Red Phosphorescent Emitters Research Articles

High EQE of 43.76% in solution-processed OLEDs operating at a wavelength of 626 nm

Simultaneously achieving high-efficiency, deep-red emission, and solution-processed organic light-emitting diodes (OLEDs) remains a huge challenge. In this work, a thermally activated delayed fluorescent (TADF) material of CzPXZ that exhibits aggregation-induced emission property and a deep-red phosphorescent emitter of Ir(dmppy)(piq)2(od) are developed to build effective energy transfer pathways by dissolving them in a non-polar organic solvent. The electroluminescent emission peaks of CzPXZ:Ir(III)-based OLEDs are located at deep-red 626 nm, demonstrating efficient energy transfer from CzPXZ to the Ir(III) complex. Furthermore, an optimized DPEPO hole-blocking layer is utilized in such Ir(III)-doped OLEDs to enhance the radiative recombination. Therefore, a high external quantum efficiency of 43.76% is achieved for CzPXZ:Ir(III)-based OLEDs. This work sheds light on the great potential of energy transfer from AIE-TADF to red phosphorescent emitters for high-efficiency, solution-processed, deep-red OLEDs.

Investigation of the Relationship between Quantum Yield, Charge-Transfer State, and Structure of the Ligands in Red-Emitting Heteroleptic Iridium(III) Complexes.

Iridium(III) organometallic complexes have been a key component in commercialization of organic light-emitting diodes, but the direct relationship between their structural features and photophysical properties has not yet been fully established. Here, combined experimental and theoretical studies are carried out to elucidate the main factors governing the quantum efficiency of red phosphorescent emitters by using two heteroleptic iridium(III) complexes with high geometrical similarity. It is found that two red-emitting heteroleptic iridium complexes differing only in the steric direction of phenylquinoline (pq) and phenylisoquinoline (piq) ligands, annotated Red-pq and Red-piq, show clearly different degrees of distortion of the ligand geometry in the excited state, which leads to the higher quantum yield of Red-piq than that of Red-pq. This larger distortion of the piq ligand causes more suppressed nonradiative decay of Red-piq than that of Red-pq which is the important factor governing the higher quantum yield of Red-piq.

Acceptor-σ-donor-σ-acceptor host material for red phosphorescent and thermally activated delayed fluorescent OLEDs

A novel bipolar host molecule di-spiro [fluorene-9,5′-quinolino [3,2,1-de] acridine-9′,9″-fluorene]-2,2″,7,7″-tetracarbonitrile (QACN) featuring an Acceptor-σ-Donor-σ-Acceptor structure was developed. Systematic investigations into its photophysical, electrochemical, and thermal properties unveiled exceptional thermal stability, a three-dimensional spatial configuration, and bipolar carrier transport capability. Employing QACN as the host material in red phosphorescent and thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) yielded a maximum external quantum efficiency of 25.3 % and 16.5 %, respectively. Compared with the commercial material TPBi (EQE = 12.0 %, λEL = 668 nm) and mCP (EQE = 14.5 %, λEL = 652 nm), the TADF OLEDs based on QACN achieved a higher EQE and a large red-shift emission (EQE = 16.5 %, λEL = 694 nm). These findings underscore the potential of QACN as an effective host material for red phosphorescent and TADF emitters and provide a new auxiliary finesse for realizing deep red and near-infrared (NIR) OLEDs.

Achieving slow efficiency roll-off and tunable spectrum in high-performance tandem WOLEDs with mixed hosts

