Photovoltaics/Amorphous/Stanford 040611



Stanford Ovshinsky, the man behind amorphous PV...
Stanford Stands for Stamina
by Donald Georgi
When the Webmaster of this publication was a tot, he would initiate a LEGOs building session by taking his entire box of individual pieces, dump them in a single pile on the floor and ponder it pensively to determine the goal for the day.  Similarly, in writing a summary of Stanford Ovshinsky,  the founder of Energy Conversion Devices (ECD), one might follow a similar pattern of dumping all the salient features, accomplishments, and ups and downs of the man on the table. Initially, there would be a huge pile of pieces in the puzzle, possibly too confusing to fit together into a big picture. Nevertheless, we will try to piece them together!

Stamina

The repeating feature which emerges from Mr. Ovshinsky’s many efforts is his stamina. From his early 30’s, Mr. Ovshinsky has focused on the field of amorphous materials and metal hydrides, sometimes developed with limited success. But, his continual perseverance and stamina have carried him to a position of renown. He has successfully contributed to technologies which improve the quality of life and leave a cleaner planet in doing so. The path has not been easy because Mr. Ovshinsky’sdevelopments have not blossomed easily and the monetary returns to provide continuation of  funding for new progress have not always appeared. However, the Ovshinsky aura has flourished over the last half century primarily because of his alternative contributions most visible in the field of amorphous materials and metal hydrides.

Semiconductor Electronics

As electronic technology converted from vacuum tubes to semiconductors in the 1950’s, practically all the growth employed crystalline structures. Although complex, these materials were rigorously understandable so that the path from discrete transistors could move to higher density integrated circuits. At that time, improvements in crystalline semiconductor fabrication happened so rapidly that the Director of R & D at Fairchild Semiconductor, Gordon MooreA, made a statement in a 1965 paper which predicted that the number of transistors on a chip had been and would continue to double annually. Years later, Carver Mead, a  pioneer in the development of the MOSFET,  coined Moore’s observation as ‘Moore’s Law’ leading to all sorts of perceptual problems such as the ‘law’ now slowing down to doubling only every two years. It has also caused ‘pundits’ to expect the law to apply to other fields such as batteries which should have energy density doubling in similar time frames.

Amorphous vs. Crystalline

Crystalline semiconductor fabrication is not something one does on their home stove on weekends. Very complicated and very expensive equipment and processes are required to produce semiconductors. As both the field and the processes developed, the complexity gave Mr. Ovshinsky reason to pursue amorphous technology as both a better and cheaper way to make semiconductors.

The conventional crystalline semiconductor begins with growing silicon crystals which are cooled slowly. Crystals have small numbers of atoms arranged in an orderly patternB. Ingots of single large crystal pure silicon are grown in a vacuum, then sliced and further processed to build the final transistor array.

Conversly, the atoms of an amorphous structure are arranged in a random array rather than an orderly pattern. To get the same melted silicon to form amorphously, it is cooled quickly, producing a noncrystalline structure. Window glass is a good example of  amorphus structure. The structure arguably could lead to fabrication technology which would allow greater sizes to be produced at lower costs. This concept was proven correct by Mr. Ovshinsky over the years.

Developing the Amorphous Technology

The amorphous semiconductor concept was to be researched and developed in Mr. Ovshinsky’s company, Energy Conversion Devices, formed in 1960 with his wife Iris. They funded the comapny with money from Stanford’s inventions. The capital requirements for this pursuit caused the business plan to change to a public company in 1964. He now had other people’s money to assist him in the quest. He also was able to attract many highly skilled people to contribute to this exciting pursuit of technology which differed from the mainstream of current thiinking.  By 1968, the company was able to announce its first amorphous switch relying on his personally coined principle “Ovonics”, a name which has since been extended to  divisions of ECD.

In the late ‘60s, Ovshinsky’s amorphous silicon challenged crystalline, but the direction of the technology was to favor crystalline. First, crystalline was already robustly established and, as previously mentioned, was rigorously understood to allow for continued technological growth. The second technological force which was to help crystalline was that the direction of semiconductors, which packed an ever increasing number of transistors in an ever shrinking space. The fabrication need was not for greater area, but for greater density in a modestly increased area, with the ability to manipulate ever smaller-sized elements. As the rest of the century unfolded, the desirability for crystalline semiconductors expanded, instead of amorphous technology.

