摘要
随着全球能源危机和环境污染的日益加剧,开发利用新型可再生清洁能源已经成为人们生产、生活,维持社会可持续发展的迫切需求。近年来开发利用太阳能,光伏发电技术已经吸引了人们的普遍关注,其中聚光光伏发电技术以其独特的优势,如较高的转换效率和较低的发电成本,已被公认为最具潜力的地面应用发电技术。聚光型多结化合物半导体太阳能电池是聚光发电技术的核心器件,具有目前最高的光电转换效率,良好的温度特性,抗辐射性能好,以及寿命长等优点。本论文主要从电池结构设计、器件制备以及可靠性三个方面对聚光多结化合物半导体太阳能电池开展研究,具体研究内容如下：
(1)开展聚光多结化合物半导体太阳能电池的理论研究和结构设计。首先将聚光Ga0.5In0.5P/GaAs/Ge三结太阳能电池分成三个子电池,分别研究pn结掺杂浓度、界面特性、载流子迁移率等参数对各子电池性能的影响,其次设计并优化各子电池结构,其中考虑到高倍聚光测试要求,着重优化了GaInP顶电池发射层结构。最后完成子电池间电流匹配的优化设计,从而获得了高性能的聚光Ga0.5In0.5P/GaAs/Ge三结太阳能电池,1000倍聚光测试下具有38.1%的转换效率。
(2)对聚光GaInP/InGaAs/Ge三结太阳能电池关键工艺技术进行研究。首先运用金属有机化学气相沉积(MOCVD)外延生长技术,不断摸索外延生长工艺,克服外延反相畴、晶格匹配以及GaInP有序化等外延问题,进而生长完成高质量、高转换效率的单结子电池；优化高掺杂超薄外延层的生长条件,获得了峰值隧穿电流密度达150A/cm2的高性能GaAs隧穿结；完成高质量、低表面缺陷的三结太阳能电池的外延生长。其次优化设计电极占空比,获得优化的电极图形；设计并制备Al2O3/TiO2双层减反膜结构,使得电池短路电流提升33.3%。最后成功制备高效聚光GaInP/InGaAs/Ge三结太阳能电池,在1000倍聚光条件下实现了39.2%的转换效率。
(3)开展聚光多结化合物半导体太阳能电池的可靠性研究。首先运用三维(3-D)等效电路模型分别考察了光照和外接恒流源情况下电池表面缺陷的缺陷行为,发现对具有表面缺陷的三结电池进行作用,使用外接恒流源的方式将比直接光照的方式使电池衰退的更快。其次依据IEC62108检验标准,对表面良好和表面缺陷样品进行快速老化实验研究,老化测试结果表明,表面良好和表面缺陷样品性能衰退均在8%以内,完全满足IEC62108检验标准要求。
(4)开展新型Ⅲ-Ⅴ族化合物半导体太阳能电池的理论研究工作。首先,研究了应力补偿多量子阱GaAs太阳能电池在不同组分多量子阱In1-xGaxAs/GaAsyP1-y结构下的电池性能,与标准GaAs电池对比发现,应力补偿多量子阱GaAs电池具有较高的短路电流,但是开路电压和填充因子相对较低,电池效率低于标准GaAs电池。另外,提出了新型应力补偿多量子阱AlGaAs/GaAs电池结构设计,模拟结果显示新型电池结构具有比标准电池更高的短路电流和开路电压,但是填充因子较应力补偿多量子阱GaAs电池和标准电池低。其次,针对无定形GaInP/InGaAs/Ge三结电池的特殊结构,研究电池在不同位错密度下的性能变化,发现当位错密度大于106/cm2时电池转换效率明显降低。同时获得不同聚光倍数下电池的性能参数,300倍聚光下电池指出了最高的转换效率。最后,开展新型InGaN太阳能电池的理论研究,优化设计n-on-p型InGaN电池结构,获得了20.8%的较高转换效率,同时讨论位错密度对InGaN电池性能的影响,发现相比于GaAs太阳能电池,InGaN太阳能电池具有更好的稳定性和耐久性。
Since the global energy crisis and environmental pollution are increasingly serious, it is vital for the sustainable development of human society to explore renewable energy. Recently, photovoltaic technology has attracted increasing attention, which can convert sunlight into electricity. Concentrator photovoltaic with its special advantages, such as higher efficiency and lower cost, has been recognized to be the most promising power generation technology for terrestrial application. Concentrator multijunction solar cell is the key of concentrator photovoltaic, and has the highest photoelectric conversion efficiency, good temperature characteristic, superior radiation-resistant property, and long lasting lifetime span, and so on. This work is mainly divided into three parts, cell design, device fabrication and reliability study for the III-V compound semiconductor concentrator multi-junction solar cells, and the main achievements are summarized as follows:
1. The theoretical study on concentrator GaInP/GaAs/Ge triple-junction solar cell was carried out. Firstly, by changing the doping concentration of p-and n-layer, interface recombination and carrier mobility, and so on, the characteristics of three subcells from concentrator GaInP/GaAs/Ge triple-junction solar cell, GalnP top cell, GaAs middle cell and Ge bottom cell, were investigated. Secondly, in order to achieve high efficiency under high concentration, the subcell structures, especially emitter structure of GaInP top cell, were designed and optimized. Finally, by complying with current matching of subcells, high-efficient concentrator Ga0.5In0.5P/GaAs/Ge triple-junction solar cell was obtained with efficiency of 38.1%at 1000 suns.
