Immobilizing Pristine and Mn-doped CdSe Quantum Dots on Carbon Nanowhiskers for Solar Energy Conversion

Srivastava, Vishwas (2013) Immobilizing Pristine and Mn-doped CdSe Quantum Dots on Carbon Nanowhiskers for Solar Energy Conversion. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS Dissertation of Vishwas Srivastava) Vishwas_Srivastava_(08MS44)-MSThesis.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Semiconductor quantum dots (QDs) due to their size quantization property have recently emerged as light harvesters for use in next generation solar cells.1 Hence, it is imperative to separate the electron-hole pair and transport the photogenerated electrons to the electrode surface.1 Employing another wide band gap semiconductor,2 or an electron acceptor shell3 are several strategies that can quickly separate the charge carriers to enhance the operation of quantum dot solar cells. Carbon nanomaterials, due to their unique electronic and structural properties can be used as efficient materials for charge separation in semiconductor nanocrystals. Both single walled and multi walled carbon nanotubes have demonstrated photovoltaic properties.4 Further, by doping optically active transition metal ions, for example, Mn2+, it is possible to tune the electronic and photophysical properties of QDs.5 The Mn d-d transition is both spin and orbitally forbidden resulting in very long lifetimes.6 In this work we have studied the effects of spiraled carbon nanowhiskers (CNWs) on the optical properties of CdSe and CdSe:Mn2+ nanocrystals. Our aim was to establish the electron or charge transfer from the excited state of CdSe and CdSe:Mn2+ nanocrystals to the Π-electron network of the CNWs and employ this system to generate photocurrent in a TiO2 sensitized solar cell. A newer approach was employed to crystallize CdSe in a zinc blende lattice and dope Mn2+ impurity into this lattice. EPR spectroscopy was employed to evaluate the doping of Mn2+ ions into CdSe QDs. X-ray diffraction (XRD), dynamic light scattering (DLS) and transmission electron microscopy (TEM) confirmed the size and structure of the QDs. CNWs were synthesized from waste mustard oils in the presence of FeNO3.9H2O catalyst and were characterized for their electrical properties by means of conductivity measurement and Raman Spectroscopy. Emission quenching and lifetime decay experiments were performed on dispersions of QDs and CNWs. Covalent as well as electrostatic interactions were employed to immobilize the QDs on the walls of CNWs and the effect of these stronger interactions was also studied by means of photoluminescence and time correlation spectroscopy. The photocurrent measurements were carried out for CdSe, CdSe:Mn2+ and CdSe-CNW samples deposited on TiO2 coated FTO electrodes. The current density and efficiency were measured and it was found that CdSe-CNW samples had an efficiency of 5.8% as compared to the 4% efficiency for CdSe QDs.

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