Alternating Copolymers Containing Bithiophene and Dialkoxynaphthalene for the Applications to Field Effect Transistor and Photovoltaic Cell: Performance and Stability

Poly(5′,5′′-bithiophene-alt-2,6-[(1,5-didecyloxy)naphthalene]) (PBDN) was synthesized from 2,6-dibromo-l,5-didecyloxynaphthalene and 1,1′-[2,2′-bithiophene]-5,5′-diylbis[1,1,1-trimethylstannane] and was used as the active layer in organic thin-film transistors (OTFTs) and organic photovoltaic cells (OPVs). The obtained PBDN was soluble in organic solvents such as chloroform, chlorobenzene, and toluene and had a weight-averaged molecular weight of 9100, with a polydispersity index of 1.31. The photoluminescence (PL) maximum of the polymer was found at 500 and 530 nm in solution and at 567 nm in the film state, respectively. The highest occupied molecular orbital (HOMO) level of PBDN was low (−5.38 eV, ultraviolet photoemission spectroscopy and cyclic voltammetry), and the solution-processed thin-film transistors (TFTs) prepared using this polymer only showed a minimal change in their performance (<15%) after air exposure for three months, thereby retaining a field-effect-transistor (FET) mobility of 0.02 cm 2 /(V s). This excellent air stability is superior to those of other solution-processed polymer-based OTFTs. Analysis of the thin-film structure by in situ grazing-incidence X-ray diffraction, near-edge X-ray absorption fine structure spectroscopy, and atomic force microscopy showed that not only the low HOMO level of PBDN but also the presence of close-packed frustrated structures in the polymer film were responsible for the superior stability of the devices. Photovoltaic performances of PBDN were also presented with a high open circuit voltage of 0.83 V and power conversion efficiency of 1.3% when blended with [6,6]-phenyl-C 61 -butyric acid methyl ester.