Solution-processed Small Molecule Research Articles

High-Efficiency Y6 Homojunction Organic Solar Cells Enabled by a Secondary Hole Transport Layer.

Y6 homojunction solar cells are prepared using the exciton/electron-blocking material poly[9,9-di-n-octylfluorene-alt-N-(4-sec-butylphenyl)diphenylamine] (TFB) as a secondary hole transport layer material in conjunction with PEDOT:PSS. Using this device architecture, a maximum power conversion efficiency (PCE) of 2.57% is achieved, which is the highest reported thus far for a solution-processed small molecule homojunction organic photovoltaic (OPV) device. The devices display an unexpectedly low thickness dependence, with the average PCE only decreasing by ≈17% when the Y6 active layer thickness is increased from 80 to 300nm. Time-resolved photoluminescence measurements show that the TFB does not contribute to charge generation through photoinduced hole or electron transfer. However, transient absorption spectroscopy on thin films of neat Y6 and a 1:1 blend of Y6:TFB shows that the TFB enhances the formation of the long-lived Y6 intermolecular charge-transfer state in the blend film. It is found that careful selection of the electron transport layer (ETL) is required to avoid unintended charge generation at the interface with Y6 so as to ensure that the device is a true homojunction. The improved efficiency of this architecture is attributed to the electron-blocking and hole-extraction effects of the TFB layer.

Air-Stable Perylene Diimide Trimer Material for N-Type Organic Electrochemical Transistors.

A new method is reported to make air-stable n-type organic mixed ionic-electronic conductor (OMIEC) films for organic electrochemical transistors (OECTs) using a solution-processable small molecule helical perylene diimide trimer, hPDI[3]-C11. Alkyl side chains are attached to the conjugated core for processability and film making, which are then cleaved via thermal annealing. After the sidechains are removed, the hPDI[3] film becomes less hydrophobic, more ordered, and has a deeper lowest unoccupied molecular orbital (LUMO). These features provide improved ionic transport, greater electronic mobility, and increased stability in air and in aqueous solution. Subsequently, hPDI[3]-H is used as the active material in OECTs and a device with a transconductance of 44 mS, volumetric capacitance of ≈250 F cm-3, µC* value of 1 F cm-1 V-1 s-1, and excellent stability (> 5 weeks) is demonstrated. As proof of their practical applications, a hPDI[3]-H-based OECTs as a glucose sensor and electrochemical inverter is utilized. The approach of side chain removal after film formation charts a path to a wide range of molecular semiconductors to be used as stable, mixed ionic-electronic conductors.

Solution‐Processable Small‐Molecule Hole Transport Material for High‐Performance QLED via Manipulating Dipole Moments

AbstractSolution‐processed quantum‐dot light‐emitting diodes (QLEDs) have achieved huge successes in terms of both efficiency and stability. However, the polymeric hole transport materials (HTMs) utilized in QLEDs, to some degree, cause severe damages to performance consistency due to their intrinsic batch‐to‐batch problems. Thus, adopting small‐molecule HTMs with definitive chemical structures and molecular weight is a plausible way to solve this issue. In this work, a new solution‐processable small‐molecule (SM)‐HTM (FLCZ) is designed and synthesized, through elaborately modifying the widely used 4,4′‐bis(carbazole‐9‐yl)‐biphenyl via manipulating dipole moments, for solution‐processed QLEDs to replace polymeric HTMs. The FLCZ‐based QLED has achieved an external quantum efficiency (EQE) of 14.63% (18.74 lm W−1, 20.01 cd A−1), approaching that of polymer HTMs‐based QLEDs. In addition, the large surface energy of 45.29 mN m−1 is also observed on the film of FLCZ, which is highly desirable for inkjet‐printed (IJP) QLED devices, results in the IJP fabricated QLED with an EQE of 10.69%. To the best of knowledge, the results of this work represent the highest device performance of conventional solution‐processed QLED based on a single SM‐HTM.

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor-Acceptor-Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization.

