Small Molecular Organic Light-emitting Diodes Research Articles

Electroactive Dyes Based on 1,8-Naphthalimide with Acetylene Linkers as Promising OLED Materials - the Relationship Between Structure and Photophysical Properties.

Four A-π-D-π-A type small organic molecules with 1,8-naphthalimide motifs were successfully synthesised. The designed compounds are built of two 1,8-naphthalimide units linked via ethynyl π-linkages with selected functionalised donor motifs i. e. 2,2'-bithiophene, fluorene, phenothiazine and carbazole derivative. The synthesis based on Sonogashira cross-coupling allowed us to obtain the presented dyes with good yields. The resulting symmetrical small molecules' optical, electrochemical and thermal properties were thoroughly investigated, and their potential applicability for the OLED devices was demonstrated. In addition, the relationship between molecular structure and properties was considered by employing experimental and theoretical studies. As a result of using various donor groups, it was possible to achieve efficient electroluminescence in the range from green (DEV4) to orange-red light (DEV3) with a maximum luminance of 3 820 cd/m2 for DEV4. Upon the insertion of an acetylene linker to the designed molecules, the free rotation of D and A fragments, and hence the effective π-electron communication within the entire molecule, is possible, which was confirmed by DFT studies. The obtained dyes are characterised by high thermal stability, reversible oxidation-reduction process, satisfactory optoelectronic properties and good solubility in organic solvents, which is advisable for the application in small molecular organic light-emitting diodes (SM-OLEDs) technology.

Slow recombination of the de-trapped carriers from doped OLEDs induced by spontaneous orientation polarization

The spontaneous orientation polarization (SOP) induced by molecular permanent dipole moment has recently been discovered and plays an important role in the photoelectric performances of small molecular organic light-emitting diodes (OLEDs). However, the effect of SOP on the transient electroluminescence (TEL) dynamics of OLEDs is still vague. Herein, we find that the generation of the spike at the TEL falling edge in doped OLEDs depends remarkably on the SOP in the devices. That is, SOP is a disadvantage for spike formation and even causes the spike to disappear totally. In addition, a long-lived EL decay was observed in the OLEDs with strong SOP. Through a series of control experiments, it has been confirmed that the SOP-induced internal electric field in doped OLEDs would slow down the recombination process of the de-trapped carriers, leading to a suppression of the TEL spike and a long-lived EL decay. Accordingly, our findings not only supplement the prerequisites for the generation of the TEL spike, i.e., with weak SOP effects in an emissive layer, but also deepen the fundamental understanding of the SOP role in the TEL dynamics process of OLEDs.

Molecular Stacking Effect on Small-Molecular Organic Light-Emitting Diodes Prepared with Solution Process.

The light-emitting layer (EML) is generally prepared by mixing the host and dopant to realize an organic light-emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs, depending on the structure of the host materials. In particular, phase separation because of π-π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (Tg) are ideal because phase separation occurs because of π-π stacking during the annealing process, if Tg is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different three-dimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties, depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing because of its limited free rotation through the internal bond shows much better device characteristics even after annealing at a higher temperature than Tg. The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.

Small molecular phosphorescent organic light-emitting diodes using a spin-coated hole blocking layer

Small molecular green phosphorescent organic light-emitting diodes (OLEDs) have been studied using a solution processed polyethyleneoxide (PEO) hole blocking layer (HBL) and a Cs2CO3/Al cathode. PEO is soluable in alcoholic solvents and allows the fabrication of multilayer OLEDs by successive spin-coating. The current efficiency of the optimized OLED with the PEO HBL increases from 18.8 cd/A to 32.1 cd/A, and the turn-on voltage reduces from 4.8 V to 3.4 V, compared with the device without the PEO HBL. Photovoltaic measurements indicate that the injection barrier for electron is reduced by inserting the PEO layer. X-ray photoelectron spectroscopy measurements further reveal that improvement in device performance is due to the partial penetration of Cs into the PEO layer.

Efficient Solution-Processed Nondoped Deep-Blue Organic Light-Emitting Diodes Based on Fluorene-Bridged Anthracene Derivatives Appended with Charge Transport Moieties

A series of fluorene-bridged anthracene derivatives appended with different charge transport moieties were synthesized, and their thermal, photophysical, and electrochemical properties were investigated. By the introduction of fluorene between two anthracene units as well as peripheral functional aryl substituents, the sophisticated compounds show a decreased tendency to crystallize and have high glass transition temperatures ranging from 165 to 229 °C. The theoretical calculations reveal that the four self-hosted blue emitters possess noncoplanar structure to suppress the intermolecular interaction in films. Solution-processed small-molecular organic light-emitting diodes featuring 1 as the emitter achieve a maximum current efficiency of 2.0 cd A–1 with Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.13), which are very close to the National Television Standards Committee’s blue standard. A facile strategy to design solution-processable highly emissive anthracene derivatives for n...

