Symposium EP08 : Advanced Polymer Semiconductors---Key Properties and High-Performance Electronics

For at least the past 10 years, printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had been already fabricated by printing techniques at industrial scale, despite the great potentiality, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications. In the case of logic integrated circuits (ICs), the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector.
In this presentation, I will give a talk on the recent progressive of my group on development of printed organic integrated circuits. I will mainly talk about on development of high performance inkjet printed unipolar and ambipolar polymer field-effect transistors (FETs), and applications to elementary organic complementary inverter, ring oscillators and various logic circuits. Several issues should be addressed including method to improve charge injection properties, reducing operating voltage, and deposition over a large area by graphic art printing processes. By development of organic semiconductors and gate dielectrics, printing processes, contact resistance, and mobility of organic semiconductors, we have obtained high field-effect mobility more than 10 cm²/Vs for both of p-channel and n-channel FETs with less than 5 V operating voltage, and ambipolar OFETs and the CMOS polymer ring oscillator showed very high operating frequency of 50 KHz. In addition, various logic circuits such as NAND, NOR, and XOR etc. were demonstrated as well.

Printed electronics has been considered for a wide range of novel applications requiring non-conventional properties such as portability, flexibility and wearability and has opened the prospect of economically convenient mass production of electronic devices, owing to the suitability of the fabrication techniques and materials for low-cost, high-throughput industrial production. However, the optimization of some relevant figures of merit still has to be addressed appropriately to credibly enable the implementation of many real applications. Primarily, a transistor operation frequency suitable for more demanding applications must be guaranteed (e.g. more than 10 MHz in the case of RFID-based item-tracking systems or driving circuitry for flexible displays). Secondly, the correct operation of circuits should be achieved at a bias voltage compatible with batteries or energy-harvesting devices. In addition, such goals should be conveyed through a low-cost, industrially scalable production flow.
Here, we fabricated flexible low-voltage, high-frequency Organic Field-Effect Transistors (OFETs) exhibiting a transition frequency of 3 MHz at a bias voltage of 10 V. Such devices were realized only via the use of solution-based techniques (i.e. inkjet printing, femtosecond laser sintering, bar-coating), identifying a process flow which is suitable for the future scale-up to mass production. The high-frequency performance of the realized devices is ascribed to the combination of the high-resolution of the laser direct-writing of electrodes with the optimal charge transport properties of bar-coated poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (P(NDI2OD-T2)), yielding a charge mobility of 0.3 cm²/Vs (at a bias of 10 V) in devices with a channel length of 1.5 µm and with 1.4-µm-wide electrodes. Concurrently, the low-voltage operation was achieved thanks to a solution-processed dielectric stack integrating a low-k and a high-k material, which results in a specific capacitance of 39 nF/cm² with a gate current leakage below 10 nA/cm².
The proof of the feasibility of such low-voltage, high-frequency devices with the use of scalable solution-based techniques is a significant step forward towards the adoption of printed electronics in applications in the fields of smart tagging, wearable devices, large-area sensors and displays.

Organic semiconductors have been extensively developed with regards to their charge transport ability and have recently been demonstrated to be a possibilities for large-area, commercial electronic photonics and electronics. In order to develop organic semiconducting materials for OTFT, there are many prerequisites such as high mobility above 0.1, which is amorphous silicon mobility. It should also have good solubility for solution process. Moreover, easy control of ambipolar transport characteristics is also useful. So far, various synthetic strategies of organic semiconductors have been reported that increase their charge carrier transport characteristics for OTFT. In this presentation, we will report design strategy of organic semiconductors for high mobility, environmental-friendly processability, and easy ambipolar transport.

Monodisperse conjugated polymer particles are accessible via dispersion polymerization protocols. The particles have diameters in the range of the wavelength of visible light and can be processed via ink-jet printing, during which they self-assemble into photonic crystals. This allows for the generation of printed and optically pumped organic lasers without the need for sophisticated nanofabrication techniques.
Integration of the particles into opto-electronic devices would enable new device architectures; however, the colloids are too large to be applied in current opto-electronic diode architectures. Because of low charge carrier mobility in conjugted polymers such devices would require high voltages and currents, which would deteriorate the light emission quality.
Here we will show the synthesis of monodisperse conjugated polymer particles with incorporated inorganic low work function nanoparticles. When the hybrid inorganic/conjugated polymer particles are integrated into a dielectric matrix, the inorganic nanoparticles can be addressed by an alternating electric field. Here they act as nanoelectrodes allowing for light emission from the conjugated polymer in a light emitting capacitor device.

Operational instability of organic field-effect transistors (OFETs) is one of the most critical obstacles to their practical use and commercialization. Prolonged operation of OFETs under an applied bias can cause a significant drop in the channel current and a continuous shift in the threshold voltage, which prohibit the normal operation of an electronic device. The bias-stress-driven electrical instabilities are attributed to charge carrier trapping inside the device. In this talk I will discuss the challenges and our progresses in understanding of charge trapping phenomena in OFETs. First, I will briefly introduce the charge trapping phenomenon and discuss how to improve it under using of polymer gate dielectrics. Effects of material characteristics of gate-dielectric polymers on the OFET operation will be focused. Next, I will introduce several approaches for analyzing charge traps at the semiconductor-dielectric interface. The latter part includes our recent studies using photoexcited charge collection spectroscopy, a novel experimental method to probe charge traps. The new spectroscopic method has been applied to investigate the relationships between charge traps and the chemical/physical structure of materials. Especially, I will overview the recently reported polymer semiconductors with high charge mobility yet low crystalline ordering, and present our recent results on bias-stress induced charge trapping for this type of polymer semiconductors. All these systematic study results on the bias instability of OFETs would contribute to unveiling the charge trapping mechanisms and to realizing the robust and practical OFETs eventually.

Recent demonstration of mobilities in excess of 10 cm2/Vs have energized research in solution deposition of polymers for thin film transistor applications. Due to the pi-stacking motif of most semiconducting polymers, the local mobility is expected to be intrinsically anisotropic. Fabrication of aligned films enables further optimization of device performance and can enable detailed structural characterization. Blade coating is an excellent prototyping tool for production deposition techniques such as slot die coating. Unlike slot die coating, a pre-metered technique, blade-coating is a self-metered technique and there exist two distinct operational regimes: the Landau-Levich or horizontal dip coating regime and the evaporative regime. We report results from blade coating a number of high performing polymer materials on unpatterned and nano-structured¹ substrates. Depending on polymer and deposition regime, aligned films can be produced on unpatterned substrates. In general, the alignment appears nucleated at the air interface as bottom contact devices exhibit isotropic transport. In all cases, nano-structured substrates produce anisotropic bottom contact devices with the polymer chain aligned along the groove direction. The aligned films typically exhibit significant (c.a. 5 times) greater mobility than isotropic films. Detailed morphological studies suggest that the improvement in transport is related to alignment of the inter-crystallite regions (tie-chains). In-situ studies indicate that the coating induced alignment is facilitated by the presence of a lyotropic liquid crystal phase. The ability to independently control the alignment at the top and bottom of the films allows the deposition of novel, twisted structures.

¹Tseng, H.-R., et al., Nano Letters 2012, 12, 6353-6357.

π-Conjugated molecules, especially for electron donor-acceptor (D-A) ones, have shown promising applications in solution-processable, large-area, low-cost and flexible electronic devices.^[1] Recently tremendous progress has been made, which have yielded high performance p-type, ambipolar and n-type semiconductors. Compared with various of electron donors, new electron acceptors were seldom reported.
For the last five years, we have developed two new electron acceptors bipyrrolylidene-2,2’(1H,1’H)-dione (BPD) and (E)-[4,40-biimidazolylidene]-5,50(1H,10H)-dione (BID), derived from Pechmann dye (an old synthetic dye synthesized in 1882), for constructing D-A conjugated molecules and polymers. Fundamental studies have shown that BPD and BID exhibit low LUMOs (~-3.7 eV) and high HOMOs (~-5.3 eV), which are beneficial for both hole and electron charge carrier transport. FET studies show that conjugated polymers or small molecules based on these two electron acceptors exhibit high p-type mobilities up to 1.4 cm²V^-1s^-1 and ambipolar mobilities up to 1.24 cm²V^-1s^-1 (hole) and 0.82 cm²V^-1s^-1 (electron).^[2] In addition, BPD-based polymer can also be used as electron donors for BHJ organic solar cells.^[3]


[1] a) Sirringhaus, H. Adv. Mater. 2014, 26, 1319; b) Sokolov, A. N.; B. C.-K. Tee, C. J. Bettinger, J. B.-H. Tok, Bao Z. Acc. Chem. Res. 2012, 45, 361; c) Guo, Y.; Yu, G.; Liu, Y. Adv. Mater., 2010, 22, 4427.
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[3] Qi, P.; Wang, Z.; Liu, Z.; Yang, S.; Yang, Y.; Yao, J.; Zhang, G.; Zhang, D. Polym. Chem. 2016, 7, 3838.

π-conjugated polymeric conductors are produced via simple solution processes on many different substrates and offer a wide variety of functionalities. Their potential is highlighted not only for a substantial conductor in solar cells and supercapacitors, but also for an energy conversion medium in thermoelectronics and spintronics devices. In the applications given above, doping is one of the most indispensable techniques to increase conductivity. Because a π-conjugated core is distorted by the addition or extraction of electrons, structural and energetic disorder is unavoidable. Most recently, a novel doping method, referred to as molecular implantation, which enables the introduction of a dopant into the semiconducting polymer network^[1] and realizes a relatively high charge concentration of up to 5×10²⁰ cm^-3 with excellent controllability of the doping level^[2]. Molecular implantation enhances the original lamellar structure and band-like transport, which is supported by the observation of Hall effect and Anderson localization. However, molecular implantation is a vacuum process, and dopants are limited to only those that are evaporative small molecules.
Here, we extend the concept of molecular implantation, and present a simpler, versatile, and efficient doping method^[3]. A standard thiophene-based polymeric semiconductor, poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3.2-b]thiophenes) (PBTTT-C14), can be doped by various dopants in solution, where dopants are not necessarily fully dissolved in the solvent. A solid-state film of PBTTT-C14 was immersed into a dopant-dispersed fluorinated solvent CT-solv 180^TM (commercially available from Asahi Glass Co.), which does not cause the damage to the polymer film. Efficient charge transfer and reasonably high electrical conductivity were demonstrated with two different molecular dopants, 2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and molybdenum tris(1-(trifluoroacetyl)-2-(trifluoromethyl)ethane- 1,2-dithiolene) (Mo(tfd-COCF₃)₃). It should be noted that even with non-evaporative, insoluble dopants such as the Mo(tfdCOCF₃)_3, a metal–organic complex, a significant increase in conductivity together with a metallic nature, i.e. observation of the Hall effect and Anderson localization, is confirmed. Comprehensive studies based on UV-vis-NIR absorption, electron spin resonance (ESR) spectroscopy, and X-ray reflectivity (XRR) measurements confirm that dopant molecules are likely stored within the lamellae in PBTTT-C14, which enables two-dimensional charge transport to be established.
[1] K. Kang, S. Watanabe, et al. Nature Materials, 2016, 15, 896
[2] R. Fujimoto, S. Watanabe, et al. Organic Electronics, 2017, 47, 139
[3] R. Fujimoto, Y. Yamashita, et al. Journal of Materials Chemistry C accepted.

Raman spectroscopy is a powerful tool to characterize electron-phonon coupling in various semiconductors. Here, we investigate the polaronic nature of two well-known high-mobility, thiophene-based polymers (PBTTT and DPPT-TT) by Raman spectroscopy combined with Density Functional Theory (DFT) calculations. Chemical and electrochemical hole doping of these polymers leads to characteristics changes in the intensity ratios of prominent Raman-active modes but no significant frequency shifts. The data suggest a localization of positive polarons on the electron-rich thienothiophene cores that are present in both polymers. DFT calculations also show that the mode intensity ratio variations are most likely caused by the local electric field that originates from negatively charged dopant molecules or electrolyte anions and the positive polaron on the polymer chain. The characteristic changes of the Raman mode intensities with the degree of doping also enable in-situ mapping of charge carrier concentration in the channel of electrolyte-gated polymer transistors with high spatial resolution.

Molecular doping of conjugated polymers is a widely explored tool for the preparation of highly conducting materials for organic thermoelectrics. While doping of thin films is well understood, there is a lack of doping strategies that can be applied to bulk materials. Hence, the up to millimeter thick architectures, which are needed to construct the legs of a thermoelectric generator, are more challenging to realize. In this talk two bulk doping strategies are presented. Foams of poly(3-hexylthiophene) can be sequentially doped without compromise in thermoelectric performance. Further, conjugated polymers with more polar oligo ethylene glycol side chains show improved compatibility with the dopant. As a result, thermally stable p-doped films with a high electrical conductivity of 100 S/cm can be prepared. The same concept is also applicable to n-type polymers, with conductivities reaching 0.3 S/cm.

The choice of dopant and the method used to introduce a dopant can greatly influence the resulting electronic and thermoelectric properties of conjugated polymers. Multiple factors contribute to these dopant dependent properties; including how the dopant affects the polymer crystallinity and film morphology, how close the dopant molecule is to the pi-conjugated backbone, and how efficiently the dopant creates polarons on the polymer. In this work we investigate the influence of large molybdenum complexes as dopants vs. smaller iron complexes on the electrical conductivity and Seebeck coefficient of several conjugated polymers. For multiple polymers we find that the large molybdenum complexes lead to electrical conductivities that are over an order of magnitude higher than that of the iron complexes at low doping concentrations. However, the electrical conductivity of the polymers doped with the molybdenum complexes saturate at much lower doping concentrations than the iron complexes. Thus, the iron complexes can lead to overall greater electrical conductivities and higher power factors than the molybdenum complexes. Through grazing incidence X-ray diffraction measurements and optical spectroscopies, we find that a major reason for the large differences in the electrical conductivities of the polymers doped with the molybdenum and iron complexes is due to significantly different effects on the film morphologies.

Improving the charge carrier mobility of solution-processable organic semiconductors is critical for the development of advanced organic thin-film transistor (OTFTs) and their application in the emerging sector of printed electronics. To this end, molecular doping can be seen as a powerful tool with the potential to resolve many of the outstanding issues that are currently preventing OTFT commercialisation. Yet the addition of dopant molecules into an organic semiconductor often disrupts its microstructure, introducing defects and therefore harming long-range charge transport. In this presentation I will discuss different doping methods that can be used to enhance the charge carrier mobility in OTFTs based on a wide range of organic semiconductors including, small-molecules, polymers and small-molecule:polymer blends, with the latter system exhibiting the most promising performance characteristics. Emphasis will be placed on the “all-round” positive effect that the inclusion of a suitable dopant has on key OTFT characteristics including contact resistance, bias-stress stability and device-to-device parameter variation.

