Photovoltaic devices based on electrochemical-chemical deposited CdS and poly3-octylthiophene thin films

Cadmium sulfide (CdS) is a well-known wide bandgap semiconductor for solar cell applications. In this work we report an electrochemical/chemical method to prepare CdS thin films on gold (Au)-coated glass substrates. 10 nm thick of titanium (Ti) film was first sputtered on glass surface to improve the adhesion between the subsequent sputtered Au film and the glass surface. Cadmium films were then electrochemically deposited on Au surface in an acidic solution with negative potential, and the obtained glass/Ti/Au/Cd samples were annealed in H₂S atmosphere to convert Cd into CdS. XRD pattern of H₂S-annealed Cd samples shows a hexagonal wurtzite phase in CdS with (0 0 2) as the preferential crystalline plane. Photovoltaic properties were clearly shown in hybrid heterojunctions of CdS and poly3-octylthiophene (P3OT) with Au as the top and the back metal contacts.     

Analysis of electrical parameters in heterojunctions based on poly 3-octylthiophene and cadmium sulfide thin films

Planar hybrid heterojunctions were built with poly 3-octylthiophene (P3OT) and chemical bath-deposited cadmium sulfide (CdS) thin films on a conductive glass substrate. The organic material, P3OT, acts as a light absorber and the inorganic one, CdS, as the electron acceptor. Two types of CdS films had been used: one is as-deposited and the other doped with HgCl₂. Heterojunctions were formed by casting a chemically synthesized P3OT solution onto CdS films. The P3OT film thickness was also varied for heterojunction studies. Current vs. potential (I–V) characterizations under dark and illumination conditions were performed for the P3OT/CdS heterojunctions under 88 mW/cm² irradiance level, which show photovoltaic effect with different open circuit voltage (V_OC) levels, being as high as 1 V for some devices. A parametric analysis of I–V curves details the effect of CdS resistivity and P3OT film thickness on series and shunt resistance of the heterojunctions.     

Physicochemical and morphological properties of spin-coated poly(3-alkylthiophene) thin films

Poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) were synthesized by direct oxidation of the respective monomers with FeCl₃ as oxidant/catalyst. It was observed that the type of monomer affected the molecular weight and polydispersity as well as degree of regioregularity of the polymer, measured by size exclusion chromatography and by ¹H nuclear magnetic resonance, respectively. Homogeneous P3HT and P3OT films were prepared by spin-coating technique with different polymer concentration in solutions. Morphology study with atomic force microscopy indicates the cluster size difference between P3HT and P3OT films. Optical absorption properties of the polymeric films were analyzed in pristine and doped state. The electrochemical characterization of the poly(3-alquilthiophenes) (P3Ats) films showed two oxidation peaks, one at 0.35 V vs. Ag/AgCl, and the second one at 0.8 V vs. Ag/AgCl. The colors of the film at these two potentials were black and blue, respectively.     

Electronic properties of poly(3-methylthiophene)

Poly(3-methylthiophene) (P3MT)-based Schottky barrier diodes were prepared and their capacitance and conductance were measured as a function of frequency. The loss tangent of these structures shows a maximum which depends on the temperature according to an Arrhenius law. An average activation energy of 0.3 eV is deduced from the position of this maximum. A similar value was found from the temperature dependence of the I–V characteristics of these diodes. In addition, the short-circuit photocurrent versus the photon energy reveals a peak at 1.66 eV which is attributed to the anti-bonding level of the polaron.     

Thermal-induced changes on the properties of spin-coated P3HT:C60 thin films for solar cell applications

The thermal transition behaviour, optical and structural properties of spin-coated P3HT:C₆₀ blended films with different C₆₀ ratios were investigated using differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA), ultraviolet-visible (UV-vis) spectroscopy, photoluminescence (PL), Fourier transform infrared absorption (FT-IR) spectroscopy and Raman spectroscopy. DSC analysis showed that the P3HT:C₆₀ blends have quite different thermal characteristics. The absorption spectra of the annealed P3HT:C₆₀ (1:1 wt%) films becomes enhanced and red shifted. This feature is evident in the photoluminescence measurements where the formation of polymer crystallites upon annealing is observed. Raman spectroscopy showed a substantial ordering in the polymer film during annealing. It was found that the performance of a P3HT:C₆₀ (1:1 wt%) device was dramatically improved by annealing.     

