D-π-A-π-D-type low band gap diketopyrrolopyrrole based small molecules containing an ethynyl-linkage: synthesis and photovoltaic propertiesElectronic supplementary information (ESI) available: Synthetic procedures and the NMR spectra of the synthesized compounds. See DOI: 10.1039/c5ra01946j

• Main Authors: Ji, Changyan, Yin, Lunxiang, Li, Kechang, Wang, Lihui, Jiang, Xueying, Sun, Yingji, Li, Yanqin
• Format: Journal Article
• Language: English
• Published: 07.04.2015
• ISSN: 2046-2069
• DOI: 10.1039/c5ra01946j

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. Two new OSMs M3 and M4 containing ethynyl-linkage have been synthesized successfully and the ethynyl-linkage effect were investigated.