Small-molecule Organic Semiconductors Research Articles

π-Conjugated Organic-Inorganic Hybrid Photoanodes: Revealing the Photochemical Behavior through In Situ X-Ray Absorption Spectroscopy.

Small-molecule organic semiconductors exhibit great potential for the photoelectrochemical oxidation of water because of their n-type semiconductor nature and their tunable bandgaps. In this work, several head-to-tail bis-coumarins were synthesized and their photophysical properties characterized. Their characteristics as n-type semiconductors were modified by varying the electronic character of substituents at positions 1 and 7, which enabled the energy level of the LUMO and the photoinduced charge-carrier-transfer efficiency to be modulated. X-Ray absorption near-edge structure (XANES) spectroscopy confirmed that the charge transfer is a crucial factor contributing to the resulting activity of the photoanode. The photoactivity of the photoanodes towards water oxidation was revealed to be governed by both the LUMO energy level and transfer efficiency of the photoinduced charge carriers. Among the studied molecules, a bis-coumarin with benzothiophenyl substituents showed the greatest potential as light absorber for photoelectrochemical water oxidation.

Crystallization Control of Organic Semiconductors during Meniscus‐Guided Coating by Blending with Polymer Binder

AbstractSmall molecule organic semiconductors (OSCs) suffer from their uncontrolled nucleation and growth during solution processing limiting their functionality in electronic devices. In this work, a new method is presented based on dip‐coating a blend consisting of OSC and insulating polymer to control the crystallization of the active film for organic field‐effect transistors. A small fraction of amorphous poly(methyl methacrylate) (PMMA) efficiently improves the crystallization of dip‐coated small molecule OSCs, α,ω‐dihexylquaterthiophene (DH4T) and diketopyrrolopyrrole‐sexithiophene (DPP6T). The maximum charge carrier mobilities of dip‐coated OSC:PMMA films are significantly higher than drop‐cast blend ones and comparable with OSC single crystals. The high charge carrier mobility originates from a continuous alignment of the crystalline films and stratified OSC and PMMA layers. The improved crystallization is attributed to two mechanisms: first, the polymer binder leads to a viscosity gradient at the meniscus during dip‐coating, facilitating the draw of solute and thus mass transport. Second, the polymer binder solidifies at the bottom layer, reducing the nucleation barrier height of small molecule OSC. The findings demonstrate that a small fraction of a polymer binder during dip‐coating efficiently improves the crystallization as well as the electronic properties of small molecule OSC films.

Synthesis of organic molecule donor for efficient organic solar cells with low acceptor content

Abstract A new planar A-D-A structured organic small molecule semiconductor (O-SMS) with dialkyl-thiophene substituted benzodithiophene (BDT) as central electron-rich core flanked by relatively electron-deficient units of [1,2,5]thiadiazolo[3,4-c]pyridine (PTz) and terminated with alkyl-bithiophene as π-conjugated end-caps, BDTDPTz, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDTDPTz possesses wider absorption spectra with an optical bandgap of 1.65 eV, lower the highest occupied molecular orbital (HOMO) energy level of −5.42 eV and highly crystalline structures in solid films. The OSCs based on BDTDPTz:PC71BM blend film with a lower PC71BM content of 40% demonstrate a power conversion efficiency (PCE) of 6.28% with a relatively higher open-circuit voltage of 0.868 V and short circuit current density of 12.83 mA cm−2. These results indicate that highly coplanar and crystalline structure of BDTDPTz can effectively reduce the content of fullerene acceptor in the active layer and then enhance the absorption and PCE of the OSCs.

Small molecule semiconductors for organic photovoltaics: a truncation approach

Solution-processable small molecule (SM) organic semiconductors for organic photovoltaics have been in the spotlight for several years. In particular, SM semiconductors have been developed in an attempt to control their film morphology, study their intermolecular interactions, and synthesize new electron donor and electron acceptor subunits. SMs have many advantages over polymers including well-defined molecular structures, monodispersity, and no batch-to-batch dependence. Although SM semiconductors can be designed by truncation from polymers, such examples have rarely been reported. Herein we designed SM semiconductors by truncating a representative polymer, Poly[4-(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b’]dithiophen-2-yl)-alt-benzo[c][1,2,5]thiadiazole] (PBDTBT). Based on density functional theory (DFT) calculations, 2,1,3-benzothiadiazole (BT) was chosen as an electron acceptor subunit instead of thieno[3,4-c]pyrrole-4,6-dione (TPD). The SM semiconductors were end-capped with pyridine derivatives. Thermal, optical and electrochemical properties of these materials were examined to confirm the degradation temperature, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energy levels, and the band gaps. In particular, the benzyloxyl pyridine-capped semiconductor (SM1) exhibited power conversion efficiency (PCE) of 1.92% which is higher than those shown by the corresponding polymer PBDTBT (0.90% and 1.71%).

