Engineering Efficiency Roll‐Off in Organic Light‐Emitting Devices

Previous studies have identified triplet‐triplet annihilation and triplet‐polaron quenching as the exciton density‐dependent mechanisms which give rise to the efficiency roll‐off observed in phosphorescent organic light‐emitting devices (OLEDs). In this work, these quenching processes are independently probed, and the impact of the exciton recombination zone width on the severity of quenching in various OLED architectures is examined directly. It is found that in devices employing a graded‐emissive layer (G‐EML) architecture the efficiency roll‐off is due to both triplet‐triplet annihilation and triplet‐polaron quenching, while in devices which employ a conventional double‐emissive layer (D‐EML) architecture, the roll‐off is dominated by triplet‐triplet annihilation. Overall, the efficiency roll‐off in G‐EML devices is found to be much less severe than in the D‐EML device. This result is well accounted for by the larger exciton recombination zone measured in G‐EML devices, which serves to reduce exciton density‐driven loss pathways at high excitation levels. Indeed, a predictive model of the device efficiency based on the quantitatively measured quenching parameters shows the role a large exciton recombination zone plays in mitigating the roll‐off.     

Modification of Exciton Lifetime by the Metal Cathode in Phosphorescent OLEDs,and Implications on Device Efficiency and Efficiency Roll‐off Behavior

Time resolved photoluminescence and electroluminescence measurements are used to study changes in the emission characteristics of materials typically used in phosphorescent organic light emitting devices (PhOLEDs). Studies on archetypical PhOLEDs with phosphorescent material, fac‐tris(2‐phenylpyridine) iridium (Ir(ppy)₃), show that the lifetime of triplet exciton is modified when in close proximity to a metal layer. Interactions with a metal layer ～30–100 nm away, as is typically the case in PhOLEDs, result in an increase in the spontaneous emission decay rate of triplet excitons, and causes the exciton lifetime to become shorter as the distance between the phosphorescent material and the metal becomes smaller. The phenomenon, possibly the result of the confined radiation field by the metal, affects device efficiency and efficiency roll‐off behavior. The results shed the light on phenomena affecting the efficiency behavior of PhOLEDs, and provide new insights for device design that can help enhance efficiency performance.     

Investigating the Role of Emissive Layer Architecture on the Exciton Recombination Zone in Organic Light‐Emitting Devices

An experimental approach to determine the spatial extent and location of the exciton recombination zone in an organic light‐emitting device (OLED) is demonstrated. This technique is applicable to a wide variety of OLED structures and is used to examine OLEDs which have a double‐ (D‐EML), mixed‐ (M‐EML), or graded‐emissive layer (G‐EML) architecture. The location of exciton recombination in an OLED is an important design parameter, as the local optical field sensed by the exciton greatly determines the efficiency and angular distribution of far‐field light extraction. The spatial extent of exciton recombination is an important parameter that can strongly impact exciton quenching and OLED efficiency, particularly under high excitation. A direct measurement of the exciton density profile is achieved through the inclusion of a thin, exciton sensitizing strip in the OLED emissive layer which locally quenches guest excitons and whose position in the emissive layer can be translated across the device to probe exciton formation. In the case of the G‐EML device architecture, an electronic model is developed to predict the location and extent of the exciton density profile by considering the drift, diffusion, and recombination of charge carriers within the device.     

