Constant Sheet Resistance Research Articles

Highly flexible and transparent colorless polyimide substrate sandwiched between plasma polymerized fluorocarbon and InGaTiO for high performance flexible perovskite solar cells

ABSTRACT We integrated transparent antireflective coatings and transparent electrodes onto flexible colorless polyimide (CPI) substrates to fabricate high-performance flexible perovskite solar cells. Multifunctional PPFC/CPI/IGTO substrates were fabricated by sputtering the optimal plasma-polymerized fluorocarbon (PPFC) antireflective coating and InGaTiO (IGTO) electrode films on both sides of the CPI substrate. By applying PPFC with a low refractive index (1.38) as an antireflective coating, the transparency of the PPFC/CPI/IGTO substrate increased by an additional 1.2%. In addition, owing to the amorphous characteristics of the PPFC and IGTO layers, the PPFC/CPI/IGTO substrate showed constant sheet resistance and transmittance change even after 10,000 cycles during the bending tests. The flexible perovskite solar cells, fabricated on the PPFC/CPI/IGTO substrate, exhibited an increase in current density of 1.48 mA/cm2 after the deposition of the PPFC antireflective coating. These results confirmed that the PPFC/CPI/IGTO substrate was durable against high-temperature treatment, flexible, and exhibited excellent electrical characteristics. This enhanced the efficiency and durability of the flexible perovskite solar cells. Moreover, the hydrophobic PPFC layer allowed the self-cleaning of inflexible perovskite solar cells. Given these attributes, the PPFC/CPI/IGTO structure has been recognized as a good choice for multifunctional substrates of flexible perovskite solar cells, presenting the potential for enhancing performance.

Conformal Passivation of Self-Cleanable, Flexible, and Transparent Polytetrafluoroethylene Thin Films on Two-Dimensional MXene and Three-Dimensional Ag Nanowire Composite Electrodes

Despite the excellent performance of MXene–Ag nanowire (MA) composite transparent conductive electrodes (TCEs), they rapidly and severely degrade in ambient and humid environments. To address this critical issue, we developed conformal polytetrafluoroethylene (PTFE) passivation thin films prepared using magnetron sputtering at room temperature. Compared to the PTFE passivated electrode, the nonpassivated bare MA-composite electrode exhibited a considerably increased sheet resistance, and the electrode flexibility and electromagnetic interference (EMI) shielding efficiency (SE) degraded rapidly over time because the moisture and impurities severely oxidized the MXene layer in the harsh testing environment. However, the MA-composite electrode with PTFE passivation shows constant sheet resistance, transmittance, flexibility, and EMI SE, even after the 85 °C–85% relative humidity (RH) test. Compared to the nonpassivated MA-electrode-based thin film heaters (TFHs), the PTFE-passivated MA-composite electrode-based TFHs exhibited improved stability and reliability, indicating that the sputtered PTFE film effectively prevented the moisture and impurity penetration of the MA-composite electrodes.

Highly Transparent, Flexible, and Hydrophobic Polytetrafluoroethylene Thin Film Passivation for ITO/AgPdCu/ITO Multilayer Electrodes

AbstractTransparent and flexible polytetrafluoroethylene (PTFE) thin film passivation for ITO/AgPdCu (APC)/ITO (IAI) electrodes is developed to prevent severe degradation and maintain the reliable performance of the flexible IAI electrodes. Also, oxygen ion beam treatment (IBT) on the surface of the IAI electrode improves the adhesion between the top ITO and PTFE passivation layer, as well as the performance of the passivation layer. The IAI electrode with PTFE passivation exhibits constant sheet resistance, transmittance, and flexibility, even after the 85 °C and 85% RH test. However, the bare IAI electrode shows a significant increase in sheet resistance and a decrease in transmittance and flexibility, owing to the incorporation of moisture and impurities into the IAI layer, and agglomeration of the APC layer. To demonstrate the feasibility of PTFE passivation, the performance of flexible and transparent thin film heaters (TFHs) with the bare IAI and PTEF/IAI samples are compared. The better stability and reliability of the PTFE/IAI‐based TFHs than the bare IAI‐based TFHs indicate that the PTFE film effectively prevents the introduction of moisture and impurities. The performance of the TFH with PTFE/IAI electrode implies that sputtered PTFE film is a promising transparent and flexible thin film passivation for high‐quality oxide‐metal‐oxide multilayer electrodes.

