Symposium EL01—Organic Semiconductors and Characterization Techniques for Emerging Electronic Devices

Organic photovoltaic devices have attracted significant interest for outdoor energy harvesting, as they have unusual properties including easy tunability of optical and electrical properties, and can be fabricated by low-cost, mass production methods. However, the potential for organic photovoltaic devices for indoor application (i.e. harvesting indoor light for self-sustainable electronics, e.g. power sensors) is much less explored. While there were a few initial studies on this topic, the power conversion efficiency of organic photovoltaic devices under indoor lighting (fluorescence lamp or white LEDs) is still low at that time. In 2016, our team revised the potential of organic photovoltaic devices for indoor applications by using the more state-of-the-art materials (at that time), which demonstrated good efficiency under indoor lighting. By exploring the design rules for indoor light harvesting, in 2018, we demonstrated a record high power conversion efficiency at that time (28% under 1000 lux fluorescent lamp: significantly better than silicon). These two crucial findings have then been leading to very active and significant amount of research on organic photovoltaic devices for indoor applications in the research community. In this talk, I will present the findings (and the design rules), and we believe that organic photovoltaic devices for indoor applications have much more promising potential to be commercialized (compared to outdoor applications), and can have significant impact on powering the "Internet of Things" for smart home, office, supermarket, buildings, etc.

Organic solar cells (OSCs) have attracted a lot of attention in the last decade. The main advantage of OSCs is low-cost fabrication process and greater versatility than inorganic cells due to their flexibility and lightweight. The active layer of OSC comprises a blend of donor and acceptor material. The most frequently used donors are conductive polymers, such as P3HT and PTB7. The most widely used acceptor is C60 fullerene derivative: phenyl-C61-butyric acid methyl ester, PC61BM.
To operate properly, the OSC structure must allow for the separation of photogenerated hole-electron pairs. The final efficiency (PCE) is strongly correlated with the effectivity of the charge separation process[1] occurring in the active layer, so the understanding of the charge transfer mechanism is necessary for further development of the OSCs. The main aim of this research was to determine the structural characteristics of fullerene derivatives important for improving the performance of OSCs.
In our study four newly synthesized fullerene C60 derivatives with different aromatic substituents and their blends with P3HT were selected to study the charge transport processes in the active layer. Two of them were functionalized with either one or two tiophene residues. Likewise the two others were functionalized with either one or two pyrene residues. To study these processes, light-induced Electron Spin Resonance (LESR) along with photoluminescence (PL) and time-resolved photoluminescence (TRPL) techniques were applied.
During illumination PC61BM:P3HT blend exhibits exceptionally intense charge separation that is evident in the LESR spectrum as prominent two lines[2]. The intensities of these two lines provide information on the number of photoexcited and separated electrons and holes in the steady-state, which is one of the decisive factors influencing later the PCE of solar cells. The LESR signal of the all studied materials also showed two characteristic lines indicating charge transfer within the blend. Interestingly, blends with the molecules having only one aromatic substituent showed much stronger LESR signal, making them more predestinated for photovoltaic applications. Molecules with two aromatic substituents showed much more modest signal.
PL and TRPL techniques were applied to trace the photo-carriers behavior inside the active layer. Radiative recombination and charge separation are two competing processes that lead to light emission and photocurrent generation, respectively. The observation of PL decay provides information on charge separation effectivity. Studies showed that the most effective PL decay is observed in blends of P3HT and fullerene derivatives with one aromatic substituent. On the other hand, blends of symmetric fullerene derivatives with P3HT did not exhibit significant PL attenuation. Therefore, it can be concluded that the separation of electrons and holes is ineffective in them.
The final step was to fabricate OSCs with an active layer made of blends of these new fullerene derivatives with P3HT, and their characterization. The results obtained with LESR and PL/TRPL techniques showed very high consistency with J-V characteristics of OSCs. Best performing were the cells with fullerene derivatives, for which blends with P3HT exhibited two prominent LESR lines and fast PL decay. However, even the best PCE values obtained for cells with new fullerene derivatives were lower than PC61BM:P3HT reference cell (0.17% vs. 2.23%). This may be caused by lower solubility of these new materials and less optimized fabrication process. Still, the correlation of carrier separation efficiency, observed in the LESR and PL/TRPL measurements, with the PCE obtained from J-V characteristics of the respective OSCs was clearly visible.
Acknowledgments: Research was co-founded by project TECHMATSTRATEG1/347431/14/NCBR/2018 (NCBiR).
[1] F. Laquai, Macromol. Rapid Comm., 36, 1001–1025, 2015
[2] C. Deibe, Adv. Mater., 22, 37, 4097–4111, 2010

In bulk heterojunction (BHJ) solar cells, the heterojunction interface between electron donor and acceptor drives the exciton-to-charge conversion, yet it also adds to energy and carrier losses. In principle, in low-bandgap non-fullerene acceptor (NFA) BHJs both electron affinity (EA) and ionization energy (IE) offsets should equally control the internal quantum efficiency (IQE). Allegedly, exciton-to-charge conversion is efficient even for close-to-zero offsets. Here, we rebut both notions and demonstrate that counterintuitively, the charge transfer from the exciton rather than the further charge separation is the limiting step controlled by the IE offset and secondly, that sizeable IE offsets are required to reach high exciton-to charge conversion efficiency. We find that efficient Förster Resonant Energy Transfer to the low bandgap acceptor precedes the charge transfer, which thus always occurs via hole transfer from the acceptor, hence the unimportance of the EA offset. We discuss the reasons for the threshold IE offset in terms of interface energetics and find that two physical parameters are sufficient to describe the evolution of the IQE with IE offset on a very large range of material systems. Our model also explain other experimental observations such as barrierless charge separation and the difficulty of observing CT states emission and absorption in NFA based systems.


Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-00835-x

The diluted organic semiconductor-insulator binary blend strategy recently has been used both in organic field electric transistors (OFETs) and polymer light emitting diodes (PLEDs) to improve (opt)electronic performance, processability, mechanical and environmental stability. The advantages shown in diluted OFETs and PLEDs potentially could be exploited in organic photovoltaics (OPVs) and thus the application of the diluted strategy in OPVs deserves attention. However, OPVs using the diluted semiconductor strategy have not yet been reported, possibly because introducing high concentration insulators (≥10%) as filler in the bulk heterojunction binary donor-acceptor blend offers additional challenges as compared to the OFET and PLED applications. For example, exciton dissociation occurring at the bulk donor/acceptor interfaces will be suppressed if thick insulator regions form inside the donor/acceptor junctions. Similarly, transport and collection of free charge carriers following exciton dissociation can be hindered by either encapsulation of a donor/acceptor domain inside an insulator shell, or by insulator material segregation to either or both electrodes forming dense interface layers. Either inefficient exciton dissociation into free charges or decreased charge transport and collection is sufficient to significantly degrade the photovoltaic properties and thus prevent application of the diluted semiconductor strategy.
Here we successfully prepared solution-processed diluted OPVs by blending the state-of-the-art donor and acceptor polymers with commonly used insulators (Sol. RRL 2020, 4, 2000261). We show that “ideal” donor/acceptor bulk interfaces and interpenetrating donor/acceptor penetrating network can be maintained in the resulting ternary blends, yielding improved OPV performance such as: an order of magnitude improved hole mobility, reduced radiative and nonradiative recombination, as well as enhanced environmental and thermal stability. In addition, when the diluted organic solar cells are upscaled in area, the cell performance is 5 times less reduced compared to the pristine undiluted organic solar cells – a crucial advantage for commercial application. Finally, thicker ternary blend films can be deposited yielding the same degree of semi-transparency in the optical spectrum as the binary donor-acceptor blends, vastly improving printability of semi-transparent OPVs. Our findings imply that interpenetrating donor-acceptor network can be maintained in diluted ternary blend and the diluted strategy can be used to construct improved ternary devices involving internal penetrating donor-acceptor heterojunctions such as OPVs and organic photo detectors.

Understanding the interplay between film morphology, photophysics, and its relation to device performance in bulk heterojunction organic photovoltaics remains challenging. Using the well-defined morphology of vapor-deposited TAPC:C₆₀ blends, from the surprisingly more efficient dilute-donor (5% TAPC) to the less efficient intermixed systems (50% TAPC), we see the morphological effects on charge generation and recombination on an ultra-fast time scale. With the use of ultrafast transient spectroscopy we are able to follow each step of the charge generation process and give an explanation for the counterintuitive efficiency trends in these organic solar cells.


First, we show that both Frenkel and Charge Transfer excitons generate photocurrent over the entire fullerene absorption range, giving advantage to the dilute-donor blend with more C₆₀.
Second, we show a novel technique for selectively monitoring interfacial and bulk C₆₀ clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation.
Third, we identify a fast (<1 ns) recombination channel, where free electrons recombine with trapped holes on isolated TAPC molecules which should harm the performance of dilute solar cells. We, however, give evidence that this recombination is mitigated when electrons are rapidly extracted in efficient devices, reminding us that more than just the active layer needs to be considered when designing efficient organic solar cells. [1]

[1] J. Phys. Chem. Lett. 2020, 11, 14, 5610–5617

Polar polythiophenes with oligoethylene glycol side chains are an emerging class of materials that attract considerable attention for energy harvesting and storage as well as bioelectronics. The polar side chains enhance the compatibility with molecular dopants and improve the electrical conductivity and thermal stability of the doped material. However, polar polythiophenes are exceedingly soft at room temperature due to a sub-zero glass transition temperature. To enhance the versatility of these polymers, strategies are needed that permit to modulate their mechanical stiffness. In this contribution, we report our efforts to copolymerize a polar polythiophene with urethane blocks. Further, we describe wet spinning of its blend with polyurethane into fibers. The resulting materials feature elasticity and stretchability, yet higher stiffness. In addition, the copolymer and blend continue to display a high degree of electrochemical activity and electrical conductivity when chemically doped.

Polymer electronics has developed into an independent field of research as conducting polymers combine high, metal-like conductivity with the generally desired properties of commodity polymers. The additional responsiveness of conjugated polymers to various stimuli such as heat, electric voltage, or humidity has now also promoted efforts to fabricate sensors and actuators for wearable devices and electronic skin (e-skin). In this work, functional properties and the stimuli-responsiveness of smart materials such as particularly poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were studied. PEDOT:PSS has become one of the most promising candidate for applications in polymer electronics for, e.g., its remarkably high conductivity, the exceptional stability in air, and green processability from a dispersion in water.^[1] In addition, the electronic conductivity of PEDOT combined with the polyelectrolytic PSS provide this polymer blend with a multi-responsivity.
The redox activity of PEDOT, for example, imparts electrochromism to PEDOT:PSS. This implies that the color can be switched through an electrochemical trigger,^[2] which is commonly used in electrochromic window or display applications. PEDOT:PSS exhibits not only a dark blue but also an almost colorless, transparent state when applied as a thin film. In one approach of this work, this could be exploited to make responsive photonic fibers.^[3] The hygroscopic nature of PSS also provides PEDOT:PSS with a dependence of various properties on the relative humidity (r.H.). This implies, for example, that electronic and ionic conductivity could be tuned and strongly depended on the water uptake of the films.^[4] Furthermore, swelling with water by absorbing humidity from the environment was used to fabricate humidity-triggered bilayer actuators.^[5] It could be demonstrated that for the tailor-made design of such actuators precise knowledge of the mechanical properties of the components as well as the corresponding dependence on r.H. is of utmost importance.^[5]

[1] A. Elschner, PEDOT: Principles and Applications of an Intrinsically Conductive Polymer, CRC Press, Boca Raton, FL, 2011.
[2] M. Wieland, C. Malacrida, Q. Yu, C. Schlewitz, L. Scapinello, A. Penoni, S. Ludwigs, Conductance and spectroscopic mapping of EDOT polymer films upon electrochemical doping. Flex. Print. Electron. 2020, 5, 14016.
[3] C. Dingler, J. D. Sandt, M. Kolle, S. Ludwigs, Structurally Amplified Optical Contrast in a PEDOT:PSS-based Electrochromic Fiber Indicator, in preparation.
[4] M. Wieland, C. Dingler, R. Merkle, J. Maier, S. Ludwigs, Humidity-Controlled Water Uptake and Conductivities in Ion and Electron Mixed Conducting Polythiophene Films. ACS Appl. Mater. Interfaces 2020, 12, 6742–6751.
[5] C. Dingler, H. Müller, E. Ghobadi, M. Wieland, D. Fauser, H. Steeb, S. Ludwigs, Humidity Dependent Rheological Behavior and Curvature Prediction of PEDOT:PSS/PDMS Bilayer Actuators, in preparation.

Electron transfer reactions at the interfaces of molecular sensitized semiconductors in electrolytes represent the basic steps in the working mechanism of several electrochemical and photo electrochemical systems for energy conversion and storage. For example, in dye sensitized solar cells, the design of interfaces that favor the desirable flow of photo-generated electronic charges towards the device contacts and that limit the rate of undesirable processes associated to energy loss is key to optimize device performance. [1] The identification of effective device design rules requires knowledge on the main factors influencing the kinetics of electron transfer. One such factor is the dielectric environment where charge transfer reactions occur. Electron transfer theories predict pronounced dependence of the rates of charge transfer between free molecules in solution on the polarity of the environment, where more polar solvents are expected to result in lower transfer rates. An appropriate description of this relation for electron transfer reactions involving molecules sensitizing semiconductors in electrolytes must account for the restricted geometry of these systems and is currently missing.

In this contribution, we present rates of electron transfer reactions involving photo-oxidized thiophene–carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes measured using transient absorption spectroscopy. [2] We explore the extent to which the dielectric properties of the surrounding medium can explain the rates of three different charge transfer processes occurring in these systems. We consider the hole hopping between molecules anchored to the semiconducting oxide surface, [3] the electron transfer from the oxide to the photo-oxidized molecule (electron-hole recombination) and the hole transfer from the sensitizing molecules and a redox couple dissolved in the electrolyte solution (regeneration). Strikingly, we observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. On the other hand, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules is observed when using solvents with low polarity. Further to these findings, we show that the activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which emphasizes the role of energetic disorder in the molecular monolayer on charge transport. Moreover, we show that the recombination rate in different solvents scales with the hole hopping rate, consistent with our previous study. [4] Based on our findings, we highlight the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes. Our study points to new strategies that allow to control electron transfer rates at interfaces of organic and hybrid photoelectrochemical systems and optoelectronic devices.

[1] A. Hagfeldt and M. Grätzel, Chem. Rev. 95, 49 (1995).
[2] D. Moia, et al. J. Phys. Chem. C 124, 6979 (2020).
[3] D. Moia et al. Chem. Sci. 5, 281 (2014).
[4] D. Moia et al. J. Am. Chem. Soc. 138, 13197 (2016).

Transparent conducting electrodes (TCEs) are one of the essential components of solar cells and other optoelectronic devices such as touch screens, electrochromic windows, flat panel displays, etc. Proper selection of specific TCEs is important to achieve efficient optoelectronic devices. A figure of merit (FOM) is an often-used quantity to rate the performance of TCEs. In this work, we have re-assessed the existing figures of merit and compare them with a new proposal from our side by applying them to specific material systems. It is demonstrated that the newly proposed FOM provides decisive information for the application specific development of material systems employed for TCEs.

Despite advances in the power-conversion efficiencies of NFA-based organic solar cells, limited operational lifetime presents a stumbling block for their commercialization. Operational lifetime is assessed by the endurance of organic solar cell performance under constant thermal and light stress. Here, we study the operational stability of the organic solar cells based on the ITIC and its derivatives. We show that a fine-grained ternary blend including ITIC-4F and ITIC-4Cl mixture enhances the thermal stability. The halogenated ITIC derivatives readily co-crystallize and forms a fine-grained ternary blend with nanometer-sized domains. This results in a stable performance at 130 ^oC for at least 205 hours. We further show that the long-range order of the NFA crystals can only partly enhance the photo-stability of the organic solar cells. And, structural stability of the non-fullerene acceptor is also necessary to improve the photo-stability of the organic solar cells. These observations are confirmed using Nuclear Magnetic Resonance spectroscopy, High Resolution- Mass Spectroscopy, AFM-IR, and GIWAXS. Our results indicate that the ternary approach enables the use of high-temperature processing protocols and emphasizes the need for a more informed structural design of NFAs and unveils routes to minimize impurities, during synthesis, which affect the photostability of the NFA and thereby solar cells.

Organic bioelectronics has experienced tremendous growth over the past two decades thanks to the expansion of the library of organic electronic materials available. Electron conducting (n-type) polymers are particularly suitable to translate biological events that involve the generation of electrons. However, n-type polymers that are stable when addressed electrically in aqueous media are relatively scarce, and the performance of existing ones lags behind their hole conducting (p-type) counterparts. Here, we report a new family of donor-acceptor type polymers based on naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene (NDI-T2) backbone where the NDI unit always bears an ethylene glycol (EG) side chain. We study how small variations in the side chains tethered to the acceptor as well as the donor unit affect the performance of the polymer films in the state-of-the-art bioelectronic device, the organic electrochemical transistor (OECT). First, we show that substitution of the T2 core with an electron withdrawing group (i.e., methoxy) or an EG side chain leads to ambipolar charge transport properties and causes significant changes in film microstructure revealed by ex situ X-ray scattering studies, which overall impairs the n-type OECT performance. We thus find that the best n-type OECT performer is the polymer that has no substitution on the T2 unit. Next, we evaluate the distance of the oxygen from the NDI unit as a design parameter by varying the length of the carbon spacer placed between the EG unit and the backbone. We find that the distance of the EG from the backbone affects the film order and crystallinity, and thus, the electron mobility. As such, we develop the best performing NDI-T2 based n-type OECT material to dates. Our work provides new guidelines for the side chain architecture of n-type polymers for OECTs and insight on the structure-performance relationships for mixed ionic-electronic conductors, crucial for devices where the film operates at the aqueous electrolyte interface.

Organic-light emitting diodes (OLEDs) are becoming increasingly complex, consisting of multiple components with hybrid architectures. Hence, there is a growing demand for a computational approach to accurately describe sub-nanoscale phenomena, while being robust and scalable to unveil the origin of emergent layer and device-scale properties. To date, the majority of computational OLED designs rely on estimating materials properties in the gas phase. However, OLED materials are applied as thin solid films where intermolecular interactions have a significant impact on overall device performance. In this work, we present a multi-tiered computational workflow to efficiently study the optoelectronic properties of OLED materials in realistic disordered thin films. A range of robust and validated methods are combined to reveal impacts of the amorphous solid-state morphologies on the static and dynamic optoelectronic properties. The hierarchical approach of this work provides a thorough mechanistic understanding of materials performance for systematic design and development of future smart OLEDs as well as emerging electronic devices.

