Charge transport through a randomly oriented multilayered network of two-dimensional (2D) Ti3C2Tx (where Tx is the surface termination and corresponds to O, OH and F) was studied using time-of-flight ...photoconductivity (TOFP) method, which is highly sensitive to the distribution of charge carrier velocities. We prepared samples comprising Ti3C2Tx with thickness of 12 nm or 6-monolayers. MXene flakes of size up to 16 μm were randomly deposited on the surface by spin-coating from water solution. Using TOFP, we have measured electron mobility that reached values up to 279 cm2/Vs and increase with electric-field in a Poole-Frenkel manner. These values are approximately 50 times higher than previously reported field-effect mobility. Interestingly, our zero-electric-field extrapolate approaches electron mobility measured using terahertz absorption method, which represents intra-flake transport. Our data suggest that macroscopic charge transport is governed by two distinct mechanisms. The high mobility values are characteristic for the intra-flake charge transport via the manifold of delocalized states. On the other hand, the observed Poole-Frenkel dependence of charge carrier mobility on the electric field is typical for the disordered materials and suggest the existence of an important contribution of inter-flake hopping to the overall charge transport.
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•Spin-coated thin-films of Ti3C2 MXene flakes on quartz.•Highly ordered orientation experimentally confirmed.•Measured charge carrier mobility up to 279 cm2/Vs.•Charge transport through randomly oriented multi-layered network of flakes.
We used time-of-flight photocurrent measurements to determine the role of grain boundaries in charge carrier transport in thin layers of methyl ammonium lead iodide (CH3NH3PbI3). The measurement ...results were compared to Kinetic Monte Carlo simulations, based on a transport model, which disentangles the transport within crystallites and hopping across grain boundaries. The observed mobilities of electrons are in the order ∼2.5 × 10−1 cm2V−1s−1. The hopping across grains is modeled with an Arrhenius-type probability rate, characterized by activation energy (Ea). It was found that the Ea estimated from the slope of a mobility-temperature dependence is in the range of ∼56–70 meV. The factors contributing to Ea are shunting pathways and the grain-size variations including energy level misalignments at the grain boundaries. These results represent a step toward a design of novel windowless organic-inorganic perovskite solar cells.
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•Time-of-flight photocurrent measurements were used to determine a role of grain boundaries in charge carrier transport.•Monte-Carlo simulations support the influence of grain sizes on carrier mobilities.•Arrhenius-type probability rate was used to estimate the activation energy for hopping across grain boundaries.•These results represent a step toward a design of novel windowless organic-inorganic perovskite solar cells.
The effect of 1-pyrenesulfonicacid sodium salt (1-PSA), tetracyanoethylene (TCNE) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) on charge transport properties of reduced graphene oxide (RGO) is ...examined by measuring the transfer characteristics of field-effect transistors and co-planar time-of-flight photocurrent technique. Evidence of p-type doping and a reduction of mobility of electrons in RGO upon deposition of these materials is observed. Time-resolved photocurrent measurements show a reduction in electron mobility even at submonolayer coverage of these materials. The variation of transit time with different coverages reveals that electron mobility decreases with increasing the surface coverage of 1-PSA, TCNE and F4-TCNQ to a certain extent, while at higher coverage the electron mobility is slightly recovered. All three molecules show the same trend in charge carrier mobility variation with coverage, but with different magnitude. Among all three molecules, 1-PSA acts as weak electron acceptor compared to TCNE and F4-TCNQ. The additional fluorine moieties in F4-TCNQ provides excellent electron withdrawing capability compared to TCNE. The experimental results are consistent with the density functional theory calculations.
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•RGO FETs, exhibit a positive shift of the charge-neutrality point due to deposition of organic semiconductor layer.•The amount of shift varies with the molecule electron affinity.•The electron mobility is quenched by the presence of F4-TCNQ and TCNE, while 1-PSA don’t affect the electron mobility.•With increasing the coverage of 1-PSA, the electron mobility drops and then slightly recovers at higher coverages.•The change in the electron mobility is due to effect of teh RGO/organic-layer interface dipole field.
Presented here is a novel and efficient method used to improve carrier mobilities of poly(3-hexylthiophene) (P3HT)-based organic field effect transistors by means of nanowire formation. The ...treatment, termed solvation, consists of depositing a small quantity of a solvent directly on top of a previously deposited semiconducting film, and allowing the solvent to evaporate slowly. Such treatment results in an increase of the saturation mobility by more than one order of magnitude, from 1.3 × 10−3 up to 3.4 × 10−2 cm2/Vs, while devices preserve their high ON/OFF ratio of ∼104. The atomic force and scanning electron microscopy studies show that solvated P3HT layers develop a network of nanowires, which exhibit increased degree of structural order, as demonstrated by micro Raman spectroscopy. The time-of-flight photoconductivity studies suggest that higher hole mobility stems from a reduced energy disorder of the transporting states in these structures.
