ABSTRACT The effect of cation on the CdSe/polysulfide photoelectrochemical (PEC) solar cell has been investigated. The currentpotential response of the polysulfide electrolyte on a platinum electrode was measured as a function of the alkali metal cation in solution. There was a -33 mV shift in the redox potential and an increase in the current when the cation was changed from Na § to Cs § The performance of the CdSe/polysulfide PEC cell with both polycrystalline and single-crystal semiconductor electrodes increased in the open-circuit potential, the short-circuit current, the fill factor, and the energy conversion efficiency when Cs polysulfide was used as compared with K or Na polysulfide. Impedance measurements were made on the electrodes in polysulfide solutions and hydroxide solutions with the different cations. The results for both electrolytes showed a negative shift in the flatband potential, an increase in the apparent charge-carrier concentration, and an increase in the frequency dispersion of the measurements with the addition of Cs +. The impedance measurements also showed a dependence on the orientation of the CdSe crystal. The effects above are related to a combination of a change in the solution and the surface of the semiconductor. The stoichiometric distribution of the polysulfide species changes with the addition of Cs + and the surface of the semiconductor changes, facilitating charge transfer across the semiconductor/ electrolyte interface. Initial work on the cadmium chalcogenide/polysulfide PEC solar cell was published by Gerischer ~ and Ellis et aI.2 A great deal of work has been performed since those original studies on improvements to the semiconductor, the surface condition of the semiconductor electrode, and the electrolyte. CdS, CdSe, CdTe and mixed CdSe~T%x 3-~ have been investigated to increase the energy-conversion efficiency and stability of the PEC cells. Several researchers have investigated cadmium chalcogenide PEC solar cells using polycrystalline electrodes. Long-term stabilities have been demonstrated with conversion efficiencies up to 5% for polycrystalline CdSe. 7-1~ The semiconductor/liquid junction solar cell is particularly well suited to low cost polycrystalline electrodes. Changes in the make-up of the polysulfide electrolyte have a large effect on the performance of the solar cell; the effect of hydroxide ion concentration and the sulfideto-sulfur ratio have been investigated by some researchers.~l~ Both the hydroxide ion and the sulfur-to-sulfide ratio affect the equilibrium concentration of the electroactive species, the stability of the solution, and the light absorption by the solution. Changing the cation in the solution leads to changes in the performance of the cell. ~6-~9 The cation is not involved directly in the redox reactions at the semiconductor electrode but the performance of the cell increases significantly when the cation in solution is changed from Na § or K § to Cs § The improvement is in the stability of the semiconductor, the open-circuit photovoltage (Voc) and the photocurrent response. The flatband potential (V~) also shifts negative with the addition of Cs*; yet the only difference between the cations is the ionic radius and thus the charge density. The change in cation can affect the surface of the semiconductor by altering the degree of adsorption, or by incorporation into the semicon-* Electrochemical Society Active Member. a Present address: IBM Corporation, Hopewell Junction, New York 12533-6531. ductor itself; or by changing the activity of the species in solution, or the distribution of the polysulfide species. The CdSe/polysulfide system is examined here. First, the current-potential response of polysulfide on a platinum electrode was measured. Impedance measurements then were made to determine how the change in cation affects the band energies and charge-carrier concentration of the semiconductor. The measurements were made in a variety of systems to separate the different effects: CdSe singlecrystal electrodes in hydroxide electrolytes with the different cations, CdSe single-crystal electrodes in polysulfide with the different cations, and with CdSe polycrystalline electrodes in polysulfide. The impedance measurements indicate changes in the performance of both single-crystal and polycrystalline CdSe/polysulfide PEC cells with the different cations. Polysulfide Electrolyte Polysulfide solutions have been studied extensively in the pulp and paper industry. The equilibrium relationships of the solution species are presented in The electroactive species and rate-limiting step at the photoanode have been the subject of several papers. 14' 24-2~ I
