Symposium EQ06—Innovative Fabrication and Processing Methods for Organic and Hybrid Electronics

Solubilization of organic semiconductors is critical to enable a wide variety of deposition technologies. However, the selection and location of solubilizing groups must be made with great caer, to avoid disrupting critical intermolecular interactions. This talk will address weak, non-covalent interactions that can be used as supramolecular synthons to induce or enhance crystallization in organic thin films. I will also describe current efforts to harness computation and machine learning to enhance the design of high-mobility organic semiconductors.

Organic semiconductors are competitive with conventional amorphous silicon due to their solution processability at room temperature, which allows for the inexpensive, high-throughput fabrication of lightweight, flexible optoelectronic and electronic devices. Organic semiconductors are applicable in optoelectronic devices such as organic light emitting diodes, organic solar cells, and organic field effect transistors. An inherent disadvantage of organic semiconductors is their propensity to adopt multiple molecular packing motifs, a phenomenon called polymorphism, which can yield highly-contrasting charge transport capabilities and optoelectronic characteristics. More specifically, singlet fission, a photophysical process whereby one exciton is converted into two, has a significant dependence on molecular packing. However, controlling the molecular packing of organic semiconductors is difficult because the underlying crystal structure is dictated by weak Van der Waals forces, and thus, variability in the molecular packing arises from even minute changes in processing conditions.

In this study, the molecular packing of TIPS-pentacene (a small molecule organic semiconductor) was finely-tuned using highly-ordered, tunable self-assembled monolayers. TIPS-pentacene thin films were deposited from solution using solution shearing and subsequently relaxed on top of five ordered self-assembled monolayers. Through this method, various interface-stabilized polymorphs of TIPS-pentacene were isolated. Grazing incidence X-ray diffraction was used to characterize the distinct crystal structures, revealing repeatable and controllable, precise structural changes spanning less than 2 percent shift from bulk crystal structure. Photoluminescence spectroscopy and UV-Vis spectroscopy were used to discern structure-property relationships. We discovered changes in the bandgap transition energies as well as relative changes to the steady-state photoluminescence, suggesting that these interface-stabilized polymorphs have significant differences in intermolecular interactions.

Singlet fission is an exciting area of study that could result in the breaking of the Shockley-Quessar limit and be the basis for next generation solar cells, and are strongly influenced by intermolecular interactions. Singlet fission rates as a function of polymorph have been studied by other researchers; however, the polymorphs had significantly different crystal structures. Here, the finely-tuned interface-stabilized polymorphs puts us in a unique position to obtain a deeper understanding of the sensitivity of singlet fission on crystal structure by experimental means through the systematic structural changes. In conclusion, this work provides a novel method to control and explore structure-property relationships through the fine tuning of the molecular packing of small molecule organic semiconductors using highly-ordered, tunable self-assembled monolayers.

In-plane alignment of conjugated polymers leads to anisotropic optoelectronic properties that can be exploited for multiple applications, including polarization sensitive photodetectors, light emitting devices, and thin film transistors. For example, intramolecular charge transport along the polymer backbone is typically favored, and thus promoting alignment can improve charge mobility. Given that the primary optical transition dipole moment is oriented along the polymer backbone, the greater alignment leads to increased polarized light sensitivity in detectors or polarized light emission in OLEDs. Seeing as performance metrics improve as the degree of alignment increases, maximizing in-plane alignment is highly desirable. A popular strategy for aligning conjugated polymers is straining the film on an elastomer, however the degree of alignment achieved is often modest. To maximize the degree of anisotropy, we leverage the unique thermomechanical behavior and molecular structure exhibited in PBnDT-FTAZ by applying a combination of strain and post-strain thermal anneal. The annealing process leads to optical dichroic ratios surpassing 30, the highest known reported dichroism in conjugated polymers indicative of the remarkable alignment achieved.

To better understand the mechanism behind this striking film alignment, we conducted a detailed structural analysis. In-situ UV-Visible spectroscopy reveals the change in absorption anisotropy when the films are annealed past a temperature associated with aggregate relaxation. In addition, the formation of strong vibronic features observed under parallel polarized light and a shift in the absorption spectra indicate a transition to improved packing order. In-situ GIWAXS scans further confirm the increased packing order, as sharp peaks form through annealing and subsequent cooling of the film. These scans unearth a unique packing arrangement, as two populations of crystalline order appear with distinct out-of-plane tilts in the lamellar stacking direction. The dramatic shift in film morphology with thermally annealing is further uncovered with AFM scans, which show the formation of distinct plateau-like domains. Finally, we report how the molecular structure of PBnDT-FTAZ lends itself to the reported packing behavior. Through this work, we uncover how molecular structure and thermomechanics drive packing behavior, which can guide conjugated polymer design to realize films with exceptional in-plane polymer alignment.

The processability and electronic properties of electroactive conjugated polymers (CPs) have become increasingly important due to the utility of these materials in numerous redox and solid-state devices. While the π-conjugated backbone structure is considered primarily responsible for imparting the core optoelectronic and electrochemical functionality of the material, solubilizing chains - appended to the backbone for processability - also play a significant role in tuning the morphological, redox, and electronic properties. To solubilize CP backbones, the side chains must be relatively long and are typically branched to induce many degrees of conformational freedom. Such side chains reduce the relative fraction of active material in the film, potentially obstructing π-π intermolecular interactions leading to increased localization of charge carriers, and compromise many of the desirable properties of the material (such as electrical conductivity). To reduce the deleterious effects of side chains, we report that post-processing side chain removal, exemplified here via ester hydrolysis, significantly increases the electrical conductivity of chemically doped CP films with a corresponding decrease in Seebeck coefficient. Using a variety of methods, we demonstrate that this change is not due to an increase in degree of doping, but an increase in charge carrier density and reduction in charge localization. Application of this method to high-performance ProDOT/EDOT copolymer structures yield significantly higher average conductivities, even outperforming any currently reported oligoether/glycol-based CP system. Finally, further structural modification yields a highly soluble material that can be processed from a wide range of solvents with an optimized conductivity of > 800 S/cm in thick films after post-processing side chain removal.

Conductive polymers rely on side-chains¹ or blending² to improve solubility and processibility to fabricate thin films showing efficient electronic charge transport. One such blend, poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), has become the most widely utilized conducting polymer because of its ease of processing, air stability, transparency, high electronic conductivity,³ and more recently for its mixed ionic and electronic conduction.⁴ However, the role of the PSS, which makes up 71% of the blend by weight, on the high conductivity is still unknown.⁵ Like other conducting polymers, PEDOT:PSS phase segregates, leading to a semi-ordered, semiconductor-rich component (PEDOT-rich grains) which are surrounded by a less conductive insulator-rich matrix (PSS-rich matrix).⁶ Here, we investigate the fundamental electronic charge transport through the insulator-rich phase to provide a more wholistic understanding of transport in PEDOT:PSS. We decrease the concentration of the conductor, PEDOT, by adding excess PSS and observe an exponential dependence of conductivity on PEDOT weight fraction. Surprisingly, a PEDOT:PSS blend consisting of < 3% PEDOT by weight displays a moderate mobility comparable to polyaniline. Using electrical characterization combined with molecular dynamics simulated microstructures, we show that inter-grain electronic transport through PSS-rich regions occurs via highly efficient tunneling between neighboring PEDOT chains. The results demonstrate the importance of the sidechain or blending polymer on efficient electronic transport and provides a framework for the molecular design of new conducting polymer systems.
[1] Meyer, D. L. et al. The Journal of Physical Chemistry C 123, 20071-20083 (2019).
[2] Abbaszadeh, D. et al. Nature Materials 15, 628-633 (2016).
[3] Worfolk, B. J. et al. Proc Natl Acad Sci U S A 112, 14138-14143 (2015).
[4] Rivnay, J. et al. Nature Reviews Materials 3, 17086 (2018).
[5] Gueye, M. N. et. al. Progress in Materials Science 108, 100616 (2020).
[6] Nardes, A. M. et al. Advanced Materials 19, 1196-1200 (2007).

Even though record organic semiconductor mobilities are reported for organic semiconductor single crystals, making thin film crystals remains challenging. We will present our efforts to understand crystal formation, and growth (epitaxy) of pinhole free films of numerous organic semiconductors with grains with dimensions of 100s of microns. We will explain how materials that undergo a transition from amorphous to crystalline correlate well with thermal properties. Homoepitaxial studies uncover evidence of point and line defect formation in these films, indicating that homoepitaxy is not always strain-free, but films always remain remarkable smooth (rms roughness never exceeds 5 nm). Transistors made out of large-grained films of rubrene display charge carrier mobility of up to 3.5 cm2 V–1 s–1, very close to single crystal values, highlighting their potential for practical application. Finally, we will show efforts in achieving heteroepitaxial growth of a different molecular material on top of a crystalline organic template.

The chemical composition and degree of order in self-assembled monolayers (SAMs) is critically important to the properties of molecular electronic devices. We report on the synthesis and electrical response of a series of molecular rectifiers based on benzalkylsilanes in the form of SAMs anchored to a silicon substrate and tested with an Eutectic Gallium-Indium (EGaIn) top electrode. Mixed SAMs were formed via the co-absorption method by incorporating controlled amounts of different aliphatic silane-based additives into the film of the host SAM. The impact on the current-voltage characteristics, breakdown voltages, and static contact angle measurements provided insights into the degree of order and uniformity of the mixed SAM films. We discovered that despite a reduced degree of order introduced within the SAMs, small amounts of (3-aminopropyl)triethoxysilane (APTES) enhanced the current rectification of several materials, whereas, the other additives (such as n-propyltriethoxysilane) decreased the current rectification. We attribute this behavior to the APTES molecules acting as a dopant, thus allowing more efficient charge transport under forward bias. On the contrary, under reverse bias, the shorter dopant molecules do not make intimate contact with the top electrode, decreasing the effective surface area and overall conductance of the monolayer. The most significant improvement was measured in (E)-1-(4-carbomethoxy-phenyl)-N-(3-(triethoxysilyl)propyl)methanimine, where a three-fold increase in the rectification was found when mixed with APTES at an optimized 2:1 ratio, yielding an average rectification ratio of 4500, one of the highest reported values to date for molecular diodes on silicon.

Sequential solution doping is a processing technique that allows a conjugated polymer film to be doped from a solution 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 newly synthesized molecular dopants that the doping level can be modeled using a simple Langmuir isotherm and fit to a single equilibrium coefficient. Analysis of the free energy derived from Keq yields a consistent thermodynamic model for molecular doping in polymers. We show that 50-75% of the polymer sites can be doped from solution. We compare Fermi level filling and electrochemical models for ionization efficiency of the dopant. In addition, analysis of the UV/vis/NIR and mid-IR spectra allows analysis of both the polaron sites and the neutral sites as a function of doping level. We show distinct differences between the spectral changes for polymers that when neutral formed H-aggregates rather than J-aggregates for both the neutral sites and the resulting polaron sites. Taken together we demonstrate a consistent and quantitative model for molecular doping that can be applied across different polymer and dopants.

Molecular doping is a powerful method to tune the electronic properties of solution-processed semiconductors based on π-conjugated polymers (CP), enabling high performance thermoelectric devices and thin film transistors. Despite the importance of doping, it is still challenging to predict how a given dopant will influence charge-carrier transport in semiconducting polymers. An accurate description of charge transport in these materials necessitates a good understanding of how polaron and bipolaron characteristics are affected by dopant counterions. We use first-principles calculations based on density functional theory (DFT) and time-dependent Density Functional theory (TD-DFT) with an optimally-tuned hybrid functional to determine various polaron characteristics in CP’s with and without dopant counterion present. We find that the coulomb interaction between the polaron and the dopant counterion dictates the polaron size and is the dominant contribution to the polaron binding energy, exceeding polarization and nuclei reorganization. Both polaron size and binding energy play an important role in charge transport. We also find that the location of the induced charge along the polymer chain follows the counterion. As such bipolaron, which leads to faster charge transport, become more stable than polarons when the distance between dopant counterions is sufficiently reduced.

In this talk, I will present on the semi-localized transport (SLoT) model and demonstrate its ability to quantitatively compare fundamental charge transport parameters in semiconducting polymer-dopant-processing systems (see Gregory, Hanus, et al., Nat. Mater. (2021)). The SLoT model spans the spectrum and captures both localized (hopping-like) and delocalized (metal-like) contributions to the observable electronic and thermoelectric properties by bridging the mathematical and physical formalisms from both polaronic transport models and the Boltzmann transport equations. Using the SLoT model, we can quantify fundamental transport parameters, such as the carrier ratio (carrier density) needed to exceed the transport edge, to what extent does localization (and its activation energy) decrease with increasing carrier density, and what is the hypothetical maximum electrical conductivity in the absence of localization. Although the SLoT model currently lacks chemical and structural predictive ability, the SLoT model provides quantitative transport parameters for each system that can be related to chemical and structural features; this enables a deeper understanding for why one system obtains different transport properties compared to another and could eventually lead to predictive ability. Lastly, I will compare the chemical, structural, and charge transport properties among several systems, which will illustrate the benefit to quantifying charge transport and developing structure-property relationships with the SLoT model.

Enhancements or tailoring of organic device performance using unique physical effects requires benchmarking measurements that can be used across labs. Specific unique physical processes, such as singlet fission or dynamic disorder, are difficult to probe directly using standard electrical measurements. To develop new metrics that reflect unique physical processes, devices with researcher-defined properties are required to show reproducibility and reliability. Controlling device and semiconductor properties through fabrication is therefore extremely important. In this talk I will cover how we use controlled fabrication to develop measurements to look into device and semiconductor physics in working organic semiconductor transistors and light emitting diodes, including dynamic disorder, contact resistance, and singlet fission/triplet fusion.

Conjugated polymers are an increasingly important materials class for organic electronics mainly because of their immense chemical versatility. Generally, side-chains are introduced to induce straight-forward processability, e.g., from solution. However, these chemical modifications also tend to affect local assembly and, in turn, specific optoelectronic properties such as charge transport. As a consequence, sample-to-sample variation can be significant. Here, we report an alternative pathway frequently used in the semiconducting small molecule area: side chain removal through post-processing hydrolysis. We selected a series of poly(dioxy thiophenes) as a model system and employed spectroscopic techniques utilizing X-ray, ultraviolet, and visible sources in combination with thermoelectric measurements to quantify the effects of side-chain removal on the extent of doping and charge transport properties. We find that cleaving the side chains does substantially alter the carrier density and increase the electrical conductivity. Additonally, through tuning of the repeat unit structure, different temperature dependencies in the electrical conductivity are found. Since X-ray photoelectron spectroscopy (XPS) suggests that tuning the repeat unit structure also affects core-level electron interactions, we will discuss the observed differences in the temperature-resolved electrical conductivity and extent of doping with the use of the semi-localized transport model. Moreover, by comparing the side-chain-cleaved systems to materials with complex, insulating side chains, we argue that the former polymers may provide a route towards semiconducting plastics of high processing reliability and a high conjugated backbone fraction.

Impedance Spectroscopy (IS) is an increasingly common technique to characterize both solid state and electrochemical systems including solar cells and light emitting diodes (LEDs). However, IS relies on a system response being linear with its input such that a time invariant impedance can be defined. This is usually achieved by a small amplitude input. However, doing so suppresses responses of the nonlinear processes which are of considerable interest to those designing and optimizing these devices, such as charge carrier recombination and space charge effects. This investigation employs the recently developed nonlinear extension to IS (NLIS) based in Fourier analysis of the measured harmonic current such that a nonlinear definition of higher harmonic admittance (inverse impedance) is established. By relating Fourier coefficients of the measured current with derivatives of the voltage specific transfer function we may extract valuable physical parameters of the system in question. Benchmark tests of this technique on systems of known transfer functions will be presented specifically measuring hole mobility from space charge and the diode ideality factor from recombination limited current regimes. Finally, NLIS is used to characterise 2-(7-{4-N,N-Bis(4-methylphenyl)aminophenyl}-2,1,3-benzothiadiazol-4-yl)methylenepropanedinitrile (DTDCPB), a promising novel intramolecular charge transfer (CT) organic semiconductor (OSC). The first known report of DTDCPB hole mobility is presented.

Here, we demonstrate that we can use vitrification at eutectic compositions as a tool to produce homogeneously intermixed, doped organic semiconductors with high electrical conductivity from solution. Doped organic semiconductors have found broad use as interlayers for organic light-emitting diodes (OLED), photovoltaic cells (OPV) and photodetectors. Doping, often, is achieved via addition of another species, an oxidizing or reducing agent – i.e., a dopant. However, many semiconducting polymer:dopant systems are not highly miscible, leading in many cases to undesirable phase separation and, thus, inhomogeneous film formation. As a consequence, polymer films are commonly doped via sequential doping methodologies, to achieve more homogenous dopant dispersion and, in turn, higher electrical conductivity. By using vitrification at eutectic compositions, we simplify the procedure towards homogeneously intermixed, high-conductivity polymer:dopant blends. We show that the highest electrical conductivities are found in the most vitrified regime, attributed to enhanced charge-carrier densities at these compositions as deduced from the spin densities measured in electron paramagnetic resonance. The question that remains is whether this eutectic vitrification also assists in keeping the thermal transport in such polymer:dopant blends low. For this purpose, we present thermal transient grating measurement data. Our work, thus, starts to provide a window towards a generally applicable structure/processing/property relationships to electron and charge transport in multicomponent macromolecular matter.

