J. Campos

Semiconductor thin films by chemical bath deposition for solar energy related applications

In this paper we present the basic concepts underlying the chemical bath deposition technique and the recipes developed in our laboratory during the past ten years for the deposition of good-quality thin films of CdS, CdSe, ZnS, ZnSe, PbS, SnS, Bi2S3, Bi2Se3, Sb2S3, CuS, CuSe, etc. Typical growth curves, and optical and electrical properties of these films are presented. The effect of annealing the films in air on their structure and composition and on the electrical properties is notable: CdS and ZnS films become conductive through a partial conversion to oxide phase; CdSe becomes photosensitive, SnS converts to SnO2, etc. The use of precipitates formed during deposition for screen printing and sintering, in polymer composites and as a source for vapor-phase deposition is presented. Some examples of the application of the films in solar energy related work are presented.

Chemically Deposited Sb[sub 2]S[sub 3] and Sb[sub 2]S[sub 3]-CuS Thin Films

Thin films of antimony sulfide have been deposited from chemical baths containing antimony trichloride and sodium thiosulfate maintained at 10 C. Upon annealing in nitrogen at 300 C for 1 h, the films become photosensitive with photo- to dark-current ratio of two to three orders of magnitude at 2 kW/m{sup 2} tungsten halogen radiation. The annealed films are crystalline with an X-ray diffraction pattern matching that of stibnite, Sb{sub 2}S{sub 3}, (JCPDS 6-0474) and show an optical bandgap of 1.78 eV. Deposition of a thin film of CuS on the antimony sulfide thin film and subsequent annealing in nitrogen at 250 C for 1 h produces films with acceptable solar control characteristics: integrated visible transmittance, T{sub vis}, 15%; integrated visible reflectance, R{sub vis}, 12%; integrated infrared transmittance, T{sub ir}, 14%; integrated infrared reflectance, R{sub ir}, 36%; and a shading coefficient of about 0.35. The X-ray diffraction patterns of the annealed Sb{sub 2}S{sub 3}-CuS thin films indicate the formation of a ternary compound with the structure of famatinite, Cu{sub 3}SbS{sub 4}.

Effect of bath temperature on the optoelectronic characteristics of chemically deposited CdS thin films

Abstract The temperature of the chemical bath used in the electroless deposition of cadmium sulphide thin films has been found to affect the deposition rate, optical transmission T (%), film morphology, photoconductivity σph, photoconductivity to dark conductivity ratio σph/σd and photocurrent decay time τ0. The results pertain to CdS films deposited from aqueous chemical baths containing a triethanolamine complex of Cd2+ ions and thiourea, in which Cd2+ and thiourea were mixed in 1:0.5 and 1:0.25 molar ratios. As the bath temperature was raised from 30 to 85 °C, the time required to deposit a thin film, of about 0.5 μm thickness, from the 1:0.5 bath was reduced from 480 to 18 min and that from the 1:0.25 bath was reduced from 1320 to 25 min. Systematic variations in the optoelectronic characteristics were also observed as functions of bath temperature: transmission T above the band gap absorption, 10%–80%; σph (300 W m-2 simulated solar radiation), 2 × 10 -3 − 2 Ω -1 cm -1 ; σph/σd, 106 − 109; τ0, 20−103 s.

Photocurrent response in chemically deposited CdS thin films

Abstract Highly photosensitive CdS thin films with photocurrent to dark current ratios (I ph /I d ) of ∼10 6 –10 7 under AM2 illumination are reported. The films are prepared by an electroless chemical deposition technique. The optical microscopy along with the optical transmission spectra of the films revealed a continuous thin film ∼ 0.3–0.5 μ m in thickness and with an overgrowth of single crystallites of about ∼10–20 μ m in diameter. The reasons for the high photosensitivity of the freshly prepared films and the increase in the I ph /I d value upon storage are discussed. The time response of the photocurrent, an aspect which is rarely mentioned in the literature on chemically coated CdS films, is explicitly presented.

Optical and electrical responses of polymeric electrochromic devices: effect of polyacid incorporation in polyaniline film

Abstract Solid electrochromic devices (ECDs) are prepared with chemically deposited electroactive polyaniline (PANI) and PANI-poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) thin films. The analysis of their electrical current responses suggests a higher redox potential value of the unmodified PANI thin film compared to that of the PANI-PAMPS ECD. Optical transmittance spectra of the devices show that the polyacid incorporation in PANI thin films reduces the switching voltage from ±2 to ±1.5 V, increases the optical contrast ratio by 10% at around 650 nm and doubles the optical response rate of the devices using LiClO4 dissolved in polymethyl methacrylate (PMMA) as a solid electrolyte. A parametrical analysis of the optical kinetic curves, together with the Butler–Volmer equation, indicates that PANI films contain two kinetically different phases: an interconnected network on which electrical conduction is predominated by percolation and a diffusion-controlled charge transfer zone. The first one contributes to the faster term and the second one to the slower term of the polymeric ECD optical responses.

