Abstract: The charge injection and transport properties of a high performance semiconducting polymer for organic photovoltaic (OPV) applications, poly[N-9"-hepta-decanyl-2,7-carbazolealt- 5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), are investigated by timeof- flight (TOF) and dark-injection space-charge-limited current (DI-SCLC) techniques. OPV cells employing PCDTBT are known to possess power conversion efficiency (PCE) exceeding 6%. While TOF probes only the hole mobilities of a thick film, DI-SCLC is shown to be useful down to a sample thickness of 200 nm, which is comparable to thicknesses used in OPV cells. We show that for pristine PCDTBT, the hole mobilities for both thick used in TOF and thin films for DI-SCLC are essentially the same, and they are in the range of 0.4-3.0x10^-4cm²/Vs at room temperature. Both poly(3,4-ethylene dioxythioplene) doped with poly(strenesulfonate) (PEDOT:PSS) and molybdenum (VI) oxide (MoO₃) form quasi-Ohmic contacts to PCDTBT with better hole injection from MoO3. Furthermore, the Gaussian Disorder Model (GDM) was employed to analyze the hopping transport of PCDTBT thin films. We show that PCDTBT possesses a relatively large energetic disorder of 129 meV, which is significantly higher than the energetic disorder of poly(3-hexylthiophene) (P3HT) processed under similar conditions. The correlation between energetic disorder and OPV device performance is addressed.

Abstract: Hole injection and transport in films (300-350nm) of poly(3-hexylthiophene) (P3HT) were investigated by dark-injection space-charge-limited current (DI-SCLC) technique. For samples with a nominally hole-only configuration of anode/P3HT/Au, the DI current transients depart significantly from the theory, and the signals cannot be used for reliable carrier mobility extraction. The origin of the departure can be attributed to electron leakage from the Au cathode. We outline a means of suppressing electron leakage by inserting an interlayer between the P3HT and the cathode. This interlayer has dual functions of blocking and trapping electrons. Using this interlayer, we obtain well-defined DI-SCLC signals for reliable carrier mobility determination. With a suitable interlayer to suppress undesirable carrier injection and transport, DI-SCLC technique should find broad applications in the transport characterization of narrow gap photovoltaic polymers.

Abstract: Organic thin-film transistor (OTFT) technique was used to investigate the effects of doping on N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB). Different transition metal oxides (TMOs) including molybdenum oxide (MoO₃), vanadium oxide (V₂O₅), tungsten oxide (WO₃) were employed as dopants. Using temperature dependent OTFT measurement, the carrier mobility and carrier concentration of the doping system can be extracted simultaneously. Generally, all TMOs form p-dopants and the conductivities increase drastically after doping. Among the TMOs, MoO₃ appears to be the most effective p-type dopant. It generates the largest free carrier concentration (1.4x10¹⁷cm^-3) and has the least activation energy (138 meV) for modest doping concentration of ~5 vol.%. Detailed carrier transport analysis indicates that the carrier mobilities were slightly reduced. It appears that the increase of free carrier concentration is the deciding factor in the conductivity enhancement in TMO-doped NPB.

Abstract: The charge transport properties of organic electronic materials are of critical importance for the understanding of the operation of organic photonic devices.We show in this study that admittance spectroscopy (AS), which is a measure of the frequency dependent capacitances of organic electronic materials, can be used to determine charge carrier mobilities and transport parameters in organic photovoltaic materials. A plot of the negative differential susceptance versus frequency yields a maximum at a frequency. The position of the maximum is related to the average carrier transit time. Using AS, we study the hole transport properties of thin films of poly(3-hexylthiophene) (P3HT) based materials for organic photovoltaic applications, including pristine P3HT, and P3HT doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We demonstrate that AS can be used to extract hole mobilities of P3HT and P3HT:PCBM with a thickness of ~350 nm. For P3HT:PCBM, copper phthalocyanine is used as an electron trapping layer to reduce the effect of negative capacitance.

Abstract: We describe how to use the thin-film transistor (TFT) technique to quantify carrier transport of amorphous organic semiconductors relevant to organic electronic devices. We have chosen several amorphous materials, including arylamine compounds, 4,4'-N,N'- dicarbazole-biphenyl (CBP), and a phosphorescent dye molecule [Ir(ppy)₃] for investigations. Generally, the field effect (FE) mobility was found to be about one order of magnitude smaller than that obtained from an independent time-of-flight (TOF) technique. For N'-diphenyl-N,N'- bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) and N,N'-Bis(3-methylphenyl)-N,N'- bis(phenyl)-9,9-spirobifluorene (spiro-TPD), the FE mobilities were found to be 1.7x10^-5 and 1.3x10^-5cm²/Vs, respectively. Temperature-dependent measurements were carried out to study the FE mobility. It was found that the energetic disorder increased in the neighborhood of a gate dielectric layer. This factor is one of the origins causing the discrepancy between TFT and TOF mobilities. We also examined how the hole transport of CBP is affected by Ir(ppy)₃ when it is doped into CBP.

Abstract: The synthesis, structural, photophysical, electrochemical and electroluminescent properties of a novel class of bifunctional molecule are reported in which the hole-transporting triarylamine and electrontransporting oxadiazole components were combined. The strongly luminescent compounds displayed good thermal and morphological stability as well as intense fluorescence both in solution and thin film at room temperature. The effects of the introduction of substituents with different electronic properties upon their absorption and emissive characteristics were correlated with theoretical calculations using density functional theory computations. The photophysics and electrochemistry of such systems were compared to those for the corresponding molecule without an oxadiazole ring. The bipolar compounds could be vacuum-sublimed and applied as emissive dopants for the fabrication of electrofluorescent, organic light-emitting devices with relatively simpler device structures, which can emit tunable colors by varying the aryl ring substituents.

