Electro‐optical Charge Trapping in Zinc Porphyrin Films on Indium Tin Oxide and  / SiO2 / Si

Abstract

A thin film‐of zinc octakis (β‐octyloxyethyl) porphyrin (ZnOOEP) is capable of trapping and detrapping charge in an appropriate electric field under irradiation. An effective charge capacity of the order of 30 μC/cm2 or 1018 charges/cm3 was obtained with sandwich structures of ITO/ZnOOEP/ITO (where ITO is indium tin oxide) under a bias of 10 V with irradiation. Cells with the configuration ITO/normalZnOOEP/SiO2/normalSi were also fabricated. When an electric field was applied across this cell in the dark, a small amount of charge was stored, as in a conventional capacitor. However, the charge stored under the same conditions with irradiation of the photoconductive ZnOOEP was three orders of magnitude higher. Trapped charge produced under illumination was not removed under short‐circuit conditions in the dark, suggesting that this phenomenon could be used for information storage.

Optoelectric Charge Trapping/Detrapping in Thin Solid Films of Organic Azo Dyes: Application of Scanning Tunneling Microscopic Tip Contact to Photoconductive Films for Data Storage

“…We describe here charge trapping and detrapping in organic photoconductive insulators under irradiation as studied with a sharp tip contact in a scanning tunneling microscope (STM). These results extend our earlier reports − of similar phenomena in a solid film of zinc octakis(doceoxyethyl)porphyrin (ZnODEP), which forms a liquid crystal phase at high temperature) . We have suggested a possible application of this effect to high-resolution information storage in a rewritable electrooptical medium.…”

“…Charge Trapping and Detrapping in Sudan I. High dark resistivity and good photoconductivity are highly desirable properties for charge storage, which is the basis of the proposed electrooptical memory as discussed previously. − Indeed, previous experimental results showed that charges could be readily trapped within such photoconductive thin films upon irradiation under the proper bias, and charges could be stored and later released by the irradiation of the sample producing a photodischarge current similar to that seen with ZnODEP. − Figure a shows the short-circuit photocurrent ( I sc ) of an initially uncharged film upon irradiation. In this case, the current from electrons generated directly by photoexcitation of Sudan I molecules or indirectly from the dissociation of excitons ejected into the ITO electrode and holes into the organic layer rapidly attains a steady-state level.…”

“…This I sc spike was seen only with the very first irradiation and was not observed with subsequent irradiation unless the sample was recharged again (Figure b) once a steady state was reached. This represents the trapping and detrapping process as observed with ZnODEP, which may be useful for high-density information storage. − When the sample was irradiated under a negative bias, electrons were injected into the Sudan I layer and trapped there. These trapped electrons were later released during irradiation under short-circuit conditions leading to the I sc spike.…”

“…Large area (up to several square millimeters) organic single-crystal thin films (∼micrometer) of porphyrin, 1-phenylazo-2-naphthol (Sudan I), and 1,4-di- p -toluidinoanthraquinone (Solvent Green 3) have been successfully grown, making it possible to characterize optically and electrically a molecular crystal at the same spot at the same time. Large orientation-dependent photo/dark conductivity and optical absorption effects, including orientation effects of color, have been observed. − High-resolution nanosecond charge trapping, which is promising for ultrahigh-density information storage, has been discovered in porphyrin photoconductive thin films. − This paper extends these studies to reversible charge trapping and detrapping in two different dye molecular crystals using the sharp tip of an STM as one electrical contact.…”

“…The basic principle of charge trapping as a means for information storage within photoconductive media, described in earlier papers, − is briefly illustrated in Figure . Upon irradiation, electron−hole pairs are generated within the medium (Figure a) and are subsequently separated by the electric field applied to the photoconductive layer (Figure b).…”