Attempts to achieve functional π-conjugated polymers without scarifying their unique processability have been of great interest, where the importance of chemical doping in such polymeric semiconductors is rediscovered in terms of energy harvesting applications. Because variation of molecular dopants is still limited to date, the best possible doping level realized by molecular doping is on the order of 10²⁰ cm^-3(approximately a few holes per ten monomer units)^[1], which is still far small for an implementation of energy harvesting devices, for example thermoelectric power generators.
Here, we demonstrate a new concept, anion exchange, to improve an efficiency of molecular doping particularly in p-type semicrystalline polymers. A solid-state thin film of standard semicrystalline semiconductor, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14) was doped simply by exposing it to acceptor dopant solution (tetracyano-2,3,5,6-tetrafluoroquinodimethane, F4-TCNQ). With the presence of an ionic compound, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), it is discovered that the F4-TCNQ radical anion intercalated in the PBTTT thin film is replaced to TFSI^– instantaneously. Spontaneous anion exchange (from F4-TCNQ^●– to TFSI^–) within the PBTTT-C14 thin film, is confirmed comprehensively by UV-Vis-NIR, FTIR and ESR spectroscopies, where a lower limit of exchange efficiency is determined to be 97.5 %. Comparison in anion exchange between various ionic compounds reveals that the exchange efficiency scales with the strength of chemical hardness of ions. Thus, control of an affinity of ions can directly modulate doping efficiency.
By optimizing chemical hardness particularly for a counter cation (Li⁺ or Na⁺) in ionic compounds, the conductivity increases up to 620 S cm^-1. The carrier concentration estimated from the Hall effect measurement reaches up to approximately 1.4 × 10²¹ cm^-3, which is to the best of our knowledge the highest value among those achieved with any other electrochemical/chemical doping methods. Owing to the high carrier concentration (one hole per one monomer unit), clear metallic signatures, such as a plateau in the temperature dependence of mobility, Pauli paramagnetism, together with Anderson localization, are confirmed unambiguously by magnetotransport and ESR measurements.
Overall results demonstrated here manifest that the porosity inherently embedded in the thin film of conducting polymers would be an ideal platform for store, conversion, and transport of molecules. Almost infinite selection of ionic compounds will help further improvements of doping concentration in conducting polymers.
[1] S. Kang, S. Watanabe et al., Nature Mater. 15, 896–902 (2016).

Due to their low-temperature solution processability, compatibility with large-area deposition techniques, intrinsic robust mechanical properties and excellent light-emitting properties, organic polymer semiconductors possess great potential as functional thin-film devices, such as field effect transistors, thermoelectric energy conversion devices and light-emitting devices. Because of these unique advantages, organic polymer films are extremely suitable for future IoT technology and large-area high-performance devices are highly required. However, in most functional devices, fine tuning of carrier concentration is key technique and, still, it is very difficult for organic polymer films to control their carrier density precisely.

Recently, we combined electrochemical doping technique with functional devices of several materials and we successfully maximized their device performance [1-8]. Here, we applied these techniques into organic polymer films and realized unique functionalities, i) insulator-to-metal transition induced by electrochemical doping, ii) electrochemically optimized thermoelectric energy conversion efficiency in organic polymer films and iii) highly functionalized light-emitting electrochemical cells based on organic polymers.

[1] Y. Yomogida, T. Takenobu, et al., Adv. Mater., 24, 4392 (2012).
[2] J. Pu, T. Takenobu, et al., Nano Lett., 12, 4013 (2012).
[3] H. Shimotani, T. Takenobu, et al., Adv. Funct. Mater., 24, 3305 (2014).
[4] K. Yanagi, T. Takenobu, et al., Nano Lett., 14, 6437-6442 (2014)
[5] Y. Kawasugi, T. Takenobu, et al., Nat. Commun., 7, 12356 (2016).
[6] J. Pu, T. Takenobu, et al., Adv. Mater., 12, 4013 (2016).
[7] J. Pu, T. Takenobu, et al., Adv. Mater., 29, 1606918 (2017).
[8] T. Sakanoue, T. Takenobu, et al., Adv. Mater., 29, 1606392 (2017).

Organic field-effect transistor (OFET) is a device that controls the current supplied to the connected other functional components such as organic light-emitting diode (OLED), organic photo-diode (OPD) or various sensors in organic electronic applications, and a fundamental unit that determines the extent of the organic electronic applications. The magnitude of the current flow from source to drain in the OFETs is proportional to the amount and the mobility of charges induced in the semiconducting organic films. Thus, the amount and the mobility of charges are of major concerns to consider for the further improvement of the performance of the OFETs. However, relatively few studies have investigated on the density of states (DOS) function that affects the amount of charge, while much research has been done on the mobility. Herein, the fundamental correlations between the DOS, charge carrier mobility and device performance in OFETs based on a series of the cyclopentadithiophene(CDT)-based Donor-Acceptor (D-A) type conjugated copolymers were investigated. The DOS of the CDT-based copolymers was extracted using the temperature-dependent experiments and the mobility measured from the corresponding OFETs was compared. Based on the results, the material structure and electrical property relationships of CDT-based D-A type copolymers will be discussed to provide the guidance for the rational design of high performance organic polymeric semiconducting materials.

The effect of solvent quality on the microstructure and organic field-effect transistor (OFET) performance of thin films of the high mobility naphthalene-diimide-thiophene copolymer P(NDI2OD-T2) is presented. A strong correlation between OFET mobility and solvent quality is observed with average electron mobility increasing from 0.21 cm²/Vs for samples spin-coated from tolerably-good solvents to 0.56 cm²/Vs for samples spin-coated from poor solvents, with a maximum electron mobility of 1.5 cm²/Vs observed for transistors processed from toluene. The variation in transistor performance with solvent quality is linked to the nature and extent of the solution aggregation of P(NDI2OD-T2) chains. Small angle X-ray scattering measurements reveal elongated rod-like aggregates up to 300 nm in length in solutions prepared using poor solvents, in contrast to more coil-like chains with radius of gyration of ~ 10 to 15 nm for solutions based on good to tolerably-poor solvents. Thin films produced from solvents of decreasing quality show an increase in the extent of correlated ordering of backbones and the degree edge-on orientation of polymer chains at the air/film interface. This work establishes the important link between solution-phase chain aggregation behavior, thin-film microstructure and transistor performance in the P(DNI2OD-T2) system.

A series of polymer semiconductors with high mobility has been developed in recent years. These polymers demonstrate p-type, n-type and ambipolar semiconducting behaviour. Among them, the most of high-performance semiconducting polymers are made of donor-acceptor structure. In this presentation,^[1-5] I will introduce our exploration in these copolymers, including design concept, synthesis of p-type, n-type and ambipolar copolymers, fabrication of OFET devices, and investigation of their electronic properties.

Zhiyuan Zhao, Zhihong Yin, Huajie Chen, Liping Zheng, Chunguang Zhu, Long Zhang, Songting Tan, Hanlin Wang, Yunlong Guo, Qingxin Tang, and Yunqi Liu, Adv. Mater., 2017, 29(4), 1602410.
Zhiyuan Zhao, Zhihong Yin, Huajie Chen, Yunlong Guo, Qinxin Tang, and Yunqi Liu, J. Mater. Chem. C, 2017, 5(11), 2892-2896.
Jie Yang, Hanlin Wang, Jinyang Chen, Jianyao Huang, Yingying Jiang, Jianqi Zhang, Longxian Shi, Yunlong Sun, Zhixiang Wei, Gui Yu, Yunlong Guo, Shuai Wang, and Yunqi Liu, Adv. Mater, 2017, 2017, 29(22), 1606162.
Jie Yang, Zhiyuan Zhao, Hua Geng, Changli Cheng, Jinyang Chen, Yunlong Sun, Longxian Shi, Yuanping Yi, Zhigang Shuai, Yunlong Guo, Shuai Wang, and Yunqi Liu, Adv. Mater., 2017, 29(36), 1702115.
Hanlin Wang, Hongtao Liu, Qiang Zhao, Zhenjie Ni, Ye Zou, Jie Yang, Lifeng Wang, Yanqiu Sun, Yunlong Guo, Wenping Hu, and Yunqi Liu, Adv. Mater., 2017, 29(32), 1701722.

Over recent years there has been tremendous progress in developing low-temperature, solution-processible conjugated polymer semiconductors that provide high charge carrier mobilities for both n-type and p-type device operation, good operational stability and other functionalities such as efficient electroluminescene, sensing or memory functions for a variety of applications. We have been interested in a family of conjugated polymers that exhibit a low degree of energetic disorder owing to a well defined planar backbone conformation in their solid state. These polymers adopt a near amorphous microstructure but exhibit high carrier mobilities > 1 cm2/Vs. We are interested in understanding the charge transport and device physics of these materials and the relationship between molecular structure, microstructure and charge diffusion. In this talk we will review the current status of their technological applications, discuss state-of-the-art in device performance and discuss the physical processes that currently limit their performance and operational reliability and how these limitations may be overcome.

Very high values of carrier mobility have been recently reported in newly developed polymer semiconductors for field-effect transistors (FETs). However, many of them show the non-ideal current-voltage characteristics that may lead to make the not suitable for direct applications in logic or driving circuits or sensing applications. We investigate how contact problems and bulk defects induce deviations from ideal current-voltage characteristics.
We derived quantitative methods to extract contact and channel properties from FETs by fast and simple characterizations. The presented methods tell how the metal-polymer contact works in FETs and reveal how carrier diffusion limits the device performance. Then we investigated how much contact resistance a polymeric FET can tolerate to allow the conventional current-voltage equations, which is derived for no contact resistance. We contend that mobility in transistors with resistive contact can be underestimated with the presence of the injection barrier, whereas mobility in transistors with gated Schottky contact can be overestimated by more than ten times. This is because the band bending and injection barrier experience a complicated evolution on account of electrostatic doping in the semiconducting polymers. For precision, carrier mobility should be presented against gate voltage and should be examined by other recommended extraction methods. [1,2,3]
Then, we demonstrate how to use bulk doping and interface modification to enhance charge injection rate as well as charge transport and to obtain almost ideal FET with advanced polymers. [4,5] In particular, when using the organic dopant bis(cyclopentadienyl)-cobalt(II) (cobaltocene, CoCp₂) at a low concentration, the FET mobility with P(NDI2DO-T2) was increased and the threshold voltage was decreased from 32.7 V to 8.8 V. Deviating from previous discoveries, we found that mobility increases first and then decreases drastically beyond a critical value of molecule ratio. Meanwhile, the intensity and width of the main peak of in-plane X-ray diffraction starts to decrease at the same critical molecule ratio, showing that dopants also induce crystallization in polymers. In addition, we found self-assembled dielectrics can provide high carrier concentrations as well as low interfacial dipolar disorders to enhance device performances.

Polymer electrolytes (e.g. ion gel) have been extensively researched for achieving high charge carrier mobility, low operation voltage and operational stability in unconventional field-effect transistors (FETs) and organic electronic devices. However, it is challenging to fabricate evaporated thin metal top gated device geometry because of the gel-like nature. Ion diffusion into the semiconducting film also interferes fundamental charge transport during operation. Here we develop a new high-capacitance polymeric solid-state ionic gate dielectrics prepared by a controlled blend consisting of the high-k and low-k polymers (e.g. P(VDF-TrFE), P(VDF-HFP), PMMA etc.) and the ion gel based on poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP) with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI] ionic liquid. Our engineered solid polymer electrolytes show huge capacitance values > 4 μF/cm², which allows high drive current at low driving voltages and also allow the direct deposition of a conductive top-gate electrode by any methods such as thermal evaporation and printing. Vacuum-metalized top gate FETs with the solid-state ionic dielectrics showed excellent carrier mobilities in a range of 3-20 cm²V^-1s^-1 for an ample set of common p-type conjugated polymers, and 2-10 cm²V^-1s^-1 for Indium gallium zinc oxide (IGZO) FET devices operating at ≤ 2 V. We also observed ambipolar charge transport in n-type conjugated polymers with electron mobility in the order of 10^-2~10^-3cm²V^-1s^-1 and a remarkable hole mobility of 0.14±0.02 cm²V^-1s^-1 in naphthalene diimide (NDI)-based conjugated polymer owing to the large hole accumulation compared to ~0.03cm²V^-1s^-1 using neat PMMA gate dielectric. These remarkable FET performances review the dependency on the ionic content, semiconductor, device dimensions, top metal gate electrode, and bulk polymer dielectric among others. Our solid-state ionic gate dielectrics allow us to achieve high charge-carrier mobility at low driving voltages and demonstrate a good candidate for realizing low-power, flexible and wearable electronics.

Blends and other multicomponent systems are used in various polymer applications to meet multiple requirements that cannot be fulfilled by a single material. In polymer optoelectronic devices it is often desirable to combine the semiconducting properties of the conjugated species with the properties of certain commodity polymers, such as mechanical robustness, pronounce hydrophobicity and low gas diffusion. Here we investigate bicomponent blends comprising high-mobility, polymeric semiconductors, such as diketopyrrolopyrrol derivatives (DPP-T-TT) and selected semicrystalline commodity polymers, and show that, owing to a highly favourable, crystallization-induced phase segregation of the two components, we can reach low-percolating network systems similar to poly(3-hexylthiophene):high-density polyethylene (HDPE) blends. Focus of the presentation will be on the effect of the insulator on the sub-threshold voltage, bias stress and On-Off ratio as well as long-term environmental device stability of such DPP-T-TT devices; examples in the field of organic photovoltaics and the growing bioelectronics area are also given.

Conjugated polymers came to an unprecedented epoch that the charge transport is limited only by small disorder within aggregated domains. Accurate evaluation of transport performance is thus vital to exploring the charge transport for further optimization of molecular design. Yet the routine method by means of the conventional field-effect transistors (FETs) may not satisfy such a requirement. Here, we show that the commonly used top-gate, bottom-contact polymer transistors suffer from the extrinsic effects in particular the ambipolar charge transport, which significantly distorts device characteristics and makes trouble to parameter extractions. By elevating contact electrodes and meanwhile incorporating a thin p-type contact doping layer, the charge injection and the channel transport can all take place at the top surface of the polymer semiconductor film, exactly alike the planar-processed Si metal-oxide-semiconductor FETs (MOSFETs). The devices are therefore termed as planar polymer transistors. Compared to the conventional counterparts, the planar polymer transistors exhibit ideal p-FET characteristics with a subthreshold slope as low as 85 mV/dec, because the planar ohmic contacts eliminate the injection barrier, the access transport, and the concurrent ambipolar conduction, i.e., better capability of exploring the charge transport in the donor-acceptor (D-A) copolymers studied.
We then, for the first time, examine quantitatively the Schottky barrier and find out that the charge injection is indeed seriously affected by the high energetic barrier. In the conventional polymer transistors, the Schottky barrier is quite high and strongly depends on the gate and drain voltages, which is the origin of many misconceptions remaining in this filed, such as mobility overestimation. For the planar polymer transistors, on the other hand, their injection barrier is much lower and is nearly independent of the gate and drain voltages, thanks to the ohmic contacts formed that enable high-efficiency field emission and/or thermionic field emission without and/or with a negligibly small barrier—typical ohmic-contact characteristics. In the subsequent transport analyses, we find that the planar polymer transistors deliver much superior reliability in fitting into the transport models as compared with the conventional rivals. However, only the planar transistors operating in the low-field regime are robust to explore the inherent transport properties due to the energetic disorder lowering by the lateral field induced by high drain voltage. Hence, this work opens up a reliable and robust approach to comprehend the delicate charge transport in conjugated polymers so as to develop high-performance semiconducting polymers for the promising plastic electronics.

