注入和复合对单层有机发光器件电致发光效率的影响

考虑电荷的注入、无序系统中载流子的输运及极化子激子的解离和复合过程,建立了单层有机电致发光器件电致发光效率的理论模型.计算并讨论了离化距离对器件复合效率以及注入和复合对器件电致发光效率的影响.结果表明:(1) 通过降低金属/有机物界面势垒可以显著提高器件的EL效率;(2)当两电极均为欧姆接触或一个电极欧姆接触,一个电极是接触限制时,在低场下EL效率由注入决定,而高场下复合起主要作用;(3)当两电极均为接触限制时,EL效率主要由注入过程来决定.此模型可以较好地解释一些实验现象.     

Fabrication and Characterization of Organic Single Crystal‐Based Light‐Emitting Devices with Improved Contact Between the Metallic Electrodes and Crystal

Organic single crystals have attracted great attention because of their advantages of high charge‐carrier mobility, high chemical purity, and potential for flexible optoelectronic devices. However, their intrinsic properties of sensitive to organic solvent and fragile result in a difficulty in the fabrication of the organic crystal‐based devices. In this work, a simple and non‐destructive technique of template stripping is employed to fabricate single‐crystal‐based organic light‐emitting devices (OLEDs). Efficient and uniform carrier injection induced by an improved contact between crystals and both top and bottom electrodes is realized, so that a homogeneous and bright electroluminescence (EL) are obtained. Highly polarized EL and even white emission is also observed. Moreover, the crystal‐based OLEDs exhibit good flexibility, and keep stable EL under a small bending radius and after repeated bending. It is expectable that this technique would support broad applications of the organic single crystals in the crystal‐based optoelectronic devices.     

Performance and stability improvement in organic photovoltaics using non-toxic PEIE interfacial layers

The improvement in current density–voltage characteristics and external quantum efficiency of organic photovoltaics with non-toxic polyethylenimine ethoxylated (PEIE) as a modified layer between the electron transport layer and the active layer were demonstrated. The mechanisms of carrier transport, photon generation current, and the carrier recombination influenced by the PEIE layer were systematically studied. An extra PEIE layer was demonstrated having a shorter relaxation time via pump-probe spectroscopy, which provides better charge transfer ability. From capacitance-voltage measurement, the effective capacitance of the device increase with an addition PEIE layer, corresponding to the increase of accumulation carriers generated at the interfaces. Furthermore, electrochemical impedance spectroscopy elucidated that the PEIE layer can reduce the carrier recombination probability in the devices. As a result, the lifetime of the devices with a PEIE layer is improved as compared to the devices without PEIE layer.     

Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombination Losses

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J_sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J_sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.     

Blue and green organic light-emitting devices with various film thicknesses for color tuning

Blue and green organic light-emitting devices with a structure of indium tin oxide (ITO) /N,N’-bis-(1-naphthyl)-N,N’- diphenyl-1,1’-biphenyl-4,4’-diamine (NPB)/aluminum(III) bis(2-methyl-8-quinolinato)4 –phenylphenolato (BAlq) /tris(8- hydroxyquinolate)-aluminum (Alq3)/Mg:Ag have been fabricated. Blue to green light emission has been achieved with the change of organic film thickness. Based on energy band diagram and charge carrier tunneling theory, it is concluded that the films of different thicknesses play a role as a color-tuning layer and the color-variable electroluminescence (EL) is ascribed to the modulation function within the charge carrier recombination zone. In the case of heterostructure devices with high performance, the observed EL spectra varies significantly with the thickness of organic films, which is resulted from the shift of recombination region site. It has not been hitherto indicated that the devices compose of identical components could be implemented to realize different color emission by changing the film thickness of functional layers.     

Insights into charge balance and its limitations in simplified phosphorescent organic light-emitting devices

Simplified phosphorescent organic light-emitting device (PHOLED), which utilizes only two organic layers, showed record-high efficiency when first introduced. It is quite surprising that this device can have such high efficiency without the use of complex carrier and exciton confinement layers that are common in the state-of-the-art PHOLEDs nowadays. Therefore, it is important to understand how good charge balance is in simplified PHOLED and why. In this work, we study the effects of altering charge balance in simplified PHOLED through means of changing layer thickness in the hole transport layer (HTL) and electron transport layer (ETL) as well as intentionally doping hole and electron traps in the HTL and ETL, respectively, on device efficiency. The results show that when using high carrier mobility charge transport materials, changing layer thickness does not impact charge balance appreciably. On the other hand, introducing charge traps in a thin layer within the HTL or ETL can, in comparison, influence charge balance more significantly, and proves to be a more effective approach for studying the factors limiting charge balance in these devices. The results reveal that simplified PHOLEDs are generally hole-rich, and that the leakage of electrons to the counter electrode is also a major mechanism behind the poor charge balance and efficiency loss in these devices. In order to optimize charge balance in simplified PHOLED, it is important to reduce hole transport in the device so that e-h ratio can be brought closer to unity, as well as eliminate electron leakage. Finally, we show that by simply using an electron blocking HTL, the efficiency of the device can be enhanced by as much as 25%, representing the highest reported for simplified PHOLEDs.     