Interest in organic electronics is driven in large part by the applications they can address, yet a complete set of device fabrication design rules is still absent for the devices intended to fulfill those roles. Progress is assisted by general guidelines such as minimizing the Schottky barrier at injection interfaces or reducing defect states in the semiconductor layer, but improvements are achieved mainly through experimental trial-and-error, which is time, cost, and material intensive, and yields incremental results most of the time. Here we use physically-based numerical simulations to explore a vast parameter space with high throughput in order to generate rules for material processing and device design, with the goal of optimizing charge mobility and injection. We then use these predictions to evaluate a “window” of device design where certain parameters can be relaxed in favor of cost effectiveness, but without a penalty in device performance. Examples include the contact interface and the quality/purity of the semiconductor, which we show can be compensated through adjustment of another parameter. We investigated the impact of the trap density of states (t-DOS) and the injection barrier on the charge carrier mobility in organic field-effect transistors (OFETs) for both high- and low-capacitance dielectric devices, in co-planar and staggered architectures. The simulations revealed that lowering the dielectric capacitance results in a device that is more permissive to an injection barrier, but that is also more sensitive to changes in the t-DOS; the reverse is true for increasing the dielectric capacitance. Additionally, the staggered architecture outperforms the co-planar structure when an injection barrier is present. These trends outlined a processing window for staggered-structure devices, wherein by tuning the dielectric capacitance one can raise the tolerance of traps or poor contacts. To test these predictions and take advantage of this processing window, we fabricated and characterized a variety of OFETs. We tuned the injection barrier via self-assembled monolayers (SAMs) while the trap density of states (t-DOS) was modified by altering the semiconductor film microstructure through processing and purity by synthesis method. Beginning with the case of OFETs with high-work function (SAM-treated) contacts and typical semiconductor processing, mobility changed very little with dielectric capacitance, in accordance with simulation: the high-capacitance devices (ca. 4 nF/cm²) had mobilities of 1.5 ± 0.1 cm²/Vs (average obtained on over 100 devices), while the low-capacitance devices (1.7 nF/cm²) had mobilities of 1.6 ± 0.3 cm²/Vs. In the low-capacitance devices, a nominal 0.4 eV injection barrier was introduced and devices still retained 80% of the average mobility value. (This is compared with a decrease in mobility of over 50% in the high-capacitance devices.) We then took advantage of the high capacitance window to fabricate OFETs with a low sensitivity to traps: mobilities varied by less than 15% even when the characteristic density of traps increased by over 40%. In summary, we have shown that the low-capacitance design window relaxes the requirement for a high contact work function, allowing the selection of more cost-effective and scalable materials, while the high-capacitance devices grant more lenient requirements for the quality of the semiconductor film.

Asymmetric optical properties of mirror electrodes in stacked microcavity OLED photonic crystals induces a Peierls bandgap in each photonic band. Here we explore the influence of the anode and cathode thicknesses on the bandgap and the bandwidth of the two sub-bands that result from the asymmetry. In the context of our device geometry, the silver anodes are shown to primarily control the Peierls bandgap and the aluminum cathodes are shown to tune the bandwidth of the upper and lower sub-bands. These effects are explained conceptually, modeled computationally, and resolved experimentally in the electroluminescence spectra.

The roughness of a series of crystalline organic thin films are measured as a function of film thickness. From these data, the growth exponent of each material is extracted and plotted against the molecular aspect ratio. We find that more three dimensional molecules (low aspect ratio) tend to remain smooth as film thickness increases, in contrast to planar or rod-like molecules (high aspect ratio) which quickly roughen. This suggests that the average Ehrlich–Schwoebel barrier at a molecular step edge for low aspect ratio molecules is smaller than it is for high aspect ratio molecules. These results point to the use of low aspect ratio molecules in crystalline organic electronics because of their ability to remain smooth as thickness is tuned.

Synthesis of an aluminum complex containing a functional mordant group will be presented. The functional vinyl group is designed to coordinate with silicon radicals on the surface of an etched silicon wafer. This process is called monolayer doping. FTIR, Raman, and NMR spectroscopy were used to analyze the different organometallic dopant compounds. A dried derivative of aluminum isopropoxide was found to have FTIR and Raman Peaks of 1612 and 3116(1/cm), which are indicative of the presence of a vinyl group. NMR spectroscopy shows peaks around 6.4ppm which is also indicative of a vinyl group on the compound. Tris(2,4-pentanediono) aluminum(III) was synthesized with aluminum chloride and acetylacetone in an aqueous solution. The reaction between the dopant and the silicon radicals creates a uniform layer of dopant-wafer bonds on the wafers surface. Silicon wafers have been successfully doping using this method. The doped wafer is washed and dried to remove any non bonded dopant from the surface of the wafer. A 70nm capping oxide is applied to the wafer using Plasma Enhanced Chemical Vapor Deposition (PECVD). The wafer is then heated to 1100 degrees Celsius for a variable amount of time between 10 to 40 seconds using Rapid Thermal Annealing (RTA). This drives the dopant into the lattice of silicon wafer and turns the wafer into a p-type semiconductor wafer. Resmap sheet resistance analysis of doped wafers has shown that there is an inverse relationship between annealing time and surface resistance.

Growth of molecular semiconductor thin films with better ordering and uniformity is crucial for devising organic optoelectronics with better performance. A highly ordered and crystalline organic semiconductor thin film can be an ideal active layer owing to its charge transport ability. However, luminescence materials become less efficient when fabricated into solid films, which is known as aggregation-caused quenching. Thus, the luminescent materials are often doped in host materials or remained amorphous to suppress the aggregate formation, even though crystalline films have higher charged carrier mobilities. To address this issue, luminescent molecular rotors have been suggested, that exhibits aggregation-induced emission behavior, which is opposite to the aggregation-caused quenching effect. Nevertheless, for some organic molecules, it is challenging to form crystalline thin films upon substrates, and their unique optoelectronic properties can be ignored.
Here, we investigate the thin films of 5,11-diphenylindeno[1,2-b]fluorene-6,12-dione (Dp-IFD) prepared by physical vapor deposition. Because pristine Dp-IFD thin films barely crystallize even after thermal treatment and have poor optical as well as electrical properties, Dp-IFD thin films have not been exploited to date. However, we discovered that crystalline Dp-IFD thin films can be readily achieved by sequential deposition of coronene seed layer and Dp-IFD molecules without breaking vacuum. Current atomic force microscope (c-AFM) measurement and space charge limited current (SCLC) devices showed that crystalline Dp-IFD can support 4 orders of magnitude higher electron mobility. Crystalline Dp-IFD thin films also exhibited brighter fluorescence emission with longer lifetime than their amorphous counterparts. Furthermore, we demonstrated versatile applications of the Dp-IFD thin films, such as organic solar cells (OSCs) and light emitting diodes (OLEDs), owing to its veiled superior optoelectronic properties.

DNA origami is a powerful method to create structurally versatile, highly addressable, and distinct nanoscale objects to facilitate the field of electronics. Controlled placement of DNA origami is essential to build functional devices like biochips, electronic circuits, sensors, and optical or photonic devices. Block copolymer (BCP) self-assembly can be utilized for next-generation lithography for the advanced nanopatterning of surfaces. Here we present a method of using a BCP as a mask to create a patterned surface, rendering it capable of selective attachment of DNA origami that will eventually become the base of the electronic device. We use a polystyrene (PS) co-poly(methylmethacrylate) (PMMA) block copolymer, PS(S-b-MMA), which has cylindrical nanodomains of PMMA. These cylindrical nanodomains can be oriented normal to the surface with controlled interfacial interactions. Exposure to UV radiation then degrades the PMMA and crosslinks the PS matrix, resulting in a nanoporous film on silicon surfaces after sonicating with acetic acid and oxygen plasma etching. We investigated the process window for forming ordered arrays of nanoscale polymer cylindrical domains perpendicular to the thin films, including the effect of substrate surface treatment, spinning conditions, annealing conditions, and film thickness. The cylindrical nanodomains that are perpendicular to the film surface have ~15 nm diameter. We will use the etched PS-PMMA thin films as a mask for gold deposition. DNA origami modified with thiol groups can then be used to attach to these surface-patterned gold islands selectively. The ability to position DNA origami in a controllable manner will facilitate the integration of DNA-based materials into nanoelectronic and sensor devices where we will deposit electrically conductive metallic or/and semiconducting materials. The electronic characterization will be performed to reveal the ability to use these devices in the future. This work will contribute to the field of using organic BCP and hybrid materials contribution for future electronic and sensor devices.

The transparent conductive electrode (TCE) based on a silver nanowire network (AgNW) has demonstrated high optical transparency, low sheet resistance and shows a range of potential applications in flexible electronics. However, the number of junctions and the junction resistance are the parameters governing the conductivity of the electrode and eventually limiting the applications of AgNW networks if left as it. In this work, the electroplating method was employed to decorate an Ag thin layer on AgNW-junctions to improve the contact resistance of the network. As a result, the AgNW-TCE on flexible PET substrate exhibits a sheet resistance as low as 10 Ohm/sq associated with a transmittance larger than 90% measured at λ = 550 nm by optimizing the experimental conditions. The contribution of the decorated-AgNWs network on the electric behavior of the electrode was also highlighted at the nanoscale using the conductive Atomic Force Microscopy (c-AFM). The obtained results allow us to propose a feasible process for the preparation of TCE using flexible displays, organic light-emitting diodes, and thin-film solar cells

Polymer semiconductors based on π-conjugated structures have shown distinct promise for the development of human-integrated electronics, owing to their solution processability for large-area fabrication, mechanical softness and even stretchability, as well as low cost. However, among the wide range of functional properties required for this application domain (including, but not limited to, biochemical sensing, chemotherapeutic property, bio/immune-compatibility, micro-patternability, stimuli-responsiveness), a number of them face synthetic challenges to be imparted onto conjugated polymers and thus combined with efficient charge-transport property. Here, we develop a “click-to-polymer” (CLIP) synthesis strategy for conjugated polymers, which enables the use of a click reaction for the facile and versatile attachment of diverse types of functional units to a pre-synthesized conjugated-polymer precursor. This can be utilized to realize both bulk and surface functionalization in/on the deposited thin films. Through demonstrating the applicability on four types of functional groups that each carry distinct emerging properties, for instance, ion conduction and immune-modulation, we show that functionalized polymers from this CLIP method can still retain good charge-carrier mobility above 0.1 square centimeters per volt per second. On the other hand, the surface functionalization does not cause any influence on the mobility. We take two of the realized polymers to showcase the capability of imparting new functions to conjugated polymers: photo-patternable property and biochemical sensing function, both of which advance the state of the art of realizing these two types of functions on conjugated polymers. In the future, we expect the expanded use of this synthesis approach will largely enrich the functional properties from conjugated polymers and facilitate the electronic-biological interfacing.
(Under revision)

Metal oxide-based nanomaterials are an important class of materials because of their unique physical and chemical properties. Various techniques are being used to synthesize metal oxide-based nanomaterials. Gaire et al.² reported binder free manganese-cobalt oxide supercapacitor electrodes with ultra-long cycle life. In this work, we employed photonic curing using PulseForge 1300 (NovaCentrix, Austinx, TX) to synthesize manganese-cobalt (Mn-Co) mixed oxide nanostructures and monitored the process of nucleation and growth during the synthesis process. A mixture of Mn and Co organometallic precursor was deposited on Pt-Si substrate using an air-spray, and processed using PulseForge in ambient conditions. To study the effect of processing conditions on nucleation and growth of nanostructures, processing parameters, such as power and number of pulses were varied while overall pulse fluence was kept constant. We varied power from 400 V to 700 V and number of pulses from 1 to 7 to achieve constant overall fluence of 16 J/cm² for each sample. A simulating tool for photonic curing based on transient 1-D heat conduction model (SimPulse) is used to simulate the temperature dependent thermal and material properties. SimPulse provided real time thermodynamic profile of Mn-Co mixed precursors as a function of thickness of the films. Our results give an insight into the nucleation and growth process of thin films as a function of energy per pulse and shows that by varying the fluence per pulse and number of pulses, nucleation, and growth changes. At high-energy fluence per pulse, the first pulse causes photodecomposition of the organometallic precursor, and subsequent pulses result nucleation and growth, and high-temperature chemical reactions to form crystalline non-equilibrium states of metal oxides within a porous film. We observed that pulse design plays a crucial role in the nucleation and growth of nanostructures, even when overall fluence is kept constant. Our study shows that photonic curing can be utilized as a technique to tune the properties of the nanostructures by controlling nucleation and growth. It provides better understanding of nanometric control of complex materials for next generation energy applications.

2. Gaire, Madhu, et al. "Ultra-long cycle life and binder-free manganese-cobalt oxide supercapacitor electrodes through photonic nanostructuring." RSC Advances 10.66 (2020): 40234-40243.

Organic-inorganic halide perovskites of the form ABX₃ have shown outstanding properties for solar cells. The popular compositions consist of mixtures of A-site cations methylammonium (MA), formamidinium (FA) and cesium, and X-site iodide and bromide ions, and are produced by solution processing. It is an open question whether solution processing could produce sufficient spatial performance uniformity for photovoltaic modules or compatibility with multilayered tandem solar cell deposition. In addition, the volatile MA cation presents long-term thermal stability issues. Here, we report the multisource vacuum deposition of FA_0.7Cs_0.3Pb(I_0.9Br_0.1)₃ perovskite thin films with high-quality morphological, structural, and optoelectronic properties. We find that the controlled addition of excess PbI₂ during the deposition is critical for achieving high performance and stability of the absorber material, and we fabricate p-i-n solar cells with a power conversion efficiency of 20.7%. Also, the operational stability test shows ∼95% of its initial efficiency after 100 hours. We also reveal the sensitivity of the deposition process to a range of parameters, including the type of substrate, annealing temperature, evaporation rates, and source purity, providing a guide for further evaporation efforts. Our results demonstrate the enormous promise for MA-free perovskite solar cells employing industry-scalable multisource evaporation processes.

Additive manufacturing has been evolving towards flexible substrates for the fabrication of printable and flexible hybrid electronic (FHE) devices and circuits. Generally flexible thin polymer or paper-based, these emerging substrates suffer from their heat sensitivity and/or low glass-transition temperatures. Photonic and laser sintering has shown great promise in maintaining the integrity of the substrates without structural degradation due to shrinkage, charring or decomposition. The rapid sintering process arises from microseconds to milliseconds at high temperatures onto printed functional ink traces, in order to reduce the thermal diffusion occurring into the thermally sensitive substrate materials. The minimum sheet resistance value of the conductive traces is then obtained after the proper sintering process. These results are supported by numerical models using finite element methods of the optical wavelength induced thermal dynamics in materials.

In this paper, flash lamp [1] and laser sintering [2] techniques show optimized sheet resistance values for inkjet and screen printing silver and copper inks from 0.2 Ω/sq on PET to 4 Ω/sq on SBS paper obtained at very high speed and spatial precision in comparison with conventional oven annealing. In addition, both these techniques provide a greater control over the electronic properties of the resulting conducting ink traces, while providing a more energy-efficient process within rapid processing times. The simulation results suggest that carefully controlling the heating rates within the ink volume is critical to avoid structural surface defects and achieve optimal sheet resistance values for the sintered functional inks. The research is presented in a perspective of a technology applications development for next generation of FHE sensors and interconnectors.

[1] B.N. Altay, V.S. Turkani, A.P. Pekarovicova, P.D. Fleming, M.Z. Atashbar, M. Bolduc, S.G. Cloutier, “One-step photonic curing of screen-printed conductive Ni flake electrodes for flexible electronics”, Scientific Reports 11 (2021) 3393.
[2] M. Bolduc, C. Trudeau, P. Beaupré, S.G. Cloutier and P. Galarneau, “Thermal Dynamic Effects using Pulse-Shaping Laser Sintering of Printed Silver Inks”, Scientific Report 8 (2018) 1418.

Top-emitting organic light-emitting diodes (TOLEDs) are preferred over bottom-emitting structures in terms of high aperture ratio and switching circuit integration. The device structure of typical TOLEDs consists of a metallic electrode on the top and another on the bottom, with organic layers sandwiched between them. Such electrodes induce a Fabry-Perot resonance, which is called the microcavity effect; it gives high device efficiency and high color purity. However, in most TOLEDs show viewing angle (v) dependence of luminance spectrum; the emission peak tends to be blue-shifted with increasing v. This phenomenon occurs strongly in white-color displays, which are achieved by combining red, green, and blue (RGB) sub-pixels: this is called white angular dependency (WAD). It distorts the color gamut as v increases, thus should be eliminated from TOLED displays.
Several methods have been introduced to reduce WAD in TOLEDs. One is to detune the thickness of the microcavity structure. However, tuning the thickness of active layers requires multiple masking processes, and reduces device efficiency. Another approach is to apply a light-scattering layer on top of the TOLEDs without otherwise changing device structure. The light that passes through the scattering layer spreads in random directions, and lights emitted at different angles are merged, so averaged light is extracted regardless of v, resulting in suppressed WAD. The light-scattering layers can be achieved by producing nanostructures on the film, but it could be easily damaged and scratched by external mechanical stimuli, resulting in degraded optical properties.
By embedding a high refractive index (RI) nanoparticles in polymer film could be a solution to get a robust property of the scattering film. In fact, there was a trade-off relation between haze and total transparency. Namely, the haze increases with the density of nanoparticle, but total transparency decreases, and vice versa. Such problems could be solved by employing a scattering center with RI = 1.0, air-gap embedding polymer film. The air-gap with RI=1.0 could act as not only a scattering center due to the difference of RIs between the air-gap and its surrounding polymer, but also optically transparent one.
Here, we design the optimal size and distribution of air-gap to suppress WAD in RGB TOLEDs, and demonstrate the air-gap embedded hazy film (AEHF). To design the air-gap, rigorous coupled-wave analysis (RCWA) was systematically calculated with respect to the various geometries of the air-gap such as size and period. Based on the simulation results, we demonstrated the AEHF by coating the viscous resin, poly(dimethylsiloxane) (PDMS), on the patterned polymer substrate. By using patterns with a high aspect ratio (AR) during the coating procedure, the geometries of embedded air-gap were increased compared with using patterns with low AR. From the several experimental data, the geometries of the air-gap which were embedded between high AR patterns were similar to the simulation results, realizing a high haze of ~100% while retaining a high average total transmittance of 88%. Consequently, the hazy film is utilized in TOLEDs and the WAD can be dramatically reduced about > 90 % in the RGB wavelength.

