Dimroth Frank

• PhD
• Head of Department at Fraunhofer Institute for Solar Energy Systems

Indium Phosphide (InP) plays a pivotal role in the semiconductor industry, particularly in the development of high-power, high-frequency optoelectronic devices that are essential for next-generation applications not only in telecommunications, data centers, and photonic systems but also in automotive technologies and consumer electronics. Despite i...

Multijunction (MJ) solar cells have demonstrated very high efficiencies (>30%) owing to the effective use of solar energy. Among these, the GaAs//CuInGaSe(CIGSe)‐based MJ solar cell is unique owing to its features, such as being lightweight owing to the combination of thin cells and allowing the use of flexible substrates such as thin metal plates...

The reduction of the series resistance in multi‐junction solar cells is of high importance for attaining peak efficiencies in concentrator photovoltaics. This study showcases strategies to reduce the sheet resistance of the uppermost subcell of a direct wafer bonded four–junction devices, since it contributes significantly to the series resistance....

Surface tension-driven self-alignment is a promising technique to align millimeter-scale components with a high accuracy of a few microns. It is based on liquid capillary forces moving and aligning a solid component on its receiving pad. Using molten solders as the liquid is a promising way to bond, connect and align a chip with its substrate. Micr...

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 pres...

The aim of this review paper is to summarize a decade of research focused on enhancing metalorganic vapor-phase epitaxy (MOVPE) growth rates of GaAs, driven by the imperative for most cost-effective and energy-efficient III–V compounds’ production. While MOVPE is renowned for producing high-quality devices, it has been constrained by production cos...

III–V/silicon tandem solar cells offer a promising avenue for high-efficiency, high-stability photovoltaics with low environmental risk.

In this work, we present the fabrication and analysis of a wafer‐bonded GaInP/GaInAsP//Si triple‐junction solar cell with 36.1% conversion efficiency under AM1.5g spectral illumination. The new cell design presents an improvement over previous III‐V//Si triple‐junction cells by the implementation of a rear‐heterojunction for the middle cell. Furthe...

Low-cost approaches for mass production of III–V-based photovoltaics are highly desired today. For the first time, this work presents industrially relevant mask and plate for front metallization of III–V-based solar cells replacing expensive photolithography. Metal contacts are fabricated by nickel (Ni) electroplating directly onto the solar cell’s...

In this work an inverted metamorphic four junction (IMM4J) solar cell with 30.9% conversion efficiency in beginning of life conditions under the AM0 (1367 W/m ² ) spectrum is presented. Additionally, our newest improved IMM3J cell, consisting of Ga 0.51 In 0.49 P/GaAs/Ga 0.73 In 0.27 As subcells, with 30.6% efficiency is also shown. The IMM4J solar...

The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. Because state-of-the-art efficiencies of single-junction solar cells are approaching the Shockley-Queisser limit, the multi-junction (MJ) solar cells are very attractive for high-efficiency solar cells. This pa...

In this work we report on a procedure to increase the reproducibility of multi-junction solar cell growth. This is achieved by applying a systematic offset to room-temperature photoluminescence measurement to match the required band gap for obtaining current match as simulated by optical modelling. We found that the difference in band gap extracted...

With miniaturization of dimensions, new manufacturing methods e.g. from microelectronics may lead to cost reduction of CPV. In this work, we focus on the thermal, electrical and optical boundaries of the micro-CPV module technology with respect to cell and lens miniaturization. Open-circuit voltage losses due to recombination at the solar cell peri...

This article summarizes the IEC compliant power rating procedure applied to a hybrid CPV/PV module of the EyeCon technology which uses III-V four-junction CPV cells in combination with bifacial c-Si PV cells. The combined power output of both solar cell types at standard test conditions reached 342 W/m², which corresponds to a world record efficien...

The IEC standard 62670-3 covering the power rating of CPV modules has been published in 2017. However, this rating procedure has already been applied for the first time at Fraunhofer ISE in Germany in the year 2014. Since then it has been used many times in Freiburg for several different CPV modules from various manufacturers. Amongst them are modu...