Highly efficient blue and warm white tandem OLEDs were designed and realized by doping blue thermally activated delayed fluorescence (TADF) emitter and red phosphorescent emitter into mixed hosts system together. Based on the optimization of charge generation unit (CGU), blue tandem device with external quantum efficiency of 30.93 % and color coordinates of (0.165, 0.365) was firstly realized. To achieve efficient and high-quality white OLEDs (WOLEDs), red phosphorescent emitter PQ2Ir (dpm) was doped into TCTA:26DCzPPy (1:1) to construct another electroluminescent (EL) unit near the anode. By optimizing the doping concentration of PQ2Ir (dpm), maximizing excitons utilization probability and controlling energy transfer processes, the optimal tandem WOLED with turn-on voltage, maximum brightness, current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 5.2 V, 29,595 cd m−2, 65.19 cd A−1, 38.94 lm W−1 and 40.46 %, respectively, was demonstrated. Excitingly, this device can still maintain an EQE up to 25.59 % at the certain brightness of 1000 cd m−2, which is important for lighting application. The correlated color temperature (CCT) of this device tunes from 4401 K at 1000 cd m−2 to 2642 K at 20,000 cd m−2, thereby accommodating various scene requirements.

Simultaneously Realizing High Efficiency and High Color Rendering Index for Hybrid White Organic Light-Emitting Diodes by Ultra-Thin Design of Delayed Fluorescence Sensitized Phosphorescent Layers.

In consideration of energy economization and light quality, concurrently attaining high external quantum efficiency (ηext ) and high color rendering index (CRI) is of high significance for the commercialization of hybrid white organic light-emitting diodes (WOLEDs) but is challenging. Herein, a blue luminescent molecule (2PCz-XT) consisting of a xanthone acceptor and two 3,6-diphenylcarbazole donors is prepared, which exhibits strong delayed fluorescence, short delayed fluorescence lifetime, and excellent electroluminescence property, and can sensitize green, orange, and red phosphorescent emitters efficiently. By employing 2PCz-XT as sensitizer and phosphorescent emitters as dopants, efficient two-color and three-color WOLED architectures with ultra-thin phosphorescent emitting layers (EMLs) are proposed and constructed. By incorporating a thin interlayer to modulate exciton recombination zone and reduce exciton loss, high-performance three-color hybrid WOLEDs are finally achieved, providing a high ηext of 26.8% and a high CRI value 83 simultaneously. Further configuration optimization realizes a long device operational lifetime. These WOLEDs with ultra-thin phosphorescent EMLs are among the state-of-the-art hybrid WOLEDs in the literature, demonstrating the success and applicability of the proposed device design for developing robust hybrid WOLEDs with superb efficiency and color quality.

Turning the energy channel of side-chain TADF polymers by monomer optimization for high-efficiency solution-processed white OLEDs

Suppressing non-radiative channel in luminescent polymer is the key factor for high device efficiency of organic light-emitting diodes (OLEDs). In this paper, we synthesized three side-chain TADF polymers, PTRZ-TRZ, PTRZ-TPA and PTRZ-Cz, to deeply investigate the relationship between energy transfer and molecular structure. The photophysical measurements show that the copolymerized monomer has significant effect on the exciton deexcitation process of TADF fragment. The rigid carbazole unit can efficiently separate the TADF units to inhibit the aggregation-caused quenching (ACQ) of polymer, while electron-donating triphenylamine unit will induce the severe charge transfer state to form low energy exciplex, which lead to severe energy leakage of the excited TADF. Moreover, according to the single-charge devices test, the ideal separation unit also can improve the capability of charge injection and carrier balance. As a result, the solution-processed OLEDs host with PTRZ-Cz achieved the external quantum efficiency (EQEmax) of 14.2%, which was much higher than that of PTRZ-TPA and PTRZ-TRZ. By doping red phosphorescent emitter Ir(MDQ)2(acac), the hybrid white OLED devices (T-P WOLEDs) based on PTRZ-Cz achieved the maximum current efficiency (CEmax), power efficiency (PEmax) and EQEmax of 46.8 cd/A, 32.6 lm/W and 20.7%, respectively, with a high CRI of 80.6. The outcomes obtained in this study can provide useful insights into the structure−property relationship for further development of efficient TADF polymer hosts for solution-processable OLEDs.