A critical discussion of the very high photoconductivity in chemically deposited cadmium sulfide thin films: Implications for solar cell technology

Cadmium sulfide thin films exhibiting very high photosensitivity: σpσd > 109 and very high photoconductivity, approximately 10 Ω−1 cm−1 (projected values for air mass (AM) 1), combined with a very long photoconductivity decay time, 13 h per decade, are described. These values represent the highest so far reported in the literature. The films were prepared from aqueous chemical baths containing a triethanolamine complex of Cd2+ ions and thiourea, mixed in different non-equimolar ratios. A critical analysis of the photoconductivity is presented. It is shown that polycrystalline thin film solar cell technology, based on chemically deposited CdS films, is highly feasible.

The effect of post-deposition treatments on morphology, structure and opto-electronic properties of chemically deposited CdS thin films

Abstract CdS thin films for opto-electronic applications were grown by chemical deposition. Characterization of early stages of growth of the films by optical and scanning electron microscopy revealed that the growth of CdS films by chemical deposition occurs as an ion-by-ion process as well as by colloidal particles of CdS adhering to the substrate. The bath parameters, such as Cd ion to ammonia, triethanolamine (TEA) and thiourea (TU) mole ratios, influenced the growth mode. The X-ray diffraction studies of the films showed the presence of some organo-mettalic impurities on the film surface, which can be removed by post-deposition etching of the films in very dilute acetic acid. The film morphology depends on the bath composition and the optical transmittance of the films can be improved by etching. A bath composition comprising a Cd:TEA:NH 3 :TU mole ratio of about 1:3.75:14.4:1 resulted in good quality films.

The structural, transport and optical properties of screen printed CuxS thick films

Abstract The structural, transport and optical properties of screen printed Cu x S thick films with possible application in photovoltaic and photothermal devices are reported. The X-ray diffraction studies show that the screen printed films are stable up to about 220°C in air and belong to the Cu x S structure. Above this temperature it decomposes mainly to CuSO 4 . The electrical conductivity depends on the sintering temperature and the amount of flux, Cu(NO 3 ) 2 , used in the paste for screen printing. The differential scanning calorimetry studies reveal the phase changes occurring during heating and pertaining to the dependence of electrical conductivity on the sintering temperature. A configuration consisting of screen printed Cu x S on chemically deposited and annealed (at 200°C) CdS thin film exhibited rectification.

A comparison of the various thermal treatments of chemically deposited bismuth sulfide thin films and the effect on the structural and electrical properties

Abstract The physicochemical transformations of chemically precipitated bismuth sulfide powder have been studied by differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD). The structural changes have been correlated to the electrical properties of chemically deposited bismuth sulfide thin films annealed at different temperatures ( T ) and pressures ( P t ). Two irreversible exothermic transformations are observed at 210 and 253°C in the powder, which correspond to the crystallization of Bi 2 S 3 , and to the crystallization and decomposition of local defects and their reaction to form Bi 2 S 3 . As in the case of the precipitate, the conductivity of chemically deposited bismuth sulfide thin films upon annealing is due to the formation of multicomponent phases assisted by further sulfur loss. The complex mechanism that enhances the film conductivity shows a compensating effect between P t and T . The temperature and pressure regime at which conductive films were obtained with Bi 2 S 3 as the only component of the crystalline phase were: 190°C under vacuum, and 250°C in the P t range of 10–1000 mbar. Under these conditions, the dark conductivity and photoconductivity of the films are in the range of 10 −2 to 3 Ω −1 cm −1 , mainly determined by the charge transport through Bi 2 S 3 crystals.

Electrically conductive CuS–poly(acrylic acid) composite coatings

Copper sulfide (CuS) powder precipitated from a chemical bath containing Cu(II) chloride and thiourea and annealed in air at 150 ±C for 1 h was dispersed in a poly(acrylic acid) aqueous solution (with additional water or propylene glycol as a dispersive agent) and cast on glass slides. Upon evaporation of the solvent, coatings of ,50 mm in thickness of a CuS-poly(acrylic acid) composite are formed. Measurement of sheet resistance sRhd indicates a percolation threshold of electrical conduction at a weight fraction [wf is wt. % of CuS to poly(acrylic acid) 1 CuS] of about 40%; the composite undergoes a transition from insulator sRh , 1013 Vd to conductive state sRh , 102 Vd. The morphology and thermal stability of the composite depend on the choice of the dispersive agent for the CuS powder; smoother and thermally stable (up to a temperature of 250 ±C) coatings are obtained when propylene glycol is used. The results on x-ray diffraction, thermogravimetric analysis, and Fourier transform infrared spectroscopy studies are given to indicate the structure and bonding mechanisms and their dependence on temperature and dispersive agents.