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The degradation and stability behavior of AISI 4130 steels have been examined under high-temperature dynamic plasma flame environments. A Fe2Al5 layer was formed as a result of aluminizing on the steel. An Al2O3 layer was produced on top of the Fe2Al5 layer, when the coated steel was oxidized at 500 °C (pre-oxidation treatment). The uncoated steels and the Fe2Al5 coated steels (i.e., the aluminized steels) showed large weight loss and complete changes in their original shapes due to their melting during the plasma flame tests at 1480 °C. However, the steels with the Al2O3 and Fe2Al5 (pre-oxidation treatment) were very stable without showing noticeable weight loss even after exposures to the plasma flame tests mainly due to the presence of the Al2O3 layer. SEM and XPS analyses were used to provide the presence of Fe2Al5 and Al2O3 phases on the surface. The effects of synthesized surface layers of the steels on high-temperature plasma flame exposure are discussed together with the surface morphology and microstructural observations.

Halide perovskite solar cells have been attracting tremendous attention as next-generation solar cell materials because of their excellent optical and electrical properties. Formamidinium lead tri-iodide (FAPbI3) exhibits the narrowest band gap among lead iodide perovskites and shows excellent thermal and chemical stability, also. However, the large-area coating of FAPbI3 needed for commercialization has not been successful because of the instability of the black phase of FAPbI3 at ambient temperature. This study presents a compositional engineering direction to control the polymorph of the FAPbI3 thin film for the shear coating processes, without halide mixing. By adopting a hot substrate above 100 oC, our shear coating process can produce the black phase FA-based halide perovskites without halide mixing. We carefully investigate the Cs-FA and MA-FA mixed lead iodide perovskites’ phase stability by combining the study with thin-film fabrication and ab initio calculations. Cs-FA mixing shows promising behaviors for stabilizing α-FAPbI3 (black phase) compared with MA-FA. Stable FA-rich perovskite films cannot be achieved via shear coating processes with MA-FA mixing. Ab initio calculations revealed that Cs-FA mixing is excellent for inhibiting phase decomposition and water incorporation. This study is the first report that FA-based halide perovskite thin films can be made with the shear coating process without MA-Br mixing. We reveal the origin of the stable film formation with Cs-FA mixing, and present future research directions for fabricating FA-based perovskite thin films using shear coating.

CdS has been known to be one of the best junction partners for Cu(In,Ga)Se2 (CIGS) in CIGS solar cells. However, the use of thick CdS buffer decreases the short-circuit current density of CIGS solar cells. There are two obstacles that limit the use of ultrathin CdS. The first is plasma damage to CIGS during the preparation of transparent conducting windows and the second is a low shunt resistance due to the direct contact between the window and CIGS via pinholes in the thin CdS buffer. In other words, to avoid plasma damage and shunt paths, we may have to use a CdS buffer that is thicker than necessary to form a high-quality CdS/CIGS junction. This work aims to determine how thin the CdS buffer can be employed without sacrificing device performance while also eliminating the above two obstacles. We investigate the effect of CdS thickness on the performance of CIGS solar cells with silver nanowire-based window layers, which can eliminate both obstacles. An approximately 13 nm thick CdS buffer allows us to achieve high short-circuit current density and fill factor values. To attain an even high open-circuit voltage, an additional CdS buffer with a thickness of 13 nm is needed. The data from this study imply that an approximately 26 nm thick CdS buffer is sufficient to form a high-quality CdS/CIGS junction.

