Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.     

An Electrical Rectifier Based on Au Nanoparticle Array Fabricated Using Direct‐Write Electron Beam Lithography

Close‐packed arrays of Au nanoparticles are produced in patterned regions by electron beam (e‐beam) lithography using a highly sensitive direct–write resist, N⁺AuCl₄^?(C₈H₁₇)₄Br. While the e–beam causes dewetting of the resist to nucleate Au nanoparticles, the following step of thermolysis aids particle growth and removal of the organic part. Thus formed arrays contain Au nanoparticles. Such arrays are patterned into ≈10 μm wide stripes between Au contact pads on SiO₂/Si substrates to realize electrical rectification. Under forward bias, the device exhibits a threshold voltage of +4.3 V and a high current rectification ratio of 3 × 10⁵, which are stable over many repetitive measurements. The threshold voltage of the rectifier can be reduced by applying an electric stress or by varying the electron dosage used for array formation. The nanoparticle rectifier element could be transferred onto flexible substrates such as PDMS, where the nanoparticle coupling is influenced by swelling of the substrate. Obviously, the nanoparticle size, shape, and the spacing in array are all important for the rectifier device performance. Based on the electrical measurements the mechanism of rectification is found to be due to switching of electrical conduction with applied bias, from short–distance tunneling to F–N type tunneling followed by transient filament formation.     

Half-wave organic-rectifiers with donor/acceptor assemblies in the molecular scale

We fabricate molecular rectifiers based on a monolayer of a donor molecule and a monolayer of an acceptor molecule in sequence. We characterize the donor/acceptor assemblies under ac voltage. We show that high-frequency half-wave rectifiers can be fabricated and characterized from the molecular assembly. From the frequency-response of the half-wave rectifiers, we study time responses of the processes that are responsible for the observed molecular rectification. We comment on the three steps of a molecular rectifier – the steps being electron-transfer to the acceptor moiety, electron-withdrawal from the donor moiety, and charge-transfer between an anionic-acceptor and a cationic-donor. The results show that the latter process is a slow one that may be due to conformational-change of the anionic-acceptors upon oxidation.     

Realizing the Potential of ZnO with Alternative Non‐Metallic Co‐Dopants as Electrode Materials for Small Molecule Optoelectronic Devices

High performance indium tin oxide (ITO)‐free small molecule organic solar cells and organic light‐emitting diodes (OLEDs) are demonstrated using optimized ZnO electrodes with alternative non‐metallic co‐dopants. The co‐doping of hydrogen and fluorine reduces the metal content of ZnO thin films, resulting in a low absorption coefficient, a high transmittance, and a low refractive index as well as the high conductivity, which are needed for the application in organic solar cells and OLEDs. While the established metal‐doped ZnO films have good electrical and optical properties, their application in organic devices is not as efficient as other alternative electrode approaches. The optimized ZnO electrodes presented here are employed in organic solar cells as well as OLEDs and allow not only the replacement of ITO, but also significantly improve the efficiency compared to lab‐standard ITO. The enhanced performance is attributed to outstanding optical properties and spontaneously nanostructured surfaces of the ZnO films with non‐metallic co‐dopants and their straightforward integration with molecular doping technology, which avoids several common drawbacks of ZnO electrodes. The observations show that optimized ZnO films with non‐metallic co‐dopants are a promising and competitive electrode for low‐cost and high performance organic solar cells and OLEDs.     

The Relationship between Structural and Electrical Characteristics in Perylenecarboxydiimide‐Based Nanoarchitectures

The controlled assembly of the prototypical n‐type organic semiconductor N,N′‐1H,1H‐perfluorobutyl dicyanoperylenecarboxydiimide (PDIF‐CN₂) into ordered nanoarchitectures and the multiscale analysis of the correlation between their structural and their electrical properties is reported. By making use of the Langmuir–Blodgett (LB) technique, monolayers of PDIF‐CN₂ arranged in upright standing molecular packing on different substrates are formed. Postdeposition thermal treatment makes it possible to trigger a reorganization into layered ultrathin crystalline nanostructures, exhibiting structural and photophysical properties similar to those of microscopic crystals obtained by solvent‐induced precipitation. The controlled engineering of these molecular architectures on surfaces enables us to identify both a dependence of the monolayer resistance on the molecular tilt angle in vertical junctions and a pronounced charge‐transport anisotropy with enhanced transport along the π–π stacking direction of the PDI core. While a charge carrier mobility for electrons as high as 10^–2 cm² V^–1 s^–1 is determined in monolayer field‐effect transistors for the in‐plane direction, being the highest yet reported value for a n‐type LB monolayer, the out‐of‐plane mobility measured by conductive atomic force microscopy in multilayered structures is found to be one order of magnitude lower.     

