Symposium EQ03—Next-Generation Organic Semiconductors—Materials, Fundamentals and Applications

Organic electronics offer a multitude of new applications, benefiting from properties like flexibility, easy manufacturing, biocompatibility, etc. While OLED displays are commercially successful, organic circuits have not yet reached broad applications, mainly since device performance is lacking. In this talk, I will present new approaches based on highly ordered organic multilayer structures /1,2/ which have the potential to increase device performance by orders of magnitude. Specifically, triclinic rubrene crystals are created by heating of amorphous layers and can be electrically doped during the epitaxial growth process to achieve high hole or electron conductivity. Analysis of the space charge limited current in these films reveals record vertical mobilities of 10cm²/Vs. To demonstrate the performance of this materials system, pin-diodes with record threshold frequency of 3.7 GHz are built, paving the way for future high-performance organic electronic devices based on carefully designed heterostructures. Furthermore, these structures allow comparatively long minority carrier diffusion lengths, allowing to realize the first organic bipolar transistor.

1. M.F. Sawatzki et al., Advanced Science 8, 2003519 (2021)
2. S.-J. Wang et al., Mat. Today. Phys. 2021, 17, 100352.

Electrical doping of semiconducting polymers requires the introduction of counterions that can have profound effects on both microstructure and electrical conduction. By understanding the interaction of ionic and electronic charge carriers in the solid state, we can gain new control over the properties of semiconducting polymers. We will discuss recent work using molecular design of polymers to find controlled routes to define ion-carrier interactions. Single ion conducting polymers, known as polymeric ionic liquids (PILs), can be used as electrolytes in organic electrochemical transistors. By appropriate molecular design of the PIL, the operation of polymer transistors can be switched from bulk conduction to interfacial conduction at an electrochemical double layer. PILs can also be used to form coacervates with charged semiconducting polymers. These unique semiconducting coacervates can be processed at high solids content (~50% wt polymer) to form thick films by blade casting. Recent work using unusual counterions in PILs to modify the electrical conductivity of polymers will also be discussed.

Organic electrochemical transistors (OECTs) have been demonstrated in a wide range of applications such as analyte detection, neural interfacing, impedance sensing and neuromorphic computing. Majority of OECTs use PEDOT:PSS and liquid electrolytes. In this talk, I will discuss the development of conjugated polyelectrolytes (CPEs) as semiconductors for OECTs. CPEs are materials that comprise a conjugated polymer backbone and charge-functionalized alkyl side chains and counterions. The anionic CPE poly[2,6-(4,4-bis-potassium butanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b’]- dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBTSO3K, or CPE-K) is part of a unique class of organic semiconducting polymers that are soluble in water and become doped in the presence of a proton source. CPE-K are used as mixed conductor materials for OECTs to replace PEDOT:PSS. CPE-based OECTs operate in either the accumulation or depletion mode. We investigate the impact of the alkyl chain lengths and the conjugated backbone on the optical property, the film morphology, the electronic and ionic conductivity and the transconductance in OCETs.

Reconfigurable organic logic circuits are promising candidates to realize new computation architectures which are not obtained in the current CMOS-based architectures. This is because these devices enable the flexible and diverse circuit designs by the electrical rearrangements of individual electronic components. This feature is expected to attain high-speed data processing, low power operation and the large-scale integration of the organic logic circuits, which are the desirable requirements in the forthcoming Internet of Things society.

In this presentation, we introduce electrically reconfigurable organic logic circuits based on a dual-gate antiambipolar transistor (DG-OAAT). The transistor exhibits a sharp increase and then decrease in the drain current despite the increase in the gate voltage [1,2]. Namely, a peak drain current, which originates from negative differential transconductance, is visible even at room temperature [3,4]. Of importance is that the peak drain current in the transistor is precisely controlled by three input voltages: bottom-gate, top-gate, and drain voltages. Based on these features, we achieved multiple logic gate operations, which coincide with AND, OR, NAND, NOR, and XOR, by just adjusting the bottom-gate and top-gate voltages in a DG-OAAT. Furthermore, the control of the drain voltage induced reversible switching of two logic gates, for instance, NAND/NOR and OR/XOR. Our device concept is therefore promising for realizing multifunctional logic circuits with a simple transistor configuration, which would lead to large-scale integration of organic logic circuits.

[1] K. Kobashi, R. Hayakawa, T, Chikyow, and Yutaka Wakayama, Adv. Electron. Mater. 3, 1700106 (2017).
[2] K. Kobashi, R. Hayakawa, T, Chikyow, and Yutaka Wakayama, ACS Appl. Matter. Interfaces 10, 2762 (2018).
[3] K. Kobashi, R. Hayakawa, T, Chikyow, and Yutaka Wakayama, J. Phys. Chem. C 122, 6943 (2018).
[4] K. Kobashi, R. Hayakawa, T, Chikyow, and Yutaka Wakayama, Nano. Lett. 18, 4355 (2018).

Organic thin film transistors (OTFTs) have gained much attention for developing flexible integrated circuits (ICs) because they are intrinsically flexible and compatible with low cost and large area fabrication process.¹ To maximize performance metrics of ICs such as the reliability, amplification gain, and operation speed, it is important to control threshold voltage (V_th) of OTFTs on the same substrate, in which the performance metrics are maximized by providing functional area patterns. This leads to improve the degree of freedom of the circuit design.
In the previous reports, V_th has been controlled by using additional gate structures,² self-assemble monolayers,³ and molecular doping.⁴ Although these techniques have each advantage for high-performance OTFTs, they often suffer from complicated patterning process or low patterning resolution that hinder the development of high-performance organic ICs. Photopatternable V_th control is a promising technique because it provides a simple patterning process, that is a UV treatment through a shadow mask, and a high patterning resolution.⁵ Here we present heterogeneous molecular patterns that enable V_th control of OTFTs on the same substrate.⁶ We use polymer gate dielectrics of poly((±)endo,exo-bicyclo[2.2.1]hept-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE).⁷ PNDPE forms heterogeneous molecular patterns though a UV treatment by transforming its chemical structure from aromatic ester into ortho-hydroxyketones via the photo-Fries rearrangement. The formation of ortho-hydroxyketones in gate dielectrics programmably control V_th of respective OTFTs on the same substrate. We fabricated PNDPE-based OTFTs on a ultraflexible 1-µm-thick parylene substrate, in which the OTFTs do not change its electric characteristics with a bending radius of 0.3 mm. The V_th of both p- and n-type OTFTs are successfully controlled at an operational voltage of 2 V by varying the dose of UV treatment, in which the programmable V_th ranges from -1.5 to +0.2 V in p-type (DNTT) OTFTs and from +0.3 to +0.5 V in n-type (TU-1) OTFTs. The minimum patterning size is less than 18 µm by measuring Fourier transform infrared spectroscopic imaging that is adequate for circuit application.
We demonstrated that the photopatternable V_th control in OTFTs maximize the performance metrics of ultraflexible circuits. We first fabricated p-type zero-V_GS load inverters and their ring oscillators and illuminated UV with load OTFTs. Consequently, the noise margin, amplification gain, and stage frequency were improved up to 80%, 1200, and 7.5 kHz respectively. All of which exhibited the highest performances among previously reported zero-V_GS-based organic circuits.^5,8,9 We then fabricated complementary inverters and ring oscillators, in which V_th of both p- and n-type OTFTs are controlled by the UV illumination. By adjusting turn-on voltage of both type of OTFTs near 0 V, the complementary inverters operated at as low as ~0.5 V or sub-nW. Heterogeneous molecular patterns demonstrated in this study helps the applications for more complex ultraflexible organic ICs for signal processing including the differential amplifier and the flip-flop circuit. In the presentation, we will discuss about detailed fabrication process and electric properties of PNDPE-based OTFTs and the ultraflexible organic ICs.

References:
[1] J. Soeda, et al., Appl. Phys. Exp., 6, 076503 (2013). [2] K. Hizu, et al., Appl. Phys. Lett., 90, 093504 (2007). [3] S. Kobayashi, et al., Nat. Mater., 3, 317-322 (2004). [4] I. Lashkov, et al., ACS Appl. Mater. Interfaces, 13, 8664-8671 (2021). [5] M. Marchl, et al., Adv. Mater., 22, 5361-5365 (2010). [6] K. Taguchi et al., Adv. Mater., 2104446, (2021) [7] A. Petritz, et al., Org. Electron.,14, 3070-3082 (2013). [8] H. Takahashi, et al., Jpn. J. Appl. Phys., 58, SBBJ04 (2019). [9] A. Petritz, et al., Adv. Funct. Mater., 28, 1804462 (2018).

High-mobility organic semiconductors are the key to develop performant devices for large-area flexible electronics applications. Traditionally, organic semiconductors are π-conjugated polymers and small molecules with a molecular structure based on sp²-hybridized carbons. This peculiarity is also common to carbon nanostructures, like graphene, carbon nanotubes and fullerenes, which have been widely explored towards opto-electronic applications.
In this talk, we report on the potential of sp-hybridized cumulenic carbon atom wires as a novel class of solution processable molecular semiconductors for organic electronics. Carbon atom wires are linear chains of sp-hybridized carbon atoms and represent the ultimate one-dimensional allotropic form of carbon. ^[1] They exhibit two possible isomeric configurations: cumulenes (sequence of double bonds) and polyynes (sequence of single and triple alternated bonds). Theoretical studies have predicted outstanding mechanical, optical and electronical properties for these structures. However, only a few experimental studies have investigated their electronic properties in solid-state devices and, so far, they have focused mainly on molecular junctions or monolayers. ^[2] Recently, we have demonstrated the first example of organic field-effect-transistor (OFET) based on short cumulenic molecules, namely tetraphenylbutatriene ([3]Ph). ^[3] Herein, we discuss on our latest achievements in understanding and optimizing charge-transport properties in solution processed thin-films of [3]Ph, deposited by a scalable large-area wire-bar coating technique. Our [3]Ph based OFETs display ideal characteristics and reliable field-effect-mobilities exceeding 0.1 cm²V^-1s^-1. Moreover, in dark conditions, they provide excellent operational stability in air, thus addressing one of the greatest concerns for employing sp-systems in concrete applications.
Finally, we unveil by Raman and Charge Modulation Spectroscopy (CMS) the peculiar charge transport features of these sp-based semiconductors. CMS allows to directly probe the spectral features induced by a periodic modulation of the charge density in the transistor channel, providing information on the transport of mobile charges in the device working conditions. ^[4]
These results hold promise for the exploitation of a new library of molecular semiconductors based on sp-hybridized conjugated systems.

[1] C. S. Casari, M. Tommasini, R. R. Tykwinski and A. Milani, Nanoscale 2016.
[2] M. R. Bryce, J. Mater. Chem. C 2021
[3] A. D. Scaccabarozzi, A. Milani, S. Peggiani, S. Pecorario, B. Sun, R. R. Tykwinski, M. Caironi, and C. S. Casari, J. Phys. Chem. Lett. 2020.
[4] Meneau, A.Y.B., Olivier, Y., Backlund, T., James, M., Breiby, D.W., Andreasen, J.W. and Sirringhaus, H., Adv. Funct. Mater. (2016).

The potential impact of organic semiconductors is vast, and for that impact to be felt they need to be incorporated into real-world applications such as OLEDs, sensors, and transistors. Many of these devices are held back by deficiencies in device design, where performance is limited by charge injection and transport across material interfaces. Having to contend with a nearly infinite selection of materials and geometries makes improvement a difficult task. To streamline the search through this vast parameter space, in this work we incorporated simulations into our experimental process in a feedback loop to explore the relationship between the injection barrier, trap density of states, dielectric capacitance, and device architecture in organic field-effect transistors (OFETs) in pursuit of a design window where select parameters become non-critical. As a result, we established a device configuration window where cost and complexity of processing can be greatly reduced without a steep reduction in performance. Our simulations revealed that the dielectric capacitance (c_i) plays an important role in determining the device mobility, and critically, has an effect that differs depending on the OFET structure. As such, increasing the dielectric capacitance yields a device that is more tolerant of trap states as long as the injection barrier is not very large; however, when an injection barrier is significant, lowering c_i yields devices that are able to function effectively with even large barriers (greater than 0.5 eV). Additionally, while for the case of high-quality contacts there is no notable difference, our results show that the staggered OFET geometry outperforms the coplanar geometry in the case of non-ideal contacts. To experimentally verify these trends, we fabricated staggered-structure OFETs with small and large injection barriers and with a range of values for c_i. For the case of a small injection barrier (nominally 0.0 eV), devices performed similarly when the dielectric capacitance was varied, as predicted by simulation. Mobility for the low-c_i devices based on indacenodithiophene-co-benzothiadiazole (C₁₆IDT-BT) was measured to be 1.6 ± 0.3 cm²/Vs (average obtained on over 100 devices), overlapping with the performance of the high-c_i devices at 1.5 ± 0.1 cm²/Vs. We then purposefully introduced an injection barrier of 0.4 eV and found that the mobility of the low-c_i devices was still 80% that of the small-barrier devices, compared to a 50% decrease for the high-c_i devices. Analyzing the contact and channel resistances and corresponding simulation data, it was discovered that the voltage drop across the source contact is more severe with a higher capacitance dielectric, leaving a reduced electric field strength in the channel and correspondingly lower device mobility. On the contrary, lowering the dielectric capacitance from 37 nF/cm² to 1.5 nF/cm² minimizes the voltage loss at the contacts, resulting in a channel mobility that is insensitive to injection barriers. We took advantage of this rule to improve the performance of carbon-based and organic contacts, which typically have the advantage of low-cost processability, but have work functions that introduce a large injection barrier. We selected a readily available graphite spray to pattern contacts that, while cheap and compatible with large-scale processing, come with the penalty of a nearly 1 eV injection barrier. By lowering the dielectric capacitance from 10 nF/cm² to 1.6 nF/cm², a one order of magnitude increase in mobility was observed. In summary, the low-c_i design offers a pathway to optimize charge injection and transport in OFETs while enabling the selection of cost-effective, scalable contact materials.

Perovskites have been intensively investigated for their use in solar cells and light-emitting diodes. However, research on their applications in thin-film transistors (TFTs) has drawn less attention despite their high intrinsic charge carrier mobility. In this study, we report the universal approaches for high-performance and reliable p-channel lead-free phenethylammonium tin iodide TFTs. These include self-passivation for grain boundary by excess phenethylammonium iodide, grain crystallisation control by adduct, and iodide vacancy passivation through oxygen treatment. We found that the grain boundary passivation can increase TFT reproducibility and reliability, and the grain size enlargement can hike the TFT performance; thus, enabling the first perovskite-based complementary inverter demonstration with n-channel indium gallium zinc oxide (IGZO) TFTs. The inverter exhibits a high gain over 30 with an excellent noise margin. This work aims to provide widely applicable and repeatable methods to make the gate more open for intensive efforts towards high-performance printed perovskite TFTs.

Conjugated polymers have long been the cornerstone of organic electronics. Despite the rapid progress in applications such as OPV, OFETs, and bioelectronics, the synthetic approaches to conjugated polymers have advanced little over the last decade. Further, the perfectly alternating donor-acceptor polymer motif is nearly ubiquitous across all applications. A narrow focus on synthetic methods and polymer architectures has primarily led to polymers produced by step-growth polycondensations (with poor atom and step economy), which also limit access to more advanced architectures and result primarily in materials with poor mechanical properties. Two avenues of our research are focused on addressing these issues and aimed at sustainable, broad scope and controlled synthesis of semiconducting polymers. The first avenue is focused on advancing Direct Arylation Polymerization (DArP) to provide controlled chain growth polymerization under sustainable and scalable conditions. Toward this end our efforts in emulsion and heterogeneous polymerization methods will be discussed. As a second avenue we are focusing on non-conjugated polymers with pendant electroactive groups. Here we capitalize on the myriad of chain growth polymerization methods available with non-conjugated polymers and the structural factors that can lead to effective charge transport in polymers substituted with molecular electroactive pendants. Specifically, our focus on the influence of stereoregularity on transport and electronic properties will be discussed. Mechanical properties will also be discussed within the context of novel architectures.

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and iodoperfluroalkylation. During the photocatalytic cycle, photoirradiated PDI is reduced with a sacrificial extrinsic reductant in a diffusion-mediated process to form the radical anion R^●-, which is further excited to the doublet excited state ²[R^●-]*; a strong reducing agent. Previous work in this area employs pendant groups attached at the imide positions of PDI that do not participate in these reactions and are only added to impart solubility. In this work, we employ electron-rich pendant groups capable of self-n-doping the PDI core when irradiated with 405 nm light. We observe radical anion formation is favored at low concentrations where chain rotation is uninhibited, while biradical formation is favored at high concentrations, which we attribute to charge stabilization between interacting chromophores. Cyclic voltammetric measurements support this view by demonstrating steric encumbrance increases the Lewis basicity of anions by disrupting counterion interactions but inhibits biradical formation by disrupting aggregation. Finally, femtosecond transient absorption reveals low wavelength excitation (405 nm) preferentially favors the formation of ²[R^●-]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state ¹[N]* in hydroxylated samples. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on electronic doping, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient catalytic systems based on PDIs.

Infrared (IR) photodetection enables applications such as biomedical imaging, environmental monitoring, industrial inspection, and more. These technologies remain dominated by inorganic semiconductors that require complex fabrication, high costs, and cryogenic cooling requirements that limit their applications within emerging technologies. Conjugated polymers offer a promising alternative offering low-cost and scalable fabrication, solution processability, room temperature operation, and many other attributes that are not available using current technologies. Here, we demonstrate how precise control of the structural and electronic properties of donor–acceptor conjugated polymers with narrow bandgaps and absorption spanning the near- to long-wave infrared (NIR-LWIR: 0.9 < λ < 15 µm), enables efficient charge photogeneration and high-performing devices that operate within these spectral regions. These solution-processable devices comprises bulk heterojunction (BHJ) photodiodes and monolithic photoconductor architectures. A key feature of BHJ materials is charge generation assisted by polymer aggregation, which is essential to compensate for the energy gap law, where non-radiative recombination rates are increased as the bandgap decreases. In contrast, single-component photoconductors can be utilized to circumvent the energy gap law in very narrow bandgap materials. Furthermore, fundamental investigations of polymer and device physics have resulted in correlations that connect intrinsic molecular features and noise with device performance, resulting in photodetectors with performance that rivals commercial inorganic systems in the IR. Collectively, these investigations have enabled new soft matter systems and device paradigms enabling optical to electrical transduction of IR light.

Organic electrochemical transistors (OECTs) have attracted significant attention in recent years as promising devices for bioelectronic applications due to their low operation voltages, high sensitivity, good biocompatibility, and operability in aqueous environments, as well as excellent signal amplification characteristics.^[1,2] The core component of an OECT is an organic mixed ionic-electronic conductor (OMIEC) channel material, commonly a conjugated polymer, which is able to efficiently conduct both electronic and ionic charge carriers.
This talk focuses on the application and design rationale of the OMIEC, presenting a novel, high-performing series of fully fused n-type mixed conduction lactam polymers p(g₇NC_nN), systematically increasing the alkyl side chain content, as an example. The vast majority of channel material polymers reported for OECTs are p-type (hole transport) materials, whilst n-type (electron-transporting) counterparts lag far behind, both in performance and availability.^[3,4] This is mainly due to the difficulty in achieving both stability and a high transconductance in aqueous conditions, for n-type polymers. Many current n-type OMIECs are donor-acceptor (D-A) co-polymers, constituting alternating electron-rich and electron-deficient units, synthesized via a transition metal-mediated cross-coupling polymerization. Whilst the D-A structure offers synthetic advantages, two shortcomings of this motif are the single bond connections, allowing rotational torsion, which gives rise to conformational disorder, and the localization of the lowest unoccupied molecular orbital (LUMO) on the electron-deficient unit, both of which inhibit electron mobility.^[5] In addition, residual toxic metallic species arising from polymerization present a limitation for the polymers to be applied in bioelectronic devices.
We exemplify the use of an acceptor-acceptor (A-A) motif by presenting the design of a high electron mobility polymer series that comprises only electron-deficient repeat units locked in-plane by double bond linkers, conferring a delocalized LUMO, and eliminating torsional twists along the backbone. On account of this structure, the series offers low-lying LUMO energy levels and is polymerized via a metal-free benign aldol condensation, improving biological compatibility.^[6] Employing these polymers as channel materials in organic electrochemical transistors affords state-of-the-art n-type performance with p(g₇NC₁₀N) recording OECT electron mobility of 1.20 × 10^-2 cm² V^-1 s^-1 and a µC* figure of merit of 1.83 F cm^-1 V^-1 s^-1.
In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) the highest thermoelectric performance is observed for p(g₇NC₄N), with a maximum electrical conductivity of 7.17 S cm^-1 and a power factor of 10.5 µW m^-1 K^-2. These results are among the highest reported for n-type polymers. Importantly, this series of fused OMIECs highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, however, a nuanced synthetic approach must be taken to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time a single polymer series within both OECT and OTE applications. Whereby the bulk nature of transport renders OECTs a more attractive platform for OTE research compared to OTFTs.
References:
[1] M. Berggren, A. Richter-Dahlfors, Adv. Mater. 2007, 19, 3201.
[2] L. Q. Flagg, et al., J. Am. Chem. Soc. 2019, 141, 4345.
[3] H. Sun, et al., J. Mater. Chem. C 2018, 6, 11778.
[4] M. Moser, et al., Adv. Funct. Mater. 2019, 29, 1807033.
[5] M. Hultell, S. Stafström, 2007, 75, 104304.
[6] A. Marks, et al., Angew. Chemie Int. Ed. 2021, 60, 9368.

It has been generally accepted that tie chains, i.e., semiconducting polymer molecules that span from one crystalline domain through the interstitial amorphous region into a secondary crystalline region, are necessary for good charge carrier tranport in semicrystalline polymer semiconductors. To examine this premise, we developed a method based on neutron scattering to characterize stress transmitters (combined tie chains and trapped entanglements) in semicrystalline polymers. The results of these measurements validated theoretical models for prediction of tie chain content, which we used to examine the linkage between tie chains and charge carrier transport in semicrystalline polymers. The results of these theoretical predictions combined with organic thin film transistor measurements showed a correlation between charge carrier transport and the fraction of semiconducting polymer molecules that spanned from one crystal through the amorphous region but not through a secondary crystal. In fact, the actual tie chain content was predicted to be incredibly low or non-existent. Recent thin film swelling measurements seem to support this latter prediction. This indicates that existing design rules likely need to be re-examined.

Molecular doping is an effective approach to increase the number of charge carriers in organic semiconductors (OSC). Charge carriers introduced into the OSC by doping, can take the form of polarons and/or bipolarons.¹ Identifying the nature of the charge carriers in doped polymers is essential to optimize the doping process. The differentiation between polarons and bipolarons in doped polymers is typically done by using electron paramagnetic resonance spectroscopy and optical absorption spectroscopy.^2–4
In this work, we use Raman spectroscopy to investigate the formation of charge carriers and differentiate the Raman signatures of polarons and bipolarons, in molecularly doped poly(3-hexylthiophene-2,5-diyl) (P3HT). Two molecular dopants are used: (1) the organic salt dimesityl borinium tetrakis(penta-fluorophenyl)borate (Mes₂B⁺ [B(C₆F₅)₄]⁻), which has been demonstrated to form bipolarons in P3HT, and (2) the Lewis acid tris(pentafluorophenyl)borane [B(C₆F₅)₃], which results in the formation of only polarons.³ We trace the evolution of the Raman spectrum of neutral P3HT once doped with increasing dopant concentration, covering the ranges which result in the formation of polarons and subsequently bipolarons. First-principles calculations on oligomer model systems are used to understand the changes observed in the experimentally measured Raman spectrum of P3HT films.
We identify the signatures related to charged P3HT segments with bipolarons, and explain their origin, by tracing the shifts in the main Raman peak which originates from the symmetric collective vibrations along the polymer backbone. In neutral P3HT, it appears at ~1438 cm^-1. In the charged segments with bipolarons, the collective backbone vibrations peak blue-shifts as compared to the case of neutral P3HT to ~1466 cm^-1, whereas it red-shifts in the case of charged segments with polarons to ~1429 cm^-1. This behavior is explained by the planarization of the charged P3HT segments with polarons, while the charged segments with bipolarons exist in disordered regions of the P3HT film due to the large dopant concentrations. Furthermore, we identify new Raman peaks associated with vibrations in the quinoid doubly charged segments in molecularly doped P3HT. Our results offer the opportunity for identifying the nature of charge carriers in molecularly doped P3HT, and potentially extending the approach to other polymer systems, while taking advantage of the simplicity, versatility and non-destructive nature of Raman spectroscopy.
1. Bredas, J. L. & Street, G. B. Polarons, Bipolarons, and Solitons in Conducting Polymers. Acc. Chem. Res. 18, 309–315 (1985).
2. Scott, J. C., Pfluger, P., Krounbi, M. T. & Street, G. B. Electron-spin-resonance studies of pyrrole polymers: Evidence for bipolarons. Phys. Rev. B 28, 2140–2145 (1983).
3. Wegner, B. et al. An Organic Borate Salt with Superior p-Doping Capability for Organic Semiconductors. Adv. Sci. 2001322, 1–15 (2020).
4. Enengl, C. et al. Doping-Induced Absorption Bands in P3HT: Polarons and Bipolarons. ChemPhysChem 17, 3836–3844 (2016).

We investigated performances of inverted green and blue phosphorescent organic light-emitting diodes (OLEDs) with red dye-doped hole transport layers (HTLs). We utilized three different materials of 2,7-bis(carbazo-9-yl)-9,9-spirobifluorene (Sprio-2CBP), di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC), and 4,4′-N,N′-dicarbazole-biphenyl (CBP) for the HTL and bis(1-phenylisoquinoline) (acetylacetonate)iridium(III) ((piq)₂Ir(acac)) was used as a red dye. The Spiro-2CBP, TAPC, and CBP have different highest-occupied-molecular-orbital (HOMO) energy levels and the HOMO energy level difference between the HTL and the red dye affects the performance of devices when the red dye is doped in HTLs. We fabricated inverted green and blue phosphorescent OLEDs with the red dye-doped HTLs as well as undoped HTLs. The driving voltages of devices with the red dye-doped HTLs were slightly higher compared with those of the devices with the undoped HTLs which might be explained by HOMO energy level differences. However, the green and blue devices with the red dye-doped HTLs showed enhanced efficiencies due to improved electron-hole charge balance compared with the devices with undoped HTLs. For example, the green and blue devices with red dye-doped CBP as an HTL have 28.4% and 88.9% increased external quantum efficiencies, respectively, compared with the devices with undoped HTLs. Moreover, the colors of the green and blue devices were rarely changed despite the insertion of the red dye-doped HTL.
Acknowledgment
This research was partly supported by the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (20015805, Development of material parts and processing technology for post InP fluorescence quantum dot) and in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1F1A1045517).

Organic molecules exhibiting thermally activated delayed fluorescence (TADF) have emerged as the promising alternatives to phosphorescent complexes in electroluminescence applications, because they can convert triplet excitons for fluorescence emission. However, broad emission profiles of the TADF molecules retard applications in high-definition displays. One of the strategies toward circumventing this drawback is hyperfluorescence. The system employs two dopants; one is a singlet exciton sensitizer, and the other is a fluorescent emitter that exhibits a narrow full-width at half maximum (FWHM). Organic TADF molecules can serve as singlet exciton sensitizers, but they cannot avoid substantial losses in electroluminescence efficiencies at high current densities due to bimolecular exciton annihilation including triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA). This roll-off originates in slow reverse intersystem crossing (rISC) rates (10⁴-10⁵ s^-1) of organic TADF molecules. Therefore, a sensitizer capable of fast singlet exciton harvest is urgently needed. In this research, we have developed hyperfluorescence electroluminescence devices employing linear Au(I) complexes as singlet exciton sensitizers. The Au(I) complexes are comprised of an N-heterocyclocarbenic ligand with sterically hindering substituents, 1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-2-ylidene (^DippBZI), and anionic 1,3,6,8-tetramethyl-9H-carbazolide (TMCz) or carbazolide (Cz) ligands. Two complexes, [Au(^DippBZI)(TMCz)] and [Au(^DippBZI)(Cz)], were synthesized through multi-step syntheses, including the Pd(0)-catalyzed C−N coupling reaction between 1,2-dibromobenezene and aniline, cyclization with triethylorthoformate and tetramethylsilylchloride, and complexation with the AuCl precursor. Their chemical structures were fully characterized with single crystal X-ray crystallography and standard spectroscopic methods, including ¹H and ¹³C{¹H} NMR spectroscopy, high-resolution mass spectrometry, and elemental analyses. Our Au(I) complexes enable high-efficiency electroluminescence because of their ultrafast singlet exciton harvest attributed to rapid rISC rates (~10⁸ s^-1). The harvested excitons can be efficiently transferred to a fluorescent emitter, ν-DABNA, with a minimal occurrence of detrimental Dexter energy transfer. We fabricated multilayer hyperfluorescence electroluminescence devices with using NDP-9 as a hole-injection layer, HT211 as a hole-transporting layer, oCBP as an electron-blocking layer, mCP-2CN as a hole-blocking layer, and DPFPO as an electron-transporting layer. A mixed host system served to improve charge balance within the emitting layer. The devices incorporating [Au(^DippBZI)(Cz)]:ν-DABNA (20 wt%:3 wt%) exhibited an external quantum efficiency (EQE) as high as 31%, and a narrow FWHM of 20 nm in the blue emission region (λ_EL = 475 nm). The EQE value is a factor of two improvement relative to an EQE (14.6%) of the control device having an organic singlet exciton sensitizer, t-DABNA. The [Au(^DippBZI)(Cz)]:ν-DABNA devices also showed a reduced roll-off in their device efficiencies, compared to the control device which was attributed to ultrafast rISC and short photoluminescence lifetime of Au(I) complexes. Our strategy demonstrates that linear Au(I) complexes can serve as promising singlet exciton sensitizer leading to high-efficiency hyperfluorescence electroluminescence device.

Emissive materials are the functional components in modern and emerging technologies such as organic light-emitting diodes (OLEDs), solid-state lighting, bio and chemical sensors, and data encryption. Metal-free purely organic phosphors (POPs), as a promising novel candidate, have brought milestone evolution to emissive materials in the past decade. Compared to their conventional metal-containing counterparts, these emitters have various advantages such as large design windows, readily tunable properties, easy processability, and reduced toxicity. However, due to limitations in contemporary design strategies, the intrinsic spin-orbit coupling (SOC) efficiency of POPs remains low and their emission lifetime is pinned in the millisecond regime.

In the recent few years, we have advanced the design rules of POPs combining a renovated interpretation of the photophysical laws regularizing their performance and a computation-driven design pipeline. A new design concept, named "HAAM", has been proposed, where the two main factors that control spin-orbit coupling (SOC) — the Heavy Atom effect and orbital Angular Momentum—are tightly coupled to maximize SOC. We also visualized orbital angular momentum descriptors in molecular design assisted by a novel set of natural transition orbital-based computation methods. Guided by this new design strategy, new POP structures have been tailor-designed with boosted intrinsic SOC efficiencies and bright room-temperature phosphorescence. Advantages of these new semiconducting materials have been realized in prototype POP-based OLEDs and various "on-off" switchable solid-state systems.

Nowadays, the photochemical conversion of CO₂ to high-value products is attracting numerous research interest. Developing artificial photocatalysts with excellent catalytic activity and long-term stability is still a challenge. This work demonstrates that solution-processable naphthalenedimide (NDI)-based conjugated polymers, PNDI-BT, PNDI-DTBT and PNDI-BP, which are copolymerization products of NDI with bithiophene (BT), 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) and biphenyl (BP), respectively, can catalyze the photochemical reduction of CO₂ to produce CO in the presence of trace amounts of water without the need for metal-containing co-catalysts or sacrificial agents. In particular, the PNDI-BP-catalyzed reaction generated CH₄ as well as CO. Results from time-resolved photoluminescence, photovoltage decay, electrochemical impedance spectroscopy and transient photocurrent response experiments indicate that PNDI-BP, which had the largest dihedral angles along the conjugated backbone, possessed the longest electron lifetime, the lowest charge-carrier recombination rate, and the smallest interfacial charge transfer resistance. Consequently, it had the best catalytic performance. Notably, PNDI-BP exhibited excellent recyclability, robust structural stability, and extremely steady catalytic activity for more than 330 hours during a photocatalytic CO₂ reaction. Furthermore, the solution-processability of the linear polymer allows the incorporation of porous substrates, which improve the reaction interface. The catalyst system of PNDI-BP@molecular sieves with H₂O/triethylamine doubled the CO yield to 214.8 μmol/g and enhanced the CH₄ yield by ~36 times to 61.4 μmol/g in an 18 hr reaction.

The potential of organic materials to carry electronic and optoelectronic properties similar to inorganic materials has given them the heed to develop sustainable, cost-effective and flexible electronics. Intensive progress in design and synthesis of organic semiconductors has disclosed various materials carrying functional mobility values to fabricate electrical appliances. However, the routes to prepare a new organic semiconductor are now nearly to saturation. Insertion of more substituents or functional groups to Π - conjugated cores can still construct new materials but synthetic methodologies has their own limitations. Hence, a new attitude to modify the properties of existing or new organic semiconductor can play a pivotal role to achieve a higher mobility value or more pertinent semiconductor.
It is evident that frontier orbitals has important role in setting the charge transport within organic materials. Therefore, a concepts or a notion to alter the energy of molecular orbitals might give a chance to enhance the molecular properties to deliver improved performance. Keeping this in mind, we have designed and synthesized a new class of semiconductors which are tailored to couple with vacuum electromagnetic field. Vacuum energy is an underlying background energy that exists in space throughout the entire Universe, in which an infinite number of states exists. Under quantum confinement, vacuum energy is quantized in a finite number of states that can be couple with energy states of molecules and produces hybrid orbitals which defines a completely new material with properties different to parent molecule.

