D–π–A–π–D-type low band gap diketopyrrolopyrrole based small molecules containing an ethynyl-linkage: synthesis and photovoltaic properties

Abstract

Two new D–π–A–π–D-type diketopyrrolopyrrole (DPP) based organic small molecules (OSMs), M3 and M4, were synthesized successfully, consisting of ethynyl as π-linkage, and containing alkylated carbazole and fluorene as terminal electron-donating groups, respectively. To investigate the triple-bond effect on the optical electronic properties, non-ethynyl analogues M1 and M2 with a single-bond-linkage were designed and synthesized as well. The relationship between molecular structure and property was thoroughly investigated by experimental and theoretical studies. In contrast, the ethynyl-linkage structural design could not only lower the highest occupied molecular orbital (HOMO) levels, but also delicately balance the relationship between the deep-lying HOMO and narrow band gap, thus improving the photovoltaic (PV) performance. As a result, compounds M3 and M4 exhibited relatively deep-lying HOMO levels relative to M1 and M2, resulting in the corresponding PV devices with an increased open-circuit voltage (V_OC) of 0.84 V and 0.98 V, with power conversion efficiency (PCE) of 1.99% and 3.10%, respectively. Whereas M1 and M2 based devices showed a V_OC of 0.46 V and 0.89 V, and a PCE of 1.48% and 2.23%, respectively. The best PV performance of M4 was primarily attributed to the deep-lying HOMO level and reasonably high hole mobility caused by the structural design with an ethynyl-linkage and a fluorene end-capping group. We found that the systematic investigation of the triple bond effect on novel OSMs could be critical for a deep understanding of the relationship between molecular structure and property. This work provided an important guide for the rational design of novel PV materials.