Analysis of the dynamic short-circuit resistance in organic bulk-heterojunction solar cells: relation to the charge carrier collection efficiency

This work studies the charge carrier collection efficiency in organic bulk-heterojunction solar cells based on polymer:fullerene blends. An equivalent circuit with a specific recombination term is proposed to describe the behavior of this type of devices. It is experimentally shown that this recombination term determines the slope of the current–voltage characteristic at the short-circuit condition. The variation of this dynamic resistance with the light intensity can be interpreted considering a dominant first-order recombination process. Finally, an analytical model under a constant electric field approximation is presented that can be used to calculate the charge carrier collection efficiency of the device. This model can be also used to estimate an effective mobility–lifetime product, which is characteristic of the quality of the active layer.     

Management of charge carriers and excitons for efficient and color-stable white phosphorescent organic light-emitting diodes with simplified structure

We have demonstrated color-stable and highly efficient simplified white phosphorescent organic light-emitting diodes. The key feature is the use of a novel approach to confine the distribution of charge carriers and excitons across the whole blue emission layer. The resulting two-color white device has the maximum power efficiency and current efficiency of 45.5 lm/W and 43.5 cd/A with a very low color shift over a wide range of luminance. By systematically investigating the working mechanisms, we found that the ambipolar charge carrier transport ability of co-host layer which ensures the distribution of excitons to form in the whole blue emission layer was the critical factors for constructing color-stable white devices. Our results show that simplified white devices based on two organic materials achieving excellent color stability are possible.     

Double‐Sided Surface Passivation of 3D Perovskite Film for High‐Efficiency Mixed‐Dimensional Perovskite Solar Cells

Defect‐mediated carrier recombination at the interfaces between perovskite and neighboring charge transport layers limits the efficiency of most state‐of‐the‐art perovskite solar cells. Passivation of interfacial defects is thus essential for attaining cell efficiencies close to the theoretical limit. In this work, a novel double‐sided passivation of 3D perovskite films is demonstrated with thin surface layers of bulky organic cation–based halide compound forming 2D layered perovskite. Highly efficient (22.77%) mixed‐dimensional perovskite devices with a remarkable open‐circuit voltage of 1.2 V are reported for a perovskite film having an optical bandgap of ≈1.6 eV. Using a combination of experimental and numerical analyses, it is shown that the double‐sided surface layers provide effective defect passivation at both the electron and hole transport layer interfaces, suppressing surface recombination on both sides of the active layer. Despite the semi‐insulating nature of the passivation layers, an increase in the fill factor of optimized cells is observed. The efficient carrier extraction is explained by incomplete surface coverage of the 2D perovskite layer, allowing charge transport through localized unpassivated regions, similar to tunnel‐oxide passivation layers used in silicon photovoltaics. Optimization of the defect passivation properties of these films has the potential to further increase cell efficiencies.     

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methylester (PC₇₁BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift–diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80–82% is feasible if the charge mobility is high enough (∼10⁻³–10⁻¹ cm²/V s).     

A charge-separated interfacial hole transport semiconductor for efficient and stable perovskite solar cells

Perovskite solar cells (PSCs) with high efficiency and high stability are still a challenge to produce although remarkable successes have been achieved since they were first reported in 2009. One strategy to effectively improve both the performance as well as the stability is to introduce an interfacial layer between perovskite and hole transport material. Herein, we report a charge-separated (CS) organic semiconductor as the interfacial layer that forms cascaded energy levels between perovskite and hole transportation material. This CS semiconductor displays high hole and electron mobilities by converting long-lived CS states in solution into permanent polarons (charged carriers) in films. Doping with iodinehydride is able to improve the surface morphology of the CS semiconductor layer. Our devices with an iodinehydride-doped CS semiconductor layer exhibit an efficiency of 17.87%, which is increased by ~25% in comparison with 14.24% of the reference devices that have no interfacial layer. This additional CS semiconductor layer also enhances the unsealed device stability by maintaining 90% of initial PCE, while the reference devices degraded by 35% at a relative humidity of 20–30%, temperature of 25 °C and ambient light for 240 h. This result reveals that the utilization of CS states is an alternative approach to construct high charge transport organic semiconductors. An interfacial semiconductor with proper energy level and a matching hole transport mobility can improve the hole extraction, speed up hole transport and suppress charge recombination of PSCs, and thus may be an effective strategy to improve their efficiency and stability.     