Highly efficient and low operation voltage hybrid solution-processed WOLEDs based on wide FWHM blue TADF and red phosphorescent emitters

Recent developments in the field of thermally activated delayed fluorescence (TADF) emitter have led to significantly improved performances of hybrid solution processing prepared white organic light-emitting diodes (s-WOLEDs). Importantly, wide coverage of electroluminescent spectra and balanced transfer of carriers are conclusive factors to obtain high quality lighting sources. This paper demonstrated high quality s-WOLEDs grounded on blue TADF and red phosphorescent emitters. In optimal WOLED, Förster energy transfer (ET) distance and ET rate were calculated to be 1.81 nm and 1.29 × 106 s−1, respectively. Also, exciplex host were employed to decrease operation voltage and modulate carriers' distribution. The fabricated optimal solution-processed warm white electroluminescence device exhibited high color rendering index of 80 and maximum luminescent efficiencies up to 29.22 cd A−1, 28.68 lm W−1 and 13.5%. Furthermore, the Commission Internationale de I'Eclairage coordinates of this device are (0.39, 0.41) at the current density of 10 mA cm−2, while the turn-on voltage of this device is 3.0 V.

High performance red and green phosphorescent emitters suitable for the BT.2020 color gamut

AbstractIn this work, we investigated the potential for phosphorescent emitters to achieve the BT.2020 color standard in displays, where the CIE coordinates for red and green are (0.709, 0.292) and (0.170, 0.797), respectively. Optical simulations were performed for both green and red top emission organic light emitting devices (OLEDs). For the green emitter, it is possible to reach (0.170, 0.785) using a spectrum with a peak wavelength (λmax) at 526 nm and a full width at half maximum (FWHM) less than 30 nm. For the red emitter, in order to achieve (0.708, 0.292) while maintaining a high current efficiency (CE), it is important to decrease the FWHM instead of red‐shifting the spectrum. Following the guidance of these simulation results, we designed and synthesized novel deep green (DGD) and deep red phosphorescent (DRD‐II) emitters. The photoluminescent (PL) spectrum of DGD shows an FWHM of 30 nm and a λmax of 523 nm. A top‐emission green OLED built using DGD reached a CE of 171 cd/A at an operating voltage of 3.3 V and a lifetime of 95% of initial brightness (LT95) > 1300 h at 10 mA/cm2 with a CIE (x, y) = (0.170, 0.777). This is, to our knowledge, the best device performance ever reported for a green phosphorescent OLED at this CIE y. The PL spectrum of DRD‐II has a λmax of 630 nm with an FWHM of 30 nm. A top‐emission red OLED built with DRD‐II achieved a CE of 59 cd/A, an operating voltage of 3.2 V and an LT95 over 20,000 h at a drive current of 10 mA/cm2 with a CIE (x, y) = (0.708, 0.292). We also studied the angular dependence of the above devices and found they were comparable to devices with commercial emitters for the Digital Cinema Initiative P3 (DCI‐P3) standard that had a wider FWHM. Combining these green and red emitters with a commercial blue OLED at (0.131, 0.046), we are able to cover 97% of the BT.2020 color gamut. The results using DGD and DRD‐II suggest that they have great potential to satisfy BT.2020 in an organic phosphorescent system.

Multifunctional behavior of a carbazole derivative: Red phosphorescent emission, aggregation-induced long-life exciton and light-emitting diode application

A D-π-A typed cyanyl-carboxylic derivative (named as CECZA) merely produced prompt fluorescence with lifetime at nanosceond scale in dilute solutions, whose solid-state luminescence exhibited 3.36 μs lifetime with 13.80 % quantum yield (QY, captured at 522 nm for powder at nanometer scale) at 298 K and 43.36 ms lifetime with 30.46 % QY (650 nm, 80 K, tiny crystals). Femtosecond transient absorption, Raman spectroscopy and quantum chemical calculation provided valid clues to reveal its excitonic transition mechanism. The results indicated that the restricted vibration of benzene ring on carbazole group and alkyl chain weakened the vibrational modes of CECZA molecule and strengthened inter-molecular interactions between adjacent molecules at low temperatures, which promoted the persistent phosphorescent emission. Due to strong UV–vis absorption, high quantum efficiency and excellent thermal stability, CECZA can be used as a potential candidate in light-emitting diode (LED) application. Combined with a commercial InGaN blue-emitting chip, CECZA-InGaN emitted daylight white light.

