Polycrystalline CdS Thin Films Research Articles

Towards halide treatment on CdS thin films for solar cell applications: An evolution to ion size impact on segregation and grain boundaries passivation

In this study, an impact of halide ion size on the physical properties of 200 nm thin thermally evaporated CdS films is explored. The ex-situ halide treatment is performed using 0.2 M solutions of halide compounds viz. CdCl2, CdI2, MgF2, MgCl2 and MgI2 separately and subsequently air annealing at 200 °C. A detailed grain growth mechanism in CdS films based upon nature and size of cations and anions to the treatment species is deeply discussed. The recrystallization responsible for modulation in structural, optical, topographical, photoluminescence, electrical, morphological and compositional properties of CdS films is explored via formation of different complexes. The XRD patterns reveal formation of polycrystalline CdS thin films having mixture of cubic (with space group F4̅ 3 m) and hexagonal phases with (220) cubic preferential orientation. Different crystallographic constants are calculated and discussed where the crystallite size is varied from 34 nm to 48 nm with treatment. Surface topographical study indicates that surface roughness is altered with the halide treatment and observed maximum for MgCl2 treated CdS thin films. The pristine and halide treated CdS thin films show Ohmic nature owing to linear current-voltage behavior. Optical study demonstrates that MgI2 treated films have higher transmittance and lower absorbance and energy band gap is measured out in 2.04–2.48 eV range. PL spectra of CdS films reveal a strong red emission band appeared at ∼685 nm corresponding to Sulfur vacancies (VS) and possible A centers (XS+VCd2−)−. FESEM exploration reveals grain growth with probable activation where maximum grain size is obtained for MgF2 treated films. The EDS patterns confirm deposition of CdS films and also integration of F, Cl, I and Mg in the associated treated CdS films. The investigated findings reveal that size of halide ions plays a decisive role in modification of the physical properties of CdS thin films which could be implicated as optical window to CdTe and CIGS based photovoltaic devices.

Preparation of radial ZnSe-CdS nano-heterojunctions through atomic layer deposition method and their optoelectronic applications

Radial 1-D nano-heterojunctions have distinct optoelectronic properties. However, their complex fabrication process is still the bottleneck of device applications. Herein, a facile atomic layer deposition (ALD) method was used to coat a polycrystalline CdS thin film with high uniformity and controllable thickness on the surface of the as-synthesized p-type ZnSe nanowires, for fabricating radial 1-D ZnSe-CdS nano-heterojunctions. The nano-heterojunctions exhibited excellent optoelectronic properties. Under blue/violet light, the nano-heterojunctions obtained a response radio of ∼5 × 103, a responsitivity of ∼1.43 A/W, a gain of ∼3.78 and a detectivity of ∼0.57 × 1012 cmHz1/2W−1 at zero bias. Furthermore, the nano-heterojunction also showed obvious photovoltaic characteristic with a power conversion efficiency of ∼0.96%. This method is expected to play an important role in nano-heterojunction construction and their device applications.

Preparation and Characterization of Polycrystalline CdS Thin Films Deposited by Chemical Bath Deposition

Preparation and Characterization of Polycrystalline CdS Thin Films Deposited by Chemical Bath Deposition

Room Temperature Pulse Electrodeposition of CdS Thin Films for Application in Solar Cells and Photoelectrochemical Cells

Polycrystalline CdS thin films comprising of hexagonal crystal structure are successfully fabricated by pulse electrodeposition using a two-electrode system at room temperature. The method utilizes tartaric acid as a complexing agent for the first time that minimizes the formation of colloidal sulfur while optimizing the deposition potential and avoids conventional post-deposition heat-treatment step. Additionally, deposition time is varied to obtain the desired thickness of CdS films suitable for application in thin films solar cells. Pulse electrodeposited CdS films are comprehensively characterized using SEM, XRD and micro-Raman analyses unveiling the stoichiometric polycrystalline hexagonal CdS films. The bandgap is determined to be around 2.4 eV from optical studies. Flat-band potential and carrier density are inferred to be −0.82 V vs SCE and 8.51 × 1018 cm−3, respectively, from the Mott-Schottky analysis. Photoelectrochemical studies show a photocurrent density of about 0.2 mA/cm2 at 0.2 V vs SCE for the optimized films, which is competent with previously reported values. These pulse electrodeposited CdS films prepared without any high-temperature treatment are of high quality with desirable properties and are suitable for application in thin films solar cells and photoelectrochemical cells.

