We have developed a wet chemical technique that is based on self-assembly from an aqueous solution to produce ultra-thin gold-silver nanowire films at the substrate-solution interface. The nanowire films can be used as transparent electrodes, with the advantage of a single step preparation directly on a substrate of choice, even on conducting polymer films. The deposition occurs at ambient conditions and the films do not require any high temperature annealing steps. The process starts with small metal seed particles that are mixed with the growth solution. The seed concentration determines the morphology of the nanowire film. The process is compatible with photolithographic patterning and with various printing techniques.

We report in this communication on the synthesis, purification and use of silver or copper nanowires to fabricate flexible transparent electrodes.[1]
We will present results on the synthesis and specific purification processes for both metals. The metallic nanowires are then used as flexible transparent electrodes through the fabrication of random networks of nanowires, either by spin-coating or spray-coating.[2-3] The performances of these electrodes are excellent, typically ~20-30 ohm/sq at 90 % transparency.
This solution-processable technique appears as a really promising alternative to ITO, with in addition the possibility to realize highly flexible electrodes (even crumpling was demonstrated on a 1.3 µm thick film with our silver nanowire based electrodes). Comparison of the stability of the electrodes will be discussed.
These electrodes have been integrated in various optoelectronic devices.
We will show that it affords a large area, low-cost deposition method, with good performances in devices such as organic photovoltaic cells, flexible touch sensors, and organic photodiodes.
We will also present results dealing with the use of such electrodes to realize transparent film heaters (TFH), with very good performances. [4]
[1] Flexible transparent conductive materials based on silver nanowire networks: A review. Langley D., Giusti G., Mayousse C., Celle C., Bellet D., Simonato J.P., Nanotechnology, 2013, in press.
[2] Improvements in Purification of Silver Nanowires by Decantation and Fabrication of Flexible Transparent Electrodes. Application to Capacitive Touch Sensors. Mayousse C., Celle C., Moreau E., Mainguet J.-F., Carella A., Simonato J.P. Nanotechnology, 2013, 24, 215501
[3] Synthesis and purification of long copper nanowires. Application to high performance Flexible Transparent Electrodes with and without PEDOT:PSS. Mayousse C., Celle C., Carella A., Simonato J.P., submitted.
[4] Highly Flexible Transparent Film Heaters Based on Random Networks of Silver Nanowires. Celle C., Mayousse C., Moreau E., Basti H., Carella A., Simonato J.P. Nano Research, 2012, 5(6): 427-33

For transparent conducting electrodes in optoelectronic devices, electrical sheet resistance and optical transmittance are two of the main criteria. Recently, metal nanowires have been demonstrated to be a promising type of transparent conducting electrode because of low sheet resistance and high transmittance. Here we incorporate a mesoscale metal wire (1-5 micrometer in diameter) into metal nanowire transparent conducting electrodes and demonstrate at least a one order of magnitude reduction in sheet resistance at a given transmittance. We realize experimentally a hybrid of mesoscale and nanoscale metal nanowires with high performance, including a sheet resistance of 0.36 Ohm/sq and transmittance of 92%. In addition, the mesoscale metal wires are applied to a wide range of transparent conducting electrodes including conducting polymers and oxides with improvement up to several orders of magnitude. The metal mesowires can be synthesized by electrospinning methods and their general applicability opens up opportunities for many transparent conducting electrode applications.

Cambrios has developed a transparent conductor material based on silver nanowires to replace the ITO layer in organic photovoltaics (OPV) device and in organic light-emitting devices (OLED).
After being deposited from a liquid suspension by conventional coating or printing methods onto a transparent substrate, these nanowires form a transparent conducting network. The sheet resistance of the resulting film is determined by the density of the nanostructures and can thus be easily controlled during the coating process.
The elimination of the ITO layer also results in a reduced microcavity effect and thus has a positive impact on the optical performance for OLED lighting devices.
The talk will focus on the use of this material as an ITO replacement for OLED devices for lighting applications and for OPV devices. We will report in detail on the fabrication and characterization of the nanowire film. The performance of ITO-free OPV and OLED devices with a nanowire anode will be discussed. We also will present the optical performance data of OLED lighting devices which show the implications of a reduced microcavity effect. In addition, we will show lifetime data for these devices which demonstrate the viability of this technology.

Performance demands on transparent conductors (TC) for transparent electrodes in current carrying and large area applications such as OLED displays, photovoltaic solar cells and solid-state lighting, continue to increase. Transparent conductive oxides (TCO) such as indium tin oxide (ITO), when deposited on heated glass (~ 200 °C) have useful electrical and optical (E-O) performance (~ 10 Ohm/sq - 88%T) for these applications. However, when ITO or another TCO, are vacuum deposited at low substrate temperature on plastic film to allow roll-to-roll processing, the E-O performance is much worse (~ 50 Ohms/sq - 85%T). Novel TC based on antireflected very thin metallic films in Polymer/Metal/Polymer (PMP) designs, were developed to address these performance limitations, yet be compatible with high speed, roll-to-roll, vacuum deposition on temperature-sensitive plastic film. The antireflection layers were high index polymers (P) rather than traditional brittle inorganic oxides, e.g., ITO or TiO2, to enable applications requiring flexible transparent electrodes. The fundamental optical limitation in using metallic thin films is the transmission loss, which occurs because of absorption in the metal. Because absorption is exponentially proportional to the product of the film thickness and its absorption coefficient; to mitigate absorption very thin metallic layers were used. However, if a metallic layer becomes too thin the absorption increases (anomalous absorption) because very thin metals tend to agglomerate forming an island type structure, which becomes discontinuous and non-conductive. To inhibit island formation various metal oxides were evaluated experimentally as a nucleating and bonding agent for the silver based thin film. First sputter depositing zinc oxide nanoclusters, “seeds”, enabled much thinner silver based layers without their becoming nonconductive, unstable or exhibiting significant absorption. Polymer/Metal/Polymer TC (P/M/P-TC) deposited by this method, have a combined E-O performance, which far exceeds the typical performance of single layer ITO films deposited with low substrate temperature, and is equal or better than high temperature processed ITO. For example, by adjusting the metal thickness, E-O performance from ~ 10 Ohms/sq - 87%T to < 40 Ohms/sq - ge; 90%T was achieved. The P/M/P-TC flexing/bending capability is superior to commercially available ITO and oxide-metal-oxide based TC on plastic substrates. P/M/P-TC demonstrated up to 50% extension without loss of conductivity and bending around a 3 mm radius of curvature without degradation. Additionally, its environmental durability, e.g., ge; 72 hours @ 65°C/95% RH, is also superior. The P/M/P-TC environmental durability, i.e., corrosion resistance, was further enhanced by adding anti-corrosion additives to the polymers, thin tin oxide nanoclusters, and/or using a silver/gold alloy metallic layer.

Transparent conductive oxides can be used for a wide range of electronics application ranging from touch screen to photovoltaic panels. The goal of this work is the design and realization of transparent conductive materials with enhanced stability under adverse environmental conditions while maintaining high conductive and optical transmittance characteristics.
We investigated heavily gallium (10wt%) doped ZnO (GZO) single layers and GZO/Ag/GZO trilayers, prepared by rf magnetron sputtering on glass substrate at room temperature. The best as-deposited GZO layer shows high transmittance (>85%) in the wavelength range of 400-1100 nm, and low resistivity (5 x10-4 #8486;cm) corresponding to a sheet resistance of 50 #8486;/square for 120 nm thick films. The trilayers fabricated with different GZO layer thickness (total thickness of 60-90 nm) exhibit excellent conductivity (5-8x10-5 #8486;cm), with sheet resistance of 7-9 #8486;/square and high transmittance (>80%), with the average absorptance of 10% in the visible region.
The long term stability of the films to moisture was tested by damp heat exposure (DH) at relative humidity of 85 % and 85 °C over 1000 h. The electrical, optical and morphological properties of the films before and after DH as functions of different GZO layer thickness were analyzed, in order to assess the optimal trilayer structure. After DH, a small degradation (< 5 %) of GZO single layer is observed. An increase in resistivity of trilayers is also occurred, related to the thickness of GZO and Ag layer.
We demonstrate here the trilayer structure with high optical transmittance, excellent conductivity and enhanced stability. Possible physical mechanisms responsible for a retarded degradation of the films will be discussed.

Alternative transparent electrodes for optoelectronic applications on flexible substrates are an actual topic in research and application. We have investigated a wide variety of electrodes, such as record conductivity PEDOT:PSS, high-performance silver and cost-efficient copper nanowire networks, state of the art carbon nanotube layers for top and bottom electrodes, doped small molecules and ultra-thin metal films. All of these technologies were successfully incorporated into highly efficient pin-type vacuum deposited organic solar cells.
Within this presentation we will give a short overview on our recent results, showing highly promising alternative electrode technologies and their successful integration into organic optoelectronic devices. Then we will focus on recent investigations on super-transparent dielectric/metal/dielectric thin-film electrodes. These can be deposited by vacuum deposition, just as our OLED or OPV devices, which makes them suitable as top or bottom electrodes.
By optimizing the growth and microstructure of thin metal films, we were able to improve the thin-film properties and the device performance significantly. We boosted the device lifetime by using a stabilizing metal oxide sandwich structure [1] and we were able to improve the silver thin-film electrode performance to the ITO-like range (Rs=19 Omega; /sq. and T= 83% at 580 nm) [2] by introducing seed layers, controlling the growth of the thin. Recently we modified the ultrathin film by co-evaporation, generating a microstructured thin-film-electrode with extraordinarily high transmittance of 84.3 % (93.0 % after substrate correction) in the visible wavelength regime combined with a low sheet resistance of 27.3 Omega; /sq. (figure of merit σopt/σdc of 186.7). The application of these high performance metal thin-film electrodes for efficient OLED and OPV devices is demonstrated.
[1] Schubert, S., Hermenau, M., Meiss, J., Müller-Meskamp, L. & Leo, K. Oxide Sandwiched Metal Thin-Film Electrodes for Long-Term Stable Organic Solar Cells. Adv. Funct. Mater. 22, 4993-4999 (2012).
[2] Schubert, S., Meiss, J., Müller-Meskamp, L. & Leo, K. Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer. Adv. Energy Mater. 3, 438-443 (2013).

Flexible organic solar cells have been promising as next generation renewable energy because of ease of processing, light-weight portability and a competitive price. Although their potentials were investigated over the past decade, sufficient power generation has not yet been realized to the industry fields due to the natural phenomenon, varying the angle of incidence depending on the sun&’s coming up over a full day. Therefore, the light harvesting at a wide range of incident angles is a key issue for solar cells. Several approaches, multilayer antireflection coatings or surface texturing structures have been previously used to reduce the surface reflection and enhance the absorption efficiency. However, thermal mismatch, instability of the thin film stacks and complicated fabrication process still remain major obstacles in developing the angle-independent solar cells with broadband wavelength.
Hereby, we present a novel way of preparing an omnidirectional electrodes with excellent transparency. Ormoclear/Ag/WO3 (OoMDi) was designed not only for an alternative to ITO but also for an omnidirectional layer independent of incident angle of lights. Employing the ormoclear with low refractive index (n = 1.45) as an outer organic (Oo) layer, insensitivity to the angles of incidence (0 ~ 90o) was acheived in the OoMDi structures. Moreover, oxygen plasma treatment on Ormoclear prior to deposition of 10-nm-thick Ag, surface plasmon coupling at the Ormoclear/Ag interface could be greatly suppressed. As a result, an optimized structure of Ormoclear/Ag/WO3 shows an high optical transmission of > 91%, low sheet resistance of ~5.2 ohm sq-1. Employing the OoMDi electrode as an anode to the PTB7:PCBM organic solar cell resulted in the significantly improvement in the power conversion efficiency up to 10 %, from 7.1% using ITO anode to 7.7% .

Nowadays, there is an increasing demand for transparent conductive electrodes (TCE) for application in optoelectronic devices [1], as flat-panel displays, organic light emitting diodes and photovoltaic cells, to name a few. In this context, dielectric-metal-dielectric (DMD) multilayer structures are promising candidates for next-generation flexible transparent electrodes [2, 3]. Compared to standard TCEs, DMD electrodes show enhanced conductivity, higher transmission of visible light, lower temperature process, reduced thickness and, consequently, significant cost reduction and improved mechanical flexibility. However, at present, DMD multilayer structures are still far from being implemented on thin film photovoltaic device technology. A crucial aspect is the TCE film patterning (scribing) for electrical isolation. This process, performed with a laser wavelength of 1064 nm, requires relatively high laser fluences and multipulse irradiation to segment the thick, typically 0.7 to 1 mu;m, ZnO:Al (AZO) front contact in the solar cell.
We demonstrate how the energy density threshold for the scribing of the transparent electrodes can be significantly reduced by replacing the standard AZO single layer with a 10 times thinner AZO/Ag/AZO multilayer structure with better electrical and optical properties [4]. Thin films of 40 nm of AZO, 10 nm of Ag and 40 nm of AZO are sequentially deposited on conventional soda lime glass substrates by RF magnetron sputtering at room temperature in argon atmosphere. Laser treatments are performed in air by a single pulsed (12 ns) Nd:YAG laser operating with an infrared (at 1064 nm), Gaussian-shaped (FWHM = 1 mm) beam. The laser power is varied to obtain fluences in the range from 1.15 to 4.6 J/cm2. Our experimental results, supported by a computer simulation of the thermal behaviour under laser irradiation, provide clear evidences of the key role played by the silver interlayer by increasing the maximum temperature reached in the structure and fastening the cool down process. These evidences can promote the use of ultra-thin AZO/Ag/AZO electrode in large-area products, such as for solar modules.

[1] Ginley, D. S., Hosono, H. & Paine, D. C. Handbook of Transparent Conductors. Springer (2010).
[2] Kim S, Lee J-L. Design of dielectric/metal/dielectric transparent electrodes for flexible electronics. J Photon Energy 2:021215 (2012).
[3] Crupi I, Boscarino S, Strano V, Mirabella S, Simone F, Terrasi A, Optimization of ZnO:Al/Ag/ZnO:Al structures for ultra-thin high-performance transparent conductive electrodes. Thin Solid Films 520, 4432 (2012).
[4] Crupi I, Boscarino S, Torrisi G, Scapellato G, Mirabella S, Piccitto G, Simone F, Terrasi A, Laser irradiation of ZnO:Al/Ag/ZnO:Al multilayers for electrical isolation in thin film photovoltaics. Nanoscale Res Lett 8:392 (2013).

Herein, we report a novel flexible transparent conducting hybrid composite film that can be utilized as a robust transparent electrode platform for thin-film optoelectronic devices.[1] This hybrid composite film (AgNW-GFRHybrimer film) is composed of a glass-fabric reinforced transparent composite film and silver nanowire (AgNW) networks which are monolithically buried on the film surface as the electrode. The resulting monolithic hybrid structure of the AgNW-GFRHybrimer film simultaneously offers an exceptionally smooth AgNW electrode topography (<2nm) and excellent thermo-mechanical performance, both of which are the most critical factors for the viable implementation of AgNW-based transparent electrode system.
In this study, we discuss the fabrication and basic feature characteristics of the AgNW-GFRHybrimer film, and demonstrate a flexible inverted organic solar cell with a ~6% efficiency (AM 1.5G at 100mWcm-2) using AgNW-GFRHybrimer film as the all-in-one substrate/electrode platform.
Reference
[1] Jin et al., Energy & Environmental Science, 2013, 6, 1811-1817

Metallic nanowires have demonstrated high optical transmission and electrical conductivity with potential for application as transparent electrodes that may be used in flexible devices. We systematically investigated the electrical and optical properties of 1D and 2D copper and silver nanowire (Cu and Ag NW) arrays as a function of diameter and pitch and compared their performance to that of Cu and Ag thin films. NWs exhibit enhanced transmission over thin films due to propagating resonance modes between NWs. For the same geometry, the transmission of Cu NW arrays is about the same as that of Ag NW arrays since the dispersion relation of propagating modes in metal nanowire arrays are independent of the metal permittivity. The sheet resistance is also comparable since the conductivity of Cu is about the same as that of Ag. Just as in Ag NWs, larger Cu NW diameters and pitches are favored for achieving higher solar transmission at a particular sheet resistance. Cu NW arrays may achieve solar transmission >90% with sheet resistances <10 Omega;/sq and figure of merit σDC/σop > 1000. The physics of the optical behaviors of NW arrays are revealed by the study of the contour plots and electric field profiles.
One of the primary concerns with the use of Cu is oxidation and we also investigated the impact of a nickel (Ni) coating, which can serve as an anti-oxidation layer, on the electrical and optical properties. A better performance is demonstrated with ordered Cu nanowires than other nanomaterials such as carbon nanotubes, graphene, and random metal nanowire networks.

With the development of optoelectronic devices, such as photovoltaic solar cells, light emitting diodes, flat panel devices, etc., transparent conductive oxide (TCO) coatings becoming more and more attractive. Traditionally indium tin oxide has been used as TCO because its high optical transparency, and good electrical properties, but it is a scarce element and expensive. Therefore new TCO materials are extensively investigated. Aluminum doped zinc oxide (AZO) is a promising alternative, which is non-toxic, low cost and environmentally friendly. To decreased the resistivity of AZO films a multilayer structure using a thin metallic film have been proposed by various authors [1-3].
In this study we report on transparent and highly conductive multilayer AZO/Ag/AZO structures grown by RF sputtering at room temperatures on glass substrates. The multilayer films were prepared by RF sputtering of ZnO:2% at. Al2O3 and silver targets, varying the total pressure, the ratio Ar/O2 and the Ag film thickness. We study the morphological and optoelectronic properties of the multilayer structure as a function of the growth parameters. The AZO/Ag/AZO electrodes exhibited superior square resistance and optical properties compared to AZO films grown at the same conditions. Furthermore, results of a CdTe solar cell using an optimized AZO/Ag/AZO as TCO are reported.
References
1-Anna Sytchkova, Maria Luisa Grilli, Antonio Rinaldi, Sylvain Vedraine, Philippe Torchio, Angela Piegari and Franccedil;ois Flory, J. Appl. Phys. 114 (2013) 094509.
2- Sebastian Wilken, Thomas Hoffman, Elizabeth von Hauff, Holger Borchert, Jurgen Parisi, Solar Energy Materials & Solar Cells 96 (2012) 141-147.
3- Jin-He Qi, Ying Li, Thanh-Tung Duong, Hyung-Jin Choi, Soon-Gil Yoon, Journal of Alloys and Compounds 556 (2013) 121-126.

