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Combining two or more junctions into a tandem solar cell promises to deliver a leap in power conversion efficiency that will help to sustain continued growth in installed photovoltaic (PV) capacity. Although tandems are now on the roadmaps of many PV manufacturers, much work remains before they are ready for mass deployment. Accelerating their development requires advances on many fronts. In this article, we outline the fundamentals and status of tandem PV, considering multiple PV technology pairings and architectures. We then present the challenges that must be overcome and a general timeline of activities that are required to translate tandems to commercial products. Our intent is to spur researchers and manufacturers to work together to address important aspects of tandem design, reliability, and scaling to enable more rapid progress toward mass production.

Reference solar cells play a crucial role in determining the performance of photovoltaic (PV) devices. In the performance calibration chain of mainstream single-junction PV technologies, common reference cell types include Si, KG-filtered Si, and GaAs cells. For emerging multijunction (MJ) PV technologies, such as CdTe/Si and perovskite/Si, Si reference cells with colored glass near-infrared-longpass (LP) filters like Schott RG-715 and 850 glass have been proposed. They offer a better spectral response that matches the bottom junctions of the emerging MJs, which could lead to lower uncertainties in performance measurements. However, this article reveals a prominent decrease in short-circuit current (ISC) during National Renewable Energy Laboratory's primary calibration over the course of minutes when using this type of LP-filtered reference cell, which could result in unacceptable measurement errors. Unlike quartz or KG filter glasses, LP colored glass filters demonstrate temperature-sensitive cut-on wavelength. When incident irradiance reaches these LP-filtered reference cells, the increased temperature due to light absorption causes a shift of the cut-on to longer wavelength. As a result, the device ISC exhibits a continuous decrease (approximately a 2.5% drop with RG850 LP filter in a 5-min duration) even when the device temperature is controlled at 25 °C. To address the temperature gradient issue between colored glass LP filter and the reference cell, we propose the direct integration of thin-film LP semiconductor layer on reference cells. This type of LP-filter-integrated cell has shown minimal temperature-related ISC variation and can serve as a more reliable reference cell source for accurate performance measurements.

Photovoltaic devices that operate under extremely high irradiances, such as laser power converters (LPCs), may also operate at elevated temperatures, even under active cooling, as the result of large temperature gradients. We demonstrate the operation of GaAs LPC devices under orders of magnitude of irradiances up to 150 W/cm2 in a monochromatic laser simulator with an active cooling stage. The steady-state open-circuit voltage (VOC) as a function of irradiance is known to droop at high irradiance as the result of junction heating, but the junction temperature can be difficult to measure by conventional methods. Fast, transient VOC measurements under these extreme operating conditions are used here to determine the junction temperature. Empirical VOC temperature coefficients of the devices at each irradiance of interest are determined and used as an integral part of this technique. We show that the thermal design of different LPC devices strongly affects the operating temperature of the junctions. Knowledge of the operating temperature can be a strong tool for understanding the nature of loss mechanisms and improving the design for the operation of photovoltaic LPCs at high irradiances. This technique can be used for laboratory devices during initial design as well as to characterize mass-produced and packaged devices for quality control.

Multijunction solar cell design is guided by both the theoretical optimal bandgap combination as well as the realistic limitations to materials with these bandgaps. For instance, triple-junction III-V multijunction solar cells commonly use GaAs as a middle cell because of its near-perfect material quality, despite its bandgap being higher than optimal for the global spectrum. Here, we modify the middle cell bandgap using thick GaInAs/GaAsP strain-balanced quantum well (QW) solar cells with excellent voltage and absorption. These high-performance QWs are incorporated into a triple-junction inverted metamorphic multijunction device consisting of a GaInP top cell, GaInAs/GaAsP QW middle cell, and lattice-mismatched GaInAs bottom cell, each of which has been highly optimized. We demonstrate triple-junction efficiencies of 39.5% and 34.2% under the AM1.5 global and AM0 space spectra, respectively, and the global efficiency is higher than previous record six-junction devices.