With efficiencies >20% on the horizon, non-toxic and massively scalable organic solar cells are approaching economic viability. In order for the technology to approach limiting performance, it is necessary to establish a more definitive and detailed model on how molecular ordering drives device dyanmics. Although many structure-property correlations can be found in the literature, it is critical to establish causal relationships to move the field forward. I will describe our advanced nanostructure characterization simultaneous to comprehensive device dyanmics that are directly connected to performance. This allows us to analyze correlated trends in structure and dyanmics to determine cause and effect more definitively. Our novel combination of synchrotron X-ray techniques allows us to derive a complete set of morphological parameters that includes details of donor-acceptor interfaces. We also have developed a photophysical measurement strategy using time-delayed collection field to quantify each fundamental stage in the charge generation process. By probing morphology on the devices themselves, our studies reveal unprecedented clarity on the effect of molecular mixing, crystallinity, and device interfaces on charge generation and extraction at operating conditions. The outcome is an increasingly resolved model of ideal device morphology. Such a model will enable new device processing strategies and help advise molecular design in pursuit of performance limits and commercialization of this exciting technology.

ShieldPower^TM polymers were developed at Phillips 66 for use in the photoactive layer of scalable organic photovoltaics. This talk will highlight their development from lab to pilot scale evaluation, focusing on the key parameters that enable ShieldPower^TM polymers to reach high power conversion efficiencies using scalable coating techniques. The polymers combine high conductivity, as determined by time-resolved microwave conductivity experiments, and good solubility to enable photoactive layers with films >200 nm in thickness to be coated from non-halogenated solvents via slot-die coating in ambient air in a pilot fabrication line resulting modules that reach 5.3% power conversion efficiency. Further tuning the polymers’ absorption properties and structures allows for blends with a stable non-fullerene electron acceptor to achieve power conversion efficiencies that remain constant over 250+ hours of illumination.

Organic photovoltaics (OPVs) had their performance greatly improved in the past decade, thanks to careful molecular design and process optimization, even approaching the efficiency of single-junction inorganic devices. However, such state-of-the-art OPVs rely heavily on complex polymers and compounds, whose synthesis is not scalable and unlikely to make it to market. Thus, the enhancement of low-cost OPV active layers, such as P3HT:PC₆₁BM, is a relevant and often overlooked topic OPV research. A common way to improve P3HT:PC₆₁BM blends is to increase its relatively narrow light absorption range by adding a ternary additive that is sensitive to complementary wavelengths. Silicon phthalocyanines (SiPc) have been shown to be efficient ternary additives due to their strong light absorptivity, high electron mobility, chemical versatility and scalability of manufacture. The addition of a small amount of soluble, axially substituted SiPc (OR-SiPc) have been reported to increase the current of P3HT:PC₆₁BM devices by 25% and the overall efficiency by as much as 50%. One of the main challenges faced when engineering new or improving existent active layers is the lack of robust design rules and property-performance relationships, leaving researchers and engineers to rely on time-consuming try-and-error approaches. In this context, we have synthesized 8 different OR-SiPc and evaluated how their properties impact their efficiency as additives in OPVs. The novel OR-SiPcs were synthesized by reacting different alkyl chlorosilanes of different lengths with Cl-SiPc. Characterization of the compounds has shown that all 8 OR-SiPcs possess similar surface energies and virtually identical optical and electronic properties, such as band gap (Ebg), ionization energy (IE), and energy levels. Alternatively, due to the different alkyl chain lengths in the axial positions, their solubility can change greatly. We were able to demonstrate that when OR-SiPc is added to P3HT:PC₆₁BM, device efficiency follows a nearly parabolic trend with respect to the additive solubility. The best performance was achieved by the additive whose solubility value was the closest to that of the donor polymer, P3HT. Based on these results and previously reported properties of the system, we have surmised that matching the solubility of the OR-SiPc and P3HT is beneficial because it favours the migration of the additive to the donor:acceptor interface, where its efficiency is maximized. OR-SiPc with solubilities below 20 mg ml^-1, for instance, tended to precipitate out of solution and form a separate crystalline phase, rather than migrating to the interface, making it less efficient at mediating the charge transfer between P3HT and PC₆₁BM. More research is needed to verify if a similar effect is observed in different systems and additives, nonetheless, this is an important step towards more robust design-rules for OPVs.

Semitransparent organic photovoltaic cells (ST-OPVs) are emerging as a solution for solar energy harvesting on building facades, rooftops and windows. However, the trade-off between power conversion efficiency (PCE) and the average photopic transmission (APT) in color-neutral devices limits their utility as attractive, power-generating windows. We demonstrate a semi-transparent OPV that achieves a high power conversion efficiency of 10.8% and visible transparency of ~50% using a simple non-fullerene acceptor (NFA) with strong near-infrared (NIR) absorption and simplified synthetic step. Contrary to expectations, stronger NIR absorption and closer molecular packing are obtained by employing an additive in these partially, instead of fully fused rigid NFAs. By combining NIR-absorbing material sets with an optical outcoupling structure that reduces trapping of visible radiation within the cell, that increases the NIR reflectivity, we overcome the trade-offs between efficiency, transparency, and device appearance. These results significantly surpass other semi-transparent solar cell technologies based on organic and other thin film materials systems, showing a promising future for ST-OPVs as power generating windows and other solar energy harvesting applications^[1].

[1] Yongxi Li, Xia Guo, Zhengxing Peng, Boning Qu, Hongping Yan, Harade Ade, Maojie Zhang^* and Stephen R. Forrest^*; Color-Neutral, Semitransparent Organic Photovoltaics. PNAS, 2020, 117, 21147-21154.

Phthalocyanine (Pc) based organic thin-film transistors (OTFTs) have been demonstrated as a powerful platform for rapid low-cost sensors of atmospheric parameters, volatile gasses, and small organic molecules including cannabinoids. However, traditional characterization and testing of new materials or analytes requires highly specialized equipment and expertise. Alternatively, solution-based screening can limit manufacturing burden, allowing for a broad range of assays, and is employed here with the goal of expediting sensor development. Spectroelectrochemistry is used to probe molecular interactions between cannabinoids (Δ⁹-tetrahydrocannabinol (THC) or cannabidiol (CBD)) with and without a cannabinoid sensitive chromophore (Fast Blue BB) against a variety of phthalocyanines (Pcs). Spectroelectrochemical changes to the Q-band region of the Pc spectra in the presence of analytes can be related to modified ratios of Pc-Pc orientations in solution. 2D-NMR is used to further elucidate unique Pc-Pc and Pc-analyte interactions, providing additional insight on atomic level interactions. Through altered Pc-Pc orientations, analytes can induce physical alterations to Pc thin-film morphology and structure, consequently, modifying electrical performance beyond charge trapping effects. Additionally, the effects of Pc peripheral and axial substitutions on analyte interactions and their relationship with film structure and performance is examined. Substitutions allow for an additional level of Pc tunability and are demonstrated to specifically further enhance OTFT sensing responses by up to 96%. Thus, with spectroelectrochemical solution-based screening, a range of potential materials for OTFT small organic molecules sensing applications can be rapidly assayed and OTFT responses can be predicted.

Polymers that exhibit both electronic and ionic conductivity have garnered attention for their numerous applications in bioelectronics, neuromorphic systems and batteries. [1] In the field of bioelectronics, the organic electrochemical transistor (OECT) takes advantage of this property to interface with biological systems and convert ionic and electronic signals. [2] Despite recent advances in its fabrication and materials choice, there is a lack of understanding on central roles that are important for the optimal operation of the OECT, like the ionic transport and the morphological impact in its performance. [1] To this end, we have characterised the electrochemical doping and dedoping of poly(3-hexylthiophene) (P3HT) thin films using in-situ time-resolved Vis-NIR spectroscopy. We are able to follow the evolution of the doping with great time resolution, and by decomposing the spectra we can identify the different species involved in the process. Furthermore, we developed a kinetic model to describe the interconversion between the species. Additionally, by changing the degree of crystallinity of the P3HT films, we are able to identify can morphology affect ionic transport in the film.

[1] Chung, J.; Khot, A.; Savoie, B. M.; Boudouris, B. W., ACS Macro Lett. 2020, 9, 646–655.
[2] Rivnay, J.; Inal, S.; Salleo, A.; Owens, R. M.; Berggren, M.; Malliaras, G. G., Nat. Rev. Mater. 2018, 3, 17086.

As the field of Organic Photovoltaics (OPVs) evolves, the need for fundamental understanding of the utilized materials grows along with it. Historically, OPVs feature fullerene-based electron acceptors, yielding efficiencies around 10-12%. Recently, efficiencies of over 17% have been achieved thanks to the use of non-fullerene accpeptors (NFAs).

In order for the overall performance of OPVs to show such improvement, the processes involved in photocurrent generation must be optimized as well. Charge transport is one of the critical processes that requires further investigation in these systems. While single-carrier devices are commonly used in order to deduce the mobility of charge carriers in the steady state, Electromodulated Differential Absorption (EDA) spectroscopy is the principal technique employeed in this project. This powerful technique can be used to qualify charge mobility on the picosecond time scale following photo-excitation, by recording the changes in electroabsorption (EA) over time with 100 femtosecond resolution. Carrying out these measurements in differing electric fields, drift and diffusion mechanisms of charge transport can be distinguished.

The highest performing OPVs of recent years, those achieving 17% photoconversion efficiencies, have featured BTP-4F as the acceptor material. For this reason, the ITIC and BTP NFA families have been chosen for this project in order to qualify how charge transport changes with different molecular packing and core structures. In addition to investigating the sterics of these systems, the electronics will also be investigated by using a series of substitutions of the IC end group featured in both NFA families. Because the focus of this project is on the NFA domains of the OPV devices, the samples used for this project were neat NFA devices.

When studying these two NFA families under EDA conditions, it becomes quite clear that charge transport is largely influenced by the cores of these molecules. Further investigation is underway to understand how the morphology of the NFA regions will affect charge transport.

N-type semiconducting polymers have been recently utilized in thermoelectric devices, however they have typically exhibited low electrical conductivities, in contrast to p-type semiconductors. This is due in particular to the n-type semiconductor’s low doping efficiency, and poor charge carrier mobility. Strategies to enhance the thermoelectric performance of n-type materials include optimizing the electron affinity (EA) with respect to the dopant to improve the doping process, increasing the charge carrier mobility through enhanced molecular packing and introducing polar side chains to improve the miscibility with dopant. Here, we report the design, synthesis and characterization of three fused electron-deficient copolymers P1-P3, incorporating electron withdrawing lactone unit along the backbone. The polymers were synthesized using metal-free aldol condensation conditions to explore the effect of acene size and side chains, on the electrical conductivity. Acene size optimization of P1 and P2 has been investigated, their performance provided a great insight into designing P3, which incorporated polar side chains. The polymers have exhibited the largest electron affinity of -4.68 eV and when n-doped with N-DMBI, electrical conductivities of up to 1 S cm^-1, Seebeck coefficients of -220 mVK^-1 and maximum Power factors of 5 mWm^-1 K^-2 were observed. Moreover, the polar nature of P3 has intrigued us to explore its performance in aqueous electrolytes of Organic electrochemical transistors (OECTs). OECT results indicated high transconductance of 0.35 S cm^-1 and [µC*] of 0.92 F cm^-1 V^-1 s^-1 , exceeding the state of the art materials. These results suggest a promising high-performance building block for both n-type organic thermoelectrics and OECTs, and also highlight that high electrical conductivities can be realized in conjugated polymers with careful molecular engineering.

Organic photovoltaics (OPVs) based on nonfullerene small molecule acceptors have been the subject of renewable energy research in the past five years. Ternary photovoltaic blends comprising a polymer donor, a nonfullerene small molecule acceptor, and a fullerene acceptor have proved to be most efficient active layers for OPV devices. However, the morphological parameters and their relations with mechanical properties and photovoltaic performance of this type of highly efficient polymer:nonfullerene:fullerene OPVs are still unknown to date.
To address this question, we characterized the thin-film microstructure and mechanical behavior of the best-performing ternary OPV active layer (PM6:N3:PC₇₁BM). Specifically, we alter the relative weight ratio of acceptors and systematically examined the composition dependence of photovoltaic performance, surface/bulk morphology, and mechanical parameters of the ternary blend films. It is shown that the addition of 20 wt% PC₇₁BM to the acceptor component results in the highest power conversion efficiency (PCE) and crack-onset strain (COS) (Figure 1a). Most notably, we discover that it is possible to predict the elastic modulus of the ternary blend films by an extended Halpin-Tsai model (Figure 1b). This work provides simple guidelines for predicting the mechanical properties of many electronic devices based on ternary blends.

The film morphology of polymer solar cells (PSCs) plays a significant role in the photovoltaic process that cannot be ignored, which associates with light-harvesting, exciton dissociation, and charge transport. In addition, the morphological stability of the PSCs is largely dependent on the amorphous-amorphous molecular interactions of the components, which can be described by the Flory-Huggins interaction parameter χ_aa. However, in the semi-crystalline blend systems, more complex thermodynamic interactions as reflected by crystal-amorphous interaction parameter (χ_ca), amorphous-crystal interaction parameter (χ_ac), and crystal-crystal interaction parameter (χ_cc), which are complementary to χ_aa. Here, we apply the Thermodynamic self-consistency theory coupled with Flory-Huggins theory to determine the various interaction parameters and establish the relationships between interaction parameter, morphology, and device performance of binary P3HT:nonfullerene blends, based on four nonfullerene small molecule acceptors (ITIC, Y6, O-IDTBR, ZY-4Cl). Moreover, the parameters related to molecular stacking are probed by grazing incidence X-ray diffraction and correlated partially with device performance. Chiefly, this study highlights the importance of both thermodynamic properties and molecular stacking to provide guidance for understanding morphology and device performance.

Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor-acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole, but the underlying mechanism remains unclear. In this talk I will present experimental results showing that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states. This process occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron-hole encounters at later times, both charge-transfer states and local emissive excitons are regenerated, thus setting up an equilibrium between local excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.

Organic thermoelectric materials have attracted much attention as promising candidates for low-cost, near-room-temperature and flexible power generators which are desirable power supplies for flexible electronics. However, molecular design strategies to realize efficient thermoelectric conversion are not well established because little is known regarding microscopic origin of their thermopower. Many of recent studies employ conventional polymer semiconductors such as poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-hexylthiophene-2,5-diyl) (P3HT) and focus on optimizing doping conditions and/or developing novel nanostructures. Aiming to break this situation, we propose a calculation method for quantitative evaluation of thermoelectric properties of organic semiconductors (OSCs) which facilitates preliminary screening of candidate materials and the exploration of novel organic thermoelectrics. We focus on small-molecule semiconductors in this study since their highly crystalline nature is advantageous for efficient thermoelectric conversion[1].
One of the present author (Ohno) and his collaborators have shown that the Seebeck coefficient (which is the most important indicator of the conversion efficiency and is defined as induced thermoelectric voltage per unit temperature difference) of crystalline OSCs can be calculated from the effective density of states (DOS) measured experimentally using the Mott formula[1]. Then the crucial point of our calculation method is the evaluation of the effective DOS of bulk small-molecule OSCs with taking account of the thermally induced structural fluctuation (which is non-negligible even at room temperature) originating from the fact that small-molecule OSCs usually form molecular solids bound by weak van der Waals force. To take account of this fluctuation, we propose to conduct classical molecular dynamics (MD) simulations followed by electronic structure calculation based on density functional theory (DFT).
To demonstrate the validity of the proposed method, we calculated the effective DOS and Seebeck coefficients of pentacene and rubrene whose thermoelectric properties are experimentally tested enough. In our MD simulations, we adopted the general AMBER force field, which describes intra- and inter-molecular interactions including van der Waals force and has been confirmed to be applicable to bulk OSC crystals. MD calculations were performed to simulate equilibrium states at the pressure of 1 atm at 300K, and 100 crystal structures were sampled every 5 ps (5000 steps) from the trajectories. For each sampled structure, DFT calculations were conducted using projector augmented wave method and Perdew-Wang 1991 (PW91) functional.
Although each of the DOS had sharp peaks owing to disorder-induced carrier localization, the effective DOS of bulk crystal were well approximated as an average of adequate number of DOS. Its exponentially decaying shape near the band edge was in good agreement with experimental results. The value of Seebeck coefficients estimated from the decay rates of the effective DOS were 0.26 mV/K for pentacene and 0.17 mV/K for rubrene. These values agree quantitatively with experimental results[2], so we expect that the proposed method is applicable to materials exploration. In the presentation, relationships between structures and thermoelectric properties will also be discussed in depth.

[1] S. Watanabe and M. Ohno et al., Phys. Rev. B 100, 241201(R) (2019). [2] K. P. Pernstich et al., Nat. Mater. 7, 321 (2008).

Organic electrochemical transistors (OECTs) developed for integration in bioelectronic devices utilise a conjugated polymer (CP) as the active material in the channel. The utility of OECTs has been demonstrated in various bioapplications including monitoring and stimulating biological cells.^1,2 In these applications, we want OECTs to be energy-efficient, requiring low threshold voltages to activate, and operating in accumulation mode such that they only use energy when switched “on”. Unfortunately, many existing CPs for aqueous OECTs lead to depletion mode operation, which requires a constant potential to keep the device “off” and so is more energy intensive. These materials also require crosslinking or post-treatment to prevent their dissolution in water,³ and their biocompatibility with certain cell lines may be limited by their high acidity.⁴

To overcome these limitations, we have synthesised a new CP, poly(ethylenedioxythiophene) bearing isopropyl-protected phosphonate groups (PEDOT-Phos), and performed its characterisation to elucidate its structure-property relationships. PEDOT-Phos is solution-processible from organic solvents, producing films which are readily proton doped with non-oxidising acids and undergo reversible electrochemistry in aqueous electrolytes. PEDOT-Phos can also be used in energy-efficient, aqueous OECTs with accumulation mode operation and low threshold voltages. These devices have the added advantages of high volumetric capacitances, good transconductances, and no requirement for additives or post-treatments. With these encouraging properties, PEDOT-Phos has great potential for emerging bioelectronic devices which combine good performance with improved energy efficiency.

References:
(1) Hopkins, J.; Travaglini, L.; Lauto, A.; Cramer, T.; Fraboni, B.; Seidel, J.; Mawad, D. Photoactive Organic Substrates for Cell Stimulation: Progress and Perspectives. Adv. Mater. Technol. 2019, 1800744, 1800744. https://doi.org/10.1002/admt.201800744.
(2) Fidanovski, K.; Mawad, D. Conjugated Polymers in Bioelectronics: Addressing the Interface Challenge. Adv. Healthc. Mater. 2019, 8 (10), 1–9. https://doi.org/10.1002/adhm.201900053.
(3) Stavrinidou, E.; Leleux, P.; Rajaona, H.; Khodagholy, D.; Rivnay, J.; Lindau, M.; Sanaur, S.; Malliaras, G. G. Direct Measurement of Ion Mobility in a Conducting Polymer. Adv. Mater. 2013, 25 (32), 4488–4493. https://doi.org/10.1002/adma.201301240.
(4) Mantione, D.; del Agua, I.; Sanchez-Sanchez, A.; Mecerreyes, D. Poly(3,4-Ethylenedioxythiophene) (PEDOT) Derivatives: Innovative Conductive Polymers for Bioelectronics. Polymers (Basel). 2017, 9 (12), 354. https://doi.org/10.3390/polym9080354.