Al-doped ZnO (AZO) is presently considered as probably the most promising transparent conductive oxide that can potentially replace indium-tin oxide. The application of ZnO to Si-based electronics and photovoltaics motivates studies of the ZnO-Si interface. Recently, we have reported on investigations of the interface between nominally undoped ZnO and moderately doped p- and n-type (100) Si (Quemener et al., J. Phys. D: Appl. Phys. 45 (2012) 315101). In the present work we have studied electrical properties of the interface between AZO and n-type Si with different crystalline orientations.
100-nm thick AZO has been deposited on (100), (110) and (111) n-type Si with a resistivity of 2-10 Ohm-cm and on a fused silica substrate by Atomic Layer Deposition (ALD) at 200^oC using trimethylaluminum, diethylzinc and water as the precursors. A 200-nm thick Al film has been e-beam evaporated on top of AZO. Circular contacts with a diameter of 400 mu;m have been defined by photolithography and etching of the exposed Al and AZO. Post processing heat treatments have been performed in the temperature range 200-500^oC for 30 min in air. The structures have been electrically characterized by current-voltage measurements and by Deep Level Transient Spectroscopy (DLTS). Hall measurements, performed for AZO deposited on fused silica, reveal the resistivity of the as-deposited film to be around 1.8x10^-3 Ohm-cm.
It is found that the AZO films form Schottky diodes for all of the studied Si orientations. The as-deposited structures show significant non-systematic variations in the diode behavior between different contacts on the same sample. However, after an initial heat treatment at 200^oC in air, no significant variations between the contacts occur. The general trend for all samples, regardless of crystallographic orientations, is that the leakage current is reduced after heat treatments, and the diode characteristics improve after annealing up to 400^oC. Annealing beyond this point reduces the rectification by limiting the forward current. For all samples the ideality factor is close to unity after annealing at 200 - 300^oC. Annealing at higher temperatures increases the ideality factors, suggesting an increased recombination in the space charge region. AZO on (110)Si exhibits the highest rectification. DLTS measurements reveal formation and annealing of electronic states after the heat treatments. The origin of these states is attributed to formation of SiO_x at the ZnO-Si interface.

We demonstrate thin metal mesh patterns for flexible devices especially for touch screen sensors. For transparent applications, the metal meshes should be composed of very fine lines whose width are less than 10 micrometers. In this paper, the ultra-fine pattern is fabricated by the reverse-offset printing method using Ag nanoparticle ink instead of photolithography. A whole conductive layer including active sensing area and routing electrodes is printed at one time. By controlling ink adhesion and cohesion, the printing quality is determined in reverse-offset printing. Also we found that the thickness of metal mesh affects the printing quality. For 100% transfer and sharp edge definition, printing process parameters like printing speed, printing pressure are optimized. The printing quality is verified using the optical transmittance. The optimized metal mesh shows an excellent agreement with the theoretical transmittance of the designed pattern. Finally, a single-layer metal mesh 5-inches touch screen sensor is fabricated on a transparent flexible plastic substrate.
Acknowledgement
This research was financially supported by the “Sensitivity touch platform development and new industrialization support program” through the Ministry of Trade, Industry & Energy(MOTIE) and Korea Institute for Advancement of Technology(KIAT).

There have been growing interests in transparent conducting oxides (TCOs) such as ZnO, In2O3, SnO2 and its compound structure due to their superior optical transparency and electrical conductivity, which leads to various optoelectronic applications. Interestingly, their metallic conductivities are also well maintained even in a form of amorphous phase, which have attracted a great deal of interests for the industrial application since the cost of fabrication of the device can be reduced. However, previous studies on amorphous TCOs reported that they absorb the light with the smaller frequency than their optical band gaps (~3 eV) and that causes the change of the device properties unintentionally. Furthermore, the device instability worsens in the case of absorbing the photon with the higher energy than 2.8 eV corresponding to blue and near UV light. In recent, the tail state from the band edge receives attention as the important source for visible light absorption because it can vary the electrical property of amorphous TCOs by leading the formation of peroxide. However, the amount of the tail states and the energy ranges of the light they can absorb remain unresolved.
In this study, we perform the first principles calculation based on the density functional theory (DFT) to investigate the optical absorption coefficient and the band gap of amorphous IGZO which is one of the representative TCOs. We model the 10 different supercells with 14 formula units of InGaZnO4 for the statistical analysis and carry out the GW calculation for the quasiparticle band structure. We find out that even though the tail states are smeared out from both of the valence and conduction band edge, the states close to the valence band edge are more extended energetically and spatially localized than conduction band edge. Microscopically, the anti-bonding between two oxygen ions that are located close together induces the valence band tail state. It is noted that the transition of the electrons between each tail state can be started from 2.3 eV that corresponds to green light. In addition, the theoretical absorption spectrum in the vicinity of blue light region agrees well with the experimental data implying that the tail states plays a critical role to the device instability under the visible light illumination stress.

We developed printable graphene ink through a solvent-exchange method. Printable graphene ink in ethanol and water free of any surfactant is dependent on matching the surface tension of the cross-solvent with the graphene surface energy. Percolative transport behavior is observed for films made of this printable ink. Optical conductivity is then calculated based on sheet resistance, optical transmittance, and thickness. Upon analyzing the ratio of dc/optical conductivity vs. flake size/layer number, we report that our dc/optical conductivity is among the highest of films based on direct deposited graphene ink. This is the first demonstration of scalable, printable, surfactant-free graphene ink derived directly from graphite.

Transparent electrode is a critical component to realize modern optoelectronic devices including display, touchpad, and solar cells. While the most commonly used transparent electrode up to date is thin metal oxide film as Indium Tin Oxide (ITO) due to its superior optical transparency and electrical conductance, its brittleness and lithographic difficulty have posed major challenges in manufacturing cost-effective flexible optoelectronic devices.
In this study, we firstly attempt to address the aforementioned issues via optimization of direct lithographic step of current generation ITO and conducting polymer films on flexible polymer substrate by direct laser scribing processes. Through rigorous parametric examination of various short pulsed lasers and commercially popular film configurations, optimized performance is demonstrated in terms of electrical isolation, minimal substrate damage, excellent visuality and low degree of film delamination for long-term stability. Additionally, thin coating layer of nanomaterials such as silver nanowires and carbon nanotubes as emerging alternative transparent electrodes, we will also report our recent progress on the order of magnitude improvement in the electrical performance by enhancing local connectivity within nanomaterials network utilizing a unique enhancement mechanism of laser field in the nanomaterials system. Based on advanced in-situ and ex-situ characterization results by optical and electron microscopic techniques, key improvement mechanisms are also discussed.

Transparent electrodes have been studied actively for various potential applications including touch panels, light-emitting diodes, solar cells, liquid crystal displays, etc. Indium tin oxide (ITO), the most widely used material in this area, has low resistivity and high transparency; however, researchers have tried to replace ITO due to its rarity and high cost. Metal mesh electrodes, one of the ITO substitutes, have shown promising properties comparable to those of ITO. Among many fabrication methods of metal meshes such as imprinting, photolithography, and ink-jet printing, etc., electrohydrodynamic (EHD) jet printing have been attracting attention because the techniques has many advantages including simple process, high resolution, and the ability to control the optical and electrical properties by adjusting metal-grid pitch and width. However, its resolution is still limited to a few microns. Also, due to EHD printing utilizes inks with metal nanoparticles, printed lines need high-temperature sintering process, and the lines with nanoparticles have worse electrical conductivity than that of lines deposited by vacuum process. Here, we developed a novel fabrication method of submicron metal lines by the combination of vacuum and EHD processes. The shadow masks are prepared by EHD process with viscous solutions such as glycerol, diffusion pump oil, and photoresist on top of glass substrate. High viscosity is important for stable jetting of shadow mask, and low vapor pressure is required not to be evaporated at the following vacuum process. Metals like Cu, Ag are then deposited by evaporation or sputtering process followed by the removal of shadow masks with a proper etchant for used materials. Nano gaps between shadows masks are formed and controlled by nano stage. The pitch and width of polymer-based lines defines width and gap of metal lines, respectively. It is expected that our method could be widely adopted for applications, which need high transparency and electrical conductivity.

Transparent conductors can be applied into a myriad of applications, ranging from touch screens, to thin-film PVs. Currently the vast majority of the market is dominated by ITO, which has developed for decades with a good tradeoff of optical transmittance and electrical conductance. However, with the increasing demands of TCs, the use of ITO will have several limitations, such as the scarce indium supply, the high cost, and brittleness. A lot of alternative TCs have been proposed and developed, such as carbon nanotubes, silver nanowires and nanoparticles, copper nanowires and nano-fibers, conductive polymers, metal meshes and recently graphene. In this talk, we will present using nanostructured thin metallic film as a transparent conductor structure. We&’ll show how we optimize the optical transmission and sheet resistance of the structure using 3D optical and electrical modeling methods. We&’ll also compare the results with other high performance transparent conductors, and indicate the possible enhancement mechanism from our nanostructure design.

We report on the successful deposition of transparent TiNx/ TiO2 hybrid conductive thin films on polycarbonate (PC) using atmospheric plasma. A specialized high-temperature precursor delivery system was employed using a titanium ethoxide precursor, helium delivery and primary plasma gas, and a nitrogen secondary plasma gas. The hybrid film chemical composition, deposition rate, optical and electrical properties as well as adhesion energy with the polycarbonate substrate were investigated as a function of plasma power and gas flow rate. The film deposited was a hybrid of amorphous TiNx and a Rutile phase of TiO2. The TiNx content increased with higher plasma power and nitrogen flow rate. The visible transmittance varied from 71% to 83% and increased as either the plasma power decreased or nitrogen flow rate decreased. The hybrid thin film resistivity was in the range of 101-105 ohm cm and a lowest 8.5×101 ohm cm resistivity was achieved without annealing. The adhesion energy on the polycarbonate ranged from 1.2 J/m2 to 8.5 J/m2 with increasing plasma power and decreasing nitrogen flow rate. We further demonstrated that annealing and adding of H2 into the primary plasma gas had a significant influence on the composition and resistivity of the hybrid film. After annealing, the resistivity of the thin film decreased to 6.1×10-1 ohm cm. Processing conditions for the optimum combination of the high optical transmittance TiO2 phase and the high electrical conductivity of the TiNx was determined. The study reveals a promising new way to fabricate transparent conductive thin film with low cost.

We developed poly (3,4-ethylene dioxylene thiophene):poly (styrene sulfonic acid) (PEDOT:PSS)-free organic solar cells (OSCs) using buffer and anode integrated Ta-doped In₂O₃ (ITaO) electrodes. To optimize the ITaO electrodes, we investigated the effect of Ta₂O₅ doping power on the electrical, optical, morphological, and structural properties of the co-sputtered ITaO films. In addition, optical properties and electronic structure for as-deposited and annealed ITaO films prepared at optimized doping power were measured by spectroscopic ellipsometer and X-ray absorption spectroscopy. The optimized ITaO film doped with 20W Ta₂O₅ radio frequency power showed sheet resistance of 17.11 Ohm/square, a transmittance of 93.45 %, and a work function of 4.9 eV, all of which are comparable to the value of conventional ITO electrodes. The conventional bulk heterojunction OSC with ITaO anode showed a power conversion efficiency (PCE) of 3.348 % similar to the OSCs (3.541%) with an ITO anode. In addition, OSCs fabricated on an ITaO electrode successfully operated without an acidic PEDOT:PSS buffer layer and showed a PCE of 2.634 %, which was much higher than the comparable no buffer OSC with an ITO anode. Therefore, co-sputtered ITaO electrodes simultaneously acting as a buffer and an anode layer can be considered promising transparent electrodes for cost-efficient and reliable OSCs because they can eliminate the use of an acidic PEDOT:PSS buffer layer.

We investigated characteristics of MoO₃-graded ITO films for use as hole injection layer (HIL) and anode integrated electrode for organic light emitting diodes (OLEDs). By combining a high work function of MoO₃ (6.2-6.6 eV) and highly conductive ITO, we fabricated high work function MoO₃-ITO multicomponents electrodes acting as HIL and anode simultaneously in OLEDs. Thin MoO₃ layer co-sputtered on the ITO films with graded content showed a low sheet resistance and a high transmittance as well as high work function acceptable values in fabrication of OLEDs. In particular, effect of MoO₃ thickness on the electrical, optical, morphological, and structural properties of MoO₃ graded ITO anodes was investigated in detail to optimize the MoO₃ graded layer. At optimized co-sputtering conditions, we obtained MoO₃ graded ITO electrodes with a low sheet resistance of 13 Ohm/square and a high optical transmittance of 83%, comparable to conventional ITO films. Due to the existence of MoO₃ on the ITO film, we can fabricate phosphorescent OLEDs without a HIL. The OLEDs fabricated on MoO₃ graded ITO electrodes showed an identical current density-voltage-luminance and efficiencies to OLEDs with HIL and ITO anode due to the low sheet resistance, high transmittance, and high work function of MoO₃ graded ITO films. This indicates that the MoO₃ graded ITO films fabricated by co-sputtering is a promising integrated electrode to remove the use of HIL in the OLEDs.

Demand for transparent electrodes with high electrical conductivity and transmittance has been recently increased. 1-D nanostructured metals have drawn great attention as a replacement of sputtered ITO. Among them, Cu nanofiber formed by electrospinning is expected to be most suitable for transparent electrode due to its high electrical performance and cost-efficiency. However, high oxidation tendency of Cu hinders the application as nanostructured form whose surface to volume ratio is high. There were previous attempts to enhance oxidation stability of Cu nanostructures by coating outer shell with ALD or plating. The limit of these approaches was that additive materials and inefficient process were needed after electrospinning and calcination.
We developed a novel method of self-forming outer protective carbon shell to prevent oxidation of electrospun Cu nanofiber. It can be realized with the atomic diffusion of original components by controlling oxygen partial pressure(P_O2) during calcination without any additional materials and processing. Electrospinning proceeded with the solution composed of copper acetate(CuAc) as metal precursor and polyvinyl alcohol(PVA) as polymer matrix. After electrospinning, calcination of as-spun nanofibers proceeded at 700°C by modulating P_O2 with the insertion of oxygen gas. Phase transformation of Cu and quantitative degree of C decomposition were analyzed by XRD and XPS.
Systematic categorization about the effect of P_O2 on calcination was established from the principle that C has higher oxidation tendency than Cu. Especially, calcination under specific P_O2 range between them induced simultaneous oxidation of C and reduction of Cu. Pressure between 1.0×10^-2 and 1.0×10^-1 Torr was suitable for this phenomenon called selective oxidation. In this range, Cu phase without oxide was observed unlike calcination under air which induces oxidation of Cu. Also, weight percent of C in nanofiber decreased with the increase of P_O2. It proves that C is decomposed by oxidation based combustion, not pyrolysis. In selective oxidation, various Cu/C nanostructures can be formed by inducing the growth of void inside nanofiber according to the degree of C decomposition. At the lowest P_O2, structure with small Cu agglomerates outside nanofiber was obtained. However, Cu/C core/shell was self-formed at 2.5×10^-2 Torr by the formation of void and inner diffusion of Cu. As P_O2 increased further, thickness of C shell gradually decreased to form hollow C nanofiber with outward Cu diffusion and agglomeration. Nanostructures dominantly affected electrical properties that core/shell structure showed electrical conductivity with sheet resistance(R_s) as 250Omega;/sq with 65% transmittance. Oxidation resistance of self-formed Cu/C core/shell nanofiber was demonstrated by within 10% increase of R_s during 28 days in atmosphere. Oxidation resistivity under high temperature and humidity condition is discussed.

We have fabricated silver nanowire transparent and flexible electrodes with a sheet resistance of 9 Omega;/sq at a transparency of 90%. Although these resistance and transparency values match that of ITO and are superior to most other alternative materials, there are issues that need to be addressed before the widespread use of silver nanowire electrodes in devices. In this presentation, two important issues will be discussed: surface roughness and the stability of the electrode under current flow.
When a film of silver nanowires is dispersed on a substrate and then annealed, the maximum peak-to-valley roughness can be 300 nm or more. This can cause shorting in organic optoelectronic devices where the thickness of the active layers deposited on the electrode are very thin. We will introduce a simple method to reduce maximum peak-to-valley heights to less than 15 nm, and the root-mean-square roughness to below 7 nm. Unlike other methods to reduce surface roughness, an additional material such as a polymer is not required.
I will then present the first study of the stability of nanowire electrodes during their use (i.e. when they are conducting current) [1]. In contrast to ITO where current conducts throughout the entire area of the film, in nanowire electrodes, transport occurs only through the metal wire pathways. Because of this, current densities in the nanowires can be very high. We will show that when silver nanowire electrodes conduct current at levels typically encountered in organic solar cells, the electrodes can fail in as little as 2 days. Suggestions to improve the stability of the electrodes will be given.
[1] H.H. Khaligh and I.A. Goldthorpe, Nanoscale Res. Lett. 8:235 (2013).

We have developed a low cost and convenient approach to fabricate ITO-comparable transparent electrodes by using solution process of silver nanowires mixed with poly peroxotitanium acid (PPT) gels. The PPT gel is applied to connect the dispersed silver nanowires to preserve its high conductivity while remaining transparency and reducing surface roughness of the transparent electrode. The silver nanowires were synthesized via a modified polyol method, and the PPT gels were prepared by sol-gel method in appropriate concentrations. After applying the PPT gels, the sheet resistance of the transparent electrodes was improved from 192 Omega;/square to 44.7 Omega;/square with a transmittance of 81 %. And the roughness (RMS) was decreased from 106.3 nm to 48.1 nm. The PPT gel also improved the reliability of the proposed electrodes, which the conductivity was remained after general atmospheric storage of 6 months. We also demonstrate an Alq3 based OLED with the proposed transparent electrodes.

New generation photovoltaic devices require indium-free transparent electrodes synthesizable at room temperature with multiple functional properties, including large surface area, mechanical flexibility and effective light management. Aluminum doped zinc oxide (AZO) has been identified as a promising low cost material to address these requirements.
To combine these properties while maintaining a fine control of optical transparency, electrical conductivity and morphology we exploit the versatility of Pulsed Laser Deposition (PLD) at room temperature. By varying the background O₂ pressure during deposition it is possible to tailor nano- and mesoscale morphology and stoichiometry, obtaining compact transparent conductors with state of the art functional properties (ρ < 5x10^-4 Omega; cm, visible T>85%) or hierarchically assembled mesoporous structures with tunable light scattering (haze factor >80%, with T>85%). The compatibility with flexible devices, ensured by room temperature deposition, has been tested by performing the depositions also on ethylene-tetrafluoroethylene (ETFE) substrates.
The goal of combining all the properties of interest into a multiply functionalized material is pursued by achieving separate control of mesoscale morphology and defect (Zn interstitial, O vacancy) concentration. One way to attempt this is chemically based, by performing depositions in a mixed Ar:O₂ atmosphere to permit clustering and hierarchical assembly (Ar partial pressure) and, simultaneously, stoichiometry control (O₂ partial pressure). This kind of approach, together with investigation of thermal treatments in vacuum, air or inert atmosphere, allows to study the role of oxygen vacancies and dopants (intentional and unintentional) in determining functional properties. With respect to this approach, the nanostructures were also characterized by Scanning Photo Electron Microscopy with synchrotron radiation in order to investigate preferential electron transport properties in the cross-plane direction, and how they are related with elemental (Al, Zn, O) distribution and local chemical bonding: the vertical conductivity along the nanotrees was studied through the charge induced shift of the core levels, and a significant Al concentration at the top of the structures (i.e., on the film surface) was discovered, almost irrespectively of deposition conditions.
Another approach consists in the synthesis of graded architectures consisting of a porous layer evolving into a compact layer, which can be obtained in a single deposition process by varying the background pressure during film growth. Optimized graded AZO films have a resistivity of the order of 10^-3 Omega; cm, a mean visible transparency of 80%, with a haze over 40%. The benefits of haze for application in photovoltaic devices have been tested by measuring a 100% increase in the absorption of a low bandgap polymer employed in organic photovoltaics (PCPDTBT) in the spectral regions where haze is maximum.

