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J-05: Poster Session
Tuesday, 20/Jun/2017:
4:00pm - 6:15pm

Location: Foyer

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Effects of AgNW Electrode and Active Layer Surface Morphology on the Performance of Multilayer Transparent and Flexible Transducers

Hiu Tung FOOK1,2, Jin Han JEON2, Pooi See LEE1

1School of Materials Science and Engineering, Nanyang Technological University, Singapore; 2Robert Bosch (SEA) Pte Ltd, Research and Technology Center, Singapore

There has been a growing interest in alternative transparent electrodes to replace conventional metal oxides especially in transparent flexible electronics as the brittleness of indium tin oxide (ITO) is a major limiting factor for device flexibility. Many nanomaterial-based transparent conductive films have been proposed as a solution. Among these materials, silver nanowire (AgNW) networks have been shown to offer high optical transmissivity and low sheet resistance close to that of ITO and is also more flexible, making it an excellent electrode for transparent flexible devices. For such metallic nanowire networks, other than high transparency and low resistance, low surface roughness and haze are especially important for optical devices.

Much of the research on AgNW electrodes have been focused on achieving minimal surface roughness and good adhesion on common flexible transparent substrates such as polyethylene terephthalate (PET). However, for multilayer devices it is important to also take into consideration the surface morphology of the electrodes on or between active layer(s) for optimal device performance. In this work, the effects of surface morphology of AgNW bottom, top and in-between electrodes on the properties and performance of multilayer electroactive polymer-based flexible and transparent transducers are investigated. The surface morphology of the active polymer layers plays a role in that of the AgNW conducting film coated on them and vice versa when a thin active layer is coated on the conducting film. This consequently affects the electrical breakdown strength of the multilayer transducer device. Furthermore, morphology control of the active polymer layer by means of processing conditions and post process treatment can also enhance the permittivity and energy density of the active layer, leading to enhanced performance of flexible transducers.


Electro-Thermochromic Devices Composed of Self Assembled Transparent Electrodes and Hydrogels

Yang ZHOU1, Michael LAYANI2, Freddy Yin Chiang BOEY1, Ilan SOKOLOV2, Shlomo MAGDASSI2, Yi LONG1

1Nanyang Technological University, Singapore; 2The Hebrew University of Jerusalem, Israel

Approximately 40% of the world’s energy consumption is attributed to the building sector, which is cause for alarm. This excessively high consumption has led to increased research and development of energy saving materials for solar thermal control applications in buildings. Chromogenic technologies such as electrochromic, photochromic, and thermochromic have been developed to create climate adaptive building facades to reduce energy consumption for air conditioning and lighting. Thermochromic materials are advantageous for smart windows as it do not require extra energy to operate and can be fully automated. Additionally, thermochromic materials offer large wavelength range transmission modulation. Electrochromic materials actively control the window transmission by electron transfer, where the opacity of window is triggered by electricity. However, the major disadvantages of conventional electrochromic materials are high cost.

We have shown a new approach to fabricate simple, low-cost electro-thermochromic devices. The device consists of a transparent conductive heater is composed of metallic NPs which self-assemble into grid or honeycomb structures and a thermochromic PNIPAm hydrogel. The solar modulating ability is 58.2%, which is higher than the best reported thermochromic materials, and the coloration and the response time is comparable to the best performing electrochromic material. The PNIPAm hydrogel films used enable fabrication of devices with up to 75% contrast, depending on the film thickness and the applied voltage. The facile fabrication method and the low cost of materials make this approach favorable for the lower end of the EC market. It can be used as a privacy window inside buildings and as external facade windows, while the high solar modulating ability ensures signi?cant energy savings with the added functionality of active control.


Electroless Deposition of Ultralong Copper Nanowires for Transparent Conductors

Micheal TAN, Mary Donnabelle LIRIO

University of the Philippines, Philippines

Bulk copper (Cu) is one of the most abundant metals found in the earth’s crust. Additionally, it can easily be extracted and efficiently recovered from end-of-life products which makes it more inexpensive. Cu is also an excellent electrical and thermal conductor, which makes it suitable for low value commercial applications such as electrical transmission wires and coin plating. The abundance of Cu coupled with its excellent conductivity makes it a very viable alternative to replace more expensive materials in the electronics industry. In particular, electrodes made from Cu nanowires are widely studied as a possible replacement to expensive indium tin oxide (ITO). ITO is commonly used as a transparent conducting layer on various devices, such as touch screen, OLEDs, photovoltaics, and various sensors.

