Small-molecule Semiconductors Research Articles

A simple and robust approach to reducing contact resistance in organic transistors

Efficient injection of charge carriers from the contacts into the semiconductor layer is crucial for achieving high-performance organic devices. The potential drop necessary to accomplish this process yields a resistance associated with the contacts, namely the contact resistance. A large contact resistance can limit the operation of devices and even lead to inaccuracies in the extraction of the device parameters. Here, we demonstrate a simple and efficient strategy for reducing the contact resistance in organic thin-film transistors by more than an order of magnitude by creating high work function domains at the surface of the injecting electrodes to promote channels of enhanced injection. We find that the method is effective for both organic small molecule and polymer semiconductors, where we achieved a contact resistance as low as 200 Ωcm and device charge carrier mobilities as high as 20 cm2V−1s−1, independent of the applied gate voltage.

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.

Enhancing the performance of solution-processed organic thin-film transistors by blending binary compatible small molecule semiconductors

Abstract Physical blending is a facile and effective way to improve the performance of solution processed organic thin-film transistors (OTFTs). Blending small molecule semiconductors with soluble polymers has been extensively studied in recent years. However, blending between binary small molecule semiconductors is rare due to the difficulty to obtain ideal thin films. Herein, we systematically investigate the blending effects on the morphologies of thin films and their field-effect performance by using two small molecule semiconductors, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, (C12-Ph-BTBT), which have the same aromatic skeleton. Molecular ordering and better crystallinity are observed in most of spin-coated blend thin films, thanks to the enhanced molecular interaction after blending. As a result, OTFTs based on blend thin films exhibit improved performance in most cases, with the highest average hole mobility about 1.5 cm2 V−1 s−1 demonstrated. Further device performance improvements are demonstrated by blending polystyrene with Ph-BTBT and C12-Ph-BTBT blends. The results here indicate that blending between small molecule semiconductors with compatible fused ring structures may be a promising strategy to enhance the performance of organic transistors.

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%).

(Invited) Understanding the Correlation of Molecule Structure and Charge Transport Properties in Organic Thin-Film Transistors

Recent development of organic semiconductor materials has led to the remarkable enhancement of mobility over 10 cm2/Vs in organic thin-film transistors (OTFTs). Despite these achievements, a thorough understanding of the correlation between molecular structure and charge transport properties is still not achieved. It is essential that we have a comprehensive picture of the structure-property relationship for these materials to further improve the mobility. However, this is a very challenging task since the resulting effects of molecule structure changes can be so manifold. In this work, we investigated the structure-property correlation by finely tuning the molecule structure of organic semiconductors and comparing the performance of TFT devices, aiming to deepen our knowledge of the structure-property relationship and to provide guidance for optimizing the synthesis of high-mobility organic semiconductors. Specifically, we have studied how the molecule length and molecule symmetry affect the charge transport properties of small-molecule semiconductors. We have also researched the effect of the donor unit length on the charge transport properties of donor-acceptor (D-A) copolymers. The work has at least clarified how these factors such as molecule length, molecule symmetry and the length of donor units influence charge transport in organic semiconductors. Our work thus is enlightening for people to understand the charge transport physics in OTFTs and it is also instructive for chemists to optimize the synthesis of high-mobility organic semiconductors.

High‐Mobility, Solution‐Processed Organic Field‐Effect Transistors from C8‐BTBT:Polystyrene Blends

AbstractOrganic field‐effect transistors based on aligned small molecule semiconductors have shown high charge carrier mobilities in excess of 10 cm2 V−1 s−1. This makes them a viable alternative to amorphous inorganic semiconductors especially if a high reproducibility can be achieved. Here, the optimization of high mobility organic field‐effect transistors based on the organic semiconductor 2,7‐dioctyl[1]benzothieno[3,2‐b] benzothiophene (C8‐BTBT) via the addition of a polymer additive to the printing solution is reported. Specifically, films and devices are compared based on solutions of the neat semiconductor and the blend with polystyrene and shear‐coated devices with excellent device characteristics and gate‐voltage‐independent mobility values reaching 12 cm2 V−1 s−1 are shown, which are the highest reported values for C8‐BTBT‐based films prepared by a scalable, solution‐based process.