Recently, silver nanowires (AgNWs) have intensively studied as an ITO substitute in the field of touch screen, light emitting device, solar cell, and flexible displays. In spite of their numerous advantages such as high conductivity, flexibility, low percolation thresholds, several issues still remain unresolved. a) Effective passivation and b) ohmic contact on junction side is considered the influential factor to produce high performance AgNWs based application.
Firstly, effective passivation using protecting layer is essential on AgNWs based film because of their poor oxidative stability by humidity, light, heat, or acidic gases. Polymer composite (ex) PEDOT:PSS, Teflon), which is one of the most well-known protecting materials, can passivate conductive nanowires. However, the problem is that protecting layer increase resistivity by openning the gap between two nanowires. In here, we propose a new approach to prepare novel AgNW transparent conductive film using aqueous graphene oxide (GO) nanosheets as a protecting layer. Hydrophilic, ultrathin, highly dispersible GO can have strong adhesion to the oxygen plasma treated PET owing to hydrophilic-hydrophilic interaction and it results to low sheet resistance, high uniformity, high transmittance, and strong mechanical stability.
Secondly, ohmic contact on junction side should be obtained. Resistivity of AgNWs&’ network mainly depends on number of junction and/or state of junction. If AgNWs are incorporated in the substrate or laid on attractive substrate, there is no problem. Junction of nanowire will be close enough so that resistivity, transmittance, mechanical property of film will be enhanced. In case of PDMS, however, it is hard to fabricate uniform and stable thin electrode on account of hydrophobic nature of that. So, we focus on modifying PDMS surface with various silane compounds to facilitate the realization of highly stretchable transparent electrode. As a result, well ordered and high density coordination-type bond between functional group of silane on PDMS surface and AgNW prevents self-healing ability of PDMS and leading to low resistivity with high transparency and good strechability.

Two-dimensional (2D) materials receive growing interest because of their unique properties as compared to their bulk counterparts. Although graphene has garnered the lion&’s share of this attention, other 2D materials beyond MoS₂ and BN are being sought out. Recently, a new family of 2D materials of early transition metal carbides and carbonitrides (Ti₂C, Ti₃C₂, Ti₃CN, V₂C, Nb₂C, Ta₄C₃ and more) known as MXenes has been discovered. Herein we show the fabrication of a 2D epitaxial Ti₃C₂ thin film, formed by the selective etching of Al from magnetron sputter-grown Ti₃AlC₂. The etching was carried out at room temperature and using two different aqueous etchants: hydrofluoric acid and ammonium hydrogen difluoride. Morphological, chemical and phase characterization for these films were carried out using XRD, XPS, TEM, and SEM. The optical transmittance, electrical resistivity, and magnetoresistance of the films were also measured. These data show that the films are up to ~ 90% transparent in the visible to infrared range, and that metallic-like conductive behavior is exhibited down to ~ 100 K. At temperatures below 100 K, resistivity of the films increased with decreasing temperature and magnetoresistance proved to be negative, due to a weak localization phenomenon characteristic of 2D films. These results illustrate the potential for the use of Ti₃C₂ thin films as transparent conductive electrodes, as well as in electrical, photonic and sensing applications.

Functional transparent conductive oxide (TCO) films have been used for various applications. Among the TCO materials, zinc oxide (ZnO) is one of superior candidates as alternative materials for ITO due to their several merits such as low cost of raw material and wide band gap energy (~ 3.7 eV).
We succeeded in preparing ZnO films by environmentally friendly solution process named “Spin-Spray”. Spin-spray method enables us to deposit a high-quality crystallized ZnO films at low substrate temperature (< 100^oC) without seed layer. As-deposited ZnO films had a high transmittance above 80 % and good film adhesion. Also, they could exhibit a high conductivity after UV illumination due to the doping of C and H which were generated by decomposing organic substance.
In this study, the effects of thermal treatment in hydrogen to the electrical conductivity of ZnO films were investigated. After UV illumination, as-deposited ZnO films indicated a very low (#8786; 1 cm² V^-1 s^-1) which carrier concentration (> 10¹⁹ cm^-3) was high. To improve the mobility, as-deposited ZnO films were thermally treated in hydrogen atmosphere before or after UV illumination.
The source solution was prepared dissolving 10 mM of Zn(NO₃)₂6H₂O in 1 L of de-ionized water and the reaction solution was prepared dissolving NH₃ and 4 mM C₆H₅Na₃O₇ in the same amount of de-ionizes water. ZnO films were deposited by spraying these solutions onto heated glass substrates at 95^oC. As-deposited ZnO films were subjected thermal treatment in hydrogen atmosphere at 100^oC for treatment time of 0.5, 1, and 2, 3 h and UV illumination by black-light-blue lamp for 24 h. The two kinds of post treatments were done in the different order as follows
P1) UV illumination → Thermal treatment, P2) Thermal treatment → UV illumination
After two kinds of processes, there were not noticeable changes in structural properties and ZnO films still exhibited a high transmittance above 80 % regardless of thermal treatment. In case of resistivity, the mobility of P1) increased, but the resistivity was increased (11 Omega;cm) due to the drastic decrease of carrier concentration (~10¹⁷ cm^-3). On the other hand, the resistivity of P2) decreased to ~10^-3 Omega;cm due to the increase of mobility (#8786; 8 cm² V^-1 s^-1) keeping the carrier concentration about ~10¹⁹ cm^-3. These improved results seemed to be attributed to the reduction of the electron trap density at grain boundary.

We investigated the effects of thickness on the electrical, optical, structural, and morphological properties of Al and Ga co-doped ZnO films (AGZO) grown by Linear Facing Target Sputtering (LFTS) for use as a transparent electrode in GaN-light emitting diodes (LEDs). Below a critical thickness of 200 nm, the resistivity and optical transmittance of the AGZO films were significantly affected by the thickness of the AGZO films. However, above a thickness of 200 nm, the AGZO films had similar resistivities and optical transmittances due to the stable columnar structure, which developed at a thickness of 200 nm. Due to the change of the growth mode with increasing thickness, the microstructure and surface morphology were also affected by the film thickness. Based on the figure of merit values, we determined that the optimized thickness of the LFTS-grown AGZO film was 200 nm, which was applied in a GaN-LED as a transparent anode. Successful operation of GaN-LEDs with an optimized AGZO film without plasma damage indicates that the LFTS-grown AGZO film is promising plasma damage-free anode for use in GaN-LEDs.

We fabricated highly transparent Ti doped In₂O₃ (TIO)/Ag nanowire(NW)/TIO (TAT) multilayer electrodes by Linear Facing Target Sputtering (LFTS) and brush-painting for used as anode flexible organic solar cells(FOSCs). And we investigated the TAT transparent anode as a function of number of brush-painting cycles. We achieved highly flexible TAT multilayer electrodes with a low sheet resistance of 9.01 Omega;/square and a high diffusive transmittance more than 80 % in visible region as well as superior mechanical stability. The effective embedment of the Ag NW network between top and bottom TIO films led to a metallic conductivity, high transparency. Due to the inserting Ag NW between TIO layers, we increased mechanical stability and confirmed by structural properties by FE-SEM, HRTEM, XRD analysis. Based on the figure of merit values, we determined that the optimized cycle of brush painting, which was applied in FOSCs. Successful operation of FOSCs with efficiency of 3.01 % indicates that TAT hybrid electrode is a promising alternative to conventional ITO electrode for high performance FOSCs.

With increasing need for flexible organic electronics, efforts are made to develop new TCE that are highly flexible and have functional properties better than indium tin oxide (ITO). Among various candidates, graphene and nanowire random networks have shown most promising properties on flexible substrates. However, electric short and nonuniform functional properties over large areas due to structural discontinuities delay their successful integration in flexible organic devices. Alternatively, we studied hybrid TCE (h-TCE) of nanowires and transparent conductive oxides (TCO). TCO coated between and over nanowires improve the nonuniform properties of nanowires, and the nanowires reinforce functional and mechanical properties. But, there is little understanding about the characteristics of charge carriers and transport mechanism of h-TCE interfacing with organic materials that are essential for designing h-TCE and interfaces in real devices. Here, we investigated the charge carriers and transport in h-TCE of Ag nanowires (AgNWs)-ITO integrated in f-OLED and compared them with those in ITO. On polyethylene terephthalate (PET) substrates, we fabricated 100 nm ITO films and Ag nanowires (AgNWs)-ITO h-TCE. In h-TCE, 30 nm-thick ITO was sputtered on AgNWs-coated PET. The wire density and degree of crystallinity of ITO were varied by controlling processing conditions. TCE is used as an anode in f-OLED. As a hole transport layer (HTL) on top of TCE, we spin-coated PEDOT:PSS. Using Hall measurement, the carrier density and mobility in ITO and h-TCE were analyzed. Chemical reaction and diffusion on the surface and across interfaces between TCE and HTL and the changes in energy structures with variations of ITO crystallinity and wire densities were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), respectively. Analysis results showed that the carrier density in TCE is a dominant factor for device performance when others are fixed, which is determined by the ITO crystallinity. The degree of crystallinity increases with wire density as AgNWs act as crystalline seeds. Thus, h-TCE showed superior electrical properties to ITO. XPS and UPS results revealed that diffusion of In and Sn into HTL and chemical reactions at interfaces are reduced with crystallinity as adhesion between ITO and AgNWs and the cohesion strength of ITO on the surface increase with crystallinity. Overall, the carrier density, mobility, and carrier transport are significantly affected by the microstructure of ITO matrix and surface morphology of h-TCE, which provides great possibilities of further improvement in the electrical properties of TCEs by controlling deposition processes, the density and alignment of AgNWs. We also successfully fabricated h-TCE making f-OLED perform significantly better than with ITO.

As electrical interconnections in various photovoltaic devices, indium tin oxide (ITO) is the most extensively used transparent conductive oxide (TCO). However, increasing cost and brittleness of ITO must be constantly addressed, with new TCE materials needed to replace ITO for the applications in flexible electronics. Among the new TCEs developed for ITO replacements, Ag nanowire networks (AgNWs) not only exhibit improved functional properties but also greater flexibility than ITO. In addition, AgNWs can be coated onto large-area flexible polymer substrates using new solution coating technologies. However, AgNWs have drawbacks such as electrical shorts at the ends of disconnected wires and nonuniform functional properties over large areas. We propose a hybrid TCE (h-TCE) using AgNWs and TCO. For the TCO and polymer substrate, we selected ITO and polyethylene terephthalate (PET), respectively, because commercial roll-type PET substrates with ITO are available as reference materials. ITO was sputtered atop the AgNWs coated PET. In AgNWs-ITO h-TCE, ITO thickness was less than a third of the commercial ITO on PET. According to our microstructural analysis, uniform ITO coatings over and between AgNWs were obtained. ITO atop the AgNWs grew into perfectly crystalline ITO (c-ITO) with AgNWs acting as crystalline seeds, and the degree of crystallinity between and over the AgNWs could be controlled by sputtering conditions. The degree of ITO crystallinity determined the functional properties and flexibility of h-TCE based on our 4-point probe station and bending test measurements. The nonuniform functional properties over large areas was significantly reduced and the electrical conductivity of h-TCE was comparable to that of AgNWs without ITO. Increasing ITO crystallinity and thickness, conductivity of h-TCE improved further, which confirmed that properly welding junctions between AgNWs by ITO reduced the local disconnections in the network, which is the critical factor for improving conductivity through networks. Like AgNWs without ITO, decrease in conductivity of h-TCE under tension at the bending radius (r) down to 5 mm was negligible. Below 5 mm, the degree of ITO crystallinity significantly affected the bending stability of h-TCEs. The cohesion strength of ITO and the adhesion between AgNWs and ITO increased with ITO crystallinity. Therefore, at a critical stress, largely amorphous ITO delaminated from interfaces with AgNWs, and AgNWs become free of the stress, which resulted in a gradual decrease in conductivity with further decreases in r below 5 mm. In contrast, AgNWs were broken with c-ITO without delamination at interfaces. Hence, conductivity abruptly decreased at a critical r when the matrix was c-ITO. We successfully fabricated f-OLEDs on h-TCE with significantly better performance than on ITO. The performance was well maintained under bending, while that on the homogeneous ITO was significantly degraded by ITO cracking.

Indium Tin Oxide (ITO) is a transparent conducting oxide that is used in flat panel displays and optoelectronics. Highly conductive and transparent ITO films are normally produced by heating the substrate to 300 degrees Celsius during deposition excluding plastics to be used as a substrate material. We investigated whether high quality ITO films can be sputtered at room temperature by using atomic oxygen instead of molecular oxygen during sputter deposition. The films were deposited by dual ion beam sputtering (DIBS) in a vacuum chamber with a background pressure less than 1.3 E-7 Torr. A high energy RF ion beam source with focused dual grid ion optics operating at 850 volts was used to create an argon beam incident at 45 degrees on the target material (RF-power: 80 Watt, 15 sccm Ar, 60 mA screen grid). During deposition, the substrate was exposed to either atomic or molecular oxygen. The atomic oxygen was created with a HD25 radical atom source of Oxford Applied Research (RF power: 200 Watt). A high voltage deflector was used to deflect the ions, so the substrate would only be exposed to neutral atoms (ratio of atomic oxygen to molecular oxygen = 0.6 [1]). Samples were made as a function of the oxygen flow rate (1-5 sccm). Glass microscope slides and (100) silicon wafers were used as substrates. All substrates were covered with a thin layer of SiO2 before depositing the ITO films and were rotated during deposition. The depositions were done at room temperature.
AFM measurements revealed that the samples had an rms roughness below 3 nm. XRD 2-theta scans showed no diffraction peaks, indicating that the crystals in our films were very small. The optical properties were measured by an M2000 Woollam spectroscopic ellipsometer and are best described by the combination of a Tauc-Lorentz contribution, a Gaussian contribution, and a Drude contribution. Film thickness and absorption spectra were calculated from the measured optical data. A lower absorption was observed in films sputtered at larger oxygen flow rates and films sputtered with atomic oxygen.
The electrical properties of the ITO films were measured by linear four point probe using a Jandel 4pp setup employing silicon carbide electrodes, high input resistance, and low bias current buffer amplifiers. The resistivity of the samples sputtered with atomic oxygen appeared to be consistently lower than the resistivity for the samples made with molecular oxygen, achieving a minimum value of 2.67E-3 Ohm cm. This value is comparable to what has been obtained by others using ion beam sputtering.
The figure of merit (FOM), i.e. the ratio of the conductivity and the average optical absorption coefficient (400-800 nm), was calculated from the optical and electric properties and appeared to be 1.2 to 5 times higher for the samples sputtered with atomic oxygen. The largest value obtained for the FOM was 0.08 reciprocal Ohms.
[1] Oxford Applied Research fact-sheet HD25.

Transparent conductive thin film (TCF) is at the center of attention between various layers of materials that make up electronic devices such as Display and Solar cell. Especially, The Indium Tin Oxide (ITO) layer has been applied to variety of display and electronics components such as flexible displays, smart windows, touch panels, solar cells, etc.
However, ITO films cannot meet the requirements for low cost electronics device due to its expensive raw materials. In recent years, several candidates to replace ITO films has heavily been investigated for transparent conductive material with surface resistance 100Omega;/square or less and more than 85% transmission.
In this study, we have investigated the possibilities of conductive oxide and Silver nanowire multilayer as transparent conductive thin films for replacing ITO film and simultaneous satisfying requirements for flexible electronic devices. These multilayer structured TCF were fabricated that consists of the multiple AZO conductive oxide layers and inserted silver nanowires, in which sliver nanowire was coated with dispersion of sliver nanowire.
At first, AZO(ZnO 97 wt% : Al2O3 3wt%) layer was deposited by RF plasma sputtering of about 50 nm on the substrate . On top of the conductive oxide layer, silver nanowires layer was coated by printing method. Finally, the 50nm AZO(ZnO 97 wt% : Al2O3 3wt%) film deposited on top of the silver nanowires layer to form multilayer transparent conductive thin films. The properties of this multilayered TFC have shown large variation by changing content of silver nanowires at 0.5, 1.0, 2.0 wt%
Characterizations of multilayer transparent conductive thin films were analyzed by surface resistivity and transmittance, surface flatness using 4-point probe, UV-Vis, SPM, SEM.
Acknowledgement : This research was financially supported by the Ministry of Trade, Industry & Energy(MOTIE), Korea Institute for Advancement Of Technology(KIAT) (project No. R0002275)
[1] I. Crupi, S. Boscarino, V. Strano, S. Mirabella, F. Simone, A. Terrasi, Thin Solid Films 520(2012), 2012, 2, pp.4432-4435.
[2] Jong-Wook Lim, Da-YoungCho, Jihoon-Kim, Seok-InNa, Han-KiKim, Solar Energy Materials & Solar Cells 107 (2012),
2012, 8, pp.348-354.
[3] Hong-Chol Chae, Chang-Hyun Baeg, and Joo-Wha Hong, Kor. J. Met. Mater., Vol. 49, No. 2, pp. 192~196
[4] Liangbing Hu, Han Sun Kim, Jung-Yong Lee, Peter Peumans, and Yi Cui, ACS Nano 2010, 4, 11, pp.2955-2963
[5] Ya-Jun Zhang, Hong-Bea Kim, and Sang-Yeol Lee, J. KIEEME, Vol. 26, No. 7, July 2013, pp. 510-514.

Transparent conducting electrode (TCE) is one of key materials for optoelectronic devices such as flat panel displays and thin film solar cells. Although metal oxide (ITO or Al, Ga-doped ZnO) thin films are currently adopted in those industries, the issues of cost and flexibility drive to develop alternatives. In this purpose, carbon-based nanomaterials such as CNT and graphene attracted much attention. Recently, Ag nanowire is emerging as a prominent material as TCE. There are many papers reporting successful fabrications of electronic devices adopting Ag NWs electrodes. We have also successfully demonstrated flexible organic solar cells using Ag NWs [M. Song et al, Adv. Funct. Mater. 2013, 23, 4177]. The results were quite promising to show the potential of OPV devices as next generation solar cells, being cost-effective and flexible. Further enhancement of solar cell efficiency can be expected with the design of electrode and the incorporation of plasmonic effects. In this analogy, we are trying to use metallic nanomesh as TCE for OPV devices. Several types of Ag nanomesh were designed and prepared on PES substrates by transfer printing method. Our preliminary results show that Ag nanomesh with a line width of 150 nm and line spacing of 1.6 mu;m has a good performance as TCE comparable to ITO and Ag NWs. The inverted-type OPV cells using a low band gap polymer (PTB7/PC71BM) blend exhibit a power conversion efficiency of 7.2%. More scientific analysis including plasmonic effects will be given at the presentation.

