In recent years, inkjet printing is increasingly used as a flexible and digital patterning technique in order to deposit functional materials for the manufacturing of microelectronic devices[1] including organic light emitting diodes (OLED),[2] organic photovoltaics (OPV),[3] radio frequency identification tags (RFID)[4] and organic radical batteries (ORB).[5] Due to its minimal waste generation, its efficient handling of expensive materials and its mask-less processing, inkjet printing represents a widely accepted alternative to conventional patterning methods, such as spin-coating, vapor deposition and photolithography.
In the manufacturing process of flexible electronics, inkjet printing can be used for thin film deposition of active materials like conjugated polymers, metal ion containing small molecules as well as composites of redox active polymers and carbon nanoparticles.
Apart from the active layer materials, metal precursors, such as silver nanoparticle inks, are printed onto flexible substrates in order to provide electrical conductive antennas, electrodes, contacts and interconnects. However, silver nanoparticle inks are not conductive after printing, which is due to the presence of organic stabilizers that surround the single particles in order to prevent agglomeration in the ink and ensure processability.[6] Therefore, a post-printing procedure is required to remove the organic material from the particle surfaces, merge the particles to a continuous network, hence yielding electrical conductive structures. This procedure is called sintering. The conventional thermal sintering requires temperatures above 200 °C and is therefore not compatible with the thermo-sensitive foils used as substrate materials. Therefore, alternative approaches, like exposure to a non-thermal plasma, are used to successfully sinter inkjet-printed silver nanoparticle inks without affecting the substrate materials.[7] This approach allows the production of electrically conductive components on flexible substrates via inkjet-printing.
[1] J. Perelaer, P. J. Smith, D. Mager, D. Soltman, S. K. Volkman, V. Subramanian, J. G. Korvink, U. S. Schubert, J. Mater. Chem. 2010, 20, 8446-8453.
[2] A. Teichler, J. Perelaer, U. S. Schubert, J. Mater. Chem. C 2013, 1, 1910-1925.
[3] A. Teichler, R. Eckardt, S. Hoeppener, C. Friebe, J. Perelaer, A. Senes, M. Morana, C. J. Brabec, U. S. Schubert, Adv. Energy Mater. 2011, 1, 105-114.
[4] V. Subramanian, P. C. Chang, J. B. Lee, S. E. Molesa, S. K. Volkman, IEEE. T. Compon. Pack. T. 2005, 28, 742-747.
[5] T. Janoschka, M. D. Hager, U. S. Schubert, Adv. Mater. 2012, 24, 6397-6409.
[6] S. Magdassi, A. Bassa, Y. Vinetsky, A. Kamyshny, Chem. Mater. 2003, 15, 2208-2217.
[7] S. Wünscher, S. Stumpf, A. Teichler, O. Pabst, J. Perelaer, E. Beckert, U. S. Schubert, J. Mater. Chem. 2012, 22, 24569-24576.

Current and future flexible electronics applications require material deposition, patterning, and post-deposition processes that are rapid, low-cost, and compatible with heat-sensitive plastic substrates. We present a simple technique to deposit lines of high electrical conductivity without any thermal treatment during or post-deposition. We inkjet printed a water-based silver salt (AgNO3) solution followed by short exposure to the argon plasma generated by a reactive ion etcher. A skin-like layer of silver forms on the surface of a printed line with an average thickness and composition dictated by plasma treatment conditions. This layer exhibits optimum electrical resistivity twice that of bulk silver. The technique holds the potential for simple patterned metallization of electronic components on flexible substrates.

Invisible Ag-grid transparent electrodes (TEs) have been prepared by Electrohydrodynamic (EHD) jet printing using Ag nano-particle inks. Ag-grid width less than 10 mu;m was achieved by the EHD jet printing, which was invisible to the naked eye. Ag-grid line-to-line distance (pitch) was modulated in order to investigate the electrical and optical properties of the EHD jet-printed Ag-grid TEs. The decrease in the sheet resistance at the expense of the transmittance was observed as the Ag-grid pitch decreased. The figure of merit of Ag-grid TEs with various Ag-grid pitches was investigated in order to determine the optimum pitch condition for both electrical and optical properties. With 150 mu;m Ag-grid pitch, the EHD jet-printed Ag-grid TE has the sheet resistance of 4.87 #8486;/sq and the transmittance of 81.75 % after annealing at 200 oC under near-infrared (NIR). Ag filling factor (FF) was defined to predict the electrical and optical properties of Ag-grid TEs. It was found that the measured electrical and optical properties were well simulated by the theoretical equations incorporating FF. The EHD jet-printed invisible Ag-grid transparent electrode with good electrical and optical properties implies its promising application to the printed optoelectronic devices. The optimized Ag-grid transparent electrode is employed to the fabrication of optoelectronic devices and EMI (electromigration interference) sheilding layer.

Monolithic interconnects in photovoltaic modules connect adjacent cells in series, enabling their optimal power output by providing voltage addition, while also controlling current density to minimize ohmic loss. These interconnects are typically formed sequentially, involving multiple deposition & scribing steps, and widths of about 300 to 500 mu;m are common. The collective impact of shading & dead-zone area comprised by the interconnects in modules results in a reduction of power output by about 5 to 10 %. Thus, as one reduces the interconnect width, more power can be obtained. This work introduces an alternative interconnect formation method employing ink-jet printed metal and dielectric features, which we demonstrate on copper indium gallium diselenide solar cells. Ink-jet printed poly-vinyl-phenol (PVP) and silver (Ag) serve as the interconnect dielectric and metal in exemplary structures, respectively. A variety of other, more abundant, economical, selections can also be used. We demonstrate voltage addition with printed PVP/Ag interconnects, and aspects affecting fill factor and current density are discussed. We also show that printed interconnects can be made significantly narrower than those made by conventional practice. Furthermore, an apparatus that integrates scribing with interconnect material deposition allows the structures to be made in a single pass - after all large area solar cell material deposition steps; streamlining the process to make interconnects can reduce solar panel manufacturing costs substantially.