The performance of optoelectronic devices based on conjugated polymers is critically dependent upon molecular packing; however, the paracrystalline nature of these materials limits the amount of information that can be extracted from conventional X-ray diffraction. Resonant diffraction (also known as anomalous diffraction) occurs when the X-ray energy used coincides with an X-ray absorption edge in one of the constituent elements in the sample. The rapid changes in diffraction intensity that occur as the X-ray energy is varied across an absorption edge provide additional information that is lost in a conventional nonresonant experiment. Taking advantage of the fact that many conjugated polymers contain sulfur as heteroatoms, this work reveals pronounced resonant diffraction effects at the sulfur K-edge with a particular focus on the well-studied electron transporting polymer poly([N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)), P(NDI2OD-T2). The observed behavior is found to be consistent with the theory of resonant diffraction, and by simulating the energy-dependent peak intensity based on proposed crystal structures for P(NDI2OD-T2), we find that resonant diffraction can discriminate between different crystalline packing structures. The utilization of resonant diffraction opens up a new way to unlock important microstructural information about conjugated polymers for which only a handful of diffraction peaks are typically available.

Among many promising organic semiconducting materials, dinaphthothienothiophene (DNTT) shows outstanding device performances in terms of charge transport and air stability, which exceeds those of the conventional prototypical material of pentacene. Although many reports demonstrate the excellent device performances of DNTT, a comprehensive understanding of thin film growth is lacking. In fact, the thin-film structure has long been believed to be identical to the single-crystal structure. In other words, polymorphs of DNTT have never been observed so far.
In the present study, the thickness-dependent structural evolution is revealed by means of high-resolution X-ray diffraction. This technique apparently reveals the existence of the “thin-film” phase [1], which has long been missed by the conventional analytical limit. This is also supported by infrared spectroscopy. The present study strongly suggests that a thickness-dependent structural change is a common phenomenon for rod-like organic molecules including DNTT. This study is believed to provide an in-depth understanding of thin film growth of common organic semiconductors.

[1] Shioya, N. et al., Appl. Phys. Express 2020, 13, 095505.

Control of the molecular orientation in organic semiconductor (OSC) films is essential to improve the performance of organic electronic devices. However, specific parameters for controlling molecular orientation have not yet been fully understood. In this study, we focus on the impact of growth temperature on molecular orientation in OSC films. We used pentacene on SiO₂ as a model system and applied p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) for investigating the variation of molecular orientation. The molecular orientation of films varied with growth temperature from lying (molecules orient parallel to the substrate) to standing (molecules orient perpendicular to the substrate) orientation. This change suggests that the formation of a standing orientation is thermally activated compared with the lying state. The nucleation of standing-oriented islands occurs by molecular self-assembly at sufficiently high temperatures. Conversely, molecules deposit in a lying state at low temperatures owing to hinderance of the kinetic barrier to reorientation from lying to standing states. We defined a collective orientation barrier (COB) as an energy barrier for the reorientation processes throughout the growth. We estimated the COB from an Arrhenius plot of the probability to form a standing orientation, derived from the dichroic ratio measured by pMAIRS. We found that the COB in the growth of pentacene on the SiO₂ system was approximately 0.02 eV. The COB is a key parameter for determining the molecular orientation and can be utilized for the control of molecular orientation of OSC films to improve the performance of organic electronic devices.

Developing highly conducting and air-stable n-type semiconductors have proven to be extremely challenging, despite intensive research efforts and a need for better-performing materials. Not only would highly conducting interlayers facilitate charge extraction, respectively injection, when used in organic solar cells or light-emitting diodes, but also accelerate the development of emerging plastic electronic technologies like organic thermoelectrics. Thermoelectric generators convert heat into usable electricity, due to the Seebeck effect. Constructing an efficient thermoelectric generator based on organic semiconductors, however, has proven difficult, primarily due to a significant shortage of suitable materials. While organic p-type semiconductors are abundant and relatively easy to dope, there is a shortage of suitable n-type materials and appropriate dopants. One approach that has proven popular in recent years is extrinsic doping of conjugated polymers. By mixing molecular dopants into a polymer matrix, it is possible to enhance the overall conductivity of the semiconductor to a certain degree, before morphological instabilities take over and cause microscopic phase separation of the dopant and polymer matrix, significantly limiting the doping efficiency.

In this paper, we will present a new approach to intrinsic organic conductors, namely organometallic coordination polymers. We will discuss how the choice of ligand and coordinating metal cation affect the electrical properties of the material and demonstrate how both the ligand and metal cation can be exploited to control the electrical characteristics. We will discuss how material purification affects the overall performance and elucidate the charge transport mechanism in this material class. Contrary to extrinsically doped semiconducting polymers, the organometallic coordination polymers exhibit excellent ambient and morphological stability and could pave the way to a new class of robust organic conductors for plastic electronic applications.

The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’.
Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material [1]. This thermoelectric material exhibits an excellent electrical conductivity of >10 S/cm and an ultralow thermal conductivity of <0.1 W/mK, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials.

[1] J. Liu, B. van der Zee, R. Alessandri, S. Sami, J. Dong, M.I. Nugraha, A.J. Barker, S. Rousseva, L. Qiu, X. Qiu, N. Klasen, R.C. Chiechi, D. Baran, M. Caironi, G. Portale, R.W. A. Havenith, S.J. Marrink, J.C. Hummelen, and L.J.A. Koster, Nature Comm. 11, 5694 (2020).

The use of X-ray radiation detectors has gradually increased over the past couple of decades in a wide variety of applications ranging from medical diagnostics, industrial inspection, scientific research, to cultural heritage preservation. However, the object of investigation in most situations is of complex shape and geometry, while the detectors available to study them are rigid and flat. Such incompatibility between the object and detector results in numerous issues including vignetting and distortion of images. As a result, there is a growing requirement for conformable or curved detectors. Organic-inorganic hybrid (OIH) semiconductors are one of the most rapidly advancing class of novel materials for X-ray detection, that can be effortlessly printed on flexible substrates by using low-temperature wet deposition techniques. Furthermore, the use of OIH semiconductors as a soft matrix is expected to enable the fabrication of conformable detectors. Among them, the OIH detector concept that involves X-ray stopping bismuth oxide (Bi₂O₃) nanoparticles (NPs) incorporated into an organic bulk heterojunction (BHJ) consisting of p-type polymer Poly(3-hexylthiophene-2,5-diyl) (P3HT) and the n-type [6,6]-Phenyl C71 butyric acid methyl ester (PC₇₀BM) is highly versatile due to its exceptional characteristics such as high broadband sensitivity, ultra-low dark currents, and fast response times^[1,2]. However, P3HT is generally regarded as a polymer that can crystallise depending on its molecular weight, and thereby influence the mechanical characteristics of the detector such as bendability, which is vital for conformable detectors. However, there has been very little work reported so far in this regard, especially for thick devices typically required for X-ray detection. In this study, we emphasize the importance of tuning the molecular weight of the P3HT polymer on achieving a perfect balance between the detector performance and bendability characteristics. Through fabricating conformable hybrid detectors with P3HT molecular weights in the range of 25-60 kDa, we demonstrate that there is a strong correlation between the detector performance, bendability characteristics, and crystallinity of the film. In particular, we observe the suitability of higher P3HT molecular weights for achieving detectors with a higher level of mechanical flexibility under repeated bending cycles. These observations are explained based on the internal microstructure of these films which results in higher mechanical cohesion within the film. This study highlights the importance of understanding and optimizing fundamental material properties towards designing highly conformable detectors, a fact that has not been reported in depth so far.
References
[1] M. P. A. Nanayakkara, L. Matjačić, S. Wood, F. Richheimer, F. A. Castro, S. Jenatsch, S. Züfle, R. Kilbride, A. J. Parnell, M. G. Masteghin, H. M. Thirimanne, A. Nisbet, K. D. G. I. Jayawardena, S. R. P. Silva, Adv. Funct. Mater. 2020, 2008482.
[2] H. M. Thirimanne, K. D. G. I. Jayawardena, A. J. Parnell, R. M. I. Bandara, A. Karalasingam, S. Pani, J. E. Huerdler, D. G. Lidzey, S. F. Tedde, A. Nisbet, C. A. Mills, & S. R. P. Silva, Nat. Commun. 2018, 9, 2926.

The low-frequency lattice dynamics of small-molecule organic semiconductors have a significant impact on their transport properties. Many current studies do not take into account the possible strong anharmonic behavior of the lattice dynamics in these materials. In my presentation, I will show that similar molecules can have significantly different degrees and expressions of anharmonic lattice dynamics. I will do that by presenting the lattice dynamics of BTBT, ditBu-BTBT, diC8-BTBT, diPh-BTBT, and DNTT single crystals measured by polarization-orientation (PO) Raman combined with temperature-dependent Raman in the low-frequency range. I will show an expression of strongly anharmonic lattice dynamics in BTBT indicated by the change of the PO Raman dependence of the lattice vibrations as the temperature is increased. This anharmonic expression is almost completely suppressed in the other crystals. I will also show other expressions of anharmonicity in the form of an order-to-disorder phase transition in ditBu-BTBT and a polymorphic phase transition in diC8-BTBT. I will discuss the physical origin of these anharmonic expressions, addressing their large-amplitude motion and their anharmonic potential energy surface. I will argue that these expressions of anharmonicity are not captured within the quasi-harmonic framework. I will also talk about the effect of these anharmonic expressions on the nonlocal electron-phonon interactions, and their decoupled nature.

Meniscus-guided coating methods, such as zone casting, dip coating and solution shearing, are scalable laboratory models for large-area solution coating of functional materials for thin-film electronics. Unfortunately, the general lack of understanding of how the coating parameters affect the dry-film morphology upholds trial-and-error experimentation and delays lab-to-fab translation. In this contribution we present a modelling study focusing on the prediction of dry-film morphologies produced by meniscus-guided coating of a crystallizing small molecular solute. Our model reveals how the interplay between coating velocity and evaporation rate determines the crystalline domain size, shape anisotropy and regularity. If coating is fast, evaporation drives the system quickly past supersaturation, giving isotropic domain structures. If coating is slow, depletion due to crystallization stretches domains in the coating direction. The predicted morphologies have been experimentally confirmed by zone-casting experiments of the organic semiconductor 4-tolyl-bithiophenyl-diketopyrrolopyrrole. Although here we considered a molecular solute, our model can be applied broadly to polymers and organic–inorganic hybrids such as perovskites.

Both, poly(3-hexylthiophene) (P3HT) and poly{[N,N’-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5’-(2,2’-bithiophene) (P(NDI2OD-T2)), can be regarded as the work-horses of the organic electronics community.
Based on our earlier crystallization studies on P3HT¹ my talk will give an overview about our recent findings on structure-function relationships of (P(NDI2OD-T2)². The involvement of liquid-crystalline phases, in particular nematic-like preordering in highly concentrated solutions is demonstrated. Solvent vapor annealing is shown to lead to large area alignment in spherulite-like superstructures with semi-crystalline structures. Particularly nice is the applicability of blade-coating to achieve highly anisotropic fiber-containing films where both the bulk and the surface morphology can be determined.³ These findings show completely different growth behavior than previously developed for P3HT.¹
Using such well-defined morphologies we can access detailed structural insights into chemical and electrochemical doping mechanisms.^4,5 An alternative way of inducing charge carriers in the bulk polymer film is ex-situ electrochemical doping.⁶ Here the charge carriers are generated inside a three-electrode setup in electrolyte by applying a static electrochemical doping potential. Our experiments show that the generated charged species are stable in the solid state enabling 4-line probe conductivity and absorption measurements. An advantage of our electrochemical doping technique is the ability to fine-tune the charge carrier density inside the polymer film and hence control the resulting conductivity. Conductivities as high as 220 S/cm over large electrochemical potential windows can be reached for P3HT films. Maximum conductivities in P(NDI2OD-T2) films are limited to 10^-3 to 10^-2 S/cm which is ascribed to a mixed valence conductivity mechanism in such conjugated redox polymer films.⁴

1) E.J.W. Crossland, K. Tremel, F. Fischer, K. Rahimi, G. Reiter, U. Steiner, S. Ludwigs, Adv. Mater. 2012, 24, 839.
2) Y. M. Gross, and S. Ludwigs, Synthetic Metals 2019, 253, 73.
3) D. Trefz, Y. M. Gross, C. Dingler, R. Tkachov, A. Hamidi-Sakr, A. Kiriy, C. R. McNeill, M. Brinkmann, S. Ludwigs, Macromolecules 2019, 52, 43.
4) Y. M. Gross, D. Trefz, C. Dingler, D. Bauer, V. Vijayakumar, V. Untilova, L. Biniek, M. Brinkmann, S. Ludwigs, Chem. Mater. 2019, 31, 3542.
5) K. Bruchlos, D. Trefz, A. Hamidi-Sakr, M. Brinkmann, J. Heinze, A. Ruff, S. Ludwigs, Electrochim. Acta 2018, 269, 299.
6) D. Neusser, C. Malacrida, M. Kern, Y. Gross, J. van Slageren, S. Ludwigs, Chem. Mater. 2020, 32, 6003.

Imaging of polymers by transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM) remains a challenge due to the low contrast between domains and sensitivity to the electron beam. Recent advances in instrumentation for electron microscopy have aimed to push the resolution limit, leading to remarkable instruments capable of imaging at 0.5 Å. But, when imaging soft materials, the resolution is often limited by the amount of dose the material can handle rather than the instrumental resolution. Despite the strong constraints placed by radiation sensitivity, recent developments in electron microscopes have the potential to advance polymer electron microscopy. For example, we have used monochromatated sources for spectroscopy and imaging based on differences in the valence electronic structure of conjugated polymers, electron tomography to map nanoscale inhomogeneity in polymer density, and direct electron detectors to minimize the required dose for imaging and thereby image lattice planes in crystalline polymers. Key to these efforts is characterization of the mechanism for radiation damage in the TEM, which suggests new strategies to minimize damage, and thereby push the resolution limit down to 3.6 Å (for example, for PffBT4T-20D). Altogether, the field of polymer electron microscopy is poised to make significant advances in the near future.

Organic semiconducting materials have already been commercialized for applications such as organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), and photovoltaic devices. Recent work in the Howard Katz group at Johns Hopkins University has produced two novel organic doped n-type polymer semiconducting thermoelectric materials, PNDIC1TVT and PBDOPVTT, with σ values of 0.2 S cm-1 and 0.75 S cm-1, respectively, and power factors (PF) of 67 μW m-1 K-2 and 58.1 μW m-1 K-2, respectively. A doped form of PNDIC1TVT appears to have the highest power factor value yet reported for a solution-processed conjugated organic-doped n- type polymer thermoelectric material. However, the molecular origin of this enhancement is uncertain, motivating a computational investigation of this system.
We used density functional theory to probe the nature, structure, and properties of an adduct forming between fluoride anion, an established but electrochemically unlikely n-dopant, and a model electron-accepting naphthalenetetracarboxylic diimide (NTCDI) subunit as a representative n-type conjugated polymer segment, to investigate the hypothesis that an adduct may act as an intermediate in the fluoride doping mechanism of the semiconducting polymer. Our contribution uses the ORCA code with Ahlrichs’ def2-TZVP basis set with a B97-D3 functional, based on our previous investigations and recommendations by Grimme (Phys. Chem. Chem. Phys., 2011). We confirmed adduct formation between the two species and identified three positionally unique sites at which a fluoride dopant was capable of datively bonding to the NTCDI, most strongly and somewhat surprisingly at the carbonyl carbons. Subsequent simulations explored the impact of positive counterions, cesium and tetramethylammonium. Our results confirmed the lack of experimental observations of cesium-containing adducts, which we predict would not form from a binding energy standpoint. We also found that the adducts had an ionization potential that was nearly 1 eV less than the dopants themselves, indicating a greater likelihood that the adduct was the source of doping rather than the pure fluoride salts. These results suggest that a fluoride ion-based Meisenheimer adduct could be a particularly effective fluoride intermediate to act as a dopant for n-type polymers and might be used for high-performance n-type organic thermoelectrics. We also conducted an analogous study on adduct structures using electron-donating thiophene oligomer segments and scandium triflate and tris(pentafluorophenyl)borane Lewis acids, which could also serve as p-dopants. Our results showed that the doped adduct electron affinities were at most 0.5 eV more negative than the pure Lewis acids, meaning that considering dielectric and entropic effects, they could serve as polymer p-dopants. This study strongly supports the hypothesis that adduct formation is a plausible mechanism for conjugated polymer doping by dopants that do not appear to have sufficient electrochemical driving force on their own, and suggests that adduct dopants could also contribute to useful density of states profiles for polymer thermoelectrics.

Conjugated polymers are an interesting class of materials for electrochemical devices since their electrochemical properties can be tuned by chemical design strategies to enable redox activity in various electrolytes. For example, redox activity in aqueous electrolytes can be achieved by attaching polar side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes.^[1] While this approach has been beneficial for achieving low apparent redox potentials in aqueous solution, little is known about the relationship between water uptake during the electrochemical charging process of conjugated polymers and their redox-potentials and stabilities, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes is harming the reversibility of electrochemical processes and results in mechanical instabilities of the polymer electrodes, requiring control of the swelling through side-chain engineering. We show that small changes of the side chain composition can significantly increase the redox-stability of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. We further study the charging process by employing density functional theory calculations to explain shifts of the electrochemical redox-potentials of water-swollen polymer films. Our results show that water uptake is beneficial for achieving low redox-potentials. Additionally, we investigate the affinity of the charged polymer to undergo unwanted side reactions with the electrolyte and find a high retention of the charged, reduced, conjugated polymers in pH neutral aqueous electrolytes, but only in the absence of molecular oxygen. Our work shows the importance of chemical design strategies for achieving high redox-stabilities for conjugated polymers in aqueous electrolytes.
[1] A. Giovannitti, I. P. Maria, D. Hanifi, M. J. Donahue, D. Bryant, K. J. Barth, B. E. Makdah, A. Savva, D. Moia, M. Zetek, P. R. F. Barnes, O. G. Reid, S. Inal, G. Rumbles, G. G. Malliaras, J. Nelson, J. Rivnay, I. McCulloch, Chem. Mater. 2018, 30, 2945.

The use of semiconducting polymers continues to grow despite the lack of a robust theory for how chemical doping, polaron formation, and spatial inhomogeneity localize charge carriers in these materials. Charge transport in semiconducting polymers ranges from localized (hopping-like) to delocalized (metal-like), and no quantitative model exists to fully capture this spectrum of transport behavior and its dependency on charge carrier density. In this study, we develop and validate a charge transport model that quantifies charge carrier localization and can effectively capture the localized to delocalized transition. With the archetypal P3HT-FeCl₃ polymer-dopant system, we measure the temperature dependent electrical conductivity, Seebeck coefficient, quantify the extent of oxidation using XPS, and show that the transport function prefactor and the localization activation energy are functions of the charge carrier concentration ratio (i.e., the ratio of oxidized thiophene rings to total thiophene rings). From these observations, we develop a semi-localized transport (SLoT) model, which captures both localized and delocalized transport effects with broad application in describing the electro-thermal transport of many chemically doped polymers. By applying the SLoT model to published data on semiconducting polymers and carbon nanotube systems we demonstrate its broad applicability and utility. We are able to determine system dependent parameters such as (i) the maximum localization energy of the system in the low doping limit, (ii) how this localization energy changes with doping (iii) the amount of dopant required to achieve metal-like conductivity, and (iv) the hypothetical conductivity a system could have in the absence of localization effects. Ultimately, for the first time, this proposed SLoT model captures both localized and delocalized transport, and thereby improves our ability to predict and tailor electronic properties of doped-semiconducting polymers.

