Giant Surface Potential Research Articles

Suppression of Initial Degradation via an Interfacial Charge-Induced Overshooting Effect in Solution-Processed Organic Light-Emitting Diodes.

As the chemical stability of organic materials in organic light-emitting diodes (OLEDs) greatly impacts devices' lifetime, a thoughtful and advanced design of materials and device structures is necessary. In our work, we have achieved lifetime enhancement at its initial stage for solution-processed OLEDs. This improvement was realized through the implementation of a double electron transporting layer (dETL) composed of 2-[4-(9,10-dinaphthalen-2-yl-anthracen-2-yl)-phenyl]-1-phenyl-1H-benzoimidazole (ET) and hydroxyquinolinolato-lithium (Liq). A giant surface potential was generated at the surface of a constituent electron transport layer (ETL) that contained a higher concentration of Liq with high polarity. This giant surface potential simultaneously promoted the injection of trapped/accumulated electrons through the interface within dETL and the injection of holes from the anode, generating more exciton recombination events and ultimately enhancing efficiency by 133.0% and lifetime LT95 (luminance dropped by 5%) by 300% with an overshooting effect. Additionally, the degradation at the emitting layer was mitigated by shifting the degradation zone to the dETL, which was evidenced by laser desorption/ionization-time-of-flight (LDI-TOF) mass spectroscopy.

Impact of Spontaneous Orientational Polarization on Triplet-Triplet Upconversion-Based Blue Organic Light-Emitting Diodes.

The spontaneous orientation polarization (SOP) of a permanent dipole moment of the molecule induces a giant surface potential (GSP) in an organic semiconductor film, and GSP is expected to be a crucial parameter for understanding the operational mechanism of organic light-emitting diodes (OLEDs). This study demonstrates that the voltage-dependent migration of a carrier recombination zone induced by a polar electron transporting layer (ETL) having a positive SOP causes a decline in the overall performance of the OLED in triplet-triplet upconversion (TTU) based on OLEDs. Specifically, the TTU efficiency in an OLED with 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as the ETL decreased by 20% due to the reduction of electrically generated triplet exciton density. This decrease resulted in a lower external electroluminescence (EL) quantum efficiency (EQE) of 5.4% at 1 mA cm-2, while the OLED with a nonpolar ETL resulted in an EQE of around 8.1% at 1 mA cm-2. We confirmed a shift in the recombination zone from the current density dependence of the EL spectra in the OLEDs. Our results indicate that the fixed carrier recombination zone near a hole transport layer and an emitting layer (HTL/EML) strongly enhanced the TTU process, while the polar EML/ETL interface induced the migration of the recombination zone depending on voltage, resulting in the decrease of triplet exciton density.

21‐1: Invited Paper: Analysis of Capacitance Characteristics of Highly Efficient Blue OLEDs by Impedance Spectroscopy

Blue phosphorescent materials typically have larger capacitance compared with commercialized blue fluorescent OLEDs. To improve capacitance‐related image quality issues, we need to reduce capacitance of devices without sacrificing other device's properties such as efficiency, lifetime, and operating voltage. In this study, we analyzed devices with phosphor sensitized TADF (PST) emission devices by impedance spectroscopy and revealed that the capacitance is largely affected by giant surface potential (GSP) of consisting layers. The influence of GSP on capacitance is sensitively changed by not only GSP value but also the position of GSP layers in the device. We revealed the effect of GSP of HTL, BIL etc. and explained the mechanism underlying on this phenomena based on device simulation.

Spontaneous orientation polarization of flavonoids

Spontaneous orientation polarization (SOP) is macroscopic electric polarization that is attributed to a constant orientational degree of dipole moments of polar molecules on average. The phenomenon has been found in small molecules like H2O at low temperatures and π-conjugated molecules employed in organic light-emitting diodes. In this study, we demonstrate that a thin film of baicalein, a flavonoid compound found in natural products, exhibits SOP and resultant giant surface potential (GSP) exceeding 5500 mV at a film thickness of 100 nm. Vacuum-deposition of baicalein under high vacuum results in smooth and amorphous films, which enables the generation of GSP with a slope of 57 mV/nm in air, a value comparable to the representative of an organic semiconductor showing GSP, tris(8-hydroxyquinoline)aluminum(III) (Alq3). We also found the superior photostability of a baicalein film compared to an Alq3 film. These findings highlight the potential of baicalein in new applications to organic electronics.