In the past few years, several properties of organic layers used for thermally damaged top-emission organic light-emitting diodes (TEOLEDs) have been investigated. However, the surface morphology of organic layers has not been fully explored yet. In this work, surface morphology of stacked layers and cathode used for thermally damaged TEOLEDs are evaluated. Thermal damage is caused by the exposure temperature obtained from the Cu deposition on TEOLEDs, which affects device performance. Because of the low glass transition temperature (T_g), the phosphorescent host material 4,4′bis(N-carbazolyl)-1,1′-biphenyl (CBP) is rapidly degraded to a large extent, changing its surface morphology, optical and geometrical properties.
A severe morphological change due to thermal damage occurred in the excitons blocking layer, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), even below its T_g, owing to the intense variation in the refractive index. Therefore, both the optical and surface morphological properties of TCTA were affected under thermal damage, whereas only the surface morphological properties of the hole-transport layer, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), were changed. However, the optical properties of the thick NPB remain constant despite a lower T_g than that of TCTA. Unexpectedly, increasing temperature has an effect on the hole-transport layer (NPB), the excitons blocking layer (TCTA) and the overall electroluminescence spectra and micro-cavity without affecting device performance much. The surface morphology of upper electrode (Mg:Ag) was cracked due to the difference between the thermal expansion coefficients. Moreover, electrical property of cathode was also affected by thermal damage with increasing its sheet resistance. The wrinkle structure from the full-stacked organic layer was observed, which greatly affected to the micro-cavity and optical properties. Hence, this study reveals that besides T_g, the surface morphologies and thicknesses of the organic layers are also important factors in the annealing process and play a vital role in causing thermal damage to TEOLEDs. To avoid film morphological changes, these findings highlight the need of adopting thermally stable materials in TEOLED devices.

Fiber rather than other structures and morphologies such as films or membranes is a promising path for wearable electronic systems, because this long and flexible fiber can be weaved or knitted easily, seamlessly into multifunctional fabrics. Among various fiber-based sensors, piezoelectric fibers, based on polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)), are especially attractive, since they are sensitive and reliable, and show high durability in a compact structure. Recently a novel fabrication technique, named thermal drawing (TD) technique, has been frequently performed to fabricate piezoelectric fibers. TD of multimaterials having different flowability at drawing temperature requires amorphous cladding such as polycarbonate (PC) or poly(ether-sulfone) (PES), of which melting temperature is higher than the other materials, to stabilize the TD process. However, these claddings greatly reduce the mechanical flexibility of fibers at room temperature, and it is difficult to selectively remove them for additional treatments such as electrode interconnection.
In this study, we propose a highly amorphous P(VDF-HFP) copolymer as the cladding material for piezoelectric sensor fibers, of which melting temperature and yield strain are known to be 122 °C and about 18%, respectively. Preform fabrication must be preceded to draw fiber through the TD process. P(VDF-TrFE) film is sandwiched between two electrically conductive composite sheets made of carbon black (CB) and polypropylene (PP), and the piezoelectric device is cladded in P(VDF-HFP) copolymer. Incompatibility of flow characteristics at drawing temperature among P(VDF-HFP), P(VDF-TrFE), and CB/PP composite requires optimization of preform geometry and TD parameters. The preform is transformed into a fiber of length more than 50 m via the TD process. The drawn fiber is stored at 140 °C for 12 hours to increase the crystallinity of the P(VDF-TrFE) polymer. The high solubility of P(VDF-HFP) to polar solvents such as acetone facilitates electrical interconnection through selective etching. The drawn piezoelectric fiber can be directly poled through the two CB/PP electrodes with the electric field of 40 MV/m for 45 minutes. The fabricated piezoelectric fiber produces about 5 V (V_pp) under 5% tensile strain and 0.5 V (V_pp) under bending deformation with a radius of curvature of 10 mm. Furthermore, the piezoelectric fiber can withstand large bending deformation of 1 mm radius of curvature without any severe degradation in the output voltage.

The solution-processed organic light-emitting diode (s-OLED) is being actively studied for its advantages such as low-cost fabrication and large area application, but the efficiency and stability of s-OLEDs are still lag behind those of the evaporated devices (e-OLEDs). Many articles point out the film aggregation and pinholes as the main reason for the inferior properties of s-OLEDs, but here we highlight the chemical influence of solvents on the organic materials composing the emissive layer (EML). The investigation on the device performance of s-OLEDs is conducted with two different solvents dissolving CBP:Ir(ppy)₃—chloroform (CF) and tetrahydrofuran (THF). Comparing the device performance of s-OLEDs with e-OLEDs, the CF-based device exhibits remarkably degraded properties whereas THF-based device shows less deviation from e-OLEDs. With the Fourier-transform infrared spectroscopy (FTIR) measurement, we analyze the device degradation of CF-based devices in detail. The FTIR results strongly imply that the residual CF solvent can accelerate the chemical degradation of the emissive layer, changing the bond length and force constant of CBP:Ir(ppy)₃ film. Although further researches are required to perfectly unveil the mechanisms of CF-related degradation, our work will provide useful insights into realizing the commercialization of s-OLEDs.

In the past decades, metal oxide semiconductors have received significant attention due to their unprecedented properties to be used in applications like electronics, optoelectronics, electrochemistry, sensors, and catalysis. Among them, vanadium pentoxide (V₂O₅) shows promising thermal, chemical and electrochromic properties as it is the most stable phase within other vanadium-oxides with an oxidation state of 5. Electronic applications of V₂O₅ films necessitate deposition of thin, homogeneous and crack-free layers thus they show low resistance and high carrier diffusion lengths. To date, various methods were used to deposit V₂O₅ thin films such as thermal evaporation, chemical vapor deposition, sputtering, sol-gel process and spray pyrolysis. Ultrasonic spray deposition (USD) method is another technique that allows the deposition of thin films over large areas. Moreover, this method is significant candidate to deposit thin films due to its good properties such as high material utilization, relatively low-cost processing and non-vacuum requirement. Besides, it allows repeatable build-up of high-purity thin films. In this work, V₂O₅ thin films were deposited onto pre-heated fluorine-doped tin oxide/glass substrates using USD method. The effect of post deposition annealing temperature on the structural, morphological, optical and electrochromic properties of V₂O₅ thin films have been systematically investigated. Upon annealing over 450 °C, all films showed crystalline morphology while as-deposited films were in amorphous nature. XPS analyses demonstrated the oxidation state of vanadium ions of all deposited films following post deposition annealing was +5. Films have shown visible transmittance with optical band gap values in the range of 2.57 to 2.66 eV as a function of annealing temperatures. Deposited V₂O₅ thin films were also electrochemically tested. Cyclic voltammetry (CV) results with a fast scan rate of 100 mV/s showed that the color of the electrodes was initially yellow and then turned into blue and green at the reduced state for the annealed films. CV results proved the multichromic nature of V₂O₅ thin films. The cycling performance of the V₂O₅ thin films was assessed by assembling an asymmetric cell, which showed promising cyclic stability up to 1000 cycles. Results provided herein indicated that USD method is highly suitable for the deposition of functional V₂O₅ thin films over large areas.

In recent years, immense efforts in the flexible electronics field have led to unprecedented progress and to devices of ever increasing performance. Despite these advances, new opportunities are sought in order to widen the applications of organic-based technologies and expand their functionalities and features. For this purpose, use of multicomponent systems seems an interesting approach in view of, e.g., increasing the mechanical flexibility and stability of organic electronic products as well as introducing other features such as self-encapsulation. One specific strategy is based on blending polymeric insulators with organic semiconductors; which has led to a desired improvement of the mechanical properties of organic devices, producing in certain scenarios robust and stable architectures. Here we discuss the working principle of semiconductor:insulator blends, examining the different approaches that have recently been reported in literature. We illustrate how organic field-effect transistors (OFET)s and organic solar cells (OPV)s can be fabricated with such systems without detrimental effects on the resulting device characteristics even at high contents of the insulator. Furthermore, we review the various properties that can be enhanced and/or manipulated by blending including air stability, mechanical toughness, H- vs. J-aggregation, etc.

Additively manufacturing light emitting diode (LED) arrays entirely on 3D printers will enable a new and unprecedented capability to create large scale displays in a freeform manner, untethering the process from highly specialized microelectronic fabrication facilities. Fully 3D printing optoelectronics further allows for a high adaptability to varying device form factors and fabrication scales. However, current material systems and printing methods for optoelectronics present several issues to fully 3D printed LED arrays. First, the fabrication of electrodes and encapsulation layers composed of metal-oxides or solid-state metals requires sputtering or vapor deposition processes that are not compatible with 3D printing platforms. Second, active layers that are printed by solution deposition possess a high degree of layer nonuniformity as a result of the directional mass transport within the printed droplets driven by the capillary flow. Lastly, creating repeatable and stable polymer-metal junctions between the emissive layer and cathode at room temperature has proven to be challenging on 3D printers, which should also set the stage for mechanically and electrically interfacing the cathode arrays with spatially structured interconnecting wires.

Here we report a novel device configuration and a multimodal 3D printing methodology that leads to fully printed, encapsulated and highly flexible organic LED (OLED) displays. To solve the previous printability issue for electrodes and encapsulation materials, we select inks that are in the states of solution, liquid, paste and resin. While the electrodes are extrusion printed with metallic nanoparticles and a eutectic liquid metal, the active layer is spray printed with a semiconducting polymer, poly2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV). Compared to extrusion printed devices, OLEDs with spray printed active layers demonstrated an improved irradiance and lifetime, attributable to the reduced barrier to charge transport, along with tunable electrical characteristics via the layer thickness. Further, by leveraging the yield-stress behavior of the oxide shell that wraps the liquid metal droplets, we implement a mechanical compression process to reconfigure the morphology of the cathode array and yield an improved contact area for the polymer-metal junction. The repeatable reconfiguration process creates a spatially uniform liquid metal array to be interfaced with the extrusion printed top interconnects. Finally, the device was encapsulated with polydimethylsiloxane (PDMS) that is cast into an extrusion printed silicone holder to form a flexible and transparent top layer. This novel device structure and 3D printing approach lead to a proof-of-concept demonstration of a flexible 8×8 OLED display with a 100% pixel working rate.

Stretchable touch sensors are attractive for a number of applications including wearable electronics and soft-robotics. In flexible and stretchable devices, all materials including conductors, semiconductors, and insulators must all be effectively integrated to avoid mechanical failure during repeated loading. While there have been a number of touch sensors demonstrated, they often lack mechanical stability necessary for long term application. Here, we introduce an all-solution processed stretchable touch sensor array that employs embedded Ag NW electrodes and a porous elastomer dielectric with stable operation under a strain range of over 50% and a large pressure sensitivity. Ag nanowires (NWs) are chosen as the electrodes as they provide high electrical conductivity and mechanical durability. However, achieving high pattern resolution can be a challenge. We demonstrate an effective processing strategy of printing Ag NWs through electrohydrodynamic (EHD) jet printing to achieve precise and complex patterns. The EHD printed Ag NWs were then embedded in a thin polyimide (PI) substrate providing physically stable electrodes. The Styrene-ethylene-butylene-styrene (SEBS) gate dielectric is printed on the PI substrate followed by polymer semiconductor film and finally the Ag NW source/drain electrodes. Touch sensing was introduce into the devices by forming a nano-porous SEBS layer employing a breath figure method. Through introducing the porous SEBS the transistors become effective touch sensors. We demonstrate that the porous structure can be tuned to vary pressure sensitivity and operating range. We then transform the ultraflexible touch sensors to stretchable touch sensors through employing a kirigami cut pattern to the PI substrate through laser ablation. One strength of this approach that the EHD can be used to pattern the electrodes to compliment the kirigami cut pattern. The resulting transistors show consistent performance during hundreds of cycles of bending or stretching. In addition, the porous SEBS touch sensors can sense a wide range of pressure from 80 Pa to 19 kPa.
In summary, in this talk we highlight the novel EHD Ag NW printing process, the formation of nano-porous SEBS thin films, and kirigami cut design and processing to achieve physically robust stretchable touch sensors. Using our unique processing methods, we are able to demonstrate an ultra-thin flexible pressure sensing transistor array. The combination of EHD printing and porous SEBS dielectric hold great potential for achieving high performance touch sensors with excellent sensitivity and mechanical robustness.

One reason why semiconducting (π-conjugated) polymers are amenable to the manufacture of large-area devices is because they are solution processable, making them compatible with ink-based printing or roll-to-roll deposition processes. However, the majority of literature on the mechanical properties of conjugated polymers deposit films by spin coating, a non-continuous, non-scalable process. Different deposition processes inherently have different processing conditions (and likewise, there exists a semi-infinite permutation of processing parameters) that affect the physical properties of the resulting polymer film. These morphological characteristics in turn affect the mechanical and electronic properties of the conjugated polymer film. It is currently unclear how different scalable deposition processes affect the mechanical properties, electronic properties, and morphologies of conjugated polymer films. In this talk, we compare poly(3-heptylthiophene) (P3HpT) films deposited by three roll-to-roll deposition processes – interfacial spreading (i.e., “floating film-transfer method”), solution shearing (i.e., “blade coating”), and spray coating – to those produced by spin coating. We find that the deposition process significantly affects the mechanical and electronic properties of the resulting films. We attribute these differences to changes in the amorphous and crystalline morphologies of the P3HpT films, which holds significant implications for considering how conjugated polymer films are deposited for different applications.

Light trapping with textured surfaces is an important technique in photovoltaics used to increase the optical path length without increasing the thickness of the absorber. However, the use of texture limits the application of this strategy in organic and tandem silicon/perovskite photovoltaics, among others. This limitation arises because, while there are a variety of methods available for forming thin films of electronic polymers, very few of them can form thin and conformal coatings on surfaces bearing relief structures on the micron scale. Of the methods that are capable of conformal coverage of such topography, most are applicable only to coatings that can be polymerized in situ—for example, by chemical vapor deposition—and are, thus, not amenable to polymers with complex molecular structures, such as π-conjugated (semiconducting) polymers. This presentation describes a method termed solid-phase deposition (SPD). The SPD process is a variant of water transfer printing whereby a thin film of a semiconducting polymer is suspended on water and taken up by a substrate bearing micron-scale relief structures (in this case, random pyramids such as those used for light management in silicon photovoltaics). Under ambient conditions, solid films that are sufficiently compliant (thin, ductile, or of low modulus) can coat these surfaces readily. Stiffer films comprising higher modulus polymers can be made amenable to the process by the application of heat or solvent vapor. We successfully formed coatings from films of poly(3-alkylthiophenes) spanning the range from glassy (alkyl = butyl) to rubbery (alkyl = heptyl), along with the low-bandgap polymers DPP-DTT and PTB7-Th, on textured silicon and indium tin oxide (ITO) surfaces.

Understanding soil and plant properties in real-time and at high-spatial resolution is critical for optimizing agricultural input use, and for evaluating soil and plant health. Obtaining such information in real-time and at high spatiotemporal resolution can be a challenge due to limited functionality and high costs when large numbers of devices are used, thereby limiting management approaches and potentially leading to excess input and energy use. As such novel instrumentation that can directly sample soil at high spatial density (10s-100s of meters) is needed to capture current conditions and enable optimized management strategies.

In this report three device types for monitoring soil and plant properties will be described. 1) Capacitive moisture and potentiometric nitrate sensors fabricated from biodegradable materials (conductors, substrates, encapsulants, etc.). The use of transient biodegradable materials enables large numbers of devices to be widely distributed in a field without the need for maintance or collection. 2) Fuse-like microbial activity sensors based on biodegradable materials that enable in-situ determination of the activity of microbes in soils which is important for understanding soil health. 3) Ion selective organic electrochemical transistors (OECTs) for determining the concentration of target ions of interest direclty in whole plant sap. In all of these cases the sensors are fabricated using printing techniques, primarily screen and ink-jet printing, as additive manufacutuing approaches readily enable creation of large numbers of devices, are compatible with a wide range of materials, and assists integration into complete measurement systems.

Thin-film photovoltaics with functional components on the order of a few microns, present an avenue towards realizing additive power onto any surface of interest without excessive addition in weight and topography. To date, demonstrations of such ultra-thin photovoltaics have been limited to small-scale devices on glass substrates, fabricated through vacuum deposition or with only some of the device layers solution-processed. Furthermore, such ultra-thin devices show limited mechanical integrity for everyday human handling. The solution-processability of such device architectures presents an avenue for integration into rapid manufacturing processes and their translation from lab-scale demonstrations into consumer ready technologies. In this work we demonstrate ultra-thin organic photovoltaic (OPV) modules produced fully through scalable solution-based processes (slot-die coating and screen printing). We further demonstrate integration onto light-weight (<15 g/m²) and high-strength (5kN/m tensile strength) composite fabrics, resulting in large-area (> 10cm x 10cm) prototypes of durable fabric-PVs capable of withstanding over 500 roll-up cycles. Such processing presents a path towards realizing solar technologies with sufficient mechanical integrity for applications requiring ultra-lightweight form-factors including tents, sails, drones, micro-robots and wearables.

Organic material-based conformable electronics are optimal candidates for components of bioelectronic circuits due to their inherent flexibility and soft nature. We have shown that internally ion-gated organic electrochemical transistors (IGTs) operate by leveraging ion reservoirs inside their channels to dramatically improve the operation speed and integration capacity of this ion driven transistor. In order to establish IGTs as versatile building blocks of bioelectronics, it is critical to establish fabrication strategies to enable IGT-based high-density integrated circuits for creation of electronic components such as operational amplifiers, multiplexers and logic gates. Here, we report a fabrication process that enables creation of IGTs with channel length of < 500nm, that have high transconductance (> 10 mS) and high speed (< 1 µs). This vertical fabrication strategy allows effective isolation of adjacent transistors, minimizing potential for cross-talk and gate current leakage. Furthermore, it is possible to encapsulate the circuit using long-term implantable grade (parylene C) material. Furthermore, we developed a micron-scale perforation strategy to ensure intact hydration paths for transistor channels while maintaining a low leakage current. We used these transistors to develop high density (IGTs)-based digital logics, voltage amplifiers, operational amplifiers and multiplexers. Overall, the high speed and transconductance of IGTs along their ability to be densely fabricated with minimal crosstalk facilitates their applicability to a broad range of flexible and conformable electronics.