Unassisted CO2 Reduction In article number 2201062, Bruce S. Brunschwig, Thomas Hannappel, Harry A. Atwater, and co‐workers report a monolithic integrated photovoltaic‐driven electrochemical device for unassisted CO2 reduction with 5.9% direct solar‐to‐CO conversion efficiency and 50 h stability. The prism designs featuring structures to redirect s...

Sprayed transparent conductive oxides (TCOs) are an interesting alternative to sputtered TCOs for many applications due to the possible high throughput and a simple, atmospheric pressure process of spray deposition. In this work, the growth mechanism of sprayed ZnO:In was analyzed by transmission Kikuchi diffraction (TKD) analysis of the thin film'...

Monolithic integrated photovoltaic‐driven electrochemical (PV‐EC) artificial photosynthesis is reported for unassisted CO2 reduction. The PV‐EC structures employ triple junction photoelectrodes with a front mounted semitransparent catalyst layer as a photocathode. The catalyst layer is comprised of an array of microscale triangular metallic prisms...

The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. As single-junction solar cells are limited to 30–32% conversion efficiency under 1-sun, multi-junction or tandem solar cells are expected to contribute to higher performances. The II–VI compound, chalcopyrite,...

Multijunction solar cells (MJSCs) have attracted attention as next-generation solar cells. In particular, GaAs//Si-based MJSCs are highly efficient with low cost and are expected to gain new applications, such as on-vehicle integrations. In this article, we examined a highly efficient In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xli...

Multijunction (MJ) solar cells have attracted attention as next-generation solar cells. In particular, III–V-based MJ solar cells connected with heterogeneous cells, such as GaAs//Si and GaAs//CIGSe (CuIn $_1−y$ Ga $_y$ Se $_2$ ), are expected to be highly efficient and low cost. In this article, we demonstrate a highly efficient InGaP/Al $_0.06...

III–V//Si multijunction solar cells offer a pathway to increase the power conversion efficiency beyond the fundamental Auger limit of silicon single‐junctions. In this work, we demonstrate how the efficiency of a two‐terminal wafer‐bonded III–V//Si triple‐junction solar cell is increased from 34.1 % to 35.9 % under an AM1.5g spectrum, by optimising...

We propose an easy and low-cost bonding method for III-V//Si tandem solar cells, based on conductive sol-gel-derived and hybrid materials. The design enables current matching and photon recycling. First prototypes have been demonstrated successfully.

This article describes the successful integration of a passivated, highly reflecting Ge rear side into a III-V multijunction solar cell. The use of lowly doped Ge and the new rear side leads to the aimed increase in Ge cell current up to 1.6 mA.cm−2, demonstrated by external quantum efficiency and I–V measurements. For the contact formation, two di...

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...

This article focuses on the material properties of two III-V semiconductors, AlGaAs and GaInAsP, and their usage as middle cell absorber materials in a wafer-bonded III-V//Si triple-junction solar cell. To this end single-junction solar cells were grown epitaxially lattice matched on GaAs wafers using metalorganic vapor phase epitaxy. By optimizing...

In order to realize a clean energy society by using renewable energies, high-performance solar cells are a very attractive proposition. The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. As state-of-the-art of single-junction solar cells are approaching the Sho...

III-V based solar cells have demonstrated highest conversion efficiencies. To enable further integration into the photovoltaic industry cost reduction is required. We investigate the processing of GaAs solar cells via patterned epitaxial lift-off and direct rear side plating. Fast lift-off times of 2:20 h for four-inch wafers with 8.0×8.5 mm² lift-...

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...

Multijunction (MJ) solar cells achieve high efficiencies by effectively utilizing the solar spectrum. Previously, we have developed III‐V MJ solar cells using smart stack technology, a mechanical stacking technology that uses a Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//CuxIn1−yGaySe2 three‐junction solar cell by applying...

With silicon-based tandem solar cells nearing commercialization, the significance of predicting the outdoor performance of tandem solar devices is growing. Although several models exist to calculate the outdoor performance, the availability of high-resolution irradiance data is limited. Therefore, we study the influence of the resolution of the irr...

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...

