Hole-only Devices Research Articles

Dependence of the stability of organic light-emitting diodes on driving mode

We investigated the effects of pulsed current (PC) and direct current (DC) driving modes on the stability of organic light-emitting diodes with and without hole-injection layers (HILs). Two different HIL materials were used: copper phthalocyanine (CuPc) and 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MDTATA). It was found that the half-lives of devices using PC driving modes were different from those of comparable devices using DC driving modes. For the devices without HILs, with CuPc HILs and with MTDATA HILs, the half-lives of the devices were changed by factors of 1.91, 1.41 and 0.86, respectively, when operated in PC rather than DC driving modes. Our analysis of the electrical characteristics of the corresponding hole-only devices showed that the number of holes injected into devices was greatly reduced by inserting an m-MTDATA layer compared with other designs. The results indicate that different ratios of injected electrons and holes can be obtained in these devices. Moreover, these ratios play a dominant role in the dependence of the stability of the device on the driving mode.

Enhanced charge-carrier injection caused by molecular orientation

Current densities of hole-only devices based on alpha-sexithiophene (α-6T) and N-N′-diphenyl-N-N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) increase 42 times at 1 V by rubbing α-6T with a Nylon cloth. Results of absorption spectroscopy with normal and oblique light incidence reveal that the rubbing induces a change from standing to lying orientations of α-6T in a film surface region. The increased current densities are attributed to enhanced α-6T-to-α-NPD hole injection due to the lying orientation.

P‐181: Device Parameters Determination by Numerical Model Fitting for Organic Light‐Emitting Diodes (OLEDs) using Impedance Spectroscopy Measurement

AbstractWe have developed a numerical fitting methodology which enables the determination of device parameters for OLEDs from impedance spectroscopy (IS) measurement, temperature dependence measurement and dark injection space charge limited current (DI‐SCLC) measurements. In our method, carrier mobility is estimated from DI‐SCLC, density of state (DOS) and interface state density are obtained from model fitting of real/imaginary part of capacitance measured by IS, barrier height of interface is estimated by modified Schottky model fitting of temperature dependence current‐voltage (I—V) measurements. Using this approach we have determined barrier heights, DOS, interface state densities, carrier mobilities and Richardson factors for each interface and layer in OLED devices. In case of the ITO/NPD interface, barrier height, DOS, interface state density and ent methods for extraction of device parameters such as carrier mobility, density of state (DOS), barrier height.1–17In these methodologies impedance spectroscopy (IS) is a useful tool for evaluating relaxation, transport and injection in various organic devices.4,9–17 Usually IS measurements allow the determination of equivalent circuits, DOS and carrier mobility of the organic semiconductor layers.In order to determine the carrier injection behaviour, we propose a numerical model and model fitting method included interface state density (Dit) for IS measurement results of hole‐only devices (HOD) and electron‐only devices (EOD). Therefore, a recombination model is also proposed for the negative capacitance behaviour of OLEDs.Naito et. al. 18have also proposed a numerical model included DOS of organic semiconductor for OLEDs. We obtain DOS from Naito's model and Dit from our model. Dit is expected to be a key parameter of the device degradation mechanism.

Charge Transport Study of 2,2',7,7'-Tetrakis(N,N-di-4-methoxyphenyl amino)-9,9'-spirobifluorene Using Impedance Spectroscopy

The conduction mechanism in 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenyl amino)-9,9'-spirobifluoren (spiro-MeOTAD) has been studied by impedance spectroscopy (IS) by using indium tin oxide (ITO)/spiro-MeOTAD/gold (Au) hole only device. The device could be modelled by an equivalent parallel RC network with a low contact resistance Rs in series, in which the bulk geometrical capacitance above resonance frequency of spiro-MeOTAD remains unchanged while the bulk resistance Rp changes with bias voltage. Upon increasing the bias voltage the conductivity of deposited film has increased by one order of magnitude (i.e., from ∼10-9 to ∼10-8 Ω-1 cm-1) whereas the dielectric relaxation time (τ) decreased from 1.7 ×10-4 to 1.2 ×10-5 s. Capacitance spectra of hole-only spiro-MeOTAD devices indicate the validity of the Space charge limited current (SCLC) model with localised states distributed in energy and field-dependent mobility.

