High-performance diketopyrrolopyrrole-based organic field-effect transistors for flexible gas sensors

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm²/V s on a glass substrate and 2.98 ± 0.19 cm²/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μ_FET, V_Th and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH₃ gas sensors. As the concentration of NH₃ increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼10⁷ to ∼10⁹ Ω at V_SD = −5 V and V_GS = −5 V.     

High mobility ambipolar organic field-effect transistors with a nonplanar heterojunction structure

Ambipolar organic field-effect transistors (OFETs) based on a bilayer structure of highly crystalline small molecules, n-type α,ω-diperfluorohexylquaterthiophene (DFH-4T) and p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are investigated. By employing DFH-4T/DNTT as the bottom/top layers and appropriate high work function (WF) electrodes in a bottom-gate, top-contact configuration, the superior ambipolar characteristics with matched electron and hole mobilities of 1–1.1 cm² V⁻¹ s⁻¹ are achieved. Intriguingly, this high-performance device exhibits a unique feature of an extremely rough, nonplanar heterojunction in the DFH-4T/DNTT combination and a large electron injection barrier from the high WF electrodes to DFH-4T, suggesting some underlying mechanisms for the effective charge transport and injection. The electrical and structural analyses reveal that the crystal packing of the bottom DFH-4T layer supports the growth of a high-quality DNTT crystal network for high-mobility hole transport upon the nonplanar heterojunction, and also enables the formation of an enlarged organic/metal contact surface for efficient electron injection from the high WF electrodes, as the key attributes leading to an overall excellent ambipolar behavior. The effect of intrinsic charge accumulation at the heterojunction interface on the ambipolar conduction is also discussed. Furthermore, a complementary-like inverter constructed with two DFH-4T/DNTT ambipolar OFETs is demonstrated, which shows a gain of 30.     

Methylated conjugated polymers based on diketopyrrolopyrrole and dithienothiophene for high performance field-effect transistors

In this work, a series of conjugated polymers based on diketopyrrolopyrrole (DPP) and dithienothiophene were designed for application in field-effect transistors (FETs). Owing to the synthetic nature of DPP units, the DPP polymers here contain different aromatic linkers with thiophene and methylthiophene, resulting in non-methylated and methylated DPP polymers. Methylated DPP polymers were found to show good crystalline properties and provide high hole mobilties up to 5.32 cm² V⁻¹ s⁻¹ in FETs, while non-methylated polymer exhibits a hole mobility of 3.16 cm² V⁻¹ s⁻¹. Especially, the polymer containing asymmetric linkers presents “face-on” orientation in thin films but provides the highest mobility. Our results reveal that the polymers incorporating methyl units as side chains can be used to realize high carrier mobility in FETs.     

Photo- and electroluminescence of ambipolar,high-mobility,donor-acceptor polymers

Donor-acceptor polymers with narrow bandgaps are promising materials for bulk heterojunction solar cells and high-mobility field-effect transistors. They also emit light in the near-infrared. Here we investigate and compare the photoluminescence and electroluminescence properties of different narrow bandgap (<1.5 eV) donor-acceptor polymers with diketopyrrolopyrrole (DPP), isoindigo (IGT) and benzodipyrrolidone (BPT) cores, respectively. All of them show near-infrared photoluminescence quantum yields of 0.03–0.09% that decrease with decreasing bandgap. Bottom-contact/top-gate field-effect transistors show ambipolar charge transport with hole and electron mobilities between 0.02 and 0.7 cm² V⁻¹ s⁻¹ and near-infrared electroluminescence. Their external quantum efficiencies reach up to 0.001%. The effect of polaron quenching and other reasons for the low electroluminescence efficiency of these high mobility polymers are investigated.     