39.1: Invited Paper: High Performance Red and Green Phosphorescent Emitters Suitable for BT.2020 Color Gamut

We developed a novel, deep red phosphorescent emitter (DRD‐II), with a photoluminescence (PL) spectrum as narrow as 30 nm at full‐width‐half‐maximum (FWHM). A top‐emission red organic light emitting device (OLED) built with DRD‐II achieved a current efficiency (CE) of 59 cd/A, operating voltage of 3.2 V and lifetime to 95% of initial brightness (LT95) over 20,000 h at a drive current of 10 mA/cm2 with CIE (x, y) = (0.707, 0.293). We also developed a green phosphorescent organic emitter (GD) whose PL spectrum shows FWHM of 30 nm and peak wavelength (λmax) of 523 nm. A top‐emission green OLED was built with this GD and reached CE of 171 cd/A, voltage of 3.3 V and LT95 > 1,300 h at 10 mA/cm2 with CIE (x, y) = (0.169, 0.777). This is, to our knowledge, the best efficiency and lifetime combination ever reported for a green OLED at this CIE y. Combining these red and green emitters with a commercial blue OLED at CIE (0.131, 0.046), we are able to cover 97% of the BT.2020 color gamut. Through optical simulation, we also provide guidance to further improve red and green emitters. It is found that a red emitter with smaller FWHM not only maintains saturated color at a shorter peak wavelength but also gains luminance efficiency owing to the elimination of longer wavelengths. On the other hand, the simulation demonstrates that it is indeed challenging for phosphorescent green emitters to satisfy BT.2020. Nevertheless, it is possible to reach CIE y = 0.785 with a narrow line shape and minimal shoulder peak.

Improving the Performance of Red Organic Light-Emitting Transistors by Utilizing a High-k Organic/Inorganic Bilayer Dielectric.

Integration of electrical switching and light emission in a single unit makes organic light-emitting transistors (OLETs) highly promising multifunctional devices for next-generation active-matrix flat-panel displays and related applications. Here, high-performance red OLETs are fabricated in a multilayer configuration that incorporates a zirconia (ZrOx)/cross-linked poly(vinyl alcohol) (C-PVA) bilayer as a dielectric. The developed organic/inorganic bilayer dielectric renders high dielectric constant as well as improved dielectric/semiconductor interface quality, contributing to enhanced carrier mobility and high current density. In addition, an efficient red phosphorescent organic emitter doped in a bihost system is employed as the emitting layer for an effective exciton formation and light generation. Consequently, our optimized red OLETs displayed a high brightness of 16 470 cd m-2 and a peak external quantum efficiency of 11.9% under a low gate and source-drain voltage of -24 V. To further boost the device performance, an electron-blocking layer is introduced for ameliorated charge-carrier balance and hence suppressed exciton-charge quenching, which resulted in an improved maximum brightness of 20 030 cd m-2. We anticipate that the new device optimization approaches proposed in this work would spur further development of efficient OLETs with high brightness and curtailed efficiency roll-off.

Efficient hybrid white organic light-emitting diodes with superior color stability and slow efficiency roll-off based on ultrathin emitting layer structures

In this work, high-performance hybrid white organic light-emitting diodes (WOLEDs) with high efficiencies, slow roll-off and excellent color stability were realized by strategically inserting ultrathin red phosphorescent emitter film into bluish-green thermally activated delayed fluorescent (TADF) light-emitting layer (EML). Experimental results demonstrated that the introduction of ultrathin phosphor film helps to harvest superfluous triplet excitons within bluish-green EML and relieve local exciton accumulation-induced quenching, thereby guaranteeing high exciton utilization probability. Besides, the strong holes' trapping of ultrathin layer leads to improved carriers balance, thus realizing stable recombination zone. Finally, the optimal hybrid WOLED obtained the maximum current efficiency (ηc), power efficiency (ηp) and external quantum efficiency (EQE) of 63.84 cd A−1, 66.06 lm W−1 and 28.16%, respectively. At the brightness of 1000 cd m−2, ηc, ηp and EQE as high as 57.14 cd A−1, 49.88 lm W−1 and 25.23%, respectively, can still be remained by the same device, manifesting slow efficiency roll-off. In addition, the resulting WOLED displayed excellent color stability with slightly shifted Commission Internationale de l’Eclairage coordinates (CIEx,y) from (0.43, 0.44) at 1000 cd m−2 to (0.41, 0.43) at 10000 cd m−2.

