Designing quantum dot solids for optoelectronic devices through matrix engineering
Colloidal quantum dot (CQD) solids represent a class of materials that allows one to control the optical and electronic properties due to their unique size-dependent properties with special electronic and optoelectronic device applications. Unfortunately, integration of these materials into high performing devices such as transistors and solar cells have been challenging due to: 1) uncontrolled environmental stability 2) lack of accurate control over charge carrier type and mobility 3) poor device operational stability and 4) limited experimental methods to probe the density of states in these materials in order to understand fundamental electronic and optical properties. In this thesis, we demonstrate the ability to stabilize and improve the environmental stability of these materials with amorphous Al2O3 (a-alumina). More importantly, we can accurately engineer the carrier type and mobility by varying the thickness of the alumina. Through a combination of small, compact inorganic ligands and the ability to passivate surface electronic traps, air-stable, high electron mobility PbSe QD field-effect transistors (FET) are obtained.
We then show that we can also improve transistor device operational stability through an in-vacuo ligand exchange with H2S gas introduced in an atomic layer deposition (ALD) chamber. We find that this method is universal when volatile ligands are used. Possible mechanisms for device instability will be discussed such as proton migration and trap passivation. Using an optimized film preparation, this work will be the first demonstration of a QD FET with an electron mobility greater than 10 cm2 V-1 s-1 that is also operationally stable.
Finally, we introduce a unique transmission spectroscopy technique of field-effect transistors to electrostatically probe induced charge carriers in PbSe QD films. With this technique we resolve occupation of quantized states of the quantum dots rather than the matrix or interfacial states. This platform is used to test fundamental transport models as it relates to disordered semiconductors such as QDs. From this technique, we can draw important conclusions about charge transport at room temperature. This novel experimental method can be extended to other experimental setups such as photoluminescence and photoconductivity in order to understand how to rationally improve the electronic properties of QD films.