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This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.

Metal oxides as ZnO provide an interesting alternative for conventional low work function metals as electron injection layer in organic light-emitting diodes (OLEDs). However, for most state-of-the-art OLED materials the high work function of ZnO leads to a large injection barrier for electrons. As a result the electron current in the OLED is largely limited by the contact, leading to a strong reduction of the conversion efficiency. Here we demonstrate that by depositing an amorphous ZnO layer as cathode in an inverted polymer LED, the electron injection can be strongly enhanced by electrical conditioning. For suited polymers comparable conversion efficiencies of the conventional and inverted PLEDs can be achieved. (C) 2012 Elsevier B.V. All rights reserved.

The charge transport and recombination in white-emitting polymer light- emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single-carrier devices the effect of the green- and red-emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap-assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single-layer white-emitting PLEDs can be explained by the different voltage dependencies of trap-assisted and bimolecular recombination. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

N-type doping of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) with decamethylcobaltocene (DMC) strongly improves the electron transport due to filling of the electron traps. Unexpectedly, the n-type doping simultaneously suppresses the hole transport in MEH-PPV. We demonstrate that this strong reduction of the hole transport originates from unionized DMC molecules that act as hole traps. This hole trapping effect explains why the current of a DMC-doped MEH-PPV polymer light-emitting diode is orders of magnitude lower than that of the undoped device. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1745-1749, 2011

We demonstrate a solution processed bi-layer PLED based on poly(p-phenylene vinylene) derivatives using orthogonal solvents. To lower the voltage drop the hole transport layer (HTL) based on poly[2,5-bis(2'-ethylhexyloxy)-co-2,5-bis(butoxy)-1,4-phenylenevinylene] (BEH/BB-PPV (1: 3)) is doped with tetracyano-tetrafluoro-quinodimethane (F4TCNQ). The conductivity of BEH/BB-PPV (1: 3) was observed to increase by two orders of magnitude upon doping with F4TCNQ. The doped HTL was observed to lower the operating voltage of a double layer PLED, but suffers from additional quenching by the dopant at higher voltages due to the lack of an electron blocking functionality. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The charge transport in blue light-emitting polyspirobifluorene is investigated by both steady-state current-voltage measurements and transient electroluminescence. Both measurement techniques yield consistent results and show that the hole transport is space-charge limited. The electron current is found to be governed by a high intrinsic mobility in combination with electron traps. Numerical simulations on light-emitting diodes reveal a shift in the recombination zone from the cathode to the anode with increasing bias. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Here we report on a combined photoluminescence and morphological study of a polymer-polymer blend composed of a copolymer of derivatives of polyspirobifluorene and polyfluorene (PBFF) and a derivative of polyphenylene vinylene (MDMO-PPV). Evidence of partial Forster energy transfer from PBFF to MDMO-PPV is revealed by steady-state and time-resolved photoluminescence. Atomic force microscopy (AFM) on blends with different weight ratios of host (PBFF) and guest (MDMO-PPV) polymers shows that in blends with a MDMO-PPV concentration higher than 10% two phases are present. The nature of the phases is identified by means of scanning near-field optical microscopy (SNOM). Using this technique the topography and the local photoluminescence of the thin film are simultaneously mapped with a spatial resolution of ∼100 nm. The local measurements reveal that the PL signal of MDMO-PPV is present in the whole surface, but with variations of the PL intensity and spectral shape. The correlation of the SNOM, time-resolved and steady-state photoluminescence measurements allows us to identify the two phases, one rich in PBFF and the other rich in MDMO-PPV. © 2011 The Royal Society of Chemistry.

The charge transport in a polyspirobifluorene derivative with copolymerized N,N,N',N'-tetraaryldiamino biphenyl (TAD) hole transport units is investigated as a function of the TAD content. For TAD concentrations larger than 5%, guest-to-guest transport is observed. It is demonstrated that in this regime the charge carrier density dependent mobility can be described consistently with the extended Gaussian disorder model, with a density of hopping sites which is proportional to the TAD concentration and comparable to the molecular density. VC 2011 American Institute of Physics.

The recombination processes in poly(p -phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate. © 2011 American Institute of Physics.

The operation of blue light-emitting diodes based on polyspirobifluorene with a varying number of N,N,N′,N′ tetraaryldiamino biphenyl (TAD) hole-transport units (HTUs) is investigated. Assuming that the electron transport is not affected by the incorporation of TAD units, model calculations predict that a concentration of 5% HTU leads to an optimal efficiency for this blue-emitting polymer. However, experimentally an optimum performance is achieved for 10% TAD HTUs. Analysis of the transport and recombination shows that polymer light-emitting diodes with 5%, 7.5%, and 12.5% TAD units follow the predicted behavior. The enhanced performance of the polymer with 10% TAD originates from a decrease in the number of electron traps, which is typically a factor of three lower than the universal value found in many polymers. This reduced number of traps leads to a reduction of nonradiative recombination and exciton quenching at the cathode.

