Chemical Bath Deposited Cadmium Sulfide Research Articles

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells.

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu2 ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30nm). Interfacial engineering with LGO improved the Voc of the CZTSSe TFSCs from 466 to 502mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Thermal evaporation–deposited hexagonal CdS buffer layer with improved quality, enlarged band gap, and reduced band gap offset to boost performance of Sb2(S,Se)3 solar cells

Chemical bath–deposited cadmium sulfide (CBD-CdS) is the most widely used buffer layer material for antimony selenosulfide (Sb2(S,Se)3) photovoltaic devices, but its medium band gap, large band gap offset with transparent conducting oxides, and surface defects must be addressed to improve its applicability. Hexagonal CdS films (60 nm thick) prepared using the thermal evaporation method were used as the buffer layers of Sb2(S,Se)3 photovoltaic devices. Compared with CBD-CdS (60 nm thick), thermal evaporation–deposited CdS (TED-CdS) films have a larger band gap and fewer surface defects. The Sb2(S,Se)3 film deposited on TED-CdS is more compact, and its 1D ribbons are stacked more vertically to the substrate. The band gap offset between CdS and fluorine-doped tin oxide glass decreases from 0.76 eV (CBD-CdS) to 0.60 eV (TED-CdS). Therefore, suppressed electron trap density and charge recombination, as well as improved light harvesting, are achieved. By replacing CBD-CdS with TED-CdS, the best device efficiency increases from 6.14 % to 7.16 %, with the open circuit voltage, short-circuit current density, and fill factor increasing from 0.660 V, 15.96 mA/cm2, and 58.37 % to 0.664 V, 17.39 mA/cm2, and 62.85 %, respectively. The device stability and reproducibility are also improved.

Tunability and Graded Energy Band Gap of Chemical Bath Deposited Cadmium Sulfide (CdS) Thin Film for Optoelectronic Applications

CdS thin films have continued to receive scientific and technological attention due to their potential applications in efficient solar energy conversion and utilization in device fabrication. In this research, CdS thin films were deposited on indium doped tin oxide (ITO) substrates of dimension 2.3 by 2.4 cm2. Three different aqueous bath solutions of CdS were formed by increasing the concentration of cadmium acetate as a source of cadmium while the concentration of ammonium chloride and thiourea as a source of sulfur remained constant in the reaction bath as against the usual convention to ascertain the strength of the constituents in the reaction. The energy band gap of the films decreases with increase in the concentration of cadmium as a constituent of the bath while the films optical transmittance was found to increase with increase in concentration. This indicates that the energy band gap of the films can be predetermined by the choice of the constituent of the concentration in the chemical bath deposition technique (CBD). The increase in the transmittance for both as deposited and annealed CdS confirms the suitability of the films as window layer device, solar cell and optoelectronic applications.

Temperature-dependent structural, morphological and optical properties of chemical bath-deposited CdS films

Cadmium sulfide (CdS) films were grown on top of the fluorine-doped tin oxide (FTO) coated glass substrates at four different bath temperatures by chemical bath deposition (CBD) technique. X-ray diffraction (XRD), ultraviolet–visible (UV–Vis) spectrometry, energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscope (FESEM), photoluminescence (PL) and Fourier-transform infrared (FTIR) analyses were employed to examine the morphological and optical properties of deposited films. XRD studies confirmed the cubic crystalline nature of the deposited films which improved with increase in deposition temperature, and EDX along with FESEM study showed that the films were smooth and uniform with appropriate chemical composition. Band-gap values of the CBD CdS films calculated by UV–Vis spectrometry were found to be in the range of 2.32–2.39 eV. A strong blue band and a weak green band were observed in PL emission spectra. C–S, Cd-S and S–C–N vibrations were found by FTIR spectral analysis.

Polymer/Fullerene Blend Solar Cells with Cadmium Sulfide Thin Film as an Alternative Hole-Blocking Layer.