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules TRA-IID-TRA and TRA-TIID-TRA comprising an electron-donating triarylamine (TRA) and two different electron-withdrawing cores, isoindigo (IID) and thienoisoindigo (TIID), respectively, were synthesized and characterized. Replacing the phenylene rings of central IID A with thiophene gives a TIID core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that TRA-TIID-TRA exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like TRA-TIID-TRA crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm2 V-1 s-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm2 V-1 s-1) of TRA-IID-TRA with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.

Solution-processable star-shaped small molecules for efficient organic RRAM by induced conductive filament mechanism

Highly efficient RRAM based on the star-shaped small molecules is achieved by induced conductive filaments.

Solution-processable small molecule based all-organic ultraviolet photodetector

A tetramethoxyfluorene derivative (TMD) based small molecule (C53H54O4) has been used as a donor in a bulk-heterojunction donor-acceptor blend system to realize room temperature solution-processed ultraviolet (UV) photodetector. A systematic opto-electronic study to optimize the weight ratio of the blend of TMD and a common acceptor [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) is presented in this report. The fabricated detector is found to cover the most harmful UVA region while transmitting nearly the entire visible spectrum of light. The responsivity of 6 × 10−2 A/W, specific detectivity of 9.36 × 1011Jones and photosensitivity of 105, which are on par with the existing UV photodetectors reported in the literature, are obtained at the wavelength 394 nm under zero bias condition.

Synthesis of 4,4′-Dibutyl-5,5′-di(thiophen-3-yl)-2,2′-bithiazole for applications toward organic electronic material

Except a few examples, solution-processable π-conjugated small molecules with both positive and negative type doping abilities are rare. This is a very important requirement for synthesizing ambipolar organic electronic materials. So, here a new bithiazole based small molecule 4,4′-dibutyl-5,5′-di(thiophen-3-yl)-2,2′-bithiazole (3TBTz) was synthesized and characterized by 1H NMR, 13C NMR and HRMS. An optimized (DFT-6-31G(d)-B3LYP) cation radical of 3TBTz attained completely planar geometry from a non-planar parent molecule optimized at the same level. Both, computationally (TD-DFT/CAM-B3LYP) and experimentally obtained UV–vis spectra found a decent match. The theoretically obtained reorganization energy for 3TBTz claimed superiority in comparison to its terthiophene analogue. The ambipolar red/ox profile observed in cyclic voltammetry was supported by DFT calculation.

Effect of low-dose doping of small-molecules on the surface potential and carrier balance of TADF OLEDs

Solution-Processed organic light emitting diodes (OLEDs) have been intensively studied due to the simple preparation process. However, the devices made of small molecular luminescent materials always suffer from the reduce of the carrier transport capacity by the influence of molecular agglomeration. In this work, small-molecules material 4,4',4''-tris(N-carbazolyl)-triphenylamine (TCTA) is successfully introduced into the molecular aggregation site formed by traditional host material 1,3-Di-9-carbazolylbenzene (mCP), improving the charge mobility of the emitting layer. Doped mCP film has more compact molecular packing, which makes it has better charge transfer performance as a solid solvent of 2,3,4,6-tetrakis(9H-carbazole-9-yl)−5-fluorobenzonitrile (4CzFCN). Besides, 3 wt% of TCTA is beneficial to optimized electrochemical doping, lowering the surface potential of emitting layer and promoting holes injection. Doped host-guest system device exhibits a maximum current efficiency (CEmax = 29.6 cd/A) which has improved about 60% compared to the device with host of pure mCP (CEmax = 18.5 cd/A). The findings may provide a strategy for the future research on the film structure of solution-processed small molecule OLEDs.