Electrochemically-driven ion insertion into conjugated polymers causes molecular structural changes that impact how ionic and electronic charge carriers move through the polymer matrix. Couplings between structural changes, ion motion, and electronic transport affect the performance of devices that rely on the mixed ionic electron conductivity of conjugated polymers, such as electrochemical capacitors, bioelectronics, and neuromorphic computing. Typically, the crystal structures of conjugated polymers change continuously upon ion insertion. Yet, using in situ X-ray scattering, we find that conjugated polymers can undergo reversible first order structural phase transitions upon electrochemically-driven ion insertion. We also use nanoscale infrared imaging to characterize phase separation between ion-rich and ion-poor regions of the polymer during electrochemical doping. We find that structural phase transitions have important implications for the charging mechanism of conjugated polymers for electrochemical capacitors.

Doping conjugated polymers is an effective way to tune their electronic properties for thin-film electronics applications. Chemical doping of semiconducting polymers involves the introduction of a strong electron acceptor or donor molecules that can undergo charge transfer with the polymer. The charge transfer reaction creates electrical carriers on the polymer chain (usually positive polarons a.k.a. holes) while the dopant molecules remain in the film and serves as a counterion. Unfortunately, strong electrostatic attraction between counter-anions and most dopants will localize the polarons and reduce their mobility. Here, we employ a new strategy, utilizing substituted icosahedral dodecaborane (DDB) clusters as molecular dopants for conjugated polymers. DDBs provide a unique system in which the redox potential of the dopant can be rationally tuned via modification of the substituents without significant change to the size or shape of the dopant molecule. These clusters thus allow us to disentangle the effects of energetic offset on the production of free and trapped carriers in DDB-doped poly-3-hexylthiophene (P3HT) films. We find that the propensity for charge transfer is directly proportional to the energetic offset between P3HT and the DDB dopant. More importantly, by designing DDB clusters to have a high redox potential and good steric protection of the core-localized electron density, highly delocalized polarons with mobilities equivalent to films doped with no anions present are obtained. P3HT films doped with these boron clusters have conductivities and polaron mobilities roughly an order of magnitude higher than films doped with conventional small-molecule dopants such as 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F₄TCNQ) at the same doping level. The spectral shape of the IR-region absorption for our DDB-doped polymer film closely matches the calculated theoretical spectrum for the anion at infinite distance from the polaron. At the same time, grazing incidence diffraction indicates that, despite their large size, DDB clusters can intercalate within the polymer crystallites to create highly crystalline dope polymer films. We therefore conclude that these DDB clusters are able to effectively spatially separate polarons and counterion, and that this separation is a critically important aspect for producing high carrier mobilities in doped conjugated polymers.

Skin-inspired electronic materials need to combine excellent electronic properties with additional requirements, such as stretchability, biodegradability and self-healability. In order to satisfy all the requirements, controlling multi-scale morphology through material design and processing is crucial. In this talk, I will present examples of such material systems.

In this seminar, we introduce a new light emitting device combining the benefits of organic semiconductors, such as defect tolerance and chemical diversity, with the enhanced optical properties of plasmonic systems. Excitons formed in the organic layers of our novel LED architecture are intentionally coupled to the surface plasmon mode of a nearby metal electrode for decay rate enhancement and subsequently converted to photons in free space via a nanoparticle-based out-coupling scheme. Our architecture achieves a two-fold increase in operational stability at a given brightness compared to a conventional OLED while simultaneously outcoupling 8% of photons from the device. Our innovative approach augments materials design allowing for parallel development of increases in efficiency and stability and is applicable to all commercial applications of OLEDs, which include lighting panels, televisions, and mobile displays.

The gap in performance between organic and inorganic transistors is getting closer due to continuous improvements in charge carrier mobility and device designs. With record cutoff frequencys exceeding 150 MHz, organic transistors become increasingly attractive for high-frequency applications on flexible substrates, e.g., wireless communication.
However, a severe disadvantage of the state-of-the-art organic thin-film transistor (OTFT) technology is the lack of a self-aligned transistor architecture as routinely employed in silicon-based technology. In consequence, organic thin-film transistors suffer from large overlap capacitances reducing the frequency of device operation and causing large device-to-device variation due to the required alignment step. Hence, the development of a self-aligned organic transistor architecture based on a scalable and reliable process bears great potential for the improvement of printed and flexible circuits.
Here we propose a pseudo-self-aligned OTFT in a coplanar top-gate configuration in order to reduce parasitic capacitances originating from the source-gate and drain-gate overlap. Furthermore, these device are fabricated by conventional photolithography and etching processes and do not require unreliable processes such as lift-off.
The self-alignment is achieved by employing a spacing layer between the source and drain and the gate electrode. This spacer reduces the specific capacitance in this overlap region by about a factor of 10 from around 15nF/cm² to 0.13nF/cm². While maintaining the overlap length L_OV, required for alignment during lithography, this reduces the total gate capacitance. A total device capacitance reduction from 1.34fF/µm to 0.4578fF/µm (normalized by channel width) for a channel length and overlap of L=L_OV=3µm can be estimated.

Photodetectors (PDs) find their applicability in several technological fields. In medical applications low signals need to be amplified, requiring external circuits, which increases the final price. In addition to low cost, flexibility and lightweight devices provided by organic materials, photomultiplication has been achieved in organic PDs, leading to remarkable external quantum efficiencies (EQEs). Drawbacks of such systems are the high dark currents (J_D), lack of spectral selectivity and high operation voltages, due to large injection barriers in reverse direction. By controlling the acceptor amount and with the aid of appropriated blocking layers, we force electrons to accumulate in the acceptor phase of ZnPc:C₆₀ blends, close to the cathode interface. Due to the increased field caused by the accumulated photocharges in this region a band bending arises, effectively increasing the injection rate across the barrier. Under reverse bias, holes are able to tunnel through it, allowing for high hole injection. Because they can easily cross the device through the ZnPc phase, while electrons cannot easily reach the contact, an EQE of 200% is achieved in the visible range, under -5V. Simultaneously, under dark conditions, the band bending is absent and the J_D follows that of common photodiodes with values of 10^-4 mA.cm^-2. Switching injection only by illumination leads to an on/off ratio of 10⁵. Controlling the trapping character of the acceptor phase allowed us to achieve photomultiplication in vacuum processed devices, leading to EQE higher than 100%. This results open the possibility to explore high responsivity controlled by the applied bias in the visible range for future applications in organic PDs.

Polythiophenes are among the most studied organic semiconductors and they have found widespread use as the active material in organic field-effect transistors, bioelectronic sensors and as the electron donor component in organic photovoltaic devices. Here, we investigate how subtle structural modifications of archetypical polythiophenes such as poly(3-hexylthiophene) affect the optical, structural and electrical properties of the resulting polymer thin films. We furthermore discuss the implications of these structural modification for binary systems incorporating molecular dopants and outline molecular design strategies for controlling doping efficiency and electrical conductivity.

Silicon photodiodes (SiPDs) are the foundation of light-detection technology but emerging applications such as wearable biometric monitoring, biomedical imaging, distributed ionizing-radiation detection, and others, require photodetectors with mechanical properties and form factors that are beyond those physically possible with SiPDs. Novel organic and hybrid semiconductors processed from solution at low temperatures have the potential to overcome current limitations of traditional SiPDs and to enable next-generation photodetector technology. Although the performance of photodiodes (PDs) based on these novel semiconductors has been reported to be similar or better than that of low-noise SiPDs, the claimed performance is in question because the approximations used to estimate relevant metrics are often not verified (1,2).
Questions in the characterization of PDs arise because the noise equivalent power (NEP) is seldom measured directly. Instead, it is extrapolated from responsivity values commonly measured with a low measurement bandwidth and at a high optical power that is much larger than the NEP. In addition, the electronic noise is not measured directly either. In this work, we present a reassessment of the characterization of PDs including direct measurements of the responsivity as a function of optical power, electronic noise, and a detailed derivation of the parameters that are relevant for the accurate estimation of a photodetector’s performance.
Insights derived from a detailed characterization, enable optimization of organic photodiodes (OPDs) based on polymeric bulk heterojunctions achieving a level of performance that within the visible spectral range, rivals that of low-noise SiPDs in all metrics, except response time. A comparison between small-area SiPDs and OPDs yields comparable median electronic noise currents of 37 fA, yielding comparable median noise equivalent power (NEP) values at 525 nm of ca. 200 fW and median specific detectivity (D*) values of 2.1×10¹² cm Hz^1/2/W at a low measurement bandwidth of 1.5 Hz. Champion OPDs show a D* value as high as 8×10¹² cm Hz^1/2/W. Detailed measurements of the electronic noise yields a reassessment of the physical origin of the different electronic noise contributions in OPDs, potential paths for optimization, and suggest that these OPDs could reach an extrapolated white-noise limited NEP_ex,wn of 2 fW and a D*_ex,wn of 1.6×B^1/2×10¹⁴ cm Hz^1/2/W. This remarkable performance arises from the selection of photoactive layer materials and by the optimization of a device-geometry that does not make use charge-blocking layers.
The superior design possibilities of OPDs in terms of area, form factor, weight, and custom shapes are also demonstrated by fabricating on glass devices with an area of 0.9 cm² that achieve a D* of 2×10¹² cm Hz^1/2/W and flexible OPDs with an area of 1 cm² that yield a D* of 1.1×10¹² cm Hz^1/2/W, both of which are comparable to small area OPDs on glass. Advantages of OPDs are further illustrated and quantified in a biometric monitoring application that uses ring-shaped, large-area, flexible OPDs, while maintaining low-noise SiPD-level performance.

[1] C. Fuentes-Hernandez, et al., Science, 370, 698 (2020).
[2] Y. Fang, et al., Nature Photonics, 13,1 (2019).

We report the syntheis and develpoment of benzodithiophene-based copolymers with conjugated indolin-2-one side chains (PTIBDT), and its fluorine derivetive (PTIFBDT) with a donor backbone and acceptor side chains (Type II D-A polymer) as wide band gap donor polymers for efficient non-fullerene organic solar cells with a large open circuit voltageas up to (1 eV) opposed to the conventional D-A polymers having both donor and acceptor units on backbone (Type I D-A polymers) with limited open circuit voltage values. PTIBDT and the flourinated derivative PTIFBDT having a backbone consisting of benzodithiophene donor units and side chains containing indolin-2-one acceptor units were synthesized in a simple inexpensive route with a low-lying HOMO energy level of -5.59, -5.60 eV and a wide band gap of 1.91, 1.89 eV, respectively. In addition, both PTIBDT and PTIFBDT showed typical p-type semiconductor performance with ITIC were used as donor and acceptor to form a blend active layer, the best OSC device showed a short circuit current density ( J_sc) of 15.60 and 13.70 mAcm^-2, a an open voltage (V_oc ) of 0.97 and 1.00 V, and a fill factor of 0.60 and 0.59, resulting in a power conversion efficiency (PCE) of up to 8.00 and 7.70 %, respectively.

We introduce a convenient method for analyzing bulk heterojunction (BHJ) organic photovoltaics (OPVs) by calculating its “bulk quantum efficiency” (BQE), a quantity related to the recombination losses only within the BHJ, from device J-V characteristics. By testing the method against experimental results for both vacuum- and solution-processed OPVs with various BHJ, buffer layers and interface layers compositions, we show that direct measurement of BQE isolates the properties of the BHJ from device edges (i.e., buffer layers, interfaces and electrodes). The BQE analysis is then applied to study different degradation mechanisms in OPVs with various active layer materials and device structures. In solution-processed OPVs with non-fullerene acceptors (NFAs), we show that the primary mechanism for degradation is externally induced by the ZnO buffer layer. By inserting a self-assembled monolayer at the BHJ/ZnO interface, the degradation is significantly suppressed and the solution-processed NFA BHJs are stabilized. We also introduce an equation to quantitatively describe the degradation of BQE.

Solution processed organic bulk heterojunction (BHJ) solar cells offer a promising route towards lightweight, cost-effective, and flexible photovoltaics. Although the BHJ’s nanoscale interpenetrating donor-acceptor network results in efficient charge photogeneration, it also creates complex three-dimensional pathways for collecting charge. Despite the three-dimensional nature of the charge transport, the importance of lateral pathways is generally ignored. In this work, we examine the extent of lateral current spreading during out-of-plane charge transport across BHJ active layers for insight into the impact of lateral pathways on charge collection. We first introduce a procedure, using conductive atomic force microscopy, to quantify lateral current spreading during out-of-plane charge transport. This method was used to study the dependence of lateral spreading on molecular orientation (face-on versus edge-on), molecule type (small molecule versus polymer), and film type (single component versus BHJ). Generally, charge transport is favored along the pi-pi stacking direction; hence, a face-on (edge-on) orientation normally favors vertical (lateral) charge transport. We observed in P3HT:PCBM active layers that lateral hole spreading was proportional to the edge-on to face-on population ratio of the P3HT, as determined by grazing-incidence X-ray diffraction. Interestingly, BHJ films with the greatest content of face-on P3HT did not have the highest out-of-plane current. Instead, an intermediate edge-on to face-on ratio exhibited a 20% higher out-of-plane current. We expect that the edge-on P3HT stacking plays an essential role in allowing holes to circumvent acceptor domains in which hole transport is excluded, enabling three-dimensional charge percolation. Predominantly edge-on P3HT, however, led to excessive lateral spreading and inefficient out-of-plane charge transport. We also observed that the polymer donor P3HT led to more lateral current spreading than the small molecule donor p-DTS(FBTTh₂)₂, owing to charge transport along the polymer backbone of P3HT. Less lateral current spreading was observed in BHJs compared to pure films, due to a disruption of lateral hole transport by the acceptor domains. These experiments provide insight into the effects of film morphology on charge percolation in BHJs, which will help promote efficient charge collection in organic solar cells.

Scaling-up and commercialization of organic solar cells (OSC) using high throughput deposition techniques requires thicknesses >500 nm for reproducibility. Yet most OSC materials rapidly lose efficiency as thickness increases due to the recombination prior to extraction of charges. A novel electron-donating polymer NT812 exhibits efficient, micron thick solar cells when paired with a fullerene acceptor due to highly suppressed bimolecular recombination relative to diffusion-limited Langevin rate. In this system, bimolecular recombination depends on the blend ratio and solvent additives, pointing towards the role of nano-morphology. We investigate the morphological origins of the bimolecular recombination in blends of NT812. By combining results from a suite of X-ray techniques, we are able to quantify not only the crystallinity, domain size and purity but also the sharpness of the donor-acceptor interfaces, which are found to sharpen when introducing a plasticizing solvent additive. We find that all the investigated devices possess pure domains of polymer fibrils and fullerene aggregates regardless of the recombination reduction factor. This indicates that pure phases are insufficient for non-Langevin recombination. However, large well-ordered conduits free of molecular mixing and in particular, sharp interfaces help to maintain opposite charges away from one another and keep percolation pathways clear for enhanced charge transfer (CT) state dissociation rate. This structure-morphology relationship will be key to the successful commercialization of printed OSCs at scale.

Because of their ability to combine detection and processing into a single element, phototransistors have the potential to be a disruptive optical detection technology. The complexity of commercial applications (as defined by number of vertical processing steps) can be reduced significantly when using phototransistors compared to photodiodes, for example. In addition, phototransistors have a gate-voltage-tunable responsivity (something for which there exists no analog in 2-terminal devices), with high gain from small changes in incident optical power density.

Organic phototransistors (OPTs) in particular offer the opportunity to develop devices with well-defined, tailored, optical response spectra. One of the most significant drawbacks to OPTs however is the complexity of the relationship between photocurrent and incident optical power density. In addition to the choice of two applied voltages (drain and gate), optically induced shifts in threshold voltage often lead to behavior that is difficult to predict and quantify.

In this work we have varied the composition of polymer:fullerene bulk heterojunction (BHJ) OPTs, in order to identify the respective role played by direct charge generation and optically induced shifts in threshold voltage. The current-voltage behavior of blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) were studied under illumination at various intensities. It is demonstrated that even one percent (by weight) of P3HT in PCBM produces an appreciable response while remaining highly optically transparent to the excitation wavelength, making these organic phototransistors highly attractive for optically transparent detectors.

P-type organic semiconductors typically demonstrate higher mobilities and better chemical and environmental stability compared to n-type materials which are often air sensitive and insoluble. The development of soluble, stable, high performing n-type materials is critical for the fabrication of organic electronic devices for many of the novel applications organic semiconductors allow for. Due to their high degree of intermolecular stacking, crystallinity, chemical and mechanical versatility, and compatibility with solution processed fabrication technique, axially-substituted metal phthalocyanines (MPcs) are promising materials for applications in organic thin-film transistors (OTFTs). MPcs contain multiple functional sites, including the core, periphery and axial positions, allowing to chemically tune molecules to improve device performance. MPcs with tetravalent metal centers, such as silicon and tin phthalocyanines (SiPcs and SnPcs), are among the highest performing materials in this family, however remain relatively unexplored compared to commonly used divalent metal MPcs. Seven varying lengths of branched chain alkyl silane groups were incorporated into SiPc or SnPc at the axial positions to create fourteen solution processable n-type organic semiconductors, ten of which are novel materials never before used in OTFTs. Atomic force microscopy (AFM) images of the films show that variations in alkyl chain length and metal center result in drastic changes in film morphology and corresponding OTFT performance. OTFTs fabricated with the SnPc derivatives exhibited average mobilities in the range of 0.02 – 0.70 x 10^-2 cm² V^-1 s^-1, with four of the seven compounds outperforming their SiPc counterparts whose mobilities ranged from 0.01 – 2.8 x 10^-2 cm² V^-1 s^-1. The relationship between two key fabrication parameters, thermal annealing and spin time, on thin-film microstructure and OTFT performance was investigated through the electrical characterization of the highest performing material, bis(tri-n-butylsilyl oxide) SiPc, and related to grazing-incidence wide-angle x-ray scattering (GIWAXS) and x-ray diffraction (XRD) data. Thermal annealing increased film crystallinity and altered the orientation of the SiPc molecule relative to the substrate, whereas spin time solely effected film crystallinity, thus exhibiting how these fabrication parameters can be used to change different aspects of the thin-film microstructure and modify device performance. The results of this work show the effectiveness of axial substitution as a strategy to control crystal packing and charge transport properties of SiPcs and SnPcs, and the potential of these materials as solution processable low cost n-type semiconductors.

