Andreas W. Bett

• Professor
• Managing Director at Fraunhofer Institute for Solar Energy Systems

Optical power transfer and optical data communication are versatile technologies with numerous applications, utilizing both fiber-coupled systems and free-space optical links. The integration of these technologies into a single link simplifies system design while combining the benefits of simultaneous power delivery and data communication for recei...

Following the impressive efficiencies achieved for two‐terminal perovskite/silicon dual–junction solar cells, perovskite/perovskite/silicon triple‐junction cells have now gained attention and are rapidly developing. In a two‐terminal triple‐junction cell, maximizing the open‐circuit voltage (VOC) is not straightforward as it requires understanding...

Double‐sided textured silicon solar cells with micrometer‐sized pyramid structure are used as bottom cells in monolithic tandem structures to decrease reflection losses. As top cell material, frequently perovskites are used. In this work, various additives are investigated to enhance the perovskite absorber quality, as a top cell fabricated using t...

A major renewable-energy milestone occurred in 2022: Photovoltaics (PV) exceeded a global installed capacity of 1 TW dc. But despite considerable growth and cost reduction over time, PV is still a small part of global electricity generation (4 to 5% for 2022), and the window is increasingly closing to take action at scale to cut greenhouse gas (GHG...

The measurement of the current-voltage (IV) characteristics is the most important step for quality control and optimization of the fabrication process in research and industrial production of crystalline silicon solar cells. We propose a methodology to determine the IV characteristics of silicon solar cells in a contactless way. We summarize the th...

The MorphoColor concept for highly efficient colored solar modules aims to improve the acceptance of those modules in visible areas like roofs and facades. The core of the MorphoColor technology is the combination of a structured substrate with a spectrally selective reflecting layer stack. The work presented within this manuscript investigates the...

Die neue Bundesregierung hat die einmalige Chance, aber auch die Verpflichtung, der Klima- und Energiepolitik einen dringend benötigten Neustart zu geben. Dieser muss konsequent am Ziel der Treibhausgasneutralität ausgerichtet sein. Folgende Leitlinien sollten diesen Aufbruch prägen: • Zuvorderst gilt es, den Klimaschutz beim nationalen politischen...

A thin‐film GaAs/AlGaAs rear‐heterojunction photonic power converter with a back reflector is presented by Henning Helmers and co‐workers in article number 2100113. Photon recycling is leveraged to increase the excess carrier density and consequently the output voltage. Optical resonance yields highest absorptance for near‐bandgap photons. Thereby...

We present recent results achieved in the field of photonic power conversion, i.e. monochromatic light to electricity conversion, using photovoltaic cells. Based on a thin-film processing approach we leverage photon recycling and optical resonance effects with a GaAs/AlGaAs rear-heterojunction photovoltaic cell. A back reflector yields increased ef...

For solar cells operating under the broad band solar spectrum, the photovoltaic conversion efficiency is fundamentally limited by transmission and thermalization losses. For monochromatic light these losses can be minimized by matching the photon energy and the absorber material’s bandgap energy. Further, for high crystal quality direct semiconduct...

In this paper, we present a stepped architecture optimized for current matching in high‐voltage laser power converter photovoltaic (PV) cells. The integrated series/parallel connection in stepped PV cells combines the advantages of well‐known multijunction and multisegment approaches with respect to current matching, whereas their specific drawback...

For concentrator photovoltaic modules, a promising approach is the miniaturization of the solar cell and optics. In solar cells, the perimeter is a source for the recombination of minority charge carriers, which leads to a voltage loss. With decreasing cell size, the ratio between perimeter and solar cell area increases and therefore the perimeter...

III-V based multi-junction solar cells with 4 to 6 junctions reach the highest efficiencies for the conversion of solar energy today. We present the latest results on the development of a GaInP/GaAs/GaInAs//GaSb four-junction solar cell. The implemented devices show an efficiency of 43.8 ± 2.6 % at a concentration of 796 suns and the efficiency dec...

