Photogeneration Yield Research Articles

Photosensitive polymer semiconductors

Models of photogeneration of charge carriers and structural regularities of the photoeffect are analyzed for homologous series of the main classes of photosensitive polymer semiconductors and their donor-acceptor complexes with different sensitizers. The relationship between the chemical structure of polymers and their photophysical properties is studied on the basis of an analysis of the effect of the energy and spatial structure of the monomeric unit of a polymer and the complex molecules on the spectra of the quantum yield of carrier photogeneration. The existence of general regularities of changes in the quantum yield with changes in the parameters of the molecular structure is ascertained for the main classes of polymer semiconductors and donor-acceptor complexes. Comparison of the experimental results with the calculations on the basis of the structure-sensitive model of Onsager photogeneration made it possible to estimate the initial distances and the degree of charge transfer in the excited state of complex molecules and relate the obtained values of the dipole moments to the hyperpolarizability in nonlinear optical media formed on the basis of these polymers and complexes. The ascertained structural regularities of the photophysical processes make it possible to predict the ultimate quantum yield of photoprocesses in polymer organic semiconductors and their molecular complexes, as well as the ultimate value of photosensitivity of recording media on their basis.

Exciton dissociation in conjugated polymers

AbstractIn conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field‐assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field‐independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.

Photosensitivity of new photoconductive polymers based on ruthenium-biquinolyl complexes

A set of new metal-polymer complexes has been synthesized on the basis of polymers containing biquinolyl units in the main chain and Ru(bPy2)Cl2. The spectral and mechanical properties of these complexes were studied, as well as their photosensitivity in the temperature range from 20 to 150°C. It was shown that the photosensitivity of synthesized polymers is 2×104 cm2 J−1 and the quantum yield of carrier photogeneration is η=0.025.

Why is exciton dissociation so efficient at the interface between a conjugated polymer and an electron acceptor?

Although doping of a conjugated polymer by electron acceptors strongly facilitates exciton dissociation into geminate pairs of carriers, the yield of free carrier photogeneration can be high only at high doping levels, that is, in polymer/acceptor blends. We suggest a model that explains how excitons can efficiently dissociate into free carriers at an intrinsic polymer/acceptor interface despite the Coulomb interaction between the charges within precursor geminate pairs.

Synthesis, Structure, and Photophysical Properties of Nanocomposites in the “Cu+ Organic Complex–SiO2” System

The possibility of synthesizing a series of nanohybrids in the “Cu+ organic complex–SiO2” system (with different contents of the organic component) by the sol–gel method is analyzed. It is shown that the organic component at a content of 10% provides optimum conditions for the formation of a nanocomposite in this system. The photophysical properties of the nanocomposites synthesized are investigated, and the photosensitivity and quantum yield of charge carrier photogeneration are determined.

Charge carrier photogeneration yield in amorphous materials with long-range potential fluctuations

Random intrinsic electrostatic potentials cause local intrinsic electric fields that must affect the Onsager probability of geminate pair dissociation. If the intrinsic fluctuating field is strong enough the dissociation yield could be close to unity even at weak external electric fields and at low temperatures. An analytic model describing the effect of intrinsic field and potential fluctuations on the carrier photogeneration yield is formulated and compared to observed behavior in a-Si:H and a-Se.

Generation of singlet oxygen with anionic aluminum phthalocyanines in water

The dependence of the quantum yield of photogeneration of singlet oxygen with aluminum sulfophthalocyanines on the degree of their sulfonation was studied. Experiments showed that Fotosens preparation containing, on the average, about three sulfo groups in the macroring in aqueous buffer solution (pH 7.4) exists in the monomeric form and sensitizes singlet oxygen with a quantum yield ϕΔ 0.42±0.06. Aluminum tetra-4-sulfophthalocyanine forms dimers, and correspondingly ϕΔ is lower: 0.22±0.03. Sulfonated aluminum phthalocyanine containing, on the average, about two sulfo groups in the macroring is aggregated to a significant extent. Addition of Triton X-100 detergent results in partial deaggregation of the dye, and ϕΔ increases from ∼0.01 to 0.15±0.02. In the series including previously unknown aluminum phthalocyanines containing residues of phosphonic acids or their esters, and also aluminum and zinc octacarboxyphthalocyanines, the influence of the dye structure on the quantum yield of generation of singlet oxygen suggests participation of axial hydroxy groups in associate formation. These associates are not manifested spectroscopically and generate singlet oxygen inefficiently. The quantum yields ϕΔ for the monomers and associates are 0.3 and 0.1, respectively.