Luminescent Efficiency in Single-layer Organic Electrophosphorescent Devices

Based on the charge injection and recombination processes and the triplet-triplet annihilation process, a model to calculate the electro.luminescent（EL） efficiency is presented. The influences of the applied electric field on the injection efficiency, recombination efficiency and electroluminescent efficiency are discussed. It is found that： （1） The injection efficiency is increasing while the recombination efficiency is decreasing with the applied electric field increasing. （2） The EL efficiency is enhanced at low electric field slowly but is decreasing at high electric field with the increase of applied voltage. （3） The EL efficiency is decreasing with the increase of the host-guest molecular distance （R）. So, it is concluded that the EL efficiency in single-layer organic electrophosphorescent devices is dominated by injection efficiency at lower electric field and recombination efficiency at higher electric field.     

Bilayer tellurene-metal interfaces

Tellurene, an emerging two-dimensional chain-like semiconductor, stands out for its high switch ratio, carrier mobility and excellent stability in air. Directly contacting the 2D semiconductor materials with metal electrodes is a feasible doping means to inject carriers. However, Schottky barrier often arises at the metal–semiconductors interface, impeding the transport of carriers. Herein, we investigate the interfacial properties of BL tellurene by contacting with various metals including graphene by using ab initio calculations and quantum transport simulations. Vertical Schottky barriers take place in Ag, Al, Au and Cu electrodes according to the maintenance of the noncontact tellurene layer band structure. Besides, a p-type vertical Schottky contact is formed due to the van der Waals interaction for graphene electrode. As for the lateral direction, p-type Schottky contacts take shape for bulk metal electrodes(hole Schottky barrier heights(SBHs) ranging from 0.19 to 0.35 eV). Strong Fermi level pinning takes place with a pinning factor of 0.02. Notably, a desirable p-type quasi-Ohmic contact is developed for graphene electrode with a hole SBH of 0.08 eV. Our work sheds light on the interfacial properties of BL tellurene based transistors and could guide the experimental selections on electrodes.     

Improved carrier balance and polarized in-plane light emission at full-channel area in ambipolar heterostructure polymer light-emitting transistors

We report polarized in-plane emission at full-channel area in ambipolar heterostructure organic light-emitting transistors (OLETs) utilizing bilayer oriented fluorene-type polymers. The present work demonstrates that carrier balance between bilayers is a key factor in achieving in-plane emission at almost full-channel area when hole transport is dominant in the upper layer, which acts as an electron blocking layer, and electrons are injected into the lower layer. Hole accumulation in the upper layer affects electron injection from the electrode to the lower layer and the probability of recombination of holes in the upper layer with electrons in the lower layer near the bilayer interface. For OLETs having an oriented bilayer with the channel direction parallel to orientation of the polymer chains, a uniform in-plane emission pattern, an increased electroluminescence intensity, and a high external quantum efficiency can be achieved by improving the hole mobility in the upper layer and optimizing the carrier balance. These results are expected to be very useful for the development of surface micro-light sources.     

High current conduction with high mobility by non-radiative charge recombination interfaces in organic semiconductor devices

We report a unique non-radiative p-n-p junction structure to provide high current conduction with high mobility in organic semiconductor devices. The current conduction was improved by increasing p-n junctions made with intrinsic p-type hole transport layer and n-type electron transport layer. The excellent hole mobility of 5.3 × 10^?1 cm²/V s in this p-n-p device configuration is measured by the space charge limited current method with an electric field of 0.3 MV/cm. Enhanced current conduction of 248% at 4.0 V was observed in fluorescent blue organic light-emitting diodes with introduction of non-radiative p-n-p-n-p junction interfaces. Thereupon, the power efficiency at 1000 cd/m² was improved by 22% and the driving voltage also was reduced by 17%, compared to that of no interface device. Such high current conduction with high mobility is attributed to the carrier recombination at p-n-p interfaces through coulombic interaction. This non-radiative p-n-p junction structure suggested in this report can be very useful for many practical organic semiconductor device applications.     

Numerical Analysis on Current Transport Characteristics in Single Layer Organic Electroluminescent Devices

A new model to describe I-V characteristics of organic light-emitting devices (OLEDs) is developed based on experimental results. The dependence of I-V characteristics on energy barrier, trap density and carrier mobility is analyzed. The result shows that this model combines the Fowler-Nordheim tunnel theory and the trap charge limited current theory with exponential trap distribution (TCL), and it describes the current transport characteristics of OLEDs more comprehensively. The I-V characteristics follow Fowler-Nordheim theory when the energy barrier is high, the trap density is small and the carrier mobility is large.In other cases they follow the TCL theory.     

