Work-function-controlled operation mode transition between photodiode and photoconductor modes in organic photodetectors

The organic photodetectors (OPDs) can be proposed in two different ways: a photodiode system or a photoconductor type, where the external quantum efficiency (EQE) cannot exceed 100% in the former one but it can in the latter one. Whether the electrode-active layer contacts can inject at least one kind of carriers is the critical factor to determine the specific photodetector type. Therefore, the role of electrode/semiconductor contacts in the OPDs needs to be fully understood. In this work, we demonstrate the high performance OPDs based on PTB7:PC₇₁BM bulk heterojunction with the different top metal electrodes (Ag, Au, Al) by incorporating the PFN modified ITO electrode. Ultraviolet photoelectron spectroscopy shows that the work function of ITO is effectively reduced from 4.7 eV to 4.1 eV due to the PFN dipole layer. Combined with current density-voltage characteristics and the EQE, the results indicate the devices show different working modes: a photodiode or photoconductor type OPDs. For the Ag or Au electrode device, the photogenerated charges can rapidly transport to the corresponding electrodes and then to be collected under reverse bias, which meets the photodiode type. However, the Al electrode device with more than 100% EQE presents the photoconductor mode, which is contributed to that the photogenerated charges accumulate around the interface between the PFN layer and the active layer, and then create a tunneling charge injection under reverse bias. Although both the proposed photodiode and photoconductor devices exhibit large signal-to-noise ratio (10⁴∼10⁵), fast response (∼μs), and low working voltage (−0.5 V), these two type devices show their unique strengths due to the different working modes. The photodiode devices have higher detectivity (more than 10¹³ Jones) and broader linear dynamic range (over 120 dB), while the photoconductor ones possess more than 100% EQE and the better responsivity (0.56 A/W). This work may pave a way to obtain the desirable working mode of the photodetectors by simply tuning the electrode work function.     

Photophysical Study of DPPTT‐T/PC70BM Blends and Solar Devices as a Function of Fullerene Loading: An Insight into EQE Limitations of DPP‐Based Polymers

Diketopyrrolopyrrole (DPP)‐based polymers have been consistently used for the fabrication of solar cell devices and transistors due to the existence of intermolecular short contacts, resulting in high electron and hole mobilities. However, they also often show limited external quantum efficiencies (EQEs). In this contribution, the authors analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation, and recombination kinetics in thin films and solar devices of a DPP‐based donor polymer, DPPTT‐T (thieno[3,2‐b]thiophene‐diketopyrrolopyrrole copolymer) blended with varying weight fractions of the fullerene acceptor PC₇₀BM. From the correlations between photoluminescence quenching, transient absorption studies, and EQE measurements, it is concluded that the main limitation of photon‐to‐charge conversion in DPPTT‐T/PC₇₀BM devices is poor exciton dissociation. This exciton quenching limit is related not only to the low affinity/miscibility of the materials, as confirmed by wide angle X‐ray diffraction diffraction and transmission electron microscopy data, but also to the relatively short DPPTT‐T singlet exciton lifetime, possibly associated with high nonradiative losses. A further strategy to improve EQE in this class of polymers without sacrificing the good extraction properties in optimized blends is therefore to limit those nonradiative decay processes.     

En Route to Wide Area Emitting Organic Light-Emitting Transistors for Intrinsic Drive-Integrated Display Applications: A Comprehensive Review

Organic light-emitting transistors (OLET) evolved from the fusion of the switching functionality of field-effect transistors (FET) with the light-emitting characteristics of organic light-emitting diode (OLED) that can simplify the active-matrix pixel device architecture and hence offer a promising pathway for future flat panel and flexible display technology. This review systematically analyzes the key device/molecular engineering tactics that assist in improving the electrode edge narrow emission to wide-area emission for display applications via three different topics, that is, narrow to wide-area emission, vertical architecture, and impact of high-κ dielectric on the device performance. Source–drain electrode engineering such as symmetric/asymmetric, planar/non-planar arrangement, semitransparent nature, multilayer approach comprising charge transport, and work function modification layers enable widening the emission zone. Vertical OLET architecture offers short channel lengths with a high aperture ratio, pixel type area emission, and stable light-emitting area. Transistors utilizing high-κ dielectric materials have assisted in lowering the operating voltage, enhancing luminance and air stability. The promising development in achieving wide-area emission provides a solid basis for constructing OLET research toward display applications; however, it relies on developing highly luminescent and fast charge transporting materials, suitable semitransparent source/drain electrodes, high-κ -dielectrics, and device architectural engineering.     

