Evaporated CdS 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.

Fabrication of 4.9% efficient Cu2ZnSnS4 solar cell using electron-beam evaporated CdS buffer layer

The chemical bath deposition process of depositing CdS buffer layer was replaced with electron-beam evaporation method, full vacuum preparation process, which is more environmentally friendly and non-solution contact, was achieved to prepare Cu2ZnSnS4 (CZTS) solar cells. First, the influence of growth rate on the structural, optical, morphological, electrical properties of electron-beam evaporated CdS films was investigated. CdS thin films were deposited at growth rates of about 3, 6, 9, 12 Å/s, respectively. The results revealed that CdS thin films deposited at lower rate showed better crystallinity, higher transmittance, lower carrier concentrations, preferred for the application in CZTS solar cells. Then, kesterite Cu2ZnSnS4 thin film solar cells using the optimal CdS thin films as a buffer layer are fabricated, and a best efficiency of 4.87% was demonstrated. The statistical distribution of device performance parameters of 150 solar cells using electron-beam evaporated CdS buffer layer was also demonstrated.

Preparation of CuInSe2 films on large grain CdS films for superstrate-type solar cells

Superstrate-type CuInSe2 solar cells were fabricated by preparation of CuInSe2 films on CdS films with large grains. The CdS films with large grains were obtained by annealing of evaporated films with CdS: CdCl2 powders. The horizontal grain size of the annealed CdS films was about 3 times as large as the film thickness. The orientation of the CuInSe2 films on the CdS films was evaluated by X-ray diffraction. The CuInSe2 films on the annealed CdS films had a preferred orientation with (102) or (110) plane parallel to the substrate, while that on the evaporated CdS films highly oriented along direction. The grains of the CuInSe2 films grew along several directions on the different CdS gains. As a result of the measurement of I–V characteristics under illumination, the cells using the large grain CdS films had higher Jsc than that using the evaporated CdS films. The best cell performance obtained was Voc=0.357 V, Jsc=32.2 mA/cm2 and FF=0.582 in 0.075 cm2 area. The conversion efficiency of the cell was 6.71%.

Bistable optical device realization with thin Cds films

A thin (6 μm) CdS film thermally laser induced optical device (LIOD) was realized at 210 K using the 514.5 nm argon laser line. It is demonstrated for the first time that the observation of equally contrasted all optical (all-OB) and hybrid (HB) bistable loops is possible in thin CdS films. Furthermore, it is pointed out that the photocurrent dependence on temperature is different in sprayed and evaporated CdS films resulting in opposite HB loop fashions: clockwise for the sprayed and anti-clockwise for the evaporated thin films.

13.4% efficient thin-film CdS/CdTe solar cells

Cadmium telluride is a promising thin-film photovoltaic material as shown by the more than 10% efficient CdS/CdTe heterojunction solar cells. In this work, thin-film CdS/CdTe solar cells have been prepared using CdS films grown from an aqueous solution and p-CdTe films deposited by close-spaced sublimation (CSS). The properties of CdS films deposited from an ammonical solution of a Cd-salt, an ammonium salt, and thiourea have been controlled by optimizing the temperature and composition of the solution. The solution-grown CdS films have a high photoconductivity ratio, and its optical transmission is superior to that of vacuum evaporated CdS films. The properties of p-CdTe films deposited by CSS have been optimized by controlling the temperature and composition of the source material, and the substrate temperature. The properties of CdS/CdTe heterojunctions have been studied; junction photovoltage spectroscopy is used for the qualitative comparison of junction characteristics. Solar cells of 1-cm2 area with an AM 1.5 efficiency of 13.4% are reported.

ＣｄＳ蒸着膜を用いた光機能素子

ＣｄＳ蒸着膜を用いた光機能素子

Electrical transport behaviour of CdS/CuGaSe 2 thin film heterojunction

n-CdS/p-CuGaSe 2 thin film heterojunctions have been prepared by laser evaporating CuGaSe 2 onto vacuum evaporated CdS films. The dark current-voltage characteristics have been measured at different temperatures. They show that current transport is dominated by tunneling mechanism below room temperature while recombination of charge carriers is observed for temperatures above 303 K. The C-V measurements are also studied and the results are discussed.