Organic electronics have attracted considerable interest over the last decades promising an alternative to conventional, inorganic electronics platforms. The extensive research in the field led to great advances in the understanding of charge transport mechanisms of organic materials and remarkable enhancement of field-effect transistors (OFETs) charge carrier mobilities, which exceeded the benchmark value of 10 cm² V^-1 s^-1.^1–3 Among other materials, small molecules demonstrated outstanding charge transport properties due to their highly ordered crystalline microstructure, however they lack of the easy processability typical of polymers. The latter are indeed easily cast in thin films with industrially scalable solution based printing techniques, in spite of lower charge carrier mobilities. To fully exploit the touted potential of organic materials, a promising solution is represented by the employment of multicomponent systems in which polymer and small molecules are blended together. Different examples have been reported exploiting this strategy leading to the production of high mobility OFETs.² Despite it looks clear that optimal morphologies of these blends can lead to noticeable enhancement of charge carrier mobility when compared to their neat materials counterparts cast from solution, the charge transport mechanism is still somehow uncertain.
In this work we employ a blend system comprising the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) small molecule and the indacenodithiophene-benzothiadiazole (C₁₆IDT-BT) polymer, which has been reported showing a hole mobility exceeding 13 cm² V^-1 s^-1.² We investigate the charge transport mechanisms of this blend showing that despite a negative mobility-temperature dependence, which could be a hint of band-like transport, the polymer is active in the charge transport and it does not simply act as a filler. We employ an optical spectroscopy technique, charge modulation spectroscopy (CMS), to show the characteristic polaronic absorptions and bleaching features of IDTBT and BTBT induced by the accumulated charge carriers in the blend device. The contributions of the semiconducting polymer and the small molecule to the charge transport is clarified by coupling CMS with a confocal scanning microscope and mapping the charge carrier distribution.
We believe that a more comprehensive understanding of the charge transport mechanism of this system is not only useful to clarify the working principle of already studied blend devices, but also to engineer other multicomponent systems.

¹ B.H. Lee, G.C. Bazan, and A.J. Heeger, Adv. Mater. 28, 57 (2016).
² A.F. Paterson, N.D. Treat, W. Zhang, Z. Fei, G. Wyatt-Moon, H. Faber, G. Vourlias, P.A. Patsalas, O. Solomeshch, N. Tessler, M. Heeney, and T.D. Anthopoulos, Adv. Mater. 28, 7791 (2016).
³ C. Liu, T. Minari, X. Lu, A. Kumatani, K. Takimiya, and K. Tsukagoshi, Adv. Mater. 23, 523 (2011).

All-solution processed organic field effect transistors (OFETs) have the possibility of revolutionizing low-cost electronics thanks to the potential large area production, compatibility with flexible substrates and easy implementation in emerging technologies. OFETs are the basic building blocks for any plastic logic circuits but they are still showing limited speed of operation, thus limiting their applications. Most of the efforts in the last years have been dedicated to increase the organic semiconductor mobility, since it is an essential feature to improve the speed of OFETs. However the intrinsic high performances of the materials could not be completely exploited in short-channel transistors though they are highly attractive because of their high-frequency operation.
The main problem is the large contact resistance (R_c) between the electrodes and channel region, that is significantly larger compared to the channel resistances (R_ch) in short channel transistors with a channel length of few microns. For this reason, injection in short channel OFETs is a challenging issue and a wide number of doping techniques and surface modifications of contact metals have been investigated, achieving modestly small R_c. In addition, in bottom-contacts top-gate transistor ( BCTG ) the overlap between bottom electrodes and top gate induce a parasitic capacitance contribution to the depleted channel capacitance that limit the frequency operation of OFETs. To overcome these problems, the use of the split-gate architecture, adapted from inorganic MOSFETs, has been recently successfully applied to OFETs, obtaining operation frequency in the order of 10 MHz, achieved by adopting self-aligned photolithographic steps.[1] Using this peculiar structure, R_c can be reduced and kept constant by the split-gate voltage and the parasitic capacitance contributes can be minimized by controlling the gate electrodes overlap.
Here we report a BCTG low-voltage all-printed split-gate OFETs architecture, able to achieve hundreds of kHz regime, lower than previous reported but obtain through high resolution ink-jet printing technique, with a completely mask-less procedure. Split-gates are printed over the perylene dielectric layer, aligned with bottom contact Source and Drain electrodes, while the main-gate is printed above a second dielectric layer, minimizing the overlap with below electrodes. We can thus control carriers injection by controlling the split-gate potential and drastically reduce parasitic capacitance minimizing the gate overlap.
While a further downscaling of the device will be required to achieve higher frequency, the facile and high scalable printing process could open the possibility for the realization of high speed organic circuits, considering that the split-gate architecture is effective for both p- and n-type OFETs.

[1] Uemura, Adv. Mater. 2014, 26

The presentation will give an overview about current activities in my team on the manipulation of thin films of conducting polymers. This includes structural control on the molecular and mesoscopic scale by controlled polymerization techniques and by directed crystallization, e.g. with solvent vapor annealing. We are particularly interested in electronic and electrochemical devices which deal with chemical or electrochemical doping of the polymer films, such as thermoelectric devices, battery or actuator applications. Both p-type^[1] and n-type polymers^[2] are in the focus of research as well as measurements of electronic and ionic conductivity.
Coming from classical poly(3-hexythiophene) (P3HT)^[1] we have for example synthesized regioregular polythiophenes which bear functional pendant moieties in the side chains. Triphenylamine pendant groups have shown high potential as crosslinking units in films.^[3],[4] Both electrochemical and chemical crosslinking have been studied with in-situ spectroscopy. The approach to chemically crosslink and simultaneously dope polymer films is highlighted as new approach to stable solvent-resistant doped polymer films. Pending sulfonate or imidazolium ions in the side chains on the other hand impart the polythiophene polymers with ionic conductivity.^[5] We have experimental evidence for mixed conductivity in these conjugated polyelectrolytes. Similar to P3HT the electronic conductivity can be tuned over a wide range from insulating to around 2 S/cm by external doping with strong electron acceptors. The ionic conductivity is shown to strongly depend on the degree of water uptake. Impedance spectroscopy under controlled water vapor atmospheres and temperature give insights in the ionic conductivity with maximum measured values of 10^-2 S/cm.
References
[1] S. Ludwigs, Editor, P3HT-revisited Adv. Polym. Sci., Springer, Heidelberg (2014) & G.L. Schulz, S. Ludwigs, Adv. Funct. Mater. 27 (2017) 1603083.
[2] Y. Gross, D. Trefz, R. Tkachov, V. Untilova, M. Brinkmann, G.L. Schulz, S. Ludwigs, Macromolecules 50 (2017) 5353-5366. & K. Tremel, F.S.U. Fischer, N. Kayunkid, R. Di Pietro, R. Tkachov, A. Kiriy, D. Neher, S. Ludwigs, M. Brinkmann, Adv.Energy Mater. (2014) 1301659.
[3] O. Yurchenko, J. Heinze, S. Ludwigs, ChemPhysChem 11 (2010) 1637-1640.
[4] P. Reinold, K. Bruchlos, S. Ludwigs, Simultaneous doping and crosslinking of polythiophene films, (2017) under revision.
[5] R. Merkle, P. Gutbrod, P. Reinold, M. Katzmaier, R. Tkachov, J. Maier, S. Ludwigs, Polymer (2017) under revision.

Solution-processable high mobility semiconductors, such as polymers and single walled carbon nanotubes (s-SWCNTs), offer a concrete opportunity to develop high-performance flexible electronics. The possibility to formulate stable dispersions of carbon nanotubes by non-covalent functionalization through conjugated polymers in a wide range of solvents allows the adoption of cheap solution-based deposition techniques like inkjet printing, suitable for low-cost and scalable fabrication processes. A deep knowledge of the charge transport mechanism in the printed networks is crucial in order to address an efficient processing and realize high-performance field-effect transistors. Unfortunately, it is still unclear how the polydispersity of the semiconducting nanotubes and their interaction with the functionalizing polymer affect charge injection and transport in random or semialigned networks.
In this work, we study High Pressure Carbon Monoxide (HiPCO) CNT networks wrapped by PCPDTBT (Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4 b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]), demonstrating the possibility to tune the ambipolar behaviour in field-effect devices by varying the number of printing passes, yielding to effective charge mobilities as high as 20 cm²V ⁻¹s⁻¹ for holes and 8 cm²V ⁻¹s⁻¹ for electrons.
The different temperature dependence of mobility observed with variable printing passes - ranging from a bandlike to a thermally activated transport - denotes a not trivial interplay between polymer and nanotubes in defining the charge transport in the FET channel.
Such mutual interplay is further investigated via Charge Modulation Spectroscopy (CMS) measurements, providing a mean to singularly probe the charge transport properties of the different components in an operating device.

The willingness to fabricate transistors capable of high frequency operation is spurred by possible applications like high-resolution flexible displays or devices able to communicate via wireless. This, together with high throughput manufacturing methods such as roll-to-roll coating and inkjet printing, allow organic electronics to be an attractive viable way to make low cost electronics.
So far, a record frequency of transition of 27 MHz is achieved for transistors with lithographic contacts and evaporated semiconductor, while of 20 MHz for transistors fabricated without using masks in the production flow. Nevertheless, the maximum operational frequency of OFETs is going to increase thanks to constant improvements in polymers charge carrier mobility. For such high frequencies, the direct measurement of transistor performances becomes not trivial because of parasitic contributions or the occurrence of resonance.
In this work, a setup for scattering parameter measurement is installed, allowing reliable measurements up to 10 GHz. A process compatible with S-parameters measurement is demonstrated. OFETs are realized through a mask-less approach, combining a fs-laser process for the sintering of high resolution metal electrodes and suitable deposition technique of high mobility polymer semiconductor. For the first time, all-direct-written OFET frequency behavior is characterized using S-parameters, showing transition frequencies falling above the MHz.

π-conjugated polymers have attracted enormous attention in recent years, thanks to the affordable cost, mechanical flexibility and chemical tunability. Unlike conventional inorganic materials, which usually form highly crystalline structures with long-range order, π-conjugated polymers arrange themselves in a complicated manner with noticeable amount of amorphous and low crystalline aggregates in the solid state. Besides, the crystalline domains in polymer thin films are usually revealed to be imperfect and with finite size (thereafter called crystallites). Within each crystallite, the molecules arrange in distinct ways along each crystallographic axis as a result of the intrinsically anisotropic molecular structure and localized electron distribution within molecules. Accordingly, devices made of π-conjugated polymers exhibit highly anisotropic performance along different directions. For example, organic field-effect transistors (OFET) may exhibit electron mobility as high as 1 cm²/Vs parallel with the substrate, with several orders of magnitude lower mobility perpendicular to the substrate. On the other hand, organic solar cells (OSCs) can exhibit high mobility along direction perpendicular to the substrate while the mobility parallel to the substrate is much lower. The device performance is also revealed to be highly dependent on post-processing condition, such as the thermal annealing temperature. In order to understand the different device performance, investigation and elucidation of the crystallite structure and its thermal expansion behaviors along each crystallographic direction become inevitable.
By utilizing synchrotron-based in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS), the diffraction patterns of the crystallites within films as thin as several tens of nanometers can be readily collected in less than 1 s with sufficiently high counts. In this study, two chemically similar semiconducting polymers - Poly(3-hexylthiophene-2,5-diyl) (P3HT) and Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) – have been systematically investigated as a function of temperature. Specifically, the linear thermal expansion coefficient (LTEC) along different crystallographic directions were extracted from the plots of d-spacing of lamellar stacking and π-π stacking as a function of temperature. In addition, crystallite size, lattice disorder and thin film texture across wide range of temperature have also been quantified. The finding of varying LTEC across different temperature ranges along π-π crystallographic direction (effective for charge hopping) serves a guideline of determining the optimal thermal annealing temperature for the post-process of organic electronic devices, such as OFETs and OPVs. We believe that the in-situ GIWAXS investigation of semiconducting polymer thin films is an effective way to build meaningful morphology-process-performance correlation as a guideline for further device optimization.

Organic electronics utilize low-cost, solution-processable, and readily tunable semiconducting organic materials in a variety of applications. One common way to tune the important electronic properties of this class of materials is through molecular doping, that is, oxidizing or reducing the organic semiconductor to create an appreciable quantity of equilibrium charge carriers via polaron formation. To accomplish this doping, small organic or inorganic species must be incorporated into the polymer, causing changes in the molecular ordering. This molecular order is intimately linked with electrical properties in semiconducting polymers, making it essential to understand how incorporation of dopants change their physical structures. In this work, we discuss the properties of molecularly doped films of poly-3-hexylthiophene (P3HT) as the crystallinity of the polymer is systematically varied. The dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is introduced using sequential processing (SqP), a method where the polymer is first deposited and then the dopant is introduced in a second step that involves swelling, without dissolving the polymer layer.
Using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS), optical spectroscopy, electrical measurements, and theoretical modeling, we examine the molecular ordering of doped polymer films. The GIWAXS data shows that both the degree of crystallinity and orientation can be maintained after doping with F4TCNQ via SqP, so that more crystalline pure polymer films lead to more crystalline doped films. Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity while AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm² V^-1 s^-1 in the most crystalline samples. Temperature-dependent conductivity measurements show that polaron mobility in SqP-doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity. Theoretical modeling of the polaron absorption suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. GIWAXS further showed that regardless of the P3HT crystallinity, doping led to an increase of the lamellar lattice spacing, a decrease in the π-stack spacing, and a loss of registry in the along-the-chain direction. With these observations, corroborated by the theoretical modeling, the GIWAXS data suggest that the F4TCNQ anions reside in the P3HT lamellae between the side chains and in the amorphous regions of the film, but do not π-stack within the polymer crystallites.

