Thermally Stable Hole‐Transporting Materials Based upon a Fluorene Core

We report a new class of diamine hole‐transporting materials (HTMs) based upon a fluorene core. Using a fluorene core, rather than a biphenyl group, leads to enhanced thermal stability, as evidenced by glass‐transition (T_g) temperatures as high as 161 °C for N,N′‐iminostilbenyl‐4,4′‐fluorene (ISF). The fluorene‐based HTMs have lower ionization potentials (I_p) than their biphenyl analogs, which leads to more efficient injection of holes from the indium tin oxide (ITO) anode, and higher quantum efficiencies. Devices prepared with fluorene‐based HTMs were operated under thermal stress. The failure of an organic light‐emitting diode (OLED) under thermal stress has a direct correlation with the thermal stability of the HTM that is in contact with the ITO anode. OLEDs based on ISF are stable to over 140 °C.     

Improved photovoltaic characteristics of organic cells with heterointerface layer as a hole-extraction layer inserted between ITO anode and donor layer

We have fabricated an improved organic photovoltaic (OPV) cell in which organic heterointerface layer is inserted between indium-tin-oxide (ITO) anode and copper-phthalocyanine (CuPc) donor layer in the conventional OPV cell of ITO/CuPc/fullerene (C₆₀)/bathophenanthroline (Bphen)/Al to enhance the power conversion efficiency (PCE) and fill factor (FF). The inserted ITO-buffer layer consists of electron-transporting layer (ETL) and hole-transporting layer (HTL). We have changed the ETL and HTL materials variously and also changed their layer thickness variously. It is confirmed that ETL materials with higher LUMO level than the work function of ITO give low PCE and FF. All the double layer buffers give higher PCE than a single layer buffer of TAPC. The highest PCE of 1.67% and FF of 0.57% are obtained from an ITO buffer consisted of 3 nm thick ETL of hexadecafkluoro-copper-phthalocyanine (F₁₆CuPc) and 3 nm thick HTL of 1,1-bis-(4-methyl-phenyl)-aminophenylcyclohexane (TAPC). This PCE is 1.64 times higher than PCE of the cell without ITO buffer and 2.98 times higher than PCE of the cell with single layer ITO buffer of TAPC. PCE is found to increase with increasing energy difference (ΔE) between the HOMO level of HTL and LUMO level of F₁₆CuPc in a range of ΔE < 0.6 eV. From the ΔE dependence of PCE, it is suggested that electrons moved from ITO to the LUMO level of the electron-transporting F₁₆CuPc are recombined, at the F₁₆CuPc/HTL-interface, with holes transported from CuPc to the HOMO level of HTL in the double layer ITO buffer ETL, leading to efficient extraction of holes photo-generated in CuPc donor layer.     

Reducing the driving voltage of organic light-emitting diodes by inserting a transparent ultrathin layer of oxidized silver as a hole-injecting layer

Organic light-emitting diodes (OLEDs) containing a transparent ultrathin layer of oxidized silver as a hole-injecting layer, placed between a indium–tin-oxide (ITO) electrode and the hole-transporting layer, were fabricated, and their electrical and luminescent properties were investigated. The OLEDs had a structure that consisted of an ITO layer; followed by an ultrathin Ag layer oxidized by a ultraviolet (UV)–ozone surface treatment; a N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) layer; a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped 9,10-diphenylanthracene (DPA) layer; a tris-(8-hydroxyquinoline) aluminum (Alq₃) layer; a lithium fluoride (LiF) layer; and a Al layer. The operating voltages of the OLEDs with the oxidized Ag (i.e., AgO_x) layer were drastically lower than those of the layer-free OLEDs, because the AgO_x layer, which had high oxidizability, contributed to hole injection as it oxidized the surface of the α-NPD layer. However, the external quantum efficiency of the OLEDs with the AgO_x layer was lower than that of the AgO_x layer-free OLEDs, suggesting that the carrier balance (i.e., the balance between the holes and electrons) became uneven in the emission layer, owing to the insertion of the AgO_x layer. It was assumed that this imbalance resulted from the number of holes in the emission layer being higher because of the increase in hole injection in the AgO_x layer.     