Dual use of anionic azobenzene derivative as dispersant and dopant for carbon nanotubes for enhanced thermal stability of transparent conductive films

Air- and thermally stable carbon nanotube (CNT)-based transparent conductive films have been developed by a facile wet-coating method using an anionic azobenzene derivative with sulfonate group that was synthesized by a simple one step process. The conductive film showed low sheet resistance (122 Ω/sq at T = 81%) without any post-treatment processes such as the removal of dispersant or acid treatment, because the azobenzene derivative served both dispersant and dopant roles in the CNT films. The conductive films showed constant sheet resistance under various thermal and environmental conditions, such as in air at room temperature over an 8-month period, in air at 250 °C for 1 h, and in vacuo at 100 °C for 1 h. This stability comes from the strong intermolecular interactions between the azobenzene derivative and CNT. Thermally less susceptible property of the azobenzene derived from the substituent size also reduces the thermal desorption of the azobenzene from the CNT surface, supporting the stability of transparent conductive films at relatively high temperatures.

Highly oxidation-resistant silver nanowires by CxFy polymers using plasma treatment

We demonstrate plasma-treated Ag nanowires (NWs) as flexible transparent electrode materials with enhanced long-term stability against oxidation even in a high humidity environment (80% humidity, 20 °C). Through a simple fluorocarbon (C4F8 or C4F6) plasma treatment method, a CxFy protective polymer was sufficiently cross-linked and attached on the surface of the AgNWs strongly and uniformly. Even though C4F8 and C4F6 activate differently on the AgNW surface due to the different dissociated radicals formed in the plasma, it was found that the CxFy protective polymers obtained by both chemicals work similarly as a protective layer for transparent conductive electrodes; a nearly constant sheet resistance ratio (Rs/Ro) of 1.6 was found for AgNWs treated with C4F8 and C4F6 plasmas, while the AgNWs without the plasma treatment exhibited a ratio of 176.2 after 36 days in a harsh environment. It is believed that the fluorocarbon plasma treatment can be used as a key method for ensuring long-term oxidation stability in numerous electronic applications including flexible solar cells utilizing various types of metallic nanowires.

Transparent and flexible Sb-doped SnO2 films with a nanoscale AgTi alloyed interlayer for heat generation and shielding applications.

Transparent and flexible Sb-doped SnO2 (ATO) films with a nanoscale AgTi alloyed interlayer were fabricated for use as plasma damage-free, indium-free, thermally stable electrodes for high performance heat generating films and shielding films in smart windows. The AgTi alloy-inserted ATO film on a PET substrate showed a low sheet resistance of 6.91 ohm per square and a high optical transmittance of 90.24% without thermal annealing or intentional substrate heating. Even after deformation using an outer bending radius of 4 mm, the ATO film with a AgTi interlayer showed a constant sheet resistance due to the mechanical robustness of the AgTi interlayer. Furthermore, the AgTi-inserted ATO film showed a constant resistance even after annealing at 500 °C, unlike the Ag-inserted ATO films. Furthermore, we demonstrated the feasibility of the AgTi-inserted ATO films as transparent heat generating films and shielding films for smart windows. The effective heat generation and shield performance of the ATO/Ag–Ti/ATO multilayer suggests that the multi-functional ATO/Ag–Ti/ATO films can be used to create energy-efficient smart windows for building energy management systems and automobiles.