New diketopyrrolopyrrole-based organic dyes for highly efficient dye-sensitized solar cells

Three diketopyrrolopyrrole (DPP) dyes (ICD-3, ICD-4 and ICD-5) with a D-π-A conjugation were designed and synthesized, where a symmetric phenyl-DPP-phenyl unit was used to connect a substituted diphenylamine and a thienyl acrylic acid, and two n-hexyl or 2-ethyl-hexyl chains were introduced on the periphery of the DPP macrocycle. The dyes were characterized by photophysical, electrochemical, and density functional theory calculations. Among the three dyes, the ICD-5-based DSC afforded the best photovoltaic performance: a short circuit photocurrent density (J_sc) of 16.34 mA/cm², an open circuit voltage (V_oc) of 753 mV, and a fill factor (FF) of 0.74, corresponding to an overall conversion efficiency (η) of 9.10% using I⁻/I₃⁻ redox couple-based liquid electrolyte under AM 1.5 conditions. The experimental results demonstrate that the DPP-based sensitizer is a promising option for DSCs, and rational molecular engineering is crucial for constructing highly efficient charge transfer sensitizers.     

Triphenylamine modified bis-diketopyrrolopyrrole molecular donor materials with extended conjugation for bulk heterojunction solar cells

Two new solution-processable enlarged π-conjugated donor–acceptor (D–A) organic small molecules consisting of dialkoxysubstituted benzo[1,2-b:4,5-b′]dithiophene (BDT) or dioctyltertthiophene (3T) as the central donor units, diketopyrrolopyrrole (DPP) as the acceptor unit and triphenylamine (TPA) as the terminal conjugated segment, TPA–DPP–BDT and TPA–DPP–3T, were designed and synthesized. Both small molecules possess broad absorption ranging from 300 to 800 nm with an optical band at approximately 1.50 eV and relatively low HOMO energy levels from −5.12 to approximately −5.16 eV. Expectedly, the UV–Vis absorption onset (810 nm) of TPA–DPP–BDT is largely red-shifted (60 nm) relative to that (750 nm) of previously reported BDT(TDPP)₂, which consists of BDT and DPP units. Unlike most of the TPA based molecules, strong molecular aggregation was observed in the solid state for both small molecules. In addition, atomic force microscopy (AFM) and X-ray diffraction (XRD) investigations indicated that TPA–DPP–3T and TPA–DPP–BDT exhibit good miscibility with fullerene derivatives. The organic solar cells based on TPA–DPP–BDT/PC₆₁BM(1:1) demonstrated power conversion efficiencies as high as 4.04% with a short-circuit current density (J_sc) of 11.40 mA cm⁻² and a fill factor (FF) of 53.2% when the active layer of the cell was annealed at 130 °C for 10 min.     

In-plane light emission of organic light-emitting transistors with bilayer structure using ambipolar semiconducting polymers

The concept of using an ambipolar bilayer semiconducting heterostructure in organic light-emitting transistors (OLETs) is introduced to provide a new approach to achieve surface emission. The properties of top-gate-type bilayer OLETs with ambipolar materials based on two types of fluorene-type polymers used as an emissive layer and an electron blocking layer are investigated. Line-shaped yellow–green emission occurs near a hole-injection electrode. When hole transport is dominant in the upper layer which acts as an electron blocking layer, and electrons are injected into the lower layer, an in-plane light-emitting pattern is observed. The measured in-plane emission zone confirms that both hole and electron transport are determined to occur mainly along the different organic layers between the source and drain electrodes, and an in-plane recombination zone of electrons and holes exists near the bilayer organic interface. This work is anticipated to be useful for the development of in-plane light-emitting transistors.     

Investigation of VOPcPhO as an acceptor material for bulk heterojunction solar cells

In this study, we have successfully demonstrated a new system of donor–acceptor blend for bulk heterojunction solar cells of poly(3-hexylthiophene) (P3HT) by using vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO) as acceptor material. A broad absorption over the whole visible range (450–750 nm) is achieved. Utilizing this blend system in solar cell fabrication, ITO/PEDOT:PSS/P3HT:VOPcPhO/Al solar cells have been fabricated and characterized in open air. A maximum power conversation efficiency up to 1.09% has been recorded. To confirm the charge transport, the electron and hole mobility of VoPcPhO has been measured. The results show that the VoPcPhO has bipolar transport and can act as an electron as well as hole transporting material. The electron mobility is comparable with hole mobility.     

