**2.2 Use of the CdS films in photovoltaic cells**

382 Solar Cells – Thin-Film Technologies

The thin film semiconductor properties largely depend on fabrication technology. Therefore development of actual methods, which would allow an influence on material parameters in the synthesis process and to obtain coating with the set properties, is an important scientific and technological problem. Recently the methods based on chemical processes dominate in the technology of metal sulfides thin films semiconductor. The semiconductor films with a thickness from several tenth of nanometers to hundreds of microns can be fabricated by a large number of so-called thin-film and thick-film methods. For large area in ground conditions aplication of thin-film solar cells crucial are not only their energy characteristics, but also their economic indicators. This causes use of bough thin film and thick-film technology methods for satisfying of such requirements as: fabrication simplicity, low cost, ability to create homogeneous films with a large area, controlling the deposition process, and ability to obtain the films with preferred structural, physical, chemical and

The deposition methods for wide range of semiconductors in detail are considered in literature (Aven & Prener, 1967, Chopra & Das, 1983, Green, 1998, Möller, 1993, Sze, 1981, Vossen & Kern, 1978,). We will consider only those methods that are used for cadmium sulfide films fabrication and are the best for solar cells producing. Thin film deposition process consists of three stages: 1) obtaining of substance in the form of atoms, molecules or ions; 2) transfer of these particles through an intermediate medium; 3) condensation of the particles on substrate. The methods of thin films fabrication are classified in several ways. Depending on the film grown phase are four methods of films deposition: 1) from the vapor phase; 2) from the liquid phase; 3) from the hydrothermal solutions; 4) from the solid phase. Depending on which way the vapour particle were obtained: using physical (thermal or ion sputtering), chemical or electrochemical processes, it is possible to classify deposition methods: physical vapor deposition; chemical vapor deposition; chemical deposition from the solution; electrochemical deposition. On the basis of physical and chemical vapor deposition were developed combined methods, such as: reactive evaporation, reactive ion sputtering and plasma deposition. Among the nonvacuum deposition methods of cadmium sulfide thin films for inexpensive solar cells with a large area perspective are: chemical deposition from baths (CBD), electrochemical deposition, mesh-screen printing, pyrolysis and pulverization followed by pyrolysis. Selection of the films deposition method first of all are specified by structural, mechanical and physical parameters, which should have thin-

Although, cadmium sulfide is the most widely studied thin film semiconductor material, interest of researchers to it is stable, and the number of scientific publications increasing all the time. Changing the deposition conditions drasticly alter electrical properties of CdS thin films. CdS films, obtained by vacuum evaporation have specific resistance 1•103 Om•cm and carrier concentration of 1016-1018 cm-3. Films always have n-type conductivity, that explains their structure deviation from stoichiometry, by sulfur vacancies and cadmium excess. Electrical properties of the films are largely depended from the concentration ratio of Cd and S atoms in the evaporation process and the presence of doping impurities. Electrical properties of CdS films, fabricated by pulverization followed by pyrolysis, are determined mainly by the peculiarities the chemisorption process of oxygen on grain boundaries, which accompanied by concentration decreaseing and charge carriers mobility. Due to presence of

**2. Deposition of CdS thin films and structures based on** 

**2.1 Fabrication methods** 

electrooptical properties.

film sample.

Edmund Becquerel, a French experimental physicist, discovered the photo-voltaic efect in 1839 while experimenting with an electrolytic cell, made up of two metal electrodes placed in an electricity-conducting solution. He observed that current increased when the electrolytic cell was exposed to light (Becquerel, 1839). Then in 1873 Willoughby Smith discovered the photoconductivity of selenium. The rst selenium cell was made in 1877 (Adams, 1877), and ve years later Fritts (Fahrenbruch & Bube, 1983) described the rst solar cell made from selenium wafers. By 1914 solar conversion eficiencies of about 1 % were achieved with the selenium cell after it was nally realized that an energy barrier was involved both in this cell and in the copper/copper oxide cell.

