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Investigations of the Interplay of Molecule Structures, Crystal Packing, Solar Cell, and Field-Effect Transistor Characteristics in Novel Polymeric Semiconductors

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Title
Investigations of the Interplay of Molecule Structures, Crystal Packing, Solar Cell, and Field-Effect Transistor Characteristics in Novel Polymeric Semiconductors
Author
Kim, Yiho
Advisor
Yang, Changduk
Keywords
Polymer solar cell; quinoxaline polymer; nanoscale morphology control; processing additives; organic field effect transistor; naphthalene diimide polymer; geometric features; electron-donating strength; hybrid siloxane chain; balanced face-on to edge-on texture ratio
Issue Date
2015-02
Publisher
Graduate School of UNIST
Abstract
In the quest to improve the performance of polymer solar cells (PSCs) with a view to realizing economically viable, various solvent additives such as 1,8-octanedithiol (ODT), 1,8-diiodooctane (DIO), diphenylether (DPE) and 1-chloronaphthalene (CN) are used in easily obtainable poly(2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-dyl-alt-thiophene-2,5-diyl) (TQ1)-based systems with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as an acceptor to optimize the active layer nanomorphology. Utilizing a combination of X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM), we find that the addition of 5% (v/v) CN leads to smoother films, less heterogeneous surface features, and well-distributed TQ1:PC71BM phases, resulting in more balanced charge transport in the devices and a highly efficient power conversion efficiency (PCE) of 7.08%. This is a record for quinoxaline-based PCSs and also comparable the hitherto reported highest efficiency of the PSCs in single junction devices. In addition, the PSCs using an inverted device structure show a respectable PCE of 5.83% with high stability to ambient exposure, maintaining over 80% of its initial PCE, even after storage in air for more than 1 month. A family of naphthalene diimide (NDI)-based donor (D)-acceptor (A) copolymers having various acene- (benzene (Bz), naphthalene (Np), and pyrene (Py)) and heteroacene-type (selenophene (Se) and thiophene (Th)) donor rings has been designed and synthesized, for the purpose of understanding understand structure-property relationships on the subject of the structural factor systematically and electron-donating capability of the donor portions for applications in organic field-effect transistors (OFETs) fabricated by NDI-based copolymers to have been synthesized following our new design strategy. The resulting copolymers are categorized by the lack or existence of the heteroatoms in the donor framework, and also can be classified into ‘thiophene-free’ D-A copolymers (PNDI-Bz, PNDI-Np, PNDI-Py, and PNDI-Se) and thiophene-containing copolymer (PNDI-Th). When considering from the results of optical and electronic properties, the determination that the empirical electron-donating strength of donor co-units is in the order of Bz < Np < Py < Th < Se. Even though the resulting copolymers show the similarity of the LUMO levels (-3.73 ~ -3.82 eV), the dominant contribution of NDI unit to the polymer backbone lead to that the HOMO levels are sensitive to the relative electron-donating ability and shown to primarily influence whether unipolar n-channel (PNDI-Bz and PNDI-Np) or ambipolar charge transport (PNDI-Py, PNDI-Se, and PNDI-Th) is observed in OFETs of the NDI-based copolymers. Interestingly, the best OFET performance is observed in the acene-based centrosymmetric copolymer PNDI-Np (5.6310-2 cm2V-1s-1), regardless of the strong electron donors toward efficient intramolecular charge transfer (ICT) when compared to those of the other copolymers with axisymmetric units. Thus, the present work highlights that the geometric features of the donors in NDI D-A copolymers strongly reflect the carrier mobility dynamics rather than inserting electron-rich donor moieties into the backbone to lower the band gap and further strengthen ICT. Structure-property relationships associated with a hybrid siloxane-terminated hexyl chain (SiC6), photophysics, molecular packing, thin-film morphology, and charge carrier transport are reported for two novel naphthalene diimide (NDI)-based polymers; P(NDI2SiC6-T2) consists of NDI and bithiophene (T2) repeating units, while for P(NDI2SiC6-TVT), the (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene (TVT) units are introduced into the NDI-based backbone. The analysis of the optical spectra shows that the pre-aggregation of these polymers in solution is highly sensitive to the choice of solvent, such that the films prepared by using different solvents can be 'tuned' with regard to their degrees and types of the aggregates. In-depth morphology investigations (atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD), and near-edge X-ray absorption fine structure (NEXAFS)) combined with device optimization studies are used to probe the interplay between molecular structure, molecular packing, and OFET mobility. It is found that the polymer films cast as a coating from chloroform (CF) solvent favor a mixed face-on and edge-on orientation, while 1-chloronaphthalene (CN)-cast films favor an almost entirely edge-on orientation, resulting in a difference in mobility between CF- and CN-cast devices. Within this work, the annealed P(NDI2SiC6-T2) device fabricated from CF, despite showing a less densely packed organization, shows the highest electron mobility of up to 1.04 cm2/V∙s due to a highly balanced face-on to edge-on ratio. This work, for the first time, advances our understanding for how the balanced face-on to edge-on ratio plays a dramatic role in facilitating charge transport, opening a new charge-transport mechanism in electronic devices.
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Department of Energy Engineering
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