Charge Carriers In Poly Research Articles

Direct Observation of Structure and Dynamics of Photogenerated Charge Carriers in Poly(3-hexylthiophene) Films by Femtosecond Time-Resolved Near-IR Inverse Raman Spectroscopy.

The initial charge separation process of conjugated polymers is one of the key factors for understanding their conductivity. The structure of photogenerated transients in conjugated polymers can be observed by resonance Raman spectroscopy in the near-IR region because they exhibit characteristic low-energy transitions. Here, we investigate the structure and dynamics of photogenerated transients in a regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film, as well as in a pristine P3HT film, using femtosecond time-resolved resonance inverse Raman spectroscopy in the near-IR region. The transient inverse Raman spectrum of the pristine P3HT film at 50 ps suggests coexistence of neutral and charged excitations, whereas that of the P3HT:PCBM blend film at 50 ps suggests formation of positive polarons with a different structure from those in an FeCl3-doped P3HT film. Time-resolved near-IR inverse Raman spectra of the blend film clearly show the absence of charge separation between P3HT and PCBM within the instrument response time of our spectrometer, while they indicate two independent pathways of the polaron formation with time constants of 0.3 and 10 ps.

Conductivity and dielectric permittivity studies of KI‐based nanocomposite (PEO/PMMA/KI/I2/ZnO nanorods) polymer solid electrolytes

In this present study, we have studied the relaxation behaviour and dynamics of charge carriers in poly(ethylene oxide; PEO)/poly(methyl methacrylate; PMMA) blend with potassium based iodide–triiodide redox couple as polymer solid electrolyte dispersed with different concentration of ZnO nanorods as fillers. Various composition of potassium iodide (KI)‐based nanocomposite (PEO/PMMA/KI/I2/x wt% ZnO nanorods, x = 0, 2, 4, 6, 8, and 10) polymer solid electrolytes (NCPSEs) films were prepared by solution casting technique. Structural properties of the prepared solid electrolytes are studied by X‐ray diffraction and Fourier transform infrared spectroscopy. Among the prepared NCPSEs, 6 wt% sample of solid electrolyte showed high conductivity of 4.698 × 10−5 S/cm. The relaxation and dynamics of charge carriers are studied by analyzing complex conductivity and dielectric permittivity through power law along with the contribution of electrode polarization and Havriliak–Negami formalism, respectively, for the prepared polymer solid electrolytes to find out their usefulness for dye‐sensitized solar cell applications. © 2018 Society of Plastics Engineers POLYM. COMPOS., 40:2919–2928, 2019. © 2018 Society of Plastics Engineers

Dynamics and relaxation of charge carriers in poly(methylmethacrylate)-lithium salt based polymer electrolytes plasticized with ethylene carbonate

In this paper, we have studied the dynamics and relaxation of charge carriers in poly(methylmethacrylate)-lithium salt based polymer electrolytes plasticized with ethylene carbonate. Structural and thermal properties have been examined using X-ray diffraction and differential scanning calorimetry, respectively. We have analyzed the complex conductivity spectra by using power law model coupled with the contribution of electrode polarization at low frequencies and high temperatures. The temperature dependence of the ionic conductivity and crossover frequency exhibits Vogel-Tammann-Fulcher type behavior indicating a strong coupling between the ionic and the polymer chain segmental motions. The scaling of the ac conductivity indicates that relaxation dynamics of charge carriers follows a common mechanism for all temperatures and ethylene carbonate concentrations. The analysis of the ac conductivity also shows the existence of a nearly constant loss in these polymer electrolytes at low temperatures and high frequencies. The fraction of free anions and ion pairs in polymer electrolyte have been obtained from the analysis of Fourier transform infrared spectra. It is observed that these quantities influence the behavior of the composition dependence of the ionic conductivity.

Dynamics and relaxation of charge carriers in poly(methylmethacrylate)-based polymer electrolytes embedded with ionic liquid.

In the present paper, we have studied dynamics and relaxation of the charge carriers in polymethylmethacrylate-lithium bis(trifluoromethane sulfonyl)imide polymer electrolytes embedded with 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid. We have analyzed the frequency dependent conductivity spectra using the random free-energy barrier model coupled with the contribution of electrode polarization in the low frequency region. The temperature dependence of ionic conductivity, and relaxation time obtained from the analysis of the spectra exhibits Vogel-Tammann-Fulcher type behavior. The Barton-Nakajima-Namikawa relation is consistent with the results obtained from the random free-energy barrier model. The scaling of ac conductivity spectra has been performed to understand the effect of temperature as well as the composition on the relaxation mechanism. The analysis of the ac conductivity also clearly indicates the existence of a nearly constant loss phenomenon at low temperatures or at high frequencies.

