Charge Transport Physics of Organic Semiconductors at High Carrier Densities

Abstract

The charge transport physics of organic semiconductors has been studied intensively since the discovery of electrical conduction in doped polyacetylene by Heeger, MacDiarmid, and Shirakawa. While the charge transport physics in this class of materials at low carrier densities of less than 10¹⁸ to 10¹⁹ cm¯³ has been relatively well established, the physics at much higher carrier densities of 10²⁰ to 10²¹ cm¯³ remains poorly understood. In this transport regime there is on the order of one charge per molecular repeat unit, and naturally the transport is highly correlated because of the many-body charge-charge and charge-ion interactions. This thesis focuses on the physics of organic semiconductors at such high carrier densities. Our understanding is mainly guided by charge transport studies, for instance the evolution of electrical conductivity and Seebeck coefficient with carrier density and temperature. Structural and spectroscopic characterisations have been performed to complement the transport insights. We first demonstrate that correlated transport physics can be studied over a very wide range of carrier concentrations in p-type, organic electrochemical transistors (OECTs). In the donor-acceptor copolymer indacenodithiophene-co-benzothiadiazole we show that it is possible to remove all electrons from the highest occupied molecular orbital and to reversibly access transport in the second-highest occupied molecular orbital. By adding a second, field-effect gate electrode to the OECT, additional electrons or holes can be injected at set charge states. Under conditions where ionic motions are frozen, the measured field-effect transfer characteristics reveal a frozen Coulomb gap, whereby the non-equilibrium transport signatures of the charge carriers can be investigated. We attribute the formation of the frozen Coulomb gap to the freezing of the ionic motions. We demonstrate that these many-body effects are not specific to any particular polymer system. Analogous transport signatures are seen in experiments on diketopyrrolopyrrole-based donor-acceptor and polythiophene-based pure-donor systems. Through systematic studies of the three polymer systems, we describe how distinctions in their electronic structures and microstructures are reflected in the details of their transport signatures. The apparent generality of many-body effects in doped conjugated polymers is significant, as we demonstrate superior transport coefficients in the non-equilibrium states of the frozen Coulomb gap. We show that chain alignment is an independent strategy of enhancing electrical conductivity without compromising the Seebeck coefficient. This conductivity increase is consistently seen across a wide range of carrier concentrations in the diketopyrrolopyrrole-based and polythiophene-based polymers, such that it is a potential complement to further enhance the non-equilibrium transport coefficients. Aside from doped linear polymers, we explore the charge transport physics of two-dimensional metal-organic frameworks (MOFs), as a novel class of organic conductors. We report enhanced electrical conductivities associated with improved crystallinity in a novel phase of mixed-metal benzenehexathiol-based MOFs. We show similarly metallic thermopowers regardless of the exact composition of the metal sites in these benzenehexathiol-based MOFs, which suggest that their electrical conductivities are largely governed by structural disorder.