NIR-sensing ambipolar organic phototransistors with conjugated terpolymer layers based on diketopyrrolopyrrole-benzothiadiazole-naphthalenediimide comonomer units

Abstract

Here we report ambipolar organic phototransistors (OPTRs), which can be operated in both p-channel and n-channel modes, with sensing channel layers of a conjugated terpolymer, poly[4-(5-(4-(5-(6-methoxy-7-methyl-5-(octyloxy)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-2-(2-methyldodecyl)-5-(2-octyldodecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thiophen-2-yl)-9-methyl-2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone] (PDPP-8OBT-NDI) that was synthesized by Stille coupling reaction of three comonomers. The PDPP-8OBT-NDI films showed a broadband optical absorption up to 1100 nm and good thermal stability. The organic field-effect transistors (OFETs) with PDPP-8OBT-NDI layers exhibited quite well-balanced ambipolar characteristics at the gate and drain voltage range of ±10 V in the dark, as supported by the drain current ratio (DCR) of 1.09–1.2. Upon illumination with NIR light (wavelength (λ) = 810 and 905 nm), the drain current of the devices was sensitively changed with the incident NIR light in the presence of a noticeable threshold voltage shift, even though the ambipolar characteristics of the OPTRs were marginally increased up to DCR = 1.27. The photosensitivity of the OPTRs with 80 nm-thick PDPP-8OBT-NDI layers reached ca. 665% (p-channel) and 622% (n-channel) at λ = 810 nm, while slightly lower photosensitivity of ca. 413% (p-channel) and 373% (n-channel) was measured at λ = 905 nm. The flexible OPTRs with 80 nm-thick PDPP-8OBT-NDI layers delivered good ambipolar characteristics upon continuous NIR light illumination and reproducible photocurrent responses upon optical on/off modulations of NIR light (λ = 905 nm).