Transparent conductive materials: from fundamental understanding to applications

Resume : The seminar will discuss scaling and evolution of connectivity in inorganic nanowire networks exposed to different stimuli. Network junctions control the overall properties of any network and we show that all networks evolve connectivity in specific ways. In particular, we describe how junctions evolve from capacitor-like to resistor-like barriers in response to an applied stimulus and how the network self-selects connectivity paths by choosing the lowest barrier junctions. Continued stressing causes the selected junctions to strengthen and ultimately co-opts neighbouring junctions to reinforce the overall response to the stimulus. By engineering these junctions it is possible to control the properties and response of the network. Transparent materials with arbitrarily controlled connectivity and conductivity are demonstrated as are device with arbitrarily controlled multi-level memory, and single junctions capable of a learning response.

Resume : Amorphous Indium-Galium-Zinc-Oxyde (a-IGZO) is a promising semiconducting material for large area electronics applications, displays and imagers. It combines both a sizable mobility, a good stability and a very low off current compared to conventional amorphous silicon (a-Si). However, little is known on its fundamental properties and how they could be potentially engineered. In this paper, the electronic properties of stoichiometric and defective a-IGZO will be investigated using first-principles simulations. An emphasis will be set on the statistical distribution of the energetic and of the electronic signatures of neutral and charged oxygen vacancies. In opposition to the results reported in literature, we find that oxygen vacancies do not always display a negative-U behavior but can rather adopt a smoother charge transition. Also, a non-negligible number of vacancies (~50 %) does not induce defective states in the band gap. We will illustrate that this difference with respect to previous works, is bound to the limitations of the dimensions of the models used. Finally, we will show that some oxygen vacancies adopt a meta-stable behavior upon charge loading due to the complexity of the energetic landscape built during the amorphization process.

Resume : In the optoelectronics, especially in thin-film photovoltaics, the transparent conductive oxides (TCO) play an important role to minimize the front contact optical losses. The TCOs used as a front contact needs to have low resistivity (<1 mOhmcm), high optical transparency in the broad wavelength region (>80% at 400-1000 nm) and high temperature stability. In the case of using nano-imprinted lithography for reproduction of the morphologies the good electrical and optical properties should be obtained at low deposition or post-annealed temperatures. To translate the morphology, at which the light is efficiently scattered, to the internal interfaces of the solar cells very low thickness of TCO is required (<200 nm). One of such candidates is Ga doped SnO2:In (ITO), which exhibits better optical and similar electrical properties to ITO 90/10, additionally it has lower indium content. A multicomponent Ga-In-Sn oxide target with Ga:In:Sn = 4:64:32 metal ratio was prepared, that was used in RF sputtering system for deposition of high quality GITO thin-film layers on glass. In this study, we will focus on electrical (specific resistivity, mobility and carrier concentration) and optical properties (thickness and refractive index) as a function of deposition and annealing parameters. The results of optical and electrical characterization of the two best GITOs will be presented, one globally optimized and one optimized with the respect to the low temperature deposition prerequisite.

Resume : This work demonstrates a one-step process to synthesize uniformly dispersed hybrid nanomaterial containing silver nanowires (AgNWs) coated with p-type reduced graphene (p-rGO). The hybrid nanomaterials were coated onto a polyethylene terephthalate (PET) substrate for preparing high performance flexible transparent conductive films (TCFs). The p-rGO plays the role of bridging discrete AgNWs, which provides more electron holes and lowers the resistance of the contacted AgNWs; as a result enhancs the electrical conductivity without sacrificing too much transparence of the TCFs. Additionally, the p-rGO also improves the adhesion between AgNWs and substrate by covering the AgNWs on substrate tightly. The study also shows that coating of the hybrid nanomaterials on the PET substrate demonstrates exceptional optoelectronic properties with a transmittance of 94.68% (at a wavelength of 550 nm) and a sheet resistance of 25.0 Ω/sq; furthermore, no significant variation in electric resistance can be detected even the film was subjected to a bend loading with a radius of curvature of 5.0 mm or the film was loaded with a reciprocal tension or compression for 1000 cycles. The study shows that the fabricated flexible TCFs have the potential to replace indium tin oxide film in the optoelectronic industry.

