As solar cells from direct semiconductors improve, i.e. become more radiative, luminescent coupling becomes more and more relevant. This has a strong impact on artifacts in EQE measurements of multi-junction solar cells, which e.g. is a challenge, when characterizing III-V//Si multi-junction solar cells. By measuring the response of these cells under varying illumination conditions, this effect can be understood and EQE measurements can be corrected.
The standard method to measure subcell external quantum efficiency (EQE) for multi-junction photovoltaics (MJPV) uses light biasing to bring each subcell into current limitation. This method is suitable when each subcell absorbs in a different wavelength range. However, isolating individual subcells via light biasing is difficult for semitransparent subcells with overlapping absorptance, as in MJPV designed for monochromatic irradiance in power-by-light systems. For these cells, the standard measurement approach falls short. Here, we present an alternative technique that incorporates a negative bias voltage to overcome this limitation. We demonstrate subcell EQE measurements in MJPV devices with up to six GaAs subcells.
Photonic power converters (PPCs) are photovoltaic cells that convert monochromatic light into electric power. The impact of luminescent coupling (LC) on InGaAs-based PPCs is studied. Multi-junction PPCs are simulated using an experimentally validated drift-diffusion model, and the contribution of LC is quantified. Up to 85% of the photons emitted across the InGaAs layers are re-absorbed in the dual-junction device considered. This number increases to 96% when a back reflector is included due to improved light management. Interference effects produced by multiple reflections are examined as a function of the emission angle.
We have developed a machine learning empowered computational framework to facilitate design space exploration for optoelectronic devices. In this work, we apply dimensionality reduction and clustering machine learning algorithms to identify optimal ten-junction C-band photonic power converter (PPC) designs. We outline our framework, design optimization procedure, calibrated optoelectronic model, and experimental calibration devices. We report on top performing device designs for on-substrate and flat back-reflector architectures. We comment on the design sensitivity for these PPCs and on the applicability of dimensionality reduction and clustering algorithms to assist in optoelectronic device design.
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.
We demonstrated recently a record 19.9%-efficient GaAs solar cell with an absorber thickness of only 200 nm. Our next step is to optimize the device to reach a 25% efficiency. In this contribution we will present our latest simulation and experimental results based on an extensive analysis of the optical and electrical losses. The benefits brought by the contacts optimization and the improvement of the nanostructured design at the rear side of the solar cell will be emphasized.
Direct wafer-bonding after argon-beam surface activation is a low temperature process, which allows for the monolithic integration of various materials including Si, Ge, III-V compound semiconductors, SiC or Al2O3 etc.
The process requires smooth wafer surfaces with RMS roughnesses < 1 nm and minimal particle contaminations, which is usually achieved by chemical-mechanical polishing. These wafers are sputtered with Ar in ultra-high vacuum (< 3 x 10-6 Pa) to remove few nanometers of oxides and contaminants. The process results in a thin amorphous surface layer with dangling bonds. Subsequently, the wafers are pressed together so that covalent bonds are formed, permanently joining the materials.
As no intermediate layers are applied, the approach enables a high optical transparency together with mechanical stability as well as highest electrical and thermal conductivity. The process parameters are optimized for various material to obtain electrical bond resistances < 5 mΩcm2. Even in multi-junction cells operated at a few hundred suns with current densities of ~5 A/cm2, these resistances do not significantly limit the cell efficiencies. These unique characteristics of the resulting wafer-bonds make the technique promising for a wide range of innovations in photonics or power electronics.
We apply direct wafer-bonding in the fabrication of various concepts for III-V based multi-junction solar cells reaching highest efficiencies. Examples are a wafer-bonded GaInP/GaAs//GaInAsP/GaInAs solar cell that exhibits an efficiency of 46.1 % at 312 suns as well as a GaInP/GaAs/GaInAs//GaSb solar cell with 43.8 % efficiency at 796 suns. Further, the process enables the monolithic integration of III-V materials on Si, at which a record efficiency of 34.1 % at 1 sun could be recently achieved with a GaInP/AlGaAs//Si solar cell.
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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