New III-V Device Concept Combines Two Structures

NREL combine combine two structures of III-V materials into one with the potential of significant cost savings

Researchers have made progress in the world of high-efficiency III-V multijunction solar cells. The 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.

Figure 1. The "graded buffer Bragg reflector" (GBBR) combines two complex structures (GB and BR) into one thinner unit, enabling novel multijunction devices that may surpass current efficiency records with similar cost.

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 devicesPDF, such as a 3-junction inverted metamorphic device with 37.9% 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% efficiency under the AM1.5 global spectrum and 32.4% 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% power conversion efficiency under the global spectrum—which is about 2% absolute better than the current record efficiency."

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% reflectance by increasing the refractive-index contrasting pairs, which provide the reflection in a BR, shown in Figure 2. 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%) 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.

Figure 2. Reflectance from three "graded buffer Bragg reflectors" with a varied number of contrasting refractive-index pairs.

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.

BYD And KOSTAL Announce Strategic Partnership For Energy Storage Solutions
Amarenco France's Number One In Rooftop Solar After Latest Tender Results
Trina Solar Supplies Modules To Ukraine's Largest Solar Power Plant
First “plug And Play” Power-enhancing Ribbons For PV Interconnection
NREL Cuts Cost Of III-V Tandem Cells
Strong Solar Performance Triggers Record Low Fossil Fuel Generation In Third Quarter
BayWa R.e. Receives The National Energy Globe Award For Its Solar Project In Zambia
RES And VBB Celebrate Construction Progress Of 10 MW Storage Project
World Bank To Invest $1 Billion Towards Battery Storage
Solar Might Become EU’s Fastest-Growing Energy Source Due To Elimination Of Tariffs
Organic Solar Hits New Landmark
REDEN Solar Completes The Refinancing Of Its Iberian Solar PV Assets
BayWa R.e. Partners With Mexico’s Largest Distributor Of PV Products
Welsh Researchers Supersize Perovskite Solar Technology
The Time To Invest In Emission Control And Renewable Energy Is Now
New III-V Device Concept Combines Two Structures
Spottitt Launches Cloud-Based Geospatial Data Analysis On DNV GL’s Veracity Platform
Ensibo Adopts QOS O&M Software To Manage Of 120 MW PV Portfolio
Fenix Reach 150,000 Zambians In Nine Months
ABB Connects Sweden´s Largest Solar Park To The Grid
SMA Surpasses 1 GW Installed Solar In Latin America
K-Space Grows In-line Metrology Revenue
InnoEnergy Start-ups Have Created 1,741 Jobs
Trina Solar Supplies Modules To Ukraine's Largest Solar Power Plant

Search the news archive

To close this popup you can press escape or click the close icon.
Register - Step 1

You may choose to subscribe to the Solar + Power Magazine, the Solar + Power Newsletter, or both. You may also request additional information if required, before submitting your application.

Please subscribe me to:


You chose the industry type of "Other"

Please enter the industry that you work in:
Please enter the industry that you work in: