Lattice-matched Multijunction Solar Cells Research Articles

Bandgap energy model for GaInNAsSb/GaAs alloys with high N content and strain influence

Bandgap energy of dilute nitride GaInNAsSb/GaAs alloys with N compositions as high as 8% are estimated using a method based on band anti-crossing model used for GaNAs/GaNSb/InNAs/InNSb ternary compounds. The parametrization of the model is defined by fitting with experimental composition and bandgap energy values employing a differential evolution algorithm. The effects of lattice strain on the bandgap energy are taken into account by the model resulting in an accurate prediction of the bandgap energy with an average deviation of only 12 meV compared to the experimental data. The model provides a useful tool for accurate determination of bandgap energies of dilute nitrides, including narrow bandgap, i.e. ∼0.7 eV GaInNAsSb alloys, which are becoming increasingly relevant in the development of high-efficiency lattice-matched multijunction solar cells.

Advances in Dilute Nitride Multi-Junction Solar Cells for Space Power Applications

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.

Performace of Dilute Nitride Triple Junction Space Solar Cell Grown by MBE

Dilute nitride arsenide antimonide compounds offer widely tailorable band-gaps, ranging from 0.8 eV to 1.4 eV, for the development of lattice-matched multijunction solar cells with three or more junctions. Here we report on the performance of GaInP/GaAs/GaInNAsSb solar cell grown by molecular beam epitaxy. An efficiency of 27% under AM0 conditions is demonstrated. In addition, the cell was measured at different temperatures. The short circuit current density exhibited a temperature coefficient of 0.006 mA/cm 2 /°C while the corresponding slope for the open circuit voltage was −6.8 mV/°C. Further efficiency improvement, up to 32%, is projected by better current balancing and structural optimization.

High-efficiency AlGaInP solar cells grown by molecular beam epitaxy

AlGaInP is an ideal material for ultra-high efficiency, lattice-matched multi-junction solar cells grown by molecular beam epitaxy (MBE) because it can be grown lattice-matched to GaAs with a wide 1.9–2.2 eV bandgap. Despite this potential, AlGaInP grown by molecular beam epitaxy (MBE) has yet to be fully explored, with the initial 2.0 eV devices suffering from poor performance due to low minority carrier diffusion lengths in both the emitter and base regions of the solar cell. In this work, we show that implementing an AlGaInP graded layer to introduce a drift field near the front surface of the device enabled greatly improved internal quantum efficiency (IQE) across all wavelengths. In addition, optimizing growth conditions and post-growth annealing improved the long-wavelength IQE and the open-circuit voltage of the cells, corresponding to a 3× increase in diffusion length in the base. Taken together, this work demonstrates greatly improved IQE, attaining peak values of 95%, combined with an uncoated AM1.5G efficiency of 10.9%, double that of previously reported MBE-grown devices.

Comparison of single junction AlGaInP and GaInP solar cells grown by molecular beam epitaxy

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 significantly lower internal quantum efficiency (IQE) values than Ga0.51In0.49P cells due to recombination at the emitter/window layer interface. A thin emitter design is experimentally shown to be highly effective in improving IQE, particularly at short wavelengths. Our work shows that with further optimization of both cell structure and growth conditions, MBE-grown (AlxGa1−x)0.51In0.49P will be a promising wide-bandgap candidate material for high-efficiency, lattice-matched multi-junction solar cells.

Carrier Dynamics in Quantum-Dot Multijunction Solar Cells Under Concentration

The key performance metrics of quantum-dot (QD)- lattice-matched multijunction solar cells (MJSCs) composed of InGaP/(In)GaAs/Ge with InAs/GaAs QDs are explored under high-concentration illumination with a focus on the carrier dynamics in the QD layers of the middle subcell. An effective medium approach is used to describe generation and recombination in the QD system, including carrier escape and capture from the weakly confining quantum well and the QD states. At a concentration of 1000 suns, simulations indicate that the specific QD MJSC studied outperforms a standard MJSC by 1.1% in relative efficiency operating at 25 °C. However, this gain in efficiency is highly dependent on the confinement potentials of the wetting layer, as well as the resulting current mismatch between the top and middle subcells when carrier escape rates from within the wetting layer confinements are reduced.

Towards an optimized all lattice-matched InAlAs/InGaAsP/InGaAs multijunction solar cell with efficiency >50%

An approach for an all lattice-matched multijunction solar cell optimized design is presented with 5.807 Å lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations show that a (1.93 eV)In0.37Al0.63As/(1.39 eV)In0.38Ga0.62As0.57P0.43/(0.94 eV)In0.38Ga0.62As 3-junction solar cell can achieve efficiencies >51% under 100-suns illumination (with Voc = 3.34 V). As a key proof of concept, an equivalent 3-junction solar cell lattice-matched to InP was fabricated and tested. The independently connected single junction solar cells were also tested in a spectrum splitting configuration, showing similar performance to a monolithic tandem device, with Voc = 1.8 V.

Using Dilute Nitrides to Achieve Record Solar Cell Efficiencies

ABSTRACTHigh quality dilute nitride subcells for multijunction solar cells are achieved using GaInNAsSb. The effects on device performance of Sb composition, strain and purity of the GaInNAsSb material are discussed. New world records in efficiency have been set with lattice-matched InGaP/GaAs/GaInNAsSb triple junction solar cells and a roadmap to 50% efficiency with lattice-matched multijunction solar cells using GaInNAsSb is shown.

Lattice-matched multijunction solar cells employing a 1 eV GaInNAsSb bottom cell

Over the past 15 years, tremendous effort has been put forth into increasing the efficiency of multijunction solar cells used in concentrator photovoltaics (CPV) systems. Because the cell represents a small portion of the total CPV system cost, modest improvements in cell efficiencies have a large impact on reducing the levelized cost of electricity. To address the need for higher efficiency CPV cells, Solar Junction is currently manufacturing a lattice-matched multijunction solar cell that employs a 1 eV GaInNAsSb bottom subcell in lieu of the traditional Ge subcell. Our standard 5.5 mm 2 product has recently achieved a world record in power conversion efficiency. In this article, we report on the design, performance, and integration of this high-performance cell. The importance of overdriving the GaInNAsSb bottom subcell is discussed in addition to the optimization of the anti-reflection coating used for cells under glass secondary optics.