Semiconductor Thin Film

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One economical and easy energy source is solar cells. Professor A. B. M Obaidul Islam, who is very famous in working with semiconductor thin film, accepts me as a researcher for M.Phil. degree. He tried to find a method for solar cell application that will be easier and very low cost compared to the conventional method. Thin film heterojunction solar cells play an important role in high-efficiency devices in solar energy conversion. Many transparent n-type semiconducting films have been employed as window materials, and these are deposited on p-type semiconductors, giving various types of heterojunctions. However, the most difficult and expensive part is to make a p-type thin film on a large volume, usually deposited by the vacuum evaporation technique in a small volume. The new method should be designed to deposit thin films over large areas with very low cost. We took a project with fellowship that was funded by the Bose Center for Advance Study and Research in Natural Science, Bangladesh. This is a very competitive and demandable fellowship program. I was a research fellow for three years from July 2000 to June 2003 with an initial fund. I was the key researcher among our research group, where I supervised three Master's students. Finally, a low-cost chemical bath deposition (CBD) method was developed to prepare copper selenide (Cu2-xSe) thin films for solar cell applications. Cu2-xSe is an extrinsic p-type semiconductor with a forbidden energy gap of 1.1-1.29 eV, which is near the optimum value for solar cell applications.

 

A low-cost and simple chemical bath deposition (CBD) method was used for the preparation of Cu2-xSe and Cu3Se2 thin films on glass substrates. We successfully deposit thin films on a large area of glass substrates on a bath containing a solution of copper, selenium and solvent. Different thin films were prepared by adjusting the bath parameters of pH value, deposition time, temperature of the solution and the ratios of the mixing compositions between copper and selenium in the reaction bath. The structural, electrical and optical properties of these films were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The annealing effect of the as-deposited films was studied at 250°C in air, and the film became ordered with structures, whereas a disordered surface was obtained for the as-deposited film. Annealing these films above 250°C in air results in the loss of selenium, which was confirmed using XPS and absorption studies. All the films (as-deposited and annealed) showed p-type conductivity detected by Hall measurement. Optical studies by UV spectrophotometry suggested an optimum value of the energy gap for the most useful solar cell. From the study, it was revealed that the chemical bath deposition technique can be used to prepare thin films of Cu2-xSe and Cu3Se2, and these films can be used in various purposes of thin film technology, especially for solar cell applications. The efficiency of the solar cell by making a junction with an n-type conduction film was found to be reasonable for practical applications. Further improvement of the cell is now being investigated at the Semiconductor Technology Research Center (STRC) at Dhaka, Bangladesh.

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We also employed vacuum evaporation technique to deposit thin films on glass and indium tin oxide (ITO) coated glass substrate. The electrical and optical properties of vacuum evaporated copper selenide films were also extensively studied. The resistivity of as-deposited and annealed films were in the range of (1.5-2.3)×10-5 W-m. All the films showed p-type conductivity. Transmittance and reflectance were found in the range 1–80% and 2–32% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of ~107 m-1. A comparative study has been performed between the films prepared by vacuum evaporation and chemical bath deposition technique and found identical results.