In this paper, we present our experience in designing FLATCON® concentrator modules. The FLATCON® technology uses silicone-on-glass (SoG) Fresnel lenses and solar cells mounted to a metal heat distributor on a glass rear plate. During the past two decades, various FLATCON® module designs have been investigated. The first FLATCON® modules consisted...

Solar energy has the potential to play a central role in the future global energy system because of the scale of the solar resource, its predictability, and its ubiquitous nature. Global installed solar photovoltaic (PV) capacity exceeded 500 GW at the end of 2018, and an estimated additional 500 GW of PV capacity is projected to be installed by 20...

Optical power transmission (also called power-by-light) enables transmission of electrical power without electron transport through a copper cable. At the base, a laser converts electrical power into monochromatic laser light, which is then transmitted through free space (optical wireless power transmission) or using an optical fiber (power-over-fi...

A method for distributed electrical modeling of complete photovoltaic monolithic interconnected modules (MIMs) with complex geometrical and electrical features is developed and applied to study electrical power losses of a six‐segment GaAs‐based MIM laser power converter (LPC). The model considers spatial dependence of resistive and recombination l...

The cover image is based on the Research Article Comprehensive Electrical Loss Analysis of Monolithic Interconnected Multi‐Segment Laser Power Converters by Rok Kimovec et al., https://doi.org/10.1002/pip.3075.

Concentrator photovoltaics (CPV) is a special high efficiency system technology in the world of PV-technologies. The idea of CPV is to use optical light concentrators to increase the incident power on solar cells. The solar cell area is comparatively tiny, thus saving expensive semiconductor materials and allowing the use of more sophisticated and...

In this work, we propose and realize three different design strategies to implement an optical cavity in GaAs thin film solar cells in order to confine its internal luminescence and hence to exploit photon recycling. The strategies are based on the definition of a highly reflective and very conductive back side, whereas front side light extraction...

Monolithic multi-junction solar cells made on active silicon substrates are a promising pathway for low-cost high-efficiency devices. We present results of GaInP/GaAs/Si triple-junction solar cells, fabricated by direct growth on silicon in a metal–organic vapor phase epitaxy reactor using a GaAs $_{y}$ P $_{1-}$ $_{y}$ buffer structure to overc...

The strong growth of the PV sector is accompanied by high cost pressure, accelerated innovation cycles and dynamic deployment, clearly indicating that the quality of PV products and the holistic economy of PV electricity deserve special attention. PV is expected to deliver electricity at low LCOE, Energy Pay-Back Time (EPBT) and Product Environment...

The 44.19 MWp concentrator photovoltaic (CPV) power plant CPV 1 in Touwsrivier, South Africa is three years in operation. Lessons learnt for optimizing the power production are presented. The plant configuration is described and the key steps for reliable and efficient operation and maintenance are discussed. By introducing structured operation and...

In this paper, the development of a HCPV mirror module design with passive heat dissipation and a geometrical concentration of 1000x is discussed. To benchmark the mirror module, the expected performance is compared to the well-known lens based FLATCON® module. Therefore, the electrical performance and acceptance angle are evaluated. Moreover, a de...

In the version of this Article originally published, in the ‘Rear-side light trapping’ paragraph of the Methods section, the values of depth and fill factor were incorrectly given as 350 nm and 50%, respectively; instead, the values should have read 250 nm and 60%. This has now been corrected.

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...

In the version of this Article originally published, in the legend in Fig. 5a, the blue, green and red lines were incorrectly labelled as GaAs, Si and GaInP, respectively; instead, the labels should have read, respectively, GaInP, GaAs and Si. This has now been corrected.

Power loss mechanisms in small area monolithic-interconnected photovoltaic modules (MIM) are described and evaluated. Optical and electrical losses are quantified and individual loss components are derived for loss mechanisms of small area radial (radius = 1 mm) pie-shaped six-segment GaAs MIM laser power converter. At low monochromatic homogeneous...

In the design of concentrator photovoltaic modules, numerous components need to be optimised to achieve the highest performance of the complete system while simultaneously maintaining low costs for the manufacturing processes. In this paper, we present a systematic analysis of different designs of concentrator photovoltaic modules using mirror opti...