Inkjet printing has evolved from a graphic printing technology into a fabrication technology for creating complex structural and functional materials. Compared with photolithography, inkjet printing is an additive process, requiring no photomask preparation or etching steps and thus offers advantages in terms of flexibility, scalability, speed, and costs. To date, inkjet printing is capable of depositing a wide range of materials (e.g., metals, conducting polymers, and ceramics) onto substrates, such as paper and plastics, to produce flexible electronics and even 3-D objects. The challenge to achieve better resolution and consistency, however, remains in controlling the formation and precise deposition of the ink droplets carrying the functional materials (e.g., nanoparticles and pre-polymer). To overcome this challenge, we must first understand the fluid properties of the ink and corresponding drop breakup induced by surface tension, or Plateau-Rayleigh instability. In this presentation, we will present our research in (i) establishing relationships between the fluid properties of silver ink and the printing performance and (ii) exploring the minimum feature size attainable with inkjet printing.

Printing of metal based inks on plastic substrates is a promising approach towards the mass production of highly conductive structures for electronic devices using roll-to-roll manufacturing techniques. In order for the entire process to occur at high speed, however, also fast and efficient post-deposition treatment technologies for drying and sintering are required. Recently, the exposure of the wet ink deposits to short, but intense pulses of visible light (photonic flash sintering), has been presented as a highly interesting alternative to more conventional approaches like hot air treatment (thermal sintering). The advantages of photonic flash sintering are the localised heating of only the ink deposits, and the much higher processing speeds which can be attained. In addition, process parameters like light intensity and spectral distribution, pulse length and flashing frequency can be specifically adapted for the absorption and thermal properties of each individual ink-substrate combination. This approach allows to apply this technology on a R2R basis, enabling inexpensive and highly efficient high-throughput production.
During the sintering process, however, the ink properties can change drastically, as the solvents evaporate, the ink layer shrinks, and its reflectivity and thermal conductivity increase. As a consequence, optimal photonic sintering conditions are expected to vary as a function of the sintering degree. Using a single setting of flashing parameters during the entire process might therefore not be the most ideal strategy in terms of manufacturing speed, energy efficiency and final performance of the product. Instead, process optimisation should take into account these effects by adjusting the flashing parameters accordingly. Using a combination of experimental measurements (e. g. in situ temperature monitoring of an ink line exposed to light flashes) and theoretical calculations, we have been able to develop sintering protocols which vary the applied conditions in a well-considered manner. As a simple example, controlled solvent evaporation can be achieved by applying a number of rather gentle drying flashes, followed by very few high power pulses to achieve sintering of the dried ink. This approach has the advantage of limiting high peak temperatures to a minimum, circumventing foil damage, while at the same time avoiding sudden and uncontrolled boiling of the solvent, which would otherwise lead to ink splashing.
A similar approach is also possible using a R2R setup, where the flashing parameters of different lamps in a sequential arrangement can be individually optimised considering the sintering degree of the ink passing underneath them.

To fabricate cost-effective solar cells, it is imperative to develop a low cost transparent electrode with low resistivity and high transparency. Although crystalline indium tin oxide (ITO) has been widely adopted as a transparent electrode in solar cells, it is an undesirable material for use in low cost solar cells because of the scarcity of indium and its high deposition cost. Silver nanowires (AgNWs) network films have recently attracted substantial interest as a transparent conducting material. Transparent electrodes composed of random AgNW networks can be readily achieved by simple and scalable solution. However, the AgNWs film is easy to undergo local oxidation and melting on a heated substrate in the atmosphere, which adversely affects the conductivity of the AgNWs film. In addition, the low carrier collecting efficiency of AgNW films could pose another hurdle. The limited contact area of AgNWs with n-type or buffer layers is incapable of effectively collecting the charge carrier generated at the p-n junction. Here, we propose a sandwich composite electrode structure of Al doped ZnO (AZO)/AgNWs/AZO fabricated. The AZO/AgNW/AZO composite structure is suitable for cost-effective large area fabrication, because it involves relatively low-cost materials, and it is prepared by scalable solution processes. We also demonstrated the similar approach involving copper nanowire (CuNW) in form of AZO/CuNW/AZO. Copper nanowires (CuNWs)-network film is a promising alternative to the conventional indium tin oxide as a transparent conductor. However, thermal instability and easy oxidation tendency poses a hurdle for the practical applications of CuNWs films. We present highly oxidation-resistive copper nanowire based transparent composite electrodes with high transparency and conductivity as well as flexibility. Chemical treatment effectively removes both the organic capping molecule and the surface oxide/hydroxide from the CuNWs, allowing direct contact between the nanowires. This chemical approach enables us to fabricate the transparent electrodes with unprecedented properties (19.8 Omega;/sq and 88.7 % at 550 nm) at room temperature without any atmospheric control. Furthermore, the embedded structure of CuNWs with Al-doped ZnO (AZO) dramatically improves the thermal stability (up to ~140oC) and the oxidation resistance of CuNWs. Our AZO/CuNWs/AZO composite electrodes exhibit high transparency (83.9 % at 550 nm) and low sheet resistance (35.9 Omega;/sq), maintaining the properties even with the bending number of 1,300 under a bending radius of 2.5 mm. By being implemented in Cu(In1-x,Gax)(S,Se)2 thin film solar cell, this composite electrode demonstrate the substantial potential as a low-cost (Ag, In-free), high performance transparent electrode, showing the power conversion efficiency comparable to the conventional sputtered ITO based solar cell.

