Chlorinated Indium Tin Oxide Research Articles

A Direct surface modification strategy of ITO anodes enables high-performance organic photodetectors

A direct modification strategy of indium tin oxide (ITO) for high-performance organic photodetectors (OPDs) is developed. By using chlorinated ITO (ITO-Cl) as the anode, the OPD device exhibits superior performance than ITO/PEDOT:PSS-based device.

Effects of Cl2 plasma treatment on stability, wettability, and electrical properties of ITO for OLEDs

We investigated various characteristics of chlorinated indium tin oxide (Cl-ITO) for highly efficient organic light-emitting diodes (OLEDs): the work function, surface morphology, wetting characteristic, and hole-injection property. Via the systematic analysis of Cl-ITO, we showed that the modification of ITO using the plasma treatment method has a simple process and offers higher stability than that achieved with a solution-based modification method. We fabricated Cl-ITO with a high work function of 6.04 eV, which is 1.04 eV higher than that of pristine ITO. We verified that Cl2 plasma treatment does not affect the ITO surface morphology. The Cl halogenated surface showed a low surface polarity and influenced the growth of the organic material, especially for films as thin as ∼4 nm. Therefore, we found when using ultra-thin film on Cl-ITO for design devices, the wetting characteristic of Cl-ITO should be considered. In addition, using tris-(4-carbazoyl-9-ylphenyl)-amine resulted in increased current and power efficiencies compared with those obtained using 4,4′-N,N′-dicarbazolebiphenyl. When a carrier-only device was used, the carrier-injection characteristics of Cl-ITO were higher than those of other well-known hole-injection materials. Furthermore, the Cl-ITO OLEDs showed higher stability than OLEDs with other hole-injection materials.

Performance enhancement of single layer organic light-emitting diodes using chlorinated indium tin oxide as the anode.

Effective green single-layer organic light-emitting diodes (OLEDs) are reported with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as a dopant and chlorinated indium tin oxide (Cl–ITO) as a transparent anode. The work function of the chlorinated ITO is manipulated to be ∼5.3 eV from ∼4.7 eV for bared ITO. The improvement in anode workfunction allows the direct hole injection into the HOMO of the phosphorescent dopant. As a result, the green phosphorescent OLEDs with simple single layers can deliver a current efficiency (CE) and external quantum efficiency (EQE) as high as 33.48 cd A−1 and 10.1%, respectively.

Green and low-cost approach to modify the indium tin oxide anodes in organic light-emitting diodes by electrochemical treatment in NaCl aqueous solution

We demonstrate an environment-friendly, simple, and low energy cost approach as an alternative to conventional O2 plasma treatment to modify the surface of indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs). ITO is electrochemically treated in NaCl aqueous solution. A chlorinated ITO (Cl-ITO) electrode with a work function of 5.41eV was obtained, which is 0.66eV higher than that of pre-cleaned ITO. The increase of work function is due to the anodic oxidation reactions occurred on the surface of ITO. The power dissipation is only ∼3mW in our approach, which is five orders of magnitude lower than that of O2 plasma treatment (∼100W). We fabricated the OLEDs with the configuration of Cl-ITO/NPB(35nm)/CBP:Ir(ppy)3 (15nm, 8wt%)/TPBi:Ir(ppy)3 (10nm, 8wt%)/TPBi (10nm)/Bphen (50nm)/Cs2CO3 (2nm)/Al (100nm), where NPB is N, Ń-di-1-naphthyl-N, Ń-diphenylbenzidine, CBP is 4′-bis(carbazol-9-yl)biphenyl, TPBi is 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), Ir(ppy)3 is bis(3-phenylpyridine) iridium(III) and Bphen is 4,7-diphenyl-1,10-phenanthroline. A maximum power efficiency of 95.0lmW−1 and external quantum efficiency (EQE) of 24.2% were achieved, respectively, which was slightly higher than that of the OLED fabricated on O2-plasma-treated ITO (91.2lmW−1, EQE=23.1%).

High Brightness Fluorescent White Polymer Light-Emitting Diodes by Promoted Hole Injection via Reduced Barrier by Interfacial Dipole Imparted from Chlorinated Indium Tin Oxide to the Hole Injection Layer PEDOT:PSS.