High‐Mobility n‐Type Organic Transistors Based on a Crystallized Diketopyrrolopyrrole Derivative

A new high‐performing small molecule n‐channel semiconductor based on diketopyrrolopyrrole (DPP), 2,2′‐(5,5′‐(2,5‐bis(2‐ethylhexyl)‐3,6‐dioxo‐2,3,5,6‐tetrahydropyrrolo[3,4‐c]pyrrole‐1,4‐diyl)bis(thiophene‐5,2‐diyl))bis(methan‐1‐yl‐1‐ylidene)dimalononitrile (DPP‐T‐DCV), is successfully synthesized. The frontier molecular orbitals in this designed structure are elaborately tuned by introducing a strong electron‐accepting functionality (dicyanovinyl). The well‐defined lamellar structures of the crystals display a uniform terrace step height corresponding to a molecular monolayer in the solid‐state. As a result of this tuning and the remarkable crystallinity derived from the conformational planarity, organic field‐effect transistors (OFETs) based on dense‐packed solution‐processed single‐crystals of DPP‐T‐DCV exhibit an electron mobility (μ_e) up to 0.96 cm² V^?1 s^?1, one of the highest values yet obtained for DPP derivative‐based n‐channel OFETs. Polycrystalline OFETs show promise (with an μ_e up to 0.64 cm² V^?1 s^?1) for practical utility in organic device applications.     

Modulating the Ferromagnet/Molecule Spin Hybridization Using an Artificial Magnetoelectric

Spin‐polarized charge transfer at the interface between a ferromagnetic (FM) metal and a molecule can lead to ferromagnetic coupling and to a high spin polarization at room temperature. The magnetic properties of these interfaces can not only alter those of the ferromagnet but can also stabilize molecular spin chains with interesting opportunities toward quantum computing. With the aim to enhance an organic spintronic device's functionality, external control over this spin polarization may thus be achieved by altering the ferromagnet/molecule interface's magnetic properties. To do so, the magnetoelectric properties of an underlying ferroelectric/ferromagnetic interface are utilized. Switching the ferroelectric polarization state of a PbZr_0.2Ti_0.8O₃ (PZT) bottom layer within a PZT/Co/FePc‐based (Pc ‐ phthalocyanine) device alters the X‐ray magnetic circular dichroism of the Fe site within the phthalocyanine molecular top layer. Thus, how to electrically alter the magnetic properties of an interface with high spin polarization at room temperature is demonstrated. This expands electrical control over spin‐polarized FM/molecule interfaces, which is first demonstrated using ferroelectric molecules, to all molecular classes.     

Enhanced Performance in Polymer Solar Cells by Surface Energy Control

Enhanced performance of an inverted‐type polymer solar cell is reported by controlling the surface energy of a zinc oxide (ZnO) buffer layer, on which a photoactive layer composed of a polymer:fullerene‐derivative bulk heterojunction is formed. With the approach based on a mixed self‐assembled monolayer, the surface energy of the ZnO buffer layer can be controlled between 40 mN m^?1 and 70 mN m^?1 with negligible changes in its work function. For the given range of surface energy the power conversion efficiency increases from 3.27% to 3.70% through enhanced photocurrents. The optimized morphology obtained by surface energy control results in the enhanced photocurrent and transmission electron microscopy analysis verifies the correlation between the surface energy and the phase morphology of the bulk heterojunction. These results demonstrate that surface energy control is an effective method for further improving the performance of polymer solar cells, with potentially important implications for other organic devices containing an interface between a blended organic active layer and a buffer or an electrode layer.     

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.     

Interface engineering for enhancement in performance of organic/inorganic hybrid heterojunction diode

Interface engineering through self-assembled monolayer (SAM) is an efficient way for tailoring the work function and electronic property of surface; enhancing the charge injection efficiency and device performance for microelectronics applications. Despite this, there is lack of study on effect of interface engineering of organic/inorganic hybrid heterojunction diode through SAM. Here, we have reported the surface engineering for tailoring the surface work function, electronic property, enhancement in injection efficiency and device performance. Therefore, Zinc Oxide (ZnO) film surface was modified with SAM before formation of hybrid ZnO/poly(3-hexylthiophene) (P3HT) heterojunction diode and compared with unmodified ZnO/P3HT diode. Prior to measurement of J-V heterojunction characteristics, both interfaces were characterized using absorption spectra, grazing incidence X-ray diffraction (GIXD), scanning electron microscopy (SEM), atomic force microscopy (AFM), kelvin probe force microscopy (KPFM). The modification of ZnO with SAM prior to heterojunction formation allows the better fabrication of diodes featuring ∼10 fold enhancement in rectification ratio at ±3 V and ∼32 fold enhancement in forward current density at 3 V with advancement in electronic device parameter. The enhancement in electrical characteristics are also discussed taking into account the absorption spectra, structural analysis, surface morphology, topography, surface potential, barrier height, and the energy band diagram of SAM's modified and unmodified diodes. Our study has cemented a path to further improve the device performance and parameter for electronic applications.     

