Non-vacuum Process Research Articles

(Invited) Nanoscale Electrocrystallization: A Site-Selective Electrochemical π-Figuration of Nanocrystals for Electronic Devices

Efficient nanofabrication is still the problem awaiting solution in the nanotechnology field. Such nanofabrication should involve both economy and ecology-friendly process which produces high performance materials or devices. As seen in semiconductor industry, these fabrication technologies generally consume a large amount of energy under high-vacuum and/or high-temperature conditions. There is a strong need to switch to processes which are both eco-friendly and low-cost. Therefore pinpoint nanofabrication via eco-friendly process seems to be most promising. From the viewpoint, we have developed nanoscale electrocrystallization for nanodevice fabrication.1 This method allows everyone to grow nanocrystals, which consist of π-conjugated molecules, site-selectively within a gap between two electrodes fabricated on a substrate. In addition, this method also allows for the gap between the two electrodes to be bridged by the formed nanocrystals. · Magnetic field effect devices Magnetic field effect devices were designed by using dicyanoiron(III)phthalocyanine ([Fe(Pc)(CN)2]-) tetraphenylphosphonium (TPP+) salt as a starting material of nanoscale electrocrystallization. The electrolysis gave nanocrystals that exhibited a strong correlation between the localized spin and conduction electrons, TPP·[Fe(Pc)(CN)2]2 was obtained. A negative giant magnetoresistance was first observed as an organic nanocrystal.2 Moreover, an angular dependence of the negative giant magnetoresistance that originates from the highly oriented growth of the nanocrystals was also observed. · Electronic field effect devices Electronic field effect devices such as field-effect transistors (FETs) were designed by using materials for organic conductors. A bottom-gate type FET structure was simply obtained when a silicon substrate with oxide layer was used for an electrode substrate. The two electrochemical electrodes were reused as source and drain electrodes. FET characteristics showed current enhancement in some kinds of samples.3 The device fabrication was also demonstrated via an eco-friendly and non-vacuum process using a material printer.4 Further details of fabrication, materials, and physical properties will be reported.

Cu(In,Ga)Se2 thin film absorber layer by flash light post-treatment

Cu(In,Ga)Se2 (CIGS) thin film solar cell has immerged popular candidate as a cost-effective alternative for conventional silicon-based solar cells owing to its excellent optoelectronic properties and better thermo-chemical stability. Non-vacuum process for the preparation of CIGS absorber is always preferred for its economical benefits over the vacuum based process. In this regard, we present solution processed development of CIGS absorber layer involving pristine thin film by spray casting of nanoparticulate ink followed by photonic flash light sintering post-treatment. Influence of flash light post-treatment on structural, morphological, optical and photovoltaic properties of CIGS thin film absorber layer is presented.

An overview on electrodeposited Cu(In,Ga)(Se,S)2 thin films for photovoltaic devices

Industrial implementation of Cu(In,Ga)(Se,S)2 (CIGS) solar cells based technologies is usually based on two vacuum deposition techniques: thermal evaporation and sputtering. Despite the ability to produce high purity and high efficiency solar cells with reasonable reproducibility, they exhibit some limitations with respect to their suitability for high-throughput and large area manufacturing and a better materials using rate that would in turn reduce production costs. The sophisticated vacuum equipment requires high capital investment, decreases the flexibility to implement changes and thus it is more vulnerable to predict the desired results.The compositional control that is required in CIGS solar cells in particular is difficult to maintain during up-scaling to large area modules by means of vacuum methods. Implementation of a roll-to-roll process with the use of large, flexible substrates to decrease processing time and increase throughput would further raise the desire of cost effective and fast non-vacuum processes that omit pumping times and are inherently easy to up-scale.The historical developments and main advantages of CIGS thin film photovoltaic devicesis revised and costs considerations are argued to support the interesting approach of electrodeposition as a preferred technique for industrial CIGS substituting sputtered precursors.

