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Fast epitaxial growth is an attractive method for reducing III-V photovoltaic device cost. Here, we grow a high-performance, rear-junction GaAs solar cell at 1 µm/min via metal organic chemical vapor deposition. We integrate an epitaxial distributed Bragg reflector with the solar cell to increase the optical path length for photons near the band-edge, enhancing photon recycling without the need for epitaxial lift off. Reflectivity enhancement is evident near the band-edge, permitting up to 96% external quantum efficiency. Device results demonstrate significant performance boost, achieving 1.06 V open-circuit and an efficiency of 24.3% AM1.5G.

Micro-transfer printing has demonstrated value for device- and array-level heterogeneous integration in a variety of applications. Here, it is used instead to assemble unique substrates with the potential to enable III–V growth templates that are both low-cost and specified at any desired lattice parameter within the range of III–V materials. Critical to this, epitaxial growth crystal quality must be established as comparable to that produced on conventional epi-ready substrates. We demonstrate epitaxial regrowth on gallium arsenide membranes that were transfer printed from their source substrate and onto a target handle. Both morphology and luminescence properties were characterized. Comparing the transfer printed membrane epitaxy with epitaxy on a conventional substrate, we found recombination rates and spatial uniformity from power-dependent photoluminescence and cathodoluminescence, respectively, to yield similar results, suggesting material quality was not adversely limited by the engineered substrate.

We demonstrate a novel concept for III-V engineered substrate cost reduction that does not rely on chemical–mechanical polishing or low-throughput release layer etching. With the potential for significant reduction to III-V substrate cost, the details of this concept are provided, as well as a demonstration of III-V devices grown on a membrane removed from its source substrate and bonded to a non-native target handle. The GaAs solar cell on membrane one-sun and concentration AM1.5 efficiencies register 16.2% and 19.1%, respectively, prior to the application of anti-reflection coating. This is a discrepancy of 2% (one-sun) and 1.2% (concentration) absolute versus a nominally identical control GaAs device, indicating the capability for high performance at reduced cost.

The chapter discusses a full implementation of k·p theory for the eight bands near the energy gap in a III–V semiconductor. This model can precisely calculate the band structure for both conduction and valence bands near the energy gap. Even though only eight bands are explicitly included, the terms due to interactions with “remote” bands are present in the complete version of the theory. The chapter consider the physical meaning of the results and the effect that strain has on the semiconductor band structure. The Hamiltonians for both zinc-blende and wurtzite crystals are introduced.

Previous chapters discussed the crystal structure and bandstructure of III–V semiconductors. This chapter shifts to the book’s second major topic: electronic interactions with light. It introduces the main ideas about how light waves propagate in semiconductor crystals and induce absorption, spontaneous emission, and stimulated emission in bulk semiconductors. It also considers the differences between the electronic interactions with light in zinc-blende and wurtzite crystals and what happens as the energy gap of the semiconductor is reduced to zero or when the crystal is two-dimensional.

III–V semiconductors form crystalline structures with three-dimensional periodic arrangements of the atoms. In this chapter, we will explore the nature of the crystal lattice starting from lower dimensions and progressing to real semiconductor crystals. We also learn why we expect distinct energy bands to form in the solids that crystallize in such lattices. The carrier statistics and occupation of the bands will also be examined.

The so-called k·p theory provides an effective framework for modeling the band structures of III–V semiconductors. This approach starts with a handful of bands that are coupled in accordance with the crystal symmetry discussed in Chapter 1. This chapter describes the k·p theory and lays out the assumptions that underlie its most common versions, including Kane’s model. It also shows how the spin–orbit coupling affects the band structure of semiconductors.