Cu nanowires were successfully grown through a low temperature electroless deposition on aqueous solution for 1 h. Ethylene diamine (EDA) was utilized to promote anisotropic growth of Cu nuclei after the reduction of Cu(II) ions by hydrazine. Cu nanowires with mean diameters around 90 nm and lengths exceeding 50 μm were synthesized using 171 mM EDA at 60 oC, giving an effective aspect ratio of about 450. Without EDA, only nanoparticles are produced. The synthesis temperature was also significant in limiting nanoparticle formation. Decreasing the temperature promoted the 1 dimensional growth to nanowires

A transparent conducting electrode was made from Cu nanowire ink-coated glass substrate. The sheet resistance of the Cu nanowire glass electrode was measured to be about 9617 Ω sq-1 at 84.5% transparency for a Cu nanowire density of about 30.3 µg/cm-2. This decreases to about 197 Ω sq-1 at 61% transparency as nanowire density was increased to 151.5 µg/cm-2. Finally, a 5 mA LED was powered in a series connection using the Cu nanowire glass electrode in order to prove its viability.


Evaluating Conducting Network Based Transparent Electrodes from Geometrical Considerations

Ankush KUMAR, Giridhar U. KULKARNI

Jawaharlal Nehru Centre for Advanced Scientific Research, India

Conducting nanowire networks have been developed as viable alternative to existing indium tin oxide based transparent electrode (TE). While most of the literature work pertaining to theoretical analysis is focussed on networks obtained from conducting rods (mostly considering only junction resistance), hardly any attention has been paid to those made using template based methods, wherein the structure of network is neither similar to network obtained from conducting rods nor similar to well periodic geometry. Here, we have attempted an analytical treatment based on geometrical arguments and applied image analysis on practical networks to gain deeper insight into conducing networked structure particularly in relation to sheet resistance and transmittance. Many literature examples reporting networks with straight or curvilinear wires with distributions in wire width and length have been analysed by treating the networks as two-dimensional graphs and evaluating the sheet resistance based on wire density and wire width.For the purpose of evaluating active fraction of the network, the algorithm was made to distinguish and quantify current carrying backbone regions as against regions containing only dangling or isolated wires. We also study current carrying regions and current distribution as a function of wire density in these networks and identify the hot spots.

The work will be helpful in improvisation and comparison of various TEs and better understanding of electrical percolation.


High-performance Transparent and Stretchable Piezoelectric Nanocomposite Based Nanogenerators Used for Self-powered Flexible Sensors

Xiaoliang CHEN, Kaushik PARIDA, Jiaqing XIONG, Meng-Fang LIN, Pooi See LEE

School of Materials Science and Engineering, Nanyang Technological University, Singapore

Piezoelectric nanogenerators (PENGs) with high power-generation performance, high sensitivity, and good flexibility have attracted extensive interest in flexible electronics recently. Especially, integration of transparency and stretchability in PENGs offer a fascinating application in self-powered touch screen devices, transparent electronic skins and wearable power sources. Here, we demonstrate a facile, low-cost and reliable route to fabricate a piezoelectric elastic-composite nanogenerator with transparent and stretchable characteristics using Barium Titanate (BaTiO3) nanoparticles embedded in PDMS flexible matrix. The piezoelectric device was fabricated by spraying highly conductive and transparent Ag nanowires electrodes onto both sides of the robust composite sensing thin layer. The piezoelectric performance under different modes of deformation such as vertical force, bending and stretching are characterized. Under periodic mechanical impact, stable electricity was repeatedly generated from the PENG and used to drive some low-power electronic devices to work continuously. Finally, the transparent and stretchable piezoelectric devices were successfully demonstrated as highly sensitive wearable sensors for detecting some tiny human activities including breath, heartbeat pulse, and finger movements, which shows their potential application in health monitoring.


