arXiv subjects

Martijn Kemerink

Publications and source records attributed to Martijn Kemerink.

At least 19 recordsLinked to original sources

arXiv2026

Homojunction-induced thermopower enhancement in polymer films

It has been more than twenty years since conductive polymers began to receive attention as an emerging thermoelectric material. However, the trade-off between electrical conductivity (σ) and thermopower (S) has proven to be a major challenge that has obstructed their use in actual devices. Here we report the discovery that the thermopower of the p- and n-type legs of organic thermogenerators can be substantially enhanced, without significant deterioration of σ, by constructing an in-plane segmented structure consisting of a homojunction with different doping levels on either side. In such segmented layers, the S is abnormally higher than the average value of the constituent parts when applying a forward temperature gradient (heating the heavily doped counterpart), while it is lower upon a reverse temperature gradient. Typically, for a two-stage segmented film of p-type PDPP-Se, an abnormally large S of 210 uV K-1 and σ of 2.5*10^4 S m-1 are obtained, resulting in a large power factor (PF) of 1100 uW m-1 K-2 and a record ZT of 1.36 at room temperature. The enhanced thermopower is attributed to an additional voltage developed at the homojunction under heating as explained by kinetic Monte Carlo simulations. This finding provides a breakthrough approach to the modulation of thermoelectric transport properties of conductive polymers.

cond-mat.mtrl-sci↗

arXiv2026

Composition Anisotropy Drives Large Bulk Photovoltaic Fields Along the Non-polar Vertical Direction in 2D Hybrid Perovskite Ferroelectrics

The photovoltaic electric field of the bulk photovoltaic effect (BPE) reflects the intrinsic ability of ferroelectrics to separate photoexcited excitons into electrons and holes, and are essential parameters for applications such as voltage-readout photodetectors. Because polarization defines the cation-anion displacement and noncentrosymmetric axis, the polar direction is generally one of the orientations exhibiting strong bulk photovoltaic field (EBPE). Here, we report emergent BPE behavior in 2D hybrid perovskite ferroelectrics (HPFs), where EBPE can be two orders of magnitude higher along the vertical nonpolar direction than along the polar in-plane direction. Its magnitude is up to orders of magnitude higher than that of benchmark photoferroelectrics across different material systems and is the highest among 2D HPFs reported so far. This strong BPE response with emergent directional anisotropy originates from the unique coupling among the shift-current BPE mechanism, an efficient photocarrier-generating inorganic part, and an insulator-like organic part, a combination that is conflicting or inaccessible in traditional photoferroelectrics. This composition and anisotropy also produce basic BPE behavior distinct from that of typical photoferroelectrics, including a laser intensity dependent photovoltage and a nonlinear scaling of photovoltage with material dimension. We analyze and develop a series of formulas to describe the emergent photovoltage phenomena, which should be applicable to this novel 2D ferroelectrics family and to polar systems with similar anisotropy and robust photoelectric response.

cond-mat.mtrl-sci↗

arXiv2025

Strongly Electric Field Dependent Conductivity in Quantum Dot Solids

Charge transport in QD solids is typically understood as thermally activated tunneling or hopping between states that are localized on individual QDs. Here, we show that the slow relaxation that is associated with the disorder-broadened density of (localized) states leads to a strong electric field F dependence of the charge carrier mobility. We interpret the results in terms of an increased effective electronic temperature T_eff that exceeds that of the lattice. We use a heat balance model to derive an analytical expression for T_eff (F) that is similar to, and puts a physical basis under the phenomenological expression proposed by Marianer and Shklovskii [Phys. Rev. B 46, 13100 (1992)]. We apply this model to analyze the field- and temperature-dependent conductivity in ZnO QDs with varying ligand length and depletion shell thickness and find (effective) localization lengths ranging from 2 to 5 nm. Both experimental and analytical results compare favorably to numerical simulations by kinetic Monte Carlo. Due to the large value of the effective localization length, the field dependence already becomes relevant at modest fields around 1-10 V/micron, that are typical for operational conditions of photovoltaic and light emitting devices based on quantum dot solids.

cond-mat.dis-nn↗

arXiv2025

Direct vs. Indirect Measurement of the Effective Electronic Temperature in Quantum Dot Solids

One of the characteristics of disordered semiconductors is the slow thermalization of charge carriers after excitation due to photoabsorption or high electric fields. An elegant way to capture the effects of the latter on the conductivity is through a field-dependent effective electronic temperature T_eff that can significantly exceed that of the lattice. Despite its elegance, its actual use has been limited, which, at least in part, can be attributed to the concept originating from computer simulations; experimental confirmations have largely been indirect (through scaling of conductivity) and did not establish that T_eff equals the real temperature of the electron distribution. Moreover, it has hardly been tested for important classes of disordered materials, including quantum dot solids. Here, we investigate whether the effective temperature concept is applicable to quantum dot solids, using zinc oxide as relevant model system. To verify that field-driven conductivity increases indeed reflect an actual increase of the electronic temperature, we combine direct and indirect measurements of T_eff: we convert conductivity changes at high fields to an effective temperature that we show to be consistent with a direct measurement of the electronic temperature using the Seebeck effect. These results not only confirm the relevance of the effective temperature concept to quantum dot solids but also confirm its general physical reality and open the way to systematic investigations into charge carrier (de)localization in disordered media.

cond-mat.mes-hall↗

arXiv2025

Electric field-dependent conductivity as probe for charge carrier delocalization and morphology in organic semiconductors

The charge carrier localization length α is a crucial, yet often ignored parameter of conjugated polymers that exponentially influences electronic conductivity. Here, we argue it is a unique proxy of the energy landscape as determined by sample morphology and experienced by mobile charges. To determine α, we use that in disordered organic semiconductors, slow thermalization of charge carriers after excitation, e.g. by hopping in a finite electric field, can lead to an effective electronic temperature (T_eff) exceeding the lattice temperature, thereby enhancing conductivity. We experimentally probe this effect by combining temperature- and field-dependent conductivity measurements for a range of representative conjugated polymers, using different dopants, doping protocols and doping concentrations. We find that in the high-field regime (F>1E6V/m), T_eff exhibits distinct trends vs. structural order and doping level, which can be used to extract (effective) localization lengths ranging from ~1nm in fully amorphous systems to over ~10nm in highly ordered polymers. Tight-binding and kinetic Monte Carlo simulations are used to connect measured values to morphological properties and to rule out alternative explanations. Our results demonstrate that finite-field conductivity measurements provide a powerful probe of a characteristic length scale of charge transport that is complementary to conventional structural characterization.

cond-mat.dis-nn↗

arXiv2025

Direct Measurement of the Effective Electronic Temperature in Organic Semiconductors

Organic semiconductors show complex phenomena due to their high energetic disorder. A striking example is the possibility of an increased effective temperature T_eff of the charge carrier distribution relative to the lattice temperature, which results from the slow charge carrier relaxation after excitation, either by high electric field or photon absorption. The increased effective temperature has been linked to conductivity enhancements and performance increases in actual devices, but a direct observation has been lacking. Here, we utilize nanoscopic tree-terminal devices to measure the Seebeck voltage arising in a doped organic polymer semiconductor due to a field-driven enhancement of the effective electronic temperature, providing direct proof of the existence of T_eff. The results agree quantitatively with numerical predictions by a kinetic Monte Carlo model. The findings not only provide fundamental understanding but also indicate an avenue towards low-loss thermoelectric devices.

cond-mat.dis-nn↗

arXiv2025

Soft Coulomb Gap Limits the Performance of Organic Thermoelectrics

Although consensus exists that the thermoelectric properties of doped organic semiconductors result from a complex interplay between a large number of mutually dependent factors, there is no consensus on which of these are dominant, or even on how to best describe the charge and energy transport. This holds particularly in the intermediate doping regime where the optimal performance is typically observed at the roll-off in the Seebeck coefficient - conductivity (S-σ correlation, fundamentally limiting the rational advancement of organic thermoelectric materials. Here, we combine experiments on a board set of conjugated polymers with kinetic Monte Carlo simulations across varying doping levels to uncover a general transport framework. We demonstrate that the optimal thermoelectric power factor (PF_max) consistently occurs at the transition between conventional variable-range hopping (VRH) and VRH in a density of states in which a soft Coulomb gap forms at the Fermi level, as described by Efros and Shklovskii (ES-VRH). This suggests the use of high dielectric constant materials or the promotion of charge delocalization as an avenue to shift the roll-off of the S-σ} curve, which constrains PF_max, to higher doping levels and accordingly higher PF.

