Philip T. Chiu

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%.

InGaP/GaAs/InGaAs 41% concentrator cells using bi-facial epigrowth

Spire Semiconductor has demonstrated a new bi-facial epigrowth manufacturing process for InGaP/GaAs/InGaAs N/P tandem concentrator cells. NREL has verified 1cm2 cells as 41.0% efficient at 500X, and 5.5mm cells as 41.4% at 334X, AM1.5D, 25C, matching within measurement error the world record efficiency. A lattice-mismatched 0.94eV InGaAs cell is epitaxially grown on the backside of a lightly doped, N-type GaAs wafer, the epiwafer is flipped in the MOCVD reactor glove box, and 1.42eV GaAs and 1.89eV InGaP cells are grown lattice matched on the opposite wafer surface. Cells are then made using only standard III–V process steps. The bi-facial process is an alternative to the inverted metamorphic (IMM) process. It does not use epitaxial liftoff and wafer bonding as in the IMM approach but does require breaking the growth into two parts and flipping the epiwafer, which we believe is an easier task.

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.

Optically Enhanced Photon Recycling in Mechanically Stacked Multijunction Solar Cells

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.

42.3% Efficient InGaP/GaAs/InGaAs concentrators using bifacial epigrowth

Spire Semiconductor has demonstrated a new bi-facial epigrowth manufacturing process for InGaP/GaAs/InGaAs N/P tandem concentrator cells. A lattice-mismatched 0.94 eV InGaAs cell is epitaxially grown on the backside of a lightly doped, N-type GaAs wafer, the epiwafer is flipped, rinsed, and 1.42 eV GaAs and 1.89 eV InGaP cells are grown lattice matched on the opposite wafer surface. Cells are then made using only standard III-V process steps. NREL verified that 1 cm cells achieve 42.3% efficiency at 406 suns, AM1.5D, 25°C. This establishes a new world record, exceeding the previous record efficiency of 41.6% set by an InGaP/InGaAs/Ge cell. The bi-facial tandems exhibit superior performance relative to Ge based triple junction cells due to improved current matching. In addition they are simpler to fabricate than the inverted metamorphic (IMM) process because epitaxial liftoff and wafer bonding are not required.

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.

Recent progress of Spectrolab high-efficiency space solar cells

High-efficiency Inverted Metamorphic (IMM) multi-junction solar cells are being developed at Spectrolab for use in space and near-space applications. Recently, large-area (26-cm2) IMM3J cells achieved a 1-sun, AM0 conversion efficiency of 32% with an open-circuit-voltage of 3.04 V, a short-circuit current-density of 16.7 mA/cm2, and a fill factor of 0.84. In addition, IMM4J cells (1-cm2) reached a 1-sun, AM0 conversion efficiency of 33%. The 4-junction cell achieved an open-circuit-voltage of 3.42 V, a short-circuit current-density of 15.8 mA/cm2, and a fill factor of 0.82. Both the inverted metamorphic 3J and 4J cells reveal excellent component subcell quantum efficiency. Improvements in crystal growth, as well as more optimal subcell current density balance, will further raise the 1-sun, AM0 efficiency of the 3J and the 4J IMM cells to 33% and 35% respectively. The latest status of the 72-cm2 XTJ (based on 150-mm Ge substrate) Supercells will also be discussed.

High performance 5J and 6J direct bonded (SBT) space solar cells

Spectrolab has achieved a record 36% 5J direct bonded cell, measured under an AM0 space spectrum. The increase in efficiency over previous high performance 5J cells was achieved via voltage improvements to subcell 2 and 4 of the multijunction stack. Spectrolab has also successfully grown components necessary to implement a 6J direct bonded cell with an efficiency target of 38%. In particular the bottom 3J cells have been grown with bandgaps near the target values and an average voltage bandgap offset of 0.36 V.

Temperature dependence of InGaP/GaAs/InGaAs concentrators using bifacial epigrowth

Recently, Spire Semiconductor has demonstrated a bifacial InGaP/GaAs/InGaAs concentrator cell that achieved a previous record of 42.3% at 406 suns, AM1.5D, 25°C. However, as concentrators in modules typically operate at significantly higher than room temperature (∼65°C), the dependence of cell performance as a function of temperature is of great importance to module manufacturers. Here the temperature coefficients of current, voltage, and efficiency for the 42.3% bifacial tandems are presented. The 42.3% efficient cell, exhibits a Δeff/ΔT of −0.098%/°C that is higher than the that of the Ge based triple junction (−0.06%/°C), because ΔJ<inf>sc</inf>/ΔT is negative for the bifacial tandems and it is positive for the Ge based junction. Based upon temperature dependent IV and QE measurements, the negative ΔJ<inf>sc</inf>/ΔT coefficient is attributed to insufficient current in the InGaAs subjunction. Finally, we discuss improvements made to the InGaAs junction that significantly improve the ΔEff/ΔT coefficient to a value comparable to that observed for Ge based junctions.

InGaP/GaAs/InGaAs triple junction concentrators using bi-facial epigrowth

Spire Semiconductor has demonstrated a new bi-facial epigrowth manufacturing process for InGaP/GaAs/InGaAs N/P tandem concentrator cells. NREL has verified 5.5 mm cells as 41.4% at 334 suns, AM1.5D, 25°C, matching within measurement error the world record efficiency. A lattice-mismatched 0.94 eV InGaAs cell is epitaxially grown on the backside of a lightly doped, N-type GaAs wafer, the epiwafer is flipped, and 1.42 eV GaAs and 1.89 eV InGaP cells are grown lattice matched on the opposite wafer surface. Cells are then made using only standard III-V process steps. The bi-facial process is an alternative to the inverted metamorphic (IMM) process. It does not use epitaxial liftoff and wafer bonding as in the IMM approach, but does require breaking the growth into two parts and flipping the epiwafer, which we believe is an easier task.