Organic-semiconductor: Polymer-electret blends for high-performance transistors

As compared with polymer semiconductors, solution-processed small-molecule semiconductors usually have poorer film-formation properties, which induces wide variations in device performance in terms of mobility and threshold voltage, thus severely limiting their commercial applications. In this work, we propose an easily accessible method to improve the performance and reduce the variability of small-molecule organic field-effect transistors (OFETs) by blending organic semiconductors with an insulator polymer, which is subsequently post-treated by gate stress to generate an electret. By blending the organic semiconductor 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT) with the insulator polystyrene, uniform transport layers with vertically phase segregated morphology are obtained, from which the mobility and threshold voltage of OFETs are largely manipulated. The OFETs exhibit field-effect mobilities as high as 7.5 cm2·V−1·s−1 with an on/off ratio exceeding 106 in ambient conditions. This double-layer structure provides an appropriate architecture for applying gate-stress to inject charges into the insulating layer, forming an electret. The generation of the electret is thermally accelerated and thus can be easily realized under moderate gate-stress at elevated temperature (e.g., 60 °C). After cooling, the electret layer serves as a floating-gate, which not only continuously tunes the threshold voltage and field-effect mobility, but also helps minimize the contact resistances and optimize the subthreshold swing. As an application of this method, a digital inverter is built and its performance is optimized via in situ tuning of its individual transistors.

Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV–vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.

Scalable Directed Assembly of Highly Crystalline 2,7-Dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) Films.

Assembly of organic semiconductors with ordered crystal structure has been actively pursued for electronics applications such as organic field-effect transistors (OFETs). Among various film deposition methods, solution-based film growth from small molecule semiconductors is preferable because of its low material and energy consumption, low cost, and scalability. Here, we show scalable and controllable directed assembly of highly crystalline 2,7-dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) films via a dip-coating process. Self-aligned stripe patterns with tunable thickness and morphology over a centimeter scale are obtained by adjusting two governing parameters: the pulling speed of a substrate and the solution concentration. OFETs are fabricated using the C8-BTBT films assembled at various conditions. A field-effect hole mobility up to 3.99 cm2 V-1 s-1 is obtained. Owing to the highly scalable crystalline film formation, the dip-coating directed assembly process could be a great candidate for manufacturing next-generation electronics. Meanwhile, the film formation mechanism discussed in this paper could provide a general guideline to prepare other organic semiconducting films from small molecule solutions.

A Lewis acid-base chemistry approach towards narrow bandgap dye molecules

Although it usually involves troublesome and time-consuming synthesis work, molecular structure tailoring has been the mainstream tool to adjust the optical absorption and bandgap of organic semiconductors. Herein, we provided a convenient and feasible method to prepare narrow bandgap small-molecule semiconductors by Lewis acid-base chemistry and the optical absorption was adjusted by the stoichiometry of Lewis acid-base reactions. In this work, three pyridine capped diketopyrrolopyrrole (DPP) molecules, i.e., DPPPy-Ph-F, DPPPy-Ph-3F, and DPPPy-Py-F, with medium bandgap were synthesized to coordinate with Lewis acid of B(C6F5)3 (BCF). By coordinating with BCF, as demonstrated by 1H, 19F, and 11B NMR spectra, the bandgaps of these DPP molecules were depressed largely. For DPPPy-Ph-F, with two pyridine units flanking symmetrically, 2 eq of BCF converted the two pyridine nitrogens stoichiometrically to Lewis acid adducts, along with absorption bands red-shifted and bandgap depressed (1.89 vs. 1.80 eV). For DPPPy-Ph-3F, with two pyridine units flanked and trifluorobenzene capped, adding 2 eq of BCF was unable to convert the two pyridine nitrogens to BCF adducts completely, due to the strong electron-drawing ability of trifluorobenzene substituent on pyridine, reducing its basicity. After adding excess BCF to coordinate with nitrogens, the bandgaps of DPPPy-Ph-3F also decreased from 1.92 eV to 1.84 eV. For DPPPy-Py-F, with four pyridine units flanked, 4 eq of BCF were required to bind the pyridine nitrogens. Accordingly, the red-shift of absorption bands and the depression of optical bandgaps (1.96 vs. 1.75 eV) for DPPPy-Py-F were most striking among the three molecules.