cond-mat.mtrl-sci↗

arXiv2025

Ion-Exchange Doping of Semiconducting Single-Walled Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high power factors after chemical p- or n-doping. Understanding the impact of dopant counterions on charge transport and thermoelectric properties of nanotube networks is essential to further optimize doping methods and to develop better dopants. Here, we utilize ion-exchange doping to systematically vary the size of counterions in thin films of small and large diameter, polymer-sorted semiconducting SWCNTs with AuCl3 as the initial p-dopant and investigate the impact of ion size on conductivity, Seebeck coefficients and power factors. Larger anions are found to correlate with higher electrical conductivities and improved doping stability, while no significant effect on the power factors is found. Importantly, the effect of counterion size on the thermoelectric properties of dense SWCNT networks is not obscured by morphological changes upon doping. The observed trends of carrier mobilities and Seebeck coefficients can be explained by a random resistor model for the nanotube network that accounts for overlapping Coulomb potentials leading to the formation of an impurity band whose depth depends on the carrier density and counterion size. These insights can be applied more broadly to understand the thermoelectric properties of doped percolating disordered systems, including semiconducting polymers.

cond-mat.mtrl-sci↗

arXiv2025

Barkhausen noise in the columnar hexagonal organic ferroelectric BTA

Upon a polarization reversal within a ferroelectric material, one stable state changes into another which is typically described by a progression of switching events of smaller fractions of the material. These events give rise to crackling or Barkhausen noise and follow a characteristic distribution in their sizes. Barkhausen noise has been studied to better understand the switching processes of ferroelectrics and has been applied for inorganic ferroelectric materials and perovskites. In this work, we present results from kinetic Monte Carlo simulations investigating the switching process of the small organic molecular ferroelectric benzene-1,3,5-tricarboxamides (BTAs). For temperatures below 175 K and sufficiently strong structural disorder, the system exhibits self-organized critical behavior; for higher temperatures, a creep regime is entered. Our extracted power-law exponents are smaller than those typically measured in inorganic crystals and ceramics which indicates that in the more disordered material BTA larger spanning avalanches are possible. The system was experimentally investigated with a high-sensitivity setup. No Barkhausen noise was observed which is consistent with the simulated event sizes, lying several orders beneath the noise threshold of the experimental setup. This finding corroborates the notion that switching in BTA progresses along the 1D columns in the hexagonal liquid crystal lattice, with little coupling between the columns that could give rise to larger lateral avalanches.

cond-mat.mtrl-sci↗

arXiv2025

Hot Carrier Organic Solar Cells

Hot-carrier solar cells use the photon excess energy, that is, the energy exceeding the absorber bandgap, to do additional work. These devices have the potential to beat the upper limit for the photovoltaic power conversion efficiency set by near-equilibrium thermodynamics. However, since their conceptual inception in 1982, no experimental realization that works under normal operational conditions has been demonstrated, mostly due to the fast thermalization of photo-generated charges in typical semiconductor materials. Here, we use noise spectroscopy in combination with numerical modelling to show that common bulk heterojunction organic solar cells actually work as hot-carrier devices. Due to static energetic disorder, thermalization of photo-generated electrons and holes in the global density of states is slow compared to the charge carrier lifetime, leading to thermal populations of localized charge carriers that have an electronic temperature exceeding the lattice temperature. Since charge extraction takes place in a high-lying, narrow energy window around the transport energy, the latter takes the role of an energy filter. For common disorder values, this leads to enhancements in open circuit voltage of up to ~0.2 V. We show that this enhancement can be understood as a thermovoltage that is proportional to the temperature difference between the lattice and the charge populations and that comes on top of the near-equilibrium quasi-Fermi level splitting.

cond-mat.mtrl-sci↗

arXiv2024

Barkhausen noise in the organic ferroelectric copolymer P(VDF:TrFE)

Polarization reversal within a ferroelectric material is commonly described as a progression of smaller switching events, giving rise to crackling or Barkhausen noise. While studies on Barkhausen noise, and particularly the associated event size distribution, allow for better understanding of switching processes in ferroelectrics, they were not yet conducted experimentally on organic ferroelectric materials. In this work, Barkhausen noise in the organic ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF:TrFE)) is experimentally investigated under different electric fields, increasing at various rates. A weak dependence of the structure of the Barkhausen noise on both the magnitude and rise time of the applied electric field is observed, which manifests as a trend in the probability density function power-law exponents. Specifically, an increase in maximum electric field leads to an increase of the power-law exponent; increasing the rise time causes a parallel shift towards lower exponents. While these findings do not allow to conclusively confirm or refute universal self-organized critical behavior of the polarization reversal avalanches in P(VDF:TrFE), the exponents were found to seemingly converge to the universal value of 1.5 for fast and strong driving, suggesting the system is close to this limit.

cond-mat.mtrl-sci↗

arXiv2024

Size-Dependent Charging Energy Determines the Charge Transport in ZnO Quantum Dot Solids

Building up a solid-state material from quantum dots (QD), which are often referred to as artificial atoms, offers the potential to create new materials with unprecedented macroscopic properties. The investigation of the electronic properties of such QD assemblies has attracted attention due to the increasing applications of QD solids in both electronics and optoelectronics. In the past, charge transport in QD assemblies has been explained by a variety of mutually exclusive theories, with the Mott and Efros-Shklovskii variable range hopping models being most common. However, these theories fall short in explaining the anomalous exponents of the temperature-dependent conductivity observed in various QD materials. Here, we measure the temperature-dependent conductivity of semiconducting ZnO QDs under different UV illumination intensity. Regulating the UV intensity allows us to systematically change the effective diameter of the ZnO QDs without having to rely on cumbersome size control by synthesis. Instead, the UV level controls the width of the QD depletion shell and therefore the size distribution in the overall material. We observe exponents that systematically increase from α=0.25 to α=0.62 with increasing illumination intensity, which we interpret in terms of a charge transport being limited by the (size-dependent) charging energy of the QDs.

cond-mat.mes-hall↗

arXiv2022

Charge transport in doped conjugated polymers for organic thermoelectrics

Research on conjugated polymers for thermoelectric applications has made tremendous progress in recent years, which is accompanied by surging interest in molecular doping as a means to achieve the high electrical conductivities that are required. A detailed understanding of the complex relationship between the doping process, the structural as well as energetic properties of the polymer films, and the resulting thermoelectric behavior is slowly emerging. This review summarizes recent developments and strategies that permit enhancing the electrical conductivity of p- and n-type conjugated polymers via molecular doping. The impact of the chemical design of both the polymer and the dopant, the processing conditions, and the resulting nanostructure on the doping efficiency and stability of the doped state are discussed. Attention is paid to the interdependence of the electrical and thermal transport characteristics of semiconductor host-dopant systems and the Seebeck coefficient. Strategies that permit to improve the thermoelectric performance, such as an uniaxial alignment of the polymer backbone in both bulk and thin film geometries, manipulation of the dielectric constant of the polymer, and the variation of the dopant size, are explored. A combination of theory and experiment is predicted to yield new chemical design principles and processing schemes that will ultimately give rise to the next generation of organic thermoelectric materials.

physics.chem-ph↗

arXiv2021

General Rules for the Impact of Energetic Disorder and Mobility on Nongeminate Recombination in Phase-Separated Organic Solar Cells

