Balanced Charge Carrier Research Articles

Novel 9,9′-(1,3-phenylene)bis-9H-carbazole-containing copolymers as hole-transporting and host materials for blue phosphorescent polymer light-emitting diodes

Novel photo-crosslinkable hole-transport and host materials incorporated into multilayer blue phosphorescent polymer light-emitting diodes (Ph-PLEDs) were demonstrated in this study. The oxetane-containing copolymers, which function as hole-transport layers (HTL), could be cured by UV irradiation in the presence of a cationic photoinitiator. The composition of the two monomers was varied to yield three different hole-transporting copolymers, [Poly(9,9′-(5-(((4-(7-(4-(((3-methyloxetan-3-yl)methoxy)methyl)phenyl)octan-3-yl)benzyl)oxy)methyl)−1,3-phenylene)bis(9H-carbazole)) (P(mCP-Ox)-I, -II, and -III)]. In addition, monomer 1 was copolymerized with styrene to produce copolymer P(mCP-Ph) as a host material for bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic), a blue-emitting dopant. All mCP-based copolymers displayed high glass transition temperatures (Tg) of up to 130–140 °C and triplet energies of up to 3.00 eV. The blue Ph-PLEDs exhibited a maximum external quantum efficiency of 2.55%, in addition to a luminous efficiency of 8.75 cd A−1 when using the device configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/P(mCP-OX)-III/P(mCP-Ph):FIrpic(15 wt %)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1′′-terphenyl]-3,3′′-diyl]bispyridine/LiF/Al. The device bearing P(mCP-Ox)-III HTL, containing the highest composition of mCP unit, exhibited better performance than the other devices, which is attributed to induction of more balanced charge carriers and carrier recombination in the emissive layer. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 707–718

Balanced Carrier Mobilities: Not a Necessary Condition for High‐Efficiency Thin Organic Solar Cells as Determined by MIS‐CELIV

A novel technique based upon injection‐charge extraction by linearly increasing voltage (i‐CELIV) in a metal‐insulator‐semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors. The technique (MIS‐CELIV) allows selective measurement of both electron and hole mobilities of organic solar cells with active layers thicknesses representative of operational devices. The method is used to study the model high efficiency bulk heterojunction combination poly[N‐9′′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM) at various blend ratios. The absence of bipolar transport in PCDTBT‐and‐PC70BM‐only diodes is shown and strongly imbalanced carrier mobility is found in the most efficient “optimized” blend ratios. The mobility measurements are correlated with overall device performance and it is found that balanced and high charge carrier mobility are not necessarily required for high efficiencies in thin film organic solar cells.

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.

Highly Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes Enabled by the Direct Formation of Excitons within InP@ZnSeS Quantum Dots

We demonstrate bright, efficient, and environmentally benign InP quantum dot (QD)-based light-emitting diodes (QLEDs) through the direct charge carrier injection into QDs and the efficient radiative exciton recombination within QDs. The direct exciton formation within QDs is facilitated by an adoption of a solution-processed, thin conjugated polyelectrolyte layer, which reduces the electron injection barrier between cathode and QDs via vacuum level shift and promotes the charge carrier balance within QDs. The efficient radiative recombination of these excitons is enabled in structurally engineered InP@ZnSeS heterostructured QDs, in which excitons in the InP domain are effectively passivated by thick ZnSeS composition-gradient shells. The resulting QLEDs record 3.46% of external quantum efficiency and 3900 cd m(-2) of maximum brightness, which represent 10-fold increase in device efficiency and 5-fold increase in brightness compared with previous reports. We believe that such a comprehensive scheme in designing device architecture and the structural formulation of QDs provides a reasonable guideline for practical realization of environmentally benign, high-performance QLEDs in the future.

