Tina Sebastian

Characterization of Spray Pyrolysed CuInS2 Thin Films

An indigenously developed chemical spray pyrolysis system was used to deposit polycrystalline CuInS2 thin films. It was found that smaller spray rate results in films with better crystallinity and lower resistivity. Increase in surface roughness of the films was observed for higher spray rates. Variations in film stoichiometry with composition of spray so- lution were analyzed along with its opto-electronic and structural properties. Sulfur rich starting solution with equimolar ratio of copper and indium resulted in nearly stoichiometric p-type CuInS2. Type conversion, modification of surface morphology and wide range of opto-electronic properties could be achieved by large off-stoichiometric variations. Tem- perature dependent conductivity study was used for defect analysis and levels at 436 meV, 294 meV, 131 meV, 76 meV and 50 meV were identified. In the present work, CuInS2 thin films were prepared at different spray rates and characterized. Fixing the spray rate at an optimum value, Cu/In and S/Cu ratios were also varied over a wide range so as to study the effect on film properties. With these studies, we aimed at a better understanding of structural, optical and electrical properties as well as defect chemistry of CuInS2 thin films prepared using CSP tech- nique. 2. METHODOLOGY CuInS2 thin films were deposited on glass substrates. Cleaned glass slides were placed on a base plate (mild steel) and heater rods embedded in it facilitated heating. The sub- strate temperature was maintained with the help of a feed back circuit that controlled the current flow to the heater coil. In our indigenously developed automated spray system, tem- perature of the substrate could be varied from room tempera- ture to 723 K. During spray, substrate temperature was kept constant at 573 K with an accuracy of ±5 K. Spray head and heater with substrate were kept inside a chamber with an exhaust fan for removing gaseous byproducts and vapors of solvent. Aqueous solution, containing required quantities of CuCl2, InCl3 and Thiourea (CS (NH2)2) was sprayed onto the substrate, using compressed air as carrier gas. The carrier gas and the solution were fed into the spray nozzle at a con- stantly maintained pressure and flow rate. The carrier gas flow rate could be varied in the range 0 to 2 x 10

Photoconductivity in sprayed β-In2S3 thin films under sub-band-gap excitation of 1.96 eV

β-In2S3 thin films with a band gap of ∼2.67eV exhibited persistent photoconductivity when excited using photons with energy of 1.96 eV. The photoconductive response to extrinsic photoexcitation could be removed when the film stoichiometry was changed. Photoluminescence studies in the films revealed an emission of 1.826 eV, due to donor–acceptor pair (DAP) recombination, which was absent in the film not responding to extrinsic excitation. Hence, it was concluded that presence of the DAP was responsible for the extrinsic photoconductivity under the 1.96 eV excitation. This study can initiate further a methodology for tailoring the photoresponse of this semiconducting thin film by spatially controlling the film stoichiometry.

Transverse photothermal beam deflection technique for determining the transport properties of semiconductor thin films

Nonradiative transitions occurring in semiconductors result in thermal emissions carrying information on the material’s thermal and electronic properties. A simple one-dimensional theoretical model is devised which accounts for the photothermal signal variations due to nonradiative transitions occurring in semiconductor thinfilms. The theory was verified by determining the transport properties of p-type silicon wafer. We could get the thermal diffusivity, minority carrier lifetime, surface recombination velocity, and minority carrier mobility of CuInS2 thin films, thereby proving the efficiency and simplicity of photothermal beam deflection technique for real time characterization of semiconductor thin films. The film fabrication history, composition, and post deposition treatments play crucial role in determining the transport properties and the effect of these conditions on transport properties of the film as well as on the solar cell parameters is discussed.

Implantation assisted copper diffusion: A different approach for the preparation of CuInS2∕In2S3 p-n junction

Copper indium sulfide thin films were prepared using copper diffusion into argon ion implanted In2S3 thin films. A comparative study of copper diffusion in pristine and ion implanted In2S3 samples was also performed. It was found that copper indium sulfide formation was much better in argon implanted samples compared to that in unimplanted samples. Copper diffusion in implanted samples enabled us to prepare an In2S3∕CuInS2 solar cell. The fill factor of the cell prepared was 30.2% and the efficiency was 0.34%.

Effect of defect bands in β-In2S3 thin films

Optical absorption studies in β-In2S3 thin films of band gap 2.66 eV, prepared using chemical spray pyrolysis technique, revealed presence of a defect band which could assist absorption of sub band gap photons. Extrinsic photoconductivity under excitation of 2.33 eV was observed in these films. Photoluminescence studies revealed a green emission from the films providing a recombination path to these carriers. Temperature dependence of photoconductivity showed that the states in the defect band were continuously exchanging carriers with the conduction band which caused the photocurrent to show persistent photoconductivity. Temperature dependence of photocurrent revealed existence of shallow traps located ∼24 meV below the conduction band which played vital role in controlling the photosensitivity of the films. Temporal dependence of photoconductivity revealed decay tails which were identified to be the effect of thermal release of carriers form the shallow traps.

Room Temperature Photoluminescence Surface Mapping

Photoluminescence mapping system using tip illumination/tip collection and tip illumination/far field collection has been described and it is found that the latter collection mode is better suited for luminescence signal collection. Emphasis has been laid on the information that can be gained from the spatial measurements of luminescence emission intensity at single wavelength (topography) and different wavelengths. The results of measurement on CuInS2 thin films prepared using spray pyrolysis technique are presented. Characterization of the defects in the film has been achieved. The effect of non-uniform film deposition on the luminescence has been realized.