Calibration of multi-junction (tandem) thin film photovoltaic modules under natural sunlight
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Calibration of multi-junction (tandem) thin film photovoltaic modules under natural sunlight Georgios Tzamalis, Harald Müllejans European Commission, DG JRC, IE, Renewable Energy Unit, 21027 Ispra (Va), Italy. ABSTRACT We present a procedure for calibrating tandem thin film photovoltaic modules under natural sunlight. The distinct steps involve pre-conditioning via light soaking, spectral response measurements and I-V measurements under natural sunlight. The measurements were done using a crystalline silicon reference cell as well as two filtered reference cells designed to spectrally match the top and bottom junctions in the tandem module. INTRODUCTION Thin film technology has secured its place as a commercially viable alternative to crystalline silicon cells due to its lower material and manufacturing cost and increasing lifetime and stability. Although the efficiencies of thin film solar modules are still lower than crystalline, they can drastically improve by the ability to deposit different type of materials and alloys. One method of enhancing the efficiency of a thin film module is by creating a multijunction (e.g. tandem) thin film module. It consists of a stack of two or more semi-conductive layers with different band gaps so that the gap energy decreases from the top. The top layer, usually a-Si, converts the shorter wavelength of the solar spectrum, while the bottom layer, usually microcrystalline Si, converts the longer wavelengths more efficiently. Electrically they are connected in series, which means that the current of the module is limited by the sub-cell with the lowest photocurrent under the prevailing spectral and irradiance conditions. Usually the limiting junction is not known a-priori, thus providing an additional challenge for the measurement and final calibration of a multi-junction module. The increased complexity of a multi-junction thin film module is reflected by the fact that although several type of multijunction modules are now available in the PV market, there is no IEC standard for their calibration (even though ASTM E2236 addresses some aspects). Based on our experience at the European Solar Test Installation (ESTI) laboratory, the calibration procedure of a tandem module consists of the following steps: stabilization of the module by successive light soakings, spectral response (SR) measurements, I-V measurements under natural sunlight (with simultaneous measurement of the spectral irradiance) and final correction to standard test conditions (STC). Based on the calculation of the current limiting junction from the SR a spectral mismatch correction for the limiting junction is calculated and applied to obtain the electrical parameters at STC, i.e. at total irradiance equal to 1000 W/m² with a reference spectral irradiance for AM1.5G (as defined by IEC 60904-3) and at device temperature equal to 25ºC. We present a detailed calibration study of two multijunction PV modules of the latest technology. The measurements involved all the aforementioned steps until the final calibration valu
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