Large contribution of triplet excitons to electro-fluorescence in small molecular organic light-emitting diodes

Singlet exciton formation in working fluorescent devices was investigated by analyzing the magnetic field effects on electroluminescence intensity based on a rate model. Two magnetic field sensitive processes including charge recombination and triplet–triplet annihilation (TTA) are suggested to be involved in the generation of singlet excitons. It reveals that TTA process produces considerable extra singlets which account for as high as 19% of total singlets at 20 K in non-doped device, 34% at 20 K and 17% at room temperature in doped device, causing the total singlet generation yields exceeding the classical 0.25 spin statistics limit.

Transient electroluminescence spikes in small molecular organic light-emitting diodes

We present a comprehensive study of transient nanosecond electroluminescence (EL) spikes that exceed the dc level and microseconds-long EL tails following a bias pulse in guest-host small molecular organic light-emitting diodes (SMOLEDs), including relatively efficient devices, which elucidates carrier and exciton dynamics in such devices. The transient EL is strongly dependent, among other parameters, on device materials and structure. At low temperatures, all measured devices, with the exception of Pt octaethylporphyrin (PtOEP)-doped tris(8-hydroxyquinoline) Al (Alq${}_{3}$) SMOLEDs, exhibit the spikes at \ensuremath{\sim}70--300 ns. At room temperature (RT), however, only those with a hole injection barrier, carrier-trapping guest-host emitting layer, and no strong electron-transporting and hole-blocking layer [such as 4,7-diphenyl-1,10-phenanthroline (BPhen)] exhibit strong spikes. These narrow and appear earlier under postpulse reverse bias. To further elucidate the origin of the spikes, we monitored their dependence on the pulsed bias width and voltage, the doped layer thickness, and its location within the OLED structure. The characteristics of the microseconds-long tails were also evaluated through the effect of the postpulse voltage. A model based on the recombination of correlated charge pairs (CCPs) and on charge detrapping is presented; the model agrees well with the experimental data. The results suggest that reduced electric-field-induced dissociative quenching of singlet excitons is responsible for the spikes' amplitude exceeding the on-pulse dc EL level. The long tails are attributed to recombination of charges detrapped from a distribution of shallow, mostly host, sites, reminiscent of the detrapping and recombination processes that yield the thermally stimulated luminescence of such materials. The comprehensive transient EL measurements in guest-host devices demonstrate the generality of the strong spike phenomenon in devices with charge trapping in the emitting guest molecules. Hence the transient EL can serve as an important tool to identify the dominant emitting mechanisms in the guest-host systems.

Mechanisms of delayed fluorescence in small molecular organic light-emitting diodes studied by transient electroluminescence

Two kinds of organic light-emitting diodes with structure of ITO/NPB/Alq₃/LiF/Al (Alq₃ bilayer device) and ITO/ NPB/Alq₃:DCM/Alq₃/LiF/Al (Alq₃:DCM device) were fabricated. The mechanisms of delayed fluorescence (DF) in the two devices were studied by transient electroluminescence. These results show that the DF in the Alq₃-based bilayer device decays fast after removing the pulsed bias. We suggest that the DF in bilayer device is caused by the decay of the singlet excitons which were formed by the delayed charge injection after removing the pulsed bias. In contrast, the DF in the Alq₃:DCM device is very strong. The dependences of DF in the Alq₃:DCM device on reverse bias after removing the pulsed bias and on the pulsed width were also analyzed. We found that the detrapping and then recombination of the trapped charge can only generate a small fraction of DF intensity in Alq₃:DCM device. Large fraction of the DF intensity results from triplet-triplet annihilation process.

Patterning of pinhole free small molecular organic light-emitting films by ink-jet printing

Small molecular organic light-emitting diodes (SMOLEDs) were fabricated with an ink-jet printed film of 2-( t-butyl)-9,10-bis (20-naphthyl) anthracene (TBADN) doped with 4,4′-bis[2-{4-( N, N-diphenylamino)phenyl vinyl] (DPAVBi) as emitting layer. Dewetting behavior of ink-jet printed TBADN/DPAVBi solutions were restrained by adding cyclohexylbenzene or α-chloronaphthalene to the main solvent chlorobenzene. The high boiling point and high viscosity of cyclohexylbenzene and α-chloronaphthalene has increased the thickness of liquid films and the viscosity, which restrained the dewetting from thermodynamics and kinetics aspect. Uniform TBADN/DPAVBi films obtained by ink-jet printing from chlorobenzene/cyclohexylbenzene solution have been used in the fabrication of matrix display of SMOLEDs. The OLEDs has a turn-on voltage of 5.5 V, the maximum luminance of 289 cd/m 2 and the maximum current efficiency of 0.71 cd/A.

