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A sub-cell with bandgap of around 1 eV is required to improve the efficiency of multi-junction solar cells beyond what is possible with legacy triple-junction architectures [1]. Solar Junction Corporation has been focused since 2007 on the development and commercialization of dilute nitride materials to be used as the 1eV sub-cell in a fully lattice matched multijunction solar cell. Initial focus on the terrestrial concentrating photovoltaics (CPV) market led to Solar Junction Corp.’s achievement of multiple world records in multi-junction solar cell efficiency with its triplejunction cells on GaAs [2], [3]. These solar cells have been available as commercial products since 2010. Solar Junction Corp. has leveraged its high-quality, manufacturable dilute nitride material to develop and introduce an entirely new class of space solar cells capable of reaching over 33% AM0 conversion efficiency in a four-junction (4J) configuration lattice matched to active Ge substrates, with a clear line of sight to 36% AM0 efficiency in five- or six-junction devices that can be manufactured more cost-effectively than devices relying on metamorphic technologies. In this paper, we review the latest performance and qualification results of Solar Junction Corp.’s lattice matched 4J-on-Ge space solar cells and CIC (Cell- Interconnect-Coverglass) products incorporating GaInNAsSb dilute nitride material. We also report on the production readiness of these advanced space solar cells manufactured using an optimized hybrid Molecular Beam Epitaxy (MBE) / Metal Organic Vapor Phase Epitaxy (MOVPE) growth process.

Rapid and accurate threading dislocation density (TDD) characterization of direct-gap GaAs y P 1− y photovoltaic materials using molten KOH defect selective etching (DSE) is demonstrated. TDDs measured using molten KOH DSE show close agreement with those from both electron beam-induced current mapping and planar view transmission electron microscopy, provided TDD 7 cm −2 . H 3 PO 4 DSE is also demonstrated as an accurate method for characterizing TDD of GaP substrates. Taken together, the DSE methods described here enable TDD characterization over large areas (>10 5 µm 2 ) from substrate to GaAs y P 1− y device layer.

We report enhanced performance of InGaN solar cells grown by metalorganic chemical vapor deposition through optimization of the annealing of the epitaxial wafer before device fabrication. We varied the annealing environment gas mixtures as well as temperatures to obtain the optimized annealing condition. It was found that the major improvement of the nitride solar cell efficiency after annealing is in the increase of the Voc. In addition, annealing at the reasonably moderate temperature of 550°C in O2 environment results in the highest-efficiency InGaN solar cell devices compared with devices annealed at different temperatures and in different gas environments.

N-rich growth by NH 3 -based molecular beam epitaxy was investigated for intermediate-temperature GaN and InGaN on c-plane GaN templates. The dependences of growth mode and surface morphology on group-V overpressure, In/Ga ratio, and temperature were explored with atomic force microscopy and high resolution x-ray diffraction. Extension to an “ultra-NH 3 -rich” regime of very high NH 3 -flows showed a decreased growth rate and increased In-content for InGaN alloys for constant group III source fluxes. Rapid modulation of NH 3 overpressure, growth rate, and substrate temperature has enabled the growth of high quality, many-period InGaN/GaN superlattices, while suppressing morphological instabilities and subsequent stress relaxation.

In this chapter, we give a brief historical overview of developments in the growth of gallium nitride films by molecular beam epitaxy (MBE). Active nitrogen for GaN film growth can be supplied either by plasma-assisted MBE or ammonia-assisted MBE, and a description of equipment and procedures is given for both techniques. Growth regimes for both techniques have been well established, and in general plasma-assisted MBE is practiced in a metal-rich growth regime, and ammonia-MBE is practiced in a nitrogen-rich growth regime. We review the growth regimes and growth methods for achieving high structural and electronic quality gallium nitride and related alloys (In,Al,GaN) for advanced electronic and optoelectronic devices.

We demonstrate metamorphic 1.66 eV GaAs 0.77 P 0.23 solar cells grown by molecular beam epitaxy on exact (001) GaP/Si templates. Cascading such a cell with a 1.1 eV Si junction enables theoretical efficiencies of > 37% under the AM1.5G spectrum. We optimized the initial GaP growth on pseudomorphic GaP/Si templates to promote maximum strain relaxation with minimal nucleation of new threading dislocations. Electron beam-induced current studies of our cells reveal a threading dislocation density (TDD) of 7.8×106 cm−2, about 18% lower than our prior results on GaP/Si. The lower TDD has contributed to a low bandgap-voltage offset (W OC =E G /q-V OC ) of 0.55 V, which is 40 mV lower than our previous report, and represents significant progress for GaAs y P 1−y /GaP/Si metamorphic solar cells.

