@article{PingelZenNeheretal.2009, author = {Pingel, Patrick and Zen, Achmad and Neher, Dieter and Lieberwirth, Ingo and Wegner, Gerhard and Allard, Sybille and Scherf, Ullrich}, title = {Unexpectedly high field-effect mobility of a soluble, low molecular weight oligoquaterthiophene fraction with low polydispersity}, issn = {0947-8396}, doi = {10.1007/s00339-008-4994-0}, year = {2009}, abstract = {Layers made from soluble low molecular weight polythiophene PQT-12 with low polydispersity exhibit a highly ordered structure and charge-carrier mobilities of the order of 10(-3) cm(2)/(V s), which we attribute to its proximity to monodispersity. We propose that polydispersity is a decisive factor with regard to structure formation and transport properties of soluble low molecular weight polythiophenes.}, 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} } @article{PingelArvindKoellnetal.2016, author = {Pingel, Patrick and Arvind, Malavika and K{\"o}lln, Lisa and Steyrleuthner, Robert and Kraffert, Felix and Behrends, Jan and Janietz, Silvia and Neher, Dieter}, title = {p-Type Doping of Poly(3-hexylthiophene) with the Strong Lewis Acid Tris(pentafluorophenyl)borane}, series = {Advanced electronic materials}, volume = {2}, journal = {Advanced electronic materials}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {2199-160X}, doi = {10.1002/aelm.201600204}, pages = {7}, year = {2016}, abstract = {State-of-the-art p-type doping of organic semiconductors is usually achieved by employing strong -electron acceptors, a prominent example being tetrafluorotetracyanoquinodimethane (F(4)TCNQ). Here, doping of the semiconducting model polymer poly(3-hexylthiophene), P3HT, using the strong Lewis acid tris(pentafluorophenyl)borane (BCF) as a dopant, is investigated by admittance, conductivity, and electron paramagnetic resonance measurements. The electrical characteristics of BCF- and F(4)TCNQ-doped P3HT layers are shown to be very similar in terms of the mobile hole density and the doping efficiency. Roughly 18\% of the employed dopants create mobile holes in either F-4 TCNQ- or BCF-doped P3HT, while the majority of doping-induced holes remain strongly Coulomb-bound to the dopant anions. Despite similar hole densities, conductivity and hole mobility are higher in BCF-doped P3HT layers than in F(4)TCNQ-doped samples. This and the good solubility in many organic solvents render BCF very useful for p-type doping of organic semiconductors.}, language = {en} } @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{LuBlakesleyHimmelbergeretal.2013, author = {Lu, Guanghao and Blakesley, James C. and Himmelberger, Scott and Pingel, Patrick and Frisch, Johannes and Lieberwirth, Ingo and Salzmann, Ingo and Oehzelt, Martin and Di Pietro, Riccardo and Salleo, Alberto and Koch, Norbert and Neher, Dieter}, title = {Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors}, series = {Nature Communications}, volume = {4}, journal = {Nature Communications}, number = {1-2}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/ncomms2587}, pages = {8}, year = {2013}, abstract = {Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt\% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.}, language = {en} } @article{SchubertPreisBlakesleyetal.2013, author = {Schubert, Marcel and Preis, Eduard and Blakesley, James C. and Pingel, Patrick and Scherf, Ullrich and Neher, Dieter}, title = {Mobility relaxation and electron trapping in a donor/acceptor copolymer}, series = {Physical review : B, Condensed matter and materials physics}, volume = {87}, journal = {Physical review : B, Condensed matter and materials physics}, number = {2}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.87.024203}, pages = {12}, year = {2013}, abstract = {To address the nature of charge transport and the origin of severe (intrinsic) trapping in electron-transporting polymers, transient and steady-state charge transport measurements have been conducted on the prototype donor/acceptor copolymer poly[2,7-(9,9-dialkyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PFTBTT). A charge-generation layer technique is used to selectively address transport of the desired charge carrier type, to perform time-of-flight measurements on samples with < 200 nm thickness, and to combine the time-of-flight and the photocharge extraction by linearly increasing voltage (photo-CELIV) techniques to investigate charge carrier dynamics over a wide time range. Significant trapping of free electrons is observed in the bulk of dioctyl-substituted PFTBTT (alt-PF8TBTT), introducing a strong relaxation of the charge carrier mobility with time. We used Monte-Carlo simulation to