Organic Field Effect Transistors Based on Graphene and Hexagonal Boron Nitride Heterostructures

Enhancing the device performance of single crystal organic field effect transistors (OFETs) requires both optimized engineering of efficient injection of the carriers through the contact and improvement of the dielectric interface for reduction of traps and scattering centers. Since the accumulation and flow of charge carriers in operating organic FETs takes place in the first few layers of the semiconductor next to the dielectric, the mobility can be easily degraded by surface roughness, charge traps, and foreign molecules at the interface. Here, a novel structure for high‐performance rubrene OFETs is demonstrated that uses graphene and hexagonal boron nitride (hBN) as the contacting electrodes and gate dielectric layer, respectively. These hetero‐stacked OFETs are fabricated by lithography‐free dry‐transfer method that allows the transfer of graphene and hBN on top of an organic single crystal, forming atomically sharp interfaces and efficient charge carrier‐injection electrodes without damage or contamination. The resulting heterostructured OFETs exhibit both high mobility and low operating gate voltage, opening up new strategy to make high‐performance OFETs and great potential for flexible electronics.     

The Impact of Molecular Orientation on the Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction

The anisotropy inherent to many planar organic molecules leads to a high sensitivity of various fundamental processes to the orientation of molecules within films and at heterojunctions. Such processes include absorption, charge and exciton transport, energy levels, and charge transfer, all of which are critical to organic solar cell operation. Here,an in‐depth study of bilayer cells consisting of a donor/acceptor interface between zinc phthalocyanine (ZnPc) and fullerene (C₆₀) is conducted and devices with the typically deposited standing up (edge‐on) orientation are compared to those with ZnPc lying flat (face‐on). The face‐on ZnPc‐based device allows for an increase in all solar cell parameters, substantially increasing power conversion efficiency from 1.5% to 2.8%. Spectrally resolved photocurrent measurements reveal a >50% increase in ZnPc signal, from which only 12% is accounted for by the increase in absorption associated with the face‐on orientation. The increase in internal quantum efficiency is accounted for via an improved charge transfer. The results of this study indicate that proper consideration of the orientation between donor and acceptor needs to be taken in order to fully optimize the numerous processes required for photovoltaic energy conversion.     

Fluorinated Zinc Phthalocyanine as Donor for Efficient Vacuum‐Deposited Organic Solar Cells

Efficient single bulk heterojunction organic solar cells based on blends of a fluorinated zinc phthalocyanine as electron donor and fullerene C₆₀ as electron acceptor are reported. In comparison to the commonly used absorber zinc phthalocyanine, the fluorination of the molecule to F₄ZnPc leads to an increase in ionisation potential and subsequently to an improvement of about 170 mV in the open circuit voltage of organic solar cells, while the short circuit current density and fill factor remain nearly unchanged. Similar to ZnPc:C₆₀‐based devices, the device characteristics of F₄ZnPc:C₆₀ solar cells can be further enhanced by improving the blend layer morphology by substrate heating during deposition. F₄ZnPc is an efficient donor material that can achieve a 4.6% power conversion efficiency in single heterojunction organic solar cells.     

Decoupling the Bias‐Stress‐Induced Charge Trapping in Semiconductors and Gate‐Dielectrics of Organic Transistors Using a Double Stretched‐Exponential Formula

A novel strategy for analyzing bias‐stress effects in organic field‐effect transistors (OFETs) based on a four‐parameter double stretched‐exponential formula is reported. The formula is obtained by modifying a traditional single stretched‐exponential expression comprising two parameters (a characteristic time and a stretched‐exponential factor) that describe the bias‐stress effects. The expression yields two characteristic times and two stretched‐exponential factors, thereby separating out the contributions due to charge trapping events in the semiconductor layer‐side of the interface and the gate‐dielectric layer‐side of the interface. The validity of this method was tested by designing two model systems in which the physical properties of the semiconductor layer and the gate‐dielectric layer were varied systematically. It was found that the gate‐dielectric layer, in general, plays a more critical role than the semiconductor layer in the bias‐stress effects, possibly due to the wider distribution of the activation energy for charge trapping. Furthermore, the presence of a self‐assembled monolayer further widens the distribution of the activation energy for charge trapping in gate‐dielectric layer‐side of the interface and causes the channel current to decay rapidly in the early stages. The novel analysis method presented here enhances our understanding of charge trapping and provides rational guidelines for developing efficient OFETs with high performance.     

