Doping of thin films of semiconducting polymers provides control of their electrical conductivity and thermopower. The electrical conductivity of semiconducting polymers rises nonlinearly with the ...carrier concentration, and there is a lack of understanding of the detailed factors that lead to this behavior. We report a study of the morphological effects of doping on the electrical conductivity of poly(3-hexylthiophene) (P3HT) thin films doped with small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Resonant soft X-ray scattering shows that the morphology of films of P3HT is not strongly changed by infiltration of F4TCNQ from the vapor phase. We show that the local ordering of P3HT, the texture and form factor of crystallites, and the long-range connectivity of crystalline domains contribute to the electrical conductivity in thin films. The thermopower of films of P3HT doped with F4TCNQ from the vapor phase is not strongly enhanced relative to films doped from solution, but the electrical conductivity is significantly higher, improving the thermoelectric power factor.
Conjugated polymers and related processing techniques have been developed for organic electronic devices ranging from lightweight photovoltaics to flexible displays. These breakthroughs have recently ...been used to create organic thermoelectric materials, which have potential for wearable heating and cooling devices, and near-room-temperature energy generation. So far, the best thermoelectric materials have been inorganic compounds (such as Bi2Te3) that have relatively low Earth abundance and are fabricated through highly complex vacuum processing routes. Molecular materials and hybrid organic–inorganic materials now demonstrate figures of merit approaching those of these inorganic materials, while also exhibiting unique transport behaviours that are suggestive of optimization pathways and device geometries that were not previously possible. In this Review, we discuss recent breakthroughs for organic materials with high thermoelectric figures of merit and indicate how these materials may be incorporated into new module designs that take advantage of their mechanical and thermoelectric properties.Thermoelectrics can be used to harvest energy and control temperature. Organic semiconducting materials have thermoelectric performance comparable to many inorganic materials near room temperature. Better understanding of their performance will provide a pathway to new types of conformal thermoelectric modules.
The performance of semiconducting polymers has been steadily increasing in the last 20 years. Improved control over the microstructure of these materials and a deeper understanding of how the ...microstructure affects charge transport are partially responsible for such trend. The development and widespread use of techniques that allow to characterize the microstructure of semiconducting polymers is therefore instrumental for the advance of these materials. This article is a review of the characterization techniques that provide information used to enhance the understanding of structure/property relationships in semiconducting polymers. In particular, the applications of optical and X‐ray spectroscopy, X‐ray diffraction, and scanning probe techniques in this context are described.
Charge transport in semiconducting polymers is governed by their structure at all lengthscales, from the nanoscale crystalline structure to the mesoscale arrangement between grains and how they are connected. The characterization techniques that help understand how the microstructure of these materials affects the carrier mobility and device performance are reviewed.
The thermoelectric properties of a highperformance electron‐conducting polymer, (P(NDIOD‐T2), extrinsically doped with dihydro‐1H‐benzoimidazol‐2‐yl (NDBI) derivatives, are reported. The highest ...thermoelectric power factor that has been reported for a solution‐processed n‐type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.
Thin-film solar cells are an important source of renewable energy. The most efficient thin-film solar cells made with organic materials are blends of semiconducting polymers and fullerenes called the ...bulk heterojunction (BHJ). Efficient BHJs have a nanoscale phase-separated morphology that is formed during solution casting. This article reviews recent work to understand the nature of the phase-separation process resulting in the formation of the domains in polymer-fullerene BHJs. The BHJ is now viewed as a mixture of polymer-rich, fullerene-rich, and mixed polymer-fullerene domains. The formation of this structure can be understood through fundamental knowledge of polymer physics. The implications of this structure for charge transport and charge generation are given.
Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than ...10%. In the most extensively characterized system, the poly(3‐hexyl thiophene) (P3HT):6,6‐phenyl‐C61‐butyric‐acid‐methyl‐ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase‐pure domains form in BHJs due to immiscibility of these two components.
