Chemical Bath Deposition Of CdS Research Articles

Cd 2 + ∕ N H 3 -treatment of Cu(In,Ga)(S,Se)2: Impact on the properties of ZnO layers deposited by the ion layer gas reaction method

Cu ( In , Ga ) ( S , Se ) 2 - (“CIGSSe”) based solar cells with a ZnO layer deposited by the ion layer gas reaction (ILGAR) method yield superior efficiencies (15.0%) than the references with a chemical bath-deposited CdS buffer (14.1%). However, this high performance is only reached if the absorber is pretreated in a Cd2+- and aqueous ammonia-containing bath prior to the ILGAR-ZnO deposition. The photovoltaic as well as the dark device parameters are strongly influenced by this treatment. Scanning and transmission electron microscopy (TEM) as well as x-ray diffraction measurements reveal a different morphology and structure of ILGAR-ZnO layers on top of Cd2+∕NH3-treated and on as-deposited absorbers, indicating a considerably modified absorber surface. By energy dispersive x-ray analysis in the TEM, Cd could only be identified at the ILGAR-ZnO∕Cd2+∕NH3-treated-CIGSSe interface of the respective cross sections, if the absorber was treated in a bath with an atypically high Cd2+-concentration. In this case a Cd-containing thin layer between ZnO and CIGSSe was observed in TEM images.

Optimisation of the CBD CdS deposition parameters for ZnO/CdS/CuGaSe 2/Mo solar cells

We present an optimisation of our recipe for the CdS chemical bath deposition process as applied to solar cells based on polycrystalline CuGaSe 2 (CGSe) absorber layers prepared in two stages by physical vapour deposition. We investigate the influence of the ammonia (NH 3) and the thiourea (H 2NCSNH 2) concentration, both being constituents of the chemical bath deposition (CBD) solution, at a deposition temperature of 80 °C on the microstructural and optical properties of CdS layers and on ZnO/CdS/CuGaSe 2/Mo device parameters. The composition of the CdS layers and their thickness were determined using X-ray Fluorescence Analysis. Transmission and reflection measurements performed at 300 K were used for the calculation of absorption and optical band gap energy ( E g). The E g values of the films varied from 2.41 to 2.46 eV depending on deposition conditions. Cubic phase of the as-grown layers was identified by X-ray diffraction analysis. An improvement in the investigated solar cells efficiency was achieved when the ammonia concentration was increased and the thiourea concentration was reduced, compared to the previously used standard HMI recipe. The influence of the CBD CdS preparation recipe on the ZnO/CdS/CuGaSe 2/Mo electrical and photoelectrical properties is discussed.

The effect of oxygen on interface microstructure evolution in CdS/CdTe solar cells

AbstractMicrostructural changes at the CdS/CdTe solar cell interface where close‐spaced sublimation (CSS) is used as the growth technique to deposit the p‐type CdTe absorber layer are studied by systematic layer characterization at various stages during heterojunction growth. CdS layers grown by both chemical bath deposition (CBD) and CSS provide a basis for determining the effects of CdS crystallinity, grain size, and oxygen content on the subsequent CdTe layer. As‐grown CBD CdS films exhibit small grains and variations in optical properties attributed to film impurities. In contrast, CSS yields CdS films with good crystallinity, larger grains, and nearly ideal optical properties. The hexagonal nature of CSS‐grown CdS is seen to nucleate hexagonal CdTe during the initial stages of CdTe film growth. Cubic CdS deposited by CBD in contrast promotes cubic CdTe nucleation. Oxygen anneals in the latter case can aid hexagonal CdTe nucleation. Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) of the CdS/CdTe interface show CdS‐dependent differences in interdiffusion at the interface. This interdiffusion appears to be determined by the oxygen level in the CdS. When low‐oxygen‐containing CSS CdS films are used, sulfur diffusion is substantial, leading to significant consumption of the CdS layer. When these same films are annealed in oxygen, the consumption is reduced. Te diffusion into the CdS layer is also observed to decrease with oxygen anneals. Optical modeling shows that Te alloying with the CdS layer can greatly reduce the short‐circuit current of CdS/CdTe devices. Copyright © 2002 John Wiley & Sons, Ltd.