The modern era of photovoltaics started in 1954. In that year was reported a solar conversion efficiency of 6 % (Chapin at al., 1954) for a silicon single-crystal cell. In 1955 Western Electric began to sell commercial licenses for silicon PV technologies. Already in 1958 silicon cell efficiency under terrestrial sunlight had reached 14 %. At present, available in the market SC are mainly represented of monocrystalline silicon SC. Through hightemperature process of their formation, crystal (from ingots grown from melt by Czochralski method) and polycrystalline silicon solar cells have too high price, to be seen as a significant competitor to the formation of energy from solid fuels. Polycrystalline silicon provides lower expenses and increase production, rather than crystalline silicon. In 1998, approximately 30 % photovoltaic world production was based on the polycrystalline silicon wafers. Nowadays solar cells conversion efficiency based on monocrystalline silicon is 25 %, polycrystalline – 20 % (Green at al., 2011).

In 1954 reported 6 % solar conversion efficiency (Reynolds at al., 1954) in what later came to be understood as the cuprous sulde/cadmium sulde heterojunction (HJ). This was the rst all-thin-lm photovoltaic system to receive signicant attention. In following years the efficiency of CuxS/CdS increased up to 10 % and a number of pilot production plants were installed, but after several years of research it was realized that these solar cells have unsolvable problems of stability owing to the diffusion of copper from CuxS to CdS layers. By taking advantage of new technology, work out on CuxS/CdS, researchers have rapidly raised the effciency of the gallium arsenide based cell with 4 % efficiency (Jenny at al., 1956) to present eficiencies exceeding 27 % (Green at al., 2011).

However in the last 20 years other thin lms solar cells have taken the place of the cuprous sulde/cadmium sulde, and their eficiency have raised up to almost 20 %. The most predominant are two: copper indium gallium diselenide/cadmium sulde (Cu(In,Ga)Se2/CdS) and cadmium telluride/cadmium sulde (CdTe/CdS). The rst CdTe heterojunctions were constructed from a thin lm of n-type CdTe material and a layer of p-

Chemical Surface Deposition of CdS Ultra Thin Films from Aqueous Solutions 385

coatings for controlling the flow of sunlight was first proposed in 1989 (Nair at al., 1989). The efficiency improving of such coatings in glass vacuum tube collectors were presented in (Estrada-Gasca at al., 1993). One of the main applications of chemically deposited semiconductor films has been their use in photoelectrochemical SC, mostly CdS and CdSe films (Hass at al., 1982, Boudreau & Rauh, 1983, Rincon at al., 1998). The use of chemically deposited semiconductor films in thin SC has a short history. In the structure Mo/CuInSe2/CdS/ZnO, which showed 11% efficiency (Basol & Kapur, 1990), was by the first time used chemically deposited CdS thin film. Further structure improvement allowed to reach 17% efficiency (Tuttle at al., 1995). Chemically deposited CdS film with thickness of 50 nm has been an essential element of this structure. The biggest, confirmed today for SC based on CdS/CdTe, is 16,5% efficiency in which CdS film was chemically deposited in bath (Green at al., 2011). Entering highly resistive CdS film in *p*-CuInSe2/CdS/*n*-CdS solar cell structure deemed necessary step towards improving of the solar cells stability (Mickelsen & Chen, 1980). Performed theoretical calculations (Rothwarf, 1982) showed that the thickness of CBD CdS films should be as small as possible to increase efficiency of solar cells with its use. Therefore, chemical deposition technology, which allows to fully cover the substrate at small film thickness was selected for the fabrication of thin films and showed significantly better results (Basol at al., 1991). Efficiency of *n*-CdSe or *n*-Sb2S3 chemically deposited films with WO3 inclusions as absorber material in solar cells based on the Schottky barrier has been proved in practice. For example, elements on the Schottky barrier ITO/*n*-CdSe(5 µm)/Pt/Ni/Au (13 nm) shows Uхх=0,72 V, Iкз=14,1 mA·cm-2, fill factor 0,7, and 5,5% efficiency (Savadogo & Mandal, 1993 & 1994). Abovementioned possible applications of chemical bath deposition, particularly in solar energy conversion, provided the growing interest to chemical deposition of semiconductor thin films. Chemical deposition is perfect for producing thin films on large areas and at low temperatures, which is one of the main

requirements for the mass use of solar energy.