Indium Free Transparent Conducting Al-Sn and Al-F Co-doped ZnO Thin Films: Comparative Assessment of Electrical and Optical Parameters

Arindam MALLICK, Durga BASAK

Solid State Physics, Indian Association for the Cultivation of Science, India

An immense importance of transparent conducting oxide (TCO) has been realized due to its wide utilization in transparent electronic devices and solar cell application. Sn-doped In2O3 (ITO), commonly used as TCO material is quickly required to be substituted by a non-indium based material for cost-effectiveness. As an alternative, doped and co-doped ZnO has been established to be a suitable pick due to its attractive characteristics. Doping up to a certain limit decreases the resistivity beyond which it increases. As an aid, simultaneous doping i.e. co-doping has been attempted. In this work, we demonstrate electrical and optical parameters of cation doped (Al:AZO), cation-cation (Al-Sn:ATZO) and cation-anion (Al-F:AFZO) co-doped ZnO thin films. Thin films of 200 nm thickness were deposited via RF magnetron sputtering technique using 1 at% Al, 1 at% Al +1 at% Sn and 1 at% Al+1 at% F doped ZnO targets and annealed at 550 °C for 2 hours in vacuum. All the films retain the hexagonal wurtzite structure of ZnO with strong c-axis orientation and show very high transparency above 90% in the visible and NIR region. As opposed to expectation, ATZO film shows no enhanced carrier concentration due to interstitial occupancy of Sn4+ while AFZO film shows about four times enhanced carrier concentration as compared to AZO film attaining a value of ~9×1020 cm-3 due to the simultaneous cation and anion substitution. However, the figure of merit of the best conducting film is one order lower than the ITO. The carrier relaxation time decreases significantly in ATZO film while increases in AFZO than AZO film. A close matching of the Hall mobility value with that of the calculated optical intra-grain mobility implies impurity scattering is dominant in the films. This study for the first time shows comprehensive and comparative results of cation-cation and cation-anion co-doping in ZnO.


Low Cost Copper Inks for Electrode Applications

Rachel Lee Siew TAN1, Wai Tat KERK1, Hongyu CHEN2, Jun WEI1

1Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), Singapore; 2School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore

Despite being significantly cheaper and only 6% less conductive than silver, copper (Cu) nanostructures are seldom used as materials for preparing conductive inks. Readily oxidation of Cu is one of the major drawbacks restricting the use of Cu. The oxide layer formed on the surface of the nanostructures is not conductive, which causes high junction potentials between them and prevents them from forming a continuous conductive network. Furthermore, small nanostructure agglomerates may easily create the inhomogeneity of the ink. However, increasing the size of the nanoparticles would cause them to precipitate out due to gravity. In this work, we synthesized Cu-complex ink, which was reduced to Cu upon exposure to intense pulse light (IPL). These Cu-complexes are soluble in water for preparation of ink.


Modelling of the Size Limitation of Silver Nanowires Used as Transparent Conductor to Achieve Balance between Optical Transmittance and Electrical Conductivity

Yiyang YE, Tupei CHEN, Huakai LI, Chen XU, Jun ZHANG, Jianxun SUN

School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

Silver nanowires can be used to form transparent conductors. A model, based on the Mie theory, Beer-Lambert Law and percolation theory, is developed for judging whether a silver nanowire of certain length and diameter is suitable for a specific application (e.g., touch screen) in terms of optical transmittance and electrical conductivity. Given the diameter and length of the silver nanowire, the model is able to yield the maximum surface coverage ratio Φsmax of the mat built by this silver nanowire in order to have a direct optical transmittance of at least 90%, and the minimum surface coverage ratio Φcritical in order to be conductive. If Φsmax is larger than Φcritical, then this silver nanowire is suitable for the application. According to the percolation theory and Monte Carlo simulation, Φcritical is proportional to the ratio of the silver nanowire’s diameter to length. Thus, with increasing diameter of the silver nanowire, Φsmax decreases; and to make Φcritical smaller than Φsmax, the length of the silver nanowire has to be increased. It is shown that as the diameter of silver nanowire increases from 50 nm to 250 nm, Φsmax decreases from 16.5% to 5.2%, and the minimum length required increases from 1.74 µm to 27.6 µm.