Conjugated polymers with oligoethylene glycol side chains are excellent mixed ionic-electronic conductors, and through volumetric capacitive coupling can efficiently interconvert ionic and electronic currents. These phenomena have been exploited for electrochromic, charge storage, biological sensor, neuromorphic, and organic electrochemical transistor (OECT) applications, amongst others. The polythiophene pg2T-TT, poly(2-(3,3′-bis(2-(2-(2-methoxy ethoxy)ethoxy)ethoxy)-[2,2’-bithiophen]-5-yl)thieno[3,2-b]thiophene), with its high capacitance (C*), high hole mobility (μ), and high transconductance in OECTs (ionic-to-electronic transduction gain), has become a benchmark material for this emerging class of mixed conductors. Surprisingly, simply switching the side chain position on the bithiophene repeat unit from 3,3' to the 4,4' positions (producing pgBTTT) results in a marked improvement in mixed conducting properties. The product of pgBTTT’s electronic mobility and volumetric capacitance (μ x C*), which is the source of OECT transconductance, is double that of pg2T-TT, and represents a record high intrinsic figure of merit for enhancement mode OECT channel materials (μ x C* = 563 ± 33 F cm^-1 V^-1 s^-1).

The source of this improvement is not immediately obvious. Traditional characterization methods typically carried out on dry polymer films do not reflect the operating conditions of these polymers in OECTs or other devices that exploit their mixed conducting properties, where they are swollen by a contacting electrolyte. Therefore, in situ and operando techniques are required. While X-ray scattering has become ubiquitous for structure determination, in situ or operando scattering of films emersed in aqueous electrolyte is severely limited by electrolyte scattering and absorption. This is especially critical for conjugated polymers which are weak scatterers and most often employed in (sub-micron) thin films. To overcome these challenges, we employ a new frit-based electrochemical cell that allows electrolyte contact of the thin film from below and incident X-rays to impinge from above, separating the electrolyte and x-ray path to provide unparalleled resolution of polymer microstructure in aqueous operando conditions. Leveraging this cell we compare the grazing incidence wide angle scattering (GIWAXS) patterns of dry, hydrated, electrolyte exposed, and electrochemical potential cycling pg2T-TT and pgBTTT, monitoring both molecular d-spacing and molecular packing coherence lengths. These operando measurements deviate markedly from equivalent ex situ measurements, highlighting the limited utility of ex situ experiments in elucidating the structure of electrolyte swollen polymer mixed conductors.

While pg2T-TT and pgBTTT show similar lamellar d-spacing expansions (~0.7 Å) with hydration, upon electrolyte exposure further expansion of pg2T-TT lamellae (9.4 Å) is over four times that of pgBTTT (2.1 Å). This represents a fundamentally different degree of polymer electrolyte interaction that depends solely on the side chain regio-chemistry and gives rise to the difference in mixed conducting properties. Electrochemically doping pg2T-TT and pgBTTT produces similar pi-stack d-spacing contractions and relative lamellar expansions. UV-Vis and Raman spectroelectrochemistry was carried out to verify the similar degree of charging and nature of dopant induced charge carriers in pg2T-TT and pgBTTT.

These results highlight the sensitivity of mixed conducting properties to side chain architecture and the crucial necessity of operando measurements to establish synthetic design rules for polymer mixed conductors. Further, the frit-based operando cell provides a new powerful tool to probe the previously inaccessible structure of solvent and electrolyte swollen polymer thin films with controlled electrochemical potential. This characterization capability is key for advancing the field of polymeric mixed conductors and likely other fields as well.

Given their low-cost processing, light weight, chemical versatility, and compatibly with flexible and biological systems, organic semiconductors (OSCs) are gradually becoming an integral part of our lives as active components of various optoelectronic devices. These systems undergo considerable electronic and structural transformations during device fabrication and operation which can profoundly impact their performance and stability. Characterization techniques that can elucidate the mechanisms of the time-dependent transformations occurring in these materials and devices are necessary to guide the processing and design of high performance and stable organic devices. For example, in organic field-effect transistors (OFETs), the basic building block of applications such as displays, radio frequency identification tags, and conformable sensors, identifying the sources and mechanisms leading to operational instabilities are crucial in eliminating their impact to ultimately achieve the much-needed stability to make these devices components in real-life applications. In this presentation, we describe an efficient characterization technique that serves as a quick and reliable diagnostic tool to identify with high accuracy the environmental and operational degradation pathways occurring in OFETs during device operation. Charge carrier trapping in electronic states generated in the bandgap of the OSC during operation has been identified as the main culprit for device instabilities. Therefore, by monitoring the energetic distribution and time evolution of trap states during OFET operation, and with guidance from density functional theory (DFT) calculations, we gained access to the sources of device degradation, clarified the nature of the traps generated during operation, and the parameter space that gives rise to these traps. This information guided the development of efficient design rules for device processing and delivered robust OFETs with unparalleled operational stability in both staggered and coplanar configurations regardless of the active material (small molecule and polymeric OSCs), as confirmed by the constant mobility and exceptionally low threshold voltage shift of ΔV_th = 0.1 eV achieved under aggressive bias stress conditions for 500 min in ambient air.

While it is highly desirable to simultaneously achieve the features of good electrical/optical properties, low surface roughness, robust mechanical flexibility under continuous operation bias for practical flexible electrodes, we demonstrate an outperformed a new class of Ag nano-network composite based transparent flexible electrode by a new one-step multifunctional chemical treatment. The robust electrical stability loaded with simultaneously continuous operation bias and mechanical bending is confirmed on the treated composite electrode. The organic solar cells based on our newly developed flexible composite electrode offer impressive performances as compared to the untreated device [1].
In details, we demonstrate a flexible transparent electrode showing the figure-of-merit value over 1000 (5.4 ohm/sq sheet resistance and >94% diffused transmission at 550 nm wavelength) via the new synthesis approach, simultaneously achieving selective welding at nanowires cross junctions to form Ag nano-network, removing PVP surfactants from Ag nanowires and PSS from PEDOT:PSS. In addition, with our proposed chemical treatment, the composite electrode can be completely detached from the supporting rigid substrate with no remained residues on the substrate, exhibiting extremely smooth topography (<1 nm root-mean-square roughness and <3 nm peak-to-valley roughness). Remarkably, we demonstrate robust electrical stability under simultaneously continuous operation bias and mechanical bending for the treated electrode for the first time. The OSC device with our proposed chemical treatment has achieved the best PCE of 15.41%, which is the best performance among ITO free flexible OSCs so far.
[1] J. Kim, D. Ouyang, H. Lu, F. Ye, Y. Guo, N. Zhao, W.C.H. Choy, Adv. Energy Mater., DOI:10.1002/aenm.201903919.

In general, the active layer of a high-performance polymer solar cell (PSC) has been fabricated by mixing a polymer donor and a low molecular acceptor. On the other hand, for future flexible photovoltaics, many studies are being conducted on all-PSCs that contain blend film of donor polymer and acceptor polymer to produce an active layer. However, despite the recent high power conversion efficiency(PCE) achievements of all-PSCs, the miscibility control and morphology reproducibility between the donor and acceptor polymers of the photoactive layer using two polymers remain a concern. This problem directly affects the stability of the device efficiency. Therefore, in order to overcome these shortcomings, studies have been conducted on polymers that can have heterojunctions in the internal structure of a main chain conjugated copolymer synthesized by covalent bonding between a donor and an acceptor unit.

And indoor photovoltaic (IPV) cells receive increasing attention because of their promising applications as power sources for devices of the Internet of Things ecosystem. The essential requirement for active materials of IPVs is that their absorption spectrum should overlap with the emission spectrum of light-emitting diodes (LEDs) that are often used as indoor lamps. Since IPVs require high harvesting efficiencies mainly in the visible range, the corresponding active material must be designed and applied differently from the molecular structure of the active material used in outdoor PVs. For this reason, photoactive materials for IPVs must display a narrow absorption spectrum in the visible wavelength range and a medium bandgap.

In this study, we successfully synthesized a crystalline main chain conjugated copolymer, P(BDBT-co-NDI2T), having BDBT donor-NDI2T acceptor heterojunctions in film states. Interestingly, the polymer exhibited a strong absorption band from 500 to 650 nm, which is overlapped well with the emission spectrum of white LED used as an indoor light source. IPV produced by using the P(BDBT-co-NDI2T) synthesized in this study as an active layer, showed a high power conversion efficiency of 12.70% (@500 lux) with a short circuit current density of 49.89 mA cm^-2. The PCE of the P(BDBT-co-NDI2T) film-based IPV was twice as high as that of the blend film-based IPV and showed excellent operational stability. This result was thought to be due to a smoother surface/internal morphology of the P(BDBT-co-NDI2T) films. Additionally, kelvin probe force microscope(KPFM) data also indicated a relatively uniform distribution of the surface charge in the P(BDBT-co-NDI2T). These results may explain why P(BDBT-co-NDI2T)-based devices exhibit higher efficiency and very high operational stability compare to the blend film-based device. This study is the first time that the P(BDBT-co-NDI2T) has been applied to IPV, which suggests the potential for their widespread use in the development of indoor organic PVs in the future.

In addition to sunlight, we can see buildings (hospitals, shops, airports, offices, factories) illuminated for 24 hours, and artificial lighting is beginning to be recognized as a source of energy that needs to be harvested. Indoor light sources such as white light-emitting diodes (LEDs) can provide enough energy to operate most small wireless and low power consumption indoor electronics. Therefore, if organic photovoltaics (OPV), which are very sensitive to indoor light, are developed, they can be an ideal power source for indoor Internet of Things (IoT) wireless sensor networks due to their lightweight and mechanical flexibility. Thus, OPV has great potential to harvest energy that cannot be realized with powerful silicon-based inorganic photovoltaic cells. However, high-performance OPVs are typically fabricated using highly toxic solvents such as chlorinated aromatic solvents. These solvents can cause serious concerns about the side effects of human health in terms of germ cell mutagenicity, skin corrosion, and serious eye damage. From this point of view, the OPV material must be processed with a solvent that is harmless to the human body, and it must be designed to absorb indoor lighting to achieve high power conversion efficiency.
In this study, we introduced the fluorine-substituted benzotriazole (BTA) as a third monomeric unit into the benzo[1,2-b:4,5-b′]dithiophene (BDT-Cl)-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) alternating polymer, PM7 to increase electron-accepting property. PM7 is a famous donor polymer that shows very high performance in OPV cells. However, PM7 is only soluble in halogenated solvents such as CF, CB, and 1,2-dichlorobenzene but not soluble in a non-halogenated solvent such as toluene due to the highly rigid π-conjugated polymer with a strong aggregation effect. By introducing the BTA moiety into the PM7 as a relatively ductile unit, the terpolymer PM7-J52 (50) is successfully synthesized and it is well soluble in non-halogenated solvents, enabling environmentally friendly processing. When terpolymer was used as a donor polymer for indoor photovoltaics and processed using a non-halogenated solvent, PCE was measured as high as 11.34% in outdoor PV(AM1.5G) and 17.41% (500 lux) in indoor PV, respectively. Besides, the absorption spectrum of the blend film can be adjusted by changing the third monomer composition, which can improve the performance of PV devices using outdoor and indoor light sources. Our research has proposed a new ternary copolymer structure that can be used for fabricating outdoor or indoor PVs. In addition, the use of terpolymer has the advantage of being able to use non-halogenated solvents when fabricating PV devices.

We implemented the geometrically stretchable organic light-emitting diodes (OLEDs) by breaking the stereotype that it is impossible to implement stretchable devices with a fragile oxide-type electrode under intense 2-dimensional random area strains with molybdenum trioxide (MoO₃)/gold (Au)/MoO₃ (MAM). The devices were fabricated on the pre-strained NOA63/3M elastomer and encapsulated with NOA63 for the waterproof sandwiched structure. The transmittance of the electrode was high enough to replace the semi-transparent silver (Ag) electrode, while the resistance was still lower than that. The work function of the electrode could also be tuned through the MoO₃ dipole layer for excellent charge injection. Also, it was shown that the color coordinate did not change according to the angle. By comparing the glass- and the thick NOA63 film-based devices, it was found that the triplet-triplet annihilation was reduced through the heat sink mechanism that facilitated achieving a high exciton density. Also, significantly enhanced out-coupling was achieved by inserting silicon dioxide nanoparticles into NOA63. Consequently, the high current efficiency of ~82.5 cd/A and external quantum efficiency of ~22.6% were achieved by the horizontal emitter with a minimum efficiency roll-off.

Organic polymers those exhibit features pertinent to functioning as host materials for thermally activated delayed fluorescence (TADF) emitters have considerable potential in solution-processable organic light-emitting diodes (OLEDs), allowing simple, low-cost, and large-area applications. In particular, polymer hosts have superior characteristics, including facile functionality to introduce various electron donor and acceptor entities, ability to uniformly disperse and contain small molecular dopants, and ability to produce more smooth and homogeneous films, compared to those of their small-molecule counterparts. This research describes the design and development of three new styrene-based copolymers (ABP91, ABP73, and ABP55) bearing diphenylacridine (DPAc) as the electron donor and 2,12-di-tert-butyl-7-phenyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (TDBA) as the electron acceptor.
In particular, the three polymers were synthesized via variations in the ratio of donor (DPAc) and acceptor (TDBA) monomers. A homopolymer (DHP) containing only DPAc units was also synthesized for comparison. Non-conjugated styrene backbone was employed as a linker between the donor and acceptors to restrict the conjugation length and thus accomplish a high E_T. Subsequently, the resulting copolymers featured E_Ts values in the range of 2.73–2.75 eV.
We fabricated OLEDs via solution processing, employing a familiar t4CzIPN as TADF emitter using the following configuration: ITO/PEDOT:PSS (40 nm)/PVK (10 nm)/Host:t4CzIPN (20 nm, x wt %)/TPBi (40 nm)/LiF (0.8 nm)/Al (100 nm). The devices based on ABP91:t4CzIPN, ABP73:t4CzIPN, and ABP55:t4CzIPN achieve EQE/CE/PEs as high as 21.8%/71.4 cd A⁻¹/ 44.9 lm W⁻¹ (with CIE color coordinates of (0.29, 0.60)), 22.2%/70.1 cd A⁻¹/48.9 lm W⁻¹ (with CIE color coordinates of (0.27, 0.57)), and 19.7%/61.7 cd A⁻¹/43.1 lm W⁻¹ (with CIE color coordinates of (0.27, 0.56)), respectively. In contrast, the DHP-based devices were limited to only 14.5%/47.3 cd A⁻¹/27.4 lm W⁻¹ (with CIE color coordinates of (0.30, 0.59)) because of charge imbalance, attributed to its unipolar nature. Among the three hosts, ABP73 delivered the best performance, which is attributed to the high k_RISC, high PLQY, well-balanced charge-carrier transport ability, and homogeneous film formation with a small value of RMS roughness.
These results suggest that the development of copolymers, which consist of donor and acceptor entities in the side chain or pendent groups, is more suitable for highly efficient solution processing of TADF OLEDs. To our knowledge, this is, to date, the best performance that has ever been reported for polymer host materials in the area of TADF OLEDs. We strongly believe that this work will make a significant contribution through the introduction of a variety of copolymers that function as hosts for the development of proficient solution-processable TADF OLEDs.

Development of a simple and effective patterning method applicable to solution-processible organic luminophores over a large area is critical for cost-effective production of organic light-emitting-diode (OLED) displays. Here, we demonstrate high-resolution patterning of light-emitting polymer active layers using a highly efficient photocrosslinker (4Bx). The photocrosslinker is structured in a tetrabranched geometry, wherein a photocrosslinkable azide moiety is present at each of the four corners of the molecule, and each of these moieties can form a chemical bond with light-emitting polymer semiconductors under UV irradiation. Due to the high crosslinking efficiency of 4Bx, the use of an unprecedentedly small amount of 4Bx (0.1 wt%) allows fully crosslinking light-emitting Super Yellow polymer without degrading their photoluminescence and electroluminescence characteristics. Furthermore, precisely-defined photocrosslinked patterns of Super Yellow with feature sizes of 5 µm are formed by using p-xylene as the developing solvent that was carefully selected according to Hansen solubility parameter analysis.

There are two main ways to measure the performance of organic light-emitting diodes (OLEDs). One way is using a spectroradiometer (CS-2000) through the viewing angle and the other is the most widely used technique that involves an integrating sphere. In this study, we present a comparison of measuring the performance of OLEDs using the spectroradiometer and the integrating sphere. To improve the measurement accuracy of spectroradiometers, the viewing angles were set between -70° and +70° to avoid the out of focusing area. As the external quantum efficiency (EQE) estimated using the spectroradiometer was not coincident with that obtained using the integrating sphere method, we applied a mean estimation protocol to convert the EQE values obtained from the spectroradiometer by integrating the EQE values extracted from different viewing angles. The results were summarized for four different kinds of angular emission patterns. The data obtained from the conversion of EQE and electroluminescence (EL) of OLEDs measured with a spectroradiometer from all different angular emission patterns were similar to that measured with the integrating sphere with a small deviation. This study and the mean estimation protocol should help in understanding the accuracy of OLED measurements at a laboratory scale. As such, it is possible to reduce the recurring costs and the required time for these two measurement techniques by bypassing the integral sphere measurement

As air pollution has become an urgent issue worldwide, effective chemodetection and in situ monitoring of hazardous gases have been desired for public health. In this regard, high-performance of chemosensor have been demonstrated continuously based on conventional semiconducting materials, such as oxide semiconductors, 2-dimentional materials, and organic semiconductors (OSCs). Although oxide semiconductors based chemosensor possess high sensitivity and selectivity due to their high mobility and versatile surface functionalization, high temperature operation is a prerequisite for desired high sensitive chemodetection. On the other hand, OSCs can obtain high sensitive chemodetection at room temperature because of effective contact with target chemicals. However, OSCs cannot serve functionalization sites for selectivity, and exhibits poor recovery property by penetration of chemicals into an OSCs film. Therefore, in order to secure high sensitivity, selectivity, and ideal recovery property simultaneously, novel material design and scientific breakthrough in the field of chemosensors are strongly required.
Herein, we demonstrate an ultrasensitive, selective, and fast recovered NO₂ chemodetection based on interpenetrating polymer semiconductor nano-networks functionalized with amine based artificial chemoreceptors (NH₂-IPSNs), in which polymer semiconductor chains are interpenetrated into a ladder-like organosilica network. The IPSNs serves unreacted silanol (Si-OH) groups acting as functional points on its surface; by carefully manipulating the Si-OH groups, the NH₂-IPSNs film can be designed by simple solution process without a degradation of electrical performance. This surface engineering has been considered to be available in oxide semiconductor materials, however, through a novel approach of the IPSNs, it also could be allowable in OSCs . Consequently, the NH₂-IPSNs based chemosensor exhibited ultrahigh sensitivity (990% ppm^-1 at NO₂ 5 ppm) and superior selectivity in room temperature operation, even the sensing performance persisted in ambient environment. This is because the surface anchored NH₂ groups can build strong dipole moment interaction with NO₂ molecules. Furthermore, the surface NH₂ groups can suppress penetration of NO₂ molecules into the NH₂-IPSNs film effectively, so that fully recovery capability with fast recovery rate (240 s) was achieved. As a result, the NH₂-IPSNs based chemosensor can develop ultrasensitivity, selectivity, and ideal recovery property at room temperature simultaneously, which is attributed to our conceptual novelty to reconcile the characteristics of oxide and organic semiconductors. We believe that our rational design of new concept of semiconductor material represents a big step towards next-generation chemical monitoring system for various industry and public health.

