Daylight luminescence system for silicon solar panels based on a bias switching method

Abstract

Luminescence techniques, both electroluminescence (EL) and photoluminescence (PL), are becoming powerful tools for inspecting solar cells and photovoltaic modules, 1-7 based on the reciprocity relation between photovoltaic quantum efficiency and luminescence emission. 8,9 EL consists of luminescence emission by solar cells under forward bias, 10 thereby spatially resolving defects that affect the performance and/or durability of the modules, such as cracks, heterogeneous cell activity, failed soldering, grid defects, and dark areas in cells associated with dislocation clusters. 11-18 In contrast, PL consists of luminescence emission under excitation with light. 19-28 The difficulty involved in obtaining a uniform large-area light excitation source over the module surface has prevented it from being applied to module inspection. This problem was circumvented by using the sun as the excitation source, without having to resort to an artificial light source, for example, a laser. 19,29 PL emission depends on the quality of the material, its defects, for example, dislocations, precipitates, and cracks,

Photoluminescence imaging of silicon modules in a string

“…The reason for this is to try to obtain a better image quality, as averaging over multiple images in same condition is common procedure. [28][29][30][31] Since the system needs to return to the MPP condition after the OC condition, acquiring another set of BG images, BG 2 , is less costly than acquiring another set of PL images due to the inverter's reaction time. Another set of BG images results in another ΔPL j image.…”

“…Outdoor luminescence imaging has therefore been carried out in the dark, [17][18][19] very low light conditions [20][21][22] as well as in daylight. [23][24][25][26][27][28][29][30][31][32][33][34][35] The basis of photoluminescence (PL) imaging is charge carrier excitation with an illumination source which can be a laser, 6,21 LEDs, 2,36 or sunlight. [27][28][29][30][31]34,35 However, the reflected light from the illumination source needs to be accounted for.…”

“…[23][24][25][26][27][28][29][30][31][32][33][34][35] The basis of photoluminescence (PL) imaging is charge carrier excitation with an illumination source which can be a laser, 6,21 LEDs, 2,36 or sunlight. [27][28][29][30][31]34,35 However, the reflected light from the illumination source needs to be accounted for. If the wavelength of the reflected signal does not coincide with the emitted PL signal, reflection can be eliminated by use of optical filters.…”

“…The reflected signal needs to be subtracted. [27][28][29][30][31]34,35 The need to subtract the unwanted reflection from the module has resulted in an approach in which a pair of images in two different operating points is alternately recorded. The image recorded in the operating point in which the module emits less signal is subtracted from the image recorded in the operating point in which the module emits more signal.…”

“…A few different methods have so far been developed in order to switch a module's operating point for the purpose of PL imaging in sunlight. Using a solid state-relay 30 or a MOSFET as an on/off switch connected to the module's terminals 34 enables extremely rapid, almost instantaneous, switching and acquisition of a pair of images to be taken under nearly the same illumination conditions. Another approach is based on connecting a module or a module string to a specially developed piece of equipment.…”