Heterojunction Ambipolar Organic Transistors Fabricated by a Two‐Step Vacuum‐Deposition Process

Ambipolar organic field‐effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two‐step vacuum‐deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 °C) acts as the first (p‐type component) layer, and hexadecafluorophthalocyaninatocopper (F₁₆CuPc) deposited at room temperature (25 °C) acts as the second (n‐type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10^–4 cm² V^–1 s^–1 in air. Conductive channels are at the interface between the F₁₆CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F₁₆CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin‐film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum‐deposition process. The structure of interpenetrating networks is similar to that of the bulk heterojunction used in organic photovoltaic cells, therefore, it may be helpful in understanding the process of charge collection in organic photovoltaic cells.     

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.     

Organic-inorganic Au/PVP/ZnO/Si/Al semiconductor heterojunction characteristics

The paper reports the fabrication and characterization of a novel Au/PVP/ZnO/Si/Al semiconductor heterojunction (HJ) diode. Both inorganic n type ZnO and organic polyvinyl pyrrolidone (PVP) layers have grown by sol-gel spin-coating route at 2000 rpm. The front and back metallic contacts are thermally evaporated in a vacuum at pressure of 10^-6 Torr having a diameter of 1.5 mm and a thickness of 250 nm. The detailed analysis of the forward and reverse bias current-voltage characteristics has been provided. Consequently, many electronic parameters, such as ideality factor, rectification coefficient, carrier concentration, series resistance, are then extracted. Based upon our results a non-ideal diode behavior is revealed and ideality factor exceeds the unity (n > 4). A high rectifying (~4.6×10⁴) device is demonstrated. According to Cheung-Cheung and Norde calculation models, the barrier height and series resitance are respectively of 0.57 eV and 30 kΩ. Ohmic and space charge limited current (SCLC) conduction mechanisms are demonstrated. Such devices will find applications as solar cell, photodiode and photoconductor.     

Influences of Connecting Unit Architecture on the Performance of Tandem Organic Light‐Emitting Devices

The present work investigates the influence of the n‐type layer in the connecting unit on the performance of tandem organic light‐emitting devices (OLEDs). The n‐type layer is typically an organic electron‐transporting layer doped with reactive metals. By systematically varying the metal dopants and the electron‐transporting hosts, we have identified the important factors affecting the performance of the tandem OLEDs. Contrary to common belief, device characteristics were found to be insensitive to metal work functions, as supported by the ultraviolet photoemission spectroscopy results that the lowest unoccupied molecular orbitals of all metal‐doped n‐type layers studied here have similar energy levels. It suggests that the electron injection barriers from the connecting units are not sensitive to the metal dopant used. On the other hand, it was found that performance of the n‐type layers depends on their electrical conductivities which can be improved by using an electron‐transporting host with higher electron mobility. This effect is further modulated by the optical transparency of constituent organic layers. The efficiency of tandem OLEDs would decrease as the optical transmittance decreases.     

n‐Type Quinoidal Oligothiophene‐Based Semiconductors for Thin‐Film Transistors and Thermoelectrics

Organic electronic devices have gained immense popularity in the last 30 years owing to their increasing performance. Organic thin‐film transistors (OTFTs) are one of the basic organic electronic devices with potential industrial applications. Another class of devices called organic thermoelectric (OTE) materials can directly transform waste heat into usable electrical power without causing any pollution. p‐Type transistors outperform n‐type transistors because the latter requires a lower orbital energy level for efficient electron injection and stable electron transport under ambient conditions. Aromatic building blocks can be utilized in constructing n‐type semiconductors. Quinoidal compounds are another promising platform for optoelectronic applications because of their unique properties. Since their discovery in 1970s, quinoidal oligothiophene‐based n‐type semiconductors have drawn considerable attention as candidates for high‐performance n‐type semiconductors in OTFTs and OTEs. Herein, the development history of quinoidal oligothiophene‐based semiconductors is summarized, with a focus on the molecular design and the influence of structural modification on molecular packing and thus the device performance of the corresponding quinoidal oligothiophene‐based semiconductors. Insights on the potential of quinoidal oligothiophenes for high‐performance n‐type OTFTs and OTEs are also provided.     