Optimization of CBD CdS process in high-efficiency Cu(In,Ga)Se 2-based solar cells

We present an optimization of the CdS chemical bath deposition process as applied to high-efficiency Cu(In,Ga)Se 2 photovoltaic thin-film absorber materials. Specifically, we investigated deposition time (thickness), bath temperature (65, 80 and 90°C) and a Cd 2+ partial-electrolyte treatment of the chalcopyrite absorber prior to CdS deposition. We found that thinner CdS layers (grown on as-deposited absorbers) allowing more light to reach the junction are not necessarily conducive to higher short-circuit current density. Device performance was found to be dependent on the CdS layer thickness, but rather independent of the growth temperature. On the other hand, devices prepared from absorbers subjected to a Cd 2+ partial electrolyte treatment show that the device performance dependence on CdS thickness is somewhat alleviated, and devices incorporating thinner CdS layers are possible without loss of parameters, such as open-circuit voltage and fill factor.

Structure and morphology of chemical bath-deposited CdS films and clusters

Thin films of cadmium sulfide have been deposited on glass substrates and the structural properties of films have been investigated using scanning electron microscopy and X-ray diffraction techniques. The films consist of domains (groups of grains) and weakly bound grain clusters. The structural parameters of grains, domains and clusters and the effect of film thickness on these parameters are reported. From the measurement of lattice constants in CdS films and in free CdS clusters, it has become evident that the films on glass substrates have a tensile strain along their planes. The effect of thermal annealing on the partial relaxation of the strain is discussed.

Study of Cd-free buffer layers using In x(OH,S) y on CIGS solar cells

The alternative buffer layer material In x (OH,S) y was deposited on Cu(In,Ga)Se 2 (CIGS) thin films by chemical-bath-deposition (CBD). The impurities in In x (OH,S) y buffer layers and their atomic concentration were characterized by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses. In addition, AES was used to depth profile the samples. The band-gap energy of the deposited In x (OH,S) y was determined from optical absorption data. Both the dark- and photo-current-voltage ( I– V) characteristics of the CIGS solar cells with In x (OH,S) y buffer layers were measured, and the results were compared to the CIGS cells deposited with CBD CdS buffer layers.

Progress towards modelling the CdS chemical bath deposition process

Progress is reported towards the development of a comprehensive model useful in process design and optimisation of CdS thin-film growth. Model equations are developed for the temporal variation of reactants concentrations as well as of the precipitating solid phase, both in the bulk and on the substrate. A combination of possible elementary mechanisms is employed and the resulting system of equations is solved numerically. Computational results show that the model is consistent with available experimental data, on film thickness evolution, suggesting that it may prove very useful for optimising the CBD process with respect to its design variables. To conclude the presentation, some aspects of the deposit morphology at the early stages of its growth are discussed. It appears that these first stages determine the properties of the final film.

Modification in the chemical bath deposition apparatus, growth and characterization of CdS semiconducting thin films for photovoltaic applications

In this paper, growth and characterization of CdS thin films by Chemical Bath Deposition (CBD) technique using the reaction between CdCl 2, (NH 2) 2CS and NH 3 in an aqueous solution has been reported. The parameters actively involved in the process of deposition have been identified. A commonly available CBD system has been sucessfully modified to obtain the precious control over the pH of the solution at 90°C during the deposition and studies have been made to understand the fundamental parameters like concentrations of the solution, pH and temperature of the solution involved in the chemical bath deposition of CdS. It is confirmed that the pH of the solution plays a vital role in the quality of the CBD–CdS films. Structural, optical and electrical properties have been analysed for the as-deposited and annealed films. XRD studies on the CBD–CdS films reveal that the change in Cadmium ion concentration in the bath results in the change in crystallization from cubic phase with (1 1 1) predominant orientation to a hexagonal phase with (0 0 2) predominant orientation. The structural changes due to varying cadmium ion concentration in the bath affects the optical and electrical properties. Optimum electrical resistivity, band gap and refractive index value are observed for the annealed films deposited from 0.8 M cadmium ion concentration. The films are suitable for solar cell fabrication. Further on, annealing the samples at 350°C in H 2 for 30 min resulted in an increased diffraction intensity as well as shifts in the peak towards lower scattering angles due to enlarged CdS unit cell. This in turn brought about an increase in the lattice parameters and narrowing in the band-gap values. The results are compared with the analysis of previous work.

