n-Channel organic phototransistors with an n-type conjugated polymer based on indacenodithiophene and naphthalenediimide units

Abstract

Here we report n-channel organic phototransistors (OPTRs) with a soluble n-type conjugated polymer, as a sensing channel layer, poly(5,5,11,11-tetrakis(5-(2-ethylhexyl)thiophene-2-yl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene)-co-2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (PIDTT-NDI). The PIDTT-NDI polymer was synthesized by polymerization of the corresponding IDTT and NDI monomers via the Stille coupling reaction. The PIDTT-NDI films exhibited an optical absorption edge of ca. 900 nm and the lowest unoccupied molecular orbital (LUMO) energy of ca. −4.2 eV. The organic field-effect transistors (OFETs) with the PIDTT-NDI channel layers showed n-channel transistor behaviors and their best performances in the dark were measured at the PIDTT-NDI thickness of t = 70 nm. The photosensing test disclosed that the OPTRs with the PIDTT-NDI sensing channel layers could detect both blue (434 nm) and near infrared (754 nm) light irrespective of the PIDTT-NDI thickness, even though the best photoresponsivity (R_C) was achieved at t = 70 nm. The flexible OPTRs with the 70 nm-thick PIDTT-NDI sensing channel layers, which were fabricated using poly(ethylene naphthalene) (PEN) film substrates, could detect both blue (420 nm) and deep red (700 nm) light. The present, flexible PIDTT-NDI-based OPTRs are expected to be applied for the skin therapy systems which need the blue (400–420 nm) and red (620–700 nm) light for the acne and anti-aging treatments, respectively.