Photoconductivity Of Poly Research Articles

Persistent photoconductivity in poly(p-phenylenevinylene): Spectral response and slow relaxation.

We report the spectral response and slow decay of the steady-state photoconductivity in poly(p-phenylenevinylene) (PPV) films. The spectral response of the photoconductivity is in good agreement with that calculated from the absorption data with the assumption of rapid recombination at the surface of the film; the results indicate direct photogeneration of free charge carriers via an interband transition. The photoconductivity is, therefore, consistent with a description of the electronic structure of PPV in terms of a semiconductor band model (rather than an exciton model). The very slow stretched-exponential relaxation of the photoinduced conductivity is reminiscent of the persistent photoconductivity observed in inorganic semiconductors. By assuming that the photocurrent is carried predominantly by mobile polarons near the surface, one can construct a model for the persistent photoconductivity in which the recombination of long-lived bipolarons is inhibited in the bulk where bipolarons have a lower free energy than polarons. The persistent photoconductivity, therefore, is caused by the slow dispersive diffusion of photogenerated bipolarons to the surface where they dissociate into polarons and where both polaron transport and recombination occur.

On Photogeneration of Charge Carriers in Polymers

Abstract The experimental data on the photoconductivity of poly (arylene vinylenes) and poly-diacetylene are discussed and show that separation of e-h pairs in conjugated polymers is connected with delocalization of charges and with their 1-D motion. The probability of escaping of geminate recombination is speculated to be limited by availability of sinks which prevent the geminate recombination. It permits to account for unusual photocurrent on voltage and photocurrent on intensity dependencies observed.

Characteristics of buckminsterfullerene doped conducting polymer

Conducting polymers such as poly(3-alkylthiophene) are effectively doped with buckminsterfullerene (C 60). Change of absorption spectrum and drastic quenching of photoluminescence have been observed upon C 60 doping. On the other hand, the slight enhancement of electrical conductivity and the change of ESR by C 60 doping is not so remarkable compared with conventional strong dopants. Photoconductivity of poly(3-alkylthiophene) is enhanced and the response time becomes shorter by C 60 doping, which suggests that C 60 is a weak dopant, providing mainly photoinduced charge transfers between C 60 and the polymer. These results are explained by taking electronic energy states of poly(3-alkylthiophene) and C 60 into consideration at account of polaronic effects in both C 60 and polymer. Small enhancement of electrical conductivity and significant quenching of photoluminescence have been also observed upon C 70 doping, but change of optical absorption is less remarkable than the case of C 60 doping.

Enhanced photoconductivity of C 60 doped poly(3-alkylthiophene)

Photoconductivity of poly(3-alkylthiophene) is found to be remarkably enhanced upon C 60 doping. Excitation profile of photoconductivity shows two peaks in the spectral ranges around 650 nm and 350nm. The former peak is interpreted to be due to ππ ∗ interband excitation of poly(3-alkylthiophene) followed by electron transfer to C 60, while the latter is due to the optical excitation of C 60 molecule, accompanied by hole transfer to poly(3-alkylthiophene). In both cases, positive polarons (P +) photogenerated with increased efficiency in poly(3-alkylthiophene) chains are considered to be responsible for enhanced photoconductivity, since geminate recombination of those positive charges (P +) in poly(3-alkylthiophene) with negative charges (P c −) in C 60 is suppressed compared to intrachain P +P − recombination in undoped poly(3-alkylthiophene). The decay time of photoconduction becomes shorter upon C 60 doping.

Polaron-pair generation in poly(phenylene vinylenes).

The effects of weak magnetic fields on the photoconductivity of poly(p-phenylene vinylene) (PPV) and two derivatives, poly(1,4-phenylene-1,2-dimethoxyphenyl vinylene) (DMOP-PPV) and poly(2-phenyl-1,4-phenylene vinylene) (PPPV), were observed within the temperature range 130\char21{}350 K. These effects are attributed to the formation of interchain pairs involving a negative polaron and a positive polaron. A polaron pair is formed as a result of interchain electron transfer from a molecular exciton. The lifetime of a pair is estimated to be within the range of ${10}^{\mathrm{\ensuremath{-}}8\char21{}}$${10}^{\mathrm{\ensuremath{-}}9}$ s. Thermal dissociation of a polaron pair produces free charge carriers, and recombination of the pair regenerates a singlet or triplet exciton on a single conjugated segment of a chain.

Photoconductivity of poly((E,E)‐[6.2]paracyclophane‐1,5‐diene) and its complex with TCNE

AbstractSteady‐state photoconductivity measurements carried out for poly((E,E)‐[6.2]‐paracyclophane‐1,5‐diene) pure and sensitized with tetracyanoethylene show that doping causes a significant increase of the photoconductivity. This is due to an increase of both the hole mobility and the photogeneration efficiency. The shape of the spectral characteristic is explained by the possible existence of two paths for charge‐carrier photogeneration connected with absorption of the light by the cyclophane unit for shorter wavelengths or by CT complex for longer wavelengths. For an electric field lower than 106 V/m, the photocurrent exhibits an ohmic field dependence, and for higher fields, it becomes proportional to F1.5.

