Robyn L. Woo

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 A lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations show that a (1.93 eV)In_(0.37)Al_(0.63)As/(1.39 eV)In_(0.38)Ga_(0.62)As_(0.57)P_(0.43)/(0.94 eV)In_(0.38)Ga_(0.62)As 3-junction solar cell can achieve efficiencies >51% under 100-suns illumination (with V_(oc) = 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 V_(oc) = 1.8 V.

Direct Semiconductor Bonded 5J Cell for Space and Terrestrial Applications

Spectrolab has demonstrated a 2.2/1.7/1.4/1.05/0.73 eV 5J cell with an efficiency of 37.8% under 1 sun AM1.5G spectrum and 35.1% efficiency for 1 sun AM0. The top three junctions and bottom two junctions were grown on GaAs and InP substrates, respectively, by metal organic vapor phase epitaxy. The GaAs- and InP-based cells were then direct bonded to create a low-resistance, high-transmissive interface. Both the space and terrestrial cells have high 1 sun Voc between 4.75 and 4.78 V. Initial tests of the terrestrial cells at concentration are promising with efficiencies increasing up to 10× concentration to a maximum value close to 41%.

Wide-band-gap InAlAs solar cell for an alternative multijunction approach

We have fabricated an In_(0.52)Al_(0.48)As solar cell lattice-matched to InP with efficiency higher than 14% and maximum external quantum efficiency equal to 81%. High quality, dislocation-free In_xAl_(1−x)As alloyed layers were used to fabricate the single junction solar cell. Photoluminescence of In_xAl_(1−x)As showed good material quality and lifetime of over 200 ps. A high band gap In_(0.35)Al_(0.65)As window was used to increase light absorption within the p-n absorber layer and improve cell efficiency, despite strain. The InAlAs top cell reported here is a key building block for an InP-based three junction high efficiency solar cell consisting of InAlAs/InGaAsP/InGaAs lattice-matched to the substrate.

Development of advanced space solar cells at Spectrolab

High efficiency multi-junction solar cells utilizing inverted metamorphic1,2 and semiconductor bonding technology3 are being developed at Spectrolab for use in one-sun space and near-space applications. Recently that effort has been extended to include low-concentration space applications. This paper will review the present state-of-the-art cell technologies at Spectrolab, with an emphasis on performance characterization data at both 1-sun and low-concentration operating conditions that these cells will experience in flight‥ A cell coupon utilizing IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected and there is no performance degradation or mechanical issues after the test.

35.8% space and 38.8% terrestrial 5J direct bonded cells

Spectrolab has fabricated a direct semiconductor bonded space solar cell with an efficiency of 35.8% under the AM0 space spectrum. Using a similar technology, Spectrolab has achieved a 5-junction (5J) direct bonded terrestrial cell with a record efficiency of 38.8% under the one-sun AM1.5G terrestrial spectrum. Efforts to further improve the 5J cell efficiency have focused on development of the top 3 junctions (T3J) grown on GaAs. Experiments with top 3J isotype cells have yielded an improvement of 1% in current and 100 mV in voltage for the T3J. Spectrolab has also made significant improvements in its direct bonding process. The improved process has increased bond strengths by more than a factor of 5 and eliminated issues with large voids.

First demonstration of monolithic InP-based InAlAs/InGaAsP/InGaAs triple junction solar cells

Spectrolab has demonstrated the first lattice matched InAlAs/InGaAsP/InGaAs triple junction solar cell grown on InP substrate. X-ray diffraction characterization shows high quality solar cell materials. Preliminary 1-sun AM1.5D testing of the triple junction solar cell shows promising results with an open circuit voltage (Voc) of 1.8V, a short-circuit current density (Jsc) of 11.0 mA/cm2, a fill factor of 64.4 %, and a 1-sun AM1.5D efficiency of 13.8%. The same cell also passes 27-suns under concentration. Improvements in layer design and crystal quality of advanced features can further raise the 1-sun and concentrated AM1.5D conversion efficiency of the InP-based triple junction cell beyond 20%.

Direct Semiconductor Bonding Technology (SBT) for high efficiency III-V multi-junction solar cells

In recent years, the concept of wafer bonding has attracted significant attention since this approach allows direct integration of lattice-mismatched hetero-structure devices grown on different substrates by eliminating the major limitations of lattice matching requirements. The wafer bonding approach is particularly attractive for III-V based device structures since elimination of the lattice matching constraints allows novel device designs with otherwise impossible band-gap combinations, such as in multi-junction photo-voltaic devices with subcell band gaps tailored for optimum absorption of the solar spectrum. Several other optoelectronic applications, such as photonic crystals, resonant cavity photo-detectors and surface emitting lasers, have also extensively investigated the wafer bonding approach [1]. For III-V based photo-voltaic devices in particular, which have continued to yield the highest conversion efficiency amongst all photo-voltaic technologies [2], the possibility of integration of devices grown lattice matched to GaAs and InP with optimum band-gap combinations provides a way to break the barrier posed by lattice matching constraints and achieve even higher conversion efficiencies. Recently, fabrication of a simple 2-junction solar cell has been reported by Tanabe et al [3] via direct bonding of GaAs and InP wafers.

High efficiency Inverted Metamorphic (IMM) solar cells

High efficiency Inverted Metamorphic (IMM) multi-junction solar cells have been under development at Spectrolab for use in space and near space applications This paper reviews the present state-of-the-art of this technology at Spectrolab with an emphasis on performance characterization data at in-flight operating conditions. Large area IMM3J and IMM4J solar cells with 1X AM0 efficiency greater than 32% at 28 °C have been fabricated and characterized. Degradation factors after exposure to 1 MeV electron irradiation for both IMM3J and IMM4J technologies is presented. A coupon utilizing large area, IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected. Preliminary temperature cycling data indicate that a small coupon populated with strings of these cells suffered no degradation.

Carbon nanotube-composite wafer bonding for ultra-high efficiency III–V multijunction solar cells

Device-wafer bonding provides a platform for the implementation of ultra-high-efficiency multijunction solar cell designs, by allowing optimal subcell bandgap combinations to be attained while using only high-quality materials lattice-matched to their growth substrates. One promising new method for achieving wafer bonding is to use carbon nanotube composite thin films as the bonding agent between subcells grown on dissimilar substrates. In this paper we present the first demonstration of CNT-composite bonding of III–V materials, and evaluate its suitability for solar-cell integration in terms of optical transparency, electrical conductivity, bond uniformity and robustness, and bonded-device electrical performance. Another, relatively more mature method for device-wafer integration is that of direct semiconductor bonding technology. In order to provide a basis for comparison with CNT-bonding, we also summarize the latest achievements of the SBT solar cell development effort at Spectrolab.

InAlAs epitaxial growth for wide band gap solar cells

We demonstrate high quality InAlAs epitaxial growth by metalorganic vapor phase epitaxy and wide band gap solar cell fabrication. X-ray diffraction and transmission electron microscopy were used to characterize the crystalline quality of the epitaxial InAlAs grown. InAlAs solar cells lattice-matched to InP were grown and electrically characterized under AM 1.5 global 1-sun illumination. Window layers with different composition and, therefore, band gap energies were used to compare its effect on the overall device performance. In order to improve the electrical contact at the top window (Al-rich), an InGaAs cap layer was used. The resulting first generation of InAlAs solar cells showed an efficiency higher than 14 %, open circuit voltage of Voc = 1 V, Jsc = 19.3 mA/cm2, and maximum external quantum efficiency of 81 %.