Symposium MA02 : Organic Electronics---Processing, Microstructure and Multifunctioning

Organic semiconductors are candidates for use in next-generation thin film electronics. In many of these applications, electrical doping can increase effective carrier mobilities by filling trap states, enhance electrical conductivity by increasing the density of free charge carriers, and/or can lower barriers to charge-carrier injection or collection at electrodes. An emerging application is the use of semiconducting polymers as thermoelectric materials. The performance of thermoelectrics is related to their electrical conductivity, thermopower and thermal conductivity which are all functions of the carrier concentration. We will discuss our efforts to understand how the electrical conductivity and thermopower of semiconducting polymers are interrelated using model polymer systems including poly(3-hexylthiophene) and a thienothiophene-based polymer, PBTTT. We find that changes in processing conditions can increase the electrical conductivity by >50x at the same apparent carrier concentration, while causing smaller changes in the thermopower for PBTTT. The increase in performance can be understood by the nanoscale connectivity between ordered domains and quantitated using synchrotron-based X-ray scattering methods and temperature-dependent thermopower and transport measurements. The role of the miscibility of dopants and polymers will also be discussed as a critical factor in controlling their electrical conductivity.

In this presentation we will describe the effect of composition and doping type and content on the charge transport and light properties of both organic (ORG) and metal oxide (MO) semiconductor devices. For metal oxide semiconductors, we demonstrated that electron-donor polymers can dope several classes of MOs including In₂O₃, IZO, and IGZO. These polymer-MO blends exhibit larger electron transport by ~ 2x (e.g., ~ 8 cm²/Vs for In₂O₃ in a TFT) than those of the starting undoped MO matrices (~ 4 cm²/Vs for In₂O₃). Furthermore, we used multilayer structures with different degree of doping within each layer to achieve a 2DEG (two-dimensional electron gas) transport, which exhibit electron mobilities ~ 3x (e.g., ~ 11 cm²/Vs for In₂O₃) than the single layer and undoped matrix (~ 4 cm²/Vs). Doping of ORG semiconductors is explored in a new family of mixed conjugated/deconjugated polymers as well as for tuning light scattering in novel device SERS platforms.

Martensitic transition is a solid-state phase transition involving cooperative movement of atoms, mostly studied in metallurgy. The main characteristics are low transition barrier, ultrafast kinetics, and structural reversibility. They are rarely observed in molecular crystals, hence the origin and mechanism are largely unexplored. Here we report the discovery of martensitic transition in single crystals of two different organic semiconductors. In situ microscopy, single crystal X-ray diffraction, Raman and NMR spectroscopy and molecular simulations combined indicate that the rotating bulky side chains trigger cooperative transition, giving rise to a macroscopic, highly reversible shape memory effect. Such cooperative transition is preserved from single crystals to thin films and enables function memory effect in thin film transistor devices for the first time. By observing martensitic behavior in two systems containing bulky side chains, we establish a new molecular design rule to trigger martensitic transition and shape/function memory effect in molecular crystals. Inducing martensitic transition in organic electronic systems may find use in designing next-generation smart multifunctional materials.

Organic field effect transistors, OFETs, have attracted extensive interest over the past years thanks to the promise of realize solution-process, flexible and low-cost electronics. Despite all the efforts in optimizing OFETs performances, achieve efficient charge injection from the electrodes is still an open challenge. Such contact performance is the most seriously concern for short-channel devices, with channel length typically less than few micrometres, though they are extremely attractive because of their high-frequency operation and the possibility of high-density integration. In a typical silicon based MOFET, heavily doped regions, close to the Source-Drain electrodes, promote the charge injection. Attempting to reproduce this architecture into organic devices, a wide number of possible methods have been proposed, laying on different doping techniques but mostly of them require high vacuum evaporation and masking systems.
In this work, we present a multilayer structure for the reduction of the injection barrier in Bottom-Contact Top-Gate Field Effect Transistor. A highly doped semiconductor layer - poly{[N,N0-bis(2-octyldodecyl)-naphthalene- 1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)} (P(NDI2OD-T2)) blended with electrically insulating material, high-density polyethylene (HDPE) and doped with dihydro-1H-benzoimidazol-2-yl (N-DBI) derivative - act as injection layer while the FET depleted channel is created in an upper layer of pristine P(NDI2OD-T2) formed on top of the previous.
We show, how the addition of HDPE is crucial to overcome the doping diffusion inside the active channel region and in which way OFETs characteristics are enhanced, from contact resistance subthreshold swing. Finally, we will show how this approach can be extended also to p-type organic semiconductors, acting as a versatile method to reduce the injection barrier issue in OFETs.

Microstructure in organic semiconductor thin films has been regarded as the key factor determining the performance of the organic electronic devices. In the case of organic field effect transistors (OFETs) and organic photovoltaics (OPVs), the control of the surface characteristics of underlying substrates can govern the mesoscale and/or nanoscale ordering of the semiconductor assembled on them. Here, we present the effects of molecular orientation on the performance of OFETs and OPVs in various aspects. The correlation between molecular orientation of organic semiconductor thin film and the charge carrier mobility as well as bias stress stability of an OFET is presented. Also, the orientation-dependence of photovoltaic properties is discussed in OPVs.

Organic non-volatile resistive bistable diodes based on phase-separated blends of ferroelectric and semiconducting polymers are promising candidates for non-volatile information storage for low-cost solution processable electronics.
The technology has gone through a decade of progress. 1 kb memory arrays have been demonstrated. Despite the great progress, the underlying physics has just started to be unraveled. In this contribution, we discuss the device physics of the ferroelectric:semiconductor blend memories, and show that the device functions like a vertical ferroelectric field-effect transistor.
One of the bottlenecks impeding upscaling is stability and reliable operation of the array in air. Here we present a memory arrays fabricated with an air–stable amine-based semiconducting polymer. Memory diode fabrication and full electrical characterizations were carried out in atmospheric condition (23 °C and 45% relative humidity). Device yield was 100%. The memory diodes showed on/off ratios greater than 100, and further exhibited robust and stable performance upon continuous write-read-erase-read cycles. Moreover, we show a 4-bit memory array that is free from cross-talk with a shelf-life of several months. Demonstration of the stability and reliable air operation further strengthens the feasibility of the resistance switching in ferroelectric memory diodes for low-cost applications.

Ultrathin Organic and polymer films used for organic electronics can be prepared by wet spin coating, polymer grafting, chemical and physical vapor depositions, etc. It is important to control the microstructure and nanostructure of such films not only to control adhesion but also charge carrier and hole transport. In Organic electronic including flexible displays, the preparation of such films enables a key function in controlling device performance and stability. Herewith, we report a combination of polymer surface grafting protocols produces by electrochemical crosslinking and vapor deposition methods to nanostructure the surface leading to better device performance. This can also be achieved by patterning and controlling the surface adhesion forces between the layers. The hole transport and injection layers can also be matched with the ability to prepare more efficient devices through evaporation techniques. A focus is made on using polycarbazole based hole-transport materials.

Typical devices manufactured from π-conjugated molecular materials, such as field-effect transistors and organic solar cells, are based on architectures where the material is processed to have characteristic features from the nano- to the micrometer scale. Some finite size effects during solidification may thereby occur; however, little attention has been paid so far to these and how they may affect relevant structural features of π-conjugated systems<!--[endif]---->. We will present initial data on the structure development of a model conjugated system – i.e. 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), p-DTS(FBTTh₂)₂ – using as confinement medium anodic aluminum oxide (AAO) templates consisting of hexagonal arrays of cylindrical nanopores with diameters ranging from 25 to 400 nm. Confinement leads to a notably different phase behavior compared to the bulk. We find, for example, a pronounced depression of the crystallization temperature of p-DTS(FBTTh₂)₂, indicating a change of nucleation mechanism from heterogeneous nucleation for bulk materials to a (likely) homogeneous mechanism in confinement. Moreover, we show that the temperatures of the phase transitions are dependent on the degree of confinement. Most strikingly, a confinement-induced new thermotropic mesophase is demonstrated that is absent in the bulk material. Since the p-DTS(FBTTh₂)₂ can also be textured through use of confinement effects, a new set of tools, thus, is made available to create well-defined microstructures to establish and probe important structure/property interrelationships of this class of materials.<!--![endif]---->

The emerging flexible and printed electronics industry has arrived, but there are still a number of challenges. Some examples of the challenges unique to organic semiconductors (polymers and small molecules) include poor reproducibility and process-dependent properties, an ever-increasing catalog of new materials to characterize, the richness of the material-specific phase behavior, and the complexities in these materials of local vs. global order. The intrinsic properties of these materials provide a path to disentangling the complexities as long as the properties can be measured or predicted. Here, we discuss advances made in our laboratory in characterizing the intrinsic material properties of organic semiconductors and provide examples of how to combine these properties to disentangle the complexities associated with systems based upon the organic semiconductors.

It is beginning to be widely recognized in the organic photovoltaic community that, although fullerene derivatives are remarkable electron acceptors, their total contribution to the solar cell absorption is not large enough to continue to help move power conversion efficiencies upwards. Furthermore, the fullerene electronic structure in turn constrains the donor electronic structure and shrinks the material subspace available for electron donor and acceptor permutations. Recently, non-fullerene electron acceptors have appeared that can outcompete the fullerene in performance, which is no small feat. However, to make a rational push towards power conversion efficiencies in excess of 15%, we must come to grips with the fact that to date, there exists no detailed understanding of how the chemical structure of a small molecule influences the nanoscale morphology in the thin blend film! Insofar as the phase-separated morphology drastically affects both the charge generation yield and rates of charge transport, it is critical that the connection between chemical structure and hierarchical microstructure is made. We have used a combination of charge transport measurements, resonant elastic X-ray scattering across several X-ray absorption edges, and TEM to elucidate how the molecular π-electron contours affect the nanoscale morphology. Using small molecules with varying degrees of π-connectivity and dimensionality, we have shown that the precise conjugation geometry has a large influence not just on the molecular packing on the Angstrom scale, but also on the phase separation and network formation that this packing induces. Our results have important implications for understanding the elusive link between small-molecule chemical structure and thin film morphology.

Organic photovoltaics (OPVs) have attracted major attention over the last two decades for their potential as a cheap, renewable energy technology with unique application potential unavailable to traditional inorganic technologies. Key to the development of this technology has been an understanding and control of the thin film, bulk heterojunction morphology from the molecular to mesoscale. To yield efficient devices, charge carriers must be quickly extracted from the active layer after generation, but in a competing process, oppositely charged carriers can meet and recombine. In high performance devices, the bimolecular charge recombination process represents the dominant loss mechanism and further mitigating this loss is one of the major avenues toward continued performance improvements. However, rational chemical, formulation, and processing design to both maximize the extraction rate and minimize the recombination rate has been difficult due to gaps in fundamental understanding of which factors impact each competing process.

In this talk, I will highlight recent efforts to develop an impedance-based experimental method for quantifying the charge transport and recombination behavior in real devices under operating conditions with high accuracy.[1] I will then demonstrate how this technique can be used to provide a detailed quantification of how various fabrication process parameters impact the detailed device performance characteristics. Integrating this fabrication and measurement data into a broader OPV recombination measurement database, I will demonstrate how improved data collection and availability allow the statistical analysis of competing device models and the discovery of new trends. With increased adoption of this technique and data sharing, machine learning techniques could produce improved models that capture the complex processing-structure-property relationships and accelerate the discovery and development of new materials for OPVs.

[1] M. C. Heiber, T. Okubo, B. Luginbuhl, and T.-Q. Nguyen, “Measuring the Competition Between Charge Extraction and Recombination in Organic Solar Cells under Operating Conditions” (In Preparation)

There remain significant challenges in achieving stable, high performance organic semiconductor devices from scalable manufacturing methods. We address this using synchrotron-based X-ray scattering and a variety of optical methods to observe the structure of films in-situ as they dry. I will review our studies of systems including organic photovoltaics (OPVs) which can achieve nearly 10 % power conversion efficiency (PCE), and organic thin film transistors (OTFTs) that can achieve high (>1 cm²/Vs) hole or electron mobility. I will emphasize commonalities and differences among systems, and attempt to classify the processing behavior of systems based on traits such as diffraction strength (~crystallinity). There is a surprising diversity of solidification mechanisms among organic semiconductor systems. Some are dominated by the nanoscale crystallization of the components, whereas others appear to be dominated by liquid-liquid phase separation. I will also touch on the role of additive ink formulations. Examples of good performance can be found for systems that have many different types of final morphologies that are produced by different solidification mechanisms.