Tunable Electron Donating and Accepting Properties Achieved by Modulating the Steric Hindrance of Side Chains in A-D-A Small-Molecule Photovoltaic Materials

Modulation of the electron donating and accepting properties of organic semiconductors is an important topic in the field of organic photovoltaics. Although the small-molecule semiconductors (SMs) with an A-D-A structure can act as either electron donor or acceptor in organic photovoltaic (OPV) devices, the reason why molecules with similar conjugated structures play different roles remains unclear. In this work, we designed and synthesized two A-D-A SMs named BTCN-O and BTCN-M, which have an identical backbone and differ in the alkyl substitution position. BTCN-O and BTCN-M demonstrate similar optical absorption spectra in solution and molecular energy levels in a solid film. BTCN-O forms an ordered lamellar packed structure with compact π–π stacking, whereas BTCN-M demonstrates only a weak π–π stacking effect in solid film. We also investigated their photovoltaic properties by blending each with a polymer donor, PBDB-T, and a fullerene acceptor, PC71BM, and found that the electron donating and accepting...

Interconnecting semiconducting molecules with non-conjugated soft linkers: a way to improve film formation quality without sacrifice in charge mobility.

The formation of high quality homogeneous active component films is essential to the fabrication of many organic optoelectronic devices, especially those having a large area module. However, most small molecule (SM) semiconductors are unable to achieve such a goal via solution processing because their large and rigid π-conjugated structures generally drive them to aggregate or crystallize into inhomogeneous domains in the process. In this work, a SM semiconductor (SM-DPP6T) based on a diketopyrrolopyrrole (DPP) center with three thiophene units on both sides encounters the same problem. However, when its molecules are interconnected with soft alkyl linkers and change into a poly(rod-coil) polymer, PRC-DPP6T, the problem is solved. Compared with SM-DPP6T, PRC-DPP6T can form a high quality homogeneous film with much better uniformity and coverage on silicon wafers by spin-coating. Moreover, the so-prepared PRC-DPP6T field-effect transistors displayed a much narrower performance distribution and comparable mobility when compared with those based on SM-DPP6T (0.17 vs. 0.15 cm2 V−1 s−1). These results demonstrate that the interconnection of SM semiconductor molecules with soft non-conjugated linkers is a promising way to improve film formation quality while keeping mobility intact.

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.

Ambipolar charge transport in a bis-diketopyrrolopyrrole small molecule semiconductor with tunable energetic disorder.

Energetic disorder and activation energy are important parameters that influence the charge carrier mobility in organic semiconductors. Herein, we investigate temperature-dependent ambipolar charge transport alongside its thermal activation energy in organic field-effect transistors (OFETs) based on a diketopyrrolopyrrole (DPP) based small molecule BTDPP2. The determined energetic disorder in BTDPP2 is analogous to those of highly crystalline molecules, such as pentacene, while lower than those of widely used fullerene derivatives (PCBM) or semi-crystalline polymers, such as P3HT. We demonstrate that the energetic disorder and activation energy in BTDPP2 are both impacted by the crystallinity, which is tuned by thermal annealing; moreover, to a certain extent, these two parameters can reduce with increasing the structural order. Moreover, the energetic disorder tends to decrease when BTDPP2 is subjected to thermal annealing. Through comparing the electron transport in BTDPP2 based OFETs and vertical diodes, in which the electron densities differentiate substantially, the different activation energies are roughly described in terms of achievable carrier densities in these two devices. To the best of our knowledge, this aspect has not been addressed on the electron transport in molecular semi-conductive materials. Our results shine light on fundamental understandings of charge transport properties in solution processed small molecules holding promise for opto-electronic applications.

Direct Writing and Aligning of Small-Molecule Organic Semiconductor Crystals via "Dragging Mode" Electrohydrodynamic Jet Printing for Flexible Organic Field-Effect Transistor Arrays.

Patterning and aligning of organic small-molecule semiconductor crystals over large areas is an important issue for their commercialization and practical device applications. This Letter reports "dragging mode" electrohydrodynamic jet printing that can simultaneously achieve direct writing and aligning of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystals. Dragging mode provides favorable conditions for crystal growth with efficient controls over supply voltages and nozzle-to-substrate distances. Optimal printing speed produces millimeter-long TIPS-PEN crystals with unidirectional alignment along the printing direction. These crystals are highly crystalline with a uniform packing structure that favors lateral charge transport. Organic field-effect transistors (OFETs) based on the optimally printed TIPS-PEN crystals exhibit high field-effect mobilities up to 1.65 cm2/(V·s). We also demonstrate the feasibility of controlling pattern shapes of the crystals as well as the fabrication of printed flexible OFET arrays.

Charge-transport properties of 4-(1,2,2-tri-phenyl-vinyl)-aniline salicylaldehyde hydrazone: tight-packing induced molecular 'hardening'.

