226 research outputs found
Charge Carrier Mobility in Organic Mixed Ionic–Electronic Conductors by the Electrolyte-Gated van der Pauw Method
Organic mixed ionic–electronic conductors (OMIECs) combine electronic semiconductor functionality with ionic conductivity, biocompatibility, and electrochemical stability in water and are currently investigated as the active material in devices for bioelectronics, neuromorphic computing, as well as energy conversion and storage. Operation speed of such devices depends on fast electronic transport in OMIECs. However, due to contact resistance problems, reliable measurements of electronic mobility are difficult to achieve in this class of materials. To address the problem, the electrolyte-gated van der Pauw (EgVDP) method is introduced for the simple and accurate determination of the electrical characteristics of OMIEC thin films, independent of contact effects. The technique is applied to the most widespread OMIEC blend, poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS). By comparing with organic electrochemical transistor (OECT) measurements, it is found that gate voltage dependent contact resistance effects lead to systematic errors in OECT based transport characterization. These observations confirm that a contact-independent technique is crucial for the proper characterization of OMIECs, and the EgVDP method reveals to be a simple, elegant, but effective technique for this scope
Fully Textile X-Ray Detectors Based on Fabric-Embedded Perovskite Crystals
The interest and thrust for wearable ionizing radiation dosimeters are rapidly growing, stimulated by a large number of different applications impacting on humankind, spanning from medicine to civil security and space missions. Lead halide perovskites are considered one of the most promising classes of novel materials for X-ray detectors due to their superior electronic and detection performance coupled with compatibility with solution-based printing processes, allowing fabrication onto flexible substrates. It is reported on fully textile perovskite-based direct X-ray detectors, where the photoactive layer is constituted by a silk-satin fabric functionalized with methylammonium lead bromide perovskite crystals embedded in the textile. The reliability of the proposed fabrication process, based on simple and low-tech deposition techniques adaptable to industrial printing technologies for textiles, is assessed by realizing different detector's architectures that exhibit comparable detection performances. Sensitivity values up to (12.2 +/- 0.6) mu C Gy(-1) cm(-2) and a limit of detection down to 3 mu Gy s(-1) are achieved, and low bias operation (down to 1 V) is demonstrated, validating wearable applications. Further, fully textile pixelated matrix X-ray sensors are implemented and tested, providing the proof of principle for large-area scalability
In Situ Force Microscopy to Investigate Fracture in Stretchable Electronics: Insights on Local Surface Mechanics and Conductivity
Stretchable conductors are of crucial relevance for emerging technologies
such as wearable electronics, low-invasive bioelectronic implants, or soft actuators for
robotics. A critical issue for their development regards the understanding of defect
formation and fracture of conducting pathways during stress−strain cycles. Here we present
a combination of atomic force microscopy (AFM) methods that provides multichannel
images of surface morphology, conductivity, and elastic modulus during sample
deformation. To develop the method, we investigate in detail the mechanical interactions
between the AFM tip and a stretched, free-standing thin film sample. Our findings reveal
the conditions to avoid artifacts related to sample bending modes or resonant excitations.
As an example, we analyze strain effects in thin gold films deposited on a soft silicone
substrate. Our technique allows one to observe the details of microcrack opening during
tensile strain and their impact on local current transport and surface mechanics. We find
that although the film fractures into separate fragments, at higher strain a current transport
is sustained by a tunneling mechanism. The microscopic observation of local defect formation and their correlation to local
conductivity will provide insight into the design of more robust and fatigue resistant stretchable conductors
Direct X-ray photoconversion in flexible organic thin film devices operated below 1 v
The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 10 4 and 77,000 nC mGy 1 cm 3, respectively
AC amplification gain in organic electrochemical transistors for impedance-based single cell sensors
Research on electrolyte-gated and organic electrochemical transistor (OECT) architectures is motivated by the prospect of a highly biocompatible interface capable of amplifying bioelectronic signals at the site of detection. Despite many demonstrations in these directions, a quantitative model for OECTs as impedance biosensors is still lacking. We overcome this issue by introducing a model experiment where we simulate the detection of a single cell by the impedance sensing of a dielectric microparticle. The highly reproducible experiment allows us to study the impact of transistor geometry and operation conditions on device sensitivity. With the data we rationalize a mathematical model that provides clear guidelines for the optimization of OECTs as single cell sensors, and we verify the quantitative predictions in an in-vitro experiment. In the optimized geometry, the OECT-based impedance sensor allows to record single cell adhesion and detachment transients, showing a maximum gain of 20.2±0.9 dB with respect to a single electrode-based impedance sensor
