Quantum Efficiency For Photogeneration Research Articles

Polymorph-induced photosensitivity change in titanylphthalocyanine revealed by the charge transfer integral

Abstract The crystal form of semiconductor materials is keenly correlated with the photosensitivity of optoelectronic devices. Thus, understanding the crystal form-dependent photosensitivity mechanism is critical. In this work, the microemulsion phase transfer method was adopted to prepare α- and β-titanylphthalocyanine (TiOPc NPs) with an average diameter of 35 nm. The photosensitivity (E 1/2) of α-TiOPc NPs was 2.73 times better than that of β-TiOPc NPs, which was characterized by photoconductors under the same measurement conditions. DFT was performed to explain the relationship between crystal form and photosensitivity by systematically calculating the charge transfer integrals for all possible dimers in the two different crystal forms. The hole and electron reorganization energies of TiOPc were respectively calculated to be 53.5 and 271.5 meV, revealing TiOPc to be a typical p-type semiconductor. The calculated total hole transfer mobility (μ +) ratio (2.83) of α- to β-TiOPc was almost identical to the experimental E 1/2 ratio (2.73) and the calculated photogeneration quantum efficiency (η e-h) ratio (2.23). In addition, the optimum hole transfer routes in the crystal of α- and β-TiOPc were all along with the [1 0 0] crystal orientation, which was determined by the calculated μ +. A high charge transfer mobility leads to a high photosensitive TiOPc crystal. Consequently, these results indicate that the selected theoretical calculation method is reasonable for indirectly explaining the relationship between crystal form and photosensitivity. The TiOPc molecular solid-state arrangements, namely, the crystal forms of TiOPc, have a strong influence on the charge transport behavior, which in turn, affects its photosensitivity.

Reprint of: Ultrafast photoinduced electron transfer in conducting polymer–buckminsterfullerene composites

We report time-resolved photoinduced absorption and time-resolved photoconductivity in conducting polymer-C60 composites. Photoinduced electron transfer occurs at times <1ps. Two subgap features are observed in the photoinduced absorption of pure poly (3-octylthiophene) (P3OT); these form earlier than 1ps and decay with τ<5ps. P3OT mixed with C60 shows similar spectral features at early times (<1ps); however, a new spectrum evolves after ≈1ps as a result of electron transfer from P3OT to C60. Ultrafast photoinduced electron transfer improves the quantum efficiency for photogeneration of charge carriers; in the ps domain the photoconductivity of the conducting polymer host is enhanced by more than an order of magnitude upon mixing in a few percent C60.

Spectroscopic study of polyphenylquinolines—materials with efficient intramolecular charge transfer

The energy structure and nature of intramolecular charge transfer have been determined for a series of synthesized 2,6-polyphenylquinolines (PPQs) containing resorcinol, oxygen, and phenylamine bridging groups between phenylquinoline cycles and possessing an arylene radical in the form of alkylated derivatives of carbazole or indolo[3,2-b]carbazole, as well as arylated derivatives of carbazole with nitro- and fluorine substituents. The analysis was based on data on their absorption spectra, photosensitivity, carrier photogeneration quantum efficiency, and luminescence. It is shown that the PPQ monomeric unit is a composite intramolecular charge-transfer complex (ICTC) between the donor arylene fragment and the acceptor quinoline cycle; the latter, in turn, is an ICTC between the donor bridging group and quinoline ring (when acceptor fluorine or nitro substituents are introduced into the carbazole structure, the complex becomes ternary), and the narrowest band gap (2.3 eV) corresponds to the PPQ having a phenylamine bridging group and a fragment of indolo[3,2-b]carbazole.

Solvent‐Stabilized Y‐Type Oxotitanium Phthalocyanine Photoconductive Nanoparticles: Preparation and Application in Single‐Layered Photoreceptors

Photoconductive Y-TiOPc nanoparticles (NPs) (dav = 3.4 nm), well dispersible in 1,2-dichloroethane, are efficiently prepared via a colloidal-solution-mediated phase-transition process. Single-layered photoreceptors composed of Y-TiOPc-NPs/insulator/Y-TiOPc-NPs quantum junctions exhibit outstanding xerographic properties by virtue of the high photogeneration quantum efficiency and high work function of the Y-TiOPc-NPs, as well as efficient light-induced electron tunneling through the junctions.

