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 103 S cm-1 and a sheet resistance 103 CI/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 (similar to 20 cm2 V-1 s-1), although doping-induced charge density is still in the moderate range of 1020 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 cm2 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.

Automated summary

Although metal or oxide conductive films are widely used, organic electrodes have advantages for next-generation organic electronics. A class of highly conductive and optically transparent polymer ultrathin layers are reported here, using model conjugated polymers as examples. These ultrathin layers have high conductivity due to the vertical phase separation of semiconductor/insulator blends and the thermally evaporated dopants. Metal-free monolithic coplanar field-effect transistors with high field-effect mobility and optical transparency are realized using the same ultrathin layers as electrodes.