Control of artificial pinning centers in REBCO coated conductors derived from the trifluoroacetate metal-organic deposition process

The metal-organic deposition (MOD) process using metal trifluoroacetate salts (TFA) has the advantages of low-cost and high-scalability for the fabrication of REBa2Cu3Oy (REBCO, RE: rare earth elements) superconducting coated conductors (CCs) with high critical current density, in principle, because of its non-vacuum process. For the magnetic applications of CCs such as motors, magnetic resonance imaging and superconducting magnetic energy storage, further improvement of superconducting performance under magnetic fields is required. However, the in-field superconducting performance of REBCO CCs derived from the TFA-MOD process had been inferior to those derived from the vapor-phase process. In order to improve the in-field performance, the size control of the artificial pinning centers has been known as an effective way. In the early stage, the BaZrO3 (BZO) material, which was one of the effective materials in the CCs by the vapor-phase process, was also introduced in the TFA-MOD-derived CCs. The unique feature of the BZO material in the TFA-MOD process is the shape. The BZO in the TFA-MOD process formed the particle shape, although in the vapor-phase process it has a rod shape with a long axis elongating along the thickness direction. In addition, a special heat treatment for refining the BZO particles was developed, which is called the ‘interim heat treatment’. This heating profile made the in-field characteristics higher, although they were still lower than those of the vapor-phase process. Then, the new MOD process including ‘ultra-thin once coating’ was recently developed for further refinement of the BZO particles. The characteristics of the new TFA-MOD-derived CCs in magnetic fields have become compatible with those of the CCs derived from the vapor-phase process.

Mechanochemically Synthesized SnS Nanocrystals: Impact of Nonstoichiometry on Phase Purity and Solar Cell Performance

We demonstrate nontoxic, earth-abundant light-absorbing SnS thin films fabricated by a low-cost, environmentally friendly nonvacuum process. SnS nanocrystals (NCs) are mechanochemically synthesized from elemental powders without the use of any other additives or solvents. To investigate the effect of the Sn-to-S stoichiometric ratio on the crystalline phase of the SnS NCs, the nonstoichiometry is systematically controlled from 0.95 (Sn0.95S) to 1.05 (Sn1.05S) by adjusting the mixing ratio of the Sn and S powders. The crystallographic evolution with the milling time signifies that the formation of the SnS phase follows a mechanochemically driven self-propagation reaction mechanism. The as-synthesized SnS NCs with a stoichiometric composition (i.e., Sn1.00S) are found to contain a Sn2S3 impurity phase in a non-negligible amount, which can be subsequently eliminated by a post heat treatment at 500 °C in a reducing atmosphere. Interestingly, however, the formation of Sn2S3 during the mechanochemical synthesis...

Preparation of transparent CuI–CuBr alloy thin films by solution processing

Transparent p-type CuBr1−xIx semiconducting thin films with a variable band gap were prepared using spin coating, which is a non-vacuum process. The dependence of their structural and optical properties on the CuBr1−xIx composition was investigated. In X-ray diffraction patterns, the dominant CuBr1−xIx peak was found to shift to higher angles with decreasing mixing ratio x. The Z1,2 and Z3 exciton absorption bands were observed from 300 to 400 nm, and the absorption wavelength varied with x. The ionization potential was determined using photo yield spectroscopy, and was found to increase with increasing x. These results suggest that the valence band maximum for CuBr1−xIx depends on the composition. The results obtained in the present study allowed the energy band structure for this material to be proposed.

