Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field‐effect transistors, solar cells, and thermoelectric generators. However, many ...dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid‐state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g42T‐T)—a polythiophene—in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) is explored. Doping of p(g42T‐T) with F4TCNQ results in an electrical conductivity of up to 100 S cm−1. Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.
Molecular doped polythiophenes with polar side chains display strongly enhanced processability even at high dopant fractions. Oligo ethylene side chains prevent coagulation of the polymer dopant pairs. The resulting films have a high electrical conductivity of up to 100 S cm−1. An enhanced thermal stability compared with doped poly(3‐hexylthiophene) is demonstrated.
Thermoelectric plastics are a class of polymer-based materials that combine the ability to directly convert heat to electricity, and
vice versa
, with ease of processing. Potential applications ...include waste heat recovery, spot cooling and miniature power sources for autonomous electronics. Recent progress has led to surging interest in organic thermoelectrics. This tutorial review discusses the current trends in the field with regard to the four main building blocks of thermoelectric plastics: (1) organic semiconductors and in particular conjugated polymers, (2) dopants and counterions, (3) insulating polymers, and (4) conductive fillers. The design and synthesis of conjugated polymers that promise to show good thermoelectric properties are explored, followed by an overview of relevant structure-property relationships. Doping of conjugated polymers is discussed and its interplay with processing as well as structure formation is elucidated. The use of insulating polymers as binders or matrices is proposed, which permit the adjustment of the rheological and mechanical properties of a thermoelectric plastic. Then, nanocomposites of conductive fillers such as carbon nanotubes, graphene and inorganic nanowires in a polymer matrix are introduced. A case study examines poly(3,4-ethylenedioxythiophene) (PEDOT) based materials, which up to now have shown the most promising thermoelectric performance. Finally, a discussion of the advantages provided by bulk architectures
e.g.
for wearable applications highlights the unique advantages that thermoelectric plastics promise to offer.
Thermoelectric plastics are a class of polymer-based materials that combine the ability to directly convert heat to electricity, and
vice versa
, with ease of processing.
Future brain-machine interfaces, prosthetics, and intelligent soft robotics will require integrating artificial neuromorphic devices with biological systems. Due to their poor biocompatibility, ...circuit complexity, low energy efficiency, and operating principles fundamentally different from the ion signal modulation of biology, traditional Silicon-based neuromorphic implementations have limited bio-integration potential. Here, we report the first organic electrochemical neurons (OECNs) with ion-modulated spiking, based on all-printed complementary organic electrochemical transistors. We demonstrate facile bio-integration of OECNs with Venus Flytrap (Dionaea muscipula) to induce lobe closure upon input stimuli. The OECNs can also be integrated with all-printed organic electrochemical synapses (OECSs), exhibiting short-term plasticity with paired-pulse facilitation and long-term plasticity with retention >1000 s, facilitating Hebbian learning. These soft and flexible OECNs operate below 0.6 V and respond to multiple stimuli, defining a new vista for localized artificial neuronal systems possible to integrate with bio-signaling systems of plants, invertebrates, and vertebrates.
Organic electrochemical transistors (OECTs) hold promise for developing a variety of high‐performance (bio‐)electronic devices/circuits. While OECTs based on p‐type semiconductors have achieved ...tremendous progress in recent years, n‐type OECTs still suffer from low performance, hampering the development of power‐efficient electronics. Here, it is demonstrated that fine‐tuning the molecular weight of the rigid, ladder‐type n‐type polymer poly(benzimidazobenzophenanthroline) (BBL) by only one order of magnitude (from 4.9 to 51 kDa) enables the development of n‐type OECTs with record‐high geometry‐normalized transconductance (gm,norm ≈ 11 S cm−1) and electron mobility × volumetric capacitance (µC* ≈ 26 F cm−1 V−1 s−1), fast temporal response (0.38 ms), and low threshold voltage (0.15 V). This enhancement in OECT performance is ascribed to a more efficient intermolecular charge transport in high‐molecular‐weight BBL than in the low‐molecular‐weight counterpart. OECT‐based complementary inverters are also demonstrated with record‐high voltage gains of up to 100 V V−1 and ultralow power consumption down to 0.32 nW, depending on the supply voltage. These devices are among the best sub‐1 V complementary inverters reported to date. These findings demonstrate the importance of molecular weight in optimizing the OECT performance of rigid organic mixed ionic–electronic conductors and open for a new generation of power‐efficient organic (bio‐)electronic devices.
n‐Type organic electrochemical transistors (OECTs) underperform compared to p‐type OECTs. By tuning the molecular weight of the rigid ladder‐type poly(benzimidazobenzophenanthroline), n‐type OECTs with record‐high figures of merit are reported. OECT‐based complementary inverters are also demonstrated with high voltage gains of up to 100 V V−1 and ultralow power consumption down to 0.32 nW, depending on the supply voltage.
Doping of organic semiconductors is crucial for the operation of organic (opto)electronic and electrochemical devices. Typically, this is achieved by adding heterogeneous dopant molecules to the ...polymer bulk, often resulting in poor stability and performance due to dopant sublimation or aggregation. In small-molecule donor-acceptor systems, charge transfer can yield high and stable electrical conductivities, an approach not yet explored in all-conjugated polymer systems. Here, we report ground-state electron transfer in all-polymer donor-acceptor heterojunctions. Combining low-ionization-energy polymers with high-electron-affinity counterparts yields conducting interfaces with resistivity values five to six orders of magnitude lower than the separate single-layer polymers. The large decrease in resistivity originates from two parallel quasi-two-dimensional electron and hole distributions reaching a concentration of ∼10
cm
. Furthermore, we transfer the concept to three-dimensional bulk heterojunctions, displaying exceptional thermal stability due to the absence of molecular dopants. Our findings hold promise for electro-active composites of potential use in, for example, thermoelectrics and wearable electronics.