Effect of uniaxial compression on traps of excitons and charge carriers in poly(9-vinylcarbazole) films

The effect of uniaxial pressure (1 × 108 Pa) on the photoluminescence spectra and thermally stimulated luminescence curves of poly(9-vinylcarbazole) has been investigated in the temperature range of 5–295 K. The thermally stimulated luminescence curve of crystalline carbazole has been measured for comparison. The high-temperature wings of the thermally stimulated luminescence curves are approximated by a Gaussian function, the half-width of which characterizes the disorder of energy states of deep structural traps. It is concluded that the pressure inhibits conformational changes of polymer chains of poly(9-vinylcarbazole), which leads to the formation of sandwich-like excimers as well as to an ordering of the spatial arrangement of the side carbazolyl groups. As a result, the concentration of “excimer-forming” centers increases, whereas the degree of disorder of energy states of deep structural traps of charge carriers is reduced by almost half and remains unchanged after the depressurization.

Enhanced Charge-Carrier Transport through Shorter Carbon Nanotubes in Organic Photovoltaics

We demonstrate for the first time the efficiency improvement of organic photovoltaics by the addition of shorter multiwalled carbon nanotubes (MWNTs) generated by size sorting. The different size MWNTs were generated by size sorting a batch of carboxylated MWNTs and were introduced as charge carriers in poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction photovoltaic cells. As compared to a control with only PCBM, the addition of the long and short MWNT resulted in 12 and 34% improvement in short circuit current density (Jsc) respectively. The results indicate that length of carbon nanotubes is an important consideration in photovoltaic and possibly other nanoelectronic devices.

Photogeneration of free charge carriers in π-conjugated polymers with bulky side groups

Photogeneration of free charge carriers in poly{1-[4-(trimethylsilyl)phenyl]-2-phenylacetylene} has been studied theoretically. In our work we have utilized our data obtained previously using photoinduced discharge experiment, which showed a steep slope of the dependence of the free charge carriers photogeneration yield on the applied electric field at the low electric field limit, but no saturation plateau. For the theoretical explanation we have utilized our recently developed model (Menšík et al., 2011) [20] that generalizes the model of Arkhipov et al. (2003) [19] by adding an assumption of a resonant reversible coupling of localized excitation to charge transfer states and, consequently, and dependence of internal parameters (permittivity, effective mass of the hole, rate constants) on electric field. Compared to our previous article this work provides a refined analysis of the experimental results on a large variety of intervals of the model parameters.

Effect of excessive pressure on the drift mobility of charge carriers in poly(diphenylene phthalide) films

The electron-hole transport in poly(diphenylene phthalide) films has been investigated. The dependence of the drift mobility of charge carriers on the excessive mechanical pressure has been studied using the time-of-flight method. It has been revealed that, with an increase in the thickness of the polymer film, the dispersive transport of charge carries gives way to the quasi-dispersive transport. In thin films in the prethreshold range (i.e., before switching of the samples to the highly conductive state under excessive pressure), the electron mobility increases and exceeds the hole mobility. The experimental results have been discussed in the framework of the model describing the transport through the channels formed by metastable electron-hole pairs.

Intra-molecular mobility of holes along rod-like helical Si backbones in optically active polysilanes

Intra-molecular mobility of positive charge carriers in poly( n-hexyl-[ S]-2-methylbutylsilane) (PHMBS) films doped with N, N′-bis(2,5-di- tert-buthylphenyl)-3,4,9,10-perylenedicarboximide (PDI) is studied by time-resolved microwave conductivity (TRMC) measurements. PHMBS with optically active side chains has a rod-like tightly locked Si catenation with unlikely long persistence length compared with conventional dialkyl-substituted polysilanes. It was found that PDI is a suitable electron acceptor for PHMBS as it provides the high product of photo carrier generation yield ϕ as ∼0.08% under an excitation at 355 nm. The efficient electron transfer reaction from PHMBS to PDI gives clear conductivity transients ascribed to the positive charges on the Si catenation of PHMBS by TRMC measurements. The estimated value of isotropic mobility along the tightly locked Si catenation is 0.36 cm 2 V −1 s −1, suggesting the presence of mobile holes on the backbones of PHMBS.