CBP超薄层对有机电致磷光器件效率的影响

在空穴传输层(HTL)和发光层(EML)间插入4,4-N,N′-二咔唑基联苯(CBP)超薄层,制备了结构为ITO/NPB/CBP(xnm)/CBP:Ir(ppy)3/BCP/Alq3/LiF/Al有机电致磷光器件。与未插入CBP超薄层的器件相比,CBP超薄层的引入可以有效阻挡Ir(ppy)3的三线态能量通过Dexter能量转移到HTL的NPB中,减少无辐射能量损失,提高了器件发光效率。调整CBP薄层的厚度,当x为3nm时,器件的效率提高幅度最大,从x为0nm时的9.0cd/A提高到16.9cd/A。     

Solution‐Processed,Alkali Metal‐Salt‐Doped,Electron‐Transport Layers for High‐Performance Phosphorescent Organic Light‐Emitting Diodes

High‐performance, blue, phosphorescent organic light‐emitting diodes (PhOLEDs) are achieved by orthogonal solution‐processing of small‐molecule electron‐transport material doped with an alkali metal salt, including cesium carbonate (Cs₂CO₃) or lithium carbonate (Li₂CO₃). Blue PhOLEDs with solution‐processed 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) electron‐transport layer (ETL) doped with Cs₂CO₃ show a luminous efficiency (LE) of 35.1 cd A^?1 with an external quantum efficiency (EQE) of 17.9%, which are two‐fold higher efficiency than a BPhen ETL without a dopant. These solution‐processed blue PhOLEDs are much superior compared to devices with vacuum‐deposited BPhen ETL/alkali metal salt cathode interfacial layer. Blue PhOLEDs with solution‐processed 1,3,5‐tris(m‐pyrid‐3‐yl‐phenyl)benzene (TmPyPB) ETL doped with Cs₂CO₃ have a luminous efficiency of 37.7 cd A^?1 with an EQE of 19.0%, which is the best performance observed to date in all‐solution‐processed blue PhOLEDs. The results show that a small‐molecule ETL doped with alkali metal salt can be realized by solution‐processing to enhance overall device performance. The solution‐processed metal salt‐doped ETLs exhibit a unique rough surface morphology that facilitates enhanced charge‐injection and transport in the devices. These results demonstrate that orthogonal solution‐processing of metal salt‐doped electron‐transport materials is a promising strategy for applications in various solution‐processed multilayered organic electronic devices.     

Doping‐Free Organic Light‐Emitting Diodes with Very High Power Efficiency,Simple Device Structure,and Superior Spectral Performance

Today's state‐of‐the‐art phosphorescent organic light‐emitting diodes (PhOLEDs) must rely on the host‐guest doping technique to decrease triplet quenching and increase device efficiency. However, doping is a sophisticated device fabrication process. Here, a Pt(II)‐based complex with a near unity photoluminescence quantum yield and excellent electron transporting properties in the form of neat film is reported. Simplified doping‐free white PhOLED and yellow‐orange PhOLED based on this emitter achieve rather low operating voltages (2.2–2.4 V) and very high power efficiencies of approximately 80 lm W^?1 (yellow‐orange) and 50 lm W^?1 (white), respectively, without any light extraction enhancement. Furthermore, the efficient white device also exhibits high color stability. No color shift is observed during the entire operation of the device. Analysis of the device's operational mechanism has been postulated in terms of exciton and polaron formation and fate. It is found that using the efficient neat Pt(II)‐complex as a homogeneous emitting and electron transporting layer and an ambipolar blue emitter are determining factors for achieving such a high efficiency.     

Designing Host Materials for the Emissive Layer of Single-Layer Phosphorescent Organic Light-Emitting Diodes: Toward Simplified Organic Devices

Thanks to the tremendous effort over the last 20 years, phosphorescent organic light-emitting diodes (PhOLEDs) represent a prevalent technology. In this technology, all the high-efficiency PhOLEDs are multi-layer devices constituting, in addition to the emissive layer (EML), of a stack of functional organic layers. These layers play a crucial role in the device performance as they improve the injection, transport, and recombination of charges within the EML. Single-layer PhOLEDs (SL-PhOLEDs) represent ideal OLEDs, consisting only of the electrodes and the EML. However, reaching high-performance SL-PhOLED is far from easy, as removing the functional layers of an OLED stack dramatically decreases the performance. To achieve high SL-PhOLED efficiency, the efficient injection, transport, and recombination of charges should be insured by the EML, and particularly, by the host material. In the present exhaustive review, the different molecular design strategies are analyzed, which have been used to construct high-efficiency hosts for SL-PhOLED. The impact of the electronic properties (triplet energy, HOMO/LUMO energy, mobility etc.) on the device characteristics (threshold voltage, electroluminescent spectrum, external quantum efficiency, etc.) are discussed. This allows to draw a structure/properties/device performance relationship map of interest for the future design of functional materials for SL-PhOLEDs.     

