四元硫化物Cu2Zn(Ti,Zr,Hf)S4：一类新颖光伏材料

Quaternary sulphides Cu_2Zn(Ti,Zr,Hf)S_4,the new type of photovoltaic materials

采用第一性原理电子结构方法研究了四价过渡金属Ti,Zr和Hf替代Cu_2ZnSnS_4(CZTS)中Sn原子以及Se替代S原子所得到的四元硫族化合物的电子结构、光学性质和晶体结构的稳定性.实验上用Se替代CZTS中部分S得到的Cu_2ZnSnS_4 xSex(CZTSSe)作为光吸收材料,可以进一步提高光伏效率.我们计算表明用Se替代S后,CZTSe的价带顶明显下移,并接近Cu(In,Ga)Se_2(CIGS)价带顶位置.与CZTSe的电子结构特征一样,Cu_2Zn(Ti,Zr,Hf)S_4四元硫化物的价带顶与母体材料CZTS相比也向低能移动,并接近CIGS价带顶位置.由于高光伏效率要求窗口材料Zn O、缓冲层材料和光吸收材料的价带顶和带隙满足一定的渐进的变化关系,因此可以预见用Cu_2Zn(Ti,Zr,Hf)S_4作光吸收材料可以有效地提高甚至接近CIGS的光伏效率.通过计算弹性常数和声子谱,以及有限温度下第一性原理分子动力学模拟,发现Cu_2Zn(Ti,Zr,Hf)S_4的结构稳定性与CZTS相近.进一步计算Cu_2Zn(Ti,Zr,Hf)S_4与不同缓冲层间和窗口材料与缓冲层间的反射系数,并讨论了Zn Se,In_2S_3,ZnS作为缓冲层材料和TiO_2作为窗口材料对光伏效率可能的影响.

Based on the first-principles electronic-structure method, we study the electronic structures, optical properties, and the structural stabilities of the quaternary sulphides Cu2Zn（Ti, Zr, Hf） S4, which are obtained via substituting Ti, Zr,and Hf elements for Sn elements in Cu2 ZnSnS4（CTZS）. It is well known that the photovoltaic efficiency of CZTS（Se） will be improved if the Se atoms partially substitute S atoms in CZTS. Our results show that the valence-band top of CZTSe shifts to lower energy and accesses to the valence-band top of Cu（InGa）Se2（CIGS）. Similar to CZTSe, the valenceband tops of Cu2Zn（Ti, Zr, Hf）S4 also shift to lower energies and access to the top of valence-band of CIGS. The high photovoltaic efficiency requires the smooth changes of the valence-band top and energy gap from the window material and the buffer layer to the light-absorption layer. Thus we predict that the photovoltaic efficiency will be improved if Sn atoms are substituted, even partially, by Ti, Zr, Hf atoms in CZTS, just like Se atoms substituting S atoms in CZTS.To obtain some reliable results, we perform the calculations both of PBE functional and HSE06 functional. The changes of valence-band tops from window materials to the light-absorbed materials are similar for PBE functional and HSE06 functional. The absolute values of the valence-band tops with HSE06 are lower in energies compared with PBE functional and the gaps obtained from HSE06 are larger than the gaps from PBE. We also calculate the optical properties of different light-absorbed materials including CZTi S, CZZr S, CZHf S, CZTS and CIGS, in which we mainly focus on the reflectance of different layers from the vacuum to the light-absorbed materials, from the window layers to the buffer layers and from the buffer layers to the light-absorbed layers. For the window layers we consider the Zn O and TiO2, and for the buffer layer we consider the CdS, In2S3, ZnSe and ZnS, etc. respectively. The high-performance solar cell requires low reflectance between the window layer and the buffer layer, the buffer layer and the light-absorbed layer so as to ensure more light transmit to the light-absorbed layer. Our results of reflectance show that ZnO（TiO2）/In2S3（ZnSe）/PVM are possible multilayer structures, with PVM（photovoltaic materials） =CZTS, CIGS, CZTiS, CZZrS, CZHfS. If we replace CdS buffer layer with other n-type semiconductors, the material of the window layer must be replaced accordingly with new materials to reach the lower reflectance. The structural stability of photovoltaics is an important topic in the application of photovoltaics. Our results show that CZTiS, CZZrS and CZHfS are structure-stable at zero temperature in terms of the calculated elastic properties and phonon vibration spectrum. Based on the elastic constants and Poisson-ratio,similar to Cd Te, CIGS and CZTS, the CZTiS, CZZr S and CZHfS are ductile materials suitable to be used as the flexible solar cell. Additionally, we have performed the molecular-dynamics simulations at some finite temperatures（100, 800 and 1200 K respectively）, calculated the pair-distribution functions and angle-distribution functions. As comparison, we also perform the corresponding molecular dynamics simulations of CZTS and ZnS. Our results show that the structuralstabilities of CZTiS, CZZrS, and CZHfS are close to those of CZTS and ZnS. This means that once CZTiS, CZZrS and CZHfS are obtained experimentally, they will be stable. In summary, the novel photovoltaic materials CZTi S, CZZrS and CZHfS studied in detail in this work are potentially the high-performance photovoltaic materials for the solar cell application in the near future.