Photogeneration Quantum Yield Research Articles

Photogeneration and photovoltaic effect in poly(N-vinylcarbazole):TiO 2 bulk-heterojunction elaborated by hydrolysis–condensation reactions of TiO 2 precursors

TiO 2:poly(N-vinylcarbazole) (PVK) bulk-heterojunctions (BHJ) have been elaborated from a blend of a TiO 2 precursor with the polymer in solution. The TiO 2 precursor is hydrolyzed with the surrounding air humidity in thin film. Titanium isopropoxide [Ti( iOPr) 4] and Tetrakis (9H-carbazole-9-yl-ethyl-oxy) titanium [Ti(OeCarb) 4] were used as TiO 2 precursor. Photogeneration quantum yield in BHJ of TiO 2:PVK were determined by the surface potential decay (SPD) technique. Photovoltaic effect was studied in ITO/PEDOT:PSS/TiO 2:PVK/Al structures. It was found that the photovoltaic performance is strongly dependent on the homogeneity of the blends. The improved dispersion of the TiO 2 phase obtained with the precursor bearing a carbazole group allows efficient electron transport and fill factor enhancement of the photovoltaic devices.

Photosensitive properties of metal-containing polydisalicylidene azomethines

Photosensitive properties of new metal-containing polydisalicylidene azomethines were studied. It was shown that polymer properties are controlled by the nature of the metal atom (its electron affinity energy A a and ionic radius r i ) included in “nonclassical” polyconjugation. The photosensitivity S 0.1 of studied polymers is ∼105cm2/J, and the quantum yield of free-carrier photogeneration is η ≈ 0.10–0.15, which corresponds to the level of organometallic complexes that have found application in optoelectronics.

Synthesis of new photosensitive silicon-containing poly(salicylideneazomethynes)

Photosensitive silicon-containing poly(salicylideneazomethynes) have been synthesized. It has been shown that these polymers are characterized by the presence of intramolecular donor-acceptor interactions between valence shells of silicon and nitrogen atoms; as a result, polyconjugation appears according to the nonclassical mechanism. The photo-and electrophysical properties of the polymers have been examined. The introduction of an oxygen spacer into a polymer facilitates an increase in the photosensitivity and quantum yield of free charge carrier photogeneration, and it has almost no effect on the conductive behavior of the polymers.

Photophysical properties of silicon-containing poly[salicylidene azomethine

Photophysical properties of new silicon-containing “nonclassical” polyconjugated poly[salicylidene azomethine]s were studied and a free carrier photogeneration mechanism characteristic of them was proposed. It was shown that polymer properties are controlled by the structure of the azomethine fragment and acceptor properties of the silicon atom. Due to multiple specific donor-acceptor interactions of unshared electron pairs of azomethine fragments with vacant d orbitals of silicon atoms, polymers feature photosensitive and photoconductive properties. The estimated photosensitivity S0.1 ≈ 105 cm2/J and quantum yield of free carrier Photogeneration η ≈ 0.15 correspond to the level of “classical” polyconjugated nitrogen-containing aromatic polymers used in optoelectronics.

Photoconductivity in amorphous selenium blocking structures

AbstractPhotoconductive properties of the first practical a‐Se avalanche photosensor (hole drift mobility, photocurrent response time, quantum yield of charge photogeneration and avalanche multiplication gain) have been studied. It has been shown that hole transport and avalanche multiplication mechanisms in multilayered a‐Se blocking structure used in practical photosensors are similar to those observed in a‐Se monolayers confined between insulating layers and used for the first manifestation of avalanche effect in this material by Juska and Arlauskas, phys. stat. sol. (a) 59, 389 (1980). It is suggested that combination of high avalanche gain (up to 1000) with high quantum efficiency of charge photogeneration (almost 100%) and fast photocurrent time response (∼ 1 ns) makes a‐Se avalanche photosensor a promising candidate to replace vacuum photomultiplier tubes for application in medical radiation imaging. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of molecular order on charge carrier photogeneration in perylene derivative layer

X-ray diffraction investigations show, that in the vacuum evaporated layers of 2,9-di(pent-3yl)-anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10-tetrone (PTCDI) the PTCDI molecules may have edge-on or face-on arrangement. For the samples with different molecular arrangement the dependences of the photogeneration quantum yield on electric field were determined by using the photoinduced surface potential decay technique. The obtained results were analysed using the Onsager's model of photogeneration. The clear influence of the molecular order on the photogeneration efficiency has been found.