Amorphous PV

Photovoltaics (PV) also entered the arena after WWII as a technology which would use the newly developing crystalline semiconductor process since the basic structure of PV was a semiconductor diode. Unfortunately for PV and crystalline fabrication, PV did not thrive on getting smaller and smaller diodes in smaller packages. With PV, the area of sunlight capture is directly related to the amount of power produced, and the first solid applications, orbiting satellites, required large panels of PV to generate enough electricity to both operate the systems and charge the batteries. Because of this area requirement, crystalline PV  had a problem of material and fabrication cost, since so many single crystal cells have to be connected together to form a high power array.

In 1966, Yoshihero HamakawaC of Osaka University discovered amorphous silicon photovoltaic cells, but the Japanese did not pick up on the idea. Rather, RCA continued to pursue the technology, applying for patents and presenting its work at the 1976 IEEE Photovoltaic Specialists Conference. After that time, Solarex bought the RCA amorphous silicon R &D operation.

Adobe Photoshop ImageAmorphous solids, like common glass, are materials whose atoms are not arranged in any particular order.

Amorphous silicon has random structural characteristics with ”dangling bonds” where atoms lack a neighbor to which they can bond.  However, amorphous silicon can be deposited so that it contains a small amount of hydrogen. The result is that the hydrogen atoms combine chemically with many of the dangling bonds, essentially removing them and permitting electrons to move through the material. (Photo is courtesy of http://www.eere.energy/gov/solar/silicon.html  +

Again amorphous technology continued to devleop because of the Ovshinsky stamina. Amorphous structures had not won a significant role in transistor technology, but it just barely remained relevant by finding application to large surface areas in copy machines and rewritable materials on CDs. Although ECD licensed its patents to the companies to create amorphous semiconductor products, the financial success did not blossom.

But, Ovshinsky knew amorphous technology from his transistor pursuits, and he had an opportunity to expand his concepts for useage in PV in the ‘70s when oil embargoes from the Middle East demonstrated the U.S. energy weakness because of the instability of foreign governments. PV was one of the new energy sources, imported directly to the U.S. from the sun. In addition there was the growing need for environmentally clean fuel. Oil carries a nasty carbon atom which upon combustion is converted into excess carbon monoxide, carbon dioxide and cancer producing particulates. Pure sunlight with PV electrolyzes water directly into hydrogen fuel  without  messy carbon-oxygen interactions. Then too, useful oil in the ground will only be around for less than a hundred years, wheras sunlight is projected to be available for a few billion years.

Adobe Photoshop ImageA typical triple junction silicon solar cell is composed of three semiconductor layers stacked on top of each other.  The bottom cell absorbs the red light while the middle cell absorbs middle frequency light and the upper layer, the blue part of the spectrum  

(Photo is courtesy of http.//:www.eere.energy/gov/solar/silicon.html  +

The jewel in the amorphous concept for PV was its characteristic of lending itself to a manufacture in large areas, leading to cost effective continuous fabrication processes in which  PV material could.be deposited on a continuous roll of inert sub plate and then sliced into sheets such as those seen in the photos of the SolarMine facility. The continuous sheet has many advantages.

The first is cost improvement by limiting the number of interconnects needed per square foot. If one examines a crstalline aerospace PV panel, a myriad of small cells are seen interconnected by hand or machine. Not only are these interconnected cells expensive, but they also add a reliability problem due to cracking or breaking. Once the cell connection is broken, and the string containing the cell is no longer able to contribute to panel operation. However, in the continuous sheet, there are no minuscule interconnects, and the finished sheet is mechanically robust. If it is pierced by a foreign object, the panel can continue to function.

The next advantage is its large single cell area. Such a cell has high current, but low Voltage. In crystalline  PV, many small cells must be connected in parallel to obtain higher current.

