Improved Charge Carrier Balance Research Articles

Efficient NIR Perovskite Light-Emitting Diodes Enabled by Incorporating an Anthracene Derivative as a Bifunctional Electron Transport Layer

Exploring multifunctional charge transport materials with well-matched energy-levels alignment and compatible interfaces is deemed a potential approach toward high-performance perovskite light-emitting diodes (PeLEDs). Herein, an anthracene derivative, 1-[4-(10-[1,1′-biphenyl]-4-yl-9-anthracenyl)phenyl]-2-ethyl-1H-benzimidazole (BAEBi), is demonstrated as an efficient electron tranport layer (ETL) as well as an effective surface passivation agent for near-infrared (NIR) quasi two-dimensional (Q-2D) PeLEDs. Owing to the promising electron injection and transport capability of BAEBi, an improved charge-carrier balance is attained in rather hole-dominant PeLEDs. Meanwhile, BAEBi assisted to significantly mitigate the perovskite/ETL interfacial defects, rendering a pinhole-free smooth film with quite low roughness and ameliorated exciton lifetime. Consequently, BAEBi-containing NIR Q-2D PeLEDs manifested a low turn-on voltage of 2.5 V with a decent external quantum efficiency (EQE) of 9.2% and a peak radiance of 127 W sr–1 m–2; these values are, respectively, ∼1.5- and 15-fold higher compared to that of a commonly used ETL, 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi). Importantly, the BAEBi-incorporating champion PeLED exhibited significantly curtailed efficiency roll-off and prolonged operational stability. These findings reveal an ingenious solution to address the charge imbalance issue and suppress the interfacial defects in PeLEDs that would spur further development of this emerging technology toward efficient solid-state lighting and high-resolution displays.

Porphyrin Functionalization of CsPbBrI2/SiO2 Core–Shell Nanocrystals Enhances the Stability and Efficiency in Electroluminescent Devices

AbstractSurface ligand exchange on all‐inorganic perovskite nanocrystals of composition CsPbBrI2 reveals improved optoelectronic properties due to strong interactions of the nanocrystal with mono‐functionalized porphyrin derivatives. The interaction is verified experimentally with an array of spectroscopic measurements as well as computationally by exploiting density functional theory calculations. The enhanced current efficiency is attributed to a lowering of the charging energy by a factor of 2–3, which is determined by combining electronic and optical measurements on a selection of ligands. The coupled organic–inorganic nanostructures are successfully deployed in a light‐emitting device with higher current efficacy and improved charge carrier balance, magnifying the efficiency almost fivefold compared to the native ligand.

Simplified dopant-free color-tunable organic light-emitting diodes

Organic light-emitting diodes (OLEDs) manifest a high performance that has enabled the industrial applications of this technology; nevertheless, achieving high efficiency in these devices via introducing dopants makes the fabrication process quite complicated. In this Letter, we demonstrate color-tunable organic light-emitting diodes (CT-OLEDs) with a simplified structure that incorporates a dopant-free PN heterojunction as the emitting layer. By depositing a hole transport layer, 1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC) as the P-region and co-depositing DMIC-TRZ (LGH001) and 1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene (TmPyPB), the electron transport layer, as the N-region to construct a PN heterojunction, our optimal CT-OLED exhibited a low turn-on voltage of 2.58 V with the maximum current and power efficiencies of 17.8 cd/A and 21.5 lm/W, respectively. We found that enhancing the hole-blocking property of the N-junction for improved charge–carrier balance and higher exciton formation is beneficial to boost the device performance. CT-OLEDs based on TAPC/TmPyPB:LGH001 as PN heterojunction achieved sunlight white emission with correlated color temperature and color rendering indexes of 7765 K and 74, respectively. Importantly, the emission of these devices can be well-tuned by simply regulating the bias voltage. Furthermore, the underlying physics of our developed CT-OLEDs is thoroughly investigated.

Ultrafast endothermic transfer of non-radiative exciplex state to radiative excitons in polyfluorene random copolymer for blue electroluminescence

We report a blue-emitting random copolymer (termed modified Aryl-F8) consisting of three repeat units of polydioctylfluorene (F8), Aryl-polydioctylfluorene (Aryl-F8), and an aromatic amine comonomer unit, poly(bis-N,Ν′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) chemically linked to get an improved charge carrier balance without compromising on the photoluminescence (PL) quantum yield with respect to the Aryl-F8 homo-polymer. The measured photoluminescence quantum efficiency (∼70%) of the blue-emitting polymer is comparable to or greater than the individual monomer units. The time resolved PL spectra from the modified Aryl-F8 are similar to those of Arylated-poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB) even at a time scale of 100–250 ps, indicating an ultrafast energy transfer from the (Aryl-F8 or F8):Arylated-PFB interface to Arylated-PFB, i.e., endothermic transfer of non-radiative exciplex to a radiative molecular exciton. Furthermore, the presence of non-radiative exciplex is confirmed by the photoluminescence decay profile and temperature dependent PL spectra. The luminance efficiency achieved for the modified Aryl-F8 polymer light-emitting diodes is ∼11 cd A−1 with an external quantum efficiency (EQE) of ∼4.5%, whereas it is 0.05 cd/A with an EQE of ∼0.025% for Aryl-F8. Almost two orders of higher efficiency is achieved due to the improved charge carrier balance from the random copolymer without compromising on the photoluminescence yield.