Annealed Ag contacts to MoS2 field-effect transistors

Silver contacts to few-layer (5 to 14 layers thick) MoS2 have been studied before and after annealing. Annealing was found to be critical for reducing the contact resistance but did not degrade the operation of field-effect transistors that are part of the test structure. The contact resistance for the as-deposited samples was in the range of 0.8–3.5 kΩ μm. On the other hand, the contact resistance was reduced to 0.2–0.7 kΩ μm, evaluated at a constant sheet resistance of 32 kΩ/□, after annealing at 250 or 300 °C. The reduced contact resistance is attributed to diffusion of Ag into the MoS2 and doping, as supported by further electrical characterization of the contacts and devices.

Unusually High Optical Transmission in Ca:Ag Blend Films: High‐Performance Top Electrodes for Efficient Organic Solar Cells

Highly transparent electrodes are demonstrated based on thermally evaporated calcium:silver blend thin–films, which show unusually high transmission well above the expectations from bulk material properties and thin film optics. These electrodes exhibit a low sheet resistance of 27.3 Ω/, combined with an extraordinarily high mean transmittance of 93.0% in the visible spectral range (σdc/σopt = 186.7), superior to the commonly used inorganic electrodes made from indium tin oxide (ITO). Additionally, the metal blend electrode is flexible, showing a constant sheet resistance down to a bending radius of 10 mm and can be employed on top of organic devices without causing damage to the organic material. The spontaneously formed unique microstructure of a polycrystalline Ag network with randomly distributed nanoapertures, surrounded by a calcium shell, enables broadband transmittance enhancement due to amplified plasmonic coupling. Consequently, top‐illuminated organic solar cells using such metal blend electrodes achieve a power conversion efficiency of 7.2% (which defines a new record for top illuminated organic solar cells) and even exceed the efficiency of similar bottom‐illuminated reference solar cells (6.9%) employing common ITO electrodes.

Rolling Silver Nanowire Electrodes: Simultaneously Addressing Adhesion, Roughness, and Conductivity

Silver nanowire mesh electrodes represent a possible mass-manufacturable route toward transparent and flexible electrodes for plastic-based electronics such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and others. Here we describe a route that is based upon spray-coated silver nanowire meshes on polyethylene terephthalate (PET) sheets that are treated with a straightforward combination of heat and pressure to generate electrodes that have low sheet resistance, good optical transmission, that are topologically flat, and adhere well to the PET substrate. The silver nanowire meshes were prepared by spray-coating a solution of silver nanowires onto PET, in air at slightly elevated temperatures. The as-prepared silver nanowire electrodes are highly resistive due to the poor contact between the individual silver nanowires. Light pressure applied with a stainless steel rod, rolled over the as-sprayed silver nanowire meshes on PET with a speed of 10 cm s(-1) and a pressure of 50 psi, results in silver nanowire mesh arrays with sheet resistances of less than 20 Ω/□. Bending of these rolled nanowire meshes on PET with different radii of curvature, from 50 to 0.625 mm, showed no degradation of the conductivity of the electrodes, as shown by the constant sheet resistance before and after bending. Repeated bending (100 times) around a rod with a radius of curvature of 1 mm also showed no increase in the sheet resistance, demonstrating good adherence and no signs of delamination of the nanowire mesh array. The diffuse and direct transmittance of the silver nanowires (both rolled and as-sprayed) was measured for wavelengths from 350 to 1200 nm, and the diffuse transmission was similar to that of the PET substrate; the direct transmission decreases by about 7-8%. The silver nanowires were then incorporated into OPV devices with the following architecture: transparent electrode/PEDOT:PSS/P3HT:PC61BM/LiF/Al. While slightly lower in efficiency than the standard indium tin oxide substrate (ITO), the rolled silver nanowire electrodes had a very good device yield, showing that short circuits resulting from the silver nanowire electrodes can be successfully avoided by this rolling approach.