Time delayed collection field experiments on polymer: Fullerene bulk-heterojunction solar cells

The recombination of photogenerated charge carriers in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]:1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61 bulk-heterojunction solar cells is investigated using the time delayed collection field technique. Here the lifetime of photogenerated electrons and holes that have escaped charge recombination can be determined from current measurements using a pulsed collection voltage that is delayed with respect to the excitation pulse. At 80K, the number of long lived charge carriers decays in time according to t−α with α=0.2, practically independent of laser fluence in the range of 1–1000μJ∕cm2. For excitation density <4μJ∕cm2 the number of long lived carriers (nL) depends linearly on the fluence. At higher fluence, nL is limited by a process that occurs in the time span between generation and carrier extraction under a constant bias (−4V). Continuous background illumination reduces the lifetime of long lived carriers, probably by filling the low energy trap sites, for which a number density of 1017cm−3 is estimated. Recombination of long lived carriers at 80K in the 1–1000μs range is not limited by the (bimolecular) rate of nongeminate electron and hole encounter. Instead, the rate is limited by a monomolecular process, which may be the detrapping of one type of carrier or the recombination within a Coulombically bound electron-hole pair.

Mobilities of charge carriers in poly( o-methylaniline) and poly( o-methoxyaniline)

Mobilities of charge carriers in poly( o-methylaniline) (PMA) and poly( o-methoxyaniline) (PMOA) films are electrochemically determined over a range of oxidation stages of the polymer films. In the low oxidation region, mobilities of both the polymers are comparable to that of polyaniline (PANI). In the high oxidation region, however, mobility values systematically decrease in the order of PANI > PMA > PMOA, showing that the bulky groups present at the ortho position play a more important role in the high oxidation region. The low mobilities of charge carriers in the high oxidation region explain why PMA and PMOA are less conducting than PANI.

Investigation of charge carriers in poly(3,4-butylenedioxythiophene) (PBuDOT) by means of ESR spectroelectrochemistry

In situ ESR spectroelectrochemical studies of poly(3,4-butylenedioxythiophene) (PBuDOT) have been performed, in order to investigate more closely the species responsible for the conductivity of this polymer in the doped state. In the process of electrochemical doping of the polymer, ESR spectra at progressively changed potentials were recorded. Then the subsequent dedoping process was studied accordingly. The results reveal that PBuDOT’s ESR spectroscopic properties are markedly different form its close relative, poly(3,4-etylenedioxythiophene) (PEDOT). Firstly, the potential dependence of the spin concentration displays a clear peak-shaped transient with a gradual decrease at higher oxidation potentials. Similar behaviour is seen for the ΔB pp widths of the ESR signal, indicating that the type of interactions between paramagnetic centres is potential sensitive. The course of the reduction process of the polymer is more or less the reverse of the oxidation one, with only a slight hysteresis of the spin concentration and a barely discernible one of ΔB pp widths being observed. In PBuDOT, as in PEDOT, distinct narrow ESR lines revealing a noteworthy spin concentration in the reduced state of the polymer have also been observed. The presence of these residual spins in the dedoped polymer may indicate that the dedoping process of this polymer is indeed a slow one. The presence of trace amounts of impurities or oxygen may hinder the dedoping process, especially for thicker films.

Doping-induced change of carrier mobilities in poly(3-hexylthiophene) films with different stacking structures

Mobilities of positive charge carriers in poly(3-hexylthiophene) (PHT) films of 54%, 70%, 81%, and 97% in regioregularity were electrochemically measured over a range of doping levels. The results show that better ordered stacking structures enhance mobilities, but affect little features of the mobility change by doping. The common doping level of ca. 1% for the onset of the drastic mobility increase implies that the π–π stacking structures facilitating a charge transport in neutral or lightly doped PHT films are not responsible for the evolution of metallic conduction in PHT films.

The influence of dipolar species on charge carrier transport in a linear polysilicon

Effect of polar substituents and polar dopants on the transport of charge carriers in poly[methyl(phenyl)silanediyl] (PMPSI) was studied. The transport in undoped PMPSI can be explained within the theory of disordered polarons which postulates that the temperature and field dependence of the charge carrier mobility contain contributions both from the dynamic disorder, i.e. the polaron barrier, and from the environment-dependent static disorder. Polar moieties, chemically attached to the main polymer chain, give rise to a decrease of the on-chain mobility, due to the formation of local electronic polarization states. On the macroscopic level, these moieties and polar dopants give rise to a decrease of the drift (hopping) mobility due to a broadening of the distribution of the density of transport states as a consequence of the electrostatic interaction of the charge carrier with the dipole.