Thickness dependent absorption and polaron photogeneration in poly-(2-metoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene)

A comprehensive photocurrent spectra analysis of the ITO/PEDOT:PSS/MEH-PPV/Al devices with three different poly-(2-metoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) or MEH-PPV thin-film thicknesses is used to investigate charge carrier photogeneration in this polymer. The photocurrent is calculated based on the hole polaron drift-diffusion model including Poole-Frenkel (P-F) transport. Two mechanisms (monomolecular and Langevin-type bimolecular) for hole polaron recombination are considered separately. The MEH-PPV absorption coefficient spectra dependence on the thin film thickness is experimentally determined and included in our model. By comparing the simulated photocurrent spectra to measured data in the wide range of bias voltages and for devices with different MEH-PPV film thicknesses the polaron photogeneration quantum efficiency (θp) as a function of the electric field is extracted. The θp curves obtained for different devices are perfectly fitted by the P-F expression. It is shown that polaron photogeneration process in the thin MEH-PPV films is affected by the film thickness. A unique polaron photogeneration model which accounts for the field and thickness dependences is achieved when one of the P-F parameters is left to be thickness dependent.

Bulk photoconductive gain in pentacene thin films

Bulk photoconductive gain greater than 16 is observed in pentacene thin films deposited onto coplanar interdigitated-electrode photodetector structures. The gain is highest at low light intensity but decreases at higher light intensity due to trap filling effects. The internal photogeneration quantum efficiency is found to be independent of wavelength below the absorption edge with the onset of photocurrent yield occurring at the absorption edge of the film.

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.

Quantum Efficiency of Photogeneration in Discotic Liquid Crystals: Part 1: Temperature and Wavelength Dependence

Photocarrier generation quantum efficiencies of columnar discotic liquid crystals are determined using DC photoconduction. Photoaction spectra, of single triphenylene and binary systems comprising 50:50 stoichmetric mixtures of triphenylene with a larger triphenylene based macrocycle, demonstrate that the main generation process is due to the excitation of the smaller triphenylene molecule. Other transient photoconduction experiments have shown that carriers trap in the crystalline phase of the single systems while time of flight signals are obtained in the mesophase. Temperature dependence experiments allow carrier range to be determined in the crystalline phase for the single material and demonstrate that charge transits the binary system in both the mesophase and glassy phase.

Poly(silyens)s: Charge Carrier Photogeneration and Transport: S. Nešpůrek, A. Eckhardt Polym. Adv. Technol.12, 427–440 (2001)

AbstractThe original article to which this Erratum refers was published in Polym. Adv. Technol. 12, 427–440 (2001)In the following paragraph, ‘µ’ was inserted instead of ‘γ’.The paragraph should have read: Xerographic DischargeThe electric field (F) dependences of photogeneration efficiency as measured by the xerographic discharge method are shown in Figure 4 for PMPSi and PBMSi. The electric field and temperature (T) dependences of the photogeneration efficiency can be described using Onsager theory of geminate recombination (full lines). This theory can be used to describe one step in the photogeneration process, the thermal dissociation of ion‐pairs formed by light into free charge carriers in the external electric field. Under the assumption that the distribution of bound pairs is spherically symmetrical, the overall photogeneration efficiency ν is expressed as $\eta\,(r,F,T)=\eta_0\,\,{\textstyle\int}\,4\pi r^2f(r,F,T)g(r)dr$ where ϵ0 is the voltage and temperature independent primary quantum yield (fraction of the ion‐pair generated per photon), g(r) is the spatial distribution of the pairs, and r is the pair separation distance. Function f(r,F,T) represents the Onsager dissociation probability [9] of the ion‐pair. The best theoretical fits of the experimental data given in Figure 4 were found using the Onsager theory with the Gaussian distribution of radii [17] $g(r)=(\pi^{-3/2}$­$\gamma^{-3})$ exp (−r2/γ2) using parameters: γ = 1.3 nm, η0 = 0.45 and γ = 1.6 nm, η0 = 0.85 for PMPSi and PBMSi, respectively. According to Eq. 2 the photogeneration quantum efficiency η is also temperature dependent. The activation energy was determined as Eη = 0.05 eV for PMPSi (inset in Figure 4) and 0.04 eV for PBMSi. The temperature dependence of the photogeneration efficiency is also in good agreement with the Onsager dissociation theory (full line in inset of Figure 4).

Photogeneration Action Spectroscopy of Neutral and Charged Excitations in Films of a Ladder-Type Poly(Para-Phenylene)

The photogeneration quantum efficiency action spectra of long-lived neutral and charged excitations in films of a ladder-type poly(para-phenylene) were measured. We found that both triplet and polaron action spectra show, in addition to a step function increase at the optical gap, a monotonic rise at higher energies. For triplets this rise is explained by singlet exciton fission into triplet pairs from which the triplet exciton energy in the gap was obtained; this energy was also confirmed by measuring the weak phosphorescence band. For polarons the photogeneration increase at high energies is modeled by a novel hot electron interchain tunneling process.