Stretchable Electroluminescent Devices Based on Electronic or Ionic Conductors

Jiangxin WANG, Pooi See LEE

Nanyang Technological University, Singapore

Stretchable electronics are emerging technologies under intensive studies for the development of next-generation soft, smart and interactive electronic systems. Their capabilities to confront different demanding mechanical conditions such as flexing, twisting, stretching, or conformably wrapping onto arbitrary surfaces enable electronic applications which cannot be addressed by conventional devices. As one essential electronic component, stretchable electroluminescent (EL) device plays an important role in wearable display, soft lighting devices, conformable readout systems on skins, and biomedical imaging devices. The development of stretchable and transparent electrodes is a critical challenge under urgent demand for the fulfilment of stretchable EL devices. The electrical and mechanical stabilities of the stretchable EL devices are also required to be improved.

Here, we present a novel and simple approach to fabricate stretchable EL devices using AgNWs as the transparent and stretchable electronic conductors. The alternating-current EL material embedded in elastic polymer matrix was used as the emissive layer. The stretchable device could maintain stable emission under stretching strains up to 100%.1 After that, we developed ionic conductor as another promising electrode material for stretchable EL devices.2 Compared to the electronic conductors, the ionic conductors showed extraordinary properties in transparency, stretchability and mechanical stability. The super-elastic EL devices based on ionic conductor can be stretched up to 700% strain.2 The devices can subject to repetitive stretching test at 400% strain with fairly stable performance. The presented devices provide new opportunities for soft and wearable lighting and display applications.


[1] J. Wang, C. Yan, K. J. Chee and P. S. Lee. Advanced Materials, 27, 2876-2882, 2015.

[2] J. Wang, C. Yan, G. Cai, M. Cui, A. Lee‐Sie Eh and P. See Lee. Advanced Materials, 28, 4490-4496, 2016.


Thermally Stable and Flexible Transparent Conductor based on Patterned Silver Electrode

Juan WANG, Ke HE, Xiaodong CHEN

Nanyang Technological University, Singapore

Conductors with high transparency and flexibility are facing high demands nowadays as they are essential components for flexible touch devices. Conventional indium tin oxide-based transparent conductors suffering from issues of brittleness and high manufacturing cost. It is important to develop alternatives of indium tin oxide (ITO), with the enhanced performance at different aspects. Herein, we proposed a stamp printing technique to produce Ag grid pattern on PET flexible substrate. The as-fabricated conductor shows high transmittance of ~80% and low resistance of ~30Ω/sq which makes it a promising candidate to replace ITO in flexible touch devices. With the management of space size between the Ag grid lines, tuneable conductivity and transmittance can be successfully achieved.


Transparent and Smart Light-Emitting Diode

Xiaohu HUANG1, Shi Jie WANG1, Soo Jin CHUA2

1Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), Singapore; 2Department of Electrical and Computer Engineering, National University of Singapore, Singapore

Although remarkable progress has been achieved on light-emitting diodes (LEDs) over the past few decades, transparent and stimulus-responsive LEDs using low-cost and environmentally friendly materials will open up new opportunities for LEDs beyond traditional illumination and display. Herein, we report transparent and color-switchable micro-LEDs based on arsenic-free and indium-free wide bandgap semiconductors.[1] The LEDs emit red light under forward bias and green light under reverse bias, respectively. Such a bias-polarity-switched strategy allows for tuning the intensity of each emitted light independently with little influence on its color. Moreover, the dual-color LEDs are based on a single LED structure with two-terminal operation, which facilitates compact integration of the LEDs onto other platforms. The results pave the way towards transparent, miniaturized and smart LEDs for ultra-high-definition display, smart window, smart sensors, and etc.


[1] X. Huang, L. Zhang, S. J. Wang, D. Z. Chi, and S. J. Chua, ACS Appl. Mater. Interfaces 8, 15482 (2016).


