Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

Abstract

In this study we synthesized a series of solution-processable small molecules comprising 2,2 0 -bithiophene (BTh), terthiophene (TTh), and thiobarbituric acid (TB) units as the central core, p-conjugated spacer, and acceptor end-capping moieties, respectively, but with alkyl side-chains of different lengths presented from their central BTh units (TBTThBTh-H, TBTThBTh-C4, TBTThBTh-C8, TBTThBTh-C12). We then investigated the structure-property relationships of these compounds in terms of their packing behaviors and bulk heterojunction (BHJ) photovoltaic properties. And we found that the packing of these molecules in neat films is critically dependent of their side-chain lengths, as evidenced by the variations in their lamellar structures determined with grazing-incidence wide-angle X-ray scattering (GIWAXS). The power conversion efficiencies (PCEs) of the photovoltaic BHJ devices comprising these small molecules and PC 61 BM exhibited zigzag-shaped variations with respect to the alkyl side-chain lengths, with the PCE of devices incorporating TBTThBTh-H and TBTThBTh-C8 being higher than those of devices incorporating TBTThBTh-C4 and TBTThBTh-C12. Using GIWAXS to probe the molecular packing in the BHJ active layers, we found that the alkyl chain lengths of the small molecules had a large impact on the formation of crystallites in the BHJ films; the molecules with more uniform and shorter alkyl side-chain lengths provide stronger intermolecular interactions, being more favorable for the crystallization of these molecules.

Si-Bridged Ladder-Type Small-Molecule Acceptors for High-Performance Organic Photovoltaics

“…In addition, another weak signal for π-stacking at 1.75 Å –1 for the ArSiID film in the OP direction was also detected. We suggest that two types of π-stacking configurations coexisted within the ArSiID film because the d -spacings of these two peaks for each molecule could not be classified into a set of (010) plane . The more intense π-stacking and higher intensity of the peak near 0.37 Å –1 in the IP direction of pristine ArSiID film than that seen in ArSiID-F and ArSiID-Cl imply the higher crystallinity of ArSiID as compared to those of ArSiID-F and ArSiID-Cl .…”

Effect of Alkyl Side Chains on the Photovoltaic Performance of 2,1,3‐Benzoxadiazole‐Based (‐X‐DADAD‐)n‐Type Copolymers

“…Finally, it should be noted that lowering the HOMO energy levels of conjugated polymers via increasing the bulkiness of the side chains was observed previously . However, lack of systematic side chain variation in these studies did not allow one to reveal clearly the supramolecular nature of the observed effects.…”

Direct (Hetero)Arylation for the Synthesis of Molecular Materials: Coupling Thieno[3,4-c]pyrrole-4,6-dione with Perylene Diimide to Yield Novel Non-Fullerene Acceptors for Organic Solar Cells

“…Therefore, before OSC devices were fabricated, an investigation of various post-deposition techniques was applied to thin films of compound 1 . It has been demonstrated that compounds with linear alkyl chains promote greater organization in active layers and improve device performance [ 41 , 68 , 69 , 70 , 71 , 72 , 73 , 74 ] and therefore only compound 1 with the octyl side chain was investigated. All thin films of 1 were cast from 2-MeTHF.…”