State-of-the-art alternating semiconducting polymers, synthesized via established academic protocols, often contain homocoupling defects, causing the true structure to deviate from the anticipated perfectly alternating polymer backbone. These structural defects significantly hinder the reproducibility across different polymer batches, posing a challenge to the commercial viability of the organic semiconductor field, while simultaneously imposing performance limitations in different applications by creating defected chains, limiting the attainable molecular weight and increasing the dispersity. In this study, two synthesis methods – conventional Stille polymerization and a novel defect-free route – are employed to create the p-type accumulation mode OECT (organic electrochemical transistor) benchmark material pgBTTT. The effect of homocoupling, and its absence, is investigated by comparing the bulk properties of the two polymers and evaluating their respective OECT performances.
The true structure of alternating conjugated polymers – the state-of-the-art materials for a number of organic electronics technologies – often deviates from the idealized picture but this gets relatively limited attention. Here, we quantify the amount of homocoupling defects resulting from Stille polymerization and shed new light on the actual distribution of these structural defects in a prototype polymer material. Further, when compared to a homocoupling-free variant, these defects hinder fullerene intercalation, with a clear implication on charge-transfer absorption. This demonstrates that molecular defects may (strongly) impact polymer and blend properties and calls for increased attention for defect-free materials.
The brain can perform massively parallel information processing while consuming only ~1- 100 fJ per synaptic event. I will describe an electrochemical neuromorphic device that switches at low energy (~80 fJ), and displays a large number of distinct, non-volatile conductance states within a ~1 V operating range. The tunable resistance behaves very linearly, allowing blind updates in a neural network when operated with the proper access device. These devices also display outstanding endurance achieving over 109 switching events with very little degradation. I will describe our recent efforts at scaling and materials selection, allowing us to reach 20 ns write pulses and operation at high temperature (up to 120°C). In particular, we developed a fully lithographic process that allowed us to demonstrate sub-µm channel devices, opening the door to integration with Si driving circuitry. By carefully deuterating the electrolytes, we provide strong evidence that the secret to the high speed and low energy switching properties of these artificial synapses is the combination of electronic and protonic transport. Finally, we demonstrate that the working mechanism is quite general by fabricating and operating high-performance synapses based on MXenes. This generality is promising in terms of monolithic integration as MXenes can be chosen to be BEOL compatible with Si.
Semiconducting organic polymers are most often synthesized by linking an electron poor and an electron rich (hetero)aromatic building block via a transition metal catalyzed cross-coupling copolymerization. Researchers aiming at exploring applications and fundamental performance limits, for example for organic photovoltaics, organic photodetectors, and organic electrochemical transistors, often assume that the obtained material consists strictly of a perfect repetition of the depicted polymeric repeating unit, whereas this is likely not the case. In this contribution, we demonstrate a synthesis approach to obtain the depicted “perfect” structure of these types of polymers and the influence of material defects on the optoelectronic properties and device performance.
Carrier mobility in conjugated polymers continues to increase with recent reports of field-effect mobilities exceeding 10 cm2/V.s. Charge transport is intrinsically dependent on processes occurring across multiple lengthscales. In order to access order parameters at the molecular scale we use charge modulation spectroscopy combined with theory.1 This technique allows us to further differentiate field-induced and doping-induced charges and study how their delocalization depends on local structure and disorder. Furthermore, we study the mesoscale organization of polymers using new techniques in the transmission electron microscope. By combining 4D STEM2 and HRTEM we are able to study the microstructure across a range of length-scales in real space and reciprocal space. These techniques are used on homopolymers and donor-acceptor copolymers and allow to extract information about the microstructure that is typically not visible by inspection. Such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport in conjugated polymers.
The largest need in organic electronic devices is a universal method to produce micro- to nano-scale features for devices from semiconducting polymers cheaply, and at scale. We recently developed a solution method to optically pattern conjugated polymers with resolution that exceeds the linear Abbe diffraction limit. We examine the relationship between optical write intensity, write speed, and write wavelength on the resulting pattern fidelity. Finite element modeling reveals that nearly all patterning occurs as a result of local heating and superlinear resolution is a result of a highly non-linear dissolution rate for the polymer as a function of temperature. This result is general to any conjugated polymer. We used this new technique to fabricate P3HT/F4TCNQ nanowires We also demonstrate that a P3HT nanowires can be doped and de-doped from solution without changing the dimension of the wire.
Sequential solution doping is a processing technique that allows a conjugated polymer film to be doped from a solvent that will not dissolve the polymer. We present here a method to predict the film doping level in cm-3 from the solution concentration used to dope the film. We show using four polymers and three different and newly synthesized dopants that the doping level can me modeled using a simple Langmuir isotherm. In addition, analysis of the UV/vis spectra shows filling of the density of states. Polymers with a sharper band edge demonstrate much high conductivity for the same hole density. We analyze a series of DPP polymers and show how the polymer order changes as a function of the doping level. A second recent discovery is that the anion in sequentially doped films can be exchanged with another anion after doping. This means that the reactive molecule used to doped the polymer can be removed and replaced with a different ion that is not reactive. We present a multi-ion Langmuir isotherm model and show that the film doping level in mixed ion solutions can also be predicted.