The Hall effect is the primary method to measure carrier density, mobility and carrier type in materials. The most common method for measuring the Hall effect in semiconductors uses a DC magnetic field. The community has developed a well-defined protocol for removing spurious voltages in the measurement. This protocol depends are reversing the direction of the magnetic field. This protocol breaks down for materials with mobility < ~ 10 cm² /(V s). One method to extend Hall measurements to lower mobility is to use AC magnetic fields. In practice, the frequency of the AC fields is very low ~0.1 Hz. A major limitation with this method is the length of time for the AC field Hall measurement and the effects of self-heating of the sample. We present a new measurement protocol based on the reverse-field reciprocity theorem. The reverse-field reciprocity theorem considers a four-port network with current inputs and voltage measurements and an applied magnetic field. If a current is applied to two of the inputs (say 1 and 3) and a positive field B a voltage (V) is measured on terminals 2 and 4. This voltage can depend of the magnetic field V₁(B). If the current and voltage leads are interchanged, current on terminals 2 and 4, voltage measured between 1 and 3, V₂(B). The theorem states that V₂(B) = V₁(-B). This is a very general result; the only requirement of the material is that it is electrically linear. In particular, this means that thermoelectric voltages require special treatment. In the above example, V₁(B)- V₂(B) removes the offset voltage without physically reversing the magnetic field. One consequence of this theorem is that measurements with electromagnets, superconducting magnets and permanent magnets can be made on the same time scale. Another new application is the ability to measure hall voltage on time scales not previously accessible. Although this method is the basis of the “spinning current” method often used with hall generators for field measurement, the method presented here has been extended for material characterization, in particular the measurement of low mobility materials based of the reverse-field reciprocity theorem. With this method, hall measurements can be completed in 1 to 10 second. For measurement of materials like organics and polymers this speed is an great advantage if the material properties can change during the measurement interval.

The degradation of polymer organic solar cells and materials is one of the most pressing problems facing scientific and commercial development of organic electronics. Degradation can occur in the presence of light exposure together with external oxygen and moisture exposure. We light soak organic solar cell films and then utilize infrared (IR) spectroscopy to identify IR active vibrational modes and the atomistic changes occurring before and after degradation. We find significant measurable IR changes when light exposure is performed in the presence of oxygen or in an ambient environment.
As a prototype, we investigate the low-band-gap polymer PTB7 which has led to higher efficiency organic solar cells. After light exposure, the PTB7-PCBM blend films display significant increases of increased absorption at 1727 cm^-1 attributable to increased C=O modes. In conjunction there is a broad increase at 3240 cm^-1 attributed to increased hydroxyl (OH) groups within polymer [1]. Light soaking performed in the absence of oxygen/moisture do not lead to large changes in the IR active modes. Our ab-initio electronic structure simulations interpret these by oxidation at the α-C site of the alkyl chains in PTB7, with an irreversible rupture of the alkyl chain and formation of new C=O and C-O-H conformations at the α-C. P3HT-PCBM blends do demonstrate small changes around 2500 cm^-1 after light soaking, that may be connected to local H-motion induced rearrangements. Films exposed to the ambient atmosphere in the dark do not show IR active changes, identifying photo-excited singlet oxygen to be the detrimental factor. We will discuss the threshold of photon energies needed to observe photo-structural changes. Understanding nanoscale light-induced structural changes will open pathways to designing more stable organic materials for organic electronics.
[1] S. Shah, R. Biswas, T. Koschny, V. Dalal, Unusual Infrared Absorption Increases in Photo-degraded Organic Films, Nanoscale 9, 8665-8673 (2017).

Determination of transport properties of charge carriers gives important information for designing organic semiconductor devices such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). In this presentation, we studied the carrier transport properties -electron and hole drift mobilities, bimolecular recombination coefficients, and localized-state distributions- in working OLEDs in terms of impedance spectroscopy (IS). IS has been a powerful tool to study the carrier transport properties of thin-film devices with thicknesses of ~100 nm, typical active layer thickness of working OLEDs and OPVs.
The OLED configuration we studied was aluminum doped zinc oxide: AZO (150 nm)/polyethyleneimine: PEI/poly(9,9-dioctylfluore-ne-alt-benzothiadiazole): F8BT (300 nm)/ molybdenum oxide: MoO₃ (10 nm)/Al (50 nm). IS measurement was carried out using a Solartron 1260 impedance analyzer with a 1296 dielectric interface in the frequency sweep range from 1 Hz to 1 MHz.
The transport properties that we found in the OLEDs are as follows: the electron and hole mobilities of the OLEDs at 300 K are 10^-3 cm²V^-1s^-1 and 10^-5 cm²V^-1s^-1, respectively, and are consistent with those of F8BT in literature. The bimolecular recombination coefficients are determined from the imaginary part of complex impedance spectra above the luminescence turn-on voltage. The bimolecular recombination coefficients of the OLEDs are 10^-13-10^-12 cm³s^-1, which is 10^-3-10^-2 times lower than the Langevin recombination coefficient. The localized-tail-state distributions from the conduction band and the valence band mobility edges of F8BT are determined from the electric field dependences of the activation energy of the mobilities. Both localized-tail-state distributions are well described by Gaussian distributions.
Information concerning the carrier transport properties of F8BT is essential for the design of organic devices, for the analysis of degradation processes, and for the understanding of device physics. For instance, we showed that strong thickness dependence of the current efficiency of F8BT OLEDs is well reproduced by a device simulation (Silvaco, Atlas) using the transport properties obtained in this study as inputs for the simulation.

Gate insulators can influence the electronic properties of organic field-effect transistor (OFET) and the typical requirements for gate insulators are low process temperature below 400 K, flexibility, high electrical insulation, high heat resistance, and high organic solvent resistance.
CYTOP^TM (AGC Asahi glass) is a fluorine-containing polymer and has been used as gate insulators of top-gate OFET. A reason for this is that most organic semiconductors are not soluble to the fluorine solvent of CYTOP, and hence CYTOP can be formed on an organic semiconductor layer by a wet process with no damage to the semiconductor layer. Although a variety of CYTOPs having different terminal groups have been developed recently, OFET characteristics by using the CYTOPs have not been fully examined yet. In this presentation, we studied the electronic characteristics of OFETs with three types of CYTOPs and discussed the relationship between the electronic characteristics and the chemical structures of CYTOPs.
Top-gate bottom-contact OFETs with different CYTOP gate insulators were fabricated. Three types of CYTOPs were CTL-M, CTL-A and CTL-E. The polymer terminal groups of CTL-M, CTL-A, and CTL-E were -CONH-(CH₂)_n-Si(OEt)₃, -COOH, and -COOMe. 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) and C₆₀-fused N-Methylpyrrolidine-m-C₁₂-phenyl (C60MC12) were used as p-type and n-type semiconductors, respectively. The OFET characteristics of p-type and n-type OFETs were measured at different temperatures from room temperature to 150 K, and the gate bias stress test was also carried out in the OFETs. Interface localized-state distributions of the p-type and n-type organic semiconductors were determined from the temperature dependence of the transfer characteristics of p-type and n-type OFETs.
We found that the electrical stability, the field-effect mobility, and the interface localized-state density strongly depend on the polymer terminal groups of CYTOPs. We also found the relationship of acidity (for Lewis acids, acidity relates to the compound's ability to accept an electron pair) of the terminal groups and the OFET characteristics: higher acidity of the terminal groups of CYTOPs leads to higher density of interface localized states, and thereby to lower field-effect mobility and to poorer electrical stability.

Organic electronic materials have potential applications ranging from portable organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) to wearable sensors and actuators. The most widely used organic electronic materials are conjugated polymers that have both semiconducting and conducting variants. In these materials, the electronic performance is closely related to, and underpinned by, their mechanical performance. This is especially true for portable and wearable devices that may be subjected to large stresses from everyday use and the environment. Another complication is that most thin-film electronics require multiple layers to create a fully functioning device. A recent study by Finn et al. showed that the dominant failure mechanism in roll-to-roll printed thin-film OPVs was delamination of the device stack. The mechanical failure of these devices arise from poor cohesion in individual layers and weak adhesion between layers and the substrate. Additionally, failure can arise as a result of elastic mismatches between the layers that causes stress to concentrate at the interface leading to cracking and shorting.
Scratch testing is a method of rapidly characterizing the cohesion and adhesion of thin films and coatings. This technique is usually employed to compare the adhesion of a range of thin films to a particular substrate or to characterize the adhesion of one thin film to a range of substrates. In a progressive load scratch test, a stylus is moved across the surface of a sample with a linearly increasing load until failure occurs at critical loads, Lc_i. The Lc_i’s associated with failure of the thin films are a function of film-substrate adhesion, film thickness, loading rate, and the mechanical properties of both the substrate and the thin film. In this study, we utilize progressive load scratch testing to compare the cohesive strength and adhesion of (1) P3ATs as a function of alkyl side-chain length and (2) P3HT in a range of molecular weights. The goal of this work is to provide insight into improving the cohesion and adhesion of thin-film conjugated polymers and to report the results from a mechanical testing technique not typically applied to organic electronic materials.

The power conversion efficiencies of polymer/fullerene solar cells are critically dependent on the nanometer-scale morphologies of their active layers, which are typically processed from solution. Using synchrotron wide- and small-angle X-ray scattering, we have elucidated the intricate mechanism of the structural transitions from solutions to solid films of the crystalline polymer poly[bis(dodecyl)thiophene-thieno[3,4-c]pyrrole-4,6-dione] (PBTTPD) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), including the effect of the solvent additive 1,6-diiodohexane (DIH). We found that the local assembly of rigid-rod PBTTPD segments that formed in solution instantly and then relaxed within several hundred seconds upon cooling to room temperature from 90 °C could re-emerge and develop into seeds for subsequent crystallization of the polymer in the solid films. At room temperature (25 °C), the presence of DIH in chlorobenzene slightly enhanced the formation of local assembly PBTTPD segments in the supersaturated PBTTPD in PBTTPD/PC₇₁BM blend solution. Two cases of films were subsequently developed from these blend solutions with drop-casted and spin-coated methods. For spin-coated thin films (90 nm thick), which evolve quickly, polymer’s crystallinity and the fullerene packing in the solid-state thin films were enhanced in the case of involving DIH. Regarding the effect of DIH for processing the drop-casted thick films (2.5 μm thick), which evolve slowly, DIH has no observable effect on PBTTPD/PC₇₁BM structure. Our results provide some understanding of the mechanism behind the structural development of polymer/fullerene blends upon their transitions from solution to the solid state, as well as the key functions of the additive.

Solution-processed conjugated-polymer thin films have been investigated to apply for printed, large-area, and lightweight electronic devices. The control of aggregate structure of conjugated polymer is important to enhance the performance of their devices, because aggregate structure affects optical, charge transport, and mechanical properties. Poly(3-hexylthiophene) (P3HT) has been used as a prototypical conjugated polymer to study processing-structure-property relationships. A P3HT film is composed of mixture of H- and J-aggregates of P3HT, where H- and J-aggregates have side-by-side and head-to-tail structures, respectively. We have reported the structure and charge transport property of P3HT and hybrid films with respect to orientation,^[1] deposition technique,^[2] side chain flexibility,^[3] and uniaxial alignment.^[4] Here, we show a one-pot method to prepare a P3HT film composed of high purity H-aggregate up to ≈99 % using a solution process of drop casting of P3HT solution onto low-wettability substrate at room temperature. The component ratio of H- and J-aggregates was estimated by analyzing photoluminescence spectra of P3HT films. It is found that the component ratio is changed by three orders of magnitude by controlling solvent evaporation process using different substrates. From the results of contact angle, vibrational Raman spectra, and grazing-incidence X-ray diffraction as a function of the component ratio, the selective formation of H-aggregate is attributed to the increase of interchain π-orbital overlapping and the decrease of intrachain backbone planarity of P3HT molecules. Such a pure H-aggregate film exhibited 6-fold hole density in comparison to a conventional film, according to the results of transient photoconductivity measurements. References: [1] D. Kajiya, S. Ozawa, T. Koganezawa, and K. Saitow, J. Phys. Chem. C 119, 7987 (2015). [2] D. Kajiya and K. Saitow, Nanoscale 7, 15780 (2015). [3] D. Kajiya, T. Koganezawa, and K. Saitow, J. Phys. Chem. C 120, 23351 (2016). [4] M. Imanishi, D. Kajiya, T. Koganezawa, and K. Saitow, Sci. Rep. 7, 5141 (2017).

Split-gate ambipolar organic transistor technology has been proposed as a solution for simple and low-cost fabrication of complementary electronics. In a split gate device, the polarity of the transistor can be controlled. Depending on the voltage bias of an additional control gate, a unipolar type OTFT (p- or n-) can operate, exhibiting a large on/off current ratio. However, conventional split-gate ambipolar organic thin-film transistors suffer from operational instability including a large I-V hysteresis and high bias stress effects due to a charge trapping at the dielectric/semiconductor interface. Here we demonstrate that such issues can be solved with top-gate device geometry and non-planar split-gate architecture. The proposed device operates in a controllable unipolar n(or p)-type mode with more robust electrical characteristics including hysteresis-free I-V characteristics as well as higher bias stress stability than previous split-gate ambipolar devices. Furthermore, we also demonstrate that the device has higher hole and electron carrier mobilities.

Carbon nanofibers amongst other one dimensional nanostructures such as carbon nanotubes, nanowires have been used in wide potential applications such as sensors, high strength and light materials, supercapacitors and dye sensitized solar cells. Carbon nanofibers were prepared by electrospinning polyacrylonitrile nanofibers (PAN) followed by stabilization and carbonization. We have carried out a detailed study on the electrical transport and Hall Effect measurements on carbon nanofibers annealed at different temperatures (700 °C–1400 °C). It has been observed that the ratio of the D to G peaks in Raman spectroscopy and the full width at half maximum (FWHM) of 100% peak (26 °) in the x-ray diffraction spectra decreases as the annealing temperature increases from 700 to 1400 °C, which suggests that the crystallinity of the carbon nanofibers increases as the temperature increases. Also, the diameter of the carbon nanofibers decreases from 51.17 nm to 15.89 nm as the annealing temperature increases from 700 °C to1400 °C. These nanofibers have been found to exhibit a semiconducting behavior in the temperature range of 350-10 K with their room temperature resistivity’s varying from 10 to 50 mOhm.cm. The Hall Effect measurements show that Hall coefficient value was cm³/C at 300 K for the carbonized nanofibers that were annealed at 1200 °C. The negative sign of the Hall coefficient suggests that the charge carrier is electron. The electron carrier concentration and their mobility were calculated to be 1.1 x 10²⁰ /cm³ and 25.8 cm²/Vs, respectively. The value of electronic charge carrier concentration is almost an order of magnitude higher than the values reported for carbon nanofibers synthesized under similar conditions. Due to semiconducting nature of carbon nanofibers, superior carrier concentration, and carrier mobility, carbon nanofibers have the potential to be used in non-silicon based integrated circuits.

Conjugated polyelectrolytes (CPEs) are polymers that consist of backbones with π-delocalized electronic structures and pendant substituents with ionic functionalities. Conjugated polyelectrolytes (CPEs) have attracted significant attention as active materials in polymer optoelectronic devices, such as polymer solar cells (PSCs), polymer light-emitting diodes (PLEDs) and organic thin film transistors (OTFT). Here we report a facile route to the preparation of CPEs which enables the density of the ionic groups to be easily modulated. A family of CPEs was readily prepared by an aromatic substitution reaction on a pre-formed polymer backbone, enabling the properties to be systematically investigated without the complication of varying molecular weight and dispersity. Such an approach allows the optical and electrical properties to be finely tuned. A similar approach to the preparation of conjugated zwitterionic polymers is also reported.