Polymer/fullerene blends: Bilayers of poly(3‐hexyl thiophene) (P3HT) and 6,6‐phenyl‐C61‐butyric‐acid‐methyl‐ester (PCBM) were fabricated to investigate the miscibility and mobility of the two components during thermal annealing. It is found that aggregates and/or molecular species of PCBM are miscible and mobile in disordered P3HT without disrupting the P3HT crystallite orientation or size.
Semiconducting polymers have the potential to be used in thermoelectric devices that are lightweight, flexible, and fabricated using solution processing. Because of the structural and energetic ...disorder of these polymers, the relationship between their structure and thermoelectric properties is complex. We review how interrelated processing routes and doping methods affect the thermoelectric properties of polymers. The studies highlighted here have led to correlations between thermopower and electrical conductivity that can be described by theories under investigation. With greater understanding of the materials properties behind their performance, semiconducting polymers can be used in future power generation or cooling devices.
The heterogeneous microstructure of semicrystalline polymers complicates the relationship between their electrical conductivity and carrier concentration. Charge transport models typically describe ...conductivity with an assumption of uniform doping throughout the material. Here, the evolution in morphology and optoelectronic properties of poly(3‐hexylthiophene) (P3HT) is reported as a function of carrier concentration in an organic electrochemical transistor using a polymeric ionic liquid (PIL) as the gate insulator. Operando grazing incidence X‐ray scattering reveals that negatively charged ions from the dielectric first infiltrate the amorphous regions of the semiconductor, and then penetrate the crystalline regions at a critical carrier density of 4 × 1020 cm−3. Upon infiltration, the crystallites expand by 12% in the alkyl stacking direction and compress by 4% in the π–π stacking direction. The change in crystal structure of P3HT correlates with a sharply increasing effective carrier mobility. UV–visible spectroscopy reveals that holes induced in P3HT first reside in the crystalline regions of the polymer, which verifies that a charge carrier need not be in the same physical domain as its associated counterion. The dopant‐induced morphological changes of P3HT rationalize the dependence of mobility on carrier concentration, suggesting a phase transition of crystalline regions at high carrier concentration.
Operando grazing incidence X‐ray scattering reveals structural changes during electrochemical gating of poly(3‐hexylthiophene) transistors using a polymeric ionic liquid gate dielectric.
Organic semiconductors are emerging as a viable alternative to amorphous silicon in a range of thin‐film transistor devices. With the possibility to formulate these p‐type materials as inks and ...subsequently print into patterned devices, organic‐based transistors offer significant commercial advantages for manufacture, with initial applications such as low performance displays and simple logic being envisaged. Previous limitations of both air stability and electrical performance are now being overcome with a range of both small molecule and polymer‐based solution‐processable materials, which achieve charge carrier mobilities in excess of 0.5 cm2 V−1 s−1, a benchmark value for amorphous silicon semiconductors. Polymer semiconductors based on thienothiophene copolymers have achieved amongst the highest charge carrier mobilities in solution‐processed transistor devices. In this Progress Report, we evaluate the advances and limitations of this class of polymer in transistor devices.
Thienothiophene semiconducting polymers can exhibit a planar backbone conformation, leading to highly crystalline structures, often with good orientation and inter‐grain alignment. This thin‐film microstructure is optimal in achieving high charge‐carrier mobilities in organic field‐effect transistors.
A fundamental understanding of charge transport in polymeric semiconductors requires knowledge of how the electrical conductivity varies with carrier density. The thermopower of semiconducting ...polymers is also a complex function of carrier density making it difficult to assess structure–property relationships for the thermoelectric power factor. We examined the thermoelectric properties of poly2,5-bis(3-tetradecylthiophen-2-yl)thieno3,2-bthiophene (pBTTT-C14) by measurements of an electrochemical transistor using a polymeric ionic liquid (PIL) gate dielectric that can modulate the carrier concentration from 4 × 1018 to 3 × 1020 cm–3. As carrier density increases, so does the concentration of associated counterions, leading to a greater degree of energetic disorder within the semiconductor. Using thermopower measurements, we show experimentally that the electronic density-of-states broadens with increasing carrier density in the semiconducting polymer. The origin of a commonly observed power law relationship between thermopower and electrical conductivity is discussed and related to the changes in the electronic density-of-states upon doping.
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