Ambipolar organic phototransistors with bulk heterojunction films of p-type and n-type indacenodithienothiophene-containing conjugated polymers

Abstract

We report ambipolar organic phototransistors with polymeric sensing channel layers that consist of p-type and n-type indacenodithienothiophene (IDTT)-based conjugated polymers. The p-type IDTT-based polymer, 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-5,6-bis(octyloxy)benzo[c]-[1,2,5]thiadiazole) (PIDTT-80BT), was synthesized by Stille coupling reactions between 3,9-bis(trimethytin)-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 (2Sn-IDTT) and 4,7-dibromo-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole (2Br-8OBT). The PIDTT-8OBT films showed a broad optical absorption up to ca. 660 nm (wavelength, λ) and a highest occupied molecular orbital (HOMO) energy of −5.47 eV. Organic field-effect transistors (OFETs) with the PIDTT-8OBT channel layers were operated in p-channel modes and could detect visible light (blue, green, and orange). The PIDTT-8OBT polymer was blended with 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]phenathroline-1,3,6,8(2H,7H)-tetraone) (PIDTT-NDI), an n-type polymer, and used as a channel layer. The OFETs with the PIDTT-8OBT:PIDTT-NDI blend layers delivered ambipolar characteristics and were able to detect a full range of visible light up to λ = 750 nm, which were dependent on the blend ratios. The balanced ambipolar sensing characteristics in the dark were achieved at a PIDTT-8OBT ratio of 30 wt%. In particular, the OPTRs with the PIDTT-8OBT:PIDTT-NDI blend layers could detect a deep red light (750 nm) that is not absorbed by the PIDTT-8OBT polymer due to the photogenerated charge (hole) transfer from the PIDTT-NDI component to the PIDTT-8OBT component at their bulk heterojunctions. The present ambipolar OPTRs (PIDTT-8OBT = 70 wt%) showed a high photosensitivity of ca. 1400% in the p-channel mode and ca. 800% in the n-channel mode at λ = 610 nm, while the photosensitivity reached ca. 890% in the p-channel mode and ca. 600% in the n-channel mode at λ = 750 nm.