Resume : Delafossite type oxides including CuCrO2 have attracted considerable attention due to their unique properties, such as p-type electrical conductivity and transparency in the optical region [1]. This provides the opportunity to use n-and p-type TCOs together to develop all transparent p-n junctions which can be utilized in many optoelectronic applications. Delafossite oxides also showed promising thermoelectric, ozone sensing, photo-catalytic hydrogen evolution properties and enhanced cathodic photocurrents in p-type dye-sensitized solar cell applications [2]. Among these oxides, Mg-doped CuCrO2 has a conductivity up to 220 Scm−1, which is the largest conductivity measured in the A+M3+O2 systems [3]. Studies on the utilization of wet chemical synthesis-based techniques with flexible sample design showed that a variety of delafossite materials and sample geometries are possible with these approaches instead of using conventional solid state synthesis. Recently, it was shown that delafossite oxides could successfully be synthesized via electrospinning [4,5]. Chiun and Chen, reported on the formation of single-phase CuCrO2 wires having 434 nm diameter with vacuum annealing at 700 °C for 20 min [5]. Considering of diverse potential applications such as optoelectronic devices, sensor and catalysis applications, production of CuCrO2 nanostructures via electrospinning is an alternative method with the added advantage of allowing an easy control of material chemistry with various morphologies. This study aims to understand the fundamentals of the formation mechanism of CuCrO2 in the form of nanofibers. By the investigation of the effect of time, temperature and annealing atmosphere on the formation of single phase material, the study expects to reveal a correlation between the phase purity, size, microstructure, and optoelectronic properties for specific applications. For this purpose appropriate amounts of copper chloride, chromium nitrate and polyvinylpyrrolidone were mixed in water to obtain a viscous solution. The solution was transferred on an aluminum foil substrate in the form of nanofibers, via electrospinning method. All samples were collected at room temperature with 0.3 mL/h feeding rate under an applied voltage of 16 kV. Collected fibers were annealed under two different conditions; namely isochronal and isothermal. Isochronal experiments involved heating from room temperature to various temperatures, i.e., 400, 500, 600, 650 and 700 °C under high purity flowing nitrogen gas in a tube furnace and holding the samples at relevant temperature for 1 h. Isothermal experiments were carried out at temperatures of up to 700 °C. This involved heating the furnace to a selected temperature and loading of the sample for certain durations, i.e. 15, 30, 45 and 60 min. According to X-ray diffraction measurements, with isochronal annealing, the formation of CuCrO2 phase starts at 500 °C and at least 700 °C is necessary to obtain single phase material. Scanning electron microscopy examinations revealed the formation and coarsening of plate-like grains with increasing annealing temperature. Due to this, the fiber structure was completely disappeared after heat treating the fibers at 700 °C for 3 h. To preserve the nanofiber structure and prevent coarsening, isothermal annealing experiments were performed at 700 °C. In summary, it was shown that CuCrO2 nanofibers could successfully be synthesized via isochronal and isothermal annealing of electrospun polymer/metal composite fibers. We have found that isothermal annealing at 700 °C for 1 h in flowing nitrogen atmosphere leads to the formation of a refined structure while preserving the nanofiber morphology. We have also carried out measurements on the structural, morphological, optical and electrical properties of the fibers. Support for this work was provided by TUBITAK MAG (Project no 214M410),and Scientific Research Foundation (BAP) of Selçuk University (Project no 15201028) which we gratefully acknowledge. 1. References [1] F. A. Benko, F. P. Koffyberg, J. Phys. Chem. Solids, 45, 1984, 57-59. [2] A.N. Banerjee, K.K. Chattopadhyay, Prog. Cryst. Growth Charact. Mater. 50, 2005, 52-105. [3] R. Nagarajan, A.D. Draeseke, A.W. Sleight, J. Tate, J. Appl. Phys., 89, 2001, 8022-8025. [4] S. Zhao, M. Li, X. Liu, G. Han, Mater. Chem. Phys., 116, 2009, 615-618. [5] T. Chiun, Y. Chen, Cer. Inter., 41, 2015, doi:10.1016/j.ceramint.2015.03.224.