Abstract: The hole transport property of a widely used phosphorescent dye, tris(2-phenylpyridine)iridium or Ir(ppy)₃ was investigated by thin film transistor (TFT) technique. The field effect (FE) mobility of Ir(ppy)₃ was found to be 1.7x10^-5cm²/Vs. This value is actually comparable to NPB and CBP, two popular hole transporting materials for fluorescent and phosphorescent organic light-emitting diodes (FOLED and POLED), respectively. In addition, temperature dependent measurements were carried out to study the energetic disorder of Ir(ppy)₃. The extracted energetic disorder ~ 88 meV is comparable to those of other common amorphous organic hole transporters, which are in the range of 80-90meV. Our findings indicate that the dye can directly act as a hole transporting component in POLEDs.

Abstract:

Abstract: Oxygen or air exposure to transition metal oxides (TMOs) was demonstrated to be essential in improving the hole injection (HI) efficiency at the contact formed by TMOs and small organic hole transporter. Current-voltage (J-V) and dark-injection space-charge-limited current (DI-SCLC) techniques were used to cross-examine the TMO/organic contacts. The hole transporter under investigation was N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'biphenyl)-4,4'diamine (NPB). The improvement was attributed to the reduction in the energy barrier at TMO/NPB interface, which was a consequence of the work function enhancement of TMO by the oxidation of oxygen in air.

Abstract: We use dark-injection space-charge-limited current (DI-SCLC) and admittance spectroscopy (AS) to study charge injection and transport properties of spiro-linked arylamine compounds. Examples are spiro-linked TPD and spiro-linked NPB which are both important amorphous hole transporters for organic light-emitting diodes (OLEDs). With PEDOT:PSS as the hole-injection layer, the contact is demonstrated to be generally Ohmic. Both techniques can be used to measure the carrier mobilities of thick films (>um) of spiro-compounds. DI-SCLC can be viewed as a pulse technique whereas AS can be treated as an ac technique. For the hole mobilities evaluation of the spiro-compounds, both techniques are in general agreement to each other. However, for the case of the thinner films of less than 1um, AS is superior over the DI-SCLC technique. The advantages of AS will be highlighted. It appears that AS is broadly applicable to characterize transport properties of organic thin films.

Abstract: We show that admittance spectroscopy (AS) is a better technique than time of flight (TOF) to study the charge transport properties in dispersive materials. The hole transport properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'-diamine (NPB) doped with different traps were evaluated by AS and TOF techniques. It was found that both techniques can show clear signals for measuring the mobility of NPB doped with shallow traps. When NPB was doped with deep traps, the AS signals were still clear for mobility extraction. In sharp contrast, the TOF transients become featureless and the carrier transit time cannot be determined. The validity of AS in mobility determination was demonstrated by comparing the extracted AS to TOF mobilities. Generally, the hole mobilities extracted by these two techniques were in excellent agreement. In addition, we will demonstrate that AS can be employed to measure carrier dispersion.

Copyright Notice: Copyright (2009) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/106/083710

Abstract: The high temperature limit of hole mobility (u_oo) in N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) has been studied by time-of-flight technique. The effect of dopants on u_oo was also investigated. It was found that the u_oo is independent of the nature of dopants. The common u_oo can be applied to estimate the full temperature dependence of zero-field mobility (u_o), if u_o at one temperature is known. We demonstrate this concept by predicting the room temperature l0 of NPB-doped with copper phthalocyanine (CuPc). The mobility prediction of CuPc-doped-NPB was then verified by the classic work of Hoesterey and Letson [D.C. Hoesterey, G.M. Letson, J. Phys. Chem. Solids 24 (1963) 1609].

Abstract: A blue single-layer organic light-emitting diode (OLED) was fabricated using a common hole transporter, N,N'-diphenyl-N,N'-bis (1-naphthyl) (1,1'-biphenyl)- 4,4'diamine (NPB). By doping the NPB layer with RGB dopants, the luminance and the current efficiency have been improved enormously. The charge transport mechanism will also be examined.

Abstract: This contribution examines in details the effects of dopants on the hole transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine(NPB). Dopants for NPB are copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryle)-4H-pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran (DCM2), 2-(4-biphenyl0-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (tBu-PBD) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The effects of these dopants on the hole transport of NPB will be presented. Generally, the dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbitals relative to that of NPB. Traps are found to induce significant reduction in hole mobility. However, hole scatterers only alter the mobility slightly. Two different underlying charge transport mechanisms are proposed and then it is further examined by temperature dependent measurements.

Abstract: We demonstrate that organic thin film transistors (OTFTs) can act as an alternative tool for carrier mobility evaluation in amorphous organic electronic materials. OTFT is a three terminal device which can be operated with an active layer of film thickness thinner than 100 nm. The materials under investigation are phenylamine-based (PA) compounds, which are amorphous hole transporting materials widely used in organic light emitting diodes (OLEDs). The field effect (FE) mobilities of PA compounds (hole) were determined in a TFT configuration. For the case of N, N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'biphenyl)-4,4'diamine (NPB), the FE mobility was found to be 2x10^-5 cm²/Vs. It is about one order of magnitude smaller than that obtained from independent time-of-flight (TOF) technique (2x10^-4 cm²/Vs) using a thick film of ~ 5 um. Temperature dependent measurement was performed under temperature ranging from 235 to 360 K. The extracted energetic disorder by means of the Gaussian Disorder Model from OTFT was 85meV, which was larger than that of TOF (~74 meV). Similar observations were found in other PA compounds. The increase in the extracted disorder parameter in TFT configuration was one of the origins of the discrepancy between the FE and TOF mobility. OTFTs can be regarded as a useful tool for carrier mobility evaluation with little material consumption.

