Essential Effects on the Mobility Extraction Reliability for Organic Transistors

Abstract

The reliability of mobility has come to be a critical issue to the development of new electronics especially for organic electronics, since mobility is typically extracted from field‐effect transistors containing various extrinsic effects and overestimation is popular in the literature. Recently, this issue is emphasized and a reliability factor (r) is proposed by pioneers to gauge the mobility reported. Albeit many factors discussed, how much the extrinsic effects influence r remains unrevealed and a facile solution by using organic transistors is still lacking. Here, it is shown that the widely used extraction method based on the saturation transfer characteristics is sensitive to contact effect and temperature with r dropping to 43%, indeed leading to large mobility overestimation. By contrast, the linear‐regime methods are more reliable particularly the Y‐function method that demonstrates great reliability (r ≈ 100%) even for short‐channel transistors at ultralow temperatures. In addition, operating in saturation regime induces ambipolar conduction further deteriorating reliability if contact doping is absent. High Schottky barriers, on the other side, distort device characteristics making extraction impossible. The results of this study reveal that, aside from device optimizations, selecting a right method is essential for reliable and precise evaluation of the carrier mobility by using organic transistors.

Precise Extraction of Charge Carrier Mobility for Organic Transistors

“…The widely used metal–insulator–metal (MIM) capacitor structure, however, is not suitable for this purpose because it does not contain a semiconductor film and the measured results do not represent the OFET operational characteristics. For a more detailed discussion of the C – V measurements, please refer to the literature …”

“…Additionally, the linear fitting for mobility extraction is performed at low gate voltages, particularly around the threshold voltage, in an attempt to gain the highest possible mobility, as shown in Figure c. Apart from the causes documented in the literature, such as the contact resistance, this type of extraction may be conceptually inappropriate because the related method presumes a charge approximation that is only valid in the strong accumulation regime, i.e., at high gate voltages . Alternatively, ambipolar conduction may predominate the channel conduction at low gate voltages during measurement of the transfer characteristics in the saturation regime; see Figure d .…”

Phthalimide‐Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13%

“…Both PhI-ffBT and ffPhI-ffBT show the p-type dominating transport characteristics (Figure 3) and the OTFTs annealed at 160 °C yield the optimal performance with the highest hole mobility (µ h,OTFT ) of 0.63 and 0.93 cm 2 V −1 s −1 in the saturated regime, respectively. [63] In addition, only a slight hysteresis is observed in the transfer curves for all these phthalimide-based polymers ( Figure S7, Supporting Information), resulting in a small difference between the mobilities extracted from the forward and reverse sweeps ( Figure S9, Supporting Information). Such mobility reduction from saturated to linear regime is due to the contact resistance effect.…”

Organic thin-film transistors with flame-annealed contacts

“…The reliability factors for these two devices are both 0.93 (a reliability factor of 1 corresponds to an ideal device) [30]. This reliability factor implies that, even if bad contacts were present, the true mobility of the devices is within 7% of the measured value, and 0.93 is on par with or betters values reported for other systems [31][32][33], with the exception of single crystals [30]. Measurements on a large set of devices (over 100) resulted in a lower average threshold voltage in the flame annealed OFETs, Vth = 2.7 V ± 1.2 V, compared to Vth = 6.2 V ± 1.2 V in the control, suggesting a lower density of traps in these samples.…”