Universal electron transporting layers via mixing two homostructure molecules with different polarities for organic light-emitting diodes

In general, electron transport layer (ETL) in organic light-emtting diodes (OLEDs) consists of single component of electron transporting material (ETM) or a mixture with n-dopant such as 8-hydroxyquinolinolato-lithium (Liq). However, there exists a limit to controlling a wide range of carrier density in OLEDs according to the required characteristics of the devices due to electrically insulating property of Liq. Here, we suggest a universal strategy to construct an efficient ETL. We synthesized two ETMs, diphenyl-[4-(10-phenyl-anthracene-9-yl)-phenyl]-amine (An-Ph) and phneyl-[4-(10-phenyl-anthracene-9-yl)-phenyl]-pyridin-3-yl-amine (An-Py) that have the same core structures with different polarities in functional groups. The electrical characteristics of electron-only-devices (EODs) were investigated by space charge limited current (SCLC) modeling and impedance spectroscopy analysis. Interestingly, the homostructure type ETL composed of An-Ph and An-Py showed not only superior electron transporting capability, but also the possibility of controlling electron injection and transporting in a wide range compared to the heterostructure type ETL of An-Ph and Liq. Compared to the An-Ph-only EOD, the electron mobility in 75% An-Py-mixed homostructure EOD increased by almost 4 orders of magnitude. Such dramatic variation of electron mobility was achieved thanks to the molecular design strategy to separate charge injection and charge transport regions within a molecule, which consequently induced the giant surface potential (GSP) effect between the ETL/cathode interface. As a result, the external quantum efficiency (EQE) of blue fluorescent and phosphorescent OLEDs with the homostructure ETLs was enhanced by 28.6% and 34%, respectively, compared to that of each control device without manipulating outcoupling effects. • The anthracene-based electron transporting materials, An-Py and An-Ph with different polarities were designed and synthesized for universal electron transporting layers. • The superior charge injection property was comprehensively investigated with impedance spectroscopy and attributed to the giant surface potential in the ETL. • Compared to Liq-based ETL, the EQE of fluorescent and phosphorescent OLEDs with An-Py and An-Ph ETL was enhanced by 28.6% and 34%, respectively.

Direct visualization of electric potential distribution in organic light emitting diode by phase-shifting electron holography

We utilized phase-shifting electron holography on organic light emitting diodes consisting of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3) layers to visualize their built-in potential distribution. The bilayer showed three different electric fields, namely, the fields in the α-NPD layer, near the α-NPD/Alq3 interface, and in the Alq3 layer measured as −1.8 ± 0.4 MV m−1, −10.0 ± 2 MV m−1, 3.1 ± 0.6 MV m−1, respectively. We show that they are related to hole accumulation in the α-NPD layer, the charge carrier accumulation junction around the α-NPD/Alq3 interface, and the giant surface potential spontaneously polarized in the Alq3 layer.

What Controls the Orientation of TADF Emitters?

Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role.

Influence of intermolecular interactions on the formation of spontaneous orientation polarization in organic semiconducting films

AbstractSpontaneous orientation polarization (SOP) has been frequently observed in the evaporated films of organic light‐emitting diode materials. Because SOP modifies the charge injection and the accumulation properties of the device, understanding and controlling SOP is crucial in optimizing the performance of the device. In this study, we investigated the dominant factors for SOP formation by focusing on intermolecular interactions. We examined the giant surface potential characteristics of coevaporated films incorporating 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene (TPBi) that is a typical polar molecule exhibiting SOP. In the coevaporated films of TPBi and nonpolar molecules such as 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl and 4,4′,4″‐tris (carbazol‐9‐yl)triphenylamine, the orientation degree of the permanent dipole moment (PDM) of TPBi is significantly enhanced with diluted TPBi density, though the enhancement is weak on the film with N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine. The results indicate that the PDM interaction between polar molecules results as a negative factor for SOP formation. Furthermore, we found that SOP formation is suppressed by the surface treatment of the self‐assembled monolayer on the gold substrate, indicating a positive effect of the van der Waals interaction between the molecule and the substrate surface.

Dipolar Doping of Organic Semiconductors to Enhance Carrier Injection

Spontaneous orientation polarization and the resulting giant surface potential of many organic molecules, especially electron-transport materials, have been known for years, yet various aspects of their influence on device performance have not been investigated thoroughly. In this study, the authors apply the concept of ``dipolar doping'' (diluting a polar species in a nonpolar matrix) to systematically tune the orientation polarization of a prototypical OLED layer stack. They find that a low concentration of a polar species (within the range of usual dye doping in OLEDs) can enhance carrier injection by an order of magnitude.