Abstract: The hole transport properties of two phenylamine-based compounds were evaluated by thin film transistor (TFT) measurement and time-of-flight (TOF) technique. The compounds were N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'biphenyl)-4,4'diamine (NPB) and 4,4',4"-tris[n-(2-naphthyl)-n-phenyl-amino] triphenylamine (2TNATA). With tungsten oxide/gold as the charge injecting electrode, the field effect mobility of NPB was found to be 2.4x10^-5 cm²/V s at room temperature, which was about one order of magnitude smaller than that obtained from independent TOF experiments (3x10^-4 cm²/V s). Similar observations were found for 2TNATA. Temperature dependent measurements were carried out to study the energetic disorder of the materials. It was found that the energetic disorder was increased in the neighborhood of a gate dielectric layer.

Copyright Notice: Copyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?APL/93/083307

Abstract: The effects of dopants on the hole transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) have been studied by time of flight. Five dopants: copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyrle)-4 H-pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran [DCM2], 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are used in this study. The dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbital relative to that of NPB. Generally, traps are found to induce significant reduction in hole mobility while there is a slight reduction for scattering. Two different underlying charge transport mechanisms are proposed.

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Link: http://link.aip.org/link/?APL/92/103315

Abstract: The hole transport property of a phenylamine-based compound, 4,4',4"-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine, was independently studied by time-of-flight (TOF), dark-injection space-charged-limited-current (DI-SCLC), and thin film transistor (TFT) techniques. With UV-ozone treated gold as the injecting anode, clear DI-SCLC transient peaks were observed over a wide range of electric fields. The hole mobilities evaluated by DI-SCLC experiment were in excellent agreement with the mobilities obtained from the TOF technique. The injection contact was demonstrated to be Ohmic by an independent current-voltage (J-V) experiment. However, with the same injecting electrode, the mobility deduced from the TFT method was found to be 9.8x10^-7cm²/Vs, which was about one order of magnitude smaller than the TOF mobility (~1.2x10^-5cm²/Vs). The origin of the discrepancy is discussed.

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Link: http://link.aip.org/link/?JAP/103/093705

Abstract: N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB), a common hole transporter, was employed to fabricate single-layer organic light-emitting diodes (OLEDs). With a quasi-Ohmic anode, NPB device exhibited a bulk-limited hole current in the low-voltage region. Electron injection and light emission were clearly observed for applied voltages exceeding 4 V. In order to confine the recombination zone, intentional doping was applied to the single-layer device. After doping with perylene, the luminance and current efficiency of NPB device increased dramatically. It is expected that more efficient single-layer OLEDs can be achieved by using the doping strategy.

Copyright Notice: Copyright (2007) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?APL/90/213502

Abstract: We studied the hole injection and transporting properties of four phenylamine-based hole-transporters useful for organic light-emitting diodes. The hole-transporting properties were examined in details by time-of-flight technique whereas their hole-injecting properties by current-voltage measurements and dark-current space-charge-limited-current technique (DI-SCLC). All materials were found to exhibit essentially trap-free hole transports. We discovered that a conducting polymeric anode, PEDOT:PSS, can act effectively as an Ohmic contact for hole injection into the PA compounds for DI-SCLC experiments.

Abstract: We studied the carrier injection and transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB), a common hole transporter for organic light-emitting diodes (OLEDs). NPB was found to possess significant electron mobility from time-of-flight (TOF) measurement. With bipolar transporting ability, NPB was used to fabricate single-layer devices with a configuration of ITO/PEDOT:PSS/NPB/Ca/Ag. PEDOT:PSS was demonstrated to form a quasi-Ohmic contact to NPB by admittance spectroscopy (AS) and dark-injection space-charge-limited current (DISCLC) measurements. From current-voltage (JV) characteristics, single-layer NPB devices exhibited a bulk-limited hole current in low-voltage region. Electron injection was clearly observed at a turn-on voltage of about 4V, which coincided with the luminance-voltage measurement. In order to confine the recombination zone, dye-doped NPB layer was inserted into single-layer devices. This intentional doping technique made a notable improvement in current efficiency. The mechanisms of the doped devices were also addressed.

Abstract: The effects of dopants on the hole-transporting properties of NPB, i.e., (N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4' diamine), were studied by time-of-flight technique and admittance spectroscopy. Three dopants were chosen in this study. They were 4-dicyanmethylene-2-methyl-6-4H-pyran (DCM1), rubrene (RB), and tris-(8-hydroxyquinoline) aluminum (Alq3). It can be shown that DCM1 behaves as hole traps whereas Alq3 behaves as hole scatterers in NPB. Generally, both trapping and scattering lower hole mobilities in NPB. The hole mobilities decrease when DCM1 and Alq3 are introduced into NPB whereas the hole mobility remains nearly unchanged when RB is doped into NPB. The effect of doping on carrier dispersion is also studied.

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Link: http://link.aip.org/link/?JAP/102/093705

Abstract: The author have developed 2-methyl-9,10-di(1-naphthyl)anthracene(alpha,alpha-MADN) as an effective wide band gap host material for Forster energy transfer to the unsymmetrical mono(styryl)amine deep blue fluorescent dopant (BD-1). This guest/host emitting system, at the optimal doping concentration of 3%, can also increase the probability of carrier recombination near the hole-transport/emitting layer interface for the blue organic light emitting device which produces electroluminescence efficiencies of 3.3 cd/A and 1.3 lm/W and a deep blue CIE x,y color coordinates of (0.15, 0.13) that are 50% better than those of the traditional beta,beta-isomeric host (MADN) with the same dopant.

