Effects of alkyl chains on intermolecular packing and device performance in small molecule based organic solar cells

Two donor molecules named as DR3TDTC-C6 and DR3TDTC-C8 with n-hexyl and n-octyl alkyl chains on the central building block 7H-cyclopenta-[1,2-b:3,4-b']-dithiophene (DTC) were designed and synthesized. Both of them exhibited PCEs >4%. The photovoltaic properties of these molecules were superior to their analogous donor molecule DR3TDTC, which possess two 2-ethyl hexyl alkyl chains on the same core unit and only demonstrated a PCE of 0.75% after elaborative post-treatment. The influence of the alkyl chains on the optical, electrochemical properties, packing properties and morphology of these three molecules was systematically investigated. The results demonstrated that the difference between their device performances is mainly affected by their intermolecular packing state. This indicates that length and branch structure of alkyl chains on the central unit should be given careful consideration while designing donor molecules for small molecular organic solar cells. (C) 2017 Elsevier Ltd. All rights reserved.