The structure of a conjugated polymer and its solid-state assembly are without a doubt the most important parameters determining its properties and performance in (opto)-electronic devices. A huge amount of research has been dedicated to tuning and understanding these parameters and their implications in the basic photophysics and charge transporting behaviour. The lack of reliable high-resolution analytical techniques constitutes however a major limitation, as it hampers a better understanding of both the polymerisation process and the formation of the functional thin films used in devices.
Here, by combining vacuum electrospray deposition and high-resolution scanning tunnelling microscopy (STM) we demonstrate the ability of imaging conjugated polymers with unprecedented detail, thereby unravelling structural and self-assembly characteristics that have so far been impossible to determine.
Applying this novel technique to prototypical DPP-containing polymers, we show that sub-molecular resolution STM images allow us to precisely identify the monomer units and the solubilising alkyl side-chains in individual polymer strands. Based on this, it becomes possible to determine the molecular number distribution of the polymer by simply counting the repeat units. More importantly, we demonstrate that we can identify, precisely determine the nature, locate the position, and ascertain the number of defects in the polymer backbone. This unique insight into the structure of conjugated polymers is not attainable by any other existing analytical technique and represents a fundamental contribution to the long-discussed issue of defects as a possible source of trap sites. Furthermore, the analysis of our high-resolution images, also reveals that the frequently assumed all-trans-conformation of the monomers in the polymer backbone is actually not observed, while demonstrating that the main driver for backbone conformation and hence polymer microstructure is the maximization of alkyl side-chain interdigitation.
This work has profound implications on the wider field of polymer science, representing a first, fundamental step in tackling a major and still unresolved problem, i.e. how to precisely and reliably characterise a polymeric macromolecule.

In this presentation we will give an update on our work on organic photodetectors for curved X-ray detectors, reflective pulse oximetry and retinal implants.

We have previously reported X-ray detectors on very thin plastic substrates with medical-grade performance using a solution-processed organic bulk heterojunction photodetector. This greatly simplifies the manufacturing process, opening the door to lower-cost photodetectors. As a next step, we show their advantage when curved.

Curved surfaces are the most natural shape for photodetectors – just think of the human eyeball. As a proof-of-concept, our detector-on-plastic was curved and then integrated into a Dental Cone-beam Computer Tomography (CBCT) X-ray system. CBCT is a technology that creates three-dimensional reconstructions of objects based on two-dimensional X-ray images. The curved detector’s more uniform image quality combined with enhanced reconstruction algorithms allowed the proof-of-concept system to deliver better 3D reconstructions than previous solutions. Our detector-on-foil furthermore allows the volume of 3D X-ray imaging systems to shrink by as much as 50%.

Using a low-bandgap polymer we have realized solution-processed OPDs that efficiently absorb light up to ca. 950 nm with the aim to use them as noninvasive reflective pulse oximeters. Our approach is to develop a 16x16 sensor array capable of measuring the perfusion of the microvascular tissue by means of photoplethysmography (PPG). The use of light reflection instead of tissue transillumination, enables noninvasive monitoring from virtually any skin surface.

Many research groups are working hard to develop implants for the blind. Here, organic photodetectors on plastic offer a unique chance: their softness allows them to interface intimately with neurons so that electrical signals generated by organic (semi)conducting materials are translated into bio-signals and vice versa. We will show that it is beneficial to connect organic photovoltaic cells in series by stacking them in the vertical direction: Series-connected sub-cells decrease the areal pixel current density but the higher photovoltage improves the impedance matching to the surrounding tissue and thus effectively enhances the injected current per pixel. This opens the way to generate large electrical signals without sacrificing areal density (as is the case with silicon). These advantages present a strong case for basing retinal implants on organic photovoltaic arrays on thin, flexible plastic substrates; however, this technology is still largely to be explored.

In this study, we have developed a noise reduction technique utilizing an ultraflexible and bio-conformable organic differential amplification circuit, which has a capability of amplifying differential input signal while suppressing common-mode noise. The organic circuit was fabricated on an ultraflexible 1-μm-thick parylene foil with p-channel organic thin-film transistors (OTFTs) and thin-film capacitors, and therefore, showing highly mechanical flexibility to attach onto a soft human skin surface. To demonstrate the feasibility of improving signal integrity for biosignal monitoring, we have recorded human electrocardiogram (ECG) signal. As a result, the input signal to noise ratio of -1.7 dB was improved to 20.6 dB utilizing the differential amplification circuit.
Flexible and wearable electronic system with OTFTs is expected to play an important role in realizing next-generation medical and healthcare devices because of their mechanical flexibility and lightweight properties for a curvilinear and soft human body. With bio-conformable electronic applications, various kinds of human activities can be monitored more continuously and imperceptibly. Moreover, an essential component for such flexible electronic sensor is a signal integrity (the quality of a recorded signal) to establish reliable medical applications. For monitoring electrical signals generated by human organs such as nerves, a heart, and a brain, signal monitoring with high signal integrity is necessary. However, it is still difficult because these biosignals are on the order of microvolts to millivolts, which can be easily disturbed by noise artifacts from surrounding environments. Therefore, we have developed a noise reduction technique utilizing an ultraflexible and bio-conformable organic differential amplification circuit. With a developed organic amplifier, a differential input signal of 10 mVpp sin-wave at 1 Hz was amplified to a 500 mVpp output signal, while a common-mode input signal of 100 mVpp sin-wave at 1 Hz was attenuated to a 25 mVpp output signal. These results mean the differential to the common-mode gain ratio of 200. To demonstrate the potential of improving signal integrity for biosignal monitoring, we have recorded human electrocardiogram (ECG) signal using the ultraflexible organic amplifier. As a result, 60 Hz harmonic noise, which was superimposed on input ECG signal obtained from human skin near a heart, was suppressed: the input signal to noise ratio of -1.7 dB was improved to 20.6 dB. Our flexible organic differential amplifier circuit will demonstrate the strategy for achieving high signal integrity, realized by simultaneous signal amplification and noise reduction at the end of signal collection point. This idea has relevance not only to biomedical electronics but also to many growing fields such as structural health monitoring and smart agricultures.

Recently, advances in chemical sensing based on organic thin film transistors (OTFTs) with solution processed conjugated polymers as an active layer semiconductor have shown great improvements in sensing performance.¹ Using such OTFT sensors, wide range of analytes such as volatile aliphatic and aromatic organic compounds, explosive gases and toxic solvent vapours can be detected precisely with higher sensitivity and appropriate selectivity. The sensing performance is strongly depends on the choice of organic semiconductors as an active organic channel semiconductor and its morphology at nanoscale. Thanks to the state-of-the-art solution processable diketopyrrolopyrole (DPP) based donor-acceptor (D-A) polymers which have shown tremendous progress in high performance organic electronic devices.²

In the present work, we report the detection and identification of toxic xylene isomers in gas phase using DPP polymer based OFET devices since Xylene isomers has great importance in human safety and health.³ Our ambipolar poly(diketopyrrolopyrrole-terthiophene) based OFET can detect xylene isomers at 80 ppm level, which is below the long-term permissible exposure limit (100 ppm) by the U.S. National Institute for Occupational Safety and Health. Other organic volatile analytes such as alkanes (hexane, octane and decane) as well as aromatics (benzene and toluene) have been also detected in OFET sensors.⁴ Additionally, we have also successfully demonstrated nitrobenzene, dinitrobenzene, nitromethane, trinitrotoluene and RDX based explosive OFET sensors. The sensitivity of the OFET based device has been estimated to be 1.7 µA for ppb range of TNT, 27.5 µA for ppb range of RDX at ambient conditions.⁵ The results presented in this work signify a great potential of DPP polymers for the various types of OFET based chemical sensors.

1. T. Someya, A. Dodabalapur, J. Huang, K.C. See, H.E. Katz, Chemical and Physical Sensing by Organic Field-Effect Transistors and Related Devices, Adv Mater, 2010, 22, 3799.
2. Y. Li, P. Sonar, L. Murphy, W. Hong, High mobility diketopyrrolopyrrole (DPP)-based organic semiconductor materials for organic thin film transistors and photovoltaics, Energy Environ Sci, 2013, 6, 1684.
3. Wang, T.P. Huynh, W.W. Wu, N. Hayek, T.T. Do, J.C. Cancilla, J. S. Torrecilla, M. M. Nahid, J. M. Colwell, Oz M Gazit, S. Reddy Puniredd, C. R. McNeill, P. Sonar, H. Haick, A Highly Sensitive Diketopyrrolopyrrole-Based Ambipolar Transistor for Selective Detection and Discrimination of Xylene Isomers, Adv Mater, 2016, 28, 4012.
4. B. Wang, P. Sonar, S. Manzhos, H. Haick, Diketopyrrolopyrrole copolymers based chemical sensors for the detection and discrimination of volatile organic compounds, Sens. Actuator B-Chem., 2017, 251, 49.
5. S. G. Surya, S. S. Nagarkar, S. K. Ghosh, P. Sonar, V Ramgopal Rao, OFET based explosive sensors using diketopyrrolopyrrole and metal organic framework composite active channel material, Sens. Actuator B-Chem., 2016, 223, 113.

With the advent of the Internet of Things (IoT), strong demand has grown for flexible and wearable sensors. Particularly, sensors based on small organic molecules and polymers have recently attracted great interest due to their high potential for use in flexible, low-cost, solution-processable, large-area electronics. Functional properties of organic and polymeric active layers can be tailored by rational molecular design or surface functionalization to enhance their selectivity and sensitivity. Nanoscopically engineered organic and polymeric semiconducting materials have emerged as promising building blocks for high-performance flexible sensors. In this talk, the development of high-performance organic and polymeric semiconductors will be presented with viable approaches to selectively tune the dominant polarity of charge carriers and achieve efficient charge transport, which embrace the rational design of conjugated backbones, side-chain engineering, microstructural and morphological control. Unconventional organic and polymeric nanomaterials covering single-crystalline nanowires, nanoporous films, core-shell nanomaterials, multiple-patterned plasmonic nanostructures, and chiral supramolecules will be described with their applications in flexible and wearable sensors including photodetectors, chemical and biological sensors. In addition, the fundamental charge transport and photophysical phenomena of molecule-based active layers will be discussed.

The brain can perform massively parallel information processing while consuming only ~1- 100 fJ per synaptic event. Two-terminal memristors based on filament forming metal oxides (FFMOs) or phase change memory (PCM) materials have recently been demonstrated to function as non-volatile memory that can emulate neuronal and synaptic functions. Despite recent progress in the fabrication of device arrays however, to date no architecture has been shown to operate with the projected energy efficiency while maintaining high accuracy. A major impediment still exists at the device level, specifically, a resistive memory device has not yet been demonstrated with adequate electrical characteristics to fully realize the efficiency and performance gains of a neural architecture. I will describe a novel electrochemical neuromorphic device (ENODe) that switches at record-low energy (<0.1 fJ projected, <10 pJ measured) and voltage (< 1mV, measured), displays >500 distinct, non-volatile conductance states within a ~1 V operating range, and achieves record classification accuracy when implemented in neural network simulations. We recently showed that combined with a Si access device we are able to achieve over 10⁶ switching events with very little degradation. I will discuss the working mechanism and paths towards further improvement in performance and stability.

Great research efforts are devoted to the development of portable and wearable sensors as they could have a broad impact on real life in different fields such as point-of-care medical applications and environmental monitoring due to the ubiquity of smart technologies and wireless communication networks,. The crucial bottleneck is the development of new smart materials capable to effectively convert the chemical information into an electrical signal. Amplified transduction of ionic or electrochemical signals is nowadays achieved in sensors that consist of three electrodes operating in a transistor configuration (see Figure 1A): Source and Drain electrodes which drive an electronic current through a semiconducting channel that is coupled to the gate electrode through an ionically conductive analyte solution.
In this contribution, we propose a nano-composite material based strategy to integrate the amplified electrochemical response and sensitivity of a three-terminal organic electrochemical transistor (OECT) into a simpler two terminal device, thus paving the way for a new generation of smart sensors. To do it, we design, synthesize and exploit a new composite material based on Ag/AgCl nanoparticles and PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate)). The Ag/AgCl gate electrode, which is the transducer of the OECT, is embedded into the semiconducting polymer in the form of nanoparticles and, as a consequence, the sensor combines an intrinsically amplified response with a simple two terminal electrical connection. In order to demonstrate our strategy, we investigate the novel composite material by electrostatic force microscopy, scanning electron microscopy and electrochemical impedance spectroscopy and show that the spontaneous interaction between the NPs and Cl^- ions present in the sample solution is directly coupled to the charge transfer process into the semiconductor, so inducing a fast modulation of the channel conductivity. Consequently, the current flowing in the channel is directly related to the logarithm of Cl^- ions concentration. We show two application examples that demonstrate the efficacy and the robustness of our approach. The first regards a real-time, portable sensor for in-situ detection of salinity in water. The second is a textile sensor, obtained on a cotton yarn, for real-time sweat monitoring. The here presented sensors show an excellent reliability, as demonstrated by comparing the results obtained analyzing real-life samples with our sensors and with standard chemical analyses. Finally, our approach was successfully used to fabricate novel sensors for Br^-, I^- and S^2-, demonstrating the widespread applicability of such devices.

Simultaneous sensing and visualization of pressure provides a useful platform to obtain information about a pressurizing object, but the fabrication of such interactive sensors at the single-device level remains challenging. Here, we present a single, non-pixelated electroluminescent (EL) capacitive pressure sensing displays that allows for both sensing and visualization of pressure. Our device is based on a two-terminal capacitor with six constituent layers: top electrode/insulator/hole transport layer/emissive layer/electron transport layer/bottom electrode. Light emission upon exposure to an alternating current field between two electrodes is controlled by the capacitance change of the insulator arising from the pressure applied on top. Besides capacitive pressure sensing, our EL sensor allows for direct visualization of the static and dynamic information of position, shape, and size of a pressurizing object on a non-pixelated single device platform. Monitoring the pressurized area of an elastomeric hemisphere on a device by EL enables quantitative estimation of the Young’s modulus of the elastomer, offering a new characterization method for the mechanical properties of soft materials.

Polymeric conductor poly(3,4-ethylenedioxythiophene)(PEDOT) has wide applications as transparent and flexible electrodes in electronic devices such as solar cells, organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). The electrical conductivity is the key property of PEDOT. Previously, the electrical conductivity of PEDOT has been reported to vary from 0.1 S/cm to 8797 S/cm (single crystal nanowires). Here, the electrical conductivity of PEDOT thin films as high as 6259 S/cm has been achieved using oxidative chemical vapor deposition(oCVD) with a growth temperature of 300. The influences of deposition temperature, post-growth acid treatment and film thickness are systematically studied. X-ray diffraction (XRD) reveals a crystallite configuration transition from edge-on to face-on with increasing growth temperature and decreasing film thickness. With the measured work function, Seebeck coefficient and temperature-dependent electrical conductivity, theoretical analysis reveals a metallic nature of the highly conductive polymer film, and suggested a theoretical mobility as high as 19.3 with Kang-Snyder model based on Boltzmann transport. In the end, a rectifier based on the PEDOT-Si diode working at 13.6 MHz is demonstrated utilizing the high mobility of the PEDOT material. Wafer scale fabrication of the radio frequency rectifier is demonstrated as a powerful feature of the oCVD process.