Semiconductor quantum structures are at the core of many photonic devices such as lasers, photodetectors, solar cells etc. To appreciate why they are such a good fit to these devices, we must understand the basic features of their band structure and how they interact with incident light. This book takes the reader from the very basics of III-V semiconductors (some preparation in quantum mechanics and electromagnetism is helpful) and shows how seemingly obscure results such as detailed forms of the Hamiltonian, optical transition strengths, and recombination mechanisms follow. The reader does not need to consult other references to fully understand the material, although a few handpicked sources are listed for those who would like to deepen their knowledge further. Connections to the properties of novel materials such as graphene and transition metal dichalcogenides are pointed out, to help prepare the reader for contributing at the forefront of research. The book also supplies a complete, up-to-date database of the band parameters that enter into the calculations, along with tables of optical constants and interpolation schemes for alloys. From these foundations, the book goes on to derive the characteristics of photonic semiconductor devices (with a focus on the mid-infrared) using the same principles of building all concepts from the ground up, explaining all derivations in detail, giving quantitative examples, and laying out dimensional arguments whenever they can help the reader’s understanding. A substantial fraction of the material in this book has not appeared in print anywhere else, including journal publications.

There is a need for characterization of multi-junction solar cell based test coupons under collimated sunlight for both terrestrial and space applications. These applications include solar cell angle measurements and concentrator systems. The U.S. Naval Research Laboratory (NRL) has modified a 3-zone Spectrolab LAPSS (Large Area Pulsed Solar Simulator) to provide a tunable spectrum with a 1° collimation for testing of solar arrays. NRL has added filters and a complete measuring system capable of testing individual cells and small concentrator coupons requiring collimation comparable to the sun.

This work expands upon previous research related to transfer printed virtual substrates: a technique for generating low-cost engineered substrates that is not based on wafer reuse. Here, silicon and sapphire are evaluated as candidate low-cost handle materials, validated by metal organic chemical vapor deposition regrowth of GaAs single-junction solar cells. Resulting devices are studied via materials and electrical characterization techniques, including concentration measurements that indicate GaAs//sapphire TPVS open-circuit voltage at >10 suns comparable with identical devices grown on a prime GaAs substrate.

In this paper, we present findings on micro-concentrator photovoltaic cells composed of lattice matched subcells grown on GaAs and InP substrates, which are stacked into single, four-terminal devices using micro-transfer printing. The design, modeling, growth, fabrication and assembly of the devices will be described, and potential interconnection schemes to achieve efficient, two-terminal strings of cells with flexible current and voltage outputs and resilience to defects is discussed.

Optical concentration can improve the efficiency and reduce the cost of photovoltaic power but has traditionally been too bulky, massive, and unreliable for use in space. Here, we explore a new ultra-compact and low-mass microcell concentrating photovoltaic (µCPV) paradigm for space based on the monolithic integration of transfer-printed microscale solar cells and molded microconcentrator optics. We derive basic bounds on the compactness as a function of geometric concentration ratio and angular acceptance, and show that a simple reflective parabolic concentrator provides the best combination of specific power, angular acceptance, and overall fabrication simplicity. This architecture is simulated in detail and validated experimentally with a µCPV prototype that is less than 1.7 mm thick and operates with six, 650 µm square triple-junction microcells at a geometric concentration ratio of 18.4×. In outdoor testing, the system achieves a terrestrial power conversion efficiency of 25.8 ± 0.2% over a ±9.5° angular range, resulting in a specific power of approximately 111 W/kg. These results lay the groundwork for future space µCPV systems and establish a realistic path to exceed 350 W/kg specific power at >33% power conversion efficiency by scaling down to even smaller microcells.

Increasing epitaxial growth rate is an important path toward III–V solar cell cost reductions; however, photovoltaic device performance has been shown to degrade with increasing growth rate. In this study, gallium arsenide (GaAs) material has been deposited via metal–organic chemical vapor deposition (MOCVD) at growth rates varying between 14 and 60 μm/h. Deep-level transient spectroscopy is utilized to elucidate an exponential rise in EL2 trap density as a function of growth rate when all other growth conditions are held constant. Evidence is provided that this EL2 defect is responsible for limiting the Shockley–Read–Hall (SRH) lifetime of very high growth rate solar cells. The effect of growth temperature on devices at high growth rate is subsequently investigated as a strategy to reduce trap density and improve solar cell performance. From this investigation, EL2 trap density is suppressed, and single-junction on-substrate GaAs solar cells grown at 60 μm/h are reported with 1.01 V 1-sun open-circuit voltage and 23.8% AM1.5G efficiency.

