Ordered orientation and compact molecule packing due to coplanar backbone structure of interlayer: Improvement in fill factor for photovoltaic device

Abstract

A novel alcohol/water-soluble small molecule was obtained using a p-type planar backbone. The synthesized molecule was dissolved in organic solvents and highly polar solvents. The 3,3′-(((3,3″′-dimethyl-[2,2′:5′,2″:5″,2″′-quarterthiophene]-5,5″′-diyl)bis(4,1-phenylene))bis(oxy))bis(N,N-dimethylpropan-1-amine) (QTA) film exhibited a red-shifted spectrum compared with the solution spectrum owing to its many more planar molecular conformations in the solid state. According to X-ray diffraction (XRD) measurements, the QTA film showed sharp diffraction peaks near 3.6–11.0°, which indicates the formation of an interdigitated and ordered structure as an out-of-plane peak (1 0 0) due to the alkyl side chain of the quarter-thiophene backbone. A comprehensive analysis of the out-of-plane and in-plane XRD data suggests that a large fraction of the QTA derivatives was oriented edge-on relative to the substrate. A photovoltaic device containing QTA exhibited an open-circuit voltage of 0.85 V, current density of 15.5 mA/cm2, fill factor of 62.9%, and power-conversion efficiency of 8.4%. The photovoltaic device containing the QTA derivative exhibited improved power conversion efficiency compared with those containing PFN (8.0%) due to the ordered orientation and compact molecule packing of QTA.