Development of indium-rich InGaN epilayers for integrated tandem solar cells
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Development of indium-rich InGaN epilayers for integrated tandem solar cells A. G. Melton1, B. Kucukgok1, B-Z. Wang1,2, N. Dietz3, N. Lu1,4 and I. T. Ferguson1,3 1
Department of Electrical and Computer Engineering, UNC Charlotte, Charlotte, NC, US Department of Electric Information Engineering, Hebei UST, Hebei, P.R. China 3 Department of Physics and Astronomy, Georgia State University, Atlanta, GA, US 4 Department of Engineering Technology, UNC Charlotte, Charlotte, NC, US 2
ABSTRACT InGaN epilayers have been investigated for use in photovoltaic solar cells for the past years. At present, almost all photovoltaic device structures reported have exhibited very low short circuit currents and thus very low solar conversion efficiency. This phenomenon has been attributed to point and extended defect chemistry in InGaN epilayers (e.g. vacancies, misfit dislocations, and V-defects), as well as to spinodal decomposition of the strained InGaN wurtzite lattice system. These defects become more dominant for higher indium concentration InGaN epilayers needed for multijunction photovoltaic device structures. In this work, we will report on the growth and characterization of indium-rich InGaN epilayers that have been grown by novel MOCVD growth technology, including the growth at superatmospheric reactor pressures. INTRODUCTION The ternary InGaN material system offers a superior potential for the development of high performance photovoltaic (PV) devices with reduced cost for terrestrial applications. The primary advantage of this material system is a direct-band gap, high absorption coefficient, and the ability to tailor bandgaps ranging from 0.7 eV to 3.4 eV, covering the majority of the solar spectrum. A continuum of bandgaps can be obtained by varying the indium and gallium ratio in this ternary InGaN material system, compared to the current use of multiple group III-V materials (e.g. group III-phosphide/arsenide alloys) to achieve the same end [1]. Moreover, InGaN is one of the few material systems that provide band gaps greater than 2.4 eV, which is required for tandem solar cells with high photovoltaic conversion efficiencies. However, presently encountered scientific and technological challenges obstruct the fabrication of efficient solar cells with EG < 2.0 eV necessary for >50% conversion efficiencies [2]. InGaN solar cells have not yet realized their full transformative potential despite a significant research effort over the past 7-8 years. The reason lays in the numerous materials challenges that have to be solved in order to achieve useful efficiencies and to increase the indium incorporation in InGaN alloys so that devices with bandgaps below 2.0 eV can be produced. While high peak internal and external quantum efficiencies (IQE and EQE) have been reported, InGaN solar cell efficiencies have never exceeded 3%. High VOC’s have been reported, but JSC’s have remained very low. This is most likely due to the many crystal defects commonly observed in InGaN epilayers and poor device design such as not properly optimizi
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