Nonaqueous photocell efficiency boosted Much of the recent effort to develop photoelectrochemical cells to harness the sun's energy efficiently has fo cused on cells using aqueous elec trolytes. The results have been im pressive and some such cells operate at efficiencies approaching those of solid-state silicon solar cells—about 20%. The problem is that it is notor iously tricky to suppress photodecomposition or photopassivation of the semiconductor electrode in such cells. The use of nonaqueous electrolytes largely mitigates this problem be cause water and species derived from it are the main culprits in the dele terious reactions. However, extensive past research has shown that these cells operate at only 1 to 2% effi ciency. That's a little odd, because the same semiconductor materials oper ate at much higher efficiencies in aqueous electrolytes if decomposi tion or passivation can be suppressed. The explanation for the low ef ficiencies has been the existence of surface states at the semiconductor/ electrolyte interface that tend to trap charge carriers, thereby preventing them from participating in the de sired processes. The situation may be changing. Nonaqueous photocells using a photoanode of gallium arsenide phosphide (GaAsi_xPx) developed by researchers at Stanford University, Palo Alto, Calif., operate at 13.2% ef ficiency [Nature, 300, 733 (1982)]. The same researchers have developed a nonaqueous cell using an η-type sil icon photoanode that operates at 10.1% efficiency. These are the highest efficiency values for such cells yet reported. With support provided by the Stanford chemistry department, as sistant professor Nathan S. Lewis and undergraduate coworker Chris M. Gronet studied a series of GaAsi_ x P x photoanodes using ferrocene/ferricenium as the redox couple in acetonitrile solvent. According to Lewis, studies have shown that pure n-GaAs has a bandgap of 1.44 eV, making it ideal for solar applications. However, when used in nonaqueous electro lytes, its operating potential is low and its fill factor declines above a
very low light intensity, yielding an efficiency of only 2.4%. GaP pho toanodes, on the other hand, yield potentials of 0.8 to 1.0 V in non aqueous electrolytes, but GaP's large bandgap of 2.25 eV produces low short-circuit currents and hence low solar efficiencies. The hope was to find a combination of the two mate rials that retained the positive char acteristics of each while eliminating the negative characteristics. The electrode composition that performed best was GaAso.72Po.28· The electrode has a bandgap of 1.77 eV, raised only slightly above that of GaAs. The open-circuit voltage is 1.01 V, close to that of GaP. The cell op erates at 13.2% efficiency, even though the researchers have as yet made no effort to optimize a number of cell variables. As long as water is excluded from the cell, the pho toanode appears to be quite stable. Because of the way the electrode is fabricated, significant photocorrosion can be ruled out, Lewis says. Essentially, the electrode consists of a thin layer of a specific composition of GaAsP deposited on a GaAs sub strate. That layer is only about 5 μπ\ thick. "You can count how many electrons would be required to eat away the whole layer," Lewis says.
Several factors affect photocell performance Bandgap (E g ). Expressed in electron volts, the energy required to excite a major charge carrier (electron or hole) to the conduction band of a semicon ductor; because of the solar spectral distribution, optimum Eg is 1.44 eV. Open-circuit voltage (E oc ). Expressed in volts, the maximum potential a cell develops in operation (zero current). Short-circuit current. Expressed in milliamperes per unit area of electrode, the maximum current at a given light intensity a cell develops (zero poten tial). Fill factor. The ratio of the actual cur rent/potential characteristics of a cell at maximum power output to the ideal, which would be operation at E^ and the short-circuit current.
"It would take about 10 coulombs per sq cm. We've passed more than 2000 coulombs with no observable change in electrode performance." In related work, the researchers developed a cell with an n-silicon photoanode in a nonaqueous elec trolyte that operates at 10.1% effi ciency. Previous nonaqueous pho tocells using silicon electrodes op erate at about 1% efficiency. The work was done in collaboration with James Gibbons, a Stanford professor of electrical engineering and owner of SERA Solar Corp., Santa Clara, Calif., and George Cogan, a scientist with SERA Solar. Although the effi ciency is lower than the GaAsP cell, Lewis points out that silicon, because of its availability, is the material of choice if such photocells are ever to be commercialized. "We went into silicon knowing something fundamental from our work with GaAsP," Lewis says. Higher operating potentials could be obtained when the surface of the electrode was made more ionic. This finding was applied to silicon. "We didn't want to change the silicon by derivatizing it. We wanted to make the surface ionic, which is actually pretty easy with silicon because sili con dioxide is quite ionic," he says. The most efficient n-silicon (phosphorus-doped) photoanodes are first etched to create 1- to 2-μπι hillocks across the surface. They are then treated with hydrofluoric acid and washed with methanol to pro duce a 20-Â surface layer of silicon dioxide, which gives the surface its ionic character. Additionally, Lewis says, it's known that there are no gap states in the silicon/silicon dioxide interface that, like surface states, can trap charge carriers. The ferrocene is modified slightly so that a higher concentration can be used. The methanol solvent is dried because water acts to produce a thicker layer of silicon dioxide, which acts to insulate, or passivate, the electrode, decreasing the cell's efficiency. "The message out of our silicon work," Lewis says, "is that in a very simple system, by paying proper attention to the semiconductor interface, one can obtain efficiencies that are as good as or better than any other cell using silicon electrodes." Rudy Baum, San Francisco March 7, 1983 C&EN
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