Steven V. Abramson: An OLED Unicorn Setting a Shining Example for the Industry

Steven V. Abramson: An OLED Unicorn Setting a Shining Example for the Industry

Novel benzonitrile- and benzo[d]imidazole-based bipolar hosts for green PhOLEDs with a low turn-on voltage

In this work, three bipolar host materials combined electron-rich carbazole moiety and electron-deficient benzonitrile and benzo[d]imidazole units were firstly designed and synthesized, namely 3''-(9H-carbazol-9-yl)-5'-(1-phenyl-1H-benzo[d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-4-carbonitrile (p-PNBMCz), 3''-(9H-carbazol-9-yl)-5'-(1-phenyl-1H-benzo[d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-3-carbonitrle (m-PNBMCz) and 3''-(9H-carbazol-9-yl)-5'-(1-phenyl-1H-benzo[d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-2-carbonitrle (o-PNBMCz), respectively. The glass transition temperatures (Tg) of all the materials are above 117 °C, demonstrating a good morphological stability, and the thermal decomposition temperatures (Td) are over 436 °C, reflecting remarkable thermal properties. Besides, their HOMO separated fully with LUMO, which is conducive to both hole and electron transporting. They also possess a triplet energy level (ET) of about 2.7 eV, which qualifies them to be hosts for green and red phosphorescent emitters. Green phosphorescent organic light-emitting diodes (PhOLEDs) hosted by these three materials were manufactured and they displayed good performance with very low turn-on voltages (Von). Remarkably, the device hosted by p-PNBMCz (G1) exhibits maximal external quantum efficiencies (EQE) of 17.1% and maximal power efficiency (ηp,max) of 69.2 lm/W with a minor efficiency roll-off, the ηext still remains to be 15.9% with only a 7% roll-off percentage as the luminance reaches 1000 cd/m2.

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates.

The hexahydride complex OsH6(PiPr3)2 promotes the C–H bond activation of the 1,3-disubstituted phenyl group of the [BF4]− and [BPh4]− salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ2-C,N- or κ2-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ3-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ2-C,N- and κ2-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF4]− inhibits the azolium reactions, [BPh4]− favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C–H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

38.1: Invited Paper: High Performance Deep Red Three‐Unit Tandem OLEDs for Automotive Lighting Applications

We report high performance deep red tandem OLEDs suitable for automotive lighting applications. We developed a deep red phosphorescent emitter that has an intrinsic photoluminescent (PL) peak wavelength at 639 nm and a photoluminescent quantum yield PLQY of 0.80. A low‐voltage red host (LVRH) material was developed to match this emitter, which enables a single‐unit, bottom emission device to reach 2,000 cd/m2 with an external quantum efficiency (EQE) of 25.8%, bias voltage of 3.7 V, and LT95 of 6,000 hrs. A 3‐unit device incorporating this emissive system along with a novel charge‐generation layer (CGL) was fabricated and its device structure was optimized by tuning the hole transport layer (HTL) thickness. The device achieved a current efficiency of 39 cd/A, overall EQE of 77%, LT95 of 46,000 hrs, and bias voltage of 9.9 V at 2,000 cd/m2. This tandem device also reaches a deep red chromaticity with a peak wavelength of 641 nm, and CIE (X, Y) = (0.706, 0.293). The LT95 of the same device at 85 °C was 9,700 hr at 2,000 cd/m2. Such a deep red OLED is desirable for automotive lighting applications.