Effect of thermal annealing on the cathodoluminescence of evaporated CdS films

Cathodoluminescence (CL) of evaporated undoped CdS films was investigated before and after thermal annealing in air and in vacuum at 300 °C for several hours. Pure CdS single crystals were studied for comparison. CL spectra were obtained at room temperature using the electron microprobe technique. The two well-known (green and red) CL bands of pure CdS were observed for all samples. Annealing of the films in air decreased the green luminescence while vacuum annealing led to its increase. The red luminescence increased under both annealing conditions. The results were explained in terms of radiative transitions attributed mainly to native defects and their complexes. The role of oxygen and residual impurities on the CL of thin films was discussed. A modulation of the CL spectra was obtained in thin films owing to optical interference.

Kinetics of oxygen chemisorption and its effects on electrical conductivity of thin evaporated CdS films

The mechanism and kinetics of chemisorption of oxygen on thin evaporated films of CdS have been investigated through changes in electrical conductivity caused by chemisorbed oxygen. Electrical conductivity of CdS films, when exposed to oxygen atmosphere falls exponentially with time, with a characteristic time determined by several factors such as temperature, electric field applied to the sample, illumination, etc. Electric field applied in appropriate direction is shown to enhance oxygen chemisorption. Complete theoretical treatment for these changes has been provided and the agreement between theory and experiment is shown to be excellent.

2.15 The stoichiometry of surfaces in cadmium sulphide thin films

The sulphur to cadmium ratio in cadmium sulphide thin films has been compared to that of a clean single crystal using Auger spectroscopy. Thermally evaporated CdS films were found to be deficient in sulphur in their outer layers, but were stoichiometric after etching. Electron beam evaporated CdS films were found to be stoichiometric. It was found that baking these films at 190°C in argon or nitrogen at atmospheric pressure increased the sulphur to cadmium ratio, probably due to sulphur diffusing to the surface and being physisorbed there. From measurements of the temperature dependence of the copper sulphide layer in CdSCu 2S cells it was found that chalcocite, djurleite and diginite copper sulphide phases are present even in efficient cells. A model is proposed for the structure of the copper sulphide layer in which the copper deficient phases are the result of excess sulphur on the CdS surfaces and vary in thickness according to the degree of non-stoichiometry of the CdS surfaces.

Phase Memory Effect in Photoconductive CdS Films

A new type of memory effect which has been found by the present author is investigated in detail in evaporated CdS films at low temperatures. If the light is turned off during the periodic photocurrent oscillation, the oscillation stops and the phase of the oscillation at the moment of turning off is retained; by turning on the light again, the oscillation begins at the retained phase (Phase Memory Effect). The memory remains unchanged during the memory state even if the bias voltage is interrupted or changed in value or polarity. The memory disappears in a certain time under the condition that the specimen is illuminated under a bias voltage smaller than the threshold voltage; in this case the lifetime of the memory is inversely proportional to the light intensity. It is shown that the phase memory effect is due to the travelling high-field domain which is caused by field-quenching and is controlled by the light and/or the bias voltage.

1290. Recrystallization of thermally evaporated CdS films via an H 2S heat-treatment process : L M Fraas et al, J Appl Phys, 46 (2), 1975, 491–495

1290. Recrystallization of thermally evaporated CdS films via an H 2S heat-treatment process : L M Fraas et al, J Appl Phys, 46 (2), 1975, 491–495

Recrystallization of thermally evaporated CdS films via an H2S heat−treatment process

It is observed that CdS films, evaporated so that a preferred c−axis columnar crystalline structure exists, undergo a recrystallization of texture in a postevaporation H2S heat−treatment process. This recrystallization process does not involve heavy−metal group−IB impurity doping and takes place at low enough temperatures so that soda−lime glass substrates may be used. These recrystallized films contain crystals of tens of microns in size and possess properties nearer those of CdS crystals than is usually found in CdS films.

The effect of bulk compensation on the sorption-dependent semiconductivity of thin evaporated CdS films

The effect of compensation on the chemisorption-dependent semiconductivity of thin evaporated CdS films is described. An exponential surface state distribution is associated with the chemisorption.

Some parameters affecting the resistivity of vapour deposited CdS films

Data are presented on the influence of the evaporation boat material and of a superimposed oxide dielectric film on the resistivity of evaporated CdS films.