Silicon dominates the photovoltaic industry but the conversion efficiency of silicon single-junction solar cells is intrinsically constrained to 29.4%, and practically limited to around 27%. It is possible to overcome this limit by combining silicon with high-bandgap materials, such as III–V semiconductors, in a multi-junction device. Significant c...

This paper describes the influence of an irradiance-dependent photo-induced leakage current through a semi-insulating GaAs substrate on the performance of photovoltaic monolithically series-interconnected multisegment laser power converters. A reciprocal relation between the resistivity of a semi-insulating GaAs substrate and irradiance of monochro...

One approach to consider the prevailing spectral conditions when performing CPV module power ratings according to the standard IEC 62670-3 is based on spectral matching ratios (SMRs) determined by the means of component cell sensors. In this work, an uncertainty analysis of the SMR approach is performed based on a dataset of spectral irradiances cr...

This paper introduces an innovative and fast method to determine the external quantum efficiency (EQE). The setup was specifically designed to measure monolithic multijunction solar cells. Multijunction solar cells consist of two or more subcells exhibiting different spectral characteristics. Their spectral selective sensitivity, however, is challe...

Technologiebericht "Photovoltaik" des strategischen Leitprojektes "Trends und Perspektiven der Energieforschung" (gefördert durch das BMWi) über den Status, die Perspektiven sowie das Innovations- und Marktpotenzial von verschiedenen Energietechnologien.

This chapter discusses solar cells made of III-V semiconductors, and how they have reached efficiencies of over 46% in 2016, the highest of any photovoltaic technology to date. These high efficiencies are possible due to the ability of stacking solar cells made of different III-V semiconductors. The main focus of current research is on III-V multij...

The electrical power losses of small area six-segment monolithic interconnected GaAs laser power converter are analyzed based on a variety of measurements and an experimentally validated numerical 2.5-dimensional distributed electrical model. Input parameters for the developed model were extracted from a wide-range of independent measurements perfo...

The concentrator standard conditions are defined in the standard IEC 62670-1. Those conditions demand for spectral conditions equivalent to AM1.5d as described in IEC 60904-3. The power output of CPV modules has to be rated at the concentrator standard conditions and thus at AM1.5d spectral irradiance. According to IEC standard 62670-3 the prevaili...

The radiation hardness of AlGaAs single-junction solar cells is investigated for various n-i-p solar cell designs. The material composition in both the n-p regions is varied between 3.5% and 16% Al-content, whereas the intrinsic region has a higher Al-content between 3.5% and 23%. The beginning-of-life and end-of-life quantum efficiency and current...

Silicon solar cells featuring the highest conversion efficiencies are made from monocrystalline n-type silicon. The superior crystal quality of high-performance multicrystalline silicon (HP mc) in combination with the inherent benefits of n-type doping (higher tolerance to common impurities) should allow the fabrication of high-efficiency solar cel...

In this work, we investigated the impact of intensive heat loads on concentrator solar cells assemblies. As test samples, we employed lattice-matched and lattice-mismatched triple-junction solar cells made of GaInP/GaInAs/Ge. The thermal loads were induced by different manners. We used drying cabinets, external power supplies, and a combination of...

Blade condition monitoring systems with fiber-optic sensors attract much attention because they are resistant to lightning strikes, a major issue with increasing blade lengths. However, fiber-optic sensor systems are more complex and more expensive than their electronic counterparts. We describe a new blade condition monitoring system, which combin...

OPEN ACCESS: http://dx.doi.org/10.1016/j.ijleo.2016.11.072 This work addresses the alignment of a light spot onto photovoltaic laser power converters for power-by-light applications. A model to compare the alignment tolerances of different cell designs by means of the maximum acceptable misalignment is introduced. Monolithic interconnected multi-s...

