Engineering Asymmetric Charge Injection/Extraction to Optimize Organic Transistor Performances

The introduction of an appropriate functionality on the electrode/active layer interface has been found to be an efficient methodology to enhance the electrical performances of organic field-effect transistors (OFETs). Herein, we efficiently optimized the charge injection/extraction characteristics of source/drain (S/D) electrodes by applying an asymmetric functionalization at each individual electrode/organic semiconductor (OSC) interface. To further clarify the functionalizing effects of the electrode/OSC interface, we systematically designed five different OFETs: one with pristine S/D electrodes (denoted as pristine S/D) and the remaining ones made by symmetrically or asymmetrically functionalizing the S/D electrodes with up to two different self-assembled monolayers (SAMs) based on thiolated molecules, the strongly electron-donating thiophenol (TP) and electron-withdrawing 2,3,4,5-pentafluorobenzenethiol (PFBT). Both the S and D electrodes were functionalized with TP (denoted as TP-S/D) in one of the two symmetric cases and with PFBT in the other (PFBT-S/D). In each of the two asymmetric cases, one of the S/D electrodes was functionalized with TP and the other with PFBT (to produce PFBT-S/TP-D and TP-S/PFBT-D OFETs). The vapor-deposited p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene was used as the OSC active layer. The PFBT-S/TP-D case exhibited a field-effect mobility (μFET) of 0.86 ± 0.23 cm2 V–1 s–1, about three times better than that of the pristine S/D case (0.31 ± 0.12 cm2 V–1 s–1). On the other hand, the μFET of the TP-S/PFBT-D case (0.18 ± 0.10 cm2 V–1 s–1) was significantly lower than that of the pristine case and even lower than those of the TP-S/D (0.23 ± 0.07 cm2 V–1 s–1) and PFBT-S/D (0.58 ± 0.19 cm2 V–1 s–1) cases. These results were clearly correlated with the additional hole density, surface potential, and effective work function. In addition, the contact resistance (RC) for the asymmetric PFBT-S/TP-D case was 10-fold less than that for the TP-S/PFBT-D case and more than five times lower than that for the pristine case. The results contribute a meaningful step forward in improving the electrical performances of various organic electronics such as OFETs, inverters, solar cells, and sensors.