不同垒层厚度并掺杂的GaNAs基短周期超晶格太阳能电池与MBE生长研究

GaInNAs subcell with a roughly 1 eV band gap, which is lattice-matched to germanium substrate, could become a critical component in next-generation multijunction concentration solar cells for terrestrial applications. Fundenmantal issues limiting its practical applications are short minority diffusion length，low mobility and high series resistance which could be caused by nitrogen-related deep-level defects (poor solubility of nitrogen in InGaAs) and/or strain and composition fluctuations associated with In and N concomitance during the epitaxy growth of III-V-N dilute nitride materials. These issues finally result in low short-circuit current density that is one of key factors to design multijunction solar cells..To alleviate some of these problems, we propose a new cell structure on germanium substrate using supperlattice as a photon absorber which is a combination of two differenct GaNAs/InGaAs superlattice structures with thin and thick barrier layers, and one of superlattice structures is doped. The thick GaNAs/InGaAs superlattice necessites to have a cut-off bandedge of 1 eV based on the photoluminescence and quantum efficiency measurement. The meticulously designed structure could be benefical to reasonably-high open-circuit voltage, short-circuit current density and fill factor, as well as low series resistance for the resultant solar cells. .In the project, we predict the optical properties of such short period super-lattices (SPSLs) based on the calculation or simple model. The calculated band line-up and transition energy is plotted as a function of N concentration, barrier thickness, super-lattices period and period number.Considerable efforts will be made to carry out the material growth of these different superlattice structure with a band gap of 1 eV and reasonably-good crystal quality by MBE. Also some dependences of carrier mobility, lifetime and optical properties on growth conditions are in detail investigated. Meanwhile, post-growth annealing of these as-grown GaNAs/InGaAs superlattices is done to see its impact on the material properties and device performance of the fabricated p-i-n structure which is formed by respectively sandwiching SPSL layers by p- and n-GaAs layers. To explore the good design of superlattice which produces good photovoltaic response, bias dependent quantum efficiency and photoluminescence measurements on the designed cells are performed. The light intensity and operating temperature dependences of quantuem efficiency and series resistance etc are studied to understand how III-nitride solar cells respond to concentration ratio and elevated operating temperatures. We aim to obtain the useful information to explore and develop the potential use of GaNAs/InGaAs superlattice structures for concentrator solar cells.

Ge衬底上吸收带边为1.0eV的晶格匹配型GaInNAs基电池是高效多结聚光光伏电池的重要方案之一。目前限制效率提升的问题主要是GaInNAs材料中In、N共存引起的缺陷、厚膜中有限的少子扩散长度和较高的串联阻抗导致的低短路电流密度，这也是设计多结电池的主要考虑。本项目提出在Ge衬底上使用不同垒层厚度并掺杂的两种GaNAs/InGaAs短周期超晶格作为电池有源区，以此研究实现高短路电流密度的基础问题和相关材料的MBE生长。分析设计超晶格带隙随N浓度、垒层厚度、超晶格周期等的变化，获得1eV带边时这些参数的相互关系；研究不同超晶格类型的材料生长、载流子迁移率、少子载流子寿命、发光特性以及退火对材料和器件的影响；研究施加偏压对不同垒层厚度且掺杂后GaNAs/InGaAs超晶格光学和量子效率的影响；研究超晶格电池聚光性能变化以及加热时温度的影响规律并加以比较，深析电池聚光工作时温度的影响问题。

Ge衬底上基于1 eV子电池的多结电池在空间和地面电源方面具有重要的潜在应用，但如何实现高转换效率的1 eV子电池，尤其是工作稳定性，一直是个挑战。本项目针对Ge衬底上晶格匹配的1 eV吸收带边的稀氮氮化物子电池进行了新型的GaNAs/InGaAs超晶格设计和器件制作，在理论计算、超晶格的分束外延生长、器件退火和设计等内容进行了详细的研究。项目按照年度研究计划和预期研究结果，按计划顺利执行。通过该项目的执行，全部完成了预计发表7至10篇论文（项目执行期间共发表8篇论文，其中5篇被SCI收录）、申请3项专利（项目执行期间共申请7项专利，其中5项已授权）和指导4至6名硕士/博士研究生（项目执行期间共培养7名硕士研究生，其中1名来自法国的中国留学研究生）的研究目标。主要取得了理论上考虑多结电池电流匹配对GaNAs/InGaAs超晶格子电池的设计要求、基于等离子体损伤和表面修复平衡的高质量超晶格分子束外延生长、高性能外延薄膜和器件的退火窗口以及国际上首次实现了基于GaNAs/InGaAs超晶格有源区的1 eV太阳能电池且具有良好聚光特性等重要成果。经超晶格周期和退火条件的优化，以GaNAs/InGaAs超晶格为有源区的GaAs p-i-n结构电池在一个AM1.5 G下的短路电流密度在10 mA/cm2以上。培养的人才中有一名来自远在法国求学的中国留学生和一名博士后。最为有意义的是，毕业的一名研究生加盟格力继续从事太阳能电池的产业工作。由于取得了这些研究结果，得到了上海811空间电源研究所重大基础研究计划评审专家的高度肯定，并希望将此科学问题的解决应用于GaInNAs基高效率多结电池方面，为中国空间电源用高效重量轻的多结太阳能电池提供技术保障。另外，根据上述取得的有关GaNAs/InGaAs超晶格电池的重要结果和关键数据，将在开发基于GaNAs基超晶格的多结电池以及应用于地面能源具有明显的科学意义或应用前景。