Influence of bimolecular recombination on xerographic discharge process

The photoinduced decay of surface potential, known as xerographic discharge, is analyzed as a method of determination of photogeneration quantum yield taking into account the recombination processes. The effects of slowing down the surface potential decay with time of illumination and nonzero residual potential are usually explained by trapping of the charge carriers in the bulk of the photoconductor. However, these effects can be explained also by bimolecular recombination in the illuminated thin surface layer occurring simultaneously with the photogeneration. In this work it is assumed that as a result of the illumination the mobile positive charge carriers and immobile negative recombination centers are generated. The critical value of the recombination cross section is determined for which the nonzero residual surface potential appears. This analysis allows also to determine an influence of the accumulated recombination centers on the photocurrent decay and build up of the residual potential in a series of subsequent xerographic discharge experiments.

Charge transport in a single-layer organic photoreceptor

Discharge kinetics of a single-layer organic photoreceptor under exposure to a semi-infinite pulse of light with wavelength λ = 780 nm have been investigated both experimentally and theoretically. The results have been obtained for various light intensities in the range I = 2 × 10 16–5 × 10 17 quants/s m 2 and for various initial charging potentials in the range U 0 = 1150–280 V. By fitting experimental film potential kinetics by theoretical ones and taking into account the measured dependence of hole and electron mobilities on electric field strength E, it has been determined that an important role in charge transport is played by field-induced hole traps, whose density is proportional to E σ , where 2 ≥ σ ≥ 1.2. It has been also determined that photogeneration quantum yield is proportional to E r , where 2 > r ≥ 1.

Photogeneration and photovoltaic effect in blends of derivatives of hexabenzocoronene and perylene

Photogeneration quantum yield in layers of discotic liquid crystals (DLC, hole transport material), perylene derivative (electron transport material) and of their mixtures were determined by using surface potential decay technique. While in pure DLC only very weak photogeneration in the visible range was detected, and in pure perylene derivative only negative mobile charge carriers were photogenerated, in the mixtures a synergetic effect of photogeneration of both positive and negative mobile charge carriers was observed. This is because the excitons photogenerated at perylene derivative dissociate into electron–hole pairs, and then electrons are transported via perylene derivative while the holes are transferred onto the DLC molecules. The obtained results were correlated with performance of the photovoltaic devices with active layer made of the mixtures of the DLC and perylene derivative and of their blends with photoconducting polymer. It was found, that the power conversion efficiency of the device is dependent on the film morphology.

Charge carrier photogeneration in conjugated polymer PhPPV/R6G composite system

The spectral and polarity dependence of the quantum yield of charge carrier photo-generation was studied by steady-state photocurrent measurement in a single layer PhPPV film, double layer film of PhPPV and R6G and doped film of PhPPV with R6G The intrinsic and extrinsic charge carrier photogeneration was observed. The result indicates that the quantum efficiency of the double layer device is higher than that of single layer device under reverse bias, but it is opposite under forward bias. The yield of charge carrier photogeneration of the doped film is higher than that of the other two films at both forward and reverse bias because of the increased interface area between the electron donor and acceptor.

Photoconducting polymer nanocomposites with efficient photogeneration and bipolar transport for optoelectronic applications

Photoconducting polymer nanocomposites with high electrophotographic sensitivity for both positive (S λ +) and negative (S λ −) signs of corona charging (up to 400 m 2/J in 300–620 nm range) and high charge carrier photogeneration quantum yield (β up to 0.6) are developed on the base of p-conducting (S λ + ≫ S λ −) polymer matrices (polyimides (PI), carbazolylcontaining (CzCP) polymers) and n-conducting low molecular mass additive (perylenediimide derivative (PDID)) in aggregated form. For p-composites (PI doped with PDID at content C A up to 50% wt.) the efficient hole photogeneration in PDID absorption band is related to the observed electron transfer from PI donor chain fragments with low ionization potential (I D = 6.8 eV) to the excited PDID aggregates as acceptors (with affinity E A = 1.8–2.0 eV). Low S λ − value for p-composites is likely due to potential barrier formation between PDID particles (aggregates and clusters) involved in electron transport network. For n-composites (S λ − ≫ S λ +) (CzCP doped also with PDID) high S λ − and β values are explained by the efficient electron photogeneration via excited charge transfer complex (exciplex) between excited PDID molecule as acceptor and CzCP carbazolyl pendant as donor (with I D = 7.4 eV) as well as electron transport network formation involving PDID particles. As it is only ionization potentials of polymer donor fragments that differ essentially for p- and n-composite, the conclusion is made that charge transfer donor-acceptor interaction in the ground state may be responsible for potential barrier formation between acceptor particles embedded in donor matrix. It is found that films of bipolar sensitive composites (S λ + ≈ S λ −) (obtained by mixing p- and n-composites in solution) which have microsegregated structure are characterized by the highest S λ +, S λ − and β values. The photovoltaic effect is investigated for sandwich cells with (Al, ITO) electrodes. The best parameters are found for bipolar composite films (0.5–1.0 μm thick).