Evidence for holes beyond the recombination zone and trions in the electron transport layer of organic light-emitting diodes

Recent electroluminescence (EL) detected magnetic resonance and transient EL studies reveal the presence and role of holes that drift beyond the recombination zone and approach the cathode in small molecular organic light-emitting diodes (OLEDs) with specific materials and structures. In particular, these studies suggest that these holes are responsible for trion (i.e., a bipolaron stabilized by a counterpolaron on an adjacent molecule) formation in the electron transport layer, and may contribute to EL spikes observed at the end of a bias pulse. The significance of these holes to overall OLED performance is discussed.

Improved performance of organic light-emitting diodes with cesium chloride inside tris (8-hydroxyquinoline) aluminum

A series of small molecular organic light-emitting diodes (OLEDs) based on tris (8-hydroxyquinoline) aluminum (Alq 3) was fabricated by varying thicknesses and positions of cesium chloride (CsCl) layer inside Alq 3 layer. Both luminance and efficiency are enhanced due to the improvement of electron injection when a CsCl layer was deposited between Alq 3 and aluminum (Al). For the insertion of the CsCl layer at the 10 nm position inside Alq 3 layer away from Al cathode, the enhanced current density and luminance are attributed to the reaction between diffused Al and Cs. And the efficiency and luminance are enhanced due to the trap sites induced by the CsCl layer at the distance of 20 and 30 nm away from the Al cathode. The current density and luminance of devices, in which various thicknesses of CsCl layer was inserted at 20 nm position inside Alq 3 layer away from the Al cathode, is affected by both hole trapped and insulating layer effects.

Spectrally narrowed edge emission from leaky waveguide modes in organic light-emitting diodes

A dramatic spectral line narrowing of the edge emission at room temperature from tris(quinolinolate) Al (Alq3), N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (NPD), 4,4′-bis(2,2′-diphenyl-vinyl)-,1′-biphenyl (DPVBi), and some guest-host small molecular organic light-emitting diodes (OLEDs), fabricated on indium tin oxide (ITO)-coated glass, is described. In all but the DPVBi OLEDs, the narrowed emission band emerges above a threshold thickness of the emitting layer, and narrows down to a full width at half maximum of only 5–10 nm. The results demonstrate that this narrowed emission is due to irregular waveguide modes that leak from the ITO to the glass substrate at a grazing angle. While measurements of variable stripe length l devices exhibit an apparent weak optical gain 0≤g≤1.86 cm−1, there is no observable threshold current or bias associated with this spectral narrowing. In addition, in the phosphorescent guest-host OLEDs, there is no decrease in the emission decay time of the narrowed edge emission relative to the broad surface emission. It is suspected that the apparent weak optical gain is due to misalignment of the axis of the waveguided mode and the axis of the collection lens of the probe. However, it is not clear if such a misalignment can account for all the effects of the observed evolution of the edge-emission spectra with l.

All-solution-processed blue small molecular organic light-emitting diodes with multilayer device structure

All-solution-processed multilayer blue small molecular organic light-emitting diodes are fabricated by blade coating method. Fluorescent blue host,1-(7-(9,9′-bianthracen-10-yl)- 9,9-dioctyl-9H-fluoren-2-yl)pyrene, and blue dopant, 4,4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethene-2,1-diyl)bis(N,N-bis(4-hexylphenyl)aniline), are used to achieve good solubility and pinhole-free thin film by solution process. The multilayer device structure with hole/electron transport layer is achieved by blade coating method without the dissolution problem between layers. The efficiency of the all-solution-processed device is 4.8 cd/A at 1200 cd/m 2, close to that by thermal deposition in high vacuum chamber. The device performance is optimized with the annealing temperature of TPBi layer at 50 °C.

Effect of fabrication process on the microstructure and the efficiency of organic light-emitting diode

Small molecular organic light-emitting diodes (OLEDs) can be fabricated by spin-coating and thermo-evaporation of organic materials. In this work, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectrometry (SIMS) are used to examine the microstructure of emissive layers in organic LEDs prepared with different technique using in situ C 60 + sputtering. It is found that the Ir-containing guest molecule is enriched at the bottom-interface and a concentration gradient results from spin-coating while the opposite molecular distribution is observed with thermo-evaporation. Such a difference in microstructure affects the carrier-injection and carrier-transportation and is key to the dramatic difference (∼3.5×) in the efficiency of the resultant OLED devices.

