A Raman spectroscopic investigation of graphite oxide derived graphene AIP Advances 2, 032183 (2012) Reduced graphene oxide (rGO) has been produced using an ammonia (NH 3 ) plasma reduction method. Simultaneous nitrogen doping during the reduction process enabled a rapid and low-temperature restoration of the electrical properties of the rGO. The chemical, structural, and electrical properties of the rGO films were analyzed using x-ray photoelectron spectroscopy, Raman spectroscopy, atomic force microscopy, and conductivity measurements. The oxygen functional groups were efficiently removed, and simultaneous nitrogen doping (6%) was carried out. In addition, the surface of the rGO film was flattened. Consequently, the rGO films exhibited electrical properties comparable to those prepared via other reduction methods.
The temperature effect on a CuInGaSe 2 (CIGS) solar cell was investigated in the temperature range −10 to 80°C using direct current (DC) and alternating current (AC) characteristic analysis of CIGS solar cells. The change rates of short-circuit current density (J sc ), open-circuit voltage (V oc ), fill factor (FF) and efficiency (η) with respect to temperature were investigated. In addition, the variation of impedance, total capacitance and dynamic resistance due to the temperature change implied that the width of the depletion region in a p-n junction shrank, and the effective minority carrier lifetime (τ eff ) decreased. Consequently, J sc changed slightly due to the balance between the variation of the energy bandgap (E g ) and τ eff . However, V oc linearly decreased due to the relationship of E g when the temperature increased. From these results, it has been concluded that V oc plays an important role in the characteristics of the CIGS solar cell with change of temperature.Introduction: CuInGaSe 2 (CIGS) materials have been of interest for photovoltaic applications because of their high absorption coefficient (∼10 4 / cm), adjustable direct bandgap (1.0-1.4 eV) and high stability for light radiation [1][2][3][4]. These properties enable CIGS solar cells to have high efficiency, low cost and high stability. Currently, the efficiency of CIGS solar cells has been obtained above the value of 20% and also the efficiency in flexible CIGS solar cells has already reached a value of 17% at the research level [3,4]. A CIGS solar cell device is a hetero-junction semiconductor with an n-type CdS/ZnO layer, and this means the CIGS solar cell is sensitive to temperature like other semiconductor devices. Generally, the increase of temperature in semiconductor devices reduces the bandgap (E g ) of the semiconductor [5,6], and this variation due to the temperature change strongly affects the characteristics of semiconductor devices. Especially, the temperature influences the efficiency of the solar cell. Thus, understanding of temperature dependency is essential in order to broaden the field of application of solar cells.In this Letter, the characteristics of the CIGS solar cell with change in device temperature are investigated in terms of direct current (DC) and alternating current (AC) equivalent circuit model. First, we measured the properties of a CIGS solar cell with device temperature change at the illumination of one sun condition (AM 1.5), and CIGS solar cell parameters were evaluated using J-V and C-V analyses. From the analyses, the main factors that affect the conversion efficiency of the CIGS solar cell due to the temperature change are extracted.
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