Atom Transfer Radical Polymerization (ATRP) Catalyzed by Visible Light-Absorbed Small Molecule Organic Semiconductors.

Atom transfer radical polymerization is successfully conducted with an organic semiconductor (OS)-based visible light activated photocatalyst via a precisely designed catalytic cycle. The living nature of the polymerization by the OS type photocatalyst is proved by the linearly-increasing molecular weight with consumption of the monomer, the expected molecular weight from MALDI-TOF measurements, and the chain extending reaction, as well as block copolymer synthesis. Benefitting from clear and measurable reductive potential as well as the oxidative potential of the OS-type photocatalyst, the intrinsic demand for "living radical polymerization" is discussed and examined, which indicates the diversity and great potential of OS-type photocatalysts for organic synthesis.

Extended and Modulated Thienothiophenes for Thermally Durable and Solution-Processable Organic Semiconductors

Herein, we report the rational design of practical small-molecule organic semiconductors based on a π-electron skeleton of benzothieno[3,2-b]naphtho[2,3-b]thiophene (BTNT) whose layered herringbone (LHB) packing is intentionally modulated by the designated asymmetric substitutions with the phenyl group and normal alkyl chains. The thermal stability of the hybrid BTNT core is high enough, as it lies between those of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and benzothieno[3,2-b]benzothiophene (BTBT), although the solvent solubility for the substituted BTNT at ordinary 2,8-substituting positions by the alkyl chain and phenyl group remains extremely low. We show in the BTBT and BTNT derivatives that the tuning of the substituting position works to slightly bend the rodlike organic semiconductor molecules and thus to decrease the cohesive energy of the crystals with retention of the bilayer-type herringbone (b-LHB) packing for the asymmetric rodlike molecules. This modification eventually leads to an increase in solvent solubility, a decrease in phase transition temperature, and the suppression of liquid-crystalline phases at high temperatures. By using the substituting effect, we successfully achieve the organic semiconductors with modulated alkylated Ph-BTNT that exhibits both a sufficiently high solvent solubility and a sufficiently high thermal stability. The variation in the crystal packing also enhances the intermolecular transfer integrals along the T-shaped contacts within the intralayer herringbone packing. Spin coating of the material under ambient conditions affords high-performance bottom-gate, bottom-contact organic thin-film transistors, exhibiting high thermal durability in the device characteristics below 150 °C. The obtained devices also exhibit a higher mobility, a lower threshold voltage, and a smaller subthreshold swing, by initial thermal treatment at 140 °C, composed to those of the as-prepared films, because the thermal treatment stabilizes the b-LHB packing and thus suppresses the residual minority holes and shallow traps. These findings should be crucial in the design and development of organic semiconductor materials for practical printed electronics applications.

Stencil lithography for organic thin-film transistors with a channel length of 300 nm

For some of the more demanding applications envisioned for organic transistors, lateral device dimensions of less than 1 μm may be necessary or should at least be explored in order to evaluate the scalability of organic transistors. Using stencil lithography based on high-resolution silicon stencil masks and employing two small-molecule organic semiconductors with good long-term air stability, we have fabricated organic p-channel and n-channel transistors with a channel length of 0.3 μm. Owing to the small channel length, the transistors have large channel-width-normalized transconductances (1.5 S/m for the p-channel and 0.2 S/m for the n-channel transistors). In addition, the transistors have steep subthreshold slopes (80 and 200 mV/decade) and large on/off current ratios (106).