Generalized enhancement of charge injection in bottom contact/top gate polymer field-effect transistors with single-walled carbon nanotubes

We investigate the influence of small amounts of dispersed single-walled carbon nanotubes (SWNTs) on the contact resistance and device characteristics of bottom contact/top gate polymer field-effect transistors (FETs). Five conjugated polymers representing different classes of polymer semiconductors with different HOMO/LUMO levels are employed, namely, polythiophenes (P3HT), polyphenylenevinylenes (MDMO-PPV), polyfluorenes (F8T2), naphthalene-bis(dicarboximide) bithiophene copolymers (P(NDI2OD-T2)), and diketopyrrolo-pyrrole-bithiophene copolymers (DPPT-TT). In all cases the presence of dispersed SWNTs reduces non-ohmic contact resistance and lowers threshold and onset voltages for charge transport. In some cases inherent ambipolar charge transport in conjugated polymers (F8T2 and P(NDI2OD-T2)) is revealed. The concentration of the SWNTs within the semiconducting layer remains below the percolation limit and thus the apparent mobilities and on/off ratios are still determined by the polymer and independent of the specific type of the carbon nanotubes (metallic or semiconducting). The degree of enhancement depends both on the energy level offset between the injecting gold electrode and the HOMO/LUMO level (i.e., Schottky barrier) and the charge carrier mobility of the respective polymer. The simplicity of this injection enhancement method and its broad applicability make it a step toward high performance polymer transistors without injection limitations.     

A new dithienosilole-based oligothiophene with methyldicyanovinyl groups for high performance solution-processed organic solar cells

A new linear dithienosilole-based oligothiophene end-capped with methyl and electron-withdrawing dicyanovinyl groups, DTS(Oct)₂-(2T-DCV-Me)₂, was prepared in good yield. This oligomer exhibited broad absorption spectra in bulk down to the near-IR region with the optical edge at 900 nm, resulting in an initially high power conversion efficiency of 5.44% in solution-processed organic solar cells using PC₇₁BM as an acceptor.     

Ion-gel gated field-effect transistors with solution-processed oxide semiconductors for bioinspired artificial synapses

With the development of sciences and technologies of artificial intelligence in recent years, more and more attention is focused on the research of the synaptic devices inspired by human brain. In this paper, ion-gel coupled synaptic transistors with solution-possessed amorphous indium-zinc-oxide (In-Zn-O) thin films were demonstrated. Ion-gel dielectric provides a strong ionic/electronic coupling on the solution-processed In-Zn-O thin films, which is due to the very large electric-double-layer (EDL) capacitances (∼4.87 μF/cm²). The surface morphology, chemical composition/stoichiometry and electrical performances of In-Zn-O field-effect transistors (FETs) were analyzed as a function of annealing temperature. Most importantly, the ion-gel gated In-Zn-O FETs were used for synaptic functions simulations. The in-plane gate is used as the presynaptic input terminal and the In-Zn-O channel with source/drain electrodes is used as the postsynaptic output terminal. Mobile ions in ion-gel are regarded as neurotransmitters. Gate pluses were applied on the in-plane electrodes which is analogous to presynaptic spikes onto presynaptic membrane. Fundamental synaptic functions including excitatory postsynaptic current (EPSC), spike time-dependent EPSC, paired-pulse facilitation (PPF), and dynamic synaptic behaviors are mimicked. These results may provide a new opportunity and strategy to develop of highly functional electronic synapses and next-generation neuromorphic systems by using ion-gel gated devices with solution-processed amorphous oxide semiconductors.     

Benzothiadiazole[1,2-b:4,3-b′]dithiophene,a new ladder-type multifused block: Synthesis and photovoltaic application

A new fused building block benzothiadiazole[1,2-b:4,3-b′] dithiophene (BTDT) was prepared by covalently locking thiophene unit on both sides of benzothiadiazole (BT). On the basis of this building block, a series of conjugated copolymers containing homopolymer (P1) or electron-rich comonomers such as carbazole (P2), benzodithiophene (P3 and P4) and thiophene (P5) were obtained. All polymers have good solubility in common organic solvents. The thermal, optical, electrochemical and photovoltaic properties of the polymers were investigated systematically. The thiophene units, which were covalently fastened to the BT moiety, enlarged the planarization of the polymer backbone and thus induced stronger intermolecular π–π interaction, meanwhile, decreased the electron-withdrawing ability of the BT unit. The device based on P3:PC₇₁BM exhibited a high open-circuit voltage (V_OC) of 0.96 V and moderate power conversion efficiency (PCE) of 2.16%.     

