We report on a ternary blend of SEBS, the donor polymer poly(3-hexylthiophene-2,5-diyl), and the small-molecule acceptor Indene-C60 bisadduct that yields an elastomeric bulk-heterojunction (e-BHJ) with skin-like mechanical properties. This is, with low Young’s moduli < 10 MPa and a high strain at break ca. 190%. We also demonstrate that stretchable e-BHJ enables stretchable organic photodiodes with measured median root-mean-squared electronic noise values in the tens of femtoampere range and measured noise equivalent power values at 653 nm between 13 and 24 pW at strain values up to 60%, yielding specific detectivity values in the 100 Giga-Jones range.
In this talk, we will discuss a detailed study of the performance of organic photodiodes (OPDs) based on polymeric bulk heterojunctions. We will show that their performance is comparable with that of low-noise SiPDs in all metrics, except response time within the visible spectral range. Furthermore, OPDs present significant advantages over their inorganic counterparts since they can be fabricated on flexible substrates and their level of performance remains unprecedented even when their area is increased. Advantages of OPDs are further illustrated and quantified in a biometric monitoring application that uses ring-shaped, large-area, flexible OPDs, while maintaining low-noise SiPD-level performance. We will discuss how this remarkable performance arises from the selection of photoactive layer materials and by device-geometry optimization without charge-blocking layers.
In this talk, we will present organic photodiodes (OPDs) based on polymeric bulk heterojunctions with a level of performance that within the visible spectral range, rivals that of low-noise SiPDs in all metrics, except response time. Large-area OPDs on rigid and flexible substrates retain an unprecedented level of performance. Advantages of OPDs are further illustrated and quantified in a biometric monitoring application that uses ring-shaped, large-area, flexible OPDs, while maintaining low-noise SiPD-level performance. We will discuss how this remarkable performance arises from the selection of photoactive layer materials and by device-geometry optimization without charge-blocking layers.
We developed a simple method to improve the stability of organic field-effect transistors (OFETs) with bilayer gate dielectrics. The bilayer gate dielectric comprises an amorphous fluoropolymer (CYTOP) layer and an Al2O3-HfO2 nanolaminate (NL) grown by the atomic layer deposition (ALD) technique. In the OFETs with bilayer gate dielectrics, two aging mechanisms exist, and they cause the shifts of threshold voltage in opposite directions during long-term operation. By engineering the bilayer gate dielectric, the effects of these two mechanisms can compensate, leading to devices with remarkable operational stability that is comparable or superior to that of commercial inorganic counterparts. The NL grown by ALD shows excellent encapsulation property and improves the environmental stability of the OFETs. The devices are tested by exposing the devices to high temperature and high moisture conditions (i.e., the standard 85/85 condition, meaning 85°C and 85% relative humidity). The results of OFETs with CYTOP/NL bilayer gate dielectrics are presented and compared to those OFETs with Al2O3 gate dielectrics.
The printed electronics industry offers a paradigm change in manufacturing, cost and environmental impact when compared to the conventional semiconductor industry. Printed electronic devices are expected to be mass-produced from less-energy-demanding processes over large areas and on flexible substrates with techniques that closely resemble the well-known mass production of printed media on paper. Organic light-emitting diodes (OLEDs) offer great versatility in the design of application-specific light sources; yet examples of OLEDs fabricated on exotic substrates that differ from glass and plastic are still scarce. In this talk we will present recent examples of OLEDs fabricated on unconventional substrates, including nanocellulose and shape memory polymers.
Organic photovoltaics (OPV) can lead to a low cost and short energy payback time alternative to existing photovoltaic technologies. However, to fulfill this promise, power conversion efficiencies must be improved and simultaneously the architecture of the devices and their processing steps need to be further simplified. In the most efficient devices to date, the functions of photocurrent generation, and hole/electron collection are achieved in different layers adding complexity to the device fabrication. In this talk, we present a novel approach that yields devices in which all these functions are combined in a single layer.