We demonstrated that introducing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as a hole transport layer (HTL) on top of chlorinated indium tin oxide (Cl-ITO) anode can lead to a deeper highest occupied molecular orbital level of the HTL (promoting from 5.22 to 5.42 eV) due to the interfacial dipole imparted by the Cl-ITO, which allows barrier-free hole injection to the emitting layer with polyspirobifluorene doped with the yellow emitter rubrene and significantly prevents excitons quenching by residual chlorine radicals on the surface of Cl-ITO. By use of poly[9,9-bis(6'-(18-crown-6)methoxy)hexyl)fluorene] chelating to potassium ion (PFCn6:K+) as electron injection layer and air-stable high work function (EΦ) metal aluminum as the cathode, the performance of fluorescent white polymer light-emitting diode (WPLED) achieves the high maximum brightness (Bmax) of 61 523 cd/m2 and maximum luminance efficiency (ηL, max) of 10.3 cd/A. Replacing PFCn6:K+/Al cathode by CsF/Al, the Bmax and ηL, max are promoted to 87 615 cd/m2 (the record value in WPLED) and 11.1 cd/A, respectively.

Bipolar and Unipolar Silylene-Diphenylene σ-π Conjugated Polymer Route for Highly Efficient Electrophosphorescence.

σ-π conjugated polymer strategy is proposed for designing electroluminescent host polymers with silylene-diphenylene as the backbone repeat unit giving a high triplet energy (ET = 2.67 eV). By incorporation of high ET (3.0 eV) electron (oxadiazole, OXD) and hole (triphenyl amine, TPA) transport moieties, or TPA alone (in this case, the main chain acts as electron transport channel) as side arms on the silylene, the high ET bipolar and unipolar polymers are formed, allowing a use of iridium green phosphor (Ir(ppy)2(acac), Ir-G) (ET = 2.40 eV) as the dopant. The matching of energy levels of the dopant with the hosts, leading to charge trapping into it; and singlets and triplets of the exciplex and excimer can be harvested via energy transfer to the dopant. Using these host-guest systems as the emitting layer, chlorinated indium-tin-oxide (Cl-ITO) as the anode, and benzimidazole derivative (TPBI) as the electron transport layer, this two-layer device gives the high luminance efficiency 80.1 cd/A and external quantum efficiency 21.2%, which is the best among the report values for polymer light emitting diode (PLED) in the literatures. This example manifests that σ-π conjugated polymer strategy is a promising route for designing polymer host for efficient electrophosphorescence.

Chlorinated indium tin oxide electrode by InCl3 aqueous solution for high-performance organic light-emitting diodes

The authors develop a facile and effective method to produce the chlorinated indium tin oxide (Cl-ITO) treated by InCl3 aqueous solution and UV/ozone. The work function of the Cl-ITO achieved by this treatment is as high as 5.69 eV, which is increased by 1.09 eV compared with that of the regular ITO without any treatment. Further investigation proved that the enhancement of the work function is attributed to the formation of In-Cl bonds on the Cl-ITO surface. Green phosphorescent organic light-emitting devices based on the Cl-ITO electrodes exhibit excellent electroluminescence performance, elongating lifetime due to the improvement in hole injection.

Single layer deep blue polymer light emitting diodes with chlorinated Indium Tin Oxide after surface modification for high performance

Chlorinated-Indium Tin Oxide (Cl-ITO) has been found to have higher work function than the pristine ITO. When used as anode for polymer light-emitting diodes (PLEDs) with deep blue emitting β-phase poly(9,9-di-n-octylfluorene) (β-PFO) as the emitting layer, it allows hole injection without barrier. However, the presence of chlorine free radical on the surface of Cl-ITO leads to an exciton quenching effect. The surface modification on anode by dipping into 1% ammonium (NH4OH) aqueous solution to remove free chlorine radicals on surface can enhance device performance significantly. The single layer device Cl-ITO (treated)/β-PFO/CsF/Al gives the maximum brightness 16773cd/m2 and maximum luminance efficiency 2.40cd/A, which are higher than those with untreated Cl-ITO, (2519; 0.27) and CFx treated ITO, (7800; 1.8) and PEDOT:PSS coated ITO, (12884; 1.43). The ease of the present anode treatment allows the one-layer-only device to be a promising candidate for practical application.

The effect of carbon contamination and argon ion sputtering on the work function of chlorinated indium tin oxide

The work function of indium tin oxide (ITO) was increased by treating ITO with dichlorobenzene with UV light. Carbon contamination of the Cl-ITO was measured using X-ray Photoelectron Spectroscopy (XPS) and argon ion sputtering was used to remove the carbon from the surface. It was found that the carbon contamination from residual dichlorobenzene significantly lowered the work function of the ITO and after argon ion sputtering the work function increased to 5.8 eV. It was found that chlorination of ITO occurs after more than 6 min of UV exposure. Further sputtering of ITO resulted in the removal of the functionalized chlorine, the introduction of argon ion contaminants on the ITO decreases its work function.