Electrical conductivity and optical properties of sensitized poly[(N-benzyldiphenylamino)-methane] with crystal violet

Abstract The dark conductivity and photoconductivity of poly[( N -benzyldiphenylamino)methane] was greatly enhanced in the dark and in the visible region by the addition of a small amount of crystal violet having a molar ratio of monomer:dopant from 1:0.007 to 1:0.03. The current-voltage characteristics were studied in the dark and in the visible wavelength range, and the current increased superlinearly with the increase of doping level. The activation energies for dark conductivity and photoconductivity are 0.73-0.64eV and 0.71-0.60eV, respectively, at different doping levels. The dye, crystal violet, introduced a new photoresponse maximum at 580 nm corresponding to the absorption spectrum of crystal violet.

Photoconductivity in poly[[ o-(trimethylsilyl)phenyl]acetylene

Steady-state photoconductivity was studied in thin films of poly[[ o-(trimethylsilyl)phenyl]acetylene], or poly( o-Me 3SiPA) sandwiched between SnO 2 and metal electrodes. Both the dark current density ( J d) and photocurrent density ( J ph) depend superlinearly on the applied electric field. J ph also shows a superlinear dependence on light intensity. However, J d and J ph are only weak functions of temperature at temperatures below room temperature. We conclude that the photoconductivity in poly( o-Me 3SiPA) arises mainly from trap-modulated carrier photoemission from electrodes and the polymer is capable of effective transport of holes.

Photoconductivity of sensitized poly( N-ethyl carbazole)methane with 2,4,7-trinitrofluorenone

Photoconductivity of poly( N-ethyl carbazolyle)methane was greatly enhanced in the visible region by the addition of a small amount of 2,4,7-trinitrofluorenone having a molar ratio of monomer:dopant from 1:0.06 to 1:0.3. The current-voltage characteristics were studied in the visible wavelength range, and the current increased superlinearly with the increase of doping level. The activation energies for dark and photoconductivity are 0.244–0.202 eV and 0.238–0.194 eV, respectively, at different doping levels. The photoresponse shows two charge transfer bands corresponding to two transitions of the complex.

Electronic Conduction in Poly(p-phenylene-1,3,4-oxadiazole) Films

Dark conductivity and photoconductivity in poly(p-phenylene-1,3,4-oxadiazole) (POD) films were investigated. Above 440 K, ionic conduction was observed. An electronic process which was delicately influenced by the presence of adsorbed oxygen was found below 440 K. Large photoconductivity was observed and its wavelength and temperature dependences were examined. POD films were pyrolyzed at the temperature between 670 and 810 K to give semiconductive polymer films with the conductivity between 10−20 and 10−5 S cm−1. Variations of dark conductivity, photoconductivity and charge drift mobility with pyrolysis were investigated. Charge drift mobilities of pyrolyzed POD films were around 10−7 cm2V−1s−1 and moderately dependent on the pyrolysis temperature.

Photoconductivity of poly(N‐methyl 3‐hydroxymethyl carbazolyl acrylate‐co‐acryloyl‐3′‐hydroxypropyl‐3,5‐dinitrobenzoate)

Photoconductivity of poly(N‐methyl 3‐hydroxymethyl carbazolyl acrylate‐co‐acryloyl‐3′‐hydroxypropyl‐3,5‐dinitrobenzoate)

Photoconductivity of poly(vinyl alcohol) films with Fe3+ complexes and ESR study of their structures

AbstractWe have studied whether photoconductivity is observed in polymer films containing the first transition metal complexes. Polymers investigated were poly(vinyl alcohol) (PVA), polyacrylamide, poly(acrylic acid), poly(vinyl pyrrolidone), and polyethylene glycol. Transition metal salts used were CrCl3, MnCl2, FeCl3, CoCl2, NiCl2, FeBr3, Fe(NO3)3, Fe2(SO4)3, and Fe(ClO4)3. Only in the PVA–FeCl3 film was relatively large photoconductivity due to the photoreduction of Fe3+ observed. ESR spectra indicate that an Fe3+ ion is chelated with two in‐plane OH residues of PVA and coordinated with three Cl−ions on the meridian. UV irradiation causes an electron transfer from Cl− to Fe3+. The produced chlorine atom extracts an electron from the oxygen of a neighboring OH residue. The unpaired electron thus formed can move from one oxygen to another through hydrogen bonding. In other words, the photoconductivity is due to holes produced in the network of PVA hydrogen bonding by the reduction of Fe3+. Finally, we have tried to explain why the photoconductivity is observed only in the PVA–FeCl3 film.

