Electrical and optical properties of thin films based on poly [2-methoxy-5 (2′-ethyl hexyloxy)-1,4-phenylene vinylene] doped with acridine orange dye with possible photovoltaic applications

Conjugated polymers have high potential for efficient production of low-cost flexible optoelectronics devices such as solar cells, photodiodes and light emitting diodes. In this paper, the interaction between poly [2-methoxy-5 (2′-ethyl hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and acridine orange (AO) dye has been studied to examine the role of charge pair separation and photocurrent generation in organic photovoltaic films. The AO dye concentration has been optimized on the basis of absorption spectra and photoluminescence (PL) studies carried over the films particularly, for photovoltaic applications. For this purpose, MEH-PPV polymer was dissolved in n-butyl acetate and doped with different concentration of AO dye solution. The Current–Voltage characteristics of MEH-PPV films, prepared by spin-coating technique on ITO-coated glass substrate have been determined as a function of temperature. For the cathode contact, Al was deposited using RF sputtering technique so that the current is predominantly due to holes. The conductivity increases by two orders of magnitude as the AO dye concentration increases to 2.65 mol L⁻¹. In optical properties, we have studied the UV–visible absorption spectra, PL spectra of undoped and doped MEH-PPV films. The absorption studies show that the addition of AO dye does not significantly change (reduce) the absorption, which indicates the suitability of photon absorption. The PL spectra were recorded for MEH-PPV films, which become narrow on increasing the AO dye concentration. The PL studies indicate that up to certain concentration of AO dye in MEH-PPV films, PL intensity decreases as compare to that for undoped MEH-PPV film. The combined effect of increase in conductivity by two orders, decrease in PL intensity and no significant change in absorption makes the material suitable for photovoltaic applications.     

Temperature dependence of the conductivity in large-grained boron-doped ZnO films

The temperature dependence of the conductivity is investigated as a function of boron doping in large-grained, degenerate polycrystalline ZnO films prepared by low-pressure chemical vapor deposition. Carrier transport in undoped and lightly doped films is mainly controlled by the grain boundary; field emission through grain boundaries limits the conductivity below 90 K, while thermally activated thermoionic-field emission leads to an increase in the conductivity with the temperature near room temperature. In contrast, carrier transport in highly doped films is mainly governed by intra-grain scattering, which does not depend on the temperature for degenerate electron gases, limits the mobility below 120 K, whereas a metallic behavior (decrease in conductivity with increasing temperature) is observed at room temperature, which is linked to the ionized impurity scattering. The transition between the “semiconductor”-like and metallic-like behavior at room temperature takes place for a film with carrier concentration between 6×10¹⁹ and 9×10¹⁹ cm⁻³.     

DC conduction mechanism in polyvinyl alcohol films doped with potassium thiocyanate

The current–voltage characteristics of pure polyvinyl alcohol (PVA) films and those doped with potassium thiocyanate (KSCN) are studied as a function of film temperature and dopant concentration. The conduction mechanisms operative in the films in different temperature and voltage ranges are estimated from the behaviour of log I versus V^1/2 plots (I=current, V=voltage). For undoped (pure) films, the conduction mechanism appears to be essentially a Schottky type. On doping, there is considerable influence on the type of conduction mechanism, especially at lower temperatures. At higher temperatures, however, there is no significant effect of doping on the conduction mechanism.     

Effect of doping and heat treatment on the photoluminescence of CdS films deposited by spray pyrolysis

Large area thin films of n-type CdS were prepared by spray pyrolysis technique. The films were n-type doped by having [In/Cd] ion concentration ratio of 10⁻⁶, 10⁻⁵, 10⁻⁴, 10⁻³, and 10⁻² in the sprayed solution. These films were heat treated in N₂ atmosphere at 450°C for 1 h. The as-deposited undoped, doped, and heat treated were analyzed by photoluminescence (PL) at 5 K sample temperature. In general, the spectra displayed three main emission regions (green, yellow and red) with more than one band in each. The emission intensity is found to decrease with doping and the relative intensity of the bands is found to depend on the doping concentration level. The red band is only present in doped samples and its relative intensity is found to increase with doping. The effect of heat treatment in N₂ on the as-deposited undoped and the doped (10⁻⁴) samples on the relative intensity of the observed bands were compared and discussed. The results are compared with the electrical and morphological results and correlated with the probable changes in the concentration of shallow and deep radiative native defects and structural changes. These allow for better prediction of suitable doping and treatment conditions for good quality films.     