Specifically, we report on bulk heterojunction devices in which amine-containing polymers are first mixed in the solution together with the donor and acceptor materials that form the active layer. A single-layer coating yields a self-forming bottom electron-collection layer comprised of the amine-containing polymer (e.g. PEIE). Hole-collection is achieved by subsequent immersion of this single layer in a solution of a polyoxometalate (e.g. phosphomolybdic acid (PMA)) leading to an electrically p-doped region formed by the diffusion of the dopant molecules into the bulk. The depth of this doped region can be controlled with values up to tens of nm by varying the immersion time. Devices with a single 500 nm-thick active layer of P3HT:ICBA processed using this method yield power conversion efficiency (PCE) values of 4.8 ± 0.3% at 1 sun and demonstrate a performance level superior to that of benchmark three-layer devices with separate layers of PEIE/P3HT:ICBA/MoOx (4.1 ± 0.4%). Devices remain stable after shelf lifetime experiments carried-out at 60 °C over 280 h.
In this talk, we will discuss recent advances in the science and engineering of organic light-emitting diodes (OLEDs). First, we will focus on materials in which light emission involves the process of thermally activated delayed fluorescence (TADF). In these materials, triplet excited states can convert into optically emissive singlet excited states by reverse intersystem crossing, allowing for nearly 100% internal quantum efficiency. This process can be used to design a new class of materials that are all organic, offering a lower cost alternative to conventional electrophosphorescent materials that contain heavy and expensive elements such as Pt and Ir. We will discuss molecular design strategies and present examples of materials that can be used as emitters or hosts in the emissive layer.
In a second part of this talk, we will review recent progress in fabricating OLEDs on shape memory polymer substrates (SMPs). SMPs are mechanically active, smart materials that can exhibit a significant drop in modulus once an external stimulus such as temperature is applied. In their rubbery state upon heating, the SMP can be easily deformed by external stresses into a temporary geometric configuration that can be retained even after the stress is removed by cooling the SMP to below the glass transition temperature. Reheating the SMP causes strain relaxation within the polymer network and induces recovery of its original shape. We will discuss how these unique mechanical properties can also be extended to a new class of OLEDs.
Although the detection of photons is ubiquitous, man-made photon detectors still limits the effectiveness of applications such as light/laser detection, photography, astronomy, quantum information science, medical imaging, microscopy, communications, and others. The performance of the technologically most advanced detectors based on CMOS semiconductor technology has improved during the last decades but at the detriment of increased complexity, higher cost, limited portability and compactness, and limited area. On the other hand, nature has produced a relatively simple detector with remarkable properties: the human eye. The exploration of new paradigms in photon detection using new material platforms might therefore provide a path to further challenge the frontiers of applications enabled by light.
In this talk, we will report on the realization of solution-processed organic semiconductor visible spectrum photodetectors with a high specific detectivity above 1014 Jones, at least an order of magnitude larger than values found in photodiodes based on silicon. These detectors demonstrate a sub-pA current under reverse bias in the dark, making them suitable for detecting very low levels of light. The small dark current under reverse bias allows the characterization of these devices over 9 orders of magnitude of increasing light irradiance. The detectors are based on the device structure: tin-doped indium oxide / ethoxylated polyethylenimine / poly(3-hexylthiophene) : indene C60 bisadduct / molybdenum oxide / silver and present a path toward fabrication on flexible substrates. We will show that these detectors can operate over a large dynamic range in the self-powered photovoltaic mode where the light produces a photovoltage that can be measured directly without any external bias source. We believe that large-area flexible photodetectors with detectivity values comparable to or better than those displayed by silicon-based photodiodes will enable a wide variety of applications from the detection of radiation to non-planar imaging arrays.
Organic field-effect transistors (OFETs) have the potential to lead to low-cost flexible displays, wearable electronics, and sensors. While recent efforts have focused greatly on improving the maximum charge mobility that can be achieved in such devices, studies about the stability and reliability of such high performance devices are relatively scarce. In this talk, we will discuss the results of recent studies aimed at improving the stability of OFETs under operation and their shelf lifetime. In particular, we will focus on device architectures where the gate dielectric is engineered to act simultaneously as an environmental barrier layer.
