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For each geographical region, one of the biggest challenges in reaching a zero-carbon grid is identifying sources of electricity that match the seasonal profile of the load. Summer-dominant solar electricity generation can often be balanced by winter-dominant wind electricity generation. Together with long-duration storage, balanced solar and wind generation are well positioned to provide reliable renewable electricity. However, in some locations the wind may not complement solar energy so well. For example, currently California's wind turbines produce more electricity during summer than winter, raising the question of whether all future wind plants in California will exhibit the same seasonality. As a response to this question, in this paper, we analyzed the generation from existing California wind plants and simulated potential onshore wind resource for the whole state using a metric that reflects the relative wind resource in winter. Our results indicate that the seasonality of the wind can vary for very small spatial difference with more than half of California showing stronger wind resource in the winter compared with the summer despite the current observation of the opposite trend. This study differentiates the seasonality of potential wind resources to inform the creation of a reliable, 100%-renewable-driven grid.

The standardization of VIPV (Vehicle-integrated photovoltaic) has been driven by the international discussion among scientists and engineers, from photovoltaic research, photovoltaic industries, automobile and other industries, and testing laboratories. Since the VIPV is not flat and fixed installation, as well as a wide variety of designs, the knowledge of CPV (Concentrator Photovoltaic) technology, has been useful for developing testing technologies. This paper took two examples in the performance testing and the test for the tolerance for the partial/dynamic shading, utilizing various techniques exclusively used in CPV technologies.

When grown by metalorganic chemical vapor deposition (MOCVD), nominally undoped GaInNAs is commonly observed to have an acceptor concentration of ∼1017 cm−3. However, after annealing in the MOCVD reactor at a temperature of 650 °C, p-type GaInNAs sometimes converts to n type with an electron concentration of ∼1017 cm−3. This n-type material has a slightly higher electron mobility (∼400–450 cm2/V s) than has usually been reported for 1 eV GaInNAs. Secondary ion mass spectroscopy shows significant hydrogen and some carbon contamination of these layers. The type conversion is correlated with both the nitrogen and hydrogen concentration and is relatively insensitive to the choice of growth precursors (trimethylgallium versus triethylgallium, or unsymmetric dimethylhydrazine versus nitrogen trifluoride). The data are consistent with theoretical predictions that the donors arise from a N–H complex. Annealing in the absence of hydrogen reduces the background acceptor concentration, but does not produce an electr...

High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edge...

Photomodulation spectroscopy studies of the transitions at the Γ point of the Brillouin zone of thin films of BxGa1−xAs alloys grown by metal-organic chemical-vapor deposition are presented. A very small increase of the fundamental band gap is found in samples with B content up to 3%. Under hydrostatic pressure, the band gap increases at a rate almost identical to the pressure dependence of the GaAs band gap. In contrast to the case of N incorporation at similar concentrations into GaAs, the experimental results show that B incorporation does not cause large modifications of the conduction-band structure in BxGa1−xAs alloys.

GaAs1−xNx with x=0.2% is grown by metal–organic chemical vapor deposition with growth rates between 2 and 7 μm/h and with two gallium sources. The GaAsN grown with trimethylgallium at high growth rates shows increased carbon contamination (>1017 cm−3), low photoluminescent lifetimes (∼0.2 ns), and high background acceptor concentrations (>1017 cm−3). The GaAsN is improved if it is grown with a lower growth rate or if triethylgallium is used, resulting in lower carbon contamination (∼1016 cm−3), longer photoluminescent lifetimes (2–9 ns), and slightly lower background acceptor concentrations (

The band gap and microstructure of Ga0.5In0.5P have been shown to vary with deposition conditions. However, growth on (511)B GaAs substrates has been reported to give Ga0.5In0.5P with band gaps close to that of disordered material. It is shown here, that with appropriate selection of the growth parameters, Ga0.5In0.5P can be grown with low band gap and significant ordering on even the (511)B substrates, implying that surface steps play an important role in the ordering process. For the lattice‐matched composition, a band gap of 1.83 eV was obtained using low growth temperature (575 °C), low growth rate (0.55 μm/h), and high phosphine pressure (5 Torr).

The present work aimed at assessing and comparing the thermal performances of two different types of cell carriers exposed to natural sunlight beams concentrated up to 1,500–4,500 suns. Metallic and hybrid metal‐ceramic carriers of various dimensions, or bonded to cells of different sizes, were considered. Temperature profiles inside the carriers exposed to concentrated beams were measured using temperature sensors placed at two different locations. 3D heat transfer simulations of a carrier bonded either to the real Ge‐based solar cell or to the dummy cell instrumented for our temperature measurements showed that the measured temperatures differed by less than a couple of degrees from the real solar cell surface temperatures within a large range of concentration. Experimental results and thermal simulations confirmed the need to select a high‐conductivity carrier combined with a very efficient active device for cooling the solar cells under very high concentration. In addition, the key role played by thermal constriction in the heat transfer process was highlighted, demonstrating the importance of carefully optimizing the carrier design.