Advanced nanomaterials dispersed in solution offers opportunities to generate 2D and 3D mesoscale scaffolds with designed properties. Here I will present two areas of research to demonstrate the exciting applications. In one example, I will show 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. I show that these metal nanowire networks provide high optical transmittance at very low sheet resistance and have extreme mechanical flexibility. In another example, I show how we integrate various nanomaterials into 3D scaffolds including paper, textile and sponge.

Silver nanowire (Ag NW) networks are promising candidates for replacement of indium tin oxide (ITO). However, transparent conductors based on single walled carbon nanotube and Ag nanowire networks often suffer from “haziness” resulting from surface roughness. Thus, in addition to achieving suitable transparency and conductivity, surface roughness must be minimized if realistic implementation of Ag NW networks as transparent conductors is to be realized. In this work, we have reduced the surface roughness of the Ag NW networks to below 5 nm as compared to 54 nm for as-deposited Ag NWs through optimization of low temperature annealing treatment and planarization by poly (3,4 ethylenedioxythiophene) - poly(styrenesulfanate) (PEDOT:PSS). Using this method, we have been able to produce Ag NW networks with transmittance and sheet resistance of 87% and 11 Omega;/sq, respectively. These are some of the best values reported for non-oxide based transparent conductors. Incorporation of these smooth Ag networks into polymer light emitting diodes fabricated in our laboratory yield device characteristics that are comparable to or better than those with commercially available ITO [1].
[1] S. Coskun, E.S. Ates, H.E. Unalan, Nanotechnology 24 (2013) 125202.

The application to zinc oxide (ZnO) thin-film transistor (TFT) has been recently attracting great attention due to its good electrical properties and excellent ambient stability in the fabrication of thin film at low temperature. In this work, zinc hydroxide (Zn(OH)₂ ) was synthesized from zinc nitrate and NaOH and Zn(OH)₂/NH₄OH solution can be prepared up to 5wt% for spin-coating. As the solution concentration increased, thermal treated ZnO film thickness increased linearly. In order to check the performance of solution-processed ZnO TFT, p-doped Si substrate was used as gate electrode with 100nm SiO₂ insulator layer. 2 and 4 wt % solutions gave 30 and 50 nm ZnO layer thickness, respectively after 150^oC thermal treatment on Si substrate. With 2 wt % solution, mobility of bottom gated ZnO TFT was 0.6 while 1.17 cm²/V.s with 4 wt % solution. Furthermore, a mobility of 2.91 cm²/V.s was recorded after a different post annealing treatment. The mobility of ZnO TFT device without any passivation layer has been tested over 1 month period and showed the stable behavior. Conversion process to ZnO from Zn(OH)₂ with low annealing temperature or/and a different post annealing treatment may open new future oxide TFT applications.

Among organic piezoelectric polymers, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) promises a variety of applicability for flexible piezoelectric devices due to its piezoelectric activity, flexibility and also highly ordered piezoelectric crystalline structure. However, because of the much lower piezoelectric coefficient of P(VDF-TrFE) compared to piezoelectric ceramic materials, the practical application of this material has been restricted to sensors and actuators. An ultra-thin composite of piezoelectric nanocrystals and P(VDF-TrFE) can potentially enhance the piezoelectric properties for energy harvesting applications which has yet to be fully developed.
In this work we propose the development of a composite structure consisting of barium titanate (BTO) nanocrystals embedded into a P(VDF-TrFE) matrix which can be spin cast onto a flexible substrate. To obtain uniformly disperse nano crystals in the polymer matrix we have recently developed a new approach for the surface modification of aggregate-free, monodisperse hydrophobic BaTiO3 nanocubes which enables their stabilization in polar solutions of P(VDF-TrFE). The polymer solutions with and without BTO nanocubes were successfully spin-coated on Kapton flexible substrates to produce ultra-thin uniform films. These structures were interfaced with metal electrodes for piezoresponse characterization using photo-lithography. Piezoresponse force microscopy (PFM) was used to locally delineate polarization distribution, and piezoelectric coefficient. Moreover, piezoelectric properties and switching behavior of whole structure were investigated by integrating atomic force microcopy with ferroelectric tester. Results show higher value of piezoelectric coefficient and faster switching for composite sample.

In this talk, we will discuss silver nanoplates as potential candidates to replace nanowires as metallic inks for printed electronics. Ag nanoplates represent good examples of 2-D nanostructures with an extreme degree of anisotropy in their shape and unique features in their localized surface plasmon resonances properties. When compared with nanowires, plates have larger surface area with a strong tendency to form large, ultrathin sheets. We will introduce a new approach with the capability to produce Ag nanoplates with edge length of 40-500 nm in high yield using solvothermal and hydrothermal reactions. Most importantly, we will report a new strategy for the production of Ag-Au bimetallic nanoplates with oxide-free surfaces in high yield and large quantities. Specifically, we leverage the reduction of gold ions by a mild reducing agent to coat Ag nanoplates with an ultrathin layer of gold while preserving the plate shape and minimizing the etching of Ag by gold ions via galvanic replacement. Conceptually, the success of this reaction relies on the fact that reduction of Au precursors only occurs on the surface of Ag nanoplates and self-nucleation of Au nanoparticles can be effectively eliminated. Additionally, we demonstrated that reaction temperature can maneuver the deposition rate of atoms and the surface diffusion of atoms to promote the migration of Au atoms from corners to the top and bottom facets and the interatomic diffusion between Au, Ag to generate an alloy structure. Ultimately, we aim to minimize the amount of Au needed to decorate Ag nanoplates for an enhanced chemical stability and optimal conductivity.