2. The key techniques of growing concentrator GaInP/InGaAs/Ge triple-junction solar cell were studied. By exploring the MOCVD growth process, we solve the problems for expitial growth, such as anti-phase domains, lattice-matching, and ordering in GalnP semiconductor, and achieve the epitaxial growth of high-quality and high-efficient single-junction subcells, Ga0.51In0.49P top subcell, In0.01Ga0.99As middle subcell and Ge bottom subcell. By optimizing the growth condition for high-doping and ultrathin epitaxial layer, high-performance GaAs tunneling junction with peak tunneling current density of 150A/cm2 was obtained. And epitaxial growth of high quality triple-junction solar cell with lower surface defect was achieved. Then, investigating into the influence of the ratio of electrode width and space on characteristics of multijunction solar cells, the optimal electrode pattern was obtained. By designing and optimizing antireflection film structure, the optimal Al2O3/TiO2 dual-layer film shown superior performance, enhancing short-circuit current of solar cell by 33.3%. Finally, the high-performance GaInP/InGaAs/Ge triple-junction solar cell was obtained with efficiency of 39.2%at 1000 suns.
3. Reliability of concentrator multi-junction solar cell was studied. First, by 3-D equivalent circuit model, the characteristics of surface defect of multi-junction solar cells were studied under illumination and external constant current source, respectively, and it was found that the power degradation of multijunction solar cell with surface defect is greater under external constant current source than that under illumination. Then, according to IEC 62108 standard the accelerated aging tests of cell samples with good surface and defective surface were carried out, and the aging results showed that the performance degradation of all samples was within 8%, meeting the requirement of IEC 62108 standard.
4. The theoretical study on the novel III-V compound semiconductor solar cells was carried out. Firstly, the characteristics of strain-compensated multi-quantum well GaAs solar cells with different molar fraction In1-xGaxAs/GaAsyP1-y multi-quantum wells were simulated, and in contrast with GaAs control sample, strain-compensated multi-quantum well GaAs solar cells showed higher short-circuit current, but lower open-circuit voltage, fill factor, and efficiency. In addition, the novel strain-compensated multi-quantum well AlGaAs/GaAs solar cell was studied, and shown higher short-circuit current and open-circuit voltage, but the lowest fill factor. Secondly, considering the special structure of metamorphic GaInP/InGaAs/Ge triple-junction solar cell, the influence of dislocation density on performance parameters of solar cell was studied, and it was found that solar cell efficiency shows visual degradation with more than 106/cm2 of dislocation density. In addition, investigating into efficiency dependence of metamorphic GaInP/InGaAs/Ge triple-junction solar cell on sun concentration, the highest efficiency was obtained at 300 suns. Finally, the theoretical study on the novel InGaN solar cell was carried out, and the higher efficiency of 20.8%was obtained by using n-on-p device structure. By investigating into the influence of dislocation density on the characteristics, the greater stability and durability of InGaN solar cell was shown compared with GaAs solar cell.
引文
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