Programmable memory in organic field-effect transistor based on lead phthalocyanine

A nonvolatile organic field-effect transistor (OFET) with a polymeric electret as gate insulator and spun cast film of lead phthalocyanine (PbPc) as semiconductor channel is reported. Hysteresis induced by gate–bias stress was exploited to study nonvolatile memory effects. The observation of the hysteresis and memory window is proposed to originate from charge storage in the polymeric electret. The on state retention time for the OFET memory device is more than 5 h and the device can reproduce continuous write–read–erase–read switching cycles.     

Effect of thickness of polymer electret on charge trapping properties of pentacene-based nonvolatile field-effect transistor memory

In this report, a set of pentacene-based organic field-effect transistor (OFET) memory devices using different thicknesses (ranged from 17.8 to 100.4 nm) of Poly (N-vinylcarbazole) (PVK) as charge trapping layers were fabricated, and the dependences of thickness on charge trapping behaviors were systematically investigated. As the thickness increased, the charge trapping capacity shows a Gaussian distributed growth behavior while the surface tunneling distance demonstrates the property of exponential decrease, which is ascribed to the synergistic effects of potential redistribution of trapped charge carriers and the co-existence of direct tunneling and Fowler–Nordheim (FN) tunneling. The optimum thickness (d_ot) to possess the most efficient charge trapping properties, which means a reasonably low programming voltage and high charge trapping capacity with good bias stress stability, is approximately 40 ± 5 nm. By calculating the threshold thickness (d_th) of PVK for an ultrathin memory, we proposed a model of superficial tunneling distance to deconstruct the continuous chargeable polymer electret-based OFET memory. Our work provided a quantitative evaluation method and can improve the understanding of charge trapping process from the aspect of electret thickness.     

UV assisted non-volatile memory behaviour using Copper (II) phthalocyanine based organic field-effect transistors

In this report, we have demonstrated the optical non-volatile memory characteristics using CuPc OFET. The memory operation was comprehensively demonstrated with different programming conditions. It was found that the programming of CuPc OFET with an electric pulse at the gate terminal under UV-light photo-illumination compared to other programming conditions, could substantially increase the memory window due to massive charge trapping in the polymer electret layer, which causes shift in the device transfer characteristics from low-conduction state (“OFF state”, or logic 0) to high conduction state (“ON state”, or logic 1) at V_GS = 0V. From device operation at −50V, a memory window of greater than 45V could be achieved by applying a programming voltage of +70 V at the gate terminal under UV-light photo-illumination. Moreover, it was completely erased by applying −100 V at the gate terminal in dark.     

Novel Photoinduced Recovery of OFET Memories Based on Ambipolar Polymer Electret for Photorecorder Application

A new design concept for novel photoresponsive flash organic field‐effect transistor (OFET) memory is demonstrated by employing the carbazoledioxazine polymer (Poly CD) as an electret. Photoactive electrets that can absorb the light effectively rather than photoactive semiconductors are proposed by the “photoinduced recovery” mechanism in the literature; however, the correlation between the chemical structure and photoresponsive electrical performances is ambiguous. In this study, it is reported for the first time that the OFET memory with trapped charges can be optically recovered by a polymer electret and the working mechanism can be explained by the structural design. The highly planar Poly CD electret exhibits photoluminescence quenching in film states, resulting in the generation of sufficient excitons to eliminate trapped charges under light excitation. Additionally, the Poly CD electret with coplanar donor–acceptor moieties is suitable for both p‐channel and n‐channel semiconductors. For p‐type memory devices, a large memory window (82 V) and stable nonvolatile retention performance with high ON/OFF ratio could be obtained. The memories also display good switching reliability for voltage‐programming and light‐erasing cycles. This study provides useful information for the development of polymer‐based photoresponsive flash OFET memories and demonstrates the practical applications of photorecorder and photosensitive smart tag.     