We present molecular beam epitaxy growth of novel InAlAsSb lattice-matched to InP for use in an all lattice-matched triple-junction design. Given the unexplored nature of this material, we studied the effect of substrate temperature (T sub ) and group-V to group-III flux ratio (V/III) on surface morphology, Sb-incorporation, and peak photoluminescence (PL) energy. Decreased T sub and increased V/III ratio, i.e. reduced adatom mobility at the growth front, increased the peak PL energy, likely due to a reduction in phase separation.

GaAs is an attractive material for space solar cells due to it possessing the highest photovoltaic efficiency among semiconductors. However, material vulnerability to radiation damage is a limiting factor, which should be resolved. This can be done through the optimization of solar cell geometry. Specifically, ultra-thin GaAs solar cells possess increased radiation hardness in combination with high specific power (W/kg) making them desirable for space applications. In this research, modeling of single-junction thin GaAs based solar cells with an active device thickness < 1.6 µm is performed to evaluate cell performance at high irradiation levels (1 MeV electrons with a fluence of 1E15 e-/cm2) associated with high energy orbits. The radiation tolerance of two configurations is evaluated; conventional and rear junction (thick emitter) with a backside gold reflective coating. In addition, solar cell structures utilizing n- and p-type base are simulated. The Hovel model, modified to include the photon recycling effect, is used for modeling. The effect of radiation on GaAs electronic properties is counted through degradation of minority carrier lifetime and changes in the carrier concentration. It’s found that conventional n-emitter/p-base and the rear junction with p-type thick emitter configurations with a heavily doped absorber ~5E18 [cm-3] and thickness below 800 nm have increased radiation hardness with a degradation rate

Micro-concentrator photovoltaics (μ-CPV) may provide a cost effective means of producing terrestrial solar power with high area efficiency. By printing CPV onto a transparent substrate, a Si back panel can be used to capture the diffuse sunlight component. Models show that efficiencies above 30% can be achieved even under global normal irradiance (GNI) with a 40% diffuse component. Additionally, the power output of existing installations can be boosted by adding a transparent μ-CPV array on top.To further the development of this technology, we have created a model to analyze μ-CPV array reliability under non-ideal conditions. The adverse effects of partial shading on standard large area panels such as crystalline Silicon (c-Si) are well known, but little investigation has been done on the tolerance of the μ-CPV components. In this article a SPICE simulation is used to analyze the effects of partial shading on a μ-CPV module and evaluate various circuit layouts.

Transfer printed virtual substrates (TPVS) is a promising technique for drastic III-V substrate cost reduction. In this work, we grew a three-period stack of GaAs virtual substrates on a source wafer and transfer printed material from each individual period to a single handle sapphire substrate. We characterized metal organic chemical vapor deposition (MOCVD) GaAs solar cell regrowth performance on the printed virtual substrate. Device performance is consistent between the virtual substrate layers, however, the devices presently lag behind performance of identical solar cell structures on conventional substrates. This serves as a first report for numerous critical aspects of the TPVS concept: multi-layer source stacks, MOCVD regrowth, and device results on sapphire handle wafers.

We have developed multijunction solar cells on both GaAs and InP platforms to split the solar spectrum efficiently among subcells that maintain lattice-match to their respective source substrates. The GaAs-based dual-junction solar cell has achieved 31% AM1.5D concentrator efficiency. The InP-based triple junction solar cell has achieved 8.7% AM1.5D efficiency under concentration. Using micro-transfer printing, the wide-bandgap GaAs-based subcells will be stacked on top of the narrow-gap InP-based subcells and characterized to determine 4-terminal AM1.5D efficiency of the five-junction device. The microcells, less than 200 μm on a side, are assembled on transparent glass substrates, permitting diffuse light transmission to be collected by a silicon backplane. The approach facilitates efficient collection of both direct and diffuse sunlight for potential applications in a wide variety of climates. Major milestones in III-V device development are detailed.

In this paper, we report findings from a micro CPV module, employing 170 μm GaAs-based 2J CPV cells, assembled on glass substrates using micro-transfer printing. The CPV array uses all-glass lens arrays to focus the light with a geometric concentration ratio of 740 suns, and a bifacial, monocrystalline silicon solar cell behind the substrate to capture the diffuse component of the light. We found that the diffuse capture creates a significant performance boost over CPV alone, and study the role of bifacial capture on the overall performance. The highest combined efficiency with respect to global normal irradiance was 25.4% for the module, measured by outdoor testing in Washington, DC.

