The Potential Contribution of Photoelectrochemistry in the Global Energy Future

BRUCE PARKINSON AND JOHN TURNER

1.1 History

This chapter is directed at a realistic assessment of the role of photoelectrochemistry in the future global energy scenario. We start with a brief history of the development of photoelectrochemical solar energy conversion devices and then extrapolate to the future to offer an opinion about the contribution that photoelectrochemical devices and processes might provide.

We limit this discussion to devices that employ semiconducting materials to absorb solar energy and convert it to photoexcited charge carriers that are harnessed to produce either electrical power or chemical fuels. Photoelectrochemistry implies that the semiconductor is immersed in a solution and its properties are investigated in the dark and under illumination. The modern era of photoelectrochemistry began at Bell Laboratories during the early development of semiconducting materials for use in electronic devices. Bell Laboratories' researchers immersed various semiconductors such as Ge and TiO22 and measured their electrochemical response in the light and dark and reported on their photocorrosion and even, in the case of TiO2, the ability to evolve oxygen from water when illuminated with light of greater energy than the band gap. Other researchers, most notably Heinz Gerischer, began to publish experiments, models and theories to explain the energetics and kinetics of dark and photoinduced charge transfer at semiconductor electrolyte interfaces. However it was not until the energy crisis of the early 1970s, and the paper by Fujishima and Honda, that the connection between semiconductor photoelectrochemistry and solar energy received wide attention. Although oxygen evolution was observed as early as 1968 when illuminating a rutile electrode in solution, the application of this concept to water photoelectrolysis was first pointed out by Fujishima and Honda in a series of experiments that used the n-type semiconductor rutile form of TiO2. While rutile is stable under illumination in aqueous electrolytes, its large band gap (3.0 eV) restricts its utilization to the UV portion of the solar spectrum and thus limits its ultimate efficiency. It should also be pointed out that the conduction band of rutile is not negative enough to reduce water, and so a "pH bias" was used in early work, where the oxygen-producing side of the cell was basic with respect to the hydrogen-producing electrolyte. The simplicity of the Fujishima and Honda experiment, illuminating a rutile crystal electrode with UV light in an electrolyte to produce hydrogen and oxygen directly, and the energy crisis of the early 1970s, set off a flurry of further research in photoelectrochemistry aimed at solar energy conversion. Later work using a device with heterojunctions of III-V materials as photoelectrodes considerably increased the visible light conversion efficiency of direct water photoelectrolysis but at increased cost and decreased lifetime due to corrosion in aqueous electrolytes.

In terms of solar energy conversion efficiency, the most successful devices that followed were not water-splitting devices but rather photoelectrochemical photovoltaic cells. Here, illumination of the semiconductor/electrolyte junction drove a reversible redox reaction such as sulfur/polysulfide with no net change in the chemical composition of the electrolyte, instead of photoelectrolysis where chemicals are consumed. These early cells used various semiconducting materials, such as GaAs, CdSe, Si, and MoSe2, and reached efficiencies as high as 14% in laboratory cells and in some cases achieved respectable efficiencies with polycrystalline materials due in part to the spontaneous production of a conformal junction by the redox electrolyte. The stability of some of these devices was quite good, especially for MoSe2 and related materials, but due to both the unproven long-term stability and issues related to encapsulation of the often corrosive liquid electrolytes employed, they never became serious alternatives to solid-state solar cells. These issues, along with the increased supply and lower cost of oil in the period between 1985 and 2000, reduced both the interest and the funding for research and development of photoelectrochemical energy conversion devices. A photoelectrochemical photovoltaic device did emerge in 1991, the nanocrystalline TiO2 dye sensitized solar cell, which promises to become a contender in low-cost thin-film photovoltaic solar cell market. This cell exploited some of the main advantages of semiconductor liquid junctions; the junction forms spontaneously and is conformal even when high-aspect ratio or porous nanocrystalline networks are involved. This cell has spawned an enormous amount of research, partly because it is rather easy to construct an inexpensive device with a respectable efficiency without sophisticated equipment. However, even if this device is improved...