(Ag,Cu)(In,Ga)Se2 thin films fabricated on flexible substrates via non-vacuum process

(Ag,Cu)(In,Ga)Se2 thin films with various contents of silver ions were successfully fabricated on the stainless steel substrates using the non-vacuum spin-coating process. The solar cells with the stainless steel/Mo/ACIGS/CdS/i-ZnO/ITO structure were prepared. After selenization at 550 °C for 30 min, the monophasic chalcopyrite-structured (Ag,Cu)(In,Ga)Se2 thin films were formed. The conversion efficiency of (Ag,Cu)(In,Ga)Se2 solar cells was significantly increased from 4.34 to 6.29% when the molar ratio of Ag ions to total (Ag + Cu) ions was increased from 0 to 0.2. The increase in conversion efficiency was attributed to the liquid phase that was formed from (Ag,Cu)2In intermediate compound. The formed liquid phase facilitated the grain growth and promoted the densification of thin films. The densified thin films of (Ag,Cu)(In,Ga)Se2 suppressed the additional shunt path and decreased the value of shunt conductance (G) to 12.57 mS/cm2. In addition, the values of ideal factor (A), saturated current density (J0), and series resistance (Rs) were respectively decreased to 2.96, 0.08 mA/cm2, and 0.23 Ωcm2 due to suppressing the additional shunt path. When an excess of silver ions were doped into the CIGS thin films, the microstructures of the prepared thin films became porous, and the conversion efficiency of the solar cells was reduced. This research reveals that adding an appropriate amount of silver ions effectively improved the performance of (Ag,Cu)(In,Ga)Se2 solar cells.

Facile fabrication method of small-sized crystal silicon solar cells for ubiquitous applications and tandem device with perovskite solar cells

Small-sized single crystalline silicon solar cells (ca. 25 mm2) were fabricated by a non-vacuum process as an energy supply for small devices (ubiquitous devices: a wristwatch, desktop calculator etc.) and processed for a tandem solar-cell research. A side-edge etching procedure was performed in order to eliminate detrimental cracks to improve photovoltaic properties. Moreover, the new structure of the small-sized solar cell with side contact provides reduced shadow loss of contacts. After the structural and procedural optimization, a conversion efficiency of 16.4% was achieved by a non-vacuum process with 3 mm × 8 mm surface dimension solar cell. Finally, the photovoltaic characteristics of small silicon cells, as a function of light intensity for the ubiquitous purposes were compared with amorphous silicon solar cells. In addition, the facile processed silicon solar cell was integrated into mechanically-stacked tandem solar cells with perovskite solar cells by direct contact of TCO layers of each sub-cell. The counter electrode MoOX/IZO over HTM (spiro-OMeTAD) in top perovskite cells is physically placed on top of the bottom Si solar cell to form a series tandem configuration.

Failure of non-vacuum steam sterilization processes for dental handpieces

Dental handpieces are used in critical and semi-critical operative interventions. Although some dental professional bodies recommend that dental handpieces are sterilized between patient use there is a lack of clarity and understanding of the effectiveness of different steam sterilization processes. The internal mechanisms of dental handpieces contain narrow lumens (0.8-2.3 mm) which can impede the removal of air and ingress of saturated steam required to achieve sterilization conditions. To identify the extent of sterilization failure in dental handpieces using a non-vacuum process. In-vitro and in-vivo investigations were conducted on widely used UK bench-top steam sterilizers and three different types of dental handpieces. The sterilization process was monitored inside the lumens of dental handpieces using thermometric (TM; dataloggers), chemical indicator (CI), and biological indicator (BI) methods. All three methods of assessing achievement of sterility within dental handpieces that had been exposed to non-vacuum sterilization conditions demonstrated a significant number of failures [CI: 8/3024 (fails/no. of tests); BI: 15/3024; TM: 56/56] compared to vacuum sterilization conditions (CI: 2/1944; BI: 0/1944; TM: 0/36). The dental handpiece most likely to fail sterilization in the non-vacuum process was the surgical handpiece. Non-vacuum sterilizers located in general dental practice had a higher rate of sterilization failure (CI: 25/1620; BI: 32/1620; TM: 56/56) with no failures in vacuum process. Non-vacuum downward/gravity displacement, type N steam sterilizers are an unreliable method for sterilization of dental handpieces in general dental practice. The handpiece most likely to fail sterilization is the type most frequently used for surgical interventions.

