Spin-coated Thin Films Research Articles

Ultra-high resolution imaging of thin films and single strands of polythiophene using atomic force microscopy

Real-space images of polymers with sub-molecular resolution could provide valuable insights into the relationship between morphology and functionality of polymer optoelectronic devices, but their acquisition is problematic due to perceived limitations in atomic force microscopy (AFM). We show that individual thiophene units and the lattice of semicrystalline spin-coated films of polythiophenes (PTs) may be resolved using AFM under ambient conditions through the low-amplitude (≤ 1 nm) excitation of higher eigenmodes of a cantilever. PT strands are adsorbed on hexagonal boron nitride near-parallel to the surface in islands with lateral dimensions ~10 nm. On the surface of a spin-coated PT thin film, in which the thiophene groups are perpendicular to the interface, we resolve terminal CH3-groups in a square arrangement with a lattice constant 0.55 nm from which we can identify abrupt boundaries and also regions with more slowly varying disorder, which allow comparison with proposed models of PT domains.

A new donor-acceptor crosslinkable l-shape chromophore for NLO applications

A new nonlinear-optical (NLO) chromophore containing L-Shape 2-benzofuranyldiazene in π-electron bridge has been synthetized and its chemical and thermal properties examined. Evaluation of the second order NLO properties was performed by EFISH measurements of μβ on chloroform solution and by SHG measurements on amorphous spin-coated thin films obtained from a blend of polymer and chromophores crosslinked “in situ”. The value of 830∙10−48 esu for μβ measured on the dye and of 10 pm/V for the d33 for the poled network are in medium range for similar systems, confirming the proper employ of the molecule to build NLO active networks. The crystal structures dye and its precursor examined by single-crystal analysis show a planar geometry and bond lengths pattern confirm the push pull L-shape nature of the D-π-A systems.

Eco-Friendly Push-Coated Polymer Solar Cells with No Active Material Wastes Yield Power Conversion Efficiencies over 5.5.

Push-coating is a simple process that can be employed for extremely low-cost polymer electronic device production. Here, we demonstrate its application to the fabrication of poly(2,7-carbazole- alt-dithienylbenzothiadiazole) (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) active layers processed in air, yielding similar photovoltaic performances as thermally annealed spin-coated thin films when used in inverted polymer solar cells (PSCs). During push-coating, the polydimethylsiloxane layer temporarily traps the deposition solvent, resulting in simultaneous film formation and solvent annealing effect. This removes the necessity for a postdeposition thermal annealing step which is required for spin-coated PSCs to produce high photovoltaic performances. Optimized PSC active layers are produced with a push-coating time of 5 min at room temperature with 20 times less hazardous solvent and 40 times less active material than spin-coating. Annealed spin-coated active layers and active layers push-coated for 5 min both produce average power conversion efficiencies (PCEs) of 5.77%, while those push-coated for a shorter time of 1 min yield a slightly lower value of 5.59%. We demonstrate that, despite differences in their donor:acceptor vertical concentration gradients, unencapsulated PCDTBT:PC71BM active layers push-coated for 1 min produce PSCs with similar operational stability and upscaling capacity as thermally annealed spin-coated ones. As fast device fabrication can be achieved with short-time push-coating, we further demonstrate the potential of this deposition technique by manufacturing push-coated PSC-based semitransparent photovoltaic devices with a PCE of 4.23%, relatively neutral colors and an average visible transparency of 40.2%. Our work thus confirms that push-coating is not limited to the widely employed poly(3-hexylthiophene-2,5-diyl) but can also be used with low band gap copolymers and opens the path to low-cost and eco-friendly, yet efficient and stable PSCs.

2D positional profiling of orientation and thickness uniformity in the semiconducting polymers thin films

Abstract Harnessing the full potential of solution processable conjugated polymers (CPs) as active semiconductor elements lies in the facile thin film fabrication along with amicable control of molecular self-assembly and orientation. Probing the nature and uniformity of thin films are inevitable for fabrication of devices with high reproducibility. Herein, a new method for the fast and facile profiling of thickness and molecular orientation of large area thin films is being reported. Thin films of PBTTT-C14 fabricated by three different methods like floating film transfer method (FTM), friction transfer and spin coating were subjected to profiling of thickness and molecular orientation using 2D positional mapping system followed by fabrication of organic thin film transistors. In order to prove applicability of the mapping system on other CPs, oriented films of PQT-C12 were also prepared by FTM and optical anisotropy estimated by the mapping system (23.0) and conventional spectrophotometer (22.4) validates the performance of our positional mapping system. Spin-coated thin films of PBTTT-C14 subjected to positional profiling of the film uniformity revealed that films are non-uniform and there was a gradual increase in the thickness from center to the periphery.