Built-In Potentials Induced by Molecular Order in Amorphous Organic Thin Films.

Many molecules used to fabricate organic semiconductor devices carry an intrinsic dipole moment. Anisotropic orientation of such molecules in amorphous organic thin films during the deposition process can lead to the spontaneous buildup of an electrostatic potential perpendicular to the film. This so-called giant surface potential (GSP) effect can be exploited in organic electronics applications and was extensively studied in experiment. However, presently, an understanding of the molecular mechanism driving the orientation is lacking. Here, we model the physical vapor deposition process of seven small organic molecules employed in organic light-emitting diode applications with atomistic simulations. We are able to reproduce experimental results for a wide range of strength of the GSP effect. We find that the electrostatic interaction between the dipole moments of the molecules limits the GSP strength and identify short-range van der Waals interactions between the molecule and the surface during deposition as the driving force behind the anisotropic orientation. We furthermore show how the GSP effect influences the energy levels responsible for charge transport, which is important for the design of organic semiconductors and devices.

Evaluation of internal potential distribution and carrier extraction properties of organic solar cells through Kelvin probe and time-of-flight measurements

The carrier extraction property of a prototypical small molecule organic solar cell (OSC) composed of copper phthalocyanine (CuPc), C60, and bathocuproine (BCP) was studied on the basis of the internal potential distribution and carrier dynamics in the device. The internal potential distribution in the OSC structure at the interfaces and in the bulk region was determined by the Kelvin probe method. Significant potential gradients were found in the CuPc film on indium tin oxide and in the C60 film on CuPc, consistent with charge transfer through the contacts. Moreover, surface potential of the BCP layer grew linearly with increasing film thickness with a slope of ca. 35 mV/nm (giant surface potential: GSP), which indicated spontaneous orientation polarization in the film. The potential gradient and GSP significantly changed the built-in potential of the device. Current–voltage and modified time-of-flight measurements revealed that the BCP layer worked as an electron injection and extraction layer despite the wide energy gap. These results were discussed based on the contributions of GSP and the gap states in the BCP layer.

Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq)3] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq)3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq)3/Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices.

Negative giant surface potential of vacuum-evaporated tris(7-propyl-8-hydroxyquinolinolato) aluminum(III) [Al(7-Prq)3] film

Negative giant surface potential was realized in a vacuum-evaporated film of tris(7-propyl-8-hydroxyquinolinolato) aluminum(III) [Al(7-Prq)3]. Electroabsorption response of the film presented an inverted polarity to that of tris(8-hydroxyquinolinolato) aluminum (Alq3), suggesting opposite noncentrosymmetry of molecular orientation. Asymmetric dice model with molecular geometric effect has been proposed, and propyl substitution at 7 position of the ligands was indicated to affects the molecular posture on the surface to invert the polarity of noncentrosymmetry. Our results opened a new possibility of controlling molecular orientation in a film for device applications.

Electronic structures at organic heterojunctions of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB)-based organic light emitting diodes

Interface electronic structures of four-kinds of electron transporting or hole blocking organic materials (n-type) on a widely-used hole transporting material (p-type) in organic light emitting diodes (OLEDs), N,N′-bis (1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB), were investigated by means of photoelectron spectroscopy (PES). 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (OXD-7) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) overlayers show continuous energy shift of each overlayer-derived spectral components and the vacuum level proportional to the thickness. This energy shift is ascribed to a spontaneous building up of the electrostatic potential within the organic layers (giant surface potential; GSP). The energy shift of the overlayers induced by GSP as well as the interface vacuum level shift are adequately taken into account to determine the actual energy barrier heights of the hole conduction levels at the heterojunctions. 4,4′-bis(9-carbazolyl)biphenyl (CBP) and p-bis(triphenylsilyl) benzene (UGH2) induce band bending in the NPB film which presumably results from charge transfer (CT) to the n-type materials from NPB. Despite absence of a practical vacuum level shift and thickness dependent shift of the overlayer-derived electronic states, the CT-derived energy shift of NPB reduces the actual energy barrier height with respect to the nominal barrier height being simply interpreted from PES spectra of a thick overlayer of each material. The energy level diagrams across these ‘n-on-p’ organic–organic heterojunctions were finely determined based on the above interpretation of the PES spectra.