Resume : Recently, transparent conducting oxide films that contain a reduced amount or no indium are of considerable research interest as alternative materials for Sn doped In2O3 (ITO) transparent electrode in optoelectronic applications. Here, we discussed the structure, optical and electrical properties for amorphous In2O3-ZnO (IZO) and polycrystalline Al doped ZnO (AZO) thin films as next generation transparent electrode. The structure of IZO films can be easily tailored from amorphous to polycrystalline states by changing the sputtering deposition conditions, indicating the possibility to produce the film without internal stress between the film and flexible substrate for flexible devices. Amorphous IZO films has the similar optical bandgap to ITO films, and hard x-ray photoemission spectroscopy measurements for IZO films with various carrier densities show the existence of the bandgap narrowing effect in amorphous IZO films. We also investigated the optical bandgap and work function in polycrystalline AZO films. The optical band gap and work function showed the positive and negative linear relationships with the two-thirds power of carrier density, respectively, which are explained by Burstein-Moss effect considering nonparabolic conduction band. We also discussed the carrier transport mechanism of IZO and AZO films in detailed. [1] J. Jia, Y. Shigesato et. al, J. Appl. Phys., 113, 163702 (2013), J. Appl. Phys., 112, 013718 (2012), Appl. Phys. Express, 7, 105802 (2014).

Resume : Solution processed metal-oxides based electronic devices have attracted great deal of attention during the past few years for their potential use in next generation transparent optoelectronic devices. They have high charge-carrier mobility, high optical transparancy and excellent chemical/mechanical stability. Among the metal oxide, ZnO has a wide band gap (3.37 eV), non-toxic and environmentally stable, biocompatible, biodegradable and, at the same time, is made of zinc that is among the most abundant elements in earth's crust. Recently solution processed ZnO thin-films and nano-rods have been successfully implemented as active layers in a number of different electrolyte gated TFT configurations. We are presenting a solution processed electrolyte gated ZnO based TFT. ZnO thin films were prepared with the sol-gel spin coating method. The ZnO structure has been assessed by XRD, XPS and Raman spectroscopy. The gating of the transistor has been performed with water, Posphate Baffered saline and ionic liquids. Impedance spectroscopy has been performed to measure the capacitance of the ionic liquids. The devices gated with ionic liquid shows very high drain current with mobility as high as 4.7 cm2/Vs and Ion/Ioff ratio in the order of 103. The water gated devices was further used for biofunctionalization of ZnO active channel layer and have potential to be used for biosensing applications.

Resume : The increasing demand of transparent conducting oxides (TCO) materials for optoelectronic devices [1] has turned into a worldwide multi-billion economy that nowadays depends on the availability of ITO [ ]. Global ITO sputtering targets demand is 1,400 tons (2014) and it will increase up to 2,500 tons by 2016. However, In is a rare and expensive material, hence, there is a critical need to replace ITO with reliable alternative TCOs made of abundant compounds. The post-deposition modification of ZnO-based TCOs can be the key to produce thin films with optoelectronic properties similar to ITO and at a lower cost. Here, we present our findings for post-deposition annealing enhancement of AZO, AZOH, GZOH and ZnOH deposited by RF sputtering at room temperature. These studies comprise results of thermal annealing at atmospheric pressure, vacuum, forming gas, H2 and Ar atmospheres and H2 and Ar plasma, which lead to significant improvements of the optical, morphological and electrical properties. The post-deposition annealing leads to a gain in resistivity above 20% for AZO, AZOH and GZOH, reaching ρ≈2.4?10-4, while ZnOH showed a smaller gain of 7%. The averaged optical transmittance in the visible region is about 85% for the investigated TCOs. Such results match the properties of state-of-art ITO (ρ≈10-4 and transmittance in VIS range of 90%). 1 Antonio Vicente et al, J. Mater. Chem. A, 2015; DOI:10.1039/C5TA01752A 2 TCC: Technologies & Global Markets, AVM105A, 2014

