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Synthesis and characterization of semiconductor thin films for photovoltaic applications

Tezel, Tamer
Cadmium sulfide (CdS) thin films are very attractive materials due to their tunable optical properties and potential applications in not only photovoltaic devices but also in electronics, bio-labeling and fluorescence imaging. Recently, there is a great interest in flexible photovoltaic devices due to their unique properties such as very low weight, mechanical durability and large area applications. Organic semiconductors and their heterojunctions with inorganic materials are the most promising candidates for flexible photovoltaic applications. Preparation of CdS and Polypyrrole (PPy) semiconducting thin films on flexible polyethyleneterephtalate (PET) substrates and investigation of their morphological, structural, optical and electrical properties are the main scopes of this thesis. In the first part of the study, CdS thin films were deposited on PET via electrodeposition method. Taking the advantages of electrodeposition methods, CdS thin films with good optical and electrical properties were produced. CdS thin films were also deposited on soda-lime glass substrates in order to observe substrate effect. Scanning electron microscopy equipped with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD) and UV-vis spectrometry have been used to determine the structural and optical properties of the films deposited at various temperatures and for different deposition time intervals. For all samples, molecularly homogenous CdS films have been observed with atomic percent ratios of the Cd to S very close to 1:1. Thin films showed (002) hexagonal crystal structure around 26 (2) with average grain size 7.0 nm. CdS films have high transmittance for the wavelength greater than 500nm. Band gap energies of the films, which range between 2.74 and 2.68 eV, decreased with increasing both deposition temperature and time. For further characterization, photoelectrochemical performances and electrochemical impedance spectroscopy (EIS) of both as deposited and CuCl2 treated CdS thin films have been investigated. Later, following to the deposition of individual CdS thin films, polypyrrole thin films were produced and then heterojunctions of polypyrrole with CdS were examined. It has been observed that cadmium sulfide enhanced the photoelectrochemical properties of the polypyrrole film. Influence of the polypyrrole thin film deposition time on the photoelectrochemical properties has been also investigated in this study. Frequency dependent measurements revealed that type of charge carrier changes as a function of polypyrrole deposition time.