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 (PC₇₁BM) 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 × 10¹³ Jones at –1 V, whereas CF‐based devices showed an R of 0.349 A W⁻¹ and D* of 5.86 × 10¹² 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:PC₇₁BM devices are achieved by the inverted hierarchical arrangement of PBB and PC₇₁BM via a biased orientation of PC₇₁BM 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 PC₇₁BM toward the ZnO surface and a well‐mixed PBB:PC₇₁BM blend in Xy solvent.