卤化双钙钛矿材料在光电领域和自旋电子学领域有着非常可观的应用前景。标准卤化双钙钛矿结构的材料(HDP, A2B1+B′3+XVII6)仍然存在合成高质量薄膜困难，器件的稳定性和效率还有待提高等问题。除了HDP，层状卤化双钙钛矿(LHDP, A4MB2X12)在2017年才首次被实验合成，因其可调成分丰富，服役稳定性较好，和方便调控的电学磁学性能而吸引了人们的关注。本论文采用第一性原理计算方法，通过对HDP材料进行缺陷调控设计来实现更优越的光电性能。此外，通过理论设计和计算筛选在LHDP中预测出可能的稳定新型功能材料(透明导电体和半金属铁磁体)，对尚在起步阶段的LHDP的研究和实验合成提供了指引和启发。通过系统研究过渡金属掺杂的机理，提出改良现有n型透明导电材料的方法。以n型In2O3为例，预言出Zr, Hf和Ta是比传统的Sn更理想的掺杂元素，可以实现In2O3更高的载流子浓度和迁移率。其次，提出在透明材料的价带顶引入能级位置较高的s*反键态来增强p型导电的理论设想，并在无机非铅LHDP材料中得到了完美验证。我们从稳定性，光电性质和缺陷性质出发，建立一套p型透明导电材料的筛选原则，并通过计算筛选出适合于做p型透明导电体的材料，如Cs4CdSb2Cl12等。我们阐明了HDP中本征点缺陷和晶界的形成机制及其对载流子输运的影响，提出了提高HDP器件效率和稳定性的方案。首先，预测出得到有良好n型导电性质的Cs2AgInBr6的最佳生长条件。发现HDP中主导缺陷的快速迁移很可能会破坏其服役性能的稳定性，提出在衬底上施加压缩双轴应变可以有效抑制离子迁移。其次，通过化学势点筛选和能带设计，提出了在预先设定的生长条件下，通过某些特定的本征缺陷或者缺陷复合物的自发偏聚来钝化HDP中不利晶界的深能级缺陷态的有效方法，而且钝化后的晶界诱导的能带弯曲效应有利于载流子的分离。我们提出了优良的半金属铁磁体的理论设计思路，并在114种LHDP候选材料(Cs4M2+B3+2XVII12和Cs4M4+B2+2XVII12)中进行计算筛选，理论预测出一种LHDP材料：Cs4FePb2Cl12作为优良的室温半金属铁磁体。该材料同时具有高居里温度，宽半金属间隙和足够大的磁晶各向异性能。我们通过超超交换相互作用机制和局域磁矩的轨道占据阐明了其强铁磁性的物理机制。该工作展现了LHDP在高温自旋电子学领域应用的潜力。
Halide double perovskites have many promising and emerging applications in optoelectronics and spintronics. However, standard halide double perovskites (HDP, A2B1+B′3+XVII6) face many challenges, such as the difficulty to synthesize high-quality thin-films, the instability of the device, and the low efficiencies below their optimized values. Besides of HDP, layered halide double perovskite (LHDP, A4MB2X12) was first synthesized in 2017. Due to the rich elemental family, good performance stability and tunable electronic and magnetic properties, LHDP has attracted particular attention. Using first-principles calculations, on one hand, we provided a theoretical guidance to optimize the optoelectronic performance of HDP by defect engineering. On the other hand, through theoretical design and computational screening, several possible novel functional materials (transparent conductors and half-metallic ferromagnets) were predicted, providing important guidance and inspiration for the research of LHDP. We systemically investigated the general mechanism of transition metal doping in n-type transparent conducting oxide, and proposed the method to improve their performance. Taking n-type In2O3 as an example, it was identified that Zr, Hf and Ta were better potential doping elements than traditional Sn in In2O3 for achieving higher carrier mobility and density. The approach may also be applied to design ideal dopants in other n-type oxides. Secondly, a theoretical assumption was proposed to enhance the p-type conductivity by introducing a higher energy level s* antibonding state at the top of the valence band, which had been perfectly verified in inorganic lead-free LHDP. Based on the stability, optoelectronic properties and defect properties, we designed a set of screening principles for p-type transparent conductors, and several stable LHDP, such as Cs4CdSb2Cl12 were identified as promising p-type transparent conductors. We elucidated the formation mechanism of intrinsic point defects and grain boundaries (GBs) in HDP and their influence on carrier transport. Several possible schemes to improve the efficiency and stability of HDP devices were proposed. Firstly, the ideal chemical potential conditions to grow n-type Cs2AgInBr6 with shallow defect levels were predicted. It was found that the fast ion diffusion of the dominant defects in HDPs was likely to destroy the device stability. It was proposed that the ion diffusion can be effectively suppressed by applying compressive biaxial strain on the substrate. Secondly, through a chemical potential screening process and the theoretical design of band structures, it was proposed that under some pre-designed growth conditions, some specific intrinsic defects or defect complexes can spontaneously segregate into the GB cores, and effectively eliminate the deep-levels of the harmful GBs in HDPs. In addition, the self-passivated GBs could generate band bending, which may be beneficial for charge separation, We proposed the theoretical design principles for good half-metallic ferromagnets. Through the computational screening in 114 LHDP (Cs4M2+B3+2XVII12 and Cs4M4+B2+2XVII12) candidates, only one LHDP: Cs4FePb2Cl12 was predicted to exhibit a half-metallic ground state with high Curie temperature, wide half-metallic gap and large magnetocrystalline anisotropy energy. The physical mechanism of the strong ferromagnetism in Cs4FePb2Cl12 was elucidated through the orbital occupations of magnetic ions via super-superexchange interactions. Our finding showed the potential of LHDP for high-temperature spintronic applications.