In doped CdO films: Electrical,optical, structural and surface properties

Recently, there has been a lot of work on the production and investigation of the physical properties of Transparent Conducting Oxide (TCO) materials which have common application area in photovoltaic solar cells and some optoelectronic devices. In this work, CdO film which is a material belongs to TCO family has been produced by Ultrasonic Spray Pyrolysis technique on microscope glass substrates at the substrate temperature of 250 ± 5 °C. Electrical, optical, structural and surface properties of undoped and In doped (at 1.3 and 5%) CdO films and the effect of In doping percentage on the physical properties of CdO films have been investigated. It has been determined that electrical conductivity of CdO film is high and this value has been decreased by In doping. After the optical investigations, it has been observed that the transmittances of the films are about 30% and decreased dramatically by In doping. XRD investigations showed that, films have polycrystalline structure and good crystallinity levels. It has been found that In element hasn't got an important effect on the morphology of the films after the examination of surface micrographs. It has been determined that Cd and O elements are present in the solid film by using EDS. After all investigations, it has been concluded that In doping has an important effect on the electrical, optical, structural and surface properties of CdO films.     

Synthesis and electro-optic properties of nanosized-boron doped cadmium oxide thin films for solar cell applications

Boron doped CdO thin films were prepared by sol–gel dip coating technique. Atomic force microscopy results indicate that the boron doped CdO films have the nanostructure. The influence of the boron doping on the film growth is resulted in a change of grain size. The optical band gap of the CdO films was significantly changed by boron dopant. The refractive index dispersion of the films obeys the single oscillator model. The dispersion parameters, oscillator and dispersion energy were changed by boron dopant. The optical absorption results show that the optical band gap of the CdO film can be engineered over a wide range of 2.27–2.45 eV by introducing B dopant. For solar cell applications of the CdO film, a p-Si/1% B doped n-CdO heterojunction solar cell was fabricated and the solar cell shows the best values of open circuit voltage, V_oc = 0.37 and short circuit current density, J_sc = 0.81 mA/cm² under AM1.5 illumination, despite the fact that V_oc and J_sc are lower than those reported in the literature without using frontal grid contacts and or post-deposition annealing. It is evaluated that this work is useful as a basis search for synthesis of the nanosized-boron doped cadmium oxide thin films for solar cell applications and more competitive p-Si/n-CdO based solar cells.     

Mo-doped,Cr-Doped,and Mo–Cr codoped TiO2 thin-film photocatalysts by comparative sol-gel spin coating and ion implantation

Uniformly codoped anatase TiO₂ thin films of varying (equal) Mo and Cr concentrations (≤1.00 mol% for each dopant) were fabricated using sol-gel spin coating and deposited on fused silica substrates. All films were annealed at 450 °C for 2 h to recrystallise anatase. Undoped anatase films have been subjected to dual ion implantation for the first time, using Mo, Cr, and sequential Mo + Cr at 1 × 10¹⁴ atoms/cm². The films were characterised by GAXRD, AFM, SIMS, XPS, and UV–Vis and the performance was assessed by dye degradation. Despite the volumetric doping by sol-gel and the directional doping by ion implantation, neither method resulted in homogeneous dopant distributions. Both methods caused decreasing crystallinities and associated partial amorphisation. The XPS signal of the uniformly codoped films is dominated by undissolved dopant ions, which is not the case for the ion-implanted films. Increasing Ti valences are attributed to the fully oxidised condition of the Ti⁴⁺ ions that diffuse to the surface from Ti vacancy formation compared to the Ti valence of the bulk lattice, which contains Ti³⁺. Increasing O valence is attributed to the electronegativity of O^2?, which is higher than that of Ti⁴⁺. Detailed structural mechanisms for the solubility and energetics mechanisms involve the initial formation of Mo and Cr interstitials that fill the two voids adjacent to the central Ti ion in the TiO₆ octahedron, followed by integrated solid solubility (ISS) and intervalence/multivalence charge transfer (IVCT/MVCT). The sequential order of the last two is reversed for the two different doping methods. These two effects are likely to be the source of synergy, if any, between the two dopant ions. The photocatalytic performances of the uniformly codoped films are relatively poor and correlate well with the band gap (E_g). The performances of the ion-implanted films do not correlate with the E_g, where TiO₂–Mo performs poorly but TiO₂–Cr and TiO₂–Mo–Cr outperform the undoped film. These results are interpreted in terms of the competition between the effects of Mo doping, which causes partial amorphisation and/or blockage of active sites, and Cr doping, which may cause Mo–Cr synergism, Cr-based heterojunction formation, and/or improved charge-carrier separation owing to the surface-deposition nature of ion implantation.     

