R. C. Nelson

Sensitization of Photoconductivity in Cadmium Sulfide

The photoconductivity of cadmium sulfide in the red and near infrared can be enhanced by sensitization with cyanine dyes. There is general qualitative agreement between the results of sensitization with pinacyanole, dicyanine-A, kryptocyanine, and neocyanine, and those obtained when these dyes are used as optical sensitizers in photography, so that the two effects may be considered to be similar.Cells have been made in the form: metal electrode || dye || cadmium sulfide || metal electrode, which upon illumination show a photo-emf and steady current, the dye being positive with respect to the cadmium sulfide. A simple explanation is proposed which relates this emf to the energy difference between the conduction bands of dye and cadmium sulfide. On the basis of this picture, and assuming electron conduction in both, the conduction band of the dye must lie above that of the cadmium sulfide, and the condition for sensitization by charge carrier transfer according to the original proposal of Gurney and Mott is satisfied.

Contact Potential Difference between Sensitizing Dye and Substrate

The contact potential difference between cadmium sulfide and the sensitizing dyes pinacyanole, kryptocyanine, and neocyanine has been measured by the electron beam retardation method and found to be of the order of 0.3 v in each case. That between silver bromide and pinacyanole has also been measured to be 0.38 v, the dye in all cases having the smaller electron affinity. These results confirm the tenability of the Gurney-Mott electron transfer mechanism of dye sensitization in photography. Using the same method the electron affinity of pinacyanole has been found to be about 3.1 ev; that of silver bromide is thus approximately 3.5 ev, in agreement with previous estimates.

Energy Transfers between Sensitizer and Substrate. IV. Energy Levels in Solid Dyes

The work function, or ionization energy, for eight solid dyes has been measured. In each case it was found that this energy was equal to the sum of the electron affinity of the dye, as previously measured by the writer, and the energy required to produce a charge carrier in the solid dye. The data confirm the point of view that electron-transfer processes in dye sensitization are energetically possible.

The Ionization energy of cationic cyanine dyes

Abstract Typical photosensitizing organic dyes are studied by a comparison of their measured and calculated ionization potentials. The ionization potential of the dye, adsorbed on a substrate, is measured by determining the threshold of the external photoelectric effect from the dye. The measured values are corrected for the effect of the substrate and an ionization potential, IPexp, is obtained which is independent of the substrate, and which may be compared to the calculated values. The Hartree-Fock LCAO-MO theory, for open and closed shell systems, is used in the π electron approximation with the assumption of zero-differential overlap. A first approximation to the ionization potential, IPn, is calculated from the difference between the total π electronic energies of the ground and ionized states. A method is described to correct this value, for the assumption of a static σ electron substructure, by computing the work done in the redistribution of the σ electrons upon ionization. The ionization potential calculated for one dye cation was almost twice the measured value, IPexp. It is concluded that the dye anion cannot be neglected and the HF-SCF-LCAO-MO theory is modified to include the Coulomb interaction of the anion. The ionization potential of one neutral dye molecule was more than 2 eV higher than IPexp. Agreement with experiment is obtained by calculating the ionization potential of the dye including the effects of the nearest neighboring molecules.

Energy Transfers between Sensitizer and Substrate. III. Sensitization by Thick Dye Films

Photoconductivity can be sensitized in cadmium sulfide films by specially prepared films of pinacyanole and kryptocyanine having a thickness ~1 micron. Preparations of this sort offer considerably more opportunity for experimentation than those sensitized by monolayers of dyes. Their properties are such as to support the validity of the electron-transfer mechanism in this case.

Energy Transfers between Crystals and Adsorbed Dyes. II. Effect of Concentration on Efficiency of Sensitization

The effect of density of sensitizer molecules adsorbed on a cadmium sulfide surface on the efficiency of sensitization by them has been studied for malachite green, pinacyanole, erythrosin, and 3,3′ diethylthiacarbocyanine. The losses in the case of the first two are drastic and can be accounted for directly in terms of the properties of the solid dyes. The detailed process of electron transfer from a dye molecule or aggregate to a crystal substrate is considered briefly. It is concluded that there should in general be a loss of efficiency in passing from the state in which molecules are adsorbed singly and broadside, to that in which they are adsorbed as a close-packed aggregate.

Photoconductivity of Some Cyanine Dyes

An examination of the properties of pinacyanole, dicyanine-A, kryptocyanine, and neocyanine indicates that the cyanine dyes constitute a distinct class of photoconductive dyes which differ from those previously studied in being without a high density of monoenergetic traps. In consequence, their responses are fast and relatively independent of temperature. The characteristics of cells depend very sensitively on details of preparation, and the level of photoconductivity of a given cell depends more on the homogeneity of the dye film than on the properties of the dye. Evidence is given that the polymer form of pinacyanole is present in some films, and that a rate-modifying process is associated with light absorbed by it. It is concluded that the properties of these dyes are such as not to preclude the charge carrier transfer mechanism of optical sensitization.

Energy Transfers between Crystals and Adsorbed Dyes. I. Desensitization

The electron affinities of several desensitizing dyes have been measured and found to lie in the neighborhood of 3.5 ev, and it seems unlikely that the dye can compete with sensitivity specks for electrons at equilibrium. The dyes produced a desensitization of photoconductivity in cadmium sulfide films, indicating that they can serve as surface electron traps. A mechanism of desensitization is suggested according to which the photoelectrons are loosely bound by one or several dye molecules seriatim, in consequence of which their times of arrival at the sensitivity speck are spread out over a long enough period so that the factors which contribute to the instability of the latent image become effective, and the sensitivity is thereby reduced.

A Simple Device for Direct-Recording Spectrophotometry*

A single detector, two-beam device is described in which compensation for changes in energy and detector sensitivity is achieved by using one of the signals from the detector, a photomultiplier, to adjust the working voltage on the multiplier so as to maintain the unobstructed measuring signal constant.