NREL cuts cost of III-V tandem cells

The US National Renewable Energy Laboratory (NREL) has recently combined two structures of III-V materials into one to provide a significant cost savings. The combination has already allowed for novel, potentially record-setting, multijunction solar cells. These cells consist of several thin subcells of absorbing materials with differing physical, electrical, and optical characteristics stacked atop one another to generate electricity more efficiently than single-junction devices such as silicon solar cells.

The two structures-a 'graded buffer' and a "˜Bragg reflector'-are both complex structures already used independently in solar cells and lasers. Integrating them into one, dual-purpose structure allows devices to gain functionality without extra cost, but it requires some clever design work.

Diagrams (above) of three triple-junction PV devices show how combining the graded buffer and Bragg reflector reduce complexity and cost while still offering full benefits. The "˜good' device has only a buffer layer between two absorbing layers. The "˜better' device has both a buffer and reflector in the same spot, increasing cost. The "˜best' device has one the buffer and reflector combined into one layer, making it as thick as the "˜good' device, but with all the benefits of the "˜better' device.

Better Designs through Innovation

Graded buffers (GBs) are used to match up different-sized lattice constants (i.e., the spacing between atoms in a material's crystal framework) within a III-V device, which allows designers to access materials with a whole host of valuable, diverse properties, such as a material's bandgap -which determines the wavelength of light emitted by a light-emitting diode (LED) or laser and the wavelengths most efficiently absorbed by solar cells and detectors. Integrating materials with different lattice constants and bandgaps into a single III-V multijunction solar cell has enabled many previous world-record devicesPDF, such as a 3-junction inverted metamorphic device with 37.9 percent efficiency under the 1-sun global spectrum, shown in the left side of Figure 1.

Bragg reflectors (BRs) allow selected wavelengths of light to be reflected and are commonly used in laser devices to create an optical cavity. In multijunction solar cells, Bragg reflectors reflect light back into an adjacent subcell, which is particularly useful for materials that don't fully absorb the incoming light, such as very thin subcells or thin quantum-well layers (absorbing layers with a low bandgap). In theory, higher efficiency is possible with the structure in Figure 1 (middle figure), involving quantum wells to lower the middle-cell bandgap, a BR to increase absorption, and a GB to access lattice-mismatched GaInAs. However, both BRs and GBs are relatively thick layers of the device and are expensive to make. So, these structures come with the drawback of additional cost.

In recent work, scientists from NREL, the University of New South Wales (UNSW), and California Institute of Technology (Caltech) combined a GB and BR into one structure, termed a "˜graded buffer Bragg reflector' (GBBR). The GBBR is thinner than an independent GB and BR and enables a multijunction structure with higher potential efficiency than the current record device, but without an increase in cost, shown in the right side of Figure 1.

"III-V materials have been studied and used for solar purposes for a long time-for at least 50 years. So as a materials scientist, I'm excited that we are still innovating new III-V materials and concepts that enable novel device structures," says NREL's Ryan France, lead author of 'Multijunction Solar Cells with Graded Buffer Bragg Reflectors' recently published in IEEE's Journal of Photovoltaics.

The team studied the performance of the GBBR and integrated it into this novel multijunction structure. Initial 3-junction devices already achieve 36.5 percent efficiency under the AM1.5 global spectrum and 32.4 percent efficiency under the AM0 space spectrum, which is one of the highest-reported efficiencies for a 3-junction device under the 1-sun AM0 spectrum.

As Myles Steiner, another NREL researcher and co-author of the recent paper, points out, "Our upcoming plans use a similar device to target the development of a 3-junction cell that will reach 40 percent power conversion efficiency under the global spectrum-which is about 2 percent absolute better than the current record efficiency."

Branching into Other Applications

Over its more than 40-year history, NREL has amassed world-renowned experience in designing, fabricating, characterizing, and testing III-V multijunction solar cells. Applying this new GBBR to such solar cells has been a natural strategy.

But, as often happens, innovations in one field can directly benefit other fields. GBs access materials with a variety of bandgaps, which enable LEDs and lasers with a variety of emission wavelengths. Using a GBBR instead of a GB adds a reflector directly behind an LED or laser, which could enhance the power output or reduce the cost of these emitting devices.

In their study targeting multijunction solar cells, the team developed a GBBR with 98 percent reflectance by increasing the refractive-index contrasting pairs, which provide the reflection in a BR, shown in Figure 2 (above). This reflectance may also be useful in vertical-cavity surface-emitting lasers, where it is difficult to place a metal reflector next to the optical cavity. BRs in these laser structures currently provide super-high reflectivity (99.9 percent) alternatives to metals, and the GBBR may be useful in longer-wavelength metamorphic lasers if such a high reflectivity can be achieved.

A graph plots the reflectance of three bragg reflectors with different numbers of refractive-index pairs. The reflectors with more pairs reflect more light.

In other solar-related applications, GBs and BRs are being considered as a means to mitigate radiation damage on solar devices in outer space. Therefore, the advances by NREL, UNSW, and Caltech in combining GB and BR functions into a GBBR will likely be of interest to enhance the operational performance of space solar cells. Currently, NREL has a patent pending on the GBBR technology and hopes to make the technology available for licensing in the near future.

Like the merger of the phone and camera into an integrated, well-functioning cell phone, the graded buffer Bragg reflector has brought two solar ideas together in a much-improved, new technology that will benefit applications within various markets.