Hybrid concentrator/flat‐plate photovoltaic (CPV/flat‐plate PV) technology combines III–V multi‐junction and flat‐plate bifacial solar cells to convert direct, diffuse and rear irradiance into electricity. For the first time, this article presents a procedure to rate the power output of such modules at standard test and standard operating condition...

III‐V/Si multi‐junction solar cells are potential successors to the silicon single‐junction cell due to their efficiency potential of up to 40 % in the radiative limit [1]. In this work, latest results of epitaxially integrated GaInP/GaAs/Si triple‐junction cells are presented. To reduce parasitic absorption losses, which have limited the current d...

The growth of dilute nitrides with metal organic chemical vapor phase epitaxy (MOVPE) presents two major challenges: a low N-incorporation combined with a high impurity density of carbon and hydrogen. For the device implementation the full understanding of the growth of the metastable material and the influence of the parameters on the impurity lev...

Multijunction III–V/silicon photovoltaic cells (III–V/Si), which have achieved record conversion efficiencies, are now looking as a promising option to replace conventional silicon cells in future PV markets. As efforts to increase efficiency and reduce cost are gaining important traction, it is of equal importance to understand whether the manufac...

In this paper a method is presented to accurately and quickly interpolate a dataset of the complex refractive index of arbitrary compound semiconductors. The method is based on a parameter morphing algorithm which maps critical points of two endpoint materials with known optical parameter sets onto each other. Every desired intermediate material co...

Over the past decade, the global cumulative installed photovoltaic (PV) capacity has grown exponentially, reaching 591 GW in 2019. Rapid progress was driven in large part by improvements in solar cell and module efficiencies, reduction in manufacturing costs and the realization of levelized costs of electricity that are now generally less than othe...

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...

Recent developments have renewed the demand for solar cells with increased tolerance to radiation damage. To investigate the specific irradiation damage of 1 MeV electron irradiation in GaInAsP lattice matched to InP for varying In and P contents, a simulation based analysis is employed: by fitting the quantum efficiency and open-circuit voltage si...

The terrestrial photovoltaic market is dominated by single–junction silicon solar cell technology. However there is a fundamental efficiency limit at 29.4%. This is overcome by multi‐junction devices. Recently, a GaInP/GaAs//Si wafer bonded triple‐junction two–terminal device was presented with a 33.3% (AM1.5g) efficiency. Here it is analyzed how t...

Photovoltaic cells for conversion of wavelengths in the O-band around 1310 nm are developed. Excellent performance with an efficiency of 51.1% at elevated irradiance is demonstrated with an InGaAsP based cell grown lattice matched on InP. Under one sun irradiance a bandgap-voltage difference of only WOC=331 mV is achieved. In addition, metamorphic...

Photovoltaic cells for conversion of wavelengths around 1310 nm are developed. Excellent performance with an efficiency of 51.1% is demonstrated with a InGaAsP based cell grown lattice matched on InP. GaInAs based cells grown with a metamorphic approach on GaAs substrates are realized with impressive material quality similar to that of lattice matc...

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 achiev...

Recent developments have renewed the demand for solar cells with increased tolerance to radiation damage. To investigate the specific irradiation damage of 1 MeV electron irradiation in GaInAsP lattice matched to InP for varying In and P contents, a simulation based analysis is employed: by fitting the quantum efficiency and open-circuit voltage si...

The bulk minority carrier lifetime and interface recombination velocity (IRV) in GaInP double-heterostructures (DHs) lattice matched to GaAs are extracted using time-resolved photoluminescence (PL) measured between 300 and 500 K. Effective lifetimes show a strong dependence on temperature for samples with insufficiently strong confinement potential...

The highest solar cell efficiencies today are reached with four-junction devices under concentrated illumination. The optimal bandgap combination for realistic four-junction cells is modelled to be 1.89/1.42/1.05/0.68 eV and indeed promises for efficiencies >50%. We present the development and analysis of a wafer-bonded four-junction solar cell bas...

Manufacturing of III-V devices is a well-established technique but there is a strong demand to decrease production costs. MOVPE, as an industrial scale production method, suffers from long process times and high precursor costs. In this work, we focus on increasing the throughput of a close-coupled showerhead MOVPE reactor through higher growth rat...

