Fabrication of a novel organic polymer thin film

Fabrication of organic polymer thin films and organic semiconductors are critical for the development of sophisticated organic thin film based devices. Radio Frequency plasma polymerisation is a well developed and widely used fabrication technique for polymer thin films. This paper describes the fabrication of an organic polymer thin film from a monomer based on Lavandula angustifolia. Several polymer thin films were manufactured with thicknesses ranging from 200 nm to 2400 nm. The energy gap of the polymer thin film was measured to be 2.93 eV. The refractive index and extinction coefficient was determined to be 1.565 (at 500 nm) and 0.01 (at 500 nm) respectively. The organic polymer thin film demonstrates the possibility of an environmentally friendly, cost effective organic semiconductor.     

Development of low temperature approaches to device quality CdS: A modified geometry for solution growth of thin films and their characterisation

Cadmium sulphide thin films have been grown using a modified chemical bath deposition method with four innovative features: i) ethylenediamine was used as the complexing agent, enabling the use of low cadmium concentrations, ii) a rectangular bath geometry with heated glass plate walls was employed, iii) a low deposition temperature (30 °C) was used and iv) nitrogen gas was flowed over the substrate surface during growth. The latter two features eliminate the formation and adherence of gas bubbles on the substrate during growth, hence reducing pinhole formation. On inspection, films were found to be specularly reflective and homogeneous with no visible pinholes. Characterisation was performed by atomic force microscopy, grazing incidence X-ray diffraction, optical transmittance and photoluminescence spectroscopy. It was shown that films possessed a low surface roughness value of 5.2 nm, were highly crystalline, textured, had a grain size of 15 nm and a bandgap of 2.42 eV. Preliminary results from CdTe/CdS thin film photovoltaic devices demonstrate a notable efficiency of 9.8%.     

Optical anisotropy in single-walled carbon nanotube thin films: implications for transparent and conducting electrodes in organic photovoltaics

Optical anisotropy in single-walled carbon nanotube thin film networks is reported. We obtain the real and imaginary parts of the in-(parallel) and out-of-plane (perpendicular) complex dielectric functions of the single-walled carbon nanotube (SWNT) thin films by combining transmission measurements at several incidence angles with spectroscopic ellipsometry data on different substrates. In sparse networks, the two components of the real part of the complex dielectric constant (epsilon1 parallel and epsilon1 perpendicular) were found to differ by 1.5 at 2.25 eV photon energy. The resulting angular dependence (from 0 to 70 degrees incidence angles) of transmittance is reflected in the conversion efficiency of organic solar cells utilizing SWNT thin films as the hole conducting electrodes. Our results indicate that, in addition to the transparency and sheet resistance, factors such as the optical anisotropy must be considered for optical devices incorporating SWNT networks.     

Synthesis of radio frequency plasma polymerized non-synthetic Terpinen-4-ol thin films

Recent advancements in the area of organic polymer applications demand novel and advanced materials with desirable surface, optical and electrical properties to employ in emerging technologies. This study examines the fabrication and characterization of polymer thin films from non-synthetic Terpinen-4-ol monomer using radio frequency plasma polymerization. The optical properties, thickness and roughness of the thin films were studied in the wavelength range 200–1000 nm using ellipsometry. The polymer thin films of thickness from 100 nm to 1000 nm were fabricated and the films exhibited smooth and defect-free surfaces. At 500 nm wavelength, the refractive index and extinction coefficient were found to be 1.55 and 0.0007 respectively. The energy gap was estimated to be 2.67 eV, the value falling into the semiconducting E_g region. The obtained optical and surface properties of Terpinen-4-ol based films substantiate their candidacy as a promising low-cost material with potential applications in electronics, optics, and biomedical industries.     

