Calibrated high-efficiency testing and modelling methodologies for concentrated multi-junction solar cells

Multi-junction solar cells (MJSC) under concentrated sunlight are leading the way forward to record efficiencies (>;40%, under AM1.5D solar spectrums) and are ideally suited for utility-scale energy production. MJSC are able to achieve high efficiencies by partitioning the solar spectrum using multiple n-p junctions composed of III-V and IV group semiconductors. To reduce the cost associated with their more complex epitaxial growth processes, MJSC are coupled with concentrator optics (typical concentrations of 500-1000 suns). The University of Ottawa´s SUNLab focuses its research on improving the efficiency of concentrated high-efficiency MJSC systems. This research takes place in three crucial areas: laboratory characterization of MJSC, numerical modelling of thermal, electrical and optical properties of cells and systems, and outdoor testing of concentrated photovoltaic systems. Laboratory testing of MJSC are performed under controlled temperature and spectral irradiance conditions, which allow accurate efficiency comparisons of different solar cell designs. Flash and continuous solar simulators illuminate the solar cells with intensities of up to 1000 suns. Flash solar simulators are the standard method of characterizing solar cells under concentration, whereas testing under continuous concentrated solar radiation provides critical additional information regarding thermal reliability (see Fig. 1(a)). MJSC are composed of subcells connected electrically in series, making their performance sensitive to changes in the solar spectrum. Spectral sensitivity is quantified by performing quantum efficiency measurements that determine a subcell´s ability to extract an electron for a given incident photon. For each experimental measurement, numerical modelling assists in determining the limiting elements within a design. These numerical simulations cover the critical areas, such as the concentrating optics with complex raying tracing, current-voltage characteristics o- - f the MJSC using semiconductor device simulators, and finite element method thermal models of the MJSC on carrier. Outdoor testing of a solar system measures the efficiency under real-world conditions. The University of Ottawa´s outdoor test site has three solar tracking stations that measure the power output of a photovoltaic system while monitoring the irradiance and spectrum of the sun, which determine overall system efficiencies (see Fig. 1(b)).