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Link: http://link.aip.org/link/?APL/89/252903

Abstract: Two naphthylamine-based hole transporters, namely, N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) and 4,4',4''-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine (2TNATA), were found to possess electron transporting (ET) abilities. From time-of-flight measurements, values of electron mobilities for NPB and 2TNATA are (6-9)x10^-4 and (1-3)x10^-4 cm²/V s, respectively, under an applied electric field range of 0.04-0.8 MV/cm at 290 K. An organic light-emitting diode that employed NPB as the ET material was demonstrated. The electron conducting mechanism of NPB and 2TNATA in relation to the Marcus theory [Rev. Mod. Phys. 65, 599 (1993)] from quantum chemistry will be discussed.

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Link: http://link.aip.org/link/?APL/89/262102

Abstract: The carrier mobilities of two hole transporting organic materials were evaluated by admittance spectroscopy (AS). The materials were 4,4',4''-tris{N,-(3-methylphenyl)-N-phenylamino}triphenylamine (m-MTDATA) and N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB). They were sandwiched in a configuration of anode/organic material/cathode. The anode was either indium-tin-oxide (ITO) or poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS). It is shown that PEDOT:PSS can, respectively, form Ohmic and quasi-Ohmic contact with m-MTDATA and NPB. Using PEDOT:PSS as the anode, the average hole mobilities of m-MTDATA and NPB were extracted by AS through susceptance analysis. The results are in excellent agreement with those obtained by an independent time-of-flight (TOF) technique. With PEDOT:PSS, the application of AS for characterizing carrier mobilities can be extended to hole transporting organic materials with highest occupied molecular orbital (HOMO) energy levels down to 5.4 eV.

Abstract: The effectiveness of a novel nickel-antimony doped tin oxide electrode for electrochemical degradation of organic pollutants was investigated using 4-chlorophenol (4-CP) as a model toxic organic. The experimental results demonstrate that the optimal Ni content was at Ni:Sn=1:500 in atomic ratio in the precursor coating solution, whereas the Sb:Sn ratio was set at 8:500. Using the electrode prepared with the optimal Ni doping ratio for 4-CP degradation, the charge-based efficiencies were up to 89 microg C(-1) for 4-CP destruction and 15 microg C(-1) for TOC removal, which were considerably higher than the efficiencies observed for other electrodes. It is suggested that the enhancement of the electrode for electrochemical oxidation of organics could be attributed to the production of hydroxyl radicals in anodic water electrolysis.

Abstract: The hole transporting properties of tris (8-hydroxyquinoline) aluminum (Alq3) were investigated by time-of-flight (TOF) technique between 278 and 373 K, and under an applied field range of 0.6-1.3 MV/cm. At room temperature, the hole mobility has a value between 10^-9 and 10^-8 cm² V^-1 s^-1. The hole mobility is at least two orders of magnitude less than electron under identical preparation and measurement conditions. Generally, all hole TOF transients of Alq3 exhibit a nondispersive behavior, with a clear plateau region and a dispersion tail. Two disorder transport models, namely, the Gaussian disorder model (GDM) and the correlated disorder model (CDM), were applied to analyze the temperature and field dependent hole mobility data. The GDM, however, is found to be invalid because it fails to produce a meaningful positional disorder parameter. The CDM gives a better fit to the data, yet the model is still not satisfactory.

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Link: http://link.aip.org/link/?JAP/100/094502

Abstract: We show that admittance spectroscopy (AS) can be used to determine charge carrier mobilities and transport parameters in materials relevant to organic light-emitting diodes (OLEDs). Via computer simulation, we found that a plot of the negative differential susceptance vs frequency yields a maximum at a frequency t_r^-1. The position of the maximum t_r^-1 is related to the average carrier transit time t_dc by t_dc = 0.56 t_r^-1. Thus knowledge of t_r can be used to determine the carrier mobility in the material. Devices with the structure anode/phenylamines/Ag have been designed to evaluate their mobilities. The extracted hole mobility data from AS in pristine and doped material systems are in excellent agreement with those independently extracted from time-of-flight (TOF) technique. In addition, materials with different energy levels of highest occupied molecular orbital (HOMO), are further examined in order to study the effects of injection barrier on the extracted mobility by AS. In the case of an Ohmic hole contact (e.g. ITO or Au /m-MTDATA), the mobility data is good agreement with TOF results. However, for a non-Ohmic contact, the extracted mobility appears to be smaller. Thus AS can be used a means of evaluating the quality of electric contact between the injection electrode and the organic material.

Abstract: We demonstrate that poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) can act as an excellent hole injection material for small organic charge transporters. With PEDOT:PSS as a conducting anode, it is possible to achieve nearly Ohmic hole injection contacts to phenlyamine-based materials with HOMO values of up to 5.5 eV. In current-voltage experiment, the PEDOT:PSS anode can achieve nearly Ohmic hole injection to NPB (N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine), and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl) (1,1'-biphenyl)-4,4'diamine). Meanwhile, dark-injection space charge limited current (DI-SCLC) transients are clearly observed and are used to evaluate the charge-carrier mobility of these phenylamine compounds. The carrier mobilities extracted by DI-SCLC are in excellent agreement with independent time-of-flight (TOF) technique. It is conceivable that PEDOT:PSS can be used as a general conducting anode for the electrical characterizations of organic materials that require Ohmic hole contacts.

Abstract: 1-(4'-(5-(4-methylphenyl)-1,3,4-oxadiazol-2-yl)biphenyl-4-yl)-4,4,4-trifluorobutane-1,3-dione (MPBDTFA), was prepared with high yield. With this synthesized ligand as the first ligand and 1,10-phenanthroline (Phen) as the secondary ligand, a new europium(III) ternary complex, Eu(MPBDTFA)(3)Phen, was synthesized. The new beta-diketone and its europium(III) ternary complex were characteristized by elemental analysis, thermo-gravimetric analysis, IR and UV-visible spectroscopies. Photoluminescence measurements indicated that the energy absorbed by the organic ligands was efficiently transfered to the central Eu(3+) ions, and the complex showed intensely and characteristically red emissions due to the (5)D(0)->(7)F(j) transitions of the central Eu(3+) ions. With an electro-transporting group in molecule and highly thermal stability, the synthesized Eu(III) ternary complex is expected as a red-emitting candidate material for fabrication of organic light-emitting diodes (OLEDs).