Photovoltaic device performance of single-walled carbon nanotube and polyaniline films on n-Si: device structure analysis

In this paper, we explore the use of two organic materials that have been touted for use as photovoltaic (PV) materials: inherently conducting polymers (ICPs) and carbon nanotubes (CNTs). Due to these materials' attractive features, such as environmental stability and tunable electrical properties, our focus here is to evaluate the use of polyaniline (PANI) and single wall carbon nanotube (SWNT) films in heterojunction diode devices. The devices are characterized by electron microscopy (film morphology), current-voltage characteristics (photovoltaic behavior), and UV/visible/NIR spectroscopy (light absorption). We have found that both PANI and SWNT can be utilized as photovoltaic materials in a simple bilayer configuration with n-type Silicon: n-Si/PANI and n-Si/SWNT. It was our aim to determine how photovoltaic performance was affected utilizing both PANI and SWNT layers in multilayer devices: n-Si/PANI/SWNT and n-Si/SWNT/PANI. The short-circuit current density increased from 4.91 mA/cm(2) (n-Si/PANI) to 12.41 mA/cm(2) (n-Si/PANI/SWNT), while an increase in power conversion efficiency by ~91% was also observed. In the case of n-Si/SWNT/PANI and its corresponding device control (n-Si/SWNT), the short-circuit current density was decreased by an order of magnitude. The characteristics of the device were affected by the architecture and the findings have been attributed to the more effective transport of holes from the PANI to SWNT and less effective transport of holes from PANI to SWNT in the respective multilayer devices.     

Transparent amorphous zinc oxide thin films for NLO applications

This review focuses on the growth and optical properties of amorphous zinc oxide (ZnO) thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz and glass substrates at temperature equal to 350 K. The amorphous nature of the films was verified by X-ray diffraction. Atomic Force Microscopy was used to evaluate the surface morphology of the films. The optical characteristics of amorphous thin films have been investigated in the spectral range 190–1100 nm. Measurement of the polarized optical properties was shows a high transmissivity (80–99%) and low absorptivity (<5%) in the visible and near infrared regions at different angles of incidence. Linear optical properties were investigated by classic and Time-Resolved Photoluminescence (TRPL) measurements. Photoluminescence spectrum exhibits a strong ultraviolet emission while the visible emission is very weak. An innovative TRPL technique has enabled the measurement of the photoluminescence decay time as a function of temperature. TRPL measurements reveal a multiexponential decay behavior typical for amorphous thin films. Second and third harmonic generation measurements were performed by means of the rotational Maker fringe technique using Nd:YAG laser at 1064 nm in picosecond regime for investigations of the nonlinear optical properties. The obtained values of second and third order nonlinear susceptibilities were found to be high enough for the potential applications in the optical switching devices based on refractive index changes. Presented spectra confirm high structural and optical quality of the investigated zinc oxide thin films.     

Low-indium content bilayer transparent conducting oxide thin films as effective anodes in organic photovoltaic cells

Bilayer In-doped CdO/Sn-doped In₂O₃ (CIO/ITO) transparent conducting oxide (TCO) thin films were prepared by depositing thin ITO films by ion-assisted deposition on CIO films grown by metal-organic chemical vapor deposition. The optical, electrical, and microstructural properties of these bilayer TCO films were investigated in detail. A low sheet resistance of ~ 4.9 Ω/□ is achieved for the CIO/ITO (170/40 nm) bilayers without annealing. With a significantly lower In content (20 vs. ~ 93 at.%) and a much higher conductivity (> 12,000 vs. 3000-5000 S/cm) than commercial ITO, these bilayer films were investigated as anodes in bulk-heterojunction organic photovoltaic (OPV) devices having a poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene) + [6,6]-phenyl C₆₁ butyric acid methyl ester active layer. Device performance metrics in every way comparable to those of devices fabricated on commercial ITO are achieved, demonstrating that CIO/ITO bilayers are promising low-In content, highly conductive and transparent electrode candidates for OPV cells.     

