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

这个研究项目研究了一种使太阳能电池的效率大大提高的策略，提供了一条低成本太阳能发电的途径。这项研究考察了将两种最成功的光伏(PV)技术-来自II-VI系列半导体的碲化镉(CdTe)和晶硅(Si)太阳能电池-结合在一起形成串联结构的障碍，该结构将II-VI太阳能电池沉积在硅电池的顶部，以实现比单独使用这两种电池的效率高得多的效率。所研究的基础科学问题将直接使只有两个电触点的高效多结电池成为可能，从而极大地简化其制造并降低成本。该计划在几个不同层面上造福社会。在全球和国家层面上，高效太阳能电池技术将加快光伏的部署--目前光伏发电占世界发电量的1%以上--减少其他电力来源带来的温室气体排放和国际安全担忧，增加美国和世界各地弱势群体获得电力的机会，同时为美国创造高薪、高科技的就业机会。在教育层面，这项前沿研究为来自不同背景的学生提供关键的科学培训，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。技术该项目的目标是发展所需的科学认识，以集成II-VI顶部电池，例如具有15%mg和1.75 eV带隙能量的镉镁Te合金与晶体硅底部电池，以形成只有两个端子的整体式串联太阳能电池，以便于在光伏组件中制造和使用，目标效率超过28%。该项目研究了五个关键科学领域：1)随着带隙能量的增加，控制体寿命和晶界复合的因素；2)由于颠倒II-VI单元通常的生长顺序而引起的半导体质量的变化；3)顶部和底部单元之间互连层的组成、界面性质和沉积条件；4)化学上不同的顶部和底部单元之间的相互作用，这可能会降低硅底部单元中的少数载流子寿命；以及5)集成技术，用于演示目标效率为28%的双结CdMgTe/Si太阳能电池原型，远远超过任何一种太阳能电池材料的最高效率。科罗拉多州立大学(CSU)将用其单真空、多源沉积系统制造的CdMgTe电池对1区和2区进行研究。用时间分辨光致发光和横截面扫描电子显微镜来评价CdMgTe吸收层的质量。通过标准的电流-电压、量子效率和电容测量，以及应用CSU开发的器件分析技术，将初步分析在p型CdMgTe上沉积n型层的电池。第三区和第四区将在亚利桑那州立大学(ASU)学习。在亚利桑那州立大学建造的高寿命硅电池上，将通过溅射、等离子体增强化学气相沉积和原子层沉积来沉积不同的互连层成分和组合。评估将重点放在电气传输、光学透过率和是否适合高质量的II-VI生长等关键指标上。二次离子质谱仪、随温度变化的电流-电压和电感耦合载流子寿命测量将表征由于热平衡和与顶层单元生长相关的杂质的复合活性而引起的硅单元中的电势变化。电池集成领域5将由两所大学联合完成，目标效率为28%，这将对太阳能发电在全球经济中的渗透率产生深远影响。然而，该项目的价值超越了光伏电池，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洞察力，例如在用于高速计算的低成本硅上集成光发射器。

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.