Charge Transfer States In Titanyl Phthalocyanine Films With Polymer Binder

The results of absorption spectra (AS) and spectra of surface photovoltage (V) investigation are presented for the films obtained by the incorporation of titanyl phthalocyanine (TiOPc) in polymers. The energetic diagram of neutral and mixed excited states are drawn for all polymorphs and the content of polymorphs is estimated for TiOPc films obtained by different methods of preparation. The comparison of AS and V spectra showed that the quantum efficiency of photogeneration (β) strongly depends on the predominant polymorph type. Besides it is found that high energetic CT-states contribute to the processes of charge carriers photogeneration rather than Frenkel excitons (FE) and low energetic CT-states.

Nanoscale Size Effects on Photoconductivity of Semiconducting Polymer Thin Films

Bilayer polymer photoreceptors containing a trap-free tri-p-tolylamine-doped polycarbonate (TTA:PC) layer and a conjugated polymer layer are used to study nanoscale size effects in electronic polymers. A 3-fold enhancement of photogeneration quantum efficiency is observed as the thickness of poly(2,5-pyridylenebenzobisthiazole) (PPyBT) is reduced from 100 to 10 nm. The nanometer size effects originate from the small exciton diffusion lengths and the interfacial nature of the photocarrier generation.

Infrared photoexcitation spectroscopy in soluble derivatives of poly(p-phenylenevinylene) and composites with C 60

We report detailed photoinduced absorption spectra of soluable derivatives of poly(p-phenylenevinylene), for example MEH-PPV and BCH-PPV, and their composites with C 60, from 0.05 eV to 1.3 eV. The photoinduced absorption spectra of MEH-PPV and BCH-PPV are characterized by subgap electronic absorptions and associated infrared-active vibrational (IRAV) modes. Upon adding a few percent of C 60 into both polymers, the photoinduced absorption spectra are significantly enhanced in magnitude and new peaks are found at 1.15 eV and 1.25 eV. The photoinduced charge transfer from conducting polymer onto C 60 enhances the quantum efficiency for photogeneration of charge carriers, resulting in the enhanced IRAV modes of the conducting polymer and the appearance of 1.15 eV and 1.25 eV signatures associated with C 60.

Large enhancement of the transient and steady-state photoconductivity of conducting polymer/C60 composite films

Summary form only given. We present the transient and steady-state photoconductivity (PC) results obtained from poly[2 methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly(octylthiophene) (P3OT), and from conducting polymer films sensitized with several concentrations of C/sub 60/. Both the magnitude and the lifetime of the transient PC increase substantially on increasing the concentration of C/sub 60/. The results imply that quantum efficiency for photogeneration of charge carriers is increased by photoinduced electron transfer and that recombination is inhibited by the spatial separation of the electron and hole on the C/sub 60/ and the conducting polymer, respectively. The greater enhancement of the transient PC in MEH-PPV (nearly two orders of magnitudes with addition of a few percent of C/sub 60/) than in P3OT (nearly one order of magnitude with addition of a comparable amount of C/sub 60/) suggests a higher probability of the early-time recombination in pristine MEH-PPV. As a result of the enhancement in the photogeneration efficiency and the carrier lifetime, the spectral response of the steady-state PC in both MEH-PPV/C/sub 60/ and P30T/C/sub 60/ films shows significantly enhanced PC throughout the entire spectral range from the near infrared to the ultraviolet.

Infrared Photoexcitation Spectroscopy of Conducting Polymer and C60 Composites: Direct Evidence of Photo-Induced Electron Transfer

Abstract We report direct spectral evidence of photoinduced electron transfer from the excited state of conducting polymer onto C60 by infrared photoexcitation spectroscopy, from 0.01 eV (100 cm−1) to 1.3 eV (11,000 cm−1). The photoinduced absorption spectra of poly(3-octylthiophene) (P30T) and poly[2-methoxy, 5- (2′ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) are characterized by subgap electronic absorptions and associated infrared-active vibrational modes. Upon adding a few percent of C60 into both conducting polymers, the photoinduced absorption spectra are significantly enhanced in magnitude and two new peaks are found at 1.15 eV and 1.25 eV. The photoinduced charge transfer from conducting polymer onto C60 enhances the quantum efficiency for photogeneration of charge carriers, resulting in the enhanced photoinduced response of conducting polymer and the appearance of 1.15 eV and 1.25 eV signatures associated with C60.

Direct evidence of photoinduced electron transfer in conducting-polymer-C60 composites by infrared photoexcitation spectroscopy.