International audience; In this study, detailed temperature dependent simulations for absorption and photogenerated recombination of hot electrons are compared with experimental data for an InAs/AlAsSb multi-quantum well. The simulations describe the actual photoluminescence (PL) observations accurately; in particular, the room temperature e1-hh1 simulated transition energy of 805 meV closely matches the 798 meV transition energy of the experimental PL spectra, a difference of only 7 meV. Likewise, the expected energy separations between local maxima (p1-p2) in the simulated/experimental spectra have a difference of just 2 meV: a simulated energy separation of 31 meV compared to the experimental value of 33 meV. Utilizing a non equilibrium generalized Planck relation, a full spectrum fit enables individual carrier temperatures for both holes and electrons. This results in two very different carrier temperatures for holes and electrons: where the hole temperature, T-h, is nearly equal to the lattice temperature, T-L; while, the electron temperature, T-e, is 'hot' (i.e., T-e > T-L). Also, by fitting the experimental spectra via three different methods a 'hot' carrier temperature is associated with electrons only; all three methods yield similar 'hot' carrier temperatures.

Quaternary sputtering is a promising alternative to more established deposition methods for the fabrication of Cu(In,Ga)Se2 (CIGS) thin films for photovoltaics (PV). In this technique, a single sputtering target containing all four constituents is employed to deposit the CIGS film. Quaternary sputtering offers several advantages over other deposition methods, including excellent uniformity over large areas, high material usage, and less reliance on toxic Se precursors such as H2Se. Despite these advantages, several drawbacks remain. To date, devices fabricated by quaternary sputtering without additional selenization have been limited in efficiency to about 11%, and realizing bandgap grading in order to match the performance of the best evaporated devices presents a challenge. We discuss the prospects for quaternary sputtering as a fabrication technique for CIGS and highlight areas of research that may result in improved performance. Target fabrication and usage is reviewed. We also present results for films and devices including data for the optical constants of sputtered CIGS. Some recent previously unpublished results, including a study of impurities in CIGS sputtering targets and the first demonstration of a CIGS device on a flexible glass substrate, are discussed

InxAl1-xAs1-ySby is the only III-V material lattice-matched to InP with a direct bandgap energy range as large as ~1.45–1.80 eV, making it interesting for multiple optoelectronic applications. However, inherent material challenges in this immature quaternary alloy have thus far precluded the growth of InxAl1-xAs1-ySby with device-quality properties at the InP lattice constant. We have investigated how molecular beam epitaxy growth conditions affect the photoluminescence intensity, spectrum, temperature- and power-dependence, as well as the surface morphology of In0.23Al0.77As0.75Sb0.25, which is lattice-matched to InP with bandgap energy of ~1.68 eV as measured by ellipsometry. We find that reducing the adatom mobility, via a lower substrate temperature and higher group-V overpressure, diminishes the tendency for phase separation, but also quenches the photoluminescence intensity. These findings provide a step toward identifying an optimal growth window for realizing high-quality, wide-bandgap InxAl1-xAs1-ySby lattice-matched to InP.

Mechanically stacked tandem solar cells are a potential near-term solution for increasing the efficiency of photovoltaic modules. Practical implementation requires an interconnection approach that maximizes efficiency and minimizes complexity and cost. Connecting the top and bottom cells in a voltage-matched configuration allows two-terminal modules to be fabricated without altering the cell design or processing methods. Here, we experimentally demonstrate two-terminal voltage-matched GaInP2 / Si minimodules. The two-terminal minimodules performed just as well as four terminal configurations when voltage-matching requirements were met. The magnitude of the efficiency loss experienced by the voltage-matched minimodule when voltage-matched conditions were not met depends on whether the voltage was constrained by the GaInP2 or Si cells. Monte Carlo simulations also indicate that the two-terminal voltage-matched tandems respond to small cell-to-cell parameter variations in a similar manner as four terminal tandems.