Current progress in the formation of a CdTe/CdS photovoltaic by Electrochemical Atomic Layer Deposition (E-ALD) will be presented. E-ALD is a condensed phase equivalent of atomic layer deposition (ALD). In ALD the deposition of a compound is achieved by means of sequential surface limited reactions in which the components are deposited one atomic layer at a time. In E-ALD the individual atomic layers are formed using underpotential deposition (UPD), an electrochemical surface limited reaction. XRD results show that the CdTe layer has a strong preferred (111) orientation as deposited. EPMA results show stoichiometric CdTe, with a Cd/Te ratio of 1.02. Photoelectrochemical results indicated a band gap of 1.5 eV, with p-type conductivity achieved without annealing. Different CdS deposition methods were also investigated. CdS was deposited by chemical bath deposition (CBD), successive ionic layer absorption and reaction (SILAR) and E-ALD. Results showed that the average Cd/S ratio, determined using EPMA, was 1.07, 1.10, and 1.05 for CBD, SILAR and ALD respectively. Photoelectrochemical results show that the photoresponse for E-ALD grown CdS was the highest while CBD was the least photoactive. As exciton creation in the CdS layer is parasitic to the photovoltaic, CBD CdS was used in first attempts at PV construction. Solar cells were fabricated in the substrate configuration with Au on glass serving as the back contact, 200 nm (~600cycles) of E-ALD grown CdTe, 80 nm of CBD grown CdS, and 100 nm magnetron sputtered ITO to serve as to front contact. Cells achieve a maximum current density of 0.65 mA/cm2 when exposed to light; however appear to suffer from shunts. Work on these defects is underway.

Electroless and galvanic copper deposition are steps in manufacturing printed circuit boards. Excessive deposit stress may lead to contact failures and, for flexible circuit substrates, excessive warping. In industry, deposit stress is usually determined by observing bending of metal test strips that are varnished on one side for unilateral deposition. However, electroless processes are typically applied for insulating substrates. Differences are expected from the higher the thermal expansion of most non-conducting substrates, and the redox reaction in electroless plating may be affected by the presence of a metal substrate. Therefore results obtained with metal test strips cannot be generalized to non-conducting substrates. In the present study, time-resolved dual-leg cantilever tests were carried out with standard metal and acrylonitrile butadiene styrene (ABS) test strips for a range of electroless and galvanic plating baths. The results are compared with in-situ XRD-based strain measurements of copper films plated onto ABS.

Micro-ring resonators are designed, and a sol-gel solution process is used to fabricate them on rare-earth doped thin films. A high refractive-index titanium dioxide (nTiO2=2.45) is chosen as the waveguide matrix because it can efficiently confine the pump lightwave within the optical gain medium. Unlike a matrix with a low refractive-index that requires air-cladding underneath, our Er-TiO2 platform can be directly implemented on a silica-on-Si wafer without the need for an under-cutting process. In parallel, the concentrations of Er dopant are modified from 0.1% to 5 % to maximize the emission intensity. In addition, an Er-TiO2 micro-ring resonator with a high Q factor of 10^6 is utilized to enhance the photoluminescence efficiency by forming a whispering-gallery mode. We use finite difference time domain (FDTD) method to optimize the structures of our light emitting sol-gel resonator. Optical emission spectra and enhancement factors are characterized by confocal microscopy, and waveguide edge emission is measured between lambda; = 1.45 µm -1.65 µm. A successful sol-gel resonator can provide a light source for board level optical interconnect and three dimensional integrated photonic circuits.

3D polycarbonate (PC) nanocomposites reinforced with 1D multiwalled carbon nanotube and 2D graphene hybrid fillers covered with silver nanoparticles followed by wrapping uniform layer of PEDOT:PSS has been developed. In first stage, Graphene/MWCNT/Ag nanofiller has been assembled by acylation, amination and Ag decoration reactions before incorpoarating into PC matrix. The reaction confirmation and the effect of functionalization on the structural properties of hybrid nanofillers were studied by FTIR, TGA, Raman and UV visible spectroscopy. In order to observe the morphology of the nanofillers, SEM and TEM analysis have been carried out. In later stage Graphene/MWCNT/Ag nanofillers decorated with PEDOT:PSS and without PEDOT:PSS have been incorporated into PC matrix to study the influence of PEDOT:PSS on the electrical properties of the PC nanocomposites. It is noteworthy to mention that the PEDOT:PSS enhanced the conductivity of PC nanocomposites with increasing the concentration of Graphene/MWCNT/Ag nanofillers. The novelty of this work lies in the synergy arising from the combination of two conducting fillers with unique geometric shapes and aspect ratios as well as contribution of coating PEDOT:PSS, which have not been specifically considered previously.

The surface energy of UV-ozone treated Ba_0.7Sr_0.3TiO₃(BST) thin film derived by sol gel technology was modified by the storage environment. The variation of the surface energy and thus the contact angle allows the high-k dielectric materials to be used in both OFETs fabricated by solution processed 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPs-pentacene) as well as thermal evaporated dinaphtho[2,3-b:2prime;,3prime;-f]thieno [3,2-b]thiophene (DNTT). The TIPs-pentacene ribbons are grown by drop casting on fresh BST with a small pinner and OFET shows mobility of 0.3cm² V^-1s^-1. By loading the samples into the evaporation chamber for 5 hours, the surface energy of the BST dielectric drops from 77.03 mJ/m² to 46.25 mJ/m². The work of adhesion between the BST and DNTT is around 86mJ/m² and results in island mode growth of the DNTT molecule. The tighter packing of the DNTT layer results in an averaged mobility of 1.73cm² V^-1s^-1. The in-plane and out of plane XRD results suggest the polycrystalline structure in the DNTT film and they are comparable with the recent findings in the single crystal devices. We also fabricated saturated load and depleted load inverters based on the DNTT OFETs with different W/L ratio. The saturated load one shows full-swing performance and shows a high gain as 25.