High work function MoO2 and ReO2 contacts for p-type Si and GaN by a room-temperature non-vacuum process

High work function molybdenum (Mo) and rhenium (Re) di-oxides were studied without vacuum and thermal process to investigate the contact properties on p-type Si and GaN semiconductors. Polyvinyl alcohol (5wt% PVA) was used as a binder for the metal oxide powder. ReO2 showed lower electrical resistivity < 0.023 ± 0.004Ωcm and better contact characteristics compared to MoO2. Ohmic contacts were formed easily on p-type Si (ρ = 9.77Ωcm) using ReO2 and MoO2. The metal oxide/semiconductor Schottky diodes fabricated by forming contacts on n-type Si (ρ = 2.44Ωcm) were investigated by comparing diode parameters indicating different contact barrier properties of metal oxides. As a wide energy bandgap semiconductor, p-type GaN was utilized to investigate metal oxide contact properties on a high work function semiconductor. Ohmic contact on p-type GaN was obtained using ReO2 after the surface treatment by boiling aqua regia.

Totally Vacuum-Free Processed Crystalline Silicon Solar Cells over 17.5% Conversion Efficiency

In this work, we introduce a totally vacuum-free cost-efficient crystalline silicon solar cells. Solar cells were fabricated based on low-cost techniques including spin coating, spray pyrolysis, and screen-printing. A best efficiency of 17.51% was achieved by non-vacuum process with a basic structure of &lt;AI/p+/p−Si/n+/SiO2/TiO2/Ag&gt; CZ-Si p-type solar cells. Short circuit current density (JSC) and open circuit voltage (VOC) of the best cell were measured as 38.1 mA·cm−2 and 596.2 mV, respectively with fill factor (FF) of 77.1%. Suns-Voc measurements were carried out and the detrimental effect of the series resistance on the performance was revealed. It is concluded that higher efficiencies are achievable by the improvements of the contacts and by utilizing good quality starting wafers.

Fabrication of Cd-free CuInSe2 solar cells using wet processes

CuInSe2 has become an attractive material for use in the production of thin-film solar cells. Recently, energy conversion efficiencies over 20% have been achieved by CuInSe2 solar cells created using vacuum processes. To reduce manufacturing costs, more recent efforts have focused on non-vacuum processes. In this study, we fabricated solar cells exclusively by wet processes. CuInSe2 absorption layers and In2S3 buffer layers were electrochemically or chemically deposited, and ZnO window layers were formed using solution-based chemical methods. The deposited CuInSe2 absorption layers were heat-treated to improve crystallinity. We deposited a Cd-free buffer layer of In2S3 using a chemical bath method on the CuInSe2 absorption layers, and then ZnO window layers were deposited by a solution-based process. With this system, we successfully fabricated CuInSe2 solar cells and obtained a cell efficiency of 2.38%.

Investigating steam penetration using thermometric methods in dental handpieces with narrow internal lumens during sterilizing processes with non-vacuum or vacuum processes

Dental handpieces are required to be sterilized between patient use. Vacuum steam sterilization processes with fractionated pre/post-vacuum phases or unique cycles for specified medical devices are required for hollow instruments with internal lumens to assure successful air removal. Entrapped air will compromise achievement of required sterilization conditions. Many countries and professional organizations still advocate non-vacuum sterilization processes for these devices. To investigate non-vacuum downward/gravity displacement, type-N steam sterilization of dental handpieces, using thermometric methods to measure time to achieve sterilization temperature at different handpiece locations. Measurements at different positions within air turbines were undertaken with thermocouples and data loggers. Two examples of widely used UK benchtop steam sterilizers were tested: a non-vacuum benchtop sterilizer (Little Sister 3; Eschmann, Lancing, UK) and a vacuum benchtop sterilizer (Lisa; W&H, Bürmoos, Austria). Each sterilizer cycle was completed with three handpieces and each cycle in triplicate. A total of 140 measurements inside dental handpiece lumens were recorded. The non-vacuum process failed (time range: 0-150s) to reliably achieve sterilization temperatures within the time limit specified by the international standard (15s equilibration time). The measurement point at the base of the handpiece failed in all test runs (N=9) to meet the standard. No failures were detected with the vacuum steam sterilization type B process with fractionated pre-vacuum and post-vacuum phases. Non-vacuum downward/gravity displacement, type-N steam sterilization processes are unreliable in achieving sterilization conditions inside dental handpieces, and the base of the handpiece is the site most likely to fail.

