Bell Labs Improves Solar Cells

Date May-June, 1981

One of a long series of solar cells developed by Bell Laboratories, the liquid-junction solar cell of 11.5 percent efficiency raised the hope that liquid-junction devices could compete with solid-state devices.

Locale Murray Hill, New Jersey

Key Figures

• Adam Heller (b. 1933), member of the technical staff at Bell Laboratories
• Barry Miller (b. 1933), member of the technical staff at Bell Laboratories
• F. A. Thiel (b. 1930), member of the technical staff at Bell Laboratories

Summary of Event

Efforts to harness the Sun’s energy for human purposes are as old as history. Solar panels and passive solar heating technology is but a development of the animal skin hung at night over the hut’s open window. The solar furnace is a high-tech form of the burning glass, known for centuries. The Sun as heat source has long been used.

Use of the Sun for other kinds of energy is relatively recent. In the late 1800’s, it was found that light shining on substances such as zinc or cadmium sulfides induces a flow of electrons that can be made to do work in an external electrical circuit or be stored in a battery. Later, sunlight was used to bring about chemical reactions, notably the production of hydrogen gas, which can be stored and used later as a fuel. By processes like these, the Sun’s energy can be saved for use even when it is not shining brightly.

Utilization of solar energy for direct production of electricity requires cells made of semiconductors. These are materials in which valence (bonding) electrons are not normally free to migrate and carry an electric current, but can be excited to a conducting state with a small amount of extra energy. The standard example of a semiconductor is the element silicon, in which a bonding electron can be raised to a conducting energy state, leaving behind a positive “hole” in the body of the silicon crystal. Both electrons and holes can act as charge carriers in conduction of current.

Taking this a step further, if the silicon is “doped” with a small amount of phosphorus, the extra electron in the phosphorus atom (compared with the number in silicon) is a conducting electron, and the result is n-silicon (negative silicon, because of the extra electrons). Similarly, a p-silicon (positive silicon) can be made by doping with aluminum, which is missing an electron compared with silicon. Such a material conducts electricity via the holes, which migrate within the body of the material as electrons do in n-silicon. Compounds of a Group III and a Group V element (such as gallium arsenide, indium phosphide, and others) can also be made to show n- and p-behavior.

A silicon crystal by itself can absorb solar energy to produce an electron and a hole, but these recombine quite rapidly and do no useful work. However, a thin p-silicon layer on an n-silicon base (or vice versa) creates a potential surface that holds electron and hole apart long enough that the electron can be drawn off to flow through an external circuit. This is, at the simplest, how a solar cell works. Such a cell, employing crystalline silicon, can have a solar-to-electrical energy conversion efficiency of about 20 percent. Other materials can push efficiency close to 30 percent. The difficulty is cost: Pure, carefully “doped” crystals are expensive. Polycrystalline materials are cheaper but much less efficient.

One solution to this problem that appeared promising for a time was that of combining the semiconductor device with a chemical half-cell consisting of oxidized and reduced species in solution in water or other media, with an inert electrode-like platinum to carry current from the cell. This would give a number of advantages. The dissolved species could be oxidized and reduced forms of some inorganic ion like I- and I₃^-, S^3- and S_n^-, or vanadium(II) and vanadium(III), the system used in the cell that is the subject of this article. This would give some control over the cell’s voltage, as the concentration of dissolved ions alters the voltage of the half-cell. The potential difference between solid and solution could make separation of electrons and holes cleaner and more complete, suppressing unproductive recombination. All of this would lead to more efficient production of electricity in an external circuit.

Moreover, if the liquid half-cell contains species that can be reduced or oxidized to usable products, these can be removed and used as needed. The following are examples:

At the cathode: H⁺ + e^- → 1/2 H₂;

at the anode: Br → 1/2 Br₂ + e^-.

Other products are possible, including fuel gases like methane and ethane from inexpensive starting materials like acetic acid.

At the beginning of the 1980’s, the production of electricity was the major goal. More than a decade of development of liquid-junction cells culminated in the liquid-junction solar cell, announced in 1981 by Drs. Adam Heller, Barry Miller, and F. A. Thiel, all of Bell Laboratories (one of the major research centers in the area), in a publication in Applied Physics Letters. Technically, their cell was a p-indium phosphide/vanadium(II)-vanadium(III)-hydrochloric acid/carbon cell. Its 11.5 percent efficiency was sufficient to raise the hope that liquid-junction cells might be competitive with all-solid-state solar cells, which were well above this efficiency at the time, but only in the expensive crystal form. Other liquid-junction cells exceeded 14 percent efficiency, but only with artificial light sources. By the mid- to late 1980’s, however, solid-state crystalline cells of silicon and other elements reached efficiencies near 30 percent, and even solid-state polycrystalline and amorphous silicon, in inexpensive thin films (the kind used in solar-powered calculators), could produce electricity in the 10 to 15 percent efficiency range.

It is worth noting that all the liquid-junction work was done with crystalline semiconductors; the cost-cutting thin-film materials were not evaluated. The net result was that, for electricity production, the liquid-junction cells took a backseat to solid-state devices by the late 1980’s, and research on liquid-junction devices came to a near standstill. Tandem solid-state cells (two piggyback cells absorbing at different wavelengths in the visible spectrum) achieved more than 30 percent conversion efficiency by the early 1990’s. In addition to the double-absorption feature, these cells relied on concentration of sunlight by mirror and lens systems.