Novel organic electron injection layer for efficient and stable organic light emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) was significantly improved by introducing a novel organic composition (4,7-diphyenyl-1,10-phenanthroline (BPhen): 20wt% tetracyanoquinodimethane (TCNQ)) as an electron injection layer which could be uniformly deposited over a range of thicknesses. A highly injecting contact was obtained using 1.5nm thick film of 20wt% TCNQ mixed BPhen as the electron injection layer. High efficiency and stability of organic light emitting device are ascribed to improved electron injection and hence better charge carrier balance inside the OLEDs.

Efficiency Roll‐Off in Organic Light‐Emitting Diodes

Organic light-emitting diodes (OLEDs) have attracted much attention in research and industry thanks to their capability to emit light with high efficiency and to deliver high-quality white light that provides good color rendering. OLEDs feature homogeneous large area emission and can be produced on flexible substrates. In terms of efficiency, OLEDs can compete with highly efficient conventional light sources but their efficiency typically decreases at high brightness levels, an effect known as efficiency roll-off. In recent years, much effort has been undertaken to understand the underlying processes and to develop methods that improve the high-brightness performance of OLEDs. In this review, we summarize the current knowledge and provide a detailed description of the relevant principles, both for phosphorescent and fluorescent emitter molecules. In particular, we focus on exciton-quenching mechanisms, such as triplet-triplet annihilation, quenching by polarons, or field-induced quenching, but also discuss mechanisms such as changes in charge carrier balance. We further review methods that may reduce the roll-off and thus enable OLEDs to be used in high-brightness applications.

The improved performance of solution-processed SQ:PC71BM photovoltaic devices via MoO3 as the anode modification layer

The enhanced performance of a squaraine compound, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) as a donor, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor in solution processed organic photovoltaic devices (OPVs) via the anode modification and thermal annealing is investigated. Through the optimization of these two factors, a maximum power conversion efficiency (PCE) of 4.08% under AM 1.5G illumination (100mW/cm2) is obtained. The results show that poly (3,4-ethylenedioxythiophene):poly (styrenesulfonicacid) (PEDOT:PSS) deteriorates the performance of SQ:PC71BM based OPVs, and MoO3 is used to substitute PEDOT:PSS as an effective exciton-blocking layer. A higher open-circuit voltage (VOC) of 0.95V is reached. The barrier matching between the energy-level of MoO3 and HOMO of SQ enhances fill factor (FF) and short-circuit current density (JSC). The operation mechanism of the thermal annealing is also straightened out. The higher carrier mobility and improved contact is obtained under 70°C annealing, which results from the higher nanoscale crystallinity and the optimal scale phase separation between donor–acceptor networks. Hence the balanced charge carrier mobility increases the FF and PCE of OPVs by decreasing the charge recombination.

Improving working lifetime and efficiency of phosphor doped organic light-emitting diodes

Long working lifetime and high efficient phosphorescent organic light-emitting diode (PHOLED) in which mixed host composed of wide-band-gap based 4, 7-diphenyl-1, 10-phenanthroline (Bphen) and (4,4'-bis(carbazol-9-yl)-biphenyl) (CBP) was demonstrated. The PHOLED with structure of ITO/MoO(3)/CBP:MoO(3) (15 v%, 30 nm)/CBP(10 nm)/([50v%:50v% CBP:Bphen]: 6v% Ir(ppy)(3))(30 nm)/Bphen (40 nm)/LiF (1 nm)/Al offers a peak power efficiency of 41.6 lm/W (a peak current efficiency of 39.8 cd/A)) at a low driving voltage of 3 V which increases by 55% and 27% compared to that of corresponding single-host (SH) and double emitting layer (DML) devices, respectively. Especially very long work lifetime (3530 hs) at an initial luminance of 500 cd/m(2) of the mixed hosted device is exhibited, rising by about 4.1 and 2.46 times relative to that of corresponding SH and DML devices. High efficiency and longer working lifetime was attributed to the absence of heterojunction and balanced charge carrier transport characteristics in the mixed host based OLED structure. The more detail mechanism was also presented.