One possible pathway toward reducing the cost of III–V solar cells is to remove them from their growth substrate by spalling fracture, and then reuse the substrate for the growth of multiple cells. Here we consider the growth of III–V cells on spalled GaAs(100) substrates, which typically have faceted surfaces after spalling. To facilitate the growth of high-quality cells, these faceted surfaces should be smoothed prior to cell growth. In this study, we show that these surfaces can be smoothed during organometallic vapor-phase epitaxy growth, but the choice of epilayer material and modification of the various surfaces by impurities/dopants greatly impacts whether or not the surface becomes smooth, and how rapidly the smoothing occurs. Representative examples are presented along with a discussion of the underlying growth processes. Although this work was motivated by solar cell growth, the methods are generally applicable to the growth of any III–V device on a nonplanar substrate.

Mitigating series resistance is crucial to the efficiency of concentrator solar cells at high current density. Conventional AlGaInP junction designs for the top junction of III-V multijunction cells present a challenging tradeoff between series resistance on the one hand and current collection and voltage on the other hand. In this article we discuss the physics of a reverse heterojunction solar cell that aims to improve on this tradeoff by combining a high bandgap Al0.18Ga0.33In0.49P base and a lower bandgap (Al)GaInP emitter. The high mobility of the emitter leads to a relatively low series resistance, compared with a high bandgap homojunction cell. The electroluminescence spectrum shows emission peaks from both the emitter and base, leading to an open-circuit voltage that is not strictly dominated by either layer. The reverse heterojunction design is increasingly beneficial as the one-sun voltage increases.

Compositionally graded buffers (CGBs) are essential components of high-efficiency III–V inverted metamorphic solar cells, and high-quality CGBs with low defect densities are a requirement for high device efficiencies. In this article, we use plan-view and cross-sectional cathodoluminescence spectrum imaging (CLSI) to investigate the effect of growth conditions on the microstructure of AlyGa1-x-yInxAs CGBs with xIn > 0.30. CLSI reveals significant compositional fluctuation in these materials that correlates with the distribution of threading dislocations in the CGBs. Threading dislocations are observed to collect in regions between abrupt transitions in emission energy, implying that concomitant strain fluctuations in these regions restrict dislocation motion. Thus, the compositional fluctuation leads to elevated threading dislocation densities that degrade CGB quality. We also correlate compositional fluctuation in the CGB with compositional fluctuation in subsequently grown device layers. The effectiveness of specific growth conditions at suppressing compositional fluctuation and limiting threading dislocation density is evaluated. CGBs grown at high temperatures with high Al content exhibit the lowest defect densities. The use of a high V/III ratio suppresses compositional fluctuation and defect density. These insights lead directly to improvements in the efficiency of metamorphic Ga1-xInxAs solar cells grown on AlyGa1-x-yInxAs CGBs.

Majority carrier barriers at heterointerfaces are a common source of non-linear resistance that hinders concentrator solar cell performance. The source of a particular barrier is often unclear in a multijunction device with numerous heterointerfaces. We demonstrate Zn-dopant diffusion during inverted metamorphic multijunction (IMM) device growth to be one key cause of internal barrier formation. Using an inverted GaAs/GaAs tandem solar cell with a high temperature annealing layer grown in between each subcell, we simulate the annealing conditions of a multijunction growth in a simplified structure. Through analysis of the device by secondary ion mass spectrometry (SIMS) and electrochemical capacitance–voltage profiling, we show that annealing causes Zn to diffuse out of the top cell Ga0.5In0.5P back surface field (BSF) and accumulate in the GaAs base. Through equilibrium band modeling, we show that the resultant doping profile forms an energetic barrier to hole flow in the valence band, which correlates with fill factor losses in the current–voltage curves measured under concentration. When we, instead, employ a C-doped Al0.2Ga0.8As BSF layer in the top cell, we do not observe evidence of a heterojunction barrier. We attribute this difference to the reduced diffusivity of carbon, confirmed by SIMS, as well as more favorable valence band offsets between GaAs and Al0.2Ga0.8As. Finally, we compare 5-junction IMM cells with Al0.2Ga0.8As:C and Ga0.5In0.5P:Zn BSF layers in the GaAs third junction, respectively, and show a significantly improved device performance under concentration when Al0.2Ga0.8As:C is employed. We demonstrate the importance of designing annealing tolerance into multijunction structures that are subjected to extended annealing during growth.

