By D.A. Barber
The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) entered into a cooperative agreement with North Carolina-based RF Micro Devices RFMD) in 2009 to develop a commercial production process for an Inverted Metamorphic Multi-Junction (IMM) PV cell technology NREL had invented. After years of continued research, RFMD announced in October 2011 the formation of a new group to push IMM commercialization forward.
NREL has always had a strong research hand in III-V multi-junction PV cells and CPV systems. NREL invented the first-ever multi-junction PV cell and then worked with private manufacturers to develop the industry-standard GaInP/Ga(In)As/Ge) technology. They then went on to invent and first demonstrated the IMM solar cell and introduced it to the PV industry.
Originally developed for space-based applications, III-V multi-junction cells represent a huge improvement in performance over silicon PVs particularly for CPV. Using three different photovoltaic materials in a single cell, III-V cells extract more energy from the full range of the wavelengths in sunlight. This enables PVs to produce a considerable increase in voltage, while losing much less energy to heat. As a result, III-V cells have achieved the solar industry’s highest efficiencies. The IMM technology is a further breakthrough in performance and cost advantages of these types of cells.
IMM cells are engineered to capture energy from a major portion of the solar spectrum and be more efficient - demonstrating one of the highest reported energy conversion efficiencies at 40.8 percent and research indicates that they are likely to improve in efficiency. The record efficiency was measured under concentrated light of 326 suns, which is over 300 times the amount of sun energy that hits the Earth on a typical sunny day.
RFMD announced a year ago it had successfully achieved a significant performance milestone related to the commercialization of IMM by fabricating dual-junction PV cells that integrate gallium arsenide (GaAs) and indium gallium phosphide (InGaP) PV junctions using the company's standard six-inch semiconductor equipment. The successful fabrication of these dual-junction PV cells clears the way for RFMD to develop the triple-junction IMM structures, with the ultimate goal of developing a commercially viable process.
RFMD then announced in October 2011 the formation of a new business group, the Compound Semiconductor Group, which will operate alongside RFMD's Cellular Products Group and their Multi-Market Products Group. The new CS group will “create innovative new high-power and high-performance products utilizing the company's industry-leading gallium nitride (GaN) and gallium arsenide (GaAs) process technologies.” This will take place within RFMD's New Technology Commercialization Center, which was set up for new technology incubation in agreement with NREL related to the commercialization of GaAs-based CPV cells.
IMM Growth System
IMM higher performance is a result of the cell growth process - a process that reverses the typical sequence for triple-junction cells by depositing the upper layer first and then the lower layer. And unlike conventional PVs, IMM uses gallium indium phosphide and gallium indium arsenide. The cells are grown on a gallium arsenide wafer, which is then turned over, attached to a slim metallic foil "handle" and then the cell growth substrate is detached. NREL researchers had improved IMMs by making the middle and bottom junctions metamorphic “lattice mismatched,” so they don’t follow the even pattern of atomic spacing, and so allows for a greater conversion of sunlight.
The advantage of this process includes the ability to reuse the costly substrate material and so lower production costs. Another advantage is that the “handle” substance need not be crystalline so it can be selected for specific applications while still producing a thinner and lighter device.
Multi-junction cells also beat single-junction efficiency limits by converting each region of the spectrum, which is done by the gallium indium phosphide and gallium indium arsenide separating the solar spectrum into three equal parts to be absorbed by each of the cell’s three junctions.
Today, these triple-junction PVs are targeted for use in CPV systems to leverage their high output while minimizing total system costs. One problem that has always plagued CPV is the buildup of heat from the concentrated sunlight. The IMM production process allows the addition of a reflector on the rear surface of the cells which redirects any lost light energy back to the active PV layers where it effectively boosts the cell's power efficiency. These reflectors are also designed to discard much of the infrared radiation, which in turn keeps the PV cells cooler and operating more efficiently.
III-V Use Advances
While IMM cells are still advancing towards commercialization, other forms of III-V multi-junction cells are quickly finding their way into CPV projects.
California-based Solar Junction, a spin-off of research at Stanford University, had its multi-junction CPV cells certified at 40.9 percent efficiency last year by NREL and the company expects they will achieve efficiencies of over 50 percent in the next five years. With that advance, the company announced in February 2012 it had received $19.2 million of investments from New Enterprise Associates, Advanced Technology Ventures, Draper Fisher Jurvetson and IQE. Solar Junction says the financing will enable them to scale-up their manufacturing to fulfill current and future orders with a number of CPV companies. Solar Junction’s cells incorporate the company’s proprietary Adjustable Spectrum Lattice Matched (A-SLAM) multi-junction solar cell architecture, which “provides material bandgap tunability – particularly from 0.8 to 1.42 eV - to maximize the absorbed sunlight within CPV modules, thereby increasing the efficiency and energy harvested.”
Researchers are already looking ahead to the next generation of multi-junction PV cells. Future terrestrial cells will likely feature four or more junctions with performance potential capable of reaching over 45 percent efficiency soon, according to Boeing-Spectrolab which is also developing the technology. Spectrolab says these 4, 5, or 6-junction cells are expected to trade lower current densities for higher voltage and divide the solar spectrum more efficiently while the lower current densities will significantly reduce the resistive power loss at high concentrations when compared to the 3-junction cell. According to NREL, the maximum theoretical efficiencies for various numbers of junctions are: 1 junction, 37 percent; 2 junctions, 50 percent; 3 junctions, 56 percent; and 36 junctions, 72 percent.
The immediate future for multi-junction cells is in applications for regions of the world where CPV systems have greater advantages over conventional PV systems. Currently, this niche market for multi-junction PVs is aimed at utility-scale CPV solar farms as opposed to conventional PV panels targeting roof-top applications. It may be a small niche market compared to flat-plate PVs but it is still an important one. What these small numbers of multi-junction manufacturers will be focusing on is pressing their way into a greater share in this specialty market. But one key for multi-junction concentrator PV cell manufacturers and new start-ups may also be to develop new and very different CPV applications beyond simply utility-scale use.
At the same time, developing new optical concentration systems to further advance CPV will open yet other doors for start-ups thinking out of the box.