Molecular Engineering of Polymer Semiconductors for Electronics and Photonics

Abstract

There has been tremendous progress in the development of conjugated polymer semiconductors in the last two decades for diverse applications in organic electronics and photonics. Most notably, advances in p-type (hole-conducting) polymers have enabled the development of high-performance organic field-effect transistors (OFETs) as well as more efficient fullerene-based organic photovoltaics (OPVs). In contrast, n-type (electron-conducting) polymer semiconductors remain relatively scarce and consequently the performance of n-channel OFETs and all-polymer solar cells has lagged far behind p-channel OFETs and fullerene-based OPVs. This dissertation mainly focuses on the design and synthesis of new n-type polymer semiconductors for device applications in n-channel OFETs and non-fullerene OPVs. It aims to achieve better understanding of the relationships between molecular structure, processing, morphology, and device performance. New n-type polymer semiconductors were developed based on strong electron withdrawing naphthalene diimide (NDI) building block with various selenophene derivatives as co-monomers. The resulting highly crystalline poly(naphthalene diimide)s (PNDIs) gave the electron mobility as high as 0.24 cm2/Vs in n-channel OFET measurements in air which is comparable or even higher mobility compared to most of the p-channel transistors. Using NDI-selenophene copolymer, PNDIS-HD, as acceptor, a photovoltaic performance with a power conversion efficiency (PCE) of 3.3 % (Jsc = 7.78 mA/cm2, Voc = 0.76 V, FF = 0.55) was achieved in all-polymer solar cells, and this work has stimulated a lot of current interest in fullerene-free OPVs. In further studies in all-polymer solar cells, highly enhanced photovoltaic performance was achieved by chemical modifications of acceptor polymers and controlling self-organization kinetics of polymer/polymer blend films. From these studies, a critical role of the bulk crystallinity of acceptor polymer was revealed, and provided an important criterion for the molecular design of high performance polymer acceptors. Furthermore, all-polymer solar cells with more favorable bulk morphology by slow self-organization of polymers facilitated by room temperature film aging resulted in enhanced charge carrier mobility and photocurrent. Resulting all-polymer solar cells with PCE over 7 % showed a great potential of non-fullerene solar cells and demonstrated for the first time a viable alternative pathway to organic photovoltaics.