Image sensing technology has a great impact on our daily life as well as the entire society, such as health, safety and security, communication systems, and entertainment. The conventional optical color sensors consist of side by side arranged optical filters for three basic colors (blue, green, and red). Hence, the efficiency of such optical color sensors is limited by only 33%. In this study, a vertically stacked color sensor is investigated with perovskite alloys, which has the potential to provide the efficiency approaching 100%. The proposed optical sensor will not be limited by color Moire error or color aliasing. Perovskite materials with suitable bandgaps are determined by applying the energy shifting model and the optical constants are used for further investigations. Quantum efficiencies and spectral responsivities of the described color sensors are investigated by three-dimensional electromagnetic simulations. Investigated spectral sensitivities are further analyzed for the
Semiconducting polymers play an important role in a wide range of optical and electronic material applications. Polymer thin films that result in the highest performance typically have a complex semicrystalline morphology, indicating that considerable device improvement can be achieved through optimization of microstructural properties. However, the connection between molecular ordering and device performance is difficult to predict due to the current need for a mathematical theory of the physics that dictates charge transport in semiconducting polymers. It is experimentally suggested that efficient transport in such films occurs via connected networks of crystallites. We present an analytical and computational description of semicrystalline conjugated polymer materials that captures the impact of polymer conformation on charge transport in heterogeneous thin films. We first develop an analytical theory for the statistical behavior of a polymer emanating from a crystallite and predict the average distance to the first kink in the chain that traps a charge. We use this analysis to define the conditions for percolation and the consequent efficient transport through a semicrystalline material. We then establish a charge transport model using Monte Carlo simulations that predicts the multi-scale charge transport and crystallite connections. We approximate the thin film as a two-dimensional grid of crystallites embedded in amorphous polymer. The chain conformations in the amorphous region are determined by the wormlike chain model, and the crystallites are assigned fixed mobilities. We use this model to identify limits of charge transport at various time scales for varying fraction of crystallinity.
We investigate how post-deposition laser annealing can be used to improve structural and electronic quality of room-temperature deposited CdTe. We use continuous-wave, 1064 nm laser light to anneal CdTe solar cell stacks prior to back contact deposition. Sub-bandgap optical absorption measurements by photothermal deflection spectroscopy show a reduction of sub-bandgap defects due to the annealing process. Since the 1064 nm light is only partially absorbed, in situ monitoring of the transmitted light during laser annealing gives real-time information about changes in the material. These results reveal an evolution of electronic defect annealing and surface roughness modification with laser exposure time. This hypothesis is supported by electron microscopy. Two distinct annealing regimes emerge: one at low laser power where electronic defect annealing saturates after about one minute exposure and another at high power where structural defects are annealed after several minutes exposure. Temperatures reached during laser annealing are estimated by finite element modeling of the thermal transport due to heat generation from optical absorption.
When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.
We have fabricated high-mobility ambipolar polymer transistors and have integrated multiple transistors to demonstrate
their implementation into CMOS-like logic circuitry. The performance of a selenophene-based polymer semiconductor
PSeDPPBT is initially screened using standard long-channel field-effect transistors. The polymer exhibits high and
balanced hole and electron mobilities of ∼ 0.5 cm2/Vs and ∼ 1.0 cm2/Vs, respectively. Next, exploiting the beneficial
electronic properties of PSeDPPBT, we have fabricated ambipolar inverters, ring oscillators and logic NOR gates.
Ambipolar inverters are shown to exhibit voltage inversion with proper noise margins and no voltage loss over multiple
stages. The potential speed of ambipolar logic is demonstrated by the realization of ambipolar ring oscillators with
unprecedented performance. The feasibility to perform logic operations is demonstrated by the fabrication of ambipolar
NOR gates. The combined results, (i) no loss in voltage over multiple inverters, (ii) the unprecedented speed, and (iii) the
accomplishment of a functionally complete logic operation, demonstrate the feasibility of ambipolar logic as a reliable
substitute for complementary-based logic in order to realize cost-efficient electronics.
The use of ZnO nanowires has become a widespread topic of interest in optoelectronics. In order to correctly assess the
quality, functionality, and possible applications of such nanostructures it is important to accurately understand their
electrical and optical properties. Aluminum- and gallium-doped crystalline ZnO nanowires were synthesized using a
low-temperature solution-based process, achieving dopant densities of the order of 1020 cm-3. A non-contact optical
technique, photothermal deflection spectroscopy, is used to characterize ensembles of ZnO nanowires. By modeling the
free charge carrier absorption as a Drude metal, we are able to calculate the free carrier density and mobility.