Abstract: Poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) is used as a hole-injecting anode for small organic hole transporters in current-voltage (JV) and dark injection space-charge-limited current (DI-SCLC) experiments. The hole transporters under investigation are phenylamine-based 4,4',4"-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (MTDATA), N,N'-diphenyl-N,N'-bis(1-naphthyl) (1,1'-biphenyl)-4,4'diamine (NPB), and N,N'-diphenyl-N,N'-bis(3-methylphenyl)(1,1'-biphenyl)-4,4'diamine (TPD). Clear DI-SCLC transient peaks were observed over a wide range of electric fields in all cases. For MTDATA and NPB, hole mobilities evaluated by DI experiments are in excellent agreement with mobilties deduced from independent time-of-flight technique. It can be concluded that, for the purpose of JV and DI experiments, PEDOT:PSS forms an Ohmic contact with MTDATA and a quasi-Ohmic contact with NPB despite the relatively low-lying highest occupied molecular orbital of the latter. In the case of TPD, hole injection from PEDOT:PSS deviates substantially from Ohmic injection, leading to a lower than expected DI-extracted hole mobility. The performances of other hole-injecting anodes for DI experiments were also examined.

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Link: http://link.aip.org/link/?JAP/100/063708

Abstract: Two novel organic compounds substituted with hole transporting arylamine and electron transporting 1,3,4-oxadiazole moieties have been designed and synthesized using a facile multistep reactions with high yield. The compounds are N-{4-[5-(2-phenylquinolin-4-yl)-1,3,4-oxadiazol-2-yl]-phenyl}-N'-phenyl- N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPDOPQ) and N,N'-bis{4-[5-(2-phenylquinolin-4-yl)-1,3,4-oxadiazol-2-yl]-phenyl}- N,N'-diphenyl-1,1'-diphenyl-4,4'-diamine [TPD(OPQ)₂]. These compounds emit intensive green light with high fluorescence quantum yields of 33%~48% under 450 nm light excitation. Cyclic voltammetry measurement results imply that these two compounds have both good hole-transporting and good electron-transporting properties, and hole-transporting is predominant. TG analysis showed high thermal stability of the compounds and vacuum-coating experiments confirmed the formation of homologous films from these compounds. All the results indicate that the synthesized compounds are potentially good candidates as bipolar (hole-transporting/electron-transporting) and emitting materials for fabrication of organic light-emitting diodes.

Abstract: The charge transporting properties of t-butylated 9,10-di(2-naphthyl)anthracene (ADN) compounds have been investigated experimentally and computationally in relation to their molecular structures. The ADN compounds are found to be ambipolar with both electron and hole mobilities in the range of 1-4x10^-7 cm² V^-1 s^-1 (electric field 0.5-0.8 MV/cm). As the degree of t-butylation increases, the carrier mobility decreases progressively. The mobility reduction was examined by Marcus theory of reorganization energies. All ADN compounds possess similar reorganization energies of ~0.3 eV. The reduction of carrier mobilities with increasing t-butylation can be attributed to a decrease in the charge-transfer integral or the wavefunction overlap.

Abstract: A series of Eu ternary complexes with charge conducting secondary ligands were examined as potential light-emitting materials for organic light-emitting diodes (OLEDs). The Eu complex contains an imidazole-based secondary ligand and is electron-conducting. Simple heterojunction OLEDs can be fabricated with a structure ITO/CuPc/TPD/Eu complex/cathode where ITO = indium-tin-oxide, CuPc= copper phthalocyanine, TPD = N,N-diphenyl-N,N'-bis(3-methylphenyl)(1,1'-biphenyl)-4,4'-diamine. The Eu complex plays the dual role of electron-transporting and light-emitting layer. Sharp red emission can be observed at about 612 nm. The OLEDs have a current efficiency of about 0.5 cd/A (at about 20 mA/cm²) and a maximum luminance between 150 and 200 cd/m². The effects of various substituted imidazole moieties on the electrical properties of the on the OLEDs will be discussed. It is shown that the Eu complex with unsubstituted imidazole has electron mobility ~10^-5 cm² V^-1 s^-1, and should be a very useful material for fabricating red OLEDs with simple device structure.

Abstract: A series of donor-acceptor type ambipolar electroluminescence dyes with the general structure PQ(OXD)_nT (where n=1, 2 and 3) were prepared, in which PQ is 2-phenylquinoline, T is diphenylamine which constituted the hole transporting triphenylamine moiety with an adjacent phenyl ring, and OXD is an electron transporting 2-phenyl-1,3,4-oxadiazole repeating unit. The compounds fluoresced bluish green to green hue in solid-state, exhibited a positive solvatochromism in solution and their quantum efficiency decreased rapidly with increase in n. The materials are thermally stable with glass transition temperature (Tg) ranging from 83 C (n=1) to 130 C (n=3). Cyclic voltammetry studies indicated the HOMO remained relatively unchanged with n while the LUMO decreased (away from the vacuum level) with an increase in the number of OXD. For single layer homojunction OLEDs, highest efficiency was obtained when n=1 (max luminous 3300 cd/m² and current efficiency 0.9 cd/A), whereas for multilayer heterojunction OLEDs, best results was achieved for compounds with n=1 or 2 assuming the role of the HT layer (over 8200 cd/m² max and 2.0 cd/A). Formation of exciplexes led to significant red-shift and lower emission efficiency for the compound with n=3.