Wet treatment based interface engineering for high efficiency Cu(In,Ga)Se2 solar cells

Using atomic layer epitaxy (ALE) which is a soft deposition technique for ZnO it is possible to avoid the deposition of thick buffer layers by chemical bath deposition (CBD) and wet treatments can be almost reduced to surface treatments. In this work new electrochemical and chemical treatments have been designed with the objective of surface passivation and surface doping by using solutions with different reactivities (via pH, complexing agents, metallic cations). ZnO layers are then deposited by ALE to complete the junctions. The results show relations between the interface treatment and the cell characteristics. Efficiencies comparable and in some cases higher than those of the reference cells made with CBD CdS and sputtered ZnO have been obtained (up to 12.7% with indium treatments).

Novel approach to the chemical bath deposition of chalcogenide semiconductors

Chemical bath deposition (CBD) generates has been successfully employed for the fabrication of II–VI semiconductor thin films. Thin film polycrystalline solar cells, such as the BP Solar ‘Apollo’ CdS:CdTe heterojunction device, offer the potential for low cost solar energy conversion. The large scale exploitation of these devices is partly dependent on a reduction of the potential environmental impact of the technology. The fabrication of CdS window layers by CBD at present generates considerable Cd-containing waste. Use and disposal of cadmium-containing compounds and wastes are highly regulated in the EU and elsewhere. For CdS CBD, the extent of the desired heterogeneous reaction on the substrate surface is limited by two major factors, the competing homogeneous reaction in solution and deposition of material on the CBD reactor walls. In this paper we describe our initial successful efforts to address these problems with the development of a novel high-efficiency CdS CBD system. Chemical modelling and speciation studies have enabled us to develop a process that comprises low cadmium concentrations and eliminates ammonia (which is volatile and undesirable for large scale CBD operations). Films have been characterised by spectroscopic methods (UV–vis, PL and XPS), microscopy (SEM and TEM) and powder XRD.

Recrystallization in CdTe/CdS

Processing of CdTe/CdS solar cells requires annealing of CdS and CdTe/CdS in different ambients. It has been proven that the application of a CdCl2 treatment (or its variant) is important for high efficiency solar cells. This treatment influences the structural and interface properties of the layers. We have grown CdS layers either by a chemical bath deposition (CBD) or a high vacuum evaporation (HVE) on different transparent conducting oxides (TCO): tin oxide doped with fluorine (FTO) and indium tin oxide (ITO) coated glass substrates. The CdTe layers have been grown by a HVE method. Effects of the CdCl2 treatment on the recrystallization of CdTe and CdS have been studied with X-ray diffraction and scanning electron microscopy. An increase in the grain size of CdTe from about 0.5 to 3–7 μm, along with the loss of the preferred (111) growth orientation has been observed. The strain and recrystallization of CdTe, and intermixing of the CdTe and CdS layers strongly depend on the deposition and annealing temperatures. An optimum treatment and a minimum thickness of CBD–CdS is required for high efficiency solar cells. CdS layers and the method of their deposition also have a strong influence on the microstructure of CdTe and photovoltaic properties. Solar cells with efficiency of 11.2 and 2.5% are obtained with HVE and CBD grown CdS window layers.

