Exciton Blocking Research Articles

Visualizing the “Hidden” Triplet–Triplet Fusion Process to Fluorescence in Typical Organic/Polymer Light‐Emitting Diodes

AbstractFluorescence emission of a typical poly(9,9‐dialkylfluorene) derivative (PF Green B)‐based polymer light‐emitting diodes (PLEDs) clearly demonstrates that it partially involves the contribution of triplet excitons by the triplet–triplet fusion (TTF) process through measuring the magneto‐electroluminescence (MEL) responses of devices at different bias conditions and temperatures. The TTF process to fluorescence intrinsically correlates with the concentration, lifetime, and polaron quenching of triplet excitons, as modulated by different bias regimes and conditions. Varying the cathode configurations of PLEDs by an additional exciton blocking layer and cathode buffer layers in PLEDs, such as a thin layer of bathocuproine, tetractylammonium bromide, or poly(ethylene glycol) dimethyl ether with metal cathodes, changes the carrier dynamics and regulates the magnitude of TTF process to fluorescence. The performance of the devices has increased due to the enhanced TTF process in fluorescence, as characterized by the measurement of MEL responses. The results in this work unveil the “hidden” component of fluorescence emission, which originates from the fusion of triplet excitons to singlet excitons in typical PLEDs. The results elucidate that TTF of triplet excitons to fluorescence emission can be a practicable mechanism that contributes to enhancing the performance of devices.

A design strategy of exciton blocking materials using simulations and the analysis of device properties

A hole transporting type exciton blocking layer (hEBL) is one of the important device architecture components in phosphorescence organic light emitting diodes (PhOLEDs).

Operational Stability Analysis of Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Using the Capacitance-Voltage Method

We analyzed the degradation features by measuring the capacitance–voltage characteristics after electrically aging blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). The measurement was investigated in terms of the hole transfer layer (HTL) and electron transfer layer (ETL) structures. For the HTL, three different materials—N,N′–bis(naphthalen–1–yl)–N,N′–bis(phenyl)–benzidine (NPB), 4,4′,4″-tris(carbazol–9–yl)triphenylamine (TCTA), and 1,3–bis(carbazol–9–yl)benzene (mCP)—were used at the HTL/emission layer (EML) interface; the TCTA/EML interface had the highest stability among the interfaces. For the ETL, bis [2–(diphenylphosphino)phenyl] ether oxide (DPEPO) without further dopants was used as an exciton blocking layer (ExBL) to effectively confine the excitons at the EML. However, DPEPO has low stability and carrier mobility. Therefore, 0, 10, and 40 nm-thick ExBL devices were investigated; it was found that the 0 nm-thick ExBL device was the most stable. However, the 10 nm-thick ExBL is essential to confine the excitons at the EML, which ensures a high EL performance.

Effects of charge mobilities and exciton blocking capabilities of charge blocking layers on the performance of organic light-emitting diodes

Blue thermally activated delayed fluorescence organic light-emitting diodes (OLEDs) have been fabricated using engineered charge blocking layers. It is found that the device efficiency increases with increasing the exciton blocking capability of electron blocking layer. But the device stability decreases concomitantly, mostly because the hole mobility of electron blocking layer decreases with exciton blocking capability increasing, thereby leading to the smaller width of exciton formation zone. In addition, the device stability decreases with the electron mobility of hole blocking layer decreasing, despite nearly same exciton blocking capabilities, mainly because the width of exciton formation zone decreases with the electron mobility of hole blocking layer decreasing. The width of exciton formation zone is understood based on the transit times for holes from anode to exciton formation zone and for electrons from cathode to exciton formation zone. We provide some in-depth insight useful for the delicate design of fluorescent, phosphorescent, and thermally activated delayed fluorescence OLEDs.

Isomer engineering of universal electron transport materials for suppressed exciton quenching in organic light-emitting diodes

A regioisomer of 2,7-bis(4,6-diphenyl-1,3,5-triazin-2-yl)-9,9′-spirobi[fluorene] (m-SBFTrz), 3,6-bis(4,6-diphenyl-1,3,5-triazin-2-yl)-9,9′-spirobi[fluorene] (p-SBFTrz) was developed as a high triplet energy electron transport material by engineering the substitution position of diphenyltriazine to the spirobifluorene core. The p-SBFTrz showed great potential as an electron transport and exciton blocking materials in various devices such as thermally activated delayed fluorescence organic light-emitting diodes (OLEDs) and phosphorescence OLEDs due to increased triplet energy. A large improvement of the external quantum efficiency was achieved using the p-SBFTrz instead of m-SBFTrz by completely suppressing exciton quenching.