Charge Transport Study of 2,2',7,7'-Tetrakis(N,N-di-4-methoxyphenyl amino)-9,9'-spirobifluorene Using Impedance Spectroscopy

The conduction mechanism in 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenyl amino)-9,9'-spirobifluoren (spiro-MeOTAD) has been studied by impedance spectroscopy (IS) by using indium tin oxide (ITO)/spiro-MeOTAD/gold (Au) hole only device. The device could be modelled by an equivalent parallel RC network with a low contact resistance Rs in series, in which the bulk geometrical capacitance above resonance frequency of spiro-MeOTAD remains unchanged while the bulk resistance Rp changes with bias voltage. Upon increasing the bias voltage the conductivity of deposited film has increased by one order of magnitude (i.e., from ∼10-9 to ∼10-8 Ω-1 cm-1) whereas the dielectric relaxation time (τ) decreased from 1.7 ×10-4 to 1.2 ×10-5 s. Capacitance spectra of hole-only spiro-MeOTAD devices indicate the validity of the Space charge limited current (SCLC) model with localised states distributed in energy and field-dependent mobility.

Determination of charge carrier mobility in tris(8-hydroxy-quinolinato) aluminum by means of impedance spectroscopy measurements

The hole mobility of tris(8-hydroxy-quinolinato) aluminum (Alq 3) thin films and double layer films of a 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPB) and Alq 3, have been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement with IS rests on a theory for single-injection space-charge limited current. The NPB thin film operates as an injection layer to reduce the hole injection barrier between an injection electrode and the Alq 3 thin film. The hole mobilities of Alq 3 thin films measured with IS are identical to those measured by a time-of-flight (TOF) transient photocurrent technique. The hole drift mobility determined using IS measurements in NPB/Alq 3 double layer hole-only devices where the thickness of NPB is comparable to that of Alq 3 are also identical to the hole drift mobility of Alq 3 layer. The origin of the observation is discussed using a device simulation and the results are useful for the determination of charge carrier mobility in multilayer devices.

Optimisation of PCPDTBT solar cells using polymer synthesis with Suzuki coupling

Process optimisation for the fabrication of PCPDTBT/PCBM-based bulk heterojunction solar cells using polymers synthesised by Suzuki coupling is described. The relationship between the chemical purity of the polymers and the performance of the devices was examined and by using a combination of OFET, OPV and hole-only devices, the best polymer purification technique identified. The purified polymers were used to optimise the fabrication of solar cells by varying the donor–acceptor blend ratio, the film thickness, the drying conditions and the concentration of the dithiol process additive.

Carbazole-Based Copolymers: Effects of Conjugation Breaks and Steric Hindrance

A series of carbazole-based D-π-A copolymers were synthesized to investigate the influences of conjugation length and structural distortion on intramolecular charge transfer (CT) complexation between the donor (D) and acceptor (A) components. Carbazole presents two possible linkage sites, the 2,7- and 3,6-positions, which led to significant differences in the thermal, photophysical, electrochemical, and electrical properties of the copolymers due to the positioning of the electron-rich nitrogen atom with respect to the copolymer backbone. The copolymers were comprehensively characterized using TGA, DSC, UV−vis, and photoluminescence spectroscopy, cyclic voltammetry, and DFT calculations. P(3,6C-DTBT), which was linked by a thienyl-2′,1′,3′-benzothiadiazole (DTBT) group at the 3,6-positions of the carbazoles so as to directly involve the electron-rich nitrogen atoms in conjugation, exhibited conjugation breaks in the middle of the carbazole units. The breaks resulted in a robust coplanar structure with an extraordinarily low oxidation potential and the ability to stably generate excitons, in contrast with P(2,7C-DTBT), which was linked by DTBT at the 2,7-positions of the carbazole. Two additional hexyl substituents at the 4-position of the thiophene in the DTBT groups of P(2,7C-HDTBT) and P(3,6C-HDTBT), which were identical to P(2,7C-DTBT) and P(3,6C-DTBT), respectively, except for the presence of the substituents, introduced steric hindrance between the D and A units, thereby breaking the coplanarity. Finally, the hole mobilities of the 3,6-carbazole-based copolymers were 1 order of magnitude higher than those of 2,7-carbazole-based copolymers, measured in hole-only devices. This result indicated the presence of stable radical cations and dications at the nitrogen atoms of the copolymers. This work deepens our understanding of carbazole-based D-π-A copolymers and provides insight into the design of novel materials for optoelectronic devices.

