October 6, 2008

University of Delaware's Robert Birkmire

Energy Research News editor Eric Smalley carried out an email conversation with Robert Birkmire, a professor of materials science and engineering and physics at the University of Delaware.

Birkmire is an authority on inorganic solar cells, including crystalline silicon and amorphous and polycrystalline thin-film solar cells. He's director of the Institute of Energy Conversion, a US Department of Energy Center of Excellence for Photovoltaic Research and Education.

Birkmire grows and analyzes thin-film semiconductors for photovoltaic and opto-electronic devices. He tunes the growth process to improve film properties and device performance. He's also finding ways to use sensor technologies to improve thin-film semiconductor manufacturing.

Birkmire received the World Renewable Energy Network Pioneer Award at the World Renewable Energy Congress in 2002.

Birkmire has written more than 170 technical publications and has eight U.S. patents. He received a PhD in physics from the University of Delaware in 1976.

ERN: What are the important or significant trends you see in energy research?

RB: The efforts to develop environmentally benign sources of energy, driven by the realization that energy is the root cause of climate change and that the supply of oil is limited, is the focus of energy research.

Unfortunately, instead of being the leader, the United States has been slow to respond to the need for alternative energy sources. This is especially critical, particularly since the United States is responsible for about 25 percent of the world’s energy consumption.

ERN: What would you like to see happen? Is this different from today's national and global energy research priorities?

RB: A better global recognition of the interrelationships between energy, environment, population and security is needed to accelerate implementation of alternate energy sources.

Further, nearly 35 percent of the world’s population does not have access to electricity. Providing clean forms of electricity to underdeveloped countries needs to be a priority of the industrialized nations. Providing electricity will provide clean water supplies and improve agriculture, thus reducing famine and increasing the overall quality of life.

ERN: What's the general focus of your research, and how does it relate to energy?

RB: The Institute of Energy Conversion (IEC) at the University of Delaware has been involved in the development of thin film photovoltaic technology for over 36 years and was designated as a Department of Energy Center of Excellence for Photovoltaic Research and Education in 1992.

The mission of IEC is to develop the fundamental science and engineering base required to improve photovoltaic device performance, develop processing technologies, and effectively transfer these laboratory results to large-scale manufacturing.

Currently, our research activities include

1) Developing copper-indium-diselenide-based (CuInSe2) materials and devices on flexible substrates and wide bandgap materials and devices for improved module performance and tandem cell applications.

2) Expanding the fundamental understanding of the device operation and processing treatments on doping the cadmium telluride (CdTe), improving it’s electronic properties and formation of a low resistance back contact.

3) Developing all back contact crystalline silicon heterojunction solar cells and providing support for industries in amorphous silicon.

4) In conjunction with industry, evaluating transparent encapsulating materials for flexible photovoltaic modules.

ERN: What's your view of the major thrusts in photovoltaics research: dye-sensitized, organic semiconductor, thin-film, multijunction and variations involving nanotechnology?

RB: In my opinion organic and organic/inorganic hybrid (dye-sensitized) photovoltaics are a long way off in the future (if ever) [for] providing large amounts of electricity. However, for specialty applications that do not require high efficiencies and long lifetime this type of device could be acceptable.

Thin film technologies are emerging as real alternatives to traditional crystalline silicon technologies. First Solar has demonstrated how economies of scale can reduce the manufacturing cost of thin film cadmium telluride photovoltaics and is selling 10- to 11-percent efficient modules.

Amorphous-silicon is on the verge of a major manufacturing expansion with the entrance of Applied Materials and Oerlikon as key suppliers of manufacturing equipment. If the expansion of amorphous-silicon facilities proceeds at the announced rate, there will not be sufficient silane production capacity to accommodate the demand.

Copper-indium-selenide-based photovoltaics has demonstrated the highest efficiency, [about] 20 percent, but has been slower to develop manufacturing capacity. Copper-indium-selenide-based modules have the potential to be the highest-performing thin film module in the future.

There is renewed interest in multijunction-III-V-based devices with the recent development of over 40 percent efficient devices tested under concentration. These structures have the potential to be used in power generating arrays.