In this work, chemical bath-deposited cadmium sulfide (CdS) thin films were employed as an alternative hole-blocking layer for inverted poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. CdS films were deposited by chemical bath deposition and their thicknesses were successfully controlled by tailoring the deposition time. The influence of the CdS layer thickness on the performance of P3HT:PCBM solar cells was systematically studied. The short circuit current densities and power conversion efficiencies of P3HT:PCBM solar cells strongly increased until the thickness of the CdS layer was increased to ~70 nm. This was attributed to the suppression of the interfacial charge recombination by the CdS layer, which is consistent with the lower dark current found with the increased CdS layer thickness. A further increase of the CdS layer thickness resulted in a lower short circuit current density due to strong absorption of the CdS layer as evidenced by UV-Vis optical studies. Both the fill factor and open circuit voltage of the solar cells with a CdS layer thickness less than ~50 nm were comparatively lower, and this could be attributed to the effect of pin holes in the CdS film, which reduces the series resistance and increases the charge recombination. Under AM 1.5 illumination (100 mW/cm2) conditions, the optimized PCBM:P3HT solar cells with a chemical bath deposited a CdS layer of thickness 70 nm and showed 50% power conversion efficiency enhancement, in comparison with similar solar cells with optimized dense TiO2 of 50 nm thickness prepared by spray pyrolysis.

Effect of heat‐bias soaking on cesium fluoride‐treated CIGS thin film solar cells

AbstractThe combined effect of heating and forward bias voltage, that is, heat‐bias soaking (HBS) on cesium fluoride (CsF)‐free and CsF‐treated Cu(In1‐xGax)Se2 (CIGS) solar cells using chemical bath deposition‐cadmium sulfide (CBD‐CdS) buffer layer was investigated with varying the heating temperature, bias voltage, and biasing time. Heat bias soaking followed by heat soaking (HS) (ie, HBS/HS) treatments improved the open‐circuit voltage (VOC) and conversion efficiency for the CsF‐treated CIGS solar cell, whereas such a beneficial effect was not observed for CsF‐free CIGS solar cells. Capacitance‐voltage measurement confirmed the significantly increased carrier concentration (NCV) after HBS for the CsF‐CIGS solar cell, which is one of the possible reasons for the increased VOC. Because of the extremely high NCV, the short‐circuit current density (JSC) decreased owing to the narrower depletion width. However, the high NCV could be tuned to an appropriate value via a subsequent moderate heating procedure without biasing. As a result, the JSC loss was reduced, thereby improving the cell efficiency. These results open a new route to improve the efficiency of alkali‐treated CIGS solar cells.

Use of a New Organic Complexing and Buffer Agent for Zn(S,O) Deposition for High-Efficiency Cu(In,Ga)Se2-Based Solar Cells

Chemical-bath-deposited (CBD) Zn(S,O) is one of the most studied and promising Cd-free buffer layers for Cu(In,Ga)Se2 (CIGSe) -based solar cells and has already demonstrated its potential to lead to high-efficiency solar cells. However, CBD Zn(S,O) presents some inconveniences compared with a classic CBD cadmium sulfide (CdS) buffer. Indeed, Zn(S,O) deposition time, important ammonia concentration, and metastable behavior of the final devices are obstacle to its generalized use. A new complexing agent, i.e., morpholine, has been mixed to ammonia as a buffer and complexing agent. From a theoretical study, the mix of these leads to the formation of a majority zinc-morpholine complex instead of the classic zinc-ammonia one. In situ studies have shown an increase in the deposition rate, allowing us to reduce the concentration of the other reactants. Scanning electron microscopy and X-ray photoelectron spectroscopy measurements have shown that the deposited material presents equivalent covering properties and sulfur incorporation ([S]/([S] + [O]) ratio) to the one determined using standard formulations. Finally, CIGSe-based devices have been realized using the optimized CBD Zn(S,O) as a buffer layer. Higher efficiencies than CdS-buffered reference are measured, and no metastability is observed.

High-performance cadmium sulphide-based planar perovskite solar cell and the cadmium sulphide/perovskite interfaces

Planar heterojunction perovskite solar cell is one of the most competitive photovoltaic technologies, while charge transport materials play a crucial role. We successfully demonstrated an effective electron transport material, namely chemical bath deposited cadmium sulphide (CdS) film under low temperature, in perovskite-based solar cells. Power conversion efficiency of 16.1% has been achieved, which is comparable to that of devices based on TiO2 film prepared via low-temperature processes. Electronic impedance spectra reveal that the CdS-based device presents a higher recombination resistance than TiO2-based devices, which reduces carrier recombination and increases the open circuit voltage. The interface between CdS and perovskite was characterized with improved characteristics when compared to TiO2, e.g., efficient carrier extraction and reduced surface defect–associated degradation in the devices, which help to alleviate anomalous hysteresis and long-term instability. Furthermore, the entire device was fabricated via solution process with a processing temperature below 100°C, suggesting a promising method of further development of perovskite solar cells and commercial manufacturing.

