Hiroshi Kokado

Discussion on the mechanism of photodoping

Abstract The photo-enhanced reaction between metallic silver and vitreous chalcogenides is known as “photodoping”. Based on a series of experimental results, a model for photodoping was proposed. It was assumed in this model, that a junction barrier at the silver-chalcogenide interface worked for separating photocarriers. Holes are captured by metallic silver, and electrons are trapped by active or loosely bound chalcogen atoms after travel toward the interior of a glass layer. The Coulomb attraction field between ions thus formed is large enough to overcome the kinetic barrier in the process of silver diffusion. The square root dependence for the growth of the photodoped depth with exposure time has been explained in the light of this proposed model.

A photoelectric study on the interface between metallic silver and vitreous As2S3

Abstract The electrostatic potential at the Ague5f8As2S3 interface was investigated. In the dark, cells of a structure, Ag/As2S3/Al behaved like and an electrochemical battery. When light fell onto the cells, short-circuit currents were observed, but their appearancesvaried much, depending on the excitation wavelength and the material of the illuminated electrode. At wavelengths longer than the absorption edge of As2S3 glass, photocurrents were characteristic of the polarization current and little influenced by the external field. A model for explaining these findings was proposed on an assumption of an interface reaation between silver and vitreous As2S3 in the dark. The interfacial reaction was supposed to accompany a charge separation leading to formation of a potential barrier at the interface.

Spectroscopic and photoconductive properties of polymers containing trans-1,2-bis(9-carbazolyl) cyclobutane units sensitized by 2,4,7-trinitrofluorenone

Abstract A number of polymers containing trans-1,2-bis(9-carbazolyl)cyclobutane (DCzB) were examined for their photoconductivity in the presence of 2,4,7-trinitrofluorenone (TNF) and the results were correlated with the photophysical properties (excimer formation, energy migration, fluorescence quantum yield and fluorescence lifetime). Excimer formation by DCzB groups is sterically hindered whereas energy migration occurs efficiently in the DCzB polymers. Also, charge-transfer interaction with TNF is stronger for the DCzB polymers than for poly(9-vinylcarbazole) (PVCz). Photoconductivity of the DCzB polymers was better than that of PVCz when the charge-transfer band of the TNF-carbazole pair was irradiated, which agreed well with the photophysical observation. Although carrier generation and carrier transport processes could not be separately discussed at the moment, DCzB was shown to be a promising chromophore for photoconductive polymers.

Sensitization of ZnO photoconductivity by chemical modification of surface states

Abstract Surface States on ZnO have been investigated through their effect on photoconductivity. Low energy photons (0·7 ∼ 1·5 eV) are found to induce a feeble photocurrent in ZnO which is attributable to the direct electron injection from surface levels to the conduction band. A definite correlation has been found between the i.r. induced photocurrent and the intrinsic or the dye sensitized photocurrent excited by UV or visible radiation. It is proposed that phthalic anhydride on ZnO modifies the distribution of surface levels, by a chemical interaction with chemisorbed oxygen. Surface levels originating in chemisorbed oxygen ions are transformed by phthalic anhydride into new levels which are shallower but thermally more stable. These traps are found to enhance the photoconductivity of ZnO. The chemical nature of them and the mechanism of sensitization are discussed.

A new model for spectral sensitization of photoconduction in zinc oxide powder

Abstract Seven organic dyes were adsorbed on photoconductive zinc oxide powder and their sensitizing efficiencies were examined. The adsorption of dye induced a change in the dark conductivity, the magnitude of which appeared to represent the sensitizing power of the dye. The initial slope of sensitized photocurrent was found to be an exponential function of σ 0 , the pre-exponential factor in the conductivity equation. A model is proposed, which is consistent with all the observations. In this model, the sensitized photoinjection of electrons is assumed to occur from surface states to the bulk, depending upon the population of the surface states. A critical surface level for the spectral sensitization is introduced to account for the observed temperature effect. Surface states located below the critical energy are assumed not to contribute for the sensitization.

