Semiconducting Polymers
2.3 Tailored materials for photovoltaics
2.3.3 Charge collection interface layers in a PDPP:PCBM system
When designing organic solar cell devices, interface layers for charge extraction play a crucial role. In chapter 9 a PDPP[T]2-Ph / PC70BM blend (Figure 2-11a) is used as a photoactive layer for the investigation of various electron extraction layers in solar cell devices using various electrode combinations. Calcium (Ca), zirconium acetylacetonate (ZrAcac) and a polyfluorene derivative (PFN) are compared to a reference aluminium (Al) electrode (see Figure 2-11b) with respect to their work functions. The device architecture is depicted in Figure 2-11c and the I-V characteristics of solar cells employing the different interface layers are shown in Figure 2-11d. The use of a PFN interlayer and an aluminium electrode yields a power conversion
determination and transfer matrix simulations for the determination of parasitic absorption in the interface layers have been performed. It is found that PFN based devices with a small parasitic absorption, small leakage currents and a relatively high built-in voltage perform best.
This chapter highlights the importance of choosing suitable interlayers in organic solar cells and clearly demonstrates that it is not only the low work function of an electron extracting interlayer, but also its optical properties and charge selectivity that significantly influence the overall solar cell performance.
Figure 2-11. (a) Polymer PDPP[T]2-Ph and PC70BM used a model system in bulk heterojunction solar cells; (b) Comparative energy levels determined by UPS experiments of electron extracting interface layers used in this chapter; (c) Schematic device stack of organic solar cells prepared in this study. The photoactive layer is a bulk heterojunction of PDPP[T]2-Ph and PC70BM; (d) I-V curve for a PDPP[T]2-Ph:PC70BM solar cell under illumination, comparing different interface layers for electron extraction.
1.2
Al electrode (~ 200 nm) Ca (13 nm)
The results of this thesis were obtained in cooperation with other groups and are published or prepared as manuscripts for submission as denoted below. In the following the individual contributions of all authors are summarized and specified.
Chapter 3
This chapter was published as a full paper in Adv. Funct. Mater. 2015, 25, 2725 under the title:
“High Bulk Electron Mobility Diketopyrrolopyrrole Copolymers with Perfluorothiophene”
by Christian J. Mueller, Chetan R. Singh, Martina Fried, Sven Huettner and Mukundan Thelakkat.
I designed the materials, planned the experiments, developed the synthesis of all unpublished compounds and synthesized the monomers as well as polymers. Furthermore, I characterized all materials, prepared, optimized and analyzed all transistor devices and wrote the manuscript.
Chetan R. Singh prepared and analyzed the SCLC devices and wrote the SCLC section of the manuscript.
Martina Fried (laboratory assistant, CTA) helped with the synthesis of some monomers under my guidance.
Sven Huettner performed the bulk XRD measurements during his beamtime at the Australian Synchrotron.
Mukundan Thelakkat supervised the project and corrected the manuscript.
Chapter 4
This chapter was published as a full paper in J. Mater. Chem. C, 2015, 3, 8916 under the title:
“Influence of Fluorination in π-extended Backbone Polydiketopyrrolopyrroles on Charge Carrier Mobility and Depth-Dependent Molecular Alignment”
by Christian J. Mueller, Eliot Gann, Christopher R. McNeill and Mukundan Thelakkat.
I designed the materials, planned the experiments, synthesized and characterized all monomers and polymers. I prepared, measured, analyzed and prepared the Graphs for all GIWAXS samples. Furthermore I prepared and analyzed all transistor devices and wrote the manuscript.
Eliot Gann organized the beamtime, helped with the analysis of the GIWAXS data, wrote the GIWAXS section of the manuscript and was involved in scientific discussion.
Christopher R. McNeill and Mukundan Thelakkat supervised the project and corrected the manuscript.
