arXiv subjects

Hans Kleemann

Publications and source records attributed to Hans Kleemann.

17 recordsLinked to original sources

arXiv2026

Enhanced Photomultiplication Effect by Synergistic Integration of Hole-Blocking Layers and Trap Engineering in PM-OPDs

Photomultiplication-type organic photodetectors (PM-OPDs) promise exceptional sensitivity for weak-light detection but typically suffer from a gain-bandwidth trade-off where high external quantum efficiency (EQE) incurs large dark current and slow response times. Here, we demonstrate a fully vacuum-deposited PM-OPD architecture that mitigates these limitations by integrating hole-blocking layers low-stoichiometry molecular trap engineering. We isolate discrete trapping sites that maximize positive space-charge accumulation by introducing m-MTDATA as a dedicated hole-trapping site at a low concentration (0.5 wt\%) into a BDP-OMe:C60 bulk heterojunction. This engineered charge confinement triggers efficient field-assisted electron injection from the anode while remaining strictly below the threshold for localized percolation, effectively decoupling the photocurrent multiplication mechanism from trap-mediated dark current shunts. Consequently, the optimized device achieves a peak EQE exceeding 1100% at a reverse bias of -4 V. The optimized device exhibits a specific detectivity of 4x10^{12} Jones under -2 V reverse bias along with a cutoff frequency (f-3dB) of 22 kHz.

cond-mat.other↗

arXiv2026

Universal material basis for biocompatible printed electrolytes in Organic Electrochemical Transistors

Organic Electrochemical Transistors (OECTs) stand out for their interplay between ionic and electronic conduction, making them ideal analogues to biological synapses for neuromorphic computing and biosensing applications. Furthermore, they can be printed into integrated circuits on flexible substrates, enabling low-cost and high-throughput fabrication of complete electronic systems. However, most OECT electrolytes for integrated circuits still lack biocompatibility and suffer from rheology-related printing challenges. This paper presents a novel material basis that can be combined with an ionic liquid to fabricate an electrolyte for OECTs that only contains biocompatible materials. It allows rheological adjustments to enable the use of electrolyte in both inkjet and screen printing. Furthermore, the electrolyte is UV-curable, enabling it to transition into solid-state structures after printing. Extended ink and device lifetimes for screen-printed structures enable the fabrication of advanced OECTs that can operate in ambient air for over 30 days after fabrication. Ultimately, a fully screen-printed transistor using only biocompatible materials on a leaf substrate is shown

cond-mat.soft↗

arXiv2026

SOMA: A Single-Material Organic Multivibrator Adaptive Neuron for Fully Integrated PEDOT:PSS Neuromorphic Systems

Neuromorphic electronics and spiking neural networks (SNNs) offer energy-efficient data processing, essential for real-time and edge-computing applications. In particular, interfacing and processing biological signals require devices that combine electronic performance with ionic sensitivity, which are capabilities uniquely provided by organic electrochemical transistors (OECTs). However, realizing a simple, fully integrated OECT-based neuron with rich dynamics and adaptability remains challenging. Most reported implementations rely on current-driven operation, which complicates large-scale integration and neuron-neuron coupling due to the need for precise matching of operating currents and bias voltages. Here we present a voltage-driven neuron circuit based on a multivibrator oscillator architecture, entirely fabricated from poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The neuron exhibits tunable adaptability through an additional control input, enabling switching between burst latency and length encoding modes. We further demonstrate a hardware-implemented two-neuron unit consisting of an inhibitory and a readout neuron, where readout activity is suppressed depending on the relative timing of the inhibitory input. Finally, we demonstrate that the fabrication process is compatible with polymer dendrite growth, enabling on-chip integration of synaptic elements on the same substrate. Owing to its structural simplicity and compatibility with a single, available material, this approach offers a scalable and accessible route toward integrated OECT-based SNNs.

physics.app-ph↗

arXiv2025

Electron-Ion Coupling Breaks Energy Symmetry in Bistable Organic Electrochemical Transistors

