Delocalization Enhances Conductivity at High Doping Concentrations

Many applications of organic semiconductors require high electrical conductivities and hence high doping levels. Therefore, it is indispensable for effective material design to have an accurate understanding of the underlying transport mechanisms in this regime. In this study, own and literature experimental data that reveal a power-law relation between the conductivity and charge density of strongly p-doped conjugated polymers are combined. This behavior cannot consistently be described with conventional models for charge transport in energetically disordered materials. Here, it is shown that the observations can be explained in terms of a variable range hopping model with an energy-dependent localization length. A tight-binding model is used to quantitatively estimate of the energy-dependent localization length, which is used in an analytical variable range hopping model. In the limit of low charge densities, the model reproduces the well-known Mott variable range hopping behavior, while for high charge densities, the experimentally observed superlinear increase in conductivity with charge density is reproduced. The latter behavior occurs when the Fermi level reaches partially delocalized states. This insight can be anticipated to lead to new strategies to increase the conductivity of organic semiconductors

Chemical Doping of Conjugated Polymers with the Strong Oxidant Magic Blue

Molecular doping of organic semiconductors is a powerful tool for the optimization of organic electronic devices and organic thermoelectric materials. However, there are few redox dopants that have a sufficiently high electron affinity to allow the doping of conjugated polymers with an ionization energy of more than 5.3\ua0eV. Here, p-doping of a broad palette of conjugated polymers with high ionization energies is achieved by using the strong oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue). In particular diketopyrrolopyrrole (DPP)-based copolymers reach a conductivity of up to 100 S cm−1 and a thermoelectric power factor of 10 \ub5W m−1 K−2. Further, both electron paramagnetic resonance (EPR) as well as a combination of spectroelectrochemistry and chronoamperometry is used to estimate the charge-carrier density of the polymer PDPP-3T doped with Magic Blue. A molar attenuation coefficient of 6.0\ua0\ub1\ua00.2 7 103 m2 mol−1 is obtained for the first polaronic sub-bandgap absorption of electrochemically oxidized PDPP-3T. Comparison with chemically doped PDPP-3T suggests a charge-carrier density on the order of 1026 m−3, which yields a charge-carrier mobility of up to 0.5 cm2 V−1 s−1 for the most heavily doped material

Double Doping of a Low-Ionization-Energy Polythiophene with a Molybdenum Dithiolene Complex

Doping of organic semiconductors is crucial for tuning the charge-carrier density of conjugated polymers. The exchange of more than one electron between a monomeric dopant and an organic semiconductor allows the polaron density to be increased relative to the number of counterions that are introduced into the host matrix. Here, a molybdenum dithiolene complex with a high electron affinity of 5.5 eV is shown to accept two electrons from a polythiophene that has a low ionization energy of 4.7 eV. Double p-doping is consistent with the ability of the monoanion salt of the molybdenum dithiolene complex to dope the polymer. The transfer of two electrons to the neutral dopant was also confirmed by electron paramagnetic resonance spectroscopy since the monoanion, but not the dianion, of the molybdenum dithiolene complex features an unpaired electron. Double doping allowed an ionization efficiency of 200% to be reached, which facilitates the design of strongly doped semiconductors while lessening any counterion-induced disruption of the nanostructure

Visualisation of individual dopants in a conjugated polymer: sub-nanometre 3D spatial distribution and correlation with electrical properties

While molecular doping is ubiquitous in all branches of organic electronics, little is known about the spatial distribution of dopants, especially at molecular length scales. Moreover, a homogeneous distribution is often assumed when simulating transport properties of these materials, even though the distribution is expected to be inhomogeneous. In this study, electron tomography is used to determine the position of individual molybdenum dithiolene complexes and their three-dimensional distribution in a semiconducting polymer at the sub-nanometre scale. A heterogeneous distribution is observed, the characteristics of which depend on the dopant concentration. At 5 mol% of the molybdenum dithiolene complex, the majority of the dopant species are present as isolated molecules or small clusters up to five molecules. At 20 mol% dopant concentration and higher, the dopant species form larger nanoclusters with elongated shapes. Even in case of these larger clusters, each individual dopant species is still in contact with the surrounding polymer. The electrical conductivity first strongly increases with dopant concentration and then slightly decreases for the most highly doped samples, even though no large aggregates can be observed. The decreased conductivity is instead attributed to the increased energetic disorder and lower probability of electron transfer that originates from the increased size and size variation in dopant clusters. This study highlights the importance of detailed information concerning the dopant spatial distribution at the sub-nanometre scale in three dimensions within the organic semiconductor host. The information acquired using electron tomography may facilitate more accurate simulations of charge transport in doped organic semiconductors

Impact of oxidation-induced ordering on the electrical and mechanical properties of a polythiophene co-processed with bistriflimidic acid

