KEYWORDS: Solar cells, Gallium arsenide, External quantum efficiency, Solar concentrators, Solar energy, Energy efficiency, Photovoltaics, Compound semiconductors, Group III-V semiconductors
III-V compound semiconductors provide a high degree of flexibility in bandgap engineering and can be realized through epitaxial growth in high quality. This enables versatile spectral matching of photovoltaic absorber materials as well as the fabrication of complex layer structures of vertically stacked subcells and tunnel junctions. This work presents progress in two fields of applications of III-V photovoltaics: concentrator solar cells and photonic power converters. We present latest results in advancing solar energy conversion efficiencies to 47.6% based on a wafer-bonded four-junction concentrator solar cell. Furthermore, we provide an overview of the latest development results regarding photonic power converters, showcasing several record devices. We briefly introduce a new metallization technique using electro-plated silver for handling high currents and first 10-junction InGaAs devices for optical telecommunication wavelengths. Overall, this paper highlights the potential of III-V compound semiconductors in achieving high efficiencies and spectral matching, offering promising prospects for future applications.
Silicon based multi-junction solar cells are a promising option to overcome the theoretical efficiency limit of a silicon solar cell (29.4%). With III-V semiconductors, high bandgap materials applicable for top cells are available. For the application of such silicon based multi-junction devices, a full integration of all solar cell layers in one 2-terminal device is of great advantage. We realized a triple-junction device by wafer-bonding two III-V-based top cells onto the silicon bottom cell. However, in such a series connected solar cell system, the currents of all sub-cells need to be matched in order to achieve highest efficiencies. To fulfil the current matching condition and maximise the power output, photonic structures were investigated. The reference system without photonic structures, a triple-junction cell with identical GaInP/GaAs top cells, suffered from a current limitation by the weakly absorbing indirect semiconductor silicon bottom cell. Therefore rear side diffraction gratings manufactured by nanoimprint lithography were implemented to trap the infrared light and boost the solar cell current by more than 1 mA/cm2. Since planar passivated surfaces with an additional photonic structure (i.e. electrically planar but optically structured) were used, the optical gain could be realized without deterioration of the electrical cell properties, leading to a strong efficiency increase of 1.9% absolute. With this technology, an efficiency of 33.3% could be achieved.
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