Calamitic-Type Dipolar and Quadrupolar Chromophores with a Twisted Peripheral Handle: Structure-Property Outlook as Nonfullerene Acceptors for Binary Solar Cells

Symmetrical (14, 15) and unsymmetrical (16, 17) molecules possessing tetracyanobutadiene (TCBD) as a twisted end group on the carbazole/fluorene backbone were synthesized as nonfullerene acceptors (NFAs). Systematic structural correlation of these chromophores on the optoelectronic, electrochemical, and physicochemical properties revealed better acceptor properties for symmetrical (14, 15) compared to unsymmetrical analogues (16, 17). High molar absorptivity, deep HOMO-LUMOs, better electronaccepting capacity, along with other complementary molecular properties are some of the associated factors for the better acceptor character. In a binary solar cell configuration blended with P3HT donor, quadrupolar-type symmetrical chromophores (14, 15) displayed higher power conversion efficiencies over dipolar-type unsymmetrical chromophores (16, 17). In particular, 14 with an efficiency of 5.37% emerged as the best candidate with further enhancement to 6.30% upon additive processing. The dependence of processing on the improved photoinduced current was attributed to the better nanoscale morphology, crystallinity, and electron mobility. In terms of efficiency, it is the third highest for any TCBD-based NFA solar cells. Overall, this work validates the impact of a nonplanar TCBD auxiliary positioned within a calamitic-type pi-framework to realize alternative NFAs. Viewed from a fundamental perspective, the initial results are promising, and further fine tuning of the molecular structure could possibly form an ideal acceptor for organic photovoltaics.