We have studied characteristics of ∼2.0 eV AlGaInP and ∼1.9 eV GaInP solar cells on misoriented GaAs substrates grown by molecular beam epitaxy (MBE). Lattice-matched AlGaInP solar cells are of interest for space and concentrator photovoltaics, but there have been relatively few reports on such cells grown by MBE. Open circuit voltages for GaInP cells ranged from 1.29–1.30 V, while AlGaInP cells ranged from 1.35–1.37 V. The bandgap-voltage offset (W oc ) of GaInP cells decreased from ∼575 mV to ∼565 mV as misorientation angle increased from 0° to 10°, while W oc for AlGaInP cells remained nearly constant at 620 mV. Internal quantum efficiency (IQE) measurements show that current collection for AlGaInP cells is lower than in GaInP and indicates the need for a thinner emitter with good surface passivation; no trend in IQE was observed as a function of offcut. Although W oc for AlGaInP was slightly higher than in GaInP, we believe that further optimization will allow MBE-grown AlGaInP to be a promising candidate for future top cell applications.

We have grown, via molecular beam epitaxy (MBE), the first metamorphic In0.26Ga0.74P solar cells with a 2.19 eV direct bandgap on GaP to serve as the top cell in a multi-junction device. Calculations show that the incorporation of a 2.0-2.2 eV top cell into future 4-6 junction cells could enable efficiencies as high as 60%. GaAsxP1-x graded buffers enabled a moderate threading dislocation density of 6×106 cm-2 in the In0.26Ga0.74P solar cells. Open circuit voltages (Voc) as high as 1.42 V were observed under approximate AM1.5G illumination. Little work has been reported on the MBE growth of such high-bandgap InyGa1-yP, and we believe that this Voc can be improved through systematic optimization of growth conditions. Although these devices were not optimized for current collection, we obtain an efficiency of 3.13%, surpassing that of the best GaP solar cells. Finally, as this composition is near the direct-indirect crossover point, we analyzed the low-energy cutoff of the external quantum efficiency spectrum and infer that our In0.26Ga0.74P cells are still in the direct regime.

We demonstrate metamorphic 1.73 eV GaAs0.72P0.28 solar cells grown by molecular beam epitaxy on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Cascading such a cell with a 1.1 eV Si junction in the substrate could enable a theoretical efficiency of 37% under the AM1.5G spectrum. Electron beam-induced current studies of our cells reveal a threading dislocation density (TDD) of ~1×107 cm-2 for cells on GaP/Si, which is significantly lower than previous reports. We believe that the combination of a highly optimized GaP/Si starting substrate with a well-designed metamorphic buffer enables these relatively low TDDs. Open-circuit voltages as high as 1.10 V were obtained, leading to a bandgap-voltage offset of 0.63 V. This bandgap-voltage offset is also lower than in previous reports, in qualitative agreement with the observation of lower TDD. Direct comparison with cells on bulk GaP confirm the relation between TDD and bandgap-voltage offset, indicating that more investigations to further reduce TDD in GaAsP single-junction cells are required to fulfill the ultimate goal of dual-junction integration on Si.

Temperature-dependent carrier transport is investigated using ultrafast spectroscopy in a p-GaN/i-InGaN/n-GaN solar cell with heavily-doped layers to compensate for polarization charges at the hetero-interface. We observe a flip in the transport direction at 110 K.

Efficiencies exceeding 40% have already been achieved with GaAs-based multijunction (MJ) solar cells. In this talk, we will discuss the unique advantages and challenges of fabricating hybrid InGaN-GaAs MJ cells for ultrahigh efficiency device designs.