Virtual substrates formed by integrating high quality, micro-scale III-V epitaxial template layers with cheap handle materials using transfer printing offer a pathway to very low cost III-V growth substrates for microscale device epitaxy. However, understanding the impact of coefficient of thermal expansion (CTE) mismatch on the robustness of the bond between the handle and the epitaxially grown materials is very important for potential commercialization of virtual substrate technologies. Materials such as silicon, quartz, sapphire, and alumina ceramic offer varying CTE mismatch values and are cost effective as targets for virtual substrate handles. In this paper we describe two separate experiments that examine the reliability of the bond interface between GaAs and non-native substrates using a statistical analysis of near-IR microscope images to compare void formation before and after thermal cycling. First, multijunction GaAs-based solar cells were printed to a silicon substrate and cycled between 50-250° C. The considerable CTE mismatch existing between silicon and III-V cell materials caused problematic delamination of printed materials with existing voids. Second, unprocessed GaAs chiplets were printed to sapphire and cycled between 65-270°C. No visible delamination occurred after 20 cycles, indicating no degradation in bond quality. Successful cycling at higher temperatures, such as those observed during growth, would further indicate that transfer printed virtual substrates on low-cost handles are a suitable candidate for regrowth experiments.

Hot-carrier solar cells require absorber materials with restricted carrier thermalization pathways, in order to slow the rate of heat energy dissipation from the carrier population to the lattice, relative to the rate of carrier extraction. Absorber suitability can be characterized in terms of carrier thermalization coefficient (Q). Materials with lower Q generate steady-state hot-carrier populations at lower levels of incident solar power and, therefore, are better able to perform as hot-carrier absorbers. In this study, we evaluate Q = 2.5±0.2 W·K -1 · cm -2 for a In 0.52 AlAs/In 0.53 GaAs single-quantum-well(QW) heterostructure using photoluminescence spectroscopy. This is the lowest experimentally determined Q value for any material system studied to date. Hot-carrier solar cell simulations, using this material as an absorber yield efficiency ~39% at 2000X, which corresponds to a >5% enhancement over an equivalent single-junction thermal equilibrium device.

An InGaAsP quantum well with a type-II band alignment is studied using continuous wave power and temperature dependent photoluminescence (PL) spectroscopy. The small energy separation between the ground state and first excited state results in significant thermal carrier redistribution and excited state occupation, particularly, with increasing excitation power and temperature. This state filling is evident as a high-energy shoulder in the PL spectra, the same energy region where in the simplest Planck-description the gradient is considered inversely proportional to carrier temperature. The outcome of an excited state occupation in broadening the high-energy PL tail is to perturb the temperature extracted using this analysis; therefore, the true temperature of carriers is not properly evaluated when significant state filling occurs. In addition, broadening of the PL due to phonons at higher temperatures also distorts (or falsely increases) the non-equilibrium “hot” carrier temperature and as such should be considered when using Planck's relation. The role of these two effects is considered and their mutual effect on the analysis of the extracted hot carrier temperature discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Tunnel junctions with low differential resistance and high transparency have been grown by molecular beam epitaxy on GaSb substrates. The resulting devices have been characterized and analyzed using a combination of electrical measurements and modeling. These devices have importance to multi-junction solar cells grown on GaSb substrates. The structures contain an n-type InAs quantum well embedded in a GaSb p/n junction, exploiting the high tunnel probability at the broken gap interface between p-type GaSb and n-type InAs, whilst having a minimal impact on the transparency of the device.

Simulations were performed with LT-SPICE™ to evaluate the limiting effects of cell size, perimeter recombination, and sheet resistance on the performance of a solar cell under 500 sun light concentration. A circuit model was generated using parameters extracted from a 3 junction InGaP/GaAs/InGaAsN device and simulated with various contact configurations. The results of the simulation were then used to determine the optimal cell size for a gridless cell and the conditions under which a gridless cell is more efficient than a cell with grid fingers.