Organic thermoelectric (OTE) materials have attracted considerable attention due to their low-cost, facile solution processability, mechanical flexibility, and ability to operate at low temperature which makes them a prospective candidate for flexible/wearable thermoelectric (TE) generators. Molecular doping of conjugated polymers has been considered as an imperative process to optimize TE properties such as electrical conductivity (σ), Seebeck coefficient (S), and power factor (PF = S²σ). The TE performance of conjugated polymers can be further adjusted through molecular design of the backbone and side chain engineering. While, naturally, the backbone structure of the conjugated polymer greatly effects the electronic properties of a material and its ability to be doped, the choice of side chain connected to the backbone has a major impact on the doping of OTE materials. For example, the use of polar oligo-ethylene glycol (OEG) side chains has been proven to enhance the doping efficiency by increasing the dopant miscibility in the lamellae region of the polymer which reduces the formation of dopant aggregates. However, despite of the extensive studies on OTE materials, the relationship between structure-property of OTE materials remains elusive. Therefore, it is essential to identify the relationship between the effect of dopants, polymeric structures, and morphology to logically develop high performing OTE materials.
Here in, we have synthesized series of poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) based polymers PPDT2FBTA-12, PPDT2FBT-A14, and PPDT2FBT-A18 with different length of oligo-ethylene glycol (OEG) side chains, and investigated their TE properties. As an alternative to chemical doping, we have implicated organic electrochemical transistor (OECT) device for characterization which allowed precise monitoring of doping level through voltage control. Through introducing polar OEG side chains, we expected increased doping efficiency of PPDT2FBT series resulting in enhanced TE performance. PPDT2FBT-A12 exhibited the highest σ (∼28 S cm⁻¹) after electrochemical doping while PPDT2FBT with alkyl side chain showed that of ∼22 S cm⁻¹. The PF of PPDT2FBT-A12 was measured to be 50 times higher compared that of PPDT2FBT (2.5 vs 0.05 μW m⁻¹ K^-2). The result showed that the introduction of polar OEG side chain to the conductive polymer is an effective method to increase the σ thereby achieving higher PF of OEG substituted PPDT2FBT in OECT device.

Organic semiconductors (OSCs) have been widely studied in the past three decades fuelled by their high versatility and large number of advantages in electronics owing to the merits like solution-processability and low cost of production. The changes in the charge carrier mobilities of OSCs depends on the crystal packing as the thermal fluctuations can disrupt the molecular order which causes an effect on the charge mobility. Thus, it is crucial to investigate polymorphism and the structural aspects of crystal forms. Polymorphism can serve as a significant tool to enhance charge carrier mobility without having to modify the compound chemically.
[1]benzothieno[3,2-b]benzothiophene (BTBT) derivatives have been known for their high mobility values from the recent studies. Substitutions and functionalization at 2 and 7 positions on BTBT core with various chains have been studied to explore the impact of linear alkyl and bulky chains on the properties. We have investigated the polymorphs, their respective structural features and molecular packing arrangements of three novel BTBT derivatives with different functionalized chains of comparable length- 2,7-bis(2-(2- methoxyethoxy)ethoxy)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (OEG-BTBT), 2,7-bis(7,7- dimethyloctyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBuC6-BTBT) and 2,7- diheptylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (diC7-BTBT). These molecules have different set of chain-chain and chain-core interactions compared to one another, thereby leading to various packing arrangements. We conducted thorough analysis to understand the active role of chains for the packing, which is pivotal for attainment of more crystal forms that could be engineered to facilitate charge-transport.
Acknowledgement: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 811284.

Organic field-effect transistors (OFETs) have gained tremendous interest as the next-generation printed electronics due to their roll-to-roll conventional solution processability on light and flexible substrates, enabling large-scale and cost-effective manufacturing. Development of both p-type (hole-transporting) and n-type (electron-transporting) materials are essential for complementary circuits and diverse applications. Furthermore, these materials should exhibit high charge-carrier mobilities and stability under ambient conditions for practical applications. However, while p-types have met these metrics, n-type organic semiconductors present a critical challenge of air-induced degradation, which is detrimental to OFET performance due to depressed field-effect mobility. Such instability originates from the shallow lowest unoccupied molecular orbital (LUMO) electron-accepting energy level of n-type materials close to O₂, leading to the reaction between radical anions and atmospheric species. The previously reported secondary amine annulated perylene diimide, X1, offers green-solvent (alcohol) solubility and film processability. However, due to the presence of an electron-rich nitrogen atom, the LUMO level is further increased, leading to no OFET operation in the air. To mitigate this problem, we have synthesized X6 by installing electron-withdrawing cyano groups to the bay-and-headland positions of the X1 to significantly decrease the LUMO energy level below – 4.0 eV, which is the energy level that is known to exhibit air-stable OFET performance. This new n-type material, X6, offers solution-processability, retention of the secondary amine functionality, and air-stable OFET operations.

In the last decades, organic semiconductors (OSCs) have attracted a lot of attention due to their possible employment in solution-processed optoelectronic and electronic devices, such as organic field-effect transistors (OFETs). One of the big advantages of solution-processing is the possibility to process into flexible substrates at low cost. Organic molecular materials tend to form polymorphs, which can exhibit very different conductive, luminescent, and mechanical properties. In most cases, the control of the crystal structure is decisive to maximise the performances of the final devices.
Bis(naphthalene diimide) (NDI) derivatives are a particularly interesting family of organic materials. NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. Recently, the structure-properties relationship and the polymorphism of these molecules have gained considerable attention.
We have performed a full thermal characterization of bulk NDI-C6, which presents four bulk polymorphs (Form α, Form β, Form γ, and Form δ) and two different thermal pathways depending on whether the material is melted or not, by combining different characterization techniques and we determined the crystal structure of Form γ at 54°C from powder. The analysis of the thermal expansion principal axis shows that Form α and Form γ possess negative thermal expansion (X1) and massive positive thermal expansion (X3) which are correlated to the thermal behaviour observed. C₂F₃-Cl₄NDI polymorphism was also studied, and it was possible to identify and solve the crystal structure of two polymorphs and a solvate obtained by slow evaporation experiments with this molecule, although in organic semiconductors the appearance of solvates is very uncommon. Thin films of both NDI derivatives were produced and characterized, both structural and electrical properties, since different polymorphs can present very different behaviours. In the case of NDI-C6 a new metastable form that was not observed in bulk, called Form ε, was found by spin coating deposition of the toluene solution of NDI-C6 on the Si/SiO₂ substrate. This new phase could also be obtained by quenching NDI-C6 after the melting on a Si/SiO₂ substrate, which suggests Form ε is a surface induced polymorph.
Acknowledgement
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 811284 (UHMob).

Organic electrochemical transistors (OECTs) using ion-transporting conjugated polymers have been of great interests for large transconductance g_m that enables us to amplify small brain signals or drive heavily current-demanding electronic devices. The g_m is proportional to the gate capacitance and the carrier mobility µ. OECTs can have exceptionally large gate capacitance as the whole volume of the active layer is charged, i.e., volumetric capacitance C^*. However, among a number of conjugated polymers with high carrier mobilities, only a few numbers of them have been studied for OECTs, e.g., PEDOT:PSS, because the other conjugated polymers have low ion conductivity. Herein, we present a facile way to use donor-acceptor conjugated polymers with a high carrier mobility (>1 cm²V^-1s^-1) by fluorinating them. We synthesized series of polycyclopentadithiophene-benzothiadiazole (CDT-BT) donor-acceptor copolymers with introducing fluorine on BT groups. Added fluorine altered the molecular orientation of conjugated polymer films through strong noncovalent interaction with sulphur atoms. We achieved the enhanced charge carrier mobility (from 0.75 to 1.62 cm²V^-1s^-1) and the extended volumetric capacitance of conjugated polymers (from 38.9 to 61.7 Fcm^-3). We believe that our chemical design with simultaneous improvement both µ and C^* is crucial approach to progress high transconductance of OECT performance and to bring a potential to apply extensive high-mobility semiconductors materials in OECT technology.

Thermally activated delayed fluorescence (TADF) has become a promising candidate for blue organic light emitting diode (OLED). TADF materials harvested the triplet excitons via reverse intersystem crossing process with the help of small singlet and triplet energy gap (ΔEst), achieving 100% of internal quantum efficiency. However, they suffered from broad full width at half maximum (FWHM) due to the intra charge transfer (ICT) mechanism. Hyperfluorescence (HF) was introduced to overcome the limited TADF color purity. In HF system, TADF materials act as the assistant host and transfer the singlet excitons to narrow FWHM of final dopant (FD) via Förster energy transfer (FRET). For efficient FRET process, spectral overlap between the absorption of FD and emission spectrum of TADF is crucial requirement.
In 2019, hatakeyama et al. reported the new multi-resonance (MR) type TADF material, ν-DABNA. The ν-DABNA has attracted many interests as the FD in HF system due to its extremely narrow FWHM of 18 nm. In addition, Kwon et al. reported the stable and efficient boron based TADF materials, DBA-DI possessing the short decay time with small ΔEst of 0.03 eV. DBA-DI harvested long device lifetime over 500 hrs in LT₅₀ at initial luminance of 1,000 cd/m². However, DBA-DI exhibited the broad FWHM of 65 nm and emission peak of 491 nm. As a result, the insufficient spectral overlap between DBA-DI and ν-DABNA was expected. In this study, therefore, we modified the DBA-DI structure for blue-shifted emission, and designed pMDBA-DI and KHU-033 and obtained the excellent HF device.
In order to achieve the deeper blue emission, we attached the methyl on the para- and meta- position of DBA acceptor, weakening the acceptor ability, and developed pMDBA-DI and mMDBA-DI, respectively. Since the electrophilic methyl on meta- position can attack to electron deficient boron more effectively, accepting ability of boron became weaker and band gap was effectively increased. Eventually, mMDBA-DI exhibited the 16 nm blue shifted emission of 451 nm. Thus, we can expect the larger spectral overlap than DBA-DI.
The 30% of TADF device exhibited 29.3, 23.0 and 21.3% of high devic efficiency. The maximum emission peaks were 489, 483 and 474 nm, respectively. Both materials acquired the efficiency and blue-shifted emission color. Next, HF devices were fabricated with 1% of ν-DABNA. DBA-DI based HF exhibited the 22 nm of broader FWHM derived of the insufficient FRET. While, pMDBA- and mMDBA-DI exhibited the 19 nm of FWHM and 473 nm of emission peak with of 26.6 and 28.1% of device efficiency, respectively. Since both HF devices exhibited the similar EL spectra with ν-DABNA, we can notice the FRET was successful. Next, the device lifetimes were measured at the initial luminance of 1,000 cd/m² until LT₅₀. pMDBA- and mMDBA-DI recorded the elongated device lifetime. DBA-DI based HF device exhibited the longest device lifetime, which can be the one of the best records so far. Though they exhibited the improved emission characteristics with the help of efficient FRET process, device lifetime was poorer than DBA-DI. Which can be aggravated due to the increased ΔEst value and the following triplet-triplet, and triplet-polaron annihilation process. Thus, we designed the additional TADF material, KHU-TADF, which possess the similar TADF characteristics with DBA-DI with slightly improved emission wavelength in film. The detailed information of TADF materials and device performances will be discussed on presentation.

For display applications, a narrow full width at half maximum (FWHM) with high photoluminescence quantum efficiency (PLQY) is a key material requirement for vivid and pure color. Recently, Hatakeyama et al. reported a novel concept of boron based blue material, so called a multi-resonance (MR) type emitter. The alternately aligned boron and nitrogen atoms in a rigid aromatic framework exhibited separated HOMO and LUMO orbitals distributed atom by atom on the same planar structure, thereby, not only providing a large band-gap property such as a blue color but also remaining the high oscillating strength. In addition, MR type blue material can achieve the small stoke shift and narrow FWHM owing to rigid, symmetrical, and robust aromatic ring structure. Reported DABNA-1 and DABNA-2 series exhibited 28 nm of FWHM and the Commission International de l’Éclairage (CIE) y coordinate of ≤0.13 (deep blue color). Although there were such good material characteristics, reported operational lifetime is still very short. In this study, we analyzed the degradation causes of MR type boron emitter to consider real application and its lifetime performance was significantly improved by an electron-type host introduction in the device.
To analyze the degradation reasons of MR type boron emitter, hole-only device (HOD) and electron-only device (EOD) were fabricated. Their hole and electron stabilities were investigated for 15 hours. The voltage change of HOD was only 0.06 V, while EOD changed significantly to 0.17 V. This result indicates that boron-type emitters are susceptible to electrons. Additionally, the bond dissociation energy (BDE) was calculated through molecular simulation. The BDE of anion state (1.23 eV) was very low compared to the neutral state (3.19 eV) and the cation state (3.28 eV). Based on these two results, it is expected that the lifetime will increase if electrons are prevented to MR type boron emitters. To confirm the analysis result, several devices with different host materials were fabricated with a following structure: ITO (50 nm)/ HATCN (7 nm)/ PCBBiF (68 nm)/ Host:5% DABNA-NP-TB (25 nm)/ BPPB (30 nm)/LiF (1.5 nm)/Al (100 nm). As a reference host, a bipolar type PhPC was used together with boron-type DABNA-NP-TB emitter. To avoid electron stress of emitter, electron-type KHU-host was applied. Fabricated devices show the driving voltage (@1,000 cd/m²) of 3.9 V and 3.5 V for PhPC and KHU-host, respectively. Measured external quantum efficiencies were 6.04% and 7.03%, respectively. As the electron type KHU-host device shows better efficiency, it means that the charge balance in KHU-host is relatively good. Also measured the operational lifetime (@ 600 cd/m², LT₉₅) in PhPC device was 50 hours and KHU-host device was 85 hours. We could increase about 1.7 times with electron type KHU-host. Our results will be very useful in the real application because the device lifetime of blue MR type boron emitter by blocking of anion formation could enhance significantly in the future.

The ultimate goal of organic solar cells (OSCs) is to deliver cheap, stable, efficient, scalable, and eco-friendly solar-to-power products contributing to the global carbon neutral. However, simultaneously balancing these five critical factors of OSCs toward commercialization is extremely challenging. Herein, a green-solvent-processable and open-air-printable self-assembly strategy is demonstrated to synchronously simplify the device architecture, improve the power conversion efficiency (PCE) and enhance the shelf, thermal as well as light illumination stability of OSCs. The cathode interlayer (CIL)-free self-assembled OSCs exhibit the PCE of 15.5%, higher than that of traditional inverted OSCs of 13.0%, which is among the top values for both CIL-free self-assembled OSCs and open-air blade-coated bulk-heterojunction OSCs. The remarkable enhancements are mainly ascribed to the finely self-assembly, subtly controlled donor/acceptor aggregation rate, and delicately manipulated vertical morphology. Besides, this strategy enables 13.2% efficiency on device area of 0.98 cm2, implying its potential for scalability. These findings demonstrate that this strategy can close the lab-to-fab gap of OSCs toward commercialized cheap, stable, efficient, scalable, and eco-friendly OSCs.

Conjugated polymers (CPs), which can be operated at low temperatures below 200^οC, have attracted interest as promising thermoelectric materials due to mechanical flexibility, solution–processed, and simple manufacturing process compared to inorganic materials. However, the thermoelectric performance of CPs is still lower than that of inorganic materials. At present, researchers have carried out several strategies to achieve higher charge transport properties, such as polymer backbone engineering, polymer side–chain engineering, controlling polymer alignment, or optimizing doping conditions.[1,2] Specifically, polymer side–chain engineering can be effective in the doping process of CPs. As the side–chain species for doping CPs, oligo ethylene glycol (OEG) side–chain has been considered efficient due to its high polarity, flexibility, and ion conductive behavior compared to alkyl side–chain.[2] Therefore, because of the properties of the OEG side–chain, the OEG side–chain has better performances in charge transport. We focus on the charge transport behavior depending on the side–chain effects via thermoelectric properties. The density of state (DOS), which depends on the energetic disorder, has been considered a critical factor for the thermoelectric properties because the narrow width and higher slope indicate enhanced Seebeck coefficient.[3] Therefore, it is essential to investigate the relationship between the variation of DOS and thermoelectric properties. Also, the transition of major carriers has been reported through interpretation by energy level or status of DOS for adjusting the dopant concentration in the chemical doping method. As an alternative method, we adopt an electrochemical doping system, which could continuously and reversibly control the doping level of CPs.
In this study, we demonstrate the correlation among thermoelectric properties, charge transport behaviors, and side–chain effects using newly synthesized four semi–crystalline CPs, including alkyl side–chain attached poly[(2,5–bis(2–hexyldecyloxy)phenylene)–alt–(5,6–difluoro–4,7–di(thiophen–2 yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and OEG side–chain attached PPDT2FBTs with different side–chain lengths, via electrolyte gated thermoelectric measurement system using 1–propyl–3–methylimidazolium bis(trifluoromethanesulfonyl)imide (PMIM⁺–TFSI^-). By comparing the Seebeck coefficient and electrical conductivity relation based on the Kang–Snyder charge transport model, we identify the variation of charge transport properties by side–chain effects.[4] Also, we demonstrate that the thermoelectric properties are varied by the energetic disorder related to the status of DOS through experimental measurements. Furthermore, p– to the n–type transition of the major carrier, indicated by the sigh transition of Seebeck coefficient, are interpreted in respect of energy difference between Fermi energy level and transport edge with the variation of DOS during the electrochemical doping process. Finally, our analysis provides strategies for side–chain engineered CPs to investigate enhanced thermoelectric properties based on charge transport behaviors.

REFERENCES.
[1] Zeng, H., et al., Fabrication of Oriented n–Type Thermoelectric Polymers by Polarity Switching in a DPP–Based Donor–Acceptor Copolymer Doped with FeCl3. Advanced Electronic Materials, 2021. 7(5): p. 2000880.
[2] Meng, B., J. Liu, and L. Wang, Oligo (ethylene glycol) as side chains of conjugated polymers for optoelectronic applications. Polymer Chemistry, 2020. 11(7): p. 1261–1270.
[3] Upadhyaya, M., et al., Effects of disorder on thermoelectric properties of semiconducting polymers. Scientific reports, 2019. 9(1): p. 1–11.
[4] Kang, S.D. and G.J. Snyder, Charge-transport model for conducting polymers. Nature materials, 2017. 16(2): p. 252–257.

Uncooled infrared sensors are in urgent demand, especially to reduce cost and enhance portability for different medical and consumer electronic applications. In this work, we report a room-temperature infrared detector with an ultrabroadband spectral range, covering the shortwave infrared (SWIR 1-3 micron) and midwave infrared (MWIR 3-10 um) wavelengths. An organic bulk heterojunction (BHJ) is employed as the active layer in the diode structure. The detection for SWIR light is enabled by photovoltaic or photoconductive effect depending on the bias condition, which show a spectral limit up to 1600 nm. For the detection of MWIR wavelengths, the response stems from the bolometric effect, enabled by midgap charge generation and mobility increase induced by thermally activated hopping. The thermal detection mechanism is demonstrated for 1600 nm to 10 um. The temperature coefficient of resistance (TCR) of the organic BHJ can be as high as ~10 %/K, surpassing the organic material PEDOT (TCR of ~3 %/K) and amorphous silicon (~4 %/K), which are most frequently used for thermal detection.
In our findings, the organic BHJ layer not only enables efficient photonic interband transition by creating charge transfer states, but also is favorable of converting thermal signal to electrical signal, through abundant midgap states. The combination of photonic and bolometric response makes it easy to sense a broadband range in a single device. The results are paving the way for ultra-broadband photo detection, potentially very attractive for applications relying on IR detection.

By applying smart devices on perishable products, one could measure temperature, pH, and humidity variations to ensure the delivery of a high quality product to the consumer. However, for the moment, the large-scale production is limited by the sustainability of the devices as well as their mechanical properties. Metallic conductors such as silver or copper are used in these devices, contributing to global E-Waste. To replace metallic conductors, carbon-based materials should contribute to produce recyclable, light, flexible, low-cost and printable materials. Current state-of-the-art, poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonic acid) (PEDOT:PSS), has shown promising results as a temperature and humidity sensors, yet is not competitive with metallic conductors. Indeed, PEDOT:PSS is a highly acidic material and is highly unstable to humidity variations. It can also be hard to store and to process.

For this project, the synthesis of water processable self-doped polymers has been achieved by Direct (Hetero)Arylation Polymerization (DHAP) using water as a cosolvent. Without any post-treatments, the self-doped polymers have shown air-stable electrical conductivities up to 50 S cm^-1. It has been demonstrated that these new materials reach a one-year stable conductivity under ambient light and humidity storage conditions. Results tend to demonstrate that the oxygen most-likely contributes to the doping of the polymers under acidic conditions. These processable conducting polymers could be used in various applications such as temperature and humidity sensors, various 2D and 3D printed electronic and optical devices as well as surfactants for the dispersion of carbon nanotubes and graphene in aqueous solutions.

The advancement of modern electronics has led to the miniaturization of integrated devices and to the increase in demand for ultra-small and highly-integrated device technology. However, deep ultraviolet (DUV) lithography has a resolution limit of 30 nm and cannot achieve fine patterns down to 10 nm or less via multi-patterning. Therefore, it is necessary to introduce extreme ultraviolet (EUV) lithography, which has greater resolution and productivity than those of DUV lithography. The wavelength of an EUV light source is 13.5 nm, which is 14 times shorter than that of conventional ArF sources. Then, there is no need for multi-patterning, and the number of logic memory patterning steps is expected to be reduced by 5-6 times and dynamic random-access memory (DRAM) steps by 3 times. However, a thin photoresist layer is required due to the small depth of focus of EUV light, and this leads to patterns with low aspect ratios. Alternatively, directed self-assembly (DSA) of block copolymers (BCPs) has been considered as a promising candidate to aid EUV in developing of sub-10 nm patterns. BCPs are assembled into nanoscale line patterns on pre-defined EUV patterns. However, there are obstacles to overcome to achieve high resolution patterns with low defect densities. First, neutral brush layer coating on the substrate is required to reduce the energy difference between individual polymer blocks and to form perpendicularly oriented lamellar patterns. However, this does not fully resolve the issue as it not only requires additional processes and costs but also cannot be applied on substrates with prefabricated patterns. Secondly, block copolymers with sub-10 nm patterns have slow self-assembly kinetics, leading to increased defect occurrence and self-assembly time. Therefore, a deliberate design for block copolymers with moderately-high Flory-Huggins interaction parameter (χ) values as well as rapid kinetics is required to selectively form a self-assembled pattern on an EUV pattern with only a simple additional process. Here, we suggest a newly-designed BCP that can achieve moderately-high χ values while forming perpendicular lamellar patterns without any neutral brush layer. The key to the BCP design is to incorporate a gradient copolymer block with an Si-containing block to promote universal perpendicular alignment of lamellar morphology without additional surface treatment or a neutral brush layer. The BCP was synthesized by reversible addition-fragmentation chain-transfer polymerization to form a styrene (S)/2,3,4,5,6-pentafluorostyrene (PFS) gradient with an Si-containing 4-(tert-butyldimethylsiloxy)-styrene (4BDSS) copolymer block, (P(S-g-PFS)-b-P4BDSS). The S/PFS gradient copolymer consists of gradual composition change from the block junction region to the tail, which creates enough surface energy difference to energetically drive perpendicular lamellar formation, which is well supported by a thermodynamic model. Furthermore, an encapsulation process is performed on the EUV pattern using an Si-containing gradient BCP to improve the aspect ratio.

Recent development of organic solar cells (OSCs) using small molecular non-fullerene acceptors has achieved remarkable improvement in photovoltaic efficiency. It draws attention on the discussion on the differences of excitonic properties between conjugated polymers and small molecules. In this study, we use electroabsorption (EA) spectroscopy combined with external quantum efficiency (EQE) to investigate the excitonic properties of a series of recently reported photovoltaic polymers and small molecules. It is found that the spectral characteristics of the EA signal at the lowest absorption band of all polymers show Frenkel-exciton characteristics with little contribution from charge transfer (CT). On the other hand, for small molecular materials, such as SubNC, CuPc, and Y6, the EA signal is dominated by CT characteristics. In contrast to the polymers, the EQE spectral characteristics of the small molecular materials resemble their optical absorption profile, which is consistent with the small binding energy and free carrier generation features as observed by EA approach. Details analysis of the EA results on the differences in polarizability and dipole moment in different materials will also be discussed.

Conjugated polymers are of great interest for use in various organic electronic and optoelectronic devices because of their good charge transport properties, mechanical flexibility and compatibility with low-temperature solution processes.[1] Recently, they have been emerged as a new class of thermoelectric (TE) materials, thus opening new avenues for harvesting thermal energy by scalable and inexpensive processing routes.[2,3] Compared to their traditional inorganic counterparts (usually metal alloys such as Bi-Te, Sb-Te, and Pb-Te), most organic TE materials are environmentally friendly, lightweight, and based on earth-abundant atoms, suitable for wearable and large-area applications.[2] However, their electrical conductivities need to be improved for TE applications, while maintaining the unique advantage of organic polymers, low thermal conductivity.[3]
Molecular doping is one of the most effective route to controlling the electrical properties of conjugated polymers utilized in TE and optoelectronic devices.[3] To simplify fabrication processes, the doping can be performed by simple one-step solution mixing method instead of sequential doping, which is performed as post-treatment after making polymer films.[4] The solution mixing method has an additional advantage of the applicability to very thick polymer films, which will be critically important to improve the conductivity up to the level of commercialized materials.[5,6] However, most molecular dopants and doped polymer solutions often exhibit poor solubilities, restricting the formation of high quality films and disturbing the molecular ordering of polymer structures.[4,5]
Herein, a series of studies about the fabrication of solution-mixed organic TE materials are reported to overcome the above-mentioned issues and thus to obtain excellent TE properties. This presentation covers a wide range of conjugated polymers from donor-only (P3HT, PQT-12, and PBTTT-C12) and donor-acceptor (PCDTFBT and PCDTPT) types and two representative molecular dopants, F4TCNQ and BCF. At first, two effective pairs of donor-acceptor type polymers (PCDTFBT and PCDTPT) and a molecular dopant (F₄TCNQ) are introduced to exhibit high solution stabilities and good TE properties of the prepared films.[4] As the second topic, the Brønsted acid doping of P3HT by BCF-water complexes is reported and compared with the Lewis acid doping by BCF.[5] Interestingly, the Brønsted acid-doped P3HT films form the unique type II polymorph with interdigitated alkyl chains and reduced energetic disorders. Combined with the conformational change to the planar quinoid structure, the P3HT films exhibit superior electrical and TE properties with excellent air stabilities. Some on-going results of BCF doping for PCDTFBT and PCDTPT will also be discussed.[6] As a result, the doped polymer films exhibit outstanding TE properties with the power factor and figure-of-merit ZT of up to of 49.6 μW m^-1 K^-2 and 0.061, respectively.[4,5,6] To the best of our knowledge, these values are the highest obtained for the p-type soluble conjugated polymers processed by one-step solution mixing method. The last part of this presentation will briefly discuss a couple of strategies to improve the physical properties of organic TE materials.[7,8]

[1] J. G. Ibanez, … B. A. Frontana-Uribe Chem. Rev. 2018, 118, 4731-4816.
[2] R. Kroon, … C. Müller Chem. Soc. Rev. 2016, 45, 6147-6164.
[3] W. Zhao, … D. Zhu Chem. Soc. Rev. 2020, 49, 7210-7228.
[4] E. H. Suh, … J. Jang Nano Energy 2019, 58, 585-595.
[5] E. H. Suh, … J. Jang Adv. Energy Mater. 2020, 10, 2002521.
[6] E. H. Suh, … J. Jang in preparation.
[7] Y. J. Jeong, … J. Jang Adv. Funct. Mater. 2020, 30, 1905809.
[8] T. S. Lee, … J. Jang in preparation.

Organic light-emitting devices (OLEDs) convert electrons to photons. In order to improve the OLED efficiency, emissive molecules capable of harvesting all the electrogenerated excitons have been devised. Representative examples include phosphorescent complexes, such as cyclometalated Ir(III) and Pt(II) complexes. These complexes can rapidly convert singlet excitons into triplet excition, and subsequently facilitate strong phosphorescence transition from the harvested triplet excitons. Exciton-harvested electroluminescence has also been achieved with organic molecules with small exchange energies between the singlet and triplet excited states. These molecules exhibit device efficiencies as high as those of phosphorescent complexes, but suffer from efficiency drops at high current densities due to biexcitonic losses of long-lived triplet excitons through triplet-triplet or triplet-polaron annihilations. In this research, we have developed an exciton-harvested electroluminescence strategy based on organic hosts capable of rapid, exergonic conversion of triplet excitons into singlet excitons. The exciton harvest involves quantum chemically allowed reverses intersystem crossing (rISC) from the triplet n-p* transition (³n-p*) state to the singlet triplet p-p* transition (¹p-p*) state as the key process. The singlet excitons collected at the ¹p-p* state are subsequently transferred to a fluorescent dopant that produces strong electrofluorescence. This strategy does not require precious transition metals or thermal activation energies for exciton conversion. In addition, the rapid exciton conversion is beneficial for minimizing the hazardous biexcitonic annihilation processes. To validate this idea, eight host molecules were designed and synthesized based on biphenyl which tethered carbazole and carbonyl moieties, including acetyl and benzoyl groups, at various positions. The hosts exhibited violet-to-blue emission with peak wavelengths in the region 433-453 nm. Fluorescent dopants based on pyrene (N1,N1,N6,N6-tetrakis(4-(tert-butyl)phenyl)pyrene-1,6-diamine) and anthracene (9,10-Bis[N,N-di-(p-tolyl)-amino]anthracene) were chosen to produce exciton-harvested fluorescence emission. Thin films of the hosts doped with 3 wt % dopants exhibited photoluminescence quantum yields (PLQYs) < 0.36. Our photophysical measurements revealed that the Förster radii (31.24-27.35 Å) for the pairs of FBD-Py1 and our hosts were larger than those (24.06-20.56 Å) for the TTPA:host pairs. The energy levels of the lowest unoccupied molecular orbital (LUMO) of the hosts were found to be shallower than the LUMO of the fluorescent dopants, which suggested the negligible exciton formation within the dopants. This charge carrier recombination is pivotal for exciton harvest by rISC within the hosts. OLEDs with a configuration of ITO, PEDOT:PSS, LiF/Al/LiF were fabricated. A maximum external quantum efficiency (EQE) as high as 2.4% was recorded. Although the EQEs remained low, exciton-utilization efficiencies (EUEs) were calculated to assess the extent of exciton harvest using the relationship EUE = EQE / (PLQY × η_r × η_out) where h_r and h_out are a unitary charge balance ratio and the light-outcoupling efficiency taken as 0.2, respectively. All the devices based on our hosts exhibited EUE values greater than 25% (30-98%). The results demonstrate the effectiveness of our exciton-harvest strategy. The FBD-Py1:host devices exhibited greater EUEs than TTPA:host devices. This observance was against to the larger Förster radii of the FBD-Py1:host device, which indicates an insignificant role of energy transfer in the exciton-harvested electroluminescence efficiency.

Recently, physically unclonable function (PUF) has received special attention as a highly strong encryption technology by advantage of hardware-level key generator in protect from hacking for ubiquitous IoT era. PUF is a cryptographic electronic device which can provide a unique and intrinsic digital key for authentication and identification. To apply the PUF technology to IoT and wearable smart systems, low-cost and flexible PUF devices with high-level encryption primitives which is capable of integrating with electronic circuit system is required. Here we propose a highly strong electronic PUF based on organic thin-film transistor arrays. Organic semiconductor thin-films which were manufactured by same solution-process using the same material, all showed different and unique polycrystalline microstructures with random orientation. OTFT arrays based on this organic semiconductor thin-films provided the unique electrical current distributions for each trial samples. Cryptographic characteristics of OTFT-based PUF device including uniqueness and randomness as well as their correlation with the morphological and structural properties of organic semiconductor thin-films will be discussed.

The incorporation of thermal disorder in analyzing the thermoelectric (TE) properties of materials has traditionally only examined the influence of static thermal disorder. In soft (weakly bonded) materials, however, dynamic thermal disorder can have a large effect on carrier transport by significantly altering the energy dependences of the density-of-states (DOS) and carrier mobility (µ). Using a model that draws upon both transient localization theory (to model the energy dependences of the DOS and carrier mobility), and published angle-resolved photoemission spectroscopy data (to provide reliable estimates of the homogeneous (dynamic) and inhomogeneous (static) broadening of the electronic dispersion), we show that these energy dependences can have a large effect on the thermoelectric properties of organic thermoelectric materials.

Particularly, by analyzing TE properties as the carrier concentration (p) is increased to vary the Fermi level (E_F) across sharp changes in the DOS(E) and µ(E) related to dynamic disorder, we identify three carrier concentration regimes of thermoelectric transport. The first regime (I) is a low-mobility (hopping) conduction regime at low (e.g., intrinsic) carrier concentration, which is characterized by large reductions in the electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF) below values that are typically predicted by static disorder models (i.e, σ_dynamic~(10^-6-10^-7)×σ_static, S_dynamic~1/(1.2 -14)×S_static, PF_dynamic~(10^-6-10^-10)×PF_static). The second regime (II) is an mixed (intermediate-mobility) conduction regime at an intermediate carrier concentration (this intermediate carrier concentration being similar to that typically probed by field-effect transistors), which is characterized by a nonlinear increase in the E_F vs. log(p) characteristics that causes S to exhibit nonlinear behavior; increasing well above the conventional S vs. log(p) relationship before reaching a peak value, S_peak~1550 μV/K. The third regime (III) is a high-mobility (band) conduction regime at high carrier concentrations, where thermoelectric PF is usually maximized.

A critical carrier concentration (p_crit.~4.299×10^-4 molar ratio) is observed near S_peak (in regime II) at which thermoelectric transport transitions from trap-limited behavior (in regime I) to conventional “band-like” behavior (in regime III). Above this critical carrier concentration, σ and PF are reduced compared to the perfect crystal and static-only conditions, causing a modest drop in the power factor maximum by factors of 3 and 2.3, respectively. This reduction in PF, while not as large and consequential to TE transport as the PF reduction that occurs for low (e.g., intrinsic) carrier concentration, is found to occur in a region of high carrier concentration (p>p_crit.) that has remained largely inaccessible to experimentalists due to dopant limitations that are worsened in the presence of dynamic disorder.

The theoretical framework that was developed to arrive at these predictions is discussed, along with a critical discussion of the need to have accurate representations of the energy dependences of the density-of-states (DOS) and carrier mobility (µ) in order to obtain reliable estimates of TE properties in organic semiconductors.