Effect of doping level on physical properties of antimony doped nanocrystalline tin oxide films fabricated using low cost spray technique

Abstract

Antimony doped nanocrystalline tin oxide (SnO₂∶Sb) films have been fabricated using a low cost and simple fabrication technique using a perfume atomiser. The X-ray diffraction patterns of the antimony doped tin oxide (ATO) films show that the films are polycrystalline with tetragonal cassiterite structure having (110) plane parallel to the surface of the substrate. The preferred orientation remains constant for all the doping levels (0·5–3·0 at-%) and the degree of preferred orientation increases with increasing doping concentration in the starting solution. The intensity of the (110) plane steadily increases as the doping concentration of antimony increases. The fine quality of AFM images show that the films have homogeneous and uniform surface. The sheet resistance is found to decrease with increasing antimony doping level, attain a minimum value (6·34 Ω sq^?1) when the doping concentration is 2·5 at-% and then increase for further doping.     

Properties of BaCe1−xTixO3 materials for hydrogen electrochemical separators

In this work the structural, microstructural and electrical properties of BaCe_1−xTi_xO₃ materials were investigated. The series of materials with different titanium concentrations x (0–0.3) were prepared by solid-state reaction method. The structural studies by X-ray diffraction have shown that undoped material crystallizes in orthorhombic phase, while the increasing concentration of Ti dopant up to x = 0.2 leads to the ordering of the structure to phases with higher symmetries (tetragonal and even cubic). The estimated solubility limit was found to be not higher than 20 at.% of Ti. Microstructure observations by scanning electron microscopy and linear contraction determination have shown the strong influence of Ti dopant on microstructure and an improvement of sinterability. The DC four-probe electrical conductivity measurements accompanied by the potentiometric EMF measurements of solid-state electrochemical cells in controlled gas atmospheres (containing H₂, O₂ and H₂O) and temperatures (500–800 °C) allowed determination of the total and partial electrical conductivities of selected materials. It was found that the introduction of Ti dopant leads to a decrease in total electrical conductivity by ca. one order of magnitude compared to the undoped material, almost independently of Ti concentration. Also, the modification of transport properties after doping with titanium was determined.     

Influence of dysprosium doping on the electrical and optical properties of CdO thin films

Lightly Dy-doped CdO thin films (molar 0.5%, 1%, 2%, and 2.5%) have been prepared by a vacuum evaporation method on glass and Si wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV-vis-NIR absorption spectroscopy, and dc-electrical measurements. Experimental data indicate that Dy³⁺ doping slightly stretchy-stresses the CdO crystalline structure and changes the optical and electrical properties. The bandgap of CdO was suddenly narrowed by about 20% due to a little doping with Dy³⁺ ions. Then, as the Dy doping level was increased, the energygap was also increased. This variation was explained by the effect of Burstein-Moss energy shift (or bandgap widening effect) together with a bandgap shrinkage effect. The electrical behaviour of the samples shows that they are degenerate semiconductors. However, the 2% Dy-doped CdO sample shows an increase in its mobility by about 3.5 times, conductivity by 35 times, and carrier concentration by 10 times relative to undoped CdO film. From transparent conducting oxide point of view, Dy is sufficiently effective for CdO doping.     

Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering

Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar+O₂ atmosphere. The deposited films were characterized by studying their structural, electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (1 1 1) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (>5%) the conductivity was found to decrease.     

