Electrical characterization of mica as an insulator for organic field-effect transistors

Electrical properties of conjugated polymer films, including poly(3-hexylthiophene)-2,5-diyl (P3HT), poly(3,3-didodecylquarterthiophene) (PQT-12), and poly(triarylamine) (PTAA), on mica substrates have been studied. The test structure was similar to a standard organic field-effect transistor but with a 150-μm-thick commercially available mica gate insulator/substrate, which allowed to obtain a field-effect mobility of P3HT as high as 0.08 cm²/Vs in the linear regime in ambient air. The influence of interface treatment, thermal annealing, and measurement conditions on the electrical properties of the P3HT films has been characterized and analyzed. We also studied the time dependence of the carrier concentration and mobility before and after a thermal annealing process. The results indicate that mica is a promising insulator for organic field-effect transistors, apart from already being one of the common thin-film materials widely used in electric capacitors.     

Ultra-high mobility in defect-free poly(3-hexylthiophene-2,5-diyl) field-effect transistors through supra-molecular alignment

Performance enhancement of organic field-effect transistors (OFETs) based on solution-processable conjugated polymers (CPs) holds critical significance for the realization of cost-effective commercial applications such as organic light-emitting diode displays. One of the most critical performance parameters is the charge-carrier field-effect mobility (μ_FET) that is significantly influenced by the molecular arrangement in a CP. In this article, floating film transfer method (FTM) is utilized for the deposition of a CP––defect-free poly(3-hexylthiophene-2,5-diyl) (DF-P3HT)––which results in the formation of aligned supra-molecular assemblies. When applied as the active layer in OFET devices, μ_FET reaching as high as 8.0 cm²/V.s (6.3 cm²/V.s on average) is obtained. The value of μ_FET observed in the current study is the highest value reported so far for P3HT based OFETs (∼5 times higher as compared to when DF-P3HT is deposited using spin coating).     

Electrical transport properties in electroless-etched Si nanowire field-effect transistors

We report the electrical transport of the Si nanowires in a field-effect transistor (FET) configuration, which were synthesized from B-doped p-type Si(1 1 1) wafer by an aqueous electroless etching method based on the galvanic displacement of Si by the reduction of Ag⁺ ions on the wafer surface. The FET performance of the as-synthesized Si nanowires was investigated and compared with Ag-nanoparticles-removed Si nanowires. In addition, high-k HfO₂ gate dielectric was applied to the Si nanowires FETs, leading to the enhanced performance such as higher drain current and lower subthreshold swing.     

Enhanced performance in inverted polymer solar cells employing microwave-annealed sol-gel ZnO as electron transport layers

In this work, we propose a facile microwave-assisted approach for annealing sol-gel derived ZnO films to serve as electron transport layers (ETLs) for inverted bulk heterojunction polymer solar cells. We have demonstrated an impressive enhancement in performance for devices based on a poly (3-hexylthiophene) (P3HT): (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) system employing the microwave-annealed ZnO (ZnO (MW)) ETLs in comparison to the cases using the conventional hotplate-annealed ZnO (ZnO (HP)) ones. The better electron transport in the device with the ZnO (MW) ETL is mainly ascribed to the preferable interfacial contact as evidenced by the morphology characteristics. Furthermore, the comprehensive analyses conducted from the light intensity dependent photocurrent and photovoltage measurements, the capacitance-voltage characteristics, and the alternating current impedance spectra suggest that the utilization of the ZnO (MW) ETLs can effectively suppress trap-assisted recombination as well as charge accumulation at the interface between P3HT: PC₆₁BM layers and ZnO layers, which is responsible for the enhanced device performance.     

Memory performance and retention of an all-organic ferroelectric-like memory transistor

We have built a nonvolatile memory field-effect transistor (FET)-based on organic compounds. The gate-insulating polymer features ferroelectric-like characteristics when spun from solution into an amorphous phase. Thus, the memory transistor is built using techniques developed for organic transistors without requiring high temperature annealing steps. The memory exhibits channel resistance modulations and retention times close in performance to inorganic ferroelectric FETs (FEFETs), yet at a fraction of the cost.     

Real‐Time Investigation of Crystallization and Phase‐Segregation Dynamics in P3HT:PCBM Solar Cells During Thermal Annealing

Crystallization and phase segregation during thermal annealing lead to the increase of power‐conversion efficiency in poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) bulk‐heterojunction solar cells. An understanding of the length and time scale on which crystallization and phase segregation occur is important to improve control of the nanomorphology. Crystallization is monitored by means of grazing incidence X‐ray diffraction in real time during thermal annealing. Furthermore, the change in film density is monitored by means of ellipsometry and the evolution of carrier mobilities by means of field effect transistors, both during annealing. From the combination of such measurements with those of device performance as a function of annealing time, it is concluded that the evolution of microstructure involves two important time windows: i) A first one of about 5 minutes duration wherein crystallization of the polymer correlates with a major increase of photocurrent; ii) a second window of about 30 minutes during which the aggregation of PCBM continues, accompanied by an increase in the fill factor.     

Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.     

Three‐Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Here, an investigation of three‐dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post‐treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (η_IQE ) of the devices. To investigate the 3D BHJ morphology, novel time‐of‐flight secondary‐ion mass spectroscopy studies are employed along with conventional methods, such as UV‐vis absorption, X‐ray diffraction, and high‐resolution transmission electron microscopy studies. The η_IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high‐performance P3HT:PCBM solar cells using the optimized solvent‐annealing method is reported, and these cells show a mean power‐conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm^?2.     

In‐Situ Monitoring of the Solid‐State Microstructure Evolution of Polymer:Fullerene Blend Films Using Field‐Effect Transistors

Organic field‐effect transistors (OFETs) are used to investigate the evolution of the solid‐state microstructure of blends of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PC₆₁BM) upon annealing. Changes in the measured field‐effect mobility of holes and electrons are shown to reveal relevant information about the phase‐segregation in this system, which are in agreement with a eutectic phase behavior. Using dual‐gate OFETs and in‐situ measurements, it is demonstrated that the spatial‐ and time‐dependence of microstructural changes in such polymer:fullerene blend films can also be probed. A percolation‐theory‐based simulation is carried out to illustrate how phase‐segregation in this system is expected to lead to a substantial decrease in the electron conductivity in an OFET channel, in qualitative agreement with experimental results.     

Ultra-thin polymer gate dielectrics for top-gate polymer field-effect transistors

We have demonstrated top-gate polymer field-effect transistors (FETs) with ultra-thin (30–50 nm), room-temperature crosslinkable polymer gate dielectrics based on blending an insulating base polymer such as poly(methyl methacrylate) with an organosilane crosslinking agent, 1,6-bis(trichlorosilyl)hexane. The top-gate polymer transistors with thin gate dielectrics were operated at gate voltages less than ?8 V with a relatively high dielectric breakdown strength (>3 MV/cm) and a low leakage current (10–100 nA/mm² at 2 MV/cm). The yield of thin gate dielectrics in top-gate polymer FETs is correlated with the roughness of underlying semiconducting polymer film. High mobilities of 0.1–0.2 cm²/V s and on and off state current ratios of 10⁴ were achieved with the high performance semiconducting polymer, poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene.     

Solvent-vapor induced self-assembly of a conjugated polymer: A correlation between solvent nature and transistor performance

A systematical investigation on solvent-vapor annealing in polymer thin film transistors is performed using a thiazolothiazole-bithiazole conjugated polymer as the active layer. Film morphology, packing order and device performance are closely related to polarity and solubility parameter of the annealing solvent and annealing time. The formation of highly ordered and closely connected fibrillar domains is realized by using a solvent with similar solubility parameter and polarity to the conjugated polymer. Field-effect transistors based on pristine polymer films exhibit a highest charge carrier mobility of 0.0067 cm² V⁻¹ s⁻¹. After solvent vapor annealing with THF for 48 h, the mobility boosts up to 0.075 cm² V⁻¹ s⁻¹. This correlation between solvent polarity, solubility parameter and film morphology, packing order and mobility provides a useful guideline towards high performance polymer thin film transistors with solvent-vapor annealing method.     

Influence of annealing temperature of ZnO film as the electron transport layer on the performance of polymer solar cells

The surface morphology of ZnO films at different annealing temperatures and the performance of polymer solar cells (PSCs) with ZnO as the electron transport layer are studied. The low temperature sol-gel processed ZnO film has smoother surface than that in higher temperature, which results in the best photovoltaic performance with a power conversion efficiency (PCE) of 3.66% for P3HT:PC61BM based solar cell. With increasing annealing temperature, the photovoltaic performance first deceases and then increases. It could be ascribed to the synergy effects of interface area, the conductivity and surface energy of ZnO film and series resistance of devices.     

2D Derivative Phase Induced Growth of 3D All Inorganic Perovskite Micro–Nanowire Array Based Photodetectors

A large number of derivative phases in inorganic perovskites are reported with special structures and extraordinary performances in photoelectronic device applications. The reverse phase transition between derivative phases and perovskites usually induces recrystallization or forms mixed components. In this work, derivative phase‐induced growth of the CsPbBr₃ micro–nanowire (MW) array by utilizing phase transition of the 2D CsPb₂Br₅ phase is reported. Owing to its layered structure and phase transition, annealing of CsPb₂Br₅ at a temperature of 550 °C combined with solvent quenching leads to a templating effect to form a high‐quality CsBr MW array. Subsequent PbBr₂ deposition and the second annealing are employed to form aligned CsPbBr₃ MW arrays. Based on this method, a CsPbBr₃ MW array‐based photodetector is fabricated. The large grain size, less grain boundaries, and lower surface potential of the CsPbBr₃ MW array lead to high device performance with a responsivity of 7.66 A W^?1, a detectivity of ≈10¹² Jones, and long‐term operational stability over 1900 min.     