Improvement of the contact resistance between ITO and pentacene using various metal-oxide interlayers

Ultra-thin Al₂O₃, Ta₂O₅, and TiO₂ films were deposited on the indium tin oxide (ITO) surfaces in organic thin film transistors using the atomic layer deposition (ALD) process at room temperature, and the contact resistance was significantly improved with the increase of the dielectric constant of the interlayer. The electronic band diagrams of the pentacene/ITO structures after ALD treatment on ITO surface with various metal-oxides were measured using in situ ultra-violet photoelectron spectroscopy during the step-by-step deposition of pentacene, and these results explained the decrease of the hole injection barriers and the resulting improvement of the contact resistance between pentacene/ITO interface.     

Increase mechanism of indium-tin-oxide work function by KrF excimer laser irradiation

In this study, the increase mechanism of the indium-tin-oxide (ITO) work function (φ_w) by KrF excimer laser irradiation was investigated. From the observed x-ray photoelectron spectroscopy (XPS) results and four-point probe measurements, it is suggested that the surface chemical changes and the laser irradiation time had strong effects on the φ_w of ITO. Incorporation of oxygen atoms near the ITO surface during laser irradiation induced a peroxidic ITO surface, increasing φ_w. The induced increase of the ITO φ_w by laser irradiation could be useful for the enhancement of the hole injection in organic light emitting diodes.     

Influence of equilibrium charge reservoir formation on photo generated charge transport in TiO2/organic devices

Charge transport measurements have been performed using the photo induced charge extraction by linearly increasing voltage (photo-CELIV) technique on indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO₂/P3HT:PCBM/Cu) devices. By adjusting the offset voltage such that holes are accumulated at the ITO/TiO₂ contact we obtain space charge limited current (SCLC) extraction in the dark. Using photo-generation the current response is limited by SCLC extraction at low carrier concentrations but becomes purely recombination limited at high photo-generated carrier concentration. A 1-D drift diffusion model has been developed to simulate our results and we show that the hole blocking ITO/TiO₂ contact is responsible for the SCLC behavior. The highly reduced recombination of charges seen in P3HT:PCBM devices is necessary to obtain the large extraction current transients that are seen in the experimental measurements. By comparing the simulated dark CELIV and photo-CELIV we show that photo-generated extraction is more sensitive towards changes in the surface recombination velocity.     

MgF2缓冲层对蓝光OLED性能的影响

为了提高蓝光有机电致发光器件(OLED)的发光性能,将MgF2缓冲层插入ITO阳极与空穴传输层NPB之间,通过优化MgF2的厚度,制备了结构为ITO/MgF2(x nm)/NPB(50nm)/DPVBi:DSA-ph(30nm)/Alq3(30nm)/LiF(0.6nm)/Al(100nm)的高性能蓝光器件。实验结果表明,MgF2厚为1.0nm时,器件性能最佳,对应的器件最大电流效率达到5.51cd/A,最大亮度为23 290cd/m2(10.5V),与没有MgF2缓冲层的标准器件相比,分别提高47.3%和25.2%。对ITO表面的功函数测量结果表明,MgF2缓冲层可以有效修饰ITO表面,降低ITO与NPB之间的势垒高度差,改善空穴的注入效率,从而导致电子和空穴的注入更加平衡,激发机制更高效,实现了高性能的蓝光发射,为实现高效而稳定的全彩显示和白光照明奠定了基础。     

Interface electronic structures of organic light-emitting diodes with WO3 interlayer: A study by photoelectron spectroscopy

The energy level alignment and chemical reaction at the interface between the hole injection and transport layers in an organic light-emitting diode (OLED) structure has been studied using in-situ X-ray and ultraviolet photoelectron spectroscopy. The hole injection barrier measured by the positions of the highest occupied molecular orbital (HOMO) for N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/indium tin oxide (ITO) was estimated 1.32 eV, while that with a thin WO₃ layer inserted between the NPB and ITO was significantly lowered to 0.46 eV. This barrier height reduction is followed by a large work function change which is likely due to the formation of new interface dipole. Upon annealing the WO₃ interlayer at 350 °C, the reduction of hole injection barrier height largely disappears. This is attributed to a chemical modification occurring in the WO₃ such as oxygen vacancy formation.     