Exploiting the long-range polarizability of an electrolyte based on ion migration, electric double-layer transistors (EDLTs) can be constructed in an unconventional configuration;¹ here, the gate electrode is placed coplanarly with the device channel. For example, the current density of a transistor channel can be modulated using a gate electrode that is not located directly on top/below the channel but placed apart from the channel as long as the electrode is bridged with the channel through the electrolyte– which we refer to as the remote gating. In this paper, we demonstrate the influence of the distance factors of the electrolyte layer on the operation of EDLTs with a coplanar gate. As the promptness of the electric double-layer formation depends on the distance between the channel and the gate, the dynamic characteristics of a remote-gated transistor degrade with long distance. To suppress this degradation, we suggest using multiple coplanar floating gates bridged through ionic dielectric layers. Unlike remotely gated EDLTs that utilizes a single extended electrolyte layer, the devices with multiple segmented electrolyte layers operate effectively even when they are gated from a distance longer than 1 mm. The new device architecture provided here shows new opportunities to exploit graphene transistors.

Reference
1. Kim, B. J.; Lee, S. -K.; Kang, M. S.; Ahn, J. -H.; Cho, J. H. ACS Nano 2012, 6, (10), 8646-8651.

Quinoid platforms have attracted much attention because of their unique optical, electrical and magnetic properties, and can be applied to various organic electronics, such as organic field-effect transistors (OFETs), organic photovoltaics and organic spintronics. They exhibit low band-gap and redox amphoterism arising from stable quinoid-aromatic resonance structure and high structural planarity due to double bond formation between each ring. This quinoid structure often result in high and n-type charge carrier mobilities in OFETs. We also demonstrated high performance OFETs using quinoidal polymers recently. Interestingly, the quinoidal structure stabilizes the open-shell form with diradical upon recovery of aromaticity. The open-shell diradicaloids via introduction of quinoid building blocks into polymer require the experimental and theoretical analysis to comprehensively describe their unique optical and magnetic properties.
In this research, we will present our recent results on the investigation of structure-property relationship of an open-shell conjugated polymer embedded with stable diradicals. We designed and synthesized an azaisatin-terminated quinoid building block, azaquinoidal-bithiophene (azaQuBT), which has pyridine replacing benzene in the end of terminal isatin unit. This can lead to better coplanar conformation of polymer backbone due to suppressing the rotation between quinoid-comonomer counterpart. Therefore, diradical occurred by open-shell form of azaQuBT could be well-delocalized into the polymer backbone, leading to stable ground-state diradical characters. An open-shell conjugated polymers, PazaQuBT-T, was successfully synthesized by Stille-polymerization. We characterized material properties by UV-Vis-NIR absorption, Raman spectroscopy and electron spin resonance (ESR) and discussed.

Lightweight and flexible tactile learning machines can simultaneously detect, synaptically memorize, and subsequently learn from external stimuli acquired from the skin. This type of technology holds great interest due to its potential applications in emerging wearable and human-interactive neuromorphic electronics. In this study, an integrated intelligent tactile learning electronic skin (e-skin) based on arrays of ferroelectric field-effect transistor memories with dome-shape tactile top-gates, which can simultaneously sense and learn from a variety of tactile information, is introduced. To test this device, tactile pressure is applied to a dome-shaped top-gate that measures ferroelectric remnant polarization in a gate insulator. This results in analog conductance modulation that is dependent upon both the number and magnitude of input pressure-spikes, thus mimicking diverse tactile and essential synaptic functions. Specifically, our device exhibits excellent cycling stability between long-term potentiation and depression over the course of 10,000 continuous input pulses. Additionally, it has a low variability of only 3.18 %, resulting in high-performance and robust tactile perception learning. Our 4 × 4 device array is also able to recognize different handwritten patterns using 2-dimensional spatial learning and recognition, and this is successfully demonstrated with a high degree accuracy of 99.66 %, even after considering 10 % noise.

Conjugated polymers are attractive for rechargeable batteries owing to their reversible electrochemical redox reactions and/or intercalation abilities. However, conventional conjugated polymers are generally consisting of only organic backbones which tend to form highly reactive redox species during an electrochemical process or under light excitation, leading to undesirable side reactions. These side reactions such as irreversible oxidation occurred on the polymer backbones and its physical properties will be permanently altered, leading to degradation. This problem can be alleviated by incorporating redox-active transition metals into the conjugation system, helping to stabilize the generated redox species via coordination, or having itself undergoing a stable redox process.
Over the years, there has been an increasing interest focusing on nanostructured conjugated polymers as green and renewable electrodes for energy conversion and storage devices. Realizing an optically modulated energy storage device could unlock new applications including but not limited to variable optical attenuator, optical switches, displays, and smart windows for energy-efficient buildings. Herein, a rationally designed nanostructured coordination polymer has been prepared with the aim of realizing electrical modulated light transmission with stable electrochemical cathodic reactions. By utilizing the coordination conjugated polymer as the cathode and a zinc anode, an integrated hybrid electrochromic zinc battery device can be demonstrated with promising endurance.
From the electrochemical and optical analysis, the ΔE_(HOMO-LUMO) of the polymer was found to be ca. 1.5 eV, suggesting that the polymer is behaving like an organic semiconductor with decent conductivity even with interrupted conjugation system due to metal inclusion. Based on the classical Johnson–Mehl–Avrami–Kolmogorov (JMAK) interpretation, the coordination polymer has undergone a one-dimensional growth process which can be attributed to the judicious design of the monomeric transition metal complexes. During the designing of materials, the interplay between metal and ligands has been carefully considered from both thermodynamic and kinetic aspects in order to attain coordination systems with stable and robust redox process that is critical for both electrochromism and energy storage. Unlike the intercalation-deintercalation mechanism exhibited by metal oxides, the movement of ions flux in and out of the polymer could occur rapidly during the electrochemical reaction, offering a fast charge transfer process that is not possible in inorganic materials. The polymer-bound electrode has been configured into a hybrid device which is able to operate in an aqueous Zn electrolyte, achieving high color contrast of ca. 60 %, fast response time of <1s, and good stability for over 1000 consecutive charging-discharging cycles.

Based on novel organic semiconductors, organic solar cells are an interesting alternative to conventional silicon based solar cells as they feature new possibilities. Using wet chemical processing, they can be manufactured with large-scale production methods such as roll-to-roll printing. Consequently, the production of organic solar cells has the potential to become very cheap and easy. With in-situ grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) studies, we gain information on the kinetics of morphology formation of electrodes, blocking layers and active layers of the solar cells during processing.
In terms of large-scale usability, one of the major challenges for organic solar cells is to overcome their relatively short lifetime, as compared to their inorganic counterparts. To gain a deeper understanding of organic solar cell degradation with respect to changes in the active layer nano-morphology, we present in-situ studies on model donor-acceptor organic semiconductor based solar cells during the first hours of operation. The in-operando studies reveal information on both, its evolving current-voltage characteristics and the active layer nano-morphology. For that purpose, GISAXS / GIWAXS measurements and current-voltage (IV) tracking of the operating solar cell are performed simultaneously to gain fundamental understanding. Starting from an optimized morphology of the active layers in terms of highest efficiencies for organic solar cells, depending on the donor-acceptor organic semiconductor system, a mixing or demixing process are identified to cause changes of the morphology. The altered morphology is less optimal for charge transport through the active layer due to poor percolation in a too fine morphology or poor splitting of excitons in a too coarse morphology.

Organic-inorganic hybrid semiconductors have been identified as an emerging class of novel materials for direct conversion X-ray detection^[1,2]. Their success is attributed to a number of characteristics such as sensitivity over a broad energy range including high sensitivity for hard X-rays, low voltage operation, and conformability to complex shapes. However, previous generations of such detectors displayed dark currents that are ×1000 - 10,000 higher than the industry standard of 10 pA mm^-2, therefore hindering their potential for commercialization. Herein, we introduce a new route to achieve ultra-low dark currents that are below 10 pA mm^-2 even at electric fields as high as ∼4 V μm^-1 in hybrid detectors fabricated by incorporating high atomic number bismuth oxide nanoparticles into an organic bulk heterojunction consisting of p-type Poly(3-hexylthiophene-2,5-diyl) (P3HT) and n-type [6,6]-Phenyl C71 butyric acid methyl ester (PC₇₀BM). Such ultra-low dark currents are achieved through the enrichment of the hole selective p-type polymer near the anode contact, induced via vertical phase segregation within the hybrid film. The resulting hybrid detectors display broadband response with a record high sensitivity of ∼1.5 mC Gy^-1 cm^-2 and less than 6% variation in angular dependence response when tested under 6 MV X-rays from a clinical linear accelerator. Therefore, we anticipate that the above characteristics in combination with excellent dose and dose rate linearity, reproducibility, and long-term stability would stimulate further interest towards implementing such detectors for applications in dosimetry for medical and industrial applications.

References
[1] M. P. A. Nanayakkara, L. Matjačić, S. Wood, F. Richheimer, F. A. Castro, S. Jenatsch, S. Züfle, R. Kilbride, A. J. Parnell, M. G. Masteghin, H. M. Thirimanne, A. Nisbet, K. D. G. I. Jayawardena, S. R. P. Silva, Adv. Funct. Mater. 2020, 2008482.
[2] H. M. Thirimanne, K. D. G. I. Jayawardena, A. J. Parnell, R. M. I. Bandara, A. Karalasingam, S. Pani, J. E. Huerdler, D. G. Lidzey, S. F. Tedde, A. Nisbet, C. A. Mills, & S. R. P. Silva, Nat. Commun. 2018, 9, 2926.

The 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene (C₈O-BTBT-OC₈) molecule, one of the most promising organic semiconductors, presents two different crystalline phases: a co-facial structure (bulk phase) and a herringbone structure also defined as surface-induced phase (SIP phase).¹ From solution deposition on Si/SiO₂ always the SIP phase has been detected so far and previous works have reported that with aging at ambient conditions or by solvent vapor annealing it transforms to the thermodynamically stable bulk structure.² The Bulk C₈O-BTBT-OC₈ phase crystallizes in a triclinic P-1 system with two molecules per unit cell presenting a displaced cofacial stacking, while the SIP phase has a monoclinic P2₁/c structure with a herringbone packing.^1,3
In this work we report the stabilization of the SIP phase by blending the C₈O-BTBT-OC₈ with an insulating polymer and preparing the thin film transistors (TFTs) by the bar-assisted meniscus shearing (BAMS) technique. All the TFTs based on C₈O-BTBT-OC₈ with polystyrene (PS) of low molecular weight (i.e., 3K) and high molecular weight (i.e., 100K) present the SIP structure and display good electrical properties with a field-effect mobility close to 1 cm²/V s, a threshold voltage around 0 V and an on/off current ratio of 10⁷-10⁸. However, after 3 months in ambient conditions the C₈O-BTBT-OC₈:PS100K film retain the same morphology, while the film based on the lower molecular weight PS presents the formation of different crystal domains on top of the pristine layer. The structural inhomogeneity of the C₈O-BTBT-OC₈:PS3K was investigated by micrometric spatially resolved Lattice phonon Raman spectroscopy.⁴
In order to compare the hole transport tendencies in the Bulk and SIP polymorphs of C₈O-BTBT-OC₈, we analyzed all independent pairwise HOMO-HOMO (Highest occupied molecular orbital) intermolecular electronic couplings (J_HOMO) in the two crystal structures using density functional theory (DFT) based calculations. Bulk phase show a highly anisotropic one dimensional (1D) electronic structure with only a single dominant J_HOMO value of 42 meV in a direction parallel to the a-axis of the crystal with all other J_HOMO values being ≤ 6 meV. In contrast, the SIP polymorphic phase has two main independent intermolecular couplings with moderate J_HOMO values of 12-13 meV, which span over three different directions in the ab plane of the material, indicative of a more 2D electronic isotropy. Thus, although a larger electronic coupling is found in the Bulk phase, the electronic dimensionality in the SIP phase is enhanced.
Using the polymer blend we demonstrate the possibility to stabilize, during the time period studied of one year and a half, of the metastable SIP phase which present a 2D electronic isotropy compared with the 1D π-stacking arrangement of the bulk structure and thus to be preferred for thin film devices fabrication.⁵

[1] N. Bedoya-Martínez, B. Schrode, A. O. F. Jones, T. Salzillo, C. Ruzié, N. Demitri, Y. H. Geerts, E. Venuti, R. G. Della Valle, E. Zojer and R. Resel, J. Phys. Chem. Lett., 2017, 8, 3690–3695.
[2] B. Schrode, A. O. F. Jones, R. Resel, N. Bedoya, R. Schennach, Y. H. Geerts, C. Ruzié, M. Sferrazza, A. Brillante, T. Salzillo and E. Venuti, ChemPhysChem, 2018, 19, 993–1000.
[3] C. Ruzié, J. Karpinska, A. Laurent, L. Sanguinet, S. Hunter, T. D. Anthopoulos, V. Lemaur, J. Cornil, A. R. Kennedy, O. Fenwick, P. Samorì, G. Schweicher, B. Chattopadhyay and Y. H. Geerts, J. Mater. Chem. C, 2016, 4, 4863–4879.
[4] T. Salzillo, R. G. Della Valle, E. Venuti, A. Brillante, T. Siegrist, M. Masino, F. Mezzadri and A. Girlando, J. Phys. Chem. C, 2016, 120, 1831–1840.
[5] T. Salzillo, A. Campos, A. Babuji, R. Santiago, S. T. Bromley, C. Ocal, E. Barrena, R. Jouclas, C. Ruzie, G. Schweicher, Y. H. Geerts and M. Mas-Torrent, Adv. Funct. Mater., 2020, 2006115, 1–9.

Organic mixed conductors and electrochemical phenomena at solid-liquid interface have garnered significant attention for applications in bioelectronics, electrochromics, energy storage/generation, neuromorphic computing, and thermoelectrics. These devices operate in electrolytes that render ions mobile in the film, making the coupling between electronic and ionic charges crucial. A prime example of such devices is the organic electrochemical transistor (OECT), a transducer used commonly to monitor bioelectronic signals. In this work, I will introduce the class of materials that have been used in OECT channels. Using in operando techniques, we find that the ions enter the semiconducting polymer channel hydrated and the excess swelling of the material has a significant effect on device characteristics, which can be traced back to changes in the overall structural order. ^1,2 We show that the infiltration of the hydrated dopant ions into the polymer film irreversibly changes the polymer structure and negatively impacts the OECT mobility, as well as the efficiency, reversibility, and speed of charge generation. We conclude that minimizing swelling of the polymer films during doping is a key parameter to design fast and highly efficient ion-to-electron transduction devices. Our work highlights the importance of characterizing the properties of these films in-situ (in their electrolyte swollen state) for drawing conclusions related to materials properties/device performance, which has a tremendous impact on the performance of biosensors. Lastly, I will show an n-type conjugated polymer that, against all the current design rules, shows high device performance, and introduce a new direction for the design of mixed conductors.
¹ Savva, A et al Chem. Mater. 2019 31 (3), 927-937
² Savva, A et al Adv. Funct. Mater. 2020 30 (11), 1907657

Controlled volume change of soft materials is of interest for a wide range of applications, from actuators, microfluidics and drug delivery. Conjugated polymers can transform electrical energy into volume change when included in electrochemical devices via the exchange of ions and solvent. Recently we demonstrated a glycolated polythiophene that reversibly expands by 300% upon addressing, relative to its previous contracted state, while the first irreversible actuation can achieve values of 10000%, outperforming any other conjugated polymers. Here I will present a study on a series of polythiophenes that differ in the length and/or in the distribution of the ethylene glycol side chains and how this impacts the volume change upon electrochemical addressing. In order to get insight on how the structure and morphology affect the volume change and reveal the mechanisms involved at the nanoscale we preformed Molecular Dynamics. Finally, I will present how we can apply these materials for dynamic microfiltration.

Improving the performance of organic mixed ionic-electronic conductors (OMIECs) favors not only the development of bioelectronic devices for healthcare and neuroscience applications, but also of optoelectronic and energy storage devices.[1] The performance of OMIECs can be explored using organic electrochemical transistors (OECTs). OMIECs bearing ionic side chains (called conjugated polyelectrolytes) present promising doping/dedoping cycling stability for applicative use.[2] Higher transconductance can be reached for copolymers bearing both alkyl and ionic side chains.[3] Nonetheless, the use of polar, glycolated, side chains remains to date the most effective strategy to concomitantly enhance the transconductance and response time of OECTs.[4] In this work, we compared P3HT-like polymers composed of: 100% alkyl, 100% ionic, 100% glycolated, and 50/50% ionic/glycolated side chains. Using in operando time-resolved UV-Vis-NIR absorption spectroscopy, with a time-resolution of a few milliseconds, and multivariate curve resolution (MCR) analysis, we elucidated the underlying processes driving the electrochemical doping kinetics of these OMIECs. Our findings suggest that introducing ionic side chains in glycolated polymers is an effective and easily transferable strategy to further enhance the response time of OECTs.

[1] B. D. Paulsen, K. Tybrandt, E. Stavrinidou, J. Rivnay, Nature Materials 2020, 19, 13–26.
[2] E. Zeglio, M. M. Schmidt, M. Thelakkat, R. Gabrielsson, N. Solin, O. Inganäs, Chem. Mater. 2017, 29, 4293–4300.
[3] P. Schmode, D. Ohayon, P. M. Reichstein, A. Savva, S. Inal, M. Thelakkat, Chem. Mater. 2019, 31, 5286–5295.
[4] M. Moser, J. F. Ponder, A. Wadsworth, A. Giovannitti, I. McCulloch, Advanced Functional Materials 2019, 29, 1807033.

There has been a growing interest in aqueous formulation of conjugated polymers to achieve more environmentally benign processing routes and to widen the utilization of these polymers in bioelectronics. The widely studied conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has high electrical conductivity and aqueous processability, but suffers from ill-defined structure, poor synthetic tunability, and heavily post-processing dependent performance. Here, we demonstrate how coacervation of oppositely charged polymers can be used for formulating and processing conjugated polymers in aqueous media. By sampling a wide range of polymer concentrations and dielectric media, we obtained the mixing phase diagram of a PEDOT derivative substituted with alkoxysulfonate groups (PEDOT-S) with a polymeric ionic liquid (PIL). Using this phase diagram, we were able to find compositions that enabled the formation of a complex coacervate of these 2 polymers. This coacervate is a viscous fluid with extremely high polymer content (up to 60 wt%), and can be readily blade-coated to produce films of around 5 μm in thickness. Subsequent doping of the film with a strong acid increased the electrical conductivity of the coacervate to values that were higher than those of the neat PEDOT-S polymer. Optical spectroscopy showed that acid treatment led to a lower doping level in the coacervate than in neat PEDOT-S, suggesting a smaller charge carrier density and thus a higher carrier mobility within the coacervate. We postulate that in the coacervate, the conjugation length of PEDOT-S was enhanced due to the reduction in torsional disorder along the polymer chain upon the complexation. This work shows that polyelectrolyte complexation is a promising route to formulate conjugated polymers in aqueous media at high concentrations and that the resulting material is compatible with large-scale industrial processing protocols like blade coating. The complexation can also have beneficial impacts on charge transport through the improved control of the conformation of the conjugated polymer.