Organic semiconductor heterojunction as charge generation layer in tandem organic light-emitting diodes for high power efficiency

High-performance tandem organic light-emitting diodes (OLEDs) employing a buffer-modified C₆₀/pentacene organic semiconductor heterojunction (OHJ) as a charge generation layer (CGL) are demonstrated. The unique cooperation of charge generation, transport, and extraction processes occurred in the OHJ-based CGL remarkably reduces the operational voltage. As a result, an approximately twofold enhancement in power efficiency (21.9 lm W^?1 VS 10.1 lm W^?1) can be achieved that has previously been suggested to be difficult for tandem OLEDs. When the pentacene is replaced by zinc phthalocyanine (ZnPc), copper phthalocyanine (CuPc), or phthalocyanine (H₂Pc), a similar power efficiency improvement can be also achieved. The novel design concept of the buffer-modified OHJ-based CGL is superior to that of the conventional CGLs. The investigations on the operational mechanism are performed, from which it is found that the mobile charge carriers firstly are needed to be accumulated at both sides of the heterojunction interface and then transport along the two organic semiconductors in terms of their good carrier transport characteristics under an external electrical field, and finally inject into the corresponding electroluminescent (EL) units by the interfacial layers.     

Photomultiplication in Disordered Unipolar Organic Materials

Photomultiplication in conventional inorganic semiconductors has been known and used for decades, the underlying mechanism being multiplication by impact ionization triggered by hot carriers. Since neither carrier heating by an electric field nor avalanche multiplication are possible in strongly disordered organic solids, charge multiplication seems to be highly unlikely in these materials. However, here the photomultiplication observed in the bulk of a unipolar disordered organic semiconductor is reported. The proportion of extracted carriers to incident photons is experimentally determined to be in excess of 3000 % in a single‐layer device of the air‐stable, n‐type organic semiconductor F₁₆CuPc (Pc: phthalocyanine). This effect is explained in terms of exciton quenching by localized charges, the subsequent promotion of these detrapped charges to the high‐mobility energy band of the density‐of‐states (DOS) distribution, and subsequent slow equilibration within this broad intrinsic DOS. Such a mechanism allows multiple replenishment of the optically released charge by mobile carriers injected from an Ohmic electrode. Also shown is photomultiplication in double‐layer devices composed of layers of donor and acceptor small‐molecule materials. This result implies that, apart from exciton dissociation at a donor/acceptor interface, exciton energy transfer to trapped carriers is a complementary photoconductivity process in organic solar cells. This new insight paves the way to cheap, highly efficient organic photodetectors on flexible substrates for numerous applications.     

Air stable C60 based n-type organic field effect transistor using a perfluoropolymer insulator

Air stable n-type organic field effect transistors (OFETs) based on C₆₀ are realized using a perfluoropolymer as the gate dielectric layer. The devices showed the field-effect mobility of 0.049 cm²/V s in ambient air. Replacing the gate dielectric material by SiO₂ resulted in no transistor action in ambient air. Perfluorinated gate dielectric layer reduces interface traps significantly for the n-type semiconductor even in air.     

Surface‐Transfer Doping of Organic Semiconductors Using Functionalized Self‐Assembled Monolayers

Controlling charge doping in organic semiconductors represents one of the key challenges in organic electronics that needs to be solved in order to optimize charge transport in organic devices. Charge transfer or charge separation at the molecule/substrate interface can be used to dope the semiconductor (substrate) surface or the active molecular layers close to the interface, and this process is referred to as surface‐transfer doping. By modifying the Au(111) substrate with self‐assembled monolayers (SAMs) of aromatic thiols with strong electron‐withdrawing trifluoromethyl (CF₃) functional groups, significant electron transfer from the active organic layers (copper(II) phthalocyanine; CuPc) to the underlying CF₃‐SAM near the interface is clearly observed by synchrotron photoemission spectroscopy. The electron transfer at the CuPc/CF₃‐SAM interface leads to an electron accumulation layer in CF₃‐SAM and a depletion layer in CuPc, thereby achieving p‐type doping of the CuPc layers close to the interface. In contrast, methyl (CH₃)‐terminated SAMs do not display significant electron transfer behavior at the CuPc/CH₃‐SAM interface, suggesting that these effects can be generalized to other organic‐SAM interfaces. Angular‐dependent near‐edge X‐ray absorption fine structure (NEXAFS) measurements reveal that CuPc molecules adopt a standing‐up configuration on both SAMs, suggesting that interface charge transfer has a negligible effect on the molecular orientation of CuPc on various SAMs.     

