arXiv subjects

Nir Tessler

Publications and source records attributed to Nir Tessler.

15 recordsLinked to original sources

arXiv2024

Transient current responses of organic electrochemical transistors: Evaluating ion diffusion, chemical capacitance, and series elements

For the successful implementation of organic electrochemical transistors in neuromorphic computing, bioelectronics, and real-time sensing applications it is essential to understand the factors that influence device switching times. Here we describe a physical-electrochemical model of the transient response to a step of the gate voltage. The model incorporates (1) ion diffusion inside the channel that governs the electronic conductivity, (2) horizontal electron transport, and (3) the external elements (capacitance, ionic resistance) of the ion dynamics in the electrolyte. We find a general expression of two different time constants that determine the vertical insertion process in terms of the kinetic parameters, in addition to the electronic transit time. We highlight the central role of the chemical capacitance in determining the modulation of the lateral conductivity. The different types of response of the drain current are classified, and we discuss the significance for synaptic operation in neuromorphic circuits. The model is confirmed by detailed simulations that enable to visualize the different ions distributions and dynamics.

physics.app-ph↗

arXiv2024

Switching response and ionic hysteresis in organic electrochemical transistors

Hysteresis in organic electrochemical transistors (OECT) is a basic effect in which the measured current depends on the voltage sweep direction and velocity. This phenomenon has an important impact on different aspects of the application of OECT, such as the switching time and the synaptic properties for neuromorphic applications. Here we address the combined ionic and electronic kinetic effects that cause the dominant hysteresis effects. We use a combination of tools consisting on basic analytical models, advanced 2D drift-diffusion simulation, and the experimental measurement of a Poly(3-hexylthiophene) (P3HT) OECT, working in an accumulation mode. We develop a general transmission line model considering drift electronic transport and ionic injection and diffusion from the electrolyte. We provide a basic classification of the transient response to a voltage pulse, according to the dominant ionic or electronic relaxation time, and the correspondent hysteresis effects of the transfer curves according to the general categories of inductive and capacitive hysteresis. These are basically related to the main control phenomenon, either the vertical diffusion of ions during doping and dedoping, or the equilibration of electronic current along the channel length.

physics.app-ph↗

arXiv2023

Zn-doped P-type InAs Nanocrystal Quantum Dots

Doped heavy metal-free III-V semiconductor nanocrystal quantum dots are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals, that are of particular relevance for short wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. We present p-type doping of InAs nanocrystals with Zn-enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping was achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn2+ replacing In3+ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near IR photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short wave IR region utilizing heavy-metal free nanocrystal building blocks.

cond-mat.mes-hall↗

arXiv2021

InAs nanocrystals with robust p-type doping

Robust control over the carrier type is fundamental for the fabrication of nanocrystal-based optoelectronic devices, such as the p-n homojunction, but effective incorporation of impurities in semiconductor nanocrystals and its characterization is highly challenging due to their small size. Herein, InAs nanocrystals, post-synthetically doped with Cd, serve as a model system for successful p-type doping of originally n-type InAs nanocrystals, as demonstrated in field-effect transistors (FETs). Advanced structural analysis, using atomic resolution electron microscopy and synchrotron X-ray absorption fine structure spectroscopy reveal that Cd impurities reside near and on the nanocrystal surface acting as substitutional p-dopants replacing Indium. Commensurately, Cd-doped InAs FETs exhibited remarkable stability of their hole conduction, mobility, and hysteretic behavior over time when exposed to air, while intrinsic InAs NCs FETs were easily oxidized and their performance quickly declined. Therefore, Cd plays a dual role acting as a p-type dopant, and also protects the nanocrystals from oxidation, as evidenced directly by Xray photoelectron spectroscopy measurements of air-exposed samples of intrinsic and Cd doped InAs NCs films. This study demonstrates robust p-type doping of InAs nanocrystals, setting the stage for implementation of such doped nanocrystal systems in printed electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Surface versus Impurity Doping Contributions in InAs Nanocrystals Field Effect Transistor Performance

The electrical functionality of an array of semiconductor nanocrystals depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs nanocrystals thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped nanocrystal based field-effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NCs based FETs. As-synthesized InAs nanocrystal-based FETs show n-type characteristics assigned to the contribution of surface electrons accumulation layer that can be considered as an actual electron donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NCs FETs show enhanced n-type conduction as expected from the Cu impurities location as an interstitial n-dopant in InAs nanocrystals. The simulated curves reveal the additional contribution from electrons within an impurity sub-band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC-based FETs. It provides guidelines for control of doping of nanocrystal arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs towards their future utilization as building blocks in bottom-up prepared optoelectronic devices.

