Mind The Gap: Seeking Solar Efficiency
In 1954, three scientists at Bell Telephone Laboratories in Berkeley Heights, N.J., used an array of silicon strips to absorb sunlight and create an electrical current.
The modern solar cell was born.
Harnessing solar energy wasn’t a new idea at the time. More than a century earlier, the French physicist Edmond Becquerel discovered the photovoltaic effect, the basic process in which photons — packets of energy from the sun — are converted into energy. It is said that in 1931 Thomas Edison told industrialists Henry Ford and Harvey Firestone: “I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.” In the decades that followed the Bell Labs’ discovery, scientists across the globe were determined to create a more efficient solar cell.In the late 1970s, Hameed Naseem was one of them. The young physics student at Panjab University in Chandigarh, India, was enthralled by the notion that humanity’s future energy source could come from the sun. He carried that enthusiasm to his studies at Virginia Polytechnic Institute and State University, where, after receiving a master’s degree in physics, he switched to materials engineering science for his doctorate.
“Back then there was a lot of hoopla about solar energy,” Naseem said. “There was a lot of excitement and an emphasis on research in photovoltaics. I was part of that wave. The field became hot, and then super hot, and then very cold. Most people got out of it, but I never did. I believe it is where the future of humanity is in terms of energy. It’s a free source of energy. You don’t have to wait for millions of years for nature to convert wood or living matter into oil or gas, which we then consume in a short span of time.”
Because of the problems of global warming and the limited supply of fossil resources, future energy resources are expected to be sustainable and renewable. The U.S. Energy Department’s SunShot Initiative offers a roadmap for solar to provide 14 percent of America’s electricity by 2030 and 27 percent by 2050.
Naseem, professor of electrical engineering, believes that by the year 2100, a large portion of the world’s energy will come from solar. Right now, it stands at less than 1 percent.
“Despite so many years of singing songs about photovoltaics as a better alternative to coal, petroleum and natural gas, only a small fraction of the world’s energy is produced this way,” Naseem said. “The problem with solar energy has been its cost per kilowatt hour. We have to beat the prevailing rate of producing electricity. As long as the electricity you produce by photovoltaics is more expensive than that, people are not going to adopt it because they look at their financial bottom line.”
Naseem has spent the better part of the last four decades trying to make solar energy practical and feasible. He focuses on the refinement and improvement of solar cells for a process called photovoltaic power generation, which uses semiconductors and solar cells constructed of silicon, the second-most abundant material in the earth’s crust.
INCREASING EFFICIENCY, REDUCING PRODUCTION COSTSThe photovoltaic effect is the method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons. Solar efficiency is measured by how much power enters the solar cell (light) versus how much power comes out of it (watts), so if 100 milliwatts of light power enter a 1 by 1 centimeter solar cell and 30 milliwatts of electricity is produced, then the cell is considered 30 percent efficient. For maximum efficiency, the solar cell material must be of a dark hue, which is why solar panels almost always are black or dark blue.
Solar cells are made out of materials called semiconductors, and in each semiconductor there is a small energy range in which no electron states can exist. That range is known as a band gap, and light with energies less than the band gap escapes the device and is not converted into current.
“As you decrease the band gap, the electrical current goes up because more photons get absorbed,” Naseem said. However, electric power is the product of output current and output voltage, and the voltage produced increases the band-gap energy of the semiconductor. This can cause a decrease in solar-cell efficiency. The key to making the solar cell more efficient is to maximize the power by optimizing the band gap.
“It’s a tricky thing,” Naseem said. “It’s a balancing act.”
Through the past 25 years at the University of Arkansas, Naseem and his graduate students have found ways to increase sunlight-to-electricity conversion efficiency and reduce the cost of expensive materials needed for solar-cell production. These technological breakthroughs will decrease cost-per-watt production of solar electricity to a point at which it can compete with traditional, fossil-fuel-based methods.
Most solar-cell technology is silicon based, and there are three primary types of silicon solar cells, each named after the crystalline structure of the silicon used during fabrication:
- Mono-crystalline silicon, which has a single and continuous crystal lattice structure with practically zero defects or impurities.
- Poly-crystalline silicon, also called poly-silicon, comprises discrete grains, or crystals, of mono-crystalline silicon that create regions of highly uniform crystal structures separated by grain boundaries.
- Amorphous silicon is an entirely non-crystalline form of silicon that can be thought of as grains the size of the individual atoms.
Many commercialized solar cells incorporate amorphous silicon and poly-silicon, which have acceptable conversion efficiency and cost much less than mono-crystalline silicon.
