Spatial inhomogeneity, in the form of more (crystalline) and less (amorphous) ordered regions, is a well-established phenomenon in state-of-the-art doped organic semiconductors (OSC). At the same time, a quantitative understanding of how exactly this inhomogeneity translates into (a reduction or enhancement of) the electrical conductivity remains largely elusive. In this talk, I will address two steps towards such a formal understanding. First, I will show how the electric field dependence of the conductivity of doped OSC can be used to extract an ‘effective’ localization length for the charge carriers as a characteristic length scale of charge transport that is complementary to conventional structural characterization. Tight-binding and kinetic Monte Carlo simulations are used to connect measured values to morphological properties, in particular to the width of and the distance between energy barriers in the percolating pathway. In the second part, it will be shown experimentally how a judicious blending of different polymers can be used to remove energy barriers that (still) limit the conductivity of high-performance pure materials, giving rise to materials with conductivities up to 60 000 S/cm and thermoelectric power factor above 2000 µW/mK2. Combining experiments and simulations, the conditions under which this effect occurs are identified.

Already the drain current transients in organic electrochemical transistors hint for not yet fully understood underlaying device physics, as they show a generally slower turn on compared to turn off, with the difference between these on- and off-switching times varying over orders of magnitude between materials and driving voltages. Tracking the charge carrier concentration during switching then completes this asymmetric picture: A sharp doping front entering the device during turn on, followed by gradual bulk de-doping during turn off. We measure these concentration transients optically, using the red-shifted absorption in the doped state of the redox active polymers we investigated. Comparison of this data with drift-diffusion simulations revealed a strong dependence of this asymmetry on the broadness of the density of states, which in our simulations both modulates the carrier density dependence of hole mobilities and influences the maximum charge carrier concentration for a set gate voltage. Our simulation framework also allowed us to study the surprisingly small effect of ionic mobilities in the semiconductor on switching times, above a device specific mobility threshold. The results can be used to guide the rational design of improved devices.

The mechanical mismatch between rigid electronics and soft biological tissues presents a major challenge in the development of bioelectronic devices¹. Organic mixed ionic-electronic conductors (OMIECs), particularly conjugated polymers with oligoether side chains, offer promising solutions due to their intrinsic softness, mixed conductivity, and biocompatibility^2,3. However, their significant volume changes during electrochemical cycling—driven by ion and water ingress during oxidation and subsequent expulsion during reduction—often lead to drastic changes in stiffness, complicating device-tissue mechanical matching across redox states^4,5,6.

Here, we investigate the electromechanical response of a thienothiophene-based conjugated polymer, p(g3TT-T2), functionalized with triethylene glycol side chains. Using electrochemical nanoindentation and structural characterization, we find that this polymer exhibits only a modest increase in elastic modulus from ~100 to 200 MPa upon electrochemical oxidation. This unusual mechanical stability is attributed to a reversible increase in π–π stacking that compensates for swelling-induced softening.

Our findings demonstrate that it is possible to design OMIEC materials with stable mechanical properties across redox states, opening new possibilities for compliant, tissue-matched bioelectronic interfaces that remain mechanically invariant during operation.

Chemical doping is essential for optimizing the performance of organic semiconductor (OSC) devices. While numerous strategies have been developed, most rely on strong oxidizing or reducing agents that modify the OSC’s redox state through direct electron-transfer reactions. Although effective, these methods often introduce challenges related to film morphology, long-term stability, and limited processing compatibility. Consequently, developing efficient doping approaches that operate under mild conditions and employ weak or readily accessible dopants remains an important and widely recognized unmet need within the field. In this presentation, I will highlight our recent efforts to overcome these limitations by introducing new doping concepts for OSCs, including metal-activated and light-driven redox processes. These approaches provide improved control, broaden material compatibility, and expand the design space for stable and scalable OSC doping. Together, they point toward more sustainable, versatile, and tunable strategies for tailoring the electronic properties of organic materials across a broad range of emerging applications.