Visible light active CZTS sensitized CdS/TiO2 tandem photoanode for highly efficient photoelectrochemical hydrogen generation

A tandem photoanode consisting of Cu2ZnSnS4 (CZTS) nanoparticles layer over CdS/TiO2 heterostructure thin film on FTO substrate has been fabricated for photoelectrochemical hydrogen generation. CdS thin film has been deposited by chemical bath deposition over the spin-coated TiO2 thin film, which is followed by the deposition of CZTS nanoparticles layer by spray coating technique resulting in a CZTS/CdS/TiO2/FTO photoanode. The coating of CZTS nanoparticles layer over CdS/TiO2 films resulted in the tandem structure of different band level positions, which enhances the optical absorption in the visible region and also leads to higher separation of the photogenerated charge carriers. A substantial enhancement (39 times) in the PEC activity has been demonstrated for the CZTS/CdS/TiO2/FTO photoanode as compared to TiO2/FTO photoanode, which is due to cascade band gap from 3.2 eV to 1.7 eV and formation of heterojunction at interfaces. The enhancement in the charge transport properties of CZTS/CdS/TiO2/FTO photoanode has been confirmed from the electrochemical impedance spectroscopy (EIS) measurement. The EIS results confirm that the CZTS/CdS/TiO2/FTO photoanode exhibited lowest charge transfer resistance (Rct = 302 Ω/cm2) as compared to CdS/TiO2 (Rct = 615 Ω/cm2) and TiO2 (Rct = 1700 Ω/cm2) photoanode. A mechanism depicting the enhanced the performance of photoelectrochemical (PEC) CZTS/CdS/TiO2/FTO photoanode has been proposed.

Investigation of Self-Assembly and Charge-Transport Property of One-dimensional PDI₈-CN₂ Nanowires by Solvent-Vapor Annealing.

One-dimensional (1D) nanowires have attracted great interest, while air-stable n-type 1D nanowires still remain scarce. Herein, we present solvent-vapor annealing (SVA) made nanowires based on perylene tetracarboxylic diimide (PDI) derivative. It was found that the spin-coated thin films reorganized into nanowires distributed all over the substrate, as a result of the following solvent-vapor annealing effect. Cooperating with the atomic force microscopy and fluorescence microscopy characterization, the PDI8-CN2 molecules were supposed to conduct a long-range and entire transport to form the 1D nanowires through the SVA process, which may guarantee its potential morphology tailoring ability. In addition, the nanowire-based transistors displayed air stable electron mobility reaching to 0.15 cm2 V−1 s−1, attributing to effective in situ reassembly. Owing to the broader application of organic small-molecule nanowires, this work opens up an attractive approach for exploring new high-performance micro- and nanoelectronics.

On the UV-light-induced Desorption/Adsorption Mechanism of Atmospheric Species in Solution-processed Zinc Oxide Thin Films

AbstractZinc oxide (ZnO) is a n-type transparent semiconductor which can be processed by low cost techniques (such as spray-pyrolysis and spin-coating) and can be applied as the active layer of thin-films transistors (TFTs). The electrical properties of ZnO films are strongly affected when the device is exposed to room conditions and/or UV-light, suggesting possible applications as UV or/and gas sensors. Atmospheric oxygen molecules adsorbed on ZnO surface act as charge traps, decreasing the material conductivity. The incidence of UV-light causes an increase of the material conductivity due to the photogeneration of electron-hole pairs via direct band-to-band transitions (classic photoconductivity process) and due to the desorption of oxygen molecules, which presents a relatively slower response and is a less understood mechanism. In the current paper, we study the influence of environmental parameters, such as temperature, humidity and UV-light intensity, on the electrical properties of spin-coated ZnO thin films to understand the role of the desorption mechanism on the photoconductivity process. The analysis of the device current vs. time curves shows the existence of two light-induced desorption mechanisms: i) one which increases the electrical conductivity of the ZnO film (desorption-like process) and ii) a second one which decreases the conductivity (adsorption-like process). A Plackett-Burman design of experiment (DOE) was used to study the influence of characterization factors like UV intensity, temperature and humidity on electrical parameters obtained from the experimental curves. We observed that the desorption-like process is a first order mechanism, exhibiting desorption rate proportional to n(t), where n(t) represents the adsorbate concentration as a function of the time, whereas the adsorption-like mechanism exhibits a desorption rate proportional to the forth power of n(t).