Novel tetraarylsilan-centred compounds as single host for white organic light-emitting diodes with high efficiency and low roll-off

Novel tetraarylsilane-centred compounds (DCzSiPy and DDCzSi) have been introduced in fabricating efficient white organic light-emitting diodes. Owing to its bipolar transportation characteristics of DCzSiPy, more balanced charge carrier injected into the emitting layer is achieved when it is selected as a single host. Here, double-carbazole-based material DDCzSi is used as hole-transporting layer despite that it is previously reported as a host for blue-phosphorescent organic light-emitting diodes. Highly efficient white phosphorescent organic light-emitting diodes with single-host structures are demonstrated. The maximum current efficiency is 41.5 cd A−1 and the maximum power efficiency is 39.4 lm W−1. And the external quantum efficiency roll-off from maximum to that at 5000 cd m−2 is only 19.8%. We adjust the proportion of the blue/orange emitting layer, lower roll-off of 14.7% is obtained. However, the maximum current efficiency and power efficiency are reduced to 36.7 cd A−1 and 33.2 lm W−1, respectively.

Low driving voltage white organic light-emitting diodes with high efficiency and low efficiency roll-off

Single emission layer white organic light-emitting diodes (WOLEDs) showing high color stability, low turn-on voltage, high efficiency and low efficiency roll-off by incorporating iridium(III) bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)2(acac)) phosphors dyes have been demonstrated. Our WOLEDs without any out-coupling schemes as well as n-doping strategies show low operating voltages, low turn-on voltage (defined for voltage to obtain a luminance of 1cd/m2) of 2.35V, 79.2cd/m2 at 2.6V, 940.5cd/m2 at 3.0V and 10300cd/m2 at 4.0V, respectively, and achieve a current efficiency of 40.5cd/A, a power efficiency of 42.6lm/W at a practical brightness of 1000cd/m2, and a low efficiency roll-off 14.7% calculated from the maximum efficiency value to that of 5000cd/m2. Such improved properties are attributed to phosphors assisted carriers transport for achieving charge carrier balance in the single light-emitting layer (EML). Meanwhile the host–guest energy transfer and direct exciton formation process are two parallel pathways serve to channel the overall excitons to dopants, greatly reduced the unfavorable energy losses.

Understanding the influence of doping in efficient phosphorescent organic light-emitting diodes with an organic p–i–n homojunction

We report on the development and detailed investigation of highly efficient p–i–n phosphorescent organic light-emitting diodes (PhOLEDs) using 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) as a single organic semiconductor matrix. Following optimization of doping concentration of both the phosphorescent emitter molecule and of the p- and n-type dopants, an external quantum efficiency (EQE) of 15% and a power efficiency (PE) of 28lm/W are realized at a luminance of 1000cd/m2. These values are comparable to the state-of-the-art for conventional complex multilayered PhOLEDs. By analyzing the device characteristics (i.e. electroluminescence spectra, the current density–voltage behavior of single carrier devices, the transient electroluminescent decay, and the impedance spectroscopy response), we find that the device performance is closely linked to the charge carrier balance in the device, which in turn is governed by the interplay of the conductivities of the doped layers and the transport of each charge carrier species within the emitting layer.

Enhancement of charge carrier recombination efficiency by utilizing a hole-blocking interlayer in white OLEDs

Charge carrier balance and recombination are essential factors relating to the performance of white organic light-emitting devices (WOLEDs). In this study, we discussed the contribution of charge carrier balance in the interlayer-based WOLEDs. By varying the interlayer thickness, the mechanisms of electroluminescent spectral alteration, energy transfer, and especially, charge carrier transport and balance in the devices were investigated and revealed in detail. With a 5 nm thick interlayer tailoring charge carrier transport and recombination, WOLEDs yielded a high power efficiency, current efficiency and external quantum efficiency of 36.1 lm W−1, 47.1 cd A−1 and 18.3%, respectively. Additionally, single-carrier devices and quantitative analysis were subsequently carried out, demonstrating that the enhancement of carrier recombination efficiency corresponds to the optimization of device performance.