Probing the expanding library of polymer–dopant combinations for organic thermoelectrics is challenging because polymer chemistry, processing, and microstructure influence optoelectronic properties. This complexity slows understanding and development of general design rules that could guide performance improvements across different applications. To overcome this limitation, we present a high-throughput workflow that evaluates hundreds of doping states within a single specimen, yielding >1,200 internally consistent measurements across a few samples, while minimizing variability. Applied to highperforming n-type DPP-based host doped along the dopant NDBMI, the approach shines light on a current controversy: dopant activation is dictated by host–dopant conformational interactions, not temperature alone. For this, we fixe different molecular orientation, that span from face-on to edge-on texture. Then through our workflow we evaluate the optical and electronic properties in a step-wise manner. Our results indicate that face-on oriented films activate rapidly, maintain order, and achieve power factors up to 200 µW m^-1 K^-1 at comparatively low doping, whereas edge-on and isotropic films activate slowly, accumulate disorder, and remain limited below 20 µW m^-1 K^-1. By converting sparse snapshots into dense trajectories, the method establishes how molecular orientation governs doping efficiency and performance, and provides a generalizable platform for deriving mechanism-based design rules across emerging polymer–dopant systems.

Thermoelectrics offer the ability to harvest ubiquitous waste heat to power small wearables and Internet of Things (IoT) devices, and they can also function as self-powered heat sensors. However, traditional thermoelectric materials rely on toxic, scarce, or critical raw elements. Organic thermoelectrics (OTE) based on doped conjugated polymers could address this issue, but their performance remains insufficient for most practical applications. Although recently aligned polymers such as PBTTT and DPP have shown great promise, achieving ultra-high power factors, the poor doping stability of conjugated polymers still prevents their reliable operation under ambient conditions. This lack of stability forces researchers to fall back on PEDOT:PSS for any application resembling real-life scenarios, as it remains the only ambient-stable conducting polymer. This reliance slows progress in the field because PEDOT:PSS is already a mature material that appears close to reaching its performance ceiling. Unlocking new breakthroughs in OTEs therefore, requires identifying doping strategies that are preferably not material-specific but instead compatible with a broad range of polymer chemistries, enabling fuller exploitation of the possibilities offered by organic synthesis.

In this presentation, we introduce a doping molecule that provides higher conductivity as well as improved ambient and thermal stability compared to typical molecular dopants—and does so across many prototypical conjugated polymers. We explain the mechanism behind this universal combination of high conductivity and stability using P3HT as a benchmark, supported by Gibbs free-energy simulations of the dopant–polymer system and experimental evidence from GIWAXS (nanostructure evolution) and UV–Vis spectroscopy (doping-state evolution over time). Power factors as high as 65 μW/(m·K²) for PDPP3T and 35 μW/(m·K²) for PBTTT are achieved, remaining as high as 40 μW/(m·K²) and 15 μW/(m·K²), respectively, after 25 days of storage in a glovebox. This represents an order-of-magnitude improvement in stability over state-of-the-art dopants, pointing toward the feasibility of using encapsulated, molecularly doped conjugated polymers in real thermoelectric applications.

Doped polymer semi-conductors are promising materials in organic electronics in general and for thermoelectric and organic electrochemical transistor applications in particular. The processing of the polymer semi-conductors determines their structure and hence the way dopant molecules will be incorporated in the polymer matrix. Specifically, polymorphism is often encountered in poly(thiophenes) and determines for instance the packing of side chains (interdigitated, disordered, etc…). [1] The control of side-chain packing is of paramount importance to control dopant molecule location in the polymer semiconductor hence the charge transport properties. Herein, we investigate the impact of polymorphism of a family of PBTTT polymers on doping and the resulting thermoelectric properties in oriented thin films. We consider PBTTTs with different side chains including oligo(ethylene glycol), siloxane or alkyl side chains with some ether function. [2-6] Interestingly, by combining TEM, FTIR and polarized UV-vis-NIR spectroscopy, it is possible to uncover a correlation between the initial side chain conformation and the doping efficiency that finally impacts thermoelectric properties and corresponding stability of the doped and oriented thin films.