The photocurrent versus open-circuit voltage characteristic of individual subcells in a multijunction solar cell can be measured by illuminating the subcell with a pulse sequence of spatially homogeneous laser light. In this paper, we demonstrate the applicability of this method for four junction solar cells. Furthermore, the degradation of triple...

OPEN ACCESS: http://dx.doi.org/10.1002/pip.2814 Dual-junction GaAs laser power converters optimized for one monochromatic wavelength are presented, and their temperature dependence is experimentally evaluated. External quantum efficiency and irradiance-dependent current–voltage measurements (10 to 104 W/cm2) under monochromatic laser light (809 nm...

Four-junction solar cells are being developed for space applications as they promise higher efficiencies compared to the present GaInP/GaInAs/Ge triple-junction industry standard. There are multiple technological routes to achieve four-junction cells with the ideal bandgap combination of 1.9 eV, 1.4 eV 1.05 eV and 0.7 eV. This includes metamorphic...

Stacking III–V p-n junctions on top of wafer-based silicon solar cells is a promising way to go beyond the silicon single-junction efficiency limit. In this study, triple-junction GaInP/Al $_{x}$ Ga $_{1-}$ $_{x}$ As//Si solar cells were fabricated using surface-activated direct wafer bonding. Metal–organic-vapor-phase-epitaxy-grown GaInP/Al $_{x}$...

The spatial electroluminescence intensity and spectral measurements of photovoltaic devices under forward bias have proved to be fast and reliable characterization tools. They enable quick evaluation of material and manufacturing quality and provide information linked to local performance of photovoltaic devices in different operating conditions. I...

The first section of this chapter introduces the principles of concentrating photovoltaic (CPV) technology. Since there is a huge diversity in system designs, the most common principles are outlined and examples for CPV systems are presented. The second section discusses the most important impacts on the performance of CPV systems. In contrast to s...

In this study, secondary ion mass spectroscopy of oxygen, deep level transient spectroscopy and power dependent relative photoluminescence are compared regarding their ability to resolve differences in AlxGa1-xAs material quality. AlxGa1-xAs samples grown with two different trimethylaluminum sources showing low and high levels of oxygen contaminati...

Optical power transmission (also called power-by-light) is an elegant solution to power wireless sensor nodes. At the sensor, optical power in form of monochromatic laser light is converted to electricity by photovoltaic laser power converters. These photovoltaic cells are optimized for the monochromatic wavelength of the light — opposed to solar c...

This paper presents the project Concentrating Photovoltaic modules using advanced technologies and cells for highest efficiencies (CPVMatch), which is funded from the European Union’s Horizon 2020 research and innovation programme. V multi-junction solar cells and CPV modules. Concerning cells, novel wafer bonded four-junction solar cells made of G...

In this work a direct conversion of sunlight with an efficiency of 43.4 % is demonstrated with a CPV mini-module. The mini-module consists of an achromatic full-glass lens and a wafer-bonded four-junction solar cell mounted to a heat sink. The 43.4 % efficiency was determined at IEC 62630-1 concentrator standard test conditions. The electrical char...

In this work a combined system consisting of a CPV and thermal (CPV-T) system connected with a reverse osmosis (RO) and membrane distillation (MD) for desalination is presented. In the CPV-T electrical energy is generated. The electrical energy is partly needed to run the desalination systems (e.g. pumps). The surplus electricity can be fed into th...

A concentrator standard test condition (CSTC) efficiency of 43.4% for the direct conversion of sunlight into electricity is demonstrated with a subunit of a concentrator photovoltaics (CPV) module. This is the highest reported CSTC efficiency for CPV applications thus far. This test device consists of a four-junction solar cell and uses an achromat...

In optical power transmission systems photovoltaic cells are used as power converters for monochromatic light. In case of powering remote sensors, supplementary to the optical power link also bidirectional data transmission between the two sides of the optical link is required. It is appealing to integrate the functionalities of the optical power l...

In optical power transmission systems photovoltaic cells are used as power converters for monochromatic light. In case of powering remote sensors, supplementary to the optical power link also bidirectional data transmission between the two sides of the optical link is required. It is appealing to integrate the functionalities of the optical power l...