Tandem or multijunction solar cells are a promising method to circumvent the efficiency limit of single-junction solar cells, but there is ongoing debate over how best to interconnect the subcells in a tandem cell. In addition to four-terminal and two-terminal tandem cell architectures, a new three-terminal tandem cell architecture has recently been demonstrated, which features a standard two-terminal (front-back) circuit as well as an interdigitated back contact (IBC) circuit connected to the bottom cell. It has no middle contacts, and thus, maintains some of the simplicity of a two-terminal tandem. In this study, we measure four-terminal GaInP//Si and GaInP/GaAs//Si tandem cells in four-terminal and three-terminal configurations by connecting wires to mimic a three-terminal architecture. We demonstrate that both modes allow the same efficiencies exceeding 30% to be attained. Furthermore, we show that the IBC circuit not only collects excess power from the bottom cell, but that it can inject power into the bottom cell if it is current limiting the front-back circuit, enabling four-terminal performance in monolithic structures, regardless of which cell delivers less current.

Luminescent solar concentrators (LSCs) harness light generated by luminophores embedded in a light-trapping waveguide to concentrate onto smaller cells. LSCs can absorb both direct and diffuse sunlight, and thus can operate as flat plate receivers at a fixed tilt and with a conventional module form factor. However, current LSCs experience significant power loss through parasitic luminophore absorption and incomplete light trapping by the optical waveguide. Here, we introduce a tandem LSC device architecture that overcomes both of these limitations, consisting of a poly(lauryl methacrylate) polymer layer with embedded cadmium selenide core, cadmium sulfide shell (CdSe/CdS) quantum dot (QD) luminophores and an InGaP microcell array, which serves as high bandgap absorbers on the top of a conventional Si photovoltaic. We investigate the design space for a tandem LSC, using experimentally measured performance parameters for key components, including the InGaP microcell array, CdSe/CdS QDs, and spectrally selective waveguide filters. Using a Monte Carlo ray-tracing model, we compute the power conversion efficiency for a tandem LSC module with these components to be 29.4% under partially diffuse illumination conditions. These results indicate that a tandem LSC-on-Si architecture could significantly improve upon the efficiency of a conventional Si photovoltaic cell.

Metamorphic solar cells can have optimal bandgap combinations through the use of compositionally graded buffers, where the lattice constant is slowly varied over several microns of growth. Bragg reflectors consist of several microns of alternating layers with refractive index contrast and provide a useful internal reflection to multijunction solar cells with optically thin subcells. In this work, we implement distributed Bragg reflectors within the compositionally graded buffers of inverted metamorphic solar cells to add functionality to the buffer. The reflectance of this AlGaInAs “graded buffer Bragg reflector” is very similar to the reflectance of a similar AlGaAs Bragg reflector external to a buffer as well as the reflectance predicted by the transfer matrix model, indicating that the roughness of the buffer does not drastically reduce the reflection. Reflectance of 72%, 91%, and 98% is achieved in 2, 4, and 8 μm buffers using AlGaInAs layers that alternate between 30% and 70% aluminum content. Using a 2 μm graded buffer Bragg reflector, the 1.0-eV mismatched subcell of a GaAs/GaInAs tandem has a minor increase in threading dislocation density compared to a standard graded buffer and a small, 20 mV, loss in voltage. As the buffer is thickened, the voltage loss is recuperated and excellent subcell voltages are achieved, indicating that the Bragg reflector is not severely hindering dislocation glide. We demonstrate that the benefits of the graded buffer Bragg reflector for optically thin subcells and subcells containing quantum wells, and conclude that Bragg reflectors can effectively be implemented within graded buffers, adding functionality without adding cost.

