Molecular Engineering of Polymer Semiconductors for Electronics and Photonics
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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.
- Chemical engineering