GaAsP on Si tandem cells represent a promising path towards achieving high efficiency while leveraging the Si solar knowledge base and low-cost infrastructure. However, dislocation densities exceeding 108 cm−2 in GaAsP cells on Si have historically hampered the efficiency of such approaches. Here, we report the achievement of low threading dislocation density values of 4.0–4.6 × 106 cm−2 in GaAsP solar cells on GaP/Si, comparable with more established metamorphic solar cells on GaAs. Our GaAsP solar cells on GaP/Si exhibit high open-circuit voltage and quantum efficiency, allowing them to significantly surpass the power conversion efficiency of previous devices. The results in this work show a realistic path towards dual-junction GaAsP on Si cells with efficiencies exceeding 30%.

We report here enhanced solar energy harvesting using a hybrid solar cell with silicon solar cells (visible-infrared light) on bottom and an InGaN solar cell (UV light) on top. The InGaN solar cell with 30 QW periods has peak external quantum efficiency (EQE) of 40 % at 380 nm, an open circuit voltage (V oc ) of 2.0 V, a short circuit current (I sc ) of 0.8 mA/cm2, and fill factor of 55%. We have demonstrated that the application of an InGaN “active window” to a silicon solar cell counterbalances the encapsulation power loss typically suffered during production of a solar panel

III-nitrides have recently been demonstrated as potential photovoltaic device material particularly in the high-energy portion of the solar spectrum [1–2]. The large lattice mismatch between InN and GaN however, makes it difficult to grow good quality high In-composition InGaN films for low bandgap subcells. The integration of III-N based solar cells, which have currently been demonstrated to work well above 2.0 eV, with mature IV and III–V based solar cell technologies, which work well at bandgaps ≤ 2.0 eV, has the potential to improve the efficiency of current multi-junction solar cells. In this paper, we present the first on-wafer integration of InGaN/GaN solar cells with non-III-nitride (GaAs) solar cells.

We have investigated ∼2.0 eV (AlxGa1−x)0.51In0.49P and ∼1.9 eV Ga0.51In0.49P single junction solar cells grown on both on-axis and misoriented GaAs substrates by molecular beam epitaxy (MBE). Although lattice-matched (AlxGa1−x)0.51In0.49P solar cells are highly attractive for space and concentrator photovoltaics, there have been few reports on the MBE growth of such cells. In this work, we demonstrate open circuit voltages (Voc) ranging from 1.29 to 1.30 V for Ga0.51In0.49P cells, and 1.35–1.37 V for (AlxGa1−x)0.51In0.49P cells. Growth on misoriented substrates enabled the bandgap-voltage offset (Woc = Eg/q − Voc) of Ga0.51In0.49P cells to decrease from ∼575 mV to ∼565 mV, while that of (AlxGa1−x)0.51In0.49P cells remained nearly constant at 620 mV. The constant Woc as a function of substrate offcut for (AlxGa1−x)0.51In0.49P implies greater losses from non-radiative recombination compared with the Ga0.51In0.49P devices. In addition to larger Woc values, the (AlxGa1−x)0.51In0.49P cells exhibited significan...

Gallium phosphide (GaP) is an attractive candidate for wide-bandgap solar cell applications, possessing the largest bandgap of the III-arsenide/phosphides without aluminum. However, GaP cells to date have exhibited poor internal quantum efficiency (IQE), even for photons absorbed by direct transitions, motivating improvements in material quality and device structure. In this work, we investigated GaP solar cells grown by molecular beam epitaxy over a range of substrate temperatures, employing a much thinner emitter than in prior work. Higher growth temperatures yielded the best solar cell characteristics, indicative of increased diffusion lengths. Furthermore, the inclusion of an AlGaP window layer improved both open-circuit voltage and short wavelength IQE.

We demonstrate metamorphic ∼1.7 eV GaAsxP1−x (x = 0.71 − 0.73) solar cells on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Both n+-emitter/p-base and p+-emitter/n-base polarities are explored. Cells with n-type bases demonstrate current-voltage characteristics that are similar to p-type base cells, but with blue-shifted peak quantum efficiencies. Threading dislocation densities for cells on GaP/Si were 0.92 − 1.3 × 107 cm−2, significantly lower than previous reports but higher than cells grown on bulk GaP. An open-circuit voltage of 1.12 V was obtained for a 1.71 eV cell on Si, leading to a promising bandgap-voltage offset of 0.59 V.

