A Series of Squaraine Dyes: Effects of Side Chain and the Number of Hydroxyl Groups on Material Properties and Photovoltaic Performance

Squaraine dyes are considered an important group of photoactive materials in the field of organic photovoltaic devices. In this work, we purposely tuned the side chains and number of hydroxyl (OH) groups in a series of squaraine (SQ) dyes, i.e., SQ1−4, to investigate the effect of structural variations on the material properties as well as the performance of these dyes as donor materials in bulk heterojunction (BHJ) photovoltaic cells. The material structure and properties of these SQs were systematically characterized using various tools. Solution-processed BHJ photovoltaic cells based on these SQ compounds combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor gave efficient power conversion efficiencies greater than 4.0% under AM 1.5G solar illumination at 100 mW/cm. Our observations show that the OH groups deepened the highest occupied molecular orbital (HOMO) of the donor and thus enhanced the open-circuit voltage, whereas the linear chain improved the charge transport properties in the BHJ films. Both the side chain and the number of OH groups play important roles in determining the aggregation behavior of these SQs in solid-state films: SQ1, which contains four OH groups and branched side chains, exhibits J-aggregation because of the steric hindrance of its side chains; SQ2, which contains four OH groups and linear side chains, exhibits both H-aggregation and J-aggregation; SQ3, which contains two OH groups and linear side chains, exhibits preferential H-aggregation; SQ4, which contains linear side chains without OH groups, exhibits J-aggregation, this is most likely because of its strong intermolecular coupling and intermolecular hydrogen-bonding interactions to form a head-totail packing mode, i.e., J-aggregation. Interestingly, the absorption of J-aggregates in BHJ cells contributes to the cells’ photoresponse at long wavelengths, and thus results in higher photocurrent. Our results demonstrate a clear relationship between the molecular structures of SQ dyes and their physical properties that govern their photovoltaic performance.