Solution-processed flexible nonvolatile organic field-effect transistor memory using polymer electret

Flexible organic field-effect-transistor (OFET) memory is one of the promising candidates for next-generation wearable nonvolatile data storage due to its low price, solution-processability, light-weight, mechanically flexibility, and tunable energy level via molecular tailoring. In this paper, we report flexible nonvolatile OFET memory devices fabricated with solution-processed polystyrene-brush electret and organic semiconductor blends of p-channel 6, 13-bis-(triisopropylsilylethynyl)pentacene (TIPS-PEN) and n-channel poly-{[N,N′-bis(2- octyldodecyl)-naphthalene-1,4,5,8-bis-(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P-(NDI2OD-2T); N2200). Fabricated flexible OFET memory devices exhibited high memory window (30 V) and ON/OFF current ratio (memory ratio) over 10³. Furthermore, we obtained reliable memory ratio (~10³) over retention time of 10⁸ s, 100 times of repeated programming/erasing cycles, and 1000 times of bending tests at a radius of 3 mm.     

The effect of porous structure of PMMA tunneling dielectric layer on the performance of nonvolatile floating-gate organic field-effect transistor memory devices

In this paper, we used the low and high density porous structure of polymethylmethacrylate (PMMA) film as tunneling dielectric layer in the floating-gate organic field-effect transistor (OFET) memory devices. Compared to the thin/thick nonporous structure of PMMA tunneling layer, the porous structure of PMMA tunneling layer had positive impacts on the device performance of the floating-gate OFET memory devices. Moreover, it was found that the memory performance was also increased as pore density of PMMA film increased. The atomic force microscopy (AFM) results of both porous structure of PMMA film and pentacene film on porous structure of PMMA film revealed that high density porous structure of PMMA tunneling layer can produce larger tunneling area and more electron transfer paths between pentacene film and PMMA film, which resulted in high electron capture and release efficiency of the floating-gate OFET memory devices with porous structure of PMMA tunneling layer. In addition, our porous structure of PMMA tunneling layer as well as nonporous PMMA film has high electrical insulating property due to their semi-hollow structure film, which is favourable to maintain stable retention property. Eventually, the floating-gate OFET memory devices with high density porous structure of PMMA tunneling layer showed good nonvolatile memory properties with a large memory window of about 43 V, a high ON/OFF current ratio of about 10⁴, and stable endurance and retention properties. Our results provided a new strategy to achieve the high performance floating-gate OFET memory devices.     

Interface dipole: Effects on threshold voltage and mobility for both amorphous and poly-crystalline organic field effect transistors

We report a detailed comparison on the role of a self-assembled monolayer (SAM) of dipolar molecules on the threshold voltage and charge carrier mobility of organic field-effect transistor (OFET) made of both amorphous and polycrystalline organic semiconductors. We show that the same relationship between the threshold voltage and the dipole-induced charges in the SAM holds when both types of devices are fabricated on strictly identical base substrates. Charge carrier mobilities, almost constant for amorphous OFET, are not affected by the dipole in the SAMs, while for polycrystalline OFET (pentacene) the large variation of charge carrier mobilities is related to change in the organic film structure (mostly grain size).     

High‐Performance Nonvolatile Transistor Memories of Pentacence Using the Green Electrets of Sugar‐based Block Copolymers and Their Supramolecules

Reported here are the nonvolatile electrical characteristics of pentacene‐based organic field‐effect transistor (OFET) memory devices created from the green electrets of sugar‐based block copolymer maltoheptaose‐block‐polystyrene (MH‐b‐PS), and their supramolecules with 1‐aminopyrene (APy). The very hydrophilic and abundant‐hydroxyl MH block is employed as a charge‐trapping site, while the hydrophobic PS block serves as a matrix as well as a tunneling layer. The orientation of the MH nanodomains could be well controlled in the PS matrix with random spheres, vertical cylinders, and ordered horizontal cylinders via increasing solvent annealing time, leading to different electrical switching characteristics. The electron‐trapping ability induced by the horizontal‐cylinder MH is stronger than those of the random‐sphere and vertical‐cylinder structures, attributed to the effective contact area. The electrical memory window of the device is further improved via the supramolecules of hydrogen‐bonding 1‐aminopyrene to the MH moieties of MH‐b‐PS for enhancing the hole‐trapping ability. The optimized device using the horizontal cylinders of the supramolecule electret exhibits the excellent memory characteristics of a wide memory window (52.7 V), retention time longer than 10⁴ s with a high ON/OFF ratio of >10⁵, and stable reversibility over 200 cycles. This study reveals a new approach to achieve a high‐performance flash memory through the morphology control of sugar‐based block copolymers and their supramolecules.     