The properties of quantum well carrier escape were studied by varying barrier thicknesses in InGaN/GaN multi-quantum well solar cell devices. The dependence of the photocurrent on applied bias and temperature exhibited properties indicative of the quantum well carrier escape mechanisms of thermionic emission and tunneling, with tunneling dominating for thin barriers and high fields. Simulations using a self-consistent drift-diffusion and Schrodinger solver with analytical formulas extracted carrier escape lifetimes. By employing sufficiently thin barriers, it was found that escape lifetimes can be made small compared to recombination lifetimes, leading to high internal quantum efficiency.

The hole concentration p in Mg-doped GaN films grown by ammonia molecular beam epitaxy depends strongly on the growth temperature TGR. At TGR=760 °C, GaN:Mg films showed a hole concentration of p=1.2 × 1018 cm−3 for [Mg]=4.5×1019 cm−3, while at TGR=840 °C, p=4.4×1016 cm−3 for [Mg]=7×1019 cm−3. Post-growth annealing did not increase p. The sample grown at 760 °C exhibited a low resistivity of 0.7 Ωcm. The mobility for all the samples was around 3−7 cm2/V s. Temperature-dependent Hall measurements and secondary ion mass spectroscopy suggest that the samples grown at TGR>760 °C are compensated by an intrinsic donor rather than hydrogen.

The effects of NH3 flow, group III flux, and substrate growth temperature on indium incorporation and surface morphology have been investigated for bulk InGaN films grown by ammonia molecular beam epitaxy. The incorporation of unintentional impurity elements (H, C, O) in InGaN films was studied as a function of growth temperature for growth on polar (0001) GaN on sapphire templates, nonpolar (101¯0) bulk GaN, and semipolar (112¯2), (202¯1) bulk GaN substrates. Enhanced indium incorporation was observed on both (101¯0) and (202¯1) surfaces relative to c-plane, while reduced indium incorporation was observed on (112¯2) for co-loaded conditions. Indium incorporation was observed to increase with decreasing growth temperature for all planes, while being relatively unaffected by the group III flux rates for a 1:1 Ga:In ratio. Indium incorporation was found to increase at the expense of a decreased growth rate for higher ammonia flows; however, smooth surface morphology was consistently observed for growth on s...

The effect of doping and polarization on carrier collection is investigated for InGaN quantum well solar cells. Energy band diagram simulations of actual devices indicate that spontaneous and piezoelectric polarization sheet charges can inhibit carrier collection unless these charges are screened by sufficient doping. By increasing the doping on both sides of the active region, the polarization-induced barriers to carrier collection were eliminated and the short circuit current density was increased from 0.1 to 1.32 mA/cm2 under 1.5 sun AM1.5G equivalent illumination, leading to devices with an open circuit voltage of 1.9 V and a fill factor of 71%.

We demonstrate high quantum efficiency InGaN/GaN multiple quantum well (QW) solar cells with spectral response extending out to 520 nm. Increasing the number of QWs in the active region did not reduce the carrier collection efficiency for devices with 10, 20, and 30 QWs. Solar cells with 30 QWs and an intentionally roughened p-GaN surface exhibited a peak external quantum efficiency (EQE) of 70.9% at 390 nm, an EQE of 39.0% at 450 nm, an open circuit voltage of 1.93 V, and a short circuit current density of 2.53 mA/cm2 under 1.2 suns AM1.5G equivalent illumination.

High external quantum efficiency (EQE) p-i-n heterojunction solar cells grown by NH3-based molecular beam epitaxy are presented. EQE values including optical losses are greater than 50% with fill-factors over 72% when illuminated with a 1 sun AM0 spectrum. Optical absorption measurements in conjunction with EQE measurements indicate an internal quantum efficiency greater than 90% for the InGaN absorbing layer. By adjusting the thickness of the top p-type GaN window contact layer, it is shown that the short-wavelength (

A comprehensive analysis of forward and reverse current of Ga-face GaN p-n junctions grown by NH3 molecular beam epitaxy (NH3-MBE) on metal organic chemical vapor deposition (MOCVD) GaN:Fe on sapphire is presented. NH3-MBE has a great potential for growing low leakage vertical devices due to its nitrogen-rich growth. Diodes with the lowest n-doping of n∼3.5×1017 cm−3 exhibit an extremely low reverse current of 2.6×10−7 A/cm2 at −40 V and an ideality factor of 1.3, which is lower than the state-of-the-art Ga-face p-n junctions grown on sapphire by MOCVD. An explanation for the difficulty for observing low ideality factors in GaN is presented.