Nanotechnology is the buzzword of science these days but in photovoltaics there is no demonstration of a solar cell that takes advantage of quantum effects as the size is reduced to below [about] 30 nanometers. Nanoparticle technology may find use as a precursor material for thin film synthesis and there is potential to develop new materials for the future (beyond 2020).

ERN: Thin-film solar cells seem to be particularly important, especially for ease of manufacturing. Tell me about your lab's research in thin silicon. How does thin silicon compare to other thin-film solar cell technologies?

RB: In the 1990’s, we investigated issues related to amorphous silicon single and multijunction device fabrication in collaboration with the US industry.

Our recent efforts in thin film silicon were focused on metal induced crystallization of amorphous silicon using a cheap low temperature substrate. We are no longer pursuing this, in part because of funding, and in part because we could not see a pathway to growing a thin crystalline silicon film suitable for fabricating a solar cell with reasonable performance. Thin film silicon is the least advanced of the thin film technologies and is not an ideal material to use for thin film solar cells since it has an indirect bandgap thus requiring elaborate light trapping.

ERN: Tell me about cadmium telluride and copper indium diselenide solar cells. What's their potential and how do they compare to silicon?

RB: First Solar is the largest manufacturer of thin film cadmium telluride modules in the world. Their primary market appears to be power arrays due to the perceived potential environmental impact of the cadmium in the module.

To alleviate this issue, an insurance policy that guarantees reclamation of the modules at the end of their life is part of the module cost which is currently less than crystalline silicon modules. However, if there are alternative module technologies at nearly the same cost, this could present a major obstacle for cadmium telluride in the future.

Additionally, based on laboratory research there is not a clear path to improving the cadmium telluride module performance to 15 percent since the highest efficiency laboratory cells are [about] 16 percent.

However, copper-indium-diselenide-based modules have the demonstrated potential of reaching a 15 percent module based on laboratory cells results with [about] 20 percent efficiency but developing manufacturing has proved challenging.

Global Solar, Showa Shell, Honda and Würth Solar each have manufacturing facilities over 20 megawatts [per year] with plans to increase capacity in the next few years. Further, copper-indium-diselenide-based modules are being made on flexible substrates which can have important applications in building integrated photovoltaics and reduce transportation cost compared to glass module -- large manufacturing facilities with distributed module fabrication facilities. The unanswered question is the eventual cost of the copper-indium-diselenide-based module.

In the future, crystalline silicon will continue to dominate the high efficiency module market but Cadmium telluride and copper-indium-diselenide-based modules will be cheaper.

ERN: What's the potential for improving the efficiency of amorphous silicon solar cells?

RB: Several groups have reported small area amorphous silicon/nano-crystalline silicon or micromorph cells and mini-modules with stable efficiency greater than 10 percent so the potential is there. The challenge is to increase the deposition rate of the nano-crystalline silicon material.

ERN: Has development of traditional crystalline silicon solar cells plateaued, or is there more research to be done?

RB: There is more research and development needed. As wafers become thinner, the processing of the wafer to fabricate a device becomes more challenging, particularly, where high temperature processing is used.

The heterojunction structures currently based on amorphous silicon, where all the process is done at temperatures below 200° Celsius has the potential for higher efficiency and reduced manufacturing cost by coupling thin film and crystalline silicon technologies and is particularly attractive in an all back contact configuration.

ERN: What are the milestones to watch for in photovoltaics research?

RB: The milestones to look for in the near future would be

1) Readily available > 20 percent crystalline silicon modules.

2) Bring on-line large capacity amorphous silicon facilities.

3) A large scale copper indium diselenide module manufacturing facility at the 100 megawatt scale.

4) Any photovoltaics technology on a flexible substrate at the100 megawatt scale.

ERN: How useful are solar concentrators for photovoltaics?

RB: Solar concentrators are required for multijunction III-V solar cells because of the device size, manufacturing cost and the enhanced performance. Low-level concentration where tracking is not required could be a cost effective approach for high performance crystalline silicon solar cells.

ERN: Why is there a significant gap between photovoltaic efficiencies achieved in the laboratory and those achieved by industry?