The diversity of morphologies that can produce high performance in organic semiconductors suggests that there is no single morphological trait that can be depended on to predict performance. New measurements may be needed that can probe the nanoscale distribution of molecular orientation. I will describe our further development of resonant soft X-ray scattering, which combines principles of spectroscopy, small-angle scattering, real-space imaging, and molecular simulation to produce a molecular scale structure measurement that probes these previously inaccessible aspects of organic semiconductor morphology.

Organic semiconducting polymers have garnered interest for their numerous applications in lightweight, flexible and solution-processable devices including organic field effect transistors (OFETs), organic light-emitting diodes, organic photovoltaics, biomedical devices and sensors. In addition to tuning the chemical structure of the materials, the role of morphology has been identified as a key parameter in determining device performance. Use of conjugated polymers in optoelectronic devices requires thin films to be cast from appropriate solutions. The solubility of the polymer is, however, restricted by the strong π-π interaction and the large chain rigidity that greatly lowers the entropy of mixing. Conjugated polymers are seldom molecularly dispersed in solution even through attaching flexible short side chains to the conjugated backbone. Abundant evidence have demonstrated that the conjugated segments tend to form submicrometer aggregate domains in the solutions. The internal structure of these aggregates and the nature of the interaction leading to the aggregation has however not been addressed unequivocally. Furthermore, detailed correlations between molecular properties, solution aggregation structures, and the ultimate film performances of organic semiconductors are still largely unknown. Our study indicated that polymer chain conformation (in solution, during film formation and post treatment by solvent annealing) is important in determining the final film morphology. First, the photophysics and solution aggregation behavior of different conjugated polymers, which strongly depends on polymer molecular weight, conjugated length, solvent quality and temperature for a specific polymer system are investigated. Moreover, their determinant effects for the final morphology in the cast films will be discussed in detail.

Pentacene is one of the most extensively studied semiconducting materials for organic thin-film transistors (OTFTs) and has been still a benchmark material in this research field. Understanding the bottlenecks of carrier transport in pentacene OTFT is therefore important to maximize the performance of OTFTs using not only this benchmark material but also any semiconducting small molecules. In this presentation, the reality of the carrier transport band and traps in practical polycrystalline organic thin films is explained by making pentacene into a representative case [1]. The major topics included are as follows: grain morphology and crystal structure of pentacene thin films, energy barriers at grain boundaries, potential fluctuation in a grain, equations to express the overall carrier mobility in polycrystalline films, carrier traps at organic/insulator interface, and influence of surface chemical modification on the electronic structures.
[1] R. Matsubara et al.: Chapter 10, Mobility limiting factors in practical polycrystalline organic thin films in Electronic Processes in Organic Electronics: Bridging Nanostructure, Electronic States and Device Properties, ed. H. Ishii et al., pp. 185–225, Springer, 2014.

Cost-effective production of flexible organic memory devices requires ambient solution casting of ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). However, condensation of water vapor from the environment into the drying fluid film causes phase separation, which yields a fluid state morphology comprising polymer-rich micro-droplets in a solvent–rich medium. As a consequence, the dry polymer films are rough, which leads to a low production yield of working devices. Whereas this process of vapor-induced phase separation (VIPS) is used advantageously in the production of microporous polymer membranes, it is hence to be avoided in the manufacture of thin-film organic memory devices. Through microscopic analysis, device characterization and numerical calculations we identify a processing window for smooth films, determined by the solvent’s evaporation rate and hygroscopicity, as well as the ambient humidity. Modeling of the multi-component out-of-equilibrium phase dynamics demonstrates how microstructure and feature sizes emerge during simultaneous solvent evaporation and water condensation. The numerical simulations yield morphologies consistent with experimentally observed structures and demonstrate how domain size and phase composition depend on the relative humidity of the environment. Interestingly, calculations and experiments seem to support a scenario wherein the dominant feature size in the dry polymer film is to a large extent already determined in the early stages of demixing.

Highly crystalline thin films of organic semiconductors offer great potential for the realization of high-performance, low-cost flexible electronics. These films are commonly fabricated using meniscus guided coating techniques such as solution shearing, dip coating or zone casting. In order to increase the film crystallinity, and thereby its carrier mobility, process optimization is usually carried through trial-and-error approaches to determine the right combination of semiconductor, solvent, temperature, surface treatment, coating speed and annealing treatment. Only few reports present systematic parameter studies that convey clear understanding and provide guidelines for future experiments.

Here, we present a systematic study over the impact of three important process parameters on the zone-casting of highly crystalline thin films. First, we explore the influence of ten different substrate surface treatments on thin film morphology and electrical characteristics. These surface treatments result in largely different surface energies and therefore largely different wetting envelopes. Nevertheless, we see that, as long as the solvent is fully wetting the substrate, the surface treatment only has a negligible influence on the morphology of the coated layers, but can still have a huge impact on the electrical characteristics. From this study, guidelines for choosing the right substrate-solvent combination can be extracted. Furthermore, we show how choosing the right solute concentration can result in larger processing windows and therefore higher coating speeds. Optimized coating speeds show similar electrical performances, while the drop in performance at higher speeds is less pronounced for solutions with higher solute concentrations. Finally, we also show the impact low temperature annealing has on the OTFT performance, resulting in a significant reduction of the threshold voltage. We attribute this to a beneficial reorganization of the organic semiconductor underneath the source and drain contact.

Combining these studies with our recently developed lateral homo-epitaxial fabrication technique, we demonstrate bottom-gate top contact thin film transistors based on highly crystalline C₈-BTBT films with mobilities above 10 cm²/Vs and parameter spreads below 10% over 400 devices. These films behave almost twice as good as regularly evaporated poly-crystalline thin films, showing the potential of highly crystalline thin film transistors for large area electronics.

Recent advances in the field of organic optoelectronics have been driven primarily by the development of advanced semiconductors and an improved understanding of the structure-processing-property relationships. In this talk, I will present an alternative approach for the manufacturing of a plethora of organic optoelectronic devices using a simple and highly scalable manufacturing technique namely adhesion-lithography (a-Lith) – an innovative method that can be used to pattern symmetric or asymmetric coplanar nanogap electrodes with arbitrarily large aspect ratio. I will show how a-Lith can be combined with organic functional materials to produce a plethora of functional devices including; self-aligned gate transistors, radio frequency diodes and energy harvesting circuits, multi-colour organic light-emitting nanodiodes, photodetectors and multilevel non-volatile memory devices, to name but a few. Despite the early stage of development a-Lith can already be viewed as promising alternative to conventional nano-patterning techniques for application in large-volume nanoscale optoelectronics.

Molecular assembly, crystallization and controlled phase transition have played a central role in the development of modern electronics and energy materials. Recent years, printed electronics based on semiconducting molecular systems have emerged as a new technology platform that promise to revolutionize the electronics and clean energy industry. In contrast to traditional electronic manufacturing that requires high temperature and high vacuum, these new electronic materials can be solution printed at near ambient conditions to produce flexible, light-weight, biointegrated forms at low-cost and high-throughput. However, it remains a central challenge to control the morphology of semiconducting molecular systems across length scales. The significance of this challenge lies in the order of magnitude modulations in device performance by morphology parameters across all length scales.

This challenge arises from the fact that directed assembly approaches designed for conventional hard materials are far less effective for soft matters that exhibit high conformational complexity and weak, non-specific intermolecular interactions. On the other hand, biological systems have evolved to assemble complex molecular structures highly efficiently. We are eager to transfer the wisdom of living systems to developing printed electronics technologies as to enable next generation electronics for clean energy and healthcare. In this talk, we present new insights and strategies we recently developed for controlling multi-scale assembly and transformation of semiconducting molecules. We learned from living systems and designed bioinspired assembly processes, allowing molecules to put themselves together cooperatively into highly ordered structures otherwise not possible with significantly improved electronic properties. We discovered molecular design rules that impart dynamic and switchable electronic properties through the mechanism of molecular cooperativity – a mechanism ubiquitous in nature. These new solid-state properties could potentially enable new sensing and actuation mechanisms not possible before.

Tailoring crystal polymorph and co-assembly into bicomponent or multicomponent solids are two important strategies of crystal engineering to improve the optoelectronic properties of organic semiconductors, which is highly challenging due to the weak nondirectional intermolecular interactions in organic solids.
Herein, we report the controllable growth of different crystal phases of organic semiconductors, i.e. pentacene¹, thienoacene (BDTDT)² and phthalocyanine derivatives (TiOPc)³ through polymorph induction of surface nanogrooves, solution supersaturation or vapor transport temperature gradient. The surface nanogrooves is highly important to induce the growth of orthorhombic phase pentacene films with extremely high mobility up to 30.6 cm² V^-1 s^-1. The big discrepancies of charge transport in two different crystal polymorphs for BDTDT (8.5 vs. 18.9 cm² V^-1 s^-1) and TiOPc (0.1 vs. 26.8 cm² V^-1 s^-1) were observed clearly. In the case of BDTDT, we recognized the importance of electronic couplings of (HOMO-1) to charge transport behaviour, which is generally ignored to account for the carrier mobility. In the case of TiOPc, we demonstrated experimentally that inter-layer electronic couplings may result in a drastic decrease of charge mobilities utilizing field-effect transistors if the coupling direction perpendicular to the current direction.
Furthermore, a solid solution of two porphyrin derivatives was achieved by in-situ heating their soluble precusors mixture.⁴ Organic solar cells based on the solid solution was demonstrated for the first time, resulting in a power conversion efficiency value much higher than the devices using the single component. Last but not the least, we demonstrate the photocurrent generation of molecular heterojunction co-crystals. Theoretical calculations reveals the different charge recombination degree of these two co-crystals, indicating that molecualr stacking plays an important role in the photovoltaic effects. This work opens an avenue to develop molecular scale p-n junction cocrystals as a promising donor-acceptor contacting mode for application in organic solar cells.⁵
1. Ji, D.; Xu, X.; Jiang, L.; Amirjalayer, S.; Jiang, L.; Zhen, Y.*; Zou, Y.; Yao, Y.; Dong, H.; Yu, J.; Fuchs, H.; Hu, W., J. Am. Chem. Soc. 2017, 139, 2734.
2. He, P.; Tu, Z.; Zhao, G.; Zhen, Y.*; Geng, H.; Yi, Y.*; Wang, Z.; Zhang, H.; Xu, C.; Liu, J.; Lu, X.; Fu, X.; Zhao, Q.; Zhang, X.; Ji, D.; Jiang, L.; Dong, H.; Hu, W.*, Adv. Mater. 2015, 27, 825.
3. Zhang, Z.; Jiang, L.; Cheng, C.; Zhen, Y.*; Zhao, G.; Geng, H.; Yi, Y.*; Li, L.; Dong, H.; Shuai, Z.; Hu, W.*, Angew. Chem. Int. Ed. 2016, 55, 5206.
4. Zhen, Y.; Tanaka, H.*; Harano, K.; Okada, S.; Matsuo, Y.*; Nakamura, E.*, J. Am. Chem. Soc. 2015, 137, 2247.
5. Zhang, H.; Jiang, L.; Zhen, Y.*; Zhang, J.; Han, G.; Zhang, X.; Fu, X.; Yi, Y.*; Xu, W.*; Dong, H.; Chen, W.; Hu, W.*; Zhu, D., Adv. Electron. Mater. 2016, 2, 1500423.

Molecular orientation of organic semiconductors is crucial for its performance of their device applications such as OFET. Many orientation processes including wet ones have been investigated and the oriented growth on an alignment layer is one of the most convenient method as it has been utilized in manufacturing liquid crystal displays for a long time. The first author has been fascinated by an poly(tetrafluoroethylene)（PTFE）alignment layer since he saw a letter by Wittmann and Smith in 1991.¹ Although the unique non-sticking and poor wetting properties of PTFE is rather undesirable for applications, its two remarkable features are quite important: one is a wide variety of the materials oriented on it and the other is some cases (linear conjugated molecules such as bisazo dyes^2,3, a bisethyne³, or a bisazomethine⁴) where an extremely high degree of orientation (exceeding 0.9 of an uniaxial order) can be achieved. Hence, the purpose of this presentation is to show its unique orientation mechanism we found, thus being a theoretical model for designing a novel alignment layer for organic electronics.
We have been investigating the mechanism by a molecular dynamics (MD) simulation and have just concluded that a negatively charged shallow “atomic groove” between two adjacent PTFE chains orients linear molecules by trapping them along the the groove.³ It is formed by the weak helix of the chain and its charge is due to fluorine atoms, thus the mechanism being quite unique as the other properties of PTFE. The dynamics simulates adsorption of each single molecule on a surface consisting of eight C₃₂F₆₆ chains uniaxially aligned in a plane. The orientational order was estimated from the dichroism in their polarized absorption spectra and correlate well with those estimated from the calculations. This atomic groove effect was first discovered in 2002⁵ and furthermore some modification of the F charge recently demonstrated the significant contribution of its negative charge. Recent progress in computing ability enabled us to simulate the difference in the orientation by tiny changes in a molecule end. Such accuracy ensures the validation of the conclusion. If you can design an alignment layer showing such negatively charged atomic grooves on its surface, it will have the two features of aligned PTFE.
This work is supported by JSPS-KAKENHI(JP17K4996).
[1] J. C. Wittmann, P.Smith., Nature, 352, 414 (1991); [2] T. Tanaka, et al., Langmuir, 17, 2197 (2001); [3] T. Tanaka, M. Ishitobi, Chem. Lett., accepted (2017); [4] T. Tanaka, et al., Chem. Lett., 40, 1170(2011); [5] T. Tanaka, M. Ishitobi, J. Phys. Chem. B, 106, 564 (2002).

