Orbital-Energy Modulation of Tetrabenzoporphyrin-Derived Non-Fullerene Acceptors for Improved Open-Circuit Voltage in Organic Solar Cells

Tetrabenzoporphyrin (BP) holds attractive characteristics
for optoelectronic
applications, such as the large π-conjugated framework and high
photoabsorption capability. However, its use in organic solar cells
(OSCs) has been limited because of the extremely low solubility that
hampers direct solution processing and also the high frontier-orbital
energies that lead to low open-circuit voltage (VOC). Herein, we examine BP derivatives equipped with multiple
strongly electron-withdrawing groups for photovoltaic applications.
The derivatives are generated in thin films through a thermal precursor
approach, wherein the corresponding bicyclo[2.2.2]­octadiene-fused
porphyrin derivatives are solution-cast, and then annealed to carry
out the in situ retro-Diels–Alder reaction. The frontier-orbital
energies of the resulting derivatives are effectively stabilized as
compared to pristine BP to such a degree that they afford high VOC of up to 0.94 V when used as a donor or can
even work as a new class of nonfullerene acceptor in OSCs. Single-crystal
X-ray diffraction analyses demonstrate that the conformation of the
BP framework largely varies from being near planar to highly curved
depending on its substituents. The morphology of polymer:BP-derivative
bulk-heterojunction films prepared by the thermal precursor approach
also varies between the BP derivatives. These results can greatly
extend the scope of both molecular design and morphology control for
utilization of the BP chromophore toward achieving viable organic
optoelectronic devices.