Frank Jaiser

Bulk Electron Transport and Charge Injection in a High Mobility n‐Type Semiconducting Polymer

www.MaterialsViews.com C O M M U Bulk Electron Transport and Charge Injection in a High Mobility n-Type Semiconducting Polymer N IC A TI By Robert Steyrleuthner , Marcel Schubert , Frank Jaiser , James C. Blakesley , Zhihua Chen , Antonio Facchetti , and Dieter Neher * O N Understanding charge injection mechanisms and charge transport in organic semiconductors is of fundamental importance for the further advancement of electronic and optoelectronic devices. The charge-carrier mobility ( μ ) is one of the key performance parameters of organic-semiconductor-based functional devices such as light-emitting diodes (OLEDs), fi eld-effect transistors (OFETs) or solar cells (OSCs). Organic semiconductor charge-carrier mobility is often several orders of magnitude lower than in their inorganic counterparts and frequently is a limiting factor affecting the rate of charge injection from the device contacts, charge recombination, and photogeneration. [ 1–3 ] Several studies have addressed how the semiconductor molecule/polymer chemical structure infl uence charge transport properties. However, several questions remain addressing how charges fl ow from the electrical contact to the semiconducting material, particularly in electron-transporting polymers. It is well-known that the electronic structure of π-conjugated polymers can be tuned by the introduction of substituents with different electronegativity. [ 4 ] The majority of conjugated polymers are hole-transporting (p-type), meaning that they can accept and transport holes effi ciently, whereas the development of effi cient electron-transporting (n-type) polymers has been far more challenging. Holes are often the dominant charge carriers in several functional devices, [ 5–6 ] although no fundamental reason for the superiority of hole versus electron transport in the bulk of organic semiconductors is known. [ 7 ] Reduced electron currents in organic semiconductors are usually attributed to the presence of trap states distributed energetically below the lowest occupied molecular orbital (LUMO). [ 6 , 8–9 ] De Leeuw et al. pointed out that conducting polymers with an electron affi nity lower than ∼3 eV have a strong tendency to be oxidized by oxygen and water. [ 10 ] Such oxidative processes are known to create electron-accepting units. [ 11–14 ] Additionally, the formation of electron-accepting defects during synthesis, device fabrication, and measurement, even in controlled atmosphere, might be one of the major reasons for the common observation of traplimited electron transport. Consequently, polymeric structures

Blue polymer electrophosphorescent devices with different electron-transporting oxadiazoles

We report that the performances of blue polymer electrophosphorescent devices are crucially depending on the choice of the electron transporting material incorporated into the emissive layer. Devices with 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7) doped at ∼40wt% into a poly(vinylcarbazole) matrix exhibited significantly higher efficiencies than those with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), yielding maximum luminous and power efficiency values of 18.2 Cd∕A and 8.8 lm∕W, respectively. Time resolved photoluminescence measurements revealed a long lifetime phosphorescence component in layers with PBD, which we assign to significant triplet harvesting by this electron-transporting component.

Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye: A deep blue light-emitting composite for light-emitting diode applications

We investigated the spectral properties of light-emitting diodes based on a deep blue-emitting pyrazoloquinoline dye doped into a poly(N-vinylcarbazole)-based matrix. Even though the electroluminescence (EL) of the host is redshifted and broadened with respect to the emission of the dye, the EL spectrum becomes fully dominated by the dye emission at concentrations of ca. 2wt%. This is attributed to a competition of exciplex formation on the matrix and exciton formation on the dye.

How do disorder, reorganization, and localization influence the hole mobility in conjugated copolymers?

In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.

Energy and charge transfer in blends of dendronized perylenes with polyfluorene.

Two generations of polyphenylene dendrimers with a perylene diimide core are compared with a nondendronized tetraphenoxyperylene diimide model compound regarding their application in organic light-emitting diodes (OLEDs). Single layer devices with blends of the first and second generation dendrimers in polyfluorene are investigated as active layers in OLEDs, and the effect of dendronization on the emission color and electroluminescence intensity is studied. In photoluminescence, higher degrees of dendronization lead to a reduction in Forster transfer from the polyfluorene host to the perylene, resulting in a larger contribution of the blue host emission in the photoluminescence spectra. In electroluminescence, the dopants appear to act as active traps for electrons, resulting in a predominant generation of excitons on the dye. This gives rise to a remarkably stronger contribution of red emission in electroluminescence than in photoluminescence where energy is exchanged exclusively via Forster transfer. The pronounced color change from red to blue with higher degrees of dendronization and larger driving voltages is explained by the competition of the recombination of free electrons with holes and trapping of electrons by the emitting guest.