In this study, we tried to improve the oxidation resistance of Nb-12Si (wt%) alloys at 1200 °C or higher through pack cementation coatings. Nb-12Si (wt%) alloys were prepared by arc-melting under Ar atmosphere. When the alloys were coated using pack powder mixtures composed of Si, Al2O3 and NaF, two silicide layers composed of NbSi2 and Nb5Si3 phases were successfully produced on the substrate. The Si-pack coatings were performed with various heat treatment temperatures and time conditions. The microstructures and thickness changes of the coating layers were analyzed to determine the growth behaviors of the coating layer. The growth constant of 8.4 10–9 cm2/sec was obtained with a diffusion growth mode. In addition, in order to examine the resistance of the Si-pack coated alloys, isothermal static oxidation tests were performed at 1200 °C and higher temperatures. As a result, the oxidation resistance of the alloys was determined by protecting the surface of the alloys with silicide oxide layers formed by the silicide coatings. The uncoated specimens exhibited an abnormal weight increase due to the formation of Nb oxide. The coated specimen showed excellent oxidation resistance at 1200 °C for up to 12 hrs, while the previous reports on the same alloy verified oxidation resistance only up to 1100 °C. It appears that the excellent oxidation resistance is closely related to the NbSi2 coating layer thickness. The oxidation behaviors of the coating layers after the oxidation tests were discussed in terms of microstructural and phase analyses.

We report the relation between the catalyst patterning conditions and the intensity of the 1st order Raman active modes in Au-catalyzed GaAs nanowire bundles. We fabricated e-beam lithographically Au-patterned GaAs(111)B substrates by varying the patterning conditions (e-beam dose rate, dot-size and interdot-spacings), and grew GaAs nanowires via vapor–liquid–solid process using a solid-source molecular beam epitaxy. To understand the effects of the substrate preparation conditions and resulting morphologies on the optical characteristics of 1st order transverse optical and longitudinal optical phonon modes of GaAs, we characterized the nanowire bundles using complementary μ-Raman spectroscopy and scanning electron microscopy as a function of the e-beam dose rate (145–595 μC/cm2), inter-dot spacing (100 and 150 nm) and pattern size (100 and 150 nm). Ensembles of single crystalline GaAs nanowires covered with different Au-thickness exhibit a downshift and asymmetric broadening of the 1st order transverse optical and longitudinal optical phonon peaks relative to GaAs bulk modes. We also showed that the sensitivity of a downshift and broadening of Raman spectra are directly related to morphological and surface coverage variations in as-grown nanowires. We observed clear increases of the transverse optical and longitudinal optical intensity as well as the relatively higher peak shift and broadening of Raman spectra from the 100 nm patterning in response to the dose rate change. Strong dependence of Raman spectra of the nanowire bundles on the e-beam dose rate changes are attributed to the variations in spatial density, size, shape and random growth orientation of the wires. We have shown that the identification of the changes in GaAs longitudinal optical and Arsenic anti-site peaks is good indicators to characterize the quality of as-grown GaAs nanowires. Our finding confirms the utilization of Raman spectroscopy as a powerful tool for characterizing chemical, structural, and morphological information of as-grown nanowires within the supporting substrate.

ZnO thin films are of considerable interest because they can be customized by various coating technologies to have high electrical conductivity and high visible light transmittance. Therefore, ZnO thin films can be applied to various optoelectronic device applications such as transparent conducting thin films, solar cells and displays. In this study, ZnO rod and thin films are fabricated using aqueous chemical bath deposition (CBD), which is a low-cost method at low temperatures, and environmentally friendly. To investigate the structural, electrical and optical properties of ZnO for the presence of citrate ion, which can significantly affect crystal form of ZnO, various amounts of the citrate ion are added to the aqueous CBD ZnO reaction bath. As a result, ZnO crystals show a nanorod form without citrate, but a continuous thin film when citrate is above a certain concentration. In addition, as the citrate concentration increases, the electrical conductivity of the ZnO thin films increases, and is almost unchanged above a certain citrate concentration. Cu(In,Ga)Se2 (CIGS) solar cell substrates are used to evaluate whether aqueous CBD ZnO thin films can be applicable to real devices. The performance of aqueous CBD ZnO thin films shows performance similar to that of a sputter-deposited ZnO:Al thin film as top transparent electrodes of CIGS solar cells.