Multivalued logic circuits (MVLs) have attracted considerable attention due to their capability of handling higher density of information than conventional binary logic circuits. Recently, a wide variety of materials have been actively investigated for the realization of high-performance ternary inverters. Organic semiconductors have come out as a very promising candidate in this regard because of their intrinsic flexibility and low-cost fabrication processes [1-3]. However, most of the reported devices exhibited low noise margin and an incomplete supply voltage (V_DD) to ground swing. Moreover, high driving voltage and lack of dynamic control over the inverter output have also been the major bottlenecks for their practical implementations. In this work, we have tackled these issues by developing an antiambipolar transistor (AAT) which is based on a new combination of organic semiconductors with 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as a p-type semiconductor and PhC2H4-benzo[de]isoquinolino[1,8-gh]quinolone diimide (PhC2-BQQDI) as an n-type semiconductor_. The first advantage of these semiconductors is their high charge carrier mobility which enabled the realization of a well-balanced and full-swing ternary inverter. The inverter circuit was composed of a series circuit of the proposed AAT and an n-type PhC2-BQQDI transistor. The second advantage of the material combination is their contrasting photoresponsivity under ultraviolet (UV) light irradiation. The C8-BTBT transistor was found to be very sensitive towards UV light signal, whereas the PhC2-BQQDI transistor remained invariant under UV light. The distinct photoresponsivity enabled us to fine-tune the logic states and also to improve the noise margin of the ternary inverters. Finally, low voltage operable ternary inverters were fabricated on flexible poly (ethylene 2,6-naphthalate) (PEN) substrates by employing a high dielectric constant Hafnium oxide (HfO₂) dielectric layer. The inverters showed high robustness against the applied tensile strain up to 0.78% and displayed stable operation after 100 repeated bending cycles. Thus, the proposed devices meet all the requirements for organic ternary logic circuits.
References:
1) K. Kobashi, R. Hayakawa, T. Chikyow and Y. Wakayama, Nano Lett. 18, 4355 (2018).
2) D. Panigrahi, R. Hayakawa, K. Fuchii, Y. Yamada and Y. Wakayama, Adv. Electron. Mater. 7, 2000940 (2020).
3) D. Panigrahi, R. Hayakawa, K. Honma, K. Kanai and Y. Wakayama, Appl. Phys. Express 14, 081004 (2021).

Thermally activated delayed fluorescence (TADF) emitters are considered as being the most promising emitter materials for organic light-emitting diodes (OLEDs).^[1] This work focuses on the design and synthesis, as well as the analytical, photophysical and optoelectronic characterization of blue emitting TADF materials. We chose the twisted intramolecular charge transfer transition approach for the design of our emitters. Electron-accepting and electron-donating groups are combined using N-heterocycles, inducing a charge transfer effect, which is crucial for TADF. To achieve a suitable HOMO-LUMO overlap and enhance the dihedral angle between donor and acceptor, bulky spacing units as an interconnection were used.
In 2020, our group developed the novel, star-shaped tris(triazolo)triazine (TTT) acceptor core and decorated it with carbazole- and acridine derivatives, to behave as donating groups, such as 9,9-dimethyl-9,10-dihydroacridine and 3,6-ditert-butylcarbazole. A combinatoric synthetic route was established using three tetrazole-donor-groups reacting with cyanuric chloride and 2,6-lutidine.
These emitters displayed blue emission and a delayed fluorescence with delayed lifetimes on the order of milliseconds. High photoluminescence quantum yields (PLQY) of 79% and 80% were observed in CzSi for the 9,9-dimethyl-9,10-dihydroacridine-TTT and 3,6-ditert-butylcarbazole-TTT materials, respectively. The solution-processed OLED devices performed with an EQE_max of 11.0% and 5.8%, respectively.^[2]
The monotriazolotriazine (MTT) and bistriazolotriazine (BTT) were developed as a follow up to the TTT family. Instead of using three tetrazole units, only one unit for MTT and two units for BTT were used. These new acceptors, unlike the TTT core, are not symmetrical star-shaped. Achieved were two novel acceptors in combination with commonly known donors. The effect on the TADF properties, such as the emission color and efficiency, using different donor types and donor counts was investigated. The same synthetic protocol was applied, using the triazine core linked with one and two tetrazole-donor-groups to obtain MTT and BTT, respectively.
[1] G. Hong, X. Gan, C. Leonhardt, Dr. Z. Zhang, J. Seibert, Dr. J. M. Busch, S. Bräse, Adv. Mater. 2021, 33, 2005630.
[2] F. Hundemer, E. Crovini, Y. Wada, H. Kaji, S. Bräse, E. Zysman-Colman, Mater. Adv. 2020, 1, 2862.

Since the development of the first organic light-emitting diodes (OLEDs) over 30 years ago, there has been tremendous progress so that OLED displays are nowadays produced in a wide variety of architectures, from flat, rigid components to flexible and even foldable variants. Luminescent materials based on the principle of thermally activated delayed fluorescence (TADF) are of particular interest, especially blue TADF emitters combining both high efficiencies and long device lifetimes. Chiral materials exhibiting circularly polarized luminescence (CPL) are highly sought after for information storage, encryption of data, and to build thinner and brighter OLED devices. Therefore, our main focus relies on the design and characterization of blue TADF emitters bearing [2.2]paracyclophane as planar-chiral framework.

As demonstrated by our group, the cyclophane enables TADF efficiently in through space conjugation of donor (D) and acceptor (A), and induces higher torsion angles to bridging units when being employed as (1,4)carbazolophane donor leading to lower ΔEST and higher PLQYs.^[1-2] The ring-closure of carbazoles through C−H activation has been widely examined. During our synthesis of paracyclophane-fused carbazoles, we encountered the formation of [2]paracyclo[2](1,7)carbazolophane isomers through C−C bond breakage. These novel planar-chiral (1,7)carbazolophane building blocks offer unique structural features that significantly differ from the [2.2]paracyclophane core. The structure is confirmed by X-ray analysis while NMR spectroscopic results give insight into the aromatic character. Based on this new backbone, D-A-type TADF emitters are prepared and the influence of various substituents on the photophysical properties evaluated. Furthermore, the chiroptical and optoelectronic properties are investigated, supported additionally by computational calculations.

[1] E. Spuling, N. Sharma, I. D. W. Samuel, E. Zysman-Colman, S. Brase, Chem. Commun. (Camb.) 2018, 54, 9278-9281.
[2] N. Sharma, E. Spuling, C. M. Mattern, W. Li, O. Fuhr, Y. Tsuchiya, C. Adachi, S. Bräse, I. D. W. Samuel, E. Zysman-Colman, Chem. Sci. 2019, 10, 6689-6696.

Organic light-emitting diodes (OLEDs) are used as (colored) light source in displays of smartphones and TV screen, as well as in lighting products. Over the past years, the research on this topic increased significantly, as OLEDs show efficiency and sustainability, compared to previous technologies, such as liquid crystal displays (LCDs). Apart from that, OLEDs show high contrasts, rich colors and faster response times. Moreover, the usage of flexible materials could enable the production of new types of displays, for example rollable and foldable displays.^[1]
Metal-organic TADF emitters are of particular interest, as they combine a theoretically internal quantum yield of 100% at moderate temperatures with economically friendly materials such as copper as well as pure organic molecules. However, the OLED technology still faces some challenges. Although lots of metal-organic emitters are already developed, there is still need for new materials that show high emission quantum yields at short decay times. For red and green emitters there are already great results, whereas blue emitters still suffer from a short lifetime, due to the requirement of high energy for blue emission. Another big issue is the oxidation of Cu(I), leading to non-luminescent Cu(II) species, which causes geometrical reorganization in the excited states. To reduce this fluorescence quenching effect, a bulky rigid structure of the designed materials is advantageous.^{[1, 2]}
In our group, various novel copper(I)-complexes using different monodentate ancillary phosphine ligands as well as N,P-bridging ligands based on 2-diphenylphosphinopyridine were developed and synthesized.^[3] Thus, a large class of luminescent Cu(I) complexes bearing a butterfly-shaped Cu₂X₂ core (X = halides) were obtained. Based on this, we develop related species and compare the influence of the pyridine binding site on the emission wavelength as well as redox-stability. With this ligands novel mono-, di- and tetranuclear Cu(I)-complexes are synthesized and analyzed. To increase the solubility in common organic solvents methyl groups are introduced into the ligands. Another problem occurring in the production of OLEDs is the contamination of different OLED-layers by resolving the complexes in the process of deposition. We solve this problem by using an alkyne-azide cycloaddition to cross-link the complexes to polymers in an auto-catalyzed manner. Generally, the development of blue emitters is the main focus. The research is supported by computational calculations and photophysical characterization carried out by collaboration partners.
[1] G. Hong, X. Gan, C. Leonhardt, Z. Zhang, J. Seibert, J. M. Busch, S. Bräse, Adv. Mater. 2021, 33, 2005630.
[2] L. Bergmann, D. M. Zink, S. Bräse, T. Baumann, D. Volz, Photoluminescent Materials and Electroluminescent Devices, Springer Int. Pub., Cham, 2017, 201-239.
[3] J. M. Busch, D. M. Zink, P. Di Martino-Fumo, F. R. Rehak, P. Boden, S. Steiger, O. Fuhr, M. Nieger, W. Klopper, M. Gerhards, S. Bräse, Dalton Trans. 2019, 48, 15687-15698.

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are promising materials for organic thermoelectrics.^[1,2] In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material.^[3] While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. In this talk, we introduce how molecular doping of semiconducting polymers enables spatial compositional control for thin film FGMs. Specifically, we utilize sequential vapor doping of poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno [3,2-b]thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) to fabricate various forms of FGMs including double segmented films and continuously graded films.^[4,5] Through a combination of grazing incidence wide angle X-ray scattering (GIWAXS) and Raman spectroscopy, we are able to spatially resolve dopant compositional profiles in FGM thin films. In addition, the lateral distribution of the electronic conductivity and the Seebeck coefficient of the FGM thin films are measured in order to determine spatially averaged transport parameters. Lastly, the experimentally resolved electronic conductivity and Seebeck coefficient profiles are correlated to heat and charge transport models, which reveal theoretical limits of improved thermoelectric cooling performance.^[5] Overall, our work showcases how sequential doping is an enabling technique for applying the principles of functional grading and how it further advances the understanding of structure-transport properties and the development of more efficient organic thermoelectric materials.

[1] O. Bubnova, X. Crispin, Energy Environ. Sci., 2012, 5, 9345-9362.
[2] R. Kroon, D.A. Mengistie, D. Kiefer, J.D. Ryan, L. Yu, C. Müller, Chem. Soc. Rev., 2016, 45, 6147-6164.
[3] E. Müller, K, Zabrocki, C. Goupli, G.J. Snyder, W. Seifert, CRC Press. 2017; 46-81.
[4] T. Ma, B.X. Dong, G.L. Grocke, J. Strzalka, S.N. Patel, Macromolecules, 2021, 53, 2882-2982.
[5] T. Ma, W. Kent, B.X. Dong, S.N. Patel, Appl. Phys. Lett., 2021, 119, 013902.

In organic device applications, contact doping has been commonly used method for circumventing a high contact resistance between metal electrodes and organic semiconductors prevents an efficient charge injection and extraction, which fundamentally limits the device performance. We have recently employed an efficient doping method based on solid-state diffusion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) [1] to selectively bulk-dope contact regions of PBTTT organic field effect transistors (OFETs), which significantly enhanced the charge injection properties [2]. However, adopting such extensive contact doping method compromises the device stability of OFETs due to dopant diffusion problem, which significantly degrades the device stability by damaging the ON/OFF switching performance. In this presentation, we show a significantly improved stability of the contact doping method by incorporating “dopant-blockade molecules” in PBTTT film in order to suppress the diffusion of the dopant molecules. By carefully selecting the dopant-blockade molecules for effectively blocking the dopant diffusion paths, the ON/OFF ratio of PBTTT OFETs can be maintained over 2 months. This result will maximize the potential of contact doping in OFETs by providing a promising route toward achieving stable and low-contact-resistance OFETs.

References:
[1] K. Kang, S. Watanabe, K. Broch, A. Sepe, A. Brown, I. Nasrallah1, M. Nikolka, Z. Fei, M. Heeney, D. Matsumoto, K. Marumoto, H. Tanaka, S. Kuroda and H. Sirringhaus, Nat. Mater. 15, 896 (2016).
[2] Y. Kim, S. Chung, K. Cho, D. Harkin, W.-T. Hwang, D. Yoo, J.-K. Kim, W. Lee, Y. Song, H. Ahn, Y. Hong, H. Sirringhaus, K. Kang and T. Lee, Adv. Mater. 31, 10 1806697 (2019).
[3] Y. Kim, K. Broch, W. Lee, H. Ahn, J. Lee, D. Yoo, J. Kim, S. Chung, H. Sirringhaus, K. Kang and T. Lee, Adv. Funct. Mater. 2000058 (2020)

Established organic hole-transporting materials (HTMs) have been used for a variety of optoelectronic devices, such as light-emitting diodes, solar cells, and field-effect transistors. However, the typical hole mobility and conductivity values for such materials are relatively low. For this reason, organic HTMs either need to be doped, or the formed films should be very thin (often less than 10 nm). Unfortunately, doping could lead to reduced long-term stability, while the formation of very thin films can often be challenging.
To overcome this limitation, we have recently come up with a new concept for the formation of the hole-selective contacts, by utilizing self-assembly of certain organic compounds. Due to its self-limiting nature, the process of the film formation stops at the thickness of the single molecule, while the robust anchoring phosphonic acid group is providing a strong affinity to the oxide-based substrates.
Proof-of-concept was demonstrated in the perovskite solar cells.[1] While competitive efficiency was achieved, it was still behind that of the traditional polymeric HTM PTAA. The breakthrough was achieved, when the materials called 2PACz ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) was introduced in the tandem perovskite/CIGS solar cells, leading to the record efficiency.[2] It was possible due to the good interface with the perovskite, as well as the ability of the self-assembled monolayers (SAMs) to conformally cover the rough surface of the bottom CIGS cell. As a follow-up of this work, several important results were achieved, including record efficiency of the Si/perovskite tandem solar cell,[3] and one of the highest reported efficiency of the organic solar cell.[4]
In my presentation, I am going to discuss the opportunities, provided by the concept of the SAM-based selective contacts, as well as its limitations. In addition, possible structural changes will be discussed, that could be used for the design of the novel SAMs with specific applications.

Despite the solid establishment of silicon-based electronics, the role of organic electronics stands out in some applications where inorganic devices based get limited due to their rigidity and lack of mechanical flexibility. The ability of organic semiconductor systems to be processed at low temperatures enables the utilization of plastic substrates that possess flexible properties. These abilities actuate the utilization of variable interconnects, and substrates make them a potential substitute in the domain of display units. Transistors are the fundamental unit in electronic devices due to their ability to function as a switch leading to the manifestation of binary logic. Their implementation spans the applications of sensors and amplifiers as well. One of the critical parameters that govern field-effect transistor’s performance is its off current, leakage current, and on/off ratio. These attributes rely dominantly on the properties of the active channel material, apart from the field-effect leading to inversion or accumulation of charge carriers. Reduction of parasitic leakage assists in enhancing the on/off ratio and imparts augmented gate control by eliminating ungated regions. Organic semiconductors are generally deposition via solution processing methods such as drop casting and spin coating. However, PVD techniques are also used, but it leads to the wastage of organic semiconductors due to a low deposition yield. These material deposition methods lead to complete coverage of the semiconductor over the ungated regions. Therefore, it necessitates the need for patterning of the organic semiconductor limiting it within the channel region only. The techniques of patterning could be employed from inkjet printing, screen printing, or lithography. The former two are additive deposition of organic material; however, the resolution can be limited by the nozzle in inkjet printing and pore size for screen printing. This limitation can be addressed by utilizing lithography techniques, which are well established for sub-micron size features, currently scaling down to 5nm node, which is commercially available in the Apple A14 Bionic processing unit. This work addresses the utilization of the subtractive technique of lithography for patterning an organic semiconductor. The two lithography techniques, optical lithography, and e-beam lithography have been utilized to achieve the above-discussed patterning problem. Since lithography is well integrated with contemporary fabrication technology, this method easily integrates with existing cleanroom processes. The challenge in lithography-based patterning of polymer semiconductors starts with the step of spin coating photoresist over the semiconductor since the photoresist is dissolved in an organic solvent that can potentially affect the underlying semiconductor film. The compatibility issue of the photoresist solution and developer with the polymer semiconductor, poly(3-hexylthiophene) (P3HT), has been addressed here. The improvement in performance parameters has been characterized on bottom gated FET architectures. Also, patterning of P3HT for minimum possible feature size has been tested, including the feasibility of the scaling down process has been demonstrated.

Polyaniline has been one of the most useful and well-studied conducting polymers since the invention of its nano form. Now we have emphasized on its additive-free processing as a bulk material. We developed methods to form malleable polyaniline doughs that can be pressed, molded, and extruded, like an organic metal but lack of metal hardness. The shaped bulk can be soon solidified, forming electrical conductive and healable solid materials. Conductance and mechanical behaviors are reported herein. On the other hand, aniline oligomers are small molecules of polyaniline, possessing similar chemical properties. However, we found aniline oligomers interesting in the aspects of crystallization, protonation, oxidation states, catalysis, etc. The nature of being small molecules of aniline oligomers grants us more possibilities to tune their physical and chemical properties and further benefit processing for applications, including filtration membranes and energy storage devices.

Organic field-effect transistors (OFETs) have gained lots of attention for a variety of potential applications over the past decades. However, despite the steadily rising charge carrier mobility, inefficient charge injection/extraction characteristics induced by high scale of contact resistance are considering as a bottleneck for practical use of OFETs. In this study, we showed a facile way to minimize the access resistance in OFETs by introducing the buried electrode (BE) structure through a pressure during thermal annealing process to polymer semiconductors. The additionally applied pressure in the thermal annealing process induces the reducing contact resistance by shortening the charge-transporting distance from electrode to the bulk organic semiconductor layer, resulting in the improved charge carrier mobility. As a result, the R_C of BE structured FET is decreased from 166.1 to 65.4 kΩ compared to the pristine device. Moreover, charge carrier mobility of BE structured FET is increased from 0.026 to 0.061 cm²/Vs. Our results suggest a novel structure that easily achieves low contact resistance, leading to the practical use of short-channel OFETs.

Organic thermoelectric (TE) devices are in the spotlight as a renewable energy source for next-generation wearable devices. Conjugated polymers (CPs) are attracting attention as TE materials for wearable devices due to their non-toxicity, flexibility, and low thermal conductivity. In spite of the advantages of CPs as TE materials, CPs still have lower properties than inorganic-based materials because of relatively low σ. Recently, studies for the enhancement of σ with controlling doping level and conditions have been reported. Improvement of the Seebeck coefficient (S) is also essential for application as TE materials. These two factors exhibited a trade-off relationship depending on the carrier concentration (n), and their optimization is necessary. At present, many researchers have studied to improve the TE performance of CPs, such as a structural design, packing density of the CPs, various doping methods, and dopant [1, 2]. In particular, doping methods and types of dopants greatly influence the crystallinity and charge transport mechanism of the CPs. Maintaining the crystallinity of CPs is significant to increase charge transport properties during the doping process. Dopants are effective when they penetrate both the crystalline and amorphous regions of the CPs. Various chemical doping methods have been proposed, including the solution-mixed doping method (MxD), sequential solution doping method (SqD), , and so on. However, these doping methods collapse the crystallinity of the CPs or penetrate only the amorphous region.
In this study, we suggested a hybrid doping method (HyD) to compensate for these shortcomings. We investigated the effect of HyD through comparison with MxD and SqD. We used donor-donor type polymer, i.e. poly(4,9-bis(bis(hexylthio)methylene)-7-methyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2-(4-(bis((2-hexyldecyl)thio)methylene)-6-methyl-4H-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)) which was designed to have good charge transport properties and packing density for high TE properties. By comparing the relationship of S and σ using the Kang-Snyder charge transport model, we identified the variation of charge transport mechanism by different doping methods [3]. Finally, our work provides strategies for thermoelectric materials showing high electrical conductivity through the optimized doping method.

REFERENCE
Yeran. L et al., Degenerately Doped Semi-Crystalline Polymers for High Performance Thermoelectrics, Adv. Funct. Mater. 2020, 2006900
Meng. B et al., Oligo (ethylene glycol) as side chains of conjugated polymers for optoelectronic applications. Polymer Chemistry, 2020, 11(7), 1261-1270
S. D. Kang et al., Charge-transport model for conducting polymers, Nature Materials, 2017, 16, 252-257

In organic semiconductors (OSCs), charge carriers are primarily transported along the chain backbones, thus highly ordered organic molecules are preferable for better charge transport (1). Thus, various processes such as blade coating and dip-coating have been used for highly aligned molecules (2,3). However, these methods have their limitation of micrometer scale resolution. As organic semiconductor patterns scale down to sub-micrometers, it is expected to be more favorable for uniaxially ordered molecules due to spatial confinement. Therefore, novel process strategies to fabricate sub-micro scale organic patterns are required to improve their electrical properties.
In this study, molecular aligned patterns using DPP-DTT are simply formed by utilizing a Polydimethylsiloxane (PDMS) stamp with micrometer-sized ridge/groove geometry (4). As the solvent evaporates, the solution splits towards the sidewalls of the grooves, forming two polymer nanowires in one groove. The morphology and aligned structure of polymer patterns can be modified by varying the concentration of polymer solution. At high concentration, patterns have the residues between the peaks and a low quality alignment was formed. In contrast, at lower concentration, the solution splits perfectly with no residues, resulting in highly ordered nanopatterns. Moreover, an organic field effect transistor (OFET) based on a sub-micro polymer semiconductor pattern are fabricated into bottom gate/bottom contact structure and the effect of the solution concentration on electrical performance is also investigated and observed using spin-coated devices as a comparison group. OFETs patterned at several microscales with a high concentration solution exhibit similar mobility to spin-coated devices, whereas 1D OFET devices patterned in nanowire form exhibit improved mobility compared to spin-coated devices. These experimental results suggest novel strategy for improving charge transport characteristic of OSCs.

1. Diao, Y. et al. Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains. Nature Materials 12, (2013).
2. Niazi, M. R. et al. Solution-printed organic semiconductor blends exhibiting transport properties on par with single crystals. Nature Communications 6, (2015).
3. Li, M., An, C., Pisula, W. & Müllen, K. Alignment of organic semiconductor microstripes by two-phase dip-coating. Small 10, (2014).
4. Li, S. et al. High-resolution patterning of solution-processable materials via externally engineered pinning of capillary bridges. Nature Communications 9, (2018).

Enhancing the electrical performance of short-channel organic and polymer thin-film transistors (TFTs) by improving the frequency response or drive current capability will increase the potential use of organic and polymer TFTs in various electronic applications. The dominating and relatively large contact resistances between the metal source/drain electrodes and the semiconducting channel and related charge carrier injection issues have inhibited the realization of high-performance short-channel organic and polymer TFTs. Efforts to implement device and material designs that reduce the resistive barrier at the contact metal-semiconductor interface are vital for addressing the charge carrier injection issues prevalent in many short-channel organic and polymer TFTs. Additionally, the following short-channel effects can also be problematic for device performance: deterioration in subthreshold response resulting in high subthreshold swings, poor gate control of the drain current resulting in high off-currents, and a strong drain voltage-dependence in subthreshold and turn off characteristics due to drain-induced barrier lowering (DIBL)-like effects.
In this presentation, we show that the use of nanospike electrode designs to reduce contact resistances, and thus, significantly mitigate performance restrictions due to the resistive barrier at the contact metal-semiconductor interface and those arising from short-channel geometries. The first design we present will focus on overcoming the contact resistance problem by using field-emission electrodes to improve charge carrier injection and to enhance the charge carrier density near the source electrode. Our second design will take advantage of these capabilities but trade off some of the improvement in charge carrier injection to gain huge improvements in the subthreshold response, effectively eliminating many of those previously mentioned short-channel effects. The electric-field enhancement of the nanospike contact electrodes results in more effective charge carrier injection across the Schottky barrier between the contact metal and semiconductor, when compared to conventional flat contact electrodes. Initial studies have been performed with semiconductors pentacene and dinaphtho[2,3-b:2’,2’-f]thieno[3,2-b]thiophene (DNTT). Results from ongoing experiments with polymer semiconductors and blended films will be presented. The results show that contact resistance can be reduced using the nanospike design by at least 10× relative to the contact resistance of flat electrodes when using the same source and drain metal.
We will also highlight effective surface treatments, alternative electrode materials, and additional design components that can lead to even more improvements in charge carrier injection in short-channel organic and polymer TFTs with nanospike electrodes.

Abstract. The stepwise assembly of nanoscale layers of molecules is an attractive and versatile approach for designing functional thin films. This approach enables fabrication of materials that can hold and release charge in response to external stimuli coupled with color changes.¹ We will demonstrate the fabrication of a multilevel per bit organic memory cell composed of redox-active ruthenium, iron, and osmium polypyridyl complexes. The organic memory cell is dual function: the information can be read optically as well as electrochemically. Such coatings might have interesting applications including new read-while-write devices with both electrochemical and optical read-out
1. Hamo, Y., Lahav, M. & van der Boom, M. E. Bifunctional Nanoscale Assemblies: Multistate Electrochromics Coupled with Charge Trapping and Release. Angew. Chem. Int. Ed. 59, 2612–2617 (2020).

Organic semiconductors hold great promise as smart and environmentally-friendly materials for energy production and flexible electronics. Current research in the field is largely focused on the structure-function relationships of these promising materials.
However, at room temperature, thermal fluctuations of the atoms and molecules introduce dynamic disorder that strongly affects both the electronic and mechanical properties, rendering their exact properties unpredictable.
Since organic semiconductors are soft, their thermal fluctuations are strongly anharmonic, affecting fundamental properties of the crystals, i.e., phase transitions, thermal expansion, and thermally activated disorder.
These anharmonic effects control structural dynamics, thus significantly impacting the electronic and mechanical properties of the organic solid.
Current theoretical models use the harmonic approximation, thus, they cannot predict anharmonic effects.
This poses a persistent problem in our predictive power to design rules for new, high-performance organic semiconductors.
To overcome this challenge, we used state-of-the-art THz-Range Raman scattering to investigate how the crystal structural dynamics changes with temperature.
In my presentation I will show how chemical modifications to a core [1]benzothieno[3,2-b]benzothiophene (BTBT) crystal can be used as a sensitive proxy that reports on the effect of chemical modifications on anharmonic effects.
Using this approach, I will show that conjugated chemical modifications (diPh-BTBT and DNTT) suppress the anharmonic effects better than alkyl-based modifications (diC8-BTBT and ditBu-BTBT) and thus can improve material properties at room temperature.

Dinaphthothienothiophene (DNTT) derivatives have attracted considerable attention as an active layer for organic thin-film transistors (OTFTs) due to their high carrier mobility and high stability in air. Since the charge transport in OTFTs occurs mainly in the first few monolayers on the dielectric layer, analysis of the submonolayer structure is crucial for understanding the relationship between structure and properties. Nevertheless, the thin-film structure of DNTT has long been believed to be identical to the single-crystal structure [1]. In the present study, the thickness-dependent structural evolution of this compound on silicon is revealed by high-resolution measurements of X-ray diffraction and infrared spectroscopy. These techniques apparently identify several polymorphs depending on the thickness. The most important finding is that the compound exhibits a unique molecular structure in the first monolayer at the substrate interface. This study is believed to provide an in-depth understanding of the structure-property relationship in OFETs.
[1] Shioya, N. et al., Appl. Phys. Express 2020, 13, 095505.

The charge-transfer (CT) state can arise at the interface of charge-donating (D) and charge-accepting (A) molecular units. It is characterized by an electronic structure that is a hybrid of the two parent compounds, and thus has strong implications on the performance of optoelectronic devices, as well as fundamental processes in semiconductors like doping. Here, we report on the electronic structure and properties of two polymorphs of the CT complex DBTTF-TCNQ (dibenzotetrathiafulvalene – 7,7,8,8-tetracyanoquinodimethane), the α- and β-polymorphs. When crystallized by the physical vapor transport method, the α-polymorph forms with a rectangular facet while the β-polymorph is elliptical. We differentiated the structure of each system through a combination of X-ray photoemission spectroscopy, UV-Vis reflectance, polarized infrared spectroscopy, and selected-area electron diffraction, finding that each grows in a 1:1 D:A ratio, but with different orientations of the D and A units with respect to the crystal facets. While the π-stacking direction of α-DBTTF-TCNQ is oriented along the long crystalline axis, it is oriented along the short axis for the β-polymorph.
The effects of these structural differences are immense. Using angle-resolved photoemission spectroscopy, we observe different electronic structures for each crystal. The highest occupied molecular orbital (HOMO) for α-DBTTF-TCNQ is positioned at 0.9 eV, with HOMO-1 and HOMO-2 at 2.15 eV and 2.8 eV, respectively, while only two HOMO bands were measured for β-DBTTF-TCNQ, a HOMO at 1.15 eV and a HOMO-1 at 2.7 eV. The polymorphs vary in degree of charge transfer, too, with α-DBTTF-TCNQ having an ionicity of approximately 0.5e and β-DBTTF-TCNQ being nearly neutral. The strongest difference arises in their charge transport properties. In field-effect transistors of the same device architecture, the α-polymorph shows ambipolar charge transport with a higher electron mobility than hole mobility in the π-stacking direction (0.4 cm²V^-1s^-1 versus 0.04 cm²V^-1s^-1), while the opposite is true for β-DBTTF-TCNQ (0.03 cm²V^-1s^-1 for electrons versus 0.1 cm²V^-1s^-1 for holes). Further analysis of the sub-threshold regime of the transistor characteristics reveals a small but appreciable difference in the trap density of states between the crystals. Together, these results highlight how a varying overlap between the frontier molecular orbitals of the CT complex donor and acceptor constituents can cause varying charge-transport polarities, a result which has been theoretically predicted.

The strong relationship between solid-state order and charge transport properties in organic solids is well known; even minuscule shifts in intermolecular contacts arising from simple molecular vibrations can lead to significant changes in hole or electron mobility. I will present a substantial array of linearly-fused chromophores functionalized to induce strong, two-dimensional π-stacking interactions ideal for thin-film charge transport, and show that minuscule changes to the functional groups - for example the deletion of just a few hydrogen atoms - can completely change the crystal packing arrangement. I will also show how stronger non-covalent interactions, such as hydrogen bonding, can be detrimental to thin-film formation. I will finally present several new chromophore designed to minimize the impact of long-axis phonon modes, to substantially improve transistor performance.

The ability to control triplet excitons is of paramount importance in and beyond the field of optoelectronics. As molecular triplet excitons are dark states, triplets can generally not be accessed directly nor harvested luminescently. Conventionally, triplet excitons are controlled via heavy-metal induced spin-orbit coupling or tuning of the singlet-triplet energy gap, both of which put heavy constraints on the system design. Recently, our group demonstrated triplet exciton control in organic molecules coupled to inorganic lanthanide-doped nanoparticles via a spin-exchange coupling mechanism, allowing for triplet exciton control from both the ground and excited state. This work explores the effect of molecular design of the organic molecules on the reported spin-exchange coupling. We couple three different anthracene carboxylic acid ligands to various lanthanide-doped nanoparticles and demonstrate, using ultrafast transient absorption spectroscopy, ligand-orientation dependent intersystem crossing accelerations in the presence of lanthanides with unpaired 4f electrons. Furthermore, we show ligand-orientation dependent triplet lifetime enhancements of up to three orders of magnitude upon coordination of the organic molecules to the lanthanide-doped nanoparticles, giving rise to millisecond triplet lifetimes, and demonstrate that these lifetimes are largely unaffected by the presence of oxygen. Additionally, steady-state photoluminescence measurements revealed unexpected singlet energy transfer in the hybrid organic-lanthanide systems. Using a combination of steady-state and time-resolved photoluminescence techniques, the resulting lanthanide emission could be related to the orientation of the surface-anchored anthracene ligands on the nanoparticle surface. Current measurements are looking in more detail at the distance dependence of the spin-exchange coupling and energy transfer mechanism. Overall, these results provide new insights into a novel way of controlling triplet excitons in organic molecules and deepen our understanding of the relationship between molecular structure and photophysical properties of organic-lanthanide systems, which is essential for many optoelectronic and biomedical applications.

The low carrier mobility of organic semiconductors and the high parasitic resistance and capacitance often encountered in conventional organic Schottky diodes, hinder their deployment in emerging large-area radio frequency (RF) electronics. In this talk I will discuss how we have overcome these limitations by combining self-aligned asymmetric nanogap electrodes produced by adhesion-lithography, with a high mobility organic semiconductor and demonstrate RF Schottky diodes able to operate in the 5G frequency domain. Specifically, I will show how optimal device engineering combined with electronic doping of a high mobility polymer semiconductor yields Schottky diodes that exhibit maximum extrinsic cutoff frequencies (f_C) of >10 GHz. Our work highlights the importance of the planar nanogap architecture and paves the way for the use of organic Schottky diodes in large-area radio frequency electronics of the future.

With the promise of low-cost solution-processing and the reported mobilities surpassing the industrial standard of amorphous Si, OFETs are now close to becoming ubiquitous circuit components of flexible electronics such as wearable devices, display matrices and biosensors. The bottleneck for OFETs to outperform their inorganic counterpart is their issues with environmental, thermal and operational stability, and the general poor understanding of mechanisms involved in these processes. The instabilities can be quantified by voltage threshold shifts, changes in mobility, ON/OFF current ratio or subthreshold swing as a function of applied condition. This can result in a variety of device shortcomings such as erroneous extraction of measured quantities in sensors or an unstable pixel brightness of an OLED display. Device instabilities are widely attributed to trapping of charges due to intrinsic properties such as conformational defects, or external effects related to atmospheric conditions or induced by bias stress.
Here, we focus on the operational stability of the devices, i.e. under a prolonged bias. While a lot of research has been done into understanding and reducing the ON-state bias stress instabilities (such as addition of molecular additives [1], molecular doping [2] or encapsulation [3]), work on the OFF-state bias stress effects is scarce [4]. Since most OFET devices stay in in the depletion mode for longer times, excellent OFF-state stability is an equally important industrial requirement.
In this work, we introduce a novel method of improving OFF-state stability of solution-processed polymeric OFETs, using an anti-solvent treatment on an Indacenodithiophene-co-benzothiadiazole (IDT-BT) film. We demonstrate an improvement across both voltage threshold (4-fold) and mobility (10-fold) stability that suggests a decrease in the density of both deep and shallow charge traps. Moreover, the anti-solvent treated devices exhibit superior long-term stabilisation of the device as well as OFF-state bias stability when exposed to atmospheric species or light. We also show the microscopic and crystallographic morphology change induced by the treatment, which we believe is crucial for understanding of the underlying mechanism. Our results show a simple and effective way of minimising the OFF-state instability of OFETs that can contribute to the future advancement of organic electronics in industrial applications.
[1] M. Nikolka, I. Nasrallah, B. Rose, M. K. Ravva, K. Broch, A. Sadhanala, D. Harkin, J. Charmet, M. Hurhangee, A. Brown, S. Illig, P. Too, J. Jongman, I. McCulloch, J. L. Bredas, H. Sirringhaus, Nat. Mater. 2017, 16, 356.
[2] M. P. Hein, A. A. Zakhidov, B. Lüssem, J. Jankowski, M. L. Tietze, M. K. Riede, K. Leo, Appl. Phys. Lett. 2014, 104.
[3] H. F. Iqbal, Q. Ai, K. J. Thorley, H. Chen, I. McCulloch, C. Risko, J. E. Anthony, O. D. Jurchescu, Nat. Commun. 2021, 12, 2352.
[4] M. Kettner, M. Zhou, J. Brill, P. W. M. Blom, R. T. Weitz, ACS Appl. Mater. Interfaces 2018, 10, 35449.