Breakthrough to Non-Vacuum Deposition of Single-Crystal, Ultra-Thin, Homogeneous Nanoparticle Layers: A Better Alternative to Chemical Bath Deposition and Atomic Layer Deposition.

Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin films with the thickness of a few nanometers on rough surfaces. We present a new ”paradigm shift” non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In,Ga)Se2 (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.

Electrical properties of Cs3Sb photocathode emitters in panel device applications

Nonvacuum process technology was used to produce Cs3Sb photocathodes on substrates, and in situ panel devices were fabricated. The performance of the devices was characterized by measuring anode current as functions of the devices' operation times. An excitation light source with 475 nm wavelength was used for the photocathodes. The device has a simple diode structure, providing unique characteristics such as a large gap, vertical electron beam directionality, and resistance to surface contamination from ion bombardment and poisoning by outgassing species. Accordingly, Cs3Sb photocathodes function as flat emitters, and the emission properties of the photocathode emitters depend on the vacuum level of the devices. Improvement of current stability has been observed after conducting the electrical conditioning process to remove possible adsorbates on Cs3Sb flat emitters.

Fabrication and electrical characterization of planar lighting devices with Cs3Sb photocathode emitters

Non-vacuum process technology was used to produce Cs3Sb photocathodes on substrates, and in-situ panel devices were fabricated. The performance of the devices was characterized by measuring the anode current as functions of the devices’ operation times. An excitation light source with a 475-nm wavelength was used for the photocathodes. The device has a simple diode structure, providing unique characteristics such as a large gap, vertical electron beam directionality, and resistance to surface contamination from ion bombardment and poisoning by outgassing species. Accordingly, Cs3Sb photocathodes function as flat emitters, and the emission properties of the photocathode emitters depend on the vacuum level of the devices. An improved current stability has been observed after conducting an electrical conditioning process to remove possible adsorbates on the Cs3Sb flat emitters.

Enhanced Photocurrents with ZnS Passivated Cu(In,Ga)(Se,S)2 Photocathodes Synthesized Using a Nonvacuum Process for Solar Water Splitting.

Chalcopyrite Cu(In,Ga)(Se,S)2 (CIGS) semiconductors are potential candidates for use in photoelectrochemical (PEC) hydrogen generation due to their excellent optical absorption properties and high conduction band edge position. In the present research, CIGS thin film was successfully prepared on a transparent substrate (F:SnO2 glass) using a solution-based process and applied for a photocathode in solar water splitting, which shows control of the surface state associated with sulfurization/selenization process significantly influences on the PEC activity. A ZnS passivation surface layer was introduced, which effectively suppresses charge recombination by surface states of CIGS. The CIGS/ZnS/Pt photocathode exhibited highly enhanced PEC activity (∼24 mA·cm-2 at -0.3 V vs RHE). The performances of our CIGS photocathode on the transparent substrate were also characterized under front/back light illumination, and the incident photon to current conversion efficiency (IPCE) drastically changed depending on the illumination directions showing decreased IPCE especially under UV region with back illumination. The slow minority carrier (electron) transportation is suggested as a limiting factor for the PEC activity of the CIGS photocathode.

