Enhancement of charge transport properties of small molecule semiconductors by controlling fluorine substitution and effects on photovoltaic properties of organic solar cells and perovskite solar cells

Abstract

We prepared a series of small molecules based on 7,7'-(4,4-bis(2-ethylhexyl)-4<i>H</i>-silolo[3,2-<i>b</i>:4,5-<i>b</i>']dithiophene-2,6-diyl)bis(4-(5'-hexyl-[2,2'-bithiophene]-5-yl)benzo[<i>c</i>][1,2,5]thiadiazole) with different fluorine substitution patterns (<b>0F-4F</b>). Depending on symmetricity and numbers of fluorine atoms incorporated in the benzo[<i>c</i>][1,2,5]thiadiazole unit, they show very different optical and morphological properties in a film. <b>2F</b> and <b>4F</b>, which featured symmetric and even-numbered fluorine substitution patterns, display improved molecular packing structures and higher crystalline properties in a film compared with <b>1F</b> and <b>3F</b> and thus, <b>2F</b> achieved the highest OTFT mobility, which is followed by <b>4F</b>. In the bulk heterojunction solar cell fabricated with PC₇₁BM, <b>2F</b> achieves the highest photovoltaic performance with an 8.14% efficiency and <b>0F</b> shows the lowest efficiency of 1.28%. Moreover, the planar-type perovskite solar cell (PSC) prepared with <b>2F</b> as a dopant-free hole transport material shows a high power conversion efficiency of 14.5% due to its high charge transporting properties, which were significantly improved compared with the corresponding PSC device obtained from <b>0F</b> (8.5%). From the studies, it is demonstrated that low variation in the local dipole moment and the narrow distribution of <b>2F</b> conformers make intermolecular interactions favorable, which may effectively drive crystal formations in the solid state and thus, higher charge transport properties compared with <b>1F</b> and <b>3F</b>.