Lateral and vertical phase separation plays critical roles in the performance of the next-generation organic and hybrid electronic devices. A novel method is demonstrated here to switch between lateral and vertical phase separation in semiconducting 6,13-bis(triisopropylsilylethynyl) pentacene (TIPSE pentacene, or TP)/polymer blend films by simply varying the alkyl length of the polyacrylate polymer component. A series of polyacrylate polymers is used in this study: poly(ethyl acrylate) (PEA), poly(butylacrylate) (PBA), and poly(2-ethylhexyl acrylate) (P2EHA). The phase separation modes depend on intermolecular interactions between small molecule TIPSE pentancene and polymer additives. The blend film with a dominant vertical phase separation exhibits a significant enhancement in average mobility and performance consistency of organic thin-film transistors (OTFTs).

We have fabricated high-k dielectric films by the simple solution process incorporating TiO₂ nanoparticles into poly-4-vinyl phenol (PVP) for organic thin film transistors (OTFTs). The compatibility between polymer and nanoparticles was enhanced by introducing ligand to the surface of nanoparticles, which has a similar structure of repeating units in the polymer. No significant aggregation, even at a concentration of 31 wt %, was found in the nanocomposites, as observed by transmission electron microscopy (TEM). As a result, the nanocomposite exhibited a low leakage current density (~10^-8 A/cm²). With an increase in the concentration of TiO₂ nanoparticles added, the dielectric constant of the nanocomposites also increased proportionately as compared to that of pristine PVP. The performance of the OTFTs in terms of the charge carrier mobility, on/off ratio, threshold voltages, and hysteresis was evaluated. The relationship between the concentration of TiO₂ nanoparticles and the device performance will be discussed in detail.

Semiconductor nanostructures are ideal candidates for non-metallic plasmonic materials that operate in near-to mid-infrared (IR) range. In contrast to metal nanostructures, semiconductor nanomaterials have the advantage of possessing tunable carrier concentrations by engineering dopants, charge injection, or field effects. Here, we demonstrate that colloidal copper sulfide nanocrystals can serve as tunable plamonic building blocks. We characterize the surface plasmon response of anisotropic copper sulfide (Cu2-xS) nanodisks and triangles. We find that the surface plasmon modes of these semiconductor nanocrystals are strongly dependent on nanostructure shape and crystallographic orientation, particularly for highly anisotropic structures that possess direction-dependent carrier mobilities. We also demonstrate that these semiconductor nanocrystals can undergo strong plasmonic coupling upon assembly into superlattices. We use self-assembly to fabricate large-scale films and arrays of semiconductor nanocrystals that operate in the IR range andcan be extended to more sophisticated metamaterials and plasmonic devices.

Amorphous oxide-based semiconductor (AOS) thin-film transistors (TFTs) have attracted significant attention due to their excellent uniformity, high mobility and high transparency. Particularly, TFTs using indium gallium zinc oxide (IGZO) as an active channel have been studied intensively for their superior electrical properties and environment stability. However, Indium as the key element in the IGZO system is rare and expensive, which may be a limited factor for the AOS application. Therefore, it is required to search for a more abundant and economical candidate for the InZnO system. It was reported that Sn-Zn-O (ZTO) is promising to achieve high mobility but suffers from low on/off ratio [1]. It has been demonstrated that some proper dopants incorporated into ZTO are effective to suppress the carriers generation and to achieve high on/off ratio [2].
In this study, we investigated the effect of various metallic dopants such as Ce, Al and Y into ZTO system by using sol-gel process. The results show that Y doped ZTO present both high film quality and the improved TFT performances using Y-ZTO as the active channel layer of TFTs. Furthermore, the effect of different ratio of Y in Y-ZTO system (i.e., the mole ratios of Y-ZTO were Y:Sn:Zn=x:2:2, where x=0.1-0.8) was investigated. We prepared the Y-ZTO solution by mixing proportional yttrium nitrate hexahydrate, tin chloride dihydrate and zinc nitrate dihydrate in 2-methoxyethanol solvent. The results showed that the increasing Y ratio can cause the reduced saturation current of Y-ZTO based TFTs and the increased crystallization temperature of Y-ZTO films. The optimized Y-ZTO films without crystallization were fabricated at the mole ratio of 0.5:2:2 and at the 450 °C annealing temperature. The fabricated Y-ZTO based TFTs show the excellent devices performance such as the channel mobility of 0.23 cm^2/v.s,, subthreshold swing of 2.8V/decade, and on/off ratio of ~10^6.
Reference:
[1]Myung-Gil Kim, Mercouri G. Kanatzidis, Antonio Facchetti , Tobin J. Marks, “Low-temperature fabrication of high-performance metal oxide thin-film electronics via combustion processing”, Nature Material 10, 382-388 (2011)
[2] Seok-Jun.Seo, Hyuck Jeon.Jun , Hwan Hwang.Young, Byeong-Soo.Bae “Improved negative bias illumination instability of sol-gel gallium zinc tin oxide thin film transistors”,Applied Physics Letter,volume:99,Issue:15,pages: 152102 - 152102-3,Oct 2011

Ordered bulk heterojunction (OBHJ) hybrid photovoltaics promise high-efficiencies by ensuring alternating donor-acceptor domains at length scales commensurate with transfer-limiting dimensions. However, achieving such highly ordered nanostructures is challenging, especially in the context of large-area, low-cost fabrication. Solution-based roll-to-roll processing of polymer photovoltaics has been shown to address these issues but can not, as of yet, be applied for ordered systems in which inorganic nanocrystals serve as the superior electron-accepting material. Here, we demonstrate the achievement of solution-processed composite films of ZnO nanowires vertically arrayed in polythiophene, suitable for OBHJ photovoltaic active layers, using magnetic field-directed assembly. Cobalt-doped ZnO nanowires are synthesized in the bulk and surface modified with 1-dodecanethiol to allow dispersion in dichlorobenzene. The nanowire dispersions are added to solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT) to form hybrid organic-inorganic composite blends. Room-temperature paramagnetic properties of the Co-doped nanowires provide a handle for alignment under a 6T magnetic field. In our case, a sample rotation process, deemed rotational annealing, is required during alignment to effectively break the degeneracy associated with the negative magnetic anisotropy of the nanowires. Finally, the desired vertically-oriented configuration is locked in place as the composite film dries under controlled evaporation in the presence of the magnetic field.