Highly Efficient Simple‐Structure Blue and All‐Phosphor Warm‐White Phosphorescent Organic Light‐Emitting Diodes Enabled by Wide‐Bandgap Tetraarylsilane‐Based Functional Materials

Two host materials of {4‐[diphenyl(4‐pyridin‐3‐ylphenyl)silyl]phenyl}diphenylamine (p‐PySiTPA) and {4‐[[4‐(diphenylphosphoryl)phenyl](diphenyl)silyl]phenyl}diphenylamine (p‐POSiTPA), and an electron‐transporting material of [(diphenylsilanediyl)bis(4,1‐phenylene)]bis(diphenylphosphine) dioxide (SiDPO) are developed by incorporating appropriate charge transporting units into the tetraarylsilane skeleton. The host materials feature both high triplet energies (ca. 2.93 eV) and ambipolar charge transporting nature; the electron‐transporting material comprising diphenylphosphine oxide units and tetraphenylsilane skeleton exhibits a high triplet energy (3.21 eV) and a deep highest occupied molecular orbital (HOMO) level (‐6.47 eV). Using these tetraarylsilane‐based functional materials results in a high‐efficiency blue phosphorescent device with a three‐organic‐layer structure of 1,1‐bis[4‐[N,N‐di(p‐tolyl)‐amino]phenyl]cyclohexane (TAPC)/p‐POSiTPA: iridium(III) bis(4′,6′‐difluorophenylpyridinato)tetrakis(1‐pyrazolyl)borate (FIr6)/SiDPO that exhibits a forward‐viewing maximum external quantum efficiency (EQE) up to 22.2%. This is the first report of three‐organic‐layer FIr6‐based blue PhOLEDs with the forward‐viewing EQE over 20%, and the device performance is among the highest for FIr6‐based blue PhOLEDs even compared with the four or more than four organic‐layer devices. Furthermore, with the introduction of bis(2‐(9,9‐diethyl‐9H‐fluoren‐2‐yl)‐1‐phenyl‐1H‐benzoimidazol‐N,C³)iridium acetylacetonate [(fbi)₂Ir(acac)] as an orange emitter, an all‐phosphor warm‐white PhOLED achieves a peak power efficiency of 47.2 lm W^?1, which is close to the highest values ever reported for two‐color white PhOLEDs.     

m‐Indolocarbazole Derivative as a Universal Host Material for RGB and White Phosphorescent OLEDs

The host materials designed for highly efficient white phosphorescent organic light‐emitting diodes (PhOLEDs) with power efficiency (PE) >50 lm W^‐1 and low efficiency roll‐off are very rare. In this work, three new indolocarbazole‐based materials (ICDP, 4ICPPy, and 4ICDPy) are presented composed of 6,7‐dimethylindolo[3,2‐a]carbazole and phenyl or 4‐pyridyl group for hosting blue, green, and red phosphors. Among this three host materials, 4ICDPy‐based devices reveal the best electroluminescent performance with maximum external quantum efficiencies (EQEs) of 22.1%, 27.0%, and 25.3% for blue (FIrpic), green (fac‐Ir(ppy)₃), and red ((piq)₂Ir(acac)) PhOLEDs. A two‐color and single‐emitting‐layer white organic light‐emitting diode hosted by 4ICDPy with FIrpic and Ir(pq)₃ as dopants achieves high EQE of 20.3% and PE of 50.9 lm W^?1 with good color stability; this performance is among the best for a single‐emitting‐layer white PhOLEDs. All 4ICDPy‐based devices show low efficiency roll‐off probably due to the excellent balanced carrier transport arisen from the bipolar character of 4ICDPy.     