Polymorphism is a common feature of polymer semiconductors such as polythiophenes, and it plays a crucial role in determining their optical and electronic properties. Controlling polymorphism is thus an elegant means to probe structure-property correlations in doped polymer semiconductors. In this contribution, we focus on PBTTT bearing single-ether side chains, namely PBTTT-⁸O.^[1-3]This polymer forms two distinct polymorphs under controlled growth conditions: form 1 and form 2 are prepared by thermal annealing and solvent treatment in n-hexane of rub-aligned thin films, respectively. Polarized UV-vis-NIR spectroscopy demonstrates that backbone planarization is reduced in form 2 whereas regular π-stacking and extended planarization is present in form 1. Upon doping with the strong acceptor F₆TCNNQ, the aligned polymorphs show substantially different charge transport and thermoelectric properties. The best thermoelectric properties are observed for the doped form 1 with a very high power factor of 3.6±0.6 ×10³ µW.m^-1.K^-2 versus 2.4±0.5 ×10² µW.m^-1.K^-2 for the doped films of form 2. Notably, blends of the two polymorphs show even better TE performances than form 1.

Very strong electron donor and acceptor molecules were developed as potent dopants of organic semiconductors. Some of these can also modify the charge density (doping level) of another excitonic materials class, comprising few or monolayer two-dimensional (2D) semiconductors, such as transition metal dichalcogenides. However, the electrical and electronic nature of the supporting substrate plays a key role in these doping processes, as exemplified for mono- and bi-layer MoS₂ as 2D semiconductor. In some cases, p-doping of the 2D semiconductor is only mimicked, and the organic dopants only reduce the native n-doping of MoS₂. More commonly, charge transfer occurs between a conductive substrate and organic dopants, with the 2D semiconductor sandwiched between these two experiencing a strong electrostatic field. With depth-dependent analysis of photoemission data, it is possible to obtain the charge and electrostatic field distribution across such structures on the atomic scale. This type of remote-charge-transfer and its associated electric field may enable realizing excitonic insulator states of 2D semiconductors.

The talk will give an overview of our recent activities about design and processing of polymeric mixed-ionic-electronic conductors for opto-electronic and switchable technologies making use of electrochemical doping of conducting polymers.

Starting from p-type doping of poly(3-hexylthiophene) (P3HT) films¹ and their solid-state absorption and conductivity behavior, I will show our latest results on n-type doping of naphthalenebisimide-based copolymers (PNDI2OD-T2)² and some of its derivatives³. It is interesting to note that our data clearly show bell-shaped conductivity profiles which are indicative of localized charges and redox polymer characteristics for the n-type materials.

Within the talk I will also introduce redox polymer materials based on triphenylamine (TPA) and carbazole (CBz) redox units.⁴ Both redox units can be irreversibly linked upon (electro)chemical oxidation leading to dimer redox species. In polymer films these dimers can act as crosslinkers, the number of redox units strongly influencing their charge transport behavior. Extending our strategy from 2D thin films to 3D architectures allows us to apply 3D digital light processing which enlarges our architectural design yielding redox-active chess pattern, dot arrays or even pyramids with distinct electrochromic properties.⁵

Organic semiconductors provide distinct advantages for infrared (IR) detection, including mechanical flexibility and tunable spectral response. However, organic photodiodes are typically limited to the short-wave IR range (< 2000 nm). Here, we demonstrate that a bolometer architecture using solution- or vacuum-deposited doped organic semiconductors can extend the detection wavelengths well beyond 10 µm, achieving detectivities up to 8 × 10⁹ Jones. Although thermal detectors such as bolometers are generally slower than photonic devices, we show that minimizing the overall device thickness enables response times as fast as 41 ms—suitable for imaging. We further find that the intrinsically low thermal conductivity of (doped) organic semiconductors permits pixel sizes of 10 µm while maintaining detectivities near 10⁹ Jones, provided the total device thickness remains below 1000 nm. Device physics and performance limitations of the approach will be discussed in this presentation, underlining the strong potential of doped organic semiconductors for IR detection.

The record-high responsivity of photomultiplication organic photodetectors (PM OPDs) makes them a highly promising candidate for next-generation biometric monitoring, imaging, and emerging near-infrared (NIR) sensing technologies. Photomultiplication is achieved through charge injection at high reverse bias via optical population of electronic trap states. However, the fundamental carrier dynamics that regulate this process remain poorly understood, primarily due to the absence of direct experimental probes capable of quantifying low densities of trapped charges. In this work, a novel set of spectroscopic approaches is applied to elucidate the dynamics of carrier trapping in a new high-performance near-infrared PM OPD with specific detectivity of 5.7 x 10¹² Jones and an external quantum efficiency of 3500% under -10 V. Trap selective spectroscopical techniques reveal how the optimised ratio of donor polymer and non-fullerene acceptor of 100:16 by weight features the highest trap density at the active layer/electrode interface. By tracking the population of trapped electrons under time-resolved measurements, a relatively fast build-up (500 ns) is found and a much slower depletion of trap carrier density (100 μs to ms). To test the reproducibility of the proposed device architecture and mechanism, an alternate donor polymer is empolyed, showing comparable device performance. Finally, the utility of these devices is demonstrated through photoplethysmography measurements for the determination of the cardiac cycle. These findings underscore the potential of trap-engineered PM OPDs for high-sensitivity biomedical sensing.¹