The origin of the high work function of chlorinated indium tin oxide

The impact of halogenation, in particular Cl and F, on the work functions of indium tin oxide (ITO) surfaces was studied using density functional theory calculations. We found that a strong surface dipole layer induced by the halogen, rather than a change in the electrochemical potential (that is, Fermi level) of the ITO, led to a dramatic increase in the work function. However, the work function for F-coated ITO was lower than that of Cl-coated ITO. This result contradicts the well-known fact that F is much more electronegative than Cl. Detailed computations reveal that both electronegativity and atomic size collectively contribute to the extraordinarily high work function of Cl-ITO. Additionally, the work function increases linearly with increasing surface halogen coverage for both systems, which was consistent with experimental data. The transparent tin-doped indium oxide (ITO) is a particularly attractive material for use in the electrodes of organic electronic devices. However, ITO suffers from a low ‘work function’ — the energy required to remove an electron from the material's surface and an important characteristic of such devices, which rely on charge transport between layers of different materials. Coating the surface of ITO with halogen atoms, particularly chlorine, has recently been shown to increase the work function. Now, Yao He at Yunnan University and co-workers in China and Canada have used density functional theory calculations to explore why this occurs. The work function of ITO increases linearly as its surface becomes covered with either chlorine or fluorine — to notably higher values with chlorine — owing to charge transfer from the material to the halogen. The magnitude of the effect is influenced by the halogen's electronegativity and ionic radius; chlorine's superior efficiency arises because it strikes the correct balance between the two. The work functions of indium tin oxide terminated with Cl and F have been studied using DFT calculations. The results show that the work function of Cl-terminated ITO is much higher than that of F-terminated ITO despite the fact that F is more electronegative than Cl. Detailed analysis through visualization of the atomic-scale charge transfer at these adatom–oxide interfaces reveals that both high electronegativity and atomic size are crucial to increase the work function of ITO.

Performance enhancement of organic light-emitting diodes by chlorinated indium tin oxide in the presence of hydrogen peroxide

We demonstrate the enhanced performance in organic light-emitting diodes by chlorinated indium tin oxide (ITO) in the presence of hydrogen peroxide. We adopt the approach of UV light irradiation of the ITO with o-dichlorobenzene and hydrogen peroxide. Adding hydrogen peroxide in the system can accelerate the rate of UV photolysis of o-dichlorobenzene and reduce the UV illumination time, and the device shows the highest performance of current efficiency 4.15cd/A. Under UV radiation, hydrogen peroxide would generate hydroxyl radical HO. to attack o-dichlorobenzene, then further reducing the UV illumination time.

Polymer solar cells using chlorinated indium tin oxide electrodes with high work function as the anode

This paper reports polymer solar cells (PSCs) using chlorinated indium tin oxide (Cl-ITO) substrates with high work function as the anode. There is no conducting polymer buffer layer between the Cl-ITO anode and the active layer in these PSCs. PSCs with P3HT:PCBM (P3HT=poly(3-hexylthiophene), PCBM=[6,6]-phenyl-C 61-butyric acid methyl ester) exhibited high photovoltaic performance, short-circuit current density ( J sc) of 9.91 mA/cm 2, open-circuit voltage ( V oc) of 0.57, fill factor (FF) of 0.69, and photovoltaic efficiency of 3.90% under AM1.5G illumination immediately after the device fabrication. The photovoltaic performance is comparable to that of normal PSCs with a conducting polymer buffer layer. But the photovoltaic efficiency of PSCs with Cl-ITO degrades quickly. The photovoltaic efficiency dropped to 3.43% and 3.24% just 10 and 20 min after the device fabrication, respectively. The reasons for the photovoltaic degradation were studied by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The work function of Cl-ITO decreases with time. The decrease in the work function of Cl-ITO is attributed to Cl desorption from the surface of ITO.

Chlorinated indium tin oxide as a charge injecting electrode for admittance spectroscopy

Admittance spectroscopy (AS) is a simple technique to measure the carrier mobility of organic semiconductors, but requires an Ohmic contact for charge injection. Chlorinated indium tin oxide (Cl-ITO) is shown to be a good charge injecting electrode for AS due to its high work function of >6.1 eV. The mobility of N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) measured with Cl-ITO was found to be in excellent agreement with the value measured by time of flight. Cl-ITO is therefore an ideal electrode for AS since it possesses a high work function, but does not introduce any series impedance into the device structure.

Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility

In organic light-emitting diodes (OLEDs), a stack of multiple organic layers facilitates charge flow from the low work function [~4.7 electron volts (eV)] of the transparent electrode (tin-doped indium oxide, ITO) to the deep energy levels (~6 eV) of the active light-emitting organic materials. We demonstrate a chlorinated ITO transparent electrode with a work function of >6.1 eV that provides a direct match to the energy levels of the active light-emitting materials in state-of-the art OLEDs. A highly simplified green OLED with a maximum external quantum efficiency (EQE) of 54% and power efficiency of 230 lumens per watt using outcoupling enhancement was demonstrated, as were EQE of 50% and power efficiency of 110 lumens per watt at 10,000 candelas per square meter.