Despite the impressive advancements obtained in the last decade in the development of materials and fabrication techniques for high-performance organic electrochemical transistors (OECTs), disagreement still reigns from a modeling standpoint. New models are periodically proposed while experimental studies continue to highlight the need of a more comprehensive and fundamental theory that takes into account the electronic, ionic, physical, and chemical properties of the system. In this work, we tackle the problem starting from thermodynamic axioms and reason in terms of free energy of mixing and chemical potential. With a combination of experiments, theory, and numerical simulations, we put forth a model capable of binding the existing fragmented theoretical and experimental evidence into a single mathematical framework. We show that the OECT channel can be treated as a thermodynamic binary system and the interactions between the components are key to explaining the physics of the materials and devices. We use our model to shine a light on the electrostatic interactions occurring within the polymer matrix, and reveal that the entropy of mixing is the major driving force behind the operation of the OECT. Finally, we propose bistability as the possible mechanism causing the ubiquitous hysteretic behavior of OECTs, similarly to already shown we propose for Lithium batteries. Our approach is a bridge between the chemical processes occurring in the electrolyte/polymer system and the electronic properties of the device, thus providing key insights for targeted material design for enhanced device performances and targeted functionalities, such as neuromorphic computing, logic, and cell culture.

Organic electrochemical transistors (OECTs) rely on the transduction of ionic fluxes into electronic signals to operate, and they do so with exceptional performance.^[1] Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in particular displays high ionic and electronic mobilities, and has been shown be stable for up to three weeks.^[2] However, PEDOT:PSS suffers from being a depletion mode material, meaning that OECTs made with PEDOT:PSS are always ON, and as such, require high power consumption to turn OFF or be modulated.
Semiconducting conjugated polymers, on the other hand, give rise to accumulation (enhancement) mode OECTs with inherently high ON/OFF ratios and low power consumption. Due to this, synthetic efforts have gone towards developing a library of such materials. For applications like biological sensing, these materials should be stable in ambient and aqueous environments, that is, not degrade during the duration of the experiments.
OECT stability is commonly probed by switching the gate between ON and OFF states while biasing the drain electrode. As such, current degradation, and therefore transconductance loss, is measured. This is a good indicative of redox reversibility, but not of long-term stability, which is often overlooked. Recently, a p-type diketopyrrolopyrrole (DPP) based organic semiconducting polymer was reported to retain more than 85% its initial transconductance value over ~75 hours exposed to electrolyte.^[3] However, this is reported for a non-biological relevant anion.
We show that by tuning the solvent mixture composition used to cast a thiophene-based p-type semiconducting polymer film, not only the OECT performance is enhanced but also the device shelf-life stability is improved. A similar effect is obtained by the use of a Lewis acid in the polymer solution, despite clear morphological differences. By blending both additives, we can not only elongate the life of our devices, but also modify the figures of merit of the OECTs and tune their morphology. As such, our work provides insights into structure-property relations of semiconducting polymers for applications in bioelectronics that require high performance alongside long shelf-life.
[1] Rivnay, J. et al. Organic electrochemical transistors. Nat. Rev. Mater. 3, 17086 (2018)
[2] Kim, S.-M. et al. Influence of PEDOT:PSS crystallinity and composition on electrochemical transistor performance and long-term stability. Nat. Commun. 9, 3858 (2018).
[3] Wu, X. et al. Enhancing the Electrochemical Doping Efficiency in Diketopyrrolopyrrole-Based Polymer for Organic Electrochemical Transistors. Adv. Electron. Mater. 7, 2000701 (2021)

We report on a combined experimental-theoretical study of charge transport in two flagship organic semiconductors, namely DNTT (Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), and its alkylated derivative, C8-DNTT-C8. By combining THz spectroscopy measurements to first-principles non-adiabatic molecular dynamics simulations [1] [2], we show that both crystals display a power-law decrease of the hole mobility with increasing temperature that is characteristic of band-like transport [3] [4]. In line with earlier investigations, the RT mobility in C8-DNTT-C8 is about twice as large as that of DNTT [5], mostly owing to a favourable 2D band structure. The most remarkable result, however, is the fact that the falloff of mobility with temperature is much steeper in C8-DNTT-C8 vs DNTT. A detailed analysis of the results in the framework of transient delocalization theory reveals that this difference can be ascribed to the shape of the charge Density of States in the two materials and directly reflects the presence of thermally accessible extended states.
Acknowledgements:
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 811284.
Bibliography
[1] S. Giannini, J. Blumberger, A. Carof, “How to calculate charge mobility in molecular materials from surface hopping non-adiabatic molecular dynamics – beyond the hopping/band paradigm,” Phys. Chem. Chem. Phys., 2019.
[2] S. Giannini et al., “Quantum localization and delocalization of charge carriers in organic semiconducting crystals,” Nat. Commun., 2019.
[3] T. Nematiaram and A. Troisi, “Modeling charge transport in high-mobility molecular semiconductors: Balancing electronic structure and quantum dynamics methods with the help of experiments,” J. Chem. Phys., 2020.
[4] S. Fratini et al., “Charge transport in high-mobility conjugated polymers and molecular semiconductors,” Nat. Materials, 2020.
[5] G. Schweicher et al., “Chasing the “Killer” Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors,” Advanced Materials, 2019.

In the last years, organic electronic-based immunosensors, such as electrolyte-gated organic transistors (EGOTs), have emerged as a promising alternative strategy for the ultra-sensitive and label-free detection of biological analytes. Their intrinsic characteristics, such as high amplification, low cost, flexibility, and biocompatibility, make them the ideal candidates for point-of-care testing.
Among EGOTs, two different architectures can be distinguished: organic electrochemical transistors (OECTs) and electrolyte-gated organic field-effect transistors (EGOFETs). Both, OECTs and EGOFETs, are three-terminal devices in which the source and drain electrodes are connected by an organic semiconductor (OSC) layer, that acts as a conducting channel, and is separated by an electrolyte from the gate electrode. The application of a potential difference between gate and source (V_GS) leads to the formation of two electrical double layers, at the gate/electrolyte and electrolyte/OSC interfaces, promoting the accumulation of charge carriers in the OSC, which results in a current flow between the source and drain (I_DS) following the application of a second potential difference (V_DS).
In the present work, we report the first EGOFET-based immunosensor for the detection of neurofilament light chain (NF-L), a candidate biomarker for several neurological disorders. NF-L is a major component of the axonal cytoskeleton of the neurons and, following injuries of the central nervous system, NF-L is released into the body fluids, providing a direct indication of neural damage.
In order to endow the EGOFET device with the ability to selectively recognize NF-L, the gate electrode was used as the sensing element, and specific anti-NF-L antibodies were immobilized on the gate surface, with a potentially controlled and uniform orientation. A decrease in the output current was observed after the incubation of the gate electrode in buffered solutions containing increasing concentrations of NF-L. In addition, following the binding events occurring on the gate surface, a concomitant shift of the threshold voltage (V_th) towards more negative values, and a decrease in transconductance (g_m) were observed.
The biosensor exhibited the maximum response, defined as the normalized relative current change, in the subthreshold regime. The observed trend suggested the presence of two different regimes, which were interpretated as the simultaneous formation of two layers on the gate surface: a first antigen-antibody layer, and second layer corresponding to weak protein-protein interactions, observed at high NF-L concentrations. The successful fit using the Guggenheim-Anderson-de Boer (GAB) adsorption model and further morphological characterization of the gate electrode supported this hypothesis. In addition, a large set of control experiments was performed in order to evaluate the selectivity and specificity of the biosensor.
In summary, our EGOFET biosensor demonstrated to selectively detect NF-L in a wide dynamic range of concentrations (100 fM-10 nM), providing a label-free, rapid, and reproducible response, indicating its potential as an alternative sensing platform for the detection of NF-L in neurological disorders such as Parkinson’s disease or multiple sclerosis.
The authors acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 813863.

The double ionisation of dopants is a recent experimental observation that allows the doping efficiency (free charges per dopant molecule) to rise above 100% [1]. This is exciting because high conductivities can be achieved with fewer dopant molecules, meaning less disruption to the resulting film’s microstructure. However, the current models of doped organic semiconductors based on Fermi-Dirac statistics with two Gaussian functions to represent the host and dopant density of states, fail to explain double dopant ionisation, or indeed the analogous situation of bipolaron formation on the host. Here, we directly address the model’s shortcomings by considering the splitting of the singly-occupied frontier molecular orbitals that proceed electron transfer between host and p-dopant. This adaptation to the model is rooted in previously reported photoelectron spectroscopy measurements by Winkler et al. [2] . We test the new model over a parameter space relevant for organics, observing robust trends in the fractions of doubly-ionised, singly-ionised and neutral species. These trends show good agreement with experimental measurements of doubly-ionised p-dopants, and bipolaron formation on a p-doped host. Finally, we identify the orbital splitting as the key parameter to be either minimised for double ionisation of dopants, or maximised to avoid the formation of bipolarons on the host.
[1] Kiefer, D. et al. Double doping of conjugated polymers with monomer molecular dopants. Nature Mater 18, 149–155 (2019).
[2] Winkler, S. et al. Probing the energy levels in hole-doped molecular semiconductors. Materials Horizons 7, 427-433 (2015).

Designing doped π-conjugated polymers (CPs) for organic thermoelectrics remains a challenging pursuit, in part because of an incomplete understanding of charge-carrier transport in these disordered materials. For example, it is often assumed that charge-carrier transport in doped CPs is dominated by one type of charge carrier, either holes or electrons, as determined by whether the dopant reduces or oxidizes the polymer. However, we find that this is not always a valid assumption, as several polymers that are heavily doped with oxidizing dopants (i.e., p-type dopants) show n-type charge transport, as supported by negative Seebeck coefficients and negative (n-type) Hall voltages. Here, we show that moderate p-doping of PDPP4T with FeCl₃ leads to a p-type power factor of 24.5 µW m^-1 K^-2, while further increases in FeCl₃ doping concentration lead to an n-type power factor of 9.2 µW m^-1 K^-2 for this same PDPP4T polymer. This n-type behavior upon heavy doping with oxidizing dopants is not unique to this one CP-dopant system, but occurs with multiple dopants (FeCl₃ and NOBF₄) and with all donor-acceptor type polymers examined. A mechanistic explanation for the origin of n-type charge-carrier transport is presented based on a combination of Seebeck coefficient measurements, Hall effect measurements, density functional theory calculations, ultraviolet and inverse photoelectron spectroscopy, UV-Vis-NIR absorbance, and electron paramagnetic resonance (EPR) spectra. Ultimately, the combination of data points to the presence of hopping-type holes in the amorphous regions and delocalized electrons in the crystalline regions. With two carrier types present the Seebeck coefficient is determined by the energy-weighted contributions of each carrier type to the total electrical conductivity. This work uncovers important fundamental insight for understanding charge-carrier transport in heavily doped polymers and presents a new approach to the development of high-performance n-type organic thermoelectric materials.

Optical microcavities are photonic structures consisting of two reflective mirrors that concentrate light to small volumes within their in-between distance. They are often used to improve and modify the outcoupling of light, via Purcell enhancement, in light-emitting materials and devices. This is the so-called weak light-matter coupling regime where decay rates of excitons and photons in the cavity are faster than their interaction process, thus exciton and cavity resonances can be treated as independent entities¹. By carefully designing optical microcavities to have resonances that overlap with the excitons in terms of energy, space and polarization, the system can transition to the strong light-matter regime. There, excitons and photons give their place to new eigenstates that are called polaritons. Because polaritons inherit both exciton and microcavity-photon properties, their hybrid part-matter/part-photon nature makes them an excellent platform for studying a plethora of fascinating phenomena such as stimulated scattering, parametric amplification, Bose-Einstein condensation and superfluidity². From the practical standpoint, molecular semiconductors are a favourable material for studying polaritons because the large binging energy of their excitons allows them to exist up to and beyond room temperature.
While polaritons were initially attracted only fundamental research interest, the realization of polaritons in molecular semiconductors has given a new twist to the field and has enabled polaritons to find their niche in applied research. Notably, the ability of polaritons to change the energy landscape of a molecular system offers a novel approach towards the modification and optimization of the chemical and physical properties of organic semiconductors³. Thus, infusing polaritonics into organic optoelectronics can pave the way for addressing several challenges in organic light-emitting devices such as the low operational lifetime and low external quantum efficiency. For example, tuning a polariton state close to a singlet or triplet state can enhance intersystem crossing, enable reverse intersystem crossing, and modify the spectrum of a molecular emitter. In addition, the photonic component of polaritons inherits its delocalization character to the molecular excitons which can lead to non-radiative radiative energy transfer beyond the sub-10nm Förster radius limit. For example, this exciton delocalization can be beneficial in organic photovoltaic and light-emitting devices that are based on host-guest configurations.
In the lab conditions, we create polaritons in optical microcavities that sandwich an organic semiconductor thin film. Because polaritons decay out as photons, we use spectroscopy to study the properties of the system and a combination of theoretical tools namely coupled harmonic oscillator and transfer-matrix modelling, to further improve our devices^4–6. In this talk, I’ll give a brief introduction to polaritons, discuss the experiments for characterizing polaritonic samples, and how in my group we engineer organic light-emitting diodes (mainly thermally activated delayed fluorescence molecules) coupled with microcavities for improved efficiency.
References:
1. Frisk Kockum, A., Miranowicz, A., de Liberato, S., Savasta, S. & Nori, F., Nature Reviews Physics 2019 1:1 1, 19–40 (2019).
2. Sanvitto, D. & Kéna-Cohen, S., Nature Materials 15, 1061–1073 (2016).
3. Hertzog, M., Wang, M., Mony, J. & Börjesson, K., Chemical Society Reviews vol. 48 937–961 (2019).
4. Daskalakis, K. S., Maier, S. A. & Kéna-Cohen, S., Physical Review Letters 115, 035301 (2015).
5. Daskalakis, K., Freire Fernandez, F., Moilanen, A., van Dijken, S. & Törmä., ACS Photonics 6, 2655–2662.
6. Daskalakis, K. S., Maier, S. A., Murray, R. & Kéna-Cohen, S., Nature Materials 13, 271–278 (2014).

The controlled introduction of dopants in organic semiconductors enhances bulk conductivity and lowers contact resistance, enabling the design of high-performance devices. Considerable progress has been made over the past two decades on the synthesis of high performance molecular redox agents, enabling efficient n- and p-doping of organic semiconductors with a broad range of electron affinities (EA) and ionization energies (IE). In this talk, we report a recent study [1] using a powerful oxidant, hexacyano-trimethylene-cyclopropane (CN6-CP), in conjunction with a powerful reductant, (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium (RuCp*Mes)₂ to realize a large gap p-i-n homojunction diode based on phenyldi(pyren-2-yl)phosphine oxide (POPy₂). With a single particle gap of 3.6 eV,[2] POPy₂ has a high ionization energy (IE=5.8 eV) and a low electron affinity (EA=2.2 eV), both of which make it challenging to p- and n-dope. We use ultra-violet and inverse photoemission spectroscopies (UPS, IPES) to determine the frontier orbital energies of dopants and host. We confirm with IPES the large CN6-CP EA (5.87 eV) previously determined via cyclic voltammetry,[3] and show that this oxidant is able to p-dope POPy₂. N-doping of POPy₂ using (RuCp*Mes)₂ has previously been demonstrated [2]. Using both dopants, we build a p-i-n POPy₂ homojunction diode with record high built-in potential (2.9 V). Ambipolar injection leading to blue light emission (~450m) is demonstrated.

[1] H. L. Smith at al., (under review)
[2] X. Lin et al., Nature Materials, 16, 1209 (2017)
[3] Y. Karpov et al., Advanced Materials 28, 6003 (2016)

Organic mixed ionic electronic conductors (OMIECs), typically conjugated polymers, are promising materials for aqueous-based electrochemical transistors due to their redox-activity as well as ionic and electronic conductivity. Judicious side chain engineering enables the intercalation of ions into the bulk of the OMIEC, enabling volumetric charging. By utilizing them as the active channel material in organic electrochemical transistors (OECTs), volumetric charging of the channel allows OECTs to exhibit large transconductances, thereby enabling the amplification of minute voltage modulations such as bioelectronic and biochemical signals.¹

Controlling the threshold voltage of OECTs has been of great interest in the fields of bioelectronics and organic semiconductors for several reasons. For instance, designing an OECT with turn-on voltage close to 0 V enables low power devices. Additionally, achieving optimal amplification of sensed bioelectronic and biochemical signals requires an OECT with maximum transconductance within the potential range of the signal it is meant to amplify. Hence, various approaches to tune the OECT threshold voltage have been reported: by modifying the chemical backbone (and subsequently density of states) of the OMIEC channel,² by introducing chemical dopants into the OMIEC channel,³ or by using metal gates of discrete work functions.⁴ While effective in controlling the OECTs’ threshold voltage, these approaches have several drawbacks. Firstly, modifying the channel material may result in undesirable modifications in the OECTs’ electronic properties such as carrier mobility. Furthermore, custom-designed OMIECs require long synthetic lead times with properties such as turn-on voltage, mobility and capacitance that are not easily customizable independently. Lastly, using gate materials of discrete work functions limit the tunability of threshold voltages on finer scales.

To address these challenges, we present a versatile method to tune the threshold voltage of OECTs by chemically doping OMIEC polymer gates. Utilizing different compositions of F_nTCNQ (n=0, 1, 2, 4) as electron acceptors in glycolated p-type OMIECs of various ionization potentials tunes the work function of the polymer blends. Utilizing these doped polymers as OECT gate electrodes, we shift the threshold voltage of OECTs by up to 400mV. This enables the fine tuning of the threshold voltage and region of maximum transconductance for low power and high gain electronics. This approach presents greater flexibility in controlling the operation point of OECTs while maintaining the transport properties of the channel. Utilizing OECTs with different doped gates, we achieve unipolar inverters with gain >40. Our approach can be generalized to other F_nTCNQ – p-type polymer pairs, paving the way for more effective tuning of electronic properties of electrochemical devices.

1. Tan, S. T. M. et al. High-Gain Chemically Gated Organic Electrochemical Transistor. Adv. Funct. Mater. 31, 1–9 (2021).
2. Giovannitti, A. et al. Energetic Control of Redox-Active Polymers toward Safe Organic Bioelectronic Materials. Adv. Mater. 1908047, 1–9 (2020).
3. Keene, S. T. et al. Enhancement-Mode PEDOT:PSS Organic Electrochemical Transistors Using Molecular De-Doping. Adv. Mater. 32, (2020).
4. Doris, S. E., Pierre, A. & Street, R. A. Dynamic and Tunable Threshold Voltage in Organic Electrochemical Transistors. Adv. Mater. 30, 1–5 (2018).

One of the main fascinations behind the transport physics in polymers is the observation of a metallic state. The presence of a finite density of states at the Fermi level has been shown through multiple transport signatures, such as a finite electrical conductivity at low temperatures approaching absolute zero,[1] a Pauli contribution to the paramagnetic susceptibility,[2] a plasma oscillation of the nearly free electron gas,[1] and temperature-dependence of the Seebeck coefficient obeying Mott formula.[3, 4] While the earliest reports of these observations date back to the 1970s, up until now metallicity has been reported mostly for polymers with low degree of structural disorder and relatively high conductivity (e.g., around 1000 S/cm in specially prepared polyaniline[1] and FeCl₃ ion-exchange doped PBTTT[3], and 600 S/cm in AsF₅ doped cis-polyacetylene[5]).

In this study, we demonstrate the generality of the insulator-to-metal transition in a wide range of polymers having very different microstructures and electrical conductivities. Experimental access to an extended regime of carrier densities is made possible in an electrochemical transistor device architecture, which has recently been showed to be as effective[6] as the recently developed ion-exchange doping technique.[6, 7] We show that a clear metallic temperature-dependence of Seebeck coefficient is also present in representative polymers with low conductivity and crystallinity. Independent confirmation of the presence of the metallic states are provided through photoemission and electron paramagnetic resonance spectroscopies. We argue that metallicity is accessible in semiconducting polymers in general, as evidenced by a similar evolution of the Seebeck coefficient with temperature and carrier density, provided that a high degree of carrier density could be achieved to shift the Fermi level into the delocalized states of the energy band.

[1] Lee, K. et. al., Metallic transport in polyaniline, Nature 441, 65-68 (2006).
[2] Kang, K. et. al., 2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion, Nat. Mater 15, 896-902 (2016).
[3] Huang, Y. et. al., Design of experiment optimization of aligned polymer thermoelectrics doped by ion-exchange, Appl. Phys. Lett. 119, 11903 (2021).
[4] Watanabe, S. et. al., Validity of the Mott formula and the origin of thermopower in pi-conjugated semicrystalline polymers, Phys. Rev. B 100, 241201(R) (2019).
[5] Chiang, C. K. et. al., Electrical Conductivity in Doped Polyacetylene, Phys. Rev. Lett. 39, 1098 (1977).
[6] Jacobs, I. E. et. al., High-Efficiency Ion-Exchange Doping of Conducting Polymers, Adv. Mater, 2102988 (2021).
[7] Yamashita, Y. et. al., Efficient molecular doping of polymeric semiconductors driven by anion-exchange, Nature 572, 634-638 (2019).

The last decade has seen renewed interest in the fundamentals of doping processes in semiconducting polymers.[1] Molecular doping has been the most studied approach, in large part because films are easy to fabricate and show high conductivities. However, molecular doping is subject to several limitations: 1. disorder due morphological disruption from dopant ions; 2. limited charge densities due to insufficiently strong dopants; 3. reduced doping efficiency from charge transfer complexation; and 4. poor stability resulting from the inherent reversibility of charge transfer. These difficulties largely stem from the dual role molecular dopants must play. The neutral dopant first acts as a redox agent, while the product of this redox reaction—the ionized dopant—then acts as a compensating ion. A recently proposed method based on ion exchange allows for replacement of the dopant counter-ion with arbitrary ion from an electrolyte solution.[2,3] This separates the redox and charge compensation steps, and enables the fabrication of doped films with a range of different counterions. The improved microstructural control this process allows enables us for the first time to systematically address a longstanding but still poorly understood question: what limits the electrical conductivity at the high doping levels relevant to organic thermoelectrics? Is it the formation of charge carrier traps in the Coulomb potentials of the counterions, or is it the structural disorder in the polymer lattice?
Here[4] we present a simple framework for understanding ion exchange doping and evaluate a wide range of charge transfer dopants and electrolytes. From this systematic study, we identify a set of experimental conditions which allows for extremely high doping levels of approaching 1 charge per monomer and nearly 100% ion exchange efficiency in several different polymer classes. In each of these degenerately doped polymer systems, we achieve record or near-record conductivities: >1200 S/cm for PBTTT, 220 S/cm for P3HT, 80 S/cm for DPP-BTz, and 15 S/cm for IDTBT. Most critically, we show that in this regime conductivity is poorly correlated with ionic size, but strongly correlated with paracrystalline disorder. This observation, backed by a detailed electronic structure model that incorporates ion-hole and hole-hole interactions and a carefully parameterized model of disorder, indicates that trapping by dopant ions is negligible, and hence that the doping efficiency in these systems is near 100%. Our results imply that in this high carrier density regime, relevant to thermoelectric devices, maximizing crystalline order is the most critical factor in improving conductivity. This realization, along with the greatly improved control enabled by ion exchange, provides a clear path forward for the design of polymer thermoelectrics.
1. Jacobs, I. E., Moulé, A. J., Adv. Mater. 2017, 29, 1703063
2. Yamashita, Y. et al. Nature 2019, 572, 634–638
3. Jacobs, I. E., Adv. Mater. 2021, 2102988
4. Jacobs, I. E., et al. arXiv:2101.01714

Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excited state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells raises the maximum theoretical efficiency of a solar cell from the Schockly-Queisser Limit of 33% to around 45% by effectively harvesting the energy from high energy photons. SF has been reported and extensively studied in crystalline acenes, and more recently acene dimers to better understand the fundamental photophysics and materials requirements for SF. Incorporation of these SF materials in to functional solar cells, although demonstrating modest efficiency enhancements, has had limited success. In our efforts to produce higher efficiency printed organic solar cells we had the desire to incorporate solution processible SF materials in printed organic solar cells, however most of the reported SF materials are highly crystalline and either do not promote SF in the solid state or controlling crystallisation is difficult.
Here we report our studies of new solid-state singlet fission materials using liquid crystallinity to promote self-assembly and to pre-organise triplet host chromophores. Using design criteria outlined by Busby et al. [1], suggesting an Acceptor-Donor-Acceptor (A-D-A) structure may support SF, we have used strong p-p interactions of a fluorenyl-substituted hexabenzocoronene (FHBC) donor to promote strong self-assembly, coupled with thienyl-substituted diketopyrrolopyrrole (TDPP) as the triplet host. We needed to design our intra-molecular SF host, with i) a singlet energy around 2.0 eV if TDPP was to be our triplet host, ii) strong self-association through the core, and iii) solution processability. Thin films of the discotic liquid crystalline FHBC(TDPP)₂ material forms hexagonally packed columns and has a singlet energy level of 2.00 eV [2]. SF studies on FHBC(TDPP)₂ demonstrate a triplet yield of 150% in amorphous thin films, increasing to 170% in thermally annealed films.[3] This constitutes a new class of singlet fission materials.
[1] Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.; Zhu, X. Y.; Campos, L. M.; Sfeir, M. Y Nat. Mater., 14, 426-33. (2015)
[2] Wong, W. W. H.; Subbiah, J.; Puniredd, S. R.; Purushothaman, B.; Pisula, W.; Kirby, N.; Muellen, K.; Jones, D. J.; Holmes, A. B., J. Mater. Chem. 22 (39), 21131-21137. (2012)
[3] Saghar Masoomi-Godarzi, Maning Liu, Yasuhiro Tachibana, Valerie D. Mitchell, Lars Goerigk, Kenneth P. Ghiggino, Trevor A. Smith and David J. Jones Adv. Energy Mater. 2019, 9 (31), 1901069. DOI:https://doi.org/10.1002/aenm.201901069

Semiconducting organic polymers are most often synthesized by linking an electron poor and an electron rich (hetero)aromatic building block via a transition metal catalyzed cross-coupling copolymerization. Matrix-assisted laser desorption-ionization - time of flight (MALDI-ToF) mass spectrometry analysis shows, however, that there are a large number of homo-couplings and impurities present besides the desired perfectly alternating donor-acceptor structure, even in almost all tested commercial samples. Furthermore, the end groups are often unknown. As a result, material suppliers struggle in providing the exact same product, with its optimal properties, to their customers and batch-to-batch variability can be extensive. Researchers aiming at exploring applications and fundamental performance limits, for example for organic photovoltaics, organic photodetectors, and organic electrochemical transistors, often assume that the obtained material consists strictly of a perfect repetition of the depicted polymeric repeating unit, whereas this is likely not the case. This caveat severely hinders these research endeavors. In this contribution, we demonstrate a synthesis approach to obtain the depicted “perfect” structure of these types of polymers. The stacking behavior and morphology of the materials is investigated using small-angle X-ray diffraction techniques. We also show the influence of material defects on the optoelectronic properties and device performance.

Thermally activated delayed fluorescence (TADF) materials are a promising emerging class of systems for organic light-emitting diodes (OLEDs) due to their potential to generate light output from both singlet and triplet excitons.^[1] As a result, TADF OLEDs can attain an internal quantum efficiency close to 100% without the need for any heavy metal additive. A large number of new TADF materials are being synthesized with the goal of obtaining high quantum yields, better stability, and desired color coordinate. Important parameters for the design and performance of TADF emitters are forward and reverse intersystem crossing rates (ISC/rISC) between singlet and triplet states. The magnitude of these rates is typically determined from the prompt and delayed transient photoluminescence decay. In addition to the desired radiative photoexcited decay, these materials are also subject to loss mechanisms due to exciton-exciton annihilation, which quench the excited state population non-radiatively. Singlet-triplet annihilation (STA) and triplet-triplet annihilation (TTA) can significantly affect the TADF efficiency and lifetime. We carried out power-dependent time-resolved photoluminescence (TrPL) measurements on a model TADF emitter 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene (CzDBA) and demonstrate that this photoluminescence decay strongly depends on the initial photoexcited population density due to exciton-exciton annihilation processes. By analytical kinetic modeling of the radiative decay we extract the STA and TTA rate constants on the order of 10^-17 s^-1 and 10^-18 s^-1, respectively. Neglecting these quenching processes leads to erroneous estimates of the (reverse) intersystem crossing rates. The obtained TTA rate constant from the TrPL is in excellent agreement with the rate obtained from the efficiency roll-off of CzDBA based OLEDs.^[2] An accurate characterization method for the exciton annihilation dynamics is therefore a prerequisite for understanding the device physics of TADF-based OLEDs. We further demonstrate the broad applicability of our approach by studying the delayed fluorescence of a series of novel blue-emitting materials.

[1] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 2012, 492, 234.
[2] B. van der Zee, Y. Li, G-J. A. H. Wetzelaer, P. W. M. Blom, Adv. Optical Mater. 2021, 2100249

Flexible and stretchable electronics are of great interest given their wide range of uses, such as bioelectronics. Solution processable conjugated polymers are great candidates for the creation of flexible and stretchable electronics due to their ability to be fabricated into devices at low cost and at a large scale through different processing techniques, such as roll-to-roll printing. The semi-crystalline nature of most state-of-the-art conjugated polymer semiconductors limits their use by inhibiting their mechanical flexibility. To promote improved mechanical properties, previous studies have shown promising results by blending conjugated polymers with elastomers to produce flexible and stretchable devices while maintaining and even promoting charge transport. However, there is still not a systematic understanding of the phase behavior of such composite films and how it may be affected by different processing conditions and polymer structures. This study focuses on gaining further understanding of the elastomer’s molecular weight on the overall film morphology. The molecular weight of the elastomer was chosen given its known impact on the thermodynamics of phase separation in a two-polymer system. Diketopyrrolopyrrole (DPP)-based conjugated polymer was blended with various molecular weights of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) in solution. Composite solutions were spincast and the morphologies of these films were studied through the use of optical (UV-Vis) spectroscopy, grazing incidence wide angle x-ray scattering (GIWAXS), and atomic force microscopy (AFM) to gain a better understanding of the aggregation and planarization of the polymer backbone, crystallinity, and phase separation. The aggregation and planarization of the polymer backbone of the DPP increases with decreasing SEBS molecular weight. The effects of the SEBS’ molecular weight on the final film morphology and phase separation will be discussed in addition to their effect on the electronic properties of the material. This work will allow for the creation of a more systematic method to creating thin films with specific morphologies.

Controlling the band gap, molecular packing and absolute energy levels of conjugated polymers and oligomers is fundamentally important for their utilisation in a number of applications. Most approaches utilize the co-polymerisation of various co-monomer materials for this goal. However establishing structure-property relationships by varying co-monomer ratio can be challenging, as it is difficult to disentangle the influence of changes in the backbone chemistry from variations in the molecular weight, end-groups and polydispersity. In principle controlling such properties by post-polymerisation functionalisation is a powerful approach to tune materials properties. Provided functionalization chemistries can be developed with close to quantitative yields, then post-polymerization reactions can be used to tune the optoelectronic performance of the conjugated backbone, enabling the synthesis of libraries with consistent dispersity and length. Such reactions can also enable the introduction of sensitive groups which would not tolerate the polymerization conditions. In this talk I will present out recent work towards developing quantitative reactions for backbone modification. I will discuss the development of reactive end groups and co-monomers which can be easily modified in quantitative yield and highlight how these approaches can be utilised to make graft co-polymers which are responsive to various stimuli. The utilisation of such approaches for the development of donor polymers for bulk-heterojunctions will also be explored.

For over forty years, conjugated polymers (CPs) have been a source of enormous fundamental breakthroughs, enabling foundational insight into the nature of π-bonding and electron pairing, the creation of novel optoelectronic functionalities, and the development of commercially relevant technologies. Despite the achievement of significant technological milestones, the complex structural and energetic heterogeneities that define these materials preclude bandgap control at low energies, tailored interactions with infrared (IR) light, the study of fundamental physical phenomena, and the design and realization of new device functionalities. To address these modern challenges, we developed precision synthetic methods that enable control of the frontier orbital energetics, coplanarity of the conjugated backbone, intermolecular interactions, and many chemical, electronic, and structural features that affect electronic coherence within these π-conjugated macromolecules and enable unprecedented levels of bandgap control from 1 → 0.1 eV. We discovered that narrow bandgaps afforded through extended π-conjugation are intimately related to the coexistence of nearly degenerate electronic states. Through articulating novel mechanisms of spin alignment, topology control, and exchange, we have enabled the synthesis of neutral CPs with ground states that span the entire range from “conventional” closed-shell structures, to biradicaloids with varying degrees of open-shell character, to diradicals in both singlet (S = 0) and triplet (S = 1) spin-states. These materials exhibit weaker intramolecular electron-electron pairing and stronger electronic correlations than their closed-shell counterparts, which imparts novel phenomena not previously measured in soft-matter (polymer) systems.

The functionality of polymers is strongly controlled by their neighbour-neighbour interaction and hence how they are arranged within a functional thin film. Besides the choice of the chemical structure, one of the main factors are the processing parameters that are decisively influencing the arrangement of the polymers within the final film after deposition. It is therefore of interest to examine the opportunities for exploiting processing conditions solely based on systematic variation of external parameters. For example, the lateral phase segregation within a binary thin film can be achieved via the application of an external electric field during processing [1].
We present ways that allow structural control in final thin films using external processing parameters. This way we can strongly influence the film forming behaviour on defined timescales. Furthermore, we can demonstrate that the induced morphology changes persist and withstand different environmental influences. We systematically examine the solution phase and final film morphology with GIWAXS, UV-VIS-spectroscopy and DSC to fully characterize the effect of the external processing parameters on the various stages within the processing. Moreover, we pair these experimental results with theoretical calculations to generate a consistent picture of the fundamental processes leading to the observed behaviour on different length scales. This thorough understanding of the fundamental mechanisms in combination with a screening of several relevant, modern material systems enables us to give predictions about the exploitation of the presented approach for potential applications.
[1] Electrophoresis Assisted Printing: A Method To Control the Morphology in Organic Thin Films, S. Pröller, O. Filonik, F. Eller, S. Mansi, C. Zhu, E. Schaible, A. Hexemer, P. Müller-Buschbaum, E. M. Herzig, ACS Applied Materials & Interfaces 2020 12 (5), 5219-5225 , DOI: 10.1021/acsami.9b18064

The emerging market of the internet of things is insistently demanding for energy power solutions which are cost-effective, compact in size and biocompatible for applications in portable and healthcare technologies. In this framework Organic Thermoelectric MicroGenerators (μOTEGs) offer an interesting opportunity to replace or support conventional batteries exploiting non-toxic, flexible and conformable carbon-based materials.
These devices, relying on the Seebeck effect, are able to sustainbly convert temperature gradients (e.g. body-environment temperature difference, waste heat...) into electrical power in the order of a few mW/cm² which is enough to operate simple sensors and transmitters.