Analysis of device performance and thin-film properties of thermally damaged organic light-emitting diodes

This paper reports the variation in the optical and geometrical properties of individual organic layers to be used for thermally damaged top-emission organic light-emitting diodes (TEOLEDs). The copper deposited on the back of TEOLEDs is employed as a thermal facilitator, and a certain thermal damage occurs to the organic layers and devices. The phosphorescent host material 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) is rapidly damaged to a significant extent owing to the low glass transition temperature (T g ), which also changes its optical and geometrical surface properties. Although the optical properties of the hole transport layer, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) were changed slightly, the surface morphology was changed significantly. Despite having a higher T g , the exciton blocking layer, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), shows notable variations in optical properties and surface morphology due to heat exposure. Surprisingly, the electroluminescence spectra and micro-cavity are affected by increasing temperature without any considerable changes in device performance. Hence, this study reveals that besides T g , the surface morphologies and thicknesses of the organic layers are also important factors in the annealing process and play a vital role in causing thermal damage to TEOLEDs. • Surface morphology variations with respect to temperature in the TEOLEDs. • The variation in optical and geometrical properties of organic layers depending on the thickness. • CBP, TCTA and NPB showing notable damage to the properties by thermal exposure. • The color purity of TEOLEDs was affected by micro-cavity after thermal exposure.

12Н-quinoxaline[2,3-b]phenoxazines: Synthesis, optical, electrochemical properties and insight into photovoltaic application

A series of N-aryl derivatives of heteropentacyclic 12Н-quinoxaline[2,3-b]phenoxazine (QOPO) system were synthesized and spectral, luminescent, electrochemical and optoelectronic properties of the prepared compounds studied. The compounds are well soluble in nonpolar and polar solvents, absorb in the strongest emissive part of the solar spectrum and exhibit sufficiently high photostability at air conditions. The HOMO, LUMO energy levels of the obtained QOPOs were calculated by the DFT B3LYP/6–311++G(d,p) method and assessed using the electrochemical methodology. The double-layered p-n heterojunction organic solar cells constructed on the basis of QOPOs as electron donor components, bathocuproine as the exciton blocking layer and fullerene C60 as electron acceptor displayed promising values of photovoltaic (open circuit voltage and short-circuit current density) parameters.

Improvement of inverted planar heterojunction solar cells efficiency by using KI/Alq3 hybrid exciton blocking layer

The exciton blocking layer (EBL) as an interfacial layer is extremely critical in determining the organic photovoltaic cell (OPV) performances. Here, we studied inverted planar heterojunction solar cells PHJ-OPVs with the following configuration ITO/EBL/C60/CuPc/MoO3/Al. Upon the EBL functionality which can act as an exciton blocking layer and allows the electron collection at the cathode, we proposed the insertion of hybrid EBL consisted of KI/Alq3 thin layer. The Alq3 is known as an EBL due to its broad bandgap, whereas we found that when a thin layer of 1 nm of KI is introduced in ITO/Alq3 interface, the KI decomposed during the thermal deposition, and only potassium interacts and diffuses in the Alq3 layer, which effectively enhances the electrons collection at the ITO/C60 interfaces leading to the improvement of open-circuit voltage (Voc), and device power conversion efficiency by 36% than the device using Alq3 alone as EBL.

Power Conversion Efficiency Improvement of Planar Organic Photovoltaic Cells Using an Original Hybrid Electron-Transporting Layer.

In organic photovoltaic (OPV) cells, besides the organic active layer, the electron-transporting layer (ETL) has a primordial role in transporting electrons and blocking holes. In planar heterojunction-OPVs (PHJ-OPVs), the ETL is called the exciton blocking layer (EBL). The optimum thickness of the EBL is 9 nm. However, in the case of inverted OPVs, such thickness is too high to permit efficient electron collection, due to the fact that there is no possibility of metal diffusion in the EBL during the top metal electrode deposition. In the present work, we show that the introduction of a thin potassium layer between the indium tin oxide (ITO) cathode and the EBL increases dramatically the conductivity of the EBL. We demonstrate that K not only behaves as a simple ultrathin layer allowing for the discrimination of the charge carriers at the cathode/organic material interface but also by diffusing into the EBL, it increases its conductivity by 3 orders of magnitude, which allows us to improve the shape of the J–V characteristics and the PHJ-inverted OPV efficiency by more than 33%. Moreover, we also show that PHJ-inverted OPVs with K in their EBLs are more stable than those with Alq3 alone.