Analysis of Complete Organic Semiconductor Devices by Laser Desorption/Ionization Time‐of‐Flight Mass Spectrometry

In this contribution, it is shown that the method of laser‐desorption/ionization time‐of‐flight mass spectrometry (LDI‐TOF‐MS) is a powerful technique for analyzing complete organic devices, such as organic light‐emitting diodes (OLEDs) or organic solar cells. LDI‐TOF‐MS has the potential to analyze fully processed organic devices without special pretreatment such as dissolving the device, peeling off the metal cathode, or using additional matrix materials. Thus, devices may be analysed as they are with a minimum of measurement artefacts. It is demonstrated that the method allows an analysis of complex organic multilayer devices, their composition, and incorporated impurities. It even allows possible electrochemical reaction products caused by device degradation to be analyzed. Thus, LDI‐TOF‐MS has major advantages compared to measurements of dissolved samples. As an example, the identification of all of the materials used in a complete OLED is shown. Furthermore, a detailed chemical analysis of long‐term driven OLEDs, including the detection of degradation products, is presented. From these data, several degradation mechanisms can be distinguished.     

Interlayer Engineering with Different Host Material Properties in Blue Phosphorescent Organic Light‐Emitting Diodes

We investigated the light‐emitting performances of blue phosphorescent organic light‐emitting diodes, known as PHOLEDs, by incorporating an N,N’‐dicarbazolyl‐3,5‐benzen interlayer between the hole transporting layer and emitting layer (EML). We found that the effects of the introduced interlayer for triplet exciton confinement and hole/electron balance in the EML were exceptionally dependent on the host materials: 9‐(4‐tert‐butylphenyl)‐3.6‐bis(triphenylsilyl)‐9H‐carbazole, 9‐(4‐tert‐butylphenyl)‐3.6‐ditrityl‐9H‐carbazole, and 4,4’‐bis‐triphenylsilanyl‐biphenyl. When an appropriate interlayer and host material were combined, the peak external quantum efficiency was greatly enhanced by over 21 times from 0.79% to 17.1%. Studies on the recombination zone using a series of host materials were also conducted.     

High Solid‐State Near Infrared Emissive Organic Fluorophores from Thiadiazole[3,4‐c]Pyridine Derivatives for Efficient Simple Solution‐Processed Nondoped Near Infrared OLEDs

Many efforts have been dedicated to developing near infrared (NIR) fluorescent emitters with strong emission especially in the range of 700–1000 nm due to their potential applications in biomedical and optoelectronic fields. However, high solid state NIR emission fluorophores are still rare for applications. Herein, two efficient donor‐π‐acceptor type NIR emitters, C3HTP and C4HTP , are designed and synthesized by end‐capping two isomeric bis(n‐hexylthienyl)thiadiazole[3,4‐c]pyridines as π‐acceptor with structural bulky, electron rich tercarbazole moiety. They exhibit excellent solid state NIR emission with an emission peak at 725 nm, especially C3HTP , reaching a record high photoluminescence quantum yield (Φ_PL) of 34% for NIR organic fluorescent materials. By taking advantage of their Φ_PL values in the film state (Φ_PL = 10–34%), suitable energy levels (highest occupied molecular orbital (HOMO) level ≈ ?5.3 eV), high hole mobility (5.49 × 10^?8 cm² V^?1 s^?1) as well as good amorphous film forming ability by solution casting, they are used to fabricate a nondoped emissive layer (EML) in simple double‐layer solution processed NIR electroluminescent (EL) devices. The device containing C3HTP as the EML shows a NIR emission peaking at 726 nm and excellent EL performance with a high external quantum efficiency of 1.51%, which is the best solution processed nondoped NIR organic light‐emitting diodes reported to date. Importantly, this represents an advance in near infrared organic fluorescent materials and EL devices that meet the requirements of many applications.     

