Bridging the “green” LED gap to provide greener lighting

	Nelson Tansu

Concerns over the rising cost of oil and the environmental impact of carbon emissions have prompted a national discussion about solar, biofuels, nuclear, wind and other renewable sources of energy.
Solid-state lighting has received relatively scant attention by comparison, but Nelson Tansu says it could significantly reduce the amount of energy that human beings must produce to light houses, offices, schools and more.
Tansu, the P.C. Rossin Assistant Professor in the department of electrical and computer engineering and a faculty researcher in Lehigh’s Center for Optical Technologies (COT), notes that about one-third of the energy produced in the United States is used to generate electricity. About 22 percent of this electrical power is used for lighting.
But when this power is harnessed to light an incandescent lamp, or light bulb, more than 95 percent of it is dissipated as invisible infrared light or heat.
A fluorescent lamp is about four times more efficient than a light bulb, but it contains mercury, which can pose environmental and health concerns.
The solution to this low return on investment, says Tansu, lies in solid-state lighting, a technology that relies on light-emitting diodes (LEDs), which emit light from semiconducting materials. LEDs illuminate cell phones, car dashboards and the backlighting in the liquid crystal displays in laptops and TVs, and are also used in a growing number of traffic signals and city and highway billboards.
The LED technologies that color these electronic displays, says Tansu, have the potential to increase the energy efficiency of a typical incandescent bulb by 10 to 15 times or more, from less than 5 percent to approximately 50 percent. LED technologies also promise to achieve two to three times the energy efficiency of a fluorescent lamp.
The federal government’s Sandia National Laboratory agrees. In its website, the lab notes that white LED light now achieves twice the luminous efficacy of an incandescent bulb but only one-third that of a fluorescent lamp. In the future, however, LEDs promise to outpace incandescent lighting by 10 times while doubling the efficacy of fluorescent lighting.
“By greatly improving the efficiency of the 22 percent of electrical power that we currently use for lighting,” says Tansu, “solid-state lighting can significantly reduce the worldwide demand for energy, while still providing reliable and environmentally friendly solutions.”
Tansu and Volkmar Dierolf, an associate professor of physics, recently received a three-year grant from the U.S. Department of Energy (DOE) to study methods of improving the efficiency of white LEDs. Dierolf is also a COT faculty researcher.
The $600,000 award, which is being matched by $150,000 in additional funding from the state of Pennsylvania, is being provided through the DOE’s Solid-State Lighting Core Technology Research (CTR) Program. The agency’s goal is to develop, by 2025, inexpensive and long-lasting solid-state lighting technologies that achieve 50-percent efficiency while reproducing the spectrum of sunlight.

	Volkmar Dierolf

The competitive CTR grants were awarded this year to five U.S. research groups. Only two of the five—Lehigh’s and a group from the Georgia Institute of Technology—are based at universities. The remaining groups are led by industrial consortiums.
A green light to the future
The light generated by an LED is emitted from a semiconducting material—usually indium gallium nitride (InGaN) or gallium phosphide (GaP)—within the LED. A semiconductor is an element or compound that conducts electricity under certain conditions.
An InGaN semiconductor emits light in the blue and green portions of the spectrum, and a GaP semiconductor emits light in the red spectrum. A white LED must mix these colors in the correct proportion to produce white light. It can achieve this through wavelength conversion or through color mixing, in which multiple LEDs, each one the size of a human hair, are combined in a single lamp to produce white light.
The catch, says Tansu, is the relative inefficiency of the green light produced by an LED. When combined with red and blue LED light to produce white light, this shortcoming limits the overall “radiative efficiency” of the white LED light.
The inefficiency of green LED light, says Tansu, stems from a phenomenon called the “charge-separation effect,” which occurs when electron and electron hole carriers are spatially separated inside the nanoscale quantum-well active region of LED devices.
In nitride LEDs, says Tansu, a large internal polarization field exists in the InGaN semiconductor quantum well that is required to generate green light. But the polarization field creates an electrical field inside the active region of the semiconductor, and the electrical field in turn promotes the separation of electron and electron hole carriers in the active region.
“This leads to a reduction in the radiative transition probability rate for electron and electron hole carriers in the active region that is responsible for generating light radiation,” says Tansu.
To achieve a higher efficiency of green LED light and boost the radiative efficiency of the overall LED, Tansu is attempting to engineer a 2- or 3-nanometer structure which allows the electron and electron hole carriers to align more precisely.
“We need to engineer the spatial position of both the electron and the hole carriers to the center of the nanostructure-active region to increase the chance that they will overlap,” says Tansu. “To do this, we use quantum mechanics to design the nanostructures for engineering the electron and hole wave functions, which will result in a significantly enhanced radiative efficiency.”
Tansu reported the results of his work last May in the Proceedings of the IEEE/OSA Conference on Lasers and Electro-Optics (CLEO), which is the major international conference in the fields of photonics and optoelectronics. In August, his article “Polarization Engineering via Staggered InGaN Quantum Wells for Radiative Efficiency Enhancement of Light-Emitting Diodes” was published in Applied Physics Letters (APL). The article was co-authored with Ronald A. Arif and Yik-Khoon Ee, who are Ph.D. candidates in electrical engineering and members of Tansu’s research group.
In November, Laser Focus World, a monthly magazine that covers optoelectronic technologies for 70,000 international subscribers, also reported on Tansu’s use of staggered InGaN quantum wells to enhance the radiative efficiency of nitride LEDs. Tansu and his group, including Hongping Zhao, a Ph.D. candidate in electrical engineering, are pursuing other approaches based on type-II nitride-based quantum wells and InGaN quantum dots. The results of these studies were recently published in APL and the Journal of Crystal Growth.
In a related effort to improve the efficiency of LEDs, Tansu and his group are experimenting with arrays of microlenses to extract, with greater efficiency, the light that is generated in quantum-well nanostructures but then trapped inside the structures in part because of the large refractive-index difference between the gallium nitride and air.
The group published the results of this research in November in Applied Physics Letters. Their article, “Enhancement of Light Extraction Efficiency of InGaN Quantum Wells Light-Emitting Diodes Using SiO2 / Polystyrene Microlens Arrays”, was co-authored by Ee, Arif and Tansu, and also by James Gilchrist, assistant professor of chemical engineering at Lehigh, and Pao Kumnorkaew, a graduate student in chemical engineering. The group reported its results for the first time last May, also in the Proceedings of the IEEE/OSA Conference on Lasers and Electro-Optics (CLEO).
The group’s work with microarrays was featured in January in Laser Focus World, which said the arrays promised to improve light-extraction efficiency of LEDs while overcoming three challenges to previous extraction techniques—cost, scalability and process control.
Tansu is also collaborating with Profs. Rick Vinci and Helen Chan of materials science and engineering on research aimed at reducing defect density by decreasing the non-radiative recombination process in gallium-nitride semiconductors, which in turn will improve the radiative efficiency of the LED. This project is supported by the Division of Material Research of the National Science Foundation (NSF).
Recently, Tansu’s group also received funding from NSF’s Electronics Photonics Devices Technology research program. The total funding from both NSF programs amounts to $600,000 over three years.
Tansu has filed patent applications on the microlens array and staggered quantum wells, as well as type-II nitride-based quantum wells.
Tansu, Dierolf, Arif, Ee and Zhao are affiliated with the COT, while Gilchrist, Chan, Vinci and Kumnorkaew are affiliated with the university’s Center for Advanced Materials and Nanotechnology.