Ni/Pd/Pt掺杂的BaTi1-xCoxO3薄膜带隙工程的理论研究

To absorb and exploit the solar energy is an important avenue to solve the energy crisis and environmental problems. Since the efficiency of the solar energy transformation of the widely used photovoltaic device is too low, it is a task of top priority to manufacture high-efficiency photovoltaic devices. We plan to combine density functional theory with the dynamical mean-field theory to investigate the effects of the doping concentration of the transition-metal Co, the ratio of the Ni/Pd/Pt-substitution, the concentrations of O vacancies, the relative position of the atoms and the thickness of the BaTi1-xCoxO3 thin film on its structural stability, band gap, polarization, electronic structure and transmittance. The purpose of the program is to design a certain multilayer film. Each layer of the thin film has a direct band gap with a value ranging between 2.0 eV and 1.6 eV(The corresponding absorption peak is between 621 nm-776 nm). The unabsorbed solar light in the upper layer can be absorbed by the lower layers in the multilayer structure. Since the absorption peaks of the multilayer structure cover the main illuminance of the solar light, the designed multilayer films have high absorption efficiency. Furthermore, the designed films have good charge separation property and therefore a high solar conversion efficiency can be obtained. The information from our study has profound theoretical importance of pushing on the understanding of strong-correlated electron system. Our work is also of great value in application field with the potential of reducing the development cycle of high-efficiency photovoltaic devices.

吸收和利用太阳能是解决能源危机和环境问题的一条重要途径。目前广泛使用的光伏器件,太阳能转换效率太低，研制高效光伏器件成为当务之急。本项目结合密度泛函理论与动力学平均场理论，研究BaTi1-xCoxO3薄膜中过渡金属Co的掺杂浓度、Ni/Pd/Pt替代Co的比例、氧空位的浓度、原子的相对位置及薄膜的厚度对薄膜的结构稳定性、带隙、极性、电子结构和透光性能的影响。目的是设计一种多层膜，各层薄膜的带隙为直接带隙，带隙大小逐层减小（2.0 eV ～1.6 eV），对应的吸收峰波长从621 nm增加到776 nm。因这些吸收峰覆盖太阳光谱辐照度强的主要能谱，因此该多层膜结构对太阳光具有极高的吸收效率。设计的薄膜具有较好的电荷分离性质，因而可获得较高的太阳能转换效率。我们的研究不仅具有重要的理论价值，可推进对强关联电子体系的理解；同时具有重要的应用价值，可缩短高效光伏器件的开发周期。

用基于局域密度近似（LDA）和Heyd–Scuseria–Ernzerhof (HSE)杂化密度泛函的第一性原理方法，研究了：Co掺杂浓度对体材料BaTiO3带隙的影响；Ni/Pd/Pt替代BaTi1-xCoxO3中部分Co对其带隙的影响；氧空位对Co掺杂和(Co, Ni)/(Co, Pd)/(Co, Pt)共掺杂体系带隙的影响；各体系的极化特性。主要工作和研究成果如下：（1）用LDA方法和HSE杂化密度泛函方法计算BaTiO3的带隙，结果分别为1.74 eV和3.3 eV，后者与实验值(3.4 eV)符合得很好。（2）构建了32种掺杂体系，用LDA优化结构，HSE计算电子结构，发现有9种体系符合高效太阳能电池材料的带隙要求。其中单原子掺杂有5种：BaTi0.875Pd0.125O3与BaTi0.75Pd0.25O3带隙分别为1.85 eV和1.95 eV，BaTi0.875Co0.125O2.875、BaTi0.926Co0.074O2.926和BaTi0.963Co0.037O2.963带隙分别为1.95 eV、1.92 eV和1.90 eV；(Co, Ni)/(Co, Pd)/(Co, Pt)共掺有4种：BaTi0.75Co0.125Pd0.125O2.75带隙为1.95 eV，BaTi0.926Co0.037Ni0.037O2.926带隙为1.80 eV，BaTi0.926Co0.037Pd0.037O2.926带隙为1.86 eV，BaTi0.926Co0.037Pt0.037O2.926带隙为1.86 eV。电子结构分析表明，掺杂Pd引入的Pd_4d态提供新的导带底，比原来Ti_3d导带底更靠近费米面，从而降低了带隙。对于(Co, Ni)/(Co, Pd)/(Co,Pt)共掺杂情形，Co 3d能带稍稍跨过费米面，产生部分空穴，当氧空位存在，氧空位提供的电子填满这些空穴，则产生新的能级较高的价带顶，从而减小带隙。其中（Co, Pd）共掺杂情形，价带顶升高与导带底降低同时发生，从而带隙大大减小。（3）发现上述9种体系中有8种保留有较好的极化特性，其中(Co, Ni)/(Co, Pd)/(Co, Pt)共掺杂体系极化较强，不含Co的单掺杂体系极化较弱，表明Co对保留钛酸钡的极化性质起着非常重要的作用。本项目工作可为研制高效光伏器件提供重要的理论依据。