Large-Scale Computational Screening of Molecular Organic Semiconductors Using Crystal Structure Prediction

Predictive computational methods have the potential to significantly accelerate the discovery of new materials with targeted properties by guiding the choice of candidate materials for synthesis. Recently, a planar pyrrole-based azaphenacene molecule (pyrido[2,3-b]pyrido[3′,2′:4,5]pyrrolo[3,2-g]indole, 1) was synthesized and shown to have promising properties for charge transport, which relate to stacking of molecules in its crystal structure. Building on our methods for evaluating small-molecule organic semiconductors using crystal structure prediction, we have screened a set of 27 structural isomers of 1 to assess charge mobility in their predicted crystal structures. Machine-learning techniques are used to identify structural classes across the landscapes of all molecules and we find that, despite differences in the arrangement of hydrogen bond functionality, the predicted crystal structures of the molecules studied here can be classified into a small number of packing types. We analyze the predicted p...

Inorganic Polymer Micropillar-Based Solution Shearing of Large-Area Organic Semiconductor Thin Films with Pillar-Size-Dependent Crystal Size.

It is demonstrated that the crystal size of small-molecule organic semiconductors can be controlled during solution shearing by tuning the shape and dimensions of the micropillars on the blade. Increasing the size and spacing of the rectangular pillars increases the crystal size, resulting in higher thin-film mobility. This phenomenon is attributed as the microstructure changing the degree and density of the meniscus line curvature, thereby controlling the nucleation process. The use of allylhybridpolycarbosilane (AHPCS), an inorganic polymer, is also demonstrated as the microstructured blade for solution shearing, which has high resistance to organic solvents, can easily be microstructured via molding, and is flexible and durable. Finally, it is shown that solution shearing can be performed on a curved surface using a curved blade. These demonstrations bring solution shearing closer to industrial applications and expand its applicability to various printed flexible electronics.

Employing Pneumatic Nozzle Printing for Controlling the Crystal Growth of Small Molecule Organic Semiconductor for Field‐Effect Transistors

AbstractA pneumatic nozzle‐based printing technique that can control the crystallization of small molecule organic semiconductor thin films during deposition is reported. By tuning the printing speed and the substrate temperature, large‐sized organic crystals are grown along the printing direction, resulting in field‐effect mobility enhancement by 50 times compared to spin‐coated films for 6,13‐bis(triisopropylsilylethynyl)‐pentacene. Furthermore, silver electrodes using the technique to fabricate printed flexible inverters with a gain as high as 16 are printed.

Evidence of thermal transport anisotropy in stable glasses of vapor deposited organic molecules

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.

Real-Time Structural and Optical Study of Growth and Packing Behavior of Perylene Diimide Derivative Thin Films: Influence of Side-Chain Modification

We study the growth of two n-type small-molecule organic semiconductors from the perylene diimide family: N,N′-bis-(2-ethylhexyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIR-CN2) and N,N′-1H,1H-perfluorobutyl-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIF-CN2) whose chemical structures differ only in the imide substituents, branched alkyl chains −C8H16 and linear fluoroalkyl chains −C4F7H2, respectively. Both types of substituents introduce some degree of steric hindrance for intermolecular interactions, affecting solid-state packing during thin film formation, and thus induce specific structure-dependent optoelectronic properties in thin films. The transition from an amorphous structure to crystalline domains with strong intermolecular coupling was followed in situ and in real time during growth. We investigated the structural and morphological properties by X-ray diffraction and atomic force microscopy as a function of the substrate temperature and chemical structure. We examined the relationsh...

Role of Polymorphism and Thin-Film Morphology in Organic Semiconductors Processed by Solution Shearing

Organic semiconductors (OSCs) are promising materials for cost-effective production of electronic devices because they can be processed from solution employing high-throughput techniques. However, small-molecule OSCs are prone to structural modifications because of the presence of weak van der Waals intermolecular interactions. Hence, controlling the crystallization in these materials is pivotal to achieve high device reproducibility. In this perspective article, we focus on controlling polymorphism and morphology in small-molecule organic semiconducting thin films deposited by solution-shearing techniques compatible with roll-to-roll systems. Special attention is paid to the influence that the different experimental deposition parameters can have on thin films. Further, the main characterization techniques for thin-film structures are reviewed, highlighting the in situ characterization tools that can provide crucial insights into the crystallization mechanisms.