Organic mixed ionic-electronic conductors (OMIECs) are a class of materials that have recently drawn interest as materials for next generation biosensors, energy storage, neuromorphic computing and other organic electronic applications. This class of materials rely on counterions from an electrolyte solution to penetrate the volume of the organic layer in order to compensate injected electronic charges. Measuring the microstructure of these materials is uniquely challenging due to the essential role the electrolyte plays during operation of these types of devices. Traditional microstructure measurements are rarely performed in contact with liquid electrolytes, and therefore fail to fully characterize the most device relevant microstructure. Additionally, many of these materials are electrochemically unstable in the presence of air which limits the understanding we can gain from traditional ex-situ scattering techniques. These challenges motivate in-situ scattering experiments with both potential control and solvent present. Here we develop an in-situ grazing-incidence wide-angle x-ray scattering (GIWAXS) procedure based upon a “rolling drop” electrode, that allows us to interrogate the crystallite lattice while fully hydrated AND electrochemically doped. We explore a model series of polymers that display distinctly different swelling based upon the distribution of ethylene glycol in their side chains. Using this technique, we separate the active swelling of the polymer crystallites due to electrochemical oxidation/reduction from the passive swelling due simply to the hydration of the polymer lattice. Additionally, we use the identity of the dopant ion to change the hydrophobicity of the electrochemically doped polymer to further probe the role of hydration in this series of polymers.

The growing demand for flexible, lightweight, and stretchable electronics has led to significant interest in printed electronics such as thin-film transistors (TFTs), which are used as foundational components in many emerging organic electronic devices such as displays and sensors. These applications require TFTs that can both operate at low voltages to minimize power utilization and be effectively produced by large scale printing methods to reduce cost.

Polyelectrolyte materials produce a large electrical double-layer capacitance (EDL) that is independent of layer thickness. This large EDL capacitance is exploited to produce TFTs that operate at low voltages. These materials are also compatible with large scale production methods that have less control over layer thickness. However, the resulting devices are typically slow-responding due to the slow ion migration during electrical double-layer formation. To improve this, the material's ionic conductivity must be improved. Ideally, an optimal polyelectrolyte gating material must insulate charge, have a high conductivity for fast switching speeds, and produce a large double-layer capacitance required for reduced operating voltages.

Our study investigates structure-property relationships between polymeric structural elements, such as morphology, polymer composition, anionic salt, and the materials' physical properties of interest: double-layer capacitance and conductivity. We synthesized a matrix of diblock copolymers consisting of insulating polystyrene first block followed by a second copolymer block consisting of an ionic liquid (IL) conductive monomer and soft PEGMA monomer. We varied the IL/PEGMA composition of the second block, the length of the second block, as well as the mobile anion associated with the ionic liquid monomer. Each material was tested in capacitors to determine the EDL capacitance and conductivity of each material to investigate structure-property relationships.

We found that microphase separation is critical for increased conductivity. Also, we found that increasing the PEGMA loading of the second block greatly improved conductivity. However, PEGMA loadings over 75% solubilized the first block eliminating the microphase separation and thus the nanochannel domains essential for conductivity. The double-layer capacitance can be changed by an order of magnitude depending on the anionic salt. We have identified structural design rules showing what chemical handle influences a specific material property.

Incorporating the dielectrics into top-gate top-contact TFTs with n-type semiconductor P(NDI2OD-T2), we found the devices could be operated at exceptional low voltages (< 1 V) while maintaining mobility values comparable to literature. We have also performed frequency-dependent measurements of the TFTs to compare the properties of the polyelectrolytes with different mobile anions, finding these results correlated well with observed TFT performance.

Organic mixed ionic-electronic conductors (OMIECs) are organic conjugated materials with the ability to uptake ions from aqueous electrolyte and transduce between ionic and electronic currents. The unique properties of OMIECs make them highly attractive in range of biological, energy and neuromorphic computing applications. For these materials operating in electrolyte emersed situations, the structure and composition is non constant during operation, and is difficult to characterize. This has become a major impediment to elucidation of structure property relationships that further the development of new materials. Here, we have filled these gaps of understanding that have been neglected in previous studies by quantifying the composition with ex situ X-ray fluorescence spectroscopy (XRF) and probing the device relevant structure with ex situ and operando grazing incident X-ray scattering. PEDOT:PSS and pg2T-TT were selected as model materials representing polyelectrolyte/conjugated polymer blends and homogeneous glycolated conjugated polymers, respectively. XRF studies of doped and dedoped PEDOT:PSS revealed the effect of Donnan exclusion due to the presence of PSS poly anions, preventing anion uptake and resulting in cation-only transport. Additionally, charge and mass balances revealed that the equilibrium between the polyanion and the proton contributed significantly to the doping processes. XRF studies of doped and dedoped pg2T-TT quantified the dopant-anion concentration modulation, directly measured the minority role of cation transport, and revealed an unaticipated massive amount of charge trapping (>10²⁰ anions cm^-3) when exposed to electrolyte, that persists when reductively dedoped. The low conductivity of the reductively dedoped state confirmed the localized nature of the electronic charge counterbalancing the trapped anions. The pH dependence of dopant composition was investigated in both systems.

Ex situ grazing incident small-angle X-ray diffraction (GISAXS) confirmed the compositional homogeneity of both pg2T-TT and PEDOT:PSS polymer films. Operando grazing incident wide-angle X-ray diffraction (GIWAXS) of PEDOT:PSS, revealed the degree of polymer-electrolyte interaction resulting moderate lamellar expansions (1.6 Å) with electrolyte exposure, following a lamellar spacing relaxation upon ionic doping. Operando GIWAXS of pg2T-TT showed a moderate lamellar expansion (0.6 Å) with DI water exposure, but a much larger lamellar expansion (~10 Å) upon electrolyte exposure. The potential dependent pi stack and lamellar d-spacing were quantified from the diffraction patterns. In contrast to PEDOT:PSS, the lamellar spacing of pg2T-TT was clearly increased during doping process.

Combining these multimodal results gives new insight into these systems. For instance, the XRF and GISAXS reveal that cations are evenly distributed in PEDOT:PSS contrary to common assumptions about the heterogenous composition of PEDOT-rich and PSS-rich domains. Only when phase separation is driven by processing in strong acids, is an inhomogeneous distribution of cations observed. XRF and operando GIWAXS reveal the permanent structural modification that accompanies the high degree of charge trapping in pg2T-TT. As confirmed by GISAXS, this implies that anions must be present in both the amorphous and the crystalline regions of the polymer. These results represent the first quantitative compositional determination of OMIECs and the first operando scattering studies of weakly scattering homogenous OMIECs in aqueous conditions, and establish quantitative techniques necessary to produce meaningful structure-property relationships to drive the rational design of next-generation OMIECs.

Conjugated polymers are used in many organic electronic devices such as organic photovoltaics, organic light emitting diodes, organic field effect transistors and bioelectronics. However, fundamental characteristics of these materials, such as mechanical properties and phase behavior, and their correlation with chemical structure, are not well understood. Nevertheless, controlling mechanical properties of conjugated polymers is crucial to enabling flexible and stretchable electronics. The plateau modulus G_N^°, which is inversely related to the entanglement molecular weight M_e, can be used to predict the mechanical response of polymers for many orders of magnitude in frequency (along with the molecular weight distribution and relaxation time). The relationship between structural parameters (Kuhn length b and Kuhn monomer volume v₀) and mechanical stiffness (plateau modulus G_N^°), initially proposed by Graessley and Edwards and experimentally investigated by Everaers, while well-studied for flexible and stiff polymers, has a large gap in experimental data between the flexible and stiff regimes. This gap prevents the validation of the crossover between flexible and stiff polymers and therefore, the prediction of mechanical properties from chain structure of any polymer in this region. Given the chain architecture, including a semiflexible backbone and side chains, conjugated polymers are an ideal class of material to study this cross-over region. Using small angle neutron scattering, oscillatory shear rheology, in-situ polarized optical rheology, along with the freely rotating chain model we have shown that twelve polymers with aromatic backbones, including conjugated polymers, populate a large part of this gap. We also have shown that a few of these polymers exhibit nematic ordering which explains a lower experimental G_N^° than predicted using Everaers’ plot, as nematic polymers have fewer entanglements than isotropic polymers. Nevertheless, when isotropic, these polymers follow the proposed relationship between b, v₀, and G_N^°.

Synaptic devices with linear resistance tuning are highly sought after for parallel programming of artificial neural network (ANN) weights in neuromorphic arrays. Conventional resistive memories however suffer from nonlinear and asymmetric conductance tuning and excessive write noise that are detrimental for accelerating learning in hardware ANNs. To address these challenges, we have recently introduced electrochemical random-access memories (ECRAMs), where resistive switching is controlled by ion insertion from the electrolyte into a semiconductor channel, enabling linear resistance tuning and parallel array programming [1]. However, state-of-the-art organic ECRAMs have not yet demonstrated stable and efficient operation at elevated 20-100 ^oC temperatures required for large scale integration with Si logic.

Here, we show that semiconducting polymers in combination with ion gels enable solid-state organic ECRAMs with stable and temperature-independent operation up to 110 ^oC. With a judicious choice of materials we demonstrate ECRAMs with linear conductance tuning (over 100x distinct states), fast switching (20 ns), rapid write-read cycles (<1 µs), low voltage (±1 V) and low energy (~100 fJ/write) operation, and excellent endurance (>10⁹ write-read operations at 90 ^oC). The use of novel polymers also improves the dynamic range to >2x. We use a series of photolithographically patterned devices and quantitative modelling to demonstrate that further scaling would lead to faster than 1 GHz switching. This shows that organic ECRAMs are temperature-resilient and meet the stringent device requirements for efficient neuromorphic computing. Demonstration of such high-performance solid-state ECRAMs [2] is a fundamental step towards their integration into synaptic arrays amenable to learning and inference.

[1] E. Fuller, S. T. Keene*, A. Melianas*, et al. Science 364 (6440), 570-574 (2019)
[2] A. Melianas, et al. Science Advances 6 (27), eabb2958 (2020)

Conjugated polymers can be ideal materials for the design of efficient photovoltaic devices, batteries, thermoelectric cells, light emitting diodes and many new technologies. Moreover, a large flexibility in organic synthesis methods enables the efficient application of molecular design principles to produce superior materials. Yet, the use of computational methodologies for conjugated polymers is still limited by a lack of properly validated simulation force fields that can be used to model structures and temporal fluctuations through molecular dynamics simulations. This presentation motivates the use of neutron and x-ray scattering techniques for the development of improved molecular simulation force fields and structural parameters specifically for poly-3-alkyl-thiophene, a model conjugated polymer. Quasi-elastic neutron scattering (QENS) experiments are used along with MD simulations to quantitatively compare proposed force fields to extensive sets of experimental data. X-ray and polarized neutron diffraction are also used to correlate experimental and model-generated polymer structures. QENS validation of MD force fields presents a unique opportunity to increase the accuracy of highly uncertain parameters that are used in the simulation of conjugated polymers, including partial charges, Lennard-Jones and backbone torsion parameters. Many of these parameters are estimated from quantum mechanical calculations such as density functional theory but, unlike for force fields designed for small molecules, they are not often parameterized to experimental data. High variability is also observed in parameters for the small number of simulation force fields that have been proposed in the literature. A vision for the accelerated development of accurate force fields for these classes of materials is also proposed.

The hierarchical assembly of conjugated polymers has gained much attention due to its critical role in determining the optical/electrical/mechanical properties. The hierarchical morphology encompasses molecular scale intramolecular conformation (torsion angle, chain folds) and intermolecular ordering (π-π stacking), mesoscale domain size, orientation and connectivity, and macroscale alignment and (para)crystallinity. Such complex morphology in the solid state is fully determined by the polymer assembly pathway in the solution state, which in turn is sensitively modulated by molecular structure and processing conditions. However, molecular pictures of polymer assembly pathways remain elusive, due to the lack of detailed structural characterizations in the solution state and lack of understanding on how various factors impact the assembly pathways. In this talk, we discuss pre-aggregation and liquid-crystal mediated assembly pathways and their relationship. We develop SAXS models for in-depth analysis of the complex solution state structure of conjugated polymers complemented by imaging and spectroscopy. Further we discover a chiral, helical liquid crystal mediated assembly pathways from achiral conjugated polymers. We elucidate a hierarchical helical structure spanning the molecular scale to micron scale. We further trace the molecular origin of chiral liquid crystal assembly pathways. These studies change the way we perceive structure of conjugated polymers and may bring forth exciting new optoelectronic properties not imagined before.

Hierarchical structures are inherent to various soft material systems including biomolecules, mesogens and conjugated polymers. Structural chirality commonly results from the hierarchical organization of chiral building blocks, e.g., amyloids, M13 phage and chiral mesogen [1, 2]. Recently, it has been discovered that achiral mesogens/nanostructures can also form chiral, twisted structures due to symmetry breaking or topological defects [3-5]. This new finding can open a new degree of freedom for tuning electrical and optical properties and provide a fundamental understanding of complex supramolecular assembly and phase transition behaviors. In this work, we demonstrate chiral emergence from achiral donor-acceptor (D-A) conjugated polymers, a high-performance organic semiconductor. Our previous work demonstrated that the morphology of printed isoindigo-bithiophene-based conjugated polymer (PII-2T) films results in chiral, zigzag twinned domains despite the inherent molecular achirality [6]. We anticipate that the chirality is responsible for the multistep hierarchical assembly of achiral, torsional molecular structures. Still, several aspects regarding the nature of its multiscale organization and behavior remain a mystery. In conjugated polymers, the main intermolecular forces are π-interactions between the backbones and dispersion forces among the alkyl side chains. Such interaction often results in the presence of solution-phase aggregates and/or liquid crystalline phase. Given the importance of solvent in mediating these interactions, we also demonstrate how a solvent influences the development of mesophase and the relationship between pre-aggregation and mesophase formation. This study provides a clear map for the multiscale assembly pathway of conjugated polymers as we explore the possible hierarchical mesophase structures that can be used to enhance device performance

1. G. Nystrom et al., Nat Nanotechnol 13, 330-336 (2018)
2. W. J. Chung et al., Nature 478, 364-368 (2011)
3. L. E. Hough et al., Science 325, 456-460 (2009)
4. M. R. Tuchband et al., P Natl Acad Sci USA 116, 10698-10704 (2019)
5. G. Singh et al., Science 345, 1149-1153 (2014)
6. K. S. Park et al., Sci Adv 5, eaaw7757 (2019)

A group of organic small molecules, selected for small differences in molecular structure, are investigated for their ability to crystallize as a thin film. Each material is vapor deposited in a range of thicknesses between 20 - 80 nm and subsequently thermally annealed. The annealed films are categorized into three groups based on the crystal morphology as determined by polarized optical microscopy: platelet-forming, spherulite-forming, and those that resist crystallization. Differential scanning calorimetry is utilized to determine bulk thermal properties of these materials. We demonstrate that these thermal properties provide a reliable indicator of a material's crystallization motif. Platelet-forming materials tend to be characterized by high melting points (T_m) and a large crystallization driving force at the material's crystallization temperature (ΔG_c). The materials that resist crystallization as a thin film have small ΔG_c. These guidelines can help determine which organic molecules have a greater likelihood of growing into large-scale crystalline frameworks, a key step for improving charge carrier mobility and exciton diffusion length in organic semiconductors.

An exciting recent progress has been made in our understanding of fundamentals of charge carrier transport and photoconductivity of crystalline and polycrystalline organic semiconductors, largely achieved thanks to high-sensitivity ac-Hall effect measurements. This talk will cover one or more examples. For instance, we have elucidated the effect of grain boundaries on the Hall effect in high-performance, polycrystalline organic transistors, where it was found that capacitively charged grain boundaries may lead to an underestimated Hall mobility and an overestimated Hall carrier density [1]. In another example, the intrinsic mobility-strain relationship has been measured for the first time in organic semiconductors (here, in single crystalline rubrene). It became possible thanks to the ac-Hall effect and Raman measurements as a function of uniaxial mechanical strain applied to ultra-thin, flexible single-crystal rubrene OFETs. The study reveals a very strong, anisotropic and reversible modulation of the intrinsic (trap free) charge carrier mobility of these OFETs with strain, showing that an effective mobility of organic circuits can, in principle, be enhanced by up to ~ 80% with only 1% of compressive strain [2]. Finally, a photo-Hall effect has been measured in organic semiconductors for the first time [3]. These measurements have allowed us to directly access the mobility and density of charge carriers photogenerated at the surface and in the bulk of (ungated) organic crystals under cw photoexcitation. Our data reveal some unexpected properties of photoexcitation process, including the generation of mobile carriers of only one type (holes) and unconventional power exponents in photoconductivity. The data are also consistent with the mechanism of photoconductivity in rubrene based on an interaction of long-lived, mobile triplet excitons with surface electron traps, leading to a release of mobile holes [3].
References:
1. H. H. Choi, A. F. Paterson, M. A. Fusella, J. Panidi, O. Solomeshch, N. Tessler, M. Heeney, K. Cho, T. D. Anthopoulos, B. P. Rand and V. Podzorov, "Hall Effect in Polycrystalline Organic Semiconductors: The Effect of Grain Boundaries", Adv. Funct. Mater., 1903617 (2019).
2. H. H. Choi, H. T. Yi, J. Tsurumi, J. J. Kim, A. L. Briseno, S. Watanabe, J. Takeya, K. Cho, V. Podzorov, "A Large Anisotropic Enhancement of the Charge Carrier Mobility of Flexible Organic Transistors with Strain: A Hall Effect and Raman Study", Adv. Science, 1901824 (2019).
3. V. Bruevich, H. H. Choi, V. Podzorov, "The Photo-Hall Effect in High-Mobility Organic Semiconductors", Adv. Funct. Mater., DOI:10.1002adfm.202006178 (2020).

Metal and metalloid phthalocyanines (MPcs) are a large class of thermally stable organic semiconductor materials which can be manufactured with relatively inexpensive reagents and that have easily accessible synthetic routes. A wide variety of MPcs containing different metal cores have been incorporated into incorporated into organic light-emitting diodes (OLEDs), organic solar cells (OPVs) and organic thin-film transistors (OTFTs), including copper (CuPc) and zinc (ZnPc), trivalent MPcs (one axial group) including aluminum (AlPc-Cl) and tetravalent MPcs (two axial groups) including silicon (SiPc-Cl₂₎ and tin (SnPc-Cl₂). To date, the greatest reported mobility for an MPc-based OTFT incorporates titanyl phthalocyanine (TiOPc) as the semiconductor, which exhibits a hole mobility (µ_h) on the order of 10 cm²V^-1s^-1. Although MPcs are generally p-type, examples of n-type derivates of these materials exist including SiPcs, SnPcs and peripherally-fluorinated F₁₆-CuPc.