Control of Solid‐State Dye‐Sensitized Solar Cell Performance by Block‐Copolymer‐Directed TiO2 Synthesis

Hybrid dye‐sensitized solar cells are typically composed of mesoporous titania (TiO₂), light‐harvesting dyes, and organic molecular hole‐transporters. Correctly matching the electronic properties of the materials is critical to ensure efficient device operation. In this study, TiO₂ is synthesized in a well‐defined morphological confinement that arises from the self‐assembly of a diblock copolymer—poly(isoprene‐b‐ethylene oxide) (PI‐b‐PEO). The crystallization environment, tuned by the inorganic (TiO₂ mass) to organic (polymer) ratio, is shown to be a decisive factor in determining the distribution of sub‐bandgap electronic states and the associated electronic function in solid‐state dye‐sensitized solar cells. Interestingly, the tuning of the sub‐bandgap states does not appear to strongly influence the charge transport and recombination in the devices. However, increasing the depth and breadth of the density of sub‐bandgap states correlates well with an increase in photocurrent generation, suggesting that a high density of these sub‐bandgap states is critical for efficient photo‐induced electron transfer and charge separation.     

1‐Methyl‐2‐(anthryl)‐imidazo[4,5‐f][1,10]‐phenanthroline: A Highly Efficient Electron‐Transport Compound and a Bright Blue‐Light Emitter for Electroluminescent Devices

A new organic blue‐light emitter 1‐methyl‐2‐(anthryl)‐imidazo[4,5‐f][1,10]‐phenanthroline ( 1 ) has been synthesized and fully characterized. The utility of compound 1 as a blue‐light emitter in electroluminescent (EL) devices has been evaluated by fabricating a series of EL devices A where compound 1 functions as an emitter. The EL spectrum of device series A has the emission maximum at 481 nm with the CIE (Commission Internationale de l'Eclairage) color coordinates 0.198 and 0.284. The maximum luminance of devices in series A is 4000 cd m^–2 and the best external quantum efficiency of device series A is 1.82 %. The utility of compound 1 as an electron injection–electron transport material has been evaluated by constructing a set of EL devices B where 1 is used as either the electron‐injection layer or the electron injection–electron transport layer. The performance of device series B is compared to the standard device in which Alq₃ (tris(8‐hydroxyquinoline) aluminum) is used as the electron injection–electron transport layer. The experimental results show that the performance of 1 as an electron injection–electron transport material is considerably better than Alq₃. The stability of device series B is comparable to that of the standard Alq₃ device. The excellent performance of 1 as an electron injection/transport material may be attributed to the strong intermolecular interactions of 1 in the solid state as revealed by single‐crystal X‐ray diffraction analysis. In addition, compound 1 is a colorless material with a much larger highest occupied molecular orbital–lowest unoccupied molecular (HOMO–LUMO) gap than Alq₃, which renders it potentially useful for a wide range of applications in EL devices.     

A Multifunctional Blue‐Emitting Material Designed via Tuning Distribution of Hybridized Excited‐State for High‐Performance Blue and Host‐Sensitized OLEDs