Resume : The efficiency of TiO2 is strongly dependent on the ability to control the morphology and crystal size, defects, doping and also its crystallographic phase, which plays a key role in the functionalization of this material. TiO2 crystallizes in three polymorphs: anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic), being anatase and rutile the most investigated and extensively used so far. The characteristic physical and chemical properties associated to rutile and anatase phases, i.e. different bandgap, surface energy, dielectric constant, refractive index or mechanical properties, among others, make them specifically suitable for different applications. The anatase to rutile transition (ART) is not totally understood so far, and several issues still require to be addressed. In this work, locally promoted ART and micropatterning based on titania polymorphs have been achieved by means of controlled laser irradiation on doped (Al, Fe) TiO2 nanoparticles synthesized by a modified Pechini method. This method allows us to obtain high homogeneity in size and composition for the doped anatase TiO2 nanoparticles, for which high dopant (Al, Fe) cationic concentration were obtained avoiding phase segregation [1]. Control of the anatase to rutile transition was achieved by laser irradiation and the kinetic of this process has been studied as a function of the dopant and the irradiation conditions [2]. Raman and PL results demonstrate that the ART can be either inhibited or promoted by doping with Al or Fe, respectively. The higher the concentration of Fe in the anatase TiO2 nanoparticles, the faster the phase transformation evolves. Stabilization of anatase due to the presence of Ti3 at the surface of the nanoparticles, reduction of Fe3 to Fe2 involving oxygen deficiency during laser irradiation and formation of rutile starting at the surface of the nanoparticles have been found to influence the ART process. HRTEM results point out to the relevance of the presence of twins in the formation of rutile phase by laser irradiation. An improvement in the control of the ART has been described, as compared with other methods, such as thermal annealing for which micrometric control of local ART is not achieved. The possibility of achieving an ART micrometric patterning has been demonstrated [3], thus facing challenging performances of this material. [1] G. C. V?squez, M.A. Peche-Herrero, D. Maestre, A. Cremades, J. Ram?rez-Castellanos, J.M. Gonz?lez-Calbet, J. Piqueras. J. Phys. Chem. C, 117, 1941, (2013) [2] OEPM Patent P201400722, (2014) and OEPM Patent P201400759, (2014) [3] G. C. Vasquez, A. Peche-Herrero, D. Maestre, A. Gianoncelli, J. Ram?rez-Castellanos, A. Cremades, J. M. Gonzalez-Calbet and J. Piqueras, J. Phys. Chem C (in press).

Resume : The preparation of photofunctional hybrid thin films of intercalated chromophores in Layered double hydroxides (LDH) matrix by pulsed laser deposition (PLD) is investigated, and the functionality of the films will be examined through the optical properties introduced by the chromophore. An interesting class of chromophores is coumarins and coumarins derivates. Two systems will be studied: coumarin intercalated in MgAl-LDH and coumarin and dodecyl sulfate (DS) co-intercalated in MgAl-LDH. The introduction into the LDH gallery of a second anion, dodecyl sulfate anion aimed the suppression of the aggregation of chromophore anions. X-ray diffraction, infrared spectroscopy and UV-vis spectroscopy were the techniques used for the investigation of the functional hybrid targets and the as prepared films. The effect of the deposition condition, primarily of the laser wavelength on the optical properties of the photofunctional coumarine intercaleted - LDH films was examined.

Resume : Thin films of p-type Cr2O3:Mg have been produced by Molecular Beam Epitaxy (MBE) as well as amorphous p-type CuCrO2 deposited by a low temperature solution based method (spray pyrolysis). These thin films are deposited on top of a n-type Indium Tin Oxide substrate creating transparent pn junctions. The band alignment of these two interfaces is studied using X-ray Photoelectron Spectroscopy (XPS) and Ultra Violet Photoelectron Spectroscopy (UPS). An estimate of the band discontinuities at the interfaces of ITO/ Cr2O3:Mg and ITO/a- CuCrO2 are given. An emphasis is given to the fact that the Fermi level and work function position for both Cr2O3:Mg and a-CuCrO2 show changes with surface preparation and surface termination.