Photoconductivity of poly(vinyl alcohol) films containing Cu2+ complexes

AbstractPhotoconductivity has been measured in well‐dried poly(vinyl alcohol) (PVAl) films containing Cu2+ complexes prepared by treatment with Cu(NO3)2, CuSO4, CuCl2, and CuBr2. When the ratio [Cu2+]/[MU] ([MU] denoting the concentration of PVAl monomer residues) is greater than 0.015, illumination at the charge‐transfer (UV) band produces a strong photocurrent in PVAl‐CuCl2 and PVAl‐CuBr2, but not in PVAl‐Cu(NO3)2 or PVAl‐CuSO4. Optical absorption spectra suggest that two halide ions enter the first coordination sphere in the cis configuration. The temperature dependence of ESR indicates the existence of antiferromagnetic superexchange interaction via intervening halide ions. Hence, there exists a network structure of the PVAl‐CuCl2 or PVAl‐CuBr2 complex. The dependence of the photocurrent on the polarity of the illuminated electrode shows that the majority of carriers are holes. Holes in the network structure produced by charge transfer from halogen to Cu2+ ions are concluded to be responsible for the photoconduction.

Photoconductivity of poly(N‐acryloylcarbazole)

AbstractThe photoconductivity of poly(N‐acryloylcarbazole) (PACz), with the pendant carbazolyl group only a short distance from the skeletal chain of the polymer but separated from it by a carbonyl group, is investigated and compared with that of poly(N‐vinylcarbazole) (PVCz). There is no significant difference between PACz and PVCz in the temperature, light intensity, and spectral dependences of the photocurrent. The photoconductivity of PACz, however, is much inferior to that of PVCz and even to that of poly(N‐carbazolylethylvinylether), a representative vinyl polymer with pendant carbazolyl groups far from the skeletal chain. The poor photoconductivity of PACz is discussed in relation to the intensity of the electronic interaction between neighboring carbazolyl groups in the polymer chain and to singlet exciton migration. It is attributed mainly to an extremely low efficiency of extrinsic carrier generation via a singlet exciton, which is due to the poor electron‐donating character and the extremely short lifetime of a singlet exciton in the presence of the carbonyl group.

The molecular weight dependence on the photoconductivity of poly(N‐vinylcarbazole)

AbstractPoly(N‐vinylcarbazole), (PNVC), was prepared, fractionated by gel permeation chromatography, and then characterized by viscometry and vapour pressure osmometry. The fractionated PNVC species with relatively narrow molecular weight distributions were successfully used to measure both their electrical dark‐conductivity and photoconductivity using a surface type cell in high vacuum (ca. 10−7 mm Hg) at room temperature. A molecular weight dependent photoconductivity was found for the fractionated PNVCs with weight average molecular weights in the range of 1,2·103 to 2,4·105. This observation is in contradiction to Epping's results who has found a molecular weight independent photoconductivity in the molecular weight range of 3·105 to 7·106. Our molecular weight dependence may be well understood in terms of the interrupted overlap of the π‐electrons of adjacent carbazolyl groups at the terminal parts of the polymer chains, this effect being all the more stronger the smaller the molecular weight is.

The Photoconductivity of Poly(N-vinylcarbazole). IV. The Photoconductivity of Copolymers of N-Vinylcarbazole with Styrene, Vinyl Acetate, and N-Vinylpyrrolidone

The Photoconductivity of Poly(<i>N</i>-vinylcarbazole). IV. The Photoconductivity of Copolymers of <i>N</i>-Vinylcarbazole with Styrene, Vinyl Acetate, and <i>N</i>-Vinylpyrrolidone

Sensitizing Substance in Highly Photoconductive Poly(N-vinylcarbazole) Photochemically Treated with Polyhalides

It has been known that the photoconductivity of poly(N-vinylcarbazole)(PVK) can be much improved by treating it photochemically with some polyhalides. With the aim to understand mechanism of the high sensitivity of thus treated PVK, photochemical products were examined, mainly those in PVK treated with CBr4 or CHI3. Spectroscopically found were molecular halogen, an unidentified substance which absorbed near-ultraviolet light and a dye which was photodecomposed. Detailed investigation revealed that (1) the dye undergoes a reversible bleaching by ammonia, (2) PVK is subject to cross-linking by the photochemical treatment, (3)substitution reactions take place seemingly at the 3-position of the carbazole ring of PVK (from IR spectrum), and (4) the photoconductivity at any wavelength exhibits a definite dependence on the amount of the produced dye. The sensitizing substance was speculated to be a cationic dye with carbazole rings as the chromophore, and also with a linkage such as - CBr- which at the same timecrosslinks PVK. The high sensitizing efficiency of the dye would result from the tightly bound state of the dye to the photoconductive polymer.