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

Colorado State University, Fort Collins CO

Investigators

Abstract

Abstract Nontechnical This 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. Technical The 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.