Growth of alkyl-monosubstituted thiophene/phenylene co-oligomer crystals and their device application

We synthesized a novel small-molecule organic semiconductor, which is soluble in organic solvents at room temperature under normal pressure. We demonstrated that its high-quality crystalline films can be directly grown on substrates using various solution techniques such as solution casting, slow evaporation, and edge casting. We applied crystals to FETs with a bottom- or top-contact configuration, revealing that the carrier mobility ranged from ∼10−4 to ∼10−2 cm2 V−1 s−1.

Long-Lived Correlated Triplet Pairs in a π-Stacked Crystalline Pentacene Derivative.

Singlet fission is the spin-conserving process by which a singlet exciton splits into two triplet excitons. Singlet fission occurs via a correlated triplet pair intermediate, but direct evidence of this state has been scant, and in films of TIPS-pentacene, a small molecule organic semiconductor, even the rate of fission has been unclear. We use polarization-resolved transient absorption microscopy on individual crystalline domains of TIPS-pentacene to establish the fission rate and demonstrate that the initially created triplets remain bound for a surprisingly long time, hundreds of picoseconds, before separating. Furthermore, using a broadband probe, we show that it is possible to determine absorbance spectra of individual excited species in a crystalline solid. We find that triplet interactions perturb the absorbance, and provide evidence that triplet interaction and binding could be caused by the π-stacked geometry. Elucidating the relationship between the lattice structure and the electronic structure and dynamics has important implications for the creation of photovoltaic devices that aim to boost efficiency via singlet fission.

Engineered exciton diffusion length enhances device efficiency in small molecule photovoltaics

A post processing method of solvent vapor annealing (SVA) enhances the diffusion length (LD) and domain size in small molecule organic semiconductors which leads to an enhancement in device performance in bulk heterojunction organic photovoltaics.

Tailoring the framework of organic small molecule semiconductors towards high-performance thermoelectric composites via conglutinated carbon nanotube webs

Regulating the structure of p-type organic small molecules to generate thermoelectric composites for achieving a high power factor of 113.2 μW m−1 K−2.

How accurate are energies of HOMO and LUMO levels in small-molecule organic semiconductors determined from cyclic voltammetry or optical spectroscopy?

Examined are relations between the energies of the transport gaps (i.e., the gaps determined by UPS/IPES measurements and taken for ‘true’ HOMO-LUMO gaps), and the electrochemical gaps (determined from CV experiments) and optical gaps (determined from UV-VIS optical spectroscopy). The statistics, based on literature data for several small (non-polymeric) molecules, shows that, on average, the electrochemical measurements underestimate the gap energies by ca. 16%, and the optical gap is smaller than the transport gap by ca. 37%. These differences seriously affect the accuracy of determination of energies of HOMO and LUMO levels by indirect methods. The concordance with the results obtained from the UPS/IPES measurements can be improved by using correction factors put forward in the present paper.

Reducing Trap-Assisted Recombination in Small Organic Molecule-Based Photovoltaics by the Addition of a Conjugated Block Copolymer.

The performance of organic photovoltaics (OPVs) based on the small-molecule organic semiconductor p-DTS(FBTTh2 )2 is greatly improved by the addition of a conjugated block copolymer composed of difluoroquinoxaline and thienopyrrolodione blocks (D130). The power conversion efficiency (PCE) of the p-DTS(FBTTh2 )2 -based OPV is improved from 5.08% to 6.75% by the addition of 5 wt% D130 to the photoactive layer, which is composed of p-DTS(FBTTh2 )2 and a fullerene derivative. Current-voltage and grazing incidence wide-angle X-ray scattering analyses revealed that the addition of D130 significantly reduces the trap density of the device and changes the packing orientation of p-DTS(FBTTh2 )2 from mostly edge-on to partially face-on. These changes greatly improve the charge carrier mobility of the OPV, indicating that D130 is highly compatible with p-DTS(FBTTh2 )2 . Furthermore, the addition of D130 improve the photostability of the OPV by reducing the burn-in loss under a light soaking intensity of 1 sun. The D130-based OPV maintained 34% of its initial PCE after a light soaking test for 858 h. In contrast, the PCE of the OPV without D130 reduced to 14% of its initial efficiency in the same time period.

Large bipolaron density at organic semiconductor/electrode interfaces

Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a bipolaron sheet density >1010 cm−2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought.