Mathematical Modeling of Liquid-Junction Photovoltaic Cells III. Optimization of Cell Configurations

A one-dimensional mathematical model of the liquid-junction photovoltaic cell was coupled with primary resistance calculations to predict the optimal performance of three cell configurations. Two cells are considered in which the semiconductor is i l luminated from the electrolyte side and one in which the semiconductor is i l luminated from the current-collector side. An economic analysis is presented based upon these results. The performance of the liquidjunct ion photovoltaic cell is dependent upon the design, surface area, and placement of the counterelectrode and current collectors. Most studies of the liquid-junction photovoltaic cell have been oriented toward developing an understanding of the semiconducting electrode which characterizes the cell [see, e.g., Ref. (1-19)]. This work describes the design and optimization of liquid-junction photovoltaic devices. The advantages and problems inherent in the liquidjunct ion cell are reviewed, and a mathematical model of the liquid-junction cell (20, 21) is used to predict the optimal performance of various cell configurations. An economic analysis is presented based upon these results. The l iquid-junction photovoltaic cell has appeal because, in contrast to solid-state junctions, the junct ion between electrolyte and semiconductor is formed easily and allows use of polycrystalline semiconductors. The electrochemical nature of the cell allows both production of electricity and generation of chemical products which can be separated, stored, and recombined to recover the stored energy. These features could make the liquid-junction cell an economical alternative to solid-state photovoltalc devices for solar energy conversion. Liquid-junction cells also have the advantages that are attributed to other photovoltaic devices. Photovoltaic power plants can provide local generation of power on a small scale. The efficiency and cost of solar cells is independent of scale, and overall efficiency is improved by locating the power plant next to the load. Nuclear and fossil-fuel burning plants, in contrast, are economical only if built on a large scale (on the order of 1000 MW) (22). The design of a l iquid-junction photovoltaic cell requires selection of an appropriate semiconductor* Electrochemical Society Active Member. ~Present address: Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22901. electrolyte combination and design of an efficient cell configuration. The selection of a semiconductor is based upon the bandgap, which provides an upper limit to the conversion efficiency of the device, and the choice of electrolyte is governed by the need to limit corrosion. The optimal design of the liquid-junction photovoltaic cell is aided by use of mathematical models. Bandgap.--Photovoltaic cells rely on the unique properties of semiconductors to convert incident radiation to electrical current. The semiconductor property of interest is the moderate gap between the valence and the conduction-band energy levels. Incident photons of light with energy greater than or equal to the bandgap energy transfer their energy to valence-band electrons, producing conduction-band electrons and vacancies in the valence band. An upper limit to the efficiency of photovoltaic devices can be established, based upon the bandgap and the solar spectrum, without consideration of cell configuration. This "ultimate efficiency" is given by (11)