Determining the location of the dopant atoms in the ZnO lattice is important to determine the doping mechanisms of the
ZnO nanowires. Solid-state NMR is used to distinguish between coordination environments of the dopant atoms.
The performance of polymer field-effect transistors is highly dependent on their processing history. For instance, thermal processing plays a role in micro-structure development and consequently in device performance. A transport model was developed based on the semiconductor micro-structure where highly mobile states are located in the crystalline areas and defects and disordered regions correspond to areas where carriers are trapped. By applying this model to electrical characterization data of PQT-12 (a regio-regular polythiophene), it is found that annealing tightens the energetic distribution of the traps. Films quenched from the melt performed worse than annealed films due to an increased trap density and broader energy distribution of the traps. X-ray diffraction in grazing and specular geometry was carried out at the Stanford Synchrotron Radiation Laboratory on PQT-12 thin films to reconcile the predictions of the transport model with the micro-structure of the PQT-12 thin films. In all cases the polymer crystallites are textured with the π-stacking direction in the plane of charge transport and the rocking curves indicate the existence of a population of highly oriented crystallites. Annealing the as-spun films improves the crystallinity and texture, in agreement with the transport model. Quenching produces defects in the films, which are likely to produce traps, thereby lowering the carrier mobility.
The interface between the semiconducting polymer and the gate dielectric is one of the most critical regions of a polymeric thin film transistor. For polymeric TFTs, it is difficult to disaggregate the contributions of the electronic structure of the semiconductor and that of the dielectric because, in part, the microstructure of thin films of semiconducting polymers is strongly affected by the chemical functionality at the surface of the dielectric. We have developed a lamination technique that can be used to transfer semiconducting films formed on surfaces that yield films
with high mobility to other dielectrics. We have studied films of semiconducting polymers, such as poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene] and poly(3-hexylthiophene) using this method. The effects of self-assembled monolayers (SAMs) formed on inorganic dielectrics on device performance are discussed. Our results suggest that mobility is mainly controlled by the structure of the semiconducting film and that the threshold voltage of TFTs may be modified through the use of SAMs.
A novel jet-printing approach to fabricate thin film transistor (TFT), active matrix backplanes for x-ray imagers is described. The technique eliminates the use of photolithography and has the potential to greatly reduce the array manufacturing cost. We show how jet-printing is used to pattern the layers of the active matrix array and also to deposit semiconductor material. The technique is applied to both amorphous silicon and polymer transistors, and small prototype arrays have been fabricated and tested, including arrays with a high fill factor amorphous silicon p-i-n photodiode layer for indirect detection x-ray imaging applications. The TFT characteristics are excellent, and acquired x-ray images will be presented and compared to those from conventional TFT arrays. The printing process has been extended to flexible substrates which are important for rugged x-ray imagers, using a low temperature amorphous silicon process to accommodate the plastic substrate. Polymer TFT arrays made with jet-printed polymer solutions have also been demonstrated and we present data from arrays, and discuss options for integrating organic photodiodes or direct detection sensors. The opportunities and challenges of using polymer semiconductors in x-ray imaging arrays, are discussed and we show that the TFT performance meets the needs of radiographic imaging, although the radiation hardness and long term degradation are not sufficiently studied.
The functional lifetime of large-aperture optical components used on a laser such as the National Ignition Facility is an important engineering parameter. To predict the lifetime of fused silica transmissive optics, it is necessary to measure the rate of damage propagation as a function of fluence and understand the effects of the laser parameters. In order to begin such predictions without a large-area flat-top laser beam, damage growth experiments were conducted using a small Gaussian beam. Damage was initiated by producing mechanical flaws on the surface of the optic. Since output surface damage in transmissive topics can propagate at least two orders of magnitude faster than input surface damage, the experiments were focused on damage initiated at the output surface. The experiments showed that if damage was initiated, it could not grow at fluences below a threshold of about 5 and 8 J/cm2 at 355 and 1064 nm, respectively. When damage was able to propagate under laser irradiation, the phenomenon occurred in two stages. Initially, the damage grew both laterally and along the optical axis at a rate varying linearly with fluence. Lateral growth stopped in areas where the fluence was lower than the growth threshold. At this point, the area of damage typically filed the size of the beam and the rate of axial damage propagation significantly decreased. During this stage, laser irradiation drilled at constant rate a channel through the window. During stage I, the damage area grew much faster at 355 nm than at 1064 nm. During stage II, 355 nm light drilled four to five times faster than 1064 nm light. At given fluence at 1064 nm, the drilling rate did not change between 3 ns and 10 ns. Finally, drilling at 1064 nm produced a well-defined cylindrical damaged region while 355 nm light generated less regular clusters of cracks.