Photocarrier generation quantum yield for ionically self-assembled monolayers

We have fabricated photovoltaic cells from organic donor/acceptor couples arranged as ionically self assembled monolayers. Making use of the morphological control on the nanometer level we were able to study the influence of the device structure and layer composition on the quantum yield of charge carrier photogeneration. The photoluminescence quenching and the photocurrent spectra reveal sample composition, charge carrier photogeneration and transport properties independently.

Electrode effect on photohole generation in smectic phenylnaphthalene liquid crystalline photoconductor

The photohole generation process in the smectic phases of a liquid crystalline photoconductor, 2-(4′-octylpheny)-6-dodecyoxylnaphthalene (8-PNP-O12) with different illuminated contact electrodes was investigated through steady-state and transient photocurrent measurements. It was revealed that the photogeneration of holes was governed by two different processes according to the electrode materials: the Onsager type of photocarrier generation in the bulk and the electrode-enhanced hole photoinjection with a delay of μs when Al, and either Pt or In2O3–SnO2 electrodes are applied, respectively. In the latter process, the photogeneration yield was one order of magnitude larger than that in the bulk generation process, but decreased as the molecular ordering degraded from the smectic B phase to the smectic A phase, then to the isotropic phase. According to the spectral response and the results of additional experiments on the effect of chemical doping with electron acceptors, the photohole injection was concluded to be attributed to the exciton decay at the interface of the electrode and liquid crystal, i.e., the charge transfer from photoexcited 8-PNP-O12 molecules to the electrode. The experimental results were analyzed according to a one-dimensional Onsager model and the diffusion length of excitons in the SmB phase was determined to be 30±10 nm.

Ultrafast on-chain dissociation of hot excitons in conjugated polymers

A model describing field-assisted on-chain dissociation of hot optical excitations in conjugated polymers is developed. It rests on the notion that excess photon energy is required for the onset of intrinsic charge-carrier photogeneration. Conversion of this energy into excited vibrational modes within a conjugated polymer segment leads to temporal overheating. The vibrational heat bath is considered as the main source of the energy that allows charge carriers to escape from the potential well formed by Coulomb and external electric fields. Although the quantum yield of carrier photogeneration strongly increases with increasing external field, it reveals an anomalously weak temperature dependence in quantitative agreement with experimental data on a ladder-type poly-para-phenylene and on a polyphenylenevinylene derivative.

Photogeneration in N-carbazolyl-substituted polysilanes and their charge-transfer complexes with 2,4,7-trinitrofluorenone

Photoconductivity studies of N-carbazolyl-substituted silane homo- and copolymers and their charge-transfer complexes with 2,4,7-trinitrofluorenone (TNF) are reported. The quantum yield of photogeneration was investigated by means of xerographic discharge method. The photogeneration yields and their dependencies on electric field were analysed in terms of the Onsager model of geminate recombination and they were compared with previously obtained results for poly(N-vinylcarbazole), pure and doped with TNF.

Analysis of electric-field assisted photogeneration in polyparacyclophane doped with 2,4,7-trinitrofluorenone

A new photoconducting polymer with extended cooperative electronics effects, poly((E,E)-[6.2]paracyclophane-1,5-diene) (PDE), doped with a low molecular weight organic acceptor, 2,4,7-trinitrofluorenone (TNF), was investigated by means of steady-state photoconductivity and xerographic discharge methods. The photogeneration quantum yield in the PDE+TNF charge transfer (CT) complex for blue light is comparable with the photogeneration yield in poly(N-vinylcarbazole) (PVK) doped with TNF. It is demonstrated that the electric-field dependence of the photogeneration quantum yield in PDE+TNF systems can be described by the Braun kinetic model of dissociation of the CT complex, and not by the Onsager model of geminate recombination, which is appropriate for PVK+TNF systems.