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•Improvement in carrier mobilities, in P3HT-based organic transistors, by means of solvation treatment.•Improvement in carrier mobilities is over one order of magnitude.•Time of flight analysis also reveals improvement in mobilities.•AFM and SEM analyses reveal improvement is due formation of nanowires.
The charge mobility in a new hole transporting polymer, poly(2,6-bis(thiophene-2-yl)-3,5-dipentadecyldithieno3,2-b;2′,3′-dthiophene) (PBTDTT-15), and its blend with (6,6)-phenyl-C70-butyric acid ...methyl ester (PC70BM) in a weight ratio of 1:3 at ambient atmosphere condition was investigated using time-of-flight (TOF) photoconductivity and photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) techniques. The bulk heterojunction based photovoltaic (PV) blend (PBTDTT-15:PC70BM (1:3)) exhibited a promising power conversion efficiency (PCE) of 3.23% under air mass 1.5 global (AM 1.5G) illumination of 100mW/cm2. The charge mobility and recombination properties of the best performing cells were investigated. The hole mobility in the pure PBTDTT-15 was in the range of 4 × 10−4 cm2/(V s), which was reduced almost 5 times in the PBTDTT-15:PC70BM (1:3) blend. The PhotoCELIV transient observed for the photovoltaic (PV) blend was dominated by electrons, with the charge mobility of the order of 10−3 cm2/(V s), and a weak shoulder at a long time scale due to holes. The effective bimolecular recombination coefficient (β) obtained for the PV blend deviated significantly from the Langevin recombination coefficient (βL) indicating a phase-separated morphology. The obtained results indicate that the PBTDTT-15:PC70BM blend can be potential for organic solar cell applications.
This article investigates hole transport in poly2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV)/CdSe colloidal quantum dot (CQD) nanocomposites using a modified time-of-flight ...photoconductivity technique. The measured hole drift mobilities are analyzed in the context of Bässler's Gaussian disorder model and the correlated disorder model in order to determine the polymer internal morphology of hybrid nanocomposite thin films. This work shows that increasing the CdSe CQD concentration decreases the polymer hole mobility from ~5.9 × 10-6 cm2/Vs in an MEH-PPV film to ~8.1 × 10-8 cm2/Vs in a 20:80 (wt%) MEH-PPV:CdSe CQD nanocomposite film (measured at 25 °C and ~2 × 105 V/cm). The corresponding disorder parameters indicate increasing disruption of interchain interaction with increasing CQD concentration. This work quantifies polymer chain morphology in hybrid nanocomposite thin films and provides useful information regarding the optimal use of semiconductor nanocrystals in conjugated polymer-based optoelectronics.
The charge transport properties in a series of electroluminescent biphenylyl substituted PPV derivatives, poly(2-(3'-(p-2"-ethylhexyloxyphenyl)phenyl)-1,4-phenylenevinylene) (MP-PPV), ...poly2-(2'-phenyl-4',5'-bis(3"-methylbutoxy))phenyl-1,4-phenylene vinylene (BP-PPV) and poly{2-(4',5'-bis(3"-methylbutoxy)-2'-phenyl)phenyl-1,4-phenylene-vinylene-co-(9,9-dioctyl-2,7-fluorenylene-vinylene)} (BPPPV-PF), have been studied using a time of flight (TOF) photoconductivity technique. The TOF transients for holes in these polymers were weakly dispersive in nature with a mobility of the order of ~10⁻⁵cm²/V s at room temperature. The temperature and field dependence of charge mobility has been studied and analyzed using the disorder formalisms (Bässler's Gaussian disorder model (GDM) and correlated disorder model (CDM)). The charge transport parameters, such as mobility prefactor, energetic disorder, positional disorder, and the average intersite distance, were estimated from the fit and correlated to the molecular structure of the polymer. The microscopic charge transport parameters derived for these polymers indicate that these biphenylyl substituted PPV derivatives have very good chemical purity and structural regularity, which is similar to the reported values for structurally related fully conjugated polymers with high chemical purity.
The charge transport properties in a novel electroluminescent poly{2-(4′,5′-bis(3″-methylbutoxy)-2′-p-methoxy-phenyl)phenyl-1,4-phenylene vinylene-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} ...(BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207–300K) and electric fields (1.5×105–6.1×105V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300K of 5×10−5cm2/Vs at 2.5×105V/cm. This increased to 8.4×10−5cm2/Vs at 6.1×105V/cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler’s Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor μ∞=1.2×10−3cm2/Vs, energetic disorder parameter σ=82meV and positional disorder parameter Σ=1.73. The average inter-site separation (a=7Å) and the charge localization length (L=3.6Å) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.
The tendency to saturation in the collected charge as a function of light intensity in Langmuir-Blodgett films of poly(3-hexylthiophene) (PHT) has been studied. The tendency to saturation can have ...two reasons; that the space charge limited current regime has been reached, or influence of very fast bimolecular recombination. We have studied these two cases, and we conclude that bimolecular recombination is a probable cause for the tendency to saturation. The hole mobility in PHT can therefore possibly be estimated as high as 1 cm2/Vs in the initial stage after photoexcitation.
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