Light Emitting Diodes (LEDs) have recently gained importance in the experimental practice of novel photovoltaic (PV) devices. Tunability of peak wavelengths and the high efficiency of light emission have finally shown LEDs full potential in a variety of techniques. In this work, the following applications in the indoor characterization of PV cells and modules are presented, some of which are consolidated, while some others are totally novel. LEDs have recently been introduced as an alternative to conventional xenon or halogen based solar simulators. High intensity LEDs are now on the market and the authors show preliminary results and related challenges in the indoor characterization with a LED-based, steady state solar simulator for cells and commercial size modules. The continuous source is of fundamental importance in testing novel devices (e.g. organic PV, dye sensitized solar cells, capacitive and high-efficiency c-Si modules) where few millisecond pulsed simulators are no-longer reliable. Most importantly, the standard IEC 60904-9 Class A spectral irradiance requirement can be reached with a set of different LEDs and can be improved, for example via overlapping commercial halogen bulbs. The dependence of the electrical parameters on the Average Photon Energy (APE) is a new insight that emerged in the last years, giving interesting information for energy rating: the combination of powerful LED lamps with a conventional Class AAA large area solar simulator is also presented in this work, showing that a wide range of target APE values can be easily obtained. Comparison with the results from the outdoor field is shown. Multi-junction PV structures take also advantage of the use of coloured LEDs in experimental tools. Indeed, the standard IEC 60904-9 Class A spectral irradiance requirement can be really poor for such devices, which easily exhibit current limitation under artificial spectra and the consequent measurement artifacts that are widely studied in the literature. This work shows how the use of powerful LEDs can transform a conventional, single source pulsed solar simulator in a tunable simulator for spectral characterization of multi-junction modules, thus enhancing the spectral mismatch correction when measuring such devices. From the same point of view, the importance of LEDs as additional bias light (superimposed over a pulsed solar simulator) is also shown for a number of applications, such as spectral responsivity of multi-junction modules and the investigation of the dependence of the electrical parameters on the angle of incidence. With the revision and introduction of all the experimental challenges above, this paper represents a useful tool for any research centre dealing with the characterization of PV devices, with special interest in pre-normative techniques of measurements where standard procedure are still under discussion.
University Appl. Sciences and Arts of Southern Switzerland, University of Pavia
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