α,α′‐Diarylacenaphtho[1,2‐c]phosphole P‐Oxides: Divergent Synthesis and Application to Cathode Buffer Layers in Organic Photovoltaics

A divergent method for the synthesis of α,α′‐diarylacenaphtho[1,2‐c]phosphole P‐oxides has been established; α,α′‐dibromoacenaphtho[c]phosphole P‐oxide, which was prepared through a Ti^II‐mediated cyclization of 1,8‐bis(trimethylsilylethynyl)naphthalene, underwent a Stille coupling with three different kinds of aryltributylstannanes to afford the α,α′‐diarylacenaphtho[c]phosphole P‐oxides in moderate to good yields. X‐ray crystallographic analyses and UV/Vis absorption/fluorescence measurements have revealed that the degree of π‐conjugation, the packing motif, the electron‐accepting ability, and the thermal stability of the acenaphtho[c]phosphole π‐systems are finely tunable with the α‐aryl substituents. All the P?O and P?S derivatives exhibited high stability in their electrochemically reduced state. To use this class of arene‐fused phosphole π‐systems as n‐type semiconducting materials, we evaluated device performances of the bulk heterojunction organic photovoltaics (OPV) that consist of poly(3‐hexylthiophene), an indene‐C₇₀ bisadduct, and a cathode buffer layer. The insertion of the diarylacenaphtho[c]phosphole P‐oxides as the buffer layer was found to improve the power conversion efficiency of the polymer‐based OPV devices.