The substituents on the intermediate electron-deficient groups in small molecular acceptors result appropriate morphologies for organic solar cells

The small molecule acceptors with the structure of acceptor-donor acceptor' donor-acceptor (A−DA'D−A) boost the power conversion efficiency of organic solar cells. Compared with multifused DA'D core with an electron-deficient unit at the center, the unfused core emerges low cost and high yield. Herein, we designed and synthesized three small molecule acceptors named as TR2F-IC4F, BTOC8-IC4F and, BTOC6C8-IC4F, with an unfused core containing one benzothiadiazole (BT) or triazole (TR) unit and two cyclopentadithiophene (CPDT) units. The nearly planar geometry is realized through noncovalent F⋯H, F⋯S and S⋯O interactions. These acceptors have broad near-infrared absorption. For devices with BTOC8-IC4F, TR2F-IC4F and BTOC6C8-IC4F, the highest PCEs are only 5.81%, 5.89% and 7.55%, respectively. For devices with BTOC6C8-IC4F, the PCEs are further improved to 10.01% for ternary cells, which add PC 71 BM into devices. In this work, we found with the increase of the substituents on the intermediate electron-donating groups, the planarity of the molecules become weaken, but can obtain more compatible with the donor molecules, which could result in the different film-forming properties and morphologies in blending films. This work provides a typical example of introducing unfused core into small molecule acceptors, which is important to design new solution-processable small molecule acceptors in the future. The small molecule acceptors named as TR2F-IC4F, BTOC8-IC4Fand BTOC6C8-IC4F with different substituents on the intermediate electron-deficient groups have different film-forming properties and morphologies in blending films, which correlate with the device performance. When PC 71 BM is used as the third component, the parameters of these devices can be further increased. The power conversion efficiency of the solar cell based on BTOC6C8-IC4F is improved from 7.55% to 10.01% by using PC 71 BM as the third component. • Changing the substituents on the intermediate electron-donating groups in the acceptor molecule can obtain different film-forming properties and morphologies in blending films. • The unfused core emerges low cost and high yield and the existence of noncovalent F⋯H and S⋯O interactions, obtain the planarity of molecules. • The PCEs can further improved for ternary cells, when add PC 71 BM into devices.

On the role of solution-processed bathocuproine in high-efficiency inverted perovskite solar cells

The s-shaped J-V curve of perovskite solar cells can be eliminated using the solvent penetration (SP) process with a PCBM spacer. The experimental results show that the viscosity of the solvent used in the SP process influences the shunt and series resistances simultaneously. In addition, the fill factor of PEDOT:PSS based perovskite solar cells can be largely increased from 40.3% to 70.7% when the bathocuproine (BCP) molecules are dissolved in IPA solvent during the SP process, which significantly increases the power conversion efficiency (PCE) by 98.9%. Besides, the PCE of perovskite solar cells increases from 13.56% to 20.29% when the PEDOT:PSS film is replaced by a P3CT-Na thin film. It is noted that the absorbance spectra, photoluminescence (PL) spectra, time-resolved PL, Raman spectra and X-ray diffraction patterns demonstrate that the BCP molecules penetrate into the PCBM thin film and the PCBM/perovskite interface during the SP process, which increases the electron mobility of the PCBM thin film and passivates the interfacial MA cations in perovskite thin film, respectively. In other words, the solution-processed BCP small molecules are vertically distributed across the PCBM thin film. Our results help completely understand the entire roles of the solution-processed BCP molecules in the high-performance inverted-type perovskite solar cells.

2,2′-(Arylenedivinylene)bis-8-hydroxyquinolines exhibiting aromatic π–π stacking interactions as solution-processable p-type organic semiconductors for high-performance organic field effect transistors

A new class of solution-processable small molecule organic semiconductors that are capable of functioning at low operating voltages (∼6 V) have been developed and their structure–activity correlation has been studied using crystallographic analyses.