The electroreflectance spectrum of spontaneously ordered Ga0.5In0.5P contains two extra features at energies near 2.2 and 2.35 eV. The energies and intensities of these transitions are shown to be dependent on growth temperature and substrate misorientation. The possible origins of the two transitions are explored. The analysis shows that the transition at about 2.35 eV exhibits shifts consistent with those expected for Ga0.5In0.5P with ordered {111} planes. The second transition is shown to be unexpected and is not conclusively assigned. However, a possible assignment is identified, implying that a complete understanding of spontaneously ordered Ga0.5In0.5P may require simultaneous consideration of two different types of order.

Ordering in the CuPt structure is known to significantly reduce the band gap of Ga0.52In0.48P as well as induce a number of unusual details in its optical properties, including long, excitation‐intensity‐dependent lifetimes and an excitation‐intensity‐dependent emission energy. We report photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman measurements performed on ordered and disordered Ga0.52In0.48P. The dominant high energy emission process at low temperature in disordered Ga0.52In0.48P is established to be excitonic, but the exciton trapping energy is not unique. PLE from ordered Ga0.52In0.48P shows significant tailing of electronic states into the band gap and a ‘‘band edge’’ which depends on detection energy. The dominant radiative process in ordered Ga0.52In0.48P is not excitonic. A large increase in the Stokes shift between the absorption edge (band gap) and PL emission peak occurs when the material orders. Hence, low temperature PL is determined to be a particularly poor measure of band gap. Resonant Raman scattering is used to study optical phonons and their coupling to electronic states. We find that the resonance enhancement at the band edge occurs via localized excitons.

The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at ∼470 cm−1 (Ga–N stretch) and two or three bands at ∼3100 cm−1 (N–H stretch). The change in the Ga–N stretch absorption can be explained if the nitrogen environment is converted from NGa4 to NInGa3 after annealing. The N–H stretch is also changed after annealing, implying a second, and unrelated, structural change.

We report the epitaxial growth of zinc-blende BxGa1−x−yInyAs and BxGa1−xAs on GaAs substrates with boron concentrations (x) up to 2%–4% by atmospheric-pressure metalorganic chemical vapor deposition. The band gap of BxGa1−xAs increases by only 4–8 meV/%B with increasing boron concentration in this concentration range. We demonstrate an epitaxial BxGa1−x−yInyAs layer deposited on GaAs with a band gap of 1.34 eV that is significantly less strained than a corresponding Ga1−yInyAs layer with the same band gap.

Electrical properties of unintentionally doped p-type Ga0.95In0.05N0.013As0.987 quaternary alloys grown by metal–organic vapor-phase epitaxy have been investigated by Hall-effect and photoconductivity measurements. Persistent photoconductivity (PPC) has been observed in this material at temperatures T

We show that incorporation of nitrogen in Ga1−xInxAs to form Ga1−xInxNyAs1−y alloys leads to a splitting of the conduction band into two nonparabolic subbands. The splitting can be described in terms of an anticrossing interaction between a narrow band of localized nitrogen states and the extended conduction-band states of the semiconductor matrix. The downward shift of the lower subband edge accounts for the N-induced reduction of the fundamental band-gap energy. An analysis of the relationship between the subband splitting and the band-gap reduction demonstrates that the energetic location of the valence band is nearly independent of the N content in Ga1−xInxNyAs1−y alloys.

Theoretical efficiencies are calculated for two‐junction, series‐connected solar cells using air mass 1.5 global and direct irradiance spectra. For band‐gap combinations previously limited by a low bottom‐cell current, thinning of the top cell is shown to result in significant increases in the theoretical efficiencies. The increases are primarily due to increased short‐circuit currents, since current matching is achievable. Smaller gains are also seen in the open‐circuit voltages of the thinner cells when a low surface‐recombination velocity is assumed. Thus, a number of material combinations which previously could only be used in four‐terminal configurations can now be considered for series‐connected two‐junction solar cells.

The phenomenon of ordering in Ga0.52In0.48P is well known to reduce the optical band gap; the amount of band gap reduction is often used to measure the degree of ordering. For such measurements to be meaningful, the band gap of the random (‘‘completely disordered’’) binary alloy must be known. Values of this fundamental material parameter appearing in the literature vary by up to 40 meV, while the largest band gap reduction reported to date is only about 120 meV, i.e., within a factor of 3 of the uncertainty in one endpoint. We report here a low temperature band gap of 2.010±0.007 eV for material lattice matched to GaAs as deduced from a broad spectrum of samples believed for different reasons to contain minimal ordering. The corresponding value at 295 K is 1.910±0.008 eV.