The weak intermolecular forces inherent to molecular crystals give rise to polymorphism, or the ability for these materials to adopt multiple solid-state packing arrangements. Differences in molecular packing can result in distinct materials properties of the same compound (e.g., diamond vs. graphite), making polymorphism of general interest to the materials community. Yet, our collective understanding of this phenomenon remains inadequate. Calculations can shed light on the energetic landscape across different polymorphs but predictions for which phases are preferentially accessed remain challenging because accessibility is often dictated by kinetics (i.e., processing), as opposed to thermodynamics (i.e., energetics). In this talk, I will highlight a framework we have developed that directly correlates the presence of short intermolecular contacts with polymorphic accessibility. Subtle changes in the molecular chemistry do not significantly affect the possible solid-state packing arrangements, but do result in differences in the non-covalent intermolecular interactions that ultimately lead to preferential access to specific polymorphs. Starting with a series of core-chlorinated naphthalene diimide derivatives, we show this framework to be widely applicable to compounds ranging from molecular semiconductors to pharmaceutics, such as rotinavir, and biological building blocks, such as guanine. In this era of machine learning and materials by design, this framework can identify which polymorphs, among those available, are practically accessible and stable, thereby extending the predictive power of the solid-state properties of molecular materials prior to synthesis.

Single-molecule spectroscopy has provided unprecedented access to underlying structure-function relations of highly disordered conjugated polymers such as P3HT [1], but is often considered of only limited utility in illuminating molecular characteristics in the solid state. This perception has changed with the introduction of single-molecule solvent-vapor annealing techniques, which allow the deterministic growth of multichain aggregates – mesoscopic nanoparticles – to mimic bulk material characteristics while retaining the low degree of heterogeneity characteristic of a sub-ensemble experiment. Such single aggregates reveal that exciton generation in bulk films is an inherently intermolecular process, involving scores of polymer chains because of the prevalent process of singlet-singlet annihilation. As a consequence, even large aggregates containing many polymer chains exhibit deterministic single-photon emission in the form of photon antibunching [2]. A corollary of this effect is strong triplet-polaron quenching in large multichain aggregates, which leads to the disappearance of triplet-exciton photon correlation signatures in particles grown by controlled solvent-vapor annealing [3].

Most recently, it has become possible to directly image the growth and arrangement of multichain aggregates during annealing. Annealing promotes chain ordering, which strengthens the intrinsic J-aggregate character of polymer chains. Once the solvent molecules are removed by drying, cofacial stacking of these carefully ordered chains is encouraged, which gives rise to pronounced H-aggregate spectral characteristics. This drying process, on the level of single nanoparticles, is highly reversible and can be repeated many times to show a controlled switching between J- and H-type characteristics. Single-particle annealing therefore offers a unique window to monitoring the dynamics of solvent removal from polymer films, in real time, during processing [4].

[1] Thiessen, Vogelsang, Lupton et al., PNAS 110, 3550 (2013)
[2] Stangl, Vogelsang, Lupton et al., PNAS 112, 5560 (2015)
[3] Steiner, Lupton, Vogelsang, JACS 139, 9787 (2017)
[4] Eder, Vogelsang, Lupton et al., Nature Comm. 10.1038/s41467-017-01773-0 (2017)

Due to their ease of processability and mechanical flexibility, conjugated polymers are attractive materials for organic electronics. Though substantial progress has been made in enhancing their charge transport properties, clear design guidelines are still difficult to establish. A fundamental reason resides in the intimate connection between charge transport and morphology [1]. Morphologies of conjugated polymers are complex and diverse, and thus hard to predict and characterize. In addition, identifying the structural features that are most crucial for charge transport is not trivial. One interesting recent observation is that, to achieve high mobilities, perfect lamellar order is not necessary [2]. Morphologies with only partial lamellar order (“lamellar-like”) can either originate from the intrinsic properties of the polymer, as exemplified by the microstructures of some aggregating materials [2] and by the smectic mesophases of liquid-crystalline polymers [3], or can be induced by processing, e.g. with chain aligning techniques [4].
Here we present a simple model that enables the study of morphologies with lamellar-like order, at device-relevant length scales. Nonbonded interactions responsible for biaxiality of chain orientation and for stacking are described by anisotropic soft potentials, constructed in a top-down manner on the basis of symmetry considerations. Bonded interactions are introduced bottom-up, to capture the conformational characteristics of the polymer under study. Considering polyalkylthiophenes as a test system, we perform Monte Carlo simulations of chains of various lengths, starting from different initial configurations. Lamellar-like morphologies are obtained, either as mono- or polydomains. The type of lamellar-like order realized is characterized by computing 2D scattering patterns, which can be easily compared with experimental GIWAXS data. We conclude that the obtained morphologies correspond to a lamellar-like smectic mesophase, an organization that was also reported in experiments [3]. We analyze conformational properties, quantifying in a simple way the length of conjugated segments and identifying connectivity pathways between the lamellae. In perspective, atomistic detail can be reintroduced via backmapping, allowing for prediction of charge transport [5].
[1] S. Himmelberger, A. Salleo, MRS Commun. 2015, 5, 383.
[2] R. Noriega, J. Rivnay, K. Vandewal, F. P. V. Koch, N. Stingelin, P. Smith, M. F. Toney, A. Salleo, Nat. Mater. 2013, 12, 1038.
[3] Z. Wu, A. Petzold, T. Henze, T. Thurn-Albrecht, R. H. Lohwasser, M. Sommer, M. Thelakkat, Macromolecules 2010, 43, 4646.
[4] L. Hartmann, K. Tremel, S. Uttiya, E. Crossland, S. Ludwigs, N. Kayunkid, C. Vergnat, M. Brinkmann, Adv. Funct. Mater. 2011, 21, 4047.
[5] P. Gemünden, C. Poelking, K. Kremer, K. Daoulas, D. Andrienko, Macromol. Rapid Commun. 2015, 36, 1047.

Solution-based processing of materials for electrical doping of organic semiconductor interfaces is attractive for boosting the efficiency of organic electronic devices with multilayer structures. To simplify this process, self-doping perylene diimide (PDI)-based ionene polymers were synthesized, combining semiconductor PDI components with electrolyte dopants embedded within the polymer backbone. Functionality contained within the PDI monomers suppressed their aggregation, affording self-doping interlayers with controllable thickness when processed from solution into organic photovoltaic devices (OPVs). Optimal results for interfacial self-doping led to increased power conversion efficiencies (PCEs) of the fullerene-based OPVs from 2.62% to 10.64%, and of the non-fullerene-based OPVs from 3.34% to 10.59%. These ionene interlayers enable chemical and morphological control of interfacial doping and charge transport, demonstrating that effective conductive channels are crucial for charge transport in doped organic semiconductor films. Using the interlayers with efficient doping and charge transport, both fullerene- and non-fullerene-based OPVs were achieved PCEs exceeding 9% over interlayer thicknesses ranging from ~3 to ~40 nm.

Herein, we report our efforts on the development of new materials and devices for high performance thick film polymer solar cells (PSCs), which are more compatible for roll-to-roll printing process. A series of high mobility electron donor materials and cathode interfacial materials were designed and synthesized. The resulting PSCs based on these newly developed materials exhibited promising performance with a thick interlayer and light-harvesting layer ^[1-5]. It was also found that the introduction of highly crystalline small molecule donors into ternary PSCs is an effective way to enhance the charge transport and thus increase the active layer thickness of ternary PSCs that are more suitable for roll-to-roll production than previous thinner devices ^[6]. These works may provide new solutions for preparing both active layer and interlayer for PSCs and also make them potential candidates to be employed in high performance large area PSC module device in the future.

References
[1] Y. C. Jin, Z. M. Chen, S. Dong et al. Adv. Mater. 2016, 28, 9811.
[2] Y. Jin, L. Ying, F. Huang et al. Adv. Energy. Mater. 2017, 1700944.
[3] Z. Wu, C. Sun, S. Dong et al. J. Am. Chem. Soc. 2016, 138, 2004.
[4] C. Sun, Z. Wu, Z. Hu et al. Energy Environ Sci. 2017, 10, 1784.
[5] S. Dong, K. Zhang, Z. C, Hu et al. Adv. Mater. 2017, 29, 1701507.
[6] B. Fan, K. Zhang, X. F. Jiang et al. Adv. Mater. 2017, 10, 1606396.
[7] B. Fan, L. Ying, Z. Wang et al. Energy Environ. Sci. 2017, 10, 1243.
[8] G. C. Zhang, K. Zhang, Q. W. Yin et al. J. Am. Chem. Soc. 2017, 139, 2387.

One of the biggest attractions of the organic photovoltaics (OPV) technology is the easiness with which it can be integrated. However, despite its semitransparency and wide variety of colours, a major unresolved challenge is to fabricate optically inconspicuous organic photovoltaic modules (OPV-M) that can be integrated into visually demanding products. This is dominantly due to the visual obstruction from Z-interconnection lines inherent to module processing with classical patterning methods. We now present for the first time a solution to this problem, which utilizes a visually seamless interconnection method to elegantly minimize the conspicuity of the interconnection regime. We realize such invisible interconnects by inkjet printing highly conductive silver lines or dots, which penetrate the solar cell stack and form an electrical connection between adjacent cells. Topographical and cross-sectional images of the interconnection region are taken to illustrate how the connection is formed. Photos of the modules are also provided to showcase the stark visual difference between the classical and the new approach implemented in this work. Beside the minimal power conversion efficiencies (PCE) loss when transferred from small (0.1 cm²) single cells to 3-cell solar modules (3 cm²), we also demonstrate the effective connection between cells by special measurement structures that can precisely measure the specific interconnection resistance down to 1.5 mΩcm². Finite element (FEM) simulation further rationalizes the excellent electrical cell-to-cell connection established with this interconnection technology. We combine this technology with a variable-geometry module design into an innovative digital inkjet printing platform for the production of state-of-the art but visually attractive solar modules. The full potential of this concept is demonstrated by various fully inkjet printed semitransparent OPV-M portraits and logos with area over 84cm² and efficiencies over 3.5%.

Organic photovoltaic cells (OPVs) offer a light weight and potentially cost-effective approach for solar energy harvesting. Recent studies on solution processed, non-fullerene acceptors (NFAs) has led to single junction cell efficiencies > 13%. Here we report a tandem OPV structure with a NFA-based near infrared subcell spin-coated on a visible-absorbing subcell grown by vacuum thermal evaporation (VTE). A novel charge recombination zone completely protects the VTE layers from dissolution during deposition of the NFA-based active layer. The tandem cell achieves the power conversion efficiency of 15.0%. This work demonstrates the feasibility of combining subcells with different processing techniques for achieving efficient OPVs, providing flexibility over current multijunction designs.