We demonstrate a new solution-based coating process, centrifuge coating, for the fabrication of nanostructured conductive layers over large areas. This coating procedure can be applied to 1-dimensional nano- filaments as well as 2-dimensional nano-layers, with the added benefit of minimizing materials wastes resulting from particle re-aggregation. Alongside with fabrication improvement, a theoretical model is developed to account for the sheet resistance exhibited by layered random-network coatings such as nano-filaments and graphene; in particular, it allows the correlation of the commonly observed scaling regimes to the coating microstructure. We also show a refined setup which allows the determination of curvature- dependent sheet resistance. This simple setup can be used to minimize the discrepancy in the measured electromechanical performance for flexible electronics.

Nanostructuring the thin-film layer stack of an organic light-emitting diode (OLED) is a promising approach for emission control. Emission efficiency may be improved and light may be guided to specific emission angles using a periodic grating nanostructure. The most common approach in fabricating nanostructured OLEDs is nanostructuring the indium tin oxide (ITO) layer. The refractive index contrast between ITO and the subsequent organic layers allows for efficient grating scattering. Due to the mechanical rigidity of ITO, other transparent electrode materials need to be considered for mechanically flexible, nanostructured OLEDs. Here, we present a flexible, nanostructured OLED employing a nanostructured composite organic-inorganic base layer in combination with a conductive polymer layer.
TiO2 nanoparticles were blended into a nanoimprint polymer resist. This yields an inorganic-organic composite material suitable for nanoimprint lithography with an increased optical refractive index up to n=1.86 at a wavelength of 500 nm. The composite material was spin-coated onto flexible polycarbonate (PC) substrates and imprinted with a 460-nm period grating using UV nanoimprint lithography. Subsequently, a PEDOT:PSS conductive, transparent polymer layer was processed on this base layer. The nanostructure transfers to the conductive polymer. To demonstrate the functionality of this nanostructured, flexible electrode an organic emission layer (PPV-derivative “Super Yellow”) and a metal cathode (LiF/Al) were deposited. OLED operation is demonstrated for different bending radii. The emission spectrum is characterized as a function of emission angle using a goniophotometer. The extraction of waveguide modes is observed both in electroluminescence and photoluminescence emission spectra. The waveguide mode extraction angle is demonstrated to vary with bending radius of the flexible OLED device.
The combination of an inorganic-organic composite material with a conductive polymer transparent electrode is a promising approach for improving the performance of ITO-free, flexible OLEDs. The conductive polymer provides the electrical conductivity, while the composite material may be nanostructured and provides the high refractive index necessary for emission control. Thus, the combination of the two materials may be used to replace nanostructured ITO layers.

Indium tin oxide (ITO) has been used as a transparent conductive electrode in liquid crystal display, solar cells, light-emitting diodes and so on, as it has excellent electrical conductivity and transparency. However, ITO suffers from high cost, limited resource, poor flexibility and complex fabrication process. Because of these limitations, many studies have been focused on carbon materials such as carbon nanotube and graphene because graphene is suitable for a transparent conductive electrode due to its high transparency, carrier mobility, flexibility and scale-ability. But its low sheet resistance compared to that of ITO needs to be improved.
We used two layered graphene grown by chemical vapor deposition method on Cu-foil. A technique of layer by layer AuCl₃ (20mM) doping was previously reported to be an effective method to lower sheet resistance and maintain the stability. We investigated the doping effects on graphene with various configurations. 4-point probe, transmittance measurement, scanning electron microscopy and micro-Raman spectroscopy were performed to study optical and the electrical characteristics of the doped graphenes. We observed that the simple top-doping process is preferred because the defects may be introduced during the complex layer-by-layer doping process. The details of the results will be presented at the conference.

In this paper, we report a method to fabricate roll printed and thermal roll imprinted transparent conductive films (TCFs) using Ag metal grids and embedded grids with conductive polymer. We were development a highly TCF having a roll-to-roll printability on flexible substrates. First, a gravure-offset printing method was used to fabricate Ag electrode grid layers and a electro-spray coating method was also used to form thin polymer layers with a cell structure of substrate/ Ag grids/ PEDOT:PSS. The optical transmittance and sheet resistance of Ag grid were controlled in the range of 80 ~ 83 % and 20 ~ 100 Ohm/Sq., respectively, by controlling the line width and pitch of Ag grid (30 cm x 20 cm). Second, to fabricate a high resolution imprinted TCFs, the following steps were performed: the manufacture of an electroforming stamp mold, the fabrication of thermal roll imprint (TRI) patterned plastic substrates, patterned plastic substrates filled Ag paste using a doctor blade coating, and coated with a thin film layer of conductive polymer. As a result of measuring line widths and channel lengths of a TRI grid patterns, the linearity and uniformity characteristics for deviation and variation was good obtained. After the Ag paste was used to fill part of the patterned film with conductive polymer coating, the following parameters were obtained: a sheet resistance of 5 ~ 20 Ohm/Sq., transmittance ratio was 83 ~ 86 % at a wavelength of 550 nm (30 cm x 30 cm). We were fabrication the large area polymer solar cells by roll-to-roll compatible slot die coating and screen printing on embedded Ag grids substrates. Printed and coated polymer solar cells with a cell structure of plastic substrate/ Ag grids/ PEDOT:PSS/ ZnO/ P3HT:PCBM/ PEDOT:PSS/ Ag grids obtained the power conversion efficiency (PCE) of between 0.79 % and 1.84 % for large area devices (30 cm2).

The polyol reduction of AgNO3 has been a preferred method of synthesizing silver (Ag) nanostructures and is typically conducted by using a hot-injection process, where the Ag precursor is rapidly injected to a preheated, ethylene glycol solution containing polymeric stabilizers such as poly(vinyl pyrrolidone) and other additives. However, it is often difficult to control the crystallinity of seeds, which plays an important role in determining the morphology of the final products. Here we report that long Ag nanowires with an average length of about 20 mu;m can be synthesized in high yields by applying a heat-up process in the polyol synthesis. Electron microscopy studies revealed that multiple-twinned Ag seeds were generated preferentially during the heat-up procedure, and then grew into nanowires. We also demonstrate that these Ag nanowires can be applied as electrode materials for the fabrication of flexible and transparent organic field-effect transistors with a high hole-mobility.

We have investigated sheet resistance and transparency tunable Ag-doped In₂O₃ (IAO) films through the insertion of a nano-size Ag layer for use as transparent electrodes for organic solar cells (OSCs). Due to the high conductivity of a nano-size Ag layer, the optimized IAO/Ag/IAO film showed the lowest resistivity of 3.988×10^-5 Ohm-cm, even though it was sputtered at room temperature. Furthermore, effective antireflection of the IAO/Ag/IAO at optimized Ag thickness (10 nm) led to high optical transmittance of 84.78 %, which is higher than that of conventional ITO films. Using optimized IAO/Ag/IAO with sheet resistance of 4.988 Ohm/square and optical transmittance of 84.78 % prepared on a glass substrate, we successfully demonstrated OSCs. The successful operation of OSCs with IAO/Ag/IAO multilayer electrodes indicates that the IAO/Ag/IAO multilayer is a promising transparent electrode for large-area OSCs due to its low sheet resistance and high transparency when it is very thin. In addition, we investigated the effect of Ag thickness in the IAO/Ag/IAO electrodes on the performance of OSCs to correlate the Ag thickness and performance of OSCs.

Indium-doped tin oxide (ITO) has been the preferred transparent conductor (TC) for laboratory studies and commercial applications because of its electrical conductivity and transparency in the visible range of the spectrum. However, thin film fabrication of ITO requires high-vacuum deposition techniques and the resulting film is absorbing strongly in the ultraviolet (UV) and infrared (IR) regions of the spectrum, restricting its application space. In attempting to provide a design strategy for a viable alternative to ITO, we study a nontraditional transparent conductive oxide (TCO) material, namely ultrathin ruthenium dioxide films (~10 nm), deposited from solution phase onto optical substrates using an easy, bench-top, self-limiting process [1]. The resulting nanoscale skins of RuO₂ are broadband transparent over an uncommonly wide spectral range for an oxide, from UV to microwave, far surpassing the range of ITO [2]. Also, the deposition protocol allows for conformal, non-line-of-sight coating of structured 3D substrates and the resulting films are intrinsically conductive, exhibiting a unique combination of high carrier concentration and low carrier mobility. In this presentation, we will provide an overview of the synthetic methods and the optical, electrical and spectroelectrochemical properties of RuO₂ ultrathin films as a broadband TCO.
[1] C.N. Chervin, A.M. Lubers, K.A. Pettigrew, J.W. Long, M.A. Westgate, J.J. Fontanella, D.R. Rolison, Nano Lett.9 (2009) 2316-2321.
[2] J.W. Long, J.C. Owrutsky, C.N. Chervin, D.R. Rolison, J.S. Melinger, U.S. Patent Application 20110091723.

Reduced graphene oxide (rGO) thin films are fabricated as gate electrodes for organic thin film transistors (OTFTs) by an alternative method combining chemical and subsequent pressure-assisted thermal reduction at relatively low temperature on flexible plastic substrates. The reduction process uses a hot press to squeeze the two bodies, the chemically reduced graphene oxide (CRGO) film and the substrate, maximizing the area of direct contact. Heat flow which induced by the contact area in the reduction transfers sufficient energy to dissociate functional groups from the exfoliated graphene oxide layers. The pressure-assisted thermally reduced graphene oxide (PRGO) thin film has lower sheet resistance (~100 k#8486;/sq), lower root-mean-square (RMS) roughness (1.61 nm), higher restoration of the conjugated π-orbital system (C/O atomic ratio of 10), and smaller interlayer spacing (0.364 nm) at about 78% optical transparency compared to CRGO and thermally reduced graphene oxide (TRGO) thin films. The PRGO thin film-gated OTFT exhibits a field-effect mobility of 0.18 ± 0.04 cm2 Vminus;1 sminus;1, a threshold voltage of minus;18.18 V, and an on/off current ratio of 6.94 × 10^4.

Among various transparent conductive oxides, indium zinc oxide (IZO) is known for its higher work function than ITO, which seems preferable for the use as anode of organic light emitting diodes. IZO film is not indium-free unlike aluminum zinc oxide (AZO) film, but it is known that indium consumption can be reduced by the use of metal interlayer in the middle of IZO film. We have investigated electrical and optical properties of IZO/Ag/IZO structures prepared by magnetron sputtering under various conditions.
First, single layer of IZO (70 nm in thickness) sputtered in Ar atmosphere had electrical resistivity of 4 x 10^-4 Omega;cm, n-type conduction. That deposited in mixtures of Ar and O2 (O2 ratio:0-100%) showed high resistivity of 10^1 -10^4 Omega;cm with the increase of O2 ratios.
Then, we have fabricated multilayer structure of IZO/Ag/IZO film by sputtering. Ag thickness was mainly 10 nm. Then thickness of IZO was 30 or 40 nm each. When the IZO layer was sputtered in Ar, a multilayer with a resistivity of 5 x 10^-5 Omega;cm was obtained. Optical transmittance at a wavelength of 550 nm was 81 % and its figure of merit (FOM) was 26 x 10^-3 Omega;^-1. On the other hand, when the IZO layer was sputtered in O2, a resistivity of the multilayer was also high as 10^4 Omega;cm.
It has been reported that a multilayer consists of high resistivity oxide layer and metal interlayer showed a high conductivity. However, in our results, it is found that conductivity of a multilayer depends not only on property of metal interlayer but also on that of oxide layers.

Transparent conducting electrodes (TCEs) are an essential component for modern opto-electric devices such as solar cells, organic light emitting diodes (OLEDs), liquid crystal displays (LCSs), and touch screen panels (TSPs). Especially, random networks of metal nanowires (metalNW) are considered the most promising TCE material for the viable replacement of indium tin oxide (ITO) due to their tempting advantages of excellent opto-electrical property, good flexibility, and compatibility with low-cost solution process. In this work, we demonstrate flexible transparent conducting plastic films with surface-embedded silver or copper nanowire (metalNW-GFRHybrimer film). This film can be used as a high-performance flexible TCE platform for opto-electric devices. MetalNW-GFRHybrimer film is composed of a glass-fabric reinforced transparent composite (GFRHybrimer) film as the basal substrate and random networks of AgNW or CuNW that are monolithically buried on the film surface as the TCE. The resulting hybrid structure of the metalNW-GFRHybrimer film represents exceptionally smooth surface topology, good thermo-mechanical performance, and excellent opto-electrical performance. With the embedment of the metalNWs in thermally and chemically resistant resin matrix, excellent stability against heat, thermal oxidation, and wet chemicals was established, which is not commonly achieved with standard metalNW TCE systems on glass. [1] The electrical and optical performances of the metalNW-GFRHybrimer film were found to be strongly durable even during long-term high-temperature annealing. The chemical stability was confirmed by an extended duration time in the K2S corrosion test. The metalNW-GFRHybrimer film can be a promising platform for high-performance flexible opto-electric devices.
Key Words: Transparent conducting electrode, Metal nanowires, Glass-fabric reinforced transparent composite
[1] H. G. Im, J. Jin, J. H. Ko, J. Lee, J. Y. Lee and B. S. Bae “Flexible Transparent Conducting Composite Film Using Monolithically Embedded AgNW Electrode with Robust Performance Stability” Nanoscale accepted

Flexibility and transparency has become crucial requirements for emerging electronics including bendable and rollable displays, and flexible sensors. Graphene, a two-dimensional monolayer of sp²-bonded carbon atoms, is being considered as a future transparent and flexible electrode for those emerging frontiers due to its high optical transmittance, flexibility, and conductivity. However, the difficulty of large-area production has been the major obstacle to apply graphene at industrial production level. The roll-to-roll production by chemical vapor deposition (CVD) and transfer process provided a practical solution to large-area graphene sheet production. However, the patterning of graphene on a desired substrate is still limited, although it is an essential process for graphene device fabrication. Previously, various graphene patterning methods have been developed, including electron beam lithography, scanning probe lithography, plasma etching, catalytic etching, and nanoimprint lithography. However, these methods are costly and hardly scalable. Other lithographical methods also require complicated pre-defined masking and wet chemical etching processes. Here we present a selective pattering of graphene on flexible transparent substrates using direct near infrared (NIR) nano second pulse laser (1064nm) without using any masking and lift-off processes. We found that the polyethylene terephthalate (PET) film is totally transparent to the NIR laser, while the graphene layer absorbs the laser beam by ~2.3%, resulting in the selective removal of graphene from the irradiated region. Compared to other conventional patterning methods, the NIR pattering of graphene is expected to be more practical for applications to industrial production by enabling fast patterning of graphene on transparent flexible substrates without using complicated lithographic methods.

A ternary mixed oxide Zn1minus;xMgxO has been successfully doped with aluminium to create a range of transparent
conducting oxides (TCOs) with tunable refractive index as well as work function. Conductive material
was synthesised up to a magnesium concentration of x=0.45, although the conductivity is reduced compared
to standard ZnO:Al. The changes in band gap, work function and conductivity have been attributed to a
modified band structure and energetic position of the aluminium induced donor state.

Metal nanowire transparent conducting electrodes (TCEs) have been widely developed for their promising sheet resistance-transmittance performance, excellent mechanical flexibility, and facile synthesis method. How to lower the junction resistance without compromising optical transmittance has become the key issue in enhancing the performance. Here we have combined electrospinning and electroless deposition to synthesize interconnected, ultra-long metal nanowire networks. For both silver and copper nanowire networks, the sheet resistance-transmittance performances reach around (10 Omega;/sq, 90%), which is by far the best performance among all wet-chemistry synthesized metal nanowire TCEs. The bending tests suggest our electrolessly desposited metal nanowire TCEs are high flexible, and we also synthesize a 11-cm large sample to demonstrate it scalability. This approach opens new opportunities for flexible electronics and roll-to-roll large-scale manufacturing of transparent electrodes.

Electrode work functions greatly influence the performance of organic heterojunction (planar as well as bulk) solar cells. In such devices, a proper adjustment of the electrodes with respect to their work functions can improve the power conversion efficiencies. New low work function electrodes are highly desirable since so far widely used metals like Mg, Ca, Li or their alloys are chemically reactive and very sensitive to oxygen, upon which they easily oxidize. In case of organic electronic devices in the so-called inverted architecture, a low work function contact is placed on the bottom of the device. The bottom electrode is usually the one which is transparent. The most commonly used indium tin oxide (ITO) material for transparent electrode applications typically has work function of 4.2 - 4.8 eV or even higher. Transparent conducting aluminum-doped zinc oxide (ZnO:Al, AZO) films have the work function of 4.1 eV, which promotes their use as transparent electrodes in inverted organic solar cell devices. Additionally, by introducing magnesium into AZO films, a further decrease of the work function can be achieved due to the modification of the ZnMgO:Al band structure.
We obtained, using atomic layer deposition, AZO films with a very thin (a few nm thick) Zn1-xMgxO:Al (0 < x < 0.4) layers grown on the top of the AZO films. Here, we combine the highly conducting state ensured by the AZO layer and the low work function due to magnesium presence at the AZO surface. An additional advantage of the proposed electrode architecture is almost unchanged ZnMgO:Al film morphology, comparing to ZnO:Al surface. We test the so-obtained layers as electron injecting/hole blocking contacts to organic single layer devices as well as ZnO/organic hybrid heterojunction diodes. Based on the current-voltage and capacitance measurements we indicate the possible mechanisms of the charge injection and transport phenomena in those devices.
This work was partially supported by the Polish National Science Centre (NCN) under decision No. DEC-2012/07/D/ST3/02145, and by the European Union within the European Regional Development Fund, through the Innovative Economy grant (POIG.01.01.02-00-108/09).