We report on multijunction GaInP/GaAs photovoltaic cells with efficiencies of 29.5% at 1‐sun concentration and air mass (AM) 1.5 global and 25.7% 1‐sun, AM0. These values represent the highest efficiencies achieved by any solar cell under these illumination conditions. Three key areas in this technology are identified and discussed; the grid design, front surface passivation of the top cell, and bottom surface passivation of both cells. Aspects of cell design related to its operation under concentration are also discussed.

It has been shown that under certain growth conditions the pseudobinary semiconductor alloy GaInP shows cation site ordering into the Cu‐Pt structure, and that this ordering results in a lowering of the band gap Eg from that of the disordered alloy. The Eg lowering is known to depend on growth conditions, including the orientation of the substrate. We study the dependence of Eg on epilayer thickness for GaInP grown by metal‐organic vapor‐phase epitaxy. For epilayers grown on singular (100) substrates under growth conditions conventionally used to produce ordered material, Eg decreases dramatically with increasing epilayer thickness: Eg for a 10‐μm‐thick epilayer is ∼40 meV lower than for a 1‐μm‐thick epilayer. This dependence of Eg on thickness can be understood in terms of the recently observed faceting of the GaInP growth surface.

We report the first observation of band‐gap energy reduction in Ga0.47In0.53As deposited on (100) InP by atmospheric pressure organometallic vapor phase epitaxy due to CuPt‐type ordering. A reduction of more than 65 meV in the band‐gap energy is observed for lattice‐matched samples that show strong CuPt‐like ordering by transmission electron microscopy. By comparison samples that show no CuPt‐like ordering diffraction signatures, do not have reduced band‐gap energies. Studies of the influence of growth parameters on the band‐gap energy indicate a U‐shaped dependence on the growth temperature with a minimum around 550 °C and decreasing band‐gap energies with increasing growth rate (at a constant V/III ratio) over the range 0.5–4 μm/h.

Photon management has been shown to be a fruitful way to boost the open circuit voltage and efficiency of high quality solar cells. Metal or low-index dielectric-based back reflectors can be used to confine the reemitted photons and enhance photon recycling. Gaining access to the back of the solar cell for placing these reflectors implies having to remove the substrate, with the associated added complexity to the solar cell manufacturing. In this work, we analyze the effectiveness of a single-layer reflector placed at the back of on-substrate solar cells, and assess the photon recycling improvement as a function of the refractive index of this layer. Al2O3-based reflectors, created by lateral oxidation of an AlAs layer, are identified as a feasible choice for on-substrate solar cells, which can produce a Voc increase of around 65% of the maximum increase attainable with an ideal reflector. The experimental results obtained using prototype GaAs cell structures show a greater than two-fold increase in the external radiative efficiency and a Voc increase of ∼2% (∼18 mV), consistent with theoretical calculations. For GaAs cells with higher internal luminescence, this Voc boost is calculated to be up to 4% relative (36 mV), which directly translates into at least 4% higher relative efficiency.

The band gap of Ga0.5In0.5P is reported as a function of growth rate and growth temperature. The Ga0.5In0.5P is grown lattice matched to 2°‐off (100) GaAs substrates by atmospheric pressure organometallic chemical vapor deposition using an inlet group V/III ratio of 65. The variation of the band gap is surprisingly complex, taking five different functional forms within the two‐dimensional parameter space. These include regions in which the band gap (1) increases with growth rate, (2) decreases with growth rate, (3) is independent of both growth rate and temperature, (4) is independent of growth rate, but dependent on growth temperature, and (5) is not measurable since three‐dimensional, instead of two‐dimensional, growth is observed. The behavior can only be explained by a theory involving competing processes. One such theory is described.

A two‐terminal, monolithic cascade solar cell with an efficiency of 27.3% is reported. The device structure consists of a Ga0.5In0.5P homojunction grown epitaxially upon a GaAs homojunction, with a GaAs tunnel diode interconnect. The tandem combination of these two materials is lattice matched, and has a theoretical efficiency of 34%. The device was grown by metalorganic chemical vapor deposition at 700 °C, using trimethylgallium, trimethylindium, arsine, and phosphine as sources. The minority‐carrier transport properties of the Ga0.5In0.5P are shown to be relatively insensitive to variations of the growth temperature and phosphine overpressure. Other factors that affect the efficiency of the device are presented and discussed.

