Photovoltaic cells liquid junction

So-called wet solar cells show promise, particularly because of their relative ease of fabrication. In this type of photovoltaic cell, the junction is formed, between a semiconductor and a liquid electrolyte. No doping is required because a junction forms spontaneously when a suitable semiconductor, such as GaAs, is contacted with a suitable electrolyte, Three knotty problems (accelerated oxidation of surface of semiconductor exchange of ions between semiconductor and electrolyte forming a blocking layer and deposition of ions of impurities on the surface of the semiconductor) all have been solved and thus the concept now appears technically viable,... 

In any case, it is perceived from the above discussion that the problem of longterm chemical stability of polycrystalline semiconductor liquid junction solar cells is far from being solved. Still, as already pointed out in the early research, any practical photovoltaic and PEC device would have to be based on polycrystalline photoelectrodes. Novel approaches mostly involving specially designed PEC systems with alternative solid or gel electrolytes and, most importantly, hybrid/sensitized electrodes with properties dictated by nanophase structuring - to be discussed at the end of this chapter - promise new advances in the field. 

Gutierrez MT, Salvador P (1987) Photoelectrochemical characterization and optimization of a liquid-junction photovoltaic cell based on electrodeposited CdSe thin films Influence of anneaUng and photoetching on the physical parameters determining the cell performance. Sol Energy Mater 15 99-113... 

As it has been described in various other review articles before, the conversion efficiencies of photovoltaic cells depend on the band gap of the semiconductor used in these systems The maximum efficiency is expected for a bandgap around Eg = 1.3eV. Theoretically, efficiencies up to 30% seem to be possible . Experimental values of 20% as obtained with single crystal solid state devices have been reported " . Since the basic properties are identical for solid/solid junctions and for solid/liquid junctions the same conditions for high efficiencies are valid. Before discussing special problems of electrochemical solar cells the limiting factors in solid photovoltaic cells will be described first. 

Similar photovoltaic cells as those described above can be made with semiconductor/ liquid Junctions. The basic function of such a cell is illustrated in terms of an energy scheme in Fig. 2. The system consists of an n-type semiconductor and an inert metal... 

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

Polymer Film Coating to Stabilize Liquid-junction Photovoltaic Cells... 

Figure 7.7 Schematic representation of a liquid-junction photovoltaic cell using an n-type semiconductor. R/O is the redox couple in the electrolyte.

M. E. Orazem, Mathematical Modeling and Optimization of Liquid-Junction Photovoltaic Cells, Ph.D. dissertation. University of California, Berkeley, California (1983). 

Figure 1, The platinized Chi a cell. The Chi a-free electrode is used as a half cell in a liquid-junction photovoltaic cell. In photolytic reaction, only the platinized Chi a electrode is used in the production of molecular hydrogen and oxygen from water.

Figure 2. Effect of oxygen in the photovoltaic response of the platinized Chi a electrode, (a) Electrolyte purged with argon gas, measurements made under a positive pressure of Ar (b) O, electrolyte saturated with oxygen (c) , oxygen removed by passage of argon gas through the electrolyte solution for 30 min. The ordinate units are given in 10 electrons per incident photon. The efficiency of the liquid junction cell was calculated by dividing the photovoltaic response (electrons sec ) by the incident photoresponse (photons sec ).

The principal elements of the liquid-junction photovoltaic cell, as shown in Fig. 1, are the counterelectrode, the electrolyte, the semi-conductor-electrolyte interface, and the semiconductor. The distribution of charged species (ionic species in the electrolyte and electrons and holes in the semiconductor) is altered by the semiconductor-electrolyte interface, and an equilibrium potential gradient is formed in the semiconductor. The interfacial region may be... 

The liquid-junction photovoltaic cell has the advantages that the junction between electrolytic solution and semiconductor is formed easily and that polycrystalline semiconductors can be used. The principal disadvantage is that the semiconductor electrode tends to corrode under illumination. 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. 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.72... 

The design of a liquid-junction photovoltaic cell requires selection of an appropriate semiconductor-electrolyte combination and... 

Quantitative optimization or prediction of the performance of photoelectrochemical cell configurations requires solution of the macroscopic transport equations for the bulk phases coupled with the equations associated with the microscopic models of the interfacial regions. Coupled phenomena govern the system, and the equations describing their interaction cannot, in general, be solved analytically. Two approaches have been taken in developing a mathematical model of the liquid-junction photovoltaic cell approximate analytic solution of the governing equations and numerical solution. 

A number of computer programs related to the liquid-junction photovoltaic cell have been developed. Leary et al.205 for example, calculated carrier concentrations in polycrystalline films using a numerical solution of Poisson s equation coupled with overall charge neutrality within spherical grains. Their model was used for analysis of semiconductor gas sensors. Davis and colleagues206"208 presented a computer program which uses simultaneous calculation of surface and solution equilibrium states to obtain the equilibrium condition of electrical double layers. 

The optimal design of liquid-junction photovoltaic cells shares constraints with solid-state photovoltaic cells.25 209 Current collectors cast shadows and can reduce the amount of sunlight absorbed in the semiconductor. A constraint unique to the liquid-junction cell is the placement of the counterelectrode relative to the semiconductor-electrolyte interface. Shadows, which reduce efficiency and cause local currents in solid-state photovoltaic cells, may lead to localized corrosion in photoelectrochemical cells. Mass-transfer and kinetic limitations at the counterelectrode and resistance of the electrolyte can play important roles in the optimal design of the liquid-junction photovoltaic cell. These considerations are treated qualitatively by Parkinson.210... 

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