Performance enhancement of p-type organic thin-film transistors by surface modification of hybrid dielectrics

The quality of the dielectric/organic semiconductor interface is a critical issue, because it determines the charge transport properties in organic thin-film transistors (OTFTs). High-k organic-inorganic hybrid films have received considerable attention for their outstanding dielectric properties, including low leakage currents, high breakdown fields, and suitable band offsets against the organic semiconductor. However, Hf and Zr hybrid gate dielectrics on p-type OTFTs show poor charge transport properties in the organic semiconductor channel, due to the polaron disorder elicited by the high-k properties and the presence of the –N(CH₃)₂ polarity (hole trapper) on the dielectric/semiconductor interface. In this report, the surface of the Hf and Zr hybrid dielectrics was capped by an ultra-thin poly-1,3,5-trivinyl-1,3,5,-trimethyl-cyclosiloxane (pV3D3) layer formed via an initiated chemical vapor deposition (iCVD) process, to modify the hybrid dielectrics/semiconductor interface. The pV3D3-capped Hf and Zr hybrid OTFTs show an enhanced V_T stability while a large amount of V_T shift was observed from the Hf and Zr hybrid OTFTs. This large amount of V_T shift is attributed to the hole trap sites originated by –N(CH₃)₂ on the uncapped hybrid dielectrics. Furthermore, the p-type OTFTs with the pV3D3-capped hybrid dielectrics show a higher mobility than those with the uncapped hybrid dielectrics. The presence of the non-polar/low-k pV3D3 on the hybrids contribute to narrow the density of state (DOS) in the organic channel, improving the charge transport properties. This combined approach using the bulk layer of Hf and Zr hybrid films and the pV3D3 capping layer can overcome the limitations of single-layer hybrid dielectrics and improve the overall device performance of the OTFTs.     

The Effect of Gradual Fluorination on the Properties of FnZnPc Thin Films and FnZnPc/C60 Bilayer Photovoltaic Cells

Motivated by the possibility of modifying energy levels of a molecule without substantially changing its band gap, the impact of gradual fluorination on the optical and structural properties of zinc phthalocyanine (F_nZnPc) thin films and the electronic characteristics of F_nZnPc/C₆₀ (n = 0, 4, 8, 16) bilayer cells is investigated. UV–vis measurements reveal similar Q‐ and B‐band absorption of F_nZnPc thin films with n = 0, 4, 8, whereas for F₁₆ZnPc a different absorption pattern is detected. A correlation between structure and electronic transport is deduced. For F₄ZnPc/C₆₀ cells, the enhanced long range order supports fill factors of 55% and an increase of the short circuit current density by 18%, compared to ZnPc/C₆₀. As a parameter being sensitive to the organic/organic interface energetics, the open circuit voltage is analyzed. An enhancement of this quantity by 27% and 50% is detected for F₄ZnPc‐ and F₈ZnPc‐based devices, respectively, and is attributed to an increase of the quasi‐Fermi level splitting at the donor/acceptor interface. In contrast, for F₁₆ZnPc/C₆₀ a decrease of the open circuit voltage is observed. Complementary photoelectron spectroscopy, external quantum efficiency, and photoluminescence measurements reveal a different working principle, which is ascribed to the particular energy level alignment at the interface of the photoactive materials.     

Enhancement of Carrier Mobilities of Organic Semiconductors on Sol–Gel Dielectrics: Investigations of Molecular Organization and Interfacial Chemistry Effects