Resume : Titanium dioxide has found a wide application in novel environment- and energy-related processes such as photocatalysis for water purification, gas sensing, etc. Adding Nb or V dopants improves its electrical conductivity without decreasing its visible transmittance what makes it promising for TCO applications. The role of generation and transformation of point defects by technological processes in the electrical conduction mechanism is important for TiO2:Nb(V) film application. We report on TiO2 multilayers deposited on glass and Si substrates by the sol-gel method, focusing on the influence of 1.2 at.% Nb(V) doping on the structure of the films and their optical and electrical properties. The TiO2:Nb(V) films crystallized in anatase phase as evidenced by XRD analyses. XPS measurements revealed that Nb(V) dopants segregated from bulk to the surface. The transmittance values were around 80 % in the 400- 900 nm visible range of light. Electrical measurements of the I-V and C-V characteristics of MIS structures with the TiO2:Nb(V) films pointed out that the doping level in the films had be equal or greater than 10^16 cm^-3. The character of I-V dependences and decrease of the specific resistivity (10^4-10^5 Ohm.cm) by increasing the electrical field evidenced for bulk character of electrical conduction in the TiO2:Nb(V) films and revealed that the current through the films is space charge limited current via deep levels with energy distribution in the energy gap of TiO2.

Resume : Oxide electronics is an important emerging area, notably for the development of transparent thin film transistors (TFTs) and other complex electronic circuits. The successful application of n-type oxides to TFTs has motivated the interest in p-type oxide based semiconductors, also to be applied to TFTs or to complementary metal-oxide semiconductor (CMOS) technology. However, until now there is a lack of p-type oxide semiconductors with performance similar to that of n-type oxide. Among the different metallic oxides, Cu (I)-based oxides exhibit one of the lowest ionic character. These compounds are therefore one of the most promising candidates as p-type transparent semiconductors.Nevertheless, the band gap of 2,17eV is modest for transparent electronics applications, since the transmittance of Cu2O films is low on the visible part of the light spectrum. The incorporation of cations with large radii than Cu has been proposed as a way to achieve a higher band gap, by diminishing of three-dimensional Cu-Cu interactions, only possible with larger cations than Cu . Therefore, cation doped Cu2O thin films were grown by metal-organic chemical vapor deposition (MOCVD). The three doping elements studied (Sr2 , Sn2 and La3 ) were selected having in account theoretical predictions for the band structure and the deposition conditions of Cu2O. The study focus on thin films growth optimization combined with electronic transport analysis and optical transmittance measurements.

Resume : CuxCryOz thin-films are grown via a DLI-MOCVD (Direct Liquid Injection - Chemical Vapour Deposition) process for the substrate temperature range of [400-650oC], different precursor’s concentrations and at different oxygen partial pressure. Electrical and optical properties of thin films, and the influence of depositions’ parameters on these properties are deeply investigated. The as-deposited films present a medium degree of crystallinity, a good transmission up to 80% in the visible range with a corresponding band gap around 3.2 eV, making such films particularly appealing for transparent electronics applications. Electrical conductivity values up to 30 S/cm were measured. Thermal activation energies are in range of hundreds of meV, strongly influenced by the oxygen partial pressure. The crystallinity of the films is improved by increasing the precursor concentration and by tailoring substrate temperatures. No influence of the oxygen partial pressure on to crystallinity was observed. The band gap can also be tuned by adjusting the precursor’s concentration, the oxygen partial pressure and the substrate temperature. The conduction mechanisms are also studied and significant shifts in the activation energy of the conductivity are evidenced. Various hypotheses in terms of dopants, defects, crystallinity and polarons are discussed to interpret the conduction mechanisms of holes.by change in the polaron hopping mechanism.

Resume : The electron-transport properties of adatom-graphene system are investigated for different spatial configurations of adsorbed atoms: when they are randomly-, correlatively-, or orderly-distributed over different types of high symmetry sites with various adsorption heights. Potassium adatoms in monolayer graphene are modeled by the scattering potential adapted from the independent self-consistent ab initio calculations. The results are obtained numerically within the framework of quantum-mechanical Kubo-Greenwood formalism. A band gap may be opened only if ordered adatoms act as substitutional atoms, while there is no band gap opening for adatoms acting as interstitial atoms. The type of adsorption sites strongly affect the conductivity for random and correlated adatoms, but practically does not change the conductivity when they form ordered superstructures with equal periods. Depending on electron density and type of adsorption sites, the conductivity for correlated and ordered adatoms is found to be enhanced in dozens of times as compared to the cases of their random positions. These the correlation and ordering effects manifest weaker or stronger depending on whether adatoms act as substitutional or interstitial atoms. The conductivity approximately linearly scales with adsorption height of random or correlated adatoms, but remains practically unchanged with adequate varying of elevation of ordered adatoms.

