title: Synthesis, structure formation, charge transport and applications of semiconducting copolymers based on thieno[3,4-c]pyrrole-4,6-dione
Creators: name: Weller, Tina
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: semiconducting polymer
subject: conjugated polymer
subject: TPD
subject: organic electronic
subject: organic solar cell
subject: OFET
subject: charge carrier transport
subject: 500
subject: 530
subject: 540

description: This thesis deals with new semiconducting copolymers based on thieno[3,4-c]pyrrole-4,6-dione (TPD) for the application in organic electronics. These polymers were designed by the donor-acceptor concept, whereby all polymers have TPD as the common acceptor unit. Charge transport properties were systematically tuned by structural variation of the comonomers, especially with regards to donor strength. In order to gain a deeper understanding of structure-property relations, the thin film alignment was elucidated in grazing incidence wide angle X-ray scattering (GIWAXS) measurements. Conjugated polymers often are semicrystalline materials with a rigid polymer backbone, which interact with one another via π-π stacking and alkyl stacking. Therefore, the polymer chains can assemble into stacks, which can orient to the substrate in an edge-on, face-on or end-on manner. While an edge-on alignment corresponds to an orientation of the planar π-system perpendicular to the substrate, an alignment parallel to the substrate is referred to as face-on. The alignment in an end-on fashion is only rarely observed because here the polymer chains stand on the substrate. Consequently, the device performance can depend on the thin film alignment since the charge transport along the π-π stacking direction is more efficient than along the alkyl stacking direction. An edge-on alignment favors charge transport in an organic field effect transistor (OFET), whereas a face-on alignment promotes charge transport in a diode configuration.
In the first part of this thesis, we investigate the influence of fluorination following two different concepts. First, we studied the influence of thiophene fluorination in p-type materials based on TPD and terthiophene. In detail, the thiophenes attached to the TPD unit were decorated with additional solubilizing side chains. Upon fluorination, the energy levels are lowered due to the decreased donor strength allowing a higher open-circuit voltage in organic solar cells. Indeed, the open-circuit voltage could be increased for the fluorinated polymer/PCBM blend. However, the fill factor suffered losses resulting only in an identical power conversion efficiency compared to the non-fluorinated polymer system. More important, we additionally studied the influence of processing on thin film alignment and device performance. Both materials show a high degree of crystallinity and strongly aggregate in solution as shown in temperature dependent UV-Vis measurements. Furthermore, the nature of aggregates depends on the solvent used. Processing from chloroform results in predominantly face-on alignment, whereas processing from dichlorobenzene suppresses the formation of these face-on stacks. Consequently, higher charge transport in OFET was achieved for films processed from dichlorobenzene, while the performance in organic solar cells benefits from processing from chloroform.
The second concept of fluorination was used to facilitate electron transport by decreasing the electron density along the polymer backbone stabilizing the radical anion. Therefore, we incrementally decreased the donor strength of the 1,4-di(2-thienyl)benzene comonomer by di- and tetrafluorination of the phenyl moiety. In fact, we achieved exclusively n-channel operation in OFETs upon tetrafluorination, whereas the non- and difluorinated polymers show ambipolar charge transport behavior. Interestingly, all polymers show an end-on alignment, which is rarely observed without any grafting of the polymer chains to the substrate surface. In addition to fluorination of the phenyl group, we wanted to prove this concept for thiophene. Therefore, we synthesized a series of four copolymers containing TPD and non-fluorinated as well as fluorinated thiophene. Besides the copolymerization of TPD with thiophene derivatives, we used a TPD monomer additionally flanked with thiophene. Next to the influence of fluorination, we also studied the impact of the extended backbone. In fact, fluorination of thiophene leads to exclusively n-channel operation in OFET for the copolymer without extended backbone. All other polymers show ambipolar charge transport, whereby fluorination improves the efficiency of charge transport, especially of electrons.
The second part of the thesis deals with the incorporation of the very rigid, planar and electron-rich thienothiophene (TT) moiety into TPD copolymers in order to compare it with the simple thiophene (T) moiety. In addition to the influence of the nature of comonomer, the influence of the molecular weight in TPD copolymers carrying thiophene was studied. All polymers show highly balanced ambipolar charge transport. The charge carrier mobilities could be improved by increasing the molecular weight in TPD copolymers carrying thiophene and by substitution of thiophene with thienothiophene by one order of magnitude each. This trend correlates well with the observed thin film alignment. Whereas PTPDT with low molecular weight showed no preferential ordering of the polymer chains to the substrate, a face-on alignment could be achieved for PTPDT with higher molecular weight. A dramatic change in alignment towards edge-on fashion was caused by the thienothiophene moiety yielding very high and balanced charge transport. 
In summary, we designed and synthesized p-type, ambipolar as well as n-type materials by variation of the donor strength of the comonomer. For each class of materials, a series of copolymers was systematically studied regarding thermal, optical and electrochemical properties. More important, the charge transport properties were correlated with the thin film alignment of these polymers. Therefore, insights into structure-property relations in TPD based copolymer were gained.