Photocurrent generation for nearly intrinsic semiconductor electrodes has an extremely small quantum yield because the space charge layer is insufficient to promote the efficient separation of electron hole pairs when suitable biases are applied. However, if the bulk material can be made photoconductive, a space charge layer having a higher electric field strength will be created. It is demonstrated that thin platelets of
normalGaS
and
normalCdS
, although insulating in the dark, can be used as photoelectrodes if they are illuminated with light having energy less than the bandgap in addition to light having energy above the bandgap. With this arrangement, high quantum yields similar to those obtained for well‐doped semiconductors are obtained. The charge distribution in a semiconductor platelet under various modes of illumination is discussed.
The quantum efficiency of dye sensitization at polycrystalline SnO2 thin films can reach a high value close to ϕ = 1 if determined immediately after preparation, however it slowly drops down to low values (ϕ ∼ 0.01) after inserting the electrode into aqueous electrolyte solutions. The quantum efficiency decay rate depends on the electrolyte composition. Capacitance and cyclic voltammetry measurements indicate the formation of an insulating layer. Auger‐ and IR‐reflection spectra have been used to identify this layer as a mixed oxide/hydroxide formed by hydrolysis of the surface oxide. This layer acts as an energy barrier for the injection current from the excited dye molecule thus decreasing the sensitization quantum efficiency.
Photocurrents due to dye sensitization have been measured at silver chloride single crystals used as a thin membrane between two electrolytic contacts. Doping of the crystals with divalent cations raised the quantum yield significantly by eliminating traps for electron capture in the bulk and forming a depletion layer of Ag+‐vacancies at the contact with an Ag+ containing electrolyte which enhances the transfer of the injected electrons into the bulk. Specifically adsorbed Ag+‐ions on the crystal surface shift the energy position of the conduction band edge to more positive redox potentials, enhancing in this way the rate of electron transfer from excited states to the conduction band of AgCl, but acting on the other hand also as traps for electrons and as efficient recombination centers.
Die durch Farbstoffsensibilisierung induzierten Photoströme werden an AgCl‐Einkristallen gemessen, die eine dünne Membran zwischen 2 Elektrolytkam+ mern (eine enthält Rhodamin B) bilden.
Für dünne, im Dunklen isolierende GaS‐ und CdS‐Plättchen wird gezeigt, daß sie als Photoelektroden benutzt werden können, wenn sie durch Licht mit Energien oberhalb und unterhalb der Bandlücke bestrahlt werden.
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