To realize μOTEGs both p- and n-type semiconductors are required, the latter constituting the weak link due to poor performances in terms of electrical conductivity and air-stability, greatly limiting the employment of scalable fabrication techniques such as printing technologies. Molecular doping has therefore stemmed as one of the possible solutions to enhance charge transport. Despite the actual interaction mechanism has not yet been fully disclosed, benzimidazoles derivatives have so far emerged has one of the most promising class of organic n-type dopants due to their solution processability and compatibility with both polymers and small molecules.
Here we report on a modification of the widely used and commercially available 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) obtained by inserting an iminostilbene moiety, producing the innovative derivative named IStBI. The change in the chemical structure does not modify the electronic levels of the benzimidazole core, as measured by cyclic voltammetry, but instead acts mainly on the interactions within the semiconductor matrix. The study is conducted using the widely spread n-type polymer P(NDI2OD-T2), ensuring a large platform of comparable results available in the literature. All the processing was carried out in solution and under nitrogen-atmosphere due to the lack of air and moisture stability of the obtained semiconducting blends. Thin films, casted through spin-coating technique, have been extensively characterized in their electrical conductivity and Seebeck coefficient. After optimizing the processing conditions (i.e. solvent, dopant concentration and mixing temperature) and post-deposition thermal treatment, we obtained an electrical conductivity of 1.14 x 10^-2 S cm^-1, which is to the best of our knowledge the highest value ever reported for solution doped P(NDI2OD-T2) blends. This resulted in also an improved power factor of 4 x 10^-3 μWm^-1K^-2. From the analysis of the thermoelectric properties it is evident a reduced dopant miscibility in the polymer matrix compared to other previously reported benzimidazole derivatives but at the same time an improved doping capability.
To support these findings structural and morphological analysis have been performed through GIWAXS analysis and AFM imaging techniques respectively.

We have developed a synthetic approach that allows us to synthesise monodisperse macromolecules on the gram scale. Our original targets are represented by the structures T1-T4, which consist of three fluorene-based arms of different lengths attached to a hexa-hexyl truxene core (macromolecules T1-T4).^[1] In contrast to conjugated polymers, batches are prepared with 100% reproducibility and the products can be isolated in high purity. These attributes are extremely well valued, because subsequent work towards device optimisation (design, processing, annealing, etc) can rely on the consistent behaviour of the organic semiconductor. Such compounds have been made for purely photonic applications, such as organic lasers, downconvertors in hybrid LEDs and in visible light communications.^[2]
Since the materials have been designed to be amorphous, their charge transport properties in the bulk are extremely low. Therefore, to open up the potential of star-shaped structures for applications such as OLEDs and OLETs, we designed novel core systems to improve aggregation in the solid state whilst retaining high levels of emission. In this talk, we present the synthesis and characterisation of the new materials TriR and Ind3HBC, based on soluble fused cores of graphene fragments.
[1] A. L. Kanibolotsky, R. Berridge, P. J. Skabara, I. F. Perepichka, D. D. C. Bradley, M. Koeberg, J. Am. Chem. Soc., 2004, 126, 13695-13702
[2] A. L. Kanibolotsky, N. Laurand, M. D. Dawson, G. A. Turnbull, I. D. W. Samuel and P. J. Skabara, Acc. Chem. Res., 2019, 52, 1665-1674

The fundamental premise of organic semiconductors is that synthetic chemists can generate materials with properties “on demand”. Unfortunately, even if this became a reality, we would not know what to order!
Indeed, while organic semiconductors have been around for a while, the preeminent role of the microstructure in governing their properties is far from understood. In this talk, I will emphasize the role played by structure at different length-scales and how charge transport is a complex multi-scale phenomenon. We use charge-modulated IR spectroscopy to measure the delocalization of charges in crystallites. Correlating carrier mobility to charge delocalization highlights the importance of mesoscale film properties, such as the connectivity of aggregates by tie-chains. As a result, we study the mesoscale organization of polymers using new techniques in the transmission electron microscope and complementary X-ray diffraction measurements at the synchrotron. By combining these techniques we are able to study the microstructure across a range of length-scales in real space and reciprocal space. Microstructural analysis at these lengthscales coupled with charge transport theory allows to better understand the role of defects. Thus, such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport and in general structure-property relationships in soft materials.

Organic mixed ionic/electronic conductors (OMIECs) have drawn interest for their potential applications in diverse new technologies ranging from energy storage to biosensing. Of particular interest as sensors are organic electrochemical transistors (OECTs). Recent work has shown carboxylic acid functionalized polythiophenes are effective OECT materials. We investigate the pH dependent properties of these OECTs. We find that at low pH, the OECTs are stable to electrochemical cycling. However, at higher pH these devices display dramatic instability (~20% decay per cycle). We investigate the mechanism of this degradation via optical spectroscopy (UV-vis and FTIR). Further, we investigate the difference in charge compensation mechanism in the different electrolytes by quartz crystal microbalance (QCM). We find that at low pH oxidizing the film results in mass gain, similar to other p-type materials. However, at higher pH, oxidation of the film results in mass loss, suggesting an entirely different charge compensation mechanism.

Electronic devices have quickly become an integral part of our everyday lives, and current trends indicate that many more exciting applications will be possible in the future. This is because organic electronic materials are compatible with flexible, stretchable, and lightweight devices; moreover, they can be processed from solution using low-cost manufacturing techniques. Typically, these devices have utilized conjugated polymers as their active layer components. Despite the success of conjugated materials on multiple device fronts, the use of conjugated polymers comes with limitations. To address these limitations and expand the range of potential materials that can be used, this work evaluates an emerging class of macromolecules, radical polymers. Unlike their conjugated polymer counterparts, radical polymers are comprised of a non-conjugated backbone with stable open-shell groups at their pendant sites. Here, we developed radical polymer-based blends for mixed electron and ion conduction purposes. Mixed electron- and ion-conducting polymers have received considerable interest due to their applicability in a variety of organic electronic devices. This work developed a novel radical polymer-based blend by combining poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO), poly(poly(ethylene oxide) methyl ether methacrylate) (PPEGMA), and lithium hexafluorophosphate (LiPF₆). PPEGMA with LiPF₆ had one of the highest polymer-based room temperature ionic conductivity of 10^-4 S cm^-1. PTEO demonstrated an ability to conduct both charges and ions with an ionic conductivity of 10^-6 S cm^-1. A blend of the two polymers at equal weight ratios had a room temperature ionic conductivity of 10^-4 S cm^-1 and electronic conductivity of 10^-2 S cm^-1, which was similar to pristine PPEGMA and PTEO thin films, respectively. This similarity resulted from the formation of distinct pathways of ion (i.e., through PPEGMA domains) and electron (i.e., through PTEO domains) conduction due to microscale phase separation between the two polymers with the lithium ions mainly incorporating in the PPEGMA domains. With the addition of lithium salt, the electronic conductivity of the blend increased by 1.7 times. However, at [Li⁺]:[O] ratios higher than 0.08, the electronic conductivity suffered due to poor film quality. This work serves a template for explore the structure-property relationships in novel radical polymer-based blends for mixed conduction. By taking advantage of the phase separation in these blends, researchers can utilize these structure-property relationships to develop the next generation of mixed conduction organic electronic devices.

The current success of organic semiconductor technology is mainly driven by the development of organic light emitting diodes (OLED), which are now routinely employed in display technologies. In the last decade, however, organic photovoltaics (OPV), leveraging the impressive improvement in device efficiency and stability, have gradually moved from a lab curiosity to a niche market. Their recent success has coincided with the rapid development of effective replacements for the fullerene-based materials that have been prevalent as electron acceptor materials until recently; namely the small molecule nonfullerene acceptors (NFAs). Through strategic design, an acceptor-donor-acceptor (A-D-A) configuration afforded highly absorbing small molecules with tunable energetics, thereby allowing the achievement of record power conversion efficiencies (PCEs) in OPVs. This relatively new class of materials offer a number of opportunities to develop new areas of research. Between those, organic photodetector (OPD), a technology based on organic photodiodes and thus closely related to OPV, is one of the most exciting. Recent efforts in the field of OPD have been focused on extending broadband detection into the near-infrared (NIR) region. The early absorption cut-off of solution processed organic semiconductors presents a challenge in achieving NIR detection, however, careful tuning of their chemical structures can help extend OPD responsivity into the infrared window. Here, we discuss how to control charge carrier recombination in organic solar cells and photodetectors for NIR light-to-current conversion for high efficiency and stable devices.

Near-infrared (NIR) organic photodetectors with high detectivity are fabricated with an organic electron blocking layer (EBL) with an appropriate energy band alignment. To avoid damage to a preceding organic electron blocking layer during a subsequent coating of an organic photoactive layer, cross-linking technology using a novel photoinitiator is used for an EBL. Poly-TPD is used as an EBL due to its appropriate energy band alignment with a NIR organic sensitizing layer. A ternary blend film composed of PTB7-Th, CO_i8DFIC, and PC₇₁BM are used as a NIR sensitizing layer with strong photosensitivity in multi-spectral (UV-Visible-NIR) wavelengths of 300-1,000 nm. By measuring the optical properties of poly-TPD films, first of all, it clearly shows that cross-linked poly-TPD EBL is not washed by organic solvents used for coating a NIR sensitizing layer. The organic NIR photodetector with an EBL mixing a photoinitiator of 1 wt% shows the highest detectivity of 1.16 × 10¹³ Jones at 900 nm due to significantly low dark current (1.5 × 10^-6 mA cm-2 at -0.1 V). In this work, the effect of a photoinitiator is systematically studied using the various amounts of photoinitiator.

Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime.^[1] However, near-infrared photodetection at a high detectivity has been proven to be more challenging and, to date, the true potential of organic semiconductors in this spectral range (800–2500 nm) remains largely unexplored. We have recently shown that the main factor limiting the specific detectivity is non-radiative recombination, which is also known to be the main contributor to open-circuit voltage losses in organic photovoltaics.^[2] Based on this finding we concluded that OPDs have the potential to be a useful technology up to 2 μm, given that high external quantum efficiencies can be maintained at these low photon energies. To further elaborate on the fundamentals defining these limitations, a next-generation of ‘defect-free’ organic semiconductors is synthesized. This allows us to investigate the influence of material imperfections (end-capping, homocoupling defects, and other impurities) on the material properties and device performance. In this contribution, we give an overview of the synthetic approaches applied and the latest organic semiconductors for high performance NIR OPDs, approaching their intrinsic limits.

[1] N. Li et al., Mater. Sci. Eng., 146, 100643 (2021)
[2] S. Gielen et al., Adv. Mater., 32, 2003818 (2020)

Organic light-emitting diodes (OLEDs) serve as the active component in high-end displays and have received a considerable amount of interest because of their potential in lighting applications. However, for a more widespread application, production costs need to be lowered and the energy efficiency and lifetime need to be improved. Over the past two to three decades, OLEDs were designed with increasing complexity, using a multilayer structure consisting of typically five layers. Recently, we have developed an efficient and stable single-layer OLED based on thermally activated delayed fluorescence (TADF) [1], stepping aside from the conventional multilayer device concept that has been commonplace in the OLED field for more than two decades.
The single-layer TADF OLED has the advantage of extremely low operating voltages and high power efficiency. In addition, due to the broad recombination zone, the operational lifetime is greatly improved. However, a remaining question is how this broad emission profile affects the optical outcoupling efficiency. Here, we demonstrate that the optical outcoupling in single-layer OLEDs can be calculated with a combined optical and electrical model. The charge-recombination profile is first simulated based on the measured charge-transport properties. This recombination profile is subsequently used to calculate the optical outcoupling efficiency. It is demonstrated that an outcoupling efficiency as high as 26% can be obtained for a CzDBA single-layer OLED. The results show that single-layer devices with a broadened emission zone can achieve similar outcoupling efficiency to multilayer OLEDs with an optimized confined emission zone, establishing a route to efficient, stable, and simplified OLEDs.
[1] Kotadiya, N.B., Blom, P.W.M. & Wetzelaer, GJ.A.H. Efficient and stable single-layer organic light-emitting diodes based on thermally activated delayed fluorescence. Nat. Photonics 13, 765–769 (2019). https://doi.org/10.1038/s41566-019-0488-1

With the conversion from the use of fossil fuels to more sustainable energy sources such as sunlight, wind and water, there is a high demand for research in optimizing these sustainable sources. Solar energy harvesting has proven to be a great chance for a more sustainable and carbon emission free industry and society. The recent development of new active materials for third generation solar cells (dye-sensitized, organic and perovskite) has had an immediate effect on the research of appropriate charge transport materials. To ensure high power conversion efficiencies (PCE), the properties of charge transport materials need further investigation and optimization.
With organic hole transport materials (HTM) currently being the bottleneck in perovskite solar cells (PSC) to higher PCEs, improvements are of the utmost importance. The currently most used organic small molecule HTM, spiro-OMeTAD only shows high PCEs through insertion of dopants. The hydrophilic properties of most dopants strongly decrease the stability and lifetime of developed modules and result in the possibility of exposition to hazardous lead compounds.
We are targeting the issue of low PCEs by providing a synthetical and theoretical approach to a large library of two to four Donor-π-arms attached to different core systems like [2.2]paracyclophanes and porphyrins. Further variations include different triarylamine and carbazole derivatives as donors, thiophene and related moieties such as benzothiophene or thieno[3,2-b]thiophene as well as the associated chalcogen structures as π-linkers. By comparing the changing properties each synthesized molecule shows, the ideal HTM for any active layer can be chosen. Furthermore, finetuning of properties such as solubility, fill factor or molecule stacking can be addressed. Hereby we allow our materials to not only be implemented on the currently most used perovskite MAPbI, but in addition to be used for any emerging new active layers such as lead-free and oxide-based perovskites.

The pairing of the SMAs with optimal polymer donors (PDs) is an important issue during the development of a variety of non-fullerene small molecule acceptors (SMAs) to improve power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, a systematic investigation is conducted with the development of the SMA series, named C6OB-H, C6OB-Me, and C6OB-F, which contain distinctive terminal substituents –H, –CH3, and –F, respectively. Also, two PDs (i.e. PBDT-H, PBDT-F) are paired with these SMAs. The result shows that the PD/SMA pairs with similar terminal groups yield enhanced molecular compatibility and energetic interactions, which suppress voltage loss while improving blend morphology to enhance simultaneously the open–circuit voltage, short–circuit current, and fill factor of the OSCs. The OSC based on the PBDT-F:C6OB-F blend sharing fluorine terminal groups achieves the highest PCE of 15.2%, which outperforms those of PBDT-H:C6OB-F (10.1%) and PBDB-F:C6OB-H OSCs (11.2%). Furthermore,as active layer thicknesses being the range of 85~310 nm, the PBDT-F:C6OB-F OSC maintains high PCEs. Unfortunately, the PCE of PBDT-H:C6OB-F-based OSC already drops by 80% from 10.1% to 2.1% when the active layer thickness increases from 100 to 200 nm. For achieving high-performance OSC, the importance of designing appropriate terminal groups for both PDs and SMAs to obtain optimal molecular compatibility and blend morphology is established through this study.

Molecular doping of polymers is an effective route to increase the conductivity in organic active layers. The conductivity can be increased either by increasing the density of charges or by increasing the mobility of charges. The mobility of charges is considered the major limiting factor for molecular doping. This limitation stems from Coulomb attraction between the dopant ion and the generated charge. Hence, only 5% of generated charges contribute to the conductivity.
One option to overcome the mobility barrier is tuning the used conjugated polymer and its dielectric constant. The dielectric constant is inverse proportional to the Coulomb attraction between the dopant ion and the generated charge. A high dielectric constant is favored by polarity, i.e. the introduction of polar side chains.
Since the mobility of polarons in doped polymers is not understood fully, this work focuses on the short-range (<1nm) conductivity. The short-range conductivity of p-doped P(g₄2T-T), a polythiophene with oligoethylene glycol side chains, is investigated via THz time domain spectroscopy. We compare it to p-doped P3HT which contains only alkyl chains. The used electron acceptor is F4TCNQ. The combination of THz time domain spectroscopy and transient absorption gives insight on charge carrier density, short-range conductivity, mobility, lifetime, and delocalization of (photogenerated) charges.
The short-range conductivity of p-doped P(g₄2T-T) is >10times higher than the one of p-doped P3HT, which we relate to an increased dielectric constant of the material shielding mobile charges from Coulomb attraction to the dopant anion. We demonstrate that the light-generated species in p-doped P(g₄2T-T) recombine most probable via a polaronic transition, which indicates that fewer neutral sites are left compared to those in p-doped P3HT. Moreover, the conductivity results show that the inter-molecular charge transfer, which is a hopping-type transfer, is less limiting in the P(g₄2T-T) system.
The dielectric constant is relevant to increase the conductivity of doped polymers, by increasing the mobility of charges.

Pt(II) complexes in the form of tetradentate ligands are promising as deep blue light emitting materials for organic light emitting diodes because they exhibit high efficiency of photoluminescence and light emission energy. However, in order to increase the color purity of blue, it is necessary to avoid additional emission of longer wavelengths and red shift of the spectrum caused by strong intermolecular interactions Pt(II) complexes. In this study, a tetradentate Pt(II) complex with a large volume of adamantly group and a non-planar ligand exhibiting deep blue emission was newly reported. An adamantly group with a large volume increases the distance of intermolecular and reduces the redshift emission resulting from the strong dipole-dipole interaction. As a result, the Pt(II) complex of this structure hardly changes the emission color even when the dopant concentration is increased. OLEDs emitters containing this novel Pt(II) complex have the maximum external quantum efficiency value of 22.6%, and the ‘CIE’(Commission International de L'Eclairage) value of y less than 0.13, showing deep blue emission. This efficiency value belongs to the highest range among phosphorescent OLEDs exhibiting deep blue color (CIE y < 0.15) with Pt(II) complexes. This results show a novel approach to designing new Pt(II) complexes for deep blue OLEDs exhibiting high efficiency.

Organicfield effect transistors(OTFTs) that employ solution-processed polymer semiconductors attract significant attention internal stability of their active channel layers. In this paper, a series of new donor-acceptor copolymers based on diketopyrrolopyrrole(DPP) are synthesized by stille coupling to obtain high performance of organic thin film transistors with various additional donor units including thiophene, thiophene-vinylene-thiophene, selenophene, and selenophene-vinylene-selenophene. The characteristics of the obtained copolymer were measured by TGA and DSC for thermal stability, by CV for electrochemical property, and by UV-vis absorption for optical property. OTFT performance will be discussed using new DPP derivative.

Recently, the quinoid-type molecules and polymers have been considered as attracting semiconducting materials for organic electronics due to their interesting properties such as superior electrical properties, very low band gap and spin characteristics. For the development of quinoidal building blocks, the rational design of isomer-controllable core containing quinoid materials has been considered as an important issue since the presence of geometrical isomers limits the systematic investigation of their chemicophysical properties based on structure-property relationship.
In this work, a novel isomer-free and low-lying energy level isatin-terminated quinoid molecule incorporating extended aromatic rings in quinoid core was designed and synthesized by simple indophenine reaction, then a novel conjugated polymer using the prepared quinoidal monomer was synthesized. It was found that the developed quinoidal building block exhibited the single isomer configuration, accomplished by steric hindrance between aryl rings and electrostatic intramolecular interaction. In addition, the employment of aromatic rings in quinoid core had effects on the decrease of the electron-releasing mesomeric effect into conjugated backbone, resulting in significant red-shift (~ 200 nm) absorption and dramatic downshifting both HOMO (~ 0.3 eV) and LUMO (~ 0.6 eV) energy levels. The charge transport capability of the prepared quinoidal conjugated polymer was evaluated by fabricating the organic field-effect transistors (OFET), exhibiting the balanced ambipolar charge transport mobilites in both p- and n-channels.

Organic light emitting devices (OLEDs) with high external quantum efficiency (EQE) is indispensable for demanding solid-state lighting technologies. EQE depends on internal quantum efficiency (IQE) of the emissive layer (EML), charge balance and out coupling efficiency of the device architecture. Especially, EQE of OLEDs drops at higher luminance (called as efficiency roll-off) due to lack of exciton confinement in EML and triplet induced exciton quenching. Triplet based losses are evolved through triplet-triplet and triplet-polaron annihilation (TTA and TPA) mechanisms which cause decrement in IQE of EML. However, the inability to confine the exciton density in EML can induces a significant change in device efficiency despite having an efficient EML. Hence, it is essential to confine exciton density or recombination zone (RZ) in EML for enhanced efficiency of OLED device. In general, the change in RZ is carefully examined using a sensing layer in the EML by comparing the device electroluminescence (EL) pertaining to EML and sensing positions.
Here, we presented a simple and effective way to depict the RZ movement in blue phosphorescent OLEDs (Ph-OLEDs) by varying the thickness of EML. The changes observed in EL spectra according to various EML thicknesses were investigated and its color coordinates have been compared to expose the enhanced performance of blue Ph-OLED with efficient exciton confinement in EML. RZ shifting in Blue Ph-OLEDs have been monitored by EML thickness variation and electrical analysis. From main RZ and sub RZ, we analyzed the EL intensity and concluded that the optimal point of EML thickness could be found through EL intensity. Hence, this study would be beneficial to optimize the thickness of EML to confine the RZ and appropriate selection of the EML thickness in blue OLED is essential for achieving improved performance to realize the desired efficiency.

Tris(4-(1-phenyl-1H-benzo[d]imidazole)phenyl)phosphine oxide (TIPO) and Tris(4-(1-phenyl-1H-benzo[d]imidazole)phenyl)triazine (TBIT) were newly synthesized and applied to n-type interlayer of planar perovskite solar cells (PSCs) for effective electron transport layer. The molecule materials contained phenyl benzimidazole group which is combined with a phosphine oxide core or triazine ring core and has contributed to the improvement of charge extraction and stability. Since the constituent molecules phosphine oxide and benzimidazole (BIZ) have high polarity and strong π-electrons, the molecules trigger passivation defects to improve charge transport and flattening the surface morphology. In addition, the stability of the device was improved by introducing TIPO material as the passivation and protection layer.

An image sensor is a photoelectric conversion device using semiconductor materials used in cameras, photocopiers, and medical diagnostic devices. Currently, an image sensor uses a Si photodiode as a core conversion element for a unit pixel. However, in recent years, the demand for ultra-high-definition image sensors now requires sub-micron-sized image pixels, which cannot be realized with Si photodiodes. Since the Si photodiode has a low absorption coefficient for visible light due to an indirect band gap, an active layer of several μm, which is undesirable, must be formed thickly in order to secure a sufficient amount of light absorption. In addition, color filters are required to impart color selectivity to each pixel due to the chromatic absorptive properties of Si. Since the color filter itself absorbs the incident light to some extent, the quantum efficiency of the final device is greatly reduced. In addition, installing color filters expands the light path and increases the overall volume of the image sensor, reducing the integrity of the image pixels.
Recently, organic semiconductors have been reported to replace the role of Si in the field of photodiodes in order to overcome the disadvantages of Si. Since the organic semiconductor absorbs light through the HOMO-LUMO transition of electrons, the absorption coefficient for the entire visible light is high, so that the thickness of the active layer can be made low thickness. Also, there is the potential for color-selective absorption for R/G/B because organic semiconductors can fine-tune the absorption range. This is a great advantage compared to Si as a photoactive layer in a photodiode because it does not require a color filter.
In this work, we synthesized a donor with novel green selectivity. The general characteristics of the unit are its planar molecular structure, high atmospheric pressure stability and strong electron donating ability, all of which can be related to its excellent electrical properties. It has been found that with appropriate acceptor units and synthesis, it is possible to have a narrow absorption band for the green range.

Conjugated polymers are becoming increasingly ubiquitous in bioelectronic devices operating in aqueous electrolytes, such as the organic electrochemical transistor (OECT), thanks to their mixed ionic-electronic conductivity and processability via various means.¹ However, electron-conducting (n-type) polymers are scarce and the performance and stability of the existing n-type OECTs are poor. Here, we introduce a solution-processable doping method to improve n-type OECT performance. Using organic salt blends in the polymer solution, we improve the transconductance of the n-type OECTs, stemming from an increase in electronic charge mobility. We show that although the salt cation induces a marginal doping effect, it improves the planarity of the polymer backbone and enhances the charge delocalization. Raman and electrochemical quartz crystal micro-balance studies show that the interactions of the salt with the polymer moieties (NDI or bi-thiophene) are governed by the cation type. Finally, we demonstrate that the salt-doped devices have at least one year-long lifetime while the pristine devices have limited stability. Our work provides a new and easy route to improve n-type device performance and stability in ambient conditions, which are the main bottlenecks preventing the realization of n-type organic electronic devices.
1. Ohayon, D.; Inal, S. Organic Bioelectronics: From Functional Materials to Next-Generation Devices and Power Sources. Adv. Mater. 2020, 32, 2001439.

All-polymer solar cells (all-PSCs), an attractive class of photovoltaic cells for wearable and portable electronics, have excellent morphological and mechanical stability. Recently, a new type of polymer acceptors (P_As) composed of non-fullerene small molecule acceptors (NFSMA) capable of high charge carrier mobility and strong light absorption has been proposed to improve power conversion efficiency (PCE). However, polymerization of NFSMA significantly reduces the entropy of mixing in PSC blends by preventing efficient charge generation and formation of mixed blend domains necessary for mechanical robustness and morphological stability. A way to solve this problem is to design an efficient P_As with the same building units as the polymer donor (P_D). Here, an NFSMA-based P_As series [P(BDT2BOY5-X), (X = H, F, Cl)], obtained by copolymerization of NFSMA (Y5-2BO) and benzodithiophene (BDT), has been reported. Therefore, the all-PSC blend composed of PBDB-T P_D and P(BDT2BOY5-X) P_A containing BDT units exhibits excellent interfacial compatibility, morphological and electronic properties. As a result, PBDB-T:P(BDT2BOY5-Cl) all-PSCs showed significantly improved PCE of 10.84% compared to Y5-2BO NFSMA (PCE = 7.02%) and conventional P_A P(NDI2OD-T2) (PCE = 6.00). In addition, the improved compatibility of these all-PSCs greatly improves their thermal stability and mechanical robustness. For example, the crack onset strain (COS) and toughness of the PBDB-T:P(BDT2BOY5-Cl) blend are 15.9% and 3.24 MJ m^–3, respectively, in comparison to the PBDB-T:Y5-2BO blends at 2.21% and 0.32 MJ m^–3.

A photodetector is a very attractive and promising device, considering the development of autonomous driving and wearables in modern society. In particular, organic photodetectors outperform inorganic photodetectors in terms of mechanical flexibility, low-temperature processability, and low-cost manufacturing with the advantage of solution processing. Organic Photodetector (OPD) has attracted much attention because it is very useful for selective spectrum detection when it absorbs a broad spectrum. Therefore, OPD can be broadly divided into a visible ray region and a near-Infrared (NIR) region with the selectivity of the absorbed spectrum. NIR region that we are targeting is 760 to 3000 nm, which is a spectrum that cannot be seen by the human eye, so very interesting research is being conducted. It is being applied in fields that are indispensable for the next generation, such as optical communication and biomedical imaging.
In this work, various donor parts were designed and synthesized based on the accepting core molecule. The suitable spectrum of the material was confirmed through the UV-visible absorption spectroscopy. As a results, we designed and synthesize new organic semiconduting materials with NIR-selective absorption in 900~1000nm.

The efficiency of organic optoelectronic materials often depends on the energetic alignment of low-lying singlet and triplet states. For example, a tiny singlet-triplet gap is a prerequisite for thermally activated delayed fluorescence, shown to be a promising triplet harvesting mechanism in OLEDs. For the vast majority of organic molecules, the lowest triplet state (T1) is energetically below the lowest singlet (S1), as predicted by Hund's first rule. However, recently there has been an increasing number of reports on an emerging class of organic emitters exhibiting an inversion of the lowest singlet and triplet states. In this contribution, we will demonstrate how such inversion is a direct consequence of electronic correlation effects and review the current molecular design that enables such an inversion to occur. We will also discuss the challenges related to quantum-mechanical electronic structure calculations for such molecules, particularly demonstrating how the commonly used TD-DFT method fails to describe the state inversion.

Colloidal crystals have been investigated for the past 30 years, and a high degree of sophistication regarding their synthesis and properties has been achieved. In particular, their photonic properties based on the periodic undulation of the refractive index have been in the focus of intense research. Various external stimuli result in a variation of the photonic stopband rendering them suitable for sensing applications. Temperature is one of such external stimuli resulting in a complete loss of the photonic stopband upon thermal annealing beyond the glass transition temperature of a polymer-based colloidal crystal. This phenomenon is well-known as dry-sintering, which is accompanied by a jump in thermal conductivity.[1] The time-dependence of this transition strongly depends on the polymer composition of the colloidal ensemble. We demonstrate that the dry-sintering kinetics can be shifted by orders of magnitude when adding more and more particles with a higher glass transition temperature. Going beyond the established class of homogeneous colloidal crystals, we introduce a method to spatially vary the binary particle composition between low and high Tg particles in a controlled and gradual manner. Infusion withdrawal coating is capable of gradually changing the particle dispersion while dip-coating a glass substrate. The result is a new class of colloidal crystal, where the composition is locally varied. Such gradient colloidal crystals are suitable to respond not only to temperature but also time. Isothermal conditions, for instance, lead to a continuous, spatially progressing loos of the photonic bandgap that the bare eye can easily perceive.[2]
In this contribution, I will introduce the concept of infusion withdrawal coating to fabricate such gradient colloidal crystals and how these can be utilized as a novel and simple time-temperature sensor.
[1] Nutz, F. A.; Retsch, M., Tailor-made temperature-dependent thermal conductivity via interparticle constriction. Science Advances 2017, 3, eaao5238.
[2] Schöttle, M.; Tran, T.; Feller, T.; Retsch, M., Time-Temperature Integrating Optical Sensors Based on Gradient Colloidal Crystals. Adv. Mater. 2021, e2101948.

A modular synthesis route for purely organic thermally activated delayed fluorescence (TADF) emitters enables easy access to the modification of emission color through the choice of suitable donor and acceptor units. The development of TADF molecules that emit at longer wavelengths and show high-efficiency devices with low efficiency roll-off remains a challenge.^[1] Here, we report a three-part series of fluorinated dibenzo[a,c]phenazine (BP)-based TADF molecules, each exhibiting two donor moieties with different electron-donating strengths, namely 3,6-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-10-fluorodibenzo[a,c]phenazine (2DTCz-BP-F), 3,6-bis(9,9-dimethylacridin-10(9H)-yl)-10-fluorodibenzo[a,c]phenazine (2DMAC-BP-F) and 10,10'-(10-fluorodibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (2PXZ-BP-F). Color-tuning is accessed through the donor strength and leads to a wide color range from green to deep-red with photoluminescence maxima, λ_PL, of 505 nm, 589 nm, and 674 nm in toluene solution. These TADF emitters produce OLED devices that show excellent to moderate performance with an EQE_max of 21.8% in the case of 2DMAC-BP-F, 12.4% for 2PXZ-BP-F, and 2.1% with 2DTCz-BP-F. The electroluminescence (EL) emission maxima, λ_EL, were found to be 585 nm, 605 nm, and 518 nm in mCBP host, respectively.

[1] G. Hong, X. Gan, C. Leonhardt, Z. Zhang, J. Seibert, J. M. Busch, S. Brase, Adv. Mater. 2021, 33, e2005630.

The electrochemical properties of organic electronic materials have seen a resurgence in interest, motivated by electrochemical transistors (OECTs), biosensors, electrochromic windows, light emitting electrochemical cells, and neuromorphic computing elements. A key question uniting all devices is the mechanism of device operation. I will present a series of in-situ FTIR measurements and scan rate dependent cyclic voltammetry measurements that demonstrate that, at least in the case of P3HT with a LiClO4 electrolyte, the process is best thought of in terms of three concerted steps. They are s follows: electrolyte polarization, charge injection, and ion diffusion. In addition, the structure-property relationship that governs this process is also an important open question. While it is known that amorphous materials tend to favor ion transport as does the inclusion of ethylene oxide containing side-chains, a quantitative relationship with predictive power has yet to emerge. A third, often underappreciated question is the relationship between electrolyte gated field effect transistors (EGOFETs) where mixed ion-electron transport does not occur, and OECTs, where mixed ion-electron transport is a vital part of device function. I will present a theory that describes the distinction between EGOFETs and OECTs in terms of Flory-Huggins mixing thermodynamics with a voltage dependent enthalpy of mixing. The enthalpy of mixing provides insight into the structure-property relationship that differentiates OECTs from EGOFETs. In addition, the same factor also provides insight into the structure-property relationships that governs the rate of ion diffusion. We present a series of temperature dependent measurements that show that ion movement in P3HT follows the Vogel-Tammann-Fulcher (VTF) equation, commonly used to describe ion movement in aliphatic polymers. Substituting the enthalpy of mixing for the activation energy in the VTF equation makes predictions about the structure-property relationship governing ion movement that are borne out by experiment. This perspective, wholly based in bulk properties, offers an additional vantage point from which to understand mixed ion-electron transport, one that should enrich nanostructral investigations.