Chapter 5
This chapter was published as a full paper in the Journal of Polymer Science Part B: Polymer Physics 2016, 54, 639 under the title:
“EDOT-Diketopyrrolopyrrole Copolymers for High Bulk Hole Mobility and Near Infrared Absorption”
by Christian J. Mueller, Chetan R. Singh and Mukundan Thelakkat
I designed the polymers, planned the experiments and synthesized as well as characterized all monomers and polymers. Furthermore I prepared and analyzed all transistor devices as well as wrote the manuscript.
Chetan R. Singh prepared and analyzed all SCLC devices and wrote the SCLC paragraph of the manuscript.
Mukundan Thelakkat supervised the project and corrected the manuscript.
Chapter 6
This chapter is prepared for submission as a full paper under the title:
“Universal Molecular Orientation Control in Polydiketopyrrolopyrroles”
by Christian J. Mueller, Eliot Gann, Chetan R. Singh, Christopher R. McNeill and Mukundan Thelakkat.
I synthesized all polymers, prepared and measured the GIWAXS samples, analyzed the GIWAXS data and wrote the manuscript.
Eliot Gann organized and supervised the beamtime, helped with the analysis of the GIWAXS data, was involved in the scientific discussion and corrected the manuscript.
Chetan R. Singh measured the SCLC mobilities of the three EDOT copolymers.
Mukundan Thelakkat and Christopher R. McNeill supervised the project and corrected the manuscript.
Chapter 7
This chapter was published as a full paper in Adv. Energy. Mater. 2015, 5, 1500914 under the title:
“Diketopyrrolopyrroles with a Distinct Energy Level Cascade for Efficient Charge Carrier Generation in Organic Solar Cells”
by Christian J. Mueller, Michael Brendel, Pia Ruckdeschel, Jens Pflaum and Mukundan Thelakkat.
I planned the experiments, designed and synthesized all compounds and their precursors as well as characterized the materials and wrote the manuscript except for the device part. I furthermore conducted and analyzed the computational experiments.
Michael Brendel prepared and characterized the solar cell devices, wrote the device part of the manuscript, contributed to introduction and conclusion, was involved in the scientific discussion and corrected the manuscript.
Pia Ruckdeschel helped with the synthesis of the three compounds D1-D3 during her three week internship under my guidance.
Jens Pflaum and Mukundan Thelakkat supervised the project and corrected the manuscript.
Chapter 8
This chapter was published as a full paper in Macromolecules 2016, 49, 3749 under the title:
“Azido-Functionalized Thiophene as a Versatile Building Block To Cross-Link Low-Bandgap Polymers”
by Christian J. Mueller, Tobias Klein, Eliot Gann, Christopher R. McNeill and Mukundan Thelakkat.
I designed the materials and experiments, developed the synthesis and synthesized as well as characterized all of the materials except those stated below and P9. I planned, conducted and analyzed the GIWAXS experiments and furthermore wrote the manuscript.
Tobias Klein synthesized the azido-functionalized thiophene monomer 5 and the polymers P1-P4 during his bachelor thesis under my supervision.
Eliot Gann organized and supervised the beamtime, was involved in the scientific discussion and corrected the manuscript.
Christopher McNeill was involved in the scientific discussion and corrected the manuscript.
Mukundan Thelakkat supervised the project and corrected the manuscript.
Chapter 9
This chapter was published as a full paper in Advanced Materials Interfaces 2016, 3, 1500422 under the title:
“Influence of Electron Extracting Interface Layers in Organic Bulk-Heterojunction Solar Cells”
by Chetan R. Singh, Cheng Li, Christian J. Mueller, Sven Hüttner and Mukundan Thelakkat Chetan R. Singh planned the experiments, fabricated and characterized the solar cell devices, performed the electroabsorption spectroscopy and wrote the manuscript except of synthesis part.
Cheng Li established the electroabsorption spectroscopy set-up, was involved in the scientific discussion and corrected the manuscript.
I synthesized and characterized the PDPP polymer. I wrote parts of the manuscript concerning details on the synthesis of PDPP and its properties. Furthermore I was involved in the scientific discussion.
Sven Hüttner measured the optical constants of interlayers.
Mukundan Thelakkat supervised the project and corrected the manuscript.
