Introducing Siloxane-Terminated Side Chains in Small Molecular Donors for All-Small-Molecule Organic Solar Cells: Modulated Molecular Orientation and Enhanced Efficiency

Abstract

In this work, three small molecular donors (SMDs) S35, S35–1Si, and S35–2Si, with 3,5-difluorophenyl-substituted benzodithiophene as the central 2-dimensional unit to combine different numbers of siloxane-terminated side chain, were synthesized for all-small-molecule organic solar cells (ASM-OSCs). The three SMDs showed comparable film absorption peaks at 570 nm and optical band gaps of 1.8 eV. Relative to S35 and S35–1Si with symmetric alkyl side chains and asymmetric side chains on the central unit, respectively, the S35–2Si carrying two symmetric siloxane-terminated side chains displayed largely elevated melting and crystalline temperatures, lowered surface energy, and modulated molecular orientation. The three SMDs possessed edge-on dominated molecular orientations of their neat films; however, a big difference was found for their blend films with nonfullerene acceptor Y6. The S35:Y6 and S35–1Si:Y6 blends exhibited edge-on and face-on bimodal orientations but the S35–2Si:Y6 blend showed pure face-on orientation, indicating quite different donor:acceptor intermolecular interactions. Some large domains existed in the S35:Y6 and S35–1Si:Y6 blends, but could be suppressed by the S35–2Si:Y6 blend, leading to a more balanced charge transport. In ASM-OSCs, the two S35:Y6 and S35–1Si:Y6 active layers showed comparable power conversion efficiencies (PCE) of ∼12% but a much higher efficiency of 13.50% could be achieved with the S35–2Si:Y6 active layer. Our results suggest that the siloxane-terminated side chain is promising to regulate crystalline ability of a SMD, paving a way for high performance ASM-OSCs.

Recent progress in all-small-molecule organic photovoltaics

“…Chen's team designed and synthesized small molecules S35, S35-1Si and S35-2Si by substituting the alkylthiophene side groups of BTR with 3,5-diuorophenyl units with different numbers of siloxane-terminated side chains. 78 The introduction of siloxane-terminated side chain exhibits ignorable inuence upon the optical bandgap and absorption coef-cients. And all three small molecules deliver edge-on dominated orientations in neat lms.…”

Effect of melting temperatures on the orientation and rheology of polyacrylate/silica (PAcr/SiO2) composite microspheres in twin‐screw extruder

“…Importantly, it was clearly observed that the of composite fibers at 240°C and 250°C were higher than that of the neat PMMA. This behavior was attributed to the improvement on the entanglement of the molecular chain, 41,42 resulting from the construction of orientation structure with “head‐to‐tail” (Figure 2). Afterwards, the mechanical comparison corresponded to the rheological performance of microfiber orientation could confirm that elongation at break were greatly attributed to microfiber orientation.…”

A comparison of the positional effect of difluorination and the synergistic effect of siloxane-terminated side chains on benzodithiophene-based conjugated polymers for efficient photovoltaic application

“…The locations of the (100) peaks on the IP direction for 35Si and 23Si and (100) peaks on the OOP direction of 35Si are slightly decreased compared with the alkyl side chain substituted polymers, which is a common variation for polymer donors with a siloxaneterminated side chain. 38,42…”