Indium Tin Oxide (ITO) has been widely used as transparent conductive electrode (TCE) in optoelectronics. However, it suffers from high production costs, dwindling supplies, limited chemical and mechanical stability. Therefore, seeking for an efficient alternative to ITO as TCE is of great significance. As first step we have investigated solution-processed transparent electrodes based on electrochemical exfoliated graphene (EG)^[1], characterized by remarkable properties (C/O ratio 17.9), high yield (78%) and low cost. Uniform and smooth electrodes have been obtained by spray-coating of a high-quality EG dispersion on both rigid and flexible substrates. Compared to previous reported solution processed graphene based TCEs, our EG electrodes show improved features (i.e. sheet resistance, R_s, as low as 0.18 kΩ sq^–1).^[2] However, the organic solar cells (OSCs) based on these pristine EG electrodes exhibit power conversion efficiency (PCE) typically 40% lower than their ITO analogues. Hence, in order to enhance performances of the devices, keeping a feasible approach, we have developed a mixed-dimensional structure (1D-2D) using AgNWs and EG.^[3] Compared with pristine EG these hybrid TCEs exhibit a dramatic decrease of the sheet resistance R_s from 78 to 13.7 Ω sq^-1 combined with an optical transparency of 89%. In addition the surface roughness (RMS) reduces from 16.4 to 4.6 nm and their mechanical and chemical stability is enhanced. The spray-coated hybrid AgNWs-EG TCEs have been successfully implemented in OSCs and polymer LEDs (PLEDs), which output remarkable efficiencies that are equivalent with their commercial ITO-based counterparts. To the best of our knowledge these are the first solution-processed transparent electrodes that match the performance of ITO based TCEs.^[3]

[1] S. Yang, A. G. Ricciardulli, S. Liu, R. Dong, M. R. Lohe, A. Becker, M. A. Squillaci, P. Samori, K. Mullen, X. Feng, Angew. Chem. Int. Ed. 2017, 56, 6669.
[2] A. G. Ricciardulli, S. Yang, X. Feng, P. W. M. Blom, ACS Appl. Mater. Interfaces 2017, 9, 25412.
[3] A. G. Ricciardulli, S. Yang, G. A. H. Wetzelaer, X. Feng, P. W. M. Blom, Submitted.

There has been a rising demand for more complex chips and multi-chips integration in microfluidics. However, chip-to-chip and chip-to-world interconnections remain simple, i.e. plastic tubing, due to its very different melting processing techniques, materials properties and insufficient knowledge of fiber electronics. These challenges can be addressed by applying new device structures and new sensing materials made available by multimaterial fiber processes, which have recently emerged as a material platform for a variety of sensing modalities. In this work, we present a new strategy that integrates different functional units, such as pumping or sensing units, into the multimaterial fibers as microfluidics interconnections.

Hundreds of meters of uniform fibers with cross-sections of 2x1 mm were produced through a thermal drawing processing. The basic structure of fiber consists a 1x0.5 mm hollow core as a fluid channel inside of polycarbonate (PC) cladding. Two conductive polymer films made of carbon black doped polyethylene (CPE, 25 um) are placed adjacent to the hollow core. CPE films are electrically insulated from the fluid channel by a thin dielectric film of polycarbonate (5 um), and can be electrically connected on the fiber surfaces with silver paint.

Multiple microfluidics-related functions, such as flowrate sensor and fluid pump, have been demonstrated with this specific fiber design. Here we specifically focus on demonstrating a hybrid in-fiber multi-segment thermal flow sensor design that enables extended measurement range at high flow-rate sensitivity and low flow resistance. We start with a first-order heat-transfer analysis to establish the analytical temperature response of a long hot film in a fiber microfluidic channel. This model predicts a limited range of linear flow-rate response for a single-segment sensor, which is confirmed by measurement. We then explore multi-segment structure sensors with a significantly extended operational range in both numerical simulations and experiments.

We report integrated multi-segment fiber flow rate sensors that combines high sensitivity (0.38 V/(uLmin-1)), large dynamic range (0-200 uL/min) and small pressure drop (8 Pa at 100 uL/min), in a 1mm x 0.5mm fluidic channel embedded in a multi-material fiber. Depends on the flow rate region of interest, one or multiple appreciate segments can be measured for the best performance, which enables us to extend measurement range within a single device. Comparing to widely used metal films, new sensing materials CPE provides 20 times higher TCR (0.09 K-1) and 10⁷ times higher resistivity (0.2 Ωm), allowing us to obtain low pressure drop while maintaining high sensitivity. High sensitivity, low pressure drop and wide measurement range together qualify our fiber sensors as very promising functional microfluidics interconnections. Our work opens up many new opportunities for microfluidics research and is extremely suitable for biomedical applications.

Wearable electronics are essential for continuous monitoring of physiological.However, commercial wearable devices such as smart watches, bracelets and glasses, are poorly skin-mountable and bulky since their sensors are fabricated by rigid inorganic semiconductor materials. Thus, there exist obvious motion-relative disturbances on sensing information obtained from these sensors. One practicable solution is to adopt flexible electronics especially epidermal electronics that can be ultrathin, lightweight and stretchable. Although great progress has been made on epidermal electronics, it is evident that these availble epidermal electronics still have drawbacks such as high power consumption, sophisticated process and high cost To address these issues, solution-processable electrolyte-gated organic transistors which have low-voltage, easy-process and high sensitivity, are developed as promising wearable electronics devices. Here, we develop flexible and highly photodetectors based on electrolyte-gated organic transistors with ionogel/silver nanowire membranes. Random network of silver nanowire (AgNW), which is used as a transparent and flexible electrode, exhibits high conductivityand excellent mechanical flexibility. The ionogel/AgNW nanocomposite membrane, in which AgNW was directly embedded into an ionogel-type gate dielectric layer, shows a large capacitance (~2µF/cm²),low sheet resistance and ultra-high optical transparency (T > 90%). In the electrolyte-gated organic transistor device, we chose a heterojunction active layer formed by blending a high hole mobility and narrow-bandgap polymer semiconductor (PSC) with an inorganic quantum dots (QDs) and silver nanoparticles(AgNPs) . Thanks to the large mobility difference between PSC and QDs as well as large carrier trapping effects of AgNPs, the device exhibits high responsivity of 7.5×10⁵ AW^-1 and ultrahigh sensitivity (~7.5×10⁵). Transient photocurrent response at different light intensitiesreveals the device has shorter photoresponse time at lower light intensity, contrary to that of traditional phototransistors with oxide gate dielectrics. We suggest this difference is originated from the existing strong interactions between photogenerated hole carriers and anions that can induce extra long-lived trap states. It is noting that our device presents a large 3dB bandwidth of ~100 Hz. Therefore, our device is capable of detecting low-frequency pulse signals.To evaluate the utility for flexible electronic applications, we carried out bend-radius dependent measurements. It is noteworthy that the sensitivity remains in the level of 10⁵ when bending device to a radius as small as 2 mm. Owing to fast and air-stable operations, we believe the device can be taken as a very promising photosensing module for constructing smart wearable devices in the future.

Solution processed organic bulk heterojunctions (BHJs) are promising for enabling low-cost, green, and flexible photovoltaics. The BHJ architecture, with its nanoscale interpenetrating donor-acceptor structure, allows for ample light absorption in a thin (hundreds of nanometer thick) active layer, while satisfying the need for a high density of donor-acceptor interfaces for exciton dissociation. Nevertheless, the highly heterogeneous nanostructured active layer, with its locally varying composition, crystallinity, and miscibility between components, leads to a complex landscape for charge transport. Little is known about the impacts of local BHJ structure on charge percolation across the active layer (out-of-plane).
In this presentation, we will delineate key structural features that impact out-of-plane charge transport in small molecule:fullerene BHJ active layers comprising p-DTS(FBTTh₂)₂:PC₇₁BM. A series of 15 BHJ films with varying compositions and degrees of phase separation were characterized electrically by conductive atomic force microscopy (C-AFM) and structurally by grazing incidence x-ray diffraction (GIXD). C-AFM was used to quantify and map local hole mobility, while GIXD provided the crystallite size and population along π-π and alkyl stacking directions. These experiments show that the strongest predictor of out-of-plane charge carrier mobility across all morphologies is the in-plane π-π crystallinity within a film. This result is further supported by nanoscale hole mobility maps that show a halo of moderate hole mobility regions concentrated around high hole mobility hot spots, indicating lateral access of charge from the surrounding regions to the hot spots. Additional insight into the dependence of hole mobility on active layer composition and phase separation is provided by percolation theory. This analysis provides clues about the differences in domain connectivity that affect hole transport efficiency when the degree of phase separation is increased.

Printed electronics based on solution processable semiconducting polymers has emerged as a burgeoning technology that promises to revolutionize electronics manufacturing from transistors, sensors, solar cells, light-emitting diodes (LEDs), to medical devices. Unlike traditional electronic manufacturing that requires elevated temperature and high vacuum, organic electronics can be printed at near ambient conditions on flexible substrates to produce light-weighted, biointegrated electronics at low cost and large scale. It is well-known that substrate surface properties have a profound impact on morphology of thin films solution coated atop and the resulting solid-state properties. Therefore, it is important to establish general design rules to better understand surface-induced crystallization and develop material-agnostic methods for controlling morphology across multiple length scales at once during coating, not only to enable large-scale manufacturing of high-performance devices, but also for elucidating charge transport mechanism in conjugated polymers.

We have demonstrated that by modulating the free energy barrier to heterogeneous nucleation multiscale morphology of donor-accepter (D-A) conjugated polymers can be controlled during meniscus-guided solution coating [1, 2]. Lower nucleation free energy is associated with faster polymer crystallization addressing the disparity in time scales of polymer assembly and high-throughput coating. By conducting in-depth morphology characterizations, we concluded that surfaces with lower free energy barrier would increase thin film crystallinity, degree of molecular ordering and extent of domain alignment in synergy with unidirectional-flow. Notably, the enhanced morphology led to a significant increase in the charge carrier mobility in organic field-effect transistors along the polymer backbone as well as the pi-pi stacking direction. We further developed a free energy model, for the systems following classical nucleation theory, relating the substrate surface energy to the penalty of heterogeneous nucleation from solution in the thin film geometry. The model correctly predicts the experimental trend. The introduced generic model introduced is a significant step towards establishing design rules and understanding the critical role of substrates in determining morphology of solution coated thin films. Our methodology and mechanistic understanding have broad implications, given the importance of surface-induced crystallization across many disciplines.

References:
1- Zhang, F.*; Mohammadi, E.*; Luo, X.; Strzalka, J.; Mei, J.; Diao, Y. Lanmguir 2017, 8, 16070.
2- Mohammadi, E.; Zhao, C.; Zhang, F.; Qu, G.; Meng, Y.; Zhao, X.; Mei, J.; Zuo, J.; Shukla, D.; Diao, Y. Nat. Comm. 2017, 8, 16070.

Interpenetrating bulk-heterojunction structure with domain size of 10-20 nm is the ideal morphology for carrier generation, separation and transportation in organic solar cells. However, film morphology of p-DTS(FBTTh₂)₂/EP-PDI blend systems is always far from satisfactory for nano-structure of optimal devices. When the weight ratio of p-DTS(FBTTh₂)₂/EP-PDI is greater than 8:2 or smaller than 4:6, large phase separation structure is observed induced by p-DTS(FBTTh₂)₂ or EP-PDI crystallization. When the ratio of p-DTS(FBTTh₂)₂/EP-PDI changes from 7:3 to 5:5, no obvious phase separation can be detected. In order to obtained the interpenetrating structure with domain size of 10-20 nm, we tuned the molecule diffusion of p-DTS(FBTTh₂)₂ and EP-PDI by thermal annealing. When the annealing temperature is T < T_{m EP-PDI} + T_a or T > T_{c p-DTS(FBTTh2)2} - T_b (T is thermal annealing temperature, T_a and T_b are constants), molecule diffusion rate of both p-DTS(FBTTh₂)₂ and EP-PDI are too slow or too fast, resulting in no phase separation or large phase separation morphology. When T is between T_{m EP-PDI} + T_a and T_{c p-DTS(FBTTh2)2} - T_b, p-DTS(FBTTh₂)₂ crystallized and formed framework. Thus p-DTS(FBTTh₂)₂ crystallization inhibited massive crystallization of EP-PDI, leading to the formation of bi-continuous phase separation structure. As a result, we constructed the phase diagram according to different blend ratio and thermal annealing temperature of the p-DTS(FBTTh₂)₂/EP-PDI crystalline-crystalline small molecule blend system. Furthermore, we revealed the phase separation mechanism and summarized guideline to construct bi-continuous phase separation structure with proper domain size and phase purity. Based on the bi-continuous phase separation structure we got, high performance of 4.25% power conversion efficiency was obtained.