Silver nanowire (AgNW) networks have been adopted as a front electrode in Cu(In,Ga)Se2 (CIGS) thin film solar cells due to their low cost and compatibility with the solution process. When an AgNW network is applied to a CIGS thin film solar cell, reflection loss can increase because the CdS layer, with a relatively high refractive index (n ~ 2.5 at 550 nm), is exposed to air. To resolve the issue, we apply solution-processed ZnO nanorods to the AgNW network as an anti-reflective coating. To obtain high performance of the optical and electrical properties of the ZnO nanorod and AgNW network composite, we optimize the process parameters – the spin coating of AgNWs and the concentration of zinc nitrate and hexamethylene tetramine (HMT – to fabricate ZnO nanorods. We verify that 10 mM of zinc nitrate and HMT show the lowest reflectance and 10% cell efficiency increase when applied to CIGS thin film solar cells.

The highest efficiency of 4.225% for vapor-transport-deposited (VTD) SnS absorber/CdS heterojunction solar cells with good long-term stability over two years is reported. These improved characteristics are attributed to the reduction in interface defects of the heterojunction, which can occur either during the deposition of transparent electrodes or direct annealing of the SnS/CdS heterojunction at 300 °C. The enhanced heterojunction interface quality is supported by the substantially reduced dark saturation current density and shunt conductance of the fabricated devices measured under dark conditions. Although the SnS/CdS device exhibits efficiency over 4%, significant short-circuit current loss mainly due to recombination was revealed by quantum efficiency and optical analysis. Admittance analysis shows the presence of numerous defect densities of Sn and S vacancies (>1017 cm−3) in the VTD-grown SnS absorber. Minimizing bulk and interface defect densities of SnS absorbers will be a key issue in achieving high efficiency (>5%) of SnS-based thin-film solar cells.

We report optical phonon vibration modes in ensembles of self-catalyzed InP/InAs/InP multi core-shell one-dimensional nanostructures (nanopillars and nanocones) grown on InP(111)B substrates using liquid indium droplets as a catalyst via metal-organic chemical vapor deposition. We characterized the Raman vibration modes of InAs E1(TO), InAs A1(TO), InAs E1(LO), InP E1(TO), InP A1(LO), and InP E1(LO) from the ensemble of as-grown nanostructures. We also identified second-order Raman vibration modes, associated with InP E1(2TO), E1(LO+TO), and E1(2LO), in the InP/InAs/InP core-shell nanopillars and nanocones. Raman spectra of InP/InAs/InP nanopillars showed redshift and broadening of LO modes at low-frequency branches of InAs and InP. Due to the polar nature in groups III–V nanowires, we observed strong frequency splitting between InAs E1(TO) and InAs A1(LO) in InP/InAs/InP nanocones. The Raman resonance intensities of InP and InAs LO modes are found to be changed linearly with an excitation power. By tilting the substrate relative to the incoming laser beam, we observed strong suppression of low-frequency branch of InP and InAs LO phonon vibrations from InP/InAs/InP nanocones. The integrated intensity ratio of InP E1(TO)/E1(LO) for both nanostructures is almost constant at 0-degree tilt, but the ratio of the nanocones is dramatically increased at 30-degree tilt. Our results suggest that Raman spectroscopy characterization with a simple substrate tilting method can provide new insights into non-destructive characterization of the shape, structure, and composition of the as-grown nanostructures for the wafer-scale growth and integration processing of groups III–V semiconducting hetero-nanostructures into nanoelectronics and photonics applications.