RB: In the laboratory, there are processing conditions you can use that are not acceptable for a commercial product and, in the laboratory.

Yield is not an issue. Lab cells are aggressively optimized with some process condition “on the edge” that is not acceptable for high yield. For thin films, lab cells can use higher cost substrates that are not practical for manufacturing. Plus the size of the champion laboratory cell or module is in general smaller than a commercial module. Additionally, there are inherent area-related losses going from the laboratory cell/module to a commercial module.

ERN: How much of the US and world electricity needs can be met by photovoltaics?

RB: This is always an interesting question. In the 2020 to 2030 time frame, I think photovoltaics could generate 20 percent of the electricity. Research from the National Renewable Energy Laboratory (NREL) indicates that photovoltaics will begin to impact base-load generation at about a 10 percent level.

ERN: What are the important social questions related to energy?

RB: When over two billion people have no electricity and live in poverty -- this a big societal issue. The access to electricity improves agriculture and quality of life (water for irrigation and clean water to drink) and provides a means of communication with the world.

ERN: What are your thoughts on the state of public understanding of energy and energy research?

RB: In the US, I suspect that the average person is confused. There is no leadership at the national level and the mis-information that is spread by the existing energy companies and special interests groups is exceptionally misleading. On the other side, media hype based on press releases or other non-refereed reports can lead to unrealistic expectations such as solar cells that can be painted on your house being available soon.

ERN: What could be done to improve the pursuit of energy research in terms of business trends, politics, and/or social trends?

RB: Leadership at the national level is needed to address short- and long-term solutions to the energy future coupled with the reality that energy and the changes in climate are directly connect to burning fossil fuels. Thus, any energy plan/policy needs to be coupled to climate change and reduction of carbon dioxide (Co2) emissions.

ERN: In a perfect world how would we get our electricity?

RB: A mix of renewable energy source that have minimal impact on the environment and are safe and secure.

ERN: In terms of energy and anything affected by energy, what will be different about our world in five years? In 10? In 20?

RB: In the US, the next five years will be the transition period where energy efficiency is a primary focus of energy policy along with the emergence of clean energy technology into the marketplace.

In ten years a measurable fraction of our energy will come from renewable sources and competing technologies will start to define the energy for the future.

By 2030, there will be a clear pathway to how energy will be produced -- photovoltaics will be one of the major contributors in the energy mix.

ERN: What do you imagine you will be working on in five years? 10 years?

RB: I suspect that I will still be working in the area of photovoltaics on ways to improve the performance and on emerging technologies. One of the key areas for photovoltaics will be to address the storage issues which I have an expanding interest in becoming involved.

ERN: What got you interested in science and technology?

RB: I grew up in an exciting time in the development of science and technology. I used to watch a TV show "Watch Mister Wizard" which demonstrated the wonders of science. It was an era when Albert Einstein was a hero, the launching of Sputnik in 1957 and the inspiration of John Kennedy to go to the moon.

ERN: What's the most important piece of advice you can give to a child who shows interest in science and technology?

RB: Convince the child that science is not difficult as some make it out to be and it is fun and rewarding to understand how things work whether from a fundamental science point of view or an engineering perspective.

ERN: What's the most important piece of advice you can give to a college student who shows interest in science and technology?

RB: Go for it! If you have the interest and have the aptitude for math (the language of science and engineering), science and engineering will be a rewarding and exiting career.

ERN: What books that have some connection to science or technology have impressed you in some way, and why?

RB: There was a book by George Gamow, “Mr. Tompkins in Wonderland”, which is a great way to learn about modern physics. There is a series of books that followed on science.

ERN: Is there a particular image (or images) related to science or technology that you find particularly compelling or instructive?

RB: At IEC we deposited 110 feet of copper indium germanium diselenide (CuInGeSe2) on a 10-inch wide polyimide web coated with molybdenum in our multi-source in-line evaporator. The average cell efficiency of small area devices made on the web is [about] 10 percent. I believe that this technology has the potential to reach 15 percent at the module level and will be one of the competitive technologies for the future.

Back to ERN October 6/13, 2008



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