Mechanically-stacked tandem solar cells combining existing photovoltaic technologies present a straightforward path toward higher conversion efficiencies. Voltage-matched configurations have been proposed as a simplified approach for fabricating two terminal modules, but they are subject to performance losses compared to four terminal configurations if voltage-matching conditions are not met. Here, we demonstrate a two terminal voltage-matched GalnP 2 /Si tandem with 23 % efficiency. Through a combination of experimental measurement and simulation, we explore the impact of voltage and fill factor mismatches between the top and bottom sub-cell strings. Our results show that efficiency losses are minimized when voltage-mismatches are accommodated by designing the bottom strings to have a higher voltage. Fill factor mismatches due to differences in series resistance also lead to changes in the optimal ratio of the number of cells in the top and bottom sub-cell strings.

Transfer printing is an emerging technology for the mechanical stacking of III-V multijunction solar cells, yielding ultra high efficiency devices under concentration while avoiding existing epitaxial growth limitations. This commercial technique allows for energy harvesting of the entire solar spectrum through combinations of devices grown on GaAs, InP and GaSb substrates. In this paper, individual single junction materials outlined as transfer printing candidates in an InP-based tandem were investigated to optimize a fabrication process and to study electrical device performance. InGaAsP single junction solar cells grown by MOCVD at compositions of In 0.9 Ga 0.1 As 0.21 P 0.79 (corresponding to a 1.2 eV band gap) and In 0.76 Ga 0.24 As 0.52 P 0.48 (1.0 eV band gap) were demonstrated. Process optimization, including particular mesa isolation challenges, and device characterization are discussed.

To evaluate the effects of high temperature terrestrial ambient conditions on InGaP films for use in PV devices, InGaP with a bandgap of 1.87 eV was grown on GaAs by MOVPE. The films were annealed in ambient conditions at 400°C and 450°C for a cumulative duration of 16 hours, respectively. At intervals of 1, 6, and 16 hours of annealing, VASE measurements were taken to determine the composition and thickness of the developing oxide layer. The oxide was found to be like GaP-oxide in nature, with parallel growth rates over 16 hours of annealing time. This trend was independent of temperature. Final oxide thickness was dependent on annealing temperature, with InGaP annealed at 450° C having a thicker oxide. Surface roughness also increased after annealing, as shown by AFM.

We propose a novel methodology for III-V substrate cost reduction that does not rely on chemical-mechanical polishing (CMP) or low-throughput release layer etching. The details of this process are provided, and results of an economic model for GaAs and InP virtual substrates dictate 20x and 34x reductions to substrate cost, respectively. The impact of modeled assumptions are quantified in order to better understand the potential range of end-goal virtual substrate cost.

In this paper, we report a novel heterojunction solar cell based on sintered CdTe and CdSe nanocrystal thin films using solution-based deposition. For the absorber layer, CdTe thin films were made using a layer-by-layer deposition process consisting of spin coating colloidal CdTe nanocrystals followed by a sintering step. The optical constants for these sintered CdTe films were accurately determined through a combination of optical modeling and measurements. For the all-solution processed p–n heterojunction, we focus on CdSe nanocrystal thin films due to their excellent compatibility with solution processing. For the optimized inverted structure (glass/ITO/CdSe/CdTe/Cr/Au), a high open-circuit voltage $(V_{\rm oc})$ of 593 ± 32 mV with an efficiency of 1.9 ± 0.2% was obtained under simulated one sun illumination. These preliminary results demonstrate that this novel inverted heterojunction structure has the potential to produce a high-quality CdSe/CdTe junction for utilization in heterojunction solar cells.

In this paper, we present temperature-dependent current–voltage measurements of tunnel junctions lattice matched to InP at temperatures ranging from room temperature to 220 °C. Temperature-dependent tunneling properties were extracted by fitting the current–voltage characteristics using a simple analytical formula. Three different designs of tunnel junction were characterized, including a bulk InAlGaAs tunnel junction, an InAlGaAs tunnel junction with InAlAs cladding layers and an InGaAs/InAlGaAs quantum-well tunnel junction. Each device exhibited different temperature dependence in peak tunnel current and excess current, with the quantum-well tunnel junction exhibiting the greatest temperature sensitivity. We use a non-local tunneling model, in conjunction with a numerical drift-diffusion solver, to explain the performance improvement available by using double heterostructure cladding layers around the junction region, and use the same model to explain the observed temperature dependence of the devices. Copyright © 2014 John Wiley & Sons, Ltd.