Positive flat band voltages measured on MIS diodes consisting of evaporated CdS films

SiO 2 films with very low losses have been produced by the decomposition of Si(C 2H 5O) 4 in an r.f. glow discharge. Loss-tangents of less than 10 -4 and break-down voltages of about 1 × 10 7 V are obtainable by this method. The optical and electrical properties of these films are discussed. M-SiO 2-Si diodes with the SiO 2 film produced by the above method always show negative flat band voltages. On the other hand, M-SiO 2-CdS diodes with the CdS produced by evaporation onto a glass substrate always show positive flat band voltages. Until now this behaviour has only been found in MIS diodes with an Al 2O 3 insulating film. The influence on the properties of the MIS diodes of the SiO 2 preparation and of annealing the CdS films in a H 2S r.f. glow discharge are discussed.

Shift in Lux-Ampere and Current-Voltage Characteristics of Evaporated CdS Films, Due to the Photoelectret Effect

We have investigated the effect of the photoelectret state on lux-ampere and current-voltage characteristics of a CdS photoconductor. A shift in the lux-ampere and current-voltage characteristics is observed towards illumination and voltage axes, respectively. A percentage deviation as large as 30% is observed under polarization conditions defined by (a) an illumination intensity of 68 lx, (b) an electric field of 0.5 kV/cm, and (c) a time of 60 sec. Convergence of lux-ampere characteristics at high illumination level is explained by saturation of photopolarization with time.

Study of photoelectret phenomenon and dark depolarization current in evaporated semiconductor films of CdS

The photoelectret properties of vacuum evaporated CdS films on Pyrex glass plate have been studied. The deposition conditions were controlled to obtain high resistivity CdS films (resistivity of the order of 10 7 Ω cm). The photoelectret polarization was carried out by simultaneous application of electric field and white light illumination from tungsten lamp source. The polarization exhibits the linear dependence upon the polarizing voltage and polarizing time, with saturation effects for large values of these parameters. The polarization in films is found to be four orders of magnitude higher than the corresponding value in thick CdS crystals or powders. The dark depolarization current decay is very slow, and time for current, to decay to approximately half of its initial value varies between 1 and 8 min. This large decay time is correlated to the presence of deep trap levels in the CdS films. The study is of significance to improve the performance of thin film CdS devices.

Light sensitive CdS thin films with temperature resistant contacts

Evaporated CdS films are used as photo conductors for making an array of light sensitive resistors. The contacts and conductor tracks are made on the substrate glass prior to the evaporation of the CdS layer. These contacts have to withstand the high-temperature treatment to which the CdS layer is subjected to make it photo-sensitive. Gold contacts with a molybdenum underlayer appear to be satisfactory as for adherence to the glass and ability to withstand the high temperature treatment. The importance of adding the correct amount of oxygen during the activation process is shown once again. The change of dark current has been recorded during this process. Finally some properties are given of these light sensitive resistors a.o. light- and dark current and spectral response.

Effect of neutron irradiation on the acoustic performance of evaporated CdS films. (0.9 to 2.0 GHz transducers): Duncan, W., Hutchins, R., Richter, J., Stewart, P.A.M., Journal of Applied Physics, Vol 39, No 13 (December 1968) pp 5987–5994

(x)Ni0.4Zn0.6Fe2O4+(1−x)Ba0.6Sr0.4TiO3 composite ceramics with x=0.6, 0.7, 0.8, 0.9 and 1 were synthesized by solid state reaction method. The high dense composites have only two phases, i.e., Ni0.4Zn0.6Fe2O4 and Ba0.6Sr0.4TiO3. The permittivity ε′ of the composites decreases slightly with the frequency increasing from 3 MHz to 1 GHz. The permittivity ε′′ of the composites also shows a little increase with frequency in the 3 MHz–1 GHz range. The permeability displays a relaxation resonance within the 3 MHz–1 GHz frequency range. The permeability μ′ increases while the cut-off frequency decreases with the Ni0.4Zn0.6Fe2O4 concentration, obeying the Snoek's law μifr=constant. The permittivity ε′ of the composites decreases with Ni0.4Zn0.6Fe2O4 concentration. The composites have a relatively higher ε′ than the pure Ni0.4Zn0.6Fe2O4 at 1–10 GHz. In the frequency range of 1–10 GHz, the magnetic permeability μ′ reaches its maximum and μ′′ shows a minimum for the composite with x=0.6 in all ceramics. The permeability μ′ of the composites decreases with dc magnetic field at 1–10 GHz. The permeability shows a domain wall resonance, and the resonance frequency shifts to high frequency with the dc magnetic field. The permittivity was also influenced by the dc magnetic field due to a magnetodielectric effect.