A new I-V measurement set-up called LaserSim has been developed for photovoltaic laser power converters – photovoltaic cells optimized for monochromatic artificial light - providing irradiances of up to 89 W/cm². The evaluation of the set-up and first I-V curve and efficiency measurements are presented. Similar to flash simulators used for I-V char...

The open circuit voltages Voc of individual subcells in a multijunction solar cell are measured by illuminating a given subcell with a pulse of spatially homogeneous, nearly monochromatic light with a rising edge in the μs regime. The influence of luminescent coupling and semi-transparency on Voc is eliminated by over-illuminating all subcells belo...

Very reflective back side reflectors directly below the active region of a solar cell alter the photon absorption and carrier collection dynamics in a manner that can be exploited for enhanced efficiency. The enlarged optical path increases the short circuit current, and the confinement and (re)absorption of the photoluminescence leads to a higher...

A temperature dependent modeling study is conducted on a GaAs laser power converter to identify the optimal incident laser wavelength for optical power transmission. Furthermore, the respective temperature dependent maximal conversion efficiencies in the radiative limit as well as in a practically achievable limit are presented. The model is based...

Stacking III-V p-n junctions on top of wafer based silicon solar cells is a promising way to go beyond the silicon single-junction efficiency limit. Indeed, realistic conversion efficiencies beyond 35% under 1 sun AM1.5g conditions can be expected for a triple-junction made of GaInP/AlGaAs//Si with ~ 4 µm III-V materials on top of Si wafer. In this...

A power-dependent relative photoluminescence measurement method is developed for double-heterostructures composed of III-V semiconductors. Analyzing the data yields insight into the radiative efficiency of the absorbing layer as a function of laser intensity. Four GaAs samples of different thicknesses are characterized, and the measured data are co...

A major issue in the characterization of photovoltaic modules is the limitation of the homogenously irradiated area provided by the available sun simulators. Therefore the Stepwise Measurement Procedure for the Characterization of Large-area PV Modules (SMP) was developed recently. This concept enables the characterization of modules with aperture...

The full set of information on the study "Flexibility concepts for the German power supply in 2050. Ensuring stability in the age of renewable energies" can be found on http://www.acatech.de/de/aktuelles-presse/dossiers/dossier-stromversorgung-2050.html and the main document with the study in English is available on http://www.acatech.de/de/publika...

A method for the determination of the subcell I-V characteristics of multijunction solar cells in the presence of optical coupling is presented and applied to a Ga0.50In0.50P/Ga0.99In0.01As/Ge triple-junction solar cell. Each of the subcells is described by a two-diode model and can be illuminated by a narrowband light source externally. Optical co...

The highest solar cell conversion efficiencies are achieved with four-junction devices under concentrated sunlight illumination. Different cell architectures are under development, all targeting an ideal bandgap combination close to 1.9, 1.4, 1.0, and 0.7 eV. Wafer bonding is used in this work to combine materials with a significant lattice mismatc...

Photovoltaic power generation is a growing renewable primary energy source, expected to assume a major role as we strive toward fossil fuel free energy production. However, the photovoltaic efficiencies limit the conversion of solar radiation into useful power output. Hybrid systems extend the functionality of concentrating photovoltaics (CPV) from...

Radiatively dominated III–V semiconductor solar cells are strongly influenced by the effects of photon recycling. From a modeling standpoint, the semiconductor transport equations must account for this to predict accurate open-circuit voltages , even for cells on substrate. Using Shockley–Read–Hall (SRH) lifetimes based on internal quantum efficien...

Modeling single junction solar cells composed of III-V semiconductors such as GaAs with the effects of photon recycling yields insight into design and material criteria required for high efficiencies. For a thin-film single junction GaAs cell to reach 28.5% efficiency, simulation results using a recently developed model which accounts for photon re...

Starting on June 2011, NGCPV has been the first coordinated project between the European Commission and Japanese NEDO in order to advance in the science and technology of concentrator photovoltaics (CPV). Research has covered all relevant areas in CPV, from solar cells to complete systems, including multijunction solar cells, novel solar cell conce...