In the past, our group had demonstrated solution-processed top-gate OFETs using TIPS-pentacene and PTAA blends as a semiconductor layer with a bilayer gate dielectric layer of CYTOP/Al2O3, where the oxide layer was fabricated by atomic layer deposition, ALD. Such devices displayed high operational stability with little degradation after 20,000 on/off scan cycles or continuous operation (24 h), and high environmental stability when kept in air for more than 2 years, with unchanged carrier mobility. Using this stable device geometry, simple circuits and sensors operating in aqueous conditions were demonstrated. However, the Al2O3 layer was found to degrade due to corrosion under prolonged exposure in aqueous solutions. In this talk, we will report on the use of a nanolaminate (NL) composed of Al2O3 and HfO2 by ALD to replace the Al2O3 single layer in the bilayer gate dielectric use in top-gate OFETs. Such OFETs were found to operate under harsh condition such as immersion in water at 95 °C.
This work was funded by the Department of Energy (DOE) through the Bay Area Photovoltaics Consortium (BAPVC) under Award Number DE-EE0004946.
We demonstrate top-gate organic field-effect transistors (OFETs) with a bilayer gate dielectric and doped contacts fabricated on shape-memory polymer (SMP) substrates. SMPs exhibit large variations in Young’s modulus dependent on temperature and have the ability to fix two or more geometric configurations when a proper stimulus is applied. These unique properties make SMPs desirable for three-dimensional shape applications of OFETs. The electrical properties of OFETs on SMP substrates are presented and compared to those of OFETs on traditional glass substrates.
In this talk, we will discuss recent advances in green and white electrophosphorescent stacked organic light-emitting diodes (OLEDs) with inverted top-emitting structures. These devices combine the advantages of having inverted electrode positions, a top-emissive design, and a stacked architecture. We will also demonstrate OLEDs that are fabricated on cellulose nanocrystal substrates and discuss how the use of such naturally-derived materials can reduce the environmental footprint of organic electronic devices such as OLEDs.
We discuss a non-vacuum low-cost reverse stamping method for the realization of circuits based on top-gate organic field-effect transistors (OFETs) with a bi-layer gate dielectric. This method allows for patterning of high-k inorganic dielectric films produced by atomic layer deposition and consequently of the bilayer gate dielectric layers used in our top-gate OFETs. We demonstrate the fabrication and operation of logic inverters and ring oscillators following this approach.
We report on the nonlinear optical (NLO) transmittance and reflectance of a 20 nm-thick Ag film characterized by time-resolved
white-light continuum pump-probe experiments. The change in complex permittivity Δε(t) is extracted and is
fitted to the Drude model in the frequency domain and a two-temperature model in the time domain. A unified model is
presented that fully describes the dynamic NLO response of a thin Ag film that can be incorporated easily into the
modeling of more complex metal-dielectric multilayer structures designed to take advantage of the NLO response of Ag.
We report on the linear and nonlinear optical properties of Ag/Au multi-metal thin films and Fabry-Perot
resonator cavities. The linear optical properties of these multi-metal layers, having different mass distributions
and Ag/Au ratios with thicknesses around 15 nm, resemble those of electrically continuous metal layers. The
optical losses introduced by interband transitions in the Au layers are reduced to achieve peak transmittances
of 76 % around 550 nm. Using femtosecond-pulsed white-light continuum pump-probe experiments we show
that the nonlinear optical response of such multi-metal layers is comparable to that of neat Au thin films.
Low-finesse Fabry-Perot resonators fabricated with such multi-metal layers, combine the large NLO response
of Au with a transmittance of 60% and a spectral bandwidth that covers the visible spectral range.
We present the design, fabrication and characterization of the optical properties of one-dimensional metal-organic
photonic bandgaps (MO-PBGs) composed of a tetraphenyldiaminobiphenyl-based polymer and ultrathin electrically
continuous Cu layers. The fabricated MO-PBGs achieve a peak transmission of around 44% at 620 nm combined with
very large spectral, around 120 nm FWHM, and angular, more than 120° field-of-view, bandwidths. Using 140 fs pulses
at various wavelengths we have found up to 10 × enhancements in the nonlinear optical (NLO) properties of the MO-PBGs
when compared with the NLO response of ultrathin electrically continuous Cu layers.