In this paper we present detailed balance simulations which determine the material parameters required to produce hot carrier solar cell (HCSC) annual energy yields comparable with that of multi-junction (MJ) systems. We demonstrate that HCSCs are less spectrally sensitive than equivalent MJ devices providing significant motivation for pursuing their development. Spectral variation in a given location over the course of the day and throughout the year means that the HCSC provides more consistent power production. The HCSC can also be developed for a standard reference spectrum and still perform optimally in a variety of locations with different atmospheric conditions, unlike the location sensitive performance of MJ devices. We show that an ideal hot carrier solar cell with bandgap 0.69 eV under 2000× concentration would require a thermalization coefficient 0.1 W K − 1 cm − 2 to produce more power over the course of the year than an InGaP/GaAs/Ge triple junction device located at Solar Village in Saudi Arabia. The lowest experimentally demonstrated thermalization coefficient is 9.5 W K − 1 cm − 2 indicating that further materials development is required to achieve this target.

In this paper, a detailed analysis on the radiation response of solar cells with multi quantum wells (MQW) included in the quasi-intrinsic region between the emitter and the base layer is presented. While the primary source of radiation damage of photovoltaic devices is minority carrier lifetime reduction, we found that in the case of MQW devices, carrier removal (CR) effects are also observed. Experimental measurements and numerical simulations reveal that with increasing radiation dose, CR can cause the initially quasi-intrinsic background doping of the MQW region to become specifically n- or p-type. This can result in a significant narrowing and even the collapse of the electric field between the emitter and the base where the MQWs are located. The implications of the CR-induced modification of the electric field on the current-voltage characteristics and on the collection efficiency of carriers generated within the emitter, the MQW region, and the base are discussed for different radiation dose conditions. This paper concludes with a discussion of improved radiation hard MQW device designs.

© 2016, National Academy of Sciences. All rights reserved. Emerging classes ofconcentrator photovoltaic (CPV) modules reach efficiencies that are far greater than those of even the highest performance flat-plate PV technologies, with architectures that have the potential to provide the lowest cost of energy in locations with high direct normal irradiance (DNI). A disadvantage is their inability to effectively use diffuse sunlight, thereby constraining widespread geographic deployment and limiting performance even under the most favorable DNI conditions. This study introduces a module design that integrates capabilities in flat-plate PV directly with the most sophisticated CPV technologies, for capture of both direct and diffuse sunlight, thereby achieving efficiency in PV conversion of the global solar radiation. Specific examples of this scheme exploit commodity silicon (Si) cells integrated with two different CPV module designs, where they capture light that is not efficiently directed by the concentrator optics onto large-scale arrays of miniature multijunction (MJ) solar cells that use advanced III-V semiconductor technologies. In this CPV+scheme ("+" denotes the addition of diffuse collector), the Si and MJ cells operate independently on indirect and direct solar radiation, respectively. On-sun experimental studies of CPV+modules at latitudes of 35.9886° N (Durham, NC), 40.1125° N (Bondville, IL), and 38.9072° N (Washington, DC) show improvements in absolute module efficiencies of between 1.02% and 8.45% over values obtained using otherwise similar CPV modules, depending on weather conditions. These concepts have the potential to expand the geographic reach and improve the cost-effectiveness of the highest efficiency forms of PV power generation.

One advantage of microconcentrator photovoltaics (μ-CPV) is the range of possible circuit connection arrangements. Large numbers of cells can be connected in combinations of series and parallel strings, which can provide a means to protect the module against performance loss due to cell failure, cell shading, and other variations in cell parameters that may arise during the fabrication and wiring processes. We introduce a model to evaluate the best strategy for optimizing module reliability through circuit layout using a combination of series and parallel connected cell strings. The model uses realistic solar cell device parameters in an electrical network simulation, coupled with a Monte Carlo method to introduce defects into the array with different probabilities. We use the model to optimize the layout of a μ-CPV panel designed to be integrated with the conventional 60-cell crystalline silicon panels for diffuse light capture to produce a hybrid direct/diffuse CPV product.

