Patrick Pingel

Bio: Patrick Pingel is an academic researcher from University of Potsdam. The author has contributed to research in topics: Doping & Organic semiconductor. The author has an hindex of 20, co-authored 28 publications receiving 2210 citations. Previous affiliations of Patrick Pingel include Delft University of Technology & Humboldt University of Berlin.

Fluorinated copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells.

Abstract: A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC(70)BM 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 PC(70)BM 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.

Comprehensive picture of p -type doping of P3HT with the molecular acceptor F 4 TCNQ

Abstract: By means of optical spectroscopy, Kelvin probe, and conductivity measurements, we study the $p$-type doping of the donor polymer poly(3-hexylthiophene), P3HT, with the molecular acceptor tetrafluorotetracyanoquinodimethane, F${}_{4}$TCNQ, covering a broad range of molar doping ratios from the ppm to the percent regime. Thorough quantitative analysis of the specific near-infrared absorption bands of ionized F${}_{4}$TCNQ reveals that almost every F${}_{4}$TCNQ dopant undergoes integer charge transfer with a P3HT site. However, only about 5$%$ of these charge carrier pairs are found to dissociate and contribute a free hole for electrical conduction. The nonlinear behavior of the conductivity on doping ratio is rationalized by a numerical mobility model that accounts for the broadening of the energetic distribution of transport sites by the Coulomb potentials of ionized F${}_{4}$TCNQ dopants.

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors

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.

Intermolecular hybridization governs molecular electrical doping.

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.

Localized Charge Transfer in a Molecularly Doped Conducting Polymer

Abstract: Doped conjugated polymers can exhibit exceptionally high conductivity (>10 S cm). Here, doping refers to the formation of charge-transfer complexes (CTCs) or salts by combining appropriate pairs of donors and acceptors. Similar phenomena can be found in crystalline CTCs comprising small molecules. For several decades, the nature of charge transfer (CT) and the dimensionality of charge transport in conducting polymers and small molecule crystals have been at the focus of research. For instance, the degree of CT influences conductivity and determines whether metallic or insulating character prevails, and the strength of interchain interactions determines whether primarily 1D or 3D electronic properties prevail. In addition, disorder—on both a molecular and mesoscopic scale—has a tremendous impact on these properties. In fact, true metallic behavior of a doped conjugated polymer was demonstrated only recently by significantly improving the structural quality of thin films. Apart from the interest in fundamental phenomena occurring in such systems, recent progress in the field of organic electronics has intensified efforts toward improving the understanding of conducting polymers, as they are a key element for the successful realization of printed all-organic (opto-) electronic devices. At present, formulations of poly(ethylenedioxythiophene)/poly(styrenesulfonate) dominate applications; however, it should be interesting to develop alternative routes to conducting polymers that are not based on an aqueous dispersion and thus allow for new processing options and functionality. The most widely studied class of semiconducting polymers that can be rendered conducting upon doping (with inorganic acceptors) is based on polythiophene, which has donor character; tetrafluorotetracyanoquinodimethane (F4TCNQ) is one of the strongest known molecular electron acceptors and has been used for doping molecular organic semiconductors. However, combinations of polythiophenes and strong molecular acceptors have not yet been investigated. Here we show that mixtures of the prototypical soluble polythiophene variant poly(3-hexylthiophene) (P3HT) and F4TCNQ form CTCs with high conductivity in thin films (ca. 1 S cm). Because of the flexibility of the polymer chains and the variety of possible interchain interactions, a large number of different local conformations in P3HT/F4TCNQ CTCs can be expected, leading to large variations in the electronic structure and transport properties within a macroscopic sample. We investigated the local conformation and electronic structure of P3HT/F4TCNQ CTCs in thin films by combining X-ray absorption near edge structure (XANES) measurements with theoretical modeling using density functional theory (DFT). Most notably, we found that only one specific CTC conformation predominated, in which F4TCNQ was strongly bent out of its neutral planar form because of pronounced electron donation from P3HT chain segments. In contrast, in a related F4TCNQ/oligothiophene CT crystal, F4TCNQ remained planar because of dominant intermolecular interactions in an ordered environment. Furthermore, the energy levels of the CTC were clearly shown to be hybrids of the individual levels of the separate donor and acceptor molecules, indicating that the CT in P3HT/F4TCNQ is highly localized and does not involve significant interchain interactions. Thin films of F4TCNQ-doped P3HT prepared from solution typically exhibited a dc conductivity of 1 S cm, which corresponds to an increase in conductivity by five orders of magnitude over pristine P3HT. This increase is lower than the reported value of 30 S cm for ClO4 -doped P3HT. Interestingly, the conductivity of our P3HT/F4TCNQ films was five orders of magnitude higher than the value for dimethylquarterthiophene/F4TCNQ crystals. In these crystals intermolecular interactions are strong, whereas it was proposed that in conducting polythiophenes, conduction happens primarily along single polymer chains because of weak interchain coupling. In this communication, we relate the localized character of the CT between P3HT and F4TCNQ with the observed conductivity and explain the nature of the CT states. The XANES spectrum of neutral F4TCNQ exhibits three main spectral features: peaks A, B, and C (Fig. 1a). Using just C O M M U N IC A IO N

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Abstract: Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Abstract: The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

A general relationship between disorder, aggregation and charge transport in conjugated polymers

Abstract: Conjugated polymer chains have many degrees of conformational freedom and interact weakly with each other, resulting in complex microstructures in the solid state. Understanding charge transport in such systems, which have amorphous and ordered phases exhibiting varying degrees of order, has proved difficult owing to the contribution of electronic processes at various length scales. The growing technological appeal of these semiconductors makes such fundamental knowledge extremely important for materials and process design. We propose a unified model of how charge carriers travel in conjugated polymer films. We show that in high-molecular-weight semiconducting polymers the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for efficient long-range charge transport. This generalization explains the seemingly contradicting high performance of recently reported, poorly ordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials. The recent demonstration that highly disordered polymer films can transport charges as effectively as polycrystalline semiconductors has called into question the relationship between structural order and mobility in organic materials. It is now shown that, in high-molecular-weight polymers, efficient charge transport is allowed due to a network of interconnected aggregates that are characterized by short-range order.

Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their applications.

Abstract: 3.2. Thienothiophenes 1216 3.2.1. Thieno[3,4-b]thiophene Analogues 1216 3.2.2. Thieno[3,2-b]thiophene Analogues 1217 3.2.3. Thieno[2,3-b]thiophene Analogues 1218 3.3. , ′-Bridged Bithiophenes 1219 3.3.1. Dithienothiophene (DTT) Analogues 1220 3.3.2. Dithieno[3,2-b:2′3′-d]thiophene-4,4-dioxides 1221 3.3.3. Dithienosilole (DTS) Analogues 1221 3.3.4. Cyclopentadithiophene (CPDT) Analogues 1221 3.3.5. Nitrogen and Phosphor Atom Bridged Bithiophenes 1222