Low density of gap states and unpinned Fermi level in n -channel organic thin-film transistors

Electronic states within the band gap of a semiconductor have a significant influence on the electronic properties and the operational stability of electronic devices, especially in the case of organic semiconductors. Previous work has shown that the gap-state density in organic thin-film transistors (TFTs) is often so large that the Fermi level is pinned deep within the gap. For p-channel organic TFTs, we have recently reported that the density of gap states and hence the pinning of the Fermi level can be greatly reduced by optimizing the fabrication process. For n-channel organic TFTs, very little is known about the gap density of states and its impact on the performance and stability of the devices. Here, we report measurements of the density and distribution of gap states in n-channel TFTs based on two different organic semiconductors. We have found that the gap-state density in both semiconductors is relatively small (∼1017 cm-3) and no greater than in p-channel organic TFTs fabricated with the same technology and the same gate dielectric. This is a significant finding, since both the performance and the stability of n-channel organic TFTs are often greatly inferior to those of p-channel organic TFTs. We have also found that the gap-state density increases over time during air exposure and that this increase in gap-state density correlates with the time-dependent, air-induced decrease in electron mobility.