Knowledge of the thermal conductivity (k) of new materials is essential for their application in both electronic (high k desired) and thermoelectric (low k desired) devices. We have developed a simplified ac-photothermal apparatus [1] for measurement of the transverse (i.e. through-plane for partly aligned polymers and interlayer for layered crystals) thermal diffusivity, D = k/c, where c is the specific heat per volume, of small samples. Our technique is essentially the Fourier transform of the laser flash method. The sample, with a typical area of 5 mm^2 and heated on its front side with chopped light, is placed in the dewar of an MCT detector close to the detector, which measures the thermal radiation from the back of the sample. For optically opaque samples, an analysis of the complex frequency dependence of the detector signal gives the transverse diffusivity; results will be presented for free-standing PEDOT:PSS films and samples of cellulose nanofibrils coated with PEDOT:PSS (samples provided by X. Crispin, Linkoping U.) For samples which are not opaque, the same analysis, overlooking the finite optical absorption length, can lead to a very large overestimate of the diffusivity. On the other hand, including the effect of finite absorption in the analyses can make their results less definitive. Here we show how the technique and analysis can be adapted for less absorbing samples and present our results for TIPS-pentacene (provided by J. Anthony, U. Kentucky), correcting the value we previously presented [1]. Research supported by NSF Grant # DMR-1262261

[1] J. W. Brill, et al, J. Appl. Phys. 118, 235501 (2015)

The rapid development of the information technology industry has led to the ever-increasing demand for mass data storage that has driven research towards high-capacity memory devices. Multi-level memory (MLM) enables high-density data storage per unit area without reducing the size of memory cells, hence overcoming downscaling limitations of lithography technology as well as reducing manufacturing costs. Owing to their multi-bit storage capability, non-destructive read-out, low cost, and good compatibility with flexible substrates, non-volatile organic field-effect transistor (OFET) memory has been regarded as a highly-promising candidate for applications in next-generation, large-area printed electronics.^[1] To date, considerable efforts have been devoted to improving the charge-trapping capacity of OFET memory devices by developing functional gate dielectrics. In fact, the ability to tune the charge-trapping property within the organic semiconductor layer could also enable OFETs with memory functionality but currently has received far less attention from the research community.

In this work, non-volatile OFET memory devices based on pentacene/N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13)/pentacene trilayer organic heterostructures are demonstrated.^[2] The judiciously-controlled discontinuous n-type P13 film embedded into p-type pentacene layers can not only provide electrons in the semiconductor layer that facilitates electron trapping; it also works as charge trapping sites, which is attributed to the quantum well-like pentacene/P13/pentacene organic heterostructures. The synergistic effects of charge trapping in the discontinuous P13 film together with the gate-dielectric-modification charge-trapping layer, composed of poly(4-vinylphenol), can drastically improve the memory performance. The devices exhibit excellent non-volatile memory characteristics with stable data endurance characteristics of 3,000 programming/erasing cycles and long data retention time (extrapolated to >10 years). MLM characteristics with four stable data storage states are achieved due to the high charge capacity as well as the large memory window. Moreover, the trilayer organic heterostructures are also successfully applied to flexible non-volatile memory devices that remain excellent memory performance even after 10,000 bending cycles (bending radius = 10 mm), demonstrating their extraordinary mechanical properties. This study introduces novel organic heterostructures for the fabrication of high-capacity OFET memory devices for applications in next-generation printed electronics. More importantly, the concept of the quantum well-like organic heterostructures provides general guidelines to construct high-performance multi-bit OFET memory from the large library of organic materials to form similar energetically-favourable hetero-systems.

References
[1] Y.-H. Chou, et al., Polym. Chem., 2015, 6, 341-352.
[2] W. Li, et al., Adv. Sci., 2017, 4, 1700007.

Organic heterojunctions with high mobility are typically desired in order to achieve high performance devices such as complementary circuits, organic light emitting diodes (OLEDs) and organic solar cells. Thus, organic single crystals are ideal candidates for the junctions as they have the potential to exhibit the superior charge transport properties among organic materials with ordered molecular packing compared with their amorphous or polycrystalline counterparts. Despite the great efforts that have been made to fabricate organic single crystalline heterojunctions, there are still difficulties in fabricating organic single crystalline heterojunctions via a convenient solution method. Here, we demonstrate a facile one-step solution method to grow several organic single crystalline heterojunctions from a droplet of mixed solutions containing two types of organic semiconductor molecules. One example of the obtained heterojunctions demonstrates multiple properties like charge transport as well as charge recombination and electronic devices based on the obtained heterojunction have been made as light-emitting field-effect transistors (LEFETs).

In recent years, polymeric materials have attracted vast interest in the field of organic electronics, photonics and bioelectronics. One widespread use of π-conjugated polymers has been seen in the area of organic photovoltaics (OPVs). This is because of the promise this interesting class of ‘plastics’ offer for good processability and mechanical flexibility, and their potential to open pathways to print or coat devices onto any surface, including large-area structures. However, due to the low dielectric constant of most polymers (∼2 to ∼4), dissociation of coulombically-bound, photo-generated excitons into fully separated charges is challenging, resulting in high recombination losses and low device efficiencies [1]. We demonstrate here that blending of polymers may allow us to combine desired properties of individual components to enhance exciton generation, charge separation and transport, without the need to introduce all functionalities in a single material. We present a strategy to provide pathways that should assist to manipulate the dielectric constant of OPV blends by introducing poly(vinylene fluoride) (PVDF) [2] into donor:acceptor blends using solution deposition methods. We show that PVDF affects the phase morphology and other relevant microstructural features of the donor:acceptor system. For this purpose, we use binary (donor polymer:PVDF) and ternary (donor:acceptor:PVDF) phase diagrams constructed from differential scanning calorimetry (DSC) and visualized by vapor phase infiltration (VPI) [3] and scanning electron microscopy (SEM) techniques. We correlate photo-physical processes and device characteristics with these ternary blend microstructures, and demonstrate that by controlling device phase morphology and its dielectric properties, we gain an additional tool to manipulate device functions.

[1] Y. Tamai et al., J. Phys. Chem. Lett. 2015, 6, 3417−3428
[2] K. Asadi et al., Appl. Phys. Lett. 2011, 98, 183301
[3] S. Obuchovsky et al., Solar Energy Materials & Solar Cells. 2015, 280–283

Thin film transistors (TFTs), especially organic TFTs (OTFTs), play a key role in enabling the next-generation flexible electronics. Nevertheless, challenges remain on developing TFTs with low power consumption. A common solution to reduce power consumption is to reduce operation voltage by increasing the capacitance of a gate insulator material, because capacitance is inversely related to the voltage. To this end, ionic liquid gel (ion-gel) based solid-state electrolytes are promising replacement for traditional inorganic or organic high-k dielectric materials, because they have much higher capacitance than that of traditional dielectric materials (e.g., 1-10 µF cm^-2 versus 0.1 µF cm^-2). The high capacitance of ion-gel originates from the formation of electrical double layers (EDLs) at gate/ion-gel and ion-gel/semiconductor interfaces. However, the capacitance of an ion-gel film generally reduces rapidly at switching frequencies higher than 10-100 KHz, preventing its application in megahertz (MHz) TFTs. The high frequency limitation stems from the overall slow polarization/diffusion rate of ions within the insulating layer. The past researches, pioneered by Frisbie and Lodge groups, focused on maximizing the ionic conductivity to enhance the high frequency capacitance. On the other hand, reducing the thickness of an ion-gel insulating layer (e.g., to sub-micron) should, in theory, improve its frequency response too. But, it is extremely challenging to prepare pinhole-free sub-micron ion-gel layers via solution-based techniques. Herein, we employed a solvent-free technique, initiated chemical vapor deposition (iCVD), to first deposit ultrathin dry polymer films, followed by injecting ionic liquid into these swellable polymer films to form ion-gel films as thin as 20 nm. These ultrathin pinhole-free ion-gel films can retain a capacitance greater than 1 µF cm^-2 at a frequency up to 1 MHz, making them promising gate insulator materials for low-voltage MHz TFTs. In addition, ion-gel films obtained from this new strategy is soft substrate (e.g., plastics) compatible, and patternable, which are beneficial for fabricating soft electronics.

Two-dimensional (2D) crystal films of organic semiconductors have attracted widespread attention for large-area and low-cost flexible optoelectronics due to their unique advantages of molecular designability, excellent flexibility and solution processability. The 2D crystal films of a series of p-type organic semiconductors are grown up to centimeter size with thickness of several molecular layers by “solution epitaxy” and a carrier mobility exceed 10 cm2 V-1 s-1 is achieved in our previous work. However, it remains challenging to achieve high mobility, air-stable and solution-processed organic field-effect transistors (OFETs) based on 2D crystal films of n-type materials with a performance comparable to that of p-type organic semiconductors. Here, a modified technique is demonstrated to grow 2D single crystals of n-type organic semiconductors from solution, enabling high-mobility electron transport in OFET devices.

Design of a high-voltage organic thin film transistor (HVOTFT) with the addition of a bottom field plate have shown improvements in the critical and breakdown field via numerical simulation. Such a field plate can help push the boundaries of high-voltage operation in organic thin film transistors (OTFT) technology beyond a drain to source voltage (V_DS) of 500 V while being controlled by a low gate to source voltage V_GS. High-voltage operation is not well developed in OTFT technology, and would allow the integration of HVOTFTs with large electrostatic MEMS actuators or photovoltaic systems on glass. The HVOTFT is designed with a dual accumulation region and dual threshold voltage to minimize the electric field peak within the channel. The design is also compatible with low temperature processing, self-assembled monolayer treatments and organic semiconductor solution-processing, for low cost and flexible applications.

Stretchable electronic devices have attracted extensive interest because of their potential applications for next-generation wearable electronic devices. Intrinsically stretchable semiconducting layers are important for the fabrication of large-area and simple integration processing of electronic devices with uniform electronic properties. The stretchability of semiconducting layers can be enhanced by rational molecular structure design of the side-chain or backbone of conjugated polymers. Recently, the side-chain engineering has been focused on the field of polymeric electronics since it can greatly enhance the solubility and electrical properties of polymers. Herein, we report diketopyrrolepyrrole(DPP)-based copolymers functionalized with urethane side-chain for applications as stretchable electronics. These polymers have three different backbone units. Because of the dynamic non-covalent bonding of urethanes, the fabricated semiconducting layers showed good stretchability as well as self-healable properties. Our results demonstrate the high potentials of urethane side-chain substituted DPP-based polymers for next-generation wearable electronics.

Organic conjugated molecules have a great potential for cost-competitive, large-area and high throughput mass production on mechanically flexible substrate. In particular, small molecule, which is one of the organic conjugated molecules, have several distinct advantages such as well-defined molecular structure, high purity, and less batch-to-batch variation. During recent years, organic solar cells based on the small molecules have accomplished considerable progress, reaching the PCEs over 10%. Now, for further advances in this field, printed or halogen free fabricated small molecule solar cells should be also demonstrated. In this study, blend films of small molecules, benzodithiophene terthiophene rhodanine (BTR) and PC₇₁BM were slot-die coated using a halogen-free solvent system. Moreover, for up-scaling production, we fabricated small molecule solar cells with not only time-consuming solvent vapor annealing (SVA) treatment but also more roll-to-roll compatible solvent additive approaches. The suitability of halogen-free additive method for up-scaling production was analyzed by using UV-vis absorption, EQE, AFM, TEM, SCLC, Light intensity and 2D-GIWAXS measurements. As a result, high efficiencies of 7.46% and 6.56% were achieved from time-consuming solvent vapor annealing (SVA) treatment and roll-to-roll compatible solvent additive approaches, respectively. After successful verification of our roll-to-roll compatible additive method on small-area devices, we finally demonstrated large-area photovoltaic modules composed of 4 stripes with a total active area of 10 cm², achieving a power conversion efficiency (PCE) of 4.8%.

Atomic switches, which are operated by the formation/decomposition mechanism of conduction filaments (redox-based electrochemical reactions), have been explored as next-generation switching devices due to their low operating voltage (<0.05 V), high on/off-current ratio (>10⁶), extremely high retention time (> 10 years), and good cyclic endurance (>10⁶). Recently, for future electronic applications, such as electronic skin and flexible devices/circuits, flexible polymer materials are considered as the most promising solid electrolyte layer for high-performance flexible atomic switching devices. Wu et al.^[1] demonstrated a flexible and transparent atomic switching device with using polyethylene oxide (PEO) as an electrolyte layer, and this device showed high on/off resistance ratio (10⁵), fast switching speed (<1 μs), and long retention time (>1 week). Hosseini et al.^[2] fabricated chitosan-based atomic switching device with on/off-current ratio of 10⁵ and operation voltage of 0.6 V.
Here, we fabricated a high-performance solid polymer electrolyte (SPE) atomic switching device with using poly-4-vinylphenol (PVP) / poly(melamine-co-formaldehyde) (PMF). Our PVP/PMF atomic switches showed low SET/RESET voltages (0.25 and –0.5 V, respectively), high on/off-current ratio (10⁵), excellent cyclic endurance (>10³), and long retention time (>10⁴ sec). To accomplish these excellent bipolar switching property and device stability, we applied two key techniques: i) the adjustment of the number of cross-linked chains to modulate the Cu ion diffusion and electrical insulating property in the PVP/PMF electrolyte, and ii) the insertion of a Ti buffer layer to control the number of diffused Cu ions and supply electrons into the PVP/PMF electrolyte. By increasing the number of cross-linked chains through the adjustment of the PVP/PMF ratio, we improved the insulating property of the PVP/PMF electrolyte, decreasing the off-current level of the PVP/PMF switching device. The Ti-buffer layer supplied electrons to the PVP/PMF electrolyte and acted as a diffusion barrier for Cu ions. Thus, we dramatically improved the bipolar switching property (SET/RESET voltages ↓ and on/off-current ratio ↑) and device stability (maximum cyclic endurance ↑ and HRS/LRS retention time ↑).