Thin gallium phosphide layers were deposited on (0 0 1) Silicon surfaces via organometallic vapor phase epitaxy and characterized by electron channeling contrast imaging (ECCI). Stacking fault pyramids at a density of up to 6 × 10⁷ cm⁻² were identified in the GaP nucleation layer by varying the diffraction conditions. We show that these defects ori...

Conversion of direct, diffuse, and albedo irradiance into electricity is demonstrated with a new kind of hybrid bifacial high‐concentration photovoltaic module named bifacial EyeCon. It consists of Fresnel lenses that concentrate the direct sunlight 321x onto III‐V triple‐junction solar cells that are mounted on the front surface of p‐PERC bifacial...

Multijunction (MJ) solar cells achieve very high efficiencies by effectively utilizing the entire solar spectrum. Previously, we constructed a III‐V//Si MJ solar cell using the smart stack technology, a unique mechanical stacking technology with Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//Si three‐junction solar cell with a...

Multijunction solar cells with four junctions are expected to be the next-generation technology for both space and concentrator photovoltaic applications. Most commercial triple-junction solar cells are today grown on germanium, which also forms the bottom subcell. Extending this concept to four junctions with an additional ∼1-eV subcell was proven...

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that...

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...

A 4J CPV module achieving 41.4 % efficiency at CSTC is presented in this paper. The high efficiency is enabled through the combination of high quality achromatic full-glass lenses with high efficiency 4J solar cells. This module has been built to demonstrate the potential of CPV module technology when improving the efficiency of the optical element...

III‐V on Si multi‐junction solar cells have already exceeded the efficiency limit of Si single‐junction devices but are often challenged by expensive layer transfer techniques. Here, progress in the development of direct epitaxial growth for GaInP/GaAs/Si triple‐junction solar cells is reported. The III‐V absorbers with a total thickness of 4.9 µm...

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...

In this paper, we introduce a new kind of hybrid concentrator photovoltaic module that is capable of harvesting direct and diffuse irradiance. The concept, denominated EyeCon, uses a Fresnel lens to concentrate direct irradiance onto a primary III-V four-junction solar cell. Directly underneath, a large-area silicon cell is stacked and used as a se...

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...

We investigate the material properties of GaAsP graded buffers on both GaP and Si substrates in order to determine limitations to dislocation glide in GaAsP. Phase separation is not observed in these GaAsP buffers, but CuPtB atomic ordering is present, and has an impact on the Burgers vector distribution of gliding dislocations. Tellurium surfactan...

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...

III–V devices outperform all other solar cells in terms of efficiency. However, the manufacturing of these cells is expensive and prevents their use in a number of applications, which would benefit from the high efficiency. A major contribution to the cost is the metal–organic vapor phase epitaxy process for the III–V compounds. Increasing growth r...

This paper investigates the outdoor performance of a new kind of hybrid concentrator photovoltaic module. The concept which we name, EyeCon, uses an array of Fresnel lenses to concentrate direct irradiance onto III-V four-junction solar cells while a large-area silicon cell absorbs diffuse sunlight and also acts as a heat distributing substrate. Th...

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.

Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic...

The highest solar cell efficiencies today are reached with four-junction devices under concentrated illumination. One multi-junction cell concept with prospects of reaching highest conversion efficiencies of > 50 % is based on the materials Ga 0.50 In 0.50 P/GaAs/Ga 0.79 In 0.21 As//GaSb. The first processed cell demonstrated an efficiency of 29.1...

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.

The Hall Van-der-Pauw method is widely used to assess the electrical properties of GaSb based semiconductor layers. Semi-insulating GaSb substrates are not available, and therefore, Hall structures are generally grown on semi-insulating GaAs. The lattice mismatch of 7.8% between GaAs and GaSb results in high defect densities, which may influence th...

The development of photovoltaics as a serious means of producing renewable energy has accelerated greatly in the last ten years, with prices for silicon-based solar cell systems dropping dramatically in the last few years. The next great opportunity for photovoltaics following this competitiveness in prices will be to enhance the cell and panel eff...

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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...