Highly Conductive Ultrathin Layers of Conjugated Polymers for Metal-Free Coplanar Transistors with Single-Polymer Transport Layers

Although metal or oxide conductive films are widely used as electrodes of electronic devices, organic electrodes would be more favorable for next-generation organic electronics. Here, using some model conjugated polymers as examples, we report a class of highly conductive and optically transparent polymer ultrathin layers. Vertical phase separation of semiconductor/insulator blends leads to a highly ordered two-dimensional (2D) ultrathin layer of conjugated-polymer chains on the insulator. Afterwards, the thermally evaporated dopants on the ultrathin layer lead to a conductivity of up to 10 3 S cm –1 and a sheet resistance 10 3 Ω/square for a model conjugated polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2- b ]thiophenes) (PBTTT). The high conductivity is due to the high hole mobility (∼ 20 cm 2 V –1 s –1 ), although doping-induced charge density is still in the moderate range of 10 20 cm –3 with a 1 nm thick dopant. Metal-free monolithic coplanar field-effect transistors using the same conjugated-polymer ultrathin layer with alternatively doped regions as electrodes and a semiconductor layer are realized. The field-effect mobility of this monolithic transistor is over 2 cm 2 V –1 s –1 for PBTTT, one order higher than that of the conventional PBTTT transistor using metal electrodes. The optical transparency of the single conjugated-polymer transport layer is over 90%, demonstrating a bright future for all-organic transparent electronics.