Odd–even modulation of electrode work function with self-assembled layer: Interplay of energy barrier and tunneling distance on charge injection in organic light-emitting diodes

In this study, a series of trifluoromethyl-terminated alkanethiols with different chain length (CF 3(CH 2) n SH, n = 5, 6, 10, 11, 12 and 13), were used to form self-assembled monolayers(SAMs) on silver and gold surfaces. The SAM modulates the metal work function as a result of the different orientation of the terminal dipolar CF 3 group due to different chain tilt of the SAM on Au and Ag surfaces, respectively. The SAM-modified Ag and Au were used as the anode in the fabrication of hole-only devices as well as top-emitting organic light-emitting diodes (TOLEDs) with different hole-transporting layers. The correlation between current density/luminous efficiency and the SAM chain length/work function were analyzed to provide information on charge balance. Thus with NPB as the hole-transporting layer, where little difference between the HOMO energy level and the work function of the SAM-modified electrode, the charge injection is modulated by the chain length (tunneling distance-dependent) of the SAM used for modification, decreasing with increasing chain length. With BPAPF as the HTL, where a barrier is present, the charge injection is modulated more by the work function (injection barrier-dependent), which shows an odd–even alternation.

Interfacial charge transfer and charge generation in organic electronic devices

We have recently proposed that improvement of device performance using a buffer layer of molybdenum trioxide (MoO3) originates from interfacial charge generation at an interface of MoO3 and an organic hole-transport layer [17]. However, there is no clear experimental evidence enough to support the charge generation in our recent report. In this study, from comparison of current density–voltage characteristics of organic hole-only devices and ultraviolet/visible/near-infrared absorption spectra of composite films, we can conclude that the interfacial charge generation surly occurs to realize space-charge-limited currents of a wide variety of organic hole-transport layers. Moreover, a drastic increase in current density of a bilayer device of n-type C60 and p-type N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) by using a MoO3 layer can provide the evidence of the charge generation.

Manipulation of Work Function and Surface Free Energy of Tungsten Oxide Hole Injection Layer Modified with a Self-Assembled Monolayer

As a hole injection layer for organic devices, a tungsten oxide (WOx) thin film was vapor-deposited on an indium–tin oxide (ITO) substrate, on which a self-assembled monolayer (SAM) of either 3-aminopropyltrimethoxysilane (APS), phenyltrimethoxysilane (PTMS), or octadecyltrimethoxysilane (ODS) was prepared to modify the surface characteristics. The deposition of WOx substantially increased the ionization potential (Ip) of the substrate surface, which was effective in enhancing hole injection. The formation of SAM on WOx reduced Ip, but enabled the control of the surface free energy so as to modify the growth morphology of an organic film deposited on its surface. A hole-only device was prepared using a hole transport material of N,N '-diphenyl-N,N '-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD). In the space-charge-limited region, a high current was drawn by using an anode that has a high Ip. At low driving voltages, however, the current flow was considerably influenced by the surface free energy. It was found that the PTMS-SAM on WOx gives a satisfactory accommodation of both the work function and the surface energy.

Enhanced Driving Performance of Organic Light-Emitting Diodes with All Carrier Ohmic-Contacts

This paper elucidates the enhancement of the optoelectronic properties of organic light-emitting diodes (OLEDs) by the p-doping effect of molybdenum oxide -doped -tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA). The device performance strongly depends on both the thickness and the doping concentration of the -doped 2-TNATA. As the doping concentration is increased from 25 to 75%, the hole ohmic properties of the hole-only device with the glass/indium tin oxide -doped 2-TNATA structure were improved, due to the increase in the p-type doping effect. Also, the photoemission spectra revealed that the p-type doping effects cause the lowering of the hole-injecting barrier height, as well as the improvement of the hole conductivity. This improvement is caused by the formation of the charge transfer complex that is generated by doping into 2-TNATA, markedly increasing the number of hole carriers. With -doped 2-TNATA as a hole ohmic contact and fullerene /lithium fluoride (LiF) as an electron ohmic contact, the OLED with the glass/ITO/-doped 2-TNATA (75%, )/NPB structure showed both a maximum luminance of at and power efficiency of at about .