We report direct spectral evidence of photoinduced electron transfer from the excited state of poly(3-octylthiophene), P3OT, onto ${\mathrm{C}}_{60}$ by infrared photoexcitation spectroscopy, 0.01 eV (100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) to 1.3 eV (11 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). The photoinduced absorption spectrum of P3OT, characterized by subgap electronic absorptions and associated infrared-active vibrational modes, is significantly enhanced in magnitude upon adding a few percent of ${\mathrm{C}}_{60}$. Two new peaks are found in the photoexcited spectrum of the P3OT/${\mathrm{C}}_{60}$ composite and assigned to the allowed HOMO (${\mathit{T}}_{1\mathit{u}}$)-LUMO (${\mathit{T}}_{1\mathit{g}}$) transitions of ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ with energies, 1.15 and 1.25 eV, in good agreement with calculated values. The photoinduced charge transfer from P3OT onto ${\mathrm{C}}_{60}$ enhances the quantum efficiency for photogeneration of charge carriers resulting in charge separation in the excited state. As a result, the lifetime of the excited-state configuration, (P3OT${)}^{+}$ and ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$, is extended; thereby leading to both the enhanced photoinduced response of P3OT and the appearance of the 1.15- and 1.25-eV signatures associated with ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$.

Slope to intercept ratio of low-field photogeneration efficiency in β-metal-free phthalocyanine

Using the delayed collection field technique we measured the slope to intercept ratio of the photogeneration quantum efficiency in β-metal-free phthalocyanine. At short delay times between pulsed laser illumination and the application of the collection field slope to intercept ratio is significantly smaller than the value predicted by the Onsager theory of geminate recombination. At longer delay times, exceeding about 200 ms, the slope to intercept ratio reaches the theoretical value indicating that the time evolution of geminate recombination is significantly slowed down by traps. At higher fields the carrier generation is dominated by the field modulated yield of the geminate pairs.

Transport properties of self-supporting porous silicon

We present the results of the first transient and steady-state photoconductivity measurements on self-supporting (19 μm thick) porous silicon (PS) sample. The value of the free-carrier photogeneration quantum efficiency (η) is discussed together with the mobility-lifetime product, deduced from transient (μDτD) and steady-state photocurrent (μτ)ss. Surprisingly (μτ)ss ⪡ (μDτD). From very similar spectral dependence and the magnitude of the steady-state photocurrents measured in the sandwich and coplanar geometries, we conclude that the “tissue” controlled transport in PS is almost isotropic.

Sensitization of the photoconductivity of conducting polymers by C60: Photoinduced electron transfer.

We have investigated the effect of photoinduced electron transfer on the photoconductivity (PC) of conducting polymer-${\mathrm{C}}_{60}$ films by comparing the photoconductivity (carrier generation and carrier transport) of the conducting polymer sensitized with ${\mathrm{C}}_{60}$ with that of the conducting polymer alone. We present time-resolved transient PC results, subnanosecond to 0.5 \ensuremath{\mu}s, obtained from poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly(3-octylthiophene) (P3OT), and from conducting polymer films sensitized with several concentrations of ${\mathrm{C}}_{60}$. Both the magnitude and the lifetime of the transient PC increase substantially on increasing the concentration of ${\mathrm{C}}_{60}$. The results imply that quantum efficiency for photogeneration of charge carriers is increased by photoinduced electron transfer and that early time recombination is inhibited by the spatial separation of the electron and hole on the ${\mathrm{C}}_{60}$ and the conducting polymer, respectively. The greater enhancement of the transient PC in MEH-PPV (nearly two orders of magnitude with the addition of a few percent of ${\mathrm{C}}_{60}$) than in P3OT (nearly one order of magnitude with the addition of a comparable amount of ${\mathrm{C}}_{60}$) suggests a higher probability of early time recombination in pristine MEH-PPV. As a result of the enhancement in the photogeneration efficiency and the carrier lifetime, the spectral response of the steady-state PC in both MEH-PPV/${\mathrm{C}}_{60}$ and P3OT/${\mathrm{C}}_{60}$ films is significantly enhanced throughout the entire spectral range from the near infrared to the ultraviolet.

Ultrafast photoinduced electron transfer in conducting polymer—buckminsterfullerene composites

We report time-resolved photoinduced absorption and time-resolved photoconductivity in conducting polymer-C 60 composites. Photoinduced electron transfer occurs at times < 1 ps. Two subgap features are observed in the photoinduced absorption of pure poly(3-octylthiophene) (P3OT); these form earlier than 1 ps and decay with τ<5 ps. P3OT mixed with C 60 shows similar spectral features at early times (<1 ps); however, a new spectrum evolves after ≈1 ps as a result of electron transfer from P3OT to C 60. Ultrafast photoinduced electron transfer improves the quantum efficiency for photogeneration of charge carriers; in the ps domain the photoconductivity of the conducting polymer host is enhanced by more than an order of magnitude upon mixing in a few percent C 60.