Anisotropic Proton Conductivity of Poly(aspartic acid) Thin Films

Abstract Anisotropic proton conductivity was investigated in 60-nm-thick spin-coated thin films of partially protonated poly(aspartic acid)/sodium polyaspartate (P-Asp). The film showed large conductivity difference between in-plane and through-plane directions. The in-plane and through-plane conductivities to the substrate are 2.7 × 10−6 and 3.4 × 10−9 S cm−1, respectively, at the same relative humidity (RH) of 50% and 298 K. The proton conductivity for the in-plane direction was three orders of magnitude greater than that for the through-plane direction. The anisotropic origin is derived from the reported highly oriented structure of the P-Asp thin film. The proton conductivity exhibits no difference for the two in-plane directions between radial and circumferential directions, which can be expected as anisotropic main chain orientation by a centrifugal force during spin-coating.

Spin-coated Cu2CrSnS4 thin film: A potential candidate for thin film solar cells

In this regard for the first time, the multicomponent chalcogenide Cu2(Zn1-xCrx)SnS4 (CZCrTS) nanocrystals at (0 ≤x ≤ 1) were synthesized by the solvothermal method. Improvement of structural, optical and electrical properties in Cr-doped Cu2ZnSnS4 thin film deposited on SLG substrate by spin coating technique has been investigated. It was observed that introduced the Cr element into the Cu2ZnSnS4 nanocrystal led to increasing the disorder of cations distribution within the unit cell through increasing the tensile strain inside the unit cell resulting in a decline the unit cell volume accompanied by a decrease the particle size and also the crystallite size. It has been found that the optical band gap of the CZCrTS nanocrystals ranges from 1.54 to 1.35 eV with increasing the partial substitution of Zn element with Cr element which in turn also led to a significant improvement in the crystallinity of the Cu2ZnSnS4 film, a facilitates the grain growth and also increases the grain size where accompanied this process post-sulfurization at 450 ℃ for 30 min. In addition, it was observed that the number of grains in the Cu2CrSnS4 film (x = 1), 2.79 (nm−2), were lower than that, 6.8 (nm−2) of the Cu2ZnSnS4 film. Also, the dislocation density in the Cu2CrSnS4 film (x = 1), 0.012 (nm−2), was lower than that, 0.024 (nm−2) of the Cu2ZnSnS4 film, which resulted in decreased the defects in the grain boundaries and increasing the current from 0.34 mA for the Cu2ZnSnS4 film to 0.42 mA for the Cu2CrSnS4 film. Due to a higher absorption coefficient (>104 cm−1), a higher optical conductivity (> 23 *1012 S−1) and lower transmittance have been reported for the Cu2CrSnS4 thin film (x = 1), we believe the Cu2CrSnS4 thin film may be a good candidate can be used as an absorbent layer to sunlight in thin film solar cell.

Quasi-phase-matched third harmonic generation in organic multilayers

We report the first realization of quasi-phase-matched (QPM) third harmonic generation in isotropic polymer films. Spin-coated thin films of ethyl-violet molecules dispersed in a polymer host (EV) were used as cubic nonlinear optical media because of their transparency at both the fundamental (1230 nm) and the third harmonic (410 nm) wavelengths. A passive layer of a UV-curable material was formed to compensate the phase shift between the two light waves after propagating through each EV layer. We fabricated a series of samples with 1~4 EV layers (0~3 alternatingly coated passive layers). The third harmonic output power showed a quadratic increase with the number of layers, providing a strong evidence for successful quasi-phase-matching. A conversion efficiency of 0.15% was observed with a 190 fs pulse input.

Novel Thermal Imaging Method for Rapid Screening of Drug-Polymer Miscibility for Solid Dispersion Based Formulation Development.