The process developed by Naseem, known as top-down aluminum-induced crystallization, creates poly-silicon with crystal grains 30 times larger than grains currently produced in the photovoltaic industry. Standard poly-silicon contains grains of 0.5 to 1 micrometer, which is one-100th the diameter of a human hair. Naseem’s process yielded a grain size up to 150 micrometers, which is important because the performance of a photovoltaic device is limited primarily by defects at the boundaries of crystal grains. Increasing the size of crystal grains decreases the number of boundaries.
Further, traditional processing of silicon-based cells requires a heating temperature of 1,000 degrees Celsius to cause the silicon to reach a crystalline state. Naseem’s method of converting amorphous silicon into poly-silicon can be done at temperatures between 100 and 300 degrees Celsius, which saves time, materials and energy.
In November 2009, Douglas Hutchings, a doctoral graduate of electrical engineering, partnered with Naseem and students in the Sam M. Walton College of Business to start a company, Silicon Solar Solutions LLC, which is commercializing a process to crystallize amorphous silicon into large grain poly-silicon with unparalleled grain size and ease of processing. The company holds the licenses from the university to five patents on which the technology is based. Silicon Solar Solutions has produced prototype solar cells that meet or exceed some performance metrics of cells made by major manufacturers.
STEPS TOWARD SUCCESS
The Photovoltaics Research Lab comprises two rooms at the Arkansas Research and Technology Park in south Fayetteville. Naseem’s laboratory is part of the Green Renewable Energy Efficient Nanoplasmonic Solar Cells Center, which is comprised of five institutional partners: the University of Arkansas campuses at Fayetteville, Little Rock, Pine Bluff and Fort Smith and Philander Smith College in Little Rock.
A device sitting on a table in the corner in one of the rooms is the original vacuum deposition chamber built under Naseem’s supervision after he came to the university in 1985. Freddy Goh, the first doctoral student to study under Naseem, built the deposition system in 1987. Since then, Naseem and his students have been fabricating thin films of materials such as silicon in these chambers to observe how the materials react under different environments.
“Within a six-month period, we were able to get 7-percent efficient amorphous silicon solar cells,” Naseem said. “That was the top of the line at that time.”
Goh, after earning his doctorate, worked for Texas Instruments then founded a solar energy company in his native Singapore. A member of the Arkansas Academy of Electrical Engineers, Goh
is now the chief technology officer for one of Europe’s largest solar firms.
A few feet away from the original deposition system is the stainless steel semiconductor cluster tool. A circular array of extending vacuum chambers makes the device look like a silver mechanical octopus.
“We have one, two, three, four and five,” Naseem said, counting off the vacuum chambers. Motioning toward the front of the machine, Naseem points out a chamber called the hydrogenation chamber. That is the one in which Seth Shumate, one of Naseem’s doctoral students, discovered the self-aligned hydrogenated selective emitter for N-type solar cells. The chamber has a tungsten filament, similar to a light bulb, which heats to 1,900 degrees Celsius. When hydrogen is introduced to the chamber, it hits the surface of a tungsten filament, separating the hydrogen atoms. The single-step method increases solar power conversion efficiency and reduces the amount of silver needed to produce high-efficiency solar cells, thereby lowering material costs.
Shumate is chief technology officer for Silicon Solar Solutions.
If it is successful, the emitter would represent the single largest technology leap in solar-cell efficiency since 1974, Hutchings said. Picasolar, a graduate business plan competition team built around Shumate’s invention, took the MIT NSTAR Clean Energy Prize in Boston in May, winning $250,000. Picasolar won $313,500 in spring 2013 in graduate business plan competitions. The team is using the winnings to transform into a start-up company that will market the hydrogen selective emitter to solar cell manufacturers.
Silicon Solar Solutions shares equipment with Naseem in the Photovoltaics Research Lab, and the professor is “definitely very much involved” with the company, Hutchings said.
“I have always referred to him as the ‘Head of R&D,’” Hutchings said, referring to research and development. “He has some tremendous ideas for improving solar cells in a variety of ways and it can be a challenge to focus on those that are nearer to commercial viability. He is a very good complement to the rest of the team.”
Each advancement in his lab is a step closer to making the widespread use of solar energy a reality, Naseem said.
“As more people become aware of the problems associated with greenhouse-gas emissions, the demand for sources of clean energy goes up,” he said. “It’s very important that you continue to develop a technology that is sustainable. We have to find alternatives. This is where we are now. Now is a good time to develop solar. I predict it will take off and become a prolific and essential contributor to the nation’s power grid.”