Low-cost supercritical CO2/H2O2 treatment on ITO anodes of fluorescent organic light-emitting diodes

A simple and cost-effective approach is proposed as an alternative to conventional oxygen plasma treatment to modify surface property of Indium tin oxide (ITO) anode of a fluorescent organic light-emitting diode (OLED). This was achieved by treating the ITO anode in supercritical CO₂ (SCCO₂) fluids with hydrogen peroxide (H₂O₂). The SCCO₂/H₂O₂ treatment yielded an ITO work function of 5.35 eV after 15 min treatment at 85 °C and 4000 psi, which was significant higher than 4.8 eV of the as-cleaned ITO surface and was slightly less than 5.5 eV of the ITO surface treated by oxygen plasma. The highest work function achieved was 5.55 eV after 45 min SCCO₂/H₂O₂ treatment. The SCCO₂/H₂O₂ treatment can be used to tailor the ITO work function through changing the operation pressure of the treatment. In addition, the correlated dependence of OLED performance on the ITO anodes with and without the treatments was investigated. The maximum power efficiency of 1.94 lm/W was obtained at 17.3 mA/cm² for the device with 15 min SCCO₂/H₂O₂ treatment at 4000 psi. This power efficiency was 19.3% and 33.8% higher than those of the oxygen plasma treatment and as-clean, respectively. The improvement in device efficiency by the SCCO₂/H₂O₂ treatments can be attributed to enhanced hole injection and balance in charge carriers due to increased work function and surface energy of the ITO anodes.     

Efficient cathode contacts through Ag-doping in multifunctional strong nucleophilic electron transport layer for high performance inverted OLEDs

This paper presents an efficient and stable green inverted organic light emitting diode (IOLED) using multifunctional and strong nucleophilic quality electron transport material (1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (m-bPPhenB)) with silver (Ag) as an n-dopant. By the energy level alignment study using in-situ ultraviolet photoelectron spectroscopy measurement, negligible electron injection barrier between indium tin oxide (ITO) and Ag-doped m-bPPhenB (Φ_e ≈ 0.03 eV) is observed and the electrons can be easily tunneled from ITO into Ag-doped m-bPPhenB layer. Also, Ag dopant forms coordination bonds with phenanthroline based unit, which improves electron injection from ITO. Fabricated IOLED devices using an Ag-doped m-bPPhenB have an extremely low driving voltage of 3.6 V and external quantum efficiency of 29.0%. Such good performances of IOLED are attributed to negligible electron injection barrier at the interface between ITO and Ag-doped m-bPPhenB. The Ag-doped IOLED device also shows a good air stability owing to the stable Ag n-dopant. The doping of Ag into special electron transport layer in the IOLED structure could be applicable to various displays and lighting applications.     

The role of molybdenum oxide as anode interfacial modification in the improvement of efficiency and stability in organic light-emitting diodes

It has been experimentally found that molybdenum oxide (MoO₃) as the interfacial modification layer on indium-tin-oxide (ITO) in organic light-emitting diodes (OLEDs) significantly improves the efficiency and lifetime. In this paper, the role of MoO₃ and MoO₃ doped N,N′-di(naphthalene-1-yl)–N,N′-diphenyl-benzidine (NPB) as the interface modification layer on ITO in improvement of the efficiency and stability of OLEDs is investigated in detail by atomic force microscopy (AFM), polarized optical microscopy, transmission spectra, ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). The studies on the energy level and the morphology of the films treated at different temperatures clearly show that the MoO₃ and MoO₃:NPB on ITO can reduce the hole injection barrier, improve the interfacial stability and suppress the crystallization of hole-transporting NPB, leading to a higher efficiency and longer lifetime of OLEDs.     

Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices

Cesium pivalate ((CH₃)₃CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq₃ (50 nm)/EIL/Al was used to evaluate the electron injection efficacy of (CH₃)₃CCOOCs, the results showed (CH₃)₃CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm². More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq₃:2% C545T (40 nm)/MADN (15 nm)/(CH₃)₃CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/A with Commission Internationale d’Eclairage (CIE_x,_y) color coordinates of (x = 0.30, y = 0.65) at current density of 140 mA/cm², which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties.     