This study aimed to develop a rapid, simple, and inexpensive screening method for selecting the best polymeric candidates possessing high active pharmaceutical ingredient (API) miscibility during the early stages of formulation development of solid dispersion based pharmaceutical products. A new thermal imaging based method, thermal analysis by structural characterization (TASC), was used as a thermoptometric tool in conjunction with data analysis software to detect the melting point depression and postmelting dissolution of felodipine particles screened over thin spin-coated films of ten polymers commonly used in the pharmaceutical field. On the polymeric substrates the drug showed different degrees of melting point reduction, reflecting their different levels of polymer-drug miscibility. Using TASC to detect melting point depression is significantly (20-40 times) faster than the conventional DSC method without loss of the sensitivity of detection. The quantity of the material required for the screening is less than 1/1000th of the material used in conventional DSC tests, which significantly reduce the material wastage. Isothermal TASC tests and IR imaging confirmed the occurrence of thermal dissolution of the drug in the polymer for more miscible pairs. The real-time stability tests validate the accuracy of the polymer-drug miscibility screening results. These results demonstrate TASC as a promising screening tool for rapidly selecting the polymeric excipients for pharmaceutical formulations development.

Structural, morphological, optical and third order nonlinear optical response of spin-coated NiO thin films: An effect of N doping

The present work deals with the deposition of NiO and Nitrogen (N)-doped NiO thin films by sol-gel spin coating technique. Structural, morphological, linear and non-linear optical characteristics of undoped and N-doped (1–15 wt%) NiO films were studied. From XRD measurements, it is evident that single phase nano crystalline NiO is formed for all doping concentrations. Surface morphology study shows that higher concentration of N doped NiO thin films were of high quality and EDX mapping confirmed the doping of Nitrogen in films. The Raman spectra of the studied films were analyzed over the range of 1400-200 cm−1. The optical studies confirm that as doping increases, transparency of the film decreases (except at 10% N doping) and the band gap narrows. Nonlinear parameters such as refractive index and susceptibilities also depend on N dopant concentration. Z-scan studies viz., absorption index, nonlinear refractive index were carried out on undoped and N doped NiO samples and the results were matched with theoretical calculated values.

Enhancing the performance of solution-processed organic thin-film transistors by blending binary compatible small molecule semiconductors

Abstract Physical blending is a facile and effective way to improve the performance of solution processed organic thin-film transistors (OTFTs). Blending small molecule semiconductors with soluble polymers has been extensively studied in recent years. However, blending between binary small molecule semiconductors is rare due to the difficulty to obtain ideal thin films. Herein, we systematically investigate the blending effects on the morphologies of thin films and their field-effect performance by using two small molecule semiconductors, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, (C12-Ph-BTBT), which have the same aromatic skeleton. Molecular ordering and better crystallinity are observed in most of spin-coated blend thin films, thanks to the enhanced molecular interaction after blending. As a result, OTFTs based on blend thin films exhibit improved performance in most cases, with the highest average hole mobility about 1.5 cm2 V−1 s−1 demonstrated. Further device performance improvements are demonstrated by blending polystyrene with Ph-BTBT and C12-Ph-BTBT blends. The results here indicate that blending between small molecule semiconductors with compatible fused ring structures may be a promising strategy to enhance the performance of organic transistors.

Sulfurization of hematite Fe2O3 and anatase TiO2 by annealing in H2S

Abstract Spray-deposited hematite Fe2O3 and spin-coated anatase TiO2 thin films were annealed under H2S in order to investigate the effect of sulfurization by H2S. Sulfurized films were characterized using UV–vis spectrophotometry, X-ray diffractometry, Raman spectroscopy, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Sulfurized Fe2O3 and TiO2 films are composed of multiple phases. The Fe2O3 film undergoes phase transformation from hematite into a mixture of magnetite (Fe3O4) and pyrrhotite (FeS) at annealing temperatures above 250 °C. At 450 °C α-Fe2O3 is fully transformed into pyrite (FeS2). The TiO2 film transforms into a mixture of TiO2 and TiS2. TiS2 formation begins at temperatures above 550 °C. At 700 °C, TiO2 is completely transformed into TiS2.

Direct Observation of Conformational Relaxation of Polymer Chains at Surfaces.

Sum-frequency generation spectroscopy was employed to follow the conformation evolution of polystyrene chains at the surface of a spin-coated film in a temperature-ramping mode as well as under isothermal annealing. The conformation of surface chains in an as-cast film was observed to be in a nonequilibrium state, in accordance with reported results for polymer chains in thin spin-coated films. While the relaxation of surface nonequilibrium chains was induced by the enhanced surface mobility, the whole chain motion such as reptation might be a key factor in determining the time scale for equilibrating the surface chain conformation.