Modification in the Chemical Bath Deposition Apparatus, Growth and Characterization of CdS Semiconducting Thin Films for Photovoltaic Applications

Abstract In this paper, growth and characterization of CdS thin films by Chemical Bath Deposition (CBD) technique using the reaction between CdCl 2 , (NH 2 ) 2 CS and NH 3 in an aqueous solution has been reported. The parameters actively involved in the process of deposition have been identified. A commonly available CBD system has been sucessfully modified to obtain the precious control over the pH of the solution at 90°C during the deposition and studies have been made to understand the fundamental parameters like concentrations of the solution, pH and temperature of the solution involved in the chemical bath deposition of CdS. It is confirmed that the pH of the solution plays a vital role in the quality of the CBD–CdS films. Structural, optical and electrical properties have been analysed for the as-deposited and annealed films. XRD studies on the CBD–CdS films reveal that the change in Cadmium ion concentration in the bath results in the change in crystallization from cubic phase with (1 1 1) predominant orientation to a hexagonal phase with (0 0 2) predominant orientation. The structural changes due to varying cadmium ion concentration in the bath affects the optical and electrical properties. Optimum electrical resistivity, band gap and refractive index value are observed for the annealed films deposited from 0.8 M cadmium ion concentration. The films are suitable for solar cell fabrication. Further on, annealing the samples at 350°C in H 2 for 30 min resulted in an increased diffraction intensity as well as shifts in the peak towards lower scattering angles due to enlarged CdS unit cell. This in turn brought about an increase in the lattice parameters and narrowing in the band-gap values. The results are compared with the analysis of previous work.

Optimization of Chemical Bath‐Deposited Cadmium Sulfide on InP Using a Novel Sulfur Pretreatment

A thiourea/ammonia pretreatment followed by chemical bath deposition of cadmium sulfide was used to passivate the surface of indium phosphide (100). The pretreatment was shown by X‐ray photoelectron spectroscopy to effectively remove native oxides from the InP surface and form an indium sulfide layer. The subsequent chemical bath deposition of CdS on a sulfur‐passivated surface forms a stable layer that protects the substrate from oxidation during chemical vapor deposition of . The passivation process was optimized for electrical response by varying the reactant concentrations of the chemical bath. Reflection high‐energy electron‐diffraction (RHEED) analysis showed that the pretreatment results in a (1 × 1) surface, which reconstructs to (2 × 1) after heating in vacuo to 200°C. RHEED and atomic force microscopy showed an increase in CdS surface roughness with increasing thickness corresponding to the formation of oriented surface asperities. CdS‐passivated metal‐insulator‐semiconductor diodes exhibited a density of interface states of when calculated by the high‐low method, more than one order of magnitude lower than the of untreated metal‐insulator‐semiconductor samples. A CdS layer thickness of ∼ 10 Å was determined to yield optimal capacitance‐voltage response for all CdS deposition conditions investigated. © 1999 The Electrochemical Society. All rights reserved.

Morphological and structural studies of CBD-CdS thin films by microscopy and diffraction techniques

The influence of cadmium salt and thiourea concentrations on the morphological and structural properties of chemical bath-deposited CdS thin films has been investigated. Two different feature regimes have been distinguished: an inner continuous layer grown directly on the glass and independent on the deposition conditions, and other porous overlayer, more dependent on the chemical concentrations. Root mean square, RMS, and average roughnesses, R a, as quantified by AFM, are about 10–13 nm and 7–11 nm, respectively, for all CdS samples tested. These films are sulphur-poor, decreasing S/Cd atomic ratio from 0.82 at low cadmium salt, 1 mM, and high thiourea concentrations, 100 mM, down to 0.76 at higher [Cd 2+], 5 mM, and lower [TU], 10 mM.

Chemically and electrochemically deposited thin films for solar energy materials

Direct energy gap materials, e.g. CdTe, CuInSe 2, CuInGaSe 2, CdSe, ZnP 2 and Zn 3P 2, are the most interesting for thin-film solar cell applications. Among the various methods of preparation of these films, chemical bath deposition and electrodeposition deserve special attention because they have been shown to be inexpensive, low-temperature and non-polluting methods. Based on Pourbaix diagrams of CdS, CdTe, CuInSe 2, CdSe, etc., drawn from basic considerations, the best parameters for their electrodeposition are deduced. Theoretical considerations on the chemical-bath deposition of CdS, CdSe and Sb 2S 3 are also indicated. In particular, the role of the complexing agent and of the ligands in chemical bath deposition quality is discussed, as are the uniformity and stability of the films. The photoelectrochemical, Schottky barrier and heterojunction solar cell properties based on chemically and electrochemically deposited thin films with heteropolyacids are shown. Future trends for chemically and electrochemically deposited polycrystalline thin films are addressed. Results from very recent work done in the improvement of chemically and electrochemically deposited thin films are presented. Significant results obtained on advanced CdS/CdTe, CdS/CIS and CdS/CIGS solar cells developed by industry and by laboratory groups worldwide are indicated. Emerging low cost materials or/and less environmental hazards materials which may introduce solar cells into worldwide market are considered in the conclusion.

