In solution-processable bulk heterojunction (BHJ) organic solar cells (OSCs), the morphology of the active layer is crucial to the achievement of high efficiency, hence much research effort has been devoted to the fine-control of the morphology of photovoltaic active layers. Nevertheless, most of the current works were focused on the optimization on the post-processing conditions of the optoelectronic film. Although the morphology of the active layers also correlates highly with the molecular structure of electron donor materials, less work have been carried out to elucidate the relationship between molecular structure and morphology. Recently, some researchers have found that BHJ OSCs with isomers bearing common molecular constructive units but different linking positions could show notable different device performance, which could be ascribed to different nano-morphology caused by electron donor isomers with different linking positions. However, all the reported isomers have their subunits connected through C-C bonds with different linking positions. Although N-heterocyclic rings have been considered as quite important building blocks in photovoltaic materials, to the best of our knowledge, no report could be found concerning the difference between the C-C and C-N linked compounds in photovoltaic characteristics. In our previous works, we have developed a series of C-N linked asymmetrical squaraines (ASQs), and active layers based on them show analogous absorption and morphology properties.1, 2 Herein, we report two ASQs in which indoline moieties were integrated into the skeleton through N4 or C7 via C-N or C-C linkages. The two ASQs exhibit significant difference in morphology of ASQ/PCBM blended film. As a result, BHJ OSCs with the C-C linked ASQ as electron donor shows higher Jsc (10.50 mA cm-2) but lower FF (0.41) than that of the device based on the C-N linked ASQ (Jsc = 9.93 mA cm-2, FF = 0.47). In addition, a high PCE of 4.02% could be acquired in the device with the C-N linked ASQ as electron donor, which is among the best performance of ASQ-based solution-processed BHJ OSCs. Therefore, different integration manner of the indoline segment into the skeleton should have strong effect on the morphology of the active layers. Our findings may pave an important way for the tune of morphology through molecular tailoring, so that ASQs for BHJ OSC applications with much enhanced performance could be exploited.
Acknowledgement We acknowledge National Natural Science Foundation of China (project No. 21432005, 21190031 and 21372168) for financial support.
References
1. L. Yang, Q. Yang, D. Yang, Q. Luo, Y. Zhu, Y. Huang, Z. Lu and S. Zhao, J. Mater. Chem. A, 2014, 2, 18313-18321.
2. D. Yang, Q. Yang, L. Yang, Q. Luo, Y. Chen, Y. Zhu, Y. Huang, Z. Lu and S. Zhao, Chem. Commun., 2014, 50, 9346-9348.
a) Corresponding author’s E-mail: huangyan@scu.edu.cn; Tel (Fax): 86-28-85410059.