Electroluminescence Characteristics of a New Green-Emitting Phenylphenothiazine Derivative with Phenylbenzimidazole Substituent

A new green-emitting material with donor-acceptor architecture, 3,7-bis(1'-phenylbenzimidazole-2'-yl)-10phenylphenothiazine (BBPP) was synthesized and its thermal, optical, and electroluminescent characteristics were investigated. Organic light-emitting diodes (OLEDs) with four different multilayer structures were prepared using BBPP as an emitting layer. The optimized device with the structure of [ITO/2-TNATA (40 nm)/ BBPP (30 nm)/TPBi (30 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (100 nm)] exhibited efficient green emission. Enhanced charge carrier balance and electron mobility in the organic layers enabled the device to demonstrate a maximum luminance of 31,300 cd/m 2 , a luminous efficiency of 6.83 cd/A, and an external quantum efficiency of 1.62% with the CIE 1931 chromaticity coordinates of (0.21, 0.53) at a current density of 100 mA/ cm 2 .

Investigation of pure green-colour emission from inorganic-organic hybrid LEDs based on colloidal CdSe/ZnS quantum dots

We studied the operation characteristics of inorganic–organic hybrid light emitting devices (HLEDs) that have a green CdSe/ZnS quantum dot (QD) emitting layer (EML) and a 1,3,5–tris(N–phenylbenzimidazol–2–yl)–benzene (TPBi) hole blocking layer (HBL). A series of devices with a different thickness TPBi HBL all showed pure electroluminescence that is consistent with green photoluminescence of colloidal QDs without any colour contamination from organic layers. The best performance, which we attributed to more balanced charge carrier supply to a QD EML and efficient energy transfer from excitions in a TPBi layer to QDs, was observed from the device with a TPBi HBL of 20 nm thickness: the deep green emission with the luminous efficiency of 1.55 cd/A at standard video brightness (200 cd/m²) and the luminance as large as ~1800 cd/m².

PVK空穴传输层对Ndq3近红外发光二极管性能的影响

A three-layered near-infrared OLED with structure of ITO/PEDOT∶PSS/PVK/neodymium tris-(8-hydroxyquinoline)(Ndq3)/Al was fabricated,in which poly(N-vinylcarbazole)(PVK) was hole transport layer,and poly(3,4-ethylene dioxythiophene)∶poly(styrenesulfonate)(PEDOT∶PSS) was hole injection layer.The EL spectrum shows three emission peaks at 905,1 064,1 340 nm in the near-infrared range which are correspond to Nd3+ transitions of 4F3/2→4I9/2,4F3/2→4I11/2,4F3/2→4I13/2,respectively.The influence mechanism of functional layers was discussed with help of I-V curves,in reference with single layer device(ITO/Ndq3/Al) and double layer device(ITO/PVK/Ndq3/Al).The increase in device current corresponding to the reduction of the series resistance was observed and was related to the high conductivity of PEDOT∶PSS layer.The joint action on the hole injection barrier by PVK and PEDOT∶PSS was contributed to the improvement of the charge carriers balance of device and EL emission intensity.Also,the decrease of the ITO surface roughness was realized as one of the key issue for the performance improvement.

Phosphorescent white organic light-emitting devices with color stability and low efficiency decay by using wide band-gap interlayer

High-performance phosphorescent white organic light-emitting devices (PhWOLEDs) with color stability and low efficiency decay are demonstrated by inserting wide band-gap materials between emitting layers. The two devices with N,N′-dicarbazolyl-3,5-benzene (mCP) and p-bis(triphenylsilyl)benzene (UGH2) as the interlayer exhibit both slight Commission Internationale del’Eclairage (CIE) coordinates variations of (± 0.010, ± 0.005) and (± 0.013, ± 0.006) in a wide voltage range, and low decay in current efficiency which shifts from the peak value 35.4 cd·A−1 and 27.4 cd·A−1 to 28.8 cd·A−1 and 23.5 cd·A−1 at 40000 cd·m−2, respectively. The improvements are attributed to the charge carriers balance and the elimination of energy transfer loss by confining the carrier accumulation at the exciton formation interface through the interlayer.