Studies on Dilute Magnetic Semiconducting Co-Doped CdS Thin Films Prepared by Chemical Bath Deposition method

The Co-doped CdS polycrystalline thin films were deposited onto the glass substrates by chemical bath deposition method. All the deposited films were characterized for structural, surface morphology, elemental, optical and magnetic properties using X-ray diffractometer, scanning electron microscopy (SEM), energy dispersive X-ray, UV–Vis. NIR spectrometer and Vibrating sample magnetometer before and after doping. The thin films exhibit hexagonal wurtizite structure with (0 0 2) preferential orientation and the calculated crystallite size is 29 and 18 nm for pristine and Co-doped CdS films respectively. X-ray photoelectron spectroscopy (XPS) analysis showed the presence of Cd, S and Co peaks along with O 1s peak. The studies revealed that the surface Cd atoms of films are found to be bound either to S atoms or O atoms. SEM images showed grainy surface for pristine which gets modified under Co doping. Energy dispersive analysis using X-rays confirmed the presence of Cd, S and Co in the films. From optical transmittance spectra, the bandgap was estimated and found to be varying from 2.4 to 2.27 eV with the increase of Co concentrations. The room-temperature ferromagnetic behavior was exhibited by Co-doped CdS films.

Improvement of CdS Thin Films Optical Properties and Crystallinity by Laser Radiation

A possibility to improve the quality of polycrystalline CdS thin film by Nd:YAG laser irradiation was shown. The CdS thin film was prepared by closed space sublimation method on ITO/glass structure and irradiated by Nd:YAG laser with intensity up to 15 MW/cm2. Several evidences demonstrated the improvement of the optical properties of the CdS thin film after the laser radiation: appearance of photoluminescence band at ~2.530 eV, which is attributed to bound exciton emission; appearance of “spike” in the reflection spectrum at ~2.573 eV between upper and lower polariton branches and the increase intensity of LO phonon line at 305 cm-1about 3 times in Raman back scattering spectra. Moreover, the increase by 20% of the size of crystals in CdS after laser radiation takes place, which was proved by using X-Ray diffraction analysis.

Deposition methods and properties of polycrystalline CdS thin films

CdS thin film was used as a suitable window layer for CdS/CdTe solar cell, and the properties of CdS thin films deposited by pulsed laser deposition (PLD), chemical bath deposition (CBD) and magnetron sputtering (MS) were reported. The experimental results show that the transmittances of PLD-CdS thin films are about 85% and the band gaps are about 2.38–2.42eV. SEM results show that the surface of PLD-CdS thin film is much more compact and uniform. PLD is more suitable to prepare the CdS thin films than CBD and MS. Based on the thorough study, by using totally PLD technique, the FTO/PLD-CdS(150 nm)/CSS-CdTe solar cell (0.0707 cm2) can be prepared with an efficiency of 10.475%.

Synthesis of CdS with Large Band Gap Values by a Simple Route at Room Temperature

We report the synthesis of CdS polycrystalline thin films deposited with 0%, 0.88%, 1.78%, 2.58%, and 3.40% (volume fraction, φ) O2 in sputtering Ar gas using a radio frequency magnetron sputtering method. The obtained CdS samples were characterized by X-ray diffraction, scanning electron microscope, Raman

Improving CdS/CdTe thin film solar cell efficiency by optimizing the physical properties of CdS with the application of thermal and chemical treatments

CdS polycrystalline thin films have been used as a window layer in CdTe, Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells; the specific optical and electrical properties of the CdS-partner play an important role in the photovoltaic device performance, so that one of the problems to be solved is to reduce the pin-hole density and intergrain cavities on the CdS surface. SnO2:F/soda-lime glass substrates (FTO) were treated with HCl, Ar and O2. CdS films were deposited on FTO substrates by using the chemical bath deposition technique and then CdS thin films were thermally annealed on CdCl2, Ar atmospheres and muffle at atmospheric ambient. Optical and morphological measurements revealed that the HCl treatment improves the CdS growth because the defect levels were reduced while the grain size is increased. The processed CdS-thin films were applied on CdTe solar cells and the photovoltaic efficiency was about 12% when the CdS thin films were thermally annealed in muffle at atmospheric ambient.

Influence of CdS Thin Films Growth Related with the Substrate Properties and Conditions Used on CBD Technique

CdS polycrystalline thin films have been used as window layer in solar cells; the quality of the CdS-partner plays an important role in the photovoltaic device performance. CdS thin films were deposited using Chemical Bath Deposition (CBD) and photoassisted-CBD technique. The SnO2:F substrates used in this work were treated with HCl (0.1M) and some of themwere annealed in different atmospheres (Ar and O2). The properties of CdS thin films were influenced by the HCl treatment, position and the movement of substrates inside the reactor glass. CdS samples were deposited in areas of 4cm2, 50cm2 and 100cm2; obtaining thin films with thickness of 35 - 300nm with good crystalline quality and uniform morphology. All samplespresented optical transmittance around of 85 - 91% and average gap energy of 2.5eV.