Optimization of CdS buffer layer for high‐performance Cu2ZnSnSe4 solar cells and the effects of light soaking: elimination of crossover and red kink

AbstractA spike‐like conduction band alignment of kesterite absorbers with a CdS buffer layer is one of the key factors for high‐performance solar cells using this buffer/absorber heterojunction combination. However, it can also be the origin of fill factor and current‐reducing distortions in current–voltage curves, such as light/dark curve crossover, or an s‐like curve shape for long wavelength monochromatic illumination (red kink) if light‐dependent defect states are present in the buffer layer. In this work, we show that by changing the cadmium precursor source from sulfate to nitrate salts for the chemical bath deposited cadmium sulfide for Cu2ZnSnSnSe4/CdS heterojunction solar cells red kink can be eliminated, and crossover greatly improved (and eliminated entirely after light soaking). These improvements lead to a decrease in series resistance and an increase in fill factor and increase power conversion efficiency from 7.0% to 8.2%. We attribute this improvement to a reduction of deep level acceptor‐like traps states inside the CdS layer, which are responsible for an increase of the conduction band spike up to a current blocking value for the sulfate precursor case. Furthermore, the effects of light soaking will be discussed. Copyright © 2015 John Wiley & Sons, Ltd.

SOLUTION PROCESSING OF CADMIUM SULFIDE BUFFER LAYER AND ALUMINUM-DOPED ZINC OXIDE WINDOW LAYER FOR THIN FILMS SOLAR CELLS

Cadmium sulfide ( CdS ) and aluminum-doped zinc oxide ( Al : ZnO ) thin films are used as buffer layer and front window layer, respectively, in thin film solar cells. CdS and Al : ZnO thin films were produced using chemical bath deposition (CBD) and sol–gel technique, respectively. For CBD CdS , the effect of bath composition and temperature, dipping time and annealing temperature on film properties was investigated. The CdS films are found to be polycrystalline with metastable cubic crystal structure, dense, crack-free surface morphology and the crystallite size of either few nanometers or 12–17 nm depending on bath composition. In case of CdS films produced with 1:2 ratio of Cd and S precursors, spectrophotometer studies indicate quantum confinement effect, owing to extremely small crystallite size, with an increase in Eg value from 2.42 eV (for bulk CdS ) to ~ 3.76 eV along with a shift in the absorption edge toward ~ 330 nm wavelength. The optimum annealing temperature is 400°C beyond which film properties deteriorate through S evaporation and CdO formation. On the other hand, Al : ZnO films prepared via spin coating of precursor sols containing 0.90–1.10 at.% Al show that, with an increase in Al concentration, the average grain size increases from 28 nm to 131 nm with an associated decrease in root-mean-square roughness. The minimum value of electrical resistivity, measured for the films prepared using 0.95 at.% Al in the precursor sol, is ~ 2.7 × 10-4 Ω ⋅ cm. The electrical resistivity value rises upon further increase in Al doping level due to introduction of lattice defects and Al segregation to the grain boundary area, thus limiting electron transport through it.

Solution Based Cation-Exchange Process to Control Optoelectronic Properties of Cadmium Sulphide Thin Films

The control of optoelectronic properties of cadmium sulphide films, by a simple solution based ion exchange process, is an important aspect for various applications. The effect of post doping of certain metal ions on chemical bath deposited cadmium sulphide thin films has been studied in this work. An alkaline bath has been developed to obtain highly homogeneous CdS films on various substrates. X-ray diffraction of these films reveals that as deposited material is amorphous. The energy dispersive x-ray analysis confirms that a reasonable amount of cations can be incorporated into these films without affecting its atomic structure. Optical absorption and electrical conductivity measurements suggest that there is a clear effect of cation-exchange with respect to lead, silver and copper ions. Electrical conductivity is significantly improved by copper ions, this change is close to five orders of magnitude with respect to intrinsic films. These results clearly show that both substitutional and interstitial mechanisms are involved in the cation-exchange process of these films.