Engineering the interface energetics of solar cells by grafting molecular properties onto semiconductors

The electronic properties of semiconductor surfaces can be controlled by binding tailor-made ligands to them. Here we demonstrate that deposition of a conducting phase on the treated surface enables control of the performance of the resulting device. We describe the characteristics of the free surface of single crystals and of polycrystalline thin films of semiconductors that serve as absorbers in thin film polycrystalline, heterojunction solar cells, and report first data for actual cell structures obtained by chemical bath deposition of CdS as the window semiconductor. The trend of the characteristics observed by systematically varying the ligands suggests changes in work function rather than in band bending at the free surface, and implies that changes in band line-up, which appear to cause changes in band bending, rather than direct, ligand-induced band bending changes, dominate.

Chemical Bath Deposition of CdS Thin Films: An Approach to the Chemical Mechanism Through Study of the Film Microstructure

Many papers have been published lately on chemical bath deposition of CdS (CBD‐CdS) thin films and related materials due to the promising results obtained using CBD‐CdS for the fabrication of thin‐film solar cells. In spite of this little of the research proposes a realistic chemical mechanism for the deposition process based on the determination of kinetic parameters. In this paper we present an exhaustive study of the CBD‐CdS kinetic from which we propose a new chemical mechanism which agrees with the kinetic parameters determined supported by heterogeneous catalysis concepts. Simultaneously, the dependence of the deposited film structure on the kinetic variables is studied and the results obtained corroborate the proposed mechanism. These studies have allowed us to stablish a standard set of conditions for the fabrication of homogeneous and continuous very thin CdS films.

An x-ray photoelectron spectroscopy investigation of O impurity chemistry in CdS thin films grown by chemical bath deposition

We used x-ray photoelectron spectroscopy to investigate the chemistry of O impurity atoms in CdS thin films grown for photovoltaic purposes by chemical-bath deposition (CBD). We compared the Cd 3d photoline, O 1s photoline, Cd MNN Auger line, and O KLL Auger line taken from a CBD CdS thin film, CdS single-crystal reference, Cd metal reference, CdO reference, and Cd(OH)2 reference. This comparison showed that the O present in thin-film CBD CdS is a manifestation of H2O incorporated into the film during the CBD growth. Ar+ ion sputtering, a technique frequently used in thin-film analyses, preferentially removed S from the CBD CdS thin film and created CdS1−xOx (x∼0.04) in the surface region from the incorporated O impurity.

Morphological investigations on CdS-TCO photovoltaic window layers using atomic force microscopy

The morphology of CdS-indium tin oxide (ITO) and CdS-ZnO bilayers has been investigated by atomic force microscopy in order to obtain a better understanding of their behaviour as window coatings in solar cells. Chemical bath-deposited CdS layers with t hicknesses ranging between 0.05 and 0.12 μm and RF-magnetron-sputtered ITO and ZnO films have been independently analysed before the study of the combined materials. A CdS thickness below 0.1 μm has been found to be optimal for avoiding the adhesi on of large solution particles from the chemical bath, and thus for achieving a homogeneous CdS layer useful as a polycrystalline substrate to improve the transparent conductive oxide (TCO) grain size.

A novel cadmium free buffer layer for Cu(In,Ga)Se 2 based solar cells

Solar cells based on Cu(In,Ga)Se 2 were prepared replacing the “standard buffer layer” CdS with a In x (OH,S) y thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl 3 and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl 3 and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage ( C-V) measurements indicate that the high V ∞ values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of 15.7% (active area) using the cadmium free In x (OH,S) y buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS.