High-efficiency polymer solar cells controlled by photoelectrochemically formed ordered polythiophene active layers with various thicknesses

The effect of polythiophene (PT) with various thicknesses on photocurrent generation was investigated. The PT layer was prepared via anchor-guided photoelectrochemical polymerization of thiophene onto TiO2 layers (0.25-6 mu m-thick) covered with a sub-monolayer of tethered C207 molecules, 3-{5-[N,N-Bis(4-diphenylamino) phenylthieno [3,2-b]thiophen-2-yl}-2-cyano-acrylate acid. Although the thickness of the porous TiO2 layer is different from the ultra-thin compact TiO2 layer (150 nm), a highly ordered and uniform PT layer can also be photoelectrochemically grown onto the surface of and into the crevices of the C207-covered TiO2 nanoparticles. With a 1.5 mu m-thick PT layer separated from the cathode by the hole-transport material Spiro-OMeTAD, 2,2',7,7'-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9'-spirobifluorene, the open-circuit voltage V-oc and short-circuit current density J(sc)were measured to be 0.78 +/- 0.02 V and 14.77 +/- 0.31 mA/cm(2), respectively. The power conversion efficiency (8.41 +/- 0.31 %) of such a polythiophene-based solar cell (PTSC) is higher than those of solar cells sensitized by an ordered thin PT film (270 nm) grown onto compact TiO2 layers or by a thick PT layer grown onto coarse TiO2 layers (6 mu m). The high conversion efficiency of the PTSC is attributed to the oriented PT growth along the chain-like C207 molecules at the TiO2 nanoparticles, which facilitates facile transport of charges while effectively preventing the hole/electron recombination.