Synthesis of Semiconductor Quantum Dots, Study of Their Optical Properties and Their Application in Sensitized Solar Cells

Abstract

Requirement of clean and cheap renewable energy production by understanding and then manipulating the physics of semiconductor nanostructures to realize cost effective and efficient Nanophotovoltaic devices is the primary drive for the current research. Many advantages of semiconductor quantum dots (QDs) like higher optical absorption cross-section, wavelength tunability, reduced phonon mediated relaxation and capability for solution processing at a low cost made these very popular. Here we have reported easy, economic synthesis and fabrication procedures for QD based nano-photovoltaic devices. The main focus of this thesis work is to develop synthesis procedures with reduced synthesis steps while maintaining optimum optical and electronic transport qualities of these nanostructured devices. We have developed a simple low temperature and aqueous synthesis method for CdTe QDs. The measured photoluminescence quantum yield of the as prepared QDs is highest among the aqueous based synthesis procedures reported till now for CdTe QDs. We have used these QDs as photo-absorber in Quantum Dot Sensitized Solar Cells. However it was evident that the polysulfide electrolyte reacts with these CdTe QDs, which reduces the performance of QD sensitized cells. Further we have tried to avoid such reactive degradation using type-II heterostructure i.e. core/shell structure with CdTe as core and CdSe as shell. We then presented a quicker, economic and less toxic, single step aqueous synthesis procedure for core/shell QDs at relatively low temperature. We have shown that the structural as well as optical properties of these core/shell QDs are almost similar to that of core/shell QDs synthesized with purification of core QDs. We have demonstrated that the power conversion efficiency of QD sensitized cells depend on their shell thickness. Reasonable good efficiency (~2%) is measured for the devices fabricated using core/shell QDs having thicker shells. There might be two reasons for observing efficiency increment by using such CS QDs # 1. energy level alignment in type-II heterostructure helps in separation of photogenerated carriers and #2. shell layer helps in separating the electrolyte and core which avoids reaction between them. ixInterestingly, the efficiency of the cells also increases by ~83% with sintering of photo-anode, as this sintering helps in removing the insulating capping layer and thereby improves electronic charge transport. Many a time agglomeration of QDs due to sintering of photo-anode also reduces its photo absorption efficiency. To avoid such issues while making the QD sensitized cells, we have used uncapped i.e. ligand free AgInS 2 QDs as photo-absorber provided by the research group of Dr. Angshuman Nag, IISER Pune. The obtained efficiency using ligand free QDs is superior than any of the previous reports on capped AgInS 2 QDs based QDSSCs as it facilitates better charge transport. Apart from these studies we have also studied the dependence of hydrodynamic size on the excitation intensity using Florescence Correlation Spectroscopy in collaboration with Dr. Shivprasad Patil, IISER Pune. In this study we have observed that the estimated hydrodynamic size is significantly affected by photo-excitation intensity above a certain threshold due to blinking. Here we have calculated photo-darkened fraction and probability of QD entering into dark state upon photon absorption. We have also studied collisional broadening of E 3 excitonic resonance in absorption spectra over wide range sizes and its overall aging and temperature variation for lead sulfide QDs at room temperature. As the carrier multiplication (CM) is a direct consequence of hot- exciton physics therefore, our results on the effects of excitonic collisions on absorbance spectra may help in understanding the physics of CM for photovoltaic applications.