The dielectric‐semiconductor interfacial interactions critically influence the morphology and molecular ordering of the organic semiconductor molecules, and hence have a profound influence on mobility, threshold voltage, and other vital device characteristics of organic field‐effect transistors. In this study, p‐channel small molecule/polymer (evaporated pentacene and spin‐coated poly(3,3?;‐didodecylquarterthiophene) – PQT) and n‐channel fullerene derivative ({6}‐1‐(3‐(2‐thienylethoxycarbonyl)‐propyl)‐{5}‐1‐phenyl‐[5,6]‐C61 – TEPP‐C61) show a significant enhancement in device mobilities ranging from ～6 to ～45 times higher for all classes of semiconductors deposited on sol–gel silica gate‐dielectric than on pristine/octyltrichlorosilane (OTS)‐treated thermally grown silica. Atomic force microscopy, synchrotron X‐ray diffraction, photoluminescence/absorption, and Raman spectroscopy studies provide comprehensive evidences that sol–gel silica dielectrics‐induced enhancement in both p‐ and n‐channel organic semiconductors is attributable to better molecular ordering/packing, and hence reduced charge trapping centers due to lesser structural defects at the dielectric‐semiconductor interface.     

Electrical instability in short channel organic thin-film transistors induced by lucky-polaron mechanism

In this work we study the electrical stability under both gate bias stress and gate and drain bias stress of short channel (L = 5 μm) bottom contact/top gate OTFTs made on flexible substrate with solution-processed organic semiconductor and fluoropolymer gate dielectric. These devices show high field-effect mobility (μ_FE> 1 cm²V⁻¹s⁻¹) and excellent stability under gate bias stress (bias stress V_ds = 0V). However, after prolonged bias stress performed at high drain voltage, V_ds, the transfer characteristics show a decreased threshold voltage, degradation of the subthreshold slope and an apparent increase in the field effect mobility. Furthermore, the output characteristics show an asymmetry when measured in forward and reverse mode. These experimental results can be explained considering that the bias stress induces the damage of a small part of the device channel, localized close to the source contact. The analysis of the experimental data through 2D numerical simulations supports this explanation showing that the electrical characteristics after bias stress at high V_ds can be reproduced considering the creation of donor-like interface states and trapping of positive charge into the gate dielectric at the source end of the device channel. In order to explain this degradation mechanism, we suggest a new physical model that, assuming holes injection from the source contact into the channel in bounded polarons, envisages the defect creation at the interface near the source end of the channel induced by injection of holes that gained energy from both the high longitudinal electric fields and the polaron dissolution.     

Nanoscale Mapping of the Conductivity and Interfacial Capacitance of an Electrolyte-Gated Organic Field-Effect Transistor under Operation

Probing nanoscale electrical properties of organic semiconducting materials at the interface with an electrolyte solution under externally applied voltages is key in the field of organic bioelectronics. It is demonstrated that the conductivity and interfacial capacitance of the active channel of an electrolyte-gated organic field-effect transistor (EGOFET) under operation can be probed at the nanoscale using scanning dielectric microscopy in force detection mode in liquid environment. Local electrostatic force versus gate voltage transfer characteristics are obtained on the device and correlated with the global current–voltage transfer characteristics of the EGOFET. Nanoscale maps of the conductivity of the semiconducting channel show the dependence of the channel conductivity on the gate voltage and its variation along the channel due to the space charge limited conduction. The maps reveal very small electrical heterogeneities, which correspond to local interfacial capacitance variations due to an ultrathin non-uniform insulating layer resulting from a phase separation in the organic semiconducting blend. Present results offer insights into the transduction mechanism at the organic semiconductor/electrolyte interfaces at scales down to ≈100 nm, which can bring substantial optimization of organic electronic devices for bioelectronic applications such as electrical recording on excitable cells or label-free biosensing.     

表面修饰的ZnPc薄膜晶体管性能研究

以热生长的SiO2作为栅绝缘层,酞菁锌(ZnPc)作为有源层,研究了具有十八烷基三氯硅烷(OTS,C18H37SiCl3)/SiO2双绝缘层结构的有机薄膜晶体管(OTFT)。实验表明,采用OTS可以有效地降低SiO2栅绝缘层的表面能并改善表面的平整度,器件的场效应迁移率提高了3.5倍,漏电流从10-9A降到10-10A,阈值电压降低了5 V,开关电流比从103增加到104。结果显示,具有OTS/SiO2双绝缘层的器件结构能有效改进OTFT的性能。     

Porous Organic Field‐Effect Transistors for Enhanced Chemical Sensing Performances