Xerographic study of neat polymers containing trans-1,2-bis(9-carbazolyl)cyclobutane units

Abstract Photoconductivity of the following unsensitized polymers was evaluated by xerographic methods and discussed with reference to the polymer structures: poly(9-vinylcarbazole) (1), poly(9-ethyl-3-vinyl-carbazole) (2), poly[trans-1-(3-vinyl-9-carbazolyl)-2-(9-carbazolyl)-cyclobutane] (3), copolymers of 2 and 3 [content of 2: 91% (4), 34% (5)] and poly[trans-1-(3-acryloyloxymethyl-9-carbazolyl)-2-(9-carbazolyl)cyclobutane] (6). The initial charge decay rate under photoirradiation and the half-lifetime were in the order 5 > 1 > 3 > 2 > 6 > 4 and 5 3 6 1 2 4 , respectively. The excimer intensity increased in the order 6, 3 5 4 2 « 1 . The biscarbazolylcyclobutane unit scarcely formed excimer while the local chromophore concentration was high. These factors were favourable for both carrier generation and carrier transport. Other factors necessary for a good photoconductor were also discussed.

Barrier Properties in Electrophotographic Zinc Oxide

A number of factors are known to influence greatly the photoelectrical and the electrophotographical properties of zinc oxide powder. Some of them, such as the degree of zinc excess and the state-species and amount-of chemisorbed oxygen, are in most cases uncontrollable, in the sense that they are almost determined at the time of manufacturing, unless some drastic treatment is imposed on samples before use. While some other factors such as the surface doping can be used to improve or control behaviors of samples at the time of use, these factors seem to be intimately related to the hard-to-control ones. After a brief review of studies of the oxygen states on zinc oxide surface, a discussion is developed concerning the photoconductive properties and the electrophotographic charge acceptance of zinc oxide bearing various degrees of zinc excess. The potential barrier at the surface of microcrystals, the height of which is estimated from dark conductivities, gives a good measure for the charge acceptance. Further, based upon the conventional theory of barrier development by oxygen chemisorption, the photo-to-dark conductivity ratio is argued in terms of the donor density, the barrier height, and other factors.

Sensitized Photocurrent in Dye-Deposited Poly-(N-Vinylcarbazole)

Photo-induced charge transfer has been studied which takes place at interface of dye and poly-(N-vinylcarbazole) (PVK). The charge transfer produces an anomalous spike in photocurrent at the early stage of illumination. Its polarity is determined solely by the type of dye employed and the magnitude is independent on the external field across the double-layer of dye and PVK. Oxygen and other electron acceptors are found to suppress the spike to appear though they assist the steady photocurrent to grow. On the basis of these findings, a mechanism of dye-sensitized photoconductivity in PVK will be discussed.

A Photochemical Reaction of a Photospiran in Non-polar Solvents, and a Photographic System with High Sensitivity

1,3,3-トリメチルインドリノ-6-ニトロベンゾピリルスピランは, 無極性溶媒中紫外線照射により, メロシアニン色素形構造に 変化するが, 低溶解度のため溶液より析出され, 赤褐色微粉体として単離される。この着色粉末は熱的に安定で, しかも, 光照射 しても変化が認められない。また, 四塩化炭素中では, 紫外線照射により四塩化炭素の光分解生成物と塩と思われる黄色生成物を 生果成し, これも常温で安定で, 溶液から粉末として分離できる。これらの生成物に関し, 赤外, 紫外可視吸収スペクトルの測果から, それぞれの構造を検討した。さらに, 他のスピラン化合物も, 多ハロゲン化合物の光分解生成物と光消色性の発色成することが認められ,それらを用いる高感度の光消色性記録の可能性を明らかにした。

Photo-reversible Photography

マラカイトグリーンロイコニトリル(MGCN)のホトクロミズムを用い,光発消色記録材料の可能性について検討した。MGCNは,エチルアルコール溶液で,紫外線照射により光イオン解離し,MG+とCN-を生成,着色する。この着色は,暗所でイオンの再結合反応により減少し,元の無色の状態にもどる。電子受容性分子(EAM)例えば,TCNQ,クロルアニル,TNBの添加により,この再結合反応は阻害され,着色は安定化する。これはCN-のEAMによる捕獲のためである。さらにEAMの捕獲生成物の吸収域波長光で照射すると,一部再結合反応が再開され着色の減少が見られた。またTNBの場合は,加熱により,安定化された着色体の減少が見られた。これらの結果より,光発消色過程の可能性を検討した。