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Forschungsarbeit

Structure-Property Relationships in Novel Diketopyrrolopyrrole Materials for Organic Electronics

by Christian J. Müller (21.07.2015)

My PhD focusses on the development of new materials based on diketopyrrolopyrrole (DPP). Both low-bandgap polymers as well as small molecules are synthesized and investigated to gain more insights into structure property relationships of these materials. Instead of hunting for record efficiencies and mobilities in single devices employing a single material, I specialized in synthesizing distinct sets of materials with a systematic variation of structural subunits in order to shed light on the influence these variations have on the properties of the materials. 

This approach is exemplary shown in Figure 1 [1]. Here, the aryl flanking unit Ar on the DPP core is varied from the electron-rich thiophene over phenyl to the electron deficient pyridine. Variation of this structural unit not only influences the electron density of the resulting polymer, but also has an effect on the dihedral angle α (see generic polymer structure in Figure 1). Whereas the five-membered thiophene ring has little potential for steric hindrance, the six-membered phenyl ring causes a large dihedral backbone twist. Upon substituting the phenyl-ring with a pyridine unit, one hydrogen atom in the ring is missing providing more room for sufficient planarization of the system.

The planarization of the backbone can be taken one step further by also optimizing the dihedral angle β, which occurs between the DPP monomer and the comonomer Mco (see generic polymer structure in Figure 1).  Here we chose to utilize a fluorinated comonomer in order to provide additional non-covalent diffusive interactions between two adjacent aryl units. Here, a F⋯H coordination between the fluorine atoms in the comonomer Mco and hydrogen atoms on the DPP aryl flanking unit Ar is exploited in order to further planarize the system [2]. Moreover, fluorination not only provides additional coordination moieties, but also drastically decreases the electron density on the backbone due to the highly electronegative fluorine atoms, which is beneficial when designing electron transporting polymers.

[Bildunterschrift / Subline]: Figure 1. Synthesis of PDPP copolymers employing different aryl flanking units on the DPP core with varying electron density. Simultaneously the comonomer is fluorinated in order to further increase electron deficiency on the polymer backbone and providing a moiety for enhanced backbone planarization.

The charge transport properties of these six polymers was hence investigated in organic field effect transistors (OFET) as well as in space charge limited current (SCLC) diode devices. These two methods are based on very different charge transport models and hence give detailed information about the mobility and charge transport in these materials. Whereas charges in OFET devices are transported horizontally through a very narrow (few nanometers) channel close to the semiconductor-dielectric interface, charges in SCLC devices have to travel vertically through the semiconducting layer with thicknesses of several hundred nanometers. Therefore, mobility values obtained from OFET devices show an interfacial mobility that can be greatly influenced by modifying and optimizing the interface (e.g. via silylation or annealing of the semiconductor), whereas the mobilities obtained by SCLC show the real bulk mobility of the polymers. The latter is more meaningful when discussing the viability of materials for organic photovoltaics, because charges are generated in the bulk of the active layer of those devices and then have to travel to an electrode interface to be extracted.

In OFET devices, polymers with thiophene or phenyl as the aryl flanking unit are typically p-type materials only showing hole transport. Whereas the thiophene-flanked polymers show hole mobilities as high as µh = 0.6 cm2V-1s-1, the phenyl-flanked DPP copolymers only show very low mobilities due to their low crystallinity and hence their unsuitable alignment at the semiconductor/dielectric interface. The pyridine-flanked derivatives are exclusively n-type, showing only electron transport with electron mobililties as high as µe = 0.6 cm2V-1s-1. Upon fluorination, this n-type behavior is pronounced. For the otherwise exclusively p-type materials ambipolarity is introduced in the fluorinated copolymers (see Figure 2 a, b).

In SCLC devices (see Figure 2 c, d) all polymers show ambipolar charge transport in the bulk with a constant hole mobility µh of about 10-5 cm2V-1s-1. Electron transport is however greatly influenced by the choice of the aryl flanking unit, being the lowest for the electron-rich thiophene derivatives and the highest for the electron deficient pyridine-units. The electron mobility can be further improved by fluorination of the comonomer yielding in a record electron bulk mobility µe of 4.3 x 10-2 cm2V-1s-1 which is in the same range as PCBM which is widely used as an acceptor (i.e. electron transport material) in polymer photovoltaic devices.

[Bildunterschrift / Subline]: Figure 2. Hole (left) and electron (right) mobilities obtained from OFET devices (top) and SCLC devices (bottom).

This study gives detailed insights into the rational design of high electron mobility materials by simultaneously increasing the backbone electron deficiency as well as increasing the backbone planarization. We are currently focusing on the impact of these variations on the solid-state structure and ordering of these materials in thin films employing diverse xray techniques such as grazing incident wide angle xray scattering (GIWAXS) and near edge xray absorption fine structure spectroscopy (NEXAFS).

References

[1] C. J. Mueller, C. R. Singh, M. Fried, S. Huettner, M. Thelakkat, Adv. Funct. Mater. 2015, 25, 2725.

[2] N. E. Jackson, B. M. Savoie, K. L. Kohlstedt, M. Olvera de la Cruz, G. C. Schatz, L. X. Chen, M. A. Ratner, J. Am. Chem. Soc. 2013, 135, 10475.


Scientific Career
  • since 2011
  • Doctoral Student, Chair for Macromolecular Chemistry I, Bayreuth University
  • 2008-2009
  • Research Stay at the University of Cambridge, UK
  • 2005-2010
  • Study of Chemistry ("Diplom-Studium"), University of Marburg

Selected Publications
  • * Z. H. Fard, C. Müller, T. Harmening, R. Pottgen, S. Dehnen, Angew. Chem. Int. Ed. 2009, 48, 4441. (Journal Inside Cover)
  • * Z. H. Fard, L. Xiong, C. Müller, M. Holynska, S. Dehnen, Chem. Eur. J. 2009, 15, 6595. (VIP, Journal Cover)
  • * J. Fang, B. H. Wallikewitz, F. Gao, G. Tu, C. Müller, G. Pace, R. H. Friend, W. T. Huck, J. Am. Chem. Soc. 2011, 133, 683.
  • * L. Gong, C. Müller, M. A. Celik, G. Frenking, E. Meggers, New J. Chem. 2011, 35, 788.
  • * C. J. Mueller, C. R. Singh, M. Fried, S. Huettner, M. Thelakkat, Adv. Funct. Mater. 2015, in print.