Influence of film thickness and In-doping on physical properties of CdS thin films

Abstract Polycrystalline CdS thin films were deposited on glass substrates by close spaced sublimation technique. Samples of various thicknesses, ranging from 250 to 940 nm were obtained. The optical and electrical properties of pure CdS thin films were studied as a function of film thickness. The resistivity of as-deposited CdS films was in the order of 10 6 –10 8 Ω cm, depending upon the film thickness. In the high temperature region, carriers are transported over the grain boundaries by thermionic emission. Resistivity was reduced to the order of 10 −2 –10 1 Ω cm by the thermally diffusion of indium into CdS films, without changing the type of carriers. The annealing temperature dependence of structural, optical and electrical properties of In-doped CdS films showed that the samples annealed at 350 °C and 400 °C exhibited better results.

Current transport mechanism in CdS thin films prepared by vacuum evaporation method at substrate temperatures below room temperature

CdS thin films were deposited by vacuum deposition method at low substrate temperatures instead of the commonly used vacuum deposition at high substrate temperatures (TS>300K). The effect of low substrate temperature on the current transport mechanisms in polycrystalline CdS thin films has been studied as a function of temperature over the temperature range 100–300K. Both thermally assisted tunneling of carriers through and thermionic emission over the grain boundary potential have contributions to the conduction in the range 250–300K for the sample prepared at 300K substrate temperature. The dominant conduction mechanism of the samples prepared at 200K and 100K is determined as thermionic emission over 200K and Mott's hopping process below 200K. The Mott's hopping process is not applicable for the sample prepared at 300K.

Preparation and Properties of CdS Thin Films Deposited by Chemical Bath Deposition

CdS thin films deposited on glass substrate are prepared by chemical bath deposition using the reaction between CdSO4 and CS (NH2)2. The composition, surface morphology and structural properties of as-deposited and annealed CdS thin films were studied using scanning electron microscopy (SEM), X-ray diffractometry (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques. The results indicate that the dense, homogeneous polycrystalline CdS thin films with smooth surface can be obtained by chemical bath deposition. The CdS thin films have cubic structure and the ratio of S and Cd is 1:1 in CdS thin films. Optical properties of CdS films were measured with ultraviolet-visible spectrophotometer. The optical band gap energy (Eg) of film sample was found to be 2.31 eV.

Structural and optical analysis of CdS thin films grown by magnetron sputtering technique

Structural and optical characterizations were performed on CdS polycrystalline thin films grown by magnetron sputtering method. The thicknesses of the films were estimated to be 200-550 nm by scanning electron microscopy (SEM) measurements. X-ray diffraction (XRD) data show that CdS films are in the stable hexagonal crystalline structure. Using Debye-Scherrer's formula, the average grain size for the samples was found to be 40 ~ 45 nm. X-ray photoelectron spectroscopic (XPS) experiments indicate that the CdS samples are Cd rich in the surface region and S rich in the bulk region of the film. Photoluminescence (PL) bands, including green emission band, yellow emission band, orange emission band, and red emission band, were observed at room temperature and analyzed for the CdS films. The energy gap of the samples was also estimated and discussed from the room temperature optical transmittance spectra.

Broadening of optical transitions in polycrystalline CdS and CdTe thin films

The dielectric functions ε of polycrystalline CdS and CdTe thin films sputter deposited onto Si wafers were measured from 0.75 to 6.5 eV by in situ spectroscopic ellipsometry. Differences in ε due to processing variations are well understood using an excited carrier scattering model. For each sample, a carrier mean free path λ is defined that is found to be inversely proportional to the broadening of each of the band structure critical points (CPs) deduced from ε. The rate at which broadening occurs with λ−1 is different for each CP, enabling a carrier group speed υg to be identified for the CP. With the database for υg, ε can be analyzed to evaluate the quality of materials used in CdS/CdTe photovoltaic heterojunctions.

Swift heavy ion induced structural changes in CdS thin films possessing different microstructures: A comparative study

This study is carried out to verify the role of thin film microstructure in determining the energy relaxation processes of swift heavy ions in CdS polycrystalline thin films. Two sets of CdS thin film samples, differing in their microstructures, prepared using thermal evaporation and spray pyrolysis, are irradiated with 100 MeV Ag ions using Pelletron accelerator. It is observed that the effects produced differ significantly in the two films. For the evaporated films, defect annealing dominates for lower irradiation fluences but at higher fluences the effects due to defect creation and their migration are dominant. A transformation from the metastable cubic to hexagonal phase together with the creation of a significant amount of compressive strain is seen in these films for irradiation at the highest fluence. The optical absorption of the samples shows an increase in band gap from 2.34 eV for the as grown film to 2.43 eV for the sample irradiated at the highest fluence that is further confirmed by photoluminescence (PL) studies. In contrast, the spray deposited samples undergo a significant improvement of crystalline quality for all fluences as shown by an increase in x-ray diffraction peak intensity, sharper optical absorption edge, reduction in defect PL intensity, and removal of asymmetry in the line shape of the longitudinal optical phonon on its lower wavenumber side in Raman spectra.

