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The dielectric constant ( εr) is a fundamental material parameter that governs charge transfer processes in organic semiconductors, yet its value is often assumed rather than measured. Here, we use impedance spectroscopy to determine εr in regioregular poly(3-hexylthiophen-2,5-diyl) (P3HT) thin films p-doped with the molecular dopants hexafluoro-tetracyanonaphthoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We fit the impedance spectra using a single RC circuit model to determine the frequency-dependent capacitance and extract εr. The value of the dielectric constant increases by around two-thirds from 2.9 ± 0.1 (undoped polymer) to 4.9 ± 0.6 on the addition of one F4TCNQ molecule per 500 P3HT monomer units. In contrast, the addition of the weak dopant 7,7,8,8-tetracyanoquinodimethane (TCNQ), which does not undergo ground state charge transfer with P3HT, has no effect on the dielectric constant. Our results support the hypothesis that molecular doping has a considerable impact on the materials dielectric constant via polarizable host-dopant complexes.

Charge transport is one of the key factors in the operation of organic solar cells. Here, we investigate the electron and hole transport in the non-fullerene acceptor (NFA) IT-4F, by a combination of space-charge-limited current measurements and multiscale molecular simulations. The electron and hole mobilities are fairly balanced, amounting to 2.9 × 10−4 cm2 V−1 s−1 for electrons and 2.0 × 10−5 cm2 V−1 s−1 for holes. Orientational ordering and electronic couplings facilitate a better charge-percolating network for electrons than for holes, while ambipolarity itself is due to sufficiently high electron affinity and low ionization energy typical for narrow-gap NFAs. Our findings provide a molecular-level understanding of the balanced hole and electron transport in an archetypical NFA, which may play a key role in exciton diffusion and photogenerated hole transfer in organic solar cells.

Amorphous small-molecule organic materials are utilized in organic light emitting diodes (OLEDs), with device performance relying on appropriate chemical design. Due to the vast number of contending materials, a symbiotic experimental and simulation approach would be greatly beneficial in linking chemical structure to macroscopic material properties. We review simulation approaches proposed for predicting macroscopic properties. We then present a library of OLED hosts, containing input files, results of simulations, and experimentally measured references of quantities relevant to OLED materials. We find that there is a linear proportionality between simulated and measured glass transition temperatures, despite a quantitative disagreement. Computed ionization energies are in excellent agreement with the ultraviolet photoelectron and photoemission spectroscopy in air measurements. We also observe a linear correlation between calculated electron affinities and ionization energies and cyclic voltammetry measurements. Computed energetic disorder correlates well with thermally stimulated luminescence measurements and charge mobilities agree remarkably well with space charge–limited current measurements. For the studied host materials, we find that the energetic disorder has the greatest impact on the charge carrier mobility. Our library helps to swiftly evaluate properties of new OLED materials by providing well-defined structural building blocks. The library is public and open for improvements. We envision the library expanding and the workflow providing guidance for future OLED material design.

The degradation of polymer light-emitting diodes (PLEDs) under current stress is governed by the formation of hole traps. The presence of traps is reflected in the low-frequency response of PLEDs by a negative contribution to the capacitance that originates from trap-assisted recombination. Since the relaxation time scales with the (inverse) concentration of traps, impedance spectroscopy measurements allow for a quantitative determination of the amount of traps formed during degradation. We demonstrate that the obtained hole trap concentration is in agreement with the amount found by numerically modeling the increase in the PLED driving voltage. Impedance spectroscopy measurements are therefore useful as an in-situ characterization tool during PLED degradation, providing information on trap formation without numerical device modeling.

By diluting poly(dioctylfluorene) (PFO) with low molecular weight polystyrene (PS), electron trapping and trap-assisted recombination can be strongly suppressed. For polymer light-emitting diodes (PLEDs) consisting of a PFO:PS (1:3) blend, a doubling of the efficiency is expected owing to trap dilution. Experimentally, we observe a much larger efficiency increase of nearly an order of magnitude. The electroluminescence and photoluminescence spectra of the PFO:PS blend show a sharpening of the vibrational peaks with regard to pristine PFO with the emission maximum at 436 nm. This spectral feature is characteristic of the formation of the β-phase in PFO upon PS addition. From the analysis of the absorption spectra, the fraction of the β-phase induced by blending typically amounts to 13%. The strong PLED efficiency enhancement therefore arises from the combined effect of β-phase formation (∼4 times) and trap dilution (∼2 times).

By blending semiconducting polymers with the cross-linkable matrix ethoxylated-(4)-bisphenol-a-dimethacrylate (SR540), an insoluble layer is acquired after UV-illumination. Following this approach, a trilayer polymer light-emitting diode (PLED) consisting of a blend of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine] (poly-TPD) and SR540 as an electron-blocking layer, Super Yellow-Poly(p-phenylene vinylene) (SY-PPV) blended with SR540 as an emissive layer, and poly(9,9-di-n-octylfluorenyl-2,7-diyl) as a hole-blocking layer is fabricated from solution. The trilayer PLED shows a 23% increase in efficiency at low voltage as compared to a single layer SY-PPV PLED. However, at higher voltage, the advantage in current efficiency gradually decreases. A combined experimental and modelling study shows that the increased efficiency is not only due to the elimination of exciton quenching at the electrodes but also due to suppressed nonradiative trap-assisted recombination due to carrier confinement. At high voltages, holes can overcome the hole-blocking barrier, which explains the efficiency roll-off.