high efficiency of transformation is not represented.

compounds.

**2.4 The advantages of chemical surface deposition (CSD) over CBD** 

In the CBD process, the heat necessary to activate chemical reaction is transferred from the bath to the sample surface, inducing a heterogeneous growth of CdS on the surface and homogeneous CdS formation in the bath volume. The reaction is better in the hottest region of the bath. Therefore, for baths heated with thermal cover deposition also occurs on the walls, and bath, which heat up immersed heater, significant deposition occurs on heating element. Additionally, the solution in the bath should be actively mixed to ensure uniform thermal and chemical homogeneity and to minimize adhesion of homogeneously produced particles to the surface of CdS film. The disproportion of bath volume and that which is necessary for the formation of CdS film, leads to significant proportion of wastes with high cadmium content. Different groups of researchers put efforts for decreasing the ratio of volumes bath/surface through use of overlays. However the clear way for unification of large areas deposition with high cadmium utilization and high speed of growth, to achieve

The chemical surface deposition (CSD) technology demonstrated in this paper overcomes these limitations through use of the sample surface as a heat source and use of solution surface tension to minimize the liquid volume. The combination of heat delivery method to surface and small volume of solution leads to high utilization of cadmium and its

type copper telluride (Cu2−xTe), producing ∼7 % eficient CdTe-based thin-lm solar cell (Basol, 1990). However, these devices showed stability problems similar to those encountered with the analogous Cu2−xS/CdS solar cell, as a result of the difuusion of copper from the p layer. The lack of suitable materials with which to form heterojunctions on n-type CdTe, and the stability problems of the Cu2−xS/CdS device, stimulated investigations into p-CdTe/n-CdS junctions since the early 1970s. Adirovich (Adirovich at al., 1969) rst deposited these lms on TCO-coated glass; this is now used almost universally for CdTe/CdS cells, and is referred to as the superstrate conguration. In 1972 5-6 % eficiencies were reported (Bonnet & Rabenhorst, 1972) for a graded band gap CdSxTe1−x solar cell.

The research for CuInSe2/CdS started in the seventies, a 12 % efficiency single-crystal heterojunction p-CuInSe2/n-CdS cells were made by in 1974 (Wagner, 1975) and in 1976 was presented the rst thin lm solar cells with 4-5 % eficiency (Kazmerski at al., 1976). In the last 30 years a big development of these cells was given by the National Renewable Energy Laboratories (NREL) in U.S.A. and by the EuroCIS consortium in Europe.

Nowadays CdS among Si, Ge, CdTe, Cu(In, Ga)Se2, ZnO belongs to the widespread group of semiconductors. Beyond the attention of researchers are still many issues associated with cadmium sulfide as componenet of thin-film semiconductor devices, although the CdS is one of the most studied semiconductor materials.