An indenocarbazole-based host material for solution processable green phosphorescent organic light emitting diodes.

We designed and synthesized a new host material with a highly soluble and thermally stable indenocarbazole derivative (7,7-dimethyl-5-phenyl-2-(9-phenyl-9H-carbazol-3-yl)-5,7-dihydro-indeno[2,1-b]carbazole) that can make green phosphorescent organic light-emitting diodes (PHOLEDs) in a solution process. In particular, these are used in a blue common layer structure in which green and red-emitting layers are formed by a solution process and blue common layers are thermally evaporated. The new host material possesses excellent hole transport capability and high triplet energy (T1). Mainly we designed the hole dominant material to keep the exciton forming area away from the hole transport layer (HTL) and emitting layer (EML) interface, an interfacial mixing area to improve device performance. As a result, the greatest lifetime of 1300 hours was achieved and a high current efficiency of up to 66.3 cd A−1 was recorded when we used the optimized device structure of a 5 nm thick bipolar exciton blocking layer (B-EBL). It may be a good agreement of exciton confinement and reduced electron accumulation at the HTL and EML interface.

Analysis of the triplet exciton transfer mechanism at the heterojunctions of organic light-emitting diodes

In this study, a model was proposed to analyze the triplet exciton transfer at the heterojunction interface and emission efficiency. The experimental results obtained for four devices with and without exciton blocking layers (EBLs) were analyzed. Although the incorporation of an EBL improved the peak external quantum efficiency (EQE) due to exciton blocking, the efficiency droop behavior became stronger, and the EQE was worse than that of the reference device without an EBL at current densities over 5 mA cm−2. To understand the physical mechanism underlying this observation, a new carrier transport simulation program was developed. This program considered a multi-Gaussian shape density of states, field-dependent mobility, exciton diffusion, triplet–triplet annihilation, triplet–polaron quenching and triplet exciton transfer at the interface. The simulation results showed that although the excitons were blocked at low current density when an EBL was used, the large exciton density generated at the interface and strong annihilation behavior resulted in a strong droop behavior. The other key factor was that the EBL limits hole injection; this resulted in stronger overflow at high current density, which reduced the efficiency and exhibited such switching behavior.

Rigid Oxygen‐Bridged Boron‐Based Blue Thermally Activated Delayed Fluorescence Emitter for Organic Light‐Emitting Diode: Approach towards Satisfying High Efficiency and Long Lifetime Together

AbstractThermally activated delayed fluorescence (TADF) materials have emerged as an efficient emitter for achieving high efficiency of blue organic light emitting diodes (OLEDs). However, it is challenging to satisfy both high device efficiency and long operational lifetime together. Here, highly efficient and electrochemically stable blue TADF emitter, 5‐(5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10,15‐diphenyl‐10,15‐dihydro‐5H‐diindolo[3,2‐a:3′,2′‐c]carbazole (DBA‐DI) is designed and synthesized for high efficiency and long lifetime OLED. This emitter exhibits high photoluminescence quantum yield of 95.3%, small single‐triplet energy gap of 0.03 eV, short delayed exciton lifetime of 1.25 µs, and high bond dissociation energy (BDE). Also, phosphine oxide free high triplet energy host systems (single and mixed) and exciton blocking layer materials are analyzed using molecular and optical simulations to find an efficient host system with high BDE and suitable emission zone for high efficiency and stable OLEDs. The fabricated OLED with DBA‐DI and high triplet host exhibited a maximum external quantum efficiency (EQE) of 28.1% with blue CIE color coordinates of (0.16, 0.39) and long operational lifetime (LT50) of 329 h at the initial luminance of 1000 cd m−2. Furthermore, the mixed host‐based TADF device showed a slightly lower EQE of 26.4% and almost two times longer lifetime (LT50: 540 h) than the single host device.

Donor/Acceptor Heterojunction Organic Solar Cells

The operation and the design of organic solar cells with donor/acceptor heterojunction structure and exciton blocking layer is outlined and results of their initial development and assessment are reported. Under halogen lamp illumination with 100 mW/cm2 incident optical power density, the devices exhibits an open circuit voltage VOC = 0.45 V, a short circuit current density JSC between 2 and 2.5 mA/cm2 with a fill factor FF ≈ 50%, an external quantum efficiency (electrons/s over incident photons/s) EQE ≈ 5% and a power conversion efficiency of about 0.5%. Measurements of the photoelectrical characteristics with time are also reported, confirming that non encapsulated organic solar cells have limited stability in ambient atmosphere.

High efficiency green perovskite light-emitting diodes based on exciton blocking layer

In recent years, metal halide perovskite materials, owing to their excellent photoelectric properties including high photoluminescence quantum yield, high color purity, tunable band gap, etc., have been regarded as new-generation lighting sources and are widely used to fabricate perovskite light-emitting diodes (PeLEDs). Though great progresses have been made in recent years, neither the efficiency nor stability has not yet reached the requirements of commercialization. Thus, further improvement is needed. In this work, a small organic molecule material, namely 4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) with a wide bandgap and a good hole transport ability, is used as an exciton blocking layer by utilizing the spin-coating method to improve the stability and efficiency of PeLEDs. Highly efficient and stable CsPbBr<sub>3</sub> PeLEDs are finally realized. The physical mechanism related to the improved electroluminescence performance is investigated thoroughly. Firstly, the stepped energy level alignment is formed, since the highest occupied molecular orbital energy level (HOMO) of TAPC is located between the HOMO of (3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT: PSS) and the valence band of CsPbBr<sub>3</sub>, which is beneficial to hole injection and transport. Meanwhile, the lowest unoccupied molecular orbital level of TAPC is high enough to prevent electrons from leaking into the anode effectively and confine electrons and excitons well in the emitting layer. Secondly, the introduction of the TAPC layer can avoid the direct contact between the perovskite light emitting layer and the strong acidic layer of PEDOT:PSS, thereby eliminating the related excitons quenching, which can further increase the radiative recombination.

An efficient CsPbBr3 perovskite light-emitting diode by employing 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene as a hole and exciton blocking layer

An efficient perovkite light-emitting diode (PeLED) is fabricated by incorporating 1,3,5-tri (m-pyrid-3-yl-phenyl)benzene (TmPyPB) between the emitting layer (EML) and the electron transporting layer (ETL), where TmPyPB plays an indispensable blocking effect on both excitons and holes. It shows a maximum current efficiency of 9.37 cd/A, and the corresponding external quantum efficiency (EQE) of 2.42% with a saturated green color Commission International de I'Eclairage (CIE1993) coordinated of (0.1279, 0.7951) under a driving voltage of 6 V. Meanwhile, the operational lifetime of the TmPyPB-based PeLED is improved to about 2.6-fold to that of the one with only the traditional ETL of aluminum 8-hydroxyquinolinate (Alq3). The enhancements of electroluminescence (EL) performance are mainly attributed to better confinement of both the holes and the excitons in the EML, and more balanced carrier transportation in the TmPyPB-based PeLED. This work offers a feasible strategy to develop high performance optoelectronic devices.

Tandem white organic light-emitting diodes stacked with two symmetrical emitting units simultaneously achieving superior efficiency/CRI/color stability

Abstract In this work, a series of three and four-color tandem white organic light-emitting diodes (WOLEDs) are developed by using the optimized charge generating unit (CGU) to connect two electroluminescence (EL) units with symmetrical emitting layers, in which symmetrical emitting layers are constructed based on the mixed hosts; sandwiched between hole and electron-transporting hosts and the light emitted from two EL units that are absolutely complementary for forming white emission. All resulting tandem WOLEDs realize good white emission with maximum color rendering index (CRI) beyond 77 and 90 for three and four-color white devices and extremely high EL spectra stability, and also achieve high device efficiency with maximum external quantum efficiency (EQE) exceeding 33.10%. For example, for the optimized four-color tandem WOLED, the maximum CRI and EQE of 91 and 34.78% are demonstrated and only very slight CIE (Δx, Δy) variation of (0.002, −0.010) was observed at a wide luminance range from 170.9 cd/m2 to 13,870 cd/m2. To the best of our knowledge, this is the first tandem WOLED with only two EL units realizing such high device performances. More importantly, the proposed tandem WOLEDs here avoid introducing carrier or exciton blocking layer or using more EL units to realize high color quality white emission that provides a novel approach to develop simple, but high-performance tandem WOLEDs.

