Studies of Charge Transport and Photovoltaic Properties of Semiconducting Polymers

Abstract

Semiconducting polymers are finding growing applications in electronics and optoelectronics, including organic light-emitting diodes (OLEDs) for displays and lighting, organic field-effect transistors for flexible/foldable electronics, organic photovoltaics to tackle climate change, organic electrochemical transistors (OECTs) for biosensing and neuron-like computing, organic thermoelectrics for harvesting waste heat, and so on. Understanding of the molecular and supramolecular factors that influence the properties of single- and multi-component semiconducting polymers is central to further advancement in these and other applications. However, knowledge of such factors that limit the photovoltaic and charge transport properties of electron-conducting (n-type) conjugated polymers remains scarce and lags behind their hole-conducting (p-type) counterparts. This dissertation aims to establish the structure–morphology–property relationships of n-type semiconducting polymers with respect to charge transport and photovoltaic properties, elucidate the underlying physics, and provide quantitative guidelines for the molecular engineering of next-generation semiconducting polymers and organic electronic devices. The first objective of the dissertation is to tackle the question: what molecular and supramolecular factors govern the photovoltaic properties of all-polymer solar cells (all-PSCs), which are composed of binary blends of n-type and p-type semiconducting polymers? My studies of various binary polymer blend systems presented in Chapter 2 show that polymer molecular weight and surface energy of the blend constituents have profound influences on the blend photophysics, charge transport, and morphology, which in turn dictate the photovoltaic properties of all-PSCs. Highly efficient all-PSC devices with photon-to-electricity conversion efficiency > 10% are achieved by scalable strategies that control the blend component molecular weight and differences in surface energies. A second objective focused on elucidating the mechanism of electron transport of n-type semiconducting polymers in view of polymer chain topology and polymer chain length. In these studies (Chapter 3 and 4), I combined the field-effect transistor as the electron transport probe with model semi-flexible and model rigid-rod conjugated polymers, and diverse thin-film morphological characterization tools. I found that significant structural disorder in semi-flexible polymers imposed an upper limit on the electron mobility at a modest chain length of 45-60 repeat units. In contrast, suppressed structural disorder and favorable energetic landscapes in rigid-rod polymers enable continuous growth of electron mobility with chain length. A third objective is to probe the formation mechanism of different polaronic species in n-type semiconducting polymers upon electrochemical doping (Chapter 5). A ternary mixture of singlet polarons, singlet bipolarons, and triplet bipolarons is demonstrated for the first time to coexist in equilibrium in heavily n-doped polymers. Singlet electron-polarons are the primary charge carriers at a low to moderate doping level, and triplet electron-bipolarons are the dominating charged states at extremely high doping level.