X.Q. Liu

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.

Status of 40% production efficiency concentrator cells at Spectrolab

Multijunction solar cells based on III–V semiconductors are the most efficient solar cells in the world, and of the established photovoltaic technologies, have the greatest potential for future growth in efficiency. Champion cells with efficiency greater than 40% have been demonstrated by several groups since 2006, and in that same period, the efficiency of cells in mass production has also increased steadily. These devices offer the promise of very competitive solar power systems exploiting the high efficiency under high optical concentration. To this end, Spectrolab is conducting a multi-year program to develop solar cells with still higher efficiency and substantial cost reductions and to fully characterize and qualify them for reliable performance in the field. Development of the fourth production generation with 40% average production efficiency is nearing completion. Cell design and performance will be presented, with a summary of qualification and field test status. Progress in ongoing efforts to automate cell production for cost reduction and increased manufacturing capacity will be discussed. Development of these high-performance multijunction CPV cells is key to the emergence of CPV technology as the lowest cost solar power solution in high DNI areas.

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.

Development of space solar cells at Spectrolab

High efficiency Inverted Metamorphic (IMM) and Semiconductor Bonded Technology (SBT) multi-junction solar cells have been under development at Spectrolab for use in space and near space applications. This paper will review the present state-of-the-art of this technology at Spectrolab with an emphasis on performance characterization data at operating conditions that these solar cells will experience in flight. Solar cell current-bias characteristics under illumination (LIV) at AM0 28°C are presented along with external quantum efficiency measurements that are used to verify the X-25 solar simulator LIV short circuit current density. A mechanical and thermal stress model has been used to predict mechanical stresses on a ultra-lightweight panel assembly in orbit and will be discussed.

Environmental testing of inverted metamorphic solar cells for space

Inverted metamorphic (IMM) solar cells are being pursued for space applications. This work examines their extended performance under key environmental conditions. Electron irradiation with 1 MeV electrons found 81% of the power remained after exposure to 1e15/ cm2 fluence. Test strings mounted on rigid panels were subjected to 11,000 thermal cycles without electrical degradation. Thermal soak testing at 250 °C identified 1% of efficiency loss, equivalent to production devices. No performance loss was found after 30 days of humidity exposure. These tests verify the integrity of IMM devices, while matching measurements by NREL confirm the quality of Spectrolabs IMM testing method.

Large area plan-view transmission electron microscopy sample preparation for multijunction metamorphic solar cell devices

A focused ion beam (FIB) sample preparation technique is developed to produce very large areas of electron transparent material for plan-view transmission electron microscopy measurements from specific layers in a multi-layer device structure. An initial cross section FIB cut allows plan-view sample creation from any layer of interest, and therefore rapid characterization of the defect density as a function of depth is achievable for structures containing multiple layers, such as multi-junction metamorphic layer structures for high efficiency III-V solar cells. Uniform electron transparent plan-view areas of greater than 100 μm2 are extracted from a cross-section to facilitate observation of defect densities as low as ~106 cm-2. To demonstrate the technique, the active cell layers of two inverted metamorphic III-V solar cell structures have been imaged to reveal defect densities as high as 109 cm-2 and as low as 106 cm-2.