Elaboration and characterization of chalcogenide thin films by chemical bath deposition technique

Abstract

Chemical bath deposition, which is a very attractive method, used to produce thin films for polycrystalline solar cells such as CdS and PbS. This work was based on this method. Our objective is to provide a comprehensive study on the effect of the deposition parameters on the structural and optical properties of CdS and PbS thin films and to monitor all reactions steps leading to CdS thin films formation. For the CdS thin films the bath solution is a mixture of cadmium carbonate CdCO3 and thiourea as source of cadmium and sulphur respectively, ammonia was used as complexing agent. In this study, two sets of samples were prepared; in each set, only one parameter is varied while the rest of parameters were fixed. In the first set, a few drops of sulphuric acid were added while in the second more of sulphuric acid were added. In order to investigate the deposition time. For both sets, films were deposited for different durations ranging from 30 to 90 min at bath temperature 55°C. X-ray diffraction, UVisible spectrophotometry and Fourier transform infrared techniques were used to investigate CdS thin films and the different steps of reactions leading to CdS product. For the first set, the growth of films occurs by the mechanism ion-by-ion with low deposition rate. Hexagonal CdS thin films were obtained with (002) preferred orientation and having average crystallite size in the range of 14.3 to 30.4 nm. All the films exhibit a high transmittance in the visible range; it is in order of 70 to 92%, the values of the optical gap are between 2.42 and 2.58 eV, this according to deposition time. Studies of different solutions and steps leading to the formation of CdS products were carried. The formation mechanism of CdS thin films was proposed based on FTIR results. For the second set, the obtained CdS films in this case have cubic structure with a preferential orientation along the axis (111) and crystallite size between 24.4 and 38.9 nm. The transmittance of the films was found to be in order of 40 to 80%, the values of the optical band gap increases with deposition times from 2.19 to 2.35 eV. In this work, PbS thin lms were prepared for dierent deposition times ranging from 30 to 90 min and different lead nitrate molar concentrations (0.01, 0.05, 0.075 and 0.1) mol/l at a bath temperature of 55°C, using thiourea as source of S2- ions and TEA as complexing agent. For different deposition times, the films grow preferentially along (200) direction, with increase in deposition time, and at fixed molar concentration the transmittance remained less than 30% and the values of the optical band gap decreases from 1.8 to 1.6 eV, while the crystallite size increases from 21.9 to 27.8 nm. For various molar concentrations of lead nitrate and with time deposition equal to 60 min, it was observed that the films grow preferentially along (111) or (200) directions depending on the molar concentration. The energy band gap decreases from 1.58 to 1.37 eV and the crystallite sizes increases from 28.0 to 32.6 nm with increasing the concentration of lead nitrate.