Organic Optoelectronics Research Articles

Mixed-flow design for microfluidic printing of two-component polymer semiconductor systems

The rational creation of two-component conjugated polymer systems with high levels of phase purity in each component is challenging but crucial for realizing printed soft-matter electronics. Here, we report a mixed-flow microfluidic printing (MFMP) approach for two-component π-polymer systems that significantly elevates phase purity in bulk-heterojunction solar cells and thin-film transistors. MFMP integrates laminar and extensional flows using a specially microstructured shear blade, designed with fluid flow simulation tools to tune the flow patterns and induce shear, stretch, and pushout effects. This optimizes polymer conformation and semiconducting blend order as assessed by atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing incidence wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (R-SoXS), photovoltaic response, and field effect mobility. For printed all-polymer (poly[(5,6-difluoro-2-octyl-2H-benzotriazole-4,7-diyl)-2,5-thiophenediyl[4,8-bis[5-(2-hexyldecyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-2,5-thiophenediyl]) [J51]:(poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}) [N2200]) solar cells, this approach enhances short-circuit currents and fill factors, with power conversion efficiency increasing from 5.20% for conventional blade coating to 7.80% for MFMP. Moreover, the performance of mixed polymer ambipolar [poly(3-hexylthiophene-2,5-diyl) (P3HT):N2200] and semiconducting:insulating polymer unipolar (N2200:polystyrene) transistors is similarly enhanced, underscoring versatility for two-component π-polymer systems. Mixed-flow designs offer modalities for achieving high-performance organic optoelectronics via innovative printing methodologies.

Hydroxy-benzimidazoles as blue-green emitters: Synthesis, structural and DFT studies

The new benzimidazole ligands (3a-e) were synthesized using ethyl 4-(butylamino)-3-nitrobenzoate and substituted salicylaldehyde in the presence of sodium dithionite reagent which undergoes “one-pot’’ nitro reductive cyclization. Here benzimidazole moiety acts as an electron-acceptor (A) whereas substituted 2-hydroxyphenyl moiety acts as an electron-donor (D) unit. The molecular structures were characterised by FTIR, 1H NMR, 13C NMR, single crystal XRD and MS analysis. Optoelectronic properties were determined by UV–vis, solution and solid photoluminescence, quantum yield and lifetime. The solvent-dependent absorption and emission were studied using both polar protic and polar aprotic solvents. All the derivatives exhibited ESIPT, especially compound ethyl 1-butyl-2-(3,5-dichloro-2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (3e) displayed dual emission in both polar protic and polar aprotic solvents. The compound stability and electrochemical property were determined by thermal gravimetric analysis (TGA) and cyclic voltammetry (CV) respectively. The compounds emit intense blue-green fluorescence with high to moderate quantum yield. Also, these derivatives exhibited a high Stokes shift. The computational studies like Density-Functional Theory (DFT) and Molecular Electrostatic Potential (MEP) were conducted to provide important insights into the structure-property relationships. The crystal packing is stabilized through intermolecular hydrogen bonds (C---H…O) and intermolecular interactions (π… π). The findings of results help in developing novel ligands in the field of organic optoelectronics.

Thermal Transitions in P3HT:PC60BM Films Based on Electrical Resistance Measurements.

In this paper, we present research on thermal transition temperature determination in poly (3-hexylthiophene-2,5-diyl) (P3HT), [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM), and their blends, which are materials that are conventionally used in organic optoelectronics. Here, for the first time the results of electrical resistance measurements are explored to detect thermal transitions temperatures, such as glass transition Tg and cold crystallization Tcc of the film. To confirm these results, the variable-temperature spectroscopic ellipsometry studies of the same samples were performed. The thermal transitions temperatures obtained with electrical measurements are well suited to phase diagram, constructed on the basis of ellipsometry in our previous work. The data presented here prove that electrical resistance measurements alone are sufficient for qualitative thermal analysis, which lead to the identification of characteristic temperatures in P3HT:PC60BM films. Based on the carried studies, it can be expected that the determination of thermal transition temperatures by means of electrical resistance measurements will also apply to other semi-conducting polymer films.