Effects of the Na2CO3 dopant on electron injection and transport in organic light emitting devices

Na2CO3 is used as an n-type dopant to improve the electro-optical properties of tris(8-hydroxyquinoline)aluminum (Alq3)-based organic light-emitting diodes (OLEDs). Device properties such as the turn-on voltage, maximum luminance, and device efficiency were improved as Na2CO3 was doped. The higher mobility of Na2CO3-doped samples has been derived using space-charge-limited current measurements. Photoelectron spectroscopy results show that some electrons transfer from Na2CO3 into Alq3, which moves the Fermi level close to the lowest unoccupied molecular orbital of Alq3. Therefore, both the electron-transporting ability and electron-injection efficiency were enhanced, which improves the charge carrier balance in OLEDs and leads to better device efficiency.

Mechanisms of the improved organic photovoltaic performance by using electron-transporting CuPc derivative as anode modifying layer

The authors demonstrate the mechanisms of the improved performance of the organic photovoltaic (OPV) cells by using an electron-transporting copper phthalocyanine (CuPc) derivative, hexadecafluoro-copper-phthalocyanine (F16CuPc) as the anode buffer layer. The power conversion efficiency and long-term lifetime of the OPV cells with 3nm F16CuPc as the anode buffer layer are increased by 50% and 165%, respectively, compared to the reference cell. X-ray diffraction analysis suggests that increased crystalline of the CuPc layer is found, leading to increased hole mobility and exciton diffusion length when it is deposited on F16CuPc. As a result, a balanced charge carrier transport between CuPc and C60, decreased charge carrier recombination probability, and increased exciton dissociation efficiency can be obtained. Meanwhile, the built-in potential of the cell is also increased, which further promotes the extraction of the photogenerated charge carriers and increases the open circuit voltage of the cells. And the prolonged long-term lifetime of the cells is attributed to the suppressed formation of the reactive −O species on ITO surface and the prevented penetration of them into the organic layers.

Balanced charge carrier mobilities in bulk heterojunction organic solar cells

All solution processed ambipolar bulk heterojunction organic field effect transistors were fabricated using Au and Al as hole and electron injection contacts to study both types of charge carrier mobilities in a single device. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were mixed with different ratios. Well balanced electron and hole mobilities were obtained at 1:0.3 ratio. Organic solar cells with the same ratio yielded draft enhancement in device performance parameters, indicating optimized carrier transport in the bulk of solar cell devices.

Effect of a thermally evaporated bis (2-methyl-8-quninolinato)-4-phenylphenolate cathode buffer layer on the performance of polymer photovoltaic cells

We investigated the device characteristics of polymer photovoltaic (PV) cells based on a poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61 butyric acid methyl ester (PCBM) bulk heterojunction with a cathode buffer layer of thermally evaporated bis (2-methyl-8-quninolinato)-4-phenylphenolate (BAlq). A power conversion efficiency (PCE) of 2.46% was obtained with the insertion of a 4-nm-thick BAlq, which was ∼118% increase over that for the cell without a BAlq layer, under Air Mass 1.5 Global (AM 1.5 G) illumination, 100 mW/cm2. Moreover, we examined the charge carrier transport property, and found that the hole mobility of the cell was enhancement due to the insertion of a BAlq layer with a thickness of less than 4 nm, which accounted for the improved in the photocurrent and fill factor (FF) due to the better balance of charge carriers. Finally, the BAlq buffer layer was also demonstrated as an optical spacer that improved the optical absorption of the P3HT:PCBM layer, which accounted for the Jsc enhancement of the device.