State of the art organic solar cells exhibit power conversion efficiencies of 18 % and above. These devices benefit from the suppression of free charge recombination with regard to the Langevin-limit of charge encounter in a homogeneous medium. It has been recognized that the main cause of suppressed free charge recombination is the reformation and resplitting of charge transfer states at the interface between donor and acceptor domains. Here, we use kinetic Monte Carlo simulations to understand the interplay between free charge motion and recombination in an energetically-disordered phase-separated donor-acceptor blend. We identify conditions for encounter-dominated and resplitting-dominated recombination. In the former regime, recombination is proportional to mobility for all parameters tested and only slightly reduced with respect to the Langevin limit. In contrast, mobility is not the decisive parameter determining the non-geminate recombination coefficient k_2in the latter case where k_2 is a sole function of the morphology, CT and CS energetics and CT states decay properties. Our simulations also show that free charge encounter in the phase-separated disordered blend is determined by the average mobility of all carriers, while CT reformation and resplitting involves mostly states near the transport energy. Therefore, charge encounter is more affected by increased disorder than the resplitting of the CT state. As a consequence, for a given mobility, larger energetic disorder in combination with a higher hopping rate is preferred. These findings have important implications for the understanding of suppressed recombination in solar cells with non-fullerene acceptors which are known to exhibit lower energetic disorder than fullerenes.

physics.app-ph↗

arXiv2021

Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage ($V_\text{OC}$) is the one with most room for improvement. Existing models for the description of $V_\text{OC}$ assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe $V_\text{OC}$ of disordered organic devices. For two representative donor:acceptor blends it is shown that $V_\text{OC}$ is actually 0.1-0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced $V_\text{OC}$.

physics.app-ph↗

arXiv2021

Rubbing and Drawing: Generic Ways to Improve the Thermoelectric Power Factor of Organic Semiconductors?

Highly oriented polymer films can show considerable anisotropy in the thermoelectric properties leading to power factors beyond those predicted by the widely obeyed power law linking the thermopower $S$ and the electrical conductivity $σ$ as $S\proptoσ^{-1/4}$. This has led to encouraging practical results with respect to the electrical conductivity, notwithstanding that the conditions necessary to enhance $σ$ and $S$ simultaneously are less clear. Here, kinetic Monte Carlo simulations are used to study the impact of structural anisotropy on the thermoelectric properties of disordered organic semiconductors. We find that stretching is a suitable strategy to improve the conductivity along the direction of strain, while the effect on the power factor depends on the morphology the polymer crystallizes. In general, crystalline polymers show a simultaneous increase in $σ$ and $S$ which is not the case for amorphous polymers. Moreover, we show that the trends resulting from simulations based on variable-range hopping are in good agreement with experiments and can describe the different functional dependencies in the $S$ versus $σ$ behaviour of different directions.

cond-mat.mtrl-sci↗

arXiv2021

High thermoelectric power factor of poly(3-hexylthiophene) through in-plane alignment and doping with a molybdenum dithiolene complex

Here we report a record thermoelectric power factor of up to 160 $μ$ W m-1 K-2 for the conjugated polymer poly(3-hexylthiophene) (P3HT). This result is achieved through the combination of high-temperature rubbing of thin films together with the use of a large molybdenum dithiolene p-dopant with a high electron affinity. Comparison of the UV-vis-NIR spectra of the chemically doped samples to electrochemically oxidized material reveals an oxidation level of 10%, i.e. one polaron for every 10 repeat units. The high power factor arises due to an increase in the charge-carrier mobility and hence electrical conductivity along the rubbing direction. We conclude that P3HT, with its facile synthesis and outstanding processability, should not be ruled out as a potential thermoelectric material.

physics.app-ph↗

arXiv2021

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy dependent density of states. Extracting disorder and hopping parameters from experimental data like temperature dependent current-voltage characteristics typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) has been extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable range hopping (VRH) model that has been integrated in a freeware drift-diffusion solver. The mobility model has been calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space charge limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p-type and n-type devices. We find a critical ratio of aNN/$α$ (mean inter-site distance / localization radius) of ~3 below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45-120 meV are found, without any clear correlation with photovoltaic performance when the same active layer is used in an organic solar cell.

cond-mat.mtrl-sci↗