We report on the design and fabrication of metal-dielectric photonic band-gap structures (MDPBGs) with high
transmission and broad spectral bandwidth in the visible range. Using the complex refractive index measured by
spectroscopic ellipsometry, we have designed structures with an aperiodic thickness distribution which show a flat
passband transmission (64% ± 1.5%) over more than 150 nm within the visible spectrum. Using e-beam deposition, we
demonstrate the growth of continuous 12 nm Ag layers on Al2O3, and MDPBGs with five periods of Al2O3/Ag/Al2O3
that show more than 49% ± 2.5% transmission and at least 150 nm bandwidths. When compared to dielectric-dielectric
stacks, the use of metallic layers provides excellent out-of-band rejection, in particular beyond the IR edge where the
MDPBG acts collectively as a metallic solid. Such structures could thus be used as hot mirrors with extremely high and
broad out-of-band rejection.
We report on the trapping mechanisms in bis-triarylamine (PATPD) based polymer composites. Although exceptional stability under continuous operation has been reported in PATPD-based composites, a small degradation of the response time in photorefractive devices under continuous operation has been found when improved styrene-based
chromophores, with high figure-of-merit, are used. The accumulation of relatively large densities (~1017 cm-3)
of filled traps is observed even though to first approximation the transport manifold has the lowest ionization potential of all the moieties in the composite, so no apparent deep trapping sites are to be present. The results of spectroscopic studies where the formation of chromophore aggregates is explored and correlated with the formation of hole-trapping sites that dominate the temporal evolution of the photogenerated current density and C60 anion accumulation after several minutes of continuous operation will be presented and compared with numerical simulations considering a two-trapping site model in materials containing the chromophore DBDC.
We report on the photorefractive properties of two polymer composites that utilize a new bis-triarylamine side-chain polymer matrix. Correctly locating the frontier orbitals of the new transport manifold with respect to the HOMO levels of chromophores, allows stable continuous operation over exposure levels of more that 4 kJ/cm2 when samples are electrically biased at 57 V/μm. This operational stability is combined with video-rate compatible grating build-up times and a dynamic range that allows index modulations of 3 x 10-3 and gain coefficients on the order of 100 cm-1 at moderate fields. The thermal stability of one of the composites reported is excellent, showing no signs of phase separation even after one week at 60°C. A comparison with the stability of composites where the new matrix was replaced by PVK is also presented.
We report on an efficient and fast hybrid photorefractive polymer sensitized with CdSe quantum dots. The surface of the quantum dots was treated with 4-methylbenzenethiol. This surfactant allows the quantum dots to have an efficient photoinduced charge generation when mixed with a mixture of chromophores. The enhanced photoconductive properties lead to fast grating build-up times of 100 ms and below. In four-wave mixing experiments, overmodulation of the diffraction efficiency was observed at an applied field of 60 V/μm and gain coefficients on the order of 20 cm-1 at moderate fields.
Hole mobilities in substituted N, N'-bis-(m-tolyl)-N-N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPD) derivatives doped in polystyrene (PS), were analyzed by the time-of-flight technique to determine the effect of altering the geometric and electronic structure of TPD. Data were collected as a function of applied field and temperature to yield the energetic and positional disorder parameters defined in the disorder formalism. The impact of the molecular dipole moment on transport properties was also evaluated. The larger molecular dipole moments of the derivatives lead to an increase in the energetic disorder, which contributes to their lower mobilities. However, the dipolar disorder contribution was found to account only partially for the large differences in mobility.
We present a study on the effects of the dark conductivity on the photorefractive performance of polymers doped with styrene-based chromophores. We find that reducing dark conductivity in such composites increases diffraction efficiency and at the same time decreases the response time. We use a polymer composite based on a polyvinylcarbazole matrix doped with 4-Homopiperidinobenzylidenemalononitrile (7- DCST), sensitized with C60, and plasticized with N- ethylcarbazole (ECZ) and butyl benzyl phthalate (BBP). The reduction of the dark conductivity is achieved by coating one of the electrodes with a SiO blocking layer.
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