Amphiphilic di-block copolymers have been suggested as a versatile tool in exploring the role of domain size, domain purity and interfacial energy cascades in bulk heterojunctions. Di-block copolymers can be used to eliminate macro-phase separation while the free energy of the individual blocks can be modified to enable clean phase separation, however current synthetic routes do not normally lead to clean di-block products with contamination with homo-polymers and tri- or higher block by products. Purification can be tedious or impossible. We have previously reported excellent phase separation in P3HT-block F_TEGTBT, while the oxygenated side chains facilitate purification [1]. Here we report the synthesis of asymmetrically functionalised push-pull monomers (AFPPMs) for both p- and n-type organic semiconductors and the pseudo-living polymerisation to give amphiphilic di-block copolymers in a 1-pot synthesis of pBDTBT-b-pN2200 using sequential Stille and Suzuki-Miyaura polymerisation with a Grubbs-3 catalytic system. In this talk I will describe the synthesis of the AFPPMs, material characterisation and OSC device results.
[1] Mitchell, V. D.; Wong, W. W. H.; Thelakkat, M.; Jones, D. J., "The synthesis and purification of amphiphilic conjugated donor-acceptor block copolymers" Polym. J. 2017, 49 (1), 155-161. DOI:10.1038/pj.2016.97

The electrospinning technique is an attractive route to improve the electrical properties of conjugated polymer fibers as the stretching during the electrospinning process induces the alignment of polymer chains. The effects of various processing conditions on the structure and properties of electrospun poly(3-hexylthiophene) (P3HT) fibers will be presented. The processing of P3HT was facilitated using a high molecular weight poly(ethylene oxide) (PEO). The PEO concentration in the P3HT/PEO fibers was varied from 25% to 2.5%. The molecular weight of P3HT in the P3HT/PEO fiber was also varied from »31 kg/mol to »83 kg/mol. The effect of the aging of P3HT/PEO solution on the spinnability and electrical properties was investigated. The self-assembly of P3HT in the spinning solutions was investigated using shear-rheology. The electrospun fibers display a redshift in the photoluminescence spectra compared to the corresponding thin film. This indicates more ordered chain formation in the fiber mat. The XRD data of the electrospun fibers indicate that the P3HT chains formed crystalline domains with both interchain π-π and lamellar stackings. Electrical conductivity was measured both for fibers mat and single fiber. The single fiber electrical conductivity was higher than that of the mat. An increase in electrical conductivity for the single fiber was observed with the aging of the spinning solution. The average electrical conductivity of ~1.4*10^-5 S/cm was observed in undoped fibers obtained from 24 h aged solution of P3HT with a molecular weight of 83 kg/mol. Our results provide insights into processing-structure-property relationships for conjugated polymers.

Chemical doping is one of the most widely used methods to enhance the optical and electronic properties of conjugated polymers (CPs). Solution-based chemical doping is well adopted due to its low-cost, easy operation, and compatibility with printing methods for device fabrications. Sequential doping is the preferred solution doping method because of better CP film quality preservation and enhanced electrical conductivity. In this work, uniformly distributed poly(3-hexylthiophene) (P3HT) nanowire (NW) networks thin-film is prepared. The networks’ optical, electronic, and morphological property changes resulting from sequential doping by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) were investigated by UV-Vis spectroscopy, atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), and electrical conductivity measurements. Higher dopant solution concentration (~1 mg/mL) causes the formation of “dot-like” features on the film surface. KPFM measurements clearly differentiate these features from P3HT NWs, indicating the accumulation of dopant molecules on the surface at the excess dopant loading. At a moderate F4TCNQ dopant solution concentration of 250 μg/mL, the P3HT NW networks show little morphological differences from its pristine state, but its absorption band corresponding to doping products can be clearly observed. At even lower doping levels, a significant increase of the electrical conductivity by at least three to four magnitudes can be observed while no morphology changes can be observed from AFM measurements. Our results demonstrate that AFM and KPFM can be used to distinguish undoped, moderately doped, and excessively doped P3HT NW networks and offer direct insights into the location of dopant molecules in the doped systems.

A few years ago, we had used fullerene acceptor for solar cell. However, in comparison with fullerene acceptors, many researcher find out that non-fullerene acceptors has many advantages. First, non-fullerene acceptors are more easily fine tuned for HOMO and LUMO than fullerene acceptors. Second, optical absorbance of non-Fullerene acceptor is strong and easy to control. Also, there are other merits such as easy chemical transformation, material diversity, available cost and good stability. So, we developed efficient fused-ring electronic non-fullerene acceptor derivatives based on indacenodithieno[3,2-b]thiophene(IT) core and side chains. We used side chains in order to get lower HOMO levels and higher electron mobility. As a result, the acceptor derivative exhibits a champion power conversion efficiency (PCE) of 11.43% and a Jsc of 16.49 mA/cm². In addition, in indoor light, acceptor derivative is shown high PCE of 25.13%. By this research, we provide new non-fullerene acceptor design access method using (IT) core and side chains to realize improved performance in organic solar cells(OSCs).

As the field of organic bioelectronic progresses, one molecular design rule that has emerged for conjugated polymers (CPs) to allow for ion penetration is the inclusion of polar side chains.^1,2 This is of particular importance when these materials are used in devices such as the organic electrochemical transistor, which relies on ion penetration inside the bulk of the film to change its conductivity state. One exception to this rule, however, is the n-type ladder-type polymer poly(benzimidazobenzophenanthroline) (BBL). With no added hydrophilicity, BBL is shown to outperform the more common n-type CPs, based on donor-acceptor (D-A) moieties, in OECTs. In this work, we shed a light on the why.³ By comparing BBL to a D-A polymer, sharing a similar acceptor backbone, but with a bithiophene donor and polar side chains (named P-90), we show that BBL not only displays a better morphology for electron transport, but also a higher charge storage capacity. We also demonstrate that BBL exhibits a more efficient ion-to-electron coupling compared to that of P-90, and a contrasting swelling behavior. Our results argue that a particular hydrophilic phase is not required for hydrated ion transport as long as the polymer microstructure allows free volume for ion penetration.
1. Giovannitti, A. et al. The role of the side chain on the performance of N-type conjugated polymers in aqueous electrolytes. Chem. Mater. 30, 2945–2953 (2018).
2. Giovannitti, A. et al. Controlling the mode of operation of organic transistors through side-chain engineering. Proc. Natl. Acad. Sci. U. S. A. 113, 12017–12022 (2016).
3. Surgailis, J. et al. Mixed Conduction in an N-Type Organic Semiconductor in the Absence of Hydrophilic Side-Chains. Adv. Funct. Mater. 31, 2010165 (2021).

Emulating synaptic functions is important for realizing efficient computers. Synaptic functions can be emulated by controlling ion movement, which leads to a gradual change in conductivity. This change is similar to the excitation or depression processes of biological synapses, which are key characteristics of a neural system. However, to emulate fully functional synapses, including short-term and long-term transitions, a device should be able to control not only the direction of ion movement but also the position of ions, depending on the strength of stimuli. In this study, we developed artificial synapse devices using organic ion reservoirs in contact with an aqueous solution; these devices can control the movement as well the position of ions. When the strength of the electrical field is higher than a threshold value, the ions are absorbed into the organic ion reservoirs, which affects the ion concentration of the solution. Because this ion concentration determines the conductivity of the device, it can be gradually changed depending on the strength of the applied electric field; this functionality is similar to that found in biological synapses. Thus, the developed devices can exhibit short-term and long-term synaptic plasticity, which are key properties for memorization and computation in the brain. These organic-based devices have the potential to overcome the limitations of existing von Neumann architecture-based computing systems and can substantially advance neuromorphic computing technology. In this presentation, the proposed synaptic devices will be presented in detail.

Halide perovskites (HPs) has been conceived as resistive switching (RS) memory devices; this is due to their low operation voltage, high on/off ratio, and tunable band gap. However, the HP-based RS memory devices have several challenges owing to the short endurance of 1,000 cycles, data retention of 2×10³ s and stability of perovskite film.Herein, oxide passivated halide perovskites as made by hydrolysis and condensation processes. The SiO₂-passivated perovskite RS memory device exhibited excellent performance with an endurance of 6,000 cycles, a high on/off ratio of 10⁶, and data retention of 1.8×10⁴ s. The SiO₂-passivation layer could disturb the Ag ion migration into the halide perovskite layer by calculated the activation energy of ionic conductivity and the results of the time-of-flight secondary ion mass spectroscopy. Various oxides were used as passivation materials. Especially, the zirconium oxide-passivated devices exhibit excellent properties with an endurance of 57,000 cycles and a retention of 7.8 × 10⁴ s. The high cohesive energy of oxides effectively increased the formation voltage by retarding the Ag ion migration, leading to the improved endurance properties of the devices. This paper proposes the strategy for significantly improving the low endurance property of HP-based RS memory devices using the oxide passivation technology.

Semiconducting polymers are of interest for a variety of electronic applications due to their low-cost processing methods to create large-area, flexible, lightweight devices. However, the low conductivity and intrinsic charge carrier mobilities of this class of materials are a glaring disadvantage. By chemically doping conjugated polymers, conductivity is increased due to oxidative addition of mobile charge carriers. When doped, a positive charge is created on the polymer backbone that must be accompanied by an anion, typically derived from the oxidizing dopant. When regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) is doped with 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ), conductivity is greatly improved while largely preserving the original film quality and morphology. Because of the side-chain positioning, RR-P3HT forms a reasonably crystalline lattice with an ordered lamellar structure when cast from low vapor pressure solvent. On the other hand, regiorandom P3HT (RRa-P3HT), with irregular side-chain positioning forms a more amorphous structure. RRa-P3HT shows only a slight increase in conductivity when conventionally doped with F4TCNQ, likely because the dopant anion can sit very close to the polaron in an amorphous film, lowering the carrier mobility. Using varying crystallinities of P3HT, we aim to elucidate the role of polymer disorder on the ability of the dopant to penetrate through the film and on the resulting carrier mobility. One particularly interesting new doping method utilizes anion-exchange. In this method, a pristine polymer film is exposed to the dopant molecule in the presence of a high-concentration salt solution. Due to mass action, anions from the high-concentration salt solution exchanges with the dopant anions. We have seen a ~10x increase in conductivity when the dopant and salt are introduced to RR-P3HT and a more pronounced 400x increased when this same doping is done to RRa-P3HT. We can use grazing incidence wide-angle x-ray scattering (GIWAXS) to look at the structural changes occurring in polymer doping and anion-exchange to determine how this large change in conductivity changes the structure of the polymer. Additionally, a new counter-ion introduced via anion-exchange now resides in the polymer matrix. In this work, we have seen that with RR-P3HT, the lamellar stacking is very similar in conventional-doping and anion-exchanged materials, but the π-stacking shifts, indicating a structural change to accommodate the TFSI^- anion. Crystallinity (which correlates with the low q peak intensity), starts high and gets even higher upon doping. RRa-P3HT shows only a slight increase in conductivity when conventionally doped with F4TCNQ. When anion-exchange is applied to RRa-P3HT, however, we have been able to show increases in conductivity of 400x compared to conventionally doped film. This increase in conductivity is accompanied by a structural change from amorphous to more crystalline. Doping increases crystallinity, and thus, with a higher degree of doping due to anion-exchange, crystallinity increases tremendously when the polymer starts off amorphous.

The Organic field-effect transistors (OFETs) have been largely investigated due to their low cost manufacturing, flexibility, and lightweight, as well as, their optical and electrical characteristics, that allow its application in sensors, amplifiers, attenuators signal filtering, and high-frequency response devices, like biosensing, wearable electronics, telecommunications and other potential applications. In this work, a CIInPc flexible OFET, was manufactured and characterized to evaluate its optoelectronic and morphological properties. For the manufacturing a high-evaporation vacuum deposition was used and Ultraviolet-visible spectroscopy analysis and scanning electron microscopy where used to evaluate the morphology and optoelectronic properties. Also, a study regarding the electrical characteristics for different time-dependent wavefunction input signals, has been conducted. The latter was driven to determine its time-response characteristics, gains, phase shift and to determine whether the device function as an attenuator or an amplifier with the selected configuration. The device has been modelled to obtain the OFET operation parameters. A resulting capacitance of 567 pF was calculated. A uniform and continuous films where obtained, which guarantees an efficient charge transport. The OFET device was characterized by different input wavefunction signals, where the input voltage and frequency were changed. For all signal the output voltage is lower than that of the input voltage. Also, for the higher frequency the output voltage is decreased compared to lower frequencies. Gains decreasing variation of up to 0.05 indicating to the operating application as an attenuator. Also, a phase variation of up to 100 ° while varying the input frequency. The model resulted on a gate capacitance values between 500 and 1180 pF, and gate-drain capacitance values between 50 and 500 pF. All of this could give evidence that state of the art ClInPc flexible OFET devices could be used toward current high-performance frequency-dependent flexible applications.

There has been great interest in semiconducting polymers over the past few decades owing to the wide range of applications to which they can be tailored, including: photovoltaics (OPVs) and thermoelectrics, light emitting diodes (OLEDs), and biosensing. The conductivity and mechanical properties of semiconducting polymer films are influenced by their morphology, which is dictated not only by polymer structure, but also by processing. Solution casting and solvent annealing are typically used to prepare polymer semiconductors bearing well-ordered, conjugated backbones with semi-crystalline morphology to maximize their charge carrier mobility.
As the lack of solubility of many rigid semiconducting polymers in common organic solvents impedes their post-polymerization processing, most studies have been limited to a subset of materials with solubilizing side chains. Combined polymerization and deposition in situ of less soluble monomers (such as 1,2-ethylenedioxythiophene [EDOT]) can be achieved by oxidative polymerization using inorganic Lewis acidic oxidants, bypassing solution processing of the polymer entirely; additionally, this technique is amenable to both solution phase and vapor deposition processes. However, the mechanism of oxidation and effects of the oxidant on the structure of the resulting semi-conducting polymer films has been poorly elucidated. In this work, we prepare thin films of polythiophenes in situ by solution phase oxidative polymerization using a range of inorganic oxidants in order to structurally characterize and compare the resulting materials. Using an array of characterization techniques including impedance spectroscopy (EIS), IR/Raman and UV-vis spectroscopies, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), we demonstrate that choice of oxidant significantly affects the properties of the resulting polymer films. The rate of polymerization of the monomer, crystallinity and order in the resulting polymer film, and degree of doping plus identity of dopant all lead to significant differences in electrical conductivity. From these results, we posit that oxidant should be selected judiciously to complement the thiophene monomer being oxidatively polymerized in order to achieve films deposited in situ having high conductivity and crystallinity.

TADF OLEDs, including small molecules, dendrimers, and polymers, have been demonstrated with nearly 100% utilization of singlet and triplet excitons due to the efficient RISC processes. However, in terms of material design and photophysics, the charge transfer to induced TADF mechanism and the relaxation of the excited state in dominating the bandwidth still puzzles the scientists. Due to the intrinsic charge transfer nature of the D-A structure, the spectra of most TADF OLEDs are quite broad and structureless, which makes it challenging to fabricate the full-color active matrix displays with wide color gamuts. Some advances in TADF OLEDs with narrowed spectral profiles have been demonstrated in the last few years. Hatakeyama’s group constructed a rigid polycyclic aromatic framework based on triphenyl boron, where para-substitution of the nitrogen atom, a resonant effect was induced by the boron atom, which contributed to the excellent separation of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMO) and thus TADF features. The materials designed similarly are nowadays called multiple resonance effect-based TADF emitters (MR-TADF). When the work on multiple resonance structures evolved by the science community, another effect was discovered. In some cases, similar systems allowed the singlet excited state to be lower than the triplet excited state (singlet-triplet inversion). In such materials, there is no need for thermal up-conversion from triplet to the singlet excited state, and the RISC process should be faster than for typical TADF emitters. In theory, it should be possible to obtain a narrow emission TADF emitter with an inverted singlet-triplet gap.
Here I would like to present new donor-acceptors structures which exhibit an inverted singlet-triplet gap. Comprehensive analysis from photophysics to OLED devices revealed very promising results and efficiencies but couldn’t be supported by the theoretical models presented in the current research.

Among the range of thienoacene-based organic semiconductors, materials with an internal thieno[3,2-b]thiophene substructure, such as [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) have been successfully applied as active layers in organic field effect transistors (OFETs) giving high hole mobility. Previously, we reported the deposition by solution shearing of blends of symmetric BTBT derivatives with polystyrene (PS). It was proved that the use of blends improved the thin films processability, the OFET performance and device stability. ^1,2 Herein, we prepared OFETs of the series of the Cn-DNTT and Cn-BTBT derivatives by a simple solution shearing technique (i.e., Bar Assisted Meniscus Shearing). This was carried out by blending the active materials with polystyrene to promote film processability, reproducibility and stability. X-ray spectroscopy was used for structural identification of the films. The resulting OFET devices, fabricated and measured in inert atmosphere and environmental conditions, exhibited a p-type behavior. The materials which show a higher solubility gave rise to more homogeneous films and higher OFET performance.
References:
1. Temiño, I. et al. A Rapid, Low-Cost, and Scalable Technique for Printing State-of-the-Art Organic Field-Effect Transistors. Adv. Mater. Technol. 1, 1–7 (2016).
2. Salzillo, T. et al. Enhancing Long-Term Device Stability Using Thin Film Blends of Small Molecule Semiconductors and Insulating Polymers to Trap Surface-Induced Polymorphs. Adv. Funct. Mater. 30, (2020).

The research on flexible electrodes has been actively conducted due to the growing demand of foldable electronics. By replacing metal with conducting polymer, i.e., poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), the stiffness of conventional metallic electrodes could be overcome. However, the weak physical adhesion at the intrinsic heterojunctions remained as one of the biggest limitations, which led to interfacial peeling after repeated folding and unfolding. Herein, homojunction-based device structure was proved to be a promising candidate for completely foldable electronics with extraordinary mechanical durability. In homojunction-based polymer thin-film transistors (PTFTs), selectively doped p-type diketopyrrolopyrrole (DPP)-based semiconducting polymer films with FeCl₃ were used as source/drain electrodes. Hence, the physical interfaces between source/drain electrodes and active layer were removed, consequently eliminating the possibility of interfacial peeling. At the same time, the gradual work function (WF) change with depth was generated since the polymer films were sequentially doped using overcoating method. The WF gradient lowered the contact resistance and thus promoted charge injection. In addition, interfacial crosslinking at heterojunction could further improve the mechanical robustness at interfacial adhesion. In an effort to further improve the device performance of homojunction-based PTFTs, various other p-dopants, AuCl₃ and CuCl₂, were also employed. The operation of resulting devices varied depending on the properties of dopants—dopant size, oxidizing ability, etc—suggesting that the dopant selection plays a crucial role in the electrical performance of homojunction-based PTFTs.

Faced with exponential growth and an increasing dependence on technology, the world requires practical materials to contribute to the Internet of Things. Organic electronic materials are emerging as an alternative to silicon-based devices as they are low in cost, can exhibit a low environmental impact in regards to their synthesis and processing, are recyclable, and can produce large scale thin films on a variety of substrates. Furthermore, soluble small molecule organic semiconducting (OSC) materials can be made into thin films at low temperatures with minimal energy input via solution processing. However, while solution processing is desired, the typical solvents used are halogenated and thus are detrimental to both human health and the environment. Therefore, it is necessary to explore new OSC materials that are green solvent processable, in which a green solvent can be described as exhibiting a minimal negative impact on both human health and the environment in terms of production, usage, and disposal. While organic light-emitting diodes (OLEDs) have had success to the point of commercialization, other organic electronic materials for use in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) have lagged in their commercialization due to a lack of device performance caused by low charge carrier mobilities. Of the three different types of OSC materials (n-type, p-type, and ambipolar), p-type OSCs are considered superior due to their air-stability and high charge carrier mobilities with several reportings of over 1 cm²/Vs.
One class of semiconducting materials for OPV and OFET devices are perylene derivatives, the most popular of which are perylene diimides. However, perylene diimides are traditionally n-type OSCs due to their strongly electron withdrawing imide groups, making them unstable in air. By replacing these imide groups with four butyl ester substituents the electron withdrawing effects are lessened, resulting in an increase in the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). By doing so, this perylene butyl tetra ester becomes more suitable as a p-type OSC material. Additionally, N-annulation on the perylene core results in further destabilization of the HOMO and LUMO energies, as this pyrrolic nitrogen donates electron density into the π-orbitals οn the perylene core. In the past, N-annulated perylene tetra esters have gained attention for their potential use in OLEDs, however, despite being a semiconducting material they have not been considered for usage in OFET devices.^1,2 This is likely due to their strong emissive properties making them attractive for use in OLED devices.^1,2 However, it has been discovered that the N-annulated perylene butyl tetra ester is natively soluble in 1-butanol, a green solvent that has a favourable vapour pressure for thin-film processing. This property of green solvent processability prompted further investigation of the compound, specifically for application in OFET devices. Initial OFET devices with the N-annulated perylene butyl tetra ester processed with 1-butanol have provided hole mobilities as high as 4.5 x 10^-5 cm²/Vs. The hole mobilities are expected to be improved upon by blending with 1-butanol soluble polymers that also act as dopants to improve the morphology of the thin-films as well as decrease the Fermi level of the material for improved charge generation.
References
1. Chem. Phys. Chem. 2016, 17, 859 – 872.
2. Soft Matter, 2015, 11, 3629 – 3636.

Conducting polymers feature electrical conductivities close to those of conventional metals, simple solution processability, and mechanical flexibility, suggesting a wide range of optoelectronic applications. Based on these contents, we propose a conducting polymer based on perylene diimide (PDI) with oligoethylene glycol as a side chain. PDI is one of the electron deficient building blocks and has high electron mobility. However, due to the large dihedral angle between the co-monomer and PDI during polymerization, it affects molecular packing. Compared to the normal alkyl chain, the oligoethylene glycol (OEG) chain has several properties such as hydrophilicity, high polarity, high flexibility and ionic conductivity, so it is widely used as side chains of conjugated polymers. Introducing of OEG side chains to conjugated polymers would endow the resulting polymers with high polarity, leading to an enhanced dielectric constant and miscibility with dopant. By combining the two moieties into one, we designed and synthesized PDI(DEG)-T and PDI(DEG)-2T. We investigated the optical and electrochemical properties of the two synthesized polymers and then tried to apply for organic thermoelectric devices through molecular doping. We measured conductivity and Seebeck constant through chemical doping and electrochemical molecular doping, and made an organic thermoelectric device and tested its performance. In further research, we will discuss the relationship between molecular structure, physical properties and identify device characteristics by fabricating optimized organic thermoelectric devices.

Organic semiconductors are solution-processable, lightweight, flexible and cheap materials that potentially can be implemented in a wide range of new technologies, like organic light emitting diodes, organic field-effect transistors, organic solar cells, organic photovoltaic devices, etc. Thanks to the versatility of organic synthesis, it is easy to tune material properties through a careful molecular design. Design of smart molecules is possible by the introduction of new functionalities and the engineering of the molecular packing, in order to increase the efficiency of charge transport.
Recently, chiral supramolecular structures have raised as very interesting systems in the field of spintronics since chiral and helical structures behave as spin filters due to the phenomenon of Chirality Induced Spin Selectivity (CISS) effect. The exploitation of the CISS effect can improve material properties due to a lower charge recombination and a better charge transport because the spins move preferentially in one direction. We will report on the design, synthesis and performance of a new class of chiral molecular semiconductors for potential spintronics applications.

A new class of molecular spintronic devices can be fabricated by chemically bonding magnetic molecular channels to the electrodes of a prefabricated tunnel junction with exposed side edges. Experiments showed that the cyanide-bridged octametallic molecular cluster, [(pzTp)FeIII(CN)3]4[NiII(L)]4¬[O3SCF3]4 [(pzTp) = tetra(pyrazol-1-yl)borate; L = 1-S(acetyl)tris(pyrazolyl)decane] molecule impact depended on the type of metallic electrodes used in the tunnel junction testbed. Experimental magnetization and transport studies showed a dramatic difference in molecule response on tunnel junctions with different combinations of metallic electrodes. Transport via paramagnetic molecular channels on a tunnel junction involving paramagnetic and ferromagnetic metal electrodes was dramatically different than the suppressed current state observed on tunnel junctions involving two ferromagnetic electrodes. We conducted theoretical studies to understand the experimental data and also to explore a wide range of electrode materials on tunnel junction-based molecular spintronics devices (TJMSD). Here, we report a Monte Carlo simulation study that focuses on understanding the effect of electrodes on the magnetic and the physical properties of TJMSD. A 3D Heisenberg model of cross-junction-shaped TJMSD was used for the simulation study. Inter-atomic Heisenberg exchange coupling was varied for one TJMSD electrode to study the effects of ferromagnetic, paramagnetic, and antiferromagnetic materials. This study provides insights for designing and understanding futuristic molecular spintronics devices.

Luminescent organic molecules (LOMs) are interesting compounds, as they combine the photo-induced charge carrier generation and recombination of inorganic semiconductors with the light-weight and easy fabrication techniques common to organic materials. LOMs therefore offer the potential to be used in the development of inexpensive, flexible, and tunable organic electronic and optoelectronic devices as alternatives to inorganic semiconductors. Examples of such devices include organic light emitting diodes (LEDs), solar cells, chemical and biochemical sensors, field-effect transistors, and nonlinear optical media for lasing applications.
We present results of a novel class of LOMs, benzoyl pyraziniums, whose optical properties can be controlled by tuning the electronic and chemical structure, as well as their concentration and interaction with surrounding materials. Optical characterization of the three differently substituted (methoxy, fluorine and unsubstituted) salts of the LOMs, in solution and in thin film, exhibited interesting results. Using photoluminescent (PL) spectroscopy we find that in solution, the PL emission intensity increases with decreasing concentration, while the emission spectra shift towards higher wavelength with increasing concentration. The recombination lifetimes of all three salts are seen decrease from approximately 6 ns to 1.3 ns as solution concentration is increased from 0.1 to 100 mM. As pyridiniums are known to be capable of intramolecular charge transfer, assuming that the primary concentration dependent non-radiative decay is through this pathway, our results indicate that as concentration increases, the number of non-radiative decay pathways increase, and thus the observed decrease in PL intensity and lifetime, and the redshift in the emission peak. Given these interesting results, we next focus on the optical properties of these LOMs in solid state, via spin-casted thin films, as studies on solid films for cumulative emission under different concentrations allows us to study their behavior as composites in the mesoscale regime. While thin films also demonstrate the spectral blue-shift with decreasing concentration, the shift is only on the order of a few nanometers (nm) over the entire range of 0.1 – 100 mM. More importantly, the emission is significantly narrowed, with a full width at half maximum of barely 4 nm. Further investigations of the electrochemistry of these molecules offer unique perspectives on the charge transfer properties within them, which is used to optimize their optical emission and establish these LOMs as highly suitable candidates for organics LEDs.
Funding:
This work was supported with funding from: NSF-CREST: Center for Cellular and Biomolecular Machines at UC Merced (NSF-HRD-1547848)

Organic electrochemical transistors (OECTs) are a type of electrolyte-gated organic field effect transistor (OFET) that are favored over other OFETs to amplify biological signals due to their higher signal amplification properties (i.e. transconductance) and ability to operate in aqueous media.¹ The key component in OECTs responsible for these advantages is an organic mixed ionic electronic conductor (OMIEC), capable of undergoing bulk rather than just interfacial doping, thus enabling transport of both ionic and electronic charge carriers.
At present, the performance of n-type materials significantly lags behind their p-type counterparts, with mobilities for n-type materials typically three orders of magnitude lower.² This can generally be explained by examining the delocalization of the Lowest Unoccupied Molecular Orbital (LUMO) in n-type polymers compared to the delocalization of the Highest Occupied Molecular Orbital (HOMO) in p-type polymers. In common n-type materials, the LUMO is localized on the electron deficient component, causing electrons to become trapped and mobility to decrease. Another key concern for n-type materials in OECT applications is stability, which is an issue when the LUMO is not deep enough to retain electrons and the polymer is oxidized by ambient species. Improvements in n-type materials are necessary to allow for the creation of complementary circuits, enabling faster circuit speeds and increased operational stability.
In this vein, to study the impact of the planarity of the backbone on the electrochemical activity and OECT performance, two novel n-type polymers (P4gNDI and P4gNDTI) were synthesized. P4gNDTI employs a new acceptor unit, naphthodithiophene diimide (NDTI), a thiophene-annulated derivative of NDI, which was selected due to its high planarity. DFT calculations suggested the linkage between NDTI and the comonomer was more planar, with an energetic minimum at 180°, compared to the linkage between NDI and the comonomer, which showed a double well potential with stable conformations, neither of which were at 180°.
On account of the extended effective π-conjugation conferred by a more coplanar backbone, P4gNDTI displayed a higher electron affinity, indicating a deeper LUMO. UV-Vis spectroscopy corroborated this theory by showing a narrower bandgap than its NDI equivalent, further indicating an increased effective π-conjugation. The deeper LUMO of P4gNDTI caused the material to be significantly more stable than P4gNDI to reduction and oxidation in aqueous media. The two polymers were tested in an OECT set-up, with P4gNDTI giving a µC* value of 0.27 F V^-1 cm^-1 s^-1, two orders of magnitude larger than the P4gNDI derivative. µC* is a geometry-independent term that allows for the evaluation of a material’s performance.
The origin of this performance increase can be explained by examining the extracted mobility, which was three orders of magnitude larger for P4gNDTI, compared to P4gNDI. This superior electronic charge transport properties of P4gNDTI can be attributed to its closer molecular packing and stronger intra- and interchain interactions, evidenced by Grazing Incidence Wide Angle X-Ray Scattering (GIWAXS), where a smaller π-π stacking distance was observed in the solid state. These promising results indicate that backbone planarity has a strong influence over the OECT performance of these materials, predominantly through increasing the charge transport properties, highlighting the applicability of this design strategy in future OECT materials.
1 D. Khodagholy, et al., Nat. Commun., 2013, 4, 1–6.
2 S. Griggs, et al., J. Mater. Chem. C, 2021, 9, 8099–8128.

The development of detectors for protons and heavy particles is a long-lasting research topic not only for fundamental applications, but more recently, for monitoring energy and flow of particles in ion beam applications. Novel beam extraction technologies, like laser driven accelerators, require fast and reliable systems for monitoring the beam quality. However, the most demanding application of ion beams, for which accurate measurements are increasingly needed, is hadron therapy of cancer. In this application field ion beams, mostly proton beams, are used for the controlled treatment of cancer by focusing them onto small volumes, in order to avoid the spreading of the radiation to healthy tissues (1). Organic semiconductors have already been demonstrated as reliable detectors for ionizing radiation, and here we show the first results on their use for proton beams monitoring. Organic semiconductors can be solution deposited at low temperature onto flexible and large-area substrates. A crucial point for their dosimetry application is their water tissue equivalence in terms of absorption and, consequently, the calibration of the sensor is not needed (2-4). The here reported devices have a photoconductor structure where the active semiconducting layer is an organic thin film of microcrystalline bis(triisopropylgermylethynyl)-pentacene (TIPGe-Pn), deposited by drop casting onto two interdigitated gold electrodes in co-planar architecture (5) onto a flexible plastic substrate (i.e. Polyethylene terephthalate (PET). The detector response under proton irradiation was tested using a 5 MeV beam with currents typically in the 1-100 pA range. During the irradiation, the sensors were polarized at low voltages (<1 V). By exploiting the structure of this sensor, two different operation modes can be effectively used: (i) real-time mode, based on the interaction between the proton beam and the organic thin film; and (ii) the integration-mode, based on the persistent current baseline shift, that scales proportionally to the total absorbed dose. The dynamic of the induced current peak is a fingerprint for the detection of ionizing radiation by thin-film organic semiconductor-based sensors. As recently discussed (3,4,6), the current induced in organic thin film following exposure to ionizing radiation is increased by an inner mechanism of amplification (i.e. the photoconductive gain effect (3)) mediated by trap states. The Integration-mode operation is due to the generation of charges in polymeric foils following proton irradiation is an effect known in the literature (7) and is well justified in the here presented devices, fabricated onto a 125 µm thick plastic substrate absorbing a consistent fraction of the proton energy (about 1590 keV per H⁺). We suggest that such proton-induced charges, accumulated in the plastic substrate, act as a bottom-gate effect for the organic semiconductor layer, increasing its electrical conductivity. The best sensitivity obtained by this new class of detectors is S = (5.15 ± 0.13) pC Gy^-1 and a Limit of Detection down to (30 ± 6) cGy s^-1 has been calculated. The sensors demonstrate a stable and reproducible response to proton beams. To the best of our knowledge this is the first study showing such a behavior in an organic device, demonstrating the potentiality of this new class of materials as flexible, portable and human-tissue equivalent proton detectors.
[1] M. Durante, J. S. Loeffler, Nat. Rev. Clin. Oncol. 7 (2010), pp. 37–43.
[2] L. Basiricò,et al., Nature Communications, vol. 7, p. 13063, (2016)
[3] L. Basiricò et al., Front. Phys. 8, 1–11 (2020).
[4] L. Basiricò, Aet al. Adv. Mater. Technol. 2000475 (2020),
[5] I.Fratelli et al. Science Advances, l7 16 (2021)
[6] I. Temiño et al., Nat. Commun. 11, 1–10 (2020).
[7] R. Mishra et al.,. Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 168, 59–64 (2000)

Ionizing radiation is the foundation of modern society with rapid growth in medical diagnostics and treatment, space exploration, nuclear energy, and border security. However, increased use in recent decades has correlated to higher radiation exposures, with harmful effects to the health of workers and patients. Active monitoring has now become compulsory in many countries to instantaneously detect, evaluate, and correct for any deviations from the planned exposure. Ideal sensors for active monitoring must be positioned on the body and operated at low voltages to record the total biologically relevant radiation dose absorbed across a wide range of spectra and radiation sources, termed tissue equivalence. These devices should also be relatively transparent to ionizing radiation to minimize impact on protocols.
Printed organic semiconductors are to date the only flexible technology capable of active monitoring as a wearable, tissue equivalent sensor. However, they have long been thought of as poor materials to sense ionizing radiation due to low sensitivity and poor tolerance towards high energy radiation. Here we develop a new system to solve these issues, reporting the optoelectronic, x-ray response, and radiation tolerance of an organic photodetector fabricated with solution-based inks prepared with polymer donor P3HT and non-fullerene acceptor o-IDTBR. The optoelectronic properties facilitated a high operating efficiency under x-rays without requiring any external bias when coupled with a plastic scintillator as a safe alternative to high voltage devices. Challenges with background luminescence from the polyethylene substrates commonly used for printed electronics required development of devices onto an alternative flexible substrate, polyamide (Kapton). P3HT:o-IDTBR performance was found to be both higher and more stable on the Kapton substrates than P3HT:PCBM. The tissue equivalence was determined by measuring the energy dependence of the detector responses across low and high x-ray energies. The response of devices coupled with the plastic scintillator matched the theoretical output of the plastic scintillator, validating the energy independence and tissue equivalence of the materials whilst commercial silicon detectors exhibited a 7-fold over-response compared to human tissue at low energies.
The devices also exhibited temporal responses approaching sub-microsecond time scales, the fastest ever observed for organic semiconductors as radiation detectors. High energy x-rays were produced from a Varian Clinac 2liX linear accelerator at 6 MV (<E>=1.2 MeV) with a pulse width of 3.6 µs, with individual pulses able to be resolved with temporal resolution directly comparable to state-of-the-art silicon radiation detectors. This result established organic semiconductors as the first ever ultrafast flexible devices for ionizing radiation with a tissue equivalent response.
The radiolucency was experimentally verified to be 99.8%, enabling operation as a transparent dosimeter with negligible perturbance of radiation beams. Device stability as a function of cumulative ionizing radiation dose past 5 kGy demonstrated P3HT:o-IDTBR was significantly more radiation tolerant than expected. P3HT:PCBM devices exhibited a 98.5% decrease in short circuit current density compared to just an 18.4% for P3HT:o-IDTBR under a radiation exposure equivalent to 10 years in a typical medical clinic. Mechanistic studies employing electrochemical impedance spectroscopy revealed photocurrent degradation occurred primarily in the P3HT polymer via formation of deep trapping sites in the P3HT:PCBM blends however, blends with o-IDTBR were more effective at preventing radiation induced damage. The reported devices provided a successful demonstration of the first fully organic ionizing radiation detectors, with further optimization of the scintillator required to enhance the sensitivity and tissue equivalence.