High-speed continuous deposition of solution-processed perovskite thin films with controlled microstructural morphology has been intensively pursued toward practical commercialization of high-efficiency perovskite solar cells. In order to meet this urgent demand in a timely manner, we present a rapid (~5 mm·s−1) scalable meniscus-controlled coating method to deposit large-grained (>100 μm) perovskite thin films over large area. Shearing movement of the meniscus of perovskite solution, confined by the hydrophobic-treated top and temperature-controlled bottom plates enables a rapid crystallization of perovskite grains, also facilitating the continuous influx of solutes at the meniscus for continuous film growth with less consumption of perovskite solution. Optical and structural analyses revealed that the rapid-shear-deposited perovskite thin films possess a large-grained, densely-packed and highly-crystalline phase. Consequently, perovskite solar cell based this film exhibits the power conversion efficiency of 10.9% even without compositional engineering and additive controls. Therefore, it can be claimed that this rapid shearing deposition of perovskite solution paves a way to achieve a cost-competitive production of the high-efficiency solar cell.

Transparent conducting electrodes are essential components in various optoelectrical devices. Although indium tin oxide thin films have been widely used for transparent conducting electrodes, silver nanowire network is a promising alternative to indium tin oxide thin films owing to its lower processing cost and greater suitability for flexible device application. In order to widen the application of silver nanowire network, the electrical conductance has to be improved while maintaining high optical transparency. In this study, we report the enhancement of the electrical conductance of silver nanowire network transparent electrodes by copper electrodeposition on the silver nanowire networks. The electrodeposited copper lowered the sheet resistance of the silver nanowire networks from 21.9 Ω/□ to 12.6 Ω/□. We perform detailed X-ray diffraction analysis revealing the effect of the amount of electrodeposited copper-shell on the sheet resistance of the core-shell(silver/copper) nanowire network transparent electrodes. From the relationship between the cross-sectional area of the copper-shell and the sheet resistance of the transparent electrodes, we deduce the electrical resistivity of electrodeposited copper to be approximately 4.5 times that of copper bulk.

We report the effect of morphology and substrate of self-catalyzed indium phosphide (InP) nanostructures on phonon vibration modes. Using liquid indium as a catalyst, we grew self-catalyzed InP nanocones and nanopillars on single crystal substrates of InP(111)B, Si(111), and Si(100) via metal-organic chemical vapor epitaxy. Due to crystal symmetry breaking in one-dimensional nanostructure, longitudinal-optical (LO) and transverse-optical (TO) phonon modes are clearly resolved with the strong anisotropic behavior. Broadening and downshift of LO phonon modes are found to be sensitive to the morphology (i.e., aspect ratio and surface-to-volume ratio) and crystal structure (i.e., Wurtzite and Zinc Blende) of the as-grown nanostructures. This work demonstrated that Raman spectroscopy provides statistical insights on the quality of as-grown nanostructures (i.e., growth orientation, crystal structures, and the presence of structural defects) without destroying samples.

Silver nanowire (AgNW) electrodes have been employed in solar cells as transparent conducting electrodes (TCEs). When a AgNW electrode is used as a network-type TCE in solar cells, photogenerated charge carriers must laterally travel to reach AgNW networks. If the empty space of the network is larger than the lateral collection length of the charge carriers ( L lc ), a significant fraction of the charge carriers will be lost. Therefore, determination of L lc is essential for choosing and/or designing a suitable network for solar cells. Here, we develop a method to relate the lateral photocurrent measured from a specially designed pattern to the L lc value. We apply this method to Cu(In,Ga)Se2 thin-film solar cells with AgNW TCEs. The L lc value was determined to be ≈ 26 μm in the CdS/CIGS/Mo structure under 1-sun illumination. The measured lateral collection length is much larger than the normal spacing between AgNWs, indicating a high lateral collection efficiency in AgNW TCE-based CIGS thin-film solar cells.

The authors have recently reported the enhanced thermal stability of silver nanowire (AgNW) network transparent electrodes by electrodeposition method [1]. AgNW networks are known to break into droplets at elevated temperatures (spherodization temperature) that are still much lower than the bulk Ag melting temperature. This phenomenon is known as Rayleigh instability. As the diameter of individual AgNW in the network increases by electrodeposited Ag on the AgNW surface, the thermal stability of AgNW network can be enhanced. Here, we provide the data on the spherodization temperature depending on AgNW diameter. We also report the calculated activation energy required to induce the spherodization of AgNW network.

