Evaluations on Current-matching of Multi-junction Photovoltaics under Outdoor Conditions

Key differences of precisely measuring currents on wafer based solar cells and multi-junction solar cells are shortly described at the beginning of the paper. In the next section, the impact of precise Standard Test Condition (STC) measurements on the energy yield prediction by PV simulation software is reviewed, with special mention of the Staebler-Wronski Effect. Solar modules containing amorphous silicon absorbers perform in moderate latitudes in a cyclic way: hot season with higher output and cool season with lower output. Both, spectral variations of the irradiance as well as temperature rise during irradiance are reported in literature contributing to this cycling behaviour, which finally is not an issue but a feature of this technology. In consequence, a ‘design to geography’ approach is suggested for module manufacturers, allowing for higher energy yields even without considerably increasing nameplate power under STC. Standardisation, which highlights the energy yield in addition to STC nameplate power, is identified as urgently needed. A better understanding of amorphous silicon based multi-junction PV modules can only be gained, if more spectrally resolved data acquisition is performed and evaluated for high and low geographic latitudes. Instead of a ‘static’ temperature coefficient, amorphous silicon modules are better characterised with the ‘effective’ temperature coefficient which can become positive e.g. in building integrated heat-insulating PV elements.