ZnO and ZnO based alloys are among the most promising candidates as Transparent Conductive Oxides (TCOs) for large scale applications such as windows films/active layers for solar cells and displays.¹ However, despite the wide research on these materials and that most of the CuIn_1-xGa_xSe₂ (CIS and CIGS) based photovoltaic modules are already based on ZnO as a conducting front contact,² the role of commonly used dopant impurities (at extremely high concentrations suitable for TCOs applications) on the materials properties and stability are not fully clarified and still under investigation.
As an example, it has been shown that Al introduces non-equilibrium defects into ZnO films.³ Similarly, the thermally induced conductivity degradation of Ga doped ZnO has been attributed to the Ga atoms instability that relax to octahedral coordination after air annealing at already relatively low temperatures (~420 °C).⁴
We have recently shown that Si acts similarly as Al with respect to the electrical and structural properties of ZnO.⁵ Here, we present a comparative study of the annealing effects on magnetron sputtered thin films of ZnO nominally undoped and doped with Si to ~1.5 at.% and Al to ~2 at.%, as measured by Rutherford Backscattering Spectrometry (RBS) and corresponding to a resistivity of ~2 x10^-3 #8486; x cm as extracted from room temperature Hall measurements (RTH). The films were prepared on quartz substrates by RF co-sputtering at 400 °C and subsequently annealed in air for 1 h in the 550 °C -1050 °C range. The comparison between the structural characteristics of the annealed films clearly indicate that Al and Si inhibit the increase in the grain size as extracted from X-ray diffraction (XRD) measurements. Photoluminescence Spectroscopy (PL) confirmed these findings indicating that Si and Al are retarding a substantial onset of the near band edge emission at annealing temperatures above ~800 °C. However, RTH measurements reveal that the films experience a dramatic increase in resistivity of ~4 orders of magnitude already after the 550 °C annealing step possibly due to chemisorption of Oxygen at the grain boundaries and/or increase in the number of compensating Zinc vacancies (V_Zn).^6,7
1) K. Ellmer, A. Klein, and B. Rech, Transparent Conductive Zinc Oxide (Springer-Verlag Berlin Heidelberg, 2008).
2) F. J. Pern, R. Noufi, B. To, C. DeHart, X. Li, and S. H. Glick (2008), Conference Paper NREL/CP-520-42792.
3) H. Ryoken, I. Sakaguchi, T. Ohgaki, N. Ohashi, Y. Adachi, and H. Haneda, Mater. Res. Soc. Symp. Proc. 829 (2005).
4) J. A. Sans, G. Martnez-Criado, J. Pellicer-Porres, J. F.Sagrave;nchez-Royo, and A. Segura, Appl. Phys. Lett. 91, 221904 (2007).
5) R. Schifano, M. Schofield, L. Vines, S. Diplas, E. V. Monakhov, and B. G. Svensson
IOP Conf. Series: Materials Science and Engineering 34, 012007 (2012)
6) S. Major, A. Banerjee, and K. L. Chopra, Thin Solid Films, 122, 31, (1984).
7) S. Lany and A. Zunger, Phys. Rev. Lett., 98, 0405501, (2007).

Because of the increasing demands in large-area optoelectronic devices such as organic light emitting diodes, touch screen panels and thin film solar cells, novel transparent conductors are being extensively studied in order to replace wide band gap transparent conductive oxides (TCO), indium tin oxide (ITO) in particular, which suffers from various problems including its scarcity, huge waste of target material and fragile nature. The potential alternatives of ITO include conducting polymers and carbon-based materials such as PEDOT:PSS, graphene and CNT, however, their performance highly depends on the preparation and often does not meet the requirements for many applications in terms of conductivity and stability. On the other hand, metal NW transparent conductor, or metal NW percolation network, has become a markedly mature technology in terms of material synthesis and fabrication process, yet very little work has been done on developing patterning techniques for such metal NW based transparent conductors.
In this study, we demonstrate a simple patterning method for metal NW percolation network by using 355 nm nanosecond (ns) pulsed laser ablation. This method is basically conducted in ambient condition without using any conventional patterning processes such as photolithography or chemical etching, which are considered as time consuming, multistep and expensive. Pulsed laser ablation has long been utilized as a facile and handy tool for direct patterning, and we confirmed that metal NW also can be effectively ablated by ns pulsed laser ablation at low threshold fluence even at a single shot. Being a direct writing method, no premade mask is needed to create an arbitrary pattern, while the processing speed has been increased significantly in this study by using galvanometer mirror to obtain a rapid scanning (>100 mm/s) of focused laser beam. By scanning the focused laser spot at low (1 KHz) or high pulse repetition (20 KHz) rates, a microhole array with ~15 mu;m hole diameter or a continuous ablation line at 5~50 mu;m width is readily produced on the metal NW percolation network to tune the properties of the resultant percolation network or create a desired pattern at micro-scale. It is also found that metal NW based electrode line prepared by the ablation process substantiates that the general relation for a conducting thin film fails at a narrow width, which should be considered for the applications that requires a high resolution patterns. Finally, through a simple capacitive touch sensor demonstration, it is confirmed that the selective laser ablation can be directly applied for the device fabrication which requires a patterned transparent conductor.

We reported on transparent MoO₃-Zn doped In₂O₃ (IZO) electrodes fabricated by co-sputtering for use as an anode in organic solar cells (OSCs). By adjusting RF and DC power of MoO₃ and IZO targets during co-sputtering, we fabricated the MoO₃-IZO electrode with graded content of the MoO₃ on the IZO films. In particular, we investigated the thickness effect of graded MoO₃ layer on the electrical, optical, structural and morphological properties of MoO₃-IZO multicomponent electrodes. At optimized MoO₃ thickness of 20 nm, the MoO₃ graded IZO electrode showed a sheet resistance of 40.22 Ohm/square and optical transmittance of 70.1 %. Without degradation of electrical and optical properties of IZO films, we obtained MoO₃ graded IZO films for use as an anode for OSCs. To investigate the feasibility of MoO₃ graded IZO films, we fabricated conventional P3HT:PCBM based OSCs with MoO₃ graded IZO as a function of MoO₃ thickness. The OSC fabricated on the MoO₃ graded IZO anode showed a fill factor of 66.53 %, a short circuit current of 8.121 mA/cm², an open circuit voltage of 0.592 V, and a power conversion efficiency of 3.2 % comparable to OSC fabricated on ITO anode. We correlated the performance of OSCs with a MoO₃ thickness on the IZO film and suggested a possible mechanism to explain the effect of MoO₃ graded layer.

We investigated characteristics of ITO/Ag-Pd-Cu (APC)/ITO multilayer electrodes prepared by direct current magnetron sputtering for use as an anode in organic solar cells (OSCs). To optimize the sheet resistance and optical transmittance of ITO/APC/ITO multilayer, we fabricated the ITO/APC/ITO multilayer at a fixed ITO thickness of 30 nm as a function of APC thickness. At optimized APC thickness of 12 nm, the ITO/APC/ITO multilayer exhibited a sheet resistance of 6 Omega;/square and optical transmittance of 84.15 % at a wavelength of 550 nm which is comparable to conventional ITO/Ag/ITO multilayer. However, the APC-based ITO multilayer showed a higher average transmittance in a visible region than the Ag-based ITO multilayer. The higher average transmittance of ITO/APC/ITO multilayer indicated the multilayer is suitable anode for organic solar cells with P3HT:PCBM active layer. OSCs fabricated on the optimized ITO/ACP/ITO multilayer exhibited a better performance with a fill factor of 64.815 %, a short circuit current of 8.107 mA/cm², an open circuit voltage of 0.59 V, and power conversion efficiency (3.101 %) than OSC with ITO/Ag/ITO multilayer. Therefore, ITO/APC/ITO multilayer can replace Ag-based ITO/Ag/ITO multilayer in fabrication of OSCs due to its high optical transmittance in a wide range wavelength and low sheet resistance.

Oxide thin film transistors (TFTs) have attracted considerable interest for gate diver and pixel switching devices of the active matrix (AM) liquid crystal display (LCD) and organic light emitting diode (OLED) display because of their high field effect mobility, transparency in visible light region, and low temperature processing below 300oC. Recently, oxide TFTs with polycrystalline In-Ga-O(IGO) channel layer reported by Ebata. et. al. showed a amazing field effect mobility of 39.1 cm2/Vs. The reason having high field effect mobility of IGO TFTs is because In2O3 has a bixbyite structure in which linear chains of edge sharing InO6 octahedral are isotropic.
In this work, we investigated the characteristics and the effects of oxygen partial pressure significantly changed the IGO thin-films and IGO TFTs transfer characteristics. IGO thin-film were fabricated by rf-magnetron sputtering with different oxygen partial pressure (O2/(Ar+O2), Opp)ratios. IGO thin film Varies depending on the oxygen partial pressure of 0.1%, 1%, 3%, 5%, 10% have been some significant changes in the electrical characteristics. Also the IGO TFTs VTH value conspicuously shifted in the positive direction, from -8 to 11V as the Opp increased from 1% to 10%. At Opp was 5%, IGO TFTs showed a high drain current on/off ratio of ~10^8, a field-effect mobility of 84cm2/Vs, a threshold voltage of 1.5V, and a subthreshold slpe(SS) of 0.2V/decade from log(IDS) vs VGS.

Indium tin oxide (ITO)-free organic solar cells were fabricated with highly conductive and transparent MoO₃/Au/MoO₃-PEDOT:PSS composite electrode. The solution process MoO₃-PEDOT:PSS composite film on MoO₃/Au enhance both the of transmittance and conductivity of the electrode. The transmittance of anodes and power conversion efficiency, (PCEs) were optimized with thickness of MoO₃-PEDOT:PSS composite film. Transmittance was maximized at 80% at 550 nm wavelength using 30 nm bottom MoO₃ layer and solution process 30 nm top MoO₃-PEDOT:PSS composite layer of a 12 nm Au layer. The MoO₃-PEDOT:PSS composite film characterized with Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). Power conversion efficiency was maximized at 2.81% which is comparable to that of ITO based conventional organic solar cell, PCE (2.89%). The improvement of PCEs due to the incorporation composite in multilayer electrode is attributed to the smooth surface, high reflectance index (2.45), lower work function of the electrode (4.9 eV) close to ITO (4.7 eV) and high conductivity. The MoO₃/Au/MoO₃-PEDOT:PSS electrode was shown to be a promising replacement of ITO for use in low-cost optoelectronic devices.

As the market of transparent electrode is growing, researchers developed transparent electrodes with new materials and fabrication process. Many materials and processes to make a highly transparent and highly conductive transparent electrode are introduced but they all have their own pros and cons. Metal-based films are researched well and the new material, graphene is uprising for next-generation transparent electrode material. Metallic nanostructure has very low surface resistance, while it has big hole for transparency. Graphene flake film is easily fabricated by solution-based process and has good uniformity but has very low electric conductivity. With these two materials, this paper suggests a hybrid-type transparent electrode with copper grid and graphene film.
Copper grid is fabricated by two processes; lift-off process and PDMS nano-imprinting transfer pro-cess while PDMS process was not very successful. Graphene flake film is fabricated by vacuum filtration process with AAO filter and Teflon filter. Teflon filter filtration process was not stabled. With these two ma-terials and processes, hybrid transparent electrode was developed. It has 75% transparency and 40Omega;/sq sur-face resistance. This result is not very effective in comparison with pre-existed ITO film (with >85% trans-parency and <20Omega;/sq surface resistance). This paper shows its possibility to be a flexible transparent elec-trode with high performance over than ITO film.
Keywords: transparent electrode, metal grid, graphene, touch screen, solar cell

Silver nanowire based transparent electrodes hold several important advantages over traditional transparent conductive oxides such as indium tin oxide. They are more compatible with flexible substrates and can be deposited at ambient temperatures and pressures. Here, we will discuss a computer controlled spray deposition technique we have developed and optimized for room temperature deposition. This technique has allowed us to deposit a transparent conductor with good performance (14 ohm/sq and peak light transmission of 93%) directly onto a sensitive organic solar cell with no heating steps. This process has enabled us to produce semi-transparent solar cells suitable for inclusion into a hybrid tandem photovoltaic device stack.
Lifting the low temperature deposition restriction allows for significantly improved electrode performance. By re-tuning our spray deposition system for a heated glass substrate (100°C) and adding an additional laser based post processing step, we are able to deposit uniform large area films which pass over 92% of incident light while maintaining a low sheet resistance of 6.3 ohm/sq, surpassing the highest performing solution processed transparent conductors reported in literature today.

Copper nanowire (CuNW) networks are one of the most promising candidates to replace indium tin oxide films as the premier transparent conducting electrode (TCEs) due to its high sheet resistance(Rs) -transmittance(T) performance, superior mechanical flexibility and low lost. However, the chemical activity of CuNWs causes a substantial increase in the Rs after thermal oxidation or chemical corrosion, which may undermine its applicability. In this work, we utilize atomic layer deposition(ALD) to coat a passivation layer onto electrospun copper nanowires and remarkably enhance their durability. The passivation layer is composed of 20-nm-thick aluminum-doped zinc oxide (AZO) for the inner layer and 1-nm-thick aluminum oxide for the outer layer. Without changing the optical transmittance, the passivated CuNW TCE shows an resistance increase of only 10% after thermal oxidation at 160 °C in dry air and 80 °C in humid air with 80% relative humidity, whereas the bare CuNWs quickly become insulating. In addition, the coating and baking of the acidic PEDOT:PSS layer increases the Rs of bare CuNW by 6 orders of magnitude, while the passivated CuNWs show an 18% increase. Our work demonstrates that this ALD method can greatly enhance the reliability of CuNW TCE and thus provide a practical solution for the degradation problem of metal nanowire TCEs.

Hybrid photovoltaic devices incorporating inorganic and organic materials are receiving great interest as an approach to next-generation photovoltaics. These technologies aim to combine the advantages of different material systems to provide better efficiency, more cost efficient manufacturing, or both. Silicon/Organic Heterojunctions (SOH) are attractive because these heterojunctions can be fabricated at temperatures < 100°C, without p-n junctions in the silicon, using simple methods such as spin-coating [1,2]. In comparison, conventional crystalline silicon solar cells require p-n junctions that are fabricated at temperatures higher than 800°C [3]. In this abstract we report a record 12.7% AM1.5 power efficiency for this type of cell, enabled in our work by a spray-coated nanowire-enhanced transparent conductor, with a sheet resistance as low as 10Omega;/square.
The basic cell structure consists of the conducting organic layer PEDOT:PSS spin-coated on a p-type silicon (100)wafer. A sparse metal contact (to admit light) and a uniform ohmic back contact complete the structure. The field from the SOH separates electron-hole pairs created in the silicon, and the high LUMO (- 3.3 eV) blocks electrons from entering the PEDOT from the silicon, leading to high open circuit voltages (~ 0.59V). To lower the series resistance from the PEDOT, a silver nanowire mesh is applied on top before a minimal metal contact (<1% contact pad). Silver nanowire meshes are increasingly of interest as a flexible transparent conductor for organic PV (5-7). Deposition of AgNW networks has been demonstrated via spin-coating, drop-casting, or spray deposition on top of a polymer film . The networks typically must utilize some process to join AgNW junctions to enhance conductivity such as thermal annealing, , pressure, or laser fusing[7].
In this work, Ag NW networks were applied via spray coating for silicon-organic heterojunction PV. First, we demonstrate the performance of the spray deposited AgNW films (with any subsequent annealing) is comparable or better than the, spin-cast laser-fused [7] or thermally-annealed AgNW films . The conduction and transmission properties of these films are are typically 10 Omega;/square sheet resistance for a wire density yielding 75% transmission at 600nm wavelength, vs 100 Ohms/sq for the PEDOT:PSS alone. Without any thermal/laser process, any degradation of the polymer layer is avoided. Spray deposition has recently been shown to provide a “fusing” effect simply due to the kinetic energy of the AgNWs during the deposition process [4].. Second, utilizing spray-coated AgNW films, we report a Si/PEDOT:PSS solar cell with an AM1.5 power conversion efficiency of 12.7%. The high conductivity of the nanowires allows both sparse metal contact coverage while maintaining a very high fill factor of 72.6% and allowing an Isc of 29.6 mA/cm2.

The printed electronics has obtained enormous attention over the last few decades due to its low cost, and various applications such as flexible display, organic solar cells, wearable display, and flexible battery. Since these printed electronics use flexible substrates that have low-melting point, flexibility, and low cost, solution-based noble metallic nanoparticles (Au, Ag, etc) inks are widely employed. However, these noble metals are very expensive to be commercialized. Therefore, copper nanoparticles are recently proposed as an alternative for high electrical conductivity and low cost. However, it is difficult to sinter the Cu nanoparticles because it is easily oxidized in the room temperature. Therefore, a flash light sintering method with poly(N-vinylpyrrolidone) (PVP) coated Cu nanoparticles has been investigated. The oxide shells of the copper nanoparticles can be eliminated by combinational function of flash light and PVP evaporation in the room temperature and ambient condition. Meanwhile, the flexible electrical devices fabricated on the polymer flexible substrates via printed electronics should maintain their electrical functions under severe mechanical bending and twisting and stretching conditions. To satisfy these needs, several studies have been conducted to increase reliability of sintered silver nanoparticles thin film by mixing various materials such as CNT, graphene, and nanowires. However, there is no study result to increase a reliability of the sintered Cu nanoparticles thin film. Therefore, in this study, Cu nanowires were added in Cu nanoparticles-ink to improve reliability and electrical conductivity under the mechanical fatigue loading. The Cu nanowires (NW) /nanoparticles (NP)-ink was printed on the polyimide substrates and sintered by flash light irradiation in the room temperature and ambient condition. The reliability of the flash light sintered Cu NW/NP thin film was investigated by measuring resistance variation during outer-bending fatigue tests varying the bending radius, the weight fraction of Cu NW and the flash light irradiation conditions. It was found that the flash light sintered Cu NW/NP thin film showed much higher reliability by adding only 5 wt% of Cu NW (The resistance change (ΔR/R0) of the Cu NW/NP film was 4.196 while that of the Cu NP film was 92.751 at 103 cycles under the 7 mm of the bending fatigue radius). X-ray diffraction, scanning electron microscope, and in-situ sheet resistance measurement were conducted to study deeply the mechanism of reliability improvement of sintered Cu nanowires/nanoparticles thin film.

Transparent electrodes are important for solar cells, display, touch screen and solid state lighting. Indium tin oxides are the dominating transparent electrode technology although the scarcity of indium and the sputtering processing can limit their future application. I will present novel metal nanowire networks as transparent conducting electrodes to replace the existing indium tin oxides. Metal nanowires with diameters smaller than and with separations larger than the wavelength of the light can allow the sunlight pass through without significant reflection or scattering back. We show that these metal nanowire networks provide very competitive optical transmittance at very low sheet resistance. The low cost processing makes them attractive for future large area optoelectronic applications.

Discussed will be methods to make transparent electrodes, de-icing films and resistive memories using graphene and its derivatives. This will include the use of spray-on films from graphene nanoribbons made by splitting carbon nanotubes, nanotube reinforced graphene called rebar graphene, and fabricated graphene nanoribbons by a top-down method called meniscus-mask lithography. Applications in flexible transparent electrodes will be discussed, as well as in the use of RF and optically transparent de-icing films for radomes, phased array antenna, automotive and construction glass.

