Charge Transport and Photo-Physical Studies in Conjugated Polymers, Hybrid Nanocomposites and Devices

Abstract

The main motivation of this thesis is derived from the fact that physics of disordered systems like conjugated polymer has yet not achieved as concrete understanding as ordered and crystalline systems such as inorganic semiconductors. Through the work done in this thesis, several efforts have been made in order to understand basic charge transport (hopping, current injection) phenomena and photo-physical properties (photoluminescence quenching, absorption, photoconductivity) in conjugated polymer and their hybrid composites. The thesis consists of 7 chapters. Chapter 1 discusses the background knowledge and information of the general properties of conjugated polymers, quantum dots and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the temperature and field dependent anisotropic charge transport in polypyrrole. Chapter 4 presents an idea to probe and correlate disorder and transport properties using impedance and Raman spectroscopy. Chapter 5 mainly talks about the doping level dependent photophysical and electrical properties of poly(3-hexylthiophene). Chapter 6 reveals the charge transport phenomena in hybrid composites of poly(3,4-ethyldioxythiophene):polysterene sulfonate (PEDOT:PSS) and cadmium telluride quantum dots. Chapter 1: Conjugated polymers and their hybrid systems are easily processible and cost effective material having huge scope for advanced materials of the future. Although variable range hopping (VRH) is widely accepted to model charge transport in π-conjugated systems, but at very low temperatures, high fields, high carrier concentrations one need to explore other models. Conjugated polymers are anisotropic intrinsically. Therefore, anisotropic charge transport can provide basic insights about the physics of charge hopping. Quantum dots, and their hybrid nanocomposites with semiconducting polymers receiving a huge attention for light emission and photovoltaic purposes. It is important to learn about the charge injection,barrier heights, etc. in order to achieve efficient hybrid devices. Chapter 2: Synthesis of the samples, both conjugated polymers and quantum dots, and fabrication of hybrid devices is an important and integral part of this thesis. An Electropolymerization technique is used for making polymer samples on conducting substrates. This is quite interesting because one can tune doping level, disorder and thickness simultaneously. Hydrothermal process is adopted to get highly aqua-dispersible quantum dots. Samples are characterized by different techniques like Raman spectroscopy, energy dispersive spectroscopy. Photoluminescence, UV-Vis absorption, transmission electron microscopy and atomic force microscopy are used to explore several properties of the polymer and hybrid nanocomposites. Chapter 3: It is known that conjugated polymers are intrinsically one–dimensional materials. Therefore it is important to learn anisotropic behavior of these complex systems. Hence, a comparison of electronic transport to their morphology has been carried out and role of carrier density and disorder is discussed further. Both in-plane and out-of-plane charge transport is studied in electrochemically deposited polypyrrole on platinum. Strong anisotropy is observed in the system which is correlated to granular morphology. Field dependence of anisotropic conductivity is also explored. Field scaling analysis shows that all field dependent curves of conductance at different temperatures can fall on to single master curve. Glazman – Matveev model is used to describe nonlinear conduction in field dependence and nonlinearity exponent is estimated. Disorder and carrier density along with the morphological structure like length and orientation of polymer chains with stacking arrangement of different layers in PPy films play an important role in governing the anisotropy in transport properties. Chapter 4: Two different techniques, namely impedance and Raman spectroscopies are used to probe disorder and transport properties in the polypyrrole. An effort is made to correlate the transport properties to the morphology by probing disorder via two different spectroscopic techniques. Frequency dependence of both real and imaginary part has shown that disorder and inhomogeneity varies in different PPy devices, which thus affect the transport properties like conductivity and mobility. Mobility values along the thickness direction for each sample reveal the impact of disorder on out-of¬plane geometry. A circuit based on consideration of the distributed relaxation times, is

successfully used to obtain the best fit for the Cole–Cole plot of various PPy devices. FWHM of the de-convoluted peaks of Raman spectra is attributed to the change in distribution of the conjugation length in the PPy films. Chapter 5: The main focus of this chapter is the qualitative exploration of different photo-physical and electrical properties of electropolymerized poly(3-hexylthiophene) and their dependence on doping level. Photoluminescence quenching, band edge shifting in absorption spectra, electrochromic effect, significant enhancement in photocurrent at optimum doping level, two relaxation behaviors in reactance spectra and presence of negative capacitance at low frequencies are distinct features which are observed in poly(3-hexylthiophene) in this work. Quenching in photoluminescence intensity is attributed to charge transfer occurring between polymer chains and dopant ions. Two semicircles in the Cole-Cole plots refer to two type of relaxation process occurring in bulk layer and at interface. Frequency response of capacitance at higher bias and lo side of frequency shows a negative capacitance due to the relaxation mechanism associated with the space-charge effect. Chapter 6: Synthesis of quantum dots and fabrication of hybrid devices is one of the catchy parts of this chapter. Huge quenching photoluminescence intensity and very high increment (~ 400 %) in photocurrent clearly depict the charge transfer at molecular level. Temperature dependent current–voltage characteristics show the absence of thermionic emission since the barrier height is more than the thermal energy of the carriers. Further analysis confirms that the charge carrier injection of ITO/PPCdTe3/Al device is controlled by tunneling processes. The hybrid system has shown a peculiar transition from direct tunneling to Fowler–Nordheim tunneling mechanism which is because of the change in shape of the barrier height from trapezoidal to triangular type with increase in applied electric field. Chapter 7: The conclusions of the different works presented in this thesis are coherently summarized in this thesis. Thoughts and prospective for future directions are also summed up.