Highly Efficient and Stable Hybrid White Organic Light Emitting Diodes with Controllable Exciton Behavior by a Mixed Bipolar Interlayer**Supported by the National Natural Science Foundation of China under Grant No 91441201.

Highly efficient and stable hybrid white organic light-emitting diodes (HWOLEDs) with a mixed bipolar interlayer between fluorescent blue and phosphorescent yellow emitting layers are demonstrated. The bipolar interlayer is a mixture of p-type diphenyl (10-phenyl-10H-spiro [acridine-9,9′-fuoren]-3′-yl) phosphine oxide and n-type 2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole). The electroluminance and Commission Internationale de l'Eclairage (CIE1931) coordinates' characteristics can be modulated easily by adjusting the ratio of the hole-predominated material to the electron-predominated material in the interlayer. The hybrid WOLED with a p-type:n-type ratio of 1:3 shows a maximum current efficiency and power efficiency of 61.1 cd/A and 55.8 lm/W, respectively, with warm white CIE coordinates of (0.34, 0.43). The excellent efficiency and adaptive CIE coordinates are attributed to the mixed interlayer with improved charge carrier balance, optimized exciton distribution, and enhanced harvesting of singlet and triplet excitons.

Effect of bulk and planar heterojunctions based charge generation layers on the performance of tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs) were fabricated using organic planar and bulk heterojunctions based charge generation layers (CGLs), which were composed of cobalt phthalocyanine (CoPc) and fullerene (C60). The electroluminescent (EL) characteristics of these two kinds of devices were systematically studied. The results showed that, compared to the corresponding devices with planar heterojunction (PHJ) based CGL, the tandem OLEDs with bulk heterojunction (BHJ) based CGL exhibited a dramatic improvement of performance. By investigating the electrical characteristics of CGLs, it was found that more hetero-interfaces introduced in the BHJ blend were beneficial for generating more interfacial dipoles and charge carriers, and the optimized charge transport pathways were favorable to promote both electron and hole mobilities. As a result, the improved charge carrier balance led to the efficiency enhancement of device performance. The results demonstrated the advantageous effect of BHJ blend film for the rational design of CGLs on the realization of high OLEDs performance.

Thick polymer light-emitting diodes with very high power efficiency using Ohmic charge-injection layers

We demonstrate the optimization of single-layer polymer LED structures with active layers with thicknesses of the order of 1 micron. By using a combined approach of the addition of MoO3, as a bottom hole-injection layer, and the incorporation of such thick active layers, exceptionally high performance metrics are achieved. In particular, brightnesses of 1000 cd m−2 at driving voltages of only 6.8 V, corresponding to a power efficiency of 7.8 lm W−1, a current efficiency of 17.2 cd A−1, and external quantum efficiency of 5.6%, are reported for devices based on F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]). A side-by-side comparison, between the standard LED structure and hybrid structures, demonstrates that with MoO3 as bottom hole-injection layer, the electron and hole charge carriers are both giving space-charge limited current for both carriers due to the Ohmic contacts. The devices hence show improved charge carrier balance, and, most importantly, high brightness at low operational voltage. Such thick active-layer devices with high performance metrics, in addition to improved engineering and processing tolerances, are thus especially important for application to high-throughput device fabrication methods.

Synthesis and Application of Photolithographically Patternable Deep Blue Emitting Poly(3,6-Dimethoxy-9,9-dialkylsilafluorene)s

Poly(silafluorene)s (PSFs) are promising light-emitting materials with brilliant solid-state blue luminescence, high quantum efficiency, excellent solubility, and improved thermal and chemical stability. PSFs are reported to have high electron affinity and conductivity originating from σ*-π* conjugation between the σ*-antibonding orbital of the exocyclic Si-C bond and the π* antibonding orbital of the butadiene fragment, a promising characteristic for improved charge carrier balance in OLEDs. In this paper, we present a protocol for photopatterning derivatives of poly(3,6-dimethoxy-9,9-dialkylsilafluorenes) with resolutions exceeding 10 μm. The procedure begins by converting polymers (Mn = 50-55 kg/mol, PDI = 1.8) with cyclohexenyl and norbornenyl containing side chains to their respective epoxides using the Prilezhaev reaction and m-chloroperoxybenzoic acid (m-CPBA). Using the I-line (365 nm) of a Karl Suss MA6 mask aligner, a 1 s UV light exposure of the photoacid generator (PAG) bis(4-tert-butylphenyl)iodonium hexafluoro-phosphate (DtBPI-PF6) generates sufficient protons to catalyze epoxide ring-opening and form a bridging network of covalent C-O bonds which renders the material insoluble in developing solvents such as toluene or THF. The resultant cross-linked material possess characteristic blue photoluminescence with solid state quantum yields >80%. Polymer films have excellent transparency (with a measured Eg ≈ 3.0 eV). Energy levels determined using cyclic voltammetry were -5.7 and -2.7 eV for HOMO and LUMO, respectively. Additionally, several device applications are demonstrated which incorporate cross-linked films. These include examples of solid state lasing in the region of 420-450 nm from cross-linked films on second order corrugated silica substrates (Λ = 200 nm). OLEDs were also prepared with a cross-linked emitting layer as part of a trilayer device which we report to have a maximum external quantum efficiency of 3.2% at 33 mA/cm(2) and a stable blue-violet emission with an electroluminescence maximum at 410 nm. Photopatternable PSF epoxides are also shown to be efficient hosts for Förster energy transfer and we provide examples of pattern layers incorporating small molecule emitters which emit in both the red and green while blue emission of the host is effectively suppressed.