Abstract: Four fluorine-containing X-branched oligophenylenes, X-OPP(5)-Fn, n = 1,2,3 and X-OPP(5)-CF₃, have been synthesized and studied as potential hole-blocking materials. These molecular materials exhibit a low HOMO energy level (ca. 6.5 eV) and a large energy gap (ca. 3.5 eV), indicating that they can be employed as effective hole-blockers in blue-emitting organic EL devices. Four-layer organic EL devices consisting of CuPc as a hole injection layer, alpha-NPD as a hole-transporting and emitting material, X-OPP(5)-Fn or X-OPP(5)-CF₃ as a hole-blocker, and Alq₃ as an electron-transporter sandwiched between indium tin oxide (ITO) and LiF/Al electrodes were fabricated, and their performance was examined and compared with the widely used hole-blocker bathocuproine (BCP), the non-fluorine-containing X-branched analogue X-OPP(5)-H and the X-branched analogue with one less phenyl ring, X-OPP(3)-F₂. The preliminary results show that these new molecular materials, especially X-OPP(5)-F₂, are excellent hole-blockers for blue-emitting organic EL devices.

Abstract: The charge transport properties of three tertiary-butyl (t-Bu) substituted anthracene derivatives (ADN), critical blue host materials for organic light-emitting diodes (OLEDs), have been investigated experimentally and computationally. From time-of-flight (TOF) measurements, all ADN compounds exhibit ambipolar characters. The hole and electron mobilities are in the range (1-5)x10^-7 cm² V^-1 s^-1 under an external applied field of about 1 MV cm^-1. Un-substituted ADN has the highest carrier mobilities while heavily t-Bu substituted ADN has the least. The electron and hole conducting properties of are consistent with ab initio calculation, which indicates that the frontier orbitals are localized mainly on the anthracene moiety. t-Bu substitutions in ADN increase the hopping path lengths among the molecules and hence reduce the electron and hole mobilities. The results demonstrate that t-Bu substitution is an effective means of engineering the conductivity of organic charge transporter for OLED applications.

Abstract: We examine the feasibility of admittance spectroscopy (AS) and susceptance analysis in the determination of the charge-carrier mobility in an organic material. The complex admittance of the material is analyzed as a function of frequency in AS. We found that the susceptance, which is the imaginary part of the complex admittance, is related to the carrier transport properties of the materials. A plot of the computer-simulated negative differential susceptance versus frequency yields a maximum at a frequency tau_r^-1. The position of the maximum tau_r^-1 is related to the average carrier transit time tau_dc by tau_dc=0.56 tau_r. Thus, knowledge of tau_r can be used to determine the carrier mobility in the material. Devices with the structure ITO/4,4',4'-tris[N,-(3-methylphenyl)-N-phenylamino] triphenylamine/Ag have been designed to investigate the validity of the susceptance analysis in the hole mobility determination. The hole mobilities were measured both as functions of the electric field and the temperature. The hole mobility data extracted by susceptance analysis were in excellent agreement with those independently obtained from time-of-flight (TOF) measurements. Using the temperature dependence results, we further analyzed the mobility data by the Gaussian disorder model (GDM). The GDM disorder parameters are also in good agreement with those determined from TOF.

Copyright Notice: Copyright (2006) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/99/013706

Abstract: A series of fluorescent dyes consisted of a thiophene unit, an 1,3,4-oxadiazole unit and four different arylamine moieties were prepared using a facile multi-steps synthetic route with high yield. The four arylamine structures studied were triphenylamine, N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine, diphenyl(I-naphthyl)amine and 9-phenylcarbazole. Experimental results shown that their HOMOs varied from 5.21 to 5.73 eV strongly affected by the arylamine chemistry while their LUMOs remained relatively unchanged. Their corresponding emission colors ranged from UV (393 nm) to bluish green (483 nm). In general, the thiophene unit enhanced the overall thermal stability of the compounds. According to cyclic voltammetry, the compounds are predominantly hole-transporting while OLED results indicated cpd 10 possess both hole and electron transport properties. Single layer OLED fabricated from 10 resulted in ca. 2000 cd/m² (luminous intensity) and 1.10 cd/A (current efficiency) max, whereas, a multilayer OLED using 10 as the hole transporting layer achieved over 7400 cd/m² and 2.3 cd/A max.

Abstract: A series of electroluminescent and conductive phenyl ring-substituted poly(N-phenyl-N-styryl-1-naphthylamine) (P-XNPA, where X = -H (P01), -OCH3 (P02) and -F (P03)) polymers were prepared. The polymers exhibit a high hole mobility, an excellent solubility and a good thermal stability. Their emission color, ionization potential (IP), quantum efficiency, glass transition temperature (T-g), efficiency and stability were all found to be related to the properties of the phenyl substituted side group. As an example, the hole mobility of poly(4-MeONPA) (P02) is 10^-5 cm² V^-1 s^-1 at room temperature, compared to that of poly (N-vinylcarbazole) (PVK) being 10^-7 cm² V^-1 s^-1. P02 emits blue light at 450nm and has a high thermal stability at an onset decomposition temperature above 420 degrees C and T-g of 132 degrees C. A simple organic light-emitting diode (OLED) device fabricated from the same polymer has a maximum luminance of 588 cd/m² at 175 mA/cm².

Abstract: The effects of nitrogen, oxygen, and moisture on the electron mobilities of tris(8-hydroxyquinoline) aluminum (Alq3) have been examined by time-of-flight (TOF) technique at room temperature under an applied electric-field range of 0.3-1 MV/cm. Our results suggest that both oxygen and water molecules can impede electron conduction in Alq3. Detailed analysis suggests that water molecules induce electron traps even at very low dosages. Oxygen interacts with Alq3 at high dosages and results in highly dispersive TOF signals. Moisture appears to be more detrimental than oxygen in hindering electron transport in Alq3.

