Structure–Property Relationship Study of Substitution Effects on Isoindigo-Based Model Compounds as Electron Donors in Organic Solar Cells

• Published in: ACS applied materials & interfaces 2014-08, Vol.6 (16), p.14533-14542
• Main Authors: Ren, Yi, Hiszpanski, Anna M, Whittaker-Brooks, Luisa, Loo, Yueh-Lin
• Format: Article
• Language: English
• Subjects: Electrons; Indoles - chemistry; Photochemistry
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/am503812f

We designed and synthesized a series of isoindigo-based derivatives to investigate how chemical structure modification at both the 6,6′- and 5,5′-positions of the core with electron-rich and electron-poor moieties affect photophysical and redox properties as well as their solid-state organization. Our studies reveal that 6,6′-substitution on the isoindigo core results in a stronger intramolecular charge transfer band due to strong electronic coupling between the 6,6′-substituent and the core, whereas 5,5′-substitution induces a weaker CT band that is more sensitive to the electronic nature of the substituents. In the solid state, 6,6′-derivatives generally form J-aggregates, whereas 5,5′-derivatives form H-aggregates. With only two branched ethylhexyl side chains, the 6,6′-derivatives form organized lamellar structures in the solid state. The incorporation of electron-rich benzothiophene, BT, substituents further enhances ordering, likely because of strong intermolecular donor–acceptor interactions between the BT substituent and the electron-poor isoindigo core on neighboring compounds. Collectively, the enhanced photophysical properties and solid-state organization of the 6,6′-benzothiophene substituted isoindigo derivative compared to the other isoindigo derivatives examined in this study resulted in solar cells with higher power conversion efficiencies when blended with a fullerene derivative.