Andreea Boca

Progress of inverted metamorphic III–V solar cell development at Spectrolab

Inverted metamorphic (IMM) solar cells based on III–V materials have the potential to achieve solar conversion efficiencies that are significantly higher than todays state of the art solar cells which are based on the 3-junction GaInP/GaInAs/Ge design. The 3J IMM device architecture based on (Al)GaInP/GaInAs/GaInAs, for example, allows for a higher voltage solar cell by replacing the low bandgap Ge (0.67 eV) from the conventional 3J structure with the higher bandgap (∼1 eV) metamorphic GaInAs. The inverted growth simply allows the lattice-matched junctions (i.e., (Al)GaInP/GaInAs) to be grown first on the growth substrate, thereby minimizing or shielding them from the defects that arise from the metamorphic layers. Spectrolab has demonstrated 30.5% AM0 efficiency based on the 3J IMM cell architecture grown on a Ge substrate, with Voc = 2.963V, Jsc = 16.9 mA/cm2, and FF = 82.5%. In addition, 4J IMM cells have been demonstrated with Voc of 4.072 V and AM0 efficiency approaching 25%. With additional development, demonstrating 33% AM0 efficiency is expected in the near future. However, the IMM devices demand more complex processing requirements than conventional solar cells, and we demonstrate the capability to fabricate large area solar cells from standard Ge solar cell substrates.

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.

Semiconductor-bonded III–V multijunction space solar cells

Boeing-Spectrolab recently demonstrated monolithic 5-junction space solar cells using direct semiconductor-bonding technique. The direct-bonded 5-junction cells consist of (Al)GaInP, AlGa(In)As, Ga(In)As, GaInPAs, and GaIn(P)As subcells deposited on GaAs or Ge and InP substrates. Large-area, high-mechanical strength, and low-electrical resistance direct-bonded interface was achieved to support the high-efficiency solar cell structure. Preliminary 1-sun AM0 testing of the 5-junction cells showed encouraging results. One of the direct-bonded solar cell achieved an open-circuit-voltage of 4.7V, a short-circuit current-density of 11.7 mA/cm2, a fill factor of 0.79, and an efficiency of 31.7%. Spectral response measurement of the five-junction cell revealed excellent external quantum efficiency performance for each subcell and across the direct-bonded interface. Improvements in crystal growth and current density allocation among subcells can further raise the 1-sun, AM0 conversion efficiency of the direct-bonded 5-junction cell to 35 – 40%.

High-efficiency multijunction photovoltaics for low-cost solar electricity

Multijunction solar cells divide the solar spectrum into smaller slices, delivering experimental efficiencies over 40%, and enabling theoretical efficiency over 60%. These high efficiency cells have a powerful effect on the cost effectiveness of new concentrator photovoltaic systems now being deployed around the world, making this technology one of the most viable options for plentiful solar-generated electricity.

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.

Prismatic covers for boosting the efficiency of high-concentration PV systems

Prismatic cover experiments were performed on GaInP/Ga(In)As/Ge triple-junction concentrator solar cells having grid designs with up to 39% metal coverage. The devices were characterized both before and after the application of optically-clear silicone covers patterned with cylindrical-lens microstructures. In solar-simulator LIV measurements at 1 sun and at 500 suns, we observed a boost in the efficiency of prism-covered cells, relative to before covering, of up to 34%, depending on the grid coverage fraction. The spectral response of the cells before and after covering was also characterized, and the observed boost in external quantum efficiency was found to be consistent with the LIV performance gains. Also studied were the lateral tolerances allowed by the prismatic covers, for alignment to the underlying cell grid structure, as a function of the maximum angle of incidence of the light impinging onto the cell. Ray-tracing simulations were performed, and found to be consistent with preliminary angular-dependence spectral response measurements done under monochromatic illumination.

GaInP/GaAs dual junction solar cells on Ge/Si epitaxial templates

In this study, we report synthesis of large area (≫ 2 cm2) crack-free GaInP/GaAs double junction solar cells on 50 mm diameter Ge/Si templates fabricated using wafer bonding and ion implantation induced layer transfer techniques. Defect removal from the template film and film surface prior to epitaxial growth was found to be critical to achievement of high open circuit voltage and efficiency. Cells grown on templates prepared with chemical mechanical polishing in addition a wet chemical etch show comparable performance to control devices grown on bulk Ge substrates. Current-voltage (I–V) data under AM 1.5 illumination indicate that the short circuit current is comparable in templated and control cells, but the open circuit voltage is slightly lower (2.08V vs. 2.16V). Spectral response measurements indicate a drop in open circuit voltage due to a slight lowering of the top GaInP cell band gap. The drop in band gap is due to a difference in the indium composition in the two samples caused by the different miscut (9° vs. 6°) of the two kinds of substrates.

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.

In-flight performance of III-V multi-junction solar cells from the Forward Technology Solar Cell Experiment

The Materials on the International Space Station Experiments (MISSE) present a unique opportunity in space science by offering a low-cost platform to expose materials directly to the space environment on the International Space Station (ISS). MISSE experiments consist of a “suitcase” like package known as the “Passive Experiment Carrier” (PEC) that can be carried by astronauts and mounted externally to the ISS. The 5th MISSE payload (MISSE-5) contained both passive and active experiments. The Forward Technology Solar Cell Experiment (FTSCE) on MISSE-5 measured current-voltage (I–V) characteristics on 36 solar cells of various types. Over 1500 I–V curves were recorded on each cell during a 13-month period. This paper analyses the results for all the III–V multi-junction cells flown, including state-of-the-art space qualified cells and next generation metamorphic cells.

High-irradiance high-temperature vacuum testing of the Solar Probe Plus array design

The Solar Probe Plus (SPP) spacecraft will fly further into the Suns corona than any previous mission, reaching a minimum perihelion at 9.5 solar radii from the center of the Sun. The solar arrays powering the spacecraft will operate under unusually high irradiances and temperatures. The array design, material choices, and necessary test facilities for SPP are therefore quite different from those used on traditional space panels. This paper gives an overview of the high-irradiance high-temperature vacuum (HIHT-Vac) reliability testing completed to date at Emcore on three small-scale coupons representing two competing SPP-array technologies. Both technologies successfully passed the HIHT-Vac test with no measurable performance, visual or mechanical degradation, reaching a key milestone in the development of the SPP array.