| dc.description.abstract | Electrical Conduction Mechanism in Conjugated Conducting Polymer Polypyrrole
In this thesis, the mechanism of electrical conduction in conjugated conducting polymer polypyrrole has been studied in high and low doping limits. Conducting polymers are very complex systems in which disorder, interchain interaction, and doping level play important roles. Dopant-induced metal–insulator transition and crossover from 3D to 1D variable range hopping (VRH) conduction in polypyrrole–PF? have been studied using different low-temperature transport property measurements.
Methodology
A new methodology was adopted to vary the doping level in polypyrrole. A fully doped polypyrrole sample was prepared from a single polymerization by galvanostatic electrochemical method at –40?°C. By cutting this sample into six pieces without peeling off from the electrode and by dedoping, a batch of samples having different doping levels was prepared. In this way, two batches of samples were prepared:
First batch: more samples on the higher doping level side
Second batch: more samples on the lower doping level side
Structural Analysis
X-ray diffraction patterns taken on three samples from the first batch were fitted with Gaussian peaks around 12°, 20°, and 25°.
The lower-angle peak is assigned to intrachain ordering
The higher-angle peak is assigned to interchain ordering
This partially crystalline structure was analyzed by considering the monoclinic structure of polypyrrole. The interchain crystalline domain length was estimated to be ~20?Å. X-ray diffraction studies show a decrease in interchain order with a decrease in doping level. With decreasing doping level, polymer chains bend along their axis, making the structure loosely packed. Thus, dopant species play an important role in parallel alignment of polymer chains and hence more order.
Electrical Conduction
For high doping level samples (first batch), preliminary characterization by the resistivity ratio Pr = ?(1.2?K)/?(300?K) and W vs T plot shows a continuous transition from metal to insulator via a critical regime. Thus, as the doping level in polypyrrole is varied, a metal–insulator transition is observed.
From X-ray diffraction data, it is clear that interchain disorder increases with decreasing doping level. In the metallic regime, as electronic properties are very sensitive to disorder, the increase in interchain disorder induced by dedoping is responsible for the observed metal–insulator transition.
Metallic side: Temperature dependence of conductivity analyzed using localization–interaction model of electrical conduction in disordered electronic systems. Electron–electron interaction is the dominant mechanism.
Insulating side: Samples show variable range hopping (VRH) conduction and a crossover from 3D-VRH to 1D-VRH.
For the second batch, more samples in the insulating regime were prepared to study hopping conduction in detail. With decreasing doping level, interchain disorder increases, driving the system toward insulating behavior. Thus, to explain electrical conduction in polypyrrole by varying doping level, both disorder and interchain charge transfer must be considered.
Metallic side: More sensitive to disorder
Insulating side: More sensitive to interchain charge transfer
Therefore, doping-level-induced transition is not just due to change in doping level but also due to increase in disorder with decreasing doping level.
Magnetoconductance and Thermopower
Negative magnetoconductance and universal scaling observed for metallic samples confirm electron–electron interaction as the dominant mechanism.
At low fields: Magnetoconductance shows ?B dependence
At higher fields: Shows B² dependence, characteristic of localization–interaction model
Thermopower is positive for all samples as holes are the charge carriers. With decreasing conductivity, thermopower increases monotonically.
Metallic samples: Linear temperature dependence, most metallic sample shows electron–phonon enhancement contribution
Insulating side: Becomes more nonlinear due to hopping conduction
Hopping Conduction and Dimensionality Transition
In the second batch (insulating side), hopping conduction was studied in detail. With decreasing doping level, a transition in dimensionality of VRH conduction mechanism from 3D to 1D was observed. This change is attributed to the role of dopant ions in interchain charge transfer.
Higher doping: More interchain interaction, dopant ions contribute to DOS ? 3D-VRH
Lower doping: Fewer dopant ions, less interchain hopping ? 1D-VRH
As expressions for temperature dependence of conductivity for different mechanisms (Efros–Shklovskii VRH, tunneling conduction in granular metals, and 1D-VRH) are similar, temperature dependence alone cannot confirm the mechanism. However, electric field dependence of conductivity confirms hopping conduction.
Nonlinear Electrical Conduction
Nonlinear conduction studied by I–V characteristics measurements. Electric field dependence of conductance confirms transition from 3D-VRH to 1D-VRH.
Space-charge-limited conduction (SCLC) observed at low temperature
From SCLC: Mobility and carrier density obtained
Most probable hopping distance estimated from electric field dependence of resistance in low-field region
Summary
Electrical conduction in conducting polymer polypyrrole has been studied at various doping levels in high and low doping limits. For higher doping level samples, the observed metal–insulator transition is not just due to decrease in carrier density but is driven by increase in interchain disorder induced by removal of dopant species.The decrease in doping level reduces the three-dimensional delocalization of charge carriers, which is necessary for the metallic state. For the lower doping level samples, the observed transition in dimensionality of variable range hopping conduction is attributed to the decrease in interchain charge transfer due to reduced doping. The dopant ions provide sites for interchain hopping of charge carriers.
Overall, this study helps in understanding the role played by the extent of doping level on the electrical properties of conducting polymers. It explains how dopant ions influence structural order and how electrical properties are affected in high and low doping regimes in conducting polymers like polypyrrole.
Future Scope
1. AC Conductivity
It will be interesting to study the AC conductivity in samples showing the transition in dimensionality of variable range hopping conduction. As the time scales of carrier transport along the chain and between the chains are different, the AC conductivity or frequency dependence will be able to separate these two processes. The AC conductivity, ?(?), provides information about processes occurring at time scales ? ? ??¹, while DC conductivity is sensitive to the slowest processes.
2. Thermal Properties
In the case of conducting polymers, not much attention has been paid to thermal properties like specific heat and thermal conductivity. It will be of interest to see whether the thermal properties of metallic conducting polymers resemble those of metals. For metals, the expression that combines electrical conductivity and thermal conductivity is the Wiedemann–Franz law, which can be tested for metallic conducting polymers as well. | |