#### **2.3 Peculiarities of chemical bath deposition (CBD)**

CBD technology consist of the deposition of semiconductor films on a substrate immersed in solution containing metal ions and hydroxide, sulfide or selenide ions source. The first work on CBD is dated 1910 and concerns to the PbS thin films deposition (Houser & Beisalski, 1910). Basic principles underlying the CBD of semiconductor films and earlier studies in this field were presented in the review article (Hass at al., 1982), which encouraged many researchers to begin work in this direction. Further progress in this area is presented in review article (Lokhande, 1991), where references are given for 35 compounds produced by the mentioned method, and other related links. Chemical reactions and CBD details for many compounds were listed in the next paper (Grozdanov, 1994). The number of materials which can be produce by CBD, greatly increased, partly due to the possibility of producing multilayer film structures by this method with subsequent annealing, which stimulates crosboundary diffusion of metal ions and thereby motivates fabrication of new materials with high thermal stability. For example, crossboundary diffusion of CBD coatings PbS*/*CuS and ZnS*/*CuS leads to materials such as Pb*x*Cu*y*S*z* and Zn*x*Cu*y*S*z* with *p*-type conductivity and thermal stability up to 573 K (Huang at al., 1994). Annealing of Bi2S3*/*CuS coatings at temperatures 523-573 K leads to formation of new Cu3BiS3 compounds with *p*-type conductivity (Nair at al., 1997). In recent years we counted approximately 120 CBD semiconductor compouns.

Among the first applications of CBD semiconductor films were photodetectors based on PbS and PbSe (Bode at al., 1996). Although the chemically precipitated CdS films were made back in the 60's of last century, for photodetectors were used CdS layers, obtained by screen printing and sintering (Wolf, 1975). Chemically deposited CdSe films are fully suitable for use in photodetectors (Svechnikov & Kaganovich, 1980). At late 70's and early 80-ies the main direction in chemical bath deposition technology was deposition of thin films for use in solar energy conversion. One of the first developments in this area was the coating producing that absorbs sunlight (Reddy at al., 1987), and its use in glass vacuum tube collectors (Estrada-Gasca at al., 1992). Application of the chemically deposited films in

type copper telluride (Cu2−xTe), producing ∼7 % eficient CdTe-based thin-lm solar cell (Basol, 1990). However, these devices showed stability problems similar to those encountered with the analogous Cu2−xS/CdS solar cell, as a result of the difuusion of copper from the p layer. The lack of suitable materials with which to form heterojunctions on n-type CdTe, and the stability problems of the Cu2−xS/CdS device, stimulated investigations into p-CdTe/n-CdS junctions since the early 1970s. Adirovich (Adirovich at al., 1969) rst deposited these lms on TCO-coated glass; this is now used almost universally for CdTe/CdS cells, and is referred to as the superstrate conguration. In 1972 5-6 % eficiencies were reported (Bonnet & Rabenhorst, 1972) for a graded band gap CdSxTe1−x solar cell. The research for CuInSe2/CdS started in the seventies, a 12 % efficiency single-crystal heterojunction p-CuInSe2/n-CdS cells were made by in 1974 (Wagner, 1975) and in 1976 was presented the rst thin lm solar cells with 4-5 % eficiency (Kazmerski at al., 1976). In the last 30 years a big development of these cells was given by the National Renewable Energy

Nowadays CdS among Si, Ge, CdTe, Cu(In, Ga)Se2, ZnO belongs to the widespread group of semiconductors. Beyond the attention of researchers are still many issues associated with cadmium sulfide as componenet of thin-film semiconductor devices, although the CdS is

CBD technology consist of the deposition of semiconductor films on a substrate immersed in solution containing metal ions and hydroxide, sulfide or selenide ions source. The first work on CBD is dated 1910 and concerns to the PbS thin films deposition (Houser & Beisalski, 1910). Basic principles underlying the CBD of semiconductor films and earlier studies in this field were presented in the review article (Hass at al., 1982), which encouraged many researchers to begin work in this direction. Further progress in this area is presented in review article (Lokhande, 1991), where references are given for 35 compounds produced by the mentioned method, and other related links. Chemical reactions and CBD details for many compounds were listed in the next paper (Grozdanov, 1994). The number of materials which can be produce by CBD, greatly increased, partly due to the possibility of producing multilayer film structures by this method with subsequent annealing, which stimulates crosboundary diffusion of metal ions and thereby motivates fabrication of new materials with high thermal stability. For example, crossboundary diffusion of CBD coatings PbS*/*CuS and ZnS*/*CuS leads to materials such as Pb*x*Cu*y*S*z* and Zn*x*Cu*y*S*z* with *p*-type conductivity and thermal stability up to 573 K (Huang at al., 1994). Annealing of Bi2S3*/*CuS coatings at temperatures 523-573 K leads to formation of new Cu3BiS3 compounds with *p*-type conductivity (Nair at al., 1997). In recent years we counted approximately 120 CBD