Optical and dielectrical properties enhancement of composite films based on MEH-PPV matrix and NBD-Cl small organic molecules

The solution-processable small organic molecules have provoked the interest for their applications in cost-effective optoelectronic devices. This work presents the incorporation of the small organic molecules of 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) into the MEH-PPV polymer matrix to enhance the performance of charge transport. The absorption and photoluminescence spectra were used to investigate the optical properties of the composite films. An optimal weight ratio of (1:0.5) of MEH-PPV:NBD-Cl is found to achieve the best efficiency of charge transfer. For the same composition, the device based on ITO/MEH-PPV:NBD-Cl/Al shows an improvement in current levels and reduction in threshold voltage. Additionally, by fitting the experimental impedance spectroscopy data, informations about the resistances and mobilities were extracted. The effective charge mobility are 1.62 ×10-3 cm² V−1s−1.

Tuning Charge Transport in PVDF-Based Organic Ferroelectric Transistors: Status and Outlook.

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor-acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

An extended-gate type organic transistor with a solution-processable small molecule semiconductor capable of detecting glutathione in water

We herein report glutathione (GSH) detection in water using an organic field-effect transistor (OFET) with a solution-processable small molecule semiconductor. The fabricated OFET operated reproducibly at below ∣3∣ V. The sensing portion for GSH in the OFET is an extended-gate electrode made of gold (Au). Owing to the strong binding of GSH with Au through chemisorption, we have successfully observed a shift of threshold voltage of the OFET upon addition of GSH in an aqueous solution. Because GSH is important for our body, the device would be applied to various research fields including molecular biology, analytical chemistry, and medical diagnostics.

Fluorene based amorphous hole transporting materials for solution processed organic light-emitting diodes

Abstract For an organic light-emitting diode (OLED), enhancing hole injection into the emissive layer for charge balance is a priority to achieve efficient device performance, while solution processing enables organic devices to be fabricated cost-effectively with a large area-size via continuous roll-to-roll manufacturing. However, a limited number of small molecules-based hole-transporting materials (HTMs) were reported with the solution process feasibility and high performance. Here, we demonstrate a series of low cost, efficient and solution processable small molecule HTMs, namely, 2,7-Di(4-fluorophenyl)-9,9-diethylfluorene (Fl-PyEyF), 2,7-Di(3,5-difluorophenyl)-9,9diethylfluorene (di-Fl-PyEyF) and 2,7-Di(2,4,6-trifluorophenyl)-9,9-diethylfluorene (tri-FlPyEyF), designed by using two fluorophenyl, difluorophenyl or trifluorophenyl fragments as common end capping groups with 9,9-Diethylfluorenes cores, respectively, for highly efficient OLEDs. The glass transition temperatures of the molecules were estimated to be higher than 80 °C, which can provide morphologically stable amorphous thin films. All the molecules possess good solubility in common organic solvents. Moreover, all the synthesized molecules have not only appropriate highest occupied molecular orbital energy levels for good hole injection ability, but also sufficient lowest unoccupied molecular orbital for electron blocking capability, adequate ionization potentials (6.15 eV) and suitable triplet energies, which make them suitable hole transporting materials. The current efficiency of a conventional iridium(III) bis(4phenylthieno[3,2-c]pyridinato-N,C-2՜) acetylacetonate based phosphorescent yellow OLED device increases from 40.2 to 61.0 cd/A, an increment of 51.7% by substituting the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the tri-Fl-PyEyF counterpart. These findings suggest that the reported materials can serve as potential molecular HTMs in solution-processed OLED.

Solution processable small molecules as efficient electron transport layers in organic optoelectronic devices

The coated QPSMs form the interfacial dipole between the active layer and the cathode, which can induce the tuned WF of Al, leading to the enhanced electron transport in OSCs and OLEDs.

Carbazole-based green and blue-BODIPY dyads and triads as donors for bulk heterojunction organic solar cells.

Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). Their optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37-1.48 × 105 M-1 cm-1) in solutions with absorption maxima at 586 nm for mono-styryl groups and at 672 nm for di-styryl groups. The introduction of the styryl moieties results in a large bathochromic shift and a significant decrease in the HOMO-LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from -4.99 to -5.16 eV as determined from cyclic voltammetry measurements. Cyclic voltammetry measurements and theoretical calculations demonstrate that the frontier molecular orbital levels of these compounds match with those of PC71BM as the acceptor, supporting their application as donor materials in solution-processed small molecule bulk heterojunction (BHJ) organic solar cells. After the optimization of the active layer, B2:PC71BM and B3:PC71BM based organic solar cells showed an overall power conversion efficiency of 6.41% and 7.47%, respectively. The higher PCE of the B3-based OSC is ascribed to the more balanced charge transport and exciton dissociation, better crystallinity and molecular packing.

Highly Efficient Deep-Blue Electroluminescence Based on a Solution-Processable A-π-D-π-A Oligo(p-phenyleneethynylene) Small Molecule.

The development of solution-processable fluorescent small molecules with highly efficient deep-blue electroluminescence is of growing interest for organic light-emitting diode (OLED) applications. However, high-performance deep-blue fluorescent emitters with external quantum efficiencies (EQEs) over 5% are still scarce in OLEDs. Herein, a novel highly soluble oligo(p-phenyleneethynylene)-based small molecule, 1,4-bis((2-cyanophenyl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-CNPE), is designed, synthesized, and fully characterized as a wide band gap (2.98 eV) and highly fluorescent (ΦPL = 0.90 (solution) and 0.51 (solid-state)) deep-blue emitter. The new molecule is functionalized with cyano (-CN)/2-ethylhexyloxy (-OCH2CH(C2H5)C4H9) electron-withdrawing/-donating substituents, and ethynylene is used as a π-spacer to form an acceptor (A)-π-donor (D)-π-acceptor (A) molecular architecture with hybridized local and charge transfer (HLCT) excited states. Physicochemical and optoelectronic characterizations of the new emitter were performed in detail, and the single-crystal structure was determined. The new molecule adopts a nearly coplanar π-conjugated framework packed via intermolecular "C-H···π" and "C-H···N" hydrogen bonding interactions without any π-π stacking. The OLED device based on 2EHO-CNPE shows an EQEmax of 7.06% (EQE = 6.30% at 200 cd/m2) and a maximum current efficiency (CEmax) of 5.91 cd/A (CE = 5.34 cd/A at 200 cd/m2) with a deep-blue emission at CIE of (0.15, 0.09). The electroluminescence performances achieved here are among the highest reported to date for a solution-processed deep-blue fluorescent small molecule, and, to the best of our knowledge, it is the first time that a deep-blue OLED is reported based on the oligo(p-phenyleneethynylene) π-framework. TDDFT calculations point to facile reverse intersystem crossing (RISC) processes in 2EHO-CNPE from high-lying triplet states to the first singlet excited state (T2/T3 → S1) (hot-exciton channels) that enable a high radiative exciton yield (ηr ∼ 69%) breaking the theoretical limit of 25% in conventional fluorescent OLEDs. These results demonstrate that properly designed fluorescent oligo(p-phenyleneethynylenes) can be a key player in high-performance deep-blue OLEDs.

Role of Oxide/Metal Bilayer Electrodes in Solution Processed Organic Field Effect Transistors

High performance, air stable and solution-processed small molecule 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) based organic field-effect transistors (OFETs) with various electrode configurations were studied in detail. The contact resistance of OFET devices with Ag, Au, WO3/Ag, MoO3/Ag, WO3/Au, and MoO3/Au were compared. Reduced contact resistance and consequently improved performance were observed in OFET devices with oxide interlayers compared to the devices with bare metal electrodes. The best oxide/metal combination was determined. The possible mechanisms for enhanced electrical properties were explained by favorable morphological and electronic structure of organic/metal oxide/metal interfaces.

Negligible Energy Loss During Charge Generation in Small-Molecule/Fullerene Bulk-Heterojunction Solar Cells Leads to Open-Circuit Voltage over 1.10 V

Solution-processable small molecules (SMs) that can serve as donors in bulk-heterojunction (BHJ) solar cells are practical alternatives to their polymer counterparts. However, SM–fullerene blends c...