【作者】 王欣；

【导师】 李来才； 刘向洋；

【作者基本信息】 四川师范大学 ， 化学， 2022， 硕士

【摘要】 太阳能电池的光电转换效率与光诱导界面电荷转移效率密切相关,通过构建新型异质结构使其具有高效的界面电荷转移速率,将对提高太阳能电池的性能有不可忽视的意义。近些年来,半导体类型的零维量子点（0D-QD）结构与单层二维过渡金属硫化物（2D-TMDs）由于它们优异的光电性质,得到了理论和实验学家的广泛关注。近些年来,科学家们发现将这两类新型光电材料通过范德华相互作用相结合而形成的异质结结构,不仅可以充分发挥两类结构各自的优势,还可能带来一些新的、更为优异的光电性能。因此,设计新型太阳能电池时可以利用理论计算的方法深入探究该类异质结的几何结构、电子结构以及相关的光电性能。然而,相比于相关的实验研究,对该类体系的理论研究还相对较少。在诸多半导体量子点中,硒化镉（CdSe）量子点得到了理论和实验学家的广泛研究。该类量子点不仅可以有效的吸收太阳光,还可以通过改变尺寸大小,或者与ZnS等结合形成核-壳结构来调控相应的电子结构性质。因此,基于CdSe量子点与二维过渡金属硫化物形成的范德华异质结的性质研究在近些年来引起了科学家的广泛关注。然而,此前的理论和实验研究主要集中在Mo S₂或WS₂与CdSe量子点形成的异质结性质,除了这两种结构外,还存在着其他几种常见的二维过渡金属硫化物如Mo Se₂,Mo Te₂,WSe₂等可能与CdSe量子点形成新的范德华异质结,相关研究仍然较少。此外,对于基于CdSe的核-壳结构量子点与二维过渡金属硫化物形成范德华异质结的理论研究更是极为缺乏。因此,为了进一步阐明该类体系的性质,在本文中,我们构建了基于两种CdSe量子点(纯Cd₃₃Se₃₃以及核-壳结构Cd₆Se₆-Zn₂₇S₂₇量子点)和四种不同二维过渡金属硫化物（Mo S₂,Mo Se₂,Mo Te₂,WSe₂）形成的八个范德华异质结结构。在上述模型的基础上,利用密度泛函理论计算,深入探究了这些体系的几何结构和电子结构,对优化后的结构计算层间距离和吸附能、电荷密度和差分电荷密度、态密度和带隙,我们发现这些异质结都可以稳定存在,结合过程放热且界面之间通过范德华相互作用结合,而没有共价键和电荷重叠,电荷交换非常小。有利于电子-空穴分离的II型异质结中,WSe₂/CdSe和WSe₂/CdSe-ZnS有更理想的光伏性能。详细讨论并比较了其光电性能和可能发生的电子转移或空穴转移,从中筛选了可能具有优异性能的太阳能电池结构的异质结,本文为今后基于密度泛函计算合理设计具有更好光电性能的新型范德华异质结提供了一定的理论参考。更多还原

【Abstract】 The photoelectric conversion efficiency of solar cells is closely related to the photoinduced interfacial charge transfer efficiency.It is of great significance to improve the performance of solar cells by constructing new heterostructures with high interfacial charge transfer rates.In recent years,semiconductor zero-dimensional quantum dots（0D-QD）and monolayer two-dimensional transition metal sulfides（2D-TMDs）have attracted extensive attention from theorists and experimentalists due to their excellent photoelectric properties.In recent years,scientists have found that the heterojunction structure formed by combining these two kinds of novel photoelectric materials through van der Waals interaction can not only maximize the advantages of both types of structures,but also may bring some new and better photoelectric properties.Therefore,the geometric structure,electronic structure and photoelectric properties of the heterojunction can be further explored by theoretical calculation when designing new solar cells.However,compared with the relevant experimental research,the theoretical research on this kind of system is relatively rare.Among many semiconductor quantum dots,CdSe quantums dot have been studied extensively by theorists and experimentalists.This kind of quantum dots can not only absorb sunlight effectively,but also adjust the corresponding electronic structure properties by changing the size or combining with ZnS to form core-shell structure.Therefore,studies on the properties of van der Waals heterojunctions based on CdSe quantum dots and two-dimensional transition metal sulfides have attracted extensive attention of scientists in recent years.However,previous theoretical and experimental studies mainly focused on the properties of heterojunctions formed by Mo S₂ or WS₂ and CdSe quantum dots.In addition to these two structures,several other common two-dimensional transition metal sulfide,such as Mo Se₂,Mo Te₂,WSe₂,etc.,may form new van der Waals heterojunctions with CdSe quantum dots,but there are still few relevant studies.In addition,theoretical studies on the formation of van der Waals heterojunctions by core-shell quantum dots based on CdSe and two-dimensional transition metal sulfides are extremely scarce.Therefore,in order to further clarify the properties of this kind of system,we constructed the eight van der Waals heterojunction structures formed by two kinds of CdSe quantum dots(pure Cd₃₃Se₃₃ and core-shell Cd₆Se₆-Zn₂₇S₂₇ quantum dots)and four different two-dimensional transition metal sulfides（Mo S₂,Mo Se₂,Mo Te₂,WSe₂）in this paper.On the basis of the above model,using density functional theory calculation,the geometric structure and electronic structure of these systems are deeply explored,and their photoelectric properties are discussed and compared in detail by calculating the interlayer distance and adsorption energy,charge density and differential charge density,state density and band gap of the optimized structure.We find that these heterojunctions can exist stably,the bonding process is exothermic,and the interface is bonded by van der Waals interaction,without covalent bond and charge overlap,and the charge exchange is very small.In the type II heterojunction which is beneficial to electron-hole separation,WSe₂/CdSe and WSe₂/CdSe-ZnS have better photovoltaic performance.The photoelectric properties and possible electron transfer or hole transfer are discussed and compared in detail,and the heterojunction which may have excellent performance in solar cell structure is screened.This paper provides a theoretical foundation for the rational design of novel van der Waals heterojunctions formed by0D-QDs and 2D-TMDs with better optoelectronic properties in the future.更多还原