This talk is about an alternative strategy to prepare functional oxide thin films by solution processing at very low temperatures (< 350 degree C) compatible with integration with polymeric substrates for flexible electronics. The method is based on the combination of Seeded Diphasic Sol Gel (SDSG) precursors with Photo Chemical Solution Deposition (PCSD) methodology.
There is a current demand for cost-efficient, portable, high-tech electronic devices that is pushing the integration of active oxide thin layers with flexible substrates. However, the crystallization temperature is a key parameter in the preparation of these functional oxide thin layers. For example many of the perovskite thin films crystallize at temperatures well above 600 degree C, which degrade underlying electronics, semiconductor substrate or their metallization layers. This constitutes a major drawback for integration of functional oxide films with polymeric substrates. Low temperature synthesis of these functional oxide films, such as ferroelectrics is then of paramount importance.
In this work we report our solution base alternative method of fabrication of ferroelectric crystalline metal oxide thin films, namely lead zirconate titanate (PZT) with well-defined properties at crystallization temperatures lower than 350 degree C. The combination of the nucleation of the crystalline phase in the films at low temperatures, by the photo-activation of the precursors chemistry, in addition to the simultaneous promotion of the crystallization, by introducing nanocrystalline nucleus, allows the preparation of crystalline PZT films at temperatures <350 degree C with well-defined dielectric and ferroelectric response. In this talk we demonstrate the concept on the most important multifunctional oxide, PZT, but this methodoly is in fact a processing platform that can be used for many other functional oxide layers.

Silver nanoparticle inks are increasingly applied for the manufacture of inkjet-printed electrically conductive patterns. In order to obtain a high conductivity the printed, liquid patterns have to be functionalized by an appropriate post-deposition treatment step. This includes the evaporation of solvents, the removal of additives and the growing-together of the nanoparticles. Modern post-treatment methods using e.g. microwaves, intense pulsed light [1] or adopted infrared radiation, are nevertheless at the basis of the thermal process. The thermal treatment e.g. in furnaces or on heating plates, is applicable for a great variety of inks and ensures an efficient sintering without major technical efforts. It has been studied intensively whereby the reports mainly focused on the reducing of the resistivity by controlling the parameters of the thermal treatment. Our researches exceed this comparatively studies by investigate multi-layered systems, their manufacturing and post-treatment.
However, the functionality of a pattern is a result not only of the post-treatment, but of a comprehensive set of parameters of the overall workflow. Therefore we report on the influence of the parameters of the prepress step, the printing step and the post-treatment step on the functionality of the functional patterns.
In the experiments two silver nanoparticle inks were inkjet printed on a rigid and a flexible substrate. In the respective workflow steps the following parameters were varied:
(1) Prepress: The digital images were prepared to print patterns with a shape comparable to the shape of butterflies. Here the geometry of the patterns was varied.
(2) Printing: The geometry of the printed pattern was compared to the geometry of the digital images to consider effects of spreading. The different drying behaviors of both inks were investigated. In addition, the number of layers which were printed on top of each other was varied.
(3) Post-Treatment: The sintering temperatures and times were varied.
The patterns were studied by the application of versatile technologies: The morphology of the patterns is investigated by profilometry and optical microscopy; the microstructure is analyzed by scanning electron microscopy and X-ray diffraction; the electrical characteristics were determined by the measurement of the resistances.
The results show comprehensively the relation between the parameters of the workflow steps, occurring effects during the manufacture and the resulting microstructure and functionality of the patterns. The knowledge of these parameters enables us to control the industrial manufacturing of comparable conductive patterns.
[1] N. Marjanovic et al. ”Inkjet printing and low temperature sintering of CuO and CdS as functional electronic layers and Schottky diodes”, J. Mater. Chem. 21, 2011, 13634-13639.

Functional materials that can retain their conductivity over large ranges of mechanical strain are in demand for applications such as flexible electronic devices (displays, antennas, transistors, solar cells), smart textiles and polymer-based actuators. The current state of the art stretchable conductive materials include wavy metal structures placed on flexible substrates, composites made by backfilling networks of conductive nanomaterials with polymers, elastomeric fiber networks doped with conductive nanoparticles and patterned flexible substrates filled with liquid metal. Each approach has its own advantages and shortcomings. All reported approaches require fabrication on a planar substrate and subsequent patterning. Even though these materials can be fitted onto 3D substrates after fabrication by taking advantage of their flexibility, overall conductivity is sacrificed for conformal coverage. Here we report a method to fabricate and pattern highly conductive circuits conformally on 3D substrates. This method results in a stretchable conductor that consists of a rubber fiber mat-silver nanoparticle composite. The fiber mat is spun from a poly(styrene-block-isoprene-block-styrene) (SIS) toluene solution using solution blow spinning onto 3D substrates. The rubbery block of the block copolymer, isoprene, results in conformal coating of the substrate and elasticity. To establish conductivity, a silver precursor solution is applied to a pre-spun mat and nucleated to form conductive pathways around and inside the fiber, creating a percolating network of nanoparticles. The resulting composite has a uniform conductivity of 410 S/cm at zero applied strain, and retains its conductivity up to 100 S/cm at 100% strain. Conductive paths are formed on the fiber mat by spray coating the silver precursor solution through a patterned mask. The mat is released after the deposition of the precursor solution, then the silver particles are nucleated to form the conductive pathways. This allows for flexible conductive patterns to be directly transferred onto 3D substrates without loss of conductivity. We believe this method can be beneficial for fabrication of non-planar electronic devices such as 3D antenna geometries and spherical cameras in flexible form.