Molecular dopants are essential to dope semiconducting polymers for any device application. We introduce new highly soluble high electron affinity (EA) molecular dopants for solution doping of high ionization energy (IE) semiconducting polymers. Our new dopants are based on the parent molecule CN6-CP, that has a record EA of 5.87 eV, but is difficult to process. We demonstrate once and twice substituted ester analogs with EA of 5.75 eV and 5.61 eV but high solubility and solution processability. We demonstrate the use of these dopants with a series of alternating diketopyrrolopyrrole (DPP) polymers. All polymers achieve a sequentially doped conductivity of >15 S/cm. We developed a model to measure the doping level and hole density based on measurement of the UV/vis/NIR absorbance. We show doping of up to 80% of sites in the polymer, which prompts an important discussion of the maximum doping level for a conjugated polymer. We present a population model that is consistent across multiple polymers with multiple different molecular dopants. This presentation will reveal new synthesis, excellent materials properties, and a consistent method to quantify doping level across different semiconducting polymers.

Copper(I) thiocyanate (CuSCN) has emerged as a versatile p-type/hole-transporting semiconductor with extensive applications across a diverse range of electronic and optoelectronic devices. Considering the structure of CuSCN, it is a 3D coordination polymer (CP), in which the Cu⁺ centers are bridged by the SCN^– ligands. The development of electrical conductivity and electronic applications of CPs, among which metal-organic frameworks (MOFs) are also included, is currently gaining a great deal of research interest; however, only a handful of practical device applications have been realized. In this respect, the expansive range of devices in which CuSCN can be employed emphasizes its uniqueness, which can be attributed to the favorable hole-transporting characteristics and solution-processability.
Expanding from CuSCN, the development of new materials based on the modifications of the ligand or the metal center will be presented in this talk. For the former, adding co-ligands to CuSCN can alter the crystal structure and electronic properties drastically. In fact, diethyl sulfide (DES), which is commonly used as a solvent to process thin films of CuSCN, also acts as a co-ligand and coordinates strongly to Cu⁺. Treating CuSCN with antisolvents can affect CuSCN-DES interaction and film properties, which ultimately can improve device performance further. Furthermore, adjusting the metal center also strongly affects the chemical, physical, and electronic properties. Mixing Cu⁺ and Zn²⁺ in the thiocyanate network can yield a new structure with melting behavior, glass transition, and tunable energy levels. Tin(II) thiocyanate [Sn(SCN)₂] is also a wide band gap semiconductor and has been recently employed as an ultrathin interlayer in organic photovoltaics (OPVs). This talk aims to present the recent investigations into the development of semiconductors and devices based on metal thiocyanates and show some examples for future advancements that could be applied to the broader field of CPs.

Over the last decade, significant progress has been made in developing redox-active polymeric organic semiconductors for electrochemical applications in aqueous electrolytes. Next to performance improvements, the investigation of chemical structures with improved electrochemical stability paved the way for exciting electrochemical applications, including electrochemical transistors [1,2], neuromorphic computing [4], electrochemical sensors [5], and energy storage devices [6]. While tuning energy levels (via backbone engineering) and the local environment (via side-chain engineering) are successful strategies for improving the electronic and ionic charge transport properties of polymeric OSCs in electrochemical devices, little is known about the reason why some polymeric OSCs achieve a higher performance than others.
In my talk, I will discuss our recent findings on in-situ electrochemical characterization techniques where we investigated the origin of high-performance of state-of-the-art polymers based on polythiophene with hydrophilic side chains in electrochemical transistors [1,2]. Our findings show that aggregate formation in polymeric OSCs is beneficial for achieving high electronic charge carrier mobilities (leading to high transconductances (g_m)), but only if these aggregates are electrochemically accessible on fast time scales. Our finding suggests that the role of the side-chain expands beyond controlling the morphology and enabling interactions with solvent molecules from the electrolyte. When chosen accordingly, side chains can provide a framework for reversibly accessing ordered aggregates with electrochemical doping, yielding highly mobile electronic charge carriers and high transconductances Finally, I will discuss strategies for utilizing this concept to improve the performance of hole-transporting conjugated polymers for electrochemical devices.
[1] A. Giovannitti, D.-T. Sbircea, S. Inal, et. al., Proc. Natl. Acad. Sci. 2016, 113, 12017. [2] M. Moser, T. C. Hidalgo, J. Surgailis, et. al. Adv. Mater. 2020, 32, 2002748. [3] A. Giovannitti, K. J. Thorley, C. B. Nielsen, et. al., Adv. Funct. Mater. 2018, 28, 1706325. [4] A. Melianas, T. J. Quill, G. LeCroy, et. al., Sci. Adv. 2020, 6. [5] S. T. M. Tan, A. Giovannitti, A. Melianas, et. al., Adv. Funct. Mater. 2021, 31, 2010868. [6] A. A. Szumska, I. P. Maria, L. Q. Flagg, et. al. J. Am. Chem. Soc. 2021, 143, 14795.

Electrolyte Gated Organic Transistors (EGOTs) are rapidly emerging as one of the architectures of choice for label-free biosensing, characterized by their high sensitivity, low-voltage operation, low-cost fabrication, flexibility, and biocompatibility. The main feature of EGOTs is that they operate in an aqueous environment, using an electrolyte containing the analyte as dielectric between the gate electrode and the semiconductive organic channel. We fabricated two EGOT immunosensors for the detection of pro-inflammatory cytokine Interleukin-6 (IL-6) based on Organic Electrochemical Transistor (OECT) and Electrolyte Gated Organic Field Effect Transistors (EGOFET), using PEDOT:PSS and TIPS-Pentacene as channel material, respectively. Both architectures were operated as potentiometric biosensors, in which the gate electrode was functionalized with the biorecognition unit (anti-IL6 antibody). In order to quantify the biosensor response, we constructed a dose curve, which was fitted to adsorption isotherms, under the hypothesis of dynamic equilibrium between the biorecognition unit and the target analyte. Our study shows that Frumkin isotherm describes better the dose curve obtained by either EGOFET or OECT, compared to the most typically used Langmuir and Hill isotherms. Frumkin isotherm provides a physical view of the competing phenomena at the gate/electrolyte interface, characterized by the electrostatic repulsions between the antibody/antigen couples at the surface, which increase with the surface coverage at the functionalized gate. Moreover, application of Frumkin model allows estimation of binding constants: the corresponding analysis of free energy unifies the results obtained by both EGOT architectures, suggesting that the response is due to the binding events at the gate/electrolyte interface, and independent of the transduction mechanism. Therefore, EGOT can be used not only as biosensors for analytical purposes but also as a tool for investigating fundamental aspects of biorecognition phenomena. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 813863.

The development of bioelectronic technologies is driven by biomedical applications for new therapeutic and diagnostic tools. However, there are many important questions in plant biology that remain unanswered due to lack of sophisticate tools. Furthermore, the climate change and growing population calls for plants with increased tolerance to biotic and abiotic stress and plants with higher productivity. In my group we are developing organic bioelectronic technologies for sensing and actuation in plants that overcome limitations of conventional methods used in plant science. In my talk I will present our recent advancements of interfacing bioelectronic tools with plant model systems. We demonstrate enchanced plant resistance to drought via electronically controlled delivery of biomolecules with the organic electronic ion pump. Furthermore, with OECT implantable sensors we reveal previously uncharacterized sugars dynamics in trees and finally we present a novel bioelectronic platforms for stimulation of plant growth. Our results highlight the potential of bioelectronics in elucidating and enhancing plant processes but also for their application in agriculture.

Recent progresses in the design and synthesis of organic mixed ion-electron conductors are actively contributing to the development of new exciting technologies based on organic electrochemical transistors (OECTs). Mass-scale production of large-area circuitry^1–3, neuromorphic and neuroprosthetic devices^4–6, and advanced biosensors^7–9 are only some of the remarkable developments achieved by this technology just in the past few years. However, until recently, relatively few examples of OECT-based microwave devices have been proposed, and generally with modest performances compared to other existing technologies. In 2021, Bonacchini and Omenetto have introduced a novel hybrid device configuration in which PEDOT:PSS-based OECTs electrically tune the electromagnetic properties of a number of metallic resonant microwave structures, called metadevices, obtaining excellent amplitude and frequency tuning performances in the sub-5GHz range¹⁰.
In this work, we build upon the results previously achieved with PEDOT:PSS-based OECTs by exploring a diverse set of mixed ion-electron conducting polymer electrolytes. We demonstrate that the gate-controlled free charge carriers in these semiconductors are able to effectively respond to fast lateral voltage/current oscillations (0.5-5 GHz), a necessary condition for the realization of tunable microwave resonators. Moreover, the chemical design of the polymers enables transistors with either p-type or n-type behaviors, in depletion or enhancement mode, thus allowing to finely engineer the voltage range and the modus operandi of the metadevices. To highlight the technological potential of this new approach to reconfigurable microwave photonics, we demonstrate a wireless and battery-less electrochemical sensor in which an OECT-tuned microwave resonator is used to transduce the response of a coplanar enzymatic reaction cell. Finally, we also report a set of multifunctional microwave devices where the integration of different combinations of p-type and n-type OECTs on the very same metadevice platform enables unique microwave modulation schemes, such as selective frequency and/or amplitude modulation, depending on the chosen driving voltages.
We believe these results open to a new avenue of research for organic mixed ion-electron conductors in microwave devices, with potential impacts also at higher frequency spectra, such as the tens of GHz and the THz range. Further research efforts in this direction could lead to a new generation of reconfigurable photonic technologies that benefit of the many desirable properties of organic semiconductors, such as the ease of processability on large-area flexible substrates. Moreover, these results suggest interesting opportunities for the convergence of metadevice systems with other more established OECT-based technologies, leading to new concepts in neuromorphic photonics and wireless biointerfaces.
References:
1. Andersson Ersman, P. et al. Nat. Commun. 2019 101 10, 1–9 (2019).
2. Sun, H. et al. Adv. Mater. 30, 1704916 (2018).
3. Zabihipour, M. et al. Adv. Mater. Technol. 2100555 (2021). doi:10.1002/ADMT.202100555
4. Cea, C. et al. Nat. Mater. 19, 679–686 (2020).
5. Keene, S. T. et al. Nat. Mater. 19, 969–973 (2020).
6. Fuller, E. J. et al. Science (80-. ). 364, 570–574 (2019).
7. Ohayon, D. et al. Nat. Mater. 19, 456–463 (2020).
8. Guo, K. et al. Nat. Biomed. Eng. 2021 57 5, 666–677 (2021).
9. Tan, S. T. M. et al. Adv. Funct. Mater. 2010868, 1–9 (2021).
10. Bonacchini, G. E. & Omenetto, F. G. Nat. Electron. 2021 46 4, 424–428 (2021).

The shift in research volume for many emerging technologies towards perovskite-inspired materials over the past decade has been remarkable – particularly in the photovoltaic (PV) research space. Here, a plethora of compositionally tuned metal-halide species have materialised as high performing light absorbers capable of achieving performance metrics that were, until recently, almost unimaginable.
Substitutional doping of the perovskite active layer in perovskite solar cells (PSCs) is a widely adopted strategy capable of achieving enhancements in a number of device performance metrics. Despite such fundamental improvements there remain several challenges associated with improving lifetime and stability of PSCs. For example, it is well known that various extrinsic factors including moisture, heat, bias, oxygen and light result in perovskite degradation. To overcome such issues numerous sophisticated strategies have been proposed, including additive engineering, incorporating superoxide scavengers and metal–organic frameworks (MOFs), introducing interlayers and halide substitutional doping. Of these strategies, additive engineering is seen as an effective means of improving stability and often, in parallel, improving device PCE.
Here we outline additive engineering using a variety of organic small molecules, including naturally occurring species, paying particular attention to the performance changes induced by additive addition. We consider the interplay between additive incorporation and active layer microstructure-morphology with measured PV parameters to derive key relationships between additive properties and device behaviour. Using secondary ion mass spectrometry, we investigate where such additives reside in the active layer and collectively use our findings to explain key relationships between structure, composition and performance of additive engineered perovskite solar cells.

So far the molecular energy level offset of a type II heterojunction formed at the donor–acceptor interface in organic solar cells is believed to be the main driving force for photoinduced charge transfer. Typically, this offset is determined through measuring the molecular energy levels of the individual donor and acceptor materials in solution or film by means of cyclic voltammetry (CV). However, transferring such derived molecular energy levels into blend films of the same materials is obviously limited and not adequate in all cases. In the present work, we present material combinations in which a non-fullerene acceptor failed to achieve successful charge transfer, despite the fact that energy levels obtained by CV on single materials indicated a type II heterojunction. We present and discuss cases of impeded as well as eased charge transfers. Particularly, charge transfer may even be obtained via characteristic Fano-resonances, which have been discovered to occur for non-fullerene acceptors and some blends with them.

Efficient charge generation through doping is key to high performance organic thermoelectric devices. While modulation doping is a widely used doping method in inorganic semiconductors where a heavily doped wide band gap semiconductor is brought in contact with a narrow band gap semiconductor, this technique has not been well studied and exploited in organic semiconductors. Here, we investigate the charge and thermoelectric transport in modulation doped large area rubrene thin-film crystals with different crystal phases. We show that modulation doping allows achieving superior doping efficiencies even for high doping densities, when conventional bulk doping runs into the reserve regime. Modulation-doped orthorhombic rubrene achieve much improved thermoelectric power factors, exceeding 20 µWm-1K-2 at 80 °C. Theoretical simulations provide insights to the energy landscape of the heterostructures and its influence of the qualitative trends of the Seebeck coefficient. Our results show that modulation doping together of high-mobility crystalline organic semiconductor films are a promising strategy for achieving high-performance organic thermoelectrics.

In bulk heterojunction organic solar cells, the energetic landscape at the donor-acceptor interface provides the driving force for charge separation. The mechanism leading to efficient charge separation in fullerene-based blends has been intensively investigated, however with the recent advent of high-efficiency non-fullerene acceptors (NFAs) now surpassing 19% power conversion efficiency, the previous findings have to be revisited for NFA-based systems. In this presentation, I will discuss our latest insights into the photophysical processes governing charge separation, recombination, and energetic (voltage) losses in novel NFA-based systems studied by steady-state and advanced transient spectroscopy techniques. I will address the question, how the interfacial energy offsets control exciton dissociation and charge separation in binary and ternary blends of polymer or small molecular donors with novel NFAs, including photoactive systems using state-of-the-art Y-type acceptors. Generally, it appears that it is primarily the ionization energy (IE) offset that limits the exciton-to-charge transfer (CT) state conversion in many NFA-based systems, while the subsequent separation of the CT state into free charges is barrier-less. Sizeable IE offsets of 0.4-0.5 eV are required to ensure quantitative exciton-to-CT state conversion. The underlying reasons for this observation and the implications for future donor and acceptor material design strategies will be discussed.
Refs.: Nat. Mater. 2021, 20 (3), 378-384; Adv. Energy Mater. 2021, 11 (28), 2100839; Adv. Energy Mater. 2021, 2102363;

Triplet-triplet annihilation (TTA) is one of the primary contributors to efficiency roll-off and permanent device degradation in phosphorescent organic light-emitting diodes (OLEDs). The two limiting case models used to quantify this quenching mechanism are multi-step Dexter and single-step Förster, which respectively assume ideal Fickian diffusion or perfect trapping of triplet excitons. For device relevant guest doping levels (typically 5-12vol%), significant diffusion and trapping of excitons exist, so neither model fits experimental data well. We introduce an intermediate TTA model, which is a weighed average of the limiting cases of pure radiative decay and multi-step Dexter TTA. Kinetic Monte-Carlo simulations and time-resolved photoluminescence measurements of an archetype host-guest system demonstrate that our intermediate model provides significantly improved fits with more realistic physical values, is more robust to variations in experimental conditions, and provides additional insight into the TTA processes.

Organic mixed ionic-electronic conductors (OMIECs) are attracting significant interest for a broad range of applications, spanning neuromorphic, bioelectronic and healthcare technologies, sensors, energy storage devices, display technologies and the Internet of Things. Electrodes, made from metals or other conductive materials, are a universal requirement for electronic devices and OMIEC-based devices. The process of charge carrier injection and extraction between electrodes and the active layer has a strong influence over device performance: on one hand, efficient injection/extraction mechanisms enable good performance, effective communication with devices, and transfer information appropriately, for the eventual circuit-integration of these emergent devices and materials. On the other hand, poor injection/extraction may result in devices that do not switch on or function as well as they could do. The role of such interfaces within devices utilizing traditional semiconductors as active layers have been explored extensively over the years. Currently, there are fewer studies investigating the role of parasitic resistance at the electrode/OMIEC interface, including limited energy-level descriptions, which are critical for device design and optimization. Here, the role of contacts in OMIEC-based devices are explored, for novel functionality and improved performance.

Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable a range of devices. Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. One consequence of this single bond link is that the aromatic repeat units have the freedom to twist with respect to each other, introducing energetic disorder. We present design and synthesis strategies to restrict or even eliminate this tortional disorder through enhancement of non-covalent interactions. Additionally, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. Polymers with very large electron affinities can be synthesised by this method, resulting in air stable electron transport, demonstrated in solution processed organic thin film transistors. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers.

Fabrication of wearable medical sensors heavily relies on conventional semiconductor vacuum processing. We have adopted the unique manufacturing capabilities of printed electronics and designed wearable medical devices that are soft, lightweight, and skin-like. These soft and conformable sensors significantly improve the signal-to-noise ratio (SNR) by establishing a high-fidelity sensor-skin interface. Over the past 8 years we have used different printing techniques for fabricating wearable medical sensors in two sensing modalities: bioelectronic and biophotonic. In bioelectronic sensing, we have designed and fabricated flexible and inkjet-printed gold electrode arrays which were implemented in a smart bandage for early-detection of pressure ulcers. Recently, the efficacy of the electrodes is demonstrated on conformal surfaces and on the skin to record
electrocardiography (ECG) and electromyography (EMG) signals. Hybrid systems that combine flexible sensors with rigid computational components on a separate substrate provide a real-time analysis of physiological signals, with the implementation of machine-learning models for signal processing. In biophotonic sensing, we have demonstrated a flexible and printed sensor array composed of organic light-emitting diodes and organic photodiodes, which senses reflected light from tissue to determine the oxygen saturation. We use the reflectance oximeter array beyond the conventional sensing locations. The sensor is implemented to measure oxygen saturation on the forehead with 1.1% mean error and to create 2D oxygenation maps of adult forearms under pressure-cuff–induced ischemia. In addition, we present mathematical models to determine oxygenation in the presence and absence of a pulsatile arterial blood signal.

In organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF), non-emissive triplet excitons are converted to emissive singlet excitons via reverse intersystem crossing (rISC), enabling a 100% internal quantum efficiency. Despite the impressive progress in efficiency, understanding of the device physics of TADF OLEDs is still in early development. To model the operation of TADF based OLEDs, quantification of the triplet population is a prerequisite. We present a numerical drift-diffusion model for TADF OLEDs that next to singlet and triplet generation also includes the positional dependence of intersystem crossing (ISC), rISC and triplet-triplet annihilation (TTA). As experimental model system, we use a single-layer OLED based on the yellow TADF emitter 9,10-bis(4-(9H-carbazol-9-yl)−2,6-dimethylphenyl)−9,10-diboraanthracene (CzDBA) that possesses nearly trap-free transport and a high photoluminescence quantum yield. Our model accurately describes the voltage dependence of the current density and external quantum efficiency (EQE), both as a function of temperature and active layer thickness. Our model reveals that the steep increase in EQE at low voltage originates from emissive trap states, whereas the efficiency decrease at high voltage (roll-off) is dominated by TTA, with a temperature independent rate constant of 7±3 × 10^-18 m³ s^-1. The model allows us to quantitatively disentangle the various contributions of direct and trap-assisted recombination as well as recombination following rISC to the EQE, providing a useful tool for further optimization of TADF OLEDs.

Organic semiconducting polymers are being considered for a wide range of commercially relevant applications. From thermoelectric materials (imagine a medical monitor that does not need a battery) to flexible, large-area solar cells (a benefit of solution processability), these common-element materials could contribute to a greener, more sustainable future. Doping these materials with small molecules increases their charge carrier density and thus their conductivity, increasing their utility. This presentation will demonstrate the feasibility of p-doping organic semiconducting polymers with novel types of complexes, namely zwitterionic Wheland-type adducts, as models for mechanistic intermediates in already established doping processes and as dopants in their own right.
We will discuss Wheland adducts comprised of tris(pentafluorophenyl)borane (TPFB), a Lewis acid, and small-molecule conjugated systems. We obtained experimental results revealing the possibility of p¬-doping commercially available polymers (PBTTT, PNTz4T, and PTAA) with Wheland-type complexes. While TPFB alone can, indeed, dope these polymers itself and lead to high conductivities, we found that the Seebeck coefficients (voltage differences per degree of temperature difference) of Wheland adduct-doped polymers were higher than those doped with TPFB alone, by up to an order of magnitude. Moreover, conductivities obtained by adding intact adducts depended on the structures of the adducts, and were different from values obtained by adding the adduct components sequentially.
We used an ab initio computational chemistry approach to probe the nature, structure, and properties of these Wheland-type dopants and their effects on the polymers. This grounded their experimental findings in theory-driven calculations that helped elucidate the adduct dopants’ structural and electronic properties. We used the ORCA density functional theory (DFT) code with a def2-TZVP basis set, a B97-D3 functional, and an implicitly modeled acetonitrile solvent, choices that were based on our previous work and as recommended by Grimme (Phys. Chem. Chem. Phys., 2011). We will show that Wheland-type adducts are energetically favored to form for all the TPFB:conjugated molecule pairs we studied. We will also show that the electron affinities of these adducts nearly exactly matches that of TPFB alone, providing insight into why these complexes can act as p-dopants. Finally, we will demonstrate that some charge transfer is plausible between polymers and dopants through a comparative study of their respective ionization energies and Coulombic effects. This study strongly supports the hypothesis that Wheland-type adducts act as p-dopants for organic semiconducting polymers.

A monolithic ternary logic transistor based on a vertically stacked double n-type semiconductor heterostructure is presented. Incorporation of the organic heterostructure into the conventional metal-oxide-semiconductor field-effect transistor (MOSFET) architecture induces the generation of stable multiple logic states in the device; these states can be further optimized to be equiprobable and distinctive, which are the most desirable and requisite properties for multivalued logic devices. A systematic investigation reveals that the electrical properties of the device are governed by not only the conventional field-effect charge transport but also the field-effect charge tunneling at the heterointerfaces, and thus, an intermediate state can be finely tuned by independently controlling the transition between the onsets of these two mechanisms. The achieved device performance agrees with the results of a numerical simulation based on a pseudo-metal–insulator–metal model; the obtained findings therefore provide rational criteria for material selection in a simple energetic perspective. The operation of various ternary logic circuits based on the optimized multistate heterojunction transistors, including the NMIN and NMAX gates, is also demonstrated.

IoT (Internet of Things) is thought to be one possible key-aspect for a future technological revolution, in which specific functionality are cost-effectively integrated in daily objects, creating an extended network of interacting devices via wireless communication. This would be very helpful in many application-fields, ranging from health-care to distributed-sensing. In this scenario, organic electronics has the potential to be a promising technology tanks to its compatibility with simple large-area and low-cost fabrication processes. Moreover, the possibility of using flexible and conformable substrates is highly desirable to realize wearable and light-weight devices. However, to ensure wireless communications one has to face many challenges. In particular, to maximize operational frequency of a single organic transistor it is highly desirable downscale device dimensions to reduce unwanted parasitism and increase channel transconductance, since they directly depend on channel and overlap length. Unfortunately, this is not trivial since many undesired phenomena will occur at small scale, among which contact limitations are maybe the most relevant, reducing the overall performances and nullifying all the results obtained in many years in improving transport properties. In this work it is demonstrated the successful use of a solution-processed dopant injection layer in short channel and low overlap (L_C = 2um, L_OV = 3um) field-effect transistors in combination with C₈-BTBT: C₁₆-IDT-BT (1:4) blend as active material, which is among the best performing solution processed poly-crystalline existing organic semiconductors, yet to be exploited in downscaled devices. This approach was successful to reduce contact resistance, therefore allowing to retain good mobility in short channel devices, reaching values as high as 3 cm²/Vs in downscaled structures with a suitable geometry for high-frequency. This fundamental step, in combination with direct-writing approaches for device fabrication, is fundamental for the development of next-generation organic field-effect transistors operating in the UHF regime, which was thought for a long time to be out of reach for organic electronics, especially for solutions based approaches.

Spin doublet radical organic semiconductors can show near unity luminescence yield from their lowest energy excited state and are attractive as the emissive component in organic light-emitting diodes (OLEDs).

Here we report our recent measurements of direct, rapid, spin-allowed energy transfer from triplet excitons generated within a closed-shell organic host to a doublet chromophore. We use a carbene-metal-amide (CMA-CF₃) as a model host, since following photoexcitation it undergoes extremely fast intersystem crossing to set up a population of triplet excitons within a few picoseconds. We track the subsequent energy transfer to the TTM-3PCz radical using transient absorption and temperature-dependent transient photoluminescence spectroscopies. These show that direct triplet-to-doublet energy transfer is the dominant channel that accounts for over 90% of all radical emission. OLEDs based on the CMA-CF₃:TTM-3PCz blend show improved device characteristics compared to radical OLEDs without triplet-enhanced energy transfer.

Our design overcomes triplet-imposed performance limits for optoelectronics by activating spin-allowed triplet-doublet transfer on picosecond–nanosecond timescales, with light emission obtained orders of magnitude faster than derived from conventional triplet(-singlet) management technologies.

This method allows photophysical studies to reflect the working mechanisms in OLEDs under operating conditions by mimicking spin statistics under electrical charge injection, which may be a powerful tool for the wider organics community.

Quinoidal compounds have attracted much attention due to their unique optical, electrochemical, electrical and magnetic properties, and considered as promising candidates for a wide range of organic electronics, such as organic field-effect transistors (OFETs), organic thermoelectrics and next-generation organic spintronics. They exhibit low band-gap with NIR absorption and redox amphoterism arising from stable quinoid-aromatic resonance form, and high structural planarity because of double bond linkage between each rings. Reported various quinoidal molecules and polymers demonstrated high charge carrier mobility up to > 5 cm²/Vs for both hole and electron in OFET devices.
In terms of the electronic state, the quinoid has a closed-shell structure. When the energetically stable quinoid form is extended, this structure is transformed to open-shell diradical form at the either end due to the energy gain by total aromatization. The diradical contributes to increase the electrical conductivity by acting as a carrier, and exhibits high spin-induced magnetic behaviors due to stable triplet states with parallel spins.
In this presentation, we will present our recent results on the investigation of charge transport and spin-magnetic properties of open-shell and closed-shell quinoidal conjugated polymers. We designed and synthesized i) two quinoidal conjugated polymers with different length of quinoidal core and ii) open-shell azaquinoidal polymer with stable radicals. The quinoidal polymers show low band-gap and amphoteric redox behavior. The OFET devices based on these open-shell quinodal polymers demonstrate high charge carrier mobilities. We performed DFT calculations to predict diradical character index (y₀) and singlet-triplet energy gap, and spin distribution. Moreover, we characterized and discussed their spin-magnetic properties via Raman spectroscopy, electron spin resonance (ESR) and magnetic property measurement system (MPMS). The quinoidal conjugated polymer exhibits ferromagnetism with magnetic hysteresis loop at low temperature as well as room temperature.

Organic photovoltaic technologies have been steadily improving in the past decade due to rapid evolutions in materials design, device stacks and processing strategies. Due to their low-cost manufacturing potential, high efficiency and non-toxic nature, several market-ready applications are now emerging for this technology. For instance, building integrated photovoltaics, photovoltaic modules for indoor energy harvesting, or automotive applications. In this presentation we will review some of the advancements in the field that have enabled this technology to reach new heights in the past decade and will also present on some recent results obtained in collaboration with our development partners in the field of organic photovoltaics.

Electronic and optoelectronic devices, solar cells, LED, and transistor and so on, work by the motion of charge carriers. In particular, the carrier transport properties at interfaces between two materials play a role for device performance. To clarify the properties at the interface (e.g. organic heterointerface), we have performed femtosecond photoemission electron microscopy (fs-PEEM) experiments to image the dynamics of conducting electrons with time, space, and energy resolution [1, 2]. In this contribution, we will present our recent achievements on investigating organic semiconductor materials and devices using fs-PEEM.
A first example is a result of tracing the electron motion across an organic heterostructure composed of pentacene and fullerene, which is a typical structure of organic solar cells. Recombination times of photogenerated electrons in pentacene and in fullerene were individually estimated. Furthermore, the electron motion across the interface was visualized. The estimated transporting time across the interface was as short as 8 ps [3].
Organic light emitting diode (OLED) using thermally activated delayed fluorescence is a candidate of next generation high efficiency OLED. However, the efficiency degradation occurs when it is in the solid states. Mount of loss was estimated by comparing the recombination dynamics of photogenerated electrons measured by fs-PEEM with one by time-resolved photo luminescence experiments.
Finally, we are going to present operando observation of organic transistors. Antiambipolar transistors (AAT) with high on/off current ratio at room temperature has been demonstrated by fabricating defect-free and direct-contact organic heterointerfaces [5,6]. The AAT was consisted of a partially stacked p- (α-6T) and n-type (PTCDI) layers at around the middle of the transistor channel, source and drain electrodes on opposite side of the channel, respectively, and a gate electrode on the back side. Although, the AAT shown Λ-shape I-V curves, the mechanism of charge transport is not clear. Operando observation of AAT was performed by taking PEEM images by sweeping the gate voltage, while constant source-drain voltage was applied. We visualized that influence of depletion layer created at the interface works as a valve for the current.
References:
[1] Keiki Fukumoto, Yuki Yamada, Ken Onda, and Shin-ya Koshihara, Appl. Phys. Lett. 104, 053117 (2014).
[2] Shin-ya Koshihara, and Keiki Fukumoto, WO2018/159272, published on 2018.09.07.
[3] Masato Iwasawa, Ryohei Tsuruta, Yasuo Nakayama, Masahiro Sasaki, Takuya Hosokai, Sunghee Lee, Keiki Fukumoto, Yoichi Yamada, J. Phys. Chem. C 124, 13572 (2020).
[4] Yusuke Fukami, Masato Iwasawa, Masahiro Sasaki, Takuya Hosokai, Hajime Nakanotani, Chihaya Adachi, Keiki Fukumoto and Yoichi Yamada, Adv. Opt. Mater. 2100619 (2021).
[5] Yutaka Wakayama, and Ryoma Hayakawa, Avd. Func. Mater. 30, 1903724 (2020).
[6] Kazuyoshi Kobashi, Ryoma Hayakawa, Toyohiro Chikyow, and Yutaka Wakayama, Nanoletters 18, 4355 (2018).

The fabrication of bulk heterojunction organic solar cells by a sequential deposition of the donor and acceptor materials draws increasing attention from the scientific community due to its ease and compatability with large-scale production. Understanding the impact of sequential processing on the stratification in the active layer requires a methodology that can probe vertical compositional variations on a nm scale. In this talk, I will demonstrate how ultraviolet photoemission spectroscopy depth profiling can be utlized to examine the vertical distribution of donor and acceptor materials within the active layers of sequentially deposited organic solar cells. Focusing on two different material systems, I will show that despite both cases following the same order of deposition, i.e. the acceptor is deposited on top of the donor, in one system an excess of acceptor component is formed at the top of the film, while in the other an excess of donor. Correspondingly, in the latter case, the donor surface enrichment leads to the formation of an extraction barrier, resulting in a poor fill factor. This issue can be resolved by the addition of an electron extraction layer that effectively dissolves the excess donor, leading to a significantly improved photovotlaic performance.

Abstract：
Piezoelectric materials are extensively used in the application of energy harvesting and sensing devices and they have recently attracted much attention by the scientific community. Among them, piezoelectric polymers have special advantages, such as lightweight, deformability, and flexibility, which make them have the potential to become soft electronics. PVDF has become the most widely investigated piezoelectric polymer material because of its excellent characteristics (like chemical resistance, UV stability, mechanical strength) and acceptable price. Electrospinning can produce nano- or micro-fiber membranes with high surface area; moreover, the mechanical stretching and electric polarization during the process can in situ improve the piezoelectric property of PVDF. Therefore, electrospinning becomes an effective and simple method for preparing a piezoelectric-related PVDF membrane.¹ For most catalysts, the limited catalytic efficiency and difficulty in recovering astrict their development and practical applications. When the catalysts are combined with a matrix, the catalysts are easily recycled and reused, meantime the uniform distribution of catalysts on substate can reduce aggregation and increase the surface area of catalysts. In addition, when a piezoelectric material is applied as the matrix, since the piezoelectric field formed in the piezoelectric material can accelerate the migration of electrons and reduce the recombination between electrons and holes, the catalytic efficiency can be improved.²
The current work aims to show the production, characterization, and photocatalytic performance of nanostructured membranes based on electrospun PVDF matrix and TiO₂ catalysts. PVDF-TiO₂ core-sheath nanofiber membrane is prepared by coaxial electrospinning with PVDF solution and TiO₂ solution as the core and sheath. Two steps are carried out in order to investigate the piezo-catalytic activity of the proposed nanofibrous membranes:
● Evaluation of TiO₂ solutions with different TiO₂ concentrations, solvents, and additions in order to find the proper one for achieving good morphology PVDF-TiO₂ core-sheath nanofibers with a uniform TiO₂ coverage;
● Evaluation of the core-polymer solution: PVDF, P(VDF-TrFE), PAN are used as core solutions in coaxial electrospinning for investigating the performance of core-sheath nanofiber membranes with different piezoelectric properties.
All the obtained membranes are characterized in terms of morphology, piezo-catalytic activity, stability, and potential service life. The obtained results allowed us to consider the proposed approach as a valid solution in the development of new advanced solutions for wastewater treatment.