By having the amorphous cell built, on a structurally economical and rigid substrate, the balance of plant (BOP) expenses of mounting and connecting such panels are reduced. Note the simple ruggedness of the supports in the SolarMine photos.

Business and Technical Challenges

One of the first limitations of amorphous PV materials is its  limited efficiency in converting sunlight into electricity. Early on, the amorphous PV efficiency was in the 6% ballpark, while commercial crystalline operated at double that efficiency. This meant that despite the economies of manufacturability, the amorphous material had to overcome a 50% shortcoming in dollars per Watt. As new ways of  increasing efficiency are  found such as triple layer devices which capture greater amounts of solar energy in each sandwiched layer, the amorphous PV efficiency has increased so that lab versions are now operating at 13% efficiency. This higher efficiency should be made available to commercial production. Of course, the competetion, crystalline PV, has also increased efficiency with multilayer construction and points to a possible 39% efficiency at an unknown cost. Amorphous silicon has one more challenge; the popular amorphous hydrogenated   alpha silicon PV materials suffer from the “Staebler-Wronski” effect which is a continual time based degradation of electrical properties, poor performance in the red portion of the spectrum, and low hole mobility limit amorphous pv efficiencyD.

Adobe Photoshop ImageA separate factor sneaks in to support amorphous PV which surprisingly has nothing to do with economics, but everything to do with aesthetics. The amorphous fabrication substrate can be adjusted to produce attractive appearing PV sheets which look like roof shingles and are flexible enough to be handled similarly. In appearance, to the casual observer, the amorphous PV roof may be undetectable from a regular roof   a factor which has already been instrumental in the choice of amorphous in some installations. +

Ovshinsky pursued the amorphous PV in the early ‘70s and between 1976 and 2003 had used his stamina to attract $290 million for PV development. For ECD, this battle between amorphous and crystalline for market acceptance and dominance is more sophisticated than just efficiency or dollars per Watt when considering both balance of plant costs, maintenance costs and lifetime. The proof of the elimination of light induced cell degradation and poor manufacturing yield will be displayed not just in the laboratory, but in final costs and long term performance in PV fields and rooftops. If one type of PV material costs significantly more to install, or maintain, or drops original output too steeply over the years, the total cost may favor other PV types. Installations such as SolarMine at the ChevronTexaco field will provide real world data for amorphous PV.

Metal Hydride Batteries

The other half of the Ovshinsky stamina has been applied to metal hydrides. The names of Ovshinsky, Ovonics, ECD and Nickel-metal hydride batteries continually intertwine. This did not come easily. The catalysts for the creation of Nickel-metal hydride chemistry are most likely the Nickel-cadmium and Nickel-hydrogen batteries. Nickel-cadmium has a long and satisfactory development from pocket plate to sintered construction  in the Twentieth Century, providing popular rechargeable performance in everything from space vehicles to portable tools. In the last part of the 20th Century, Nickel-hydrogen provided such excellent performance that it continues to dominate space vehicle battery selection to this day. The Nickel-hydrogen with its positive electrode construction of nickel hydroxide is also found in the Nickel-cadmium cell, but the cadmium anode material is replaced with gaseous hydrogen which is oxidized to water by the OH- ion during discharge. The choice of hydrogen gas necessitates a costly pressure vessel for the construction. (Unfortunately, due to language understanding limitations, many Nickel-metal hydride batteries are often confusedly referred to by Asian sources as Nickel-hydrogen batteries.)

If the hudrogen of a Nickel-hydrogen battery could be stored in another form, the pressure enclosure requirement would be reduced or removed. That is exactly what the Nickel-metal hydride battery offers. Storing the hydrogen in a metal hydride, which on discharge is oxidized to the metal and water, produces electrons. On recharge, the hydrogen in the water combines with the metal so that pressure does not build and the enclosure does not require extensive pressure handling ability. An added advantage of the hydride is that the metal hydride has a higher energy density than the cadmium electrode which results in a higher capacity and longer service life than the Nickel-cadmium battery.

