In this paper we report on the synthesis and development of vanadium oxide precursor flexographic ink for the printing of hole-transporting layers in organic solar cells. For the synthesis of vanadium oxide inks, a sol-gel methodology was utilized. By modifying the vanadium alkoxide precursor with a right type of coordinating ligands a stable and flexoprintable ink has been successfully developed. Flexo-printing afforded smooth and uniform vanadium oxide sol-gel films on top of PCDTBT:PC70BM films. The conversion of the synthesized sol-gel film into a corresponding vanadium oxide layer was followed by DSC/TGA and XPS analyses. The inks were used for the fabrication of inverted organic solar cells by flexo-printing. Power conversion efficiencies ranging between 3.5 % and 4.5 % were achieved, which are slightly lower than the reference cells using vacuum-deposited MoO3 as the hole-transporting layers.
The ablative decomposition of GaN films induced with a XeCl excimer laser ((lambda) equals 308 nm) was investigated for a potentially low-damage surface planarization process. Samples, 2-micrometers -thick, were grown on (0001) sapphire by ammonia molecular beam epitaxy. They had a characteristic micro-hillock type surface morphology with a roughness, averaged over 5 x 5 micrometers 2 area, typically, of 13 nm. Following the laser irradiation, this roughness could be reduced to 3.6 nm. The results indicate that the ablation process follows the Lambert-Beer's law, with an absorption coefficient of 3 x 105 cm-1. The experiment was carried out with relatively short pulses ((tau) equals 10 ns), which appear to be responsible for the observed onset of the laser-induced decomposition of GaN and surface planarization at significantly smaller laser fluences than reported in the literature. The ability to carry out decomposition of GaN with low laser fluences is of practical importance for achieving a low-damage GaN planarization process and/or intentional delaminating of this material from the sapphire substrate by the back side irradiation technique.
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