A damage morphology study was performed with a 355 nm, 8-ns Nd:YAG laser on synthetic UV-grade fused silica to determine the effects of post-polish chemical etching on laser-induced damage, compare damage morphologies of cleaved and polished surfaces, and understand the effects of the hydrolyzed surface layer and water-crack interactions. The samples were polished, then chemically etched in a buffered HF solution to remove 45, 90, 135, and 180 nm of surface material. Another set of sample was cleaved and soaked in boiling distilled water for 1 second and 1 hour. All the samples were irradiated at damaging fluences and characterized by Normarski optical microscopy and scanning electron microscope. Damage was initiated at micro-pits on both input and output surface of the polished fused silica sample. At higher fluences, the micro-pits generated cracks on the surface. Laser damage of the etched fused silica surface shoed that the real density of micro-pits decreased with etched thickness. SIMS analysis of the polished surface showed significant trace contamination levels within a 50 nm surface layer. Micro-pits formation also appeared after irradiating cleaved fused silica surfaces at damaging fluences. Linear damage tracks corresponding cleaving cracks were often observed on cleaved surfaces. Soaking cleaved samples in water produced wide laser damage tracks.
Scratches of measured width were produced on the surface of a IV grade fused silica window using a diamond tip. Two scratch morphologies were observed: plastic and brittle. The scratches were irradiated with a 355 nm laser pulse. The laser-induced damage threshold (LIDT) of the unscratched output surface was 15 J/cm2 at 3-ns. The LIDT of the scratched surface as a function of scratch width was then measured for both input and output surface scratches. Input surface scratches of width smaller than 10 micrometers did not influence the LIDT of the silica window. On the output surface, 7 $mUm wide scratches lowered the LIDT by a factor of two. For larger scratches, the LIDT reached an asymptotic value of 5 J/cm2 on both input and output surface. Possible reasons for this LIDT drop could be electric field enhancement in the cracks below the scratch, the presence of contamination particles in the scratch, or the weakening of the material because of existing mechanical flaws.
Assuming the observed scaling of laser damage threshold fluence with the (almost) square root of pulse duration is due to thermal conduction, we develop a formalism for directly comparing pulses of different shapes and durations. We find, for example, that a top hat pulse leads to 15% higher temperature (presumably 15% lower damage threshold) than a Gaussian pulse of the same fluence. We also find that the damage threshold of the expected NIF type pulse should be estimated from a Gaussian pulse with the same peak intensity. We find that the deviation of the scaling of damage threshold from square root of pulse duration has contributions from both the small but finite size of laser energy absorbers and from the temperature dependence of thermal properties. Keywords: laser damage, damage threshold, scaling, thermal conduction
Multilayer coatings manufactured from metallic hafnium and silica sources by reactive electron beam deposition, are being developed for high fluence optics in a fusion lasers with a wavelength of 1053 nm and a 3 ns pulse length. Damage threshold studies have revealed a correlation between laser damage and nodular defects, but interestingly laser damage is also present in nodule-free regions. Photothermal studies of optical coatings reveal the existence of defects with strong optical absorption in nodule-free regions of the coating. A variety of microscopic techniques were employed to characterize the defects for a better understanding of the thermal properties of nodular defects and role of thermal defects in laser damage. Photothermal microscopy, utilizing the surface thermal lensing technique, was used to map the thermal characteristics of 3 mm X 3 mm areas of the coatings. High resolution subaperture scans, with a 1 micrometers step size and a 3 micrometers pump beam diameter, were conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy was used to visually identify defects and characterize their topography. The defects were then irradiated to determine the role of nodular and thermal defects in limiting the damage threshold of the multilayer.
Starting from the absorption of laser energy at a subsurface nanoparticle in fused silica, we simulate the consequent buildup of stresses and resulting mechanical material damage. The simulation indicates the formation of micropits with size comparable to a wavelength, similar to experimental observation. Possible mechanism for enhanced local light absorption are discussed.
A morphology study was conducted on fused silica surfaces damaged by single pulse laser irradiation at 355 nm. The physical characterization of the surface showed that cracks initiate at ellipsoidal pits on both input and output surfaces. The size of the pit increases with laser pulse-length and the orientation of the ellipse is perpendicular to the electric field. The pits are less than 300 nm deep. Cracks initiate along the main axis of the ellipse. The morphology of the cracks is different for input vs. output surfaces. The output surface crack exhibits a shell- like morphology typical of localized compressive stress. The input surface crack shows, on the other hand, a star-like pattern. These differences seem to be caused by differences in plasma propagation on the surface. The extent of damage and the number of shells or branches increases with increasing fluence. A molten morphology can be observed at the center of the cracks for fluences above the pit formation threshold. This indicates that local temperatures can be in excess of several thousand degrees C.
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