Tailoring the structure of interface between polymer and ceramic nanoparticles is considered as one of the key approaches to achieve high energy density in next generation dielectric capacitors. To achieve high energy density, we have attached surface engineered barium titanate nanoparticles (~100 nm) to polymer by forming covalent bonds between the nanoparticles and polymer matrix. Nanoparticles used for this work are surface engineered by using a three-step process. The first step of hydroxylation attaches -OH groups which are the reactive sites for the second step of silane treatment. The third step of monomer grafting ensures the formation of covalent bond between the nanoparticles and monomers and provides higher mobility to reactive terminations on nanoparticle surface. Thiol-ene matrix is prepared by a radical mediated process from a mixture of monomers Pentaerythritoltetrakis (3-mercaptopropionate), 2,4,6 Triallyloxy-1,3,5- triazine and 1,3- Diisopropenylbenzene. The monomers and nanoparticles undergo click reactions without the need of photo-initiator when the mixture is processed with an intense pulsed light from a xenon flash lamp. We compare two different routes of surface engineering: an alkene-ended silane (3- Acryloxypropyl trimethoxysilane) treatment followed by thiol monomer grafting, and a thiol ended silane (3 Mercaptopropyl trimethoxysilane) treatment followed by alkene monomer grafting. Nanocomposites with significantly higher electrical breakdown (~5 MV/cm) are prepared which implies an improved interface with reduced defects. The as prepared nanocomposites show energy density as high as 26 J/cm³ while the dielectric loss is maintained below 0.2. The results from this study show that interface can be improved by increasing the surface density and mobility of the reactive terminations on nanoparticle surface. Photonic curing process utilized in this work is a roll-to-roll amenable, rapid, and high-throughput method. The ability to tailor the interface to obtain desired properties while preparing nanocomposites in large scale using a cost-effective method provides an easy route to an industrial scale method for the preparation of high-energy density nanocomposites.

The design of appropriate dielectrics plays a crucial role in the performance of organic field-effect transistors (FETs). Along with the active semiconductor layer, the dielectric-semiconductor interface dictates charge transport properties in FETs. Polymer ferroelectric dielectrics with their high dielectric constants are attractive for low-operating voltage FETs. Another route for enhancing the dielectric constant of polymer dielectrics is via the incorporation of semiconducting and insulating nanoparticles. Amongst magnetic nanoparticles, cobalt ferrite (CoFe₂O₄ or CFO) has received a lot of interest in sensing and biomedical applications. Its insulating property is attractive in polymer gate dielectrics. CFO magnetic nanocrystals, soluble in organic solvents, were synthesized by a thermal decomposition method and coated with polymers such as poly(vinyl alcohol) (PVA). Improved performance of organic FETs (both small molecule and polymer) using CFO incorporated non-ferroelectric dielectrics are observed. In particular, the threshold voltage and the subthreshold swing are lowered in pentacene and other organic FETs using CFO incorporated cross-linked poly(4-vinyl phenol) (PVP) dielectric. The application of an external magnetic field allows for another parameter to tune the device performance. We will further discuss a nonlinear optical probe for investigating carrier transport in such FETs.

This work was supported by National Science Foundation under Grant No. ECCS-1707588

A microcavity OLED, consisting of a conventional OLED stack with two metallic mirror electrodes, shows narrow-band emission centered around specific peak resonant wavelengths. These cavity modes are analogous to the energy states found in any resonator system, including musical instrument strings and 1-dimensional quantum square wells. The first three cavity modes are directly pumped by varying the thickness of the microcavity structure. The impact of the location of the dipole emitter within the microcavity on outcoupling efficiency and polarization is explored. The microcavity OLED is then used as a unit cell in a planar, 1-dimensional photonic crystal. Stacking N microcavities splits the resonant modes into N discrete states; a photonic band centered at the single microcavity state. Devices are fabricated by thermal evaporation with in-vacuo shadow mask transfers to enable parallel-connected unit cells. The photonic density of states is controlled by varying the thickness of the semistransparent metal mirror electrodes. A photonic Peierls bandgap is tuned by varying only alternate mirror thicknesses. Device parameters, including N, the thickness of the semi-transparent metal electrodes, and dipole emission position are correlated to emission properties such as peak wavelength, FWHM, and Q-factor for each of the photonic states. The experimental results are guided by a predictive computational modelling tool, which is critically important for the complex-architecture devices.

Film morphology plays a major role in polymer LED device performance. As an example, blue LEDs fabricated using polyfluorene (PFO) have shown an increase in luminance efficiency with the presence of minimal crystalline domains of a specific phase.[1] Previous work has demonstrated that PFO deposition via resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) promotes this crystalline β-phase formation in PFO while maintaining device-quality film surfaces.[2] RIR-MAPLE is a unique variation of pulsed laser deposition featuring an Er:YAG laser (peak wavelength of 2.94 microns) that is resonantly absorbed by the hydroxyl bond vibrational mode.[3] Therefore, PFO deposition occurs by formation of a frozen “oil-in-water” emulsion target in which the polymer is dissolved in primary solvent trichlorobenzene (TCB), and dispersed within the water matrix that resonantly absorbs the incident laser energy. Phenol is added as a secondary solvent to lower the emulsion vapor pressure, and minimal surfactant is added to help emulsion formation. The resonant laser absorption sublimates the water-ice matrix and transfers intact emulsified polymer particles directly from the complex emulsion to the substrate without degradation. Therefore, the morphology of the complex emulsion determines the film morphology. [2,4] In this work, the impact of the complex emulsion morphology on LED device performance is investigated.

Given that the complex emulsion target used in RIR-MAPLE contains multiple components (polymer, primary solvent, secondary solvent, surfactant, and water), the existing two-phase emulsion theory cannot predict accurately the resultant morphology. Empirical studies of RIR-MAPLE deposition have shown that an emulsion recipe with a concentration of 0.83 mg/ml PFO in water yields device-quality, pinhole-free films containing around 6% crystalline β-phase.[2] Two different emulsion recipes that maintain this specific polymer concentration in water but change the total water content in the emulsion were developed. The emulsion recipe with high water content contains 5mg/mL PFO in TCB with emulsion ratio of 1:0.5:6 (TCB:phenol:water), and low water content recipe has 2.5 mg/mL PFO in TCB with emulsion ratio of 1:0.25:3 (TCB:phenol:water). Despite having a drastic difference in emulsified particle size (dynamic light scattering), two outcoming films showed similar surface roughness and morphology determined by atomic force microscopy.[4] Further, cryo-transmission electron microscopy images of the frozen emulsions showed that the observed difference in particle sizes resulted from large agglomerations of emulsified particles occurred in the low water content emulsion.[4] While these differences in emulsion morphology did not impact the overall film morphology, it is not known how these features could impact LED performance. It is anticipated that the networked emulsified particles of the low-water content emulsions will yield better LED performance due to better charge injection.

Therefore, this work will investigate the impact of PFO complex emulsion morphology, as determined by the overall water content, on LED performance for active region films deposited by RIR-MAPLE. The LEDs will be fabricated using the following device heterostructure: ITO/PEDOT:PSS/PFO/LiF/Al. LED device characterization will include current density vs. voltage, luminous flux, and electroluminescence spectra. This work will reveal the relationship between polymer emulsion morphology and PFO LED device performance, thereby providing a guide for future RIR-MAPLE-deposited PFO devices. This work is supported by the National Science Foundation under Grant No. NSF CMMI-1727572.

Reference:
[1] Peet. J, et al., Adv. Mat., 20(10), p 3938, 2008
[2] Ferguson, S; et. al., J. Electron. Mat, vol. 48(5), 2019
[3] A. D. Stiff-Roberts and W. Ge, App. Phy. Review, vol. 4, pp. 041303, 2017
[4] S. Ferguson, et. al., Electronic Materials Conference, Ann Arbor, MI, June 2019

Flexible electronics have attracted a significant attention in the recent years due to their high demands in a variety of fields such as medical field, sports field, cosmetic field and so on. A number of manufacturing technologies have been developed to fabricate the flexible electronics based on different functional materials. However, there are quite a few challenges in current techniques such as high costs, time consuming and difficult scale-up process, making it hard to produce a large amount of high-quality flexible electronics. Here we reported a novel Corona-enabled Electrostatic Printing (CEP) technology to manufacture flexible electronics in an ultrafast manner of which the printing time can be down to 100 ms. A detail mechanism study is shown in this paper to demonstrate the printing principle of CEP by using simulations and experimental studies. With the help of CEP, a variety of functional material such as graphene, PEDOT: PSS and CNT were able to be fast printed to fabricate flexible multifunctional sensors that can detect strain, temperature and humidity signals. And CEP could also be integrated with roll-to-roll (R2R) process for ultra-fast mass production. The binder-free feature of CEP enables a fast, simple and pure printing process, producing high quality flexible electronics such as graphene-based sensors that possess high sensitivity and acoustic signal detection capabilities.

p-Doping organic semiconductors with high ionization energy (IE) remains challenging, as these materials require dopants with correspondingly high electron affinity (EA). Recently, an organic molecular p-type dopant, hexacyano-trimethylene-cyclopropane (CN6-CP) was synthesized and shown to have an EA of 5.87 eV measured by cyclic voltammetry (CV), almost 300 meV higher than other known high EA molecular dopants. In this work, we measure the EA of CN6-CP using inverse photoelectron spectroscopy (IPES) and confirm a high value of 5.88 eV in condensed phase. We then use CN6-CP to dope phenyldi(pyren-2-yl)phosphine oxide (POPy₂), a wide energy gap (3.6 eV), large IE (5.87 eV), organic semiconductor and demonstrate a more than two order of magnitude increase in film conductivity. Using CN6-CP and (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium (RuCp*Mes)₂, a highly reducing n-dopant capable of n-doping a low EA (2.2. eV) material such as POPy₂, we build a rectifying p-i-n homojunction with a 2.9 eV built-in potential. We demonstrate blue emission from this p-i-n structure, indicating both hole and electron injection into this large gap diode through the n- and p-doped regions.

Organic thin-film transistors (TFTs) are being developed for flexible electronics applications, such as rollable or bendable active-matrix displays and conformable sensor arrays. The deposition and patterning of the various transistor components (gate electrode, gate dielectric, source/drain contacts, organic-semiconductor layer) can be accomplished using a wide range of methods, which are often categorized into vacuum-based and solution-based approaches. All of these have real or perceived advantages and drawbacks, which are often related to the anticipated costs of large-scale commercial manufacturing. A lot about these can be learned by analyzing the present-day commercial manufacturing of active-matrix organic light-emitting diode (AMOLED) displays for mobile phones and television sets. All of the OLED and TFT components in these displays are deposited by vacuum-based methods, while patterning is accomplished by optical lithography (TFT stack, anodes, interconnects) or stencil lithography (OLED semiconductors and cathodes). The only process steps involving the use of liquid organic solvents are those in which the surface is coated with polyimide (to create the flexible substrate) or photoresist (in preparation for optical lithography). By keeping the use of organic solvents, which are often toxic, carcinogenic and environmentally harmful, to an absolute minimum, the costs of proper solvent management and disposal and thus the total manufacturing costs are kept as low as possible. The commercial manufacturing of organic TFTs based on small-molecule semiconductors can potentially be even less expensive, since these TFTs can be fabricated entirely without organic solvents, simply by depositing and patterning all materials in vacuum directly on the flexible substrate. Following this general proposition, organic TFTs as well as unipolar and complementary circuits with good static and dynamic performance can successfully be fabricated on a variety of flexible substrates, including plastics and paper.

Evaporative self-assembly of semiconducting polymers has been studied extensively in order to attain a facile and low-cost means of preparing hierarchical micrometer and nanometer-scale features. Yet, solvent evaporation is often a time consuming kinetic process that limits any potential for practical applications toward organic and printed electronics. In this presentation, we introduce a novel high-throughput and continuous printing technique for semiconducting polymer via a modified doctor blading technique with oscillatory meniscus motion, termed meniscus-oscillated self-assembly (MOSA). The oscillatory meniscus motion of the roller regulates a periodicity of the stick-slip phenomenon between the roller and polymer solution that can continuously output semiconducting polymer line arrays. These printed lines demonstrate highly defined boundaries and drastically enhanced crystallinity via shear-induced crystallization. This novel continuous printing technique has the capacity to pave a new path forward for practical applications in printed electronics.

Semiconducting Polymers (SPs) have received widespread attention due to their promising qualities like superior absorbance/emission, easy chemical tunablity, low-temperature solution processing, lightweight and flexible substrates, and low environmental toxicity. A significant obstacle for the industrial development of SPs is the lack of a patterning technology that is inexpensive, rapid and viable and capable of producing sub-micron features. Photomask lithography is impossible because the SPs cannot withstand the processing steps. The Moule group recently developed a new photopatterning concept that enables micropatterning of SPs. We present a novel solution based optical patterning method that is compatible with any non-cross linked SP, termed Projection Photothermal Lithography (PPL). We have built a lab scale PPL microscope and demonstrated rapid (∼ 4cm²hr⁻¹), large single exposure area (0.21 mm²), sub-μm patterns can be obtained optically. Selective polymer domains are removed as a photo-induced temperature gradient enables selective dissolution. We estimate the commercial patterning throughput of (∼ 5 m²hr⁻¹) can be obtained through optimization of optical components.

Ambipolar transistors, i.e. transistors with symmetrical n- and p-type performances, open new venues for the design and integration of high-density, efficient and versatile circuits for advanced technologies. Their performance requires two processes: efficient injection of holes and electrons from the metal electrodes into the semiconductor; and transport of both carriers through the semiconductor. Organic semiconductors (OSC) support ambipolar transport, but charge injection is strongly asymmetric due to inherent misalignment of the electrode work function with both conducting levels of the OSC. Here we will introduce a new electrode concept capable of efficiently injecting both type of charge carriers into OSCs. The electrode has a mosaic-like structure composed of islands of two metals with high and low work functions, in this case Al and Au, respectively. Under suitable applied bias the Au (Al) domains in direct contact with the OSC allow efficient hole (electron) injection into the HOMO (LUMO) level. Implementing this electrode as both source and drain in organic filed effect transistors (OFET) lead to fully balanced ambipolar performance while maintaining high ON/OFF ratios. We then used the ambipolar OFETs to significantly simplify circuit design and fabrication of digital and analogue elements, i.e. digital inverter and an analogue phase shifter, composed of one type of transistor only. Finally, we demonstrate that a single ambipolar OFET can replace several unipolar transistors to fabricate digital transmission gate circuits that utilize one transistor only. The new electrode design concept can include other metal combinations and compositions to balance ambipolar injection, and the use of the mosaic electrodes can be extended to other electronic devices that require ambipolar charge injection such as light emitting transistors, memory devices etc.

Small area (~0.1cm²) hybrid organolead-halide perovskite solar cells have recently approached efficiencies near that of single crystalline silicon cells. Scalable methods such as slot-die coating and spray coating offer the possibility of high quality large-area perovskite deposition at these performance levels; however, resistive losses across the transparent front electrode highlight the need for alternative architectures and designs. A variety of alternative designs are explored—gridline, module, and hybrid designs—to mitigate front electrode resistive losses, employing laser-scribing to achieve both controlled gridline deposition and monolithically integrated series interconnections. We have developed a laser scribing mechanism to achieve gridline patterning and removal of the electrodes and active material utilizing a single wavelength (1064nm) pulsed laser at ultrafast processing speeds (>1m/s). This process coupled with improved module design enables high throughput device and module development.

Our work first develops the primary scribing mechanism employed: TCO-based lift-off. We demonstrate the unique characteristics of this scribing method as it interacts exclusively with the front electrode transparent conducting oxide (TCO) material to perform all the necessary scribes between both the gridline and module architectures. With a single wavelength source, controlled removal of either the front electrode or active layers is achieved to perform each scribe step: front electrode patterning (gridline patterning, module P1), creating the series interconnection channel (module P2), and rear electrode patterning (module P3). The unique lift-off mechanism results in particularly improved P2 performance via reduced contact resistance and increased short-circuit current and fill factor compared to traditional P2 ablation mechanisms that often require ultra-short laser pulses to avoid residual thermal damage to the perovskite layer. By avoiding interactions with the perovskite entirely, the TCO-based lift-off mechanism is compatible with lower cost, longer pulse duration laser systems and is capable of complete device and module patterning on a single system.

Following the development of a uniquely broadly applicable laser-scribing mechanism, gridline, module, and hybrid designs are considered for large-area perovskite photovoltaic applications. Resistive losses of each of the dominant components (front electrode, gridlines, scribed interconnections) are evaluated against geometric design considerations to find the optimal device and module geometries. Reductions in resistive and geometric losses motivate the selection of an optimal architecture, though alternative designs are considered for potential compatibility with projected processing challenges.

In this work, we present our recent progress on the photonic-induced conversion of new semiconducting ink materials and their use for the fabrication of new hybrid opto-electronic sensor platforms. First, we will present the manufacturing-ready photonic curing process and its key advantages for high-speed treatment of large surfaces compared with standard laser-induced conversion. Most importantly, simulations will be used to demonstrate how photonic sintering can achieve high-temperature conversion of the functional inks without compromising the structural integrity of low-temperature substrates.
Then, we will illustrate how slurries of silicon-based particles can also be sintered by photonic curing. Altogether, we will describe how we used a thorough understanding of the chemistry combined with a cutting-edge photonic curing expertise to enable novel sensor designs and their integration in fully-autonomous flexible hybrid electronic sensor platforms. Most importantly, we will show how we modified the chemistry of a sol-gel TiO₂ precursor in order to generate high concentrations of oxygen vacancies and describe how this modified chemistry can allow a conversion from amorphous to anatase and rutile polymorphs at room temperature and in ambient environment using low-power photonic irradiation [1]. As a result, unique multi-morphic film architectures are achieved using digital additive manufacturing strategies coupled with both low-power laser irradiation and photonic curing [2].
In turn, these unique metal-oxide film structures enable new printable devices architectures for a wide range of applications from biomedical to energy harvesting. We will show a few concrete examples in this presentation [4-5].