Characterization and third harmonic generation calculations of undoped and doped spin-coated multilayered CuO thin films

The paper presents the physical characterizations of undoped and (Sn, Mn, Fe and In) doped Cupric Oxide (CuO) thin films prepared by sol-gel spin-coating process. The first aim of the work is to investigate the relation between the electrical resistivity and the third order nonlinear optical susceptibility of pure deposited CuO films, second is to explore the impact of introducing diverse dopants on the nonlinear optical responses. In this study, we demonstrate that we can control the electrical and also the nonlinear optical properties of the obtained structure using a different number of deposited layers. Moreover, the influence of doping due to variations in the microstructure and morphology is examined and discussed.

Ultrasensitive Heterojunctions of Graphene and 2D Perovskites Reveal Spontaneous Iodide Loss

Summary Despite the demonstrated high efficiency of perovskite solar cells and light-emitting devices, the understanding of the intrinsic stability of perovskites is far from complete. In this work, using an ultrasensitive, exfoliated 2D perovskite single-crystal sheet/graphene heterostructure device, we reveal spontaneous iodide loss as an important degradation pathway of 2D perovskite single crystals, which n-dopes the perovskite semiconductor by generating positively charged iodide vacancies. Furthermore, we show that covering perovskites with graphene can suppress the iodide loss, significantly improving perovskite stability. A perovskite phototransistor is demonstrated with a graphene/2D perovskite/graphene structure, which shows no degradation after 75 days. Our work not only provides important insights for future stable perovskite optoelectronic device development, but also demonstrates the potential of graphene as a promising sensitive diagnostic tool for device and material degradation studies.

Deposition and characterization of spin-coated n-type ZnO thin film for potential window layer of solar cell

Spin-coating technique has been utilized to deposit ZnO thin films on ITO substrates. The dependence of the structural, optical, and electrical properties of the prepared films on both the sol–pH values and annealing temperatures has been investigated. X-ray diffraction analysis has revealed that the optimal sol–pH value was found to be 8 at annealing temperature 450 °C. At these conditions, the optical transmission and reflection data have also shown a clear enhancement in both the morphology and the grain size of the films. In addition, the analysis of I–V and C–V measurements of the Schottky Au/ZnO diode has shown a high agreement with both the investigated structural and optical properties. Furthermore, a new empirical method based on the calculated electrical parameters in the context of both the thermionic emission and Mott–Schottky models has been proposed to evaluate the approximate value of the electron effective mass in the conduction band of the ZnO (m* = 0.224 mo).

Characterization of low-temperature solution-processed LiCoO2 thin-film cathode with molecular weight control of polyvinylpyrrolidone

Numerous LiCoO2 cathode thin-films have been prepared for all-solid-state thin-film battery applications. Preparation methods include sputtering, pulsed laser deposition, electron beam evaporation, screen printing, and spin coating. When compared to vacuum deposition methods, the solution-processed fabrication methods could result in significant cost savings and large-area production. Unfortunately, high-temperature annealing (Tanneal > 700 °C) of LiCoO2 cathode thin film is usually crucial for achieving a high performance layered phase. Here, we report on phase control of spin-coated LiCoO2 thin film by varying the molecular weight of polyvinylpyrrolidone (PVP) at low temperature. We could achieve thick and smooth film morphology by optimizing the PVP molecular weight. This method could be a promising cathode layer for low-cost thin-film batteries.

Effects of coating cycles on spin-coated indium nitride thin films

ABSTRACTIndium nitride (InN) thin films were prepared on aluminium nitride/p-silicon (111) substrates via sol–gel spin coating followed by nitridation. The InN thin films deposited at different coating cycles (10, 20 and 30) were analysed, and the growth mechanism was discussed in depth. The film thickness was enhanced with increasing number of coating cycle. X-ray diffraction showed that the crystalline structure of InN was improved at 20 coating cycles and started to degrade at 30 cycles. The diffusion path of the active nitrogen atoms into the deeper film was limited by the low growth temperature and led to gradual surface saturation. The film deposited at 20 cycles showed densely packed InN grains. Through the proposed approach, an InN optical band gap of 1.71 eV was achieved. It can be deduced that the variation in the number of coating cycles has significantly affected the film properties.