Organic field-effect transistors (OFETs) are being extensively studied in order to fully exploit the advantages of organic electronics, with the aim of developing low cost, large area, flexible electronic systems. One of the main factors hampering the diffusion of these devices into the consumer market is the high operating voltage, in the order of tens of volts, which negatively affects their power consumption and long term operational stability. In order to obtain transistors operating at low voltage, it is necessary to increase the gate capacitance per unit area. Two main strategies can be adopted, increasing the dielectric constant of the dielectric material employed or reducing the thickness of the dielectric layer.
In this work, we focus on the exploitation of thin dielectric films for the development of low voltage, transparent, flexible OFETs. A top-gate bottom-contact (TG/BC) configuration has been adopted, with PEDOT:PSS source and drain electrodes ink-jet printed on a flexible polyethylene naphthalate (PEN) substrate. Poly[2,5-bis(7-decylnonadecyl)pyrrolo[3,4-c]pyrrole-1,4(2 H,5 H)-dione-(E)-1,2-di(2,2′-bithiophen-5-yl)ethene] (29-DPP-TVT) and poly([N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)) (P(NDI2OD-T2)) have been used as p- and n-type semiconductors, both patterned by ink-jet printing. As gate dielectric, a thin film of parylene C has been employed both alone and in multi-layered structures with common low-k dielectrics. In all cases, the overall thickness of the dielectric layer is around 100nm.
After properly optimizing the process, we were able to obtain OFETs being operated at voltages lower than 10V. Despite the reduced thickness of the dielectric layer, very low gate leakage current density and high break-down voltage have been achieved. An array of optimized devices has been fabricated, and the yield, uniformity and flexibility have been tested, together with the long-term stability of the transistors.
CMOS inverter logic gates with balanced output characteristics have been fabricated, thanks to proper tailoring of the transistors’ channel widths. Additionally, ring oscillators and flip-flops have been produced, thus demonstrating the applicability of our devices as starting point in order to develop more complex complementary integrated circuits.

Solution-processed polymer field-effect transitors (FETs) have been considered for many novel applications towards large area and flexible electronics, since they can enable pervasive integration of electronic functionalities through high-throughput printing technologies in all sorts of appliances, their portability and wearability. Applications are countless: personal devices (e.g. wearable health monitoring devices) to large-area sensors (e.g. electronic skin, bio-medical devices), and smart tagging of products with radio-frequency identification tags. A strong driving force has come from flexible and/or rollable displays deployable on demand, to be integrated with portable and wearable devices. However, printed FETs fabricated with scalable tools fail to achieve the minimum speed required for example to drive high-resolution displays or to read the signal from a real-time imager, where a transition frequency (f_T), i.e. the highest device operative frequency, above 10 MHz is required. In this contribution I will review our recent progress in increasing f_T in polymer and polymer-hybrids based devices by combining only printing and laser-based direct-writing techniques. In particular, fine control of the microstructure of the pristine or blended semiconducting phase and uniaxial alignment of polymer chains allows to overcome temperature activation of mobility, and to boost charge-carrier mobility in FETs. Along with a drastic reduction of capacitive parasitism, we demonstrate the possibility to achieve MHz operation in all-organic transistors on plastic foils, and tens of MHz regime in fast direct-written devices employing metallic electrodes. Efforts to reduce the operation voltages down to batteries compatible values, while retaining high operational frequency, will also be reported.

Improvements in organic electronic material synthesis and device performance have enabled printed organic electronics for systems applications. However, circuit design requires adapting to the performance limitations of current organic thin film transistors (TFT), particularly printed devices. This work focuses on the design and implementation of a multi-sensor smart tag with printed organic TFT circuits on a flexible substrate. The goal is a circuit that self-cycles through multiple sensor outputs connecting them to a single analog-to-digital conversion channel, and designed to minimize the effects of TFT variability. N- and p-type semiconductors are inkjet printed on flexible substrates to form logic gates in conjunction with a high-k PVDF dielectric to reduce voltage operation. Based on device simulation, an astable RC vibrator is used to feed a clock and anti-clock signal into a master-slave flip flop connected as a frequency divider. The output signals from the vibrator and frequency divider show rail-to-rail operation, which address a multiplexer that selects the sensor circuit voltage output. The operation of the vibrator, which only requires a power supply, enables a continuous cycling of the outputs. The performance of the circuit is reasonably insensitive to TFT variation. The output voltage of this circuit is very stable since the operation point of the sensor circuit is independent of the performance of the OTFTs. This system is integrated with humidity, temperature and light sensors to demonstrate performance as a sensor tag.

Hemodynamic response (HR), which is associated directly with cardio-cerebrovascular diseases, such as hypertension, ischemia and Alzheimer’s disease, is a vital indicator of human health. In order to realize daily monitoring of HR with a compact, light-weight, and low-cost device, high-sensitivity optical sensing technologies have been developed in recent years. We have previously demonstrated that organic bulk heterojunction phototransistors can serve as an efficient light receiver in a flexible near-infrared (NIR) photoplethysmography (PPG) sensor [1]. The tunable and ultrahigh responsivity enables pre-amplifier-free, low-power tracking of HR, outperforming commercial PPG sensors. However, the reported single-wavelength phototransistor cannot separately extract physiologic parameters such as the concentration change of hemoglobin (Hb) and oxyhemoglobin (HbO₂), which requires selective detection of more wavelengths. Also, the bias stress-induced instability limits the application of the device for long-duration monitoring. In this work, we introduce a novel device fabrication technique − blade-assisted spin coating (BAS), which enables dual-wavelength selective detection on a single chip (which contains two phototransistors) while maintaining high light responsivity to each individual wavelength. Furthermore, we greatly enhanced the device stability via a charge-selective electrode (CSE) that can suppress electron injection and thus minimize charge trapping at the semiconductor/dielectric interface. As a result, multiday stable monitoring of HR signals with dual-wavelength channels was achieved. The study shed light on a new application direction where organic transistors may have technological advantages and ultimately lead to performance improvement.
[1] H. Xu, J. Liu, J. Zhang, G. Zhou, N. Luo, N. Zhao, Adv. Mater. 2017, 29, 1700975. https://doi.org/10.1002/adma.201700975

Organic electrochemical transistors (OECTs) have been shown to be promising devices for amplification of electrical signals and selective sensing of ions and biologically important molecules in an aqueous environment, and thus have potential to be utilised in bioelectronic applications. The sensitivity, selectivity and intensity of the response of this device is determined by the organic semiconducting polymer employed as the active layer. Until now, most OECTs have been fabricated with commercially available conducting poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) as the active layer, and therefore operated in depletion mode with limited modulation. This work presents the design of new organic semiconducting materials which demonstrate significant improvements in OECT performance, through operation in accumulation mode, with high transconductance and low operating voltage.
We discuss here the design, synthesis and performance of novel intrinsic semiconducting polymers for efficient accumulation mode OECT devices. Key aspects such as ion and charge transport in the bulk semiconductor and operational voltage and stability of the devices are addressed in order to elucidate important structure-property relationships. A range of new semiconducting polymers, designed to exhibit facile electrochemical doping of either holes or electrons, facilitate ion penetration and migration, as well as have aqueous compatibility are reported. Optimisation of a series of polymer parameters including electrochemical doping, charge carrier mobility and capacitance are discussed. This approach leads to the design of polymers that can outperform state-of-the-art PEDOT:PSS based depletion mode devices with peak transconductances above 20 mS, peak currents in the mA regime, on/off ratios above 105 and excellent switching times below 1 ms. In addition, we demonstrate that polymers with sufficiently high electron affinities and low ionisation potentials can achieve charge carrier ambipolarity, with both p and n-type device operation. Analysis by spectroelectrochemical measurements as well as electric impedance spectroscopy demonstrate a capacitance per volume unit (C*) of 397 F/cm^3.

The performance of organic semiconductors, i.e., the carrier mobility is highly dependent on the packing structures in the solid state rather than the molecular properties, e.g., the energy levels of the frontier molecular orbitals. In our study we used alkylthio groups to functionalize thienoacenes to selectively tune their packing structures and therefore semiconducting properties. Firstly, benzo[1,2-b:4,5-b’]dithiophene (BDT) was used as a model molecule, and simple methylthio groups were introduced to the α- (α-MT-BDT) or β-positions (β-MT-BDT). We found that α-MT-BDT adopted herringbone stacking and “end-on” orientation, while β-MT-BDT showed rubrene-like “pitched” π-stacking and “edge-on” orientation. As a result, the mobility of β-MT-BDT derived from the single-crystal organic field-effect transistors (SC-OFETs) was one order of magnitude higher than that of α-MT-BDT [1]. Continued with BDT as the model molecule, we put other different alkylthio groups to the BDT core with the sulfur atoms from the alkylthio groups connected to α- or β-positions. A series of alkylthio-ring modified BDT derivatives, benzo[1,2-b,4,5-b’]bis(4H-5,6-dihydrothieno[2,3-b]thiopyran (α-HTPBDT), benzo[1,2-b,4,5-b’]bis(4H-5,6-dihydrothieno[3,2-b]thiopyran (β-HTPBDT), bis(ethylenedithio)benzo[1,2-b:4,5-b’]dithiophene (BEDT-BDT) and bis(methylenedithio)benzo[1,2-b:4,5-b’]dithiophene (BMDT-BDT) were designed and synthesized. These molecules showed similar trends of packing, molecular orientation on the substrate and OFETs properties to those of MT-BDTs regarding to the position of the sulfur atoms in the alkylthio groups. Methylthionation was also successfully done in larger π-extended molecular backbone systems, e.g. naphtho[2,3-b:6,7-b’]dithiophenes (NDT) and anthra[2,3-b:6,7-b’]dithiophene (ADT). Our research presents a systematic study to tune the packing and molecular orientation on the substrate in thienoacenes and effective molecular modification strategies to develop high performance semiconducting materials in OFETs.
[1] Wang et al., Chem. Commun. 2017, 53, 9594-9597.

Vinyl linkage has been introduced to D-A conjugated polymers back bone to improve their charge carrier mobility in organic field effect transistors and exhibits promising results. However, due to the configurational (trans-cis) and conformational isomerization nature for vinylene, usually only one vinyl linker is fused in the repeating unit for those high performance conjugated polymer. It is believed the isomerization from vinylene would lead to packing disorder in the solid state. Probability of the isomerization rises along with the increasing of vinyl ratio in the repeating unit. Therefore a low crystallinity in a multi-vinyl fused conjugated polymer is inevitable, which would affect the interchain charge transporting and lower the charge carrier mobility. As a matter of fact, only a few multi-vinyl flanked D-A conjugated polymers have been studied. In our work, we present a series of multi vinyl flanked conjugated polymers based on benzothiadiazole and oligothiophene, some of which have exhibit hole mobility over 2 cm² V^-1 s^-1 under relatively low crystalline conditions in the solid state. Meanwhile, the conformational effects from vinylene are studied via single crystal and GIXRD. The effect of vinyl ratio in the repeating unit is also investigated. The results manifest that indeed the existence of multiple vinylene in polymer backbone would result in lowered crystallinity, but on the other hand they can also improve the co-planarity of the polymer backbone and provide extended conjugation length, which improve the intra-chain charge transport. Thus, some conjugated polymers with low crystallinity and high charge carrier mobility can be generated from this multi-vinyl linked method, and such might be useful in designing materials for future applications in flexible devices.

The semiconducting properties of conjugated molecular systems are determined by the intermolecular/interchain pi-pi interactions and ordered packing. But, more and more studies manifest that apart from the conjugated backbones the side alkyl chains can dictate the self-organization of the polymer owing to the need for the side chains to form low-energy, space-filling structures. In many cases, the attractive van der Waals interactions between the alkyl side chains can exert an important influence on the interchain packing and backbone conformation, and as a result the thin film microstructure and charge transporting are affected. In this presentation, I will introduce our recent explorations in this aspect, including conjugated D-A polymers with both branching and linear alkyl chains and these with functional groups such amino and urea. The results demonstrate that semiconducting performance of these conjugated polymers can be largely improved by varying the side chains, and side-chain engineering becomes a new strategy for development of high performance of organic/polymeric semiconductors.

References

Hewei Luo, Chenmin Yu, Zitong Liu, Guanxin Zhang, Hua Geng, Yuanping Yi, Katharina Broch, Yuanyuan Hu, Aditya Sadhanala, Lang Jiang, Penglin Qi, Zhengxu Cai, Henning Sirringhaus, Deqing Zhang, Sci. Adv., 2016, 2 (5), e1600076.
Jingjing Yao, Chenmin Yu, Zitong Liu, Hewei Luo, Yang Yang, Guanxin Zhang, Deqing Zhang, J. Am. Chem. Soc. 2016, 138 (1), 173−185.
Yang Yang, Guanxin Zhang, Hewei Luo, Jingjing Yao, Zitong Liu, Deqing Zhang, ACS Appl. Mater. Interfaces, 2016, 8 (6), 3635-3643.
Si-Fen Yang, Zi-Tong Liu, Zheng-Xu Cai, Matthew J. Dyson, Natalie Stingelin, Wei Chen, Hua-Jun Ju, Guan-Xin Zhang, De-Qing Zhang, Adv. Sci. 2017, 1700048.
Si-Fen Yang, Xiao Zhang, Peng-Lei Chen, Zi-Tong Liu, Jian-Wu Tian, Guan-Xin Zhang, De-Qing Zhang, Adv. Elec. Mater. 2017, DOI:10.1002/aelm.201700120.
Jing Ma, Zitong Liu, Zhijie Wang, Yizhou Yang, Guanxin Zhang, Xisha Zhang, Deqing Zhang, Mater. Chem. Frontiers, 2017, DOI: 10.1039/C7QM00307B

Various efforts have been made to increase the efficiency of organic solar cells by using CNTs, having excellent electrical and structural characteristics. One of the main drawbacks of CNT-free organic solar cells is that they absorb light only in a small wavelength range, resulting in low overall power conversion efficiency. Since CNTs vary in their chirality and energy band gaps accordingly, CNTs can be combined with organic solar cells to absorb light in a wider wavelength range. When making devices, if the metallic single-walled carbon nanotubes (m-SWNTs) present in semiconducting single-walled carbon nanotubes (sc-SWNTs)-conductive polymer composite, device will be short circuited due to the m-SWNT. Thus, the process that separates highly pure sc-SWNT from the SWNT mixture before the binding process of the SWNT with the conductive polymer is essential. But this approach requires tedious and laborious experimental steps (i.e. sonication, chemical reaction, centrifugation, post-annealing treatment, etc.) to get pure sc-SWNT.
In this study, we demonstrate that semiconducting polymer with appropriate length of side chain can selectively suspends only sc-SWNT. Under appropriate experimental conditions, polymer selectively combines with sc-SWNT, having similar circumference length with side chain length of polymer. Then, sc-SWNT-polymer composite can be easily separated from m-SWNT using mild centrifugation. We were able to simplify the conventional SWNT separation and semiconducting SWNT-conductive polymer bonding process in one step.
In this study, HipCO SWNT and CoMoCAT SWNT were used as SWNT, Poly (3-dodecylthiophene-2,5-diyl) (P3DDT) and P8TBTz-C₁₂ were used as a conductive polymer, and toluene as a solvent.
Absorption measurement clearly showed that m-SWNT was clearly removed from polymer-sc-SWNT solution obtained from experiment. Using this process, low cost, high performance CNT applied organic solar cell would be expected.