Elucidating the role of current injection on the influence of open-circuit voltage in small-molecule organic photovoltaic devices: From the aspects of charge transfer and electroluminescent spectrum

We demonstrate that the open-circuit voltage (V_OC) of organic photovoltaic (OPV) devices composed of rubrene and C₆₀ can be considerably different when the anode and active layer are changed. Two types of anodes and active layers were compared. In plasma-treated indium-tin-oxide (ITO) OPV devices, the parameter V_OC exhibits an improvement from 0.68 V to 0.76 V when the device structure is varied from a bilayer to a mixed structure. However, in the OPV devices that use ITO/MoO₃ as the anode, a similar V_OC is observed regardless of the device structure. A series of temperature-dependent measurements are conducted to investigate these results. The calculation of barrier height at the rubrene/C₆₀ (or rubrene:C₆₀) interface yields the prediction of V_OC, suggesting that an excess energetic loss occurs in the mixed structures. The electroluminescent (EL) spectra of these devices show that the mixed structure can completely quench the EL of rubrene single layer. A broad band of the charge transfer (CT) emission is observed clearly. A temperature-dependent measurement for the extracting injection barrier is conducted and shows that the mixed structure is favorable for the hole current injection. The CT properties are obtained using the external quantum efficiency and EL spectra of the OPV devices. We find that the nonradiative recombination loss is highly correlated with the injected current; the lower the injection barrier induced the less the nonradiative recombination loss. Therefore, the parameter V_OC can be improved when the injected current is increased.     

Highly efficient blue polyfluorenes using blending materials as hole transport layer

Efficient blue polyfluorenes have been generated by incorporating the hole transport material N-([1,1′-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)- phenyl)-9H-fluoren-2-amine (BCFN) into poly(9,9-dioctylfluorene) (PFO) as an emissive layer. BCFN has an appropriate highest occupied molecular orbital (HOMO) energy level and high hole transport/electron barrier properties, which can effectively reduce the hole injection barrier and improve the charge carrier injection and transport. These properties resulted in a significant improvement in the electroluminescent (EL) performance of PFO. To further improve the EL performance of PFO, the blend hole transport layer, PVK [Poly(N-vinylcarbazole)]:BCFN with weight ratio of 3:7, was inserted between the PEDOT:PSS and the emissive layer. The blend hole transport layer effectively reduced exciton quenching and markedly decreased the hole injected barrier. A maximum luminous efficiency (LE_max) of 4.31 cd A⁻¹ was obtained with the CIE coordinates of (0.17, 0.13). The device maintained a LE_max of 4.27 cd A⁻¹ at a luminance of 1000 cd m⁻². In addition, stable EL spectra were obtained and were nearly identical when the applied voltage was increased from 5 to 11 V. These results indicate that blending the appropriate hole transport material can be an efficient method to improve device performance based on the large band gap of blue-lighting materials.     

Quantifying charge transportation for stable organic solar cells with a novel aluminum acetylacetone electron collection layer

Elucidating charge transportation mechanism for low-temperature solution-processed charge collection layer is crucial for efficient and stable organic solar cells (OSCs). Herein, we present a facile method to fabricate Al(acac)₃ film and integrate it as electron collection layer (ECL) in OSCs. The PM6:Y6-based OSCs with Al(acac)₃ ECL achieve a higher efficiency of 16.12% as compared to the control devices with a 10.90% efficiency. Importantly, the Al(acac)₃-based devices present superior operational stability, which retains 60% of the initial PCE after 100 h continuous illumination in air. Investigations on optics and carrier recombination dynamics reveal that the improved performance benefits from high up to 98% transparency of Al(acac)₃ ECL, optimized interfacial contact and enhanced exciton dissociation ratio from 85.10% to 97.51%. Further studies on quantified interface resistances during the internal charge transfer indirectly reflect the enhancement of stability. These results highlight the promising prospect of Al(acac)₃ ECL in processing compatible, operationally stable high efficiency OSCs.     

Highly Efficient Three Primary Color Organic Single‐Crystal Light‐Emitting Devices with Balanced Carrier Injection and Transport

Organic single crystals have a great potential in the field of organic optoelectronics because of their advantages of high carrier mobility and high thermal stability. However, the application of the organic single crystals in light‐emitting devices (OLEDs) has been limited by single‐layered structure with unbalanced carrier injection and transport. Here, fabrication of a multilayered‐structure crystal‐based OLED constitutes a major step toward balanced carrier injection and transport by introducing an anodic buffer layer and electron transport layer into the device structure. Three primary color single‐crystal‐based OLEDs based on the multilayered structure and molecular doping exhibit a maximum luminance and current efficiency of 820 cd cm^?2 and 0.9 cd A^?1, respectively, which are the highest performance to date for organic single‐crystal‐based OLEDs. This work paves the way toward high‐performance organic optoelectronic devices based on the organic single crystals.