Inverted organic solar cells with non-clustering bathocuproine (BCP) cathode interlayers obtained by fullerene doping

Bathocuproine (BCP) is a well-studied cathode interlayer in organic photovoltaic (OPV) devices, where it for standard device configurations has demonstrated improved electron extraction as well as exciton blocking properties, leading to high device efficiencies. For inverted devices, however, BCP interlayers has shown to lead to device failure, mainly due to the clustering of BCP molecules on indium tin oxide (ITO) surfaces, which is a significant problem during scale-up of the OPV devices. In this work, we introduce C70 doped BCP thin films as cathode interlayers in inverted OPV devices. We demonstrate that the interlayer forms smooth films on ITO surfaces, resulting from the introduction of C70 molecules into the BCP film, and that these films possess both improved electron extraction as well exciton blocking properties, as evidenced by electron-only devices and photoluminescence studies, respectively. Importantly, the improved cathode interlayers leads to well-functioning large area (100 mm2) devices, showing a device yield of 100%. This is in strong contrast to inverted devices based on pure BCP layers. These results are founded by the effective suppression of BCP clustering from C70, along with the electron transport and exciton blocking properties of the two materials, which thus presents a route for its integration as an interlayer material towards up-scaled inverted OPV devices.

Master equation study of excitonic processes limiting the luminous efficacy in phosphorescent organic light-emitting diodes

We use a master equation model for numerical simulations of triplet loss mechanisms in phosphorescent organic light-emitting diodes (OLEDs). In addition to the IV characteristics, which were successfully simulated in our previous work, the measurements of luminous efficacy (LE) for the OLEDs with various doping profiles of the emitter fac-tris(2-phenylpyridine)iridium (III) (Ir(ppy)3) can be reproduced based on a single set of excitonic parameters, which is a strong indication of the validity of the model. In order to minimize the strong Dexter diffusion of triplets out of the emission layer (EML) and the nonradiative decays, an additional exciton blocking layer and stronger exciton confinement on guest molecules are investigated in the simulations to improve the device efficiency. With the modifications, the LE of the lowest-doped OLED with 4% Ir(ppy)3 can be more than twofold increased at a luminance up to 20000cd/m2. However, due to the enhanced triplet-triplet annihilation, the roll-off in electrophosphorescence efficiencies under high current injections of the upgraded devices becomes stronger, which can nevertheless be suppressed by a higher emitter concentration in the EML.

Efficiency enhancement in fluorescent deep-blue OLEDs by boosting singlet exciton generation through triplet fusion and charge recombination rate

Abstract We report fluorescent deep-blue organic light emitting diodes (OLEDs) with significantly high efficiency by enhancing singlet exciton generation via triplet-triplet annihilation (TTA). Here, electron transport layers (ETL) and exciton blocking layers (EBL) with different characteristics (especially mobility) are applied to increase singlet harvesting (using TTA process) through appropriate charge carrier injection in the emissive layer. OLED with high electron mobility ETL, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and high hole mobility EBL, 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine (PCzAc) exhibit relatively high external quantum efficiency of 9.18% with deep-blue color coordinates of (0.135, 0.115) at the luminance of 1000 cd/m2 and low driving voltage (4.5 V). The transient electroluminescence measurements are conducted to investigate the relation between prompt fluorescence from the initial singlet excitons and delayed fluorescence from the TTA process. This analysis shows significant enhancement in the singlet exciton generation by TTA process and charge recombination rate.

Degradation pathways in standard and inverted DBP-C70 based organic solar cells

Achieving long-term stability in organic solar cells is a remaining bottleneck for the commercialization of this otherwise highly appealing technology. In this work, we study the performance and stability differences in standard and inverted DBP/C70 based organic solar cells. Differences in the charge-transfer state properties of inverted and standard configuration DBP/C70 solar cells are revealed by sensitive external quantum efficiency measurements, leading to differences in the open-circuit voltages of the devices. The degradation of standard and inverted solar cell configurations at ISOS aging test conditions (ISOS-D-3 and ISOS-T-3) was investigated and compared. The results indicate that the performance drop in the small molecule bilayer solar cells is less related to changes at the D-A interface, suggesting also a pronounced morphological stability, and instead, in the case of inverted cells, dominated by degradation at the electron transport layer (ETL) bathocuproine (BCP). Photoluminescence measurements, electron-only-device characteristics, and stability measurements show improved exciton blocking, electron transport properties and a higher stability for BCP/Ag ETL stacks, giving rise to inverted devices with enhanced performance and device stability.