The interplay between the nanostructure of a doped polythiophene with oligoether side chains and its electrical as well as mechanical properties is investigated. The degree of order of the polymer is found to strongly vary when co-processed with bistriflimidic acid (H-TFSI). The neat polythiophene as well as strongly oxidized material are largely disordered while intermediate concentrations of H-TFSI give rise to a high degree of π-stacking. The structural disorder of strongly oxidized material correlates with a decrease in the kinetic fragility with H-TFSI concentration, suggesting that positive interactions between TFSI anions and the polymer reduce the ability to crystallize. The electrical conductivity as well as the Young\u27s modulus first increase upon the addition of 4-10 mol% of H-TFSI, while the loss of π-stacking observed for strongly oxidized material more significantly affects the latter. As a result, material comprising 25 mol% H-TFSI displays an electrical conductivity of 58 S cm−1 but features a relatively low Young\u27s modulus of only 80 MPa. Decoupling of the electrical and mechanical properties of doped conjugated polymers may allow the design of soft conductors that are in high demand for wearable electronics and bioelectronics

Tuning of the elastic modulus of a soft polythiophene through molecular doping

Molecular doping of a polythiophene with oligoethylene glycol side chains is found to strongly modulate not only the electrical but also the mechanical properties of the polymer. An oxidation level of up to 18% results in an electrical conductivity of more than 52 S cm(-1) and at the same time significantly enhances the elastic modulus from 8 to more than 200 MPa and toughness from 0.5 to 5.1 MJ m(-3). These changes arise because molecular doping strongly influences the glass transition temperature T-g and the degree of pi-stacking of the polymer, as indicated by both X-ray diffraction and molecular dynamics simulations. Surprisingly, a comparison of doped materials containing mono- or dianions reveals that - for a comparable oxidation level - the presence of multivalent counterions has little effect on the stiffness. Evidently, molecular doping is a powerful tool that can be used for the design of mechanically robust conducting materials, which may find use within the field of flexible and stretchable electronics

Hexanary blends: a strategy towards thermally stable organic photovoltaics

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 \ub0C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells

Efficiency of molecular doping of conjugated polymers

In recent years, organic semiconductors have become known as a promising alternative to traditional inorganic materials. Conjugated polymers are one of the most studied classes of organic semiconductors. Molecular doping is a powerful tool to tune their optical, electronic and –possibly– even the mechanical properties. The addition of molecular dopants introduces positive or negative charges to the polymer, so called polarons. However, factors such as dopant aggregation, the energetic offset between the polymer and dopant, as well as less-than-integer charge transfer limits the efficiency of the charge transfer process. Additionally, each polaron-dopant ion pair is strongly bound. Hence, only a fraction of holes is able become free charges that contribute to electrical transport. In order to develop materials that show improved doping efficiency, it is paramount to measure the number of (bound and free) polarons. This thesis explores different methods that allow to assess the polaron density and concepts to improve the efficiency of molecular doping. Spectro-electrochemistry combined with chronoamperometry can be used to measure the polaron attenuation coefficient. If the polaron attenuation coefficient is known, it is possible to estimate the polaron density using UV-vis-NIR spectroscopy. Fourier transfer infrared (FTIR) spectroscopy can be used to estimate the polaron density for polymers doped with quinodimethane type monomeric dopants. Further it is shown that electron paramagnetic resonance (EPR) spectroscopy can be used to estimate polaron density and confirm that the dopant Mo(tfd-COCF3)3 can undergo two charge transfer events, i.e. double doping, with a low ionization energy conjugated polymer

Chemical Doping of Conjugated Polymers with the Strong Oxidant Magic Blue

Molecular doping of organic semiconductors is a powerful tool for the optimization of organic electronic devices and organic thermoelectric materials. However, there are few redox dopants that have a sufficiently high electron affinity to allow the doping of conjugated polymers with an ionization energy of more than 5.3\ua0eV. Here, p-doping of a broad palette of conjugated polymers with high ionization energies is achieved by using the strong oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue). In particular diketopyrrolopyrrole (DPP)-based copolymers reach a conductivity of up to 100 S cm−1 and a thermoelectric power factor of 10 \ub5W m−1 K−2. Further, both electron paramagnetic resonance (EPR) as well as a combination of spectroelectrochemistry and chronoamperometry is used to estimate the charge-carrier density of the polymer PDPP-3T doped with Magic Blue. A molar attenuation coefficient of 6.0\ua0\ub1\ua00.2 7 103 m2 mol−1 is obtained for the first polaronic sub-bandgap absorption of electrochemically oxidized PDPP-3T. Comparison with chemically doped PDPP-3T suggests a charge-carrier density on the order of 1026 m−3, which yields a charge-carrier mobility of up to 0.5 cm2 V−1 s−1 for the most heavily doped material