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.

A data fusion approach to optimize compositional stability of halide perovskites

Summary Search for resource-efficient materials in vast compositional spaces is an outstanding challenge in creating environmentally stable perovskite semiconductors. We demonstrate a physics-constrained sequential learning framework to subsequently identify the most stable alloyed organic-inorganic perovskites. We fuse data from high-throughput degradation tests and first-principle calculations of phase thermodynamics into an end-to-end Bayesian optimization algorithm using probabilistic constraints. By sampling just 1.8% of the discretized CsxMAyFA1−x−yPbI3 (MA, methylammonium; FA, formamidinium) compositional space, perovskites centered at Cs0.17MA0.03FA0.80PbI3 show minimal optical change under increased temperature, moisture, and illumination with >17-fold stability improvement over MAPbI3. The thin films have 3-fold improved stability compared with state-of-the-art multi-halide Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, translating into enhanced solar cell stability without compromising conversion efficiency. Synchrotron-based X-ray scattering validates the suppression of chemical decomposition and minority phase formation achieved using fewer elements and a maximum of 8% MA. We anticipate that this data fusion approach can be extended to guide materials discovery for a wide range of multinary systems.

Efficient small molecular organic light emitting diode with graphene cathode covered by a Sm layer with nano-hollows and n-doped by Bphen:Cs2CO3 in the hollows

Graphene is a favorable candidate for electrodes of organic light emitting diodes (OLEDs). Graphene has quite a high work function of ∼4.5 eV, and has been extensively studied when used as anodes of OLEDs. In order to use graphene as a cathode, the electron injection barrier between the graphene cathode and the electron transport layer has to be low enough. Using 4,7-diphenyl-1,10-phenanthroline (Bphen):Cs2CO3 to n-dope graphene is a very good method, but the electron injection barrier between the n-doped graphene and Bphen:Cs2CO3 is still too high to be ∼1.0 eV. In this work, in order to further reduce the electron injection barrier, a novel method is suggested. On the graphene cathode, a Sm layer with a lot of nano-hollows, and subsequently a layer of Bphen:Cs2CO3, are deposited. The Bphen:Cs2CO3 can n-dope graphene in the nano-hollows, and the Fermi level of the graphene rises. The nano Sm layer is very easily oxidized. Oxygen adsorbed on the surface of graphene may react with Sm to form an O−–Sm+ dipole layer. On the areas of the Sm oxide dipole layer without nano-hollows, the electron injection barrier can be further lowered by the dipole layer. Electrons tend to mainly inject through the lower electron barrier where the dipole layer exists. Based on this idea, an effective inverted small molecular OLED with the structure of graphene/1 nm Sm layer with a lot of nano-hollows/Bphen:Cs2CO3/Alq3:C545T/NPB/MoO3/Al is presented. The maximum current efficiency and maximum power efficiency of the OLED with a 1 nm Sm layer are about two and three times of those of the reference OLED without any Sm layer, respectively.

Design of efficient molecular organic light-emitting diodes by a high-throughput virtual screening and experimental approach.

Virtual screening is becoming a ground-breaking tool for molecular discovery due to the exponential growth of available computer time and constant improvement of simulation and machine learning techniques. We report an integrated organic functional material design process that incorporates theoretical insight, quantum chemistry, cheminformatics, machine learning, industrial expertise, organic synthesis, molecular characterization, device fabrication and optoelectronic testing. After exploring a search space of 1.6 million molecules and screening over 400,000 of them using time-dependent density functional theory, we identified thousands of promising novel organic light-emitting diode molecules across the visible spectrum. Our team collaboratively selected the best candidates from this set. The experimentally determined external quantum efficiencies for these synthesized candidates were as large as22%.

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.

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.

塗布法により作製されたリン光低分子EL素子(発光型/非発光型ディスプレイ合同研究会)

塗布法により作製されたリン光低分子EL素子(発光型/非発光型ディスプレイ合同研究会)

Theoretical Study on Photophysical Properties of Multifunctional Electroluminescent Molecules with Different π-Conjugated Bridges

The photophysics of a series of molecular organic light-emitting diodes (OLEDs) has been studied by theoretical calculation. These molecular OLEDs have been integrated by an electron- and hole-transporting components as well as an emitting components into the donor-pi-acceptor (D-pi-A) structures: 2-carbazolyl-7-dimesitylboryl-9,9-diethylfluorene (1), trans-4'-N-carbazolyl-4-dimesitylborylstilbene (2), and trans-2-[(4'-N-carbazolyl)styryl]-5-dimesitylborylthiophene (3). To reveal the relationship between the structures and properties of these multifunctional electroluminescent materials, the ground- and excited-state geometries were optimized at the B3LYP/6-31G(d), HF/6-31G(d), and CIS/6-31G(d) levels, respectively. The ionization potentials and electron affinities were computed. The mobilities of hole and electron in these compounds were studied computationally based on the Marcus electron transfer theory. The lowest excitation energies (E(g)) and the maximum absorption and emission wavelengths of these compounds were calculated by time-dependent density functional theory methods. The solvent effect on the emission spectra of these compounds was considered by a polarizable continuum model. As a result of these calculations, it was concluded that the electron injections of these compounds are much easier than Mes(2)B[p-4,4'-biphenyl-NPh(1-naphthyl)], and the diethylfluorene-based compound has higher electron mobility and better equilibrium properties as compared to the stilbene-based and styrylthiophene-based compounds.

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.