阴极蒸镀和隔离层对有机发光二极管性能的影响

制备了简单结构的有机发光二极管(OLED)ITO/NPB/Alq3/Al/Ag。实验结果表明,快速蒸镀法制备的Ag阴极越厚,器件性能越差,而慢速蒸镀200nmAg阴极时器件性能也较差。在Alq3与Al阴极之间插入BCP/C60/LiF隔离层后,即使快速蒸镀法制备的Ag厚达280nm,器件的最大电流密度、最大亮度和最大电流效率仍分别高达248.6mA/cm2、5380.7cd/m2和3.52cd/A。隔离层不仅保护NPB和Alq3基本不被玻璃化,还很好地与Alq3和Al阴极匹配,大大提高了器件性能。     

Highly efficient quasi-two dimensional perovskite light-emitting diodes by phase tuning

Tetraphenylphosphonium chloride (TPPCl) is used as an additive in the antisolvent for preparing the quasi-two Dimensional (quasi-2D) perovskite film. This strategy is not only beneficial for the morphology formation but also the phase tuning of the quasi-2D perovskite film. Highly efficient and stable perovskite light-emitting diodes (PeLEDs) were achieved with the maximum luminance of 35,000 cd/m², the maximum current efficiency of 48.0 cd/A and the maximum external quantum efficiency (EQE) of 12.42%. Due to the reduced exciton quenching rate, improved charge carrier injection and transport ability, the electroluminescent performance of the TPPCl-based PeLEDs has been enhanced.     

Operational lifetime improvement of solution-processed OLEDs: Effect of exciton formation region and degradation analysis by impedance spectroscopy

It is known that the lifetime of the organic light-emitting diode device manufactured by the solution process is deteriorated due to the problem of mixing the interface between the hole transport layer and the emitting layer. We found that moving the recombination area away from the interface mixing zone as above doubled the efficiency (22.8 cd/A → 51.5 cd/A, 7.6% EQE → 14.6% EQE) and tripled the lifetime (14 h → 42 h). The reason was mainly attributed to the suppression of degradation due to exciton-polaron quenching at the mixing interface. Especially, the degradation of solution-processed devices has not been widely discussed. Therefore, in this study, we investigated thoroughly the deterioration of those devices by impedance spectroscopy and molecular simulation. The trap sites were revealed to present at the mixing zone when a large amount of charges accumulating there. These traps potentially resulted from the fragment of molecules undergoing the bond dissociation due to quenching of exciton and negative charge. Furthermore, by fitting Cole-Cole plots, we observed that the most stress region after half-lifetime test was the mixing zone. To confirm which molecules have a high possibility to dissociate, we calculated the bond dissociation energy of the possible dissociated bonds. The dissociation of host molecules from the anionic excited state due to quenching of exciton and negative polaron played the main role in device degradation. Meanwhile, the dissociation of hole transport molecules in the anionic charged states could also affect device lifetime by slow degradation.     

Management of Host–Guest Triplet Exciton Distribution for Stable,High-Efficiency,Low Roll-Off Solution-Processed Blue Organic Light-Emitting Diodes by Employing Triplet-Energy-Mediated Hosts

High-quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll-off blue solution-processed organic light-emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T₁ state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high-performance blue OLED devices. The smaller T₁ energy gap between host and guest allows the quenching of long-lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet-triplet and triplet-triplet annihilations. Triplet-energy-mediated hosts with high enough T₁ and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQE_max) of 20.8%, long operational lifetime (T₅₀ of 398.3 h @ 500 cd m⁻²), and negligible efficiency roll-off (EQE of 20.1% @ 1000 cd m⁻²) are achieved for bluish-green TADF OLEDs. Additionally introducing a narrowband emission multiple-resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQE_max of 23.1%, suppressed roll-off and extended lifetime of 456.3 h are achieved for the sky-blue sensitized OLEDs at the same brightness.     

Achieving Above 60% External Quantum Efficiency in Organic Light‐Emitting Devices Using ITO‐Free Low‐Index Transparent Electrode and Emitters with Preferential Horizontal Emitting Dipoles

Comprehensive theoretical and experimental studies are reported on organic light‐emitting devices (OLEDs) adopting either the conventional high‐index indium tin oxide (ITO) electrode or the low‐index conducting polymer electrode, either isotropic emitters or emitters having preferentially horizontal emitting dipoles, and different layer structures. Intriguingly, with the use of low‐index electrode in the device, in addition to the known suppression of waveguided modes, the surface plasmon modes can also be effectively suppressed with larger emitter‐to‐metal distances yet with better immunity to accompanied increase of the competing waveguided modes (induced by thicker organic layers) as in the ITO device. As a result, overall coupling efficiencies of OLED internal radiation into substrates can be significantly enhanced over those with ITO electrodes. Through effective extraction of radiation within substrates, green phosphorescent OLEDs adopting both the low‐index ITO‐free electrode and the preferentially horizontal dipole emitter (with a horizontal dipole ratio of 76%) achieve a high external quantum efficiency (EQE) of up to ≈64%. The simulation also predicts that very high EQEs of ≥80% are possible with highly horizontal dipole emitters for all red/green/blue/white OLEDs, clearly revealing the potential of combining low‐index transparent electrodes and horizontal dipole emitters for high‐efficiency OLEDs.     