Barium Hydroxide as an Interlayer Between Zinc Oxide and a Luminescent Conjugated Polymer for Light‐Emitting Diodes

AbstractA study of hybrid light‐emitting diodes (HyLEDs) fabricated with and without solution‐processible Cs2CO3 and Ba(OH)2 inorganic interlayers is presented. The interlayers are deposited between a zinc oxide electron‐injection layer and a fluorescent emissive polymer poly(9‐dioctyl fluorine–alt‐benzothiadiazole) (F8BT) layer, with a thermally evaporated MoO3/Au layer used as top anode contact. In comparison to Cs2CO3, the Ba(OH)2 interlayer shows improved charge carrier balance in bipolar devices and reduced exciton quenching in photoluminance studies at the ZnO/Ba(OH)2/F8BT interface compared to the Cs2CO3 interlayer. A luminance efficiency of ≈28 cd A−1 (external quantum efficiency (EQE) ≈ 9%) is achieved for ≈1.2 μm thick single F8BT layer based HyLEDs. Enhanced out‐coupling with the aid of a hemispherical lens allows further efficiency improvement by a factor of 1.7, increasing the luminance efficiency to ≈47cd A−1, corresponding to an EQE of 15%. The photovoltaic response of these structures is also studied to gain an insight into the effects of interfacial properties on the photoinduced charge generation and back‐recombination, which reveal that Ba(OH)2 acts as better hole blocking layer than the Cs2CO3 interlayer.

Effect of Charge Carrier Transport Property and Energy Level of Host Material on Phosphorescent White Organic Light-Emitting Device

Phosphorescent white organic light-emitting devices (PhWOLEDs) with a double emitting layer (EML) structure were fabricated. The EMLs were doped with blue and yellow phosphors of iridium(III) bis[4,6-(difluorophenyl)-pyridinato-N,C2'] picolinate and [2-(4-tert-butylphenyl)benzothiazolato-N,C2'] iridium (acetylacetonate). Comparing the performance of PhWOLEDs with the host of yellow EML varying from p- to n-type materials, the results showed that the PhWOLED fabricated with 4,7-diphenyl-1,10-phenanthroline (BPhen) as the yellow host had the highest performance, exhibiting a peak current efficiency of 31.7 cd/A, a peak power efficiency of 17.1 lm/W, and the Commission Internationale del'Eclairage coordinates of (0.33, 0.41) at a bias of 9 V. The enhancement was attributed to the improved charge carrier balance, broadened recombination region, and adequate energy levels between the BPhen host and the dopants, which made distinct roles for the dopants to effectively harvest the charge carriers.

Effect of Solution Processed Hole Injection Layers on Device Performance of Phosphorescent Green OLEDs

We have studied the effect of hole injection layers (HILs) on the performance of phosphorescent green organic light-emitting diodes (OLEDs). The PEDOT:PSS and other new PEDOT-based materials of HIL 1.3N and HIL 3.3N were used as HILs. The OLED with HIL 1.3N, having the work function of 6.08 eV, exhibits low driving voltage and high efficiency because of the improved hole injection. The position of emission zone, studied by analyzing the electroluminescence spectra, is found to be at the center of the emission layer (EML), while the emission zone is near the interface for the OLEDs with PEDOT:PSS and HIL 3.3N. This is due to the improved charge carrier balance within the EML caused by the efficient HIL of HIL 1.3N.

High performance blue light-emitting diodes based on conjugated polymer blends

We report on the fabrication and properties of single layer blue light-emitting diodes (LEDs) based on conjugated polymer blends of emissive and hole transport materials. Despite the fact that the photoluminescence quantum efficiency of the blend is lower compared to that of both the host and the guest polymers, an enhancement in both the electroluminescence quantum and power efficiency is seen for the blend. This observation indicates that the hole transporting material leads to a significantly improved hole injection and thus a greatly improved charge carrier balance factor. Optimised single layer blue LEDs showed a maximum brightness of 6000 Cd/m 2 at 14 V and a maximum external ELQE of 1% (2.1Cd/A) at 15 Cd/m 2 corresponding to a power efficiency of 1 1m/W.