In state-of-the-art blue phosphorescent organic light-emitting diode (PHOLED) device architectures, electrons and holes are injected into the emissive layer, where they are carried by the emitting and hole transporting units, respectively. Using transient electroluminescence measurements, we disentangle the contribution of the electrons and holes on the transport and efficiency of both pristine and degraded PHOLEDs. By varying the concentration of hole transporting units, we show that for pristine PHOLEDs, the transport is electron dominated. Furthermore, degradation of the PHOLEDs upon electrical aging is not related to the hole transport but is governed by a decrease in the electron transport due to the formation of electron traps.

Solar cells based on blends of regioregular poly(3-hexylthiophene) as a donor and a commercial perylene diimide derivative as a acceptor are investigated. Thermal annealing of the devices results in increased photocurrent generation but is simultaneously accompanied by a loss in open-circuit voltage. It is shown that the decreased open-circuit voltage is caused by the formation of a cathode barrier, as evidenced by injection-limited electron currents, a reduced built-in voltage, and a weaker light-intensity dependence of the open-circuit voltage. Device simulations show that the increased photocurrent after annealing is the result of an increased electron mobility.

The quenching of excitons at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) PEDOT:PSS) anode in blue polyalkoxyspirobifluorene-arylamine polymer light-emitting diodes is investigated. Due to the combination of a higher electron mobility and the presence of electron traps, the recombination zone shifts from the cathode to the anode with increasing voltage. The exciton quenching at the anode at higher voltages leads to an efficiency roll-off. The voltage dependence of the luminous efficiency is reproduced by a drift-diffusion model under the condition that quenching of excitons at the PEDOT:PSS anode and metallic cathode is of equal strength. Experimentally, the efficiency roll-off at high voltages due to anode quenching is eliminated by the use of an electron-blocking layer between the anode and the light-emitting polymer. cop. 2014 AIP Publishing LLC.

The intermixing of two emissive layers in a four-layer solution-processed polymeric light-emitting diode with a hole injection, two emissive layers, and one hole-blocking layer is investigated. The relative emission of both emissive layers is measured and compared to a calculated recombination profile across the device using drift-diffusion simulations. A good agreement between the measured and calculated relative emission was found, supporting that there is no intermixing in the two emissive materials.

The effect of contact barriers on the light-intensity dependence of the open-circuit voltage of organic solar cells is investigated in experiments and simulations. Reduced light-intensity dependence is found when the open-circuit voltage surpasses the built-in voltage, leading to a slope of kT/2q for a device with one non-ohmic contact and a slope of zero for a device with two non-ohmic contacts. The reduced light-intensity dependence of the open-circuit voltage is not caused by entering a contact-recombination-limited regime but by the absence of band bending in the vicinity of a non-ohmic contact.

The current–voltage characteristics of light-emitting devices containing thin films of poly(dialkoxy-p-phenylene vinylene) (PPV) incorporated with silicon dioxide nanoparticles have been investigated. It is demonstrated that the current enhancement of the devices containing composite layers can be modeled by assuming that the effective thickness of the composite layers is about half of their actual thickness. Field-effect measurements reveal that the mobility of the charge carriers in PPV is not significantly changed by the incorporation of nanoparticles.

Non-volatile memories operating at low voltage are indispensable for flexible micro-electronic applications. To that end, ferroelectric delta-PVDF films were investigated as a function of layer thickness down to 10 nm ultra-thin films. Capacitors were fabricated using PEDOT:PSS as non-reactive electrode. Full polarization reversal was obtained at an unprecedented voltage below 5V. The remanent polarization of 7 mu C/cm(2) and coercive field of 120 MV/m are independent of layer thickness, demonstrating that ferroelectric switching in delta-PVDF is extrinsic, dominated by inhomogeneous nucleation and growth. The ease of processing of delta-PVDF allowed to determine a lower limit of the critical ferroelectric thickness. (C) 2013 AIP Publishing LLC.

The trap-limited electron currents in poly(p-phenylene vinylene) (PPV) derivatives can be modeled using a Gaussian trap distribution that is positioned approximately 0.75 eV below the lowest unoccupied molecular orbital (LUMO) of PPV. Photothermal deflection spectroscopy measurements and internal photo-emission spectroscopy measurements confirm the claim of a Gaussian shaped trap distribution centered at 0.75 eV below the LUMO of PPV. Additionally, two PPV derivatives that differ in the number of conformational defects incorporated during synthesis exhibit identical electron trapping behavior, showing that the traps do not originate from extrinsic impurities of the synthesis or defects in the polymer chains. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3656713]

Staggered bottom-contact top-gate organic ferroelectric field-effect transistors are fabricated with poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) as ferroelectric gate and poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine] as semiconductor. Polarization reversal of the ferroelectric gate is monitored by displacement transients in the gate current. By varying both the source and drain biases and by using fully and partially polarized transistors, we show that conductance switching only requires polarization of P(VDF-TrFE) at the source electrode. Polarization at the drain is irrelevant and does not impede charge extraction. © 2011 American Institute of Physics.