Hot Exciton Dissociation in a Conjugated Polymer

A model has been developed to treat field-assisted ultrafast on-chain dissociation of optical excitations in a conjugated polymer. Conversion of the excess photon energy above the singlet exciton into the vibrational heat bath of a polymer segment is considered as a main source of the energy required for charges to escape from a potential well formed by a superposition of the Coulomb and external electric fields. The calculated yield of carrier photogeneration increases strongly with increasing field but is independent of the temperature in good quantitative agreement with data obtained by Barth et al. [Chem. Phys. Lett. 288, 147 (1998)] on weakly disordered conjugated ladder-type poly-phenylene.

Photoconduction in Langmuir–Blodgett films of octasubstituted metal-free phthalocyanine molecules

In-plane steady-state photoconduction in Langmuir-Blodgett (LB) films of octasubstituted metal-free phthalocyanine molecules have been studied over a temperature range between 78 and 293K. Deep-level traps localised at grain boundaries of multicrystalline films are believed to be responsible for both dark conduction and photoconduction. Slow and fast relaxation mechanisms are identified from measurements of transient currents. Small changes of substituting chains in the molecule are found to have significant influences on rate and quantum yield of photogeneration of charge carriers in LB films.

Temperature-independent quantum yield of carrier photogeneration in weakly disordered conjugated polymers

A model of field-assisted intrasegmental dissociation of optical excitations in weakly disordered conjugated polymers is formulated. The model accounts for both over-barrier separation of charge carriers and tunneling dissociation of excitations in the course of their relaxation from an initial higher excited state to the lowest excited state. The model offers an explanation of recent observations of charge carrier photogeneration which show a strong field dependence but, remarkably, virtually no temperature dependence.

Influence of Solvent Polarity and Proticity on the Photochemical Properties of Norfloxacin

The fluoroquinolone antibacterial norfloxacin (NF) is a moderate photosensitizer of singlet molecular oxygen (1O2). We have studied photosensitization by NF as a function of medium polarity and proticity in solvent mixtures. We have used 1,4-dioxane and propylene carbonate mixtures to keep proticity constant while modulating polarity, and water/D2O and ethylene carbonate mixtures to alter proticity without large changes in polarity. The absorption spectrum of NF was little affected by solvent changes, as compared to the fluorescence spectrum that exhibited as much as a 50 nm blue-shift, e.g. 1,4-dioxane versus D2O. The quantum yield of NF fluorescence saturated at an almost 10 times higher value (approximately 0.14) when proticity was increased by added water, up to 0.2 mol fraction, to ethylene carbonate. Less pronounced, the increasing polarity in 1,4-dioxane/propylene carbonate mixtures affected the fluorescence yield much less. Norfloxacin produces 1O2 and is able to quench 1O2. The rate constant for 1O2 quenching is 4.5 x 10(7) M-1 s-1 in propylene carbonate but decreases ca four times in D2O. The quantum yield of 1O2 photogeneration was also up to five times higher in solvents that were both protic and polar than vice versa. Our data show that NF is more photochemically active in an environment that is both protic and polar. This suggests the involvement of polar excited state(s) and possible proton/hydrogen transfer during photoexcitation. Similar processes may initiate the phototoxic response reported in some patients treated with the fluoroquinolone drugs. The phototoxicity of NF and other fluoroquinolone antibiotics may strongly depend on their localization in hydrophilic or hydrophobic cell/tissue regions.