Investigation of an efficient YbF 3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq 3)-based organic light-emitting diodes (OLEDs) using YbF 3 as the electron injection layer have been investigated. With an YbF 3 (3.0 nm)/Al cathode, the device with Alq 3 as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq 3-cathode interface could be well covered by YbF 3 at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq 3 and Al, and to reduce the destruction of Alq 3 by Al.

Small molecular and polymer organic light-emitting diodes based on novel iridium complex phosphor

Molecular organic light-emitting devices (MOLED) and polymer devices (POLED) were fabricated using a novel iridium complex bis[2-(4′-tert-butylphenyl)-1-phenyl-1H-benzoimidazole-N,C2′] iridium (III) (acetylacetonate) phosphor as an emissive dye doped in two different host materials. Electroluminescent (EL) property, current density–voltage–luminance, and current efficiency characteristics of two kinds of devices were measured. The results showed that MOLED has a maximum current efficiency of 22.9cd/A at a luminance of 163cd/m2, which is higher than that of 7.6cd/A at 371cd/m2 for POLED. MOLED and POLED exhibited a maximum luminance of 13,500 and 4181cd/m2, respectively, with a full width at half-maximum of MOLED spectra is 68nm, which is much narrower than 116nm of POLED spectra.

Efficient small molecular and polymer organic devices using bis[2-(4- tert-butylphenyl)benzothiazolato-N,C 2′] iridium (III) (acetylacetonate) dye as emitter

Small molecular organic light-emitting diodes (MOLED) and polymer organic light-emitting diodes (POLED) were fabricated with yellow light emission phosphorescent dye bis[2-(4- tert-butylphenyl)benzothiazolato-N,C 2′] iridium (III) (acetylacetonate) doped in different hosts. The electroluminescent (EL) spectra of both devices shown two peaks generated from iridium dye but the position of main peak changed and became broader for POLED. The maximum luminance of 10,500 cd/m 2 achieved at 12.5 V for MOLED is higher than maximum luminance of 9996 cd/m 2 at 20 V for POLED. The maximum power efficiency of small molecular device is 6.4 lm/W, which is higher than 2.3 lm/W of polymer device, but the efficiency of both devices will roll off at large current density.

Spectrally narrowed edge emission from organic light-emitting diodes

A dramatic spectrally narrowed edge emission (SNEE) from small molecular organic light-emitting diodes at room temperature, with a full width at half maximum of 5–10nm, is described. The results show that this emission is due to irregular waveguide modes that leak from the indium tin oxide anode to the glass substrate at a grazing angle. Measurements of variable stripe length devices exhibit an apparent weak optical gain, but there is no observable threshold bias associated with this SNEE. Hence this apparent “optical gain” is suspected to result from misalignment of the propagating leaky waveguide mode and the collecting optics.

Organic oxide/Al composite cathode in small molecular organic light-emitting diodes

This study addresses the feasibility of using an organic oxide/Al composite cathode to fabricate the small molecular organic light-emitting diodes (OLEDs). A supplementary organic buffer film is placed at the interface between the tris(8-hydroxyquinoline) aluminum (Alq3) and the organic oxide/Al complex layers. Incorporating the rubrene/poly(ethylene glycol) dimethyl ether (PEGDE) buffer layers into the composite cathode structure markedly improves the performance of devices. The luminous efficiencies of Alq3-based OLEDs biased at ∼100mA∕cm2 are 4.8 and 5.1cd∕A for rubrene (50Å)/PEGDE (15Å)∕Al and rubrene (50Å)/PEGDE (15Å)∕LiF (5Å)∕Al cathode devices, and 1.3 and 3.8cd∕A for devices with Al and LiF (5Å)∕Al cathodes, respectively.

Spin and magnetic field effects in organic semiconductor devices

The authors describe three spin and magnetic field effects in organic semiconductor devices: First, injection, transport and detection of spin-polarised carriers using an organic semiconductor as the spacer layer in a spin-valve structure, yielding low-temperature giant magnetoresistance effects as large as 40%. Secondly, spin-dependent exciton formation: pairs of electrons and holes show different reaction rates (the reaction products being spin singlet or triplet excitons, respectively) dependent on whether they recombine in spin-parallel or spin-antiparallel orientation. It is believed that this effect ultimately determines the maximum possible electroluminescent efficiency of organic light-emitting diodes (OLEDs). And, finally, a large magnetoresistance (MR) effect in OLEDs in weak magnetic fields that reaches up to 10% at fields of 10 mT at room temperature. Negative MR is usually observed, but positive MR can also be achieved under certain operation conditions. The authors present an extensive experimental characterisation of this effect in both polymer and small molecular OLEDs. The last two effects do not, to the authors' best knowledge, occur in inorganic semiconductor devices and are therefore related to the peculiarities of organic semiconductor physics. The authors discuss their findings, contrasting organic and inorganic semiconductor physics, respectively.