Organic electrochemical transistors are extensively studied for applications ranging from bioelectronics to analog and neuromorphic computing. Despite significant advances, the fundamental interactions between the polymer semiconductor channel and the electrolyte, which critically determine the device performance, remain underexplored. Here, we examine the coupling between the benchmark semiconductor PEDOT:PSS and an ionic liquid to explain the bistable and non-volatile behavior observed in OECTs. Using X-ray scattering and spectroscopy techniques, we demonstrate how the electrolyte modifies the channel composition, enhances molecular order, and reshapes the energetic landscape. Notably, the observed bistability arises from asymmetric and path-dependent energetics during doping and dedoping, resulting in two distinct paths, driven by a direct interaction between the electronic and ionic charge carriers. These findings highlight the electrolyte's role in tuning charge carrier dynamics, positioning it as a powerful yet underutilized lever for enabling novel device functionalities.

cond-mat.mtrl-sci↗

arXiv2025

Metallization of leaf-derived lignocellulose scaffolds for high performance flexible electronics and oligodynamic disinfection

Vascular tubules in natural leaves form quasi-fractal networks that can be metallized. Traditional metallization techniques for these lignocellulose structures are complex, involving metal sputtering, nanoparticle solutions, or multiple chemical pretreatments. Here we present a novel, facile, and reliable method for metallizing leaf-derived lignocellulose scaffolds using silver microparticles. The method achieves properties on-par with the state-of-the-art, such as broadband optical transmittance of over 80%, sheet resistances below 1 Ohm/sq., and a current-carrying capacity exceeding 6 A over a 2.5 x 2.5 cm^2 quasi-fractal electrode. We also demonstrate copper electrodeposition as a cost-effective approach towards fabricating such conductive, biomimetic quasi-fractals. Additionally, we show that these metallized structures can effectively eliminate pathogenic microorganisms like fecal coliforms and E. coli, which are bacterial indicators of microbiological contamination of water. We finally show that these oligodynamic properties can be significantly enhanced with a small externally applied voltage, indicating the noteworthy potential of such structures for water purification and pollution control.

physics.app-ph↗

arXiv2024

Infrasound Detection Using Polymer Networks in Liquid Films

Recording and analyzing infrasound signals is essential to study natural phenomena, such as earthquakes, weather, or avalanches, but also has practical importance in aviation industry, optimization of wind turbines, and many more. However, the detection of faint infrasound signals is still a significant challenge as the transducers (e.g., mechanical, optical, and piezoelectric) are either difficult to integrate or do not provide sufficient sensitivity. Here, we propose an alternative principle to detecting infrasound, which is based on a free-standing liquid film covered with a polymeric organic mixed ionic-electronic conductor (OMIEC). This polymer is capable of conducting ions, as well as electrons/holes and serves as a direct electronic infrasound transducer due to the sensitivity of OMIECs to the local ion concentration in the liquid. We specifically address the detection of acoustic excitations within the infrasound regime and the dependency of the networks reaction to these stimuli on its conductive properties, mainly its impedance spectrum. The resulting sensor has a sensitivity of 613 muV Pa-1 and a power consumption of about 340 nW at 100 mHz, which puts these systems well within the range of commercial infrasound detectors, while offering significant advantages in terms of device complexity and integration.

physics.app-ph↗

arXiv2024

Bistable Organic Electrochemical Transistors: Enthalpy vs. Entropy

Organic electrochemical transistors (OECTs) underpin a range of emerging technologies, from bioelectronics to neuromorphic computing, owing to their unique coupling of electronic and ionic charge carriers. In this context, various OECT systems exhibit significant hysteresis in their transfer curve, which is frequently leveraged to achieve non-volatility. Meanwhile, a general understanding of its physical origin is missing. Here, we introduce a thermodynamic framework that readily explains the emergence of bistable OECT operation via the interplay of enthalpy and entropy. We validate this model through temperature-resolved characterizations, material manipulation, and thermal imaging. Further, we reveal deviations from Boltzmann statistics for the subthreshold swing and reinterpret existing literature. Capitalizing on these findings, we finally demonstrate a single-OECT Schmitt trigger, thus compacting a multi-component circuit into a single device. These insights provide a fundamental advance for OECT physics and its application in non-conventional computing, where symmetry-breaking phenomena are pivotal to unlock new paradigms of information processing.

physics.app-ph↗

arXiv2024

Leaf Electronics: Nature-Based Substrates and Electrodes for Organic Electronic Applications