Solution‐Processed Transparent Superhydrophobic Protection Layers for Enhancing the Device Reliability of Flexible Organic Optoelectronics

AbstractOrganic materials and devices have attracted great attention for implementation of flexible and transparent electronics applications. However, further easy‐to‐manage organic devices with acceptable environmental reliability in open air are desirable. Specifically, because water‐based threats and particle contamination can degrade the functions of organic optoelectronics, introducing a superhydrophobic protection layer onto organic devices, which can eliminate issues via excellent water repellency, is necessary. In this study, surface‐engineered TiO2 nanoparticles dispersed in a highly fluorinated solvent are deposited on organic devices using organo‐compatible solution processing. The optimization of the TiO2 nanoparticle layer, such as the surface roughness and thickness of the film, enables the realization of a transparent superhydrophobic layer; therefore, the film can be utilized in transparent organic optoelectronics, especially in phototransistor applications. The transparent superhydrophobic layer exhibits good water repellency without critical delamination issues even during mechanical deformation, such as bending and stretching tests. Flexible organic phototransistors with the transparent superhydrophobic layer show a self‐cleaning ability against harmful contaminants on the topmost surface, achieved by dropping water droplets. This work can provide a feasible solution to maintaining transparent and flexible organic devices with improved environmental reliability.

Direct Observation of Aggregation‐Induced Emission Mechanism

AbstractThe mechanism of aggregation‐induced emission, which overcomes the common aggregation‐caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward–Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Direct Observation of Aggregation-Induced Emission Mechanism.

The mechanism of aggregation-induced emission, which overcomes the common aggregation-caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward-Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Unexpected Stacking Transformation from H- to J- and X-Aggregated Co-Crystals Leading to Tunable Mechanical and Optoelectronic Properties

Molecular stacking modes, generally classified as H-, J-, and X-aggregation play a key role in determining the optoelectronic properties of organic crystals. However, the control of stacking transitions of a specific molecule is an unmet challenge and a priori prediction of performance in different stacking modes is currently not possible. Herein, unexpected stacking transition from H- to J- and X- aggregation is observed in the crystal structure of a minimalistic aromatic molecule, 4,4’-bipyridine, upon co-assembled with N-acetyl-L-alanine, a non-aromatic amino acid derivative. This stacking transition improves the physicochemical properties of the co-crystals, including a large red-shifted emission, enhanced chirality, improved thermal stability and mechanical properties. While the single crystal of 4,4’-Bpy shows good optical waveguiding and piezoelectric properties, the co-crystal has a significantly lower band-gap and improved conductivity. This demonstrates the first example of H- to J- and X- packing transition of a minimalistic model by co-crystal aggregation and provides a proof-of-concept strategy for organic optoelectronics design.

A new polymorph of zinc-phthalocyanine and its optical properties

Zinc-phthalocyanine has great potential for the applications in band-structure engineering, organic solar cells and photodynamic therapy. The optical and structural properties of zinc-phthalocyanine have been studied extensively previously. Here we report we have fabricated the nanowires of zinc-phthalocyanines (the η-phase) and investigated their crystal structures and optical properties. The in-situ x-ray-diffraction experiments show that zinc-phthalocyanine nanowires grown at the low-temperature region have a distinct crystal structure as compared with the zinc-phthalocyanine polymorphs reported previously. We have also fitted the x-ray diffraction data and obtained the crystal-structure parameters, which further suggests that the nanowire phase has a new crystal structure. The UV–Vis experiments suggest similar qualitative feature in both the Q-band and the B-band, but slightly different quantitative perspectives. As explained by using the first-principles calculations, a mixture of intra-molecular and charge-transfer excitations can account for the splitting of the Q-band. Our work can provide a solid foundation for the further applications of zinc-phthalocyanines in organic optoelectronics.

Two-Dimensional Exciton Diffusion in an HJ-Aggregate of Naphthobisoxadiazole-Based Copolymer Films

Understanding exciton diffusion properties is essential for organic optoelectronics such as organic solar cells (OSCs) because excitons in semiconducting polymer solids need to migrate as quasipart...

1]Benzothieno[3,2‐b][1]benzothiophene‐Phthalocyanine Derivatives: A Subclass of Solution‐Processable Electron‐Rich Hole Transport Materials

The [1]benzothieno[3,2-b][1]benzothiophene (BTBT) planar system was used to functionalize the phthalocyanine ring aiming at synthesizing novel electron-rich π-conjugated macrocycles. The resulting ZnPc-BTBT and ZnPc-(BTBT)4 derivatives are the first two examples of a phthalocyanine subclass having potential use as solution-processable p-type organic semiconductors. In particular, the combination of experimental characterizations and theoretical calculations suggests compatible energy level alignments with mixed halide hybrid perovskite-based devices. Furthermore, ZnPc-(BTBT)4 features a high aggregation tendency, a useful tool to design compact molecular films. When tested as hole transport materials in perovskite solar cells under 100 mA cm-2 standard AM 1.5G solar illumination, ZnPc-(BTBT)4 gave power conversion efficiencies as high as 14.13 %, irrespective of the doping process generally required to achieve high photovoltaic performances. This work is a first step toward a new phthalocyanine core engineerization to obtain robust, yet more efficient and cost-effective materials for organic electronics and optoelectronics.