The trapping,detrapping, and transport of the ambipolar charges in the electret of Polystyrene/C60 blend films

The electrons and holes were injected into the blend electrets of polystyrene and C₆₀ (PS/C₆₀) by adjusting the biases of conductive atomic force microscopy probe. We visualized the charges trapping, release, diffusion, and retention processes of the PS/C₆₀ electrets by utilizing the Kelvin Probe Force Microscopy (KPFM), and found that the localization and retention abilities of the ambipolar charges are enhanced with the increase of C₆₀ content, indicating that blending C₆₀ in PS matrix is a promising method for the charge trapping layer in transistor memory devices. Furthermore, we discussed the storage and diffusion mechanisms, and speculated that the interface of C₆₀ and PS in the blend electrets and repulsive force between charge clusters around C₆₀ are the important factors for the novel storage effect of the blend electret.     

Fine tuning hole injection for high-performance polyfluorene-based blue emitting device by adjusting work function of anode via deposition of CFx layer with proper ionization potential on indium tin oxide

By introducing CF_x thin film as hole injection layer on top of indium tin oxide (ITO) anode via plasma polymerization of CHF₃, the device with poly(9,9-dioctylfluorene) (PFO) as emitting layer, ITO/CF_x(35 W)/PFO/CsF/Ca/Al, is prepared. At the optimal C/F atom ratio using the radio frequency power 35 W, the device performance is optimal having the maximum current efficiency 3.1 cd/A and maximum brightness 8400 cd/m². This is attributed to a better balance between hole and electron fluxes, resulting from a decrease in hole injection barrier as manifested by ultraviolet photoelectron spectroscopy and scanning surface potential microscopy.     

MoO3修饰氧化石墨烯作为空穴注入层影响研究

研究了MoO₃修饰氧化石墨烯(GO)作为空穴注入层的影响。采用旋涂的方法制备了GO, 再真空蒸镀修饰层MoO₃,得到了空穴注入能力强和透过率高的复合薄膜。MoO₃的厚分 别采用0、3、5和8nm。通过优化MoO₃的厚度发现,当MoO₃的厚为5nm时,复合薄膜 的透过率达到最大值,在 550nm的光波长下透光率为88%,且此时采用 复合薄膜作为空穴注入层制备的结构为 ITO/GO/MoO₃(5nm)/NPB(40nm)/Alq₃(40nm)/LiF(1nm)/Al(100nm)的有机电致发光器件(OLED)性能 最佳。通过对OLED进一步的优化,改变Alq₃的厚度,分别取50、60和70nm,测量其电压 、电流、亮度、色坐标和电致发光(EL)光谱等参数发现,当Alq₃的厚为50nm时器件性能最 佳。最终制备了结构为ITO/GO/MoO₃(5nm)/NPB(50nm)/Alq₃(50nm)/LiF(1nm)/Al(100 nm)的OLED,在电压为10V时,最大电流效率达到5.87cd/A,与GO单独作为空穴注入层制备的器件相比,提高了50%。     

Gettering by heat thermal processing: application in crystalline silicon solar cells

The aim of this work is to getter unwanted impurities from solar grade crystalline silicon (Si) wafers and then to enhance their electronic properties. This was done by forming a sacrificial porous silicon (PS) layer on both sides of the Si wafers and by performing infrared (IR) thermal annealing treatments (at around 950 °C) in a SiCl₄/N₂ controlled atmosphere. The process allows concentrating unwanted impurities in the PS layer and near the PS/silicon interface. These treatments reduce the resistivity by about two orders of magnitude at a depth of about 40 μm and improve the minority carrier diffusion length from 75 to 210 μm. This gettering method was also tested on silicon wafers where grooved fingers and back contacts were achieved using a chemical vapor etching (CVE) method. Front buried metallic contacts and small holes for local back surface field were then achieved after the gettering stage in order to realize silicon solar cells. It was shown that the photovoltaic parameters of gettered silicon solar cells were improved as regard to ungettered ones.     

