Spectroscopy of Charge Carriers at Ionic Liquid/Semiconductor Interfaces

The interface of semiconductors plays an important role in all semiconductor devices - often defining the device's properties. Ionic liquids are typically employed as gating dielectrics to achieve high charge densities at semiconductor surfaces. Such ionic liquid/semiconductor interfaces are usually studied using electrical transport techniques, which often have the drawback of requiring modeling to achieve an understanding of the species involved in the devices at the ionic liquid/semiconductor interfaces. Through the use of infra-red and optical spectroscopy this work seeks to uncover the nature of charge carriers surface of both organic semiconductors, specifically rubrene, and two dimensional semiconductors, specifically monolayer MoS2, and how these charges interact in the presence of mobile ions. I show that for rubrene infra-red spectroscopy reveals with the formation of an ionic liquid/rubrene interface the rubrene surface becomes intrinsically hole doped. Additionally, that when rubrene is gated the ionic liquid to achieve high charge densities in rubrene there is a saturation of the conductive species resulting in a lowering of hole mobility. In the case of monolayer MoS2 photoluminescence spectroscopy shows that forming the ionic liquid/MoS2 interface results in mobile ions screen the charged defects in the MoS2 increasing the photoluminescence intensity

Lead halide perovskites: Crystal-liquid duality, phonon glass electron crystals, and large polaron formation

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. This “crystal-liquid” duality implies that lead halide perovskites belong to phonon glass electron crystals, a class of materials believed to make the most efficient thermoelectrics. We show that the crystal-liquid duality and the resulting dielectric response are responsible for large polaron formation and screening of charge carriers, leading to defect tolerance, moderate charge carrier mobility, and radiative recombination properties. Large polaron formation, along with the phonon glass character, may also explain the marked reduction in hot carrier cooling rates in these materials