Actualizing full singlet exciton yield via a reverse intersystem crossing from the high‐lying triplet state to singlet state, namely, “hot exciton” mechanism, holds great potential for high‐performance fluorescent organic light‐emitting diodes (OLEDs). However, incorporating comprehensive insights into the mechanism and effective molecular design strategies still remains challenging. Herein, three blue emitters (CNNPI, 2TriPE‐CNNPI, and 2CzPh‐CNNPI) with a distinct local excited (LE) state and charge‐transfer (CT) state distributions in excited states are designed and synthesized. They show prominent hybridized local and charge‐transfer (HLCT) states and aggregation‐induced emission enhancement properties. The “hot exciton” mechanism based on these emitters reveals that a balanced LE/CT distribution can simultaneously boost photoluminescence efficiency and exciton utilization. In particular, a nearly 100% exciton utilization is achieved in the electroluminescence (EL) process of 2CzPh‐CNNPI. Moreover, employing 2CzPh‐CNNPI as the emitter, emissive dopant, and sensitizing host, respectively, the EL performances of the corresponding nondoped pure‐blue, doped deep‐blue, and HLCT‐sensitized fluorescent OLEDs are among the most efficient OLEDs with a “hot exciton” mechanism to date. These results could shed light on the design principles for “hot exciton” materials and inspire the development of next‐generation high‐performance OLEDs.     

New Solution‐Processable Electron Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Several new solution‐processable organic semiconductors based on dendritic oligoquinolines were synthesized and were used as electron‐transport and hole‐blocking materials to realize highly efficient blue phosphorescent organic light‐emitting diodes (PhOLEDs). Various substitutions on the quinoline rings while keeping the central meta‐linked tris(quinolin‐2‐yl)benzene gave electron transport materials that combined wide energy gap (>3.3 eV), moderate electron affinity (2.55‐2.8 eV), and deep HOMO energy level (<‐6.08 eV) with electron mobility as high as 3.3 × 10^?3 cm² V^?1 s^?1. Polymer‐based PhOLEDs with iridium (III) bis(4,6‐(di‐fluorophenyl)pyridinato‐N,C^2′)picolinate (FIrpic) blue triplet emitter and solution‐processed oligoquinolines as the electron‐transport layers (ETLs) gave luminous efficiency of 30.5 cd A^?1 at a brightness of 4130 cd m^?2 with an external quantum efficiency (EQE) of 16.0%. Blue PhOLEDs incorporating solution‐deposited ETLs were over two‐fold more efficient than those containing vacuum‐deposited ETLs. Atomic force microscopy imaging shows that the solution‐deposited oligoquinoline ETLs formed vertically oriented nanopillars and rough surfaces that enable good ETL/cathode contacts, eliminating the need for cathode interfacial materials (LiF, CsF). These solution‐processed blue PhOLEDs have the highest performance observed to date in polymer‐based blue PhOLEDs.     

The Root Causes of the Limited Stability of Solution‐Coated Small‐Molecule Organic Light‐Emitting Devices: Faster Host Aggregation by Exciton–Polaron Interactions

The degradation mechanism is compared in organic light‐emitting devices (OLEDs) fabricated by solution‐coating to that in vacuum‐deposited OLEDs. Devices comprising various host materials made by vacuum‐deposition or solution‐coating are investigated. Changes in devices electroluminescence (EL) spectra during prolonged electrical driving are compared and analyzed. Hole‐only devices are also utilized, and employed to study the effects of charges and excitons, separately and combined. The results reveal that the faster degradation of solution‐processed devices relative to their vacuum‐deposited counterparts under electrical stress is due to a faster aggregation of the host materials. Interactions between excitons and polarons in the emitting layers of the devices induce this aggregation phenomenon. Although this phenomenon affects both vacuum‐deposited and solution‐coated emitting layers, it is found to occur much faster in the later. The findings shed light on the root causes of the limited stability of solution‐processed OLEDs.     

