Diketopyrrolopyrrole Organic Thin‐Film Transistors: Impact of Alkyl Substituents and Tolerance of Ethylhexyl Stereoisomers

Bis(thiophen‐2‐yl)‐diketopyrrolopyrrole (DPP) dyes bearing various alkyl substituents at the amide positions (n‐butyl, n‐pentyl, n‐hexyl, n‐heptyl, n‐octyl, 2‐ethylhexyl) and chlorine (Cl), bromine (Br), or cyano (CN) substituents at the thiophene positions have been synthesized and investigated with regard to their molecular and semiconducting properties. Intense absorption, strong fluorescence, and reversible oxidation and reduction processes are common to all of these dyes. Their characterization as organic semiconductors in vacuum‐processed thin‐film transistors reveals p‐channel operation with field‐effect mobilities ranging from 0.01 to 0.7 cm² V^?1 s^?1. The highest mobility is found for the DPP dyes bearing the 2‐ethylhexyl substituents, which is surprising, considering that as a result of the chiral substituents, this material is a mixture of (R,R), (S,S), and (R,S) stereoisomers. The high carrier mobility in the films of the DPPs bearing stereoisomerically inhomogeneous ethylhexyl groups is rationalized here by single‐crystal X‐ray diffraction (XRD) analysis in combination with XRD and atomic force microscopy studies on thin films, which reveal the presence of slightly different 2D layer arrangements for the n‐alkyl and the 2‐ethylhexyl derivatives. For the cyano‐substituted DPPs possessing the lowest LUMO levels, ambipolar transport characteristics are observed.     

Printed Sub‐2 V Gel‐Electrolyte‐Gated Polymer Transistors and Circuits

The fabrication and characterization of printed ion‐gel‐gated poly(3‐hexylthiophene) (P3HT) transistors and integrated circuits is reported, with emphasis on demonstrating both function and performance at supply voltages below 2 V. The key to achieving fast sub‐2 V operation is an unusual gel electrolyte based on an ionic liquid and a gelating block copolymer. This gel electrolyte serves as the gate dielectric and has both a short polarization response time (<1 ms) and a large specific capacitance (>10 µF cm^?2), which leads simultaneously to high output conductance (>2 mS mm^?1), low threshold voltage (<1 V) and high inverter switching frequencies (1–10 kHz). Aerosol‐jet‐printed inverters, ring oscillators, NAND gates, and flip‐flop circuits are demonstrated. The five‐stage ring oscillator operates at frequencies up to 150 Hz, corresponding to a propagation delay of 0.7 ms per stage. These printed gel electrolyte gated circuits compare favorably with other reported printed circuits that often require much larger operating voltages. Materials factors influencing the performance of the devices are discussed.     

The Influence of Film Morphology in High‐Mobility Small‐Molecule:Polymer Blend Organic Transistors

Organic field‐effect transistors (OFETs) based upon blends of small molecular semiconductors and polymers show promise for high performance organic electronics applications. Here the charge transport characteristics of high mobility p‐channel organic transistors based on 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene:poly(triarylamine) blend films are investigated. By simple alteration of the film processing conditions two distinct film microstructures can be obtained: one characterized by small spherulitic grains (SG) and one by large grains (LG). Charge transport measurements reveal thermally activated hole transport in both SG and LG film microstructures with two distinct temperature regimes. For temperatures >115 K, gate voltage dependent activation energies (E_A) in the range of 25–60 meV are derived. At temperatures <115 K, the activation energies are smaller and typically in the range 5–30 meV. For both film microstructures hole transport appears to be dominated by trapping at the grain boundaries. Estimates of the trap densities suggests that LG films with fewer grain boundaries are characterized by a reduced number of traps that are less energetically disordered but deeper in energy than for small SG films. The effects of source and drain electrode treatment with self‐assembled monolayers (SAMs) on current injection is also investigated. Fluorinated thiol SAMs were found to alter the work function of gold electrodes by up to ～1 eV leading to a lower contact resistance. However, charge transport analysis suggests that electrode work function is not the only parameter to consider for efficient charge injection.     