Microwave photoconductivity and polaron formation in poly[methyl(phenyl)silylene

The photogeneration of charge carriers in poly[methyl(phenyl)silylene] (PMPSi) is a multistage process including, photoinduced electron transfer and relaxation of the charge-transfer state which results in ion (polaron) pair formation. The radical ions are usually detected by transient optical absorption. Covalently linked molecular dipoles or Coulomb-correlated ion-pairs, and free and shallow localized carriers can also be detected by the method of time-resolved microwave conductivity (TRMC). This method is based on the proportionality of the relative change in the reflected microwave power and the “induced conductivity” due to dipolar transitions that are caused by the absorption of a light. The formation of radical ion-pairs in poly[methyl(phenyl)silylene] upon light excitation was studied by the abovementioned techniques.

Charge Injection into Thin Conjugated Polymer Films

We review several results on the charge injection into thin conjugated polymer films and discuss tunnelling and space-charge limited current models. The analysis of the currently available models reveals an apparent divergence in the literature which we try to eliminate via the consideration of both effects, tunnelling and space-charge formation, simultaneously. We analyse experimental results using a tunnelling model, which includes the contribution to the electronic potential originated by the space-charge effects due to the low mobility of charge carriers in conjugated polymers. As a consequence, the empty levels accessible for charge carriers in the polymer are shifted and the potential barrier for charge injection at the electrode/polymer interface is modified. This modification implies in a different tunnelling current and consequently in different current versus voltage dependences. We use our results to estimate the mobility of negative charge carriers in poly (p-phenylene vinylene) and to determine the trap centre concentration in the polymer layer of devices which present hysteresis in the current versus voltage curves.

Carrier Trapping Centers in Polysilane Film

Anomalous behavior of trapping and release of charge carriers in poly(methylphenylsilane) films has been discovered by thermally stimulated current spectroscopy. The carrier trapping occurs effectively at room temperature but not at 77 K. Trapped carriers are released at some specific temperatures which do not depend on the heating rate in the measurements. No trapped carrier is released at room temperature. Differential scanning calorimetry indicates that the onset temperatures of the carrier release correspond to the endothermic peaks. Such carrier trapping and release are not observed for polysilanes with saturated hydrocarbon side group but with aromatic side group. Theses results suggest that the observed phenomena are related to the motion of the aromatic substituents.

Model of the photogeneration of free charge carriers in poly(N-vinylcarbazole)

A time solution of the excimer–ion pair system is presented here with a special attention to the time asymptote. The influence of the parameters of the excimer–ion pair on the photogeneration of free charge carrier process together with a prolonged lifetime of ion pairs was discussed. A large number of mutual excimer–ion pairs transitions were also confirmed. © 1998 John Wiley & Sons, Ltd.

Enhancement of carrier mobilities in poly(3-methylthiophene) by an electrochemical doping

A combination of potential-step chronocoulometry and in situ conductivity measurement leads to a successful evaluation of mobilities of positive charge carriers in poly(3-methylthiophene) at room temperature over a wide range of a doping level from 0.02 to 23%. A carrier mobility of 2 × 10 −5 cm 2 V −1 s −1 at a lightly doped region compares well with the one obtained with the time-of-flight technique. The mobility starts to increase at around 0.1% doping level and it reaches 0.13 cm 2 V −1 s −1. The drastic mobility enhancement over four orders of magnitude provides a strong indication that a conduction mechanism is switched from a hopping to metallic transport.

Trap distribution for charge carriers in poly(paraphenylene vinylene) (PPV) and its substituted derivative DPOP-PPV.

In this paper, we present measurements of the charge-carrier transport in the conjugated polymer poly(paraphenylenevinylene) (PPV) and its substituted derivative poly(1,4-phenylene-1,2-diphen- oxyphenyl vinylene) (DPOP-PPV) by using the time-of-flight technique. A method for evaluating the experiments, based on Fourier transform techniques, is presented to calculate the density of localized states from the measured photocurrent within a multiple trapping model. The transport properties of DPOP-PPV can be described by a conventional hopping mechanism, where nearly every monomer acts as a trap. At room temperature, the effective mobility for holes in DPOP-PPV is in the range of ${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{2}$/V s, whereas for PPV, a value of less than ${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{cm}}^{2}$/V s can be estimated.