Thermodynamic Property–Performance Relationships in Silicon Phthalocyanine-Based Organic Photovoltaics

Axially substituted silicon phthalocyanines [(R3SiO)2-SiPc] have recently emerged as promising alternatives to fullerenes in organic photovoltaics (OPVs), with advances in both molecular design and device fabrication resulting in a fourfold improvement in efficiency, bringing these materials closer to commercial viability. Further refinements in SiPc-based OPVs can only be achieved through exploration of their physical properties and correlation with performance metrics. In this work, we have synthesized seven (R3SiO)2-SiPc derivatives and paired them with P3HT in OPV devices to explore how both the fundamental thermodynamic properties of the SiPcs and the blend morphology affect device performance. From these studies, (R3SiO)2-SiPc derivatives with smaller critical radius (rc) values correlated to higher power conversion efficiencies (PCEs) in devices due to their tendency to form smaller domains in the active layer. Higher values for the Flory–Huggins miscibility parameter also correlated to higher PCEs due to the formation of unadulterated, sharper domains. Both properties are dependent upon axial substituent size, with values minimized by smaller axial substituents, serving as a guideline for the molecular design of SiPc-based non-fullerene acceptors (NFAs). Moreover, the majority of the (R3SiO)2-SiPc materials reported herein outperformed the fullerene-based reference device, with one derivative resulting in a device with a PCE greater than 4%.