Transparent Structures for Fabrication of Optoelectronic Devices

Michael LAYANI1, Lioz ETGAR1,2, Shlomo MAGDASSI1,2, Stav RAHMANY2

1Nanyang Technological University, Singapore; 2The Hebrew University of Jerusalem, Israel

Grid patterns fabrication is showing great promise as a mean to obtain transparent electrodes and structures of functional materials. Typical method to obtain such structures include PDMS stamping and photolithography based processes. However, these processes require sophisticated equipment and are considered not very compatible for large scale industrial applications. Self assembly of nanoparticles into grid patterns using mesh-assisted method, has shown great promise for a simple, low-cost, high throughput for fabrication of large scale transparent electrodes 1 , 2 . Here we report on fabrication of transparent electrodes composed of silver nanoparticles, and of semitransparent solar cells composed of organo-metal halide perovskite 3 , 4 . The transparent electrodes were utilized for fabricationof flexiuble electroluminescent devices. For solar cells, sequential deposition of the perovskite precursors was performed at ambient conditions, while in the first step PbI 2 islands are formed, and in the second step the PbI 2 islands react selectively with methylammonium iodide, resulting in a perovskite periodic islands pattern. The resulting solar cells, show a power conversion efficiency of 7.3% with 20% transparency.


Tuning Native Donors to Achieve High Conductivity in Al Doped ZnO Films as Transparent Conducting Oxide Substrate

Shuvaraj GHOSH, Arindam MALLICK, Durga BASAK

Solid State Physics, Indian Association for the Cultivation of Science, India

Sn:In2O3 (ITO) and F:SnO2 (FTO) which are expensive and scant, are widely used as transparent conducting oxide (TCO) electrodes for various modern see-through devices. Huge consumption of In for transparent electronic industry will lead to its shortage in near future besides the present problem of price escalation. Realising the importance of alternative TCOs without content, impurity doped specially Al doped wide band gap (3.3 eV) ZnO (AZO) being popular harnessed owing to its low cost, ease to prepare, high abundance and non-toxicity of the components. However, increasing Al doping level beyond a certain level, degrades the conductivity of the ZnO due to impurity scattering. Thus, to improve conductivity of AZO films further we attempt to control the native donors defects such as Zn interstitial (Zni). AZO films of various thicknesses are grown on glass substrates via RF sputtering technique using a 1 wt% Al doped ZnO target and annealed at higher temperature with Zn vapour in reducing ambient. Resultant highly c-axis oriented AZO films are phase pure and more than 90% transparent in the visible region. Electrical measurements show that the carrier concentration increases as Zn vapour concentration increases, the highest being 1.6x1021 cm-3 as compared to 5.9x1020 cm-3 for without Zn treatment implying excess Zn diffusion into AZO and formation of Zni defects as confirmed by secondary ion mass spectroscopy. The lowest sheet resistance of 350 nm AZO film is 8.8 Ω/sq with a FOM value of 6.3x10-2-1 which are akin to commercial ITO or FTO substrates. Further details on growth, electrical transport parameters and formation mechanism of Zni donors will be offered during the presentation. This study shows not only fundamentals aspects of defects in ZnO but also provide a way to trickily engineer AZO layer for use in highly transparent electronic and optoelectric devices.


Ultraviolet Photodetector Based on n-IGZO / p-NiO Heterojunction with Ag Nanowires and Al-doped ZnO as Transparent Electrodes

Jianxun SUN, Huakai LI, Tupei CHEN, Jun ZHANG, Yiyang YE, Chen XU

School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

An ultraviolet photodetector based on n-IGZO / p-NiO heterojunction with Ag nanowires and Al-doped ZnO as transparent electrodes is fabricated and characterized in this work. The heterojunction of the photodetector is formed by the deposition of 170 nm n-type indium gallium zinc oxide (n-IGZO) layer and 130 nm p-type nickel oxide (p-NiO) layer onto the commercial AZO glass by the RF magnetron sputtering deposition process. The top electrode of the photodetector is formed by spray coating of silver nanowires. A peak responsivity of 0.024 A/W is observed for the photodetector at the wavelength of ~370 nm with the full width at half maximum (FWHM) of smaller than 30 nm, showing a high spectrum selectivity. Good repeatability and fast response have been achieved in the light intensity range of 4.75 – 32.7 mWcm-2 at the UV wavelength of 365 nm.

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