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 x 10(13) Jones at -1 V, whereas CF-based devices showed an R of 0.349 A W-1 and D* of 5.86 x 10(12) Jones. More importantly, Xy-processed OPDs exhibited an ultrafast signal response time of 3 mu s, which is much faster than that of CF-processed OPDs (42 mu s). 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.