Enhancing the Performance of Pentacene-Based Organic Thin Film Transistors by Inserting Stacked N,N\aku '-Diphenyl-N,N\aku '-bis(1-naphthyl-phenyl)-(1,1'-biphenyl)-4,4'-diamine and Tris(8-hydroxyquinolino)-aluminum Buffer Layers

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Organic thin-film transistors (OTFTs) were fabricated using various buffer layers between the active layer and source/drain electrodes. The device structure was glass/indium--tin oxide (ITO)/poly(methyl methacrylate) (PMMA)/pentacene/buffer layer/Ag (source/drain). N,N\aku '-diphenyl-N,N\aku '-bis(1-naphthyl-phenyl)-(1,1'-biphenyl)-4,4'-diamine (NPB), tris(8-hydroxyquinolino)-aluminum (Alq3), Alq3/NPB, and NPB/Alq3 films were used as the buffer layers, respectively. The OTFTs using stacked NPB/Alq3 as a buffer layer exhibited a field-effect mobility of 0.31 cm2 V-1 s-1, on--off drain current ratio of 6.7\times 10^{5}, and threshold voltage of -16.8 V. Additionally, the interface mechanism and contact resistance were determined by ultraviolet photoelectron spectroscopy (UPS) and the transmission line method (TLM). Experimental results indicate that a low energy barrier between the electrode and pentacene enhances the ability of holes to transfer from an electrode to pentacene. Moreover, inserting a buffer layer between the electrode and pentacene reduces the contact resistance. Such an improvement is attributed to the weak interface dipole at the interface of the active layer and electrodes.
2012-04-25