Among the first applications of CBD semiconductor films were photodetectors based on PbS and PbSe (Bode at al., 1996). Although the chemically precipitated CdS films were made back in the 60's of last century, for photodetectors were used CdS layers, obtained by screen printing and sintering (Wolf, 1975). Chemically deposited CdSe films are fully suitable for use in photodetectors (Svechnikov & Kaganovich, 1980). At late 70's and early 80-ies the main direction in chemical bath deposition technology was deposition of thin films for use in solar energy conversion. One of the first developments in this area was the coating producing that absorbs sunlight (Reddy at al., 1987), and its use in glass vacuum tube collectors (Estrada-Gasca at al., 1992). Application of the chemically deposited films in

Laboratories (NREL) in U.S.A. and by the EuroCIS consortium in Europe.

one of the most studied semiconductor materials.

semiconductor compouns.

**2.3 Peculiarities of chemical bath deposition (CBD)** 

coatings for controlling the flow of sunlight was first proposed in 1989 (Nair at al., 1989). The efficiency improving of such coatings in glass vacuum tube collectors were presented in (Estrada-Gasca at al., 1993). One of the main applications of chemically deposited semiconductor films has been their use in photoelectrochemical SC, mostly CdS and CdSe films (Hass at al., 1982, Boudreau & Rauh, 1983, Rincon at al., 1998). The use of chemically deposited semiconductor films in thin SC has a short history. In the structure Mo/CuInSe2/CdS/ZnO, which showed 11% efficiency (Basol & Kapur, 1990), was by the first time used chemically deposited CdS thin film. Further structure improvement allowed to reach 17% efficiency (Tuttle at al., 1995). Chemically deposited CdS film with thickness of 50 nm has been an essential element of this structure. The biggest, confirmed today for SC based on CdS/CdTe, is 16,5% efficiency in which CdS film was chemically deposited in bath (Green at al., 2011). Entering highly resistive CdS film in *p*-CuInSe2/CdS/*n*-CdS solar cell structure deemed necessary step towards improving of the solar cells stability (Mickelsen & Chen, 1980). Performed theoretical calculations (Rothwarf, 1982) showed that the thickness of CBD CdS films should be as small as possible to increase efficiency of solar cells with its use. Therefore, chemical deposition technology, which allows to fully cover the substrate at small film thickness was selected for the fabrication of thin films and showed significantly better results (Basol at al., 1991). Efficiency of *n*-CdSe or *n*-Sb2S3 chemically deposited films with WO3 inclusions as absorber material in solar cells based on the Schottky barrier has been proved in practice. For example, elements on the Schottky barrier ITO/*n*-CdSe(5 µm)/Pt/Ni/Au (13 nm) shows Uхх=0,72 V, Iкз=14,1 mA·cm-2, fill factor 0,7, and 5,5% efficiency (Savadogo & Mandal, 1993 & 1994). Abovementioned possible applications of chemical bath deposition, particularly in solar energy conversion, provided the growing interest to chemical deposition of semiconductor thin films. Chemical deposition is perfect for producing thin films on large areas and at low temperatures, which is one of the main requirements for the mass use of solar energy.