Exciton and Polaron Quenching in Doping‐Free Phosphorescent Organic Light‐Emitting Diodes from a Pt(II)‐Based Fast Phosphor

By introducing a neat Pt(II)‐based phosphor with a remarkably short decay lifetime, a simplified doping‐free phosphorescent organic light‐emitting diode (OLED) with a forward viewing external quantum efficiency (EQE) and power efficiency of 20.3 ± 0.5% and 63.0 ± 0.4 lm W^?1, respectively, is demonstrated. A quantitative analysis of how triplet‐triplet annihilation (TTA) and triplet‐polaron annihilation (TPA) affect the device EQE roll‐off at high current densities is performed. The contributions from loss of charge balance associated with charge leakage and field‐induced exciton dissociation are found negligible. The rate constants k_TTA and k_TPA are determined by time‐resolved photoluminescence experiments of a thin film and an electrically‐driven unipolar device, respectively. Using the parameters extracted experimentally, the EQE is modeled versus electric current characteristics of the OLEDs by taking both TTA and TPA into account. Based on this model, the impacts of the emitter lifetime, quenching rate constants, and exciton formation zone upon device efficiency are analyzed. It is found that the short lifetime of the neat emitter is key for the reduction of triplet quenching.     

Light extraction enhancement in organic light-emitting diodes through polyimide/porous silica hybrid films

In this work, light extraction efficiency of organic light-emitting diodes (OLEDs) with a spin-coated polyimide/porous silica hybrids were enhanced. The polyimide/porous silica thin films (C1 and C2) were accomplished by spin coating a hybrid film composed of a polyimide-silica composite blended with various amount of porous silica nanoparticles into the opposite site of ITO glass. The optical, thermal, and morphology properties were controlled by adding various amount of porous silica. The incorporation of light extraction layer improved the maximum external quantum efficiency (EQE) by as much as 24.8% based on integrating sphere measurement condition when compared to that of a reference device without a light extraction layer. Furthermore, the device utilizing the light extraction layer showed identical EL spectra as the reference device did. Additionally, the optical emission distribution of the device was close to the Lambertian profile. The polyimide/porous silica hybrids demonstrated in this work are useful and efficient for OLED device for the enhancement of EL performance, indicating the potential in a wide range of applications, such as displays, lightings and so on.     

接触电极功函数对反式锡基钙钛矿太阳能电池性能的影响

因反式锡基钙钛矿太阳能电池可避免J-V迟滞以及铅元素,基于SCAPS-1D设计结构为ITO/HTL/CH3NH3SnI3/PCBM/back-contact的反式锡基钙钛矿太阳能电池器件.其中NiO、Cu2O以及P3HT分别作为空穴传输层,探讨导电玻璃ITO功函数在4.6?5.0 eV范围内电池性能的变化,并分析Al、...     

Visible-light photoelectrocatalytic degradation of rhodamine B over planar devices using a multi-walled carbon Nanotube-TiO2 composite

Planar devices were prepared via immobilizing multiwalled carbon nanotubes (MWCNTs) and titanium dioxide (TiO₂) sequentially on a glass substrate precovered with double symmetric-striped indium tin oxide (ITO) electrodes, which were denoted as the composite device of ITO/MWCNTs-TiO₂/ITO. The MWCNTs-TiO₂ composite film was grown into a bulk heterojunction structure, thereby offering the efficient exciton dissociation and charge transport. Compared with the blank device of ITO/TiO₂/ITO, the composite device of ITO/MWCNTs-TiO₂/ITO exhibited a much stronger visible response and promoted photo-electrocatalytic (PEC) degradation of rhodamine B 9.6 times under visible light irradiation. In addition, the composite device remained stable while performing PEC degradations of rhodamine B five times.     

Copper(I) Complex as Sensitizer Enables High-Performance Organic Light-Emitting Diodes with Very Low Efficiency Roll-Off

Organic light-emitting diodes (OLEDs) utilizing purely organic thermally activated delayed fluorescence (TADF) sensitizers have recently achieved high efficiencies and narrow-band emissions. However, these devices still face intractable challenges of severe efficiency roll-off at practical luminance and finite operational lifetime. Herein, a carbene-Cu(I)-amide complex, (MAC*)Cu(Cz), is demonstrated as a TADF sensitizer for both fluorescent and TADF OLEDs. The (MAC*)Cu(Cz)-sensitized fluorescent OLED not only achieves a high external quantum efficiency (EQE) of 14.6% with an extremely low efficiency roll-off of 12% at the high luminance of 10 000 nits, but also delivers a 15 times longer operational lifetime than that of the non-sensitized reference device. More importantly, utilizing the (MAC*)Cu(Cz) sensitizer in the multi-resonance (MR) TADF OLED results in a record-high EQE of 26.5% together with a full-width at half maximum of 46 nm and an emission peak at 566 nm. This value is the state-of-the-art efficiency for yellow-emitting MR-TADF OLEDs. The photophysical analysis proved that the fast reverse intersystem crossing process of (MAC*)Cu(Cz) is the key factor to suppress triplet exciton involved quenching at high luminance. This finding firstly demonstrates the use of Cu(I) complex as an efficient TADF sensitizer and paves the way for practical applications of TADF sensitized OLEDs.     

High efficiency and non-color-changing orange organic light emitting diodes with red and green emitting layers

We report a high performance orange organic light-emitting diode (OLED) where red and green phosphorescent dyes are doped in an exciplex forming co-host as separate red and green emitting layers (EMLs). The OLED shows a maximum external quantum efficiency (EQE) of 22.8%, a low roll-off of efficiency with an EQE of 19.6% at 10,000 cd/m², and good orange color with a CIE coordinate of (0.442, 0.529) and no color change from 1000 to 10,000 cd/m². The exciplex forming co-host system distributes the recombination zone all over the EMLs and reduces the triplet exciton quenching processes.     

Solution‐Processed Thermally Activated Delayed Fluorescent OLED with High EQE as 31% Using High Triplet Energy Crosslinkable Hole Transport Materials

Solution‐processed organic light‐emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) material as emitter have attracted much attention because of their low cost and high performance. However, exciton quench at the interface between the hole injection layer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and emitting layer (EML) in devices can lead to low device performance. Here, a novel high triplet energy (2.89 eV) and crosslinkable hole‐transporting material grafted with oxetane groups, N,N‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy)hexyloxy)phenyl)‐3,5‐di(9H‐carbazol‐9‐yl)benzenamine (Oxe‐DCDPA)), as crosslinked hole transport layer (HTL) into the interface of PEDOT:PSS layer and EML is proposed for prevention of exciton quenching, and among the reported devices with single HTL in solution‐processed TADF‐OLED, the highest external quantum efficiency (EQE)/luminous efficiency (η_L) of 26.1%/94.8 cd A^?1 and 24.0%/74.0 cd A^?1 are achieved for green emission (DACT‐II as emitter) and bluish‐green emission (DMAC‐TRZ as emitter), respectively. Further improvement, using double HTLs, composed of N,N′‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy))‐hexylphenyl)‐N,N′‐diphenyl‐4,4′‐diamine with high hole mobility and Oxe‐DCDPA with high triplet energy, leads to the highest EQE/η_L of 30.8%/111.9 cd A^?1 and 27.2%/83.8 cd A^?1 for green emission and bluish‐green emission, respectively. These two devices show the high maximum brightness of 81 100 and 70 000 cd m^?2, respectively.     

Single layer deep blue polymer light emitting diodes with chlorinated Indium Tin Oxide after surface modification for high performance

Chlorinated-Indium Tin Oxide (Cl-ITO) has been found to have higher work function than the pristine ITO. When used as anode for polymer light-emitting diodes (PLEDs) with deep blue emitting β-phase poly(9,9-di-n-octylfluorene) (β-PFO) as the emitting layer, it allows hole injection without barrier. However, the presence of chlorine free radical on the surface of Cl-ITO leads to an exciton quenching effect. The surface modification on anode by dipping into 1% ammonium (NH₄OH) aqueous solution to remove free chlorine radicals on surface can enhance device performance significantly. The single layer device Cl-ITO (treated)/β-PFO/CsF/Al gives the maximum brightness 16773 cd/m² and maximum luminance efficiency 2.40 cd/A, which are higher than those with untreated Cl-ITO, (2519; 0.27) and CFx treated ITO, (7800; 1.8) and PEDOT:PSS coated ITO, (12884; 1.43). The ease of the present anode treatment allows the one-layer-only device to be a promising candidate for practical application.     