In this paper we extend the analytical drift-diffusion model, or Hovel model, to model the electrical characteristics of solar cells incorporating a back mirror. We use a compact summation approach to derive modified optical generation functions in Homojunction solar cells, considering both coherent and incoherent reflections from the back reflector. These modified generation functions are then used to derive analytical formulae for the current-voltage characteristics of mirrored solar cells. We simulate the quantum efficiency of a simple GaAs np diode with a planar gold back reflector, and compare the results with the standard Hovel model using a generation function given by the Beer-Lambert law. Finally, we use the model to simulate the performance of a real GaAs solar cell device fabricated using an epitaxial-lift-off procedure, demonstrating excellent agreement between the simulated and measured characteristics.

We have developed an InGaP solar cell structure capable of operating at 450°C under N 2 ambient. This structure has been annealed for over 70h without degradation in room temperature performance. This type of structure has applications in hybrid solar energy plants which combine photovoltaic and thermal collection systems to maximize overall conversion efficiency. We anticipate that this device will be able to achieve up to 17% efficiency at 400°C and 500x concentration based on simulations with incorporated optical and electrical high-temperature semiconductor parameters.

The application of concentrating photovoltaics (CPV) for space applications has been pursued in the past. Recent trends toward lower launch cost into higher radiation orbits make the merits of CPV more persuasive. This paper presents preliminary work toward the development of CPV designs for space that are based on microcells with up to 6 junctions. Some basic microcell CPV design concepts are presented along with preliminary cell design which leverages microtransfer printing to enable 6 junctions. The benefits of microcells are explored with results in thermal modeling, performance in LILT conditions and radiation hardness. Experimental results for prototype designs are presented.

Transfer printing is a formidable technique to stack multijunction solar cells while avoiding existing epitaxial limitations. Thus, a larger portion of the solar spectrum can be harvested through careful design optimization. In this process, metal contacts are embedded within a highly conductive lateral conduction layer (LCL) on the lower subcells. A readily available low bandgap candidate for a multijunction stack includes a GaSb solar cell with a 1.0 eV Al 0.2 Ga 0.8 Sb LCL. Processing of a bare AlGaSb layer is difficult, as standard photolithography processes and wet etch chemistries leave the surface excessively roughened and inhibit good ohmic contact. Several dry etch recipes are investigated to etch the AlGaSb LCL while leaving a smooth surface. A recipe based on a gas mixture of BCl 3 /Ar was determined to be a viable candidate for dry etching of AlGaSb and used to fabricate a GaSb solar cell with recessed contacts in an AlGaSb LCL.

Transfer printing is a uniquely enabling technology for the heterogeneous integration of III-V materials grown on dissimilar substrates. In this paper, we present experimental results for a mechanically stacked tandem cell using GaAs and GaSb-based materials capable of harvesting the entire solar spectrum with 44.5% efficiency. We also present the latest results toward developing an ultra-high performance heterogeneous cell, integrating materials grown on GaAs, InP and GaSb platforms.

We have demonstrated for the first time a proof of concept four junction solar cell grown on an InP substrate. The split top cell design uses two 1.45 eV InAlAs subcells in series to take advantage of the large portion of the solar spectrum with energies higher than this bandgap. The prototype cell has an open circuit voltage of 2.44 V and short circuit current density of 1.8 mA/cm2. Further optimization may allow this cell to have performance comparable with high quality triple-junction GaAs cells. This architecture may be useful in other systems where high quality wide bandgaps are not available or in materials systems where carrier collection is strongly limited by the diffusion length within the material.

The balance of photogeneration and recombination gives rise to an optimum bandgap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and, therefore, places an upper limit on the voltage that can be attained. Introducing additional nonradiative recombination results in a loss in voltage that can only be compensated for by moving to higher bandgaps. Consequently, the optimal bandgap for solar energy conversion will rise with increasing nonradiative recombination rate. This balance was recognized by Shockley and Queisser for single-junction solar cells and is here extended to multijunction solar cells. A rise in optimal bandgaps has been observed in simulated single-, double-, and triple-junction devices as nonradiative recombination increases. Optimal bandgaps between excellent and poor diode quality devices are shown to differ by 100s of meV under 1-sun illumination with both terrestrial and extraterrestrial spectra but exhibit no significant change at high concentration due to the dominance of the radiative component in the recombination dynamics.