Dry growth of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

Abstract

Dry method for monolayer deposition of n-octylphosphonic acid (C 8 PA) on the surface of aluminium oxide (AlO x ) is presented. Vacuum thermal evaporation is employed to deposit initial thickness corresponding to several C 8 PA monolayers, followed by a thermal desorption of the physisorbed C 8 PA molecules. AlO x functionalized with such C 8 PA monolayer exhibits leakage current density of ~ 10

Structural changes in vapour-assembled n-octylphosphonic acid monolayer with post-deposition annealing: Correlation with bias-induced transistor instability

“…Finally, Hill at al [2] performed similar experiment and achieved the best transistor performance for C 8 PA monolayer. In addition, they obtained similar mobilities and threshold voltages for transistors with C 6 PA and C 14 PA monolayers, even though the grain size of pentacene grown on these two monolayers was vastly different.…”

“…For FTIR, a blanket layer of aluminium was thermally evaporated and UV/ozone oxidation was used to form thin AlO x [11] with low surface roughness of ~0.48 nm [8]. C 8 PA was thermally evaporated on top of AlO x and subsequently heated to 160C for 25, 90 and 210 minutes.…”

“…Although there is a general agreement that alkyl phosphonic acids with shorter aliphatic tails form more disordered monolayers, the corresponding transistor performance varies widely. While studying the effect of the aliphatic chain length (C 6 PA-C 18 PA) on thin-film transistors, Acton at al [6] obtained the highest mobility and low threshold voltage for HfO 2 /C n PA/pentacene transistors using C 8 PA. This coincided with the largest pentacene grain size and a large disorder between aliphatic tails of C 8 PA monolayer.…”

“…We have recently developed a solution-free growth of n-octylphosphonic acid (C 8 PA) monolayer using vacuum thermal evaporation [8,9]. In this process, a thickness equal to a few monolayers of n-octylphosphonic acid is deposited on aluminium oxide and the physisorbed molecules are subsequently removed by thermal heating.…”

“…When such a monolayer is implemented in low-voltage organic thin-film transistors (OTFTs) based on pentacene, the performance of the asfabricated transistors substantially changes with the duration of the desorption/postdeposition annealing of C 8 PA monolayer [8], pointing to the importance of the monolayer microstructure in the charge carrier transport within the transistor. Shorter post-deposition annealing can also result in C 8 PA thickness larger than a monolayer (cca 1.5 monolayers) [8] that is mimicking the results obtained during the solution self-assembly [10]. Consequently, such dry process can be used as a tool for correlating the structural changes in the C 8 PA monolayer with the short-and long-term transistor performance, while eliminating the effect of possible solvent traces.…”