Highly Efficient Simplified Organic Light-Emitting Diodes Utilizing F4-TCNQ as an Anode Buffer Layer

We demonstrate that the electroluminescent performances of organic light-emitting diodes (OLEDs) are significantly improved by evaporating a thin F4-TCNQ film as an anode buffer layer on the ITO anode. The optimum Alq3-based OLEDs with F4-TCNQ buffer layer exhibit a lower turn-on voltage of 2.6 V, a higher brightness of 39820 cd/m2 at 13 V, and a higher current efficiency of 5.96 cd/A at 6 V, which are obviously superior to those of the conventional device (turn-on voltage of 4.1 V, brightness of 18230 cd/m2 at 13 V, and maximum current efficiency of 2.74 cd/A at 10 V). Furthermore, the buffered devices with F4-TCNQ as the buffer layer could not only increase the efficiency but also simplify the fabrication process compared with the p-doped devices in which F4-TCNQ is doped into β-NPB as p-HTL (3.11 cd/A at 7V). The reason why the current efficiency of the p-doped devices is lower than that of the buffered devices is analyzed based on the concept of doping, the measurement of absorption and photoluminescence spectra of the organic materials, and the current density-voltage characteristics of the corresponding hole-only devices.

Characterization of the hole transport and electrical properties in poly(9,9-dioctylfluorene)

A systematic study of the hole transport and electrical properties in blue-emitting polymers as poly(9,9-dioctylfluorene) (PFO) has been performed. We show that the temperature dependent and thickness dependent current density versus voltage characteristics of PFO hole-only devices can be accurately described using our recently introduced improved mobility model based on both the Arrhenius temperature dependence and non-Arrhenius temperature dependence. Within the improved model, the mobility depends on three important physical quantities: temperature, carrier density, and electric field. For the polymer studied, we find the width of the density of states σ=0.115 eV and the lattice constant a=1.2 nm. Furthermore, we show that the boundary carrier density has an important effect on the current density versus voltage characteristics. Too large or too small values of the boundary carrier density lead to incorrect current density versus voltage characteristics. The numerically calculated carrier density is a decreasing function of distance from the interface. The numerically calculated electric field is an increasing function of distance. Both the maximum of carrier density and minimum of electric field appear near the interface.

Comparative Study on Hole Transport in N, N′-bis(naphthalen-1-yl)-N,N′-bis(pheny) Benzidine and 4, 4′, 4"-tri(N-carbazolyl)triphenylamine

Hole transport characteristics in N,N′-bis(naphthalen-1-yl)-N,N′-bis(pheny) benzidine (NPB) and 4,4′, 4"-tri(N-carbazolyl)triphenylamine (TCTA) are comparatively investigated. The current density-voltage (J – V) characteristics of hole-only devices based on NPB and TCTA at different temperatures and thicknesses show that the hole current is dominated by the bulk conduction with an exponential trap distribution. Detailed analyses of the J – V characteristics give the trap densities Nt of (6.3 ± 0.3) × 1018 and (1.9 ± 0.02) × 1018 cm3, characteristic trap depths of 135 ± 6 and 117 ± 5 meV, hole mobilities of (8.1 ± 0.5) × 10−5 and (1.9 ± 0.1) × 10−4 cm2 V−1 s−1 for NPB and TCTA, respectively. It is found that TCTA exhibits higher hole mobility. Obviously, this is directly related to the lower trap density and shallow trap depth in TCTA films, leading to good charge carrier transport.

Bright electroluminescent devices with tunable spectra obtained by strictly controlling the doping concentration of electron injection sensitizer

In this paper, we report an efficient strategy to design bright blue and blue–green electroluminescent (EL) devices by slightly doping tris(8-hydroxyquinoline) aluminum (Alq3) into N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) as the light-emitting layer (EML). Bright EL devices with tunable spectra were obtained by strictly controlling the doping concentration of Alq3. With increasing current density, EL efficiencies of these devices increase first and then decrease gradually after reaching the maximum. Analyzing the current density–voltage (J−V) characteristics of hole-only and electron-only devices, we found the presence of Alq3 molecules in EML not only facilitates the injection of electrons from hole block layer (HBL) into EML but also stays the transport of holes in EML, thus causing significant enhancement of EL efficiency and brightness due to improved carriers balance and broadening of recombination zone. More interestingly, the doping concentration of Alq3 strongly influences the injection and transport processes of electrons, thus determining the distribution of holes and electrons on NPB and Alq3 molecules.