Tunable optical and nano-scale electrical properties of WO<Subscript>3</Subscript> and Ag-WO<Subscript>3</Subscript> nanocomposite thin films

WO₃ and Ag-WO₃ nanocomposite thin films have been synthesized from pure WO₃ and Ag-WO₃ composite pressed powder targets submitted to pulses generated by a frequency quadrupled Nd:yttrium aluminium garnet (YAG; λ = 266 nm, τ ~ 5 ns, ν = 10 Hz) laser source. The irradiations were performed in low pressure oxygen atmosphere. The obtained results proved the possibility to tailor the synthesized thin films optical properties in the UV–Visible spectral region and their nano-scale electrical characteristics through the process parameters, as ambient oxygen pressure value during the thin films deposition and Ag concentration of the Ag-WO₃ composite targets. The tunable optical and electrical features allow for the creation of new materials for future applications as photocatalysts, transparent conducting electrodes, electrochromic or chemical and biological sensor devices.     

Modification of optical and electrical properties of ITO using a thin Al capping layer

In this study, the work function, transmittance, and resistivity of indium tin oxide (ITO) thin films were successfully modified by depositing an Al capping layer on top of ITO with subsequent thermal annealing. The 5 nm thick Al layer was deposited by a conventional dc magnetron sputtering method and the layer was converted into an aluminum oxinitride by subjecting the sample to rapid thermal annealing (RTA) under a nitrogen atmosphere. The films exhibited a high transmittance of 86% on average within the visible wavelength region with an average resistivity value of 7.9 × 10^− 4 Ω cm. Heat-treating the Al/ITO films via RTA resulted in the decrease of the optical band gap from that of bare ITO. In addition, the films showed red-shift phenomena due to their decreased band gaps when the heat-treatment temperature was increased. The resultant electrical and optical characteristics can be explained by the formation of aluminum oxinitride on the surface of the ITO films. The work function of the heat-treated films increased by up to 0.26 eV from that of a bare ITO film. The increase of the work function predicts the reduction of the hole-injection barrier in organic light-emitting diode (OLED) devices and the eventual use of these films could provide much improved efficiency of devices.     

Impact of various irradiation photon energies and gamma doses on the optical properties of ZnSe nanostructure thin film

ZnSe thin films were prepared by thermal evaporation technique under high vacuum (10⁻⁶ Torr) at 300 K and different film thickness. The structure of thin films was measured using grazing incident in-plane X-ray diffraction (GIIXD) and shows single phase zinc blende structure. The particle sizes of the deposited films were estimated for low film thickness by TEM and high film thickness by GIIXD. The particle size of ZnSe films was decreased from ~8.53 to 3.93 nm as film thickness lowered from 200 to 20 nm which ensures the nanocrystalline structure. The optical transmission (T) and reflection (R) in the wavelength range 190–2,500 nm for irradiated and unirradiated ZnSe thin films under investigation were measured. The effect of irradiation of different energies in range (0.1–1.25 MeV) from X-ray, ¹³⁷Cs and ⁶⁰Co irradiation sources were studied for ZnSe thin films of 100 and 200 nm thicknesses. The dependence of the absorption spectra and refractive index were investigated for different energies irradiation sources. The ZnSe films show direct allowed interband transition. The effect of particle size of nanocrystalline ZnSe thin films for unirradiated and irradiated by gamma (γ) doses from ¹³⁷Cs on the optical properties was studied. Both the optical energy bandwidth and absorption coefficient (α) were found to be (γ) dose dependent.     

Inkjet printing of single-walled carbon nanotube/RuO<Subscript>2</Subscript> nanowire supercapacitors on cloth fabrics and flexible substrates

Single-walled carbon nanotube (SWNT) thin film electrodes have been printed on flexible substrates and cloth fabrics by using SWNT inks and an off-the-shelf inkjet printer, with features of controlled pattern geometry (0.4–6 cm²), location, controllable thickness (20–200 nm), and tunable electrical conductivity. The as-printed SWNT films were then sandwiched together with a piece of printable polymer electrolyte to form flexible and wearable supercapacitors, which displayed good capacitive behavior even after 1,000 charge/discharge cycles. Furthermore, a simple and efficient route to produce ruthenium oxide (RuO₂) nanowire/SWNT hybrid films has been developed, and it was found that the knee frequency of the hybrid thin film electrodes can reach 1,500 Hz, which is much higher than the knee frequency of the bare SWNT electrodes (˜158 Hz). In addition, with the integration of RuO₂ nanowires, the performance of the printed SWNT supercapacitor was significantly improved in terms of its specific capacitance of 138 F/g, power density of 96 kW/kg, and energy density of 18.8 Wh/kg. The results indicate the potential of printable energy storage devices and their significant promise for application in wearable energy storage devices.      