Charge Carrier Photogeneration and Transport in Organic Photoconducting Polymer Nanocomposites Based on Polyimides

ABSTRACT On the base of photo- and thermostable polyimides (PI) and carbazolyl-containing polymers (CzCP) possessing predominantly p-photoconductivity and doped with perylenediimide derivative (PDID) as an acceptor with n-conductivity, the efficient bipolar polymer nanocomposites are developed. The efficient bipolar transport is obtained for microsegregated triple composites (PI-CzCP-PDID and others) in which there are p- and n-microregions with efficient charge photogeneration (quantum yield up to 0.6) at the extended microphase interface and high photoelectrical sensitivity (up to 400 m2/J) in the UV and visible ranges. The photovoltaic effect is investigated for sandwich cells with (Al, ITO) electrodes. The best parameters are found for triple composite films (0.7–1 μm thickness).

Photosensitivity of thin films of semiconductor nanocomposites based on metal-organic complexes with Cu+ and Ru2+

The photosensitivity of nanocomposites in a system constituted by an organic transition metal complex and silica was studied. It is shown that the photosensitivity of a nanocomposite is determined by the valence and the ionization potential of the metal atom, which is equal to (3–8)×104 cm2/J (quantum yield of carrier photogeneration is 0.03–0.05) for the Cu+ complex and 5×103 cm2/J for the Ru2+ complex. The possibility of controlling the magnitudes and spectra of both the photosensitivity and luminescence of thin films of nanocomposites based on these complexes by varying the structure of metal complexes and the fact that the quantum efficiency of photogeneration can be raised by introducing transport molecules into the structure of a nanocomposite gives reason to believe that photosensitive and electroluminescent devices can be created on the basis of these nanocomposites.

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.

Mechanisms controlling the efficiency of polymer solar cells

To improve the efficiency of polymer solar cells, it is vital to understand which mechanisms control the current–voltage characteristics of a given device. Temperature and light intensity dependence of the main solar cell parameters are very informative for analyzing losses. We report on the current–voltage characteristics and the external photogeneration quantum yield of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al as well as of ITO/PEDOT:PSS/P3HT:PCBM/Al devices investigated in the broad temperature range 120–325 K under variable illumination, between 0.02 and 100 mW/cm2. We discuss the recombination on traps and the low mobility of charge carriers caused by poor morphology of active layers as possible mechanisms limiting the efficiency of these devices.

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.

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.

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.

Molecular aggregate formation and microphase segregation effects on the photoelectrical and photovoltaic properties of polyimide-perylenediimide composite films

The strong increase of electrophotographic sensitivity, charge carrier photogeneration quantum yield and improved photovoltaic characteristics (sandwich cells with ITO, Al electrodes) are found for films of photoconductive soluble polyimides based on doubled triphenylamine heavily doped by the soluble perylenediimides (with content up to 50-60% wt). It is shown that the effect is due to perylenediimide aggregates of some tens nanometers size which are formed during the film casting at high solvent evaporation rate. The further increase of photoelectric and photovoltaic parameters is achieved by using the films of polymer blends of polyimides with polyetherimides (Ultem-1000 type) also doped with soluble perylenediimides. In this case the effect is due to microphase segregation in doped polymer blend films caused by the partial incompatibility of polymers, that results in the formation of microregions with n- and p-types of photoconductivity contacting with each other and possible appearance of p-n bulk microheterojunctions.

Photogeneration and photovoltaic properties of polyimides with extended charge delocalisation in polymer chains

The photogeneration and photovoltaic properties of soluble polyimides seria that contain monotype electron donor units with gradually increased size in their chains, had been studied. It is shown that photogeneration quantum yield is increased with initially formed positive charge delocalisation increasing with simultaneous rising of photovoltaic activity. The spectral sensitization and magnetic field spin effects had been studied also.