High‐Performance Top‐Gated Organic Field‐Effect Transistor Memory using Electrets for Monolithic Printed Flexible NAND Flash Memory

High‐performance top‐gated organic field‐effect transistor (OFET) memory devices using electrets and their applications to flexible printed organic NAND flash are reported. The OFETs based on an inkjet‐printed p‐type polymer semiconductor with efficiently chargeable dielectric poly(2‐vinylnaphthalene) (PVN) and high‐k blocking gate dielectric poly(vinylidenefluoride‐trifluoroethylene) (P(VDF‐TrFE)) shows excellent non‐volatile memory characteristics. The superior memory characteristics originate mainly from reversible charge trapping and detrapping in the PVN electret layer efficiently in low‐k/high‐k bilayered dielectrics. A strategy is devised for the successful development of monolithically inkjet‐printed flexible organic NAND flash memory through the proper selection of the polymer electrets (PVN or PS), where PVN/‐ and PS/P(VDF‐TrFE) devices are used as non‐volatile memory cells and ground‐ and bit‐line select transistors, respectively. Electrical simulations reveal that the flexible printed organic NAND flash can be possible to program, read, and erase all memory cells in the memory array repeatedly without affecting the non‐selected memory cells.     

Stabilization of space charge polarization in ion-dispersed gate dielectric layer of organic transistors by ultraviolet illumination for write-once read-many memory

In this work, a new type organic field effect transistor (OFET) based write-once read-many memory (WORM) device was developed. The device uses an ultraviolet (UV) cross-linkable matrix polymer mixed with ionic compounds to form an ion-dispersed gate dielectric layer. Under an applied gate voltage bias, migration of cations and anions in opposite directions forms space charge polarization in the gate dielectric layer, resulting in change of the electrical characteristics. It is shown that, with UV illumination to cross-link the matrix polymer, the formed space charge polarization can be stabilized. Therefore, the OFET can be operated as a WORM with the applied voltage bias to define the polarization and in turn the stored data, and the UV illumination to stabilize the stored data.     

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.     

Sensitivity enhancement of a flexible MEMS strain sensor by a field effect transistor in an all organic approach

We report a low-cost piezoresistive nanocomposite based organic micro electro mechanical system (MEMS) strain sensor that has been combined to an organic field effect transistor (OFET) with the objective of amplifying the sensitivity of the sensor. When the MEMS cantilever is strained by a mechanical deflection, the resulting variation of resistivity influences the gate voltage (V_GS) of the OFET and, hence, changes the drain current (I_DS) of the transistor. The present combination allows an enhancement of sensitivity to strain by a factor 3.7, compared to the direct detection of resistance changes of the nanocomposite. As a consequence, a low limit of detection of 24 ppm has been estimated in terms of strain transduction efficiency. Furthermore, the organic microsystem exhibits a short response time and operates reversibly with an excellent robustness.     

一种廉价电极的并五苯场效应晶体管

采用底栅顶接触结构,研究制备了以并五苯为有源层、聚甲基丙烯酸甲酯(PMMA)为绝缘层的全有机场效应晶体管(OFET),其中绝缘层采用溶液旋涂法制备,电极采用MoO3/Al双层电极。与传统采用单一Au为电极的器件相比,采用双层电极的器件性能大幅提高,经测试,器件的迁移率达到了0.133cm2/Vs,开关电流比可以达到2.61×105。对采用MoO3修饰层提高性能的作用机理进行了详细论证。     

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.     

Influence of Electric Field on Microstructures of Pentacene Thin‐Films in Field‐Effect Transistors

We report on electric‐field‐induced irreversible structural modifications in pentacene thin films after long‐term operation of organic field‐effect transistor (OFET) devices. Micro‐Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source‐to‐drain field that parallels the a–b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long‐term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long‐term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long‐term operations, including enhanced field‐effect mobility, improved on/off current ratio, sharp sub‐threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source‐to‐drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed.     

