Spontaneously Induced Hierarchical Structure by Surface Energy in Novel Conjugated Polymer‐Based Ultrafast‐Response Organic Photodetectors

Abstract A novel conjugated polymer, poly(4‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b']dithiophen‐2‐yl)‐2,6‐dioctylbenzo[1,2‐d:4,5‐d']bis(oxazole)) (PBB), is synthesized for use in ultrafast‐response organic photodetectors (OPDs). The PBB synthesis is considered facile because a monomer could be isolated without column chromatography purification. The excellent solubility of PBB allows easy dissolution in both halogenated and nonhalogenated solvents, facilitating the fabrication of p‐n heterojunction OPDs based‐on PBB and 6,6‐phenyl‐C71‐butyric acid methyl ester (PC 71 BM) using either chloroform (CF) or o ‐xylene (Xy). Notably, Xy‐processed OPDs show superior performance with a responsivity ( R ) of 0.385 A W −1 and specific detectivity ( D *) of 1.33 × 10 13 Jones at –1 V, whereas CF‐based devices showed an R of 0.349 A W −1 and D * of 5.86 × 10 12 Jones. More importantly, Xy‐processed OPDs exhibited an ultrafast signal response time of 3 µs, which is much faster than that of CF‐processed OPDs (42 µs). These superior static and dynamic properties of PBB:PC 71 BM devices are achieved by the inverted hierarchical arrangement of PBB and PC 71 BM via a biased orientation of PC 71 BM toward the ZnO layer as well as a homogenous nanomorphology. This ideal morphology is spontaneously induced due to the difference in interfacial energy between PBB and PC 71 BM toward the ZnO surface and a well‐mixed PBB:PC 71 BM blend in Xy solvent.