(Electronics and Photonics Division Award Address) Let There be Light... – Research at the Intersection of Electronics and Photonics

Electronics and photonics have a long and successful history of combining to provide many devices and products that are at the heart of the modern technological world, ranging form solar cells to lasers to digital displays. In this presentation I will first give a brief historical overview of the interplay and interdependence of electronics and photonics. The properties and effects that are commonly exploited in opto/electronics fall primarily in two areas: (a) the effect of incident light (or more generally radiation) due to optical absorption/transmission/reflection and the conversion of photons to electrical charge that can be collected; (b) the injection of electrons & holes leading to the emission of photons. Then I will review selected examples from my research in optoelectronic materials and devices in more or less chronological order over the past 40+ years. An early example [1] in the light-to-charge was integration of infrared detectors using narrow bandgap semiconductors (such as InSb and HgCdTe) with silicon charge coupled devices that performed data storage and image processing functions on arrays of pixels. A major research topic in my group in the charge-to-light category has for many years been electroluminescence [2] from wide bandgap semiconductors (primarily GaN and AlN) doped with rare earth elements (Er, Eu, Tm, etc.). Narrow linewidth emission can occur at wavelengths from near UV to visible to IR, depending on the electronic transitions of the selected RE element and the bandgap of the host material. In the past 10 years, my materials interest switched from inorganic to organic semiconductors and biomaterials, but with a continuing strong opto/electronic component. Currently, a major research focus is in developing simple, low cost and intuitive diagnostic devices for point-of-care use. These devices analyze the composition of biofluids (e.g. blood, sweat, saliva, etc.) to provide a 1st order indication of specific aspects the individual’s health status, such as blood coagulation, stress, etc. The use of light in PoC and other diagnostic devices is widespread. Most common and simplest approach is colorimetric detection in ambient light, where a change of color in a region of the device indicates the presence of a specific biomolecule associated with a given medical condition. Somewhat more complex diagnostic devices use fluorescence of molecules attached to biomarkers to indicate their presence. In our own research, biofluid samples are typically transported by capillary flow to a location where their key properties are measured by a combination of optical emission [3], transmission and absorption [4]. The presentation will conclude with a look forward to other applications where photonics and electronics could join forces.[1] A.J. Steckl, et al., “Application of Charge Coupled Devices to Infrared Detection and Imaging,” Proc. of IEEE, 63(1), 67, Jan 1975 ( doi.org/10.1109/PROC.1975.9708 ).[2] (a) A.J. Steckl and J.M. Zavada, “Optoelectronic Properties and Applications of Rare-Earth-Doped GaN”, Mat. Res. Soc. Bulletin, 24(9), 33, Sep 1999 doi.org/10.1557/S0883769400053045; (b) A. J. Steckl, et al., Invited Paper, “Rare-earth-doped GaN: Growth, Properties and Fabrication of Electroluminescent Devices”, IEEE J. Selected Topics in Quantum Electronics, 8 (4), 749 Jul 2002 doi.org/10.1109/JSTQE.2002.801690 [3] V. Venkatraman and A. J. Steckl, “Quantitative Detection in Lateral Flow Immunoassay Using Integrated Organic Optoelectronics”, IEEE Sensors, 17 (24), 8343, Dec 2017 (doi.org/10.1109/JSEN.2017.2764178 ).[4] P. Ray and A. J. Steckl, “Label-Free Optical Detection of Multiple Stress Biomarkers in Sweat, Plasma, Urine and Saliva”, ACS Sensors 4, 1346, 2019. (doi.org/10.1021/acssensors.9b00301). Figure 1

(Invited) Blood, Sweat and Tears... – Sensing of Human Performance Biomarkers in Bodily Fluids Using Point-of-Use Devices