Hole transport in the organic small molecule material α-NPD: evidence for the presence of correlated disorder

In this paper the hole mobility in the amorphous small molecule material N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD), which is frequently used in organic light-emitting diodes, is studied. From an analysis of the temperature and layer thickness dependence of the steady-state current density in sandwich-type α-NPD-based hole-only devices, it is found that a conventional mobility model assuming a Poole–Frenkel type field dependence and neglecting the carrier density dependence is not appropriate. Consistent descriptions with equal quality are obtained within the framework of two forms of the Gaussian disorder model (GDM and CDM), within which the presence of energetic disorder is described by a Gaussian density of states and within which spatial correlations between the site energies are absent or are included, respectively. Both models contain a carrier density dependence of the mobility. Based on a comparison of the site densities as obtained from both models with the molecular density, we argue that the analysis provides evidence for the presence of correlated disorder.

UV-ozone-treated ultra-thin NaF film as anode buffer layer on organic light emitting devices

An ultra-thin NaF film was thermally deposited between ITO and NPB as the buffer layer and then treated with the ultraviolet (UV) ozone, in the fabrication of organic light emitting diodes (ITO/NaF/NPB/Alq(3)/LiF/Al) to study its effect on hole-injection properties. The treatment drastically transforms the role of NaF film from hole-blocking to hole-injecting. This transformation is elucidated using hole-only devices, energy band measurement, surface energy, surface polarity, and X-ray photoelectron spectra. With the optimal thickness (3 nm) of the UV-ozone-treated NaF layer, the device performance is significantly improved, with a turn-on voltage, maximum luminance, and maximum current efficiency of 2.5 V, 15700 cd/m(2), and 4.9 cd/A, respectively. Results show that NaF film is not only a hole-blocking layer, but also a promising hole-injecting layer after UV-ozone treatment.

Enhancing Efficiency of Organic Light-Emitting Diodes Using a Carbon-Nanotube-Doped Hole Injection Layer

A nanocomposite layer consisting of multiwalled carbon nanotube (MWCNT)-doped poly(3,4,-ethylene dioxythiphene):poly(styrene sulfonic acid) (PEDOT:PSS) was employed as a hole injection layer (HIL) in an organic light-emitting diode (OLED). The structure of the OLED is glass/indium-tin oxide (ITO)/MWCNT-doped PEDOT:PSS/PEDOT:PSS/tris(8-hydroxyquinolinato)aluminum (Alq3)/LiF/Al. The luminous efficiency of the OLED is as high as 2.1 cd/A, which is 70% higher than that of a conventional device without an MWCNTs-doped HIL. The MWCNTs in the PEDOT:PSS act as a hole-blocking material that results in a reduction in holes transport ability, and therefore, in a balance between electron and hole mobilities, leading to enhanced luminescent performance. The mechanism was well demonstrated by through investigations by atomic force microscopy (AFM) of the film surface morphology and carrier injection properties of hole-only devices.

Relation between the built-in voltage in organic light-emitting diodes and the zero-field voltage as measured by electroabsorption

Change of title: On the determination of the built-in voltage in organic light-emitting diodes from electro-absorption measurements. For developing understanding of the current density onset voltage and injection barriers in organic light-emitting diodes (OLEDs), a precise determination of the built-in voltage, Vbi, is of crucial importance. Commonly, Vbi is assumed to be equal to the voltage V0,EA at which in an electroabsorption (EA) experiment the reflection of light at the OLED is found to become insensitive to a small voltage modulation. However, this assumption is shown to lead to significant errors for devices with well-injecting contacts. From an analysis of EA experiments for hole-only devices containing a polyfluorene-based copolymer, it is shown that V0,EA may be interpreted as an effective current density onset voltage, agreeing with the commonly accepted picture, but that for these devices Vbi is ~0.5 V larger than V0,EA. This is found to be consistent with predictions of V0,EA from model calculations of the electric field and light-absorption profiles in the semiconducting layer.