Comparative Study of Annealed and High Temperature Grown ITO and AZO Films for Solar Energy Applications
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Comparative Study of Annealed and High Temperature Grown ITO and AZO Films for Solar Energy Applications Diego Alonso-Álvarez1, Lourdes Ferre Llin2, Alexander Mellor1, Douglas J. Paul2 and Nicholas J. Ekins-Daukes1 1 Imperial College London, Department of Physics, London SW7 2AZ, United Kingdom 2 University of Glasgow, School of Engineering, Glasgow G12 8LT, United Kingdom ABSTRACT We present the optical and electrical properties of ITO and AZO films fabricated directly on silicon substrates under several growth and annealing temperatures, as well as their potential performance when used as low emissivity coatings in hybrid photovoltaic-thermal systems. We use broadband spectroscopic ellipsometry measurements (from 300 nm to 20 µm) to obtain a consistent model for the permittivity of each of the films. The best performance is found using the properties of the ITO film grown at 250 ºC, with a state of the art resistivity of 0.2 mΩ-cm and an optimized thickness of 75 nm which leads to an estimated 50% increase in the extracted power compared to a standard diffused silicon solar cell. The Hall mobility and resistivity measurements of all the films are also provided, complementing and supporting the observed optical properties. INTRODUCTION Transparent conductive oxides (TCO) are an integral part of modern electronic devices [1]. They are an essential component of displays and touchscreens in tablets and mobile phones. They are also key in light emitting diodes as well as in photovoltaics (PV), forming the front electrode in many solar cell technologies, such as in thin film devices (CdTe, CIGS and organic) or crystalline silicon heterojunction (SHJ) solar cells [2]-[4]. For solar energy, harvesting TCOs must have (a) very high transparency over the solar spectral range, and at the same time they must possess (b) excellent electrical transport properties. A more recent application is their use as low emissivity coatings for hybrid PV-thermal (PV-T) solar systems to keep the heat inside the solar cell and rise its temperature. In this case a third property, (c) low emissivity in the thermal range (5-20 µm), is also required. On the other hand, the electrical power produced by photovoltaic devices is fundamentally lower at the usual working temperatures of hybrid PVT systems (60-100 ºC), dropping at around 0.42 %/ºC in typical silicon solar cells with respect the specifications at 25 ºC. However, the choice of a photovoltaic technology with low thermal coefficients (such as SHJ or CdTe) and the use of solar cells designed to work at high temperature should minimize the detrimental effect. Nowadays, PV-T systems which operate at higher working temperatures are based on evacuated tubes, where the solar cells are directly attached to the heat exchanger without any type of encapsulation. In those systems, convective losses are mostly suppressed and radiative losses can be reduced by tuning the properties (thicknesses of the layers, composition and dopant densities) of the coatings deposited directly into the solar cells. In t
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