Graphene is a strong contender material for the replacement of indium tin oxide (ITO) as the transparent conductor of choice for electronic applications due to its exceptional electrical and optical properties [1]. For practical manufacturing applications, large scale production of graphene materials is necessary. To produce large quantities of graphene materials, reduction of exfoliated graphene oxide sheets is favoured because it is a solution phase method with potential low cost. The resulting graphene oxide suspensions can be processed as graphene inks and deposited to form graphene films via large scale and low cost solution process such as inkjet-printing. In this work, we present a study of conductive reduced graphene oxide films produced by inkjet-printing. Highly stable graphene ink (up to 6 months) was prepared by dispersing graphene oxide in water with a stabilizing surfactant at pH asymp; 10 by adding ammonia. This was subsequently reduced in solution-phase using hydrazine monohydrate. Printed film electrical and optical properties are shown to be strongly dependent on the mean flake size used in the ink. By using large area size of graphene oxide sheets and adjusting the number of printing layers films with electrical sheet resistance of 6 kOmega;/sq and optical transparency of 65% could be achieved. These properties corresponded to a ratio between the DC (σDC) and optical (σOp) conductivities (σDC/σOp) of 0.13, which was comparable with solution processed pristine-graphene films that have been reported previously [2]. This indicates that the flake size of the ink is at least of equal importance as the quality of the graphene in determining printed transparent film properties.
[1] S.-K. Bae, H.-K. Kim, Y.B. Lee, et al: “Roll-to-Roll Production of 30-inch Graphene Films for Transparent Electrodes”, Nature Nanotechnology, 5 (8), 574-578 (2010).
[2] S. De, P.J. King, M. Lotya, et al: “Flexible, Transparent, Conducting Films of Randomly Stacked Graphene from Surfactant-Stabilized, Oxide-Free Graphene Dispersions”, Small, 6 (3), 458-464 (2010).

Monolayer graphene, single-atom sheets of carbon atoms arranged in a honeycomb pattern is an attractive candidate due to its high transparency (97.7%) and flexibility. However the intrinsic sheet resistance of graphene (~ 6.5 kilo-ohms per square) is two orders of magnitude too high for transparent electrode applications. Further, the sensitivity of the electrical properties of monolayer graphene to ambient adsorbates or organic processing residue (from transfer or patterning) represents a serious obstacle to exploitation of this novel nanomaterial. Molecular functionalization offers a route to overcome these difficulties, through passivation and/or controlled adsorbate doping via charge transfer.
We report a systematic study of molecular functionalization of graphene comprising optical microscopy, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, together with initial electrical characterization results of as-fabricated and post-functionalized graphene field-effect devices. Candidate molecules screened included both p-dopants and n-dopants and deposition techniques include evaporation, spin-coating and drop-casting.
Initial data on exfoliated graphene indicate that films formed by evaporation of small molecules, e.g. tetracyanoquinodimethane (TCNQ, a p-dopant) grow by nucleation and coalescence of ultra-thin islands (< 2 nm layer thicknesses). Efficient coupling between TCNQ and graphene is evidenced by molecular signatures observed in Raman data acquired from ultra-thin films deposited on both exfoliated graphene and graphene grown using chemical vapor deposition (CVD) on copper foil. Initial data from micron-scale field-effect devices fabricated from CVD graphene show net p-doping after evaporation of TCNQ, albeit with reduced mobility (ie increased sheet resistance).
This work was supported by the European Commission under the FP7 project GO-NEXTs (309201), and by the Irish Government HEA PRTLI programmes (INSPIRE & TYFFANI)

We measure simultaneous in situ optical transmittance spectra and electrical transport properties of few-layer graphene (FLG) nanostructures upon electrochemical lithiation/delithiation. Reversible Li-intercalation stages and a two-phase boundary are observed optically. Due to the unusual electronic structure of FLG, upon intercalation we observe a simultaneous increase of both optical transmittance (up to 55% for 60-80 layers) and DC conductivity (up to two orders of magnitude), strikingly different from other materials. Transmission as high as 91.7% for sheet resistance of 3.0 Omega;/sq is achieved, corresponding to a figure of merit (FOM) σ_dc/σ_opt = 1400, five times higher than any previously demonstrated for a continuous transparent electrode. Our techniques can enable investigation of other aspects of intercalation in nanostructures, for example intercalation dynamics and solid-electrolyte interface formation

Among the range of intensely studied emerging transparent electrodes, graphene grown by scalable chemical vapor deposition (CVD) shows great promise as its two-dimensional lattice structure offers high flexibility, mechanical robustness, chemical inertness combined with unusual electrical and optical properties.[1,2] Although CVD grown graphene has a high electronic mobility, the intrinsically low charge carrier concentration limits the overall conductivity. Thus, stable doping has become a key challenge to turn graphene into a highly conductive material which is suitable for organic electronic applications such as organic light emitting diodes (OLED). In addition to high conductivity for OLEDs it is also crucial to efficiently inject charge carriers from the graphene electrode into the device.
We report on charge transfer doping using various organic and inorganic molecules. It is shown that in particular transition metal oxides, such as MoO3, V2O5 and WO3 are very promising air stable p-type dopants which can lower the sheet resistance of graphene to values below 50 Omega;/sq. Such materials can also form an optimum band alignment with graphene thus allowing efficient charge injection. The integration of doped graphene in an OLED and its opto-electronic performance is discussed in detail. We show that our modified graphene electrode leads to an OLED device efficiency that is above that of an equivalent state of the art indium tin oxide electrode.
Authors acknowledge funding from the EC project Grafol.
[1] Kidambi et al., J Phys Chem C (2012) DOI 10.1021/jp303597m
[2] Weatherup et al., ACS Nano (2012) DOI 10.1021/nn303674g

In this talk, I will summarize the recent research on synthesis, characterization and applications of low-dimensional nanomaterials in my group. I will introduce the synthesis and characterization of novel low-dimensional nanomaterials, such as 2D graphene-based composites including the first-time synthesized hexagonal-close packed (hcp) Au nanostructures on graphene oxide and the epitaxial growth of Pd, Pt and Ag nanostructures on solution-processable MoS2 nanoshees at ambient conditions, single- or few-layer metal dichalcogenides nanosheets, and large-amount, uniform, ultrathin metal sulfide nanocrystals. Then I will demonstrate the applications of these novel nanomaterials in chemical and bio-sensors, solar cells, water splitting, electric devices, memory devices, conductive electrodes, etc.

Highly transparent and conductive thin films were assembled as a potential indium tin oxide (ITO) replacement using layer-by-layer (LbL) assembly with di-walled carbon nanotubes (DWNTs), sodium deoxycholate (DOC) as a stabilizer, and poly(diallyldimethyl ammonium chloride) [PDDA]. This assembly of DOC-stabilized CNTs and PDDA grows linearly as a function of bilayers deposited, with transparency (>84% T) and electrical conductivity (~300 Omega;/sq) at a thickness of 23.5 nm (at 20 bilayers). Moreover, exposure to nitric acid vapor was able to further reduce sheet resistance in this film, down to 104 Omega;/sq, due to the removal of insulating material and charge transfer doping. The optoelectronic performance of a 5 BL DWNT LbL film is much better than most other CNT thin films and capable of ITO replacement. Additionally, the bending and electrochemical stability suggest that these films could be a useful electrode for a variety of flexible electronics applications. Similar films have been prepared using graphene oxide (GO) in place of nanotubes. Thermal reduction of these films results in a combination of electrical conductivity, from GO transitioning to rGO, and high gas barrier properties, which may be interesting for electronics packaging.

Networks of single-walled carbon nanotubes (SWNTs) are a promising replacement for indium tin oxide (ITO) transparent electrodes due to their exceptional mechanical strength and flexibility, the great abundance of carbon, and the ability to deposit them from inks using additive, low-temperature printing processes. SWNT networks have historically had poorer conductivity-absorptivity ratios than ITO because the typical ink fabrication method, dispersing SWNTs in water with the aid of sonication, damages and shortens the nanotubes. Here, a reductive dissolution method—liquid ammonia reduction of the SWNTs to form a nanotubide salt, followed by spontaneous, sonication-free dissolution in a polar organic solvent—is employed to produce solutions of individual, unbroken SWNTs. Using these solutions we have produced transparent electrodes with significantly higher performance than those made from aqueous dispersion, which are now competitive with ITO transparent electrodes on plastic substrates. For example, SWNT films with transmittance of 89% at 550 nm wavelength and sheet resistance of 130Omega;/square have been fabricated by reductive dissolution, compared to 81% transmittance and 130Omega;/square for the aqueous dispersion. These SWNT transparent electrodes have been successfully integrated into organic solar cells using two different sets of active layer materials, with power conversion efficiencies reaching 3%. For thin, fast-drying active layers, SWNT-based solar cells had lower device yield and performance than ITO-based solar cells due to lower shunt resistance, while for thick, slow-drying active layers, the yield and performance of SWNT-based solar cells was very similar to that of ITO. Additionally, flexible solar cells using SWNT transparent electrodes on plastic substrates have been fabricated and show comparable performance to solar cells made on glass.

We have designed a hybrid device comprised of an organic photovoltaic (OPV) monolithically attached to a supercapacitor via a common transparent carbon nanotube (CNT) electrode. This structure may also be viewed as an electrochemically gated OPV in which the gate voltage gradually shifts a resistor-like device into a high efficiency photovoltaic. Unlike typical gated structures, our initial results suggest that gating changes the work function of the metallic common CNT electrode rather than injecting charges into the semiconducting layers. This supposition is based on measurements of the shift in the work function of carbon nanotubes charged in aqueous electrolytes was carried out at our institute by A. Kuznetsov, A.A. Zakhidov and D.S. Suh, which showed that the modulation of work function of the CNT is almost directly proportional to the applied voltage. This shift in work function allows us to modulate the injection/collection barrier between organic semiconducting layers and the carbon nanotubes to the extent of changing the naturally anode like CNT into a cathode in OPV devices.
In this presentation we will discuss our recent results, starting from work on ‘Electrochemically gated organic photovoltaics with tunable carbon nanotube electrodes&’ recently published in Applied Physics Letters and progressing to recent experiments which seek to better explain the phenomena. Specifically, we are interested in determining whether the electrochemical charging in the OPV device extends to the semiconducting photoactive layers or is constrained entirely to the carbon nanotube electrodes. In addition, we are also examining other applications of this architecture. For this case, we will describe an electrochemically-gated, parallel tandem, organic photovoltaic device, which features two photoactive layers in addition to the supercapacitive cell. This device can be produced entirely in ambient conditions via spin-coating and lamination, and avoids many processing difficulties associated with tandem architectures.

The use of transparent electrodes in electronic devices that harvest and generate light, such as solar cells, light-emitting diodes, photoelectrochemical cells, requires both positive (p-type) and negative (n-type) contacts, one of which has to be transparent. Historically, development of p-type transparent conductors has been difficult, thus majority of these technologies have to use n-type transparent electrodes. If a p-type transparent electrode could be designed, it would lead to both new engineering solutions and entirely new applications. In this talk, I will review the history and the recent advances in the field of p-type transparent electrodes and discuss the potential paths forward.
The field of p-type transparent electrodes started with Ni- [1] and Cu-based oxides [2], which even in single crystal form [3] have moderate performance. Recent advances in oxides based on Ir [4], Rh [5], Co [6], Mn [7], Cr [8] and other transition metals [9] diversified the field. In parallel, sulfide [10], oxide-sulfide [11] and sulfide-fluoride [12] p-type transparent conductors have emerged as an alternative to widely studied oxides. Despite this significant recent progress, p-type electrodes still underperform compared to their n-type counterparts.
A mainstream engineering strategy to address the performance limitation of p-type transparent electrodes is to use a thin p-type interfacial contact layer between n-type transparent electrode and active layer of an optoelectronic device. This strategy has been very successful for hole injection/transport layers in OLED [13] and OPV [14,15] technologies. Currently it proliferates into other areas, such as inorganic thin film solar cells [16], organic small-molecule photovoltaics [17], dye-sensitized cells and crystalline Si photovoltaics.
An alternative scientific strategy to accelerate the progress in the field of p-type transparent electrodes is to first understand the origins of high electrical conduction in traditional n-type transparent electrodes such as In2O3 [18] and ZnO [19] and then use these new scientific insights to design completely new p-type transparent electrodes. Two unexpected design principles emerge from this direction: namely surface- [18] and non-equilibrium [19] enhancements of electrical conductivity. It remains to be seen if any of these ideas can be used to design better p-type transparent electrodes.
[1] TSF 236, 27 (1993)
[2] Nature 389, 939 (1997)
[3] PRB 80, 165206 (2011)
[4] APL 90, 021903 (2007)
[5] APL 80, 1207 (2002)
[6] PRB 85, 085204 (2012)
[7] DOI: 10.1002/adfm.201300807
[8] APL 99, 111910 (2011)
[9] AFM 21, 4493 (2011)
[10] TSF 517, 2473 (2009)
[11] APL 77, 2701 (2000)
[12] TSF 518, 5494 (2010)]
[13] Org. Electr. 9, 890 (2008)
[14] PNAS 105, 2783 (2007)
[15] MRS Comm 1, 23 (2011)
[16] APL 96, 162110 (2010)
[17] JAP 107, 103713 (2010)
[18] PRL 108, 016802 (2012)
[19] APL, in press (2013)

Transparent conductive oxides (TCOs) and transparent oxide semiconductors (TOSs) have a long history since 1950s. The material design concept for TCOs looks almost established, i.e., ionic oxides p-block metals with an electronic configuration of (n-1)d10ns0 and a spatial spread of ns orbitals which is enough to have large overlap with neighboring metal ns orbitals irrespective of intervening oxygen ion1). Concretely, most of the TCOs have been realized in the material systems of In2O3-SnO2-CdO-Ga2O3-ZnO. Materials based on light metal oxides such as Al2O3 and SiO2 have not been regarded as the candidates of TCOs. We reported high electronic conductivity in 12CaO7Al2O3 (C12A7) 2) which had been a typical insulator and this discovery was followed by transparent conductivity in cubic SrGeO3 in 2011.3) These two materials are TCOs realized by a new material design concept ,i.e. utilizing conduction band formed by 3D-connected nanospace and superdegeneracy at the bottom of conduction band.
The former concept led to 2D-electride, Ca2N, which may be regarded as a bulk crystal form of 2D-electron gas.4)
As for TOS, the striking advances are seen in transparent amorphous oxide semiconductors (TAOS) in science and technology due to strong demand for active layer materials in thin film transistors (TFTs). In-Ga-Zn-O (IGZO) TFTs, which was first reported in and 2003 and 2004,4) has adopted to drive high resolution displays of new iPad and 55inch OLEV-TV. This is a first mass production of TOS family. The major reasons for this adoption are high electron mobility (an order of larger than that of a-Si:H) and easy fabrication process. A major advance in TOS-TFTs is realization of p-channel TFTs and subsequent fabrication of C-MOS using ambipolar SnO. 6)
In this talk, I review these progresses viewed from electronic state and comprehensive band lineup of these materials.7)
1)MRS Bull,25,28(2000),2) Nature 419, 462 (2002) 3) Nat. Com., 2, 470 (2011),
4) Nature, 494, 336, (2013), 5) Science, 300, 1269, (2003), Nature 432, 488 (2004); (review) Sci.Tech.Adv.Mat. 11, 044305(2010), 6) Adv. Mater., 23, 3431 (2011). 7) Jpn. J. Appl. Phys.52, 090001(2013).

The field concerning transparent conducting oxides (TCO) has recently been attracting much attention in the research for an alternative material to indium tin oxide (ITO) and for the development of oxide nanostructures in order to reach new functional properties not intrinsically present in the compact form of the TCO (e.g. advanced light trapping, large surface area, highly directional conductivity). We here present a study on Ta doped TiO₂ (TaTO), belonging to a promising new class of TCOs recently discovered [1, 2].
TaTO nanostructured thin films were deposited on soda lime glass substrates via nanosecond Pulsed Laser Deposition (PLD). The deposition process is performed at room temperature and then followed by a vacuum annealing (p < 4x10^-5 Pa) to obtain single phase polycrystalline anatase. We investigated the effect of oxygen partial pressure during film synthesis and we were able to obtain compact (1-2.25 Pa range) and nanoporous-nanostructured (5-15 Pa range) thin films with a tree-like architecture. The role of the annealing temperature, time and cooling rate was analyzed in terms of structure, morphology and optoelectronic properties on both compact and nanoporous structures. The best electrical properties were obtained for deposition in oxygen pressure of 1.25 Pa coupled with a vacuum annealing at 550°C for 1 hour. The resistivity obtained for a 150 nm TaTO film is 5.9x10^-4 Omega;cm, with a mean transmittance in the visible range of 76.5%. As a matter of fact, this values are strongly competitive with the best results of the most studied Nb doped TiO₂. Moreover we have investigated the possibility to characterize the transport properties of the single tree-like nanostructures of TaTO by Scanning Photo Electron Microscopy with synchrotron radiation [3]. Preliminary results indicate an effective resistivity for the individual nano-structures lower than 10³ Omega;cm.
Exploiting a variation of the oxygen background pressure during the deposition process we are able to realize a fully TiO₂-based hierarchically grown TCO-photoanode (compact + porous TaTO double layer), that we aim to test in dye sensitized/hybrid solar cells. Using such approach we want to combine the low resistivity of the compact TaTO bottom layer and the high surface area, light scattering properties and directional conductivity of the top layer tree-like architecture. Moreover we show how the transmittance of the double layered structure can be enhanced with respect to the single compact layer.
1. Furubayashi, Y. Appl. Phys. Lett., 2005. 86(25): p. 252101.
2. Hitosugi, T. Jpn. J. Appl. Phys. Part 2-Letters & Express Letters, 2005. 44(33-36): p. L1063-L1065.
3. Jabeen, F. Nano Research, 2010. 3(10): p. 706-713.

Doped zinc oxide is a key material in various optoelectronic devices. On lab scale the material has shown to be only slightly inferior in terms of electrical conductivity to indium tin oxide (ITO), at comparable optical properties. In practice, the properties of doped ZnO unfortunately lag those of ITO to some extent, both in terms of conductivity and transmission. Apart from the extensively studied electrical properties, a less examined problem is the considerable band tailing for low temperature deposition of doped ZnO.
A suitable way to adjust electrical and optical properties is the application of thermal post-deposition treatments including at least one step where the ZnO:Al is protected by a thin protective layer. This method leads not just to a considerably enhanced mobility but to significantly reduced sub-bandgap absorption, as well. For applications in thin film solar cells this has a direct impact on device performance since optical losses in the front contact are directly linked with reduced spectral response. However, although absorption tails below the band gap are well known for ZnO:Al prepared by various deposition techniques a deep understanding of the underlying mechanisms is still missing.
We studied the appearance of such tails in non-reactively sputtered ZnO:Al thin films in respect to deposition conditions and applied annealing treatments. Here, we modeled the sub-band gap absorption using Urbach tails, with an energy width Eu, and analyzed them regarding the influence of structural properties and variation of point defect concentration by additional supply of oxygen, nitrogen and hydrogen during deposition. Next to optical spectroscopy we employed Raman measurements for determination of crystallinity and to study the influence of impurity incorporation on the presence of additional resonances. Up to now we were able to show a direct correlation of decreasing Urbach energies with lower crystalline disorder.
In this work special emphasis was put on the influence of impurities on the Urbach energy. It is usually assumed, that optical transmission is heavily influenced by film stoichiometry and that increasing deposition temperature has a beneficial effect on transmission due to the evaporation of excess zinc. Here, the oxygen supply during growth was increased and had little effect on Urbach energies, indicating that stoichiometry deviation is not the main reason for band tails after low temperature deposition.
Nitrogen on the other hand is always present in small quantities in production sputtering coaters and is known to lead to a yellowish to brownish colouring of ZnO when purposely added to the process. Therefore, nitrogen addition was investigated for various deposition conditions and post-deposition treatments.
Finally, it is investigated whether a passivation of optically active defects can be reached by addition of hydrogen to the process.