The kinetics of acid doping of the semiconductor regioregular poly-3-hexylthiophene with vaporized chlorosilane have been investigated using field-effect transistors. The dopant density has been derived as a function of temperature and exposure time from the shift in the pinch-off voltage, being the gate bias where current starts to flow. The doping kinetics are perfectly described by empirical stretched exponential time dependence with a saturation dopant density of 1±0.5× 1026 m-3 and a thermally activated relaxation time. We show that a similar relationship holds for previously reported kinetics of poly-thienylene-vinylene doped with molecular oxygen.

The effect of electron traps on the performance of bulk heterojunction organic solar cells made of poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-1,4-phenylene vinylene] blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) is investigated. By deliberately introducing 7,7,8,8-tetracyanoquinodimethane, which is a stronger electron accepting molecule than PCBM, the electron transport in the solar cells becomes trap limited. We demonstrate that the open circuit voltage and its light-intensity dependence are strongly affected by recombination of holes with trapped electrons. Depending on the amount of traps, their recombination strength, and the light intensity, the losses due to trap-assisted recombination can even dominate over the intrinsic bimolecular recombination. (c) 2007 American Institute of Physics.

The quenching of excitons at the metallic cathode of a polymer light-emitting diode (PLED) has been investigated by time-resolved photoluminescence. The decay of the luminescence is analyzed including both nonradiative energy transfer to the metallic cathode and exciton diffusion. Incorporation of the resulting exciton density profiles into a PLED device model consistently describes the reduction of the quantum efficiency at low bias voltage. (c) 2005 Americian Institute of Physics.

We present a systematic study of the influence of the processing conditions on the charge-carrier mobility in hole-only diodes and field-effect transistors (FETs) based on alkoxy-substituted poly(p-phenylene vinylene) (PPV). It is demonstrated that by chemical modification from asymmetrically to fully symmetrically substituted PPVs the mobility in both types of devices can be significantly improved. Furthermore, for symmetrical PPVs the mobility is strongly dependent on processing conditions, such as choice of solvents and annealing conditions. The increase in mobility is accompanied by a strong enhancement of the anisotropy in the charge transport. Ultimately, mobility of up to 10(-2) cm(2)/Vs in FETs and 10(-5) cm(2)/Vs in hole-only diodes have been achieved.

The material transport phenomena of hole machining, which occur during optical recording, are described for metal films such as elemental tellurium and its alloys. It is shown that the temperature gradient resulting from a focused laser beam creates a minimum in the surface tension at the hottest point of the molten spot. Consequently, a shear stress pulls material from the center of the melt towards the edge, forming a rim. It is demonstrated that the film is thin enough to have all material pulled away, thereby forming a hole and that this occurs in a time frame of about 10–50 ns. Also the cross sectional shape of the rim is modeled. Experimental information corroborating this model is obtained by investigating dropouts, which are created by recording close to the threshold. (A dropout is defined as a missing hole; i.e., it is intended to machine a hole but it fails to open.) The surface profile of dropouts indicates that material has retracted from the center to form a rim. Also, it is shown that dropo...

We report on real time measurements of the hole formation process in Te‐Se alloy films. The results tend to support a three‐step model. First, there is an initiation stage. Energy is absorbed in advance of hole formation. The precise mechanism for hole initiation depend quite sensitively on writing conditions. In some situations the evidence supports a blister formation model. After the hole is initiated, its boundary then propagates out, driven by interfacial energies. Finally, in the terminal phase of hole opening, the continued propagation of the hole boundary becomes limited by the propagation of the melt boundary, until the film material solidifies.

Light‐emitting InP diodes were made by liquid‐phase epitaxy. The most efficient diodes result when the n‐type layers are grown from a Sn‐doped melt, while the p‐type layers are grown from the same melt, but overcompensated with Zn. External quantum efficiencies up to 0.75% at room temperature and 11.7% at 77°K were observed in uncoated diodes. Threshold current densities for stimulated emission were as low as 750 A/cm2 at 77°K. Spontaneous emission can be observed normal to the p‐n junction.

Electroluminescence in GaSb liquid‐phase epitaxial diodes has been studied. Optical and electrical measurements were done between 65° and 300°K. Two emission bands are observed, which are both due to transitions between the conduction band and different acceptor levels. The peak photon energy of band I is stationary with applied voltage and is due to a thermal injection mechanism. The low‐energy band II shifts with applied voltage and is described by theories of photon‐assisted tunneling. It is demonstrated that for both emission bands the intensities and peak photon energies are directly related to the applied voltage.

The optical‐recording characteristics of Te‐trilayer structures are compared to those of single Te films on PMMA substrates. The results demonstrate that the single Te films are more sensitive using optical pulses ranging from 25 to 500 nsec, with otherwise identical recording and readout characteristics. A thermal analysis of the optical structures confirms such a sensitivity behavior.