David Lackner

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure.

Photosynthesis is natures route to convert intermittent solar irradiation into storable energy, while its use for an industrial energy supply is impaired by low efficiency. Artificial photosynthesis provides a promising alternative for efficient robust carbon-neutral renewable energy generation. The approach of direct hydrogen generation by photoelectrochemical water splitting utilizes customized tandem absorber structures to mimic the Z-scheme of natural photosynthesis. Here a combined chemical surface transformation of a tandem structure and catalyst deposition at ambient temperature yields photocurrents approaching the theoretical limit of the absorber and results in a solar-to-hydrogen efficiency of 14%. The potentiostatically assisted photoelectrode efficiency is 17%. Present benchmarks for integrated systems are clearly exceeded. Details of the in situ interface transformation, the electronic improvement and chemical passivation are presented. The surface functionalization procedure is widely applicable and can be precisely controlled, allowing further developments of high-efficiency robust hydrogen generators.

Monolithic Two-Terminal III–V//Si Triple-Junction Solar Cells With 30.2% Efficiency Under 1-Sun AM1.5g

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/AlxGa1-xAs//Si solar cells were fabricated using surface-activated direct wafer bonding. Metal-organic-vapor-phase-epitaxy-grown GaInP/AlxGa1-xAs top cells are bonded at low temperature to independently prepared wafer-based silicon cells. n-Si//n-GaAs interfaces were investigated and achieved bulk-like bond strength, high transparency, and conductivity homogeneously over 4-inch wafer area. We used transfer-matrix optical modeling to identify the best design options to reach current-matched two-terminal devices with different mid-cell bandgaps (1.42, 1.47, and 1.52 eV). Solar cells were fabricated accordingly and calibrated under AM1.5g 1-sun conditions. An improved Si back-side passivation process is presented, leading to a current density of 12.4 mA/cm2 (AM1.5g), measured for a flat Si cell below GaAs. The best 4 cm2 GaInP/GaAs//Si triple-junction cell reaches 30.2% 1-sun efficiency.

Gallium Phosphide Window Layer for Silicon Solar Cells

The integration of III-V compound semiconductors on a silicon bottom cell offers the opportunity to form two- and three-junction solar cells with a conversion efficiency exceeding 30%. This paper reports on the progress in the heteroepitaxial nucleation of gallium phosphide (GaP) on silicon, which allows the fabrication of a silicon bottom cell with front-surface passivation by a thin single-crystalline GaP window layer. GaP has a low lattice-mismatch to Si and an indirect bandgap energy of 2.26 eV, which leads to low absorption. At the same time, GaP can be doped with silicon to form an n-type contact layer. In this publication, we investigate n-Si/p-Si homojunction solar cells with a GaP window and contact layer. Metal-organic vapor phase epitaxy was used to deposit the 60-nm GaP window layer with a low density of antiphase boundaries at the heterointerface and without misfit dislocations. Open-circuit voltages of up to 634 mV have been obtained under 1-sun AM1.5g conditions for devices without antireflective coatings.

Four-Junction Wafer-Bonded Concentrator Solar Cells

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 mismatch. Three cell architectures are presented using the same two top junctions of GaInP/GaAs but different infrared absorbers based on Germanium, GaSb, or GaInAs on InP. The modeled efficiency potential at 500 suns is in the range of 49-54% for all three devices, but the highest efficiency is expected for the InP-based cell. An efficiency of 46% at 508 suns was already measured by AIST in Japan for a GaInP/GaAs//GaInAsP/GaInAs solar cell and represents the highest independently confirmed efficiency today. Solar cells on Ge and GaSb are in the development phase at Fraunhofer ISE, and the first demonstration of functional devices is presented in this paper.

Investigations on Al

Solar cells based on Al_xGa_1-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 Al_xGa_1-xAs solar cells with defect concentrations up to 1 × 10¹⁴ cm^-3 and less determined by deep level transient spectroscopy. However, for the n-doped material, inevitable DX-centers exist. These dopant-related defects limit the performance of the investigated Al_xGa_1-xAs solar cells with x ≥ 0.20. To overcome this issue, the n-doped Al_xGa_1-xAs As emitter can be replaced by an n-doped Ga_0.51In_0.49P heteroemitter. This heterojunction solar cell again shows overall defect concentrations below 1 × 10¹⁴ cm^-3 and significantly improved cell characteristics.

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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 corrected for dependencies of carrier concentration and photon recycling. This correction procedure is described and discussed in detail in order to determine the materials Shockley-Read-Hall lifetime as a function of excitation intensity. The procedure assumes 100% internal radiative efficiency under the highest injection conditions, and we show this leads to less than 0.5% uncertainty. The resulting GaAs material demonstrates a 5.7 ± 0.5 ns nonradiative lifetime across all samples of similar doping (2–3 × 10^(17) cm^(−3)) for an injected excess carrier concentration below 4 × 10^(12) cm^(−3). This increases considerably up to longer than 1 μs under high injection levels due to a trap saturation effect. The method is also shown to give insight into bulk and interface recombination.

Ga

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetim...

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III-V multijunction solar cells grown on a Si solar cell are an attractive approach to reduce the cost of high-efficiency solar cells. When using the Si wafer as an active solar cell, it is crucial to avoid degradation of the minority carrier lifetime in the Si during the metal-organic vapor phase epitaxy (MOVPE) process. After heating a Si wafer in a MOVPE reactor under a H2 atmosphere, we observed a strong degradation of its lifetime. By analyzing the annealed samples with photoluminescence and quasi-steady-state photoconductance, we found an iron contamination of up to 2 × 1012 cm-3. The measured iron concentration could be identified as the major source for the decrease of the minority carrier lifetimes of the Si wafers. By using diffusion barriers, we identified the graphite susceptor as the main source of the iron contamination. This knowledge offers pathways to preserve the minority carrier lifetime in the Si wafers during the MOVPE process.

As Solar Cells Grown by MOVPE

Hydrogen produced from solar energy has a high potential as a storage medium to buffer the fluctuations of renewable energy sources. The direct combination of concentrator photovoltaics with an electrolyzer has the capability to produce Hydrogen from sunlight at high efficiency. For this, the individual components have to be adjusted carefully and optimized with the final system in mind. This paper focuses on the solar cell development and shows first results of the combined module of a solar cell and an electrolyzer. The solar cell used for hydrogen production is a metamorphic GaInP/GaInAs dual-junction solar cell. This tandem cell reaches a maximum efficiency of 34.2% under concentration. The performance of the combined module shows a strong influence of temperature and DNI.

Nonradiative lifetime extraction using power-dependent relative photoluminescence of III-V semiconductor double-heterostructures

Photoelectrochemical solar fuel generation is evolving steadily towards devices mature for applications, driven by the development of efficient multi-junction devices. The crucial characteristics deciding over feasibility of an application are efficiency and stability. Benchmarking and reporting routines for these characteristics are, however, not yet on a level of standardisation as in the photovoltaic community, mainly due to the intricacies of the photoelectrochemical dimension. We discuss best practice considerations for benchmarking and propose an alternative efficiency definition that includes stability. Furthermore, we analyse the effects of spectral shaping and anti-reflection properties introduced by catalyst nanoparticles and their impact on design criteria for direct solar fuel generation in monolithic devices.