EAGER: TDM Solar Cells: Collaborative Research: Monolithic 2-Junction Polycrystalline II-VI / Silicon Solar Cells

【摘要】本研究项目旨在研究一种使太阳能电池效率显著提高的策略，为低成本太阳能发电提供一条途径。该研究考察了将两种最成功的商业光伏（PV）技术——来自半导体II-VI家族的碲化镉（CdTe）和晶体硅（Si）太阳能电池——结合起来形成串联结构的障碍，该结构与沉积在硅电池顶部的II-VI太阳能电池形成串联结构，其效率远高于单独使用任何一种电池。所研究的基本科学问题将直接实现只有两个电触点的高效多结电池，大大简化其制造并降低成本。该计划在几个不同的层面上对社会有益。在全球和国家层面上，高效太阳能电池技术将加速光伏发电的部署——目前光伏发电占世界电力的1%以上——减少温室气体排放，减少伴随其他电力来源而来的国际安全问题，并为美国和世界各地的弱势群体增加电力供应，同时为美国创造高薪、高科技的就业机会。在教育层面上，这项前沿研究为来自不同背景的学生提供了关键的科学培训，为他们提供进入快速增长的美国光伏劳动力所需的技能和经验。技术上，该项目的目标是发展所需的科学理解集成II-VI顶部电池，例如含有15% Mg和1.75 ev带隙能量的CdMgTe合金，与晶体硅底部电池形成一个单片串联太阳能电池，只有两个终端，便于制造和使用在光伏组件中，目标效率超过28%。该项目研究了五个关键的科学领域：1)CdMgTe的体寿命和晶界复合的控制因素，因为它的带隙能量比CdTe的带隙能量增加；2) II-VI电池正常生长顺序颠倒导致半导体质量变化；3)上下电池间互连层的组成、界面性质及沉积条件；4)化学性质不同的顶、底电池之间的相互作用可能降低硅底电池的少数载流子寿命；5)集成技术，用于演示原型2结CdMgTe/Si太阳能电池，目标效率为28%，远高于任何一种太阳能电池材料的最高效率。区域1和2将在科罗拉多州立大学（CSU）进行研究，使用其单真空、多源沉积系统制造的CdMgTe电池。CdMgTe吸收层的质量将通过时间分辨光致发光和横断面扫描电镜进行评价。在p型CdMgTe上沉积n型层的电池将主要通过标准电流电压，量子效率和电容测量以及CSU开发的器件分析技术的应用进行分析。区域3和4将在亚利桑那州立大学（ASU）学习。通过溅射、等离子体增强化学气相沉积和原子层沉积，在亚利桑那州立大学制造的高寿命硅电池上沉积不同的互连层成分和组合。评估将集中在电传输、光学透射率和高质量II-VI生长的适用性等关键指标上。由于硅电池的热收支和与顶部电池生长相关的杂质的重组活性，硅电池中的电位变化将通过二次离子质谱、温度相关的电流电压和电感耦合载流子寿命测量来表征。区域5，电池集成，将由两所大学共同完成，目标效率为28%，这将深刻影响太阳能发电在全球经济中的渗透。然而，该项目的价值超出了光伏电池，揭示了低成本多晶半导体缺陷的基本原理，并为其他应用提供了见解，例如用于高速计算的低成本硅上的光发射器集成。

AbstractNontechnicalThis research program investigates a strategy to make solar cells dramatically more efficient, providing a route to lower-cost solar electricity. The research examines barriers to combining the two most commercially successful photovoltaic (PV) technologies - cadmium telluride (CdTe) from the II-VI family of semiconductors and crystalline silicon (Si) solar cells - to form a tandem structure with the II-VI solar cell deposited on top of the Si cell for a much higher efficiency than either cell individually. The fundamental scientific issues investigated will directly enable high-efficiency multijunction cells with only two electrical contacts, greatly simplifying their manufacture and lowering costs. The program benefits society on several different levels. On a global and national level, the high-efficiency solar cell technologies would accelerate deployment of photovoltaics - which produce over 1% of the world's electricity today - reducing greenhouse gas emission and international security concerns that accompany other electricity sources, and increasing access to electrical power for disadvantaged populations in the U.S. and around the world, while creating high-paying, high-technology U.S. jobs. On an educational level, this cutting-edge research provides key scientific training for students from many backgrounds to give them the skills and experience necessary to enter the rapidly-growing U.S. photovoltaics workforce. TechnicalThe goals of the project are to develop the scientific understanding needed to integrate a II-VI top cell, for example the CdMgTe alloy with 15% Mg and 1.75-eV band-gap energy, with a crystalline silicon bottom cell to form a monolithic tandem solar cell with only two terminals for ease of manufacture and use in PV modules, and with a target efficiency over 28%. The project investigates five key scientific areas: 1) factors controlling the bulk lifetime and grain-boundary recombination of CdMgTe as its band-gap energy is increased from that of CdTe; 2) changes in semiconductor quality caused by reversing the usual growth order of II-VI cells; 3) composition, interface properties, and deposition conditions of the interconnection layers between the top and bottom cells; 4) interactions between the chemically different top and bottom cells that may degrade minority-carrier lifetime in the Si bottom cell; and 5) integration technology for demonstrating prototype 2-junction CdMgTe/Si solar cells with a target efficiency of 28%, well above the highest efficiencies of either solar cell material alone. Areas 1 and 2 will be investigated at Colorado State University (CSU) with CdMgTe cells made with its single-vacuum, multi-source deposition system. The quality of the CdMgTe absorber layer will be evaluated by time-resolved photoluminescence and cross-sectional scanning electron microscopy. Cells deposited with the n-type layer over the p-type CdMgTe will be primarily analyzed through standard current-voltage, quantum-efficiency, and capacitance measurements and the application of device-analysis techniques developed at CSU. Areas 3 and 4 will be studied at Arizona State University (ASU). Varying interconnection layer compositions and combinations will be deposited by sputtering, plasma-enhanced chemical vapor deposition, and atomic-layer deposition, on high-lifetime Si cells built at ASU. Evaluation will focus on the key metrics of electrical transport, optical transmittance, and suitability for high-quality II-VI growth on top. Potential changes in the Si cell due to the thermal budget and recombination activity of impurities associated with top cell growth will be characterized by secondary-ion mass spectroscopy, temperature-dependent current-voltage, and inductively-coupled carrier-lifetime measurements. Area 5, cell integration, will be accomplished jointly by the two universities with target efficiency of 28%, which would profoundly impact the penetration of solar-generated electricity in the global economy. The value of the project, however, extends beyond photovoltaic cells, shedding light on the fundamentals of defects in low-cost polycrystalline semiconductors, and providing insight on other applications such as integration of light emitters on low-cost silicon for high-speed computing.