REFERENCES
[1] Wu et al. Adv. Funct. Mater. 2011, 21, 93-99.
[2] Hosseini et al. ACS Nano 2015, 9, 419-426.

OPVs with high efficiencies at present rely on fullerene-based acceptors, which suffer from several issues such as high synthetic cost, morphological instability, limited visible light absorption, and poor electronic tunability. In this presentation, we have synthesized a series of small molecule acceptors (DFDE-R and DFDO-R) comprised of electron-rich dithienosilole (D) and electron-deficient difluorobenzodiathiazole (F), benzodiathiazole-connected, 3-ethylrhodanine (R) units, and alkyl chains of 2-ethylhexyl (E) and octyl (O) groups. However, the polymer:DFD-R devices showed low PCEs mainly due to the high HOMO energy levels of the small molecule acceptors. To lower the energy level of the HOMO, modifications to the backbone structure were made by replacing 2FBT with difluorobenzene (FBz). Inverted BHJ devices employing polymer donors and newly synthesized DFBz-R acceptors exhibited higher PCEs relative to that of polymer:DFD-R devices. Furthermore, these new non-fullerene acceptors are synthetically scalable and stable under ambient conditions. Our investigation on the various synthesized molecules revealed the structure-photovoltaic properties, which can guide in synthesis of new high-performance non-fullerene small molecules for the advancement in the field of organic solar cells.

We report the synthesis of a narrow-band-gap, anionic conjugated polyelectrolyte, PCPDT-FBT-SO₃K and PCPDT-F₂BT-SO₃K for applying to the hole-transporting layer (HTL). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been extensively used as HTL in bulk heterojunction (BHJ) solar cells. However, its anisotropic electrical conduction and intrinsic acidic nature limit the device performance and stability. To overcome these drawbacks of PEDOT:PSS, PCPDT-BT-SO₃K was synthesized which has properties of homogeneous electrical conduction, neutral PH. Though PCPDT-BT-SO₃K enhanced power conversion efficiency (PCE), organic photovoltaic device using PCPDT-BT-SO₃K displayed reduced an open-circuit voltage (V_oc) compared with device using PEDOT:PSS as HTL. By contrast, newly synthesized PCPDT-FBT-SO₃K and PCPDT-F₂BT-SO₃K polymers increasing work function of ITO substrates exhibited higher PCEs with higher V_oc than PCPDT-BT-SO₃K polymer. Overall, this work demonstrate the chemical tuning of conjugated polyelectrolytes can improve the photovoltaic performance of organic solar cells.

The pure C₆₀ molecules tend to closely pack into hexagonal close-packing (HCP) and face-centered cubic (FCC) single crystals and the FCC structure is energetically favorable due to larger cohesive energies. Its cubic lattice often leads to polyhedral shapes in crystallization but not extended thin plates, which impede the application of C₆₀ single crystals in electronic devices. By forming solvates, solvent-assisted methods have been proven to be effective for controlling the growth habit of C₆₀ molecules into the desired extended ribbon morphologies that are suitable for thin-film devices such as field-effect transistors (FETs). However, for electronic application, the residual solvent molecules in crystal lattice not only impede the transport of charge carriers between semiconductor molecules, but also complicate the electronic environment due to polarization. Thus, it is essential to obtain C₆₀ single crystals with desirable morphologies, while at the same time, the adverse effect of residual solvent is minimized. Here, well aligned C₆₀ ribbon-like single crystals were grown from mixed solvent of m-xylene and carbon tetrachloride. After thermal treatment, the solvent molecules were extracted from the solvated structure. With partially evolving into HCP structure, FETs based on the C₆₀ ribbons showed decreased threshold voltages between 9.09 and 24.33 V with a moderate average electron mobility of 2.19 ± 0.78 cm² V^-1 s^-1.

Single crystal dielectrics are ideal candidates for high-performance organic field effect transistors due to the ordered molecular packing and smooth surface. However, it doesn’t obtain much attention due to the lack of facile preparation methods. Here, solution-grown smooth polymer single crystal with width in tens of microns are obtained and used as the dielectric for the organic field effect transistors (OFETs). The interface between the semiconductor and the polymer single crystal dielectrics are more favorable to the charge carrier transport than spin-coated polymer thin-film dielectrics, exhibiting much higher charge mobility. With 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) single crystal as semiconductor layer, the average OFET mobility reaches up to 1.70±0.97 cm²V^-1s^-1 with a highest value of 4.95 cm²V^-1s^-1. This result demonstrated that structural ordering of the polymer dielectrics is critical to further advance the high-performance OFET devices.

Organic semiconductors (OSCs) have incredible prospects for next-generation, flexible electronic devices including bioelectronics, opto-electronics, energy harvesting and storage. They are flexible, biocompatible, printable, low cost, efficient and sustainable semiconductor materials with hybrid electrical and ionic conductivities, which can expand the functionality and accessibility of electronics beyond silicon.
Yet many fundamental challenges still exist in understanding the morphology, microstructure and charge transport processes of OSCs, with intimate connection to processing techniques and control over doping mechanisms. Firstly, solution processing prohibits definitive control over microstructure, which is fundamental for controlling electrical and ionic transport properties necessary for the development and advancement of new technologies. Second, OSCs generally suffer from poor electrical conductivities due to a combination of low carriers and low mobility, which limits overall efficiencies. Hence understanding different doping mechanisms and their influence on microstructure and charge transport properties is important for increased efficiencies of organic devices. To compete with their inorganic counterparts, characterization of the structural, electronic, and charge transport attributes of these organic semiconductor materials with respect to processing parameters and doping mechanism plays an important role for device engineering.
This effort considers the two questions of interest: (i) How does the microstructure change with chemical and electrochemical doping mechanisms and (ii) how do these changes in microstructure impact the electronic and charge transport properties. Addressing these questions will also allow us to connect the electronic and structural properties with fabrication processes and doping, to control the structural properties, and link their electronic properties with emphasis on charge transport.
Synchrotron based high energy X-ray scattering techniques, specifically grazing incidence wide angle X-ray scattering (GIWAXS) and small angle X-ray scattering (SAXS) are ideal techniques to measure intra- and inter-molecular spacings and phase segregation effects. In this work, GIWAXS was used to study the microstructural features ranging from sub angstrom to nanometer length scales, a key regime for transport in organic materials. GISAXS was used to study the structures greater than the nanometer length scale to understand phase segregations due to doping mechanisms. Electronic properties are measured using spectroscopy and correlated with electrochemical density of states. Transport properties are analyzed using cyclic voltammetry, mobility and conductivity measurements. Collectively, these measurements allow us to design organic semiconductor systems with controllable behaviors for next-generation technologies.

The printing-based patterned-layer formation of electrode metal layers are one of the key printed electronics technologies. Recently, we developed a groundbreaking electrode printing technique, called as “surface photoreactive nanometal printing (SuPR-NaP)”, which allows easy manufacture of ultrafine conductive silver patterns with a high submicron resolution [1]. This technique is based on the unique chemisorption effect of silver nanoparticles (AgNPs) on a photo-activated patterned polymer surface that is manufactured by masked ultraviolet irradiation of a perfluoropolymer, Cytop (Asahi Glass Co., Ltd.), layers. The SuPR-NaP technique allows us to produce ultrafine electrode patterns with minimum line width of 0.8 micron, by which the mass production of transparent conductive electrodes is now undertaken for the use as touch screen sensors. The next important target of the printing technique is to use and accommodate the printed ultrafine electrode patterns for the production of organic thin-film transistor (OTFT) arrays on flexible plastic substrates.
In this presentation, we will present and discuss low-voltage operation of OTFTs composed of ultrafine printed source/drain electrodes produced by the SuPR-NaP technique [2]. For this purpose, we utilized the highly-insulating Cytop layer, not only for producing the photo-activated patterned surface for the SuPR-NaP technique, but also as the gate dielectric layer for producing bottom-gate, bottom contact OTFTs [3]. To realize the low-voltage operation of OTFTs, we systematically examined the thickness, voltage-durability and capacitance of the thin Cytop layers. We found that the ultrathin thickness as thin as 26 nm is useful to obtain high-voltage durability up to 2.4 MV/cm and large-capacitance of 97 nF/cm², even after the top printed silver electrodes are deposited by the SuPR-NaP technique. Based on the examination, we successfully operated polycrystalline pentacene OTFTs using the ultrathin Cytop layer, at low voltage less than 2 V with negligible hysteresis characteristics. Additionally, we found that the surface modification of the printed silver electrodes with pentafluorobenzenethiol is effective to suppress the contact resistance and to enhance the drain current. We will discuss the stable low-voltage operation of the OTFTs in terms of the formation of high-quality semiconductor/gate insulator interface with use of the ultra-hydrophobic nature of the surfaces of ultrathin Cytop layers.
[1] T. Yamada et al., Nat. Commun. 7, 11402 (2016). [2] G. Kitahara et al., Organic Electron. 50, 426 (2017). [3] K. Aoshima et al., Organic Electron. 41, 137 (2017).

Hydroxyl groups are generally treated as electron traps in organic field-effect transistors (OFETs), which make it difficult to achieve high electron transport at the interface of organic hydroxyl-containing dielectrics and organic semiconductors. Here, we adopted the drop-pinned crystallization (DPC) method to obtain well-aligned C₆₀ single-crystal ribbons on different hydroxyl-containing polymer dielectrics, such as polyvinyl alcohol (PVA), poly 4-vinyl phenol (PVP), and got a high electron mobility (more than 1 cm² V^-1 s^-1). Single crystals used here were good candidates for investigating the intrinsic electrical properties of organic semiconductors owing to high orientation and purity. Also, the ambipolar performance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) was firstly observed in PVP dielectric FET devices (average hole mobility: 0.436 ± 0.266 cm² V^-1 s^-1, average electron mobility: 4.58 × 10^-2 ± 4.82 × 10^-2 cm² V^-1 s^-1). These findings reveal that well-aligned single crystals can show excellent electron transport even on the surface of organic hydroxyl-containing dielectrics.

Over recent years several new classes of conjugated polymer and molecular semiconductors have shown promise as high mobility materials for organic field-effect transistors. We have been interested in understanding their fundamental charge transport physics and the underpinning relationship between molecular structure and charge transport properties. Information about their electronic structure can be gained from measurements of fundamental transport coefficients such as Hall effect and Seebeck coefficient. In particular, the Seebeck coefficient provides direct insight into the density of states available for transport and complements measurements of the carrier mobility as a function of temperature and carrier concentration. In this presentation we will review our current understanding of the charge transport and thermoelectric properties of these materials and discuss these in the context of applications in electronics, optoelectronics and thermoelectrics.

We report our recent progress in printed organic nanofiber-based neuromorphic and deformable electronics. Precisely controlled organic nanofibers are promising elements for upcoming innovative electronics such as brain-inspired computation and memory, light-weight wearable electronics and biomedical electronics based on their advantages of low-cost solution printing process, high speed and large scale fabrication, high-resolution (< 1μm) patterning, and so on. We reported organic nanofiber-based neuromorphic synapses which emulated important working principles of a biological synapse, e.g., excitatory post-synaptic current (EPSC), inhibitory post-synaptic current (IPSC), paired-pulse facilitation (PPF), short-term plasticity (STP), long-term plasticity (LTP) and spike-timing dependent plasticity (STDP). Electrochemical synaptic transistor arrays with nano-feature size were produced based on aligned organic semiconducting nanofibers and ion-gel dielectric which mimic fiber-like morphology of neurons and biological synaptic cleft. These properties are promising for neuromorphic computation and memory, and the devices would serve as building blocks of future neuromorphic systems. We also recently developed aligned organic semiconducting nanofiber-based deformable electronics which are impervious to mechanical influence when mounted on the surface of dynamically-changing soft matter. Our deformable field-effect transistors can be easily deformed by applied strains (both 100% tensile and compressive strains). The mechanical durability of nanofiber can be further significantly increased by simply re-engineering the geometric structure of the nanofiber. The deformable transistors withstood 100% uniaxial stretching with minimal change of electrical properties, even after a 3D volume change (> 1700% and back to original state) of a rubber balloon. The deformable transistors robustly operated on a mechanically-dynamic soft matter surface e.g. a pulsating balloon that mimics a beating animal heart, which demonstrates potential of the deformable transistor for future biomedical applications.

