Carrier transport in benzodithiophene-dimer field-effect transistors with pentacene crystallinity control layers

Abstract

The carrier transport in the benzodithiophene ͑BDT͒-dimer films plays significant role for high carrier mobility of BDT-dimer field-effect transistors ͑FETs͒ using very thin pentacene layers as crystallinity control layers ͑CCLs͒. When the grains of the BDT-dimer films are small, the carrier mobility is not only low but also depends on the average thickness of the pentacene-CCLs. However, it is independent from the thickness of the CCLs when the grain size of the BDT-dimer films is large enough, and becomes as high as 0.76 cm 2 V −1 s −1 . This result shows that the high carrier mobility of pentacene-CCL/BDT-dimer FETs is due to the carrier transport in the BDT-dimer films.Organic field-effect transistors ͑FETs͒ have been intensively studied because they are promising for light-weight, flexible, large-area, and low-cost electronics. 1 Although pentacene FETs show carrier mobility as high as that of amorphous silicon FETs, 2,3 they rapidly degrade in air because pentacene is easily oxidized. 4,5 Therefore, we have been developing FETs using benzodithiophene ͑BDT͒ dimers, of which molecular structure is shown in Fig. 1͑a͒, because they are stable in air. 5-7 In our previous study, we have significantly improved the crystallinity of vacuum-evaporated BDT-dimer films by using very thin pentacene films of ϳ90% surface coverage as crystallinity control layers ͑CCLs͒, and fabricated high-mobility pentacene-CCL/BDTdimer FETs, which showed maximum hole mobility of 1.2 cm 2 V −1 s −1 . 8 However, it remained to be a question that the high mobility of the pentacene-CCL/BDT-dimer FETs is due to the BDT-dimer films or to the pentacene-CCLs because of irreproducibility of the fabricated FETs. In this letter, we report on the dependence of the carrier mobility on the grain size of the BDT-dimer films. This result shows that the high mobility of pentacene-CCL/BDT-dimer FETs is due to the carrier transport in the BDT-dimer films.A schematic illustration of the pentacene-CCL/BDTdimer FETs is shown in Fig. 1͑b͒. A 60-nm-thick poly͑methyl-methacrylate͒ ͑PMMA͒ buffer layer was spincoated on a 300-nm-thick thermally oxidized SiO 2 layer formed on an n + -Si ͑001͒ substrate. The sub-monolayer-thick pentacene-CCL and the 30-nm-thick BDT-dimer film were successively deposited on the PMMA buffer layer by evaporation under a pressure of ϳ5 ϫ 10 −5 Pa at room temperature. The deposition rate of the pentacene-CCLs was 0.01 nm/s, and that of the BDT-dimer films was varied between 0.002 and 0.018 nm/s. The film thickness was measured using a quartz crystal thickness monitor, and the indicated "nominal thickness" was used as the reference of the average thickness of the deposited organic films. The sample was, then, transferred to another vacuum chamber through air to deposit 30-nm-thick gold films through a shadow mask to define the source and drain electrodes. The channel length and width of the FETs were 20-60 m and 2 mm, respectively. The surface morphology of the organic films was observed by atomic force microscopy ͑AFM͒; height images we...