Hormones produced by glands in the endocrine system and neurotransmitters produced by the nervous system control many bodily functions. The concentrations of these molecules in the body are an indication of its state, hence the use of the term biomarker. Excess concentrations of biomarkers, such as cortisol, serotonin, epinephrine, dopamine, are released by the body in response to a variety of conditions - emotional state (euphoria, stress), disease, etc. The development of simple, low-cost modalities for point-of-use (PoU) measurements of biomarkers levels in various bodily fluids (blood, urine, sweat, saliva) as opposed to conventional hospital or lab settings is receiving increasing attention. The presentation starts with a review of the basic properties of primary stress-related biomarkers: origin in the body (i.e. if they are produced as hormones, neurotransmitters or both), chemical composition, molecular weight (small/medium size molecules and polymers, ranging from ~100Da to ~100kDa), hydro- or lipo-philic nature. Next, a review of the published literature is presented regarding the concentration of these biomarkers found in several bodily fluids that can serve as the medium for determination of the condition of the subject: blood, urine, saliva, sweat and, to a lesser degree, interstitial tissue fluid. The concentration of various biomarkers in most fluids covers a range of 5-6 orders of magnitude, from 100s of ng/mL (~1µM) down to a few pg/mL (sub 1pM). Mechanisms and materials for point-of-use biomarker sensors are summarized and key properties are reviewed. Illustrative examples from the literature are discussed for several sensor device categories, including lateral flow (immuno)assay devices and microfluidic devices. Selected methods for detecting these biomarkers are reviewed, including antibody- and aptamer-based colorimetric assays, electrochemical and optical detection. Finally, the presentation outlines key challenges of the field and provides a look ahead to future prospects.A.J. Steckl and P. Ray, “A Review of Stress Biomarkers Concentrations in Bodily Fluids and Their Point-of-Use Detection”, ACS Sensors, 3 (10), 2025-2044, Sept 2018 (10.1021/acssensors.8b00726).V. Venkatraman and A.J. Steckl, “Quantitative Detection in Lateral Flow Immunoassay using Integrated Organic Optoelectronics”, IEEE Sensors, 17 (24), 8343, Dec 2017 (10.1109/JSEN.2017.2764178 ).P. Ray and A.J. Steckl, “Label-Free Optical Detection of Multiple Stress Biomarkers in Sweat, Plasma, Urine and Saliva”, ACS Sensors 4, 2019. (10.1021/acssensors.9b00301)S. Dalirirad and A.J. Steckl, “Aptamer-Based Lateral Flow Assay for Point of Care Cortisol Detection in Sweat”, Sensors & Actuators 283, 79-86, Jan 2019 (https://doi.org/10.1016/j.snb.2018.11.161). Figure 1

Hierarchical Self-Assembly of Perylene Diimide (PDI) Crystals.

Controlling molecular self-assembly of organic semiconductors is a key factor in enhancing the performance of organic electronics and optoelectronics. However, unlike various p-type organic semiconductors, it has proven elusive to control molecular self-assembly with about tens of nm dimensions using n-type organic semiconductors including perylene diimide (PDI), which is the most promising alternative to fullerene derivatives, without using an additional synthetic method or additives, thus far. Here, we developed a simple self-assembling method for the hierarchical self-assembly of PDI crystals with nanometer-to-micrometer scale features using pristine PDI-C8 without using an additional synthetic method or additive. Interestingly, we observed crystalline and optical properties of self-assembled PDI crystals depending on their size and structural features. In addition, we fabricated p-n junctions composed of PDI and poly(3-hexylthiophene) (P3HT), where the p-n junctions had coassembled and blended nanomorphologies, and confirmed that coassembled nanomorphologies enabled more effective energy transfer than the blended nanomorphologies.

Structure-Dependent Charge Transfer in Molecular Perylene-Based Donor/Acceptor Systems and Role of Side Chains

In organic electronics and optoelectronics several crucial physical processes are related to charge transfer (CT) effects. In this work, we investigate mixing behavior and intermolecular coupling o...

Effect of Vibronic Coupling on Correlated Triplet Pair Formation in the Singlet Fission Process of Linked Tetracene Dimers.

Tetracene-based singlet fission (SF) materials show application prospects as triplet sensitizers in organic optoelectronics. SF involves internal conversion from photoexcited singlet states 1(S1S0) to correlated triplet pair states 1(T1T1). We derive an expression for the internal conversion rate on the basis of the Fermi golden rule with an artificial Lorentzian broadening. The internal conversion rate depends on the interstate vibronic couplings (VCs) and energy difference (ΔESF) between 1(S1S0) and 1(T1T1). Therefore, understanding the interplay between interstate VCs and ΔESF is necessary to reveal how the structure-property relationship affects the SF efficiency. Here, we propose a method to quantitatively analyze interstate VCs between 1(S1S0) and 1(T1T1). We apply this method to SF in ortho-, meta-, and para-bis(ethynyltetracenyl)benzene and identify an effect of interstate VCs on the 1(T1T1) formation rate. The interstate VCs of the meta dimer are remarkably weak, which reasonably explains the experimentally obtained slow 1(T1T1) formation rate. The weak VCs result from a very small overlap density between 1(S1S0) and 1(T1T1) of the meta dimer. Furthermore, we investigate structure-dependence of the 1(T1T1) formation rate of the para dimer and find that the para dimer shows large VCs and small ΔESF when the rotational angle between the two tetracene units is large, which leads to the faster 1(T1T1) formation rate than those of the ortho and meta dimers. The rotation of the tetracene units is the origin of the experimentally observed fast 1(T1T1) formation rate of the para dimer.