Organic solar cells are thought to suffer from poor thermal stability of the active layer nanostructure, a common belief that is based on the extensive work that has been carried out on ...fullerene-based systems. We show that a widely studied non-fullerene acceptor, the indacenodithienothiophene-based acceptor ITIC, crystallizes in a profoundly different way as compared to fullerenes. Although fullerenes are frozen below the glass-transition temperature T g of the photovoltaic blend, ITIC can undergo a glass-crystal transition considerably below its high T g of ∼180 °C. Nanoscopic crystallites of a low-temperature polymorph are able to form through a diffusion-limited crystallization process. The resulting fine-grained nanostructure does not evolve further with time and hence is characterized by a high degree of thermal stability. Instead, above T g, the low temperature polymorph melts, and micrometer-sized crystals of a high-temperature polymorph develop, enabled by more rapid diffusion and hence long-range mass transport. This leads to the same detrimental decrease in photovoltaic performance that is known to occur also in the case of fullerene-based blends. Besides explaining the superior thermal stability of non-fullerene blends at relatively high temperatures, our work introduces a new rationale for the design of bulk heterojunctions that is not based on the selection of high-T g materials per se but diffusion-limited crystallization. The planar structure of ITIC and potentially other non-fullerene acceptors readily facilitates the desired glass-crystal transition, which constitutes a significant advantage over fullerenes, and may pave the way for truly stable organic solar cells.
Foaming of plastics allows for extensive tuning of mechanical and physicochemical properties. Utilizing the foam architecture for plastic semiconductors can be used to improve ingression of external ...molecular species that govern the operation of organic electronic devices. In case of plastic thermoelectrics, utilizing solid semiconductors with realistic (millimeter (mm)‐thick) dimensions does not permit sequential doping—while sequential doping offers the higher thermoelectric performance compared to other methods—because this doping methodology is diffusion limited. In this work, a fabrication process for poly(3‐hexylthiophene) (P3HT) foams is presented, based on a combination of salt leaching and thermally induced phase separation. The obtained micro‐ and nanoporous architecture permits rapid and uniform doping of mm‐thick foams with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane, while thick solid P3HT structures suffer from protracted doping times and a dopant‐depleted central region. Importantly, the thermoelectric performance of a P3HT foam is largely retained when normalized with regard to the quantity of used material.
P3HT foams are fabricated through a combination of thermally induced phase separation and salt leaching. The foam architecture shows vastly improved dopant uptake compared to solid P3HT, enabling the superior sequential doping sequence for millimeter‐thick semiconductor structures. The thermoelectric performance of P3HT foams is largely retained per material quantity, implying that foams are attractive for plastic thermoelectrics.
The thermoelectric power factor of a broad range of organic semiconductors scales with their electrical conductivity according to a widely obeyed power law, and therefore, strategies that permit this ...empirical trend to be surpassed are highly sought after. Here, tensile drawing of the conjugated polymer poly(3-hexylthiophene) (P3HT) is employed to create free-standing films with a high degree of uniaxial alignment. Along the direction of orientation, sequential doping with a molybdenum tris(dithiolene) complex leads to a 5-fold enhancement of the power factor beyond the predicted value, reaching up to 16 μW m–1 K–2 for a conductivity of about 13 S cm–1. Neither stretching nor doping affect the glass transition temperature of P3HT, giving rise to robust free-standing materials that are of interest for the design of flexible thermoelectric devices.
We demonstrate that voltage losses due to both radiative and non-radiative recombination of charge carriers are strongly dependent on D/A phase separation. By processing the active layer with various ...solvent additives, we create distinct morphologies that lead to significantly different device open-circuit voltages ( V OC ), even though the charge transfer state energy ( E CT ) of the D/A blend remains rather constant. We find that radiative recombination losses are significantly increased for a finely intermixed morphology, due to the large D/A interface area. This leads to a total recombination loss of E CT − qV OC ≈ 0.7 eV. However, considerably smaller losses (0.5 eV), due to suppressed non-radiative recombination, are possible in solar cells where the D/A materials are organized to only allow for selective charge carrier extraction. Using a drift diffusion model, we show that the origin of the reduced non-radiative recombination losses is related to an effect which has not been considered for ‘optimized’ solar cells – the suppression of minority carrier diffusion to the ‘wrong’ contact. Our results suggest that the built-in field is not sufficiently strong even in ‘optimized’ organic solar cells and that selective carrier extraction is critical for further improvements in V OC .
During the last decade the field of polymer photovoltaics has seen a tremendous improvement in both device efficiency and understanding of the underlying physical processes. One has come to a point ...in which the prototypical large bandgap material system P3HT:PCBM is nearing optimal device performance. In order to enhance efficiencies even further, research activities for new materials are needed with better aligned energy levels. One interesting approach is by narrowing the donor bandgap to enhance light absorption. Recent developments on small band gap (<2.0 eV) materials for photovoltaic applications are reviewed. First, an introduction is given regarding the processes governing the exciton dissociation, charge transport requirements, energy level engineering of both donor and acceptor materials, and other parameters determining the photovoltaic performance. The focus is on polymeric donor materials, which are subdivided by the type of monomeric units that constitute the backbone. Finally, the synthetic methods and conditions, processing of the devices, and the device performances are summarized.
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