cond-mat.mes-hall↗

arXiv2021

Evidence for ultra long range energy transfer in organic photovoltaic donor-acceptor three dimensional films

We report an ultra long range energy transfer in a layered donor:spacer:acceptor structures, which consist of the typical organic photovoltaic material system of P3HT as the donor and PCBM as the acceptor. By varying the thicknesses of spacer and acceptor layers we show that the energy transfer is to the full volume of the acceptor and not just to its nearest interface, and we find an effective energy transfer range on the order of about 100nm. Our efforts to elucidate the origin of this process, both theoretically and experimentally, are discussed as well. Although it was recently implied that an exceptionally large energy transfer may take place in this material system, this is the first time, to the best of our knowledge, that the energy transfer mechanism is characterized in a quantitative way and compared to the common models. Our analysis offers new prospects for the familiar photovoltaic bi-layer configurations, which may be very efficient if the ultra long range energy transfer is utilized through a suitable architecture.

physics.app-ph↗

arXiv2020

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by Embedding Molecular Dipoles within their Hole-Blocking Layer

Engineering the energetics of perovskite photovoltaic devices through the deliberate introduction of dipoles to control the built-in potential of the devices offers the opportunity to enhance their performance without the need to modify the active layer itself. In this work, we demonstrate how the incorporation of molecular dipoles into the bathocuproine (BCP) hole-blocking layer of inverted perovskite solar cells improves the device open-circuit voltage (VOC) and consequently, its performance. We explore a series of four thiaazulenic derivatives that exhibit increasing dipole moments and demonstrate that these molecules can be introduced into the solution-processed BCP layer to effectively increase the built-in potential within the device, without altering any of the other device layers. As a result the VOC of the devices is enhanced by up to 130 mV with larger dipoles resulting in higher VOCs. To investigate the limitations of this approach, we employ numerical device simulations that demonstrate that the highest dipole derivatives used in this work eliminate all limitations on the VOC stemming from the built-in potential of the device.

physics.app-ph↗

arXiv2020

Insights from Device Modeling of Perovskite Solar Cells

In this perspective, we explore the insights into the device physics of perovskite solar cells gained from modeling and simulation of these devices. We discuss a range of factors that influence the modeling of perovskite solar cells, including the role of ions, dielectric constant, density of states, and spatial distribution of recombination losses. By focusing on the effect of non-ideal energetic alignment in perovskite photovoltaic devices, we demonstrate a unique feature in low recombination perovskite materials - the formation of an interfacial, primarily electronic, self-induced dipole that results in a significant increase in the built-in potential and device open-circuit voltage. Finally, we discuss the future directions of device modeling in the field of perovskite photovoltaics, describing some of the outstanding open questions in which device simulations can serve as a particularly powerful tool for future advancements in the field.

physics.app-ph↗

arXiv2020

Dipolar Hole-Blocking Layers for Inverted Perovskite Solar Cells: Effects of Aggregation and Electron Transport Levels

Herein, we report on the synthesis and investigation of two triazino-isoquinoline tetrafluoroborate electrolytes as hole-blocking layers in methylammonium triiodide perovskite photovoltaic devices with fullerene electron extraction layer. We find that increasing the thickness of the dipolar hole-blocking layer results in a gradual increase in the open-circuit voltage suggesting that aggregation of the molecules can enhance the dipole induced by the layer. This finding is confirmed by theoretical calculations demonstrating that while both molecules exhibit a similar dipole moment in their isolated state, this dipole is significantly enhanced when they aggregate. Ultra-violet photoemission spectroscopy measurements show that both derivatives exhibit a high ionisation potential of 7 eV, in agreement with their effective hole-blocking nature demonstrated by the devices. However, each of the molecules shows a different electron affinity due to the increased conjugation of one of the derivatives. While the change in electron transport level between the two derivatives is as high as 0.3 eV, the difference in the open-circuit voltage of both types of devices is negligible, suggesting that the electron transport level plays only a minor role in determining the open-circuit voltage of the device. Numerical device simulations confirm that the increase in built-in potential, arising from the high dipole of the electrolyte layer, compensates for the non-ideal energetic alignment of the charge transport levels, resulting in high VOC for a range of electron transport levels. Our study demonstrates that the application of small molecule electrolytes as hole-blocking layer in inverted architecture perovskite solar cells is a powerful tool to enhance the open-circuit voltage of the devices and provides useful guidelines for designing future generations of such compounds.