Temperature dependence of the optical transitions in electrodeposited Cu2O thin films

Cu₂O films on flexible copper and molybdenum (Mo) substrates were prepared by electrodeposition form an alkaline bath. The as-deposited films were p-type and the XRD analysis revealed that the film contains only the Cu₂O phase. The thickness of the films was calculated from the interference fringes in the reflection spectra. The Au/Cu₂O Schottky diodes were prepared by sputtering a 15 nm thick layer of very pure gold onto the Cu₂O films on Mo substrate. The probable optical transitions near the band edge were calculated from the spectral response of the device. The band gap calculated at various temperatures show a linear dependence on temperature and the absolute zero value of the band gap is deduced as 2.206 eV. The 2.493 eV direct transition observed in the room temperature shows a temperature dependence. Evidence of phonon assisted indirect transitions were observed at various temperature regions.     

Electrochemical and electrochromic response of poly(thiophene-3-acetic acid) films

Thiophene-3-acetic acid has been polymerized in chloroform by a chemical method using FeCl₃ as oxidant. The films were prepared casting the solubilized polymer on ITO electrodes and studied using cyclic voltammetry, chronoamperometry and spectroelectrochemistry. During the potential sweep, an electrochromic process is observed in which the film color changes from red to black. High electrochromic efficiency was observed for more than 600 cycles, although it decreases to 73% of the initial value. Until 264 cycles, the electrochromic efficiency at 750 nm is stable and the value is 242 cm² C⁻¹.     

Optical and photoconductive properties of SnS thin films prepared by electron beam evaporation

Thin films of SnS have been prepared by electron beam evaporation. The films represent Herzbergite orthorhombic structure, established by their XRD patterns. The band gap energy and type of optical transitions were determined from transmission spectra and an optical band gap of E_g(tr)=1.23 eV for indirect transitions and E_g(tr)=1.38 eV for direct transitions were estimated. Using the dependence of photoconductivity from wavelength, a band gap of E_g(ph)=1.2 eV was determined as well. A thermal band gap of E_g(T)=1.29 eV was evaluated from the temperature dependence of the dark resistivity, and admixture level with activation energies (0.25 and 0.36 eV) were found. Roughness of the surface of SnS thin films was evaluated using atomic force microscopy.     

Influence of photo-induced degradation on the optoelectronic properties of regioregular poly(3-hexylthiophene)

Recently, head-to-tail regioregular poly(3-hexylthiophene) (P3HT) has been widely used as an active material in fabricating polymer optoelectronic devices. This study employs UV-vis absorption, photoluminescence (PL), X-ray diffraction (XRD) and the space-charge limited current (SCLC) model to elucidate the effect of light illumination on the optic and optoelectronic properties of P3HT. The degraded performance of P3HT, such as low absorbance, PL emission, and charge-carrier mobility is caused mostly by a reduction in the degree of conjugated structure and the lower crystallinity.     

Hydrogen annealing induced physical properties of ZnTe thin films

The ZnTe material has an unprecedented role in the fabrication of high efficiency CdTe thin film solar cells and optimization of hydrogen annealing induced physical properties of ZnTe films is next required step. Consequently, in the present work, the impact of Hydrogen annealing temperature on the structural, optical, electrical, topographical, morphological, and compositional properties of ZnTe films is explored. The ZnTe thin films (having 300 nm thickness) are grown via electron-beam evaporation technique on glass and ITO substrates followed by annealing at different temperatures under a Hydrogen atmosphere. The ZnTe films are found to crystallize in cubic phase with (111) predominant peak having crystallite size in the range of 19–28 nm, whereas annealed films demonstrated lower optical transmittance vis-à-vis to pristine films. The PL spectra exhibit two luminescence peaks with a stronger band at ～351 nm and a weaker band at ～450 nm. Ohmic behavior of ZnTe films is assured through I–V characteristics, while the AFM images revealed hill-like surface topographies. The FESEM image of pristine films demonstrated a homogeneous surface comprising spherical grains whereas annealed films have spherical, stone, and blisters like morphologies. The EDS patterns assured the Te element richness as well as successful ZnTe films deposition. The observed findings signify that the Hydrogen annealing at different temperatures notably modified the physical properties of ZnTe films.     

Fabrication and photovoltaic properties of multilayered thin films designed by layer-by-layer assembly of poly(p-phenylenevinylene)s

Ultrathin films of poly(p-phenylenevinylene) (PPV) were fabricated by the layer-by-layer (LbL) deposition technique with a cationic PPV precursor and an anionic PPV. The hole mobility of the PPV-based LbL film was as high as 4×10⁻⁴ cm² V⁻¹ s⁻¹. Multilayered polymer solar cells with the PPV-based LbL film as a light-harvesting layer were fabricated by a combination of the spincoating and the LbL deposition technique, exhibiting a power conversion efficiency of 0.28% under AM1.5G simulated solar illumination with 100 mW cm⁻². The high performance is ascribed to the high light-harvesting efficiency and hole mobility of the PPV-based LbL layer.     