High Bulk Electron Mobility Diketopyrrolo-pyrrole Copolymers with Perfluorothiophene Christian J. Mueller, Chetan R. Singh, Martina Fried, Sven Huettner and Mukundan Thelakkat
An excellent bulk electron mobility of 4.3 × cm2V-1s-1 was obtained for diketopyrrolo[3,4-c]pyrrole copolymers by systematically tuning the diffusive non-bonding heteroatom interactions and dihedral angles between the aryl flanking units, DPP core and comonomer. Differences in crystalline packing, absorption, energy levels and charge carrier properties are
comparatively studied in a series of copolymers. Page 83
Influence of fluorination in π-extended backbone polydiketo-pyrrolo[3,4-c]pyrroles on charge carrier mobility and depth-dependent molecular alignment
Christian J. Mueller, Eliot Gann, Christopher R. McNeill and Mukundan Thelakkat
The degree of fluorination in π-extended polydiketopyrrolo-pyrroles is correlated with semiconductor properties in transistors and an improved molecular alignment in thin films using
depth-dependent grazing incidence X-ray scattering. Page 143
EDOT-Diketopyrrolopyrrole Copolymers for High Bulk Hole Mobility and Near Infrared Absorption
Christian J. Mueller, Chetan R. Singh and M. Thelakkat
An excellent bulk hole mobility of 2.9 × 10-4 cm2V-1s-1 and extended absorption in the near-infrared was achieved in diketopyrrolopyrrole (DPP) copolymers by incorporation of 3,4-ethylenedioxythiophene (EDOT) as an extremely electron rich moiety. By tailoring energy levels, the band gap of crystalline silicon combined with excellent bulk hole transport properties can be achieved.
Page 197
Structure property relationship - charge carrier mobilities
bulk electron mobility
SCLC electron mobility µe [cm2V-1s-1]
As-cast
Universal Molecular Orientation Control
Christian J. Mueller, Eliot Gann, Chetan R. Singh, Christopher R. McNeill and Mukundan Thelakkat
We show that full control over the mode and degree of orientation in semiconducting low-bandgap polymers is accessible by incorporation and exploitation of diffusive non-covalent interactions between adjacent aryl units in the polymer backbone. Using polydiketopyrrolopyrroles as a model system we find that the mode of orientation can be controlled via the aryl flanking unit of the diketopyrrolopyrrole core and the degree of orientation can be driven to perfection by incorporation of moieties which allow for non-covalent diffusive interactions into the
comonomers. Page 227
Diketopyrrolopyrroles with a Distinct Energy Level Cascade for Efficient Charge Carrier Generation in Organic Solar Cells
Christian J. Mueller, Michael Brendel, Pia Ruckdeschel, Jens Pflaum and Mukundan Thelakkat
Tailored low molecular weight diketopyrrolopyrrole compounds with a precise energy level offset are synthesized by tuning the electron deficiency on the terminal aryl unit.
Application of these compounds in cascade solar cells in combination with C60 leads to drastically increased short circuit current densities and power conversion efficiencies.
Page 261
Azido-Functionalized Thiophene as a Versatile Building Block To Cross-Link Low-Bandgap polymers
Christian J. Mueller, Tobias Klein, Eliot Gann, Christopher R. McNeill, Mukundan Thelakkat
We unveil a concept for the design of crosslinkable semiconducting polymers that is based on a modular tercopolymerization and stands out by its low synthetic effort, easy accessibility and its broad range of applications. It is shown that thermally stable blend morphologies and solvent resistant films can
be obtained with low degrees of functionalization. Page 295
Influence of Electron Extracting Interface Layers in Organic Bulk-Heterojunction Solar Cells
Chetan R. Singh, Cheng Li, Christian J. Mueller, Sven Hüttner and Mukundan Thelakkat
The influence of different electron extracting interlayers including Ca, ZrAcac, and PFN is investigated in normal organic solar cell geometry using electroabsorption measurements and transfer matrix simulations. It is shown that the solar cell performance is influenced by different parameters such as diode turn-on voltage, leakage currents, built-in voltages and parasitic absorption.
Page 351