Alternating dithienobenzoxadiazole-based conjugated polymers for field-effect transistors and polymer solar cells

Three new alternating copolymers derived from dithienobenzoxadizole (DTfBO) and different thiophene-based π-spacers, including terthiophene, quarterthiophene, and dithienyl flanked thienothiophene, were successfully synthesized. The DTfBO-based polymers possess optical band-gaps in the range of 1.84–1.89 eV and exhibit relatively deep HOMO levels between −5.36 eV and −5.50 eV. Due to strong interchain aggregation, DTfBO-based polymers could not be well dissolved in chlorobenzene at room temperature, but they could be processed with hot chlorobenzene solutions of ∼100 °C. Evolutions of UV absorption spectra of polymer solutions during heating process could differentiate their different aggregation ability, among which a repeating unit based on a DTfBO and a terthiophene could supply the strongest inter-chain interaction. Notably, the three DTfBO-based polymers displayed high field-effect hole mobilities between 0.21 and 0.54 cm²/(V s). In polymer solar cells (PSCs) with the three polymers as the donors, high open-circuit voltages between 0.87 and 0.93 V could be realized. For active layer thickness of 80 nm, the PSCs displayed power conversion efficiency (PCE) between 2.85% and 5.07%. A very high fill factor of 75.4% was achieved for the polymer comprising dithienyl flanked thienothiophene. With thicker ative layers of 250 nm, the three DTfBO-based polymers exhibited comparable PCEs of ∼5.61% due to obviously increased short-circuit currents. Our results suggest that DTfBO, a big coplanar heterocycle, is a promising building block to construct high mobility conjugated polymers for efficient thick-film PSCs.     

Fabrication and characterization of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors

We present our first application of the neutral cluster beam deposition (NCBD) method to fabricate bilayer heterojunction-based organic light-emitting field-effect transistors (OLEFETs) by superimposing two layers of α,ω-dihexylsexithiophene (DH6T) and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) successively. Based upon well-balanced ambipolarity (hole and electron field-effect mobilities of 2.22 × 10⁻² and 2.78 × 10⁻² cm²/Vs), the air-stable OLEFETs have demonstrated good field-effect characteristics, stress-free operational stability and electroluminescence under ambient condition.     

Low-voltage p-channel, n-channel and ambipolar organic thin-film transistors based on an ultrathin inorganic/polymer hybrid gate dielectric layer

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated low-voltage operating (below 3V) p-channel, n-channel and ambipolar OTFTs based on pentacene or/and C₆₀ as the active layers, respectively, with an ultrathin AlO_X/poly(methyl methacrylate co glycidyl methacrylate) (P(MMA–GMA)) hybrid layer as the gate dielectric. Benefited from the enhanced crystallinity of C₆₀ layer and greatly reduced density of electron trapping states at the interface of channel/dielectric due to the insertion of ultrathin pentacene layer between C₆₀ and P(MMA–GMA), high electron mobility can be achieved in present pentacene/C₆₀ heterostructure based ambipolar OTFTs. The effect of the thickness of pentacene layer and the deposition sequence of pentacene and C₆₀ on the device performance of OTFTs was studied. The highest electron mobility of 3.50 cm²/V s and hole mobility of 0.25 cm²/V s were achieved in the ambipolar OTFT with a pentacene (3.0 nm)/C₆₀ (30 nm) heterostructure.     

Charge transport and density of trap states in balanced high mobility ambipolar organic thin-film transistors

We report on charge transport and density of trap states (trap DOS) in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer thin-film transistors. This semiconductor possesses high electron and hole field-effect mobilities of up to 0.6 cm²/V-s. Temperature and gate-bias dependent field-effect mobility measurements are employed to extract the activation energies and trap DOS to understand its unique high mobility balanced ambipolar charge transport properties. The symmetry between the electron and hole transport characteristics, parameters and activation energies is remarkable. We believe that our work is the first charge transport study of an ambipolar organic/polymer based field-effect transistor with room temperature mobility higher than 0.1 cm²/V-s in both electrons and holes.     