We demonstrate the ability to fabricate high-quality nanoscale electrical contact between silver nanowires (AgNWs) and underlying semiconducting layers in chalcogenide thin film solar cells. AgNW electrodes have attracted many interests due to their ability for low temperature solution processing. However, they have a drawback that the interfacial defects can be generated between AgNWs and underlying rugged semiconductor layers making it difficult to form high-quality junction. To enhance the junction properties, conducting matrix layers have been adapted. Yet, the issues regarding the AgNW/semiconductor junction have not been fully resolved. We developed a facile method to form robust nanoscale contact between AgNWs and semiconducting thin films to achieve high performance chalcogenide thin film solar cells. The method is to deposit an ultra-thin semiconductor layer on devices using aqueous chemical bath deposition. The chemical bath deposition has capability to effectively fill even nanoscale gap and to form chemically stable bonds as well as an intimate junction. As a proof of concept, a CdS layer (~ 10 nm) was deposited using the chemical bath deposition on Cu(In,Ga)Se2 (CIGS) solar cells with a structure of AgNW/CdS/CIGS/Mo/Glass. We also identified that the key factor governing the current-voltage characteristic is the electrical contact between the AgNW electrode and the CdS buffer layer in CIGS thin film solar cells. The power conversion efficiency of the CIGS cell was dramatically improved from 4.9% to 14.2% owing to high-quality AgNW-CdS electrical contact produced by chemical bath deposition of the additional CdS layer as thin as 10 nm.

Ti-6Al-4V(Ti64) alloy has excellent mechanical properties and corrosion resistances, but poor oxidation properties at high temperatures. It is necessary to improve the oxidation resistances of the alloy in order to expand its application in the aerospace and automotive fields. This study aimed to improve oxidation resistances through diffusion coatings on the surface of the alloy. Aluminide coatings on Ti64 alloys have been investigated via pack aluminium process at temperature ranges of 400–800 °C. Al and/or TiAl3 layers were formed at the interface between the Ti64 alloy and the Al coating layer, when the Ti64 alloys were heat-treated at 400 and 500 °C. TiAl3 layer was manufactured on Ti64 alloys at 600, 700 and 800 °C. The growth of the coated TiAl3 layer followed diffusional growth manner, and the estimated value of the nucleation barrier was the lowest among the intermediate phases of the Ti-Al system. The coating kinetics were discussed together with free energy diagram observations. The activation energy for growth of the TiAl3 layer was estimated as ~82 kJ/mol in the experimental temperature ranges. At the same time, isothermal oxidation tests of the bare Ti64 alloy and the aluminized Ti64 alloys were performed at 1000 °C. By confirming the diffusion pathway of the Ti-Al-O ternary system, the oxidation behaviours and resistances of the alloys were compared and analysed.

To achieve a surface-enhanced Raman spectroscopy (SERS) sensor that is easy to make and use, we propose periodic gold nanotrench arrays, which can be fabricated without surface contamination and intricate sensor alignment. Deep and narrow plasmonic nanotrenches for amplifying local electromagnetic fields were reliably generated on a wafer-scale substrate by nanoimprint lithography and two successive oblique-angle depositions. Electromagnetic simulations and Raman measurements show that the proposed plasmonic nanostructures function as SERS sensors, enabling nanomolar sensitivity. Furthermore, we successfully confirmed the microRNA detection capability of the proposed nanostructures to demonstrate their promising potential and feasibility for use in biomedical diagnostic sensors.

