution process in which the situation of CuGaS2 is the most significant. The lattice distortion is mainly caused by the different electro-negativity and size of radius of alternative elements. Si and ZnS (or CuGaS2 and CZTS) are relatively very similar on space group and chemical binding features.For the electronic structure, the valence band maximum (VBM) and the conduction band minimum (CBM) of Si are consists of 3p states. While in the case of ZnS, CuGaS2 and CZTS, the VBM and CBM are consists of the hybridized states between the corresponding metal atoms and anion. The position of the VBM and CBM will offset with the increase of hybrid intensity. The main reason for the change of electronic structure is the variations of the micro-structure and chemical binding feature.For the optical property, the situation of Si is very different from that of ZnS, CuGaS2, and CZTS. The light absorption method of Si is dominated by indirect band gap absorption while the direct band gap absorption predominates in the case of ZnS, CuGaS2, and CZTS. Moreover, for ZnS, CuGaS2, and CZTS, it shows some similarity and deviation in optical property, in which the optical absorption threshold shifts from the UV region to the visible light region gradually. All the four diamond-like semiconductors have low reflectivity in the visible, in which CZTS quaternary compounds semiconductor has excellent absorbency in the visible region.(2) In four crystal structure of CZTS, the zinc blende-kesterite CZTS has most stable structure. In the electronic structure, the valence band is divided into two independent parts. the fundamental band gap of CZTS is mainly determined by the place of the isolated conduction band. Four crystal structure have no obvious difference on optical properties and mechanical properties. In the visible region, the zinc blende-kesterite CZTS has more optical absorption properties in solar cell application. Four crystal structure all conforms to Born stability condition in mechanics and have high Poisson’s ratio showing good toughness.(3) Na-doped kesterite CZTS models have made the crystal lattice tiny distortion and volume expansion. In its doping system, Na impurity favors occupation of the interstitial sites. The doping effects of Na in CZTS are mainly exhibited by the following aspects:energy band shifting, energy band broadening or narrowing, and effective mass of holes on the top of valence band reduction. After adulteration CZTS into Na, the optical absorption properties can be improved within most wavelength range (except for the narrow range of 440-530 nm). The Na-doped helps to improve photovoltaic properties of CZTS-based thin film solar cells.The innovation of this article is to illustrate the property change microphysics mechanism of the crystal structure of diamond-like semiconductors with similar structure maintained, through the comparative study on the structure and property of the crystal structure of diamond-like semiconductors. By analyzing the structure and property of four different CZTS crystal structure, the article finds out the key factor to decide the band gap and theoretically finds out the best structure to produce CZTS thin film solar cell. Meanwhile Na-adoped is used as the example to discuss the doping mechanism and effect. Then it specifically explains the root reason why Na-adoped can enhance the property of CZTS thin film solar cell, and it has provides theoretical reference for further improving CZTS photoelectric efficiency.