Photoinduced current transient spectroscopy technique applied to the study of point defects in polycrystalline CdS thin films

CdS thin films of variable thickness (between 160 and 1200 nm) were prepared using rf magnetron sputtering. X-ray diffraction measurements showed that the films have hexagonal structure and that the crystallites are preferentially oriented with the ⟨002⟩ axis perpendicular to the substrate surface. The results of electrical conductivity measurements as a function of film thickness and of temperature provide evidence that the conductivity is controlled by a thermally activated mobility in the presence of an intergrain barrier. The room temperature barrier height ϕ decreases with the increase in film thickness. Values of ϕ between 0 and 0.25 eV were determined. Photoinduced current transient spectroscopy performed on five samples having different thicknesses showed the presence of 11 traps with activation energies in the range 0.08–1.06 eV; deeper traps being observed on thinner films. By comparison with literature results, seven traps are attributed to native defects and foreign impurities (mainly Cu, Au, and Ag). Four other traps, not previously observed, are attributed to residual defects. The observation that deeper traps are detected in samples with larger barrier heights has been discussed and interpreted in terms of the energy band profile near the grain boundary.

Effect of oxygen on CdS polycrystalline thin films prepared in ambient of Ar and O2 by close spaced sublimation technology

In this paper，CdS polycrystalline thin films were prepared with the close-spaced sublimation （CSS） technology. In the deposition process，a controlled ambient of Ar and O2 mixture was used. The effect of oxygen on the microstructure，composition，morphology as well as optical and electrical properties of CdS thin films were studied systemically by XRD，XPS，AFM，UV-VIS and four-probe array method. The results showed that the CSS-grown CdS thin film has the hexagonal structure with crystallite size about 40 nm and the composition of them were rich in S. The impurity oxygen atoms mixed into CdS thin films and partly formed CdO，and with the increase of O2 concentration the chemical composition tinds more closely to the ideal stoichiometric proportion of CdO，the optical gap of thin films broadened and the ratio of photoconductivity to darkconductivity increased. Fortunately these effects are benefitial to CdS thin films for the photovoltaic application as a window layer.Additionally，it view of CdS/CdTe interface，we investigated the influence of oxygen on the cross-section morphology of CdS/CdTe thin films and the spectral response （SR） of the device. Then the results indicated that with the increase of O2 concentration in deposition ambient，the interdiffusion between CdS and CdTe decreased，and the SR of device in the range from 500 nm to 600 nm increased.We believe that the oxygen in CSS-grown CdS thin films plays a very important role in two aspects，firstly in improving the quality of CdS thin films for photovoltaic application，secondly in optimizing the CdS/CdTe interfacial properties in the CdTe solar cells.

Thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth (GREG) technique using photoacoustic methods

In this work we report the study of the thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth technique. CdS films were grown on pyrex glass substrates. These studies were carried out using an open photoacoustic cell made out of an electret microphone. From X-ray diffraction, atomic force microscope and photoluminescence measurements we observed polycrystalline CdS films with good morphology and crystalline quality. We obtained a thermal diffusivity coefficient of our samples with values ranging from 3.15 to 3.89 × 10 − 2 cm 2/s. For comparison, we measured a value of 1.0 × 10 − 2 cm 2/s for the thermal diffusivity coefficient of a CdS single crystal. We measured an energy gap value of 2.42 eV for our samples by using a photoacoustic spectroscopy system.

The Effect of Ionizing Radiations on the Structural, Electrical and Optical Properties of AIIBVI Polycrystalline Thin Films Used as Solar Cell Materials

AbstractThe effects of irradiation with high-energy protons (3 MeV, up to a fluency of 1013 protons/cm2), on structural, electrical and optical properties of polycrystalline CdS and CdTe thin films have been investigated. XRD investigation has revealed that the films contain wurtzite-type CdS, (001) preferentially oriented in the growth direction, and cubic phase CdTe, respectively. The defects induced by ionizing radiations have been studied by thermally stimulated currents spectroscopy (TSC). Parameters of identified defect levels were determined and their possible origin is discussed.