Supermagnetron plasma CVD of highly effective a-CNx:H electron-transport and hole-blocking films suited to Au/a-CNx:H/p-Si photovoltaic cells

Hydrogenated carbon nitride (a-CN_x:H) films (0-500 nm) were deposited on p-Si wafers to make Au/a-CN_x:H/p-Si photovoltaic cells using i-C₄H₁₀/N₂ supermagnetron plasma chemical vapor deposition. At a lower electrode RF power (LORF) of 50 W and an upper electrode RF power (UPRF) of 50-800 W, hard a-CN_x:H films with optical band gaps of 0.7-1.0 eV were formed. At a film thickness of 25 nm (UPRF of 500 W), the open circuit voltage and short circuit current density were 247 mV and 2.62 mA/cm², respectively. The highest energy conversion efficiency was 0.29%. The appearance of the photovoltaic phenomenon was found to be due to the electron-transport and hole-blocking effect of thin a-CN_x:H film.     

Effect of thermal coupling on the electronic properties of hydrogenated amorphous silicon thin films deposited by electron cyclotron resonance

In photovoltaic devices, rather thin intrinsic layers of good quality materials are required and high deposition rates are a key point for a cost-effective mass production. In a previous study we have shown that good quality amorphous silicon (a-Si:H) films can be deposited by matrix distributed electron cyclotron resonance (MDECR) plasma CVD at very high deposition rates (∼ 2.5 nm/s). However, only thick films (> 1 μm) exhibited good transport properties. A very poor thermal coupling between the substrate holder and the substrate is the main reason for such a behaviour. We present here experimental data which support this conclusion as well as the improved transport and defect-related properties of new very thin a-Si:H samples (thickness around 0.3 μm) deposited at a higher temperature than the previous ones.     

Spray pyrolyzed β-In2S3 thin films: Effect of postdeposition annealing

Indium sulfide thin films prepared using spray pyrolysis, with In/S ratio 2/3 in the solution, were annealed in vacuum at 300 and 400 °C. The effect of this treatment on properties of the films was studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, optical absorption, transmission and electrical measurements. Optical constants of the films were calculated using the envelope method. Annealing did not affect the optical properties of the film much, but the resistivity of the films showed a drastic decrease and the grain size increased. In₂S₃ thin films have potential use as buffer layer in photovoltaic heterojunction devices.     

Influence of target to substrate distance on the sputtered CuCl film properties

CuCl is a potential candidate for UV optoelectronic devices due to its superior optical properties and lattice matching with Si. Stoichiometric CuCl thin films of polycrystalline nature were grown by RF magnetron sputtering technique. The effect of varying the target to substrate distance on the compositional, structural and optoelectronic properties of the sputtered films was analysed. A critical target to substrate distance (d_ts) was observed and the film properties were clearly different above and below this distance. Based on the film properties, the optimum spacing of d_ts = 6 cm was found to yield stoichiometric and high optical quality films. The existence of more than one chemical bonding state was identified in nonstoichiometric, chlorine rich, films by analysing the Cu 2p_3/2 core level XPS spectra. Chlorine rich samples were found to show a noticeable emission from deep levels at ∼ 515 nm in cathodoluminescence (CL) spectroscopy. An exciton mediated sharp UV luminescence (385 nm) emission was realized at room temperature in the stoichiometric CuCl thin films.     

Properties of nanocrystalline zinc oxide thin films prepared by thermal decomposition of electrodeposited zinc peroxide

Zinc peroxide thin films were electrodeposited from aqueous solution at room temperature using H₂O₂ as the oxidation agent. Nanocrystalline zinc oxide thin films were then obtained from thermal decomposition of zinc peroxide thin films. The grain sizes of ZnO through thermal decomposition of ZnO₂ at 200 °C, 300 °C and 400 °C were estimated from the peak width of ZnO(110) obtained from X-ray diffraction and were 6.3 nm, 9.1 nm and 12.9 nm, respectively. The optical properties of zinc oxide thin films have been studied. The photoluminescence results indicate that ZnO thin films have low Stokes blue shift (about 110 meV) and low oxygen vacancies.     