physics.app-ph↗

arXiv2019

Nanoparticulate Metal Oxide Top Electrode Interface Modification Improves the Thermal Stability of Inverted Perovskite Photovoltaics

Solution processed γ-Fe2O3 nanoparticles via the solvothermal colloidal synthesis in conjunction with ligand-exchange method are used for interface modification of the top electrode in inverted perovskite solar cells. In comparison to more conventional top electrodes such as PC(70)BM/Al and PC(70)BM/AZO/Al, we show that incorporation of a γ-Fe2O3 provides an alternative solution processed top electrode (PC(70)BM/γ-Fe2O3/Al) that not only results in comparable power conversion efficiencies but also improved thermal stability of inverted perovskite photovoltaics. The origin of improved stability of inverted perovskite solar cells incorporating PC(70)BM/ γ-Fe2O3/Al under accelerated heat lifetime conditions is attributed to the acidic surface nature of γ-Fe2O3 and reduced charge trapped density within PC(70)BM/ γ-Fe2O3/Al top electrode interfaces.

cond-mat.mtrl-sci↗

arXiv2019

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by up to 120 mV using π-Extended Phosphoniumfluorene Electrolytes as Hole Blocking Layers

Four π-extended phosphoniumfluorene electrolytes (π-PFEs) are introduced as hole-blocking layers (HBL) in inverted architecture planar perovskite solar cells (PVSCs) with the structure of ITO/PEDOT:PSS/MAPbI3/PCBM/HBL/Ag. The deep-lying highest occupied molecular orbital (HOMO) energy level of the π-PFEs effectively blocks holes, decreasing contact recombination. We demonstrate that the incorporation of π-PFEs introduces a dipole moment at the PCBM/Ag interface, resulting in a significant enhancement of the built-in potential of the device. This enhancement results in an increase in the open-circuit voltage of the device by up to 120 mV, when compared to the commonly used bathocuproine HBL. The results are confirmed both experimentally and by numerical simulation. Our work demonstrates that interfacial engineering of the transport layer/contact interface by small molecule electrolytes is a promising route to suppress non-radiative recombination in perovskite devices and compensate for a non-ideal energetic alignment at the hole-transport layer/perovskite interface.

physics.app-ph↗

arXiv2012

Mobility and energy-transport in degenerate hopping systems

Revisiting charge transport in degenerate hopping systems we present a modification to the drift diffusion equation where instead of employing the generalized Einstein relation we add an energy flux term thus solving several inconsistencies. This leads also to the conclusion that, contrary to common belief, a constant quasi-Fermi level does not necessarily ensure zero electrical current. While we revisit the drift diffusion process in the context of degenerate hopping systems, a considerable part of the argumentation put forward can be applied generally to degenerate systems.

cond-mat.mes-hall↗

arXiv2004

Generalized Einstein relation for disordered semiconductors - implications for device performance

The ratio between mobility and diffusion parameters is derived for a Gaussian-like density of states. This steady-state analysis is expected to be applicable to a wide range of organic materials (polymers or small molecules) as it relies on the existence of quasi-equilibrium only. Our analysis shows that there is an inherent dependence of the transport in trap-free disordered organic-materials on the charge density. The implications for the contact phenomena and exciton generation rate in light emitting diodes as well as channel-width in field-effect transistors is discussed.

cond-mat.stat-mech↗

arXiv2004

Charge Transport in Non-Diluted Conjugated Polymers

We present a unified calculation method for variable range hopping transport with a varying charge concentration and a varying applied electrical field. We demonstrate that the major differences between the transport properties measured at high concentration and low concentration can be explained within this framework. In particular the difference between the measured mobility, and mobility activation energy in polymer field effect transistors and polymer light emitting diodes is explained. A theoretical method to extract the charge carrier density of state from the transport measurements of non-diluted materials is proposed.

cond-mat.stat-mech↗

arXiv2002

Self-consistent analisys of the contact phenomena in low-mobility semiconductors

Self-consistent solution of charge injection and transport in low mobility LEDs is reported . We show that explicit description of the contact region under the same premise as transport equation is needed to accurately evaluate the current-voltage characteristics of polymer or small-molecule based LEDs . The result is compared to widely used models, which treat the contact region in implicit manner .

cond-mat.soft↗