Directed assembly of nanoelements has been used to fabricate flexible devices for diverse applications. The challenge in using such techniques consists of developing highly scalable, high-rate (fast) assembly techniques for placing nanoelements precisely. Fluidic assembly, which employs the interfacial capillary forces, is a method used for assembling nanoelements onto many substrates (regardless of conductivity). However, fluidic assembly is a very slow process since it is diffusion limited. During fluidic assembly, nanoelement migration toward substrate is only driven by the concentration gradient between the assembly region and bulk solution. Here, we introduce a new, high-rate electro-fluidic assembly technique that enables nanoelement assembly directly onto polyimide surfaces up to 10mu;m thick. The significance of this technique is that the assembly process is 100 times faster than fluidic assembly. In the electro-fluidic assembly process, highly doped silicon is used beneath the insulating film as a conductive layer, and the electric field is applied through this layer. Under the influence of the applied field, nanoelements move towards the template and assemble onto desired regions. After assembly, the polyimide film can be easily peeled off from the silicon substrate. A dip coater is used to provide precise and constant pulling speed of the template from the solution. Polystyrene latex nanoparticles (50 nm-100 nm-200 nm), silica particles (30 nm), quantum dots (5 nm) and single walled carbon nanotubes have been successfully assembled. This approach could lead to many novel ways to advance various applications.

To improve the properties of high temperature polymers such as Polyimides which are processed at temperatures of about ge;350°C, organically modified layered silicates with enhanced thermal stabilities to withstand high processing temperatures and prevent premature decomposition were synthesized. The effect of pH and concentration (Cation exchange capacity and percentage composition) on the thermal stability of the organically modified layered silicates was investigated. Two main groups of OMLS were processed; those with only one surfactant (2-Dimethylaminoethyl triphenyl phosphonium bromide (DAETPB)) and those with 2 surfactants (DAETPB and 4, 4 oxydianiline (ODA)). The formation of the OMLS was confirmed by X-ray diffraction analysis (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), Thermo gravimetric analysis and differential thermo gravimetric analysis (TG-DTA) as well as dispersibility analysis. In the case of single swelling agent OMLS nonacidified ones had higher thermal stability (385°C) than their acidified counterparts (330°C). For the double swelling agents increase in concentration resulted in increase in thermal stability with DAETPB: ODA (60:60) producing the highest onset degradation temperature at 500°C compared to the single swelling agents.

Doping rare-earth (RE) ions into semiconductor nanocrystals (NCs) host is attractive, as it allows efficient coupling between the f electrons of the RE ions and the host s electrons. This enables the possibility for both electrical and optical excitation of RE ions via semiconductor host. However, the doping of semiconductor nanocrystals is difficult due to self-purification effect, resulting in the segregation of dopant to the surface of nanocrystals.[1] Recently, we have developed different strategies for promoting native defects in ZnO QDs.[2,3] These native defects present in nanocrystals can trap dopants at the surface of nanocrystals by forming a chemical bond. [4] This turned our attention towards defect rich ZnO quantum dots (QDs) for RE ion doping.
In this work, we report microwave based synthesis of Eu3+ doped ZnO QDs. The rapid dielectric heating by microwave facilitates spontaneous crystallization of ZnO QDs at higher temperature in the presence of Eu3+ ions. This enables simultaneous incorporation of native defect along with Eu3+ doping in the host QDs. The size characterization by HR-TEM shows that the prepared QDs are in the quantum size range. The Eu3+ doping of QDs was confirmed by elemental imaging in DF-STEM mode. The doped QDs when photo excited at its band gap shows native defects emission along with emission from 5D1-->7FJ (J= 1,2,3,4) transition of Eu3+ ions.
REFERNCES:
[1] Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J., Doping semiconductor nanocrystals. Nature 2005, 436 (7047), 91-94.
[2] Asok, A.; Gandhi, M. N.; Kulkarni, A. R., Enhanced visible photoluminescence in ZnO quantum dots by promotion of oxygen vacancy formation. Nanoscale 2012, 4 (16), 4943-4946.
[3] Asok, A.; Kulkarni, A. R.; Gandhi, M. N., Microwave accelerated one-minute synthesis of luminescent ZnO quantum dots. AIP Conference Proceedings 2013, 1512 (1), 404-405.
[4] Hu, L.; Huang, J.; He, H.; Zhu, L.; Liu, S.; Jin, Y.; Sun, L.; Ye, Z., Dual-donor (Zni and VO) mediated ferromagnetism in copper-doped ZnO micron-scale polycrystalline films: a thermally driven defect modulation process. Nanoscale 2013, 5 (9), 3918-3930.