Compositionally graded buffers (CGBs) have enabled the development of inverted metamorphic multijunction solar cells that contain three or more junctions and exhibit record efficiencies. Typically, the CGB is grown thick with a constant strain grading rate, because that is the most straightforward way to minimize the threading dislocation density (TDD) in device active regions. The CGB growth represents a significant expense, in terms of materials consumption and capital cost, however, hampering the economic viability of metamorphic devices. This work explores strategies for thinning Ga1-xInxP CGBs from GaAs to InP while minimizing performance loss, as evaluated by the open-circuit voltage ( $V_{{\text{OC}}}$) of ∼0.74 eV Ga0.47In0.53 As solar cells. Using the Ga1-xInxP CGB as a model, we demonstrate that slower grading is necessary in certain regions of the grade where dislocation nucleation rates are higher. Grading must be more gradual in these regions to suppress TDD. Grading rates can be increased in noncritical areas where dislocation nucleation is lower, permitting a thinner buffer. We demonstrate the relationship between VOC and TDD, establishing tradeoffs between thickness and performance. Through careful design, we demonstrate a CGB with a 40% thickness reduction (2 μm) achieved with a $V_{{\text{OC}}}$ decrease of only 7.5% (25 mV).

We propose practical six-junction (6J) inverted metamorphic multijunction (IMM) concentrator solar cell designs with the potential to exceed 50% efficiency using moderately high quality junction materials. We demonstrate the top three junctions and their monolithic integration lattice matched to GaAs using 2.1-eV AlGaInP, 1.7-eV AlGaAs or GaInAsP, and 1.4-eV GaAs with external radiative efficiencies >0.1%. We demonstrate tunnel junctions with peak tunneling current >400 A/cm2 that are transparent to <;2.1-eV light. We compare the bottom three GaInAs(p) junctions with bandgaps of 1.2, 1.0, and 0.7 eV grown on InP and transparent metamorphic grades with low dislocation densities. The solution to an integration challenge resulting from Zn diffusion in the GaAs junction is illustrated in a five-junction IMM. Excellent 1-sun performance is demonstrated in a complete 6J IMM device with VOC = 5.15 V, and a promising pathway toward >50% efficiency at high concentrations is presented.

The development of compositionally graded buffer layers (CGBs) with enhanced transparency would enable novel five and six junction solar cells, with efficiencies approaching 50% under high concentration. We demonstrate highly transparent grades between the GaAs and InP lattice constants on both A- and B-miscut GaAs substrates, employing AlxGayIn1-x-yAs and highly Se-doped Burstein-Moss (BM) shifted GaxIn 1-xP. Transparency to >810 and >890 nm wavelengths is demonstrated with BM-shifted GaxIn1-xP on B-miscut substrates and AlxGayIn1-x-yAs/GaxIn1-xP(Se) combined grades on A-miscut substrates, respectively. 0.74 eV GaInAs solar cells grown on these transparent CGBs exhibit Woc = 0.41 V at mA/ cm2, performance comparable with the state-of-the-art GaxIn1-xP grade employed in the four-junction-inverted metamorphic multijunction (IMM) cell. A GaAs/0.74cV GaInAs tandem cell was grown with a transparent BM-shifted GaxIn1-xP CGB to verify the CGB performance in a multijunction device structure. Quantum efficiency measurements indicate that the CGB is completely transparent to photons below the GaAs bandedge, validating its use in 4-6 junction IMM devices with a single-graded buffer. This tandem represents a highly efficient two-junction band gap combination, achieving 29.6% ± 1.2% efficiency under the AM1.5 global spectrum, demonstrating how the additional transparency enables new device structures.