The thin‐film structures of chemical sensors based on conventional organic field‐effect transistors (OFETs) can limit the sensitivity of the devices toward chemical vapors, because charge carriers in OFETs are usually concentrated within a few molecular layers at the bottom of the organic semiconductor (OSC) film near the dielectric/semiconductor interface. Chemical vapor molecules have to diffuse through the OSC films before they can interact with charge carriers in the OFET conduction channel. It has been demonstrated that OFET ammonia sensors with porous OSC films can be fabricated by a simple vacuum freeze‐drying template method. The resulted devices can have ammonia sensitivity not only much higher than the pristine OFETs with thin‐film structure but also better than any previously reported OFET sensors, to the best of our knowledge. The porous OFETs show a relative sensitivity as high as 340% ppm^?1 upon exposure to 10 parts per billion (ppb) NH₃. In addition, the devices also exhibit decent selectivity and stability. This general and simple strategy can be applied to a wide range of OFET chemical sensors to improve the device sensitivity.     

The Role of OTS Density on Pentacene and C60 Nucleation,Thin Film Growth,and Transistor Performance

In organic thin film transistors (OTFTs), charge transport occurs in the first few monolayers of the semiconductor near the semiconductor/dielectric interface. Previous work has investigated the roles of dielectric surface energy, roughness, and chemical functionality on performance. However, large discrepancies in performance, even with apparently identical surface treatments, indicate that additional surface parameters must be identified and controlled in order to optimize OTFTs. Here, a crystalline, dense octadecylsilane (OTS) surface modification layer is found that promotes two‐dimensional semiconductor growth. Higher mobility is consistently achieved for films deposited on crystalline OTS compared to on disordered OTS, with mobilities as high as 5.3 and 2.3 cm² V^?1 s^?1 for C₆₀ and pentacene, respectively. This is a significant step toward morphological control of organic semiconductors which is directly linked to their thin film charge carrier transport.     

Charge trapping in organic transistor memories: On the role of electrons and holes

In this work, we study charge trapping in organic transistor memories with a polymeric insulator as gate dielectric. We found that the mechanism of charge trapping is tunneling from the semiconductor channel into the gate dielectric. Depending on the semiconductor and its processing, charge trapping can result in large bi-directional threshold voltage shifts, in case the semiconductor is ambipolar, or in shifts in only one direction (unipolar semiconductor). These results indicate that optimal memory performance requires charge carriers of both polarities, because the most efficient method to lower the programming field is by overwriting a trapped charge by an injected charge of opposite polarity.     

Controllable organic nanofiber network crystal room temperature NO2 sensor

We reported an organic room temperature (RT) NO₂ sensor based on zinc phthalocyanine (ZnPc) nanofiber network. Compared with traditional polycrystalline ZnPc film devices, the sensors with ZnPc nanofiber network as the sensitive layer exhibit much better recovery characteristics, which could almost recover without any treatments at room temperature. In ZnPc nanofiber network, ultra-thin ZnPc single-crystal fibers not only improve charge transport but also make the charge exchanging process between NO₂ and sensitive materials easier. It shortens the response and recovery time and stabilizes the baseline of devices. In addition, we also optimized the sensors by varying the scale of the ZnPc nanofiber. The device performance is obviously improved when the scale of the ZnPc nanofiber becomes smaller. The device could naturally recover and the baseline achieves zero drift. It is attributed to the expansion of the ratio of surface area-to-volume (A/V).     

Investigation of turn-on voltage shift in organic ferroelectric transistor with high polarity gate dielectric

Large positive shifts of turn-on voltage V_to were observed in ferroelectric organic thin film transistor using P(VDF-TrFE) copolymer (57–43 mol%) as gate insulator during OFF to ON state sweeping. The shift of the transfer characteristic up to +25 V is attributed to the accumulation of mobile charge carriers (holes) in pentacene layer even during the device OFF state. The observed phenomena were first discussed on the basis of a negative surface potential created by the dipole field of a polar dielectric and trap states in an organic semiconductor layer. It was however found that these were unable to fully address the observed strong V_to shift due to the presence of large polarization in the P(VDF-TrFE) layer. A mechanism of negative polarization-compensating charges which are injected to the insulator region next to the semiconductor layer was proposed and examined to understand the phenomenon. The turn-on voltage is found to change with different magnitude of positive voltage pulses, and corresponds to different amount of charges injected for compensation. Time measurement of drain current shows a transient decaying behavior when gate bias is switched from positive to negative polarity which confirms the trapping of negative charges in the insulator.