#### **2.4 The advantages of chemical surface deposition (CSD) over CBD**

In the CBD process, the heat necessary to activate chemical reaction is transferred from the bath to the sample surface, inducing a heterogeneous growth of CdS on the surface and homogeneous CdS formation in the bath volume. The reaction is better in the hottest region of the bath. Therefore, for baths heated with thermal cover deposition also occurs on the walls, and bath, which heat up immersed heater, significant deposition occurs on heating element. Additionally, the solution in the bath should be actively mixed to ensure uniform thermal and chemical homogeneity and to minimize adhesion of homogeneously produced particles to the surface of CdS film. The disproportion of bath volume and that which is necessary for the formation of CdS film, leads to significant proportion of wastes with high cadmium content. Different groups of researchers put efforts for decreasing the ratio of volumes bath/surface through use of overlays. However the clear way for unification of large areas deposition with high cadmium utilization and high speed of growth, to achieve high efficiency of transformation is not represented.

The chemical surface deposition (CSD) technology demonstrated in this paper overcomes these limitations through use of the sample surface as a heat source and use of solution surface tension to minimize the liquid volume. The combination of heat delivery method to surface and small volume of solution leads to high utilization of cadmium and its compounds.

Chemical Surface Deposition of CdS Ultra Thin Films from Aqueous Solutions 387

Deposition of thin CdS films from the aqueous solutions through the stage of cadmium tetramin [*Cd*(*NH*3)4] 2+ complex ion formation, which reduces the overall speed of reaction

The sulphides films deposition from thiocarbamid coordination compounds has some chemical peculiarities. Depending on the nature and the salt solution composition may be dominated different coordination forms, and with thiourea molecules in complex inner sphere may contain anions Cl-, Br-, J-, and SO42- under certain conditions. Thus, the cadmium atoms close environment are atoms of sulfur, halogens and oxygen, and at the thermal decomposition part of the Cd-Hal or Cd-O bonds are stored and in the sulfide lattice are formed HalS• and OS•defects. In conjunction with the substrate the thiocarbamid complexes orientation on active centers of its surface is observed. The complex particles that can interact with active centers on the substrate are the link that provides sulfide link with the substrate. The nature of this interaction determines the nature of film adhesion. In the case of cadmium sulfide deposition on quartz or glass substrates the active centers are sylanolane groups (≡SiOH) which interact with halide or mixed hydroxide complexes. In result of such interaction the CdOSi oxygen bridges are created. This explains the good adhesion of the cadmium sulfide films deposited from thiocarbamid coordination compounds to glass

In CSD, a solution at ambient temperature containing the desired reactants is applied to a pretreated surface. Glass or ITO/glass (16×20 mm) substrates, CdTe (10×10 mm) and Si (30×20 mm) wafers were used in the entire work. After that sample with working solution is heated and endured for a given temperature (Fig. 1). To ensure uniformity of heating plate

**Heating** 

2+ + CS(NH2)2 + OH- CdS + 4N3 + H+

**glass CdS** 

+(NH2)CO

<sup>4</sup> 34 2 *Cd NH OH Cd NH H O* 4 [ ( )] 4 (6)

3 4 2 2 <sup>3</sup> 2 2 [ ( )] ( ) *Cd NH NH CS OH CdS NH H NH CO* 4 () (8)

3 4 <sup>3</sup> [ ( )] *Cd NH S CdS NH* <sup>4</sup> (7)

2 2

2 2

and prevents Cd(OH)2 formation by the heterogeneous mechanism.

2

substrates (Palatnik & Sorokin, 1978).

**3.2 Chemical surfact deposition of CdS thin films** 

**solution:**  Cd2+ + CS(NH2)2 + NH4OH

Fig. 1. Scheme of CdS films thin chemical surface deposition

[Cd(NH3)4]

In general form:

This paper describes CSD technology of CdS thin films from aqueous solutions of cadmium salts CdSO4, CdCl2, CdI2. The properties of CdS films deposited on glass and ITO/glass from the nature of the initial salt and solar cells based on CdTe/CdS with CSD CdS films as windows was investigated.