Quaternary GaInAsP solar cells with a bandgap of ~1.7 eV offer an attractive Al-free alternative to AlGaAs solar cells for integration in next generation of III-V multijunction solar cells with five or more junctions. Development of a high quality 1.7 eV solar cell is also highly sought for III-V/Si tandem solar cells. In this work, we systematically investigate the impact of varying base thicknesses and doping concentrations on the carrier collection and performance of 1.7 eV GaInAsP solar cells. The photoresponse of these cells is found to be very sensitive to p-type zinc doping concentration in the base layer. Prototype 1.7 eV GaInAsP n-i-p solar cell designs are demonstrated that leverage enhanced depletion width as an effective method to achieve peak quantum efficiency exceeding 90%. We also show the importance of optimal i-layer thickness as a critical parameter to reduce the drop in fill-factor (FF) due to field-aided collection. Furthermore, we demonstrate substantial improvement in the cell performance when the GaInAsP base layer is grown at 650 °C instead of 600 °C. The best GaInAsP solar cell (Eg ~ 1.65 eV) in this study achieved JSC of 21.1 mA/cm2, VOC of 1.18 V, FF of 83.8%, and an efficiency of 20.8 ± 1% under AM1.5D spectrum (21.5 ± 1% under AM1.5G spectrum). These results highlight the potential of Al-free GaInAsP solar cells for integration in the next generation of III-V multijunction solar cells.

In this paper, we study the performance of 2.0 eV Al0.12Ga0.39In0.49P and 1.4 eV GaAs solar cells over a temperature range of 25-400°C. The temperature-dependent J01 and J02 dark currents are extracted by fitting current-voltage measurements to a two-diode model. We find that the intrinsic carrier concentration ni dominates the temperature dependence of the dark currents, open-circuit voltage, and cell efficiency. To study the impact of temperature on the photocurrent and bandgap of the solar cells, we measure the quantum efficiency and illuminated current-voltage characteristics of the devices up to 400°C. As the temperature is increased, we observe no degradation to the internal quantum efficiency and a decrease in the bandgap. These two factors drive an increase in the short-circuit current density at high temperatures. Finally, we measure the devices at concentrations ranging from ~30 to 1500 suns and observe n = 1 recombination characteristics across the entire temperature range. These findings should be a valuable guide to the design of any system that requires high-temperature solar cell operation.

Multijunction solar cells can be fabricated by mechanically bonding together component cells that are grown separately. Here, we present four-junction four-terminal mechanical stacks composed of GaInP/GaAs tandems grown on GaAs substrates and GaInAsP/GaInAs tandems grown on InP substrates. The component cells were bonded together with a low-index transparent epoxy that acts as an angularly selective reflector to the GaAs bandedge luminescence, while simultaneously transmitting nearly all of the subbandgap light. As determined by electroluminescence measurements and optical modeling, the GaAs subcell demonstrates a higher internal radiative limit and, thus, higher subcell voltage, compared with GaAs subcells without the epoxy reflector. The best cells demonstrate 38.8 ± 1.0% efficiency under the global spectrum at 1000 W/m2 and ~ 42% under the direct spectrum at ~100 suns. Eliminating the series resistance is the key challenge for further improving the concentrator cells.

Inverted metamorphic multijunction solar cells have been demonstrated to be a pathway to achieve the highest photovoltaic (PV) conversion efficiencies. Attaining high-quality lattice-mismatched (metamorphic) semiconductor devices is challenging. However, recent improvements to compositionally graded buffer epitaxy and junction structures have led to the achievement of high-quality metamorphic solar cells exhibiting internal luminescence efficiencies over 90%. For this high material quality, photon recycling is significant, and therefore, the optical environment of the solar cell becomes important. In this paper, we first present recent progress and performance results for 1- and 0.7-eV GaInAs solar cells grown on GaAs substrates. Then, an electrooptical model is used to assess the potential performance improvements in current metamorphic solar cells under different realizable design scenarios. The results show that the quality of 1-eV subcells is such that further improving its electronic quality does not produce significant $V_{{\rm oc}}$ increases in the four-junction inverted metamorphic subcells, unless a back reflector is used to enhance photon recycling, which would significantly complicate the structure. Conversely, improving the electronic quality of the 0.7-eV subcell would lead to significant $V_{{\rm oc}}$ boosts, driving the progress of four-junction inverted metamorphic solar cells.