Photon-energy-dependent light effects in organic nano-floating-gate nonvolatile memories

A pentacene-based organic field-effect transistor nonvolatile memory, in which polystyrene covered Au nanoparticles act as the nano-floating-gate, is probed under different illumination conditions. The memory window can be greatly enlarged upon illumination depending on incident photon energy and intensity, and two light effects are proposed and discussed. The minority multiplication effect enhances the minority carrier tunneling into the nano-floating-gate, resulting in the remarkable positive V_T shift. The excitation-induced injection effect is strongly photon energy dependent, and it is responsible for the significant negative V_T shift. Appropriate illumination is favorable for reducing the programming/erasing voltage of organic nano-floating-gate nonvolatile memories.     

Controllable Shifts in Threshold Voltage of Top‐Gate Polymer Field‐Effect Transistors for Applications in Organic Nano Floating Gate Memory

Organic field‐effect transistor (FET) memory is an emerging technology with the potential to realize light‐weight, low‐cost, flexible charge storage media. Here, solution‐processed poly[9,9‐dioctylfluorenyl‐2,7‐diyl]‐co‐(bithiophene)] (F8T2) nano floating gate memory (NFGM) with a top‐gate/bottom‐contact device configuration is reported. A reversible shift in the threshold voltage (V_Th) and reliable memory characteristics was achieved by the incorporation of thin Au nanoparticles (NPs) as charge storage sites for negative charges (electrons) at the interface between polystyrene and cross‐linked poly(4‐vinylphenol). The F8T2 NFGM showed relatively high field‐effect mobility (µ_FET) (0.02 cm² V^?1 s^?1) for an amorphous semiconducting polymer with a large memory window (ca. 30 V), a high on/off ratio (more than 10⁴) during writing and erasing with an operation voltage of 80 V of gate bias in a relatively short timescale (less than 1 s), and a retention time of a few hours. This top‐gated polymer NFGM could be used as an organic transistor memory element for organic flash memory.     

Morphology control of tunneling dielectric towards high-performance organic field-effect transistor nonvolatile memory

We report high-performance organic field-effect transistor nonvolatile memory based on nano-floating-gate, which shows a large memory window of about 70 V, high ON/OFF ratio of reading current over 10⁵ after 1 week storage, high field-effect mobility of 0.6 cm²/V s, and good programming/erasing/reading endurance. The devices incorporate Au nanoparticles and polystyrene layer on top to form the nano-floating-gate, and we demonstrate that the morphology control of the tunneling dielectric is critically significant to improve the memory performance. The optimized tunneling dielectric morphology is favorable to the efficient charge tunneling, reliable charge storage and high-quality organic film growth.     

Molecular-scale charge trap medium for organic non-volatile memory transistors

In this work, we introduce a molecular-scale charge trap medium for an organic non-volatile memory transistor (ONVMTs). We use two different types of small molecules, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), which have the same triphenylene cores with either hydroxyl or methoxy end groups. The thickness of the small molecule charge trap layer was sophisticatedly controlled using the thermal evaporation method. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analysis revealed that there were negligible differences in the chemical structures of both small molecules before and after thermal deposition process. The ONVMTs with a 1-nm-thick HHTP charge trap layer showed a large hysteresis window, approximately 20 V, under a double sweep of the gate bias between 40 V and −40 V. The HMTP-based structure showed a negligible memory window, which implied that the hydroxyl groups affected hysteresis. The number of trapped charges on the HHTP charge trap layer was measured to be 4.21 × 10¹² cm⁻². By varying the thickness of the molecular-scale charge trap medium, it was determined that the most efficient charge trapping thickness of HHTP charge trap layer was approximately 5 nm.     

Electrochromic display cells driven by an electrolyte-gated organic field-effect transistor

Monolithic integration of an organic field-effect transistor (OFET) and an organic electrochromic display cell operating at around 1 V is reported. This was achieved by utilising a common patterned layer of poly(styrenesulfonic acid) (PSSH). In the OFET, PSSH served as the electric double layer capacitors between the gate and the organic semiconductor channel. In the electrochromic pixel, PSSH was included as the electrolyte and transports protons from and to the electrochromic layer upon switching. The enhancement mode OFET enables a relatively faster updating speed, of the display cell, and provides a much simpler addressing and updating scheme as compared to smart pixels including a depletion mode electrochemical transistor.