@phdthesis{Pingel2013,
  author    = {Pingel, Patrick},
  title     = {Morphology, charge transport properties, and molecular doping of thiophene-based organic semiconducting thin films},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-69805},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {Organic semiconductors combine the benefits of organic materials, i.e., low-cost production, mechanical flexibility, lightweight, and robustness, with the fundamental semiconductor properties light absorption, emission, and electrical conductivity. This class of material has several advantages over conventional inorganic semiconductors that have led, for instance, to the commercialization of organic light-emitting diodes which can nowadays be found in the displays of TVs and smartphones. Moreover, organic semiconductors will possibly lead to new electronic applications which rely on the unique mechanical and electrical properties of these materials. In order to push the development and the success of organic semiconductors forward, it is essential to understand the fundamental processes in these materials. This thesis concentrates on understanding how the charge transport in thiophene-based semiconductor layers depends on the layer morphology and how the charge transport properties can be intentionally modified by doping these layers with a strong electron acceptor. By means of optical spectroscopy, the layer morphologies of poly(3-hexylthiophene), P3HT, P3HT-fullerene bulk heterojunction blends, and oligomeric polyquaterthiophene, oligo-PQT-12, are studied as a function of temperature, molecular weight, and processing conditions. The analyses rely on the decomposition of the absorption contributions from the ordered and the disordered parts of the layers. The ordered-phase spectra are analyzed using Spano's model. It is figured out that the fraction of aggregated chains and the interconnectivity of these domains is fundamental to a high charge carrier mobility. In P3HT layers, such structures can be grown with high-molecular weight, long P3HT chains. Low and medium molecular weight P3HT layers do also contain a significant amount of chain aggregates with high intragrain mobility; however, intergranular connectivity and, therefore, efficient macroscopic charge transport are absent. In P3HT-fullerene blend layers, a highly crystalline morphology that favors the hole transport and the solar cell efficiency can be induced by annealing procedures and the choice of a high-boiling point processing solvent. Based on scanning near-field and polarization optical microscopy, the morphology of oligo-PQT-12 layers is found to be highly crystalline which explains the rather high field-effect mobility in this material as compared to low molecular weight polythiophene fractions. On the other hand, crystalline dislocations and grain boundaries are identified which clearly limit the charge carrier mobility in oligo-PQT-12 layers. The charge transport properties of organic semiconductors can be widely tuned by molecular doping. Indeed, molecular doping is a key to highly efficient organic light-emitting diodes and solar cells. Despite this vital role, it is still not understood how mobile charge carriers are induced into the bulk semiconductor upon the doping process. This thesis contains a detailed study of the doping mechanism and the electrical properties of P3HT layers which have been p-doped by the strong molecular acceptor tetrafluorotetracyanoquinodimethane, F4TCNQ. The density of doping-induced mobile holes, their mobility, and the electrical conductivity are characterized in a broad range of acceptor concentrations. A long-standing debate on the nature of the charge transfer between P3HT and F4TCNQ is resolved by showing that almost every F4TCNQ acceptor undergoes a full-electron charge transfer with a P3HT site. However, only 5\% of these charge transfer pairs can dissociate and induce a mobile hole into P3HT which contributes electrical conduction. Moreover, it is shown that the left-behind F4TCNQ ions broaden the density-of-states distribution for the doping-induced mobile holes, which is due to the longrange Coulomb attraction in the low-permittivity organic semiconductors.},
  language  = {en}
}
@article{TurnerPingelSteyrleuthneretal.2011,
  author    = {Turner, Sarah T. and Pingel, Patrick and Steyrleuthner, Robert and Crossland, Edward J. W. and Ludwigs, Sabine and Neher, Dieter},
  title     = {Quantitative analysis of bulk heterojunction films using linear absorption spectroscopy and solar cell performance},
  series = {Advanced functional materials},
  volume    = {21},
  journal   = {Advanced functional materials},
  number    = {24},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1616-301X},
  doi       = {10.1002/adfm.201101583},
  pages     = {4640 -- 4652},
  year      = {2011},
  abstract  = {A fundamental understanding of the relationship between the bulk morphology and device performance is required for the further development of bulk heterojunction organic solar cells. Here, non-optimized (chloroform cast) and nearly optimized (solvent-annealed o-dichlorobenzene cast) P3HT:PCBM blend films treated over a range of annealing temperatures are studied via optical and photovoltaic device measurements. Parameters related to the P3HT aggregate morphology in the blend are obtained through a recently established analytical model developed by F. C. Spano for the absorption of weakly interacting H-aggregates. Thermally induced changes are related to the glass transition range of the blend. In the chloroform prepared devices, the improvement in device efficiency upon annealing within the glass transition range can be attributed to the growth of P3HT aggregates, an overall increase in the percentage of chain crystallinity, and a concurrent increase in the hole mobilities. Films treated above the glass transition range show an increase in efficiency and fill factor not only associated with the change in chain crystallinity, but also with a decrease in the energetic disorder. On the other hand, the properties of the P3HT phase in the solvent-annealed o-dichlorobenzene cast blends are almost indistinguishable from those of the corresponding pristine P3HT layer and are only weakly affected by thermal annealing. Apparently, slow drying of the blend allows the P3HT chains to crystallize into large domains with low degrees of intra- and interchain disorder. This morphology appears to be most favorable for the efficient generation and extraction of charges.},
  language  = {en}
}