We quantitatively and analytically investigate the properties of buffer/window junctions and their effects on the energy band alignment and the current-voltage characteristics of Cu(In,Ga)Se2 (CIGS) thin film solar cells with solution processed silver nanowire (AgNW) composite window layers. AgNWs are generally embedded in a moderately conductive matrix layer to ensure lateral collection efficiency of charge carriers photogenerated in the lateral gaps present between AgNWs. Studies on the junctions between a buffer and AgNW-composite window layers and their effects on the performances of CIGS thin film solar cells have seldom been addressed. Here, we show that solution processed AgNW-composite window layers could induce defect states at the buffer/window interface, resulting in poor energy band alignment impeding carrier transport in the solar cells. On the basis of our analysis, we suggest an analytical expression of nmatrixDi2≥3.46×10−5e cm to avoid losses in the power conversion efficiency of the solar cells. nmatrix is the carrier concentration in a matrix layer embedding AgNWs, Di is the negative defect density at the buffer/window interface, and e is the relative dielectric constant of the matrix layer embedding AgNWs.

We report the systematic investigation of the surface optical phonon modes in Au-catalyzed GaAs nanowires grown on an Au pre-patterned GaAs(111)B substrate using μ-Raman spectroscopy. We employed electron-beam dose rate as a control parameter during the substrate patterning step for adjusting the nanowire base diameter and coverage, which are independent from the nanowire growth conditions. We have experimentally studied the effect of the fill factor and average diameter on the surface optical phonon modes and explained the red-shift and broadening of the surface optical phonon frequencies by employing the dielectric continuum model. The surface optical phonon mode shift is exhibited to be sensitive to fill factor, rather than base diameter. The decrease in the average diameter from 280 nm to 180 nm results in the asymmetric broadening and red-shift of the surface optical phonon frequency (~1.83 cm−1) but the theoretical calculation from the isolated single nanowire-based dielectric continuum model cannot solely explain the behaviors of the surface optical phonon mode. In contrast, the change in the fill factor from 0.01 to 0.83 results in a shift of the surface optical phonon frequency (~6.5 cm−1) from the GaAs bulk value. The red-shift and asymmetric broadening of the surface optical phonons, in an agreement with the Maxwell-Garnett approximation, are consequences of dipolar interaction of randomly aligned neighboring nanowires and the polar nature of GaAs nanowire bundles. This work suggests the pre-patterning parameter dependent surface optical phonon characteristics of GaAs nanowire bundles which are of great importance in the nondestructive characterization of low-dimensional opto-electronic materials and devices.

We report the relation between the catalyst patterning conditions and the intensity of the 1st order Raman active modes in Au-catalyzed GaAs nanowire bundles. We fabricated e-beam lithographically Au-patterned GaAs(111)B substrates by varying the patterning conditions (e-beam dose rate, dot-size and interdot-spacings), and grew GaAs nanowires via vapor-liquid-solid process using a solid-source molecular beam epitaxy. To understand the effects of the substrate preparation conditions and resulting morphologies on the optical characteristics of 1st order transverse optical and longitudinal optical phonon modes of GaAs, we characterized the nanowire bundles using complementary

Silver nanowire (AgNW) networks have demonstrated high optical and electrical properties, even better than those of indium tin oxide thin films, and are expected to be a next-generation transparent conducting electrode (TCE). Enhanced electrical and optical properties are achieved when the diameter of the AgNWs in the network is fairly small, that is, typically less than 30 nm. However, when AgNWs with such small diameters are used in the network, stability issues arise. One method to resolve the stability issues is to increase the diameter of the AgNWs, but the use of AgNWs with large diameters has the disadvantage of causing a rough surface morphology. In this work, we resolve all of the aforementioned issues with AgNW TCEs by the electrodeposition of Ag onto as-spin-coated thin AgNW TCEs. The electrodeposition of Ag offers many advantages, including the precise adjustment of the AgNW diameter and wire-to-wire welding to improve the junction conductance while minimizing the increase in protrusion height because of the overlap of AgNWs upon increasing the diameter. In addition, Ag electrodeposition on AgNW TCEs can provide higher conductance than that of as-spin-coated AgNW TCEs at the same transparency because of the reduced junction resistance, which generates a superior figure of merit. We applied the electrodeposited (ED) AgNW network to a Cu(In,Ga)Se2 thin-film solar cell and compared the device performance to a device with a standard sputtered transparent conducting oxide (TCO). The cell fabricated by the electrodeposition method showed nearly equal performance to that of a cell with the sputtered TCO. We expect that ED AgNW networks can be used as high-performance and robust TCEs for various optoelectronic applications.

