Fabrication and characterization of p⁺-i-p⁺ type organic thin film transistors with electrodes of highly doped polymer

Organic thin film transistors (OTFTs) have been explored because of their advantageous features such as light-weight, flexible, and large-area. For more practical application of organic electronic devices, it is very important to realize OTFTs that are composed only of organic materials. In this paper, we have fabricated p⁺-i-p⁺ type of OTFTs in which an intrinsic (i) regioregular poly (3-hexylthiophene) (P3HT) layer is used as the active layer and highly doped p-type (p⁺) P3HT is used as the source and drain electrodes. The 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) was used as the p-type dopant. A fabricating method of p⁺-i-p⁺ OTFTs has been developed by using SiO₂ and aluminum films as capping layers for micro-scaled patterning of the p⁺-P3HT electrodes. The characteristics of the OTFTs were examined using the photoelectron spectroscopy and electrical measurements. We demonstrated that the fabricated p⁺-i-p⁺ OTFTs work with carrier injection through a built-in potential at p⁺/i interfaces. We found that the p⁺-i-p⁺ OTFTs exhibit better FET characteristics than the conventional P3HT-OTFT with metal (Au) electrodes, indicating that the influence of a carrier injection barrier at the interface between the electrode and the active layer was suppressed by replacing the metal electrodes with p⁺-P3HT layers.