Reactive bonding mediated high mass loading of individualized single-walled carbon nanotubes in an elastomeric polymer

A reactive chemical bonding strategy was developed for the incorporation of a high mass loading of individual single-wall carbon nanotubes (SWCNTs) into an elastomeric matrix using a reactive ionic liquid as a linker. This method simultaneously prevented the agglomeration of SWCNTs and caused strong interfacial bonding, while the electronic properties of the SWCNTs remained intact. As a result, the high conductivity of the carbon nanotubes (CNTs) and the flexibility of the elastomeric matrix were retained, producing optimum electrical and mechanical properties. A composite material with a loading of 20 wt% SWCNTs was fabricated with excellent mechanical properties and a high conductivity (9500 S m(-1)). The method could be used to form transparent thin conductive films that could tolerate over 800 bend cycles at a bending angle of 180° while maintaining a constant sheet resistance.

Self-aligned top-gate amorphous gallium indium zinc oxide thin film transistors

We have demonstrated a self-aligned top-gate amorphous gallium indium zinc oxide thin film transistor (a-GIZO TFT). It had a field effect mobility of 5 cm2/V s, a threshold voltage of 0.2 V, and a subthreshold swing of 0.2 V/decade. Ar plasma was treated on the source/drain region of the a-GIZO active layer to reduce the series resistance. After Ar plasma treatment, the surface of the source/drain region was divided into In-rich and In-deficient regions. The a-GIZO TFT also had a constant sheet resistance of 1 kΩ/◻ for a film thickness of over 40 nm. The interface between the source/drain Mo metal and the Ar plasma-treated a-GIZO indicated a good Ohmic contact and a contact resistivity of 50 μΩ cm2.

Effects of various Co/TiN and Co/Ti layer stacks and the salicide rapid thermal process conditions on cobalt silicide formation

The effects of cap layer type (Ti or TiN) and its thickness, Co thickness and rapid thermal processing (RTP) temperature on cobalt silicide formation are investigated by a combination of electrical and optical measurements. Various Co/TiN and Co/Ti layer stacks (thicknesses 8–20 nm per layer) were deposited on (100) Si substrates. The first RTP step (RTP1) was performed by isochronal annealing at various temperatures between 400 and 600 °C for 30 s. It was observed that the temperature range for constant sheet resistance (Rs) values after the first RTP step (RTP1 process window) is smaller for the Co/TiN layer stacks than it is for the Co/Ti layer stacks. After the subsequent selective etch step the second RTP step (RTP2) was performed at 800 °C for 30 s. Rs after RTP2 strongly depends on the initial Co thickness and its uniformity for both systems if the RTP1 temperature was above 470 °C. For the Co/TiN layer stacks the final Rs results are not influenced by the RTP1 temperature or its uniformity (above 470 °C). In this case silicidation is independent of the cap thickness. But in the Co/Ti system the reactive Ti influences the silicidation process by reducing the amount of available Co in a manner that depends on the RTP1 temperature and the Ti cap thickness.

TiB2as a Diffusion Barrier for Cu/<si> Metallization

AbstractTiB2, films deposited by co-sputtering from a boron and a TiB, target are evaluated as the diffusion barrier for Cu metallization. Material characteristics of the TiB, films and metallurgical interactions of the Cu/TiB2/<Si> system annealed at 400−700°C for 30 min, in a 80%Ar+20%H2 flow, were investigated by glancing angle X-ray diffraction, Auger electron spectroscopy (AES), and scanning electron microscopy (SEM). Sheet resistance was measured for electrical characterization.The composition and resistivity of the sputtered TiB1 films varied with the bias applied on the substrate. To obtain a low film resistivity, a negative bias of 200V was applied during sputtering. The resulting TiB2 film is nanocrystalline with a resistivity of 300 μΩcm. After copper deposition, the Cu/TiB2/<Si> samples have a constant sheet resistance after annealing up to 600°C for 30min. The overall sheet resistance of the sample increases by five orders of magnitude after annealing at 700°C, and scanning electron micrographs reveal that the sample surface is severely deteriorated after annealing at 700°C.