Characterization of Zn1?xMgxO Films Prepared by the Sol–Gel Process and Their Application for Thin-Film Transistors

Transparent semiconductor thin films of Zn_1−x Mg _x O (0 ≤ x ≤ 0.36) were prepared using a sol–gel process; the crystallinity levels, microstructures, and optical properties affected by Mg content were studied. The experimental results showed that addition of Mg species in ZnO films markedly decreased the surface roughness and improved transparency in the visible range. A Zn_1−x Mg _x O film with an x-value of 0.2 exhibited the best average transmittance, namely 93.7%, and a root-mean-square (RMS) roughness of 1.63 nm. Therefore, thin-film transistors (TFTs) with a Zn_0.8Mg_0.2O active channel layer were fabricated and found to have n-type enhancement mode. The Zn_0.8Mg_0.2O TFT had a field-effect mobility of 0.1 cm²/V s, threshold voltage of 6.0 V, and drain current on/off ratio of more than 10⁷.     

A new diketopyrrolopyrrole-based co-polymer for ambipolar field-effect transistors and solar cells

A new thieno[3,2-b]thiophenediketopyrrolopyrrole-benzo[1,2-b:4,5-b′]dithiophene based narrow optical gap co-polymer (PTTDPP-BDT) has been synthesized and characterized for field-effect transistors and solar cells. In field-effect transistors the polymer exhibited ambipolar charge transport behaviour with maximum hole and electron mobilities of 10⁻³ cm² V⁻¹ s⁻¹ and 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. The respectable charge transporting properties of the polymer were consistent with X-ray diffraction measurements that showed close molecular packing in the solid state. The difference in hole and electron mobilities was explained by density functional theory calculations, which showed that the highest occupied molecular orbital was delocalized along the polymer backbone with the lowest unoccupied molecular orbital localized on the bis(thieno[3,2-b]thiophene)diketopyrrolopyrrole units. Bulk heterojunction photovoltaic devices with the fullerene acceptor PC₇₀BM were fabricated and delivered a maximum conversion efficiency of 3.3% under AM1.5G illumination.     

Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.     

Dithienocarbazole‐Based Ladder‐Type Heptacyclic Arenes with Silicon,Carbon, and Nitrogen Bridges: Synthesis,Molecular Properties,Field‐Effect Transistors,and Photovoltaic Applications

A new class of ladder‐type dithienosilolo‐carbazole ( DTSC ), dithienopyrrolo‐carbazole ( DTPC ), and dithienocyclopenta‐carbazole ( DTCC ) units is developed in which two outer thiophene subunits are covalently fastened to the central 2,7‐carbazole cores by silicon, nitrogen, and carbon bridges, respectively. The heptacyclic multifused monomers are polymerized with the benzothiadiazole ( BT ) acceptor by palladium‐catalyzed cross‐coupling to afford three alternating donor‐acceptor copolymers poly(dithienosilolo‐carbazole‐alt‐benzothiadiazole) ( PDTSCBT) , poly(dithienocyclopenta‐carbazole‐alt‐benzothiadiazole) ( PDTCCBT), and poly(dithienopyrrolo‐carbazole‐alt‐benzothiadiazole) ( PDTPCBT) . The silole units in DTSC possess electron‐accepting ability that lowers the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PDTSCBT , whereas stronger electron‐donating ability of the pyrrole moiety in DTPC increases the HOMO and LUMO energy levels of PDTPCBT . The optical bandgaps (E_g^opt) deduced from the absorption edges of thin film spectra are in the following order: PDTSCBT (1.83 eV) > PDTCCBT (1.64 eV) > PDTPCBT (1.50 eV). This result indicated that the donor strength of the heptacyclic arenes is in the order: DTPC > DTCC > DTSC . The devices based on PDTSCBT and PDTCCBT exhibited high hole mobilities of 0.073 and 0.110 cm² V^?1 s^?1, respectively, which are among the highest performance from the OFET devices based on the amorphous donor‐acceptor copolymers. The bulk heterojunction photovoltaic device using PDTSCBT as the p‐type material delivered a promising efficiency of 5.2% with an enhanced open circuit voltage, V_oc, of 0.82 V.     