Based on first-principles calculations, the relationship between molecular packing and charge-transport parameters has been investigated and analysed in detail. It is found that the crystal packing forces in the flexible organic molecule 4-(1,2,2-triphenylvinyl)-aniline salicylaldehyde hydrazone (A) can apparently overcome the dynamic intramolecular rotations and the intramolecular steric repulsion, effectively enhancing the molecular rigidity and decreasing the internal reorganization energy. The conducting properties of A have also been simulated within the framework of hopping models, and the calculation results show that the intrinsic electron mobility in A is much higher than the corresponding intrinsic hole mobility. These theoretical investigations provide guidance for the efficient and targeted control of the molecular packing and charge-transport properties of organic small-molecule semiconductors and conjugated polymeric materials.

Angle-Dependent Photoluminescence Spectroscopy of Solution-Processed Organic Semiconducting Nanobelts

We report an anomalous anisotropy in photoluminescence (PL) from crystalline nanobelt of an organic small-molecule semiconductor, 6,13-dichloropentacene (DCP). Large-area well-aligned DCP nanobelt arrays are readily formed by self-assembly through solution method utilizing the strong anisotropic interactions between molecules. The absorption spectrum of the arrays suggests the formation of both intramolecular exciton and intermolecular exciton. However, the results of angle-dependent PL spectroscopy indicate that the PL arises only from the relaxation of intramolecular exciton, which has an optical transition dipole moment with an angle of 115° with the long-axis of the nanobelts. The angular dependence of PL signals follows a quartic rule (IPL(θ) ∝ cos4(θ – 115)) and agrees well with the optical selection rule of individual DCP molecules. The measured polarization ratio ρ from the individual nanobelts is on average 0.91 ± 0.02, superior to that of prior-art organic semiconductors. These results provide new insights into exciton behavior in 1D π–π stacking organic semiconductors and demonstrate DCP’s great potential in the photodetectors and optical switches for large-scale organic optoelectronics.

Polarization-Induced Transport: A Comparative Study of Ferroelectric and Non-Ferroelectric Dielectric-Gated Organic Field-Effect Transistors

ABSTRACTA comparative study of ferroelectric and non-ferroelectric-gated organic field-effect transistors (FETs) have been carried out by using a small molecule semiconductor 6,13 bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) to understand the fundamental aspects of carrier transport in FET architectures. Temperature-dependent current-voltage characteristics from non-ferroelectric dielectric-gated FETs show a clear activated transport, independent of the dielectric constant. While using the ferroelectric dielectric polymer poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE), where the dielectric constant may be tuned by changing the temperature, a negative temperature coefficient of the carrier mobility is observed beyond 200 K. The polarization fluctuation dominant transport inherent to a ferroelectric dielectric in conjunction with the discrete nature of traps in TIPS-pentacene results in an effective de-trapping of the shallow trap states into more mobile states.

Biscoumarin-containing acenes as stable organic semiconductors for photocatalytic oxygen reduction to hydrogen peroxide

Organic small-molecule semiconductors can be photocatalysts in their own right, oxidizing water while reducing oxygen to hydrogen peroxide under visible light illumination.

Modulating the Surface via Polymer Brush for High‐Performance Inkjet‐Printed Organic Thin‐Film Transistors

A strategy is reported to control the morphologies of inkjet‐printed small organic molecule semiconductor by modifying printed surfaces with polymer brushes. Polystyrene (PS) brushes with different chain lengths from 1.66 to 7.35 nm are grafted onto a dielectric surface via surface‐initiated atom transfer radical polymerization to form uniform, hydrophobic surfaces. Single droplets of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐pentacene) are then inkjet‐printed onto these PS brush‐modified substrates. TIPS‐pentacene crystals are obtained with different morphologies depending on the chain length of the PS brushes. Short PS chains behave like self‐assembled monolayers on the dielectric surface such that small crystals are formed with random orientations. Longer PS chains become partially extended in the solvent altering ink spreading, contact‐line pinning, solute migration, and ultimately crystallization. Large self‐assembled crystals are generated with highly oriented structures. Organic thin film transistors are fabricated by printing single dots of TIPS‐pentacene. The device based on PS brushes of 5.14 nm exhibits the optimal device performances with an average mobility of 0.35 ± 0.23 cm2 V−1 s−1. These results demonstrate that these polymer brushes provide a route toward further understanding inkjet‐printing techniques for high‐performance organic electronics.