The need to reduce the environmental impact of inorganic electronic systems is pressing. Although the field of organic electronics provides a potential solution to this issue, research and optimization is still majorly carried out on glass or plastic substrates. Additionally, the fabrication of organic devices requiring transparent electrodes is fraught with complex techniques and expensive materials which limit widespread implementation and sustainability goals. Here, we show that the quasi-fractal lignocellulose structures extracted from natural leaves can be successfully modified to be used as biodegradable substrates as well as electrodes for optoelectronic applications. Chemically coating the microstructures of these leaf skeletons with metals results in quasi-transparent, flexible electrodes having sheet resistances below 1 ohm/sq. and a concomitant current carrying capacity as high as 6 A over a 2.5*2.5 cm2 leaf electrode, all while maintaining broadband optical transmittance values of around 80%.

physics.app-ph↗

arXiv2024

A Hybrid Process for Integration of Organic Electrochemical Transistors for High Uniformity & Reliability

Photolithography is believed to be a complementary technique to large-area printing, allowing for nanometer-scale integration and offering cost-efficiency. For organic electronics though, adapting photolithography is very challenging due to chemical incompatibilities. However, with the help of Alexander Zakhidov, orthogonal resins opened up the prospect of adapting the well-established process of photolithography for organic electronics. Here, we present a hybrid fabrication method for organic electrochemical transistors by combining orthogonal photolithography and inkjet printing, enabling high uniformity and reliability. We demonstrate how the resolution of each process affects the uniformity, and we explore the advantages of this process for device scaling and circuit integration.

physics.app-ph↗

arXiv2024

Band Structure Engineering in Highly Crystalline Organic Semiconductors

Blending of semiconductors for controlling the energy levels (band structure engineering) is an important technique, in particular, for optoelectronic applications. The underlying physics is the delocalized Bloch states, which average over the potential landscape of the blend. For organic semiconductors, it has been shown that two quite different effects, the dielectric constant and electrostatic interaction between molecules, can be used to tune the energy gap and ionization energy of disordered and weakly crystalline organic semiconductor blends. It is so far not known whether the electronic delocalization in organic crystals with large bandwidths can contribute to the energy structure engineering of the blend in a way similar to that in inorganic semiconductors. Here, we investigate the growth of highly ordered organic thin-film blends with a similar chemical structure and show the effect of band structure engineering by spectroscopic methods. We rationalize the experimental results with comprehensive theoretical simulations, showing that the delocalization is a significant effect. Our work paves the way for engineering the band structure of highly ordered organic semiconductor thin films that can be tailored for the desired optoelectronic device application.

cond-mat.mtrl-sci↗

arXiv2023

Threshold Voltage Control in Dual-Gate Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) based on Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) are a benchmark system in organic bioelectronics. In particular, the superior mechanical properties and the ionic-electronic transduction yield excellent potential for the field of implantable or wearable sensing technology. However, depletion-mode operation PEDOT:PSS-based OECTs cause high static power dissipation in electronic circuits, limiting their application in electronic systems. Hence, having control over the threshold voltage is of utmost technological importance. Here we demonstrate PEDOT:PSS-based dual-gate OECTs with solid-state electrolyte where the threshold voltage is seamlessly adjustable during operation. We show that the degree of threshold voltage tuning linearly depends on the gate capacitance, which is a straightforward approach for circuit designers to adjust the threshold voltage only by the device dimensions. The PEDOT:PSS-based dual-gate OECTs show excellent device performance and can be pushed to accumulation-mode operation, resulting in a simplified and relaxed design of complementary inverters.

physics.app-ph↗

arXiv2022

In-Liquido Computation with Electrochemical Transistors and Mixed Conductors for Intelligent Bioelectronics

Next-generation implantable computational devices require long-term stable electronic components capable of operating in, and interacting with, electrolytic surroundings without being damaged. Organic electrochemical transistors (OECTs) emerged as fitting candidates. However, while single devices feature impressive figures of merit, integrated circuits (ICs) immersed in a common electrolytes are hard to realize using electrochemical transistors, and there is no clear path forward for optimal top-down circuit design and high-density integration. The simple observation that two OECTs immersed in the same electrolytic medium will inevitably interact hampers their implementation in complex circuitry. The electrolyte's ionic conductivity connects all the devices in the liquid, producing unwanted and often unforeseeable dynamics. Minimizing or harnessing this crosstalk has been the focus of very recent studies. In this Perspective, we discuss the main challenges, trends, and opportunities for realizing OECT-based circuitry in a liquid environment that could circumnavigate the hard limits of engineering and human physiology. We analyze the most successful approaches in autonomous bioelectronics and information processing. Elaborating on the strategies to circumvent and harness device crosstalk proves that platforms capable of complex computation and even machine learning can be realized in-liquido using mixed ionic-electronic conductors.