[1] J. Benavides et al., ACS Applied Energy Materials 1, 3607 (2018)
[2] L. F. Gerlein et al., Advanced Engineering Materials 22, 1901014 (2020)
[3] D. Banerjee et al., Scientific Reports 9, 17994 (2019)
[4] D. Banerjee et al., Engineering Research Express 2, 035021 (2020)
[5] C. Trudeau et al., Flexible Electronics 4, 2136 (2020)

Organic light emitting diodes (OLED) employing two planar metallic electrodes encasing the organic layers form a microcavity etalon. This structure allows a control on the output emission characteristic of the light source resulting in a significant narrowing of the emission bandwidth and angular dispersion of light. We investigate the emission spectra using a single organic emitter molecule, by engineering the position of the dipole emitter in the microcavity structure. The position of the dipole was varied by varying the relative thickness of the electron transport layer ETL and hole transport layer HTL in fixed total thickness and fixed optical pathlength device structures. The output characteristic of a microcavity OLED is affected by its optical path length. Here we test this hypothesis that a fixed optical pathlength generates the same optical characteristics even with the changing dipole position. Varying the thickness of the ETL and HTL layers without changing the total optical pathlength of the device in principle generates the same output characteristic of light. We find that fixing the optical pathlength in the cavity functionally fixes the peak emission wavelength, but the dipole position within the cavity has strong impact on the efficiency of outcoupling of light and on the polarization.

The development of support materials is one of the most critical challenges in a wide range of fields including energy, the environment, and catalysis. This is because support materials can endow guest materials with new properties. In catalysis, the catalyst–support interaction influences the catalytic performance, so it is urgently required to create a high-affinitive sites in porous supporting media. Among the promising candidate support materials, highly porous and crystalline metal–organic frameworks (MOFs) have received considerable attention. Although many MOFs have been successfully utilized as supports, their metal sites have weak affinity for the guest material due to blockage by organic linkers. We hypothesized that the affinity could be increased by tearing the MOFs into smaller pieces based on this intrinsic limitation. Accordingly, we developed a new pathway for producing metal–organic fragments by tearing MOFs via hydrogen plasma bombardment. The hydrogen plasma exposed more metal nodes and created more delocalized electrons, resulting in high-affinity sites within the metal–organic fragments. Furthermore, metal-organic polyhedra (MOP), are distinct from MOFs, have opened up a possibility for controlling catalyst-support interactions. Here, we focus on the opportunity to use the pore structure without the hindrance to electron transfer by using MOP. As a result of this strategy, the metal–organic fragments and MOP were successfully applied as highly affinitive support.

Semiconducting polymers with satisfactory charge transport properties are demanded to meet the requirements for a vast range of applications in the field of flexible and printed electronics.¹ However, their effective integration into electronic devices is still hampered by a partial understanding of the interplay between microstructure and transport. Starting from 2009, the investigation of innovative donor-acceptor copolymers led to evidences of efficient transport in apparently poorly ordered films,² subverting the old perception of long-range crystallinity as the key for transport efficiency. Lately, evidences for well interconnected and efficient charge percolation pathways within otherwise disordered films was consistently reported in high mobility polymer field-effect transistors (FETs);³ this is made possible by polymers inherent anisotropy, enabling long-range alignment of molecular backbones within thin films microstructure and improved unidirectional film interconnectivity along the same direction. Accordingly, strong transport anisotropy is realized, where transport is maximized in the direction of backbone alignment and improved interconnectivity.⁴ Nevertheless, a substantial gap remains to be filled between emerging models for transport, mostly accounting for short range interactions, and the actual charge transport mechanism occurring within the multi-phase percolation paths of aligned FET channels.
Here we report on a post-processing methodology enabling control on those microstructural features that are critical for polymer transport, i.e. molecular alignment, degree of interconnectivity and crystalline structure. We show that a systematic lamellar thickening and improved molecular alignment can be induced upon the accurate choice of thermal post-processing temperatures, tailored to the actual thermal transitions of solution processed thin films. In fact, annealing at a temperature at which only the smaller crystals melt but the larger ones are maintained, allows for the thickening of non-molten crystals and the epitaxial growth of the molten fraction of molecules on the non-molten surface, thereby guiding molecular alignment of newly formed crystals.⁵
We show that our post-processing strategy represents an effective tool for charge percolation optimization, up to the demonstration of ideal, single crystal-like FETs characteristics and drastically reduced thermal barriers to transport. Validation tests were carried out on two paradigmatic n-type molecular systems: a diketopyrrolopyrrole-based copolymer, where electrons delocalization within crystalline domains can take place, and a series of naphthalene-diimide bithiophene copolymers, conversely characterized by a strong intra-chain charge localization at any aggregation state. Such a diverse investigation allows for a better understanding of the effect of critical chemical features for a polymeric semiconductor, like electronic structure, molecular conformation and solubilizing side chains, on charge percolation efficiency within aligned films, paving the way toward a more efficient design of polymeric semiconductors for printed electronics applications.

1 Mario Caironi and Yong-Young Noh, Large area and flexible electronics. (John Wiley & Sons, 2015); Ana Claudia Arias, et al. Chemical Reviews 110 (1), 3-24 (2010).
2 He Yan, et al. Nature 457 (7230), 679-686 (2009); Rodrigo Noriega, et al. Nature materials 12 (11), 1038-1044 (2013).
3 Brian J Eckstein, et al. Advanced Functional Materials 31 (15), 2009359 (2021); Sam Schott, et al. Advanced Materials 27 (45), 7356-7364 (2015); Alessandro Luzio, et al. Scientific reports 3 (1), 1-6 (2013); Nicola Martino, et al. ACS nano 8 (6), 5968-5978 (2014).
4 Sadir G Bucella, et al. Nature communications 6 (1), 1-10 (2015); Edward J. W. Crossland, et al. Advanced Materials 24 (6), 839-844 (2012).
5 A. Luzio, et al., Nat Comm10 (1), 1-13 (2019).

N-type dopants for organic semiconductors with reasonable ambient stability have been difficult to develop. In addition to this, dopant and organic semiconductor pairs often exhibit morphological incompatibilities. One promising, yet relatively unexplored, tactic for avoiding morphological disruption to the semiconductor after doping is to covalently bind a doping functionality to the conjugated section. Quaternary ammonium hydroxide groups have previously been bound to rylene diimides and high n-type conductivities (≈ 0.5 S/cm) and thermoelectric power factors were observed after film-formation and annealing.^{1, 2} It is likely that the active dopant is a tertiary amine,³ however multiple routes of quaternary ammonium hydroxide decomposition have been shown to lead to tertiary amine products.⁴ Any chemical transformations, such as quaternary ammonium breakdown, can have a significant effect on the morphology of the doped film and complicates any investigation into the doping mechanism.

A water-soluble and air stable naphthalene diimide molecular semiconductor, called NDI-OH, has been synthesised which is covalently bound to a quaternary ammonium hydroxide group. When thin-films were formed by drop-casting absorption features consistent with doping were observed. The chemical structure of the dopant precursor was determined for the first-time using NMR and mass spectrometry. It was seen that the ammonium hydroxide precursor is a product of ring-opening hydrolysis on the imide functionality of the naphthalene diimide. After treatment at high temperature it was found that the reverse hydrolysis reaction also took place. The dynamic nature of this ring-opening reaction was found to have a significant effect on the chemical composition of the doped films.

A steady increase in electrical conductivity was observed with increasing temperatures of drop-casting in NDI-OH thin-films. Spectroscopic structural analysis of the conducting films revealed changes to both the quaternary ammonium moiety and the NDI ring-system at higher drop-casting temperatures. These structural changes lead to large differences in film morphology, doping efficiency and therefore conductivity in films prepared at different temperatures. Film morphology was probed using a combination of atomic force microscopy and X-ray scattering. A combination of EPR and UV-vis absorption spectroscopy was also used to determine the amount and type of charge carriers generated. The results presented show that the functionality of this promising group of water-soluble conducting organics must be carefully tuned by considering atleast two different chemical transformations. The extent and direction of the dynamic hydrolysis reaction plays a role in the film morphology while the breakdown of the quaternary ammonium group contributes to the generation of the active dopant.

References:
1) Reilly, T. H.; Hains, A. W.; Chen, H.-Y.; Gregg, B. A., A Self-Doping, Advanced Energy Materials 2012, 2 (4), 455-460.
2) Russ, B.; Robb, M. J.; Brunetti, F. G.; Miller, P. L.; Perry, E. E.; Patel, S. N.; Ho, V.; Chang, W. B.; Urban, J. J.; Chabinyc, M. L.; Hawker, C. J.; Segalman, R. A., 2014, 26 (21), 3473-3477.
3) Russ, B.; Robb, M. J.; Popere, B. C.; Perry, E. E.; Mai, C.-K.; Fronk, S. L.; Patel, S. N.; Mates, T. E.; Bazan, G. C.; Urban, J. J.; Chabinyc, M. L.; Hawker, C. J.; Segalman, R. A., Chemical Science 2016, 7 (3), 1914-1919.
4) Edson, J. B.; Macomber, C. S.; Pivovar, B. S.; Boncella, J. M., Journal of Membrane Science 2012, 399-400, 49-59.

Detection and understanding of unipolar and ambipolar charge accumulation and transport is the key to understand intrinsic performance of organic field effect transistor (OFET). However, it is difficult to determine the motion of the carrier with regular electrical tests. In this paper, the generalized gated four probe (G-GFP) technique [1] is extended to employ to investigate both few-mono-layer organic small molecular semiconductors and ambipolar polymer semiconductors in transistors.
For the p-type semiconductor, we fabricated monolayer semiconductor C8-BTBT films by shearing method and investigate the impacts of film thickness, contact metal, dielectric materials, and etc. by systematically monitoring the channel potentials. For the ambipolar transistors, we fabricated iketopyropyrrole thieno [3,2-b] thiophene copolymer (DPPT-TT) and studied the motion of carriers in the ambipolar regimes. For both types of OFETs, the evolution of local potential is monitored in the channel to establish a clear quantitative image of the accumulation and transmission of carriers during device operation. Both simulation and experimental results confirm that G-GFP is a suitable and feasible method to extract the carrier mobility and to reveal the key limiting factors that induce the non-ideal effects in OFETs.[2] Also, the ambipolar device with G-GFP structure can work as a single OFET inverter [3], which shows that G-GFP can be used as an effective tool for bipolar transistor analysis and help to realize logic functions.

[1] C. Liu et al, Adv. Funct. Mater. 2019, 1901700
[2] T. Yang et al, Adv. Funct. Mater. 2019, 1903889
[3] C. Liu et al, Adv. Electron. Mater. 2021, 2001134

Organic Doping is widely used for tuning the Fermi level of organic semiconductors. The precise control in the charge carrier density facilitated by doping allows to maximize the efficiency of organic optoelectronic devices [Walzer, K. et al., Chem. Rev. 107, 1233–1271 (2007)]. However, doping concentrations used in most studies are high (in the wt.% range), which limits the doping process itself resulting to a low doping efficiency [Tietze et al., Adv. Funct. Mater. 25, 2701–2707 (2015)]. In addition, these high doping concentrations can lead to large off-currents when used to dope the channel of Organic Field-Effect Transistors [Liu et al., ACS Applied Materials & Interfaces 12, 49857 (2020)].
In this presentation, we discuss the design of a rotating shutter system that is capable of controlling doping concentrations in vacuum processed films down to the 100 ppm level [R. K. Radha Krishnan et al. Adv. Opt. Mat. in press (2021)]. The increased control over the doping concentration is used to study doping at ultra-low concentrations in C₆₀ Metal – Oxide - Semiconductor (MOS) junctions. A small-signal drift-diffusion model is presented and used to determine the doping efficiency and activation energy of the doping process. With the help of a statistical model [Tietze et al., Phys. Rev. B 86, 035320 (2012)] the observed trends are fitted and design rules for future dopant/matrix combinations with higher doping efficiency are proposed.
Based on the results of the MOS study, C₆₀ OFETs doped at the 100ppm level are designed that operate at ultra-low voltages of 800mV only and retain an on/off ratio of up to 10⁷ despite doping. The devices have low subthreshold swing in the range of 80 mV/dec and a transconductance of up to 8 mS/mm.
To demonstrate that our results can be generalized, we will discuss the effect of ultra-low doping not only on the archetypical n-type semiconductor C₆₀, but study doping of the high-performance p-type organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene) DNTT as well. Here, doping enables a balanced ambipolar device characteristic with average electron and hole mobilities of approx. . Hole and electron injecting contacts are used to study the mechanism of majority and minority charge carrier generation inside the doped films.
Overall, it will be argued that doping is a key technology for organic field-effect transistors [Lüssem, B. et al., Chem. Rev.116, 13714–13751 (2016)], and allows not only to reach high performance, but to realize new device structures and to study mechanisms of charge carrier generation in organic semiconductors as well. The use of ultra-low doping concentration broadens the potential use of doping in OFETs, which will help to move the field further towards larger scale integration.

In this study, we developed zinc oxide (ZnO) based ultraviolet (UV) photoconductor devices with low persistent photoconductivity (PPC) effect by introducing polymer-assisted sol-gel precursor with deep-UV (DUV) photoactivation. The influence of DUV photoactivation on polymer-oxide complexes was investigated by comprehensive characterization analyses. Unlike poor current recovery of conventional oxide photoconductors with ohmic-contact, the ZnO films cast with polymer-mixed precursor solution exhibited much improved PPC effects with full current recovery, especially for the films irradiated by DUV followed by thermal annealing. The polycrystalline ZnO films in the photoconductor of reduced PPC effect exhibited larger grain sizes as well as fewer defect sites deriving poor recombination of photo-excited carriers. UV photoconductors based on PEI-involved ZnO films under DUV photoactivation showed excellent device performance represented by large photocurrent (~4 μA), rapid response/recovery time (21 s and 40 s), and enhanced on-off current ratio (~ 3.7) in combination with the reduced PPC effect.

One of the important factors to improve the performance of organic field-effect transistors (OFETs) is an effective charge carrier injection from the metal electrode to organic semiconducting layer, and thus various studies have been conducted so far to improve this. In this study, the organic semiconducting layer was formed by blending the thiol SAM materials with PBTTT-C14 solution to control the nature of the charge carrier injection in the top-contact bottom gate (TCBG) structure of the OFETs, for which a spontaneously generated SAM was produced through annealing process on the electrodes. This result is simple and applicable on the TCBG-structured OFETs, which is impossible to be applied with SAM treatment until now. The field-effect mobility of PBTTT-C14 FETs blended with 1-dodecanethiol was improved from 0.025 to 0.063 cm²/Vs as compared to that without blending, and the threshold voltage was shifted from 15 to 0 V, which is closed to an ideal onset operation of the device.

Here, in the context of the polymer film on an aqueous transfer process, the value of spreading coefficient (S) plays an important role in spreading the polymer solution droplet on the base media. A high S is key to improving the polymer film uniformity and crystalline structure, which affect the organic field-effect transistor (OFET) device performance. In addition, the films floating on the base media were transferred either upward (UST) or downward (BST) to differ their solidified orientations. The differences between the UST and BST cases confirmed that the solvent in the polymer solution droplet was rapidly extracted at the interface with the base media and was slowly evaporated near the air interface with the polymer solution. The results induce rapid solidification with poor structural properties at the base media interface and slow solidification with good structural properties at the air interface. The surface properties of the polymer semiconducting films were advantageous in the vicinity of the air/polymer solution interfaces for use as a channel active layer, resulting in good OFET device performances for the UST case.

LEDs are the foundation of lighting and display products surrounding us. While they have obtained great success in the commercial market, related research and development activities remain highly active aiming to enhance factors such as energy efficiency, stability, and environmental sustainability. Currently, commercial LED products are comprised of two key components: an indium gallium nitride e LED backlight with emission centered at 450 nm to cover the blue region of the visible spectrum, and powder inorganic phosphors on top converting blue light into longer wavelengths (e.g., green and red) to tune the emission of the device. A drawback of inorganic phosphors is that scattering of the emission from the micron-sized phosphor powders leads to substantial backscattering and subsequent absorption of the emission into the LED chip, and reabsorption losses in the phosphor itself, both of which reduce the overall light output of the final LED device. Organic dyes possess environmental advantages over inorganic phosphors because they are pi-conjugated molecules made from abundant elements (C, H, N, O, etc.) and are potentially bio-sourced. In this talk, I will present dyes that have been developed from theobromine, derived from cacao beans. When blended within an industrial polymer, poly(styrene-butadiene-styrene) (SBS), their enhanced solubility enables the formation of highly transparent films, crucial for reducing scattering loss in LEDs. Furthermore, resultant dye-SBS films achieved photoluminescence quantum yields (PLQYs) of around 90% under ambient conditions. Taking advantage of their transparency and solution processability, we fabricated a waveguide with this theobromine-dye-SBS composite, which was subsequentially assembled into an edge-lit LED device of no glare and enhanced aesthetics.

Polymers are essential materials for wearable electronic systems and soft robotics as active and passive materials. We will discuss our recent work on polymer architectures that lead to control of electronic and mechanical responses of organic electronic devices. First, we will discuss how single ion conducting polymers, known as polymeric ionic liquids (PILs), can be used as electrolytes in organic electrochemical transistors. PILs provide a unique method to control the current voltage characteristics of electrolyte-gated transistors. By appropriate molecular design of the PIL, the operation of polymer transistors can be switched from conduction at an electrical double layer to conduction in the bulk by infiltration of counter ions. The behavior of transistors with PIL gates reveals the differences in the electrical conductivity of semiconducting polymers at the surface and in the bulk at high charge carrier concentrations. Second, we will discuss how polymers with a bottlebrush architecture can be used to form super-soft elastomers useful for pressure sensors. The low mechanical modulus of bottlebrush elastomers, which is comparable to that of hydrogels, allows for the formation of capacitive pressure sensors with sensitivity comparable to human touch. These studies reveal the importance of molecular design on the development of new organic electronic devices.

Organic sensors, photodiodes, and solar cells have attracted much interest in recent years for their unique properties such as flexibility, lightweight, and the low cost and ease of their fabrication processes. While not destined to replace silicon-based technology, organic-based devices provide a promising technology for wearables, windows, as well as lightweight constructions that cannot hold the current massive photovoltaic (PV) modules.

A previous work done by our group[1] presents a device engineering approach to enhance the OPVs’ performance using modulation-doping of the hole transport layer (HTL). A planer heterojunction device was investigated using two well-studied materials 1,1-bis [(di-4-tolylamino) phenyl] cyclohexane (TAPC) as a hole transporting material, C₇₀ as the accepting material, and P-type dopants, C₆₀F₄₈. The dopants were induced in a δ-doped layer of 10 nm width within the TAPC layer. The presence of the doped layer within the device induces an internal electric field at the junction leading to enhanced charge separation efficiency, reaching 50% enhancement in the current at the maximum power point and 30% at 0.8V_oc.