D‐A1‐D‐A2 Backbone Strategy for Benzobisthiadiazole Based n‐Channel Organic Transistors: Clarifying the Selenium‐Substitution Effect on the Molecular Packing and Charge Transport Properties in Electron‐Deficient Polymers

Unipolar n‐type semiconducting polymers based on the benzobisthiadiazole (BBT) unit and its heteroatom‐substituted derivatives are for the first time synthesized by the D‐A₁‐D‐A₂ polymer‐backbone design strategy. Selenium (Se) substitution is a very effective molecular design, but it has been seldom studied in n‐type polymers. In this study, within the similar conjugated framework, the Se substitution effects on the optical, electrochemical, solid‐state polymer packing, electron mobility, and air‐stability of the target unipolar n‐type polymers are unraveled. Replacing the sulfur (S) atom in the thiadiazole heterocycles with the Se atom leads to narrower bandgaps and deeper lowest unoccupied molecular orbital (LUMO) levels of the n‐type polymers. Furthermore, the Se‐substituted polymer (pSeN‐NDI) shows shorter lamellar packing distances and stronger edge‐on π–π stacking interactions than its S‐counterpart (pSN‐NDI), as observed by the two‐dimensional grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) patterns. With the deeper LUMO level and thin‐film microstructures suitable for transistors, pSeN‐NDI exhibits four‐fold higher electron mobilities (μ_e) than pSN‐NDI. However, the other Se‐containing polymer, pSeS‐NDI, forms rather amorphous film structures, which is caused by its limited thermal stability and decomposition during the thermal annealing processes, thus giving rise to a lower μ_e than its S‐counterpart (pBBT‐NDI). Most importantly, pBBT‐NDI demonstrates an electron mobility of 0.039 cm² V^?1 s^?1, which is noticeable among the unipolar n‐type polymers based on the BBT and its analogs.     

The influence of defects on device performance of MBE-grown Si homojunction and strained Si1-xGex/Si heterostructures

Different Si homojunction and strained Si_1-xGe_x/Si heterojunction diodes and bipolar transistors have been fabricated by Si-MBE. The effect of annealing on Si homojunction diodes and transistors are studied. It is found that annealing generally improves the Si device performance, such as the ideality factor and breakdown characteristics. The influence of⁶⁰Co γ irradiation on the Si_1-xGe_x/Si diode performances are investigated by studying the temperature dependence of their electrical characteristics, and the results are correlated with the quality of the MBE-films. γ irradiation causes a drop in material conductivity due to the generation of atom-displacement defects in the whole volume of the wafers and increases the defect density at hetero-interfaces. The forward I-V curves of Si_1-xGe_x/Si devices may shift towards lower or higher voltages, depending on the film quality and the irradiation dose. The increase of defect density in strained Si_1-xGe_x/Si films appears to occur easier for the films with lower quality. Electrical measurements and calculations show that the defect-associated tunneling process is important in current transport for these MBE grown Si homojunction and strained Si_1-xGe_x/Si heterojunction devices, which have initially medium film quality or have been treated by irradiation.     

Photoresponsive Transistors Based on Lead‐Free Perovskite and Carbon Nanotubes

Lead‐free perovskite materials are exhibiting bright application prospects in photodetectors (PDs) owing to their low toxicity compared with traditional lead perovskites. Unfortunately, their photoelectric performance is constrained by the relatively low charge conductivity and poor stability. In this work, photoresponsive transistors based on stable lead‐free bismuth perovskites CsBi₃I₁₀ and single‐walled carbon nanotubes (SWCNTs) are first reported. The SWCNTs significantly strengthen the dissociation and transportation of the photogenerated charge carriers, which lead to dramatically improved photoresponsivity, while a decent I_light/I_dark ratio over 10² can be maintained with gate modulation. The devices exhibit high photoresponsivity (6.0 × 10⁴ A W^?1), photodetectivity (2.46 × 10¹⁴ jones), and external quantum efficiency (1.66 × 10⁵%), which are among the best reported results in lead‐free perovskite PDs. Furthermore, the excellent stability over many other lead‐free perovskite PDs is demonstrated over 500 h of testing. More interestingly, the device also shows the application potential as a light‐stimulated synapse and its synaptic behaviors are demonstrated. In summary, the lead‐free bismuth perovskite‐based hybrid phototransistors with multifunctional performance of photodetection and light‐stimulated synapse are first demonstrated in this work.