Radiation Hardness and Defects Activity in PEA2PbBr4 Single Crystals
Metal halide perovskites (MHPs) are low-temperature processable hybrid
semiconductor materials with exceptional performances that are revolutionizing
the field of optoelectronic devices. Despite their great potential, commercial
deployment is hindered by MHPs lack of stability and durability, mainly
attributed to ions migration and chemical interactions with the device
electrodes. To address these issues, 2D layered MHPs have been investigated as
possible device interlayers or active material substitutes to reduce ion
migration and improve stability. Here we consider the 2D perovskite PEA2PbBr4
that was recently discussed as very promising candidate for X-ray direct
detection. While the increased resilience of PEA2PbBr4 detectors have already
been reported, the physical mechanisms responsible for such improvement
compared to the standard "3D" perovskites are not still fully understood. To
unravel the charge transport process in PEA2PbBr4 crystals thought to underly
the device better performance, we adapted an investigation technique previously
used on highly resistive inorganic semiconductors, called photo induced current
transient spectroscopy (PICTS). We demonstrate that PICTS can detect three
distinct trap states (T1, T2, and T3) with different activation energies, and
that the trap states evolution upon X-ray exposure can explain PEA2PbBr4
superior radiation tolerance and reduced aging effects. Overall, our results
provide essential insights into the stability and electrical characteristics of
2D perovskites and their potential application as reliable and direct X-ray
detectors
Direct X-ray photoconversion in flexible organic thin film devices operated below 1 v
The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 10^4 and 77,000 nC mGy ^(-1) cm^(-3), respectively
Selective detection of dopamine with an all PEDOT:PSS Organic Electrochemical Transistor
open6noAn all PEDOT:PSS Organic Electrochemical Transistor (OECT) has been developed and used for the selective detection of dopamine (DA) in the presence of interfering compounds (ascorbic acid, AA and uric acid, UA). The selective response has been implemented using a potentiodynamic approach, by varying the operating gate bias voltage and the scan rate.
The trans-conductance curves allow to obtain a linear calibration plot for AA, UA and DA and to separate the redox waves associated to each compound; for this purpose, the scan rate is an important parameter to achieve a good resolution. The sensitivities and limits of detection obtained with the OECT have been compared with those obtained by potential step amperometric techniques (cyclic voltammetry and differential pulse voltammetry), employing a PEDOT:PSS working electrode: our results prove that the all-PEDOT:PSS OECT sensitivities and limits of detection are comparable or even better than those obtained by DPV, a technique that employs a sophisticate potential wave and read-out system in order to maximize the performance of electrochemical sensors and that can hardly be considered a viable readout method in practical applications.openGualandi, Isacco; Tonelli, Domenica; Mariani, Federica; Scavetta, Erika; Marzocchi, Marco; Fraboni, BeatriceGualandi, Isacco; Tonelli, Domenica; Mariani, Federica; Scavetta, Erika; Marzocchi, Marco; Fraboni, Beatric
Accurate determination of band tail properties in amorphous semiconductor thin film with Kelvin Probe Force Microscopy
Amorphous oxide semiconductors are receiving significant attention due to
their relevance for large area electronics. Their disordered microscopic
structure causes the formation of band tails in the density of states (DOS)
that strongly affect charge transport properties. Bandtail properties are
crucial to understand for optimizing thin film device performance. Among the
available techniques to measure the DOS, KPFM is exceptional as it enables
precise local electronic investigations combined with microscopic imaging.
However, a model to interpret KPFM spectroscopy data on amorphous
semiconductors of finite thickness is lacking. To address this issue, we
provide an analytical solution to the Poisson's equation for a
metal-insulator-semiconductor (MIS) junction interacting with the AFM tip. The
solution enables us to fit experimental data for semiconductors with finite
thickness and obtain the DOS parameters, such as band tail width (E_t), doping
density (N_D), and flat band potential. To demonstrate our method, we perform
KPFM experiments on Indium-Gallium-Zinc Oxide (IGZO) thin film transistors
(IGZO-TFTs). DOS parameters compare well to values obtained with photocurrent
spectroscopy. We demonstrate the potentials of the developed method by
investigating the impact of ionizing radiation on DOS parameters and TFT
performance. Our results provide clear evidence that the observed shift in
threshold voltage is caused by static charge in the gate dielectric leading to
a shift in flat band potential. Band-tails and doping density are not affected
by the radiation. The developed methodology can be easily translated to
different semiconductor materials and paves the way towards quantitative
microscopic mapping of local DOS parameters in thin film devices
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