Polymer hybrid solar cells has been demonstrated that are an excellent candidates for photovoltaic devices, the polymer solar cells were prepared at room temperature by spin coater technique as the type FTO/TiO₂/PTB7:PC₇₀BM:Ag/MoO₃, the Ag nanoparticle was synthesized by chemical reduction methodology and the graphene oxide was synthesized by the modified Hummers methodology. The PTB7:PC₇₀BM was dissolved in chlorobenzene with the mixture of silver nanoparticles with 0, 0.5, 1, 3 and 5 wt%. and in the other mixture of graphene oxide of 0, 0.5, 1, 3,and 5 wt%. The homogeneous polymer ink was deposited by spin coater technique on soda lime glass substrate and FTO substrates at 400 rpm. The soda lime polymer substrates were characterized by Hall, Uv-Vis TEM, HRTEM and XRD techniques. The FTO substrates were used for make the devices and were characterized in a solar simulator in order to obtained the efficiency. The resistivity values varied as a function of silver nanoparticles concentration, 1x10⁷ to 1x10⁴ W-cm, the carrier concentration 5x10¹¹ to 7 x10¹¹ cm^-3. The best efficiency, was for the device with 1 wt% of silver nanoparticles and 1 wt% of graphene oxide.

Recently, the development of high performance non-fullerene acceptors (NFA) has made an impressive breakthrough in polymer solar cell (PSC) research. To date, many different donor polymers have been extensively studied for the NFA-based PSCs. However, only a few donor polymers have exhibited high power conversion efficiency (PCE) with the NFAs. Therefore, development of new donor polymer that can show high performance in both fullerene- and NFA-based PSCs is highly demanded.
Previously, we developed a series of donor polymers bearing 1,5-naphthyridine-2,6-dione (NTD) as a novel electron accepting building block which showed high PCE (> 9%) in fullerene-based PSC devices.^[1] Those NTDT polymers uniquely showed a large absorption coefficient, high structural coplanarity, and high crystallinity, which cooperatively contributed to the high short circuit current (Jsc) and fill factor (FF) values in thick active layer PSC. However, they showed a very low efficiency (<1%) in NFA-based PSCs due to their high crystallinity induced by linear backbone.
Herein, we present a newly designed NTD-based donor polymer (PNTD4T-o-2FB) which holds high coplanarity but with curved conformation by introducing 2,2'-(2,3-difluoro-1,4-phenylene)dithiophene as a comonomer unit. It is shown that the curved backbone conformation lowered the crystallinity but enhanced the face-on orientation of the polymer chains, which led to high PCEs in both NFA- and fullerene-based PSCs.

Reference
[1] WS Yoon, DW Kim, M-W Choi, J-M Park, SY Park, Adv. Energy. Mater. 2017, 1701467

Photovoltaics offer harvesting energy directly from sunlight hence, they are considered as an important solution to increasing demand for energy. In recent years, polymer solar cells (PSCs) have become very popular due to low-cost, light weight, flexibility, and solution processability. To reduce the fabrication cost of photovoltaic devices, the development of materials as active layers for such devices are important. Conventionally, π-conjugated polymers for optoelectronic applications can be obtained by palladium or nickel catalyzed cross coupling techniques such as Suzuki−Miyaura, Stille, Negishi, and Kumada−Corriu couplings which require several synthetic steps to prepare the metalated monomers and sometimes the synthesis and purification of such compounds is difficult. Direct arylation represents an economically attractive and ecologically benign alternative to the conventional cross-coupling reactions which provides to form carbon-carbon bonds between heteroarenes and aryl halides, which do not require organometallic intermediates thereby significantly reducing both synthetic steps, metallic by-products, and cost. Polymers comprising benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron donor tend to have promising photovoltaic properties. Its planar conjugated structure, regioregularity, easy modification and high hole mobility makes benzo[1,2-b:4,5-b′]dithiophene (BDT) one of the most successful electron-donor units for synthesis of semiconducting polymers. In this work, benzodithiophene was coupled with a series of different electron acceptors like isoindigo and benzazole to synthesize high molecular weight polymers with minimum structural defects.

TiO₂ nanoparticles (NP) involving acetyl acetone derivatives were synthesized through hydrolysis of titanium butoxide in the presence of acetyl acetone derivates and para-toluenesulfonic acid. Acetyl acetone(AcAc), 3-methylpentane-2,4-dione(MAC) and 3-phenylpentane-2,4-dione(HBS) were used as acetyl acetone derivatives. The surface of the TiO₂ NP was stabilized by the acetyl acetone derivates ligand through complexation with TiO_2.
Synthesized TiO₂ nanoparticles (NP) were applicated as electron transport layer in organic photovoltaics (OPV). OPV devices were fabricated with the conventional structure of ITO/ PEDOT:PSS/ PTB7-TH:PCBM/ TiO₂ NP/ Aluminum. All OPVs utilizing TiO₂ NP exhibited the similar power conversion efficiency (PCE) from 8.85 to 9.03%. They showed the differences in the stability during light soaking under 1sun. The OPV utilizing TiO₂-HBS NP showed PCE of 6.61% after 1000h light soaking, corresponding 73% of its initial value. OPV with TiO₂-MAC NP showed the lowest photo-stability. The PCE of the OPV was decreased to 2.22% after 1000 h light soaking that is 25% of its initial value.

Donor-Acceptor alternating conjugating copolymers with particular nanostructures have been regard as potential materials to replace inorganic semiconductors for low cost flexible optoelectronics such as transistors, solar cells, etc. However, these polymers are usually rigid and hard to process. By side chain engineering, we can not only improve the solubility but tune the conformation and self-assembly behavior of the copolymers. In the area of conjugating polymers, how polymer main chains might be packed in thin film is critical for the performance of organic optoelectronic devices.
Here, we designed and synthesized four series of donor-acceptor alternating copolymers containing alkyl side chain, oligoether side chain and semifluoroalkyl side chain through Stille Coupling reaction. The main chain is based on thieno[3,4-c]pyrrole-4,6-dione-based (TPD-based) acceptor and terthiophene-based (3T-based) donor. The bandgap of each copolymer is studied by the UV-Vis Spectroscopy and Cyclic Voltammetry. The self-assembly behaviors of copolymers are studied by Grazing Incident Wide Angle X-Ray Scattering (GI-WAXS) and Atomic Force Microscopy (AFM). Comparing the interaction and dissimilarity of semifluoroalkyl chain-alkyl chain, oligoether chain-alkyl chain and semifluoroalkyl chain-oligoether chain, we expect that the copolymers with semifluoroalkyl side chains will be easier to form nanostructures than that of alkyl side chain with reduced moisture susceptibility. The optical properties and energy band gap might be affected duo to high electronegativity of fluorine atom.
It is noteworthy that the donor-acceptor conducting copolymer with R_i chemical structure performs hexagonal cylinder structure. After annealing process, the copolymer with R_i exhibits lamellar structure and lower bandgap. According to the results of GIWAXS, the effect of phase separation leads to a higher degree of ordering in diffraction peak of hexagonal cylinder structure. This is mainly due to the advantage of replacing dodecyl side chain with oligoether side chain on the TPD, which equips copolymer with amphiphilic characteristic. By tuning the side chains on the donor-acceptor alternating conducting copolymers, we can not only tune the bandgap but also observe rare case of ordered hexagonal cylinder structure. All of these copolymers have self-assembly structure, and have the potential to be applied in the fabrication of flexible electronic devices.

Two conjugated copolymers are fabricated with donor-acceptor (D-A) and donor-acceptor-donor-acceptor (D-A1-D-A2) type molecular strategy, respectively. A classical benzothiadiazole (BT) acceptor is introduced to replace a thiophene donor unit in polymer P1 with D-A structure to achieve a narrow-bandgap terpolymer P2 with D-A1-D-A2 structure. Compared to polymer P1, the terpolymer P2 exhibits broader absorption spectrum with higher absorption coefficient, deeper the lowest unoccupied molecular orbital (LUMO) level, as well as a lower bandgap. As a result, P2-based solar cell exhibits a maximum power conversion efficiency (PCE) of 6.60%, a short-circuit current (Jsc) of 12.43 mA cm^-2, and a fill factor (FF) of 73.1 %, which is much better than those of P1-based device with a PCE of 4.70 %, a Jsc of 9.43 mA cm^-2, and a FF of 61.6 %.

The development of semiconducting conjugated polymers for organic thin film transistors (OTFTs) has been the focus of intense research efforts for their key role in plastic electronics, as well as a vision of solution processability leading to reduced costs in device fabrication relative to their inorganic counterparts. The pursuit of high-performance n-channel (electron transporting) polymer semiconductors vital to the development of robust and low-cost organic integrated circuits has faced significant challenges; mainly for poor ambient operational stability and OTFT device performance lagging far behind that of p-channel organic semiconductors (OSCs). As an alternative to the ubiquitous donor-acceptor (DA) molecular design strategy, we have fabricated a novel conjugated n-channel polymer using an all-acceptor (AA) unipolar approach. Previously, we have synthesized a high performance AA polymer, PDPP4Tz, in which bithiazole building blocks were incorporated to lower the HOMO and LUMO energy levels. PDPP4Tz showed all electron transporting mobility as high as 0.07 cm²V^-1s^-1. In order to further understand the impact of electron withdrawing moieties to conjugated polymer device performance, we have synthesized a new conjugated polymer, poly(2-(2-decyltetradecyl)-6-(5-(5’-methyl-[2,2’-bithiaol]-5-yl)-3-(5-methylpyridin-2-yl)-5-(tricosan-11-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) (DPPDPy-BTz). As an analog to benzene, the pyridine in DPPDPy-BTz is proven to lower the energy levels to achieve n-channel performance. By incorporating another electron deficient bithiazole building blocks, the preliminary electron FET mobility of the polymer is shown to be 5x10^-4 cm²V^-1s^-1. In comparison with the known DPPDPyBT, it is shown that by substituting bithiophenes with bithiazoles, we successfully lower the HOMO/LUMO energy of the DPPDPyBTz to make another all-electron transporting polymer.

In this study, a method for the formation of type II trans-crystals of highly conjugated regioregular poly(3-butylthiophene-2,5-diyl) (P3BT) and poly(3-hexylthiophene-2,5-diyl) (P3HT) through a solution crystallization process on a carbon nanotube fiber reinforcing agent has been developed. Factors such as solvent selection, polymer concentration, and vapor pressure conditions were varied to investigate mechanisms behind the assembly process for both polymer transcrystals. Macrostructures were observed to be successfully assembled under high vapor pressure conditions through characterization with optical and atomic force microscopy. Negative birefringence was observed through polarized optical microscopy as a result of lamellar alignment perpendicular to the fiber nucleating agent. Charge carrier mobility within the polymer structure is characterized by conductive atomic force microscopy and is believed to be directionally dependent based on the orientation of lamellar alignment. The facile fabrication and enhanced electrical properties of these highly ordered transcrystal structures lead to promising opportunities for their future use in flexible electronic devices.

In this talk I will summarize our recent work towards the development of high performance conjugated polymers for transistor applications. A number of different approaches will be discussed to control the polymer backbone planarity and degree of aggregation in the solid state. These include the development of novel ladder-type aromatic building blocks, the use of non-covalent interactions between adjacent aromatics and the incorporation of heavy main group elements into the main chain. Structure-property relationships will be discussed for the various approaches, and the performance of the polymers in thin-film transistor devices reported.

Directional electron-transfer events are the basis of many technologically important systems and biological processes. In this study, we demonstrate how the distance over which electron transfer occurs through organic materials can be controlled and extended. Coating of conductive surfaces with nanoscale layers of redox-active metal complexes allows the electrochemical addressing of additional but distant layers that are otherwise electrochemically silent. We also show that our composite materials can pass electrons selectively in directions that are determined by the positioning of redox-active metal complexes and the distances between them. These electron-transfer processes can be made dominantly uni- or bidirectional. Our design strategy involves 1) a set of isostructurally well-defined metal complexes with different electron affinities, 2) a scalable metal-organic spacer, and 3) a versatile assembly approach that allows systematic variation of material composition, structure, and electron transfer properties. We control the electrochemical communication between interfaces by the deposition sequence of the components and the length of the spacer, and therefore we are able to program the bulk properties of the assemblies.

We report synthesis of two highly planar conjugated polymers based on planar 4,7-bis(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (DFD) moieties: PDFDT and PDFDSe that are structured by a repeating unit of DFD-thiophene or DFD-seleonophene, respectively. Comparative study revealed that PDFDSe polymer tends more aggregating in solution phase and generates a more oxidizable conjugated system and better packed crystalline states in films with exclusive edge-on orientations. PDFDSe-based organic thin film transistor (OTFT) with Top gate bottom contact geometry performed better than PDFDT-based OTFT. With poly(methyl methacrylate) (PMMA) gate dielectric layer, the PDFDSe OTFT achieved a reasonably high hole mobility µ = 2.72 cm²V^-1s^-1 with high reproducibility. This value increases remarkably to μ = 8.52 and 20.3 cm²V^-1s^-1, respectively, for a high k-polymeric dielectric of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) and a solid-state electrolyte gate insulator (SEGI) composed of P(VDF-TrFE)/poly(vinylidene fluoride-co-hexafluroropropylene)/1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide. This improvement was attributed to the high carrier density in the semiconducting channel region, induced by the high dielectric layer capacitance. The achievement of this excellent carrier mobility demonstrates a great potential of conjugated polymers as electronic components in future electronic applications.

Polymer light-emitting diodes (PLEDs) are attractive for use in large–area displays and lighting panels, but their limited stability under current stress impedes commercialization. In spite of large efforts over the last two decades a fundamental understanding of the degradation mechanisms has not been accomplished. We demonstrate that the voltage drift of a PLED driven at constant current is caused by the formation of hole traps, which initially increases linearly with time (burn-in) and subsequently grows with the square-root of time. This transition is governed by the statistics between free and trapped holes. The mechanism for the efficiency decrease under stress, is the additional non-radiative recombination between free electrons and trapped holes. The observed trap formation rate is consistent with exciton-free hole interactions as the main mechanism behind PLED degradation. Revelation of this degradation mechanism enables us to unify the degradation of a series of poly(p-phenylene) derivatives based PLEDs.

