Air Effect on the Ideality of p‐Type Organic Field‐Effect Transistors: A Double‐Edged Sword

Abstract

Organic field-effect transistors (OFETs) often deviate from ideal behaviors in air, which masks their intrinsic properties and thus significantly impedes their practical applications. A key issue of how the presence of air affects the ideality of OFETs has not yet been fully understood. It is revealed that air atmosphere may exert a double-edged sword effect on the active semiconductor layer when determining the ideality of OFETs fabricated from p-type crystalline organic semiconductors. Upon exposing the as-fabricated device to air, water and oxygen mainly function as efficient p-type dopants for the active layer in the contact regions, enhancing charge carrier injection and consequently improving device ideality. Nevertheless, as the exposure time increases, the trapping centers for the injected minority charge carriers appear in the channel region, leading to degradation of device ideality. Inspired by the double-edged sword behavior of air, a near-ideal OFET is achieved by ingeniously utilizing the doping/positive effect and eliminating the trapping/negative effect. The effect of air on the ideality of p-type OFETs is clarified, which not only illuminates some common observations of OFETs in air but also offers useful guidance for the construction of high-performance ideal OFETs.

Improving Ideality of P‐Type Organic Field‐Effect Transistors via Preventing Undesired Minority Carrier Injection

“…Figures 1c,d schematically illustrate the BGTC OFET and the energy levels of each layer used in this research, respectively. The LUMO level and highest occupied molecular orbit (HOMO) level of dif‐TES ADT were reported at −3.04 and −5.14 eV, [ 31 ] respectively, as illustrated in Figure 1d. And the work function of thermal evaporated Au electrodes used in this research was measured to be 4.83 eV.…”

“…[41,42] When a large positive gate bias is applied, energy levels of dif-TES ADT will shift down by the gate field, hence electrons could be injected from the drain electrode to the LUMO level through thermionic field emission (as illustrated in Figure 1e). [31] It has been evidenced that these injected electrons could be readily captured by the water and oxygen absorbed in the active layer or at the dielectric surface, inducing undesired trapping effects. [21,34] Suffering from the electron injection effect, the as-fabricated device usually shows a typical double-slope behavior, exhibiting a peak transconductance (g m ) at low gate voltage (V G ) region, as shown in Figure 1f.…”

“…C10-BTBT (molecular structure is shown in the left of Figure 2a) is a type of widely used wide-bandgap OSCs, with which the fabricated OFETs have been proved to be hardly affected by the electron injection and could show near-ideal behaviors in air condition. [31,48] For this reason, it was introduced atop the dif-TES ADT MBs to block the undesired electron injection. To gain more insights into the roles of C10-BTBT layer in this device structure, the energy levels of the evaporated C10-BTBT layer were examined.…”

“…[ 22,25–29 ] For p‐type OFETs fabricated with narrow‐bandgap polymers and small molecules, recent researches have provided evidence that the observed double‐slope behaviors could also originate from undesirable electron (the minority charge carriers in p‐type OFETs) injection. [ 30–32 ] Due to the narrow bandgap of the active organic semiconductors (OSCs), electrons could be readily injected from the drain electrode when a positive gate bias is applied under device off‐state mode. These injected electrons could then be trapped by the water/oxygen migrated into the organic layer or adsorbed at the dielectric interface.…”

Suppressing Interface Strain for Eliminating Double‐Slope Behaviors: Towards Ideal Conformable Polymer Field‐Effect Transistors

“…More importantly, the nonidealized behavior not only limits the current driving capability but also reduces the switching speed of device, which severely hampers their practical applications. [9,10] Recently, great efforts have been focused on exploring the originations of the non-idealized behavior of PFETs, and concluded that trapping centers at polymer semiconductor (PSC)/ dielectric interfaces, [11][12][13][14] Coulombic interactions between charge carriers, [15][16][17] contact resistance effects, [18][19][20] and air effects [21,22] are the primary sources. Among them, the existence of trapping centers at PSC/dielectric interfaces is well accepted to explain the non-idealized behaviors of PFETs in air atmosphere.…”