This presentation describes my group’s efforts to understand the molecular and microstructural basis for the mechanical properties of organic semiconductors for organic photovoltaic (OPV) devices. Our work is motivated by two goals. The first goal is to mitigate mechanical forms of degradation of printed modules during roll-to-roll fabrication, installation, and environmental forces—i.e., wind, rain, snow, and thermal expansion and contraction. Mechanical stability is a prerequisite for inexpensive processing on flexible substrates: to encapsulate devices in glass is to surrender this advantage. The second goal is to enable the next generation of ultra-flexible and stretchable solar cells for collapsible, portable, and wearable applications, and as low-cost sources of energy—“solar tarps”—for disaster relief and for the developing world. It may seem that organic semiconductors, due to their carbon framework, are already sufficiently compliant for these applications. We have found, however, that the mechanical properties (stiffness and brittleness) occupy a wide range of values, and can be difficult to predict from molecular structure alone. We are developing an experimental and theoretical framework for how one can combine favorable charge-transport properties and mechanical compliance in organic semiconductor films. In particular, we have explored the roles of the backbone, alkyl side chain, microstructural order, the glass transition, molecular packing with fullerenes, plasticizing effects of additives, extent of separation of [60]PCBM and [70]PCBM, structural randomness in low-bandgap polymers, and reinforcement by encapsulation, on the mechanical compliance. We are exploring the applicability of semi-empirical “back-of-the-envelope” models, along with multi-scale molecular dynamics simulations, with the ultimate goal of designing electroactive organic materials whose mechanical properties can be dialed-in. We have used the insights we have developed to demonstrate several new applications for OPV that demand extreme compliance, including biaxial stretching and conformal bonding of whole devices to hemispheres, and devices with ultrathin encapsulation mounted on human skin that survive significant cyclic mechanical deformation in the outdoor environment.

Organic electronics, based on semiconducting and conducting polymers, have been extensively investigated in the past two decades and have found commercial applications in lighting panels, smartphone screens, and TV screens using organic light emitting diodes technology. Many other applications are foreseen to reach the commercial maturity in future in areas such as transistors, sensors and photovoltaics.
Organic electronic devices, apart from consumer applications, are paving the path for key applications at the interface between electronics and biology. In such applications, organic polymers are very attractive candidates, due to their distinct properties of mechanical flexibility, self-healing and mixed conduction, i.e the ability to transport both electron/holes and ionic species.
My group investigated the processing conditions leading to high electrical conductivity, long-term stability in aqueous media as well as robust mechanical properties of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS), on rigid, flexible and stretchable substrates [1-3]. We have demonstrated that stretchable PEDOT:PSS films can be achieved by adding a fluorosurfactant to the film processing mixture and by pre-stretching the substrate during film deposition. We have achieved patterning of organic materials on a wide range of substrates, using orthogonal lithography, parylene patterning and pattern transfer [4-5]. Recently we have discovered that PEDOT:PSS films can be rapidly healed with water drops after being damaged with a sharp blade [6].
My talk will deal with processing, characterization and patterning of conducting polymer films and devices for flexible, stretchable and healable electronics. I will particularly focus on the strategies to achieve films with optimized electrical conductivity and mechanical properties, on unconventional micro patterning on flexible and stretchable substrates, on the different routes to achieve films stretchability and self-healing.

F. Cicoira et al. APL Mat. 3, 014911, 2015.
F. Cicoira et al. Appl. Phys. Lett. 107,053303, 2015.
F. Cicoira, et al. Appl. Phys. Lett., in press (APL17-AR-02492R1).
F. Cicoira et al. Chem. Mater. 29, 3126-3132, 2017.
F. Cicoira et al. J. Mater. Chem. C 4, 1382–85, 2016.
F. Cicoira et al. Adv. Mater., 29, 1703098, 2017.

Stretchable electronics has expanded the application scope of the electronics and sensors particularly for health monitors, medical implants, artificial skins and human-machine interfaces. To accommodate the mechanical stretchability to nonstretchable electronic materials, the structural engineering design from special mechanical structures or architectures has been widely exploited. An alternative route to eliminating the burden of constructing dedicated architectures and the associated sophisticated fabrication processes is to build stretchable electronics from rubbery electronic materials, which have potential toward scalable manufacturing, high-density device integration, and large strain tolerance. Here, we report a low voltage operational (< -3 V) high performance rubbery electronics including a field effect transistors (FETs), integrated circuit (inverter, NOR, and NAND gate), and an active matrix with pressure sensor based on all rubbery electronic components; semiconductors, electrodes, and gate dielectrics. For the high-performance devices, the semiconductor material (P3HT nanofibril and PDMS composite) is doped with multiwall carbon nanotubes, by dry transfer to enhance the field-effect mobility. The FETs and circuits show a normal operation under the mechanical stretching strain of up to 50%. The approach to constructing integrated circuits and active matrix all from elastomeric rubbery electronic materials will move forward the advancement of stretchable electronics for a wide range of applications, such as artificial skins, biomedical implants, and wearable applications.

Stretchable and deformable materials are nowadays important for the next-generation wearable electronics since they potentially can be biocompatible and possess distinct functionalities with our body movement. Stretchable devices, such as field-effect transistors, solar cells or memories based on polymeric materials have been demonstrated. However, it is still challenging to achieve stable and high electrical performance using an intrinsically-strained active layer. A semiconducting polymer system that possesses both superior electrical behavior and high stretchability, therefore, is needed.
Side chain engineering has been focused on the field of polymeric electronics very recently since rational side chain design can significantly manipulate the solution processability, solid state molecular stacking and thin film morphology. From the view of soft electronics, we aim to achieve a ductile active layer by controlling the inter-chain packing interaction and surface morphology based on the side chain substituents.
In this talk, two side chain design strategies on isoindigo-based conjugated backbone will be presented. Firstly, carbosilane side chains have been introduced with a simple synthetic pathway to evaluate long and branched side chains in high yields. A high mobility of 8.06 cm²V^-1s^-1 was probed using a top-contact transistor device based on carbosilane side chains consistent of a 8 carbon linear spacer plus two octyl chains after branching. More interestingly, such polymers possess a relatively low tensile modulus (< 0.4 GPa) and showed a stable mobility higher than 1 cm²V^-1s^-1 even under a 60% strain, which is one of the most successful example using a single conjugated polymer for soft and intrinsically stretchable transistor applications. Furthermore, the odd-even effect have been also explored for this carbosilane side chains system to understand the relationship between side chain length, thin film deformability, and stretchable device performance. Secondly, a low glass transition temperature (i.e. -54 ^oC) poly(butyl acrylate) (PBA) side chains has been incorporated to the conjugated polymer system. The soft PBA segment offers a great opportunity to improve the mechanical property of semiconducting polymer thin films that typically contain lots of rigid conjugated rings. As a result, this set of polymers exhibited superior thin film ductility with a low tensile modulus down to 0.12 GPa, and the mobility at 60% strain could be remained almost the same as non-stretched (i.e. 0% strain) condition. Based on our achievements, both side chain system showed not only good charge transport ability but also thin film ductility under intrinsically stretching, suggesting these newly-designed materials possessed great potential for next-generation wearable electronics.

In an effort to address the fast-growing demand for data storage within the emerging sector of printed electronics and its wider deployment in the Internet of Things, solution-processable polymer-electret based charge-trapping organic field-effect transistor (OFET) memory technologies have been attracting increasing attention owing to their intriguing characteristics, including non-destructive read-out, easily integrated structures for circuitry, inexpensive fabrication processes for large-area manufacturing.^[1] However, to date a polymer electret OFET memory often requires multiple fabrication steps, including both vacuum- and solution-phase deposition techniques. This is mainly because full solution-processing is limited by solvent orthogonality during the sequential depositions of organic layers (i.e. polymer electrets and organic semiconductors). To simplify fabrication protocols, hence reducing manufacturing costs, alternative processing approaches and/or materials systems for polymer electret OFET memory are urgently needed.

Here we report a universal approach for developing an alternative semiconducting system based on organic blends using small molecules and polymer electrets and its application in non-volatile OFET memories. Such blend systems are known to self-assemble into two vertically separated components due to the phase separation occurring during the film deposition, hence forming a high mobility semiconductor layer (small-molecule) and a charge trapping layer (polymer electret). Using this approach, solution-grown 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-PEN)/polystyrene (PS) blend OFET memory devices exhibit excellent non-volatile memory characteristics with a fast switching speed (~50 µs, despite the long channel length, 30 µm, employed), low operation voltages (<15 V), and highly-stable 8-level (i.e. 3 bits) data storage characteristics. This development paves the way to high-density data storage using single memory cells and could potentially minimize the overall system cost without the need for device-size downscaling and/or complex integration.

Furthermore, we were able to demonstrate flexible, non-volatile TIPS-PEN/PS based OFET memories with superb operating characteristics, such as highly-reliable memory operation with programming/erasing cycles well in excess of 10,000. Finally, the same blend material approach is successfully extended to other high-mobility organic semiconductors such as 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) blended with PS, further demonstrating the universal nature of the proposed technology. The development of such single-step deposited charge-trapping polymer electret and organic semiconductor systems not only simplifies the manufacturing protocols but also creates a new promising direction for future research in the emerging area of printed memory systems.

Reference
[1] W.-C. Chen, Electrical Memory Materials and Devices, Royal Society of Chemistry, Cambridge, 2015.

Printed electronics promises to fabricate low-cost microelectronics on flexible substrates for applications such as flexible displays and RFID tags. In particular, inkjet printing is a popular technique to pattern organic electronic devices because patterns can be changed dynamically. Despite of its widespread use, non-idealities are commonly observed and only partially solved. Here, we demonstrate a solution to the problem of bulging lines.

After ink has been ejected from the nozzle and deposited on the substrate, ink can flow distorting the pattern. This is mainly caused by surface tension forces and the associated Laplace pressure. Traditionally, lines are printed by linearly placing drops one behind the other. By varying the drop spacing, four different regimes are observed: outward bulging of lines, scalloping, line separation and smooth lines. Unfortunately, this approach only achieves smooth lines under certain conditions and generally not at the start of a line. When a new line is started, drops form bulges that are rounded and less line-shaped to minimize surface energy. This starting bulge grows as drops are added to the end of the line due to an imbalance of pressure within the liquid line. Ink flows from a region of high pressure to a region of low pressure (the bulge). Starting bulges can be observed not only in isolated lines but also X-, T-, and L-shapes when a new segment is started e.g. in interdigitated or serpentine patterns. This reduces yield of devices such as printed transistors or requires large margins of safety.

Here, we demonstrate that by manipulating the order of drops printed, we can avoid bulges at the beginning of inkjet-printed lines. The premise of this approach is to maintain symmetry and a net force of zero on every drop added to the line. By canceling out the high pressure at either end of the line, we have removed the pressure imbalance that leads to line bulging. This is achieved by always connecting equidistant line segments or drops of equal length/volume. The basic building blocks are segments of three drops where the middle drop is printed last. We show that these segments do not become rounded unlike three-drop segments that are printed linearly, which causes bulging. Segments are successively connected without allowing pressure imbalances to form. Crucial parameters are the drop spacing within segments as well as the spacing between segments. We show how the contact angle of the ink on the substrate affects these parameters. Finally, smooth lines can be printed with this novel method. This symmetric approach can be extended to more complex two-dimensional patterns by recursively breaking them down into segments of equal length that are connected in a binary fashion.

In summary, we present a novel approach to the inkjet printing of scaled features that can be employed in printed devices. We successfully avoid undesirable line morphologies by ensuring that the pressure within lines is always balanced.

Artificial synapses are rapidly emerging for neuro-inspired electronic devices that emulate biological synapses. Mimicking biological synaptic memory functions of the brain has been a main focus in neuromorphic electronics, but mimicking the complicated human sensorimotor system that performs sequential functions related to proprioception, signal processing, and motor response is also a challenging issue especially to realize bio-mimetic soft electronics. Here, we design a photo-sensory stretchable artificial synapse that implements wireless recognition, telecommunication and muscle actuation in the similar way of biological system. The stretchable artificial synapse which incorporates a single wavy organic nanowire stably operates at 100% strain and after repeated stretching cycles; this mechanical stability is favorable for soft electronics. Our stretchable artificial synapse generated typical postsynaptic behaviors in response to presynaptic photo-stimulation and showed promising features for wireless communication as well as robust operation of biomimetic motor system of soft electronics. Our stretchable artificial synapse will provide an advance toward developing bio-inspired soft electronics, neurorobotics and electronic prostheses.

