Rare Earth Based Upconverting Materials for Solar Cell Application
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Rare Earth Based Upconverting Materials for Solar Cell Application Madhab Pokhrel*, G. A. Kumar, and Dhiraj K. Sardar, Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249-0697, USA ABSTRACT Basic spectroscopic studies of Yb and Er-doped M2O2S (M= Gd, La,Y) phosphor was reported with particular attention to its upconversion properties under 1550 nm excitation. Since the absorption spectra of Co2+overlaps with Er3+(4I13/2→4I15/2) at near infrared (NIR) region, we are proposing the concept of an efficiency enhancement of infrared upconverting phosphors as an energy converting material that could potentially improve the efficiency of Si solar cells in bifacial configuration. Different concentrations of Er3+/Co2+ phosphors were synthesized by solid state flux fusion method. The phosphor powders were well crystallized in a hexagonal shape with an average size 4 μm. Preliminary upconversion analysis of singly Er3+ and doubly Co2+/Er3+ doped Y2O2S under 980 and 1550 nm excitation indicates that the composition has to be optimized to understand the role of Co2+ in the system. KEYWORDS: Phosphors, Optical material, absorption, emission, upconversion, solar cell, upconverting layer, Cobalt (II), energy transfer *corresponding Author: [email protected] INTRODUCTION Upconversion (UC) is the spectroscopic process where a low energy photon of higher wavelength is converted into one or more high energy photons of lower wavelength.[1, 2] Spectroscopically, this is happening through the multiphoton absorption process where a low energy photon is being absorbed to the higher state by multiphoton absorption.[3, 4]More importantly, the upconversion occurs in trivalent rare earths doped materials through multiphoton absorption by real energy states through the process of ground and excited state absorption. Details on upconversion in ceramics, glass, phosphors and single crystals are already reported in the literatures.[1, 5, 6]. An apparent benefit of using upconverting layers was theoretically modeled by Trupke et al.,[7] who estimated that the upper limit of photovoltaic conversion efficiency of a single junction solar cell coupled to an ideal upconversion layer can be as high as 47.6% for nonconcentrated sunlight and 63.2% for concentrated sunlight. Additionally, other group has shown that the efficiency of a bifacial solar cell, i.e., one that absorbs light in the front and back, could be increased to almost 50% by placing an upconverting material on the reverse side.[8] Importantly, the increase in external quantum efficiency (EQE) of a commercial solar cell with an upconverting layer depends on the upconversion efficiency of the material used.[9-11] The efficiency of this process depends on the lifetime of the intermediate energy level of the ion, which itself depends on the lattice vibrations (phonons) of the host material.[3, 4] In this work, we are reporting the synthesis and optical characterization of erbium-ytterbium and erbium –cobalt doped oxysulphide phosphors as an IR energy
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