Research is being conducted on Mo- and Nb-based alloys that are used in the aerospace sector, including those used for advanced gas turbines and aircraft engines. There is a limit to using Mo, which has a high density among refractory metals, and a few studies exist describing the addition of Nb to Mo&ndash, silicide alloys. There is a lack of guidance research on the basic Nb:Mo ratio of alloys, and it is necessary to study how to improve oxidation resistance. Therefore, this study aims to improve oxidation resistance by controlling the ratio of Nb and Mo in (Nbx, Moy)Si2 coating layers with Si pack cementation coatings on Nb&ndash, Mo alloys. Static oxidation tests were carried out at 1200 &deg, C for 6 h to confirm the oxidation characteristics. As a result, a SiO2 or SiO2 + Nb2O5 ceramic protective layer was formed on the surface. After the oxidation tests, alloys with a Nb content of less than 35 at.% were found to protect the surface. The ratios of Nb and Mo in the Nb&ndash, Mo alloy and silicide coating layer were compared, and the improvement of oxidation resistance is discussed in terms of microstructural evolution.

In solar-cell technology, band-gap grading has been introduced to enhance the efficiency of charge collection, by inducing drift motion of charge carriers. Characterizing such systems accurately is tricky, though, due to technical difficulty in clearly separating carrier diffusion from drift. This study uses wavelength-dependent lateral photocurrent in a generic device structure to simultaneously characterize the electrical qualities of both the light absorber and the back surface in a graded-band-gap solar cell. The technique is readily applicable to many types of photoactive devices, because it works well without any knowledge of the actual band-gap grading.

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Recently Organic-inorganic halide perovskite (OIHP) solar cells have become one of the most promising next-generation photovoltaic materials due to their excellent electrical and optical properties. Formamidinium lead triiodide (HC(NH2)2PbI3, FAPbI3) exhibits the narrowest band gap (~1.45 eV) among Pb-based halide perovskites, which is closer to Shockley-Queisser limit than that (~1.55 eV) of Methylammonium Lead Iodide (CH3NH3PbI3, MAPbI3). Therefore, FAPbI3 has been widely adopted for recently developed highly efficient photovoltaic cells. In addition, FAPbI3 has better stability than MAPbI3, which can be easily decomposed at a high temperature or humid environment. The spin-coating has been widely adopted to make FAPbI3-based solar cells, whereas the large-area coating technologies have not been extensively studied, yet. This study presents the compositional engineering research on the polymorph control of formamidinium-based lead halide perovskites for the large-area film coating. The polymorph control of FAPbI3 has been one of the most important issues since its first appearance in photovoltaic applications. Non-perovskite structure (δ-FAPbI3) is thermodynamically more stable than the three-dimensional perovskite structure (α-FAPbI3). We adopted the shear coating method to make perovskite layers and controlled the polymorphism through A-site mixing. MA, commonly used A-site molecule, is excluded because MA can be a weak spot at the humid condition. Br mixing is not applied due to the photo-induced phase separation issue. Our results reveal that double-cation mixing (Cs-FA or Rb-FA) is not enough and triple-cation mixing (Rb-Cs-FA) cannot be avoided to make robust α-FAPbI3 for the shear coating process. We present the ternary phase diagram of (Rb-CS-FA)PbI3 to show the process window based on the XRD analysis of mixed perovskite films. The photovoltaic cell shows 15.8% power conversion efficiency (PCE) at the best compositional mixing point, which is the best PCE for FA-based perovskite solar cells fabricated by shear coating method without MA-Br mixing.