The effects of inclusion of iodine in CdTe thin films on material properties and solar cell performance

CdTe thin films were potentiostatically electrodeposited from a non-aqueous electrolytic bath containing ethylene glycol. In order to dope the CdTe using an n-type dopant according to a proposed new model, varying concentrations of iodine were added into the electrolytic bath. The resulting materials were studied for structural, morphological, optical and electrical properties. Structural analysis indicated the formation of CdTe layers without other possible phases at a concentration of 0.05 M of iodine in the bath. Optical absorption measurements have yielded a direct band gap value of 1.42±0.03 eV without showing any noticeable changes of the energy gap. Inclusion of iodine in CdTe layers have increased the electrical conductivity by a factor of 5, indicating positive n-type doping effects. The diodes of FTO/CdS/CdTe/Au structures showed improved current–voltage characteristics indicating the presence of a high potential barrier of 1.20 eV with low ideality factors around 1.40. These results demonstrate a considerable reduction of active recombination and generation centres from the structure. Although the fill factors observed are low, for the studied batches in this project, remarkable improvement of short-circuit current densities over 40 mA cm⁻² were observed together with open circuit voltage values in the range 500–700 mV. Capacitance–voltage measurements indicate a formation of a fully depleted device, desirable for photovoltaic conversion.     

Influence of the thickness on structural, optical and electrical properties of chemical bath deposited CdS thin films

Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium acetate, ammonium acetate, thiourea, and ammonium hydroxide. The XRD patterns show that the films are of hexagonal phase with preferred (0 0 2) orientation and the grain size increases with the thickness of the film. The band gap of the films was calculated from the transmittance data and it was found that the band gap decreases as the film grows in thickness. The photo-response studies indicate that the film thickness has an influence on the current decay under dark. The observed opto-electronic properties were attributed to the crystallite size and internal microstrain.     

Electrical conduction properties of flash evaporated ZincPhthalocyanine (ZnPc) thin films

Solar energy as the principle source has become very attractive, as it is abundantly available, can be tapped locally, pollution free and commercially viable over a period. Solar energy may be converted to other forms of higher-grade energy through one of several methods such as photothermal, photochemical, photoelectrochemical and photovoltaic (PV). Among these, as noted above the cleanest and most direct and efficient and of converts to electrical power is with the help of PV or solar cell devices. Photovoltaics are one of the hottest areas of research today. Nowadays organic material finds greater importance in the PV cell fabrications. Among the available organic materials ZincPhthalocyanine is a promising candidate for the solar cell applications, because of its easy synthesization and non-toxic to the environment. The major part of incident light in the visible region effectively contributes to photocarrier generation and the excited ZnPc molecules play an important role in PV effect. In this paper, we have reported electrical transport properties of flash evaporated ZincPhthalocyanine thin films have been reported. DC conduction mechanism in these films (Al–ZnPc–Al structure) was studied at different temperatures. The field dependence behaviour on activation energy and possible conduction mechanism in the ZnPc films under DC field has been discussed.     

Photoelectrochemical response and differential capacitance of poly(3-methylthiophene)

Following the theory used to study the semiconductor/electrolyte interface the differential capacitance of poly(3-methylthiophene) films has been determined from measurements with a lock-in amplifier and by electrochemical impedance spectroscopy (EIS). According to our findings, the best results were obtained by EIS because the space charge capacitance can be separated from the other capacitances. Using Mott–Schottky plots (C⁻² vs. E) we obtained the flat band potential E_fb=80 mV and the carrier density N=6×10¹⁷ cm⁻³ for the PMeT film in contact with the electrolyte, where dissolved O₂ played the role of the electron acceptor. The determined width of the depletion layer is 0.04 μm. We also investigated the photoelectrochemical response of the PMeT film. The plot of the square of the photocurrent vs. potential yields E_fb=90 mV, in good agreement with the EIS measurement. The dependence of the photocurrent with the frequency of the incident light shows that PMeT has a long response time (order of ms), compared to an inorganic semiconductor. The band gap was also determined from the photocurrent spectra. The value obtained, for a direct transition is 1.9 eV and is coincident with the value obtained from the absorption spectra.     