Electrical Characteristics of Flat Cesium Antimonide Photocathode Emitters in Panel Devices

The Cs₃Sb photocathode was formed by non-vacuum process technology. An in-situ vacuum device was fabricated successively with flat cesium antimonide photocathode emitters fabricated in a process chamber. The electrical properties of the device were characterized. Electron emission from the devices was induced by photoemitted electrons, which were accelerated by an anode electric field that was shielded from the photoemitter surface. The electrical characteristics of the devices were investigated by measuring the anode current as a function of device operation times with respect to applied anode voltages. Planar blue LED light with a 450 nm wavelength was used as an excitation source. The results showed that the cesium antimonide photocathode emitter has the potential of long lifetime with stable electron emission characteristics in panel devices. These features demonstrate that the cesium antimony photocathodes produced by non-vacuum processing technology is suitable for flat cathodes in panel device applications.

CuIn(Ga)Se2 Solar Cell Fabricated By All Wet-Processes

CuIn(Ga)Se2, I-III-IV chalcopyrite compound semiconductors, are attractive for thin film solar cells. Recently, energy conversion efficiency of CuIn(Ga)Se2 solar cells has been achieved over 20% based on vacuum process. To reduce fabrication cost, many efforts have been focused on non-vacuum process, and in this study we tried to fabricate the cells by almost all wet-processes; CuIn(Ga)Se2 absorption layers and In2S3 buffer layers were electrochemically deposited, and ZnO window layers were formed by solution based chemical methods. The deposited CuIn(Ga)Se2 absorption layers were heat-treated to improve crystallinity; however, due to surface cracks formed during heat-treatment, it was hard to adopt electrochemical method to form the absorption layers. Authors solved the problem to compensate the cracks by two-step electrodeposition process. Cd-based thin films had been used as buffer layers for a while, but nowadays many studies reported Cd-free buffer layers for CuIn(Ga)Se2 solar cells due to environmental destruction by toxicity of Cd. Therefore, we used In2S3 buffer layers forming by electrochemical method instead of Cd-bsed thin films. ZnO window layers were then deposited by solution based process in compact wire forms. We will discuss in this presentation about microstructures, chemical compositions, and light absorption abilities of each layer. And solar cell efficiency will also be presented.

The Oxide TFT with Solution Based Gate Insulator

Low temperature poly-Silicon (LTPS) thin film transistor (TFT) and Oxide TFT are developed for the better performance of the active matrix display. Recently, oxide TFT has been investigated intensively due to simple process and higher mobility than amorphous silicon TFT. Typical parameters of TFT are mobility, threshold voltage and On/Off ratio which are influenced by gate insulator. The widely used material for gate insulator is SiO2 which is rich in the earth and inexpensive. But vacuum equipments like the plasma enhanced chemical vapor deposition (PECVD) and the RF Magnetron sputter need large investment and maintenance cost than non-vacuum process. Oxide TFT of top gate structure reduces the parasitic capacitance between source-drain and gate metal. The plasma damage during deposition of gate insulator on an oxide active layer cause the defects which deteriorate TFT characteristics. However, the gate insulator by solution process reduces the defects by plasma process. SU-8 which is a negative photoresist (PR) has been being used widely in micro electro mechanical system (MEMS). It is constituted with octuple epoxy groups and includes photosensitize to react with UV light. The polymer network generated by cross-linking has mechanical and chemical stabilities and high electrical resistance. In this experiments, SU-8 was used for the gate insulator of the oxide TFT and investigated the effect of the SU-8 gate insulator on the oxide TFT. In this paper, we investigated the characteristics of gate insulator of SU-8 as changing UV exposure time to SU-8. SU-8 was coated by spin coating with 2000 rpm on a p-type Si wafer. After soft-bake for 1.5 min at 95˚C, UV of 365 nm was exposed at an energy of 320mJ/s. Exposure times were varied as 30s, 60s, 120s, and 240s. After UV exposure, hard bake was done for 20 min at 115˚C. An Al layer was deposited on SU-8, and electrical properties of the SU-8 were measured after post-annealing. Figure 1 shows the current densities of the SU-8 insulators for various exposure time from 30s to 240s. As shown in Fig. 1, the leakage currents decreased as the exposure time increased from 30s to 240s. For the gate insulator of the oxide TFT, the proper UV exposure is important. Figure 1