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.

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a Gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 1023 traps per m3 centred at an energy of ∼3.6 eV below the vacuum level, with a typical distribution width of ∼0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer. © 2012 Macmillan Publishers Limited. All rights reserved.

The electron transport in poly(p-phenylene vinylene) (PPV) derivatives blended with the air-stable n-type dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) is investigated. This dopant is activated after thin film deposition by annealing and strongly enhances the electron transport due to filling of electron traps as well as donation of free electrons to the lowest unoccupied molecular orbital (LUMO) of PPV. As a result, the electron current in a doped device exceeds the trap-free hole current. The total generated free electron density in the LUMO by the dopant typically amounts to 10(23) m(-3). (C) 2011 American Institute of Physics.

Trapping of charges is known to play an important role in the charge transport of organic semiconductors, but the role of traps in the recombination process has not been addressed. Here we show that the ideality factor of the current of organic light-emitting diodes (OLEDs) in the diffusion-dominated regime has a temperature-independent value of 2, which reveals that nonradiative trap-assisted recombination dominates the current. In contrast, the ideality factor of the light output approaches unity, demonstrating that luminance is governed by recombination of the bimolecular Langevin type. This apparent contradiction can be resolved by measuring the current and luminance ideality factor for a white-emitting polymer, where both free and trapped charge carriers recombine radiatively. With increasing bias voltage, Langevin recombination becomes dominant over trap-assisted recombination due to its stronger dependence on carrier density, leading to an enhancement in OLED efficiency. © 2011 American Physical Society.

The low electron currents in poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives and their steep voltage dependence are generally explained by trap-limited conduction in the presence of an exponential trap distribution. Here we demonstrate that the electron transport of several PPV derivatives can also be well described with a trap distribution that is Gaussianly distributed within the band gap. In contrast to the exponential distribution the trap-limited electron currents can now be modeled using the same Gaussian trap distribution for the various PPV derivatives. © 2011 American Physical Society.

The interpretation of electron currents in conjugated polymers is strongly hindered by the occurrence of hysteresis. We investigate the transport of electrons in electron-only devices based on derivatives of poly(p-phenylene vinylene) (PPV) for various hole-blocking bottom electrodes as well as purification of the polymer. The use of a variety of hole blocking bottom contacts, as metallic electrodes and n-type doped polymers, did not give any improvement in the observed hysteresis. By purification of the PPV, hysteresis free electron-only currents can be obtained. The deep traps responsible for hysteresis, with a concentration in the 1016 cm-3 range, are not responsible for the trap-limited electron transport as observed in purified PPV. © 2010 American Institute of Physics.

Characterization of the hole transport in blue-emitting polymers as poly(9,9-dioctylfluorene) (PFO) is strongly hindered by their large ionization potential of similar to 6 eV. Using common anodes as poly(3,4-ethylenedioxythiophene)/poly(styrenesulphonic acid) leads to a strongly injection limited current. We demonstrate that molybdenum trioxide forms an Ohmic hole contact on PFO, enabling the observation of a space-charge-limited current(SCLC). This allows a direct determination of the hole mobility PFO of 1.3x10(-9) m(2)/V s at room temperature, in good agreement with previously reported mobility values determined by time-of-flight measurements.

Hole transport in a blue emitting polyspirobifluorene polymer and in a white emitting polymer consisting of a polyspirobifluorene backbone and two dyes (green and red) was studied. The hole mobility was measured using the time-of-flight method as a function of the electric field and temperature in the range 10(5)-10(6) V/cm and 285-335 K, respectively. The observed temperature and electric field dependence of the hole mobility was analyzed in the framework of the Bassler disorder model. Also, steady-state current-voltage characteristics were measured over a wide range of electric fields and temperatures and the hole mobility was determined. Our measurements have shown that the hole mobility in the white emitting polymer is the same as in the blue emitting polymer. The performed disorder model analysis gives the same values for the effective energetic disorder (115 meV) and for the positional disorder (1.85) for both polymers. Therefore, we have concluded that the added green and red dyes do not act as hole traps as they have no influence on the hole mobility. It can therefore be concluded that their highest occupied molecular orbital (HOMO) levels are aligned with the HOMO level of the polyspirobifluorene backbone.

Copolymers are able to generate white light. The difference in energy levels for the different components complicates the charge transport in light-emitting devices as our measurements indicate.