Inkjet-printed Ag grid combined with Ag nanowires to form a transparent hybrid electrode for organic electronics

Networks of silver nanowires (AgNW) have been shown to facilitate high transparency, high conductivity, and good mechanical stability. However,the loose characteristic and local insulation problems due to gaps between the nanowires limit their application as electrodes. This study investigates an inkjet-printed Ag grid combined with AgNW to form a transparent hybrid electrode. The printed Ag grid on AgNW film connects the gaps between the Ag nanowires to increase the overall electric conductivity. The printed Ag-grid/AgNW hybrid electrodes have low resistivity (22.5 Ω/□) while maintaining a high transmittance (87.5%). These values are similar to standard indium tin oxide (ITO) on glass which has resistivity of 20Ω/□ and transmittance of 89% at 550 nm. In addition, these hybrid electrodes are also very flexible when fabricated on a photopolymer substrate. A spin-coating process combined with a peel-off process enable the fabrication of flexible ultra-smooth Ag-grid/AgNW electrodes. We tested the transparent and flexible electrode as the anode of a flexible organic light emitting diode (F-OLED). The light emitting layer of the F-OLED is 35 nm thick tris-(8-hydroxyquinoline) aluminum doped with 0.5% 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-I,j)quinolizin-11-one. The maximum brightness and current efficiency of the F-OLED are 10000 cd/m² and 12 cd/A, respectively, even when bent around a radius of 2 mm. The good performance of the device with Ag-grid/AgNW hybrid electrodes show that enhanced conductive inkjet-printed Ag nanoparticles combined with Ag nanowires can produce high quality electrodes for flexible organic optoelectronic devices.     

顶部发射绿色磷光OLED的制备与瞬态性能研究

用磷光材料Ir(ppy)3制备了高效率顶部发射绿色有机发光二极管(OLED),器件的结构为:ITO/Ag/NPB/Ir(ppy)3(5wt%):TPBI/TPBI/LiF/Al。研究发现与传统的无微腔结构器件相比顶部发射器件的性能有大幅度提高,其最大效率为18cd/A。通过使用F-P腔,器件的电致发光(EL)寿命由7.6μs降低为7.1μs,有效地缓解了效率随电流密度增大而下降的问题。顶部发射器件EL共振的主峰位于505nm处,发射光谱半峰宽(FWHM)窄化为23nm,色纯度为(x=0.122,y=0.671),发射光随探测角度变化较小。最后,分析了其瞬态光电性能变化原因。     

Numerical Study on Quantum Efficiency Enhancement of a Light-Emitting Diode Based on Surface Plasmon Coupling With a Quantum Well

We demonstrate the numerical study results of the enhancements of internal quantum efficiency (IQE) and external quantum efficiency (EQE) of a semiconductor quantum well when it is coupled with surface plasmons (SPs) induced on a grating interface between Ag and semiconductor. The IQE and EQE enhancements depend on the emission dipole position and the assigned intrinsic IQE. The SP dissipation in metal and the grazing-angle SP radiation lead to a significant difference between IQE and EQE. The enhancement of EQE is less significant when the intrinsic IQE becomes larger. In applying the SP coupling phenomenon to an InGaN-GaN quantum-well light-emitting diode, the efficiency enhancement is more significant in the green-red range, in which the intrinsic IQE is normally quite low.     

Enhance external quantum efficiency of organic light-emitting devices using thin transparent electrodes

High-index transparent electrodes have been one major origin of light trapping and lower light extraction in organic light emitting diodes (OLEDs). In this work, influences of the bottom transparent electrode thickness on emission properties of OLEDs are systematically studied by both simulation and experiments. Simulation shows that with substantially decreasing the thickness of the high-index indium tin oxide (ITO) electrode, waveguided modes, that otherwise would be significantly induced in regular/thicker ITO devices, can be effectively eliminated. Consequently, the overall coupling efficiencies of OLED emission into substrates can be much enhanced. Through further effective light extraction from the substrate, green phosphorescent OLEDs with a high external quantum efficiency (EQE) of up to ≈57.5% were experimentally demonstrated by adopting the very thin (20 nm) ITO electrode and preferentially horizontal dipole emitters (with a horizontal dipole ratio of 76%). The simulation further predicts that very high optical coupling efficiencies into substrates and EQEs approaching 80% are possible with further adopting purely horizontal dipole emitters and/or low-index electron transport layer (ETL) to suppress surface plasmon modes. Overall, this study clearly reveals the potential of using thin transparent electrodes for highly efficient OLEDs.