Copyright Notice: Copyright (2005) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/98/023711

Abstract: The synthesis of a novel trifunctional Pt(II) cyclometalated complex in which the hole-transporting (HT), electron-transporting (ET), and electroluminescent (EL) components are integrated into a single molecule is accomplished, and this complex can be sublimed and used for the fabrication of neat emissive layer electrophosphorescent devices.

Abstract: The effects of polar side groups (methoxy, fluoro, and chloro substitution) on the transport and luminescence properties of a series of naphthyl phenylamine (NPA) compounds were examined. The compounds are (i) N,N'-diphenyl-1-naphthylamine, (ii) N-(4-methoxyphenyl),N'-phenyl-1-naphthylamine. (iii) N-(4-chlorophenyl),N'-phenyl-1-naphthylamine, and (iv) N-(4-fluorophenyl),N'-phenyl-1-naphthylamine. Time-of-flight measurements were used to measure the hole mobilities of the NPA compounds. The hole mobilities were found to correlate with the dipole moments of the compounds. The least polar compound N,N'-diphenyl-1-naphthylamine has the highest mobility in the range of 0.5-2 x 10^-4 cm² V^-1 s^-1. whereas the most polar compound N-(4-chlorophenyl),N'-phenyl-1-naphthylamine has the lowest mobility in the range of 0.2-2 x 10^-6 cm² V^-1 s^-1 at room temperature. The dipolar disorder model proposed by Dieckmann and Young is found to be applicable to describe the data. The NPA compounds were further incorporated into a multilayer organic light-emitting diode (OLED) as the light-emitting layer. The OLEDs emit in the range 420-440 nm, and the polar side groups appear to have no clear influence on the emission wavelength of the molecules in solid state.

Abstract: The charge transporting properties of rubrene (5,6,11,12-tetraphenylnaphthacene or RB), and a new rubrene-based complex, tetra(t-butyl)-rubrene [2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene or TBRB], were examined in the form of amorphous films as functions of electric field and temperature by means of time-of-flight technique. At room temperature, the hole mobility u for RB is 7-9x10^-3cm²/Vswhereas u for the more bulky TBRB is about 2x10^-3cm²/Vs. The microscopic conduction mechanism in both materials can be modeled by the Gaussian disorder model in which hopping conduction occurs through a manifold of sites with energetic and positional disorder. The energetic disorder in RB and TBRB is almost identical and is about 78 meV in each case, and is mainly controlled by van der Waals interaction. The t-butyl groups in TBRB induce large fluctuations in the spatial separation among TBRB molecules and result in an increase in the positional disorder, and hence a reduction in the hole mobility.

Abstract: The absorption and light-emitting properties of a blue aluminum complex are investigated theoretically. The complex is bis(8- hydroxyquinolinolato) aluminum hydroxide [Al(Mq)₂OH] and emits at about 485 nm. Very bright organic light-emitting diodes can be fabricated from Al(Mq)₂OH with a maximum luminance of about 14 000 cd/m². The geometry, bonding, and frontier orbitals are examined in details by self-consistent Hartree¡VFock method and by density functional theory. Computational results indicate that the peak emission is mainly determined by the 2-methyl-8-quinolinol group and is independent of the hydroxide group. The computed absorption and emission gaps are in good agreement with experimentally determined values.

Abstract: The insertion of a thin LiF layer between Al and tris(8-hydroxyquinolino)-aluminum (Alq₃) is a significant advance in the practical development of organic light-emitting devices (OLEDs). We studied the chemistry at the buried interface of LiF/Alq₃ using X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). Dramatic changes in Al 2p, N 1s, and O 1s line shapes were observed after LiF deposition. These changes were attributed to the partial substitution of hydroxyquinoline ligands of Alq3 by F^-
, and the formation of hydroxyquinoline anions at the LiF/Alq₃ interface. The AlqFt structure of products was confirmed by ToF-SIMS.

Abstract: The time delay (t_d) in the transient electroluminescence (EL) signal of a bilayer organic light-emitting diode with a structure of indium-tin oxide /N,N'-diphenyl-N,N'-bis(3- methylphenyl)-(1,1'-biphenyl)-4,4'-diamine /tris(8-hydroxyquinoline) aluminum (Alq₃)/Al has been measured and analyzed as a function of the thickness (D) of the Alq₃ layer. For a thin layer of Alq₃ (D <180nm), it is found that t_d is affected by both the charging effect and carrier transit time through the Alq₃ layer. For a thicker layer of Alq₃ (D>200 nm), t_d approaches the intrinsic electron transit time through Alq₃ . Electron mobility of Alq₃ can be evaluated for the thick-film devices and the results are in excellent agreement with independent time-of-flight measurements. The application of transient EL in mobility measurement for C540-doped Alq3 is discussed.

Abstract: Optical time-of-flight (TOF) technique was used to measure the electron mobility of tris(8-hydroxyquinoline) aluminum (Alq₃) exposed to different gases. When freshly prepared Alq₃ sample was exposed to air, the TOF signal was observed to be highly dispersive. After annealing the sample to 370K for 12 h, the signal became non-dispersive. The field dependence of the electron mobility was found to exhibit Poole-Frenkel form between 250¡V380 K. The effects of various ambient gases (N₂, O₂, H₂O) to the electron mobility were examined. It is found that O2 acts as an extrinsic dopant and induces electron traps in bulk Alq₃.

Abstract: Hole mobilities of N0;N0-bis(3-methylphenyl)-(1; 10-biphenyl)-4; 40-diamine (TPD), TPD doped with 5, 6, 11, 12-tetraphenylnathacene (rubrene), and TPD doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryle) 4H-pyran (DCM1) were measured by a time-of-flight (TOF) technique between 200 and 300 K. Pristine TPD has a hole mobility of about 2x10^-3cm²/Vs at room temperature. The corresponding TOF time transients were non-dispersive. Upon doping, the hole mobility decreased slightly for rubrene-doped TPD, and sharply for DCM1-doped TPD. TheTOFtime transients remain non-dispersive for both kind of doped samples. The dependence of the mobility on electric field and temperature for undoped and doped TPD was investigated.