On the sub-band gap optical absorption in heat treated cadmium sulphide thin film deposited on glass by chemical bath deposition technique

The sub-band gap optical absorption in chemical bath deposited cadmium sulphide thin films annealed at different temperatures has been critically analyzed with special reference to Urbach relation. It has been found that the absorption co-efficient of the material in the sub-band gap region is nearly constant up to a certain critical value of the photon energy. However, as the photon energy exceeds the critical value, the absorption coefficient increases exponentially indicating the dominance of Urbach rule. The absorption coefficients in the constant absorption region and the Urbach region have been found to be sensitive to annealing temperature. A critical examination of the temperature dependence of the absorption coefficient indicates two different kinds of optical transitions to be operative in the sub-band gap region. After a careful analyses of SEM images, energy dispersive x-ray spectra, and the dc current-voltage characteristics, we conclude that the absorption spectra in the sub-band gap domain is possibly associated with optical transition processes involving deep levels and the grain boundary states of the material.

Properties of CdS Chemically Deposited thin films on the Effect of Ammonia Concentration

The effect of ammonia concentration on electrical properties, optical properties and structural properties of chemical bath deposited (CBD) Cadmium sulphide (CdS) thin films has been revealed. The films were prepared by using cadmium acetate as cadmium ion (Cd 2+ ) source, thiourea as sulphur ion (S 2- ) source and ammonia as the complexing agent. Ammonia concentration was changed from 0.1 M - 3.0 M. Ammonia concentration at 2.0 M uniform, dense and continuously coated films were obtained. Not only typical cadmium- pure but also unusual sulphur deficiency phenomena were observed for CBD CdS thin films. In the present investigation, the carrier concentration varied form 1.831X10 6 cm -3 to 1.026X10 6 cm -3 when ammonia concentration is changed from 0.5M to 2.5 M. The direct band gap energy at 0.5M is 1.92eV while at 2.5M is 2.65eV. The surface morphology of as deposited thin films is almost smooth and no grains were observed clearly at low molar concentration and predominant grains at the concentration of ammonia is 2.0M. By estimated Cd:S ratio value is found to be 1.04 by using EDAX. The thin film deposited at 2.0M concentration shows the highest degree crystallinity. The formation mechanism of the films with various ammonia concentrations is discussed. Keywords: Ammonia effect, CBD-CdS thin films, Optical studies, SEM, XRD

The role of Cl in the chemical bath on the properties of CdS thin films

The impact of chlorine (Cl) concentration in the chemical bath deposition (CBD) solution on the properties of post-deposition annealed cadmium sulfide (CdS) thin films on glass substrates was investigated. CBD CdS films with a thickness of 250nm were deposited from a standard bath containing cadmium sulfate, ammonia and thiourea. For doping of the CdS films with Cl, ammonium chloride was also added to the bath. The CBD process is characterized by the opportunity it provides to incorporate solution components like hydroxide group and chlorine into the lattice of growing films. CBD CdS thin films with different [Cl]/[Cd] ratios were characterized. The structural as well as electrical and photoluminescence (PL) properties of CBD CdS thin films indicated the creation of electrically and optically active defects in the CdS lattice in the deposition process. Both the incorporated hydroxychloride groups and H2 annealing play a substantial role in controlling the CdS electrical and optical properties. A good correlation between the PL and electrical properties of CBD CdS thin films was achieved and explained on the basis of a classical understanding of donor–acceptor pairs formed by the ClS+ and (VCdClS)− centers and emergence of Cd excess in CBD CdS lattice due to destruction of incorporated hydroxide groups in the annealing process.

Structural, Optical, and Electro-Optical Properties of Thermally Evaporated Tin Sulphide Layers

Thin films of tin sulphide (SnS) were deposited onto glass substrates using the thermal evaporation method. The substrate temperature, Ts was varied in the range, 280–360 °C, keeping other growth parameters constant and the effects on the chemical and physical properties of the layers deposited were investigated. The layers were observed to consist of densely packed grains, up to 9.5 µm in diameter, and X-ray diffraction studies showed the layers had a strong preferred (040) orientation. The energy band gap, determined from optical studies was found to be in the range 1.30–1.34 eV, while the optical absorption coefficient was found to be >104 cm-1 for photons with energies greater than the energy bandgap. The electrical resistivity of the films decreased with an increase of substrate temperature. Heterojunction devices were made using chemical bath deposited cadmium sulphide (CdS) as the window layer and the minority carrier diffusion length in the SnS measured to be 0.23 µm using spectral response studies.

