No abstract

The charge transport mechanism of oligo(p-phenylene ethynylene)s with lengths ranging from 0.98 to 5.11 nm was investigated using modified scanning tunneling microscopy break junction and conducting probe atomic force microscopy methods. The methods were based on observing the length dependence of molecular resistance at single molecule level and the current-voltage characteristics in a wide length distribution. An intrinsic transition from tunneling to hopping charge transport mechanism was observed near 2.75 nm. A new transitional zone was observed in the long length molecular wires compared to short ones. This was not a simple transition between direct tunneling and field emission, which may provide new insights into transport mechanism investigations. Theoretical calculations provided an essential explanation for these phenomena in terms of molecular electronic structures.

We demonstrate novel molecular design for thermally activated delayed fluorescence (TADF) polymers based on a nonconjugated polyethylene backbone with through-space charge transfer effect between pendant electron donor (D) and acceptor (A) units. Different from conventional conjugated D-A polymers with through-bond charge transfer effect, the nonconjugated architecture avoids direct conjugation between D and A units, enabling blue emission. Meanwhile, spatial π-π interaction between the physically separated D and A units results in both small singlet-triplet energy splitting (0.019 eV) and high photoluminescence quantum yield (up to 60% in film state). The resulting polymer with 5 mol % acceptor unit gives efficient blue electroluminescence with Commission Internationale de l'Eclairage coordinates of (0.176, 0.269), together with a high external quantum efficiency of 12.1% and low efficiency roll-off of 4.9% (at 1000 cd m), which represents the first example of blue TADF nonconjugated polymer.

Green‐emitting iridium dendrimers with rigid hole‐transporting carbazole dendrons are designed, synthesized, and investigated. With second‐generation dendrons, the photoluminescence quantum yield of the dendrimers is up to 87 % in solution and 45 % in a film. High‐quality films of the dendrimers are fabricated by spin‐coating, producing highly efficient, non‐doped electrophosphorescent organic light‐emitting diodes (OLEDs). With a device structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/neat dendrimer/1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene/LiF/Al, a maximum external quantum efficiency (EQE) of 10.3 % and a maximum luminous efficiency of 34.7 cd A–1 are realized. By doping the dendrimers into a carbazole‐based host, the maximum EQE can be further increased to 16.6 %. The integration of rigid hole‐transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution‐processable dendrimers for OLED applications.

A diketopyrrolopyrrole-based conjugated polymer, PDPP-4FTVT, which exhibits ambipolar transport behavior in air with hole and electron mobilities up to 3.40 and 5.86 cm(2) V(-1) s(-1), respectively, is synthesized via direct arylation polycondensation. Incorporation of F-atoms in β-positions of thiophene rings dramatically improves the efficiency of direct arylation polycondensation.

Using a "multifluorination" strategy, ambipolar donor-acceptor conjugated polymer with hole and electron mobility (μ and μ ) up to 3.94 and 3.50 cm V s , respectively, and unipolar n-type donor-acceptor conjugated polymers with μ up to 4.97 cm V s is synthesized with isoindigo as acceptor units.

Research on organic light-emitting diodes (OLEDs) has been actively pursued in last two decades because of their numerous applications in flat-panel displays and lighting sources. [1,2] Tremendous efforts have been made to improve device performance by modifying the molecular structure and optimizing the device configuration. However, device efficiency is still limited by non-emissive triplet excitons produced in OLEDs. Over the past several years, great success has been achieved in surmounting the 25 % internal quantum efficiency limit in fluorescent materials by employing phosphorescent heavy metal complexes. [3] In OLED devices based on these materials, both singlet and triplet excitons participate in light emission owing to intersystem crossing of the singlet excited states to the triplet states, and then the internal quantum efficiency can reach 100 %. [3,4] [27±29]Thanks to the pivotal works by the group of McMillin, [29±33] the relationship between the molecular structures and photophysical properties of these complexes in solution is well established. Their photoluminescence quantum yield in methylene chloride has been greatly improved, up to 0.16, by using an ether bridged phosphine ligand and a phenanthroline ligand with bulky substituents. [34,35] In the solid state, the emission intensity is expected to be even higher owing to the absence of exciplex formation, which is caused by the attack of solvent molecules.[27±29] All of the above findings indicate that these Cu . This efficiency is comparable to that of Ir III complexes in similar device structures.[13±16]The molecular structures of the Cu I complexes are depicted in Figure 1. In the general formula [Cu(NN)(PP)]BF 4 , NN stands for 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (dmp), or 2,9-di-n-butyl-1,10-phenanthroline (dnbp), and PP stands for bis[2-(diphenylphosphino)phenyl]-ether (DPEphos) or a pair of triphenylphosphine (PPh 3 ) ligands. All complexes were synthesized according to the literature, [34,35] and their purity was verified by elemental analysis. The films of 20 wt.-% Cu I complexes in poly(methyl methacrylate) (PMMA) were fabricated by spin-coating. Their optical properties are summarized in Table 1. Whereas emission in the film (504±555 nm) is significantly blue-shifted compared to the values in methylene chloride solution (560± 700 nm), the absorption maximum in the film (365±386 nm), attributed to the low-lying metal-to-ligand charge-transfer (MLCT) band, is very close to that in methylene chloride so-COMMUNICATIONS 432

By incorporating two phosphorescent dyes, namely, iridium(III)[bis(4,6‐difluorophenyl)‐pyridinato‐N,C2′]picolinate (FIrpic) for blue emission and bis(2‐(9,9‐diethyl‐9H‐fluoren‐2‐yl)‐1‐phenyl‐1H‐benzoimidazol‐N,C3)iridium(acetylacetonate) ((fbi)2Ir(acac)) for orange emission, into a single‐energy well‐like emissive layer, an extremely high‐efficiency white organic light‐emitting diode (WOLED) with excellent color stability is demonstrated. This device can achieve a peak forward‐viewing power efficiency of 42.5 lm W−1, corresponding to an external quantum efficiency (EQE) of 19.3% and a current efficiency of 52.8 cd A−1. Systematic studies of the dopants, host and dopant‐doped host films in terms of photophysical properties (including absorption, photoluminescence, and excitation spectra), transient photoluminescence, current density–voltage characteristics, and temperature‐dependent electroluminescence spectra are subsequently performed, from which it is concluded that the emission natures of FIrpic and (fbi)2Ir(acac) are, respectively, host–guest energy transfer and a direct exciton formation process. These two parallel pathways serve to channel the overall excitons to both dopants, greatly reducing unfavorable energy losses. It is noteworthy that the introduction of the multifunctional orange dopant (fbi)2Ir(acac) (serving as either hole‐trapping site or electron‐transporting channel) is essential to this concept as it can make an improved charge balance and broaden the recombination zone. Based on this unique working model, detailed studies of the slight color‐shift in this WOLED are performed. It is quantitatively proven that the competition between hole trapping on orange‐dopant sites and undisturbed hole transport across the emissive layer is the actual reason. Furthermore, a calculation of the fraction of trapped holes on (fbi)2Ir(acac) sites with voltage shows that the hole‐trapping effect of the orange dopant is decreased with increasing drive voltage, leading to a reduction of orange emission.