Charge injection and transport in quantum confined and disordered systems

Abstract

Quantum dots and conducting polymers are modern semiconductors with a high potential for applications such as lasers, LEDs, displays, solar cells etc. These applications require the controlled addition of charge carriers into the material and knowledge of the details of charge transport. This thesis describes research into the charge-injection and charge-transport properties of films of quantum dots and of conducting polymers. Experiments include the synthesis and self-assembly of PbSe nanocrystals with various (odd) shapes and the evolution of the optical absorption of these nanocrystals as their size increases. The density of states (DOS) of quantum-dot solids made from CdSe and ZnO NCs was studied with electrochemical gating; CdSe suffers from surface states while ZnO is haunted by ghost electrons. The electronic conductivity was studied experimentally and with Monte Carlo simulations. Conductivity results from electrons that hop between quantum dots. The strange thing is that at low temperature the electrons often do not hop to the nearest nanocrystal, but to one that is further away. This is called variable-range hopping (VRH). To explain the experimentally observed T-dependence of conductivity the existing VRH models have been extended with a non-resonant tunneling expression based on Einstein fluctuations. This yields exactly the temperature dependence that is observed experimentally. The DOS of the conducting polymer PPV was determined over a wide energy range. It is shown, for the first time, that the DOS is Gaussian as a result of disorder. Two Gaussian levels were found in the DOS that were identified as polaron and bipolaron formation. At the highest doping levels the absorption spectrum has the characteristics of a metal, while the conductivity remains activated and follows the Mott variable-range hopping law. Interchain hopping is the limiting factor for the hole mobility in PPV.