The direct (hetero)arylation polymerization (DHAP) reaction harnesses the single-step activation and arylation of aromatic carbon-hydrogen bonds for the efficient synthesis of conjugated polymers.[1] This palladium-catalyzed reaction allows for the formation of a bond between two sp²-hybridized carbons via the coupling of the carbon-halogen (C–X) bond of one arene (or heteroarene) with the carbon-hydrogen (C–H) bond of another. By avoiding the need for transmetalating agents used in other polymerization techniques, the number of synthesis steps is reduced, the need for expensive and often unstable reagents is minimized and the production of toxic organometallic by-products is altogether eliminated. These factors contribute to a reaction which is more favourable than traditional methods for the preparation of conjugated polymers from an industrial and an environmental perspective.

Most high-performing conjugated polymers for organic photovoltaic applications contain thiophene-based repeating units. These heterocycles possess desirable electronic features and are easily functionalized with electron-accepting or -donating moieties or with solubilizing side-chains in order to tune their electronic and physical properties. However, the issue has arisen over the selectivity of the concerted metalation-deprotonation (CMD) transition state, the key step of the direct arylation mechanism which determines the selectivity of C-H bond activation.[2] There are often multiple reactive C–H bonds on a given monomer, and if the undesired bond (the “β”-C–H bond) were to be activated, it would generate a β-defect in the resulting polymer. This may lead to a disruption in both the π-conjugation of the polymer and the supramolecular organization of the material in the solid state, factors which can contribute to reduced performance in organic electronic devices.

Given the ubiquity of thiophene-based units in conjugated polymers and the assumed issues regarding selectivity, we endeavored to study using computational techniques the direct arylation mechanism on model thiophene substrates possessing various electronic features. Using density functional theory and coupled-cluster methods, activation barriers for the CMD transition states of various C-H bonds were calculated and analyzed using the distortion/interaction model. The activating effect of a halide (C–X) on thiophene derivatives was also studied. The results suggest that there are inherent features of selectivity for electron-rich or electron-poor thiophenes, and that the location of the halide moiety greatly influences coupling selectivity by activating the undesirable β-C-H bond. We expect that these findings could guide the design of monomers amenable to high-selectivity DHAP polymerizations.

[1] Pouliot, J.-R.; Grenier, F.; Blaskovits, J. T.; Beaupré, S.; Leclerc, M., Chem. Rev. 2016, 116 (22), 14225-14274.
[2] Gorelsky, S. I.; Lapointe, D.; Fagnou, K., J. Am. Chem. Soc. 2008, 130 (33), 10848-10849.

The recent development of highly flexible and stretchable organic electronics has prompted novel applications in wearable and bio-integrated electronics where skin-like properties (stretchable, soft, self-healing) are desirable for an intimate contact with the human body. But, polymers which are conductive, mechanically compliant, biosafe and easily processable remain difficult to obtain. The commonly used conductive polyelectrolyte complex PEDOT:PSS suffers from poor mechanical properties and breaks at only 5% strain. While the use of additives (Zonyl, Triton X, ionic liquids) can improve the stretchability of PEDOT:PSS, concerns about the biocompatibility and toxicity of these additives have been raised. Another strategy to obtain stretchable conductive material is to blend PEDOT:PSS with elastomers (PDMS, polyurethane) but the poor or incompatible solubility of the elastomers makes the incorporation of the composites in electronic devices difficult. Alternatively, our approach is to use a water-soluble and intrinsically stretchable triblock copolymer comprised of polystyrene sulfonate (PSS) and poly(polyethylene glycol methyl ether acrylate) (PPEGMEA) as a matrix for the in situ oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT). While PSS-based elastomers are typically synthesized via the sulfonation of polystyrene triblock copolymers, this approach requires harsh reaction conditions and does not achieve complete sulfonation, ultimately leading to defects and poorly water-soluble polymers. Instead, we are using an aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization to obtain the well-defined copolymer PSS-b-PPEGMEA-b-PSS. This ionic triblock elastomer serves as a matrix for the polymerization of EDOT, ultimately providing a water-based formulation of stretchable, conductive polymer. After processing via casting, printing or spin-coating, the conductive films can be stretched up to 120%. We are currently investigating the use of the obtained biosafe, water-soluble and conductive elastomers in wearable strain sensors, stretchable organic solar cells and electrotactile devices.

Stepwise deposition from solution, combined with metal-ligand coordination, has served as a powerful tool for generating functional architectures on surfaces. Such systems might find many applications in molecular electronics, sensor, and solar cells. More significantly, owing to their interesting electrochromic (EC) behavior, redox-active metallo-organic assemblies are promising candidates for use in smart windows. We used a dip-coating process to generate EC molecular assemblies from metal polypyridyl complexes cross-linked with a palladium salt.^1,2 These complexes are considered ideal chromophores for fabricating EC materials, due to their excellent stability and light absorption that significantly depends on their oxidation state. Varying the number of pyridine moieties was used to control (i) the materials’ stability, (ii) color, (iii) redox-chemistry, and (iv) the film growth (i.e., linear vs. exponential). Our observations also demonstrated that minor structural differences (i.e., the pyridine-bipyridine bond order, X) at the molecular level become apparent in stability and EC properties. The molecular assemblies exhibit very high coloration efficiencies and are extremely stable. Furthermore, we demonstrate solid-state devices on flexible substrates.³

1) Shankar, S.; Lahav, M.; van der Boom, M. E. J. Am. Chem. Soc., 2015, 137, 4050–4053.
2) de Ruiter, G.; Lahav, M.; van der Boom, M. E. Acc. Chem. Res., 2014, 47, 3407–3416.
3) Elool Dov, N.; Shankar, S.; Cohen, D.; Bendikov, T.; Rehav, K.; Shimon, L.; Lahav, M.; van der Boom, M. E. J. Am. Chem. Soc., 2017, 139, 11471–11481.

Conjugated polymers are emerging as a novel class of materials for large-area solid-state energy conversion and storage applications. Their versatile chemical synthesis and easy manufacturing via low-cost processes enable new paths toward more sustainable energy landscapes. Multiple organic electronics applications require however complementary pairs of p-type (hole-transporting) and n-type (electron-transporting) conductive polymers. Unlike their p-type counterparts, n-doped conducting polymers typically suffer from a relatively low electrical conductivity (< 0.01 S/cm). Despite continuous efforts to understand charge transport mechanism in these materials and how it affects the device performance, the interplay between chemical structure, polaron delocalization length, and conductivity remains unclear. Here we show that n-doped polymers do not necessarily have to follow the typical design rules of semiconducting polymers for field-effect transistors. In contrast to undoped polymers where in fact regioregularity of the backbone and crystallinity are pursued for their beneficial effect on charge carrier mobility, polymers used in their doped state should be designed to have long polaron delocalization lengths in order to reach high conductivity. Understanding these principles will guide the design of next-generation high-conductivity polymers for energy conversion and storage applications.

In recent years products utilizing organic light-emitting diodes (OLEDs) have become commercially viable in both display and lighting applications. Much of this success stems from development of highly efficient OLEDs employing phosphorescent iridium(III) complexes as the emissive species. A common property of these emitters is their low solubility which limits their processing to relatively expensive and energy inefficient vacuum thermal evaporation.
In existing technologies the high expense of processing is overcome by the large unit cost of the current applications; smartphones and televisions. This is particularly encumbering for large-area device applications, such as lighting, as the cost of vacuum thermal evaporation increases disproportionally with device area. If OLEDs are to become feasible to be employed on a wider range of electronics then another processing technique must be adopted. Of particular interest are solution processing techniques which could allow affordable roll-to-roll processing.
Previous work has shown that functionalization of the core phosphor with branched units, commonly referred to as dendrons, can be a successful strategy to enhance solubility with the additional benefit of decreasing inter-chromaphore interactions between excited emissive species. Poly(dendrimer)s also inherit the advantages of dendrimers along with improved film forming properties. Most of the work published within the literature focuses on green-emitting species based on the archetypical iridium(III) complex, Ir(PPy)₃. Solution processable materials with deep blue- or red-emission are not as well investigated.
The work covered here describes a series deep red-emitting iridium(III) complexes functionalized with first-generation carbazolyl-dendrons; all exhibiting solution photoluminescent quantum yields in the range of 61-87% with comparable film quantum yields within a blend.
In addition to being functionalized with the same carbazolyl-dendron all iridium(III) complexes within the series utilize the 2-thienylquinoline emissive ligand. The series includes two fac-homoleptic dendrimers, two trans-N,N`, cis-C,C`-heteroleptic dendrimers and a poly(dendrimer).
The two homoleptic iridium(III) complexes are functionalized with the carbazolyl-dendrons at different positions resulting in a large variation in shape and size. The two heteroleptic iridium(III) complexes utilize the same dendronized ligands as the homoleptic complexes but with one replaced by acetoacetone. The resulting variations in shape and electronic characteristics greatly alters the photophysical and electroluminescent properties of the materials.
The poly(dendrimer) is a homopolymer synthesized by ring-opening metathesis polymerization from a norbornenyl-monomer with a pendant iridium(III) complex tethered through the emissive 2-thienylquinoline ligand and two dendronized phenylpyridine ancillary ligands (M_n=164 kDa, Dispersity= 2.4).

Ferroelectric polymers can effectively improve the photovoltaic performance of solar cells, inducing an electric field to promote the dissociation of electron-hole pairs, with the thus generated charges collected from open pores. Since such performance enhancement requires materials with a unique porous crystalline structure, we herein present a novel route to highly crystalline and porous poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films utilizing a modified breath figure method based on spin coating. The key feature of the above method is the addition of small amounts of water to the acetone/P(VDF-TrFE) solution to produce porous ferroelectric thin films which have significantly higher crystallinity values than nanostructures or films prepared by other methods. Furthermore, n-Si / poly(3,4-ethylene dioxy thiophene):poly(styrene sulfonate) hybrid solar cells with porous P(VDF-TrFE) interlayers are demonstrated to exhibit spontaneous polarization sufficient for increasing their open circuit voltages and fill factors. Finite-difference time-domain simulation reveals that the electric field due to the above spontaneous polarization increases the built-in electric field of the Schottky junction between n-Si and poly(3,4-ethylenedioxythiophene) : poly (styrenesulfonate) and reduces the reverse leakage current of the Schottky diode. Thus, the organic ferroelectric thin films with controlled porosity proposed in this study are well suited for a broad range of optoelectronic applications.

In this work we report the synthesis, purification, morphological and photovoltaic evaluation of a novel fully-conjugated donor/acceptor block copolymer system based on the P3HT-b-PFTBT scaffold. The incorporation of hydrophilic tetraethylene glycol side-chains into the PFTBT acceptor block generates an amphiphilic species whose properties provide demonstrable benefits over traditional systems. This design strategy facilitates isolation of the block copolymer from homopolymer impurities present in the reaction mixture, and we show that this purification leads to better-defined morphologies. The chemical disparity introduced between donor and acceptor blocks causes spontaneous microphase separation into well-defined domains, which we demonstrate with a combination of spectroscopy, microscopy, and X-ray scattering. The morphological advantages of this system are significant.

[1] Mitchell, V. D., Wong, W. W. H., Thelakkat, M., and Jones, D. J., "The synthesis and purification of amphiphilic conjugated donor–acceptor block copolymers," Polymer Journal, Vol. 49(1), 155-161 (2016). DOI: 10.1038/pj.2016.97
[2] Mitchell, V. D.; Gann, E.; Huettner, S.; Singh, C. R.; Subbiah, J.; Thomsen, L.; McNeill, C. R.; Thelakkat, M.; Jones, D. J., "Morphological and Device Evaluation of an Amphiphilic Block Copolymer for Organic Photovoltaic Applications" Macromolecules 2017, 50 (13), 4942-4951. DOI: 10.1021/acs.macromol.7b00377

All-polymer solar cells (all-PSCs) still exhibit lower power conversion efficiency (PCE) compared to fullerene-based polymer solar cells (PSCs) due to the large-scale phase separation leading to inefficient charge dissociation at donor/acceptor (D/A) interfaces. We controlled the (D/A) interfacial morphology in all-polymer blend film using a pentafluorobenzene-based additive (FPE) with quadrupolar electrostatic interactions between polymer donor and polymer acceptor. The FPE processed blend film exhibited a bicontinuous interpenetrating morphology without large-scale phase separation and an enhanced π–π stacking with face-on orientation. The morphology changes improved the charge carrier extraction and charge transport between D/A interfaces, resulted in an increase of short-circuit current (J_SC) and fill factor (FF) yielding a PCE of 4.5%. The photovoltaic result using FPE additive represents a 55.2% improvement over the control device (2.9% → 4.5%).

With the development of material engineering, interface modification, and advanced device processing in past decades, the power conversion efficiency (PCE) of the state-of-the-art PSCs has already reached 12% at present. we designed a novel chlorinated polymer donor PBT4T-Cl, and the PSCs using PBT4T-Cl showed a high PCE of 11.18%, which is the highest PCE of chlorinated polymer-based PSCs and is also one of the highest PCEs of FBT-based PSCs reported to date. The results demonstrated chlorine substitution of the thiophene moiety could tune-finely the HOMO energy level of the corresponding polymer as well as the charge carrier mobility and the morphologies of the blend film, eventually improving its photovoltaic properties as a donor without requiring thermal annealing. The PBT4T-Cl-based devices also showed less additive dependency than the control devices did (non-Cl-based polymer). Analysis on the GIWAXS illustrated the strong crystallinity from the blend film, and AFM and TEM measurements both revealed an optimized morphology of the spin-coated PBT4T-Cl/PC₇₁BM film without thermal annealing, all of which supported the PCE enhancement of chlorine-substituted polymer, and chlorine atoms attached to a suitable backbone would obviously promote the performance of polymer solar cells. Moreover, the chlorinated polymer-based PSCs exhibited favorable stability in the lifetime test compared with non-chlorinated analogs. Therefore, chlorination of the polymer donor is a feasible strategy to simultaneously increase the performance and stability of PSCs, eventually promoting the commercialization of PSCs.

Existing hole transport material in perovskite solar cells has only been applied to toxic solvents such as chlorobenzene due to solubility problems. These solvents result in limitation for the practical use. Therefore, substitution to non-halogen solvents is important to commercialize organic eletctronics. Here, we studied on asymmetric alkyl substitution to approach high efficient polymeric semiconductors with green processing. The concept resulted in asymmetry in the monomer itself, leading to irregularity, disturbing molecular packing. We synthesize and characterize asymmetric polymer named asy-PBTBDT and fabricate hole transport layers of perovskite solar cells using applicable green solvents from Hansen solubility parameters. The green solvent is named 2-methyl anisole that is used in food additives, which is edible and no harmful in human body. The device using asymmetric polymer leads to a PCE of around 20 % using food additive solvent (2-MA) with additives (Li-TFSI and t-BP) and a PCE of 18 % without additives Furthermore, the polymeric HTM leads to longer stability than common spiro-OMeTAD. The polymeric HTM may be a promising material to replace spiro-OMeTAD due to a green processing process for commercialization, stability and high efficiency.