Moisture Resistant Ga-Doped ZnO Films with Highly Transparent Conductivity for Use in Window Layers of Thin-Film Solar C
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Moisture Resistant Ga-Doped ZnO Films with Highly Transparent Conductivity for Use in Window Layers of Thin-Film Solar Cells H. -P. Song1, H. Makino1, S. Kishimoto1, 2 and T. Yamamoto1 Materials Design Center, Research Institute, Kochi University of Technology, Kami-City, Kochi 782-8502, Japan 2 Department of Mechanical Engineering, Kochi National College of Technology, 200-1 Otsu Monobe Nankoku-City, Kochi 783-8508, Japan 1
ABSTRACT Highly transparent conductive Ga-doped ZnO (GZO) films are one of the promising transparent conductive oxide (TCO) films for use in electrodes of flat display panels and window layers of thin film solar cells. For the ZnO-based TCO films, the stability to damp-heat environment is a crucial issue for practical applications. We will report moisture resistant GZO codoped with indium films (GZO:In) on the basis of analysis of data obtained a damp-heat test for solar cells (85°C and 85% relative humidity for 1000 hours). We used ZnO sintered targets with contents of 3 wt% Ga2O3 and 0.25 wt% In2O3 to grow GZO:In films in ion plating with direct current arc-discharge system. GZO:In films with different thicknesses (0.1-1 μm) were deposited on glass substrates at 200°C under the O2 flow rate of 15 sccm. As the film thickness increased from 0.1 to 1 μm, the resistivity and sheet resistance decreased from 4.3 μΩm to 2.6 μΩm and from 42.7 Ω/Sq. to 2.6 Ω/Sq., respectively. And the average optical transmittance (Tav) in the range from 0.4 to 1 μm decreased from ~ 86% to ~ 75%. The GZO:In film with a thickness of ~300 nm had a low sheet resistance of 10.5 Ω/Sq. and a Tav of 82.5%. After 1000 hours damp-heat (DH) test under 85°C and 85% relative humidity, the relative change of sheet resistance is 3.4% with a Hall mobility of 26.4 cm2/V.s and a Tav of 82.7% after test. The film thicker than 300 nm has a sheet resistance lower than 10 Ω/Sq. and a relative change of resistance of ~3% after DH test. INTRODUCTION ZnO-based transparent conductive oxide (TCO) films such as Al-doped or Ga-doped ZnO (AZO or GZO) films have been widely used in many applications including photovoltaic (PV) solar cells in recent years due to many advantages such as low material cost, abundant storage, and eco-friendly (non-toxicity) [1-5]. Moreover, compared to SnO2 or In2O3-based films, ZnO-based TCO films are more durable to reductive hydrogen plasma which is commonly used to fabricate the amorphous (Į-Si: H) or microcrystalline silicon (μ-Si: H) type solar cells [6]. High reliability is a crucial issue for PV solar modules which are settled outdoors. Hydrothermal operating environment is inevitable. Moisture ingress from the environment will attack the device components and cause performance degeneration or even the failure of solar module. To date large amount of research works have been performed to assess the stability of TCO materials used in PV solar cells [7-10]. Many reports show that the properties of ZnO-based TCO films were easily degenerated by a standard damp heat (DH) test (85°C and 85% relative humidity) for so
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