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

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 (Alq 3 ), Alq 3 /NPB, and NPB/Alq 3 films were used as the buffer layers, respectively. The OTFTs using stacked NPB/Alq 3 as a buffer layer exhibited a field-effect mobility of 0.31 cm 2 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.