Amorphous In-Zn-O (a-IZO) transparent conducting oxides with conductivity σ asymp; 3000 S/cm can be sputter deposited at ambient temperature, are damp heat resistant and have been demonstrated to work well as transparent contacts for CIGS PV and Epi-Si PV and are extensively used in displays. However, the high cost of ceramic In-Zn-O sputter targets has limited the widespread use of a-IZO TCOs in PV. Here, we demonstrate a new process that results in conductive and transparent a-InZnO thin films deposited via reactive sputtering from a metallic In-Zn alloy target. The highest conductivity obtained to date, σ asymp; 2100 S/cm, is an order of magnitude higher than the previous best literature result for reactively sputtered a-InZnO.
70 nm thick a-In-Zn-O thin films were grown on 2”x2” Eagle 2000 glass substrates by sputtering from a 3” diameter 87:13 In:Zn wt. % metal alloy target. DC sputtering produced the best results. Through careful control of target power density and high throughput cathode cooling, we avoided melting of the high indium content target. The total deposition pressure was 2.5 mTorr from a 20 sccm total gas flow with approximately a 4:1 Argon-O2 flow ratio. Through control of process gas composition, the sample color and conductivity could be varied from dark grey and metallic (too little oxygen); to clear and conducting (~16 - 18 % O2 in Ar); to clear/yellow and low conductivity (too much oxygen). The best samples had σ > 2000 S/cm with 90 % transparency. The X-ray diffraction measurements show the films to be amorphous. Optimizing the conductivity of a-InZnO thin films required careful In-content-specific optimization of the oxygen content of the sputter gas mix. To obtain the reproducible results necessary to enable optimizing the conductivity and transparency, it was necessary to obtain a stable surface composition on the metallic-target by pre-sputtering before deposition.

Cr2O3:(Mg,N) has been reported as a p-type transparent conducting oxide. In this contribution the effect of each precursor used for the deposition by spray-pyrolysis, will be explored and their role in determining the optical and electrical properties of Cr2O3 will be outlined. A correlation between the structural, electrical, and optical properties upon introducing nitrogen precursors has been established. In particular it has been shown that the presence of ammonium salts in the deposition environment results in less absorbing films. By combining optical measurements and NEXAFS studies, a mechanism is proposed to explain the change in the optical properties. Moreover, it is shown that the presence of the nitrate moiety in the reaction environment is necessary to improve the electrical conductivity of the deposited films. The reaction of the nitrate moiety with the ammonium moiety has been proposed as the mechanism to explain the boost in conductivity.

Al-doped ZnO (AZO) is an attractive candidate as a transparent conductive oxide (TCO) due to its improved conductivity compared to ZnO, and its high transparency in the visible range. Here, an aqueous solution-gel route for the preparation of Al-doped Zn precursors is followed. Aqueous routes have the advantage of utilizing water as the solvent, which is environmentally friendly in comparison to organic solvents such as 2-methoxyethanol which is often used in sol-gel based approaches.. Spincoating of Zn2+ precursors doped with different percentages of Al3+followed by a suitable thermal treatmentresults in transparent (> 85% in visible range), conductive films of (Al-doped) ZnO.The films are dense and show preferential (002) c-axis orientation. 2 at.% of Al-doping yields the lowest resistivityof 2.88 10-3 Omega; cm, with a mobility of 18.5 cm2/V.s and a carrier concentration of 1.17 1020 per cm3 after reductive anneal. To understand theconducting behavior, the position of the Al dopant in the ZnO lattice is studied by 27Al nuclear magnetic resonance (NMR) spectroscopy and 1H NMR. The relative quantities of substitutional tetrahedral, interstitial tetrahedral, octahedral and pentahedral coordination of Al and the chemical analysis of hydrogen in the films allows to explain the conductivity trends in the reductively treated Al-doped ZnO films. These new insights provide a basis for understanding conductivity from the level of the crystal lattice onwards.

Solution processing is an effective, low-cost means of depositing transparent conducting oxide thin films for large-area functional devices used in energy and information applications. However, the traditional solution processing methods are limited in their ability to create dense films at low-temperatures with good electrical properties because they use organic solvents, excess counter-ions, and stabilizing materials, producing porosity in the final film when combusted. Water-based nanoscale cluster precursors with minimum counterions and no organic ligands are promising “ink-precursors” for depositing high-quality and dense inorganic oxide films [1].
Here we present the synthesis of an aqueous thin-film precursor based on F-doped tin(II) hydroxide nitrate clusters and detailed characterization of these precursors for the deposition of F-doped SnO₂ thin films [2]. This precursor enables the deposition of thin films at low-temperatures, facilitating the deposition of conducting F:SnO₂ films on flexible plastic films. F acts as both a stabilizing ligand for the aqueous precursor and a functional electronic dopant. The resulting films are amorphous at low temperatures and become polycrystalline when annealed at 300 ^oC or above. Annealing at 450 ^oC causes only a 9% shrinkage in thickness and yields a dense, uniform, and crack-free film, which is consistent with the solution precursor containing minimum counterions. XPS and quartz crystal microbalance (QCM) results indicate that when the annealing temperature reaches approximately 250 ^oC, all counter ions are removed, with less than a 30% reduction in mass. The lowest electrical resistivity of 1.5 × 10^-4 #8486;m was obtained from 10 atomic percent F-doped SnO₂ films annealed at 600 ^oC in air, yielding a Hall mobility of 4.2 cm²/Vs and a carrier concentration of 9.5 × 10¹⁹ cm^-3. The films deposited on polyimide sheets showed stable electrical properties as a function of folding cycles, verifying that these films are mechanically robust and well-anchored. This new aqueous Sn precursor allows for creating variety of functional mixed-metal-oxide films by combining with other aqueous cluster species (e.g., of In, Ga, Al, Zn).
(1) Mensinger, Z. L.; Gatlin, J. T.; Meyers, S. T.; Zakharov, L. N.; Keszler, D. A.; Johnson, D. W. Angew. Chem. 2008, 47, 9484.
(2) Nadarajah, A.; Carnes, M. E.; Kast, M. G.; Johnson, D. W.; Boettcher, S. W. Chem. Mat. 2013, 25, 4080.

Toyoha Mine in Sapporo had supplied In as top of world, was closed on March 31, 2006. Before that, METI (Ministry of Economy, Trade and Industry) and Tohoku University had started to discuss planning of In substitute materials development project from 2004. I claimed advantages on ITO nanoink in this committee. By sputtering process with ITO target, only 20% ITO is deposited on the substrate with 80% formed outside. ITO target of 5 times more than apparent use for film must be prepared. If we change it to printing process, the In use reduction is expected as ca 50%, considering increase in film thickness. In "Japan Rare Metal Substitute Materials Development Project" on FY2007-2011, I was responsible as Project Sub-Leader for "Reducing Indium use in transparent conducting electrodes: ITO nano-ink." The Team was Tohoku University, ULVAC, Mitsui Mining, DOWA, and Sharp. I had also attended on U.S. - Japan Roundtable on Rare Earth Elements Research and Development for Clean Energy Technologies on November 18-19, 2010 on U.S. Department of Energy Lawrence Livermore National Laboratory so that I reported the investigation of ITO nanoink for practical use.
The key technology of ITO nanoink is to produce ITO nanoparticles precisely controlled in size, shape, and crystalline properties. However, ITO could not be prepared by hydrothermal synthesis because Sn-containing InOOH or In(OH)3 crystalline particles were quite stable without further conversion [Materials Trans., 50, 2808-2812 (2009)]. We have developed a novel solvothermal method to directly obtain ITO nanoparticles with cubic morphology on 2008 [Chem. Lett. 37, 1278-1279 (2008).]. Remarkably, highly crystalline cubic-shaped ITO nanoparticles with narrow size distribution were successfully prepared through one-step process from a mixed ethylene glycol solution of indium and tin salts [J. Materials Chemistry, 20, 8153-8157 (2010).] However, the monodispersity was considerably poor since the attempt to separate nucleation and particle growth was failed. Hence The Gel-Sol method was applied to ITO synthesis. The Gel-Sol method for the synthesis of monodispersed fine particles has been established in our laboratory for hematite, TiO2,[ J. Colloid Interface Science, 259, 43-52 (2003).], etc. The role of the initially formed gel is to supply a solute to growing particles with lower superstation to avoid spontaneous nucleation. As a result, monodispersed ITO nanoparticles with a cubic shape were fabricated by using quaternary ammonium hydroxide-assisted metal hydroxide organogels. The nanopartciels has perfect uniformity in size with beautiful shape, and perfect single crystalline structure including Sn [ Chem. Lett. 42, 738-740 (2013)]. As we were attempted to make thin film with ITO nanoink, it was successfully fabricated below 200 nm in thickness and the resistivity was drastically decreased below 1.0 x 10-3Omega;cm after heat treatments. Now GZO nanoink as substitute of ITO has also been developed.

ClearJet has developed inkjet-printed nanosilver rings to replace ITO in touch screens, providing a highly conductive, transparent solution, enabling the industry to greatly expand flexible and large-screen applications, with a unique, low-cost solution.
The Touch Screen industry has been searching for a replacement to ITO over the last few years due to limitations in conductivity, transparency, flexibility, screen size as well as patterning cost and complexity, and concerns over ITO supply. ClearJet&’s objective is to deliver a unique ink and process to address these issues, with implementation via proven industrial inkjet technology.
ClearJet uses inkjet - an additive, digital technology, meaning that a pattern can be deposited in one straightforward step. Unlike many other ITO replacement solutions, it replaces the sputtered ITO and the multi-million $ lithography, chemical etching, etc.
ClearJet deposits directly rings on the substrate - in order to break through the transparency barrier, ClearJet has developed a patented deposition technique printing rings, with a ring thickness or rim of 2-3 microns, which is not visible to the naked eye. Thus multiple rings packed with silver and touching each other form a conductive layer. But as the layer is mostly composed of holes of the thin-rimmed ring, the layer is highly transparent. Deposition by inkjet enables control of many parameters such as outer diameter (25-350 microns), rim thickness (1-5 microns).
High conductivity and transparency: The existing ITO process also results in a limited conductivity for the transparency required, while ClearJet&’s solution is based on silver, the most conducting metal and gives excellent conductivity (Rs<50#8486;/square) and transparency (>90%).
The presentation will review the ITO alternatives known in the industry and academy, exploring the advantages and disadvantages of each solution. Focusing on Clear-Jet solution, the printing process to form conductive rings, printing patterns and controlling the rings dimensions will be demonstrated.

Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide—the prototypical transparent electrode material—demonstrate excellent electronic performances, but film brittleness, low infrared transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low conductivity. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10#8486;/sq. at 90% transmission because of the high conductivity of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the number of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ~2#8486;/sq. at 90% transmission) and remarkable mechanical flexibility under both stretching and bending stresses [1]. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.
[1] H. Wu, D. Kong, Z. Ruan, P.-C. Hsu, S. Wang, Z. Yu, T. J. Carney, L. Hu, S. Fan and Y. Cui, Nature Nanotechnology 8, 421-425 (2013)

The achievement of large optical transmission in the presence of top electrodes remains an important challenge in photovoltaics and several types of optical detectors including those operating the solar blind spectral region. The need for high device speed and large charge collection efficiency leads to structures that may have as much as 50 percent of their surface area coated with metal. Here we introduce an electrode structure that makes of use shaped electrodes to achieve a theoretical transmission of 100 percent at 50 percent metal coverage across a broad spectral range. Numerical simulations of a silver interdigitated electrode with 50 percent metal coverage and using realistic optical properties demonstrate a frequency averaged optical transmission of 84 percent for normal incidence illumination with unpolarized light across an entire octave (wavelength 375 nm - 750 nm). The method doesn&’t rely on resonant effects and makes use of a structure with feature sizes that are easily accessible by current fabrication technologies. A second example study considers an interdigitated aluminum electrode at 50 percent metal areal coverage operating in the solar blind region. Simulations taking into account a realistic lossy Al optical response demonstrate transmission exceeding 90 percent from 245 nm - 300nm for TE polarized illumination and over 70 percent transmission at 250 nm under illumination with unpolarized light. An analytical description including predicted optimum design parameters for the proposed structure will be discussed.

According to resistivity and transmittance conductive oxides (TCO) are widely used in thin film optoelectronic devices. Indium-tin oxide (ITO) is the most important single layer transparent conductor. But there are technical and economic limitations for a large scale manufacturing of flexible devices. Recently, Metal-conjugated polymer hybrid structure has been actively researched to replace of ITO because the hybrid structure can produce transparent, low resistance.
In this study, We fabricated a hybrid electrode by developing Au grid and tosylate doped poly(3,4-ethylenedioxythiophene: p-toluene sulfonate) (PEDOT:PTS) connection path with optimized Au grid aperture ratio. Au grid-PEDOT:PTS was deposited on the TiO₂ which remarkably increased the transmittance. Organic solar cells fabricated on the TiO₂/Au grid-PEDOT:PTS hybrid electrode showed a power conversion efficiency of 3.88%, comparable to that of ITO-based organic solar cell (3.91%). The TiO₂/Au grid-PEDOT:PTS hybrid electrode shown to be promising replacement of ITO for use in low-cost and flexible optoelectronic devices.

Nowadays there is a growing demand for developing low-cost and industrially feasible materials and processes, which would enable the fabrication of transparent conductive electrode (TCC) at low temperatures.
We report on a simple process for creating transparent and conductive patterns at room temperature. The process is based on self assembly of metal nanoparticles to form a transparent grid with a line width of less than 5 micrometer. The grid is formed by placing a micro litter droplet of metal nanoparticles dispersion onto a mesh which is placed on a plastic foil, enabling movement of the nanoparticles before drying. During the evaporation, the particles move towards the periphery of the mesh walls due to wetting and capillary forces, and arrange along the wires of the mesh. After removal of the mesh, micron size lines having a high aspect ratio are formed. Such lines cannot be obtained by a simple printing process. While using silver nanoparticles stabilized by polyacrylic acid, sintering at room temperature is enabled by simple exposure to various electrolytes. For example, after the grid is formed, it is exposed to HCl vapors, resulting in a transparent conducting grid with a sheet resistance of less than 5#8486; per square and transparency above 85%. By adjusting parameters such as metal loading, contact angle, surface tension and environment humidity, the height and width of the lines can be controlled. It was found that this method is suitable for a variety of conductive materials.
We will demonstrate how these TCC were incorporated in electroluminescent and electrochromic devices with high stability for bending. Notably, the many steps that are required for the fabrication of the devices, do not damage the conductive transparent grid.
The whole process takes a few minutes, and does not require any special equipment for obtaining large area (>10x10cm) flexible TCC, and is performed directly on plastic substrate at low-temperature. It is therefore expected that this method can provide a good alternative for plastic ITO based TCC.

The development of organic solar cells is motivated by the low cost potential of this technology combined with the unique possibilities to integrate photovoltaic functionality in a large variety of applications. The low cost potential is based on the use of inexpensive, abundant materials and the high production speeds enabled by roll-to-roll printing and coating. Digital printing of complete device stacks on a variety of substrates provides product designers with unprecedented freedom of design for integration of photovoltaic functionality in new products.
To fully exploit the benefits of this emerging technology, all-solution processed organic solar cells need to be realized. Today however, most organic solar cells contain electrodes which are processed by sputtering or vacuum deposition (e.g. ITO, Al or Ag), thereby severely restricting the strength of the unique selling points of the technology. Here, alternatives to these electrodes, including transparent electrodes, are presented, based on solution processed current collecting grids and highly conductive PEDOT:PSS layers.
Transparent electrodes are prepared using roll-to-roll compatible technologies. Depending on the processing method, current collecting grids with sheet resistance of 15 Ohm/sq down to 1 Ohm/sq are obtained. Printed ITO-free transparent electrode are successfully demonstrated in two device configurations, i.e. conventional and inverted organic cell architectures. Design guidelines for the composite electrode are discussed.
Next step is solution-processing of the top electrode. Screen-printed and inkjet-printed silver electrodes are presented. The combination of an ITO-free bottom electrode with a solution-processed top electrode allows the manufacturing of all-solution-processed solar cells. Preliminary results obtained with these novel devices indicate very similar performance as ITO based reference devices: 2.5 % on 1 cm2 (P3HT:PCBM as photoactive material). Besides applying additive processes to form electrode structures, also a subtractive method is revealed to prepare all solution processed solar cells.
The main challenge today is the move from lab-scale devices to modules manufactured on an industrial level. We demonstrate knowledge transfer from the lab to industrial prototypes by implementing roll-to-roll printing and coating methods. Up-scaling of ITO-free organic photovoltaic (OPV) modules via printing and coating is discussed. The outcome of this work is an important step towards large-volume manufacturing of OPV on a variety of substrates with any form, any shape.

Here I will describe transparent conductors consisting of networks of silver nanowires and to a lesser extent, carbon nanotubes. Such systems have the advantage that they can be deposited over large areas and at low temperature into plastic substrates by solution phase processes such as spraying. In addition, good optical and electrical properties are found with sheet resistances below 20 Ohm/sq for T=90%. One focus of this work has been to understand what limits the electrical performance of very transparent networks. While relatively thick “bulklike” networks are controlled by the (thickness independent) conductivity of the network, this is not so simple for thin networks. We have found that, for networks with T>85% or so, percolative effects limit the electrical performance. We have used the results of percolation theory to derive a relationship between T and Rs in this percolative regime, finding it to describe experimental data very well. Interestingly, the crossover from bulklike to percolative regimes depends on the nanowire dimensions. We have found this theory very useful in understanding the limitations of such networks and have shown how the various parameters controlling the optoelectrical properties of networks vary with nanowire diameter, length and network uniformity. We have worked to extend these concepts to other applications of transparent networks. We have studied transparent supercapacitors where the electrodes consist of thin networks of carbon nanotubes. Careful measurements of the thickness dependence of the capacitance and equivalent series resistance show both properties to scale as predicted by percolation theory. Interestingly, these properties are described by different values of both percolation threshold and exponent. In addition, we have found similar results for supercapacitors where the electrodes consist of mixtures of nanotubes and exfoliated 2D oxide nanosheets. Finally, we have prepared transparent heaters from networks of silver nanowires. We have developed a model which predicts relationship between temperature, applied current and transmittance in both bulklike and percolative regimes. These models fit the data extremely well. We believe networks of 1-dimensional nanostructures will become increasingly important for applications. Understanding the relationships between network properties and performance will be critical if advances are to continue.