Self-Aligned Formation of C54 Titanium Germanosilicide Using Rapid Thermal Processing and Application to Raised, Ultrashallow Junctions

ABSTRACTIn this paper we present results on solid state reactions between Ti and Si1−xGex alloys selectively deposited onto Si (100) substrates using rapid thermal annealing (RTA) for contact applications in novel device structures. Germanium concentrations of 0%, 30%, 50%, and 100% within the reacting Si1−xGex alloy are investigated. The Si1−xGex alloys (approximately 2500 ° thick) are deposited using rapid thermal chemical vapor deposition (RTCVD). Titanium is then deposited by evaporation. Sheet resistance measurements as a function of RTA temperature (10 second anneals) provide indications of various phases that occur during the reactions through the formation of constant sheet resistance plateaus. The RTA temperature required for the formation of a minimum resistivity phase is observed to increase for increasing Ge concentrations within the reacting Si1−xGex alloy. Using x-ray diffraction we have determined that for the reactions of Ti with Si the C49 TiSi2 metastable phase forms prior to the minimum resistivity C54 TiSi2 phase. For the reactions between Ti and Ge a minimum resistivity TiGe2 phase also with the C54 structure forms, however, this phase is preceeded not by a C49 TiGe2 structure, but by a Ti6Ge5 phase. The minimum resistivity phases for Ti reactions with 30% and 50% Ge Si1−xGex, alloy reactions also have a C54 structure with unit cell dimensions varying from that of TiSi2) to TiGe2 as the Ge concentration is increased. The grain structures for the reactions are investigated by cross-sectional transmission electron microscopy (XTEM). As the Ge concentration within the reacting alloy decreases the lateral grain size for the C54 structures increases. A self-aligned germanosilicide process is identified and used to fabricate raised, ultrashallow junctions with Ti(SiGe)2 (germanosilicide) contacts. Forward and reverse bias characterization of the junctions indicate that leakage current induced during silicidation can be eliminated using raised junctions with germanosilicide contacts.

A study of undoped and molybdenum doped, polycrystalline, tin oxide thin films produced by a simple reactive evaporation technique

Uniformly thick, conductive, tin oxide thin films ranging in thickness from 3 to 85 nm were deposited by reactive evaporation of tin, in the presence of oxygen, onto heated glass substrates. The conductivity of the films could be increased by doping with molybdenum using a simple, novel technique which is described. Film resistivity was found to increase with the time of exposure to laboratory ambients subsequent to deposition. Mobility and carrier density data for the more conductive films are presented. An approximately constant sheet resistance was found for the undoped films from which it is inferred that conduction is probably dominated by a highly conductive surface layer. The sheet resistance of the doped films increased with decreasing thickness. The films produced consisted entirely of tin oxide phases as indicated by X-ray diffraction results and were highly transparent (T>85%). There was no significant difference between the optical absorption edge (3.6-3.9 eV) for doped and undoped films. A direct optical transition was found to be the dominant absorption process. The direct bandgap was found to lie in the region 4.1+or-0.1 eV.

Al‐doped Ni–Cr for temperature coefficient of resistance control in hybrid thin‐film resistors

The temperature coefficient of resistance (TCR) was tailored by doping standard 60/40 nickel–chromium resistor film to reduce the TCR from ∼100 ppm/°C to ∼0 ppm/°C. For constant sheet resistance films, TCR was a function of both substrate surface roughness and dopant concentration. TCR may be kept near zero for a wide variety of hybrid substrate materials with varied surface finishes by changing dopant concentration in the sputtering target. By controlling alloy composition to 0.1 wt. %, using powder metallurgy techniques, TCR was controlled to ±5 ppm/°C for a constant sheet resistance and invariant substrate surface finish. Because rough substrates required thicker films to obtain the same sheet resistance, the factors which controlled TCR were slightly modified. The effect was an increase in TCR for the same resistance value. Doped resistors on aluminum oxide were slightly less stable in the high temperature storage test than undoped resistors. However, because their TCR was so much lower, the change in resistance during operation of a real circuit was less than one‐fourth the change in undoped resistors used on the same circuit.