A two-armed skeleton extension strategy for the design of novel spirobifluorene-based small molecule donors

Abstract

Spirodifluorene (SF) was originally used as the core of an X-shaped four-armed molecule, and the construction of two-armed molecules with linear large conjugated planes based on SF will greatly expand the design concept of such photovoltaic materials. Taking twisted SF as the core, four novel linear conjugated two-armed small molecule donors (SMDs), SF(DPP)₂, SF(aDPP)₂, SF(DPPCz)₂ and SF(aDPPCz)₂, were designed and synthesized successfully. Herein, a two-armed extension design strategy was employed to regulate the optoelectronic properties of the materials by introducing ethynyl as the π-bridge and carbazole units as terminal units individually or simultaneously, which is completely different from traditional X-shaped materials based on SF. Considering the molecular design, SF can not only create large conjugated skeletons via an extension strategy but also has the characteristic of “protruding out of plane” in its torsional structure. Introducing an ethynyl group reduces HOMO levels and regulates molecular geometry. Introducing carbazole as the end group prolongs the conjugated skeleton and promotes the intramolecular charge transfer (ICT) process. As the most important result, the photovoltaic devices based on SF(DPPCz)₂/PC₇₁BM exhibit the highest power conversion efficiency (PCE) of 8.12% along with a short circuit current density (J_sc) of 19.60 mA cm⁻² due to the broad and strong absorption caused by the narrow band gap molecular design, accompanied by the strongest and broadest photoelectric response in the external quantum efficiency (EQE) curves and the highest hole mobility value. To our knowledge, this is the highest PCE reported so far for SF-based organic SMDs. This work provides an innovative molecular design method that extends from the two arms of molecules containing X-shaped nuclei to develop novel two-armed linear SMDs.