Organic heterostructures in organic field-effect transistors

Abstract

C onsiderable eff ort has been devoted to the development of low-cost, fl exible, large-area organic electronics for consumer products over the past two decades [1][2][3]. Organic fi eld-eff ect transistors (OFETs), important organic electronic devices, are of considerable interesting due to their wide range of potential applications, including use as display drivers and in identifi cation tags and sensors. Many methods have been developed to improve OFET performance by increasing mobility and the on/off ratio, and by reducing the threshold voltage. Th ese improvements have been achieved through the synthesis of new organic semiconductor materials, improving the device structure, controlling the deposition of crystalline organic fi lms and adopting various organic heterostructures. Recently, OFETs exhibiting mobility of the same order of magnitude as that of amorphous silicon FETs have been successfully demonstrated.Organic heterostructures have been used in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells to improve device performance. In a typical double-layer OLED structure [4], the organic heterojunction reduces the onset voltage and improves the illumination effi ciency. Organic heterojunctions have also been used to improve the power conversion effi ciency of OPV cells by an order of magnitude over single-layer cells [5]. Ambipolar OFETs, which require that both electrons and holes be accumulated and transported in the device channel depending on the applied voltage, were fi rst realized by introducing organic heterostructures as active layers [6]. It is therefore clear that organic heterostructures have an important role in the continued development of organic electronic devices. Th e introduction of organic heterostructures has signifi cantly improved device performance and allowed new functions in many applications, and so understanding the eff ects of the organic heterostructure is desirable and necessary.Th ere has been considerable focus on understanding the interfacial electronic structures of organic heterostructure consisting of amorphous or semi-crystalline organic semiconductors [7][8][9]. Various models have been proposed to predict the alignment of the vacuum level and the interface dipole in certain organic heterojunctions [8,9], and a dependence of the interface dipole on the molecular orientation has been reported [10]. In crystalline organic semiconductors, the phenomenon of band bending [11,12] has been observed at organic/inorganic and organic/organic interfaces, and band transport, in which orbital-derived electronic bands are produced due to the overlap of the π-orbitals of adjacent molecules, has also been argued [13].Th e recent discovery of high conductivity in heterojunction transistors constructed using thin crystalline fi lms of p-type copper phthalocyanine (CuPc) and n-type copper-hexadecafl uoro-phthalocyanine (F 16 CuPc) as active layers has stimulated interest in organic heterojunctions [14][15][16]. Electron-and hole-accumulation layers have been ob...

Relation between Interfacial Band‐Bending and Electronic Properties in Organic Semiconductor Pentacene

“…[1][2][3] Interfacial band-bending has been observed in many high-mobility organic semiconductor materials, which plays an important role to realize the function of semiconductor devices, or improve the device performance. [4,5] For example, ambipolar transport was improved, [6] and the threshold voltages were tuned due to the band-bending in organic heterojunctions. [7] Moreover, heterojunction films with high conductivity were also used as connecting layer in tandem devices.…”

“…The electrostatic model could be more suitable according to the relative position of work function. [4,23] Low work function, i.e., high Fermi level, represents a large electrostatic potential of electrons. Upon contact formation, electrons will flow from high Fermi level to low Fermi level, and holes will show a reverse process, which leads to the redistribution of potential and band-bending until the alignment of Fermi level.…”

Single‐Crystal C60 Needle/CuPc Nanoparticle Double Floating‐Gate for Low‐Voltage Organic Transistors Based Non‐Volatile Memory Devices

“…This indicates combined effects from the N-C 60 morphology and the charge transfer at the p/n heterojunction interface. [ 43,44 ] On the other hand, N-C 60 with a high concentration (for example, N-C 60 -17 and N-C 60 -45 devices) leads to an obvious increase in the leakage current (in other words, decrease in ON/OFF ratio) owing to the dense/ cross distributed N-C 60 , which reduces the activation energy for the hopping transport process of the charge carriers. The memory window is signifi cantly upgraded by the charge traps on going from the N-C 60 -5 (1.1 V) to the N-C 60 -7 devices (3 V), followed by a small hysteresis for the N-C 60 -17 and N-C 60 -45 (less than 1 V) devices.…”

Controlled Ambipolar Charge Transport Through a Self‐Assembled Gold Nanoparticle Monolayer

“…[21] In this respect, organic heterostructures play an important role in the continued development of organic electronic devices. [22] In organic heterojunction transistors, the charge accumulation and transport of charge carriers (holes and electrons) usually occur in different layers. Tremendous effects have been made towards developing methods to balance electron/hole mobilities, such as interface engineering, [23][24][25][26][27] material optimization, [28] special treatment, [29] or split gate technology.…”

“…[38] AFM images of C 60 and pentacene with a monolayer of Au NPs are shown in Figures 2e and g, which exhibit no sig-of traps in the pentacene layer, causing the device to operate with more positive V S for p-type operation. [22] Another reasonable explanation for this phenomenon is that the holes that are trapped in the low-mobility states of C 60 might be injected into the high-mobility states of pentacene during programming. [21] Furthermore, the existing built-in field at the heterojunction interface could also induce V S shift.…”