Biomaterials have been attracting attention as a useful building block for biocompatible and bioresorbable electronics due to its non-toxic property and solution processabillity. In this work, we report the integration of biocompatible keratin from human hair as dielectric layer for organic thin-film transistors (TFTs), with high performance, flexibility and transient property. The keratin dielectric layer exhibited a high capacitance value above 1.27 µF/cm² at 20 Hz due to the formation of electrical double layer. Fully solution processable TFTs based on p-channel poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b]dithiophen-2-yl)- alt[1,2,5]thiadiazolo[3,4-c]-pyridine] (PCDTPT) and keratin dielectric exhibited high electrical property with a saturation field-effect mobility of 0.35 cm² V^-1 s^-1 at a low gate bias of -2 V. We also successfully demonstrate flexible TFTs which exhibited good mechanical flexibility and electrical stability under bending strain. An artificial electronic synaptic PCDTPT/Keratin transistor was also realized and exhibited high-performance synaptic memory effects via simple operation of proton conduction in keratin. An added functionality of using keratin as a substrate was also presented, where similar PCDTPT TFTs with keratin dielectric were built on top of keratin substrate. Finally, we observed that our prepared devices can be degraded in ammonium hydroxide solution, establishing the feasibility of keratin layer as various components of transient electrical devices including as a substrate and dielectric layer.

Flexible smart sensing tags that combine sensing elements, electronic control circuits and usually silicon ICs in a flexible hybrid device are an important area of organic electronics research and development. Printing technology is a good fit for the fabrication of these devices because the device density and circuit speed are low, the materials are flexible and the need for several different materials favors additive manufacture. Organic TFTs provide the control logic and have been widely studied for many years. Complementary TFT circuits are preferred for most circuits and while OTFTs have improved greatly there is still a challenge to make printed complementary devices with the same structure and matched characteristics. Circuit design that mitigates device variability is needed. Organic electronic materials also provide many of the sensors, memory and actuator devices that provide functionality to a smart sensor tag. Organic electro-chemical TFTs (OECT) are important devices for high current circuits and as chemical and physiological sensors, and they provide novel electronic attributes that give dynamic circuit elements. Humidity sensors use the property that organic materials are permeable to moisture and hence change the device capacitance. The piezo- and ferro-electric polymer PVDF is used for memory devices and actuators. Examples will be given for each of these organic electronic technologies and their integration in prototype smart tags.

The performance of organic thin-film transistors (OTFTs) depends on a variety of factors including the type of semiconductor and processing conditions, the dielectric, the device structure, and the choice of electrodes. In particular, the quality of the source and drain contacts is of the utmost importance to facilitate efficient charge injection and minimize contact resistance. Here, we focus on the study of contact effects in OTFTs and their relation to processing conditions. We fabricate bottom-contact, top-gate OTFTs with pentafluorobenzenethiol (PFBT)-treated Au/Ti contacts, the organic semiconductor 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene (diF-TES ADT) and Cytop gate dielectric. We deposit the source and drain electrodes using thermal evaporation and vary the deposition rate from 0.1 Å/s to 2 Å/s, while maintaining the geometry of the device unmodified. We find that the field-effect mobility is dependent on the details of the electrode fabrication, with a maximum value being obtained for a rate of 0.5 Å/s, µ_max = 19 cm²/Vs, and gradually decreasing to a value of µ = 8 cm²/Vs, at a rate of 0.1 Å/s, and µ = 4 cm²/Vs at 2 Å/s, a value that is consistent with earlier reports obtained under similar conditions.¹ By examining the dependence of mobility on gate-source voltage, we observe an abrupt increase, followed by a plateau at the reported values, confirming that the reported mobility values are not simply an artefact of the measurement due to gated contacts. We evaluate the contact resistance through gated-TLM measurements, and find that it follows the same trend as the mobility vs. deposition rate, with the lowest value aggressively lowered to 0.5 kΩcm for 0.5 Å/s deposition rate. We discuss the factors that contribute to lowering of the contact resistance, which, in turn, gives improved device characteristics.
To determine if this drastic improvement in electrical characteristics through modifying the electrode processing is material-dependent, we further performed a similar study using another well-known semiconductor, in this case the copolymer indacenodithiophene-co-benzothiadiazole (IDTBT). Using the same device structure, we find a maximum mobility of 12 cm²/Vs at a deposition rate of 0.5 Å/s, which is approximately 4 times greater than the highest value reported in the literature for this material,² and results from a low contact resistance of 0.2 kΩcm. The vastly improved performance exhibited by these devices indicate that the deposition rate of source and drain electrodes is a salient parameter that must be accounted for in device design and is effective for both small-molecule and polymer semiconductors.
1 P. J. Diemer, et al., Appl. Phys. Lett., 2015, 107, 103303.
2 X. Zhang, et al., Nat. Commun., 2013, 4, 1–9.

In this study, we have realized threshold-voltage (V_th) controls by the selection of gate metals or the gate electrode modification with self-assembled monolayers (SAM) in organic transistors. The difference of work function between the gate metal and the organic semiconductor intrinsically varies the V_th. In other words, transistors with desired V_th can be fabricated by the modification of the gate electrodes. Using the gate modification, enhance-mode and depletion-like transistors have been realized for Al gate electrodes and SAM-treated Au gate electrodes, respectively. As a result, the V_th can be shifted for 0.95 V in 2-V-operating organic transistors, which is useful for designing organic logic circuits.
Recently, organic thin-film transistors (OTFTs) have attracted attention because of their low costs and simple processability. The technology of OTFTs is desired to be used for sensor applications in the IoT society, where the signal addressing and processing circuits are important. In such analog circuits, both low-voltage operation and highly-reliable electric characteristics are essential. Therefore, the precision and reproducible control of V_th is one of key technologies for organic analog circuits. It has been already shown by several groups that V_th can be shifted by modifying the surface of gate dielectrics with SAM or plasma treatment^1,2. Although these methods can shift V_th largely, it has been realized only on the specific inorganic dielectrics, such as silicon dioxide or aluminum oxide. Another approach for the V_th control is the modification of gate metals. This is a universal method to shift the V_th because large variety of gate metals and dielectrics are applicable.
In this study, we fabricated OTFTs with modified gate electrodes, where the metal surface is treated with polarized SAM so that the work function of the metal is changed. Various kinds of gate metals and SAM modifications are investigated in bottom-gate and top-contact transistors. A thin parylene, DNTT and Au top-contact electrodes were used for the other components in the OTFTs. As a result, we realized enhance-mode OTFTs with V_th = - 1.05V for Al gate electrodes. On the other hand, we realized depletion-like OTFTs with SAM treated Ag or Au gate electrodes. We found that SAM treatment with pentachlorobenzenthiol changes the V_th for up to 140 mV. It should be noted that only the V_th can be shifted without changing the other parameters such as mobility. These V_th control technology is useful for designing organic circuits, and we will demonstrate improved circuits designs for some analog circuits such as inverters and Pseudo CMOS technologies.


1. S. Kobayashi, et al. Nature Mater. 3, 317-322 (2004)
2. A. Kitani, et al. J. Appl. Phys. 55, 03DC03 (2016)

Typically semicrystalline conjugated polymers exhibit much higher mobility than their amorphous counterpart. The archetypal example is regio-random P3HT, with mobilities on the order of 10^-6cm²/V.s as compared to regio-regular P3HT with mobilities on the order of 0.1 cm²/V.s. The oft-cited reason for these differences is the fact that in polymer crystallites charges delocalize and take a partial 2D character, enabling higher mobility. The amount of delocalization and how it depends on structure and processing is however difficult to measure. I will show that charge modulation spectroscopy in the IR with model materials allows to determine the delocalization of the polaron in P3HT, when aided by theory. In order to extract systematic trends we will use 100% regio-regular P3HT of well-defined molecular weights (PDI~1.1). I will show how delocalization depends on molecular weight and substrate interface. Furthermore, I will compare the delocalization of charge induced by field-effect with that of charges induced by doping. We find that doping strongly suppresses delocalization, which may explain why only 5% of doping-induced charges are actually mobile.

Small molecular organic semiconductor crystals form interesting electronic systems of periodically arranged “charge clouds” whose mutual electronic coupling establishes charge and spin coherence overcoming molecular fluctuation. This presentation focuses on dynamic properties of the coherent electrons in high-mobility organic single-crystal semiconductors with systematically modified phonon scattering, which strongly restricts mobility of charge carriers in single-crystal organic transistors [1,2].
We grew only a-few-monolayer thick ultra-thin single-crystal films of decyldinaphthobenzo-dithiophene derivatives (Cn-DNBDT) on a plastic substrate so that uniaxial force can be applied by bending the samples. It is crucial to use such ultrathin crystal films to reproducibly apply strain because the surface of the plastic substrates and the semiconductor films are to be deformed in the same rate without either slippage or mechanical break. We examined the reproducible modification in crystalline structure and inter-molecular phonon vibration as the function of applied strain. At maximum, 3% strain is applied without damage to the crystal so that room-temperature mobility increased by the factor of 1.7 [2]. The measured mobility is 9.7 cm2/Vs without strain, and it significantly increases up to 16.5 cm2/Vs under 3% strain. Analysis using X-ray diffraction (XRD) measurements and density functional theory (DFT) calculations reveal the origin to be the suppression of the thermal fluctuation of the molecules rather than reduction of effective mass, which is consistent with temperature dependent measurements.
We measured spin relaxation time of the electric-field induced carriers down to 4.2 K using the large-area ultra-thin single-crystal transistors to measure electron-spin relaxiation. We found that the spin-relaxation follows the Elliott-Yafet mechanism so that the spin life time is consistently elongated at low temperatures due to reduced phonon scattering via spin-orbit coupling [3]. The result suggest that charge carrier mobility can as high as 650 cm2/Vs with minimized phonon scattering at the low temperature.
The giant strain effect and very large impact of phonon scattering in room-temperature charge transport are caused by peculiar electron-phonon coupling in the soft molecular semiconductors, which promises high-performance mechano-electronics devices such as strain sensors and vibration sensors.

[1] K. Sakai, Y. Okada, T. Uemura, J. Tsurumi, R. Häusermann, H. Matsui, T. Fukami, H. Ishii, N. Kobayashi, K. Hirose, and J. Takeya, Nature Asia Mater. 8, e252, (2016).
[2] T. Kubo, R. Häusermann, J. Tsurumi, J. Soeda, Y. Okada, Y. Yamashita, N. Akamatsu, A. Shishido, C. Mitsui, T. Okamoto, S. Yanagisawa, H. Matsui, and J. Takeya, Nature Commun. 7,11156 (2016).
[3] J. Tsurumi, H. Matsui, T. Kubo, R. Häusermann, C. Mitsui, T. Okamoto, S. Watanabe, and J. Takeya, Nature Phys. 13, 994 (2017).

In solution-processed organic bulk heterojunction (BHJ) solar cells, the purity of the phase-separated domains is known to play an important role in determining device function. While the effects of domain purity have been investigated by tuning of the BHJ morphology, such tuning typically results in several parameters (for example domain size and crystallinity) being varied at once. Here we show that by varying the time between spin-coating and the application of an anti-solvent treatment, the domain purity of the polymer-rich phase in PBDTTT-EFT:PC₇₁BM blends can be tuned while keeping other morphological parameters constant. This unique approach enables the effect of domain purity on device function to be isolated and quantified. Over the purity range explored, solar cell power conversion efficiency is observed to monotonically increase from 7.2% to 9.6% corresponding to a ~ 20% increase in domain purity. The cell fill factor is found to be most affected by changes in domain purity, with transient photovoltage measurements indicating a reduction in the rate constant of bimolecular recombination with increasing domain purity.

The molecular doping of organic semiconductors (OSCs) has recently gained significant interest due to applications in thermoelectrics, transistors, and for selective contacts in photovoltaics or LEDs (see review by Jacobs and Moule, Adv. Mater. 2017, 1703063). Taking the p-type case as an example, molecular doping is usually assumed to occur by single electron transfer from the OSC to the dopant molecule, generating an ion pair (IP). However, over the past 5 years it has become apparent that some materials—primarily small molecule OSCs—dope by a qualitatively different process. In these systems, significant orbital hybridization occurs between the dopant and organic semiconductor, generating a charge transfer complex (CTC) which accepts both electrons from the OSC highest occupied molecul