Phosphorescent white organic light-emitting diodes with stable white color depending on luminance

We fabricated simple and color-stable phosphorescent white organic light-emitting diodes (OLEDs) without an interlayer using a single host of 1,3-bis(9-carbazolyl)benzene with iridium(III) bis[(4,6-difluorophenyl) pyridinato-N,C2’]picolinate and bis(1-phenylisoquinoline)(acetylacetonate) iridium(III) as blue and red phosphorescent emitters, respectively. The CIE 1931 color coordinate difference of the white OLEDs is (0.008, 0.007) when the luminance of the device is increased from approximately 265 cd/m2 to 9156 cd/m2, which is regarded as visually indistinguishable in practice. In addition, we also measured the decay time of excitons to investigate the emission mechanism in this device using transient photoluminescence and electroluminescence techniques.

Highly-efficient solution-processed deep-red organic light-emitting diodes based on heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand

Abstract Three new deep-red heteroleptic phosphorescent iridium(III) complexes Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3), comprising cyclometalated ligand 1-pheynylisoquionoloine(piq) and heterocyclic Schiff base ancillary ligands 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol(L1), 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol (L2), and 4-(4-methoxyphenyl)imino-methyl-3-methyl-1-phenyl-1H-pyrazol-5-ol (L3) have been designed, synthesized, and characterized. All the compounds emit deep red emission with λmax values in the spectral range of 602–620 nm, high quantum yield 0.44 to 0.52 and short excited state lifetime τ (0.51–0.55 μs) due to dominant strong field ligands, resulting an efficient triplet metal-ligand charge transfer (3MLCT) excited state. Time dependent density functional theory (TD-DFT) calculations and electrochemical measurements of the compounds strongly support their genuine deep red phosphorescent emission. The combination of ancillary and cyclometalated ligands significantly influence the molecular orbitals of Ir(III) complex, leading to clearly distinct electron density distributions of the LUMO and HOMO. The compounds show good thermal stability and quantum yield, these characteristics making them an ideal candidate to exploit in phosphorescent organic light emitting diodes (PhOLEDs). Highly-efficient PhOLEDs were developed by using Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3) in solution process as deep red emitters and device composed of Ir(piq)2(L3) exhibited an excellent external quantum efficiency of 14.9% and current efficiency of 10.8 cd/A with the stable CIE coordinates of (0.67,0.33).

83‐1: Invited Paper: Optimization of High Performance Deep Red OLEDs Using Tandem Structure for Automotive Lighting Application

We report here high performance deep red OLED devices for automotive lighting applications, through optimizing the organic stacks in a tandem structure. We first developed a deep red phosphorescent emitter that has a PL peak wavelength at 639 nm. A 2‐unit device incorporating this emitter along with a novel charge‐generation layer, was tuned to achieve an external quantum efficiency of 48%, a current efficiency of 26 cd/A, LT95 of 11,000 hrs, and a bias voltage of 8.2 V at 2,000 cd/m2. This tandem device reaches a deep red chromaticity with a dominant wavelength of 625nm, and CIE (X, Y) = (0.70, 0.30), as well as a near‐Lambertian emission pattern.

Investigation of charge-transport properties in polymer/fullerene blends using transient electroluminescence technique

The charge-transport property is one of crucial factors to determine the performance of the polymer-based devices. The hole-transport properties of regioregular poly(3-hexylthiophene-2,5diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends with two blend ratios were investigated using transient electroluminescence (EL) measurements. For EL, organic light emitting diodes are fabricated with P3HT:PCBM blends as hole-transport layers. To reduce absorption, a red phosphorescent emitter with bis(1-phenylisoquinolinato-N,C2′) iridium(acetylacetonate) [(piq)2Ir(acac)] was used. Transient EL with red color is obtained in spite of P3HT:PCBM absorption and hole mobilities of P3HT:PCBM layers are calculated using delay times in transient EL signals. The hole mobility of pristine P3HT:PCBM with 1:0.8 blend ratio is approximately 6.2 × 10−5 cm2 V−1 · s−1 at approximately 615 kV cm−1, and it decreases when the PCBM ratio increases. The hole mobility of the P3HT:PCBM layer increases to approximately 1.07 × 10−4 cm2 V−1 · s−1 at approximately 615 kV cm−1 when the P3HT:PCBM blend is thermally annealed.