Very low bias stress in n-type organic single-crystal transistors

Abstract

Bias stress effects in n-channel organic field-effect transistors (OFETs) are investigated using PDIF-CN 2 single-crystal devices with Cytop gate dielectric, both under vacuum and in ambient. We find that the amount of bias stress is very small as compared to all (p-channel) OFETs reported in the literature. Stressing the PDIF-CN 2 devices by applying 80 V to the gate for up to a week results in a decrease of the source drain current of only ~1% under vacuum and ~10% in air. This remarkable stability of the devices leads to characteristic time constants, extracted by fitting the data with a stretched exponential -that are τ ~ 2·10 9 s in air and τ ~ 5·10 9 s in vacuumapproximately two orders of magnitude larger than the best values reported previously for pchannel OFETs. KEYWORDS:Bias stress, Single-crystals, organic n-type transistors. Over the last few years, high-quality n-type transistors based on organic semiconductors have been demonstrated 1 , with performance close to that of the best p-type devices 2 , and capable of operating under ambient conditions. Among these new n-type materials, Perylene Diimide molecules have received great attention, due to both their self-assembling properties and the tunability of the LUMO level, resulting in highly robust charge transport properties 3 . Currently, N,7 and 1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide)s (PDIF-CN 2 ) is probably the most attractive compound. Thin films can be deposited both by evaporation 4 and from solution 5 leading to high carrier mobility. Single crystal devices 6 exhibit outstanding properties, including the largest electron mobility values 7 reported for n-type organic FETs and band-like electron transport 8 . With these new molecular materials enabling top quality n-type FETs to be realized, it becomes important to explore all aspects of the device electrical characteristics that could limit the device performance.From a practical point of view, an important issue is the degradation of the field-effect transistor performance under prolonged application of a gate voltage (V GS ). This phenomenon, known as bias stress 9 , consists in the continuous decrease of the drain-source current (I DS ) when the transistor is driven in the accumulation regime. The bias stress effect has been widely investigated for p-channel devices. It was observed that the physical and chemical nature of the dielectric/organic interface plays a major role in determining the magnitude this effect 10,11 , but the precise microscopic origin of the phenomenon has remained elusive. Two possible mechanisms have been proposed for p-channel (hole accumulation) devices: one scenario attributes the effect to holes that are transferred from the FET channel to localized states in the gate insulator 11 ; another scenario invokes hole-assisted generation of protons in the presence of water, with the protons subsequently diffusing into the gate dielectric 10 . Both mechanisms lead to the accumulation of 3 positive charge (protons or holes) in the dielectric, which sc...

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

“…In particular, trap states, which can be induced by residual charge density in the dielectric or active layer, influence the threshold voltage, mobility, and hysteresis in the transfer characteristics. Therefore, a lot of effort have been devoted to finding a thin dielectric and organic single crystals active layer that can maintain high interface quality in combination with high gate efficiency to improve the performance of OFETs In particular p‐ or n‐types single crystal semiconducting active layers including rubrene, pentacene, tatracene, triisopropylsilylethynylpentacene (TIPS‐pentacene), poly(3‐hexylthiopene) (P3HT), N , N 0‐bis(n‐alkyl)‐(1,7 and 1,6)‐dicyanoperylene‐3,4:9,10‐bis(dicarboximide)s (PDIF‐CN2), tetracyanoquinodimethane (TCNQ), fullerene and others, have excellent device properties when combined with dielectrics such as air‐gap, polydimethylsiloxane (PDMS), CYTOP, and polymeric materials . Newly developed dielectrics can preserve the high channel mobility of single crystal active layers by eliminating the interface defects.…”

Single-Molecule Break Junctions Based on a Perylene-Diimide Cyano-Functionalized (PDI8-CN2) Derivative

“…Nowadays, PDI_CY n-type OFET, exhibiting charge carrier mobility higher than 1 cm 2 /Volt sec [ 11 , 12 ] are among the most studied devices in the emerging field of organic electronics [ 5 ] . PDI_CY semiconducting films exhibit also a strong interaction with light and the electrical response of the related devices can be so largely modified through the illumination by visible radiation [ 13 ].…”

Perylene-Diimide Molecules with Cyano Functionalization for Electron-Transporting Transistors

“…Besides analyzing the BS response of devices based on SiO 2 , in our studies, we also assessed the behavior of PDIF-CN 2 transistors employing the fluoro-polymer Cytop TM as a dielectric. In a first report [31], the BS effect was investigated for PDIF-CN 2 single-crystal transistors demonstrating how the combination between a crystalline active layer and a highly hydrophobic dielectric surface allows for achieving n-type devices with very low sensitivity to BS. For an experiment carried out in air, we found that the I DS current was decreased by only 10% under the application of a positive V GS for an entire week (≈ 6 × 10 5 s).…”

“…Different from PDI8-CN 2 , PDIF-CN 2 has been used even to grow high-quality single crystals, exhibiting remarkable performances in terms of mobility values and operation stability under ambient conditions [27][28][29]. The research group lead by Alberto Morpurgo at the University of Geneva carried out very accurate studies on PDIF-CN 2 single-crystal transistors turning this compound into the reference material for analyzing the fundamental properties of electron-transporting organic semiconductors [30,31]. PDIF-CN 2 single crystals were usually fabricated by laminating thin (<1 µm) crystals on different back-gated substrates, bearing a dielectric barrier and pre-patterned gold electrodes.…”