High quality ZnO and Ga:ZnO thin films grown onto crystalline Si (1 0 0) by RF magnetron sputtering

Undoped and 2% Ga-doped ZnO films have been deposited by RF magnetron sputtering onto single crystal Si (1 0 0) substrates equivalent to the commercial Si solar cells. The same films were also grown on amorphous silica substrates to complete their characterization. The films have been characterized by X-ray diffraction, electrical and optical measurements, X-ray photoelectron spectroscopy, Raman microspectroscopy and scanning and high-resolution transmission electron microscopy. Films present a very good quality crystalline wurtzite structure with the c-axis perpendicular to the substrate, with continuity of the (0 0 0 2) planes along the whole film, as shown by transmission electron microscopy. The doped sample shows an increase of two orders of magnitude of the electrical conductivity, an optical transmittance bigger than 85% along the visible spectrum, a diminution of the grain size in the direction parallel to the substrate and a lower surface roughness. The Ga-cations act only as substitutional impurities, they are homogeneously distributed in the whole film, maintaining the wurtzite structure and increasing the carrier density. The formation of any spurious phase or segregation of Ga₂O₃ clusters that can act as carrier traps can be discarded. The characterization results allow us to conclude that the doped film has improved electrical and optical properties with respect to the undoped one. Therefore, the Ga-doped films are very suitable candidates as transparent conducting electrodes for solar cells, displays and other photoelectronic devices.     

N‐doped titania powders prepared by different nitrogen sources and their application in quasi‐solid state dye‐sensitized solar cells

Nitrogen‐doped TiO₂ nanocrystalline particles are synthesized by a microwave‐assisted hydrothermal growth method using different amines (Dipropylamine, Diethanolamine and Ammonium hydroxide) as nitrogen sources. Characterization of the nanoparticles was performed with X‐ray diffraction, UV–vis diffuse reflectance spectroscopy, Field Emission Scanning Electron Microscopy and X‐ray Photoelectron Spectroscopy. The prepared N‐doped TiO₂ nanoparticles exhibit pure anatase phase with average diameter of 9 nm and reduced optical energy gap compared to undoped TiO₂. Immobilization of N‐doped and pure TiO₂ nanoparticles on SnO₂:F conductive glass substrates was successfully performed by using doctor‐blade technique and paste of the aforementioned nanoparticles. A series of N‐doped TiO₂ photoelectrodes with varying N dopant source and concentrations were fabricated for quasi‐solid state dye‐sensitized solar cells. The N‐doped solar cells achieve an overall conversion efficiency ranging from 4.0 to 5.7% while undoped TiO₂ showed 3.6%. The basic difference to the electrical performance of the cells is focused to the enhancement in the current density of N‐doped TiO₂‐based cells which was from 11% to 58% compared with undoped TiO₂ cells. Current densities were directly proportional with nitrogen doping level in TiO₂ lattice which differs depending on the amine source nature such as basicity differences, hydrogen bonding abilities and steric inherences. Copyright © 2013 John Wiley & Sons, Ltd.     

Visible light driven photosplitting of water using one dimensional Mg doped ZnO nanorod arrays

In the present work, we have introduced Mg doped ZnO nanorods based photoanodes for photoelectrochemical water splitting applications. Vertically aligned Mg doped ZnO nanorods were fabricated by sol-gel and hydrothermal technique. The as-prepared nanorod samples exhibited hexagonal wurtzite structure as confirmed from XRD measurements. We achieved a photocurrent density of 0.35 mA/cm² at 1.5 V vs. Ag/AgCl for 10% Mg doped ZnO photoanode which is 9 times higher than that of undoped ZnO nanorods (0.03 mA/cm²). Incorporation of Mg resulted in faster charge transport and longer life time of electrons with reduced recombination rate. Mg dopant tuned the optical band gap of ZnO and increased the carrier concentration boosting the PEC performance of the photoanodes. Since seawater is one of the most abundant natural resource on earth, we further carried out seawater splitting of 10MgZ under visible light illumination which indicated its high photostability in natural seawater for 5 h of continuous illumination.     

Selective emitter formation with a single screen-printed p-doped paste deposition using out-diffusion in an RTP-step

We present a new way to realise a selective emitter structure using a single screen-printed phosphorous paste deposition as dopant source to obtain a doping differential, with rapid thermal diffusion. The heavily doped part of the emitter is situated underneath the deposited phosphorous lines, and the lightly doped emitter in between is obtained via the gas phase, because phosphorous out-diffuses from the paste during the high-temperature step. SIMS profiles show a difference of a factor 4 in magnitude for the surface concentration between the regions underneath and beside the deposited paste. Photovoltaic results show an improvement in efficiency of 0.7% in comparison with the reference cell (homogeneous emitter), due to a 2 mA/cm² short-circuit current increase.     

Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate

We have studied differences in the interface between undoped and Al-doped ZnO thin films deposited on commercial Si solar cell substrates. The undoped ZnO film is significantly thicker than the Al-doped film for the same deposition time. An extended silicate-like interface is present in both samples. Transmission electron microscopy (TEM) and photoelectron spectroscopy (PES) probe the presence of a zinc silicate and several Si oxides in both cases. Although Al doping improves the conductivity of ZnO, we present evidence for Al segregation at the interface during deposition on the Si substrate and suggest the presence of considerable fixed charge near the oxidized Si interface layer. The induced distortion in the valence band, compared to that of undoped ZnO, could be responsible for considerable reduction in the solar cell performance.     

Low-temperature deposition of amorphous silicon solar cells

We develop amorphous silicon (a-Si:H)-based solar cells by plasma-enhanced chemical vapor deposition (PECVD) at deposition temperatures of T_s=75°C and 100°C, compatible with low-cost plastic substrates. The structural and electronic properties of low-temperature standard PECVD a-Si:H, both doped and undoped, prevent the photovoltaic application of this material. In this paper, we demonstrate how to achieve device-quality a-Si:H even at low deposition temperatures. In the first part, we show the dependence of structural and carrier transport properties on the deposition temperature. The sub-band gap absorption coefficient and the Urbach energy increase when the deposition temperature declines from T_s=150°C to 50°C, the conductivity of doped layers and mobility-lifetime product of intrinsic a-Si:H drop drastically. Therefore, in the second part we investigate the impact of increasing hydrogen dilution of the feedstock gases on the properties of low-temperature a-Si:H. We restore n-type a-Si : H device-quality conductivity while the p-type a-Si:H conductivity is still inferior. For undoped layers, we depict the hole diffusion length, the mobility-lifetime product for electrons, the Urbach energy, and sub-band gap absorption coefficient as a function of the hydrogen dilution ratio. We incorporate these optimized materials in solar cell structures of single and multilayer design and record initial efficiencies of η=6.0% at a deposition temperature of T_s=100°C, and η=3.8% at T_s=75°C. For prospective opaque polymer substrates we develop, in addition to our conventional pin cells, devices in nip design with similar performance.     

A new catalyst of AlCu@ZnO for hydrogen evolution reaction

Thin films of undoped ZnO, Al-doped ZnO, Cu-doped ZnO, and AlCu@ZnO deposited on indium tin oxide were performed by the sol-gel spin coating method. The prepared ZnO thin films were investigated for their structural and electrical properties after annealing at 500 °C for 1 h. ZnO thin films were characterized by electrochemical impedance spectroscopy, linear sweep voltammetry, scanning electron microscopy, Fourier transform infrared spectroscopy and Mott Schottky. According to the results obtained from the Nyquist diagrams of the ZnO thin films, the resistance value was found to decrease with binary doping and the resistance value was found to be lowest in AlCu@ZnO doped thin film containing 0.01 M Al and 0.1 M Cu. As ZnO thin films go to cathodic potentials, it is seen that the cathodic current value of ZnO with undoped is the lowest. It has been found that only Al and Cu doping showed less cathodic current than double doping.     

Bromine doping effect on some properties of CdS films

The suitability of bromine as an anionic dopant influencing the properties of CdS thin films is discussed in this paper. The as-deposited films were characterised respectively by XRD, SEM, UV-vis-NIR spectrophotometer and two-point probe setup. All the films appear to be polycrystalline in nature exhibiting hexagonal crystal structure with a (002) preferential growth texture. The 2θ value of the (002) plane shifts towards lower Bragg angle with doping inferring an expansion in their lattice volumes. Increased transparency and blue shift in optical band gap value are observed for the doped films. Electrical resistivity decreased with increase in Br-doping concentration. Increased transparency, widened band gap and decreased electrical resistivity values achieved confirm that Br is a suitable anionic dopant that can strongly influence the physical properties of pure CdS towards future optoelectronic devices.     

Effect of organic salt doping on the performance of single layer bulk heterojunction organic solar cell

The effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF₆) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF₆ exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm². The short circuit current density and the open circuit voltage of the doped device increased from 0.54 μA/cm² to 6.41 μA/cm² and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.