Impact of Alkyl Chain Length on Small Molecule Crystallization and Nanomorphology in Squaraine-Based Solution Processed Solar Cells

In this work, we have investigated a series of aniline-based squaraines, with varying solubilizing alkyl chains, as donor materials in bulk heterojunction (BHJ) solar cells. Although these squaraine molecules exhibit similar absorbance spectra and crystal structures, the difference in properties that drive the OPV performance becomes apparent when each squaraine is blended with PCBM. Thin film X-ray diffraction results demonstrate a disruption of squaraine crystallization in the presence of PCBM, more so for shorter side chain squaraines. As a result, the hole mobilities of BHJ films of shorter side chain squaraines show the largest drop when compared to their neat films, whereas the mobility decrease for the longer side chain counterparts is small. However, morphological studies have shown that the phase separation rapidly happens during the spin-casting process for longer side chain squaraines. Ultimately, it is the extent of phase separation that dominates the final device efficiency. Therefore, rational design can greatly be influenced as a result of our systematic materials properties overview for anilinic squaraines targeted for OPV