Under suitable stimulation some materials can exhibit electronic spin crossover (SCO) phenomena, namely a transition from a low-spin to a high-spin state. The stimulation, often a temperature variation, also results in a (thermal) hysteresis where the bistability may be used as a memory device. While such potential applications were recognised almost 20 years ago, recent efforts have turned towards the synthesis of new compounds with the SCO phenomena occurring around room temperature for the realisation of SCO in nanoscale structures, notably thin films.
In the present work we have studied the SCO phenomena from a class of polynuclear iron(II)-triazole derivatives where the chemical properties can be tailored to achieve solubility in common solvents by adding appropriate sidechains to the main backbone. Two different compounds have been studied, with short and long sidechains, to examine how cooperativity between molecules is affected by the substituents. Characterisation of thermal hysteresis via dc magnetisation measurements performed on powders, gels and pellets serves to highlight the impact of the molecular environment, i.e. presence of water or uncoordinated solvent, crystallinity and packing, on the SCO behaviour.
Solution processing of the materials has further enabled the preparation and study of stable thick and thin films, with optical characterisation confirming the retention of SCO properties. Furthermore as a first step to deploy these SCO layers inside simple electronic devices, MIM structures (metal-insulator-metal) have been fabricated and tested. Our results show the presence of a thermal hysteresis loop that faithfully reflects the SCO phenomena through the values of the dielectric function around room temperature. Our studies clearly demonstrate that in addition to the previous studies on bulk powders, electrical bistability from thin films (30-50 nm) devices can also be achieved.

Stretchable electronics has emerged as a new class of soft electronics, which is considered potential and intriguing in applications such as biomedicine and sensing system. In general, microlithography process was employed to fabricate metal interconnects in stretchable electronics, but some alternative processes can also be used. In our earlier study, we tried using aerosol jet printing process with silver nanoparticles solution as ink to simplify the conventional microlithography process. However, subsequent dry etching process for removing excess polyimide (PI) substrate material unavoidably oxidized the Ag interconnects. To improve and reduce process steps, we herein present utilizing aerosol jet printing process to generate serpentine PI pattern which could be used as the underlying stretchable substrate for conductive interconnects in stretchable electronics. With 4~8% Polyamic acid (PAA) type PI, we could produce serpentine pattern on poly(methyl methacrylate) (PMMA)-coated glass by aerosol jet inking. By tuning the deposition head sheath gas, atomizer gas, and exhaust gas flow rate, PI patterns with different thickness and width were formed, which ranged from 1 to 3um in thickness and 100 to 160um in width. For potential applications in stretchable electronics, second layer of conductive material is necessary; therefore, we also printed silver ink on the preformed PI pattern. The PI-Ag pattern could then be peeled from the carrier glass or transfer-printed onto a PDMS stamp for further fabrication process. The aerosol jet printing process provides a simpler and direct method for patterning of PI substrate layer, which not only avoids the oxidation of oxygen-sensitive materials but also facilitates the fabrication process of stretchable electronics.

Plastic photovoltaics based on bulk-heterojunction concept consists of light-harvesting conjugated polymer and electron accepting fullerenes are perceived as a promising and effective potential alternative to silicon solar cells owing to their light-weight, large area, cost-effective and flexible organic photovoltaics. The classes of transition metal phthalocyanines (MPc&’s) are interesting and their opto-electronic properties are determined by the orbital occupation of the transition metal 3d orbitals incorporated in the molecular center. In the present study transition metal phthalocyanines (CoPC, NiPC, SnPC, TiOPC, FePC) are deposited by vapor phase deposition as the stacking layers in bulk heterojunction solar cells and efficiencies were analyzed. The film thickness of these phthalocyanines are applied as 5nm, 10nm and 20nm between the PEDOT:PSS and active (P3HT:PCBM) layer to elucidate and optimize the suitable condition to augment the efficiency. Comparisons of the devices made and found that stacking TiOPC and CoPC between the PEDOT:PSS and P3HT:PCBM layer demonstrate a significantly enhanced efficiency of 2.8% (JSC of 15mA/cm2, VOC of 0.59, FF of 44) and 2.6% with 10 nm of film thickness from the normal BHJ of 1.6% ((JSC of 7 mA/cm2, VOC of 0.60, FF of 37). However, NiPC and FePC shows degrading efficiency (1.3%) which can be attributed to potential loss of optoelectric properties. The thickness of 5nm and 20 nm also show reduce in efficiency, possible excition movement in bulk heterojunction solar cell is optimum at 10 nm. The enhanced efficiency of devices with TiOPC and CoPC can be conjectured due to better opto-electrical properties and undamaged tubes. The microstructures of the heterojunction active layer were examined by using AFM, SEM, UV-Vis spectra, IV curve and EQE techniques.

Although crystalline silicon (c-Si) wafers with thicknesses on the order of about 100 mu;m continue to provide a basis for the majority of the photovoltaics industry, c-Si thickness on the order of 10 mu;m suffices to absorb a majority of incident visible light. It follows that increasing attention focuses on thinner c-Si solar cell materials as a strategy to reduce costs. In addition, strongly absorbing films with grain size comparable to their thickness are well suited for solar cell applications due to efficient carrier generation and transport properties. Our work on Si nanocrystal (nc) suspension inks and those incorporating organometallic polysilane chemicals provides a pathway to realize such materials. First we investigate the dependence of annealing drop cast Si films, derived from nc suspension inks, on temperature, nc surface modification, ambient, as well as mono- vs. polydispersity and other variables. Correlations between annealing conditions and resultant optoelectronic properties are drawn. Then we examine the influence of adding polysilane chemicals to the inks, and subsequent changes in morphological evolution during annealing. Finally the topic of doping films derived from Si inks is discussed, with implications on device formation.

Cadmium sulfide (CdS) thin films play an important role in the development of cost-effective and reliable photovoltaic devices. The material is ideal as buffer layer in the copper indium gallium selenide (CIGS) solar cells due to its suitable band gap, high transmittance and low resistivity. Among the techniques used to deposit CdS thin films, chemical bath deposition (CBD) technique is well known to be simple, low cost and convenient to be applied in industrial scale. In this paper, CdS films were deposited onto glass substrates by CBD at 60°C for 40 and 60 min from a bath containing cadmium acetate, ammonium hydroxide and thiourea. The effect of ammonium acetate addition to the bath on the structure, morphology, thickness as well as the optical and electrical properties of the CdS films was investigated. Structure and surface morphology of the prepared fi