The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by th...

We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.

The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1sun global spectrum, space spectrum, and concentrated direct spectrum at 81suns, respectively. The device consists of 1.8eV Ga0.5In0.5P, 1.4eV GaAs, and 1.0eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural “handle.” The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95V at 1sun.

The thermal annealing of nitrogen related traps in p-type InGaAsN and GaAsN is investigated by deep level transient spectroscopy (DLTS). Upon annealing, an apparent recovery of the photovoltaic properties correlates with changes in the DLTS data observed for InGaAsN and GaAsN diodes and solar cells, revealing that a nitrogen related E1 (EC−0.20eV) center has an important role in governing the solar cell performance. The large electron capture cross section (∼8.9×10−15cm2) of this center indicates that this defect may act as an efficient recombination center. Therefore, its complete removal by annealing or by some other process is essential for the high performance of GaInAsN solar cells. The internal quantum efficiency data were modeled to quantify the change in material properties associated with this improvement upon annealing. Annealed cells with indium impurity (InGaAsN) show a slightly higher photoresponse, which could be due to low scattering caused by In–N pair formation after annealing.

Dilute nitride films with a roughly 1 eV band gap can be lattice-matched to gallium arsenide and germanium, and therefore could become a critical component in next-generation multijunction solar cells. To date most dilute nitride solar cells have been plagued with poor efficiency, due in large part to short diffusion lengths. This study focuses on two techniques aimed at improving the quality of dilute nitride films grown by molecular beam epitaxy: the utilization of biased deflection plates installed in front of the nitrogen plasma source, and the introduction of antimony during growth. Results from GaInNAs cells grown with and without deflection plates, and GaInNAsSb solar cells are reported. The use of biased deflection plates during GaInNAs growth improved every aspect of solar cell performance. For the GaInNAs devices grown with deflection plates, the dark current density, open-circuit voltage, and fill factor were the best of the devices studied. The GaInNAsSb cells had the highest quantum efficienc...

We report GaInNAs grown by solid-source molecular-beam epitaxy (MBE) with background acceptor concentrations less than 1014cm−3, yielding depletion widths in excess of 3μm. GaInNAs p-i-n solar cells fabricated from this low-acceptor-concentration material show greatly increased photocurrents and internal quantum efficiencies close to unity for band gaps as low as 1.15eV. The low acceptor concentrations may be due to low levels of background impurities, such as hydrogen and carbon, in the MBE-grown layers. We discuss the dependence of the acceptor concentration on the substrate temperature used for GaInNAs growth.

We report a study comparing the device performance and the deep-level transient spectroscopy spectra of Ga1−yInyNxAs1−x diodes. It is found that a transient capacitance signal, which corresponds to a trap 0.18 to 0.25 eV below the conduction-band edge, is observed near 140 K in the nitrogen-containing samples. The existence of this transient correlates with higher dark current [lower open-circuit voltage (Voc)] and the size of the transient increases with x for x⩽0.6%. The abrupt decrease of the Voc with the addition of nitrogen is explained by the hypothesis that a close approach of the electron quasi-Fermi level to the conduction-band edge is arrested by the electron trap. The N-induced trap level reduces Voc as if it were a new conduction-band edge.

Photoluminescence of GaAs passivated with GaInP is studied over the temperature range 7–450K. Different photocarrier recombination mechanisms are identified as the temperature changes. An interface recombination velocity of less than 0.6cm∕s is measured at 300K. Lift-off processing inhibits but does not preclude laser cooling of GaAs.

Hydrogenated amorphous silicon films are made by atmospheric pressure chemical vapor deposition from polysilanes between 430 and 480 °C. The films have hole diffusion lengths up to 0.4 μm, as measured by the surface photovoltage technique. Oxygen impurities are shown to reduce the diffusion length. Stainless‐steel and crystalline silicon substrates give slightly different results, implying that contamination is introduced by the substrates.

No correlation was found between the stress in hydrogenated amorphous silicon films and the light‐induced effect, as measured by the photoconductivity. An equation is derived for calculation of the external stress applied to a film. The light‐induced degradation in a ‘‘zero‐stress’’ film (one removed from the substrate) was shown to be equivalent to that of an as‐deposited film.

The relationship between anomalous changes in the band gap and ordering of the group III sublattice of GaInP2 is discussed. X‐ray diffraction data are reported for the first time which show significant long‐range order of the CuPt type, i.e., alternating {111} Ga and In planes, in agreement with published electron diffraction data. However, the x‐ray and electron diffraction data differ as to which of the four possible orientations are observed and as to the directions of the streaks. No correlation was observed between the long‐range order parameter and the band‐gap anomalies.