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

AlGaInP solar cells with bandgaps between 1.9 and 2.2 eV are investigated for use in next-generation multijunction photovoltaic devices. This quaternary alloy is of great importance to the development of III-V solar cells with five or more junctions and for cells optimized for operation at elevated temperatures because of the high bandgaps required in these designs. In this work, we explore the conditions for the organometallic vapor-phase epitaxy growth of AlGaInP and study their effects on cell performance. Initial efforts focused on developing ~2.0-eV AlGaInP solar cells with a nominal aluminum composition of 12%. Under the direct spectrum at 1000 W/m2 (AM1.5D), the best of these samples had an open-circuit voltage of 1.59 V, a bandgap-voltage offset of 440 mV, a fill factor of 88.0%, and an efficiency of 14.8%. We then varied the aluminum composition of the alloy from 0% to 24% and were able to tune the bandgap of the AlGaInP layers from ~1.9 to ~2.2 eV. While the samples with a higher aluminum composition exhibited a reduced quantum efficiency and increased bandgap-voltage offset, the bandgap-voltage offset remained at 500 mV or less, up to a bandgap of ~2.1 eV.

The inverted metamorphic solar cell has highly tunable bandgaps, in part due to the metamorphic subcells. Using phosphide-based compositionally graded buffers, we show a wide variety of GaInAs solar cells, ranging in bandgap from 1.2 to 0.7 eV. These metamorphic subcells are all high quality and can be used for a wide variety of multijunction designs. GaInAs solar cells with 0.70 eV bandgaps are developed using an InAsP buffer that extends beyond the InP lattice constant, allowing access to an additional 2 mA/cm2 of photocurrent at AM1.5D and 25 °C. This subcell is implemented into a four-junction inverted metamorphic solar cell combined with an appropriate antireflective coating, which increases the series-connected multijunction current by 0.5 mA/cm2 with respect to designs using 0.74-eV GaInAs. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. One-sun devices achieve 35.3 ± 1.2% efficiency under the AM0 spectra and 37.8 ± 1.2% efficiency under the global spectra at 25 °C. Concentrator devices designed for elevated operating temperature achieve 45.6 ± 2.3% peak efficiency under 690× the direct spectrum and 45.2 ± 2.3% efficiency at 1000× and 25 °C. Device optimization is performed for the direct spectrum on 1-sun devices with 2% shadowing, which achieve 39.8 ± 1.2% efficiency under the direct spectrum at 1 sun, highlighting the excellent performance and bandgap tunability of the four-junction inverted metamorphic solar cell.

The emission of light from each junction in a series-connected multijunction solar cell both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n ≠ 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs of the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements.

Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high luminescence coupling to underlying subcells, both of which contribute to improved performance. Subcell voltages within two and four junction devices are measured by electroluminescence and the internal radiative efficiency is quantified as a function of recombination current using optical modeling. The performance of these concentrator multijunction devices is compared with the Shockley-Queisser detailed-balance radiative limit, as well as an internal radiative limit, which considers the effects of the actual optical environment in which a perfect junction may exist.

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of (43.8 ± 2.2)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of (42.9 ± 2.1)% at 869 suns, which is the highest concentration measured. The Voc increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.

The variability of the solar spectra in the field may reduce the annual energy yield of multijunction solar cells. It would, therefore, be desirable to implement a cell design procedure based on the maximization of the annual energy yield. In this study, we present a measurement technique to generate maps of the real performance of the solar cell for a range of light spectrum contents using a solar simulator with a computer-controllable spectral content. These performance maps are demonstrated to be a powerful tool for analyzing the characteristics of any given set of annual spectra representative of a site and their influence on the energy yield of any solar cell. The effect of luminescence coupling on buffering against variations of the spectrum and improving the annual energy yield is demonstrated using this method.

We analyze the implications of luminescent coupling on multijunction cell design and performance, using a recently developed formalism that uses the measured luminescent coupling parameters as inputs to an analytical model of the full current-voltage (J-V) characteristic of the cell. This calculation of the full J-V curve allows the determination of the cell open-circuit voltage, short-circuit current, fill factor, and efficiency in the presence of luminescent coupling. We show that luminescent coupling affects critical aspects of the cell design that include the optimal junction thicknesses and bandgaps, and affects the dependence of the cell performance on the spectral content of the light illuminating it.