Light spectrometers are highly versatile state-of-the-art measurement devices. However, using these systems, e.g., in semiconductor device characterization, creates challenging obstacles with respect to measurement time. We present a new, flexible and accurate approach to either characterize optical properties of arbitrary photosensitive devices or...

In photovoltaics (PV), sun simulators are used to reproduce outdoor conditions in a lab environment such as irradiance level, light uniformity and spectral distribution. Concentrator (C)PV applications additionally require the sun simulators to provide rays with an angular distribution similar to that of the sun rays. However, different factors in...

The quantum efficiency (QE) is an important measure for the electro-optical characterization as well as the calibration of photovoltaic (PV) devices. In this paper we present a novel experimental approach for the generation of monochromatic radiation with sun-like angular characteristics that enables external QE (EQE) measurements of concentrator p...

In this paper, we present an indoor measurement procedure for characterizing the electrical performance of large aperture photovoltaic modules. Because of the fact that sun simulators, especially for concentrator photovoltaic applications, are strongly limited in the size of the uniformly illuminated area, we developed a measurement procedure that...

An elegant solution for the power supply of sensor electronics is the application of power-by-light technology. With this technology several challenges related to conventional copper wiring are inherently overcome; benefits are galvanic isolation, the suppression of electromagnetic interference, and the possibility to combine power and bidirectiona...

Photovoltaic power generation is a growing renewable primary energy source, expected to assume a major role as we strive toward fossil fuel free energy production. However, the rather low photovoltaic efficiencies limit the conversion of solar radiation into useful power output. Hybrid systems extend the functionality of concentrating photovoltaics...

Single junction photovoltaic devices composed of direct bandgap III-V semiconductors such as GaAs can exploit the effects of photon recycling to achieve record-high open circuit voltages. Modeling such devices yields insight into the design and material criteria required to achieve high efficiencies. For a GaAs cell to reach 28 % efficiency without...

For the quaternary material (Al y Ga1− y )1− x In x As, the energy level of the silicon-related deep electron trap known as the DX-center is calculated. In addition, the composition range y(x) is derived, for which the silicon-related DX-center level is below the conduction band minimum and thus electronically active. Eventually, the result of t...

The open circuit voltage of a single subcell in a multijunction cell stack can be measured with the help of pulsed, millisecond illumination. This concept makes use of the fact that the charging of the non-illuminated cell capacitances takes place on a much longer timescale than of the illuminated one. Optical coupling introduces a photocurrent in...

Solar cells based on AlxGa1-xAs in its direct bandgap range were fabricated and analyzed. We show that state-of-the-art metalorganic vapor phase epitaxy systems and precursors are capable of growing AlxGa1-xAs solar cells with defect concentrations up to 1 × 1014 cm-3 and less determined by deep level transient spectroscopy. However, for the n-dope...

In this work CPV modules based on Fresnel lenses and using refractive secondary optical elements (SOEs) are investigated. Pure silicone as well as glass SOEs glued on top of the solar cells are explored in prototype modules. They are differently manufactured in respect to how the secondary optics was assembled. For example, units with secondary sil...

It has been known for a long time that the precise characterization of multi-junction solar cells demands spectrally tunable solar simulators. The calibration of innovative multi-junction solar cells for CPV applications now requires tunable solar simulators which provide high irradiation levels. This paper describes the commissioning and calibrati...

A network model for multi-junction solar cells has been combined with ray tracing and finite element simulations of a Fresnel lens in order to interpret experimentally derived measurement results. This combined model reveals a good agreement between simulation and measurement for advanced four-junction solar cells under a Fresnel lens when the cell...

It has been proven that the only realistic path to practical ultra-high efficiency solar cells is the monolithic multi-junction approach, i.e., to stack pn-junctions made of different semiconductor materials on top of each other. Each sub pn-junction, i.e., sub solar cell, converts a specific part of the sun’s spectrum. In this way, the energy of t...