Thin-film networks of silver nanowires (Ag NWs) pose an interesting alternative to Indium Tin Oxide (ITO) as the most commonly used material in Transparent Conducting Electrodes. They exhibit electrical and optical properties to match or even surpass those of ITO, but they also demonstrate flexibility, showing little change in conductivity after cyclic loading. Here, we study the effects of two methods of producing networks of Ag NWs and compare the results with theoretical predictions for the connectivity of random fibre networks.
It is known that as the conductivity of a network of nanowires increases with increasing density, the optical transmittance decreases, therefore a compromise between these two factors is required in order to obtain an electrode that is both transparent and conducting. Nevertheless, it is possible to keep a network conductive whilst also having a high optical transmittance by using nanowires with larger aspect ratios, since fewer nanowires are required for percolation. However, SEM and TEM images suggest that longer nanowires tend to entangle, resulting in the undesirable effect of uneven distribution of the nanowires across the substrate, which leads to some regions of high conductivity and some regions of no conductivity. It is possible that this entanglement of longer wires is due to the deposition method. In order to disentangle the nanowires, and therefore provide a more uniform coverage, high shears are required. Unfortunately, these forces may lead to nanofibre fracture, thereby changing the aspect ratio of the nanowires and affecting the percolation threshold needed to achieve conductivity.
For the deposition of Ag nanowires with a range of aspect ratios, we investigate inkjet printing and bar-coating methods, to form transparent conductive films. Bar coating induces shears that disentangle Ag nanowires but this leads to wire fragmentation and alignment, reducing conductivity. Inkjet printing, and by inference other droplet deposition methods, introduce less damage but are not effective at disentangling clumped fibres and thus only work effectively with lower aspect ratio nanowires.

Highly transparent, but still conducting electrode films are crucial for many optoelectronic devices. Indium-tin-oxide (ITO) is the most widely used transparent conducting metal oxide, but its poor physical properties make ITO unlikely to be the material of flexible optoelectronic devices. Recently, Ag nanowire (AgNW) thin film shows the potential for realization of cheap, flexible, and transparent electrodes. In the AgNW thin film, NWs collectively behave as a random metal network with high transparency, but as concentration of NWs (D) increases, there is a tradeoff between transparency and conductivity. Percolation threshold corresponds to the minimum density of NWs to form the transparent, yet conductive metal NW networks and can play an important role in the determination of the proper conditions for AgNW films to be highly transparent and conductive. Recently, terahertz (THz) spectroscopy has been extensively used to characterize the electrical conductivities of various materials. With the increase of NW density above a critical value, the NW network undergoes a transition from insulating to conducting phase and accordingly, the electrical conductivity shows the crossover from non-Drude-like to Drude-like behavior. Here, we determine the percolation threshold of silver NW (AgNW) networks by using morphological analysis combined with THz reflection spectroscopy. While the calculated shading area of AgNW is linearly proportional to the density of NWs, the spacing area enclosed by AgNWs show the percolational dependence on the density of NWs. The percolation threshold calculated from the spacing area vs. density is ~~0.105 wt% and the percolation dimension is of 1.44±0.08. In THz spectroscopy, from the divergent behavior of carrier scattering time and the increase of carrier backscattering factor, the critical NW density was determined to be 0.09 wt%, which shows an excellent agreement with the percolation threshold density of 0.105 wt% obtained from the morphological analysis.

There is an ongoing drive to replace the most common transparent conductor, indium tin oxide (ITO), with a material that gives comparable performance, but can be coated from solution at speeds orders of magnitude faster than the sputtering processes used to deposit ITO. Metals nanowires are currently the only alternative to ITO that meet these requirements, but to date there are very few reports that systematically relate the structure of metal nanowires and their networks to the properties of nanowire films. This presentation will describe the integration of modeling with experiments to predict the sheet resistance and transmittance of transparent conducting films made from metal nanowires. Nanowires with different aspect ratios were produced by controlling the number of nuclei or seeds that are present in the early stages of a nanowire synthesis. Sets of nanowires with different dimensions were made into a series of films with different area fractions of nanowires. Models were generated of nanowire networks that match the experimental values of nanowire diameter, length and area fraction. The sheet resistance from these model networks was fit to the experimental results by adjusting the one free variable, the average contact resistance between the nanowires. This method allows for the extraction of an effective contact resistance from simple sheet resistance measurements of nanowire films, and thereby can be used to quantify the extent to which various processing methods reduce the contact resistance between nanowires independent of the nanowire dimensions and network topology. Simulations of sheet resistance were combined with an empirical expression for transmittance to obtain the first fully calculated plot of optical transmittance versus sheet resistance for nanowire films. This plot quantitatively illustrates the positive effect of increasing nanowire aspect ratio on the properties of nanowire films, as well as the importance of nanowire area fraction in tuning the properties of transparent conductors based on metal nanowires. Simulated results also show how adding a small fraction of high aspect ratio nanowires to a film can greatly improve performance. These modeling results have motivated the development of a new synthesis that produces copper nanowires with aspect ratios as high as 5700 in 30 min. These nanowires were coated from solution to create films with a specular transmittance >95%T at a sheet resistance of <100 Omega; sq-1. This study demonstrates how modeling can accelerate the development of metal nanowire films as a replacement for ITO in touch screens, OLEDs, and solar cells.

The increasing use of Carbon nanotubes and metallic nanowires to fabricate electrodes and devices has led to a keen interest in the percolative nature of microscale 2D networks created from conductive sticks.
To date the modeling of such systems has included several approximations such as sticks of constant length, straight sticks and completely isotropic wire orientations. These approximations generally lead to an error in the estimation of the critical density required to achieve percolation. We have coupled Monte Carlo simulations of 2D conductive stick networks with experiments to understand the nature of these systems. The Monte Carlo simulations we perform include several new parameters that open a window into more realistic models of these 2D networks. The impact of finite scaling and length distributions are explored along with a study of the influence of junction resistances and the formation of efficient conductive pathways in an existing percolative network. These results are coupled to electrical measurements of silver nanowire networks and the observed results are explained theoretically in terms of parallel resistance formation and percolative behavior.
The results of these simulations are also employed to explore the characteristic length of percolation in a system with a density above that of the infinite systems&’ critical density. Further simulations explore the use of these systems in solar cell devices, investigating the impact of wire density on the collection efficiency of conductive stick electrodes.

Deposited networks of silver nanowires show excellent performance as transparent conductors, and have the potential to pave the way for a new generation of foldable, stretchable, and wearable electronics. Spin, spray, and blade coated nanowire electrodes are fully functional almost immediately after deposition, and can immediately compete with sputtered metal oxides in terms of sheet resistance and transmission values. However, care must be taken when integrating them into larger device structures, especially those that have grown and developed alongside metal oxide transparent electrodes and so have spent years or even decades optimizing their mutual compatibility. The use of nanocomposite electrodes composed of a silver nanowire network surrounded by a nanoparticle, sol-gel, or polymer coating can greatly enhance the versatility and functionality of silver nanowire electrodes, making them practical drop-in replacements in a number of traditional device structures. We will report on the numerous electrical, optical, and mechanical properties that can be given to a conductive nanowire network through nanocomposite formation. Device integration examples will be drawn primarily from our solar energy program, in which silver nanowire networks have come to play a prominent role.

There is an impending need for transparent but yet conducting substrates to realize the next generation consumer devices. The demand is also fuelled on other spectrum with calls for greener buildings in which, the use of smart windows - touted to be electrochromics, that require transparent and yet conducting substrate. In this paper, we illustrate our approach in fabricating solution processible hybrid transparent conductors, and their functionality in electrochromics.
Hybrid transparent conducting electrodes including polymer/metal and cellulose/metal nanoconstituents will be introduced. A transfer approach based on cellulose fibrils with the incorporation of metallic nanowires was used to synthesize the hybrid transparent conducting electrodes. Superior mechanical properties and excellent electrical conductivity can be achieved due to the enhanced interaction between the two components. The tensile strength of the resultant transparent conducting nanopaper can reach up to 20 GPa. The transparent conducting nanopaper possesses active surfaces to improve the interfacial properties with the overlay active materials with enhanced electron conduction, charge distribution, and ionic diffusion.
Assembled electrochromic device based on the hybrid transparent conducting electrodes will be demonstrated. We have extended the use of transparent conducting nanopaper for optoelectronics devices such as photodetector applications. Furthermore, the transparent conducting nanopaper can be used to fabricate stretchable devices, including a stretchable reflective electrochromics.

Although silver-nanowires demonstrate comparable electrical conductivity and opti-
cal transmittance to ITO, challenges remain. For example, silver nanowires are expensive
and in order to achieve high electrical conductivity relatively dense films are required.
Moreover, the resulting films are often hazy and require a protective coating to prevent
eventual oxidation. Numerous studies have investigated silver nanowires and graphene in-
dividually as transparent conductors, but little research has been done on hybrid systems
of the two. We report a simple, scalable and relatively inexpensive method to prepare
transparent conducting films based on silver-nanowire/graphene hybrids. We use a com-
bination of spray deposition and Langmuir-based techniques to produce ultrathin films
with controlled nanowire and graphene densities. Surface morphology of the hybrid films
was observed by SEM, AFM and optical microscopies. We demonstrate that adsorption
of graphene at nanowire junctions markedly affects the macroscopic conductivity without
significantly reducing the optical transmittance. Our optimised films which have com-
parable properties to commercial ITO contains reduced densities of silver-nanowires in
comparison to films made of pristine silver-nanowires with the same properties. The re-
sults indicate that these graphene/nanowire hybrid films may serve as a cheap replacement
for existing technologies in electronic devices.

Silver nano wire (AgNW) based Transparent Contacts (TCs) are emerging as a viable alternative to traditional Transparent Conducting Oxides (TCOs) such as Indium Tin Oxide (ITO). In addition, uniform and reproducible films can be made from liquid precursors. Hence, AgNW TCs can be used in atmospheric pressure roll-to-roll deposition process in contrast to indium based TCOs.
Our initial focus is on the properties of AgNW+PEDOT:PSS composite films deposited using spin coating technique. Two different sizes of silver nanowires have been taken for the study and categorized as long and short nanowires having average length of 32mu;m (diameter = 138nm) and 8mu;m ( diameter = 55nm) respectively. Effect of spin speed, annealing, deposition order and mechanical pressing on electrical and optical properties of bare AgNW, PEDOT:PSS and composite films of AgNW+PEDOT:PSS is thoroughly investigated. Composite films are made (i) by making homogeneous solution of AgNW and PEDOT:PSS (ii) by spin coating silver nanowire film over PEDOT:PSS film and (iii) by spin coating PEDOT:PSS film over silver nanowire film.
It is observed that effect of spin speed is substantial in all different types of films. Detailed effect of mechanical pressing on all films made using both long and short nanowires is studied. It is observed that mechanical pressing has significant effect only on bare nanowires and on composite films of type (ii) i.e. silver nanowire film over PEDOT:PSS film. Even on bare nanowire films, effect on smaller nanowire is comparatively more. It is observed that after mechanical pressing transmittance remains more or less same but sheet resistance significantly reduces. This shows that mechanical pressing reduces the contact resistance between the wires. A sheet resistance Rs = 74 Omega;/sq and transmittance T at wavelength 550nm, T550=86% is obtained for the AgNW of shorter length and Rs =10 Omega;/sq and T550=78 % for longer wires. It is observed that composite film of type (iii) where PEDOT:PSS is spun over AgNW gives the best results and Rs=10 Omega;/sq and transmittance of T550=82% is obtained. SEM pictures of these films are also taken. We plan to further investigate the roughness, diffusive transmittance and temperature dependent contact resistance of silver nanowires. The results related to these measurements will also be reported. Device testing in CIGS and/or organic photovoltaics is also planned.

The drive to develop cost effective TW scale photovoltaics has generated renewed interest in the development of earth abundant transparent conducting oxides (TCOs). In short, indium-free TCOs with performance similar to indium-tin-oxide (ITO) are strongly desired. Further, material properties beyond just the conventional metrics of high transparency and low sheet resistance must be considered in developing TCO materials for next generation PV application. In particular, tunable band gaps and tunable band edge energies are necessary to optimize electrical contact to both current and emerging earth abundant absorbers. Often, the use of an additional thin interfacial (or charge transport) layer based on TCO-like materials improves PV performance as well. Accordingly, band edge energies, work function, dopability and morphology should also be considered in selecting TCOs for any specific PV absorber. We will demonstrate this through examples taken largely from our prior and ongoing research, a few of which are summarized next. Amorphous Zinc-Tin-Oxide (a-ZTO) is a work function tunable, but low conductivity material, that has been demonstrated utility as a charge transport layer. Crystalline Ga-doped (Zn,Mg)O is a band gap tunable TCO where the conduction band minimum (CBM) energy can be tuned over a 0.3 eV range by varying the Mg content from 0 to 30%, albeit with reduced conductivity as the Mg content increases. Sputtered SnO₂ TCOs are being developed with the goal of making inexpensive and chemically robust SnO₂ a viable TCO for device applications that can not tolerate the high temperatures used in making F-doped SnO₂ by pyrolytic decomposition processes. Finally, Nb-doped anatase TiO₂ provides a potential high index of refraction TCO. Finally, our approach to TCO materials development generally begins with a broad exploration of the relevant materials and synthesis space. This is most often done using combinatorial composition gradient samples (libraries) grown by co-sputtering. The benefits and challenges of such high-throughput approaches will also be discussed.

Transparent conducting oxides (TCOs) are essential to many technologies from solar cell to transparent electronics. While n-type TCOs (using electrons as carriers) are widespread in current applications (e.g., indium tin oxides or ITO), their p-type counterparts have been much more challenging to develop and still exhibit carrier mobilities an order of magnitude lower.
The difficulties in developing high mobility p-type TCOs can be related to the intrinsically high effective masses of holes in oxides. In this talk, we will report on a high-throughput computational search for oxides with low hole effective mass, wide band gap and p-type dopability. Screening thousands of binary and ternary oxides in the Materials Project Database using state of the art ab initio techniques, we will present several unsuspected compounds with promising electronic structures. Beyond the description of those novel TCOs candidates, we will discuss and chemically rationalize our findings, highlighting several design strategies towards the development of future high mobility p-type TCOs.

ZnM₂O₄ (M=Co, Rh, Ir) spinels are considered as a class of potential p-type transparent conducting oxides (TCO). Experimentally one has shown that polycrystalline samples of ZnM₂O₄ spinels exhibit p-type conductivity. We report the formation energy of acceptor-like defects using first principles calculations with an advanced hybrid exchange-correlation functional (HSE06) within density functional theory (DFT). Due to the discrepancies between the theoretically obtained band gaps with this hybrid functional and the -scattered- experimental results, we also perform GW calculations to support the validity of the description of these spinels with the HSE06 functional. The considered defects are the cation vacancy and antisite defects, which are supposed to be the leading source of disorder in the spinel structures. We also discuss the band alignments in these spinels. The calculated formation energies indicate that the antisite defects Zn_M (Zn replacing M, M=Co, Rh, Ir) and V_Zn act as shallow acceptors in ZnCo₂O₄, ZnRh₂O₄ and ZnIr₂O₄ , which explains the experimentally observed p-type conductivity in those systems. Moreover, our systematic study indicates that the Zn_Ir antisite defect has the lowest formation energy in the group and it corroborates the highest p-type conductivity reported for ZnIr₂O₄ among the group of ZnM₂O₄ spinels. To gain further insight into factors affecting the p-type conductivity, we have also investigated the formation of localized small polarons by calculating the self-trapping energy of the holes.

Recent experimental results have shown than doped n-type BaSnO3 may be a useful high performance transparent conductor based on abundant elements. A key difference from ITO is the perovskite structure, which allows considerable flexibility in strain tuning the properties, both through substitution or alloying on the Ba site and through epitaxy. We report first principles calculations examining the effect of different strain conditions and substitution on the band gap, optical and transport properties. As expected, strain couples to the perovskite tilt systems. It also strongly affects the band gap, though not in the same way as in transition metal perovskites like SrTiO3. Specifically, the s-electron nature of the conduction band leads to a relatively greater volumetric sensitivity of the gap and optical properties relative to the tilt dependence. The band width and inferred conductivity also show substantial dependence on strain. We find that these perovskite stannates are remarkably flexible in their electronic properties in spite of the simple s-electron nature of the conduction band. This work was supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

Sulfide compounds and mixed anion oxide-sulfide materials have potential as solar absorbers or transparent contacts. Improved techniques for depositing these materials in thin-film form are necessary to obtain greater compositional and phase control. Controlling the metal-sulfur ratio in sulfides and the oxygen-sulfur ratio in oxide-sulfides is a dominant challenge to thin film growth of these material systems. Here, we report a deposition method with improved control over the sulfur content in thin-films through the addition of a radio frequency (RF) solids atom source (cracker) to a multiple-source sputtering system. This technique has enabled combinatorial growth of both sulfide and oxide-sulfide materials in thin film form.
The growth conditions for sulfur containing compounds can quickly be refined using this system for combinatorial synthesis. Co-sputtering from one or two targets provides a compositional gradient across a stationary substrate. In addition, a temperature gradient that is orthogonal to the composition gradient is induced across the substrate. An RF solids cracker is used to provide controllable amounts of activated sulfur across the entire substrate during the deposition. Typically, RF solids crackers are used in molecular beam epitaxy systems where the usual operating pressure is 10^-5 to 10^-6 Torr. Here, we employ a RF solids cracker as an addition to our sputtering system where the typical operating pressure is 3 mTorr. Together, the composition gradient, orthogonal temperature gradient and activated sulfur source can be used concurrently to control the composition and phase of the deposited thin films. For this work, all films were deposited on 2”x2” glass substrates at a chamber pressure of 3 mTorr with only argon flowing as a process gas. The temperature gradient was 485 °C to 375 °C across 2”. The film composition was measured using both Rutherford backscattering spectrometry and x-ray fluorescence. X-ray diffraction was used for structural and phase determination.
The growth of sulfides is demonstrated using Cu₂S and the growth of oxide-sulfides is shown with the Bi_xO_yS_z system. In particular, Cu₂S has been grown from both metallic Cu and ceramic Cu₂O targets. Bi_xO_yS_z films with tunable oxygen to sulfur ratios were grown from a Bi₂O₃ target. Further, the independent tuning of anion and cation ratios with this system is demonstrated by the growth of BiCuOS. The successful growth of both sulfide and oxide-sulfide compounds demonstrates the viability of this hybrid approach. These results also suggest that similar approaches with phosphides, oxide-phosphides and phosphide-sulfides would be achievable with hybrid deposition systems.
This work is supported by the U.S. Department of Energy, Office of Science, BES, under Contract No. DE-AC36-08GO28308 to NREL as part of the DOE Energy Frontier Research Center "Center for Inverse Design"