The dielectric constant of materials effect the property of the OLED

Several important materials have been used for the electron injection layer (EIL) of the organic light-emitting devices (OLEDs), such as LiF, NaCl, NaF, Al₂O₃, SiO₂, Si₃N₄, MgO, etc. LiF is the most usually used in OLED among these materials for its performance in OLED. The dielectric constant of LiF, NaCl, NaF is 9.036, 5.895 and 5.072, respectively, at 300 K [J. Fontanella, C. Andeen, D. Schuele, Phys. Rev. B 6 (1972) 582]. The thin film of these insulting layers here supply a very strong electric field to enhance the electrons injection and limit the holes injection to the emitting layer (EL). Then we kept the balance of the injected electrons and the holes, and then we got the excellent performing OLEDs.     

Electronic structures of CuI interlayers in organic electronic devices: An interfacial studies of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine/CuI and tris-(8-hydroxyquinolinato)aluminum/CuI

Electronic structures with the copper iodide (CuI) interlayer in organic electronic devices were measured and its strong electron-withdrawing properties were revealed. In situ ultraviolet and X-ray photoelectron spectroscopy showed the interfacial electronic structures of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB)/CuI/indium–tin-oxide (ITO) and tris-(8-hydroxyquinolinato)aluminum (Alq₃)/CuI/ITO as a representative hole- and electron-transport material. The large work function of the CuI interlayer induces electron transfer from both molecules and ITO to CuI. As a result, CuI dramatically reduces the hole injection barrier (HIB) from ITO to NPB and Alq₃ layers. Notably, CuI assists molecular ordering of the NPB layer, which would increase the intermolecular interaction, so would enhance the charge transport properties. Simultaneous enhancement in HIB and molecular ordering with the CuI interlayer would improve the device performance.     

有机-无机复合薄膜电致发光器件中势垒对光电性能的影响

利用无机非晶SiO2与有机聚合物PPV复合制备了异质结器件,研究了不同层之间由于能级匹配而产生的势垒对整个器件的光电性能的影响。对于单层有机器件ITO/PPV/Al及双层有机无机复合器件ITO/PPV/SiO2/Al,空穴的注入取决于ITO/PPV界面的势垒,空穴是多数载流子,发光强度主要取决于电子的注入。单层器件电子的注入能力与PPV/Al界面的势垒有关;双层器件由于引进SiO2层,提高了电子的注入能力,其发光强度和发光效率较单层器件都有改善。对于3层有机-无机复合器件ITO/SiO2/PPV/SiO2/Al,在两个方向上电子注入的势垒不同,电子的注入能力有所差别,交流激发时,当Al电极为负(ITO为正)时,器件的最大瞬时发光强度是当ITO电极为负(Al为正)时最大瞬时发光强度的1.3倍。     

Origin of Enhanced Hole Injection in Inverted Organic Devices with Electron Accepting Interlayer

Conventional organic light emitting devices have a bottom buffer interlayer placed underneath the hole transporting layer (HTL) to improve hole injection from the indium tin oxide (ITO) electrode. In this work, a substantial enhancement in hole injection efficiency is demonstrated when an electron accepting interlayer is evaporated on top of the HTL in an inverted device along with a top hole injection anode compared with the conventional device with a bottom hole injection anode. Current–voltage and space‐charge‐limited dark injection (DI‐SCLC) measurements were used to characterize the conventional and inverted N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)(1,1′biphenyl)‐4,4′diamine (NPB) hole‐only devices with either molybdenum trioxide (MoO₃) or 1,4,5,8,9,11‐hexaazatriphenylene hexacarbonitrile (HAT‐CN) as the interlayer. Both normal and inverted devices with HAT‐CN showed significantly higher injection efficiencies compared to similar devices with MoO₃, with the inverted device with HAT‐CN as the interlayer showing a hole injection efficiency close to 100%. The results from doping NPB with MoO₃ or HAT‐CN confirmed that the injection efficiency enhancements in the inverted devices were due to the enhanced charge transfer at the electron acceptor/NPB interface.