Materials and interfaces issues in pentacene/PTCDI-C8 ambipolar organic field-effect transistors with solution-based gelatin dielectric

Gelatin is a natural protein, which works well as the gate dielectric for pentacene/N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C₈) ambipolar organic field-effect transistors (OFETs) in air ambient and in vacuum. An aqueous solution process was used to form the gelatin gate dielectric film on poly(ethylene terephthalate) (PET) by spin-coating and subsequent casting. Pentacene morphology and interface roughness are two major factors affecting the electron and hole field-effect mobility (μ_FE) values of pentacene/PTCDI-C₈ ambipolar OFETs in vacuum and in air ambient. In contrast, water absorption in gelatin has higher contribution to the electron and hole μ_FE values in air ambient. The ambipolar performance of pentacene/PTCDI-C₈ ambipolar OFETs depends on their layer sequence. For example, when PTCDI-C₈ is deposited onto pentacene, i.e. in the structure of PTCDI-C₈/pentacene, unbalanced ambipolar characteristics appear. In contrast, better ambipolar performance occurs in the structure of pentacene/PTCDI-C₈. The optimum ambipolar characteristics with electron μ_FE of 0.85 cm² V⁻¹ s⁻¹ and hole μ_FE of 0.95 cm² V⁻¹ s⁻¹ occurs at the condition of pentacene (40 nm)/PTCDI-C₈ (40 nm). Surprisingly, water absorption plays a crucial role in ambipolar performance. The device performance changes tremendously in pentacene/PTCDI-C₈ ambipolar OFETs due to the removal of water out of gelatin in vacuum. The optimum ambipolar characteristics with electron μ_FE of 0.008 cm² V⁻¹ s⁻¹ and hole μ_FE of 0.007 cm² V⁻¹ s⁻¹ occurs at the condition of pentacene (65 nm)/PTCDI-C₈ (40 nm). The roles of layer sequence, relative layer thickness, and water absorption are proposed to explain the ambipolar performance.     

Facile conversion of polymer organic thin film transistors from ambipolar and p-type into unipolar n-type using polyethyleneimine (PEI)-modified electrodes

We report here a successful polarity conversion of organic thin film transistors (OTFTs) based on several polymer semiconductors with low-lying LUMO (lowest unoccupied molecular orbital) energy levels (?−4 eV) from ambipolar and even p-type into unipolar n-type devices using an ultrathin layer (∼2–5 nm) of polyethyleneimine (PEI) to modify the source and drain contacts. The work function of gold is substantially reduced with the PEI layer on its surface, which effectively suppresses the injection of holes and thus enables electron-only charge transport of these polymers in OTFTs. This general approach of electrode work function modification broadens the scope of available polymer semiconductors for use in printed electronics where n-channel OTFTs are needed.     

Improving solution-processed n-type organic field-effect transistors by transfer-printed metal/semiconductor and semiconductor/semiconductor heterojunctions

Solution-processed n-type organic field effect transistors (OFETs) are in need of proper metal contact for improving injection and mobility, as well as balanced hole mobility for building logic circuit units. We address the two distinct problems by a simple technique of transfer-printing. Transfer-printed Au contacts on a terrylene-based semiconductor (TDI) significantly reduced the inverse subthreshold slope by 5.6 V/dec and enhanced the linear mobility by over 5 times compared to evaporated Au contacts. Hence, devices with a high-work-function metal (Au) are comparable with those with low-work-function metals (Al and Ca), indicating a fundamental advantage of transfer-printed electrodes in electron injection. We also transfer-printed a poly(3-hexylthiophene) (P3HT) layer onto TDI to construct a double-channel ambipolar transistor by a solution process for the first time. The transistor exhibits balanced hole and electron mobility (3.0 × 10⁻³ and 2.8 × 10⁻³ cm² V⁻¹ s⁻¹) even in a coplanar structure with symmetric Au electrodes. The technique is especially useful for reaching intrinsic mobility of new materials, and enables significant enlargement of the material tanks for solution-processed functional heterojunction OFETs.