What about liquid-junction cells for production of chemical materials with solar energy? By their nature, these must have a liquid portion containing dissolved chemical materials. The dissolved species are oxidized or reduced to usable forms by the electron flow generated by the semiconductor part of the device. Many problems arise, the most prominent being that when the semiconductor forms an anode, it consumes itself because the holes (positive centers) that gather at the surface of the anode are powerful oxidizing agents (electron removers), and if diffusion processes do not bring chemical materials from the solution to the surface quickly enough, the holes oxidize the adjacent portion of the anode. Methods were developed to suppress this phenomenon but not to eliminate it.

Reduction reactions at the cathode are more successful because the cathode is more stable, but even here chemical limitations take over. Easily reducible species such as hydrogen ion react at the semiconductor cathode, but many metals are more difficult to reduce and do not yield to solar-induced reactions. Hydrogen production is, in fact, the most promising of the solar chemical reactions, particularly as the product, hydrogen gas, can be easily stored and used as a fuel. Research in this area continued, but at a slow pace.

Significance

Given that research on liquid-junction solar cells has fallen off and solid-state devices are receiving nearly all the attention in solar generation of electricity, Heller, Miller, and Thiel’s cell in itself is not an important landmark. Even the stacked cells of gallium arsenide and crystalline silica that broke the 30 percent efficiency barrier in 1988 have only a symbolic impact on the effort to use the Sun’s energy to generate electricity, particularly as they are far from ready for commercial application.

Taken collectively, however, these cells and a host of others over decades of development have an impact that is more than merely symbolic. They represent the inch-by-inch progress toward solar electricity as a power source competitive with grid-distributed commercial electricity. Solar electricity was first considered as a commercial possibility after the oil crisis in 1973. At that time its cost, with existing technology, was about fifteen dollars per kilowatt hour. By the late 1980’s, that figure had fallen to thirty cents per kilowatt hour, within striking distance of conventionally generated power, at six to twelve cents.

Already, solar electricity is in use in remote places where power lines cannot be brought in because of distance or expense: telecommunications relay stations, remotely operated lighthouses, and most particularly, the various space probes, vehicles, and orbiters. In some cases, solar electricity is inexpensive enough to be fed into the power grid at peak demand times when the price paid by power companies is high. Many solar power plants have been built in the United States and around the world. Solar One and Two in Barstow, California, demonstrated the feasibility of solar power generation in the 1980’s and 1990’s, with Solar Two using molten salt as a storage medium. The world’s most productive photovoltaic power stations include plants in Serpa, Portugal, in numerous locations in Germany, and in Springerville, Arizona. The United States is the second-leading solar power-producing country following Japan, and Germany is third. Other countries with significant solar energy production include Australia, the Netherlands, Spain, Italy, and France, among others. Incentives for developing this renewable source of energy continued as oil prices rose precipitously in the early twenty-first century.

Bibliography

“The Bright, Wet Look for Solar Cells.” Science News 124 (December 10, 1983): 376. One of the few mentions in the popular science press of liquid-junction cells at the time when they were regarded as competitive with solid-state.

DeMeo, Edgar A., and Roger W. Taylor. “Solar Photovoltaic Power Systems: An Electric Utility R and D Perspective.” Science 224 (Arpil 20, 1984): 245-251. A thorough discussion of solar-generated electricity from the commercial standpoint, with projections over two decades. Cost and output projections; some discussion of technologies.

Hamakawa, Yoshihiro. Thin-Film Solar Cells: Next Generation Photovoltaics and Its Applications. New York: Springer-Verlag, 2004. Covers many aspects of thin-film semiconductors. Comprehensive, with bibliography and index.

‗‗‗‗‗‗‗. “Photovoltaic Power.” Scientific American 256 (April, 1987): 86-92. A sound and thorough discussion of the use of semiconductors as solar cells. Covers both theory and materials technology, economics of solar energy, and some discussion of existing installations.

Heller, Adam. “Hydrogen-Evolving Solar Cells.” Science 223 (March 16, 1984): 1141-1148. Best discussion in this set of references of production of chemicals by liquid-junction solar cells. Very knowledgeable, if rather technical exposition of chemistry of cells. Large bibliography, well connected to points made in article.

Heller, Adam, Barry I. Miller, and F. A. Thiel. “11.5 Percent Solar Conversion Efficiency in the Photocathodically Protected P-Indium Phosphide/Vanadium (3+) Ion-Vanadium (2+) Ion-Hydrogen Chloride/Carbon Semiconductor Liquid Junction Cell.” Applied Physics Letters 38, no. 4 (1981): 282-284. Article that announced the cell that is the subject of this article. The authors’ principal innovation appears to have been an oxidative surface treatment of the indium phosphide semiconductor.

Hubbard, H. M. “Photovoltaics Today and Tomorrow.” Science 244 (April 21, 1989): 297-304. Excellent coverage of both economics and technology of solar electricity production. Large bibliography.

Markvart, Tom, and Luis Castañer, eds. Solar Cells: Materials, Manufacture, and Operation. New York: Elsevier, 2005. Covers advancements in the field of solar cell design.

Maugh, Thomas H., II. “Catalysis in Solar Energy.” Science 221 (September 30, 1983): 1358-1361. Simple and lucid explanations of both solid-state and liquid-junction cells and how they are used in producing electricity and bringing about chemical reactions.

Pool, Robert. “Solar Cells Turn 30.” Science 241 (August 19, 1988): 900-901. Announcement of Sandia National Laboratories’ tandem solar cell that broke the 30 percent conversion efficiency “barrier.” Careful description of the construction of a cell.