To extend the applicability of the design to solution-processed cells and make it universal to (almost) any donor-acceptor pair, we developed a strategy based on a cross-linkable high band-gap polymer matrix[2] where the donor material of choice can be blended at ~20w% to serve as the charge transporting/extracting level.[3] Being insoluble it can be considered as part of the substrate and the flexibility in choosing the transport material makes it universal.

We will first describe the general properties of the enhanced charge-extraction structure utilizing Poly (vinyl carbazole) bearing cinnamate pendants (PVK-cin [50%]) as the x-linkable matrix, Poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine (PTAA) as the transport/extraction material, and C₆₀F₄₈ doped PTAA as the δ-doped layer. Next, we describe the properties of a complete cell that is based on PBDB-T-2F (PM6) and Y6 (BTP-4F) as the donor-acceptor pair, respectively. [4, 5] To place the experimental results in perspective, we conduct a detailed device simulation and compare the experimentally measured enhancement to the maximum theoretical prediction.


References:
[1] H. Shekhar and N. Tessler, "15% enhancement of the photocurrent at the maximum power point of a thin-film solar cell," Sustain. Energ. Fuels, 10.1039/D0SE00836B vol. 4, no. 11, pp. 5618-5627, 2020.
[2] O. Solomeshch, V. Medvedev, P. R. Mackie, D. Cupertino, A. Razin, and N. Tessler, "Electronic formulations - Photopatterning of luminescent conjugated polymers," Advanced Functional Materials, vol. 16, no. 16, pp. 2095-2102, 2006.
[3] Y.-J. Yu et al., "p-type doping in organic light-emitting diodes based on fluorinated C60," Journal of Applied Physics, vol. 104, no. 12, p. 124505, 2008.
[4] K. Li et al., "Ternary Blended Fullerene-Free Polymer Solar Cells with 16.5% Efficiency Enabled with a Higher-LUMO-Level Acceptor to Improve Film Morphology," Advanced Energy Materials, vol. 0, no. 0, p. 1901728, 2019.
[5] T. H. Lee et al., "Efficient Exciton Diffusion in Organic Bilayer Heterojunctions with Nonfullerene Small Molecular Acceptors," ACS Energy Letters, vol. 5, no. 5, pp. 1628-1635, 2020.

The presence of extrinsic dopant molecules can have a strong impact on the photophysical or electronic properties of organic semiconductor thin film devices. For example, Extrinsic dopant molecules present at low/ medium density can greatly enhance charge transport in p-i-n OLEDs, or provide recombination sites in the emissive layer. Dopants can act as charge traps, or exciton quenchers for study of transport phenomena in organic thin films. The presence of extrinsic analyte molecules in thin film chemical sensors can either modify the properties of photoluminescence (for example through fluorescence quenching) or electrical conduction. In device studies and applications, the dopants are conventionally introduced via co-deposition with the host material; whereas in chemical sensors, the analyte molecules are absorbed from either the surrounding vapour or solution phase after the film is formed.

In this paper we present study of the process of introduction and removal of small molecules into/from thin films of semiconducting polymers. We use photoluminescence quenching chemical sensors as a model system that can provide a platform for more general study of molecule transport in polymer films. We will describe a combined approach of experiments and simulations to explore molecular doping from the vapour and solution phases; The transport of sorbed molecules under the influence of diffusion and binding processes; and the retention and thermal desorption of the dopant.

We use a computational rate equation approach to model the binding and 1-dimensional diffusion of extrinsic molecules and estimate their populations within the film. We couple Fick’s laws of diffusion to chemical binding rate equations for arbitrary analyte diffusion constants and binding rates. We relate the extrinsic dopant molecule populations within the film to the film luminescence and study the influence of the molecular interaction constants on the time-dynamics of fluorescence quenching in the film. We calculate the transport of dopant molecules through the thin film when driven by a positive or negative diffusion gradient at the film surface, observe a strong asymmetry in the fluorescence quenching signal between the in-diffusion dynamics of the dopants and the desorption of stored dopants from the film.

We compare the computational study with experiments of the sorption and transport of nitroaromatic molecules in thin films of the commercial semiconducting polymer Merck Super Yellow and non-conjugated polymers AFLAS and poly(bisphenol A-co-epichlorohydrin. We study the doping of dinitrotoluene over a range of concentrations in the polymers when introduced from acetonitrile solution and from the vapour phase. Measurements of the retention of the dopant at room temperature and at elevated temperatures are made using a combination of absorption and photoluminescence spectroscopy. Combined with the computational model these experiments provide insight to the process and time dynamics of introducing extrinsic molecular dopants in thin polymer films, their distribution and long-term stability.

Hybrid perovskites which are composed of both organic and inorganic components, have recently been found to possess excellent optical and electronic properties making them highly suitable for applications such as solar cells, light-emitting diodes, and thin film field-effect transistors (FETs) (1, 2). However, a deeper understanding of the complex charge transport in thin films of these hybrid materials is currently lacking. Here, we fabricated three-dimensional perovskite FETs in the bottom-contact top-gate and the bottom-contact bottom-gate architectures using solution processing via the anti-solvent method (3) and identified unusual transfer characteristics that result in a lower than predicted field-effect mobility as well as unwanted hysteresis. To tackle these issues, we explore the effect of perovskite stoichiometry on the transfer characteristics of both methylammonium lead iodide and complex triple-cation perovskite FETs (3, 4). Those insights are correlated with the perovskite microstructure and local compositional information obtained using characterization techniques such as photoluminescence imaging, x-ray diffraction measurements and scanning electron microscopy, to unravel the central role of iodide ions in disrupting the expected FET transfer characteristics by partially screening the applied gate field. Furthermore, for each of the architectures, we present individual strategies in the perovskite processing methodologies and post-deposition cleaning protocols (5) that successfully suppress the unwanted ion migration. Our triple-cation-composition based FET device, optimized using Design of Experiments (DOE) principles, exhibits stable transfer characteristics with n-type behavior and remarkably low hysteresis even at room temperature. It also shows a good saturation electron field-effect mobility and an on/off ratio of ~10⁴ at room temperature. Our work presents effective strategies to manage the ion migration and obtain improved device performance in organic-inorganic perovskite FETs, taking a step towards the realization of the predicted charge transport characteristics for these hybrid materials for use in different optoelectronic applications.

References
1. Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nature Photonics 2014, 8, 506-514.
2. J. Wang et al., Investigation of electrode electrochemical reactions in CH₃NH₃PbBr₃ perovskite single-crystal field-effect transistors. Advanced Materials 2019, 31, 1902618.
3. M. Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy Environ. Sci. 2016, 9, 1989-1997.
4. S. P. Senanayak et al., A general approach for hysteresis-free, operationally stable metal halide perovskite field-effect transistors. Science Advances 2020, 6, eaaz4948.
5. X.-J. She et al., A solvent-based surface cleaning and passivation technique for suppressing ionic defects in high-mobility perovskite field-effect transistors. Nature Electronics 2020, 3, 694-703.

Conjugated polymers and their use in organic electronics and have long been recognized and have made advances in display and electronic solid-state devices. The use of electropolymerization and electrodeposition have enabled the layering and formation of ultrathin functional polymer films useful for the transistor, display, and sensor devices. Utilizing control of the solution and electrochemical dielectric parameters, it is possible to tune and pattern surfaces where the polymer formed have controlled oxidation and reduction behavior which mediates the film workfunction and optical parameters. In this work, we report on the methods to prepare ultrathin dielectric and electronic materials of conjugated polymers primarily by electrochemical methods, highlighting the ability to pattern and control transport behavior up to the nanoscale. This will be demonstrated in the following: 1) electronanopatterning using SPM methods to prepare conjugated polymer network circuits, 2) electrodeposition in colloidal particle arrays and electropolymerization, and 3) electrochemically molecularly imprinted polymers. The results are devices that enable memory devices, mediated ion transport gates, and chemical and biological sensors respectively utilizing ultrathin film geometries, electrochemical control and surface characterization methods in devices.

Direct emission displays made with inorganic µLEDs can potentially challenge LCD and OLED displays with better performance, efficiency and lifetime. Markets where microLED displays are likely to first appear are large information displays requiring high brightness, home theater applications benefitting from increased dynamic range and automotive displays in which high contrast and reliability are necessary. These displays are made possible by the integration of two mainstream technologies, namely GaN LED developed for general lighting and the large area active matrix backplanes developed for LCD displays.
The unique manufacturing challenge for µLED displays is development of a technology to position and connect millions of small devices with zero defects and low cost. Conventional pick and place tools are capable of positioning µLEDs, but the throughput of serial assembly is inadequate while mass-transfer methods have difficulty changing the pixel pitch. To address these challenges eLux has developed a massively parallel Fluidic Self-align Assembly (FSA) technology which positions each µLED by capture of the device in a well structure that also contains the connecting electrodes.
FSA uses simple low-cost equipment to achieve assembly rates up to 50 million µLEDs per hour and it is anticipated that production equipment could assemble an 8K display with 100 million LEDs in a few minutes. The technology is suitable for µLEDs from 10 to 200 µm diameter offering flexibility to make a wide variety of displays with resolutions from 400 to 10 ppi or larger. Similarly, the microLED emitter area can be adjusted independently of the assembly technique to increase brightness and it is possible to produce displays over 5,000 cd/m². Fluidic assembly applies relatively low force on the device so brittle materials such as red µLEDs fabricated from AlGaInP can be assembled in the same way as blue and green emitting GaN µLEDs.
A fully integrated manufacturing system for µLED fluidic assembly will ensure that the assembled µLEDs have acceptable performance with special emphasis on eliminating shorted or high resistance devices. Testing and binning used in the general lighting industry are not practical at the micron scale, so wafers are mapped by cathodoluminescence to identify defective µLEDs. Based on the defect map, a selective harvest technique diverts bad µLEDs from the suspension so only known good die are used for assembly. FSA relies on an excess of µLEDs to ensure sufficient capture attempts for complete assembly, so it is also necessary to capture, filter and recycle unassembled µLEDs.
With a suitable choice of complementary characteristic for the device and the trap site, it is possible to use fluidic assembly techniques to position nanowires or nanoparticles as well as µLEDs. The presentation will give an overview of the integration issues with some suggestions for other possible applications and approaches.

With our marine ecosystems under threat from climate change, there is an urgent need to continuously monitor marine conditions. One key indicator is the dissolved oxygen level, but existing sensors are limited by size and costs that preclude widespread non-intrusive monitoring. We present a new dual-gate design based on printed organic electrochemical transistors (OECTs) to track dissolved oxygen concentration in seawater, a highly challenging matrix owing to its high ionic strength and multitude of chemical interferents. The sensor achieved a detection limit of 0.3 ppm dissolved oxygen concentration in seawater. The device demonstrated reliable operation over five days and was capable of monitoring oxygenation changes arising from the photosynthesis cycles of saltwater macro-algae. In addition to oxygen, the dual-gate modulation principle is generalizable to other analytes with high redox potentials such as nitrates and dissolved carbon dioxide, and it offers a new design to realize compact, highly sensitive, economical in-situ sensors for harsh marine environments.

In addition to sensor fabrication, printing approaches can lead to devices that are readily customizable and economical for everyone to explore potential benefits and create new robotic applications. We demonstrated the printing integration of multi-material actuators, with an eye toward improving the fidelity and robustness of the printing process. We achieved compact soft actuators by using liquid-crystal elastomers (LCE) patterned by extrusion. An adjustable load was attached on the probe to control the pressure applied on the film. During the LCE alignment process, the probe velocity and load were tuned for gradient controls of alignment strength with various actuation strains based on different tool path strategies. The actuated samples were recorded with a 3D scanner to compare with the original 3D model. The quantitative results displayed a good index of 84.47% in structural similarity, successfully demonstrating the high fidelity of this approach.

With efficiency now above 18%, organic solar cells are well on their way to commercial application. However, there are still major stability issues that drastically limit their lifetime. One problem that has recently become more apparent originates from the commonly used electron transport layer (ETL) zinc oxide. Although this material has very advantageous properties, such as low price, transparency and good conductivity, it has one major drawback, namely its photocatalytic activity, which degrades many good absorber materials. Typically, this problem is solved by adding a protective layer between the ETL and the absorber layer, however this results in a much more challenging manufacturing. A simpler approach would be to mix a protective compound directly into the absorbing layer. In this work we use a ternary approach by adding three different fullerene derivatives – PC₇₁BM, ICMA, and BisPCBM – to a PBDB-TF:IT-4F system in order to suppress the photocatalytic degradation of IT-4F on ZnO via the radical scavenging abilities of the fullerenes [1].
We show that already an addition of 5% fullerene can quintuple the lifetime of the solar cells under UV illumination. We attribute this improvement to a reduced photocatalytic decomposition of IT-4F in the ternary system, which results in a decreased recombination. We propose that the added fullerenes protect the IT-4F by acting as a sacrificial reagent, and thereby suppress the trap state formation. Furthermore, we show that the protective effect of the most promising fullerene ICMA is transferable to two other binary systems PBDB-TF:BTP-4F and PTB7-Th:IT-4F. Importantly, this effect can also increase the air stability of PBDB-TF:IT-4F. This work demonstrates that the addition of fullerene derivatives is a transferable and straightforward strategy to improve the stability of organic solar cells.

[1] ACS Appl. Mater. Interfaces 2021, 13, 16, 19072–19084

The reported study demonstrates a novel fabrication approach that combines the versatility of screen printing and the high resolution of aerosol jet printing to manufacture reliable and fast all-printed organic electrochemical transistors (OECTs) on PET substrates. For OECTs, both the area and physical thickness of the channels are of critical importance for rapid switching response. As a result, we take advantage of a robust screen printing process to fabricate all the layers, except for the channel that is deposited by aerosol jet printing technology. This fabrication strategy reduces the PEDOT:PSS-based channel volume by up to a factor of 15 compared to screen printed channels, giving rise to a faster switching response due to lowered device capacitance. The switching performance has been confirmed by dynamic and transfer sweep measurements for all-printed standalone OECTs, OECT-based inverters and 5-stage ring oscillator circuits. Remarkably fast switching response and propagation stage delays of just above 1 ms for all-printed OECT-based inverter and ring oscillators have been achieved, despite using a screen printed electrolyte layer solidified by UV curing. In contrast, previously reported fully screen printed OECT-based inverters and 5-stage ring oscillators exhibited propagation stage delays of ~50 ms and ~100 ms per stage, respectively [1]. Additionally, the all-printed OECT devices reveal a high ON/OFF ratio of 10³-10⁴, while keeping low switching voltages. This is evidenced by the fact that the input voltage window for the OECT-based inverter circuit was reduced to 1 V (square wave signal fluctuating between 0 V and 1 V), thereby prolonging the operational lifetime.
The results reported herein pave the way towards a cutting-edge fabrication approach that can tailor-made the device architecture to reveal the best possible match for various applications, depending on the requirements. The proposed approach was applied for the manufacturing of high-performance OECT-based devices, exemplified by OECT-based logic circuits with remarkable switching response as well as in sensor applications utilizing the high amplification in all-printed OECTs.

Acknowledgments: The research activities are funded within the BORGES project, under H2020-EU.1.3.1, grant agreement ID: 813863.

References:
[1] P. Andersson Ersman, et al. "All-printed large-scale integrated circuits based on organic electrochemical transistors." Nature Communications 10, Article number: 5053 (2019).

Organic vapor phase deposition (OVPD) is a technique for depositing active layers in organic optoelectronic devices, including OLEDs. In OVPD, organic material is evaporated into a stream of carrier gas, which transports it toward a cooled substrate, where organic vapor can be made to condense into a smooth film. This approach decouples evaporation from transport and deposition, thereby offering increased stability of the deposition rate and doping ratio compared to the classical method of vacuum thermal evaporation (VTE). Thus, OVPD is a promising technique for low-cost volume production of multi-layer tandem OLEDs, where the number of active layers can exceed 25 in number. To advance deposition process and hardware design, we analyze transport mechanisms in OVPD, identifying key conditions and dominant factors. For a given hardware configuration, we show that diffusion dominates over convection in the upstream zone at carrier gas flow rates ≤ 50 SCCM and in the downstream zone when flow rate < 400 SCCM. These parameters can be scaled to hardware of different sizes. We also show how organic material flux, which determines film production throughput, can be extrapolated from convenient laboratory-scale experiments and computational fluid dynamics simulation. We further introduce a model that predicts material utilization efficiency, independent of organic vapor concentration. Results from this model compare well with experimental values, and they present pathways for process optimization and material waste minimization.

Developing smart materials that can respond to an external stimulus is of major interest in artificial sensing devices able to read information about the physical, chemical and/or biological changes produced in our environment. Additionally, if these materials can be deposited or integrated on flexible, transparent substrates, their appeal is greatly increased.
The first [BEDT-TTF = bis(ethylenedithio)- tetrathiafulvalene based quasi-twodimensional organic superconductor β-(BEDT-TTF)₂I₃ was first reported back in 1984.^[1] Soon it became clear that ion radical salts (IRSs) derived from BEDT-TTF exhibit tuneable electronic band structures; therefore, such molecules are excellent building blocks for engineering a rich and diverse family of organic crystalline metals and semiconductors. Thanks to strong electron−electron and electron−phonon couplings, their anisotropic electronic structures exhibit many fascinating electronic and structural phase transitions, which can be controlled by external stimuli such as light, temperature, strain, pressure, and humidity, among others. Nevertheless, it is necessary to engineer these crystals into a proper material for sensing applications. This is done by forming polycrystalline layers of IRSs, derived from BEDTTTF-based conductors, in nanocomposite bilayer (BL) films.
Such systems can be further tuned by choosing the nature of the IRSs enabling high sensitivity towards strain, pressure, temperature or even contactless radiation sensing i.e. bolometers.^[2,3] In another very recent example, bilayer films, composed of conducting polycrystalline layers of two dimensional BEDT-TTF-IRSs, hydroresistive sub-micron sized crystals on top of a polymeric host matrix permit to electrically monitor relative humidity in a stable and fully reversible fashion.^[4] At the percolation threshold, fascinating novel optoelectronic properties emerge (Insulator semiconductor-like metal transition).^[5] Mechanisms of responses are discussed and correlated with fundamental properties of charge transport in these systems. This sensor platform enables combining high electrical performance of single crystals with processing properties of polymers towards a simple, low-cost and highly sensitive platform for applications in robotics, biomedicine and human health care.