Abstract: The optimum composition and film thickness determined for a single-layer organic light emitting diodes (120¡V150 cd/m² at 30 mA/cm²) has been determined in one of our previous publication [Display 21 (2001) 199]. In this following study, a series of additives was included in the original composition for the purposes of color tuning, enhancement of the device efficiency as well as the stability of the polymer thick film-organic light emitting diodes (PTF-OLED). The additives attempted including three dyes (rubrene, perylene and DCM), three sensitizers (naphthalene, anthracene and p-terphenyl), several stabilizers (antioxidant and photo-stabilizer) and a number of organic soluble salts known as phase transfer catalyst. Rubrene was found being able to improve the OLED efficiency by 3-fold and its durability by at least 60 times at a concentration of 5¡V10 pph (in part per hundred of Alq₃ replaced). The color of the OLED can also be tuned according to the additional dye used. The sensitizer naphtahlene can also improved the efficiency of the OLED by 2-fold while the effect of the other additives was less significant. In addition, the lowering of the turn-on voltage and thus a higher current efficiency resulted by the addition of a charge injection layer (lithium fluoride or calcium fluoride) in between the cathode and the organic layer. The performance of the doped PTF-OLED is within an order of magnitude compared to a heterojunction small molecule-based OLED.

Abstract: Indium tin oxide (ITO) surfaces were treated by solvent cleaning, by plasma of oxygen, argon, nitrogen and by argon ion (Ar⁺) sputtering. Angular-dependent X-ray photoelectron spectroscopy (ADXPS) and ultraviolet photoelectron spectroscopy (UPS) were used to determine the chemical composition, the chemical states and the work function after each treatment. It was found that oxygen plasma and nitrogen plasma chemically reacted with the ITO surfaces. Yet little etching of the surface can be observed after plasma treatments. Among all treatments, oxygen-plasmatreated ITO achieved the highest work function of 4.40 eV, whereas Ar⁺-sputtered ITO surface had the lowest work function of 3.90 eV. The stoichiometry of the ITO surface is shown to be the major controlling factor of the ITO work function.

Abstract: The surface chemistry of ITO thin-films before and after UV-ozone treatment was characterized using angle dependent X-ray photoelectron spectroscopy (ADXPS). After solvent cleaning, the ITO surface was covered with a thin nonconducting carbon contamination layer of ~7A. This contamination layer was removed efficiently by UV-ozone treatment, and the chemical states of the residual carbon at ITO surface after UV-ozone treatment were quite different from contaminated carbon. UV-ozone treatment modified ITO surface by introducing O^2- ions into ITO surface. The modified depth was about 50A. The modification decreased the carrier concentration at ITO surface, and thus decreased the conductivity if ITO surface.

Abstract: Organic electroluminscence (EL) devices were fabricated using a bis(2-methyl-8-quinolinato) Aluminum Hydroxide complex [Al(Mq)₂OH] as the light-emitting material. THe device exhibits bright blue EL at a peak wavelength of 485nm. A maximum luminance of about 14,000cd/m² can be achieved at a driving current density of 480 mA/cm². The efficiency of the device is about 4.6 cd/A. Possible mechanisms of EL blue shift of Al(Mq)₂OH with respect to Alq₃ are discussed.

Abstract: In this study, we first profiled model organic light-emitting diodes (OLEDs) with the structure Ag/tris(8-hydroxyquinoline) aluminium (Alq₃)/N,N-diphenyl-N,N-bis[1-naphthyl-(1,1-biphenyl]-4,4- diamine (NPB)/indium tin oxide (ITO) using dynamic SIMS. The element distribution across the multilayer and the dynamic SIMS spectra at different regions were obtained. It was shown that dynamic SIMS is a powerful tool in profilingOLEDs. Clear diffusion ofAg into the Alq₃ layer was observed. The experimental results also revealed that a distinct interface forAlq3/NPB can be obtained using dynamic SIMS.

Abstract: In the present study, model organic light-emitting diodes (OLEDs) with a structure of Ag/tris(8- hydroxyquinoline) aluminum (Alq₃)/indium tin oxide (ITO) were fabricated using vacuum evaporation and analyzed using an x-ray photoelectron spectroscopy (XPS) depth profile technique. Diffusion of Ag into the Alq₃ layer was clearly observed. Applying a positive electron field enhanced the diffusion. Compared with pure Ag, the binding energy of diffused Ag 3d_5/2 at the interface increased slightly, and the contribution of Ag to the valence-band spectra also was changed across the depth, implying a weak interaction between diffused Ag and the hydroxyquinoline ligands.

Abstract: A guest-host approach was used to fabricate a one-layer organic light emitting diode (OLED). The thick film ink approach allows the twodimensional OLED to be processed using traditional methods such as silk-screen printing. The I-V-L characteristics of the polymer thick film (PTF)-OLED were studied as a function of the device chemical compositions and physical configurations. Different polymers, hole and electron transporters, and emitters at different weight ratios were studied for its composition dependence. Device configuration also plays a significant role on its overall performance. Dependence on film thickness, electrode type, and the usage of additional charge injection layers were also investigated. The simplified one-layer device allows a straightforward interpretation for the charge-transport and recombination phenomena which shed light for its future improvement.

Abstract:

Abstract: Thin copper films were grown on glass by pulsed-laser deposition. The simultaneous in situ monitoring of the electrical resistance and optical transmittance of the growing film yielded highly reproducible and consistent data about percolation onset and film conductivity, both being useful indicators of film quality. When prepared under favorable conditions, films as thin as 1.5 nm would percolate, and became fully continuous at 5nm, with conductivity reaching 30% of that of bulk copper.