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

Abstract

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 (ß10 17 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.

Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals

“…An experimental measurement of electronic DOS is difficult. Different from the DOS evaluation based on analyses of cur rent-voltage characteristics by conventional electrical measurements [115][116][117] and using Kelvin probe force microscopy [118][119][120], UPS in principle can give more direct information on the DOS because it directly observes, for example, HOMO photoelectrons that are emitted from HOMO upon irradiation with ionizing radiation. Oberhoff et al [116] already pointed out the problem in determining the DOS from the analyses of results of electrical measurements in their paper.…”

Development of an improved Kelvin probe force microscope for accurate local potential measurements on biased electronic devices

“…As the electrical potential distribution is of particular interest for applications such as organic solar cells and organic field effect transistors (OFETs) (Palermo et al ., ), KPFM‐AM has also been extensively applied to characterise organic semiconductor‐based devices (Bürgi et al ., ; Nichols et al ., ; Puntambekar et al ., ; Bürgi et al ., ; Tal et al ., ) in order to investigate transport mechanisms and injection issues in OFETs. As the bottom‐contact structure is suited for potential measurements related to the channel charge distribution, KPFM‐AM imaging has been performed on various types of organic thin film devices based on pentacene (Hallam et al ., ; Nakamura et al ., ; Yogev et al ., ; Yogev et al ., ; Li et al ., ,b; Celle et al ., ; Wu et al ., ; Yogev & Rosenwaks, ), poly(3‐hexylthiophene) (P3HT) (Bürgi et al ., ; Liscio et al ., ; Kehrer et al ., ; Musumeci et al ., ), perylene (Luttich et al ., ) or oligothiopene (Afsharimani & Nysten, ). KPFM also facilitates the characterisation of the carrier injection process at electrode/organic semiconductor interfaces by measuring an injection potential drop (Bürgi et al ., ; Simonetti et al ., ).…”