Through the 1980s, Mr. Ovshinsky and his team at ECD repeatedly improved the constituents of the hydrogen absorbing hydride, a challenging task considering the range of alloys from rare earths to titanium/zirconium substitutes. Finding the right combination of electrical properties and manufacturability has been an ongoing challenge which has stretched ECD’s monetary resources so that maximum profitability never has been achieved. Still the metal hydrides have been constantly improved by ECD to provide higher storage density and lower costs in Nickel-metal hydride electrical characteristics. Reduced costs provide the consumer with an excellent performing, reasonably priced product.

The Killer Market

Unfortunately, the big payoff, the electric vehicle mandated by the California Air Resources Board  in the 1990s, never could achieve the range and cost requirements, so the market never materialized. Taking its place however is the hybrid vehicle which, if widely accepted, would provide a giant new market for rechargeable batteries. Nickel-metal hydride is both the current battery of choice and could extend its position as the market expands. Costs in producing both Nickel-metal hydride batteries and others such as Lithium-ion still limit their implementation, opening the door to alternatives such as fuel cells with hydrogen storage tanks. Still, hybrids which replace the  internal combustion engine with a fuel cell still might find better overall performance with a hybrid configuration which may use a Nickel-metal hydride battery.

Metal hydrides for Storage

More of Stanford’s stamina continues onward as he looks at giant markets evolving which could utilize his knowledge and development expertise with metal hydrides. There is a big quest for hydrogen powered electronic devices and transportation, for which the fuel cell is a natural contender. In the pursuit of fuel cells to power portable electronics, methanol fuel is getting the highest amount of interest, but metal hydrides could find success in hydrogen storage.  

In transportation, metal hydrides could be a major storage containment method for the hydrogen fuel. Early fuel cell powered autos used reformers to operate with gasoline, natural gas or some form of hydrocarbon fuel, but there are long term reasons for not continuing to pursue this approach. First, on board reforming is very complex, very expensive and very unreliable for the life needed in an automobile. Other reasons for not wanting gasoline or natural gas on board relate to the declining availability of oil and natural gas with escalating costs putting greater pressure on the national balance of payments. The carbon based emissions also continue to be a part of refromed fuel.

At this time in anticipation of a hydrogen economy, most transportation prototypes use pure hydrogen as the fuel which, if produced from PV or other renewables, can stop the air pollution emitted by vehicles. Hydrogen can either be stored as a gas under great pressureG, or in novel ways.

Storing compressed hydrogen gas is not cheap. It takes about 30% of the heat value in the hydrogen stored to compress it to 5000 psia.  The Ovshinsky alternative is to store the hydrogen in a metal hydride where the metal is an alloy of many metals. There are other hydride mixes which utilize lithium, or borohydratesH. By comparison, the total energy to store and recover hydrogen in  a metal hydride system is about 12.5%. Storage pressures are reduced to 400 psia. There are many factors which need to be resolved relating to fueling, temperature of operation and, of course, costs. With the decades of experience ECD has amassed in pursuing battery improvements, they are poised to understand the opportunities for metal hydrides for on board vehicle storage.

The Pioneering Spirit

The Stanford stamina continues to be displayed in these tenacious pursuits as he enters his octogenarian years. He continues to show his unwavering belief in being able to contribute new technology, methods and products to make the world a cleaner place to live. The efforts which he, his wife and his business associates have applied to amorphous materials and metal-hydrides are a part of the continuing pioneering attitude of this great nation.

Reference
A. Moore’s Law, IEEE Spectrum, December 2003, pp. 32-35
B. What is a crystalline material? http://mrsec.wisc.edu/edetc/amorphous/
C. Practical Photovoltaics, Komp, Richard, 1995, p.42
D. Amorphous and Thin-film Silicon, Nelson, et. al. NCPV and Solar Program Review Meeting 2003, p.1.
E Handbook of Batteries, Linden & Reddy, Third Edition, p. 29.1
F. Using Metal Hydride to Store Hydrogen, http://www.osti.gov/bridge
G. 10,00 PSI Milestone for Transportation Fuel Cells, Batteries Digest Newsletter, Issue 69, pp2-3
H. Sodium Boro-Hydride for Safer Fuel Cells, Batteries Digest Newsletter, Issue 71, pp 2-3


BD