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Structural, morphological and Raman studies of CdS/CdSe sensitized TiO2 nanocrystalline thin films for quantum dot sensitized solar cell applications

BACKGROUND: It is well known that quantum dot-sensitized solar cells based on nanostructured semiconductor films are considered as a promising alternative to silicon-based solar cells. The aim of this paper is to investigate the structural and morphological properties of CdS/CdSe quantum dot sensitized photoanodes based on nanocrystalline TiO2 thin films considering their performance can reach an efficiency of 2.7%. In particular, quantum dot materials such as CdSe and CdS, which typically have a couple of orders higher absorption than silicon and thin film materials can lead to higher photocurrents and are most effective with liquid electrolyte solar cells. Moreover, CdSe and CdS quantum dots have been shown to be more efficient when combined than as single species. METHODS: TiO2 thin films were prepared on fluorine tin oxide (FTO) glass via the chemical route using commercial Degussa 25 and crystallized at 550oC. Furthermore, a layer of CdS and CdSe nanoparticles was deposited on the titania film by a sequence of successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) methods. After preparation, X ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) were employed to investigate the crystallinity of the photoanodes, while Raman and micro-Raman spectroscopy were used to reconfirm the crystalline phase of the materials and sample composition. Surface morphology was investigated using scanning electron microscopy (SEM) while energy dispersive x-ray spectrometry (EDS) was used to analyze the sample stoichiometry. RESULTS: XRD performed for the TiO2 thin film before quantum dot deposition showed strong diffraction peaks at 26o, 34o, 38o and 52o indicating that the FTO substrate is polycrystalline with a mainly tetragonal SnO2 phase while peaks at 25° and 48° degrees revealed that the TiO2 layer is mainly in the anatase phase. Moreover, HR-TEM determined that the crystallite size of titania was around 20nm, while EDS of the deposited film showed that the mole ratio of O to Ti is 2.6:1. After CdS/CdSe quantum dot deposition XRD revealed that CdS exhibits a definite peak at 51° while at 42o we observed a weaker peak corresponding to the CdSe structure. EDS revealed a mole ratio of S to Cd and Se to Cd of almost 1:1 in both cases. HR-TEM showed that the CdS and CdSe quantum dots sizes ranged between 2nm to approximately 10nm. Raman spectroscopy of the TiO2 film revealed peaks at 554 cm-1 and 1092cm-1 corresponding to the FTO substrate, as well as intense peaks at 521cm-1 and 643 cm-1 related to the TiO2 anatase phase. Micro-Raman showed additional Raman active peaks at 142cm-1 and 197 cm-1 related to titania. After CdS/CdSe quantum dot deposition micro-Raman revealed an active peak at 302cm-1 corresponding to CdS while Raman active peaks at 203 cm-1 and 403cm-1 corresponded to CdSe. CONCLUSIONS: The structural properties and morphology of quantum dot sensitized photoanodes revealed that the titania thin films were highly crystalline belonging predominantly in the tetragonalanatase structure, while the CdS/CdS quantum dots were in the cubic phase. Furthermore, scanning electron microscopy (SEM) along with energy dispersive X-Ray mapping EDS showed little contamination as well as high stoichiometry for both the TiO2 substrate as well as the CdS/CdSe quantum dots. Finally, micro-Raman spectroscopy reconfirmed the crystalline phase of the materials and sample composition.

nanocrystalline ; titania ; quantum dots ; cadmium chalcogenides ; solar cells ; Raman spectroscopy

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