Temperature effect on the electrical properties of undoped and vanadium-doped ZnTe thin films

Vanadium-doped ZnTe films of composition 0 to 10 wt% V, were prepared onto glass substrate by e-beam evaporation of the element in vacuum at 8×10⁻⁴ Pa. The effects of various deposition conditions on the electrical properties of the films have been studied in detail. The deposition rate of the ZnTe films was at about 2.05 nm s⁻¹. X-ray diffraction (XRD) study shows that the as-deposited ZnTe films are amorphous in nature.The effects of temperature on the electrical properties of the ZnTe and ZnTe:V films were studied in details. The heating and cooling cycles of the samples are reversible in the investigated temperature range after successive heat treatments in air. Thickness dependence of electrical conductivity is well in conformity with the Fuchs–Sondheimer theory. Temperature dependence of electrical conductivity shows a semiconducting behavior with a spectrum of activation energy. The value of activation energy for undoped ZnTe films do agree well with earlier reported values. Dopant vanadium concentration increases the conductivity of the samples. The composition and thickness dependence of the activation energy as well as thermoelectric power studies were done in the 300–413 K temperature range. The results of d.c. conductivity and thermopower obey an activated conduction mechanism. The thermopower of undoped ZnTe films indicates a p-type conductivity. Thermopower results of ZnTe:V films also suggest that the simultaneous bipolar conduction of both carriers take place.     

Electrical resistivity of thermally evaporated bismuth telluride thin films

Semiconducting chalcogenide thin films have been receiving considerable attention in the recent years because of their wide applications in the various fields of science and technology. The studies of the electronic properties of semiconductors have been largely stimulated by attractive micro-electronic device applications. Among the various V–VI compounds, Bismuth Telluride (Bi₂Te₃) is an established low-temperature thermo electric material and is widely employed in thermoelectric generators and coolers. The present work deals with the structural and the electrical characterization of Bi₂Te₃ thin films vacuum deposited on well-cleaned glass substrates. A constant rate of deposition was maintained through out the process. To obtain uniform and homogeneous film thickness through out on all the substrates a rotary drive was employed. Quartz crystal thickness monitor was used to measure the thickness of the samples. From the X-ray diffractogram the Bi₂Te₃ films are found to be amorphous at lower thicknesses and posses hexagonal polycrystalline structure at higher thickness, having lattice parameters a=4.44 Å and c=29.40 Å. The grain size of the Bi₂Te₃ thin films before annealing and after annealing are found to be 100 and 160 Å, respectively. The micro-strain and the dislocation density are found to decrease after annealing. The thermogravimetry–differential thermal analysis (TG–DTA) studies revealed that the Bi₂Te₃ films are non-decomposable. Electrical resistivity, TCR measurements have been carried out as a function of varying temperatures in the range 303–453 K are found to show the size effect. Analyzing the size dependence of electrical resistivity it is found that the electrical resistivity is a linear function of the reciprocal of thickness of the film. The energy gap of Bi₂Te₃ thin film was calculated from the graph ln ρ vs. 1/T and it is found that the energy gap decreases with increasing thickness. From the negative values of TCR, it is inferred that Bi₂Te₃ films exhibit semiconducting behavior.     

Photoelectrochemistry of poly(3-methylthiophene), I: Surface morphology and thickness effect

Photocyclovoltammetric experiments with films of poly(3-methylthiophene) under polychromatic light irradiation show a p-type semiconductor behavior in the reduced state, with a flat band potential of 0.18 V. The photocurrent depends on film thickness and surface morphology. Films with different thickness were characterized by scanning electron microscopy, indicating a globular morphology with globule sizes changing according to the charge density used during polymer deposition. The highest photocurrent is observed for the lowest globule diameter. These films were also irradiated with monochromatic light from the electrolyte side and from the substrate side. A higher photocurrent and a photocurrent spectrum matching the absorption spectrum of the reduced form of the polymer is observed on irradiation from the electrolyte side of a 0.7 μm thick film. In contrast, for irradiation from the electrode side, the photocurrent is lower and the spectrum shows a peak at lower energy. These results were interpreted in terms of a delocalized space charge zone, different kinetics for charge transfer and mass transport across the solvent swollen polymer film and different depth of penetration of the light as a function of wavelength.     

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.     

Effect of modified ITO substrate on electrochromic properties of polyaniline films

In this work, we report the morphological and electrochromic properties of electrochemically synthesized polyaniline (PANI) thin films on bare and modified indium–tin oxide (ITO) glass substrates. In the last case, the surface of ITO glass was covered by a self-assembled monolayer of N-phenyl-γ-aminopropyl-trimethoxysilane (PAPTS). Atomic force microscopy images and perfilometry show that smoother and thinner PANI films were grown on PAPTS-modified ITO substrates. PANI-based electrochromic devices (ECDs) were assembled by using a viscous polymeric electrolyte (PE) of LiClO₄ and polymethyl methacrylate (PMMA) co-dissolved in a mixture of propylene and ethylene carbonate. The architectural design of the devices was glass/ITO/PANI/PE/ITO/glass. A dual ECD was also prepared by collocating a poly(3-methylthiophene) (P3MT) thin film as a complementary electrochromic element. The effect of the PAPTS-modified ITO substrate is reflected in a higher optical transmittance at bleach state and a little less color change at 550 nm of PANI-based ECDs.