Hailin Hu

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.

Fourier transform infrared spectroscopy studies of the reaction between polyacrylic acid and metal oxides

Abstract The chemical reactions between polyacrylic acid (PAA) and various divalent metal oxides (MO; M = Ca, Mg, Cu, Zn) were studied by Fourier transform infrared spectroscopy (FTIR). The results show clear evidence of chemical bonding between the polymer and the metals as a result of their interaction. From the energy difference between the symmetrical and the asymmetrical bands of the various possible structures, monodentate metal-carboxylate bonding was found to be the most probable in all cases. Also, from the analysis of the corresponding molar relations, the particle size of the oxides was found to play an important role in the reaction.

Fourier transform infrared spectroscopy studies of polypyrrole composite coatings

Pyrrole monomer was dispersed in polyvinyl alcohol (PVA) and polyvinyl acetate (PVAc) solutions, which were previously mixed with an iron chloride solution. The resulting mixtures were coated on substrates, and as the solution solvents were evaporated pyrrole was polymerized in the composite coatings. The very low percolation threshold values and the continuous percolation phenomena of the electrical conductivity of these coatings suggest a chain structure of the conductive PPy in the composites and a good miscibility between the conductive phase and the insulator hosts as well. FT-IR studies of the coatings suggest molecular interactions between the functional groups of the polymer matrices and PPy through the iron salt molecules, which could be the reasons for the good miscibility between the semiconductor and insulator components of the composite coatings.

Electrically conducting polyaniline–poly(acrylic acid) blends

We report an electrically conducting polyaniline–poly(acrylic acid) blend coatings prepared by mixing the emeraldine base (EB) form of polyaniline (PANI) and poly(acrylic acid) (PAA) aqueous solution. The samples show a moderate electrical conductivity σ. If they are immersed in an HCl aqueous solution, the conductivity of the samples is increased by two or three orders of magnitude and their thermal stability is also improved. Optical transmittance spectra show a complete protonation of PANI–PAA blends after immersion in HCl aqueous solution. Fourier transform infrared spectroscopy studies indicate that the better thermal stability of σ could come from the more stable protonated imine nitrogen ions. A low percolation threshold phenomenon is observed in PANI–PAA blends, from a strong interaction between the carboxylic acid groups of PAA and the nitrogen atoms of PANI.

Thin films of polyaniline–polyacrylic acid composite by chemical bath deposition

Polyaniline (PANI)–polyacrylic acid (PAA) composite thin films were deposited at room temperature on glass and polymethyl methacrylate substrates by introducing these into a freshly prepared chemical bath of HCl, PAA, aniline monomers and (NH4)2S2O8. Compared to the single PANI films, which were obtained from the same bath constitution but without PAA solution, the composite films showed a red shift at the maximum optical transmittance peak and a slightly lower electrical conductivity. Fourier transform infrared spectroscopy studies of the PANI composite thin films indicated the presence of the acid-base reaction product between PANI and PAA, as well as PAA molecule incorporation in the films. Atomic force microscopy analysis also showed morphologic and mechanical elasticity differences between the single PANI film the composite films deposited by the chemical bath method.

Polyaniline–poly(2-acrylamido-2-methyl-1-propanosulfonic acid) composite thin films: structure and properties

Abstract Polyaniline (PANI)–poly(2-acrylamido-2-methyl-1-propanosulfonic acid) (PAMPS) composite thin films are obtained by a chemical bath deposition method on conductive glass substrates. The incorporation of the polyacid in PANI, demonstrated by atomic force microscopy, ultraviolet–visible spectroscopy and Fourier transform infrared spectroscopy studies, causes changes in the electronic structure of PANI and gives a different microstructure when compared to the unmodified PANI film. The lower optical transition band gap as well as an oriented fibril microstructure of the composite film give rise to a decrease in the redox potentials of PANI, favoring a lower optical switch voltage for the PANI–PAMPS composite film-based solid electrochromic devices.

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.

Chemically stable conducting polyaniline composite coatings

Abstract Aniline monomer was dispersed in polymethyl methacrylate (PMMA) and polyvinyl carbazole (PVK) solutions. The resulting mixtures were coated on substrates, dried in toluene atmosphere and enclosed later on in a chamber containing an oxidized atmosphere of HCl and (NH4)2S2O8. Electrical and optical characterizations indicate that polyaniline (PANI) was formed in the PMMA and PVK matrixes. Sheet resistance as a function of the pH value demonstrates that both PANI–PMMA and PANI–PVK composite coatings keep their R□ almost unchanged when they are immersed in a moderate basic solution (pH∼9). Especially, PANI–PVK coatings are more stable than PANI–PMMA samples for longer time of immersion and also in solutions with a higher pH value. Scanning electron micrographs show different surface morphologies of these two composite materials suggesting a possible explanation of their chemical stability.

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.

Cadmium Sulfide Nanoparticles Synthesized by Microwave Heating for Hybrid Solar Cell Applications

Cadmium sulfide nanoparticles (CdS-n) are excellent electron acceptor for hybrid solar cell applications. However, the particle size and properties of the CdS-n products depend largely on the synthesis methodologies. In this work, CdS-n were synthetized by microwave heating using thioacetamide (TA) or thiourea (TU) as sulfur sources. The obtained CdS-n(TA) showed a random distribution of hexagonal particles and contained TA residues. The latter could originate the charge carrier recombination process and cause a low photovoltage (, 0.3 V) in the hybrid solar cells formed by the inorganic particles and poly(3-hexylthiophene) (P3HT). Under similar synthesis conditions, in contrast, CdS-n synthesized with TU consisted of spherical particles with similar size and contained carbonyl groups at their surface. CdS-n(TU) could be well dispersed in the nonpolar P3HT solution, leading to a of about 0.6–0.8 V in the resulting CdS-n(TU) : P3HT solar cells. The results of this work suggest that the reactant sources in microwave methods can affect the physicochemical properties of the obtained inorganic semiconductor nanoparticles, which finally influenced the photovoltaic performance of related hybrid solar cells.