Resume : Today, ITO is the most frequently used transparent electrode material. The limited abundance of Indium is expected to cause severe issues. In my presentation I will discuss two possible alternatives. Firstly, a hybrid approach based on a composite of a mesh of silver nanowires and a conductive metal-oxide will be demonstrated. Here, SnOx or Al:ZnO prepared at low-temperatures (100°C) will be used to fuse the wires together and also to "glue" them to the substrate. The resulting electrodes show a low sheet resistance (5.2 Ohm/sq) and high average optical transmission of 87%. Their application as transparent top-electrodes semitransparent organic solar cells will be shown. In the second part, I will introduce the first robust transparent electrodes which are at the same time gas diffusion barriers (GDBs). Generally, GDBs are inevitable to protect sensitive organic devices against ambient gases. Transparent and electrically conductive GDBs (TCGDBs) could serve as electrode and moisture barrier simultaneously. As of yet, work on TCGDBs is very limited. TCGDBs based on ZnO suffer from the severe degradation of their electrical conductivity by orders of magnitude upon exposure to damp heat conditions after very short time. We will show that these issues can be overcome by the use of tin oxide (SnOx) grown by ALD. Robust conductivities of up to 300 S/cm and extremely low water vapor transmission rates (WVTR) on the order of 10-6 g/(m2 day) can been achieved.

Resume : p-type transparent conducting materials are sought after for optoelectronic devices. Little work has been done on low cost synthesis of these materials for large area flexible substrates. In this contribution a low temperature (≈ 345°C) growth method for CuCrO2 is demonstrated by spray pyrolysis using metal-organic precursors. Smooth films were grown on glass substrates with a highest conductivity of 10 S/cm. The most conductive samples are transparent enough to have figure of merits as high as 390 μS. Remarkably despite the low crystallinity of the films, properties comparable with crystalline CuCrO2 are observed. Mobilities of the films are estimated to be 5×10-3 cm2/Vs using the small polaron hopping model. No post-annealing of the films is required in contrast to previous reports. The ability to form material at lower temperatures greatly improves prospects for applications. As this is a solution based technique it is more attractive to industry as PVD methods are slow and costly in comparison.

Resume : The valence band structure of novel p-type transparent oxides – crystalline Cr2O3:Mg and amorphous CuCrO2  – is analysed as a function of incoming photon energy. By analysing the valence band photoemission data across the Cr 3p-3d, Cu 3p-3d, and Mg 2p-3s transitions the contributions of Cu and Cr d-states and Mg s-states to the valence band structure is evaluated. The valence band of both p-type TCOs show striking similarities to measurements on crystalline CuCrO2,  highlighting the importance of the Cr-O octahedra on the electronic states at the top of the valence band.

Resume : Electronic applications of Graphene (Gr), the two dimensional carbon layer, pushes to clarify the potentialities of tuning the content and type of mobile species. Doping effects can be obtained on Gr grown by chemical vapor deposition (CVD) on metal (Cu, Ni,…) and transferred on specific substrates either during growth or by subsequent definite thermal treatments in controlled atmosphere. Many aspects of this latter procedure are still to be clarified as, for example, the role of Gr defective structures and planar morphology, or the Gr substrate features. The same thermal activated process is a matter of debate to try to clarify the nature of dopant-Gr physical bonding features. We report an experimental study by Raman and Atomic Force Microscopy of the thermal doping effects in the temperature range below 400°C on graphene grown by CVD on Cu and transferred on X/Si (where X = SiO2, Al2O3) substrates. The role of treatment atmosphere is shown highlighting a prominent role of oxygen in obtaining p-doping and vacuum in avoiding successive reactivity of entrapped species. By comparing different substrates the features of doping related to Gr and those connected to Gr substrate properties are clarified also by employing electrical characterization.