The three-junction 1.8/1.4/1.0-eV inverted metamorphic multijunction solar cell can be extended to four junctions by adding another lattice-mismatched GaInAs junction with a bandgap of 0.7 eV. However, this requires a significant amount of mismatch to GaAs substrates, i.e., 3.8%, which is difficult to obtain while maintaining high-quality material. In this paper, we perform an in-depth investigation of a GaInP compositionally graded buffer varying in composition between Ga 0.5In 0.5P and InP in order to identify limitations to dislocation glide and sources of excess dislocation formation. In situ wafer curvature, cathodoluminescence, and X-ray diffraction (XRD) are used to analyze dislocation glide; transmission electron microscopy, atomic force microscope, and XRD are used to analyze material structural properties. Composition nonuniformities and roughness are observed, and a region in the compositionally graded buffer where a significant number of excess dislocations are formed is identified. The formation of these dislocations is related to atomic ordering, which has a large influence on the dislocation behavior. Adding thickness to the region in the buffer where dislocations are formed reduces the threading dislocation density an order of magnitude. Metamorphic 0.74 eV solar cells grown on this template have internal quantum efficiency > 90% and Voc > 0.3 V with Jsc set to 13 mA/cm2, which is the expected current in a multijunction device. These results are compared with lattice-matched GaInAs/InP solar cells to evaluate the loss associated with the lattice-mismatch.

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. We demonstrate a detailed model for solar cells that calculates the external luminescent efficiency and discuss the relationship between the external and internal luminescence. The model accounts for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell, and assumes isotropic internal emission. For single-junction cells, the calculation leads to Voc, and for multijunction cells, the calculation leads to the Voc of each junction as well as the luminescent coupling constant. In both cases, the effects of the optics are most prominent in cells with high internal radiative efficiency. Exploiting good material quality and high luminescent coupling, we demonstrate a two-junction nonconcentrator cell with a conversion efficiency of (31.1 ± 0.9)% under the global spectrum.

Luminescent coupling in multijunction solar cells is the phenomenon in which a junction in forward bias radiates photons that are absorbed in and converted to photocurrent by the junction beneath the radiating one. This effect can be significant in modern high-efficiency multijunction cells. We have previously developed a combined measurement and analytical approach to characterize the short-circuit current including the effects of nonlinear coupling in terms of measurable parameters η and φ that describe the coupling strength and linearity, respectively. Here, we develop an analytical model for the full current-voltage characteristic V (J) of a multijunction cell in the presence of luminescent coupling, in terms of the η and φ parameters. We compare the model with the measured V (J) parameters of GaInP/GaAs two-junction cells that exhibit differing degrees of luminescent coupling, and show that the model well describes the measurements. We then use the model to explore the consequences of luminescent coupling on the operating parameters of an idealized two-junction cell as a function of the top-junction thickness, focusing on the open-circuit voltage, fill factor, and efficiency. The results demonstrate that the strong luminescent coupling can significantly alter the dependence of cell efficiency on junction thickness, and that consequently the well-known optical-thinning design rules must be modified.

Ultrathin solar cells reduce material usage and allow the use of lower-quality materials thanks to their one order of magnitude smaller thickness than their conventional counterparts. However, efficient photonic light-trapping is required to harvest the incident light efficiently for an otherwise insufficient absorber thickness. Quasi-random photonic crystals are predicted to have high efficient light-trapping while being more robust under angle and thickness variations than simple photonic crystals. Here we experimentally demonstrate a light-trapping solution based on quasi-random photonic crystals fabricated by polymer blend lithography. We control the average lattice parameter by modifying the spin-coating speed. We demonstrate an ultrathin GaAs cell of 260 nm with a rear quasi-random pattern with submicron features, and a Jsc = 26.4 mA/cm2and an efficiency of 22.35% under the global solar spectrum.