Efficiency enhancement of polymer solar cells by post-additional annealing treatment

We adopt the post-additional thermal annealing (PATA) process to optimize the performance of the polymer solar cells (PSCs) with an active layer composed of a blend of regioregular poly (3-hexythiophene) (RR-P3HT) and fullerenes. It is found that compared with general annealing process, the crystallinity of RR-P3HT by PATA is enhanced, and the absorption peak is raised obviously at ～500 nm after PATA. With the optimized annealing conditions, the device shows an enhancement of 31% in short circuit current density, 5% in open circuit voltage (Voc), and 11% in the power conversion efficiency (PCE) compared with that of the general annealing device.     

Reconstruction of the InSb (111)In surface as a result of sulfur adsorption

The variation in reconstruction in the InSb (111)In surface during adsorption of sulfur and annealing in ultrahigh vacuum was investigated by methods of low-energy electron diffraction and Auger electron spectroscopy. It is shown that evolution of reconstruction of the InSb (111)In surface substantially depends on the starting thickness of the adsorbed S layer on the surface. If the thickness of the S layer is only slightly larger than that of the monolayer, reconstruction (1 × 1) is formed on the surface, which transforms into reconstruction (2 × 2) during the subsequent annealing. If the S layer is several monolayers thick, this layer is initially amorphous. Annealing of such a surface at 315–325°C can lead to the formation of reconstruction (√3 × √3)-R30°, which transforms into reconstruction (2 × 2) at a higher temperature. This reconstruction is retained during further annealing until the S atoms vanish from the surface completely. It is shown for the first time that the reconstruction (√3 × √3)-R30° can form during adsorption of chalcogenide atoms on the III–V (111)III surface.     

High operational stability of solution-processed organic field-effect transistors with top-gate configuration

Electrical characteristics of top-gate field-effect transistors based on a wide range of solution-processed organic semiconductors are systematically investigated. The top-gate field-effect transistors based on different organic semiconductors—from an amorphous polymer semiconductor to a polycrystalline molecular semiconductor—exhibit higher operational stability compared with bottom-gate organic field-effect transistors reported in literature, in spite of significant difference in field-effect mobility. The correlation between charge transport and operational stability is discussed to gain insight into high operational stability of top-gate organic field-effect transistors.     

Organic nonvolatile memory devices utilizing intrinsic charge-trapping phenomena in an n-type polymer semiconductor

Charge trapping is an undesirable phenomenon and a common challenge in the operation of n-channel organic field-effect transistors. Herein, we exploit charge trapping in an n-type semiconducting poly (naphthalene diimide-alt-biselenophene) (PNDIBS) as the key operational mechanism to develop high performance, nonvolatile, electronic memory devices. The PNDIBS-based field-effect transistor memory devices were programmed at 60 V and they showed excellent charge-trapping and de-trapping characteristics, which could be cycled more than 200 times with a current ratio of 10³ between the two binary states. Programmed data could be retained for 10³ s with a memory window of 28 V. This is a record performance for n-channel organic transistor with inherent charge-trapping capability without using external charge trapping agents. However, the memory device performance was greatly reduced, as expected, when the n-type polymer semiconductor was end-capped with phenyl groups to reduce the trap density. These results show that the trap density of n-type semiconducting polymers could be engineered to control the inherent charge-trapping capability and device performance for developing high-performance low-cost memory devices.     

Cardanol-based polymeric gate dielectric for solution-processed organic field-effect transistors on flexible substrates

A cardanol-based polymer, poly(2-hydroxy-3-cardanylpropyl methacrylate) (PHCPM), was utilized as the gate dielectric of an organic field-effect transistor (OFET). PHCPM has good surface properties, appropriate gate dielectric characteristics, and good compatibility with solution-processed semiconducting polymers. The electrical properties of an FET that was prepared with natural resource-based PHCPM as a gate dielectric layer and solution-processed poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) as a semiconducting layer were investigated on flexible substrates. The flexible PBTTT-OFET device with the PCHPM gate dielectric exhibited high mobility and reliable performance, even in the bending state, without significant hysteresis.     

Hybrid Solar Cells Using a Zinc Oxide Precursor and a Conjugated Polymer

We describe a new method towards bulk‐heterojunction hybrid polymer solar cells based on composite films of zinc oxide (ZnO) and a conjugated polymer poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV). Spin‐coating diethylzinc as a ZnO precursor and MDMO‐PPV from a common solvent at 40 % humidity and annealing at 110 °C provides films in which crystalline ZnO is found to be intimately mixed with MDMO‐PPV. Photoluminescence and photoinduced spectroscopy demonstrate that photoexcitation of these hybrid composite films results in a fast and long‐lived charge transfer from the polymer as a donor to ZnO as ato be obtained n acceptor. Using the ZnO‐precursor method, hybrid polymer solar cells have been made with an estimated air‐mass of 1.5 (AM 1.5) energy conversion efficiency of 1.1 %. This new method represents a fivefold improved performance compared to similar hybrid polymer solar cells based on amorphous TiO₂.