Effect of the Solvent on the Conformation of Isolated MEH‐PPV Chains Intercalated Into SnS2

Photophysical processes in conjugated polymers are influenced by two competing effects: the extent of excited state delocalization along a chain, and the electronic interaction between chains. Experimentally, it is often difficult to separate the two because both are controlled by chain conformation. Here we demonstrate that it is possible to modify intra-chain delocalization without inducing inter-chain interactions by intercalating polymer monolayers between the sheets of an inorganic layered matrix. The red-emitting conjugated polymer, MEH-PPV, is confined to the interlayer space of layered SnS(2). The formation of isolated polymer monolayers between the SnS(2) sheets is confirmed by X-ray diffraction measurements. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the incorporated MEH-PPV chains reveal that the morphology of the incorporated chains can be varied through the choice of solvent used for chain intercalation. Incorporation from chloroform results in more extended conformations compared to intercalation from xylene. Even highly twisted conformations can be achieved when the incorporation occurs from a methanol:chloroform mixture. The PL spectra of the MEH-PPV incorporated SnS(2) nanocomposites using the different solvents are in good agreement with the PL spectra of the same solutions, indicating that the conformation of the polymer chains in the solutions is retained upon intercalation into the inorganic host. Therefore, intercalation of conjugated polymer chains into layered hosts enables the study of intra-chain photophysical processes as a function of chain conformation.

Efficient green electrophosphorescence based on ambipolar nonconjugated polymers: Evaluation of transport and emission properties

New materials for polymer organic light-emitting diodes based on a polymer matrix doped with phosphorescent dyes are presented. The matrix system is based on a polystyrene backbone bearing either electron or hole transporting units at the 4-position of each repeat unit. Random copolymers and polymer blend systems of the homopolymers are prepared, both with 62 wt.% electron transporting and 38 wt.% hole transporting moieties. Adding a green electrophosphorescent dye to the polymer matrix leads to efficient electroluminescence with a maximum current efficiency of 35 cd/A and a maximum external quantum efficiency of up to 10%. The mobilities of electrons and holes in the dye-doped copolymer, as measured by transient electroluminescence, are around 5 × 10−5 and 5 × 10−6 cm2/Vs, respectively, while the blend of the two homopolymers exhibits slightly lower mobilities of both types of carriers. Despite the pronounced imbalance of charge transport, the device performance is almost entirely limited by the phosphor...

Polymer light emitting diodes based on LiF/Al composite cathode

We report on the performances of polymer light emitting diodes utilizing LiF/Al composite cathodes. Among the devices with the composite cathodes of different concentration of LiF, the layer by layer device is the most efficient, which is different from the results based on small molecule organic light diode devices. The different performances of the devices can be ascribed to the variation of the electron injection barrier heights at the polymer/cathode interface, as suggested by photovoltaic experiments.

Boron dipyrromethene (BODIPY) with meso-perfluorinated alkyl substituents as near infrared donors in organic solar cells

Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at ∼800 nm with absorption coefficient of over 280 000 cm−1. Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at ∼790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm−2. Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor–acceptor interface were comparable in the range of 0.14–0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate Voc of 0.76 V.

Fluorenone defects in polyfluorene-based light-emitting diodes: emission properties and device performance

The spectral characteristics of polyfluorene (PF) based light-emitting diodes (LEDs) are discussed. First, conditions that facilitate photo-oxidation of PF are investigated. We show that dense chain packing and addition of hole-trapping moieties lead to increased defect formation. Second, devices containing either a defined low concentration of keto-defects or of the polymer poly(9,9-octylfluorene-co-benzothiadiazole) (F8BT) are presented. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green and blue emission components. It is shown that either blending of hole-transporting molecules into the emission layer at low concentration or the incorporation of a suitable hole-transporting layer reduces the green emission in the PF:F8BT blend, similar to what is observed for the keto-containing PF layer. We conclude that photo-oxidation leads to the formation of keto-defects that mainly constitute weakly-emissive electron traps.