cond-mat.mtrl-sci↗

arXiv2021

Novel Concepts for Organic Transistors: Physics, Device Design, and Applications

Beyond conventional organic thin-film transistors, this thesis explores possible paths for the fourth wave of organic electronics. In this context, mixed ionic-electronic conductors and organic electro-chemical transistors (OECTs) are identified as highly promising approaches for electronic bio-interfaces enabling ultra-sensitive detection of biological signals. Furthermore, these systems show fundamental properties of biological synapses, namely the synaptic plasticity, which renders the possibility to build brain-inspired, neuromorphic networks enabling highly efficient computing. In particular, the combination of OECTs acting as sensor units and self-learning neural networks at once enables the development of intelligent tags for medical applications. Overall, this thesis adds substantially new insight into the field of organic electronics and draws a vision towards further research and applications. The advancements in the field of vertical organic transistors open new perspectives for the implementation of organic transistors in high-resolution AMOLED displays or radio-frequency identification tags. Furthermore, the exploration of OECTs for neuromorphic computing will create a whole new research field across the disciplines of physics, material, and computer science.

cs.ET↗

arXiv2021

Structural evolution and on-demand growth of artificial synapses via field-directed polymerization

Interconnectivity, fault tolerance, and dynamic evolution of the circuitry are long sought-after objectives of bio-inspired engineering. Here, we propose dendritic transistors composed of organic semiconductors as building blocks for neuromorphic computing. These devices, owning to their voltage-triggered growth and resemblance to neural structures, respond to action potentials to achieve complex brain-like features, such as Pavlovian learning, pattern recognition, and spike-timing-dependent plasticity. The dynamic formation of the connections is reminiscent of a biological learning mechanism known as synaptogenesis, and it is carried out by an electrochemical reaction that we name field-directed polymerization. We employ it to dendritic connections and, by modulating the growth parameters, control material properties such as the resistance and the time constants relevant for plasticity. We believe these results will inspire further research towards the complex integration of polymerized synapses for brain-inspired computing.

cs.ET↗

arXiv2019

Vertical Organic Thin-Film Transistor with Anodized Permeable Base for Very Low Leakage Current

The Organic Permeable Base Transistor (OPBT) is currently the fastest organic transistor with a transition frequency of 40 MHz. It relies on a thin aluminum base electrode to control the transistor current. This electrode is surrounded by a native oxide layer for passivation, currently created by oxidation in air. However, this process is not reliable and leads to large performance variations between samples, slow production and relatively high leakage currents. Here, we demonstrate for the first time that electrochemical anodization can be conveniently employed for the fabrication of high performance OPBTs with vastly reduced leakage currents and more controlled process parameters. Very large transmission factors of 99.9996% are achieved, while excellent on/off ratios of 5x10$^5$ and high on-currents greater than 300 mA/cm$^2$ show that the C$_{60}$ semiconductor layer can withstand the electrochemical anodization. These results make anodization an intriguing option for innovative organic transistor design.

physics.app-ph↗

arXiv2014

Determining doping efficiency and mobility from conductivity and Seebeck data of n-doped C60 layers

In this work, we introduce models for deriving lower limits for the key parameters doping efficiency, charge carrier concentration, and charge carrier mobility from conductivity data of doped organic semiconductors. The models are applied to data of thin layers of Fullerene C60 n-doped by four different n-dopants. Combining these findings with thermoelectric Seebeck data, the energetic position of the transport level can be narrowed down and trends for the absolute values are derived.

cond-mat.mtrl-sci↗

arXiv2010

Organic Zener Diodes: Tunneling across the Gap in Organic Semiconductor Materials

Organic Zener diodes with a precisely adjustable reverse breakdown from -3 V to -15 V without any influence on the forward current-voltage curve are realized. This is accomplished by controlling the width of the charge depletion zone in a pin-diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer. The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the HOMO-LUMO gap of neigh- boring molecules. The experimental data are confirmed by a minimal Hamiltonian model approach, including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region.

cond-mat.mtrl-sci↗