Negative giant surface potential of peeled Alq 3 thin film

In order to observe negative giant surface potential (gSP), vacuum-evaporated tris(8-hydroxyquinolinolato) aluminum (Alq 3) thin film on a polytetrafluoroethylene (PTFE) substrate was peeled off by an adhesive carbon tape in dark condition. Surface potential on the backside was negative, and the absolute values were about two-thirds of the surface potential values (positive values) of the films deposited on ITO with the same thickness. Our results indicate that the gSP does not originate in interfacial or bulk charge, but in anisotropic alignment of dipole moment of Alq 3 molecules in the film. With this simple procedure, we can realize Alq 3 film with opposite molecular alignment, which can be useful in device application.

Long-Term Relaxation of Molecular Orientation in Vacuum-Deposited ALQ3 Thin Films

Long-term measurement of noncentrosymmetric molecular orientation of vacuum-evaporated tris(8-hydroxyquinolinato) aluminum (III) (Alq3) thin film was performed by means of first-order electroabsorption (EA) spectroscopy. This molecular orientation is responsible for the giant surface potential of Alq3 film, and it might be utilized to nonlinear optical devices. Although the EA response presented reduction, the relaxation time constant was on the order of year. Possible mechanisms for the reduction are discussed.

Photoinduced reduction and pattern preservation of giant surface potential on tris(8-hydroxyquinolinato) aluminum(III) thin films

The characteristics and mechanisms of photoinduced reduction in giant surface potential (gSP) on tris(8-hydroxyquinolinato) aluminum(III) (Alq3) thin films were investigated and discussed from the results of the reduction dependences on exposure time/intensity and the preservation of photopatterned gSP. The reduction dependences are explained well with numerical-model calculation of surface potential reduction due to the drift of photoexcited carriers assuming the Poole–Frenkel formula for electron mobility. The preservation of patterned gSPs suggests the existence of deep traps involved in the carrier-drift mechanism.

Influence of Organic Overlayer on Giant Surface Potential of Alq3 Film

We examined influence of an organic overlayer deposition on giant surface potential (GSP) of tris-(8-hydroxyquinolinato)aluminum (III) (Alq3) using Kelvin probe method. Before deposition of organic overlayer, surface potential (SP) of an Alq3 film on a silicon wafer was +13 ± 2 V under the dark condition. When we deposited copper phthalocyanine (CuPc) on the Alq3 film, the SP value remained the same at first and then rapidly decreased with time. It is suggested that the charge transportation through the CuPc layers causes the reduction of SP value. On the other hand, the SP value did not change by deposition of n-hexatriacontane. These results indicate that the GSP possibly vanishes without the photoabsorption of the Alq3 molecules by organic overlayer deposition, depending on the electronic/optoelectronic properties of the overlayer.

Characterization of light-erasable giant surface potential built up in evaporated Alq 3 thin films

Recently high and persistent spontaneous buildup of a surface potential (SP) upon vacuum deposition of tris (8-hydroxyquinolinato) aluminum (III) (Alq 3), which is widely used for organic light emitting devices, on a gold substrate under dark condition (28 V for 560-nm-thick Alq 3 film) has been reported. The removal of the giant surface potential by visible light irradiation has also been reported. These properties of Alq 3 film suggest potential applications to various organic devices such as memory. In this study, we investigated the retention time of the giant potential and the mechanism of the light-induced depolarization in order to discuss the feasibility of the device applications. The observed decay rate of the surface potential in vacuum condition was roughly 10% loss in 10 years, which is enough for memory devices. As to the decay rate by light-irradiation, the observed rate was successfully reproduced by a theoretical simulation based on the photo-induced randomization of oriented Alq 3 molecules.

Spontaneous buildup of giant surface potential by the deposition of Alq 3

Large and persistent surface potential (SP) was obtained in the film of tris(8-hydroxyquinolinato) aluminum (III) (Alq 3 ) prepared on a Au substrate by vacuum deposition. Under experimental condition without light irradiation, the SP changed continuously with increasing the thickness of Alq 3 , after abrupt shift at the interface. The value of the SP reached 28 V at the 560 nm-deposition of Alq 3 . This phenomenon was also observed in the Alq 3 film on an Al substrate. Preferential orientation of the Alq 3 molecules with a permanent dipole moment is suggested as an origin of this giant surface potential of the films, which corresponds to the buildup of the dipole layers in the film. Results of the second harmonic generation measurements supported this model. When we deposited Alq, under a usual experimental condition with the irradiation of the visible light, only abrupt vacuum level shift was observed and there was no further change of the SP. The giant surface potential was easily recovered by irradiation of the visible light for 1hour. These results indicate that the light irradiation during the film growth would affect the film structure or performance of organic electronic devices.