Molecular doping of conjugated polymers (CPs) is a powerful strategy to enhance charge transport and improve the performance of organic electronic devices, particularly organic thermoelectrics. Doped donor–acceptor (D–A) conjugated polymers, which feature a tunable energy offset between the Fermi level and the transport band, can achieve high electrical conductivity (σ) while maintaining a favorable Seebeck coefficient (S). However, while doping has led to remarkable improvements in device efficiency, the thermal stability of chemically doped D–A polymers—a critical factor for long-term device operation—remains poorly understood.

In this study, we systematically investigated the dopant size-dependent thermal stability of a diketopyrrolopyrrole-thiophene (DPP–T) donor–acceptor copolymer, using two molecular p-type dopants with distinct sizes: F₄TCNQ and Mo(tfd-CO₂Me)₃. Temperature-dependent UV–Vis–NIR spectroscopy revealed that DPP–T/F₄TCNQ exhibits significant dedoping under thermal stress, whereas DPP–T/Mo(tfd-CO₂Me)₃ maintains stable optical and electrical properties at elevated temperatures. Although F₄TCNQ doping provides higher initial in-plane conductivity, its conductivity decreases by more than an order of magnitude after annealing at 120 °C for 30 minutes, while the Mo-based doped polymer remains unchanged under the same conditions. Thermogravimetric analysis ruled out dopant sublimation as the primary degradation pathway, suggesting that dopant phase separation and migration drive the observed instability. This mechanism was corroborated by X-ray scattering and nanoscale infrared microscopy and spectroscopy (AFM–IR), which revealed dopant redistribution within the film microstructure.

The insights from this work underscore the importance of dopant molecular design for achieving thermally robust charge transport in doped conjugated polymers. For practical thermoelectric applications, device modules must operate continuously at elevated temperatures where conventional dopants fail to remain stable. Our findings highlight that incorporating sterically bulky dopants can suppress diffusion and phase segregation, providing a rational path toward durable, high-performance organic thermoelectric systems. By establishing the direct link between dopant size, molecular interactions, and long-term stability, this study advances the design principles needed to translate organic thermoelectrics from laboratory demonstrations to real-world energy applications.

Mixed ionic-electronic conduction, the simultaneous transport of both electronic and ionic species, is essential for a wide range of established and emerging technologies. During device operation, OMIECs undergo significant structural evolution in response to ion insertion and electrochemical doping, including swelling, phase segregation, and conformational rearrangements, which in turn modulate mixed conduction behavior. These complex structural evolutions stem primarily from the intrinsically heterogeneous morphology of OMIECs, which features a delicate interplay between long-range ordered crystalline and disordered amorphous phases. Each domains playing a critical role in mediating mixed conduction and dynamic response under bias. Calorimetric measurements, therefore, present a vital opportunity. By probing thermal transitions and segmental relaxation, they offer a direct means to investigate the structural evolution of both the crystalline and amorphous phases, addressing a critical knowledge gap in the field. We developed an integrated setup combining a temperature controller, lock-in amplifier, potentiostat and measurement tube to an E-chip calorimetry. Leveraging this setup, our E-chip calorimetry enables three complementary measurement modes: temperature-dependent scans to probe thermal transitions, frequency-dependent measurements to determine fragility index and isothermal measurements to monitor ion motion. Using this setup, we investigated the structural evolution of PEDOT:PSS and homogeneous OMIECs in their dry, swollen, and electrochemically doped states. In addition, we designed a special device architecture to enable operando calorimetric measurements. We observed non-uniform ion distribution within heterogeneous structures during swelling and (de)doping, leading to unique evolution patterns of segment motion and fragility in amorphous fractions. During swelling, the Tg of the PSS-rich phase is significantly reduced and approaches room temperature, while the Tg of the PEDOT-rich amorphous phase remains relatively high values. Fragility measurements indicate that swelling does not substantially alter chain rigidity of PEDOT segments, indicating a planar chain conformational in glassy phase. This enables efficient ion uptake within PSS-rich phases while preserving the hierarchical ordered structure in PEDOT-rich phases. In contrast, during dedoping, the Tg of the PSS-rich phase shows minimal change, while ions intercalate into the PEDOT-rich phase. This causes substantial fluctuations in the segmental dynamics of the PEDOT-rich amorphous phase and disrupts its chain conformational order, thereby enhancing dedoping efficiency. Specifically, films with a lower Tg and a higher m with more fibrillar morphology and loose structure, show more pronounced synergistic effect between segmental dynamics and fragility during operation. These characteristics promote both segmental motion-mediated transport in the PSS-rich phase and free-volume-mediated ion hopping in the PEDOT-rich phase, while supporting hierarchical charge carrier transport, providing the key physical descriptions for guiding the high-performance sample design. These findings elucidate the role of amorphous phase in OMIECs and underscore the potential of operando chip calorimetry in uncovering structure-property relationships in electroactive polymers.