Additional file 1: Figure S1. InP(111)B “nanoisland” reference substrate, treated with high temperature deposition of InP nanostructure, exhibiting preferred lateral growth to vertical growth. Figure S2. Effect of excitation power on Raman spectra of InP/InAs/InP nanopillar and nanocone for two substrate tilting angles (0 and 30 degrees).

This work was supported by NSF Grants DMR 1207213, DMR1400432, and EFRI-2DARE 1433490, and by LEAST-STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA and by SRC NRI SWAN. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Nos. NRF-2018R1C1B5085644 and NRF-2019R1A2B5B02070657).

Metal nanowire networks represent a promising candidate for the rapid fabrication of transparent electrodes with high transmission and low sheet-resistance values at very low deposition temperatures. A commonly encountered challenge in the formation of conductive nanowire electrodes is establishing high-quality electronic contact between nanowires to facilitate long-range current transport through the network. A new system involving nanowire ligand removal and replacement with a semiconducting sol-gel tin oxide matrix has enabled the fabrication of high-performance transparent electrodes at dramatically reduced temperatures with minimal need for post-deposition treatment.

Sn–Ag alloys are gaining importance among microelectronics manufacturers because of their potential benefits, e.g., low melting point, excellent wetting to common metallic substrates, and superior mechanical properties required for microelectronic packaging devices. In this work, Sn–Ag alloy was co-deposited from acidic citric acid bath on copper substrates. The plating solution was composed of SnSO4, Ag2SO4, thiourea, and citric acid as a supporting electrolyte. Further, the influence of current density on the Ag content of the coatings was studied. The results indicate that the citric acid bath produces bright Sn–Ag deposits over a wide range of current densities. The grains of the deposits obtained were bigger at lower current densities (10–30 mA cm−2) and become finer at higher current densities (40–50 mA cm−2). Initially, the coatings were very hard and become soft as the current density increases gradually due to the decreased amount of Ag in the deposit which modified the coating solderability.

Metal halide perovskites (MHPs) are one of the promising candidates for next-generation photovoltaic materials. Since the Miyasa group first adopt methylammonium lead tri-iodides as a sensitizer of dye-sensitized solar cells, the efficiency of MHP solar cells has improved at an unprecedented speed. According to the recent report of national renewable energy laboratory, the certified power conversion efficiency (PCE) of MHP solar cells exceeded 23%. However, stability and toxicity are primary concerns for MHP solar cell commercialization. Among various efforts to develop Pb-free perovskite solar cells, Sn-based materials have shown the PCE. However, the stability of Sn-based perovskites is worse than Pb-based solar cells, and the multivalent features of Sn are considered to be the origin of the unstable performance. Compositional engineering and additive control can be the effective ways to develop highly stable and efficient Pb-free perovskite solar cells. This study presents the effects of compositional engineering and various additives on the stabilization of Pb-free MHP materials via ab initio thermodynamics calculations. The concept of the ‘anti-stabilizer’ for the proper selection of the preferred phase is suggested by our previous report[1]. We present that the anti-stabilizing concept can be successfully applied to stabilize the proper band gap polymorph of black CsSnI3[2]and Bi-based layered perovskites. Additionally, the roles of additives including small alkali metal interstitials are discussed concerning thermodynamics. [1] J. Jeon, T. Eom, E. Lee, S. Kim, S. Kim, K.-H. Hong, H. Kim, J. Phys. Chem. C 121 (2017) 9508–9515. [2] J.H. Heo, J. Kim, H. Kim, S.H. Moon, S.H. Im, K.-H. Hong, J. Phys. Chem. Lett. 9 (2018) 6024–6031.