Nonfullerene Polymer Solar Cells based on a Perylene Monoimide Acceptor with a High Open‐Circuit Voltage of 1.3 V

Nonfullerene polymer solar cells (PSCs) are fabricated with a perylene monoimide‐based n‐type wide‐bandgap organic semiconductor PMI‐F‐PMI as an acceptor and a bithienyl‐benzodithiophene‐based wide‐bandgap copolymer PTZ1 as a donor. The PSCs based on PTZ1:PMI‐F‐PMI (2:1, w/w) with the treatment of a mixed solvent additive of 0.5% N ‐methyl pyrrolidone and 0.5% diphenyl ether demonstrate a very high open‐circuit voltage (V _oc) of 1.3 V with a higher power conversion efficiency (PCE) of 6%. The high V _oc of the PSCs is a result of the high‐lying lowest unoccupied molecular orbital (LUMO) of ?3.42 eV of the PMI‐F‐PMI acceptor and the low‐lying highest occupied molecular orbital (HOMO) of ?5.31 eV of the polymer donor. Very interestingly, the exciton dissociation efficiency in the active layer is quite high, even though the LUMO and HOMO energy differences between the donor and acceptor materials are as small as ≈0.08 and 0.19 eV, respectively. The PCE of 6% is the highest for the PSCs with a V _oc as high as 1.3 V. The results indicate that the active layer based on PTZ1/PMI‐F‐PMI can be used as the front layer in tandem PSCs for achieving high V _oc over 2 V.     

Molecular Engineering of Host Materials for High‐Performance Phosphorescent OLEDs: Zig‐Zag Conformation with 3D Gridding Packing Mode Facilitating Charge Balance and Quench Suppression

A group of bipyridine/carbazole hybrid compounds, namely m‐BPyDCz, p‐BPyDCz, m‐BPySCz, and p‐BPySCz, are designed and developed as host materials for phosphorescent organic light‐emitting diodes (PhOLEDs). By tuning the p/n molar ratio and para‐/meta‐ substitution style, scorpion‐, Y‐, Z‐, and L‐shape molecular conformations are generated. In virtue of intermolecular hydrogen bonds and π–π interaction, these compounds form different molecular packing patterns in their single crystals. Particularly the Z‐shaped m‐BPySCz achieves 3D gridding packing with regular and ordered carbazole and pyridine columns as carrier hoping channels and larger intermolecular distance, which not only guarantees charge balance but also suppresses exciton quenching. Consequently the m‐BPySCz hosted sky‐blue and green PhOLEDs exhibit high external quantum efficiencies of 27.3% and 28.0% and low efficiency roll‐offs of 8.1% (at brightness of 1000 cd m^?2 for blue) and 14.3% (at 10000 cd m^?2 for green), all superior to other analogs and many reported host materials. The excellent performance of m‐BPySCz versus its lowest molecular weight and lowest amorphous stability manifests that the molecular packing style of host material dominates to determine the overall performance of PhOLEDs and the 3D gridding packing mode of zig‐zag conformation may be one ideal strategy to eliminate efficiency roll‐off in PhOLEDs.     

Tuning Contact Resistance in Top‐Contact p‐Type and n‐Type Organic Field Effect Transistors by Self‐Generated Interlayers

Contact resistance significantly limits the performance of organic field‐effect transistors (OFETs). Positioning interlayers at the metal/organic interface can tune the effective work‐function and reduce contact resistance. Myriad techniques offer interlayer processing onto the metal pads in bottom‐contact OFETs. However, most methods are not suitable for deposition on organic films and incompatible with top‐contact OFET architectures. Here, a simple and versatile methodology is demonstrated for interlayer processing in both p‐ and n‐type devices that is also suitable for top‐contact OFETs. In this approach, judiciously selected interlayer molecules are co‐deposited as additives in the semiconducting polymer active layer. During top contact deposition, the additive molecules migrate from within the bulk film to the organic/metal interface due to additive‐metal interactions. Migration continues until a thin continuous interlayer is completed. Formation of the interlayer is confirmed by X‐ray photoelectron spectroscopy (XPS) and cross‐section scanning transmission electron microscopy (STEM), and its effect on contact resistance by device measurements and transfer line method (TLM) analysis. It is shown that self‐generated interlayers that reduce contact resistance in p‐type devices, increase that of n‐type devices, and vice versa, confirming the role of additives as interlayer materials that modulate the effective work‐function of the organic/metal interface.