The electrical conductivity and elastic modulus of doped conjugated polymers tend to increase in
tandem, which complicates the design of soft conductors. This work investigates how different dopant
counterions influence the electrical and mechanical properties of a thienothiophene copolymer with
triethylene glycol side chains. Sequential doping and proton-coupled electron-transfer were used to
prepare samples with a comparable oxidation level neutralized with different counterions. Highly
oxidized films featured a comparable electrical conductivity of about 100 S cm^-1 irrespective of the
counterion size. Dynamic mechanical analysis revealed that the choice of counterion strongly impacts
the sub-glass transition temperature, which varied from -44 to -3 ºC. As a result, the elastic modulus
at room temperature ranged from 0.05 to 0.7 GPa for materials with a comparable oxidation level.
Evidently, it is possible to decouple the electrical and mechanical properties of doped polymers,
which are governed by charge transport along the backbone and side-chain relaxation, respectively.
This insight opens up new opportunities for the design of soft conductors and more sustainable
bioelectronic and wearable devices whose various soft and rigid components could be created with the
same polymer.

In this work, we investigate the polymorphism of a new PBTTT-based polymer bearing oligo(ethylene-glycol) side chains separated from the conjugated backbone by a methylene spacer. We identified in rub-aligned films two distinct structures of the polymer, that coexist at room temperature: a mesophase and a crystalline polymorph. The mesophase has a well-planarized backbone and disordered side chains. In the crystalline phase, the side chains conformations are mostly antiperiplanar but at the expense of backbone planarity. We describe how to obtain aligned films of the pure LC and Cryst. phases and identify the structural signatures of the two polymorphs using Transmission Electron Microscopy (TEM) and Fourier Transform Infrared Spectroscopy (FTIR). These two phases were sequentially doped with the molecular acceptor F4TCNQ. The changes induced by doping, both structural and in the resulting transport/thermoelectric (TE) properties, are presented for the two polymorphs. Notably, despite being chemically identical, these two forms display markedly different thermoelectric properties and behaviours upon doping, emphasizing the importance of a strict control of the structure of organic PSCs to obtain reproducible TE properties.

This work presents a comprehensive structural and electronic investigation of surface molecular doping  in molecular doping in small-molecule organic semiconductor thin films for organic field-effect transistor (OFET), by using two electron acceptor molecules (F6TCNNQ and C60F48) molecular dopants. We present several studies with pentacene [1,2] and  benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives [3-6], demonstrating that the nature of OSC side groups in BTBT or the quality of pentacene packing at nanoscopic level influences the nucleation, ordering and distribution of the acceptor onto the OSC surface  On the other hand, the shape of the acceptor molecule (planar F6TCNNQ and bulky C60F48) has a profound impact on the doping mechanism (charge transfer vs. charge-transfer-complex formation) and the structural properties of the formed heterojunction. The implications for the performance and stability of the devices is also discussed. Collectively, these studies provide a microscopic view on the formation of the acceptor/OSC interfaces and the associated electronic effects.