Cyano‐Substituted Oligothiophenes: A New Approach to n‐Type Organic Semiconductors

A series of α,ω‐cyano oligothiophenes with three to six rings, as well as seven β,β′‐substituted cyano terthiophenes have been synthesized using a palladium‐catalyzed coupling reaction via organotin or organozinc intermediates. The structure of an oliothiophene trimer has been determined by X‐ray crystallography; its space group is monoclinic (C2/c) with four molecules per unit cell (Z = 4). The molecules adopt the π–π stacking structure. UV‐vis spectra of these materials as thin films show a bathochromic shift compared to unsubstituted oligothiophenes. These bathochromic shifts are interpreted in the light of charge transfer exciton. Cyano end‐capped sexithiophene (CN‐6T‐CN) sandwiched between various metals (metal/CN‐6T‐CN/metal), to form Schottky diode structures, were fabricated by vapor deposition. The electron injection and rectification ratio strongly depend on the metal contact, namely the work function of the metal is compatible with the electron affinity of the organic material. The current–voltage results are compatible with n‐type conduction in CN‐6T‐CN.     

异质诱导生长的酞菁铜薄膜晶体管的研究

因酞菁薄膜平面具有多电子共轭大π键结构,本文采用异质诱导的方式对酞菁薄膜的生长特性进行了改善研究。采用高掺杂硅为栅极,氧化硅为绝缘层,生长α-四噻吩或p-六联苯薄膜为异质诱导层,制备了酞菁铜有机薄膜晶体管。利用原子力显微镜研究薄膜生长特性,并对比研究了2种诱导层对薄膜晶体管性能的影响。实验结果表明:α-四噻吩上生长的酞菁铜薄膜,形貌呈片状,而p-六联苯上生长的酞菁铜薄膜,形貌呈针状,均与单层酞菁铜棒状形貌不同。同时,α-四噻吩与p-六联苯薄膜上生长酞菁铜后,两者晶体管电性能都有不同程度的提高,均比单层酞菁铜提高了1~2个数量级,表明α-四噻吩或p-六联苯对酞菁铜薄膜均有诱导效应,可以获得高性能的有机薄膜晶体管。     

Gold Nanoparticles Stabilized by Acetylene‐Functionalized Multidentate Thioether Ligands: Building Blocks for Nanoparticle Superstructures

Aiming at the formation of inorganic/organic hybrid gold nanoparticle superstructures, the design and synthesis of acetylene‐monofunctionalized multidentate thioether ligands and their ability to stabilize gold nanoparticles are presented. Rather monodisperse gold particles with diameters of about 1 nm are obtained, which are coated by a small number of ligands, each comprising a silyl‐protected acetylene. The acetylene is attached at the end of a rigid ethynylene‐phenylene unit of variable length, leading to functionalized gold nanoparticles carrying acetylenes at different distances from the nanoparticle surface. These particles are interlinked by diacetylene formation and are investigated by transmission electron microscopy and UV/vis spectroscopy, revealing the formation of nanoparticle aggregates and small superstructures such as dimers or trimers while the nanoparticles themselves retain their integrity. The interparticle distance in small nanoparticle superstructures reflects the ethynylene‐phenylene spacer length corroborating the wet chemical interlinking as the origin of these organic/inorganic hybrid structures.     

Vertically Aligned Hybrid Core/Shell Semiconductor Nanowires for Photonics Applications

A family of 1D organic/inorganic core/shell materials formed by an inner organic nanowire (ONW) conformally covered with an inorganic wide band gap semiconductor (ZnO or TiO₂) layer is presented. The developed procedure is a two‐steps vacuum methodology involving the formation of supported single crystal small‐molecule nanowires by physical vapor deposition and plasma enhance chemical vapor deposition (PECVD) of the inorganic shell. Critical characteristics of the last technique are the possibilities of low temperature and remote configuration deposition. Additionally, an initial step has to be included in order to create nucleation centers for the growth of the ONWs. The procedure and its general character in terms of the variability in organic core and inorganic shells composition and the applicability of the technique to different substrates are presented. The formation of the inorganic shell with no damage of the organic core single‐crystalline structure is demonstrated by high resolution transmission electron microscopy. The vertical alignment of the hybrid nanostructure is achieved thanks to the interaction of the 1D organic nanostructured surfaces and the glow discharge during the deposition of the inorganic shell by PECVD. The optical properties of these core/shell NWs are studied by fluorescence spectroscopy and microscopy, and their application as nanoscale waveguides in the 550–750 nm range addressed.     

有机无机复合膜发光器件的研究

针对聚合物电致发光（EL）材料缺乏可用的电子型聚合物半导体材料的现状，采用无机电子型半导体材料ZnO：Zn与空穴型聚合物材料poly（2，5bis（dodecyloxy）-phenylenevinylene）（PDDOPV）成功制备了结构为ITO／PDDOPV／ZnO：Zn／Al的有机／无机复合膜双层器件。复合膜器件的发光效率与亮度比单层器件提高了1个数量级以上，而复合膜的电流是单层器件的0．5倍。而且，聚合物／无机物复合膜器件的发光颜色随电压的增加而蓝移，其光致发光（PL）光谱也随激发波长的改变而改变，有可能形成了新的发光基团。