simulate the measured transient data and found that all measurements can be modeled with a single parameter set, with the charge transport behavior determined by multiple trapping and detrapping of electrons in an exponential trap distribution. The influence of the concomitant mobility relaxation on the transient photocurrent characteristics in photo-CELIV experiments is discussed and shown to explain subtle features that were seen in former publications but were not yet assigned to electron trapping. Comparable studies on PFTBTT copolymers with chemical modifications of the side chains and backbone suggest that the observed electron trapping is not caused by a distinct chemical species but rather is related to interchain interactions.}, language = {en} } @article{SalzmannHeimelDuhmetal.2012, author = {Salzmann, Ingo and Heimel, Georg and Duhm, Steffen and Oehzelt, Martin and Pingel, Patrick and George, Benjamin M. and Schnegg, Alexander and Lips, Klaus and Blum, Ralf-Peter and Vollmer, Antje and Koch, Norbert}, title = {Intermolecular hybridization governs molecular electrical doping}, series = {Physical review letters}, volume = {108}, journal = {Physical review letters}, number = {3}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.108.035502}, pages = {5}, year = {2012}, abstract = {Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.}, language = {en} } @article{KuehnPingelBreusingetal.2011, author = {Kuehn, Sergei and Pingel, Patrick and Breusing, Markus and Fischer, Thomas and Stumpe, Joachim and Neher, Dieter and Elsaesser, Thomas}, title = {High-Resolution Near-Field Optical Investigation of Crystalline Domains in Oligomeric PQT-12 Thin Films}, series = {Advanced functional materials}, volume = {21}, journal = {Advanced functional materials}, number = {5}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {1616-301X}, doi = {10.1002/adfm.201001978}, pages = {860 -- 868}, year = {2011}, abstract = {The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin films. Using a combination of spectroscopic methods, including scanning near-field optical microscopy, we study the domain structure and packing quality of highly crystalline thin films of oligomeric PQT-12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identification of regions with uniform molecular orientation. We find that a hierarchical order on three different length scales exists in these layers, made up of distinct well-ordered dichroic areas at the ten-micrometer-scale, which are sub-divided into domains with different molecular in-plane orientation. These serve as a template for the formation of smaller needle-like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high field-effect mobility of these layers of 10(-3) cm 2 V(-1) s(-1), whereas it is limited by the presence of domain boundaries at macroscopic distances.}, language = {en} } @article{AlbrechtJanietzSchindleretal.2012, author = {Albrecht, Steve and Janietz, Silvia and Schindler, Wolfram and Frisch, Johannes and Kurpiers, Jona and Kniepert, Juliane and Inal, Sahika and Pingel, Patrick and Fostiropoulos, Konstantinos and Koch, Norbert and Neher, Dieter}, title = {Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells}, series = {Journal of the American Chemical Society}, volume = {134}, journal = {Journal of the American Chemical Society}, number = {36}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/ja305039j}, pages = {14932 -- 14944}, year = {2012}, abstract = {A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC70BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC70BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58\% are achieved, giving a highest energy conversion efficiency of 6.16\%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.}, language = {en} } @article{LangeKniepertPingeletal.2013, author = {Lange, Ilja and Kniepert, Juliane and Pingel, Patrick and Dumsch, Ines and Allard, Sybille and Janietz, Silvia and Scherf, Ullrich and Neher, Dieter}, title = {Correlation between the open circuit voltage and the energetics of organic bulk heterojunction solar cells}, series = {The journal of physical chemistry letters}, volume = {4}, journal = {The journal of physical chemistry letters}, number = {22}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz401971e}, pages = {3865 -- 3871}, year = {2013}, abstract = {A detailed investigation of the open circuit voltage (V-OC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC70BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of V-OC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the V-OC in blends with fluorinated polymers or higher adduct fullerenes.}, language = {en} }