Morphological and stoichiometric study of chemical bath deposited CdS films by varying ammonia concentration

The influence of ammonia concentration on stoichiometric, surface morphological, and optical properties of chemical bath deposited cadmium sulphide thin films has been studied systemically. Chemical bath deposition (CBD) of CdS thin films was carried out via using cadmium acetate as the cadmium ion source, thiourea as the sulphur source and ammonia as the complexing agent. Ammonia concentration was changed from 0 to 2.5 M. At ammonia concentration greater than or equal to 0.1 M and lower than 0.6 M, nanowires or flake-like structures were obtained. At ammonia concentration ranging from 0.8 to 2.0 M, uniform, dense, and continuously coated films were obtained. The energy dispersive X-ray spectroscopy (EDXS) analysis revealed that as the ammonia concentration changed from 0.1 to 2.0 M, the Cd/S ratio in the obtained film increased from 0.24 to 2.61. Not only typical cadmium-poor but also unusual sulphur deficiency phenomena were observed for CBD CdS thin films. The films deposited with ammonia concentration of 1.0 M show the highest degree of crystallinity and closest stoichiometry Cd/S≅1, and have a preferred orientation. The direct band energy gaps of as-grown films were found to be 2.23–2.77 eV. The formation mechanism of the films with various morphologies and cadmium and sulphur deficiencies are discussed.

In Situ thickness measurements of chemical bath-deposited CdS

Chemical bath-deposited (CBD) cadmium sulfide has been a component in some of the most efficient Cu(In,Ga)Se 2-based solar cell devices. While accurate in situ measurements of the thicknesses of vacuum-based depositions can be accomplished with quartz crystal monitors, solution-based growth rates of CdS vary greatly on different substrates and are therefore difficult to measure. This work discusses the effectiveness of using an optical reflectance-based measurement of the growing film to determine the film’s thickness. On specular molybdenum films the measurement is accurate, while on the rough, poorly reflecting Cu(In,Ga)Se 2 (CIGS) films, the measurement is hampered.

Characterization of CdS thin films grown by chemical bath deposition using four different cadmium sources

A comprehensive study of the effect of cadmium sources on chemical bath deposited cadmium sulfide thin films is reported. Four different cadmium sources; cadmium sulfate, cadmium chloride, cadmium iodide, and cadmium acetate have been used. The effect of cadmium sources on film optical/electrical properties as well as film thickness, composition, crystal structure, and surface morphology was studied. Film growth rate and band gap were found to be sensitive to the Cd source used. Films were found to be highly stoichiometric when cadmium chloride and cadmium iodide were used. The lowest resistivity and highest mobility and band gap obtained were in the case of cadmium sulfate. All films were found to be cubic, regardless of the Cd salt used.

A Study of the Effect of Different Cadmium Sources on Chemical Bath Deposited CdS Thin Films

A comprehensive study of the effect of the cadmium source on chemical bath deposited cadmium sulfide thin films is reported. Four different cadmium sources, namely cadmium sulfate, cadmium chloride, cadmium iodide, and cadmium acetate have been used. Single-dip, for 15 minutes, as well as multi-dip depositions (four successive depositions for 8-12 minutes each) were carried out at 70oC on quartz substrates. The effect of changing the cadmium source on the film optical/electrical properties as well as the film thickness was studied. Hall Effect measurements were carried out for all films as well as Rutherford Back Scattering (RBS), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM).

Carrier transport properties of Al∕CdS∕GaSb structures

Low temperature chemical bath deposited cadmium sulfide (CdS) has been used as an interfacial layer on n- and p-GaSb to overcome the limitations posed by the highly unstable GaSb surface. The electrical characteristics of Al∕CdS∕GaSb structures have been studied. The current transport in these structures is governed by the barriers at the CdS∕GaSb interface with an effective series resistance from the bulk of the interfacial CdS layer. Space charge limited current in the interfacial layer gives rise to an increase in the forward turn-on voltages. These structures demonstrate excellent characteristics in the reverse bias region such as high breakdown voltages, low surface leakage current, and good reverse current saturation. Significantly, rectifying junctions on p-GaSb have been achieved with barrier height as high as ∼0.6eV and breakdown voltages of ∼4.5V.