Picosecond to millisecond photoexcitation dynamics in blends of C 60 with poly( p-phenylene vinylene) polymers

We compare photoexcitation dynamics in films of pristine and C 60-mixed 2,5-dioctyloxy poly( p-phenylene vinylene) (DOO–PPV) using ps transient and cw photoinduced absorption (PA), and PA-detected magnetic resonance (PADMR). We conclude that C 60 doping is relatively inefficient in DOO–PPV. Consequently, the instantaneous exciton photogeneration yield remains high (close to 1) even at high C 60 concentrations. However, as the C 60 concentration increases up to 10 molar percent, the stimulated emission lifetime and cw photoluminescence intensity decrease together by about an order of magnitude due to dissociation of the singlet excitons at C 60 related defect centers into intrachain polaron pairs, which were identified by PADMR.

Influence of Solvent Polarity and Proticity on the Photochemical Properties of Norfloxacin

Abstract— The fluoroquinolone antibacterial norfloxacin (NF) is a moderate photosensitizer of singlet molecular oxygen (1O2). We have studied photosensitization by NF as a function of medium polarity and proticity in solvent mixtures. We have used 1,4-dioxane and propylene carbonate mixtures to keep proticity constant while modulating polarity, and water/D2O and ethylene carbonate mixtures to alter proticity without large changes in polarity. The absorption spectrum of NF was little affected by solvent changes, as compared to the fluorescence spectrum that exhibited as much as a 50 nm blue-shift, e.g. 1,4-dioxane versus D2O. The quantum yield of NF fluorescence saturated at an almost 10 times higher value (?0.14) when proticity was increased by added water, up to 0.2 mol fraction, to ethylene carbonate. Less pronounced, the increasing polarity in 1,4-dioxane/propylene carbonate mixtures affected the fluorescence yield much less. Norfloxacin produces 1O2 and is able to quench 1O2. The rate constant for 1O2 quenching is 4.5 × 107 M−1 s−1 in propylene carbonate but decreases ca four times in D2O. The quantum yield of 1O2 photogeneration was also up to five times higher in solvents that were both protic and polar than vice versa. Our data show that NF is more photochemically active in an environment that is both protic and polar. This suggests the involvement of polar excited state(s) and possible proton/hydrogen transfer during photoexcitation. Similar processes may initiate the phototoxic response reported in some patients treated with the fluoroquinolone drugs. The phototoxicity of NF and other fluoroquinolone antibiotics may strongly depend on their localization in hydrophilic or hydrophobic cell/tissue regions.

Photochromic Properties in Cadmium Bismuth Aluminate Glasses and Its Oxidation Treatment Effect

Photochromic properties were investigated for glass in the system CdO-Bi2O3-Al2O3. Optical density increased with increasing BiO1.5 content in the glasses at fixed CdO content. The darkening degree could be effectively enhanced by oxidation treatments. The composition dependence and effect of oxidation treatment were studied by X-ray photoelectron and Raman scattering spectroscopies. With increasing BiO1.5 content the Lewis basicity of oxygen atoms increased and the fraction of covalently bonded cadmium decreased. The trap density of holes increased and the photogeneration yield decreased. The darkening degree showed a maximum around BiO1.5=13 cation%. The effect of oxidation treatment on the darkening degree may be related to production of a large amount of monovalent O- ions with holes in their 2p valence band, and to formation of electron centers in cadmium atoms.

Free Charge Carrier Formation in Polymers under Illumination

In addition to many extrinsic processes, free charge carriers in polymers can be photogenerated intrinsically via an intermediate stage of bound ion-pairs formed in consequence of band-to-band transitions or exciton autoionization. The excitation can be realized within the framework of (i) a chemical bond in the polymer backbone or (ii) a side group skeleton. An electron—hole pair with a short separation distance created by light on the same polymer chain or on the same side group usually recombines geminately very fast and does not contribute to the photocurrent. An inter- or intrachain charge transfer or transfer of the electron from the main chain to a side group is necessary to form a more stable ion-pair. Dissociation of the ion-pairs by Brownian motion subjected to a combination of Coulomb and applied electrical field, into free charge carriers, can be described in terms of the Onsager theory of geminate recombination. The photogeneration yield can be improved by an electron acceptor doping or by an electron accepting groups attached to side chains.