References:
E. B. Yagubskii, I. F. Shchegolev, V. N. Laukhin, et.al., JETP Lett., (1984), 39, 12.
E. Laukhina, R. Pfattner, L. R. Ferreras, et. al., Adv. Mater., (2009), 21, 1-5.
R. Pfattner, V. Lebedev, E. Laukhina, et.al., Adv. Electr. Mater., (2015), 1, 1500090.
R. Pfattner, E. Laukhina, L. Ferlauto, et.al, ACS Appl. Electr. Mater., (2019), 1, 1781.
R. Pfattner, et.al, (2021), submitted.

Introduction: Doped Hole (resp. Electron) Transport Layers HTL (resp. ETL) are commonly used in evaporated organic devices to achieve high work function hole contact (resp. low work function electron contact): in OLED to inject large current [1], in solar cell to increase the open circuit voltage Voc [2], and in photodetector to minimize the dark current [3]. However, the thickness of the HTL/ETL results from a delicate trade off. Indeed, on one hand, to minimize the detrimental impact of HTL/ETL on light absorption and series resistance effects, HTL/ETL must be kept as thin as possible. On the other hand, it is easier to reach an efficient work function by doping thicker layers. This later point is investigated more specifically in this work: an analytical model, numerical simulation and experiments are combined to identify this minimum acceptable thickness.

Simulation & Modeling: As it is surrounded by other materials with different work functions, the built in electric field within the HTL is never negligible, even in absence of applied voltage. In consequence, the Fermi level in ultra thin HTL layer, adjusted by doping, may differ from its (ideal) bulk value. A model has been derived to capture the impact of doping and thickness on the calculation of the HTL work function in two configurations: when the HTL is surrounded by two metals, when it is surrounded by a metal and an undoped organic semiconductor. Results, validated by numerical simulations, allows to determine the minimum acceptable thickness versus doping and electrostatic environment.

Experiments: Moreover, experiments were performed on a p-only structure (Ag/STTB:F4TCNQ (p-doped HTL)/ZnPc:C60 (active zone)/TiN) fabricated using common organic OPD/OLED materials. Samples feature different levels of doping and thickness. I-V curves has been fitted by a coherent set of physical parameters, confirming the capability of simulation of reproducing experiments. To extract the work function in situ, an innovative technic using I-Vs curves was implemented, following the work of X. Wei & al [4]. The results obtained clearly confirm the trend of the work function versus thickness curve predicted by theory. In high doped layers (1019 cm-3) a thickness of 10 nm is required to reach a suitable work function, while more than 20 nm is needed for lower doping (<1018 cm-3).
Conclusion: In summary, the impact of thickness and doping on the HTL work function in p-only template devices has been investigated in three complementary approaches: analytical modelling, numerical simulations and experiments. Results obtained proved that the obtainment of an efficient high work function HTL layer requires a minimum non negligible thickness, between 10 nm and 20 nm depending on doping level.

References: [1] C. Giebeler & al. J. Appl. Phys. 85, 608–615 (1999). [2] Y. Kinoshita & al., App Phys-92, 243309 (2008). [3] K. Kim & al., IEEE sensor, Vol 16, No 12, (2016). [4] X. Wei, M. Raikh, Z.V. Vardeny, Y. Yang, and D. Moses, Phys. Rev. B 49, 17480 (1994).

Organic photodetectors (OPDs) are candidates for numerous novel applications including wearable electronics and biometric monitoring. Despite significant development, it is still challenging to simultaneously achieve tailorable optoelectronic properties and faint light sensitivity. Here, we propose a new concept by combining a transmission cavity structure with organic absorbers to achieve tunable and narrowband OPDs via a full-vacuum process. Benefiting from this strategy, remarkable performance such as narrowband photoresponse (full width at half maximum of around 40 nm), a large tunability from visible to near-infrared region, fast response speed, and ultrahigh specific detectivity (> 10¹⁴ Jones) are simultaneously realized. To prove the potential application of this concept, multiple transmission cavity OPDs are integrated to be a miniaturized spectrometer whose performance is demonstrated. This novel concept extends the state-of-the-art OPDs, rivals silicon photodetectors, and will mature the spectroscopic photodetection for mobile applications.

Previously Brewer Science has reported on the solubilization of pristine carbon nanotubes in water, alcohols, and glycols through the use of super-acids and polyaromatic hydrocarbon derivatives. The goal of that research was to produce inks that allowed for rapid manufacturing of carbon-nanotube-based conductive thin films. These films do not require any post-processing steps, such as washing, to get rid of surfactants, and since the solubilizing functionality is non-covalently bonded to carbon nanotubes, the electronic structure of the carbon nanotube is preserved.
In addition to enabling the production of films with excellent conductivity, these dispersions of carbon nanotubes also provide us with a unique environment to further explore reactivity for carbon nanotube functionalization. The lack of surfactants means that the surface of the tube is still accessible as a reactive surface, and since these carbon nanotubes are still highly conductive, further functionalization doesn’t degrade the electrical properties.
Carbon nanotubes have been exploited for their sensitivity to a variety of analytes, including gases; however, gas selectivity can be a challenge. In order to improve selectivity, carbon nanotubes can be functionalized through various means to target specific analytes. Using the unique reaction environment that Brewer Science has developed, we have successfully coated carbon nanotubes with a disordered tin oxide layer characterized as mixed oxides by Raman spectroscopy to target combustible analytes. Other metal oxides, including vanadium and titanium, have been reacted similarly. SEM images of films of the functionalized tube sets demonstrates that the metal oxide is deposited on the carbon nanotube surface, and minimal free metal oxide is observed.
The resulting inks can be screen-printed between electrodes, allowing rapid manufacture of hundreds of devices. A thin layer of palladium is deposited as a catalyst, and with electronics developed by Brewer Science, the tin oxide-doped carbon nanotube films easily detect 100 ppm of carbon monoxide in our screening efforts. This same sensor, as printed, does not respond to other gases typical of tin oxide, such as methane. A sensor manufactured without either the tin oxide or the palladium does not respond to carbon monoxide.

Imperfect surface coverage, pinholes and other defects in solution-processed pervoskite (CsPbBr₃) thin films limit the radiative efficiency and stability of corresponding perovskite light-emitting diodes (LEDs). Significant effort on refinement of various deposition techniques, chemical compositions, functionalization and surface treatments have been reported in literature. In this work, we report on use of two inexpensive polymers - poly(ethylene) glycol (PEG) and polyvinylpyrrollidone (PVP) as a composite matrix for CsPbBr₃ to enhance surface coverage and uniformity of the deposited films in LEDs. A transparent perovskite precursor solution was prepared by mixing CsBr (99.999%, Sigma-Aldrich) and PbBr₂ (99.99%, Sigma-Aldrich) in DMSO with an optimized molar concentration ratio of 1.5:1 (0.2M) in a nitrogen glove box (Inert PureLab). After filtering, this solution was blended with separate PEG and PVP solutions (in dimethylsulfoxide (DMSO) and dimethylformamide (DMF), respectively, concentration > 5mg/ml) with a 6% net loading of the polymer solution to form the active ink. The ink was spin-coated on glass for film characterization. An orthorhombic phase (JCPDS(06-153-3063)) for CsPbBr₃ was confirmed using X-ray diffraction (Rigaku Miniflex, Cu-Kα, 1.54Å), with the elemental composition confirmed using energy dispersive X-ray analysis (EDX). UV-Vis spectrometry (Perkin-Elmer) was used to infer a band gap of 2.38±0.015 eV. Photoluminescence spectra (Horiba Labram, with a 325 nm source) yield a single highly intense peak at 520 nm (FWHM: 20 nm). For polymer LED fabrication, indium tin oxide (ITO) coated glass substrates were cleaned in a standard multistep process prior to spincoating poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, Baytron PVP), dehydration bake in air (120C), followed by spin-coating of perovskite ink under nitrogen, and an anneal at 75C. Samples were transferred under nitrogen for thermal evaporation at 2.3x10^-6 Torr (Angstrom Engineering) of 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, 300 A), LiF (8A) and Al (1000 A) to complete the devices with a shadow-masked top contact of area 0.3 x 0.3 cm². Control devices (no polymer additive) were processed at the same time under identical conditions. Current-voltage (J-V) and light-intensity-voltage (L-V) characteristics were acquired using a Keithley 2450 SMU and a calibrated Si photodiode. The test (CsPbBr₃-PEG-PVP) perovskite device exhibited a lower turn-on voltage (4.1V vs 3.2V), higher luminance (392 cd/m² vs 1070 cd/m²), current efficiency (0.150 cd/A vs 0.254 cd/A), when compared to the control device. Electroluminescence spectroscopy measurements (Ocean Optics HR4000) reveal in an emission at 518 nm (FWHM=17 nm) with Commission Internationale de l'éclairage (CIE1931) coordinates (0.13,0.75), in close agreement with measured PL. To study the likely source of these enhancements, we carried out comparative film analysis of test and control devices using field-emission scanning electron microscopy (FESEM, JEOL JSM-7800F Prime) and atomic force microscopy (AFM, Asylum Research MFP3D-SA). A reduced crystal grain size (204.1 nm to 101.1 nm) and nearly 100% surface coverage (compared to 81% found for control devices) found using FESEM suggests that the improved overall morphology and surface coverage plays a significant role in the improved performance. Further, a reduction in surface roughness (5.8 nm to 3.2 nm) and formation of a pinhole-free film is observed to increase yield by 20%. We are currently carrying out initial measurement of device lifetimes under accelerated test conditions under nitrogen and in air (after encapsulation) to further elucidate the relative prevalence of various device failure mechanisms in the test and control LEDs. We expect that these enhanced devices would find extensive applications in personal lighting, and transparent flexible display applications.

Optoelectronic devices based on thin-film materials such as organic polymers and small molecules, colloidal quantum dots (CQDs), and perovskites have been demonstrated as a promising route to bring disruptions in various consumer and industrial domains. Thin-film photodetectors are emerging as new imaging technology, thanks to their versatile large-area and room-temperature fabrication. Highly sensitive organic photodetectors (OPDs) enable flexible image sensors and co-integration with OLED displays. However, no organic semiconductors have been synthesized so far that can offer competitive quantum efficiency in infrared beyond 1 μm wavelength – the same limitation associated with silicon photodiodes. On the other hand, the facile bandgap tunability of CQDs made them a promising alternative to conventional infrared image sensors based on compound semiconductors. Thanks to a possibility for monolithic integration with CMOS electronics, CQDs offer low-cost, high-throughput fabrication and unprecedented resolution in infrared, which is expected to enhance the development of industries such as autonomous driving, augmented/virtual reality, and others.
Here, we present our work in which we combine the two thin-film technologies in a single photodetector in order to leverage the best features of both, such as high visible sensitivity of OPDs and high infrared sensitivity of CQDs. The photodetector contains two stacked photodiodes connected in a back-to-back serial configuration. Each photodiode can be activated separately by selecting the bias voltage polarity, offering a multispectral capability. One photodiode is based on a solution-processed organic bulk heterojunction made of PTB7-Th and IECO-4F, and it exhibits sensitivity up to 1 μm wavelength, with high external quantum efficiency (EQE) of more than 60% at 700 nm and a low dark current density of 30 nA/cm² at 1 V reverse bias. The other photodiode is based on solution-processed PbS CQDs with their size tuned for absorption up to 1600 nm and it exhibits a dark current density of 800 nA/cm² at 1 V. We perform optical simulations based on a transfer matrix method and find that an intermediate thin silver layer can reduce the optical crosstalk between the two channels. Moreover, this thin film enables tuning of the CQD photodetector’s response thanks to a strong optical cavity created with the back metal electrode, giving rise to a high EQE of more than 20% at different wavelengths in the 1-1.6 μm range. Transient measurements reveal a fast switching speed between the two photodiodes in the order of 10 μs.
By combining solution processing and physical vapor deposition, we developed a room-temperature, large-area compatible process that allows us to integrate polymers, CQDs, metal oxides, and metals in a single device. To ensure efficient charge carrier transport and low levels of dark current, we carefully engineered the device stack by controlling the surface passivation of CQDs and the energetic alignment at the interfaces. The presented photodetector is a two-terminal device that is compatible with the developed methods for integration of thin films with CMOS readout electronics, whereas its vertically stacked structure provides high spatial resolution for imaging. The presented photodetector has the potential to enable low-cost, high-resolution multispectral imaging and demonstrates how thin-film technologies can be exploited for new applications and functionalities.

Conjugated polymers are an enabling technology for numerous optoelectronic, bioelectronic, and energy harvesting/storage applications. The conductivity of several solution processable conjugated polymers, and therefore the performance of many of the corresponding devices, can be enhanced by molecular doping. For practical applications, both hole-transporting (p-type) and electron-transporting (n-type) conducting polymers are usually required. Unlike p-doped polymers that exhibit conductivities typically > 1000 S/cm, most n-doped polymers suffer from conductivity values < 1 S/cm. Various polymer design strategies, including planarization and stiffening of the polymer backbone, engineering of the donor–acceptor character, and control of the molecular dopant counterion-polymer side-chain miscibility are being explored. Despite great progress, n-doped conducting polymers do not yet meet the performance comparable to the best p-doped polymers. Here, we constructed highly ordered polymeric films, with thickness approaching unit cell dimension, and investigated their electrical properties. We used Langmuir-Schaeffer (LS) deposition technique to manufacture highly oriented polymeric monolayers with a low degree of defects. We used a combination of scanning probe microscopies, optical spectroscopy, GIWAXS, NEXAFS and electrical measurements to characterize the polymeric energetics, structural, and thermoelectric properties. We optimized the electrical performances of the ultra-thin films by n-doping them with strong reducing agents, reaching n-type conductivities of 4 S/cm, and a power factor of 1.5 µW/mK².

Metallopolymers are a new generation of hybrid materials containing a transition metal complex and a π-conjugated backbone. Ruthenium complexes between transition metal complexes are extremely important with their stable redox transitions and electrochromic properties. Herein, monomers containing the redox active [Ru(bpy)₂pytri]²⁺ complex and various polymerizable thiophene derivatives such as SNS, EDOT and ProDOT were synthesized via click chemistry. As a result of electrochemical polymerization of monomers, new electrochromic metallopolymers based on polythiophene were obtained.
It was determined that polythiophene derivatives, which consist of the same complex unit and different polymerizable units, exhibit different band gaps and color properties depending on the backbone of the polymers. The band gaps of PSNS, PEDOT and PProDOT were calculated 2.44 eV, 1.84 eV and 1.94 eV, respectively. The neutral colors of the PSNS-Ru, PEDOT-Ru and PProDOT-Ru metallopolymers were determined yellow, green and orange, respectively. The switching times of PSNS-Ru, PEDOT-Ru and PProDOT-Ru were determined as 5.14 s, 2.52 s and 3.70 s, respectively.
All polymers showed their respective redox activities of both conjugated main backbone and metal complex indicating the effective manipulation of redox behavior of conducting polymers through engineering of the monomer structure.
***This work was financially supported by the National Science Foundation of Turkey (TUBITAK Project No: 215Z400) and Akdeniz University Research Funds (FDK-2019-4934).

In the field of organic electronic devices, it is generally unusual to use polar solvent in organic solvent-based active solution for improved efficiency and stability. Residual water in active solution has also been reported to adversely affect organic electronic devices. For blocking the contact of moisture, many researchers prepare and store organic electronic devices under nitrogen (N₂) atmosphere. However, we found that various hydrophilic solvents, such as water, methanol and isopropanol, as additives have an unexpectedly positive effect for enhanced efficiency and stability of solar cells. In this study, the D-A blend films (ref-1: PTB7-Th:PC₆₁BM, ref-2: PTB7-Th:EH-IDTBR and ref-3: PM6:Y6) with water treatment were slot-die coated and optimized for up-scaling production. Furthermore, we confirmed that the bulk heterojunction morphology and power conversion efficiency depend on the quantity (0~20 v/v%) of water introduced as a solvent additive. The effects of water addition were analyzed by using AFM, TEM, FT-IR and 2D GIWAXS measurements. As a result, enhanced efficiencies of 12% (ref-1/WT), 11% (ref-2/WT) and 12% (ref-3/WT) compared with efficiency of reference cells were achieved from the water addition. In addition, the improved stability (RH 60 % and room temperature in air) of the water additive incorporated devices was demonstrated as compared with reference cells, showing the excellent potential of eco-friendly and morphology-controllable additive for the printable organic solar cells.

Electrical injection organic solid-state lasers (OSL) promise easily processable devices for chemical sensing, point-of-care diagnostic and applications requiring wavelength tunable sources. While significant progress has been made with OSL's, persistent issues such as optical losses and short operational lifetimes in the devices prevent the realization of practical application. To circumvent these issues, novel injection architectures have been proposed. One such structure is the ambipolar field-effect transistor (FET).

In our work, mode-solver calculations demonstrate that improvements to optical confinement are possible in FET geometries, by using high refractive index cladding layers. Optical experiments show an increase in amplified spontaneous emission (ASE) efficiency and lowered ASE thresholds, which we attribute predominantly to the increased confinement. The results suggest that the structure can be potentially used to improve lasing characteristics for both optically pumped, and electrical injection organic lasers where thin, low refractive index active materials are required.

Organic Field-Effect Transistors (OFETs) have been developed for different applications including bio-sensors, flexible memory, display drivers and etc. To enhance the functionality of OFET devices, downsizing the OFETs and increases their packing density are required. However, the traditional scaling-down fabrication method demands more involved fabrication processes such as photolithography, deep X-ray lithography, or e-beam lithography. To avoid these complicated processes, the current work presents an approach to downsize the OFETs by using the pre-stressed polystyrene (PS) shrink film substrates. It is a brand-new strategy for downsizing and improves the electrical performance of organic drivers. This method only needed common equipment (such as the oven or hot plate) and provided time effective downsizing fabrication process. After heating the film to 160 ^oC, over the glass transition temperature of PS, the shrink film will release its pre-loaded stress and the OFET devices will follow the shrink film substrates to shrink. The projective area of the device was reduced by 75%. The wrinkled structure can amplify the electrical field at the channel area and reduce the subthreshold voltage of the transistor. By using the AlO_x dielectric layer, the subthreshold voltage of the transistor is shifted from -1.44 V (before shrinking) to -0.18 V (after shrinking) with the subthreshold swing of 74 mV/dec and intrinsic gain of 4.151 x 10⁴. This study shows that fabricates OFET devices onto the shrink film substrate can provide a low cost and time-saving scale down method and the wrinkled structure would be one of the designs for the low voltage operated OFETs.