Activating Intersystem Crossing and Aggregation Coupling by CN-Substitution for Efficient Organic Ultralong Room Temperature Phosphorescence

Organic ultralong room-temperature phosphorescence (OURTP) has boomed recent advances of organic optoelectronics with the significant breakthrough in facilitating the intersystem crossing and stabi...

A hybrid P3HT-Graphene interface for efficient photostimulation of neurons

Graphene conductive properties have been long exploited in the field of organic photovoltaics and optoelectronics by the scientific community worldwide. We engineered and characterized a hybrid biointerface in which graphene is coupled with photosensitive polymers, and tested its ability to elicit light-triggered neural activity modulation in primary neurons and blind retina explants. We designed such a graphene-based device by modifying a photoactive P3HT-based retinal interface, previously reported to rescue light sensitivity in blind rodents, with a CVD graphene layer replacing the conductive PEDOT:PSS layer to enhance charge separation. The new graphene-based device was characterized for its electrochemical features and for the ability to photostimulate primary neurons and blind retina explants, while preserving biocompatibility. Light-triggered responses, recorded by patch-clamp in vitro or MEA ex vivo, show a stronger light-transduction efficiency when the neurons are interfaced with the graphene-based device with respect to the PEDOT:PSS-based one. The possibility to ameliorate flexible photo-stimulating devices via the insertion of graphene, paves the way for potential biomedical applications of graphene-based neuronal interfaces in the context of retinal implants.

Organic UV-Sensitive Phototransistors Based on Distriphenylamineethynylpyrene Derivatives with Ultra-High Detectivity Approaching 1018.

Organic photodetectors with UV-sensitivity are of great potential for various optoelectronic applications. Integration of high charge carrier mobility, long exciton diffusion length as well as unique UV-sensitivity for active materials is crucial for construction of UV-sensitive devices with high performance, however, very few organic semiconductors can integrate these properties simultaneously. Herein, two novel organic semiconductors containing large steric hindrance triphenylamine groups, 1,6-distriphenylamineethynylpyrene (1,6-DTEP) and 2,7-distriphenylamineethynylpyrene (2,7-DTEP) are designed and synthesized. It demonstrates that the single crystals of both 1,6-DTEP and 2,7-DTEP exhibit superior integrated optoelectronic properties of high charge carrier mobility, unique UV absorption, high photoluminescence quantum yields as well as small exciton binding energies. Organic phototransistors constructed using 1,6-DTEP and 2,7-DTEP single crystals show ultrasensitive performance with ultra-high photoresponsivity of 2.86 × 106 and 1.04 × 105 A W-1 , detectivity (D*) of above 1.49 × 1018 and 5.28 × 1016 Jones under 370 nm light illumination, respectively. It indicates the great potential of 1,6-DTEP and 2,7-DTEP-based phototransistors for organic UV-photodetector applications and also provides a new design strategy to develop series of better performance UV photoelectric organic materials for related research in organic optoelectronics.

Full Solution-Processed Fabrication of Conductive Hybrid Paper Electrodes for Organic Optoelectronics

There is growing interest in the development of “green” electronics fabricated from “natural” materials, which could lead to renewable, sustainable, and potentially inexpensive organic optoelectron...

A DNA and Self-Doped Conjugated Polyelectrolyte Assembled for Organic Optoelectronics and Bioelectronics.

Deoxyribonucleic acid (DNA) and a self-doped conjugated polyelectrolyte, poly(4-(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl-methoxy)-1-butanesulfonic acid (PEDOT-S), are assembled for organic optoelectronics and bioelectronics. The DNA's helix-coil phase transition in water is studied as a function of composition by thermo-optical analysis. DNA and PEDOT-S are functionalized by using a surfactant, cetyltrimethylammonium chloride (CTMA), and DNA:CTMA, PEDOT-S:CTMA, and DNA:CTMA:PEDOT-S:CTMA complexes were characterized regarding thermal, optical, morphological, and structural properties. Finally, DNA and DNA:PEDOT-S mixtures are processed in water for fabricating organized films through brushing. The electrical properties of these films are characterized using an interdigitated electrode. The films show an electronic conductivity of ∼10-6-10-5 S/cm in a range of semiconductors.