Reference:
(1) Azzaz, C. M.; Mattoso, L. H. C.; Demarquette, N. R.; Zednik, R. J. Polyvinylidene fluoride nanofibers obtained by electrospinning and blowspinning: Electrospinning enhances the piezoelectric β-phase – myth or reality? Journal of Applied Polymer Science 2021, 138, 49959.
(2) Liang, Z.; Yan, C.-F.; Rtimi, S.; Bandara, J. Piezoelectric materials for catalytic/photocatalytic removal of pollutants: Recent advances and outlook. Applied Catalysis B: Environmental 2019, 241, 256-269.

Following the commercial success and ubiquitous application of organic light-emitting diodes (OLEDs), on-going research into light-emitting (macro-)molecules seeks to uncover and advance further functionalities these materials, motivated by emerging application concepts in optoelectronics, fluorescent bioprobes, molecular imaging, coatings, and numerous others. In the established paradigm, the design of next-generation fluorophores is generally restricted to π-conjugated aromatic (macro-)molecules. Recent years, however, are witnessing a growing research interest in light-emitting molecules that are not based on extended conjugated π-systems.[1–3] This largely unexplored class of materials promises to overcome some of the principal limitations of conventional materials related to their cytotoxiticity, high intramolecular rigidity, non-biocompatibility and limited solubility. Key for progress, therefore, is exploring and understanding the nuances of ‘through-space conjugation’ (alternatively termed ‘homoconjugation’) that enables luminescence in the visible spectral range.
Poly(phenylene methylene) (PPM) has recently emerged as a promising multifunctional polymer with a unique range of physico-chemical properties, including thermal stability, resistance to oxidizing agents, excellent barrier properties, and high hydrophobicity.[4,5] Arguably, given the presence of an electronically insulating methylene group separating adjacent phenylene groups, its most remarkable characteristics are related to photoluminescence (PL). Specifically, PPM exhibits photoluminescence in the 400–600 nm range, solid-state PL quantum yield >41% and an unusually long solid-state PL lifetime of >8 ns.[4]
In this contribution we present a detailed spectroscopic study of poly(phenylene methylene) and its derivative poly(2,4,6-trimethylphenylene methylene) in the form of isotropic films and oriented melt-drawn fibers, and provide compelling evidence for the occurrence of homoconjugation. In particular, polarized Raman and PL spectroscopy of melt-drawn fibers reveal a preferentially perpendicular orientation of the phenylene rings relative to the fiber axis and, simultaneously, a preferentially parallel orientation of the transition dipole moment. Moreover, PL spectroscopy under hydrostatic pressures up to 8 GPa reveals a 4-fold increase in PL intensity and an apparent absence of excimer emission, both of which are highly atypical of π-conjugated emitters.[6]
Coupled with straightforward synthesis for yielding high-molar-mass products and excellent processability into films, foams and microparticles, PPM holds promise for light-emitting applications due to it reduced susceptibility to the usual non-radiative decay pathways. Given the generality of underlying principles, we hope that this work will stimulate further research into non-conventional light-emitting materials with hitherto unexplored optoelectronic properties.
[1] H. Zhang et al., J. Am. Chem. Soc. 2017, 139, 16264; [2] D. P. Chatterjee et al., ACS Omega 2020, 5, 30747; [3] L. Liao et al., Mater. Horiz. 2020, 7, 1605; [4] A. Braendle et al., J. Polym. Sci., Part B: Polym. Phys. 2020, 55, 707; [5] M. F. D’Elia et al., Coatings 2018, 8, 274; [6] A. Perevedentsev et al., Macromolecules 2020, 53, 7519.

Multifunctional devices are branch of technology that always garners interest, especially if it can be simply produced and provide uses for various potential applications. In addition, fabric devices are part of a growing field of technology that can impact many significant applications from medical devices to environmental filters and devices for novel enjoyment. To develop multifunctional fabric devices, conjugated polymers show great promise due to their excellent mechanical flexibility, in addition to their high electrical conductivity. One particular polymeric material that has been utilized is poly(3,4-ethylenedioxythiophene) (PEDOT). PEDOT is a promising conjugated polymer due to its excellent electrical conductivity and stability for use in fabric devices and has shown excellent versatility in the development of wearable healthcare monitoring devices, due deposition method called oxidative chemical vapor deposition (oCVD).

The oCVD technique is a deposition process that uses vapor-phase polymerization of a monomer (EDOT), and an oxidizer (FeCl₃). The polymerization occurs in a fully-sealed vacuum reactor, while controlled substrate and reactor heating, as well as controlled metering of the source monomer, allows the deposition of polymer thin films with fully controlled thicknesses ranging between ~10-1000nm depending on the deposition time. In addition, the mild chamber environment (<250°C) allows the deposition of PEDOT onto almost any substrate, including fabrics. In comparison to traditional solution based processing, the oCVD technique produces a film with superior traits that lend themselves to the development of fabric devices. PEDOT thin films deposited from an oCVD reactor have the ability to conformally coat complex geometries including fabric threading, as well as maintain a high conductivity and a tunable bandgap. In addition, thanks to the lack of solvents involved for doping and wettability purposes, PEDOT films deposited via oCVD do not damage the resulting substrate and allow PEDOT to maintain certain traits which otherwise would not have been realized. One trait in particular is hydrophobicity of PEDOT films. Due to this, oCVD PEDOT has the benefit to be used in a new range of applications including the potential as an oil and water separation filter. Conventional solution-based PEDOT:PSS is not considered hydrophobic due to the presence of the polystyrene-sulfonate (PSS) solvent. Therefore, PEDOT:PSS has not been used for applications requiring hydrophobicity, as even with additional treatment PEDOT:PSS was not able to reach a hydrophobic state.

What will be presented is a novel first time use of oCVD deposited PEDOT fabrics utilized in a multifunctional role of oil and water separation as well as a metallic-ion detection sensor. For the oil and water separation, oCVD PEDOT is deposited on three different fabric types in two different thicknesses where the efficiency and rate of separation between water and three oil types is analyzed including synthetic motor oil, household vegetable oil, and n-Hexane, achieving efficiencies as high >90% Water droplet contact angle measurements are utilized to analyze the hydrophobic surface of the PEDOT film compared to the bare fabric counterpart, showing clear hydrophobicity with contact angles of ~130°. Scanning electron microscopy (SEM) imagery is used to determine the effect of pore size on the oil and water separation data. In addition to oil and water separation, and PEDOT coated fabric sensor is developed to detect metallic-ions in water. The showcasing of oCVD PEDOT fabric devices utilized in the applications of metallic-ion detection and oil and water separation are expected to significantly contribute to the realization of multifunctional fabric devices for use in industrial and environmental applications.

Chemical doping is an effective strategy for tuning the electronic properties of organic semiconducting polymers (SCP), enabling high performance thermoelectric devices, thin film transistors, light-emitting diodes, and photovoltaics. A rather simple, yet lingering question in dopant selection is “Is it better to have larger or smaller molecules as dopants?”. Larger dopants are thought to be advantageous due to a lower Coulomb interaction with polarons in SCP, whereas smaller dopants may be advantageous due to their ability to intercalate in crystalline phases, with minimal morphology disruption and larger dopant concentrations that may be accommodated per unit volume. We use first-principles calculations based on density functional theory (DFT) and time-dependent Density Functional theory (TD-DFT) with an optimally-tuned hybrid functional to determine polaron sizes and binding energy in SCP’s for dopants of different sizes. The coulomb interaction between the polaron and the dopant counterion dictates the polaron size and is the dominant contribution to the polaron binding energy. We find that large dopants are generally associated with larger polaron sizes and reduced polaron binding energy, which are both advantageous for charge mobility. We also compute optical absorption spectra of doped SCP single chains and crystalline lamellae and compare with experiments on semicrystalline and amorphous films to determine whether large dopants penetrate crystalline aggregates.

Organic mixed ionic-electronic conductors (OMIECs) are currently in the spotlight of research for diverse applications. Among all, organic electrochemical transistors (OECTs) stand out as organic synaptic devices for integrated bioelectronics, leading to neuromorphic computing applications, thanks to the ionic and electronic interactions in OMIECs. However, the interplay between ionic and electronic transport is usually seen as unfavorable in OECT-based circuits. From the circuit design perspective, it is complicated to include the ionic and electronic interactions in the differential equations and predict the circuit response. Therefore, it is vital to understand the interplay between ionic and electronic conduction for engineering OMIEC-based circuits. Here, we show an OECT with photo-patternable solid electrolyte advancing towards integrated circuits. We discuss how the ionic and electronic interaction influences the OECT-based circuit response and how alterable device parameters can be employed to adjust the circuit performance. In particular, we demonstrate how the biasing conditions govern the time constant of OECTs. In addition, we discuss how a dual-gate OECT architecture can be employed to tune the threshold voltage of OECTs and control the ionic and electronic charge transport in the electrolyte and OMIECs. The electrical analysis of solid-state electrolyte OECTs provides new insights stimulating higher investigations on ionic and electronic interactions and coupled transport properties in OMIECs and paves the foundation towards the integration of OECTs for advanced applications.

We address the nature of electrochemically induced polarons in conjugated polymers and their evolution as a function of electrochemical potential. We investigate the photo-physical mechanism driving their dynamics and coupling to the local environment by means of transient absorption and Raman spectroscopies synergistically performed in situ throughout the electrochemical doping process. We employ two prototypical system to benchmark the experimental observations -- one is an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymer and the other is a poly(3-hexylthiolhene) derivative, where the alkyl sidechains have been terminated by a hydroxyl group. The changes embedded in both linear and transient absorption features show that while in the case of the ProDOT copolymer the neutral transition and the polaron have distinct ground states, in the case of the P3HT derivatives the ground state is shared between these two species. In the case of the ProDOT copolymer we also identify a precursor electronic state with charge-transfer (CT) character that precedes polaron formation and bulk electronic conductivity and it is instead absent in the case of the P3HT derivative. These marked differences can be rationalized in the context of a very different polar environment pertaining to these two classes of polymers, where the P3HT derivatives show a lower polarity with respect to the ProDOT family. This work provides insight into the energetic landscape of a heterogeneous polymer-electrolyte system and demonstrates how such coupling depends on environmental parameters, such as polymer structure, electrolyte composition, and environmental polarity.

Skin-like electronic healthcare patches, which enable the seamless monitoring of physiological information with conformal skin contact, are expected to be the next-generation healthcare gadgets beyond the watch- and band-type health trackers. Although many healthcare patches have been reported, they require external devices for monitoring the physiological information, and it is challenging to develop a fully integrated standalone healthcare patch in which the skin-conformable sensor is integrated with a stretchable and wearable display with sufficient pixels to display the biometric information.
Although conventional approaches for fabricating stretchable, skin-conformable sensors and displays (e.g. manual transfer printing, prestraining technique, and serpentine metal interconnects) impart stretchability to flexible or rigid materials and devices, they are not applicable in the reliable mass production of high-resolution stretchable optoelectronic sensors and displays. Therefore, devising a novel approach to develop electronic materials and new device architectures with inherent mechanical stretchability is imperative for realising portable and wireless wearable healthcare devices.
In this study, we will discuss a fully integrated standalone SHP that can serve as an ideal platform with high-yield manufacturability and the possibility of commercialisation for the development of next-generation electronic skin healthcare applications. Our work could inspire strategies for developing high-resolution stretchable optoelectronic devices such as stretchable displays and optical sensors. Therefore, this work paves an exciting path towards the design of mechanically robust and high-performance stretchable and wearable electronics. The novel stretchable materials and rational system design strategy employed in this study can be extended to other types of stretchable organic/inorganic materials, as well as other deformable electronic devices.

Research around inexpensive and renewable energy devices have become a generational challenge of our time. To that end, solar photovoltaic technology has advanced tremendously in recent years. However, the high upfront cost remains a major barrier for widespread adoption. Architectures which use n-Si as the absorber and Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the front transparent carrier-selective contact have recently been able to achieve efficiencies exceeding 16%. However, the characteristics and properties of PEDOT:PSS especially with various popular additives has not been fully understood. Triton X-100 and DMSO have been popularly used as a surfactant to assist in the spin-coating process and to improve the conductivity of PEDOT:PSS[2-4]. As a transparent front contact, the optical characteristics of the PEDOT:PSS layer is an important factor in improving the performance of the solar cell. This study uses UV-vis spectrophotometry to show the effect of these dopants on the photo-absorbance of the later. A low temperature four-probe method was also used to study the charge transport mechanism of PEDOT:PSS.
PEDOT:PSS (Clevios FHC Solar by Heraeus) with 0, 5 and 7% (by wt.) of DMSO or 0, 3, 5, 7% Triton X-100 was deposited on quartz and silicon substrates and spin-coated at 3000 rpm. They were then annealed under argon gas in an electric furnace at 130°C for 15 min and allowed to cooldown to room temperature, forming a thin film of about 100 nm. Samples were also prepared for four-probe conductance measurements using helium cooling from 4 K to 300 K.
The addition Triton X-100 resulted in strong absorption peaks at 277nm, which correlated to that of the phenyl group in X-100. Having a boiling point at 270°C, Triton X-100 does not evaporate easily during annealing. As PEDOT:PSS is usually stable only till 200°C, it is not possible to remove X-100 without potentially damaging the integrity if the film itself. Higher doses of the additive predictably displayed higher absorption peaks. This phenomenon is a likely cause of parasitic absorption in the UV region. It is likely to be a factor in the drop in the external quantum efficiency (EQE) observed in hybrid solar cells[2-4], when PEDOT:PSS is used as the top transparent absorber layer.
The addition of DMSO visibly reduced the absorption peak associated with PSS (226 nm). DMSO also weakens the coulombic attraction between PEDOT core and PSS shell in the PEDOT:PSS crystalline structure. This helps to remove the PSS shell, allowing for bigger grains of PEDOT to form. This rearrangement brings about more interchain interactions between the conducting polymers.
Resistance measurements at low temperatures (5-50K) were compared using the variable range hopping model. Among the curves fitted to Mott’s variable range hopping equation[5], the curve for Mott's 3D hopping showed the highest R - squared value. However, it is interesting to note that the data seemed to have a better linearity for a for a fitting parameter (β) of 0.1. This suggests the presence of another charge transport model that has more metal-like characteristics than simple variable range hopping. More information will be shared in the presentation.
We think PEDOT:PSS is a promising candidate for use in organic-inorganic hybrid solar cell designs. Spin coated PEDOT:PSS films display a strong metal-like charge transport mechanism. DMSO works to enhance conductance and even improves transparency, while the surfactant Triton X-100 negatively affects performance of PEDOT:PSS as a front contact. Parasitic absorbance of light needs to be considered when using additives to enhance other properties.
References:
1. Yang et al., ACS Energy Lett. (2017) 2, 3, 556–562
2. Sara Jackle et al., Scientific Reports (2017) 7, 2170
3. Sung-Soo Yoon et al., Adv. Energy Mater. (2018) 8, 1702655
4. Yuqiang Liu et al., ACS Nano (2016) 10, 704-712
5. A M Nardes et al., Phys. Rev. B (2007) 76, 16027-16034

The quest that what future of the electronic devices holds and to what extent can their dimensions be reduced in the near future is catching a lot of interest in the field of molecular electronics. It’s been four decades that Molecular Electronics has been around and now recently the properties and opportunities of single-molecule began to explore. The dimensions of the traditional transistors have reached less than 10nm in research devices as compared to a few decades before when the gate length of the transistor was used to 10µm approximately. The dramatically shrunk and further miniaturization of electronic devices has been challenging due to the lack of understanding of transport mechanism or technique limitations but to overcome that the use of molecules of size 8 with the order of 1nm can be synthesized to make the devices that can accomplish a variety of electronic tasks like in memory, switching, rectification, conducting wires and transistors. Hence, the solid-state device can be partly replaced by the molecular device which has the potential to replace in the coming future. On the basis of the properties inherent in the individual or ensemble molecule which lead in creating a functional electrical circuit is the core concept of molecular electronics. Keeping the goal of shrinking the electrical circuit due to the finding in electron transport through individual molecules presents the idea to look beyond the traditional Complementary Metal Oxide Semiconductor (CMOS) technology.
In the project entitled "Carrier transport in junctions between molecules and 2D materials" I work to scale down the minimum feature size of the electronic devices. The concept to build a device within a molecular scale by exploring the properties of 2D materials is one of our primary goals. This work demonstrates the fabrication of the Vertical Molecular Field-Effect transistor in which the molecular layer of desired thickness is been used to form a junction. The thickness of the molecular film can be tuned by the use of Langmuir Blodgett techniques which we have used for making a homogeneous and uniform film of the molecule over graphene. The study of layer formation and deposition of tunable thickness (i.e. 1nm to several nm) molecular layers is achieved in this project. The Brewster Angle microscopy technique for imagining the Langmuir Blodgett film during its different stages has been used. The study of molecular junction formation was confirmed by electrochemical impedance spectroscopy. The characteristics of molecular field-effect transistors have been studied in this project and it has been observed that the current-voltage measurements of the devices show the decrease in current on further increase in voltage in a device. This is termed Negative Differential Resistance (NDR) and NDR peak are observed at 0.15V and 0.11V respectively with 5.2nA and 2.6nA are the Peak-to-Valley Current Ratio (PVCR) for the devices. Hundreds of molecular vertical FET devices have been measured and the device performance was assessed according to output and transfer characteristics. The formation of junction between graphene -molecule and graphene-SiO2 was also observed. As NDR has been observed in the devices of different channel lengths which show that our devices have a significant NDR and this can be further used for various applications in logic gates and memory devices.

Organic semiconductors, such as semiconducting polymers and molecules, are predominately semi-crystalline or polycrystalline materials. The crystalline moieties of these structures often possess enhanced intermolecular charge transferring which leads to extended electron delocalization. For this reason, the crystals of organic semiconductors are mainly responsible for high charge transport mobilities and high extinction coefficient in photon absorption. Hence, the crystalline structures as well as their formation mechanisms are of great interest for organic semiconducting devices such as organic field-effect transistors as well as organic solar cells. Here, crystallization kinetics of several organic semiconductors will be examined using classic materials science models where nucleation and crystal growth kinetics are inspected. Furthermore, the obtained insights were applied into constructing high performance multi-blend organic solar cells and high charge transport mobility bulk structures.

Understanding the mechanisms of carrier injection into organic semiconductors is of pivotal importance in the future development of organic devices such as organic light-emitting diodes (OLEDs), organic solar cells and organic thin-film transistors. Since the work function (WF) of stable electrodes such as indium tin oxide is generally over 4.5 eV, it has been relatively easy to examine hole injection mechanisms between such electrodes and organic semiconductors with an ionization potential of about 5 eV. On the other hand, since the electron affinity of organic semiconductors suitable for optoelectronic devices is generally less than 3 eV, it has been difficult to inject electrons from such stable electrodes. Thus, alkali metals capable of donating electrons to the unoccupied states of organic semiconductors owing to their low WF have been widely used as the electron injection layer (EIL), especially in OLEDs. Because such alkali elements are highly chemically reactive and easily oxidize in the presence of ambient oxygen and moisture, it has been difficult to examine electron injection mechanisms using alkali metals.
The method of fabricating a low-work-function (WF) electrode without the use of alkali elements has been the subject of intense study in recent years. Zhou et al. reported that electrodes fabricated utilizing polyethyleneimine had a WF of about 3 eV by [1]. Very recently, an electrode with a WF of 2.4 eV was fabricated by Tang et al. using multivalent anions. [2]. However, an electrode with lower WF is required to gain a comprehensive understanding of the mechanisms of electron injection into organic semiconductors, since the electron affinity of materials used in OLEDs is less than 2.5 eV.
We have recently succeeded in fabricating several low-WF electrodes by utilizing organic bases and gained a complete understanding of the mechanisms of electron injection into organic semiconductors [3–5]. It has been demonstrated that the formation of hydrogen bonds between nitrogen in an amidine derivative and other organic semiconductors reduces the WF to about 3 eV [3]. We have also reported that the WF of electrodes can be tuned to 2.4 eV by utilizing the coordination reaction of phenanthroline derivatives with metals [4]. The electron injection material found in the most recent study is a superbase that can reduce the WF near an Al cathode to about 2.0 eV through both the coordination reaction and the formation of hydrogen bonds [5]. This ultralow WF, which is comparable to that of Cs, allows direct electron injection into various organic semiconductors including materials used for OLEDs. The superbase found in our study can be alternative to reactive materials that had been essential for organic optoelectronic devices.
[1] Y. Zhou et al., Science vol.336, p.327 (2012).
[2] C. G. Tang et al., Nature vol.573, p.519 (2019)
[3] H. Fukagawa et al., Adv. Mater. vol.31, e1904201 (2019).
[4] H. Fukagawa et al., Nat. Commun. vol.11, p.3700 (2020).
[5] T. Sasaki and H. Fukagawa et al., Nat. Commun. vol.12, p.2706 (2021).

Carbon neutral is now required to secure our future quality of life. Reduction of amount of carbon dioxide as well as production of CO₂-free energy resources by utilizing renewable energy such as sunlight have thus been targets of intensive researches worldwide. Photocatalytic hydrogen evolution is one of technologies to produce molecular hydrogen for energy use via water splitting by free charges generated in semiconducting materials. Organic semiconductor and its junction with either dissimilar organic or inorganic semiconductor exhibit photocatalytic behaviors under visible-light irradiation [1]. Among organic photocatalysts, graphitic carbon nitride (g-CN) has attracted considerable attentions owing to facile preparation, visible light absorption (optical gap = 2.7 eV), and high chemical stability [2]. Applications of g-CN cover photocatalytic hydrogen evolution and CO₂ reduction, and photodegradation of organic molecules [3], in addition to a fluorescent material for light emitting diode [4] and moisture-sensitive actuator [5]. Surface of g-CN is crucial in a photocatalytic reaction because free charges generated react with adsorbates. An interface with a dissimilar material facilitates exciton dissociation and/or recombination of counter charges in Z-scheme reaction. Behavior of free charges is governed by energies of frontier orbitals at surface and interface. Thus, we addressed energetics of g-CN and its interface with a co-catalyst by utilizing a well-oriented g-CN film. We determined the HOMO and LUMO levels of melon, a zig-zag polymer of heptazine units, via direct and inverse photoemission spectroscopies. Their energetic positions were found to be favorable for water splitting [6]. Valence and core level photoemission spectra clarified that Z-scheme photocatalytic reaction is possible at the interface between melon and MoO_x because of a huge built-in-potential caused by charge transfer from melon to MoO_x [7]. In our talk, after briefly reviewing these findings, we will present that a commercial glass slide used as a substrate introduces cyanamide moiety into a g-CN film [8]. Reaction mechanism, basic properties of the modified g-CN film and effect of the chemical modification on photocatalytic activity in the film form will be discussed.
References
[1]Kosco et al., Adv. Energy Mater., 10, 2001935 (2020)
[2]Wang et al., Nat. Mater., 8, 76 (2009)
[3]Ong et al., Chem. Rev., 116, 7159 (2016)
[4]He et al., Mater. Today, 22, 1369 (2018)
[5]Arazoe et al., Nat. Mater., 15, 1084 (2016)
[6]Akaike et al., Chem. Mater., 30, 2341 (2018)
[7]Kawase et al., Appl. Catal. B, 273, 119068 (2020)
[8]in preparation.

Organic photodiodes (OPDs) have attracted wide attention in light sensing and imaging with ever-growing demands for flexibility and light weight. However, they usually suffer from poor device performance, especially from high reverse dark current (J_d) compared to inorganic counterpart. Since more evidence of trap states has been shown in organic semiconductors, the high dark current in OPDs starts being ascribed to the trap states. The simulated dark current based on Shockley-Read-Hall (SRH) theory can be close the experimental dark current with extra parameters, while the direct experimental evidence of dark current related to trap states as well as their energetic positions and origins are still not unambiguously known.
Here, OPDs based on a variety of donor-acceptor (D-A) bulk heterojunction (BHJ) blends that show ultralow J_d down to 10⁻⁹ mA cm⁻² have been investigated by temperature dependent dark current and ultra-sensitive external quantum efficiency (EQE) measurement. We find that J_d is thermally activated with activation energy (E_a) ranging from 0.67 to 0.98 eV, depending on the D-A materials used as active layer. These values are less than the effective bandgap (E_e,g) of BHJs by 0.3-0.4 eV but larger than E_e,g/2, where E_e,g is defined as the energy difference between the highest occupied molecular orbital (HOMO) of donor and lowest unoccupied molecular orbital (LUMO) of acceptor. Additionally, introducing C₆₀ hole blocking layer in BHJ devices between cathode and active layer does not change the J_d and E_a, while for the polymer-only layer with extra C₆₀ layer forming a bilayer device the J_d increases dramatically and E_a reduces compared to the polymer-only device. This suggests that the dark current in BHJs is not derived from the injection current from contact but from the thermal generation inside active layer which is sub-bandgap states related (E_a<E_e,g). On the other hand, ultrasensitive EQE measurements performed on the same D-A BHJs verify the existence the sub-bandgap trap states by low-energy absorption features. Considering the cavity effect in sub-bandgap EQE spectra, the radiative saturation dark current J₀^R(T) is calculated by integrating sub-bandgap EQE spectra and blackbody spectra at various temperatures, leading to a calculated activation energy extremely close to the experimental E_a with deviation less than 0.07 eV. Two independent methods corroborate the same energetic step related to sub-bandgap trap states for the first time. In combination with the activation energy and sub-bandgap EQE spectra of donor-only and acceptor-only devices, the energetic positions of sub-bandgap trap state in BHJs were located at 0.3-0.4 eV above the HOMO level of donors. Therefore, a clear generation mechanism of dark current is described by the trap state-mediated thermal generation process: the electron is thermally excited from HOMO level to the sub-bandgap trap states in polymer first and then excited from trap state to LUMO level of acceptor at D-A interface, either thermally (under dark conditions) or optically (in the EQE measurement).
Our results show that photodiodes can serve as a good platform to investigate the sub-bandgap trap states in organic semiconductors and, more importantly, provide several strategies to further suppress the dark current in OPDs. For instance, material enigneering lowers the trap state density in donor, device layout engineering eliminates cavity effects and careful selection of D-A combination keeps the trap states position in polymer away from E_e,g/2. All these new understanding can further push the limit of ultra-low dark current in OPDs achieved here, and also pave the path to obtain better performance in near infrared OPDs.

Atropine is one of the most widely used essential medicines for treating heart and eye diseases approved by World Health Organization, whereas its overdosing leads to poisoning and even death. The conventional methods (high performance liquid chromatography, gas chromatography) for detecting atropine are highly dependent on complicated devices and the skill of operators. Meanwhile, molecularly imprinted polymers (MIPs) possessing specific pores are known as the attractive technology for the highly selective detection of target molecules. For exploring low-cost and easy-to-use applications of MIPs for atropine, herein we aim at developing a MIP-attached extended gate-type organic thin-film transistor (MIP-OTFT) for the detection of atropine. First, we operated the DFT optimization of the pre-polymerized molecular complex between monomers (2-(dimethylamino)ethyl methacrylate and N-isopropylacrylamide) and (S)-atropine. The hydrogen bonds in the complex could be clearly observed. Thus, the monomers were copolymerized through radical polymerization and cross-linked by N,N'-methylenebisacrylamide. We characterized the MIP with various methods including cyclic voltammetry (CV), differential pulse voltammetry (DPV), etc. In the DPV measurement, the peak current of the MIP electrode showed a stepwise decrease with increasing the concentration of (S)-atropine. Finally, we combined the MIP electrode with the OTFT for realizing sensitive detection in an aqueous solution (limit of detection: 85 nM).

As a core material for the realization of the 3rd generation OLED after phosphorescence, numerous pure organic-based thermal activated delayed fluorescence (TADF) emitters capable of converting non-emissive triplet excitons (T₁) into emissive singlet (S₁) excitons and exhibiting up to 100% internal quantum efficiency (IQE) have been studied in recent years. In general, in order to express the above TADF phenomenon in a specific molecular structure, efficient reverse intersystem crossing (RISC) must be guaranteed, and an important prerequisite is the small difference between singlet and triplet energies (ΔE_ST).
Recently, some boron-containing emitters have been proven to exhibit high efficiency when applied to TADF-OLED as an EML material because they satisfy the above properties. We adopted a robust and symmetrical oxygen-bridged boron acceptor structure in the emitting molecular design of this study. The presence of two oxygen atoms and boron forms a planar and rigid structure that is expected to enhance the electron interaction between donor and acceptor and suppress the non-radiative decay process to improve photoluminescence quantum yield (PLQY).
New design strategy of tethering two Cz-derivatives to acceptors through a benzene core is proposed. Among the various Cz-derivatives, benzofurocarbazole (BFCz) and indenocarbazole (ICz) have been used owing to their relatively strong donability and high molecular weight with rigid conformation. The use of boron-based emitters containing BFCz and ICz moieties is expected to afford excellent solution processability, good film-forming properties, and high TADF-OLED performance.
As a result, the Cz-based fluorescent emitter (BCz) is converted to a charge-transfer complex (BBFCZ and BICz) and the HOMO-LUMO gap decreases with increasing donor strength, but the corresponding T₁ energies of Cz, DBF, and ICz remain nearly constant, resulting in a small ΔE_ST, decreasing from 0.32, 0.13, to 0.10 eV, respectively. The k_RISC values of Cz, DBF, and ICz -based emitters were determined to be 2.31×10⁵ s⁻¹, 3.06×10⁵ s⁻¹, and 3.60×10⁵ s⁻¹, respectively. [1]
BCz, BBFCz, and BICz exhibited highly deep-blue emission at 414, 416, and 424 nm, respectively. Their various properties were systematically studied using theoretical calculations and time-resolved photoluminescence experiments. Non-doped solution-processed deep-blue OLEDs using BICz displayed a significantly high external quantum efficiency (EQE) of 10.11%, as compared to the BCz- and BBFCz-based devices. Notably, as the donor moiety changes from BCz to BICz, the luminescence mechanism changes from fluorescence to TADF. Accordingly, the use of BICz as an emitter significantly improved the device performance of the deep-blue OLED.

[1] J. Hwang, H. Kang, J. –E. Jeong, H. Y. Woo, M. J. Cho, S. park, D. H. Choi, Chem. Eng. J. 2021, 416, 129185.

Thermally activated delayed fluorescence (TADF) materials has been researched to replace heavy-atom phosphorescent emitters and blue fluorescent emitters. In this study, we report a novel TADF materials substituted by ortho-biphenyl unit in multi-carbazole TADF molecule. The new TADF emitter, 4mCz-BP is bearing four dimethylcarbazole donors in benzonitrile acceptor core to trigger TADF effect. The emitter exhibits a bluish-green light of a 491 nm wavelength as a peak intensity and a high photoluminescence quantum yield (PLAQ) of 95% in mCBP host. We used a time dependent density theory (TD-DFT) calculation method to define ortho-biphenyl substitution effect by comparing with similar multi-carbazole TADF materials 4CzBN and 4mCzBN. A local excited triplet state (³LE) with an energy (2.81 eV) close to the lowest singlet charge-transfer state (¹CT) can forms at the biphenyl of 4mCzBN-BP. However, because of the planarization of the biphenyl unit at triplet state and its large steric caused by nearby carbazole donors, ³LE at the biphenyl unit can be formed by a subset of feasible conformations of 4mCzBN-BP molecules in the mCBP host, leading to a multiexponential decay of the delayed fluorescence. The device also record a long operational lifetime (LT50) of 750 hours with organic light emitting diodes based on 4mCzBN-BP.

The development of thermally activated delayed fluorescent (TADF) emitters has greatly improved the efficiency of the fluorescent organic light emitting diodes (OLEDs). We report two highly efficient deep blue thermally activated delayed fluorescence emitters, TDBA–SAF and DBA–SAB, containing spiro acridine fluorene and spiro biacridine donor units with an oxygen bridged boron acceptor unit, respectively. The OLEDs emit deep blue color and blue color with the peak wavelength of 456 nm for TDBA–SAF and 472 nm for DBA–SAB, respectively. Both devices exhibited a narrow emission spectrum with FWHM of 55 and 62 nm due to the rigidity of the boron acceptor unit. The blue TADF device with TDBA–SAF and DBA–SAB in the DPEPO host exhibited a maximum EQE of 28.2% and 25.7% with CIE coordinates of (0.142, 0.090) and (0.144, 0.212), respectively.

It is extremely scarce that near-infrared organic light-emitting diodes (NIR OLEDs) have been actually applied to various fields such as sensors, night-vision displays, or phototherapy owing to device stability and reliability. Therefore, Realizing a novel deep red to NIR (DR/NIR) emitter for the high-performance DR/NIR OLED has become a vital research area. In this report, a novel thienothiophene-isoquinoline(ttiq)-based Ir(III) complex DR/NIR emitter with narrow full width half maximum (FWHM, 38 nm), a shallow highest occupied molecular orbital (HOMO) energy level is designed and synthesized. The best device based on a new Ir(III) complex yields record-high radiant emittance (> 5mWcm^-2) at low voltage (6 V), low external quantum efficiency (EQE) roll-off, low driving voltage (2.5–6 V), and stable operational lifetime for biomedical application with an emission peak wavelength of 696 nm. From all perspectives, this is notably an outstanding performance among other reported Ir(III)-based DR/NIR OLEDs. Moreover, DR/NIR OLEDs are applied to the biomedical field and an in vitro experiment shows an increase in cell proliferation effect of up to 24% under diverse conditions

We have studied the composition and nanoscale multi-orientational molecular ordering in a bulk heterojunction (BHJ) blend films of the well-known donor polymer PTB7-Th and narrow bandgap non-fullerene acceptor (NFA) IEICO-4F. Blend films cast from chlorobenzene (CB) with 3 vol% of 1-chloronaphthalene (CN) show a dramatic morphological instability transitioning from fine-scale nano-domains to large micron-sized phase-separated domains when either the spin-coating speed or spin-coating duration is reduced below a certain threshold. Interestingly, we observe a decrease in the oscillator strength corresponding to IEICO-4F for films with a coarse morphology compared to films with a fine-scale morphology, which we explain using structural studies. From micro-Raman spectroscopic studies, we find the predominant localization of IEICO-4F within the large circular microscopic domains, while the surrounding matrix is rich in PTB7-Th. Moreover, for these films we observe a capping layer of PTB7-Th at the centre of domain. The molecular orientation within PTB7-Th:IEICO-4F blend films has also been visualized by combining cryo-electron microscopy (EM) with X-ray scattering studies. We find that molecules are oriented differently at the edge and at the center of these domains, a feature that has not been observed previously in spin-coated films containing NFAs. Finally, we find that while varying the processing conditions drastically affects the morphology, the corresponding change in device performance is relatively low, which is in contrast to a general notion of organic photovoltaic systems such as those based on polymer/fullerene blends.