Influence of gamma radiation on the optical properties of ZnSe nanocrystalline thin films

The effect of gamma (γ) irradiation on the absorption spectra and the optical energy bandwidth of ZnSe nanocrystalline thin films have been studied. Thin films of different thicknesses from 20 to 120 nm were deposited by Inert gas condensation technique at constant temperature of 300 K and under pressure 2 × 10⁻³ Torr of Argon gas flow. The optical transmission (T) and optical reflection (R) in the wavelength range 190–2,500 nm of ZnSe nanocrystalline thin films were measure for unirradiated and irradiated films. The dependence of the absorption coefficient α on photon energy hν was determined for different γ-doses irradiated films. The ZnSe thin films show direct allowed interband transition by γ-doses. Both the absorption coefficient (α) and optical energy bandwidth were found to be γ-dose dependent. The optical energy band width has been decreased by irradiated of γ-doses. The E_gn values of irradiated thin films by 34.5 Gy of γ-doses were recovered to nearly their initial values after 100 days at 300 K.     

Influence of thermal annealing on microstructural,morphological, optical properties and surface electronic structure of copper oxide thin films

In this study, effect of the post-deposition thermal annealing on copper oxide thin films has been systemically investigated. The copper oxide thin films were chemically deposited on glass substrates by spin-coating. Samples were annealed in air at atmospheric pressure and at different temperatures ranging from 200 to 600°C. The microstructural, morphological, optical properties and surface electronic structure of the thin films have been studied by diagnostic techniques such as X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV–VIS) absorption spectroscopy, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The thickness of the films was about 520 nm. Crystallinity and grain size was found to improve with annealing temperature. The optical bandgap of the samples was found to be in between 1.93 and 2.08 eV. Cupric oxide (CuO), cuprous oxide (Cu₂O) and copper hydroxide (Cu(OH)₂) phases were observed on the surface of as-deposited and 600 °C annealed thin films and relative concentrations of these three phases were found to depend on annealing temperature. A complete characterization reported herein allowed us to better understand the surface properties of copper oxide thin films which could then be used as active layers in optoelectronic devices such as solar cells and photodetectors.     

Optical properties of solution-processable semiconducting TiOx thin films for solar cell and other applications

The optical properties of solution-processable semiconducting titanium suboxide (TiOx) thin films were investigated as a function of wavelength (350-800 nm) using ellipsometric and optical reflection technique. The variation of refractive index under different thermal annealing conditions (room temperature to 900 °C) was studied. The increase in refractive index with high-temperature thermal annealing process was observed, allowing the opportunity to obtain refractive index values from 1.77 to 2.57 at a wavelength of 600 nm. The x-ray diffraction and atomic force microscopy studies indicate that the index variation is due to the TiOx phase, density, and morphology changes under thermal annealing. The TiOx thin films have applications in organic and inorganic solar cells as well as other optical and photonic devices. We show that TiOx thin films can be used as an effective antireflection layer for Si solar cells.     

Antireflection and band gap extension effects of ZnO nanocrystalline films grown on ITO-coated glasses by low temperature process

Zinc oxide (ZnO) nanocrystalline films coated on indium tin oxides (ITO, 90:10 wt%) glasses were prepared by low temperature process. The thin films were composed of uniform nanoparticles with average diameter around 8.4 nm. All samples exhibited excellent optical antireflective phenomena, and the maximum transmission reached 92.8% for the sample spin coated at 1500 rpm at 453 nm, improved by 21.5%. The antireflective results were explained by the coherence theory. And the antireflective effects were induced by the ITO and ZnO films. The calculated thicknesses of the ZnO films agreed well with the experimental results. The theoretical calculated band gap from the average diameter of ZnO nanoparticles was also well consistent with the experimental ones obtained from the optical transmission spectra. This result was promising for applications in organic solar cells.