Solution processing is a facile fabrication method for large area, low-cost organic field-effect transistors (OFETs). Highly crystalline organic thin film by solution shearing offers a decent mobility, on/off ratio and subthreshold swing which are all important parameters to advance the OFET applications. To increase the device operation stability, self-assembled monolayers (SAMs) with low surface energy are usually used to eliminate the interface trap states locating at the semiconductor-dielectric interface. However, low surface energy which impedes solution wetting is a big challenge for the growth of highly crystalline thin film. To overcome the limitation, we replace the traditional fabrication method of in-situ growth of organic thin film by a transfer method. The high crystalline monolayer was grown on a high surface energy template substrate and transferred onto the low surface energy dielectric by using a temporary carrier. Both the morphology and the electrical property of organic monolayer remained undamaged during the transfer process and high mobility of 10 cm²V^-1s^-1 on SiO₂ was maintained. We further demonstrated a flexible, low-operation voltage OFET device with zero turn-on voltage, high on-off ratio 10⁷, and high mobility of 7 cm²V^-1s^-1, by transferring the high-quality active layer onto low surface-energy fluorinated-based SAM modified high-κ AlO_x dielectric and laminating source-drain electrodes. The transferred OFETs show a significant improvement against the bias stress effect. The device only showed 200mV threshold voltage shift after 10000s bias stress test. The fabrication method extended the application of highly crystalline organic active layer to flexible and conformal organic electronics.

Two-dimensional (2D) atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, there are still a number of technical challenges in achieving active layers with mono or bi-layer thicknesses while the electrical performance does not deteriorate. Here, we focus on organic field-effect transistors (OFETs) and fabricate them by a layer-by-layer transfer method with bottom-to-top 2D layers. The graphene bottom gate is atomically smooth and has few nanometers in thickness, which affords a pristine bottom interface for constructing upper 2D layers. High-quality C₁₀DNTT molecular crystal is deposited on fluorophlogopite mica by blade coating method, with precisely controlled thickness down to monolayer. Moreover, the van der Waal electrode is applied to ensure a clean and non-destructive interface for metal-semiconductor contact. As a result, the 2D OFET has a channel down to 1 mm length and the total thickness of the device is less than 200 nm. Simultaneously, the 2D OFET with ultra-thin fluorophlogopite mica dielectric shows high carrier mobility up to 12 cm²V^-1 s^-1, operating voltage within 1 V and decent subthreshold swing of 100 mV dec^-1. These electrical properties and the thickness allow them to conformally adhere onto non-planar surfaces for multifunctional applications, including electrophysiological signal detection on human. Our work unveils an exciting new class of two-dimensional OFETs for flexible electronic and photonic applications.

Organic memory transistor (OMT) is a promising candidate for the next generation memory devices towards human-machine interface applications. In this study, we developed nonvolatile optical memory transistor with [1]Benzothieno[3,2-b]benzothiophene (BTBT)-based p-type organic semiconductor as the active layer. We found that the polymorphs density of the active layer is corresponding directly to the OMT memory performance. The processing parameters, including deposition temperature (T), surface energy (γ), surface roughness (r) and film thickness (t), are pivotal to achieve fine control over the polymorphs of the deposited organic active layers and therefore, investigations of these parameters are highly desired. Here, we explore the effects of the processing factors mentioned above on the polymorphs morphology of the active layer under thermal evaporation and the memory performance of OMT. It is verified that T and γ are key parameters connected to the density of thin film polymorphs; r and t are also important in tuning the polymorphs density. Combining with the high-k dielectric material Al₂O₃, the memory transistor can operate at a low voltage during optical programming and electrical erasing process. Additionally, the transistor shows a field effect mobility up to 2.5 cm²V^-1s^-1 and excellent data retention for more than 12 hours with a current ratio higher than 10⁴ between the two binary states, which is a promising value for the OMT. Our findings provide valuable information in understanding the polymorphs morphology and the corresponding memory performance of BTBT-based organic semiconductor under thermal evaporation, which is believed to be with high potential in approaching next generation neuromorphic devices.

Microfluidics for cell trapping and pairing has become a very powerful platform for in vitro cell analysis. In many topics like the immune system and cancer biology, etc, using microfluidics chips to trapping cells and analyzing their behavior is very effective. Dielectrophoresis (DEP) with its compatibility with living cells is one of the mostly used method for cell manipulation. By integrating a FET sensor array into the DEP-chip, we can realize the combination of cell trapping and sensing. By using the organic field-effect transistors (OFETs) sensor array to measure the impedance of cells, we can monitor whether every trapping points which are designed and fabricated in the chip have successfully trapped a cell and the cell status (alive or dead). After identifying the successfully tapping, we can continue with subsequent manipulation like cell fusion, cell lysis, and DNA detection. . It will be a very powerful platform for bio- and biomedical science.

Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic-inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus they were not considered suitable for electrochemical processing until now. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and electrochemical doping of the films with in situ diagnostic capabilities.

In this study we display electrochromic properties of PEDOT derivatives containing Ru(II) complex which were synthesized through electrochemical homopolymerization of EDOT-Ru and copolymerization of EDOT-Ru with EDOT. All the metallopolymers revealed reversible electrochemical activity both in anodic and cathodic regions. Optoelectronic properties of metallopolymers were investigated by cyclic voltammetry, spectroelectrochemistry, kinetic and colorimetry measurements.

The band gap, optical contrast, switching time and coloration efficiency of the homopolymer (PEDOT-Ru) were determined as 1.77 eV, 33.72 %, 1.39 s and 156.21 cm²/C, respectively. PEDOT-Ru exhibited multichromic behavior, displaying purple, green, yellow and transparent colors. The studies on copolymerization demonstrated that the electrochromic properties of the copolymers could be changed through comonomer feed ratio. All copolymers displayed multichromic behavior. The neutral state colors of P(EDOT-Ru-co-EDOT)₁, P(EDOT-Ru-co-EDOT)₂ and P(EDOT-Ru-co-EDOT)₃ copolymers were appeared as green, dark green and blue, respectively.

***This work was financially supported by the National Science Foundation of Turkey (TUBITAK Project No: 215Z400) and Akdeniz University Research Funds (FDK-2019-4934).

The field of semiconducting π-conjugated polymers (SCPs) have undergone fast development in recent decades due to their various applications in organic electronics, such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), thermoelectrics and organic field-effect transistors (OFETs). In order to accomplish the high performance devices, a number of organic molecular building blocks based on aromatic rings have been designed and synthesized. Recently, the quinoid-type molecules have been considered as a promising candidate for active materials of organic electronics due to their superior electrical properties derived from more electron delocalized atmosphere facilitating efficient charge transport.
In this work, a novel quinoidal building block incorporating 3,4-ethylenedithiothiophene (EDTT) inducing intermolecular interaction via sulfur-sulfur interactions were designed and successfully synthesized. In addition to such intermolecular attraction, it has been reported that the conjugated molecules containing thioalkyl group show unique optoelectronic properties compared to molecules containing oxyalkyl group. Therefore, quinoidal polymeric semiconductors PmQEDTT-T2 and PmQEDOT-T2 that is 3,4-ethylenedioxythiophene (EDOT) incorporated structure instead of EDTT were copolymerized by Stille polymerization. The prepared quinoidal conjugated polymers exhibited significantly different properties such as electronic, optical properties and vibronic structures. Organic field-effect transistors were fabricated using PmQEDOT-T2 and PmQEDTT-T2 polymers as active layers, the highest hole carrier mobilities of 0.13 and 0.02 cm²/Vs were obtained, respectively. In this study, we will explain the substitution effect of atoms on quinoidal conjugated polymers in relation to microstructural analyses and molecular packing structures of thin films as well as thermal, optical, electrical and electrochemical properties.

Electrochemical wearable biosensors are suitable for diverse skin-attachable applications owing to their high performance, inherent miniaturization, and low cost [1], [2]. Incorporating flexible and stretchy materials enables electrochemical biosensors to impact a wide range of developing applications, including electronic skins [3], smart sensor bandages [4], and implantable medical devices [5]. Flexible/stretchable sensors need to be capable of accommodating various mechanical disturbances such as large bending, twisting and stretching, while retaining their performance for numerous cycles [6]. Though extensive efforts have been made, the performance of such flexible/stretchable electrodes under dynamic deformations issue need further studies , not only in scientific studies but also in any practical device applications.
Here, we study a flexible glucose sensor which can be subjected to various lateral strains via stretching, while providing reliable electrochemical sensing of biomarkers on a patient’s skin, undeterred by daily activity and repetitive impacts. Vertically aligned carbon nanotubes (CNTs) grown via chemical vapor deposition (CVD) are embedded onto suspended polydimethylsiloxane (PDMS). The CNTs are coated with a conjugated polymer transducer layer via electropolymerization of polypyrrole (PPy), doped with glucose oxidase (GOx). The sensor is exposed to iterative concentrations of glucose, and GOx reacts in the presence of glucose, resulting in a concentration-dependent change of measurable current. For this sensor, we have developed CNT-based sensing electrodes on flexible substrates that exhibit reliable and stable performance under stretching up to 75%. Skin-attachable devices for continuous monitoring of patients will directly benefit the medical field, with potential applications towards smart sensor bandages, critical wound monitoring, and glucose detection for diabetes.
[1] A. J. Bandodkar and J. Wang, “Non-invasive wearable electrochemical sensors: A review,” Trends Biotechnol., vol. 32, no. 7, pp. 363–371, 2014.
[2] J. R. Windmiller and J. Wang, “Wearable Electrochemical Sensors and Biosensors: A Review,” Electroanalysis, vol. 25, no. 1, pp. 29–46, 2013.
[3] D. Chen and Q. Pei, “Electronic muscles and skins: a review of soft sensors and actuators,” Chem. Rev., vol. 117, no. 17, pp. 11239–11268, 2017.
[4] H. Derakhshandeh, S. S. Kashaf, F. Aghabaglou, I. O. Ghanavati, and A. Tamayol, “Smart bandages: The future of wound care,” Trends Biotechnol., vol. 36, no. 12, pp. 1259–1274, 2018.
[5] G. A. Aitchison, D. W. L. Hukins, J. J. Parry, D. E. T. Shepherd, and S. G. Trotman, “A review of the design process for implantable orthopedic medical devices,” Open Biomed. Eng. J., vol. 3, p. 21, 2009.
[6] P. Lukowicz, T. Kirstein, and G. Tröster, “Wearable systems for health care applications,” Methods Inf. Med., vol. 43, no. 3, pp. 232–238, 2004.

Flexible electronics requires conformable substrates for a wide range of applications [1]. Conventional high-performance electronic materials such as silicon are not flexible, whereas many flexible materials, such as conducting polymers, are often characterized by poor electric properties [2]. Carbon nanotubes (CNTs) are promising owing to their excellent electronic properties and flexibility owing to their small diameter.
Here, we present stretchable electrodes composed of opposing layers of vertically aligned CNTs (VACNTs) partially embedded in a polydimethylsiloxane (PDMS) substrate; the tips of CNTs are partially embedded into PDMS. This unique synthesis permitted a rapid and facile integration of a flexible and stretchable platform for sensors and energy storage. VACNTs were grown using atmospheric-pressure chemical vapor deposition (APCVD) on a Si/SiO2 substrate and transferred onto PDMS as a stretchable electrode. A liquid mixture of PDMS base and curing agent were mixed with a ratio of 10:1 and degassed under reduced pressure in a vacuum environment to remove gas bubbles. The liquid PDMS was heated on a hot plate at 65 °C for approximately 30 min to a state of partially cured PDMS. The CNT carpet was placed face-to-face onto the partially cured PDMS, and PDMS infiltrated between each individual carbon nanotube, owing to the capillary effect and the viscoelastic property of PDMS. As a result, tips of interwoven CNTs were embedded into PDMS. The CNT−PDMS sample was left in an ambient condition to fully cure PDMS. Two such CNT-PDMS electrodes were placed face-to-face and change of resistance occurs due to the external pressure applied orthogonal to the surface.
Here, we demonstrate a pressure sensor and a pseudocapacitor using VACNT/PDMS. To detect the pressure, two CNT-PDMS electrodes are arranged in a face-to-face configuration, so the increased pressure is directly proportional to a detectable change in resistance, enabled by increased contact between the opposing electrode surfaces. The measured resistance was maintained at stretching up to 180%, with a rapid response time during loading and unloading. The sensor showed a stable performance for 10,000 loading/unloading cycles with the resistance retention of 82%. As a proof-of-concept, the sensor was successfully tested for measuring biological signals of a person, including pulse signature, muscle flexing, and step count/intensity. Towards an energy storage application, the CNT-PDMS electrode underwent electropolymerization of dodecylbenzene sulfonate polypyrrole (PPy(DBS)) to conformally coat individual CNTs, followed by deposition of a gel electrolyte, to create an all-solid state pseudocapacitor. In addition to high stretchability and stability, the pseudocapacitor exhibited a stable cyclic behavior up to 10,000 cycles of galvanostatic charge/discharge measurement. These novel CNT-based flexible electrodes demonstrated towards flexible sensor and energy storage devices . are very promising for wearable applications. This work will directly impact the development of biomedical and e-skin applications.

References
[1] Wong, W. S. & Salleo, A. Flexible electronics: materials and applications. 11, (Springer Science & Business Media, 2009).
[2] Nyholm, L., Nyström, G., Mihranyan, A. & Strømme, M. Adv. Mater. 23, 3751–3769 (2011).

The advancement in technology in the field of energy has contributed by different kinds of materials. Which includes polymer composites, metal derivatives, etc. There is a considerable increase in the usage of metal and its different structural derivatives. Heavy metals have been widely used for device applications. Hence contamination of natural resources by heavy metals is a major issue. Different sources like textile effluents, electronics industry, mining, industrial wastewater, are contributory. It causes adverse effects on health once it enters the food chain. So, great attention is required for the detection and removal of these metal ions.
Several methods have been explored for the removal and detection of heavy metals. Methods of removal include Adsorption, Ion exchange, Electrodialysis, Nanofiltration, etc. For the detection, electrochemical and spectroscopy methods, etc have been used. Materials like carbon nanostructures, polymer attached platforms, conjugated polymer, metal-organic frameworks (MOF’s), covalent organic frameworks (COF’s), etc. are act as receptors for metal ions. Further tunability of structure and functionalization could enhance the sensitivity. The current emerging materials of interest include framework structures like MOFs and COF s due to their ease of synthesis, scalability, stability, selectivity, and sensitivity. These porous architectures hold a high surface area. The change in electronic properties upon adsorption can be attributed to a sensing mechanism.
Zeolite imidazolium frameworks are a group of MOF’s which are widely used for the adsorption of a wide range of analytes. Electrochemical device-based sensors based on MOF’s are least explored. Modification of these frameworks has been done in numerous ways such as functionalization with various ligands, intercalation of ZIF in other MOF’s, nanocomposite with graphene oxide, CNT, graphene, and other materials has shown improved sensing.
In this study, ZIF- 67 has been chosen as a sensing platform for lead ion detection. It is a cobalt metal-centered methyl imidazolium-based framework. Already it has shown good adsorption towards lead ions. Integrating it into a sensing platform is quite challenging. Cyclic voltammetric study in KCl/ K₄Fe(CN)₆ exhibit ZIF- 67 holds good catalytic behavior and sensitivity towards lead ions. Modification of ZIF- 67 with 3-aminopropyl trimethoxysilane could increase sensitivity and selectivity due to the amine group's affinity towards lead ions. The sensitivity enhances with the incorporation of aminomethane siloxane compared to neat ZIF. The synthesized ZIF-67/ 3-aminopropyl trimethoxysilane was fabricated into a carbon paste electrode for further electrochemical sensing mechanism studies for lead ions detection.

Neural probe application has gotten more and more attention due to its advantages in microscale, material biocompatibility and flexible fabrication technology. Varies kind of biocompatible materials and electrochemical deposition methods were adopted to develop the neural probes so that it can minimize damage to the brain tissue and reduce the impedance for in-vivo applications. Here we designed a highly flexible gold electrode array, using SU8 and Parylene as supporting layer, coated with PEDOT:PSS to reduce the impedance and pHEMA to increase the bio-compatibility. We characterized the electrical performance of the probe for signal quality before in-vivo testing and then the probe was employed in the rat for signal acquisition and bio-compatibility test. The length of the probe was tailor made for applying to different brain regions, including primary motor cortex, hippocampus, paraventricular thalamus and cerebellum, of the rat to record the single neuronal spike and local field potential. From both acute recording in anesthesia rats and 3 months long-term chronic recording in awake animals with implantation, our probes shown a good quality with the signal to noise ratio up to 20. The post-experiment dissection and immunohistochemistry staining illustrated a minor tissue damage and immune response comparing to the commercial metal electrode arrays. The proposed probe can be applied to variety of brain regions which provide a broad application in animal electrophysiological investigation and potential clinical usage.

Photovoltaic technologies depend largely on transparent conductive electrodes (TCEs). TCEs should, in theory, have the highest light transmission and conductivity. Both traits, however, must be balanced. The selection of the best TCE depends on the photovoltaic material system and is evaluated using so-called figures-of-merit (FOM). A novel and exact FOM is introduced here that explicitly analyzes the impact of TCEs on photovoltaic performance. This unique FOM has several important properties: normalization to the theoretically ultimately attainable photovoltaic performance, proportionality to the solar device's potential power output, and, thus, significant direction for the development of advanced TCEs. Here we evaluated TCE requirements for various photovoltaic material systems in relation to the spectral range in which they operate. Additionally, we also reassessed and compared a variety of realized state-of-the-art semitransparent electrodes.