SiPcs have recently shown promise as n-type or ambipolar organic semiconductors with various derivatives. The highest reported electron mobility (µ_e) of SiPcs is ≈0.5 cm²V^-1s^-1 using pentafluoro-phenoxy silicon phthalocyanine (F₁₀-SiPc), indicating that high-performance SiPcs not only compare to but exceed other metal phthalocyanines (MPcs) in n-type OTFTs and can rival state-of-the-art organic organic semiconductors. Despite this excellent potential, only a few structure-property relationships have been established to determine the impact of the axial substituent on the electrical performance of SiPc-based OTFTs. For example, previous density functional theory (DFT) modelling data indicated that µ_e and other charge transport properties in fluorophenoxy SiPcs are dependant on the axis of transport and the number of substituted fluorine atoms on the axial pendant. Additionally, the choice of axial group affects SiPc packing in the single crystal, often changing the π-π stacking distance, herringbone angle, and degree of molecular overlap.

In this work, structure-property relationships for phenoxy-SiPcs were developed by fabricating bottom-gate top-contact OTFTs employing 11 derivatives. Characterization of thin-film properties in addition to electrical performance in devices was completed. Thin-film characterization techniques included x-ray diffraction (XRD), atomic force microscopy (AFM) Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and DFT modelling. One significant trend observed from electrical characterization was that increasing the electron-withdrawing character of axial pendant groups as characterized by their Hammett Parameter led to a reduction in threshold voltage, indicating that these substituents play a role beyond solid state packing and influencing morphology of thin-films. It was also determined that low molecular weight pendant groups contributed to a greater degree of crystalline order of SiPc materials in thin-films. These findings dictated the design of three additional novel SiPcs with phenoxy pendant group Hammett Parameters in the range of +0.66 to + 0.997. These materials exhibited threshold voltages as low as 4.8 V, exceeding the record low threshold voltage of 7.8 V reported for F₁₀-SiPc-based OTFTs fabricated in an identical architecture.

Many organic electronic devices, such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs), contain blends of molecules. The morphology, interface structure, impurities, and degradation pathways are all of great interest in improving device performance. Conventional analysis techniques, such as transmission electron microscopy, often provide critical but limited information in these materials. Atom probe tomography (APT) collects the mass-to-charge state of every ion detected with nanometer-scale resolution, providing previously inaccessible information that enables evaluation of nanometer-scale morphology, impurity analysis, and understanding degradation pathways in molecular organic electronic materials blends.

A wide variety of molecules can be analyzed with APT, ranging from fullerenes (e.g., C₆₀) to metalorganics (e.g., tris(2-phenylpyridine)iridium or Ir(ppy)₃) without indication of fragmentation. Sub-nanometer spatial resolution, sub-Dalton mass discrimination, and the ability to identify impurities or degradation products below 0.01 atomic% have been demonstrated [1,2]. We will introduce the APT technique, discuss techniques for creating APT samples, typical analysis conditions for organic small molecules, and highlight structure-property relationships in OLED and OPV devices enabled by APT analysis.

References:
[1] A.P. Proudian, M.B. Jaskot, C. Lyiza, D. R. Diercks, B.P. Gorman, and J.D. Zimmerman, Effect of Diels−Alder Reaction in C₆₀-Tetracene Photovoltaic Devices, NanoLetters 16, 6086-6091 (2016)
[2] A.P. Proudian, M.B. Jaskot, D.R. Diercks, B.P. Gorman, J.D. Zimmerman, Atom Probe Tomography of Molecular Organic Materials: Sub-Dalton Nanometer-Scale Quantification, Chemistry of Materials 31, 2241-2247 (2019).
[3] The work was supported in part by grant DE-SC0018021 funded by the U.S. Department of Energy, Office of Science Early Career program.

Advances in printing techniques have given rise to low-cost, large-area, flexible electronics and systems. These printing methods have been used to fabricate organic electronics including light-emitting diodes (OLEDs), photodiodes (OPDs), solar cells (OSCs), and thin-film transistors (OTFTs) for various applications such as flexible displays, imagers, photovoltaics, smart tags, and logic circuits. However, in applications that require dual functionality, such as optoelectronic sensors with a combination of light sources and detectors, devices are fabricated separately in multiple processing steps. Additionally, the devices are often stacked together, reducing the mechanical flexibility of the overall system. To address these limitations, we utilize blade-coating in conjunction with surface-energy-patterning (SEP) to simultaneously deposit OLED and OPD films side-by-side on a single flexible substrate. This printing technique allows us to manufacture functionally distinct devices, while retaining the ability to individually optimize each device via ink parameters. The film thickness and device performance are compared and characterized for concentrations of 8, 10, and 12 mg/ml for OLEDs and 20, 35, and 50 mg/ml for OPDs. We chose the optimal ink concentration for the blade-coating process based on desired device performance – luminance for OLEDs and EQE for OPDs. The optimized OLEDs exhibit an average turn-on voltage of 3.4 V, and average luminance of 7000 cd/m² at 8 V. At 1000 cd/m², the current density, EQE and luminous efficacy of the OLEDs is 60.8 mA/cm², 1.35% and 9.5 lm/W with peak emission at 612 nm. OPDs exhibit EQEs of over 30% over the visible spectrum, and up to 45% in the near infrared region. The OPDs have a low dark current of 65.5 pA/cm², a cutoff frequency of 300 kHz, and LDR of over 120 dB.

Characterizing the morphology of polymer blends is critical for improving the performance of next-generation sensors, photovoltaics, and pharmaceuticals. Unfortunately, blends containing materials that share similar chemical composition remain challenging to measure by traditional scanning probe microscopy due to poor topographical contrast. Coupling infrared spectroscopy with atomic-force microscopy, infrared nanospectroscopy (AFM-IR) overcomes this contrast barrier by distinguishing blended components by their unique infrared absorption response. While AFM-IR remains a robust technique for measuring surface morphology, the versatility of the technique has been largely unexplored. Herein, AFM-IR is used to determine blend composition using fundamental infrared spectroscopy theory for a series of immiscible and miscible blends. Second, nanoscale phase separation in isotope blends is explored using AFM-IR and confirmed by resonant soft X-ray scattering and small angle neutron scattering. These investigations highlight the widely unexplored potential of AFM-IR and its application for probing low-contrast, polymer blend morphology.

Organic semiconductors (OSCs) are a rapidly emerging field in molecular electronics. Specifically, single-crystalline OSCs have received considerable attention due to the unique properties associated with long-range order, solution processing, absence of grain boundaries, and extremely low defect density [1] leading to a wide range of multi-functional materials (e.g., field-effect transistors, light-emitting diodes, photovoltaics). However, the lack of strategies to control and promote π-stacking in single crystals of OSCs has limited their diversification, property tunability, and application. While covalent modification of OSCs has arguably received the most attention [2], supramolecular modification of OSCs through weak, non-covalent interactions with additional molecules (i.e., coformers) has been largely unexplored [3]. The resulting single-phase multicomponent solids (i.e., cocrystals) typically display enhanced or modulated properties compared to the parent OSC.

In this contribution, we introduce the use of halophenols as coformers to promote π-stacking of a pyridyl-containing anthracene through hydrogen-bonding. Specifically, solution cocrystallization of the anthracene OSC results in single crystals with modulated π-stacking and is accompanied with significant changes in the photophysical properties. We envisage the supramolecular control of π-stacking in single crystals of OSCs with suitable coformers could lead to a rapid diversification of multicomponent and multifunctional crystalline electronics with enhanced and tailored properties (e.g., flexible electronics).

References:
[1] Zhang, X., Dong, H., Hu, W., Adv. Mater. 2018, 30, 1801048.
[2] Anthony, J., Angew. Chem. Int. Ed., 2008, 47, 452.
[3] Ray, K. K., Campillo-Alvarado, G., Morales-Rojas, H., Höpfl, H., MacGillivray, L. R., Tivanski, A. V., Cryst. Growth Des. 2020, 20, 3.

Understanding morphology is fundamental to revealing the structure-property relationships of solids. Disordered materials are of particular interest since their morphology is rarely in the lowest energy, equilibrium state, and hence can be complex and metastable, or even unstable over time. Organic molecular solids are particularly important disordered materials since they are bonded by relatively weak van der Waals (vdW) forces. Here, we reveal the detailed nanoscale morphology within archetype organic electronic thin films using Fourier plane imaging microscopy (FIM). By depositing phosphorescent dye molecules at strategic positions within a host organic thin film, their luminescence provides high resolution, depth and area-dependent structural maps of the host. In this work, the FIM-plus-dye molecule combination is used to create 3D morphological maps of changes arising from thermal annealing in a stacked bilayer film comprising CBP (4,4′-Bis(N-carbazolyl)-1,1′-biphenyl) and TPBi (2,2',2"-1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), and at the interfaces. The volume resolution of the measurements is at the Ångstrom scale in the direction normal to the film plane that is limited only by the flatness of the predeposited film and the ability to accurately determine layer thickness during deposition, and has a resolution of approximately half the visible wavelength (~200 nm) within the plane. Our results provide a powerful tool for the high-resolution 3D morphological imaging of materials employed in a wide range of optoelectronic applications.

Wearable electronics are a growing branch of technology that can have a large impact on various types of applications. The potential for the creation of new wearable devices from the most important medical applications to ones for novel enjoyment are extreme. In order to develop these wearable electronic devices, traditional inorganic conductive materials do not lend themselves to this type of application due to their mechanically brittle nature. Instead, polymeric materials with better mechanical flexibility are more advantageous to the development of wearable electronic components. Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most promising conjugated polymers due to its high electrical conductivity as well as the environmental stability. PEDOT has been integrated in many electronic and optoelectronic devices such as organic thin film transistors (TFTs), solar cells and sensors. Due to these demonstrated performance together with mechanical flexibility, PEDOT has been garnering much attention more recently for potential use in wearable electronic applications.

For the development of wearable electronic devices with PEDOT, challenges remain particularly for clothing applications. Sustainable mechanical flexibility should be achieved over repeated folding and bending cycles as naturally occur in our daily use of clothing. Further, breathability, one of the principle functions of clothes should not be sacrificed after the coating of PEDOT.

In this presentation, in order to mitigate the challenges, oxidative chemical vapor deposition (oCVD) will be utilized, which uses mild thermal energy to generate a polymerization between a vaporized source material and an oxidizer in a vacuum chamber. We have investigated the electrical and structural stability of oCVD PEDOT and other thiophene-based polymers[1-3] and these polymers have been integrated in electrochromic devices[4]; TFTs[3, 5]; organic solar cells[6-8]; and chemical sensors[9-11]. In comparison to more conventional solution-based processes, oCVD lends itself to certain traits that are more beneficial when applied to the creation of wearable electronic devices and thin film deposition on fabric substrates. Films deposited from the use of oCVD better conform to the target fabric substrate. With the achieved excellent conformality, the fabric coated with oCVD PEDOT is still able to maintain its flexible properties as well as breathability, and can effectively be used in next generation devices on fabrics that work as clothing as well as electronics. In addition, no solvents are required for the oCVD deposition and therefore substrate distortion or damage is minimized.

What will be presented is a comparison of various properties of PEDOT thin films on both glass and fabric substrates as well as Si and glass substrates. Thin film deposition on Si and glass substrates is used as a control to compare the performance of oCVD PEDOT on fabrics. Specifically performance in regard to electrical, mechanical, surface, and chemical properties will be discussed. To mimic daily use in clothing, mechanical bending tests of oCVD PEDOT were conducted and excellent performance stability was observed over 150 times bending cycles. General electrical properties include an analysis of conductivity, carrier density, and carrier mobility utilizing Hall Effect measurements system and four-point probe. AFM and SEM are used to determine and compare surface and cross-sectional microstructures. FTIR and UV-Vis spectroscopy are used to compare optical properties and chemical bonding information. Our findings on vapor-processed conjugated polymers on fabrics with high conductivity and mechanical flexibility is expected to significantly contribute to the realization of high performance and sustainable wearable clothing electronic devices that require enhanced uniformity, flexibility, and breathability.

Organic electrochemical transistor materials face a design tension between optimizing for ion mobility and for electronic mobility. These devices transduce ion uptake into electrical current, thereby requiring high ion mobility for efficient electrochemical doping and rapid turn-on kinetics, and high electronic mobility for maximum transconductance. Here we explore one method to improve operational kinetics in a high hole mobility conjugated polymer (poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)], DPP-DTT) by using a nanowire morphology. For equivalent thicknesses, the DPP-DTT nanowire films exhibit consistently faster spectroelectrochemical kinetics in KPF6 solution (~6-10X faster) compared to a standard DPP-DTT film. Nanowire devices also exhibit favorable shifts in the threshold voltage, making the nanowire architecture both faster and energetically easier to dope compared to neat films. We use nanoinfrared imaging via photoinduced force microscopy to show that these films exhibit ion diffusion throughout the film, suggesting there is a volumetric benefit to the nanowire architecture. We propose that, for higher-mobility materials that are often hydrophobic, casting the active layer in nanowire form may offer faster kinetics and possibly lower threshold voltage while maintaining desirable device performance, thereby providing one method to optimize ion mobility without sacrificing electronic mobility.

Interest in organic electronics is driven in large part by the applications they can address, yet a complete set of design rules is still absent for devices intending 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. Here we use physically-based numerical simulations to explore a vast parameter space with a high throughput in order to generate rules for material processing and device design, and optimize charge mobility and injection. For this purpose, the organic field-effect transistor (OFET) was selected as an experimental platform since current densities in OFETs (~10⁴ A cm^-3) are much higher than in other organic devices, thereby allowing the exploration of charge injection and transport in the most extreme case. We have investigated the impact of the density of trap states and the injection barrier on the charge carrier mobility for both thick- and thin-dielectric devices, in a co-planar and staggered architecture. The simulations revealed that the performance of thin-dielectric devices (10 nm) is insensitive to large changes in the density of traps, while an injection barrier has a significant impact on mobility: only a 15% decrease in mobility was observed when raising the trap density by an order of magnitude in both architectures, but 60% and 95% decreases were observed when a 0.5 eV injection barrier was introduced in the staggered and co-planar devices, respectively. On the other hand, employing a thick dielectric (1000 nm) had the opposite trend: changes in the density of traps produced large reductions in mobility (by 60%), but the same 0.5 eV injection barrier decreases mobility by only 20% and 40% in the staggered and co-planar structures, respectively. Our results suggest that the staggered structure and the thick dielectric are more tolerant to injection barriers. All these effects were also found to scale with the intrinsic mobility of the semiconductor. To test these predictions, we have fabricated and characterized a variety of OFETs and tuned the injection barrier via self-assembled monolayers (SAMs), oxide interlayers, and contact material selection, while the density of states was modified through altering film microstructure by processing. Notably, the experimental results agree well with the simulations, validating its use to predict and guide device design. With regards to the quality of the semiconductor and the contact/semiconductor interface, these results reevaluate the requirements necessary for optimizing OFETs. For example, given a good quality semiconductor with a low trap density, improving the injection barrier is not essential if a sufficiently thick dielectric is employed in the staggered architecture, allowing the selection of a more cost-effective and scalable contact material. Further parameter exploration combining both simulation and experiment will fine-tune the model and expand its predictive power.

Cooperativity has long been used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structure transition involves simultaneous, concerted displacement of molecules in a crystalline material, in stark contrast to the typical nucleation and growth mechanism occurring in a molecule-by-molecule fashion that often disrupt the material structural integrity. Cooperative transitions have acquired much attention in the research community for its low transition barrier, ultrafast kinetics, and structural reversibility. On the other hand, cooperative transition is rarely observed in molecular crystals and its molecular origin is not well understood. In 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type semiconductor our research has shown prolific diversity in polymorphism and transition behavior resulting in 5 obtainable polymorphs showing large variations in electronic and optical behaviors. Along with the diversity of structure, both a cooperative and nucleation and growth transition has been reported simultaneously in the same system through 2 different thermally activated phase transitions. In situ microscopy, single crystal and grazing incidence X-ray diffraction, and Raman spectroscopy suggest a reorientation the alkyl side chains results in a cooperative transition behavior. While in stark contrast, we find the nucleation and growth mechanism occurs through a combination of side chain melting and increased core interactions resulting from a sudden increase in biradical nature of 2DQTT-o-B and is confirmed through in situ electron paramagnetic resonance. This is the first time, to our knowledge, that biradical interactions result in a structural change. Finally, we harness the cooperative phase transition behavior for a novel thermally actuated switching device. Through studying these fundamental mechanisms, we may establish design rules to rationally obtain polymorphic behavior for novel electronic applications.

The next generation of organic-based electronic devices will require composite materials synergistically engineered to exploit novel and multi functionalities in a single device structure. Such multifunctional organic materials are attractive for various applications, including electronic skin, smart textiles, and self-healing batteries. Organic Field Effect Transistors (OFETs) have historically suffered from the lack of a single seamless material acting as both dielectric and channel. Typical OFET devices have defects and active electrical impurities due to the quality of the dielectric/semiconductor interface critical for the device operation, which is the primary source of electrical instabilities and reliability issues. Such lack of quality control in the dielectric/semiconductor interface has until now, prevented, the commercialization of solid-state organic transistors. Here, we report a comprehensive investigation of process-friendly multilayered organic transistors without material discontinuities between dielectric and channel. The transistor design makes use of the high anisotropic electrical behavior of multilayered polyethylenimine/poly(acrylic acid) (PEI/PAA) composite thin films doped with reduced graphene oxide (rGO) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). The films were fabricated by self-assembled layer-by-layer (LbL) deposition technique, exploiting the spontaneous physical interactions that allow for the natural combination of various materials in a multilayered structure. Conductive layers formed by rGO nanosheets and the PEDOT:PSS nanofibers are dynamically ordered in a specific direction due to electrostatic interactions in the LbL film fabrication, creating highly anisotropic pathways. The in-plane film conductivity is 7 orders of magnitude higher than the conductivity normal to the multilayered planes. Temperature dependent measurements confirm the strong anisotropic electrical behavior. The charge transport mechanism is weekly thermal activated (E_A = 33.4 meV) along the aligned conductive phases while the charge transport normal to the plane is fitted with variable ranging hopping having an activation energy of 1.0 eV. The LbL films’ high anisotropic electrical conduction is used to fabricate a novel field effect transistor where the same multilayered structure operates simultaneously as both dielectric and conductive channel. The transistor presents a low power consumption as it operates at the sub voltage regime. The dielectric part shows a high specific capacitance of 18 µF/cm², and the semiconductor presents an n-type carrier transport having a field effect mobility of 4.0 cm²V^-1s^-1. Moreover, the transistor threshold voltage (V_th) is zero, which shows that electrical active traps, which can charge upon the applied gate field and shield the external applied voltage are not active or do not exist. The entire applied gate field contributes to an increasing channel current, as expected in trap free devices. The results point to the development of a simple and efficient approach to fabricate solid-state OFETs relying on the high anisotropy of the LbL films, enhancing its potential for commercial applications.