Visible-Light-Responsive High-Detectivity Organic Photodetectors with a 1 μm Thick Active Layer

Organic photodetectors (OPDs) are attracting attention for use in flexible and portable electronic applications such as image sensors, remote sensing, optical communications, and medical sensors because of their strong photon responsivity in thin films over a broad range of wavelengths. In particular, the efficient photon-to-current conversion of OPDs under visible light allows their use in indirect X-ray detectors using scintillators to convert X-rays to visible light. The polymer poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5- b']dithiophene- co-5-(2-hexyldecyl)-1,3-bis(6-octylthieno[3,2- b]thiophen-2-yl)-4 H-thieno[3,4- c]pyrrole-4,6(5 H)-dione) (PBDTT-8ttTPD) shows strong absorption bands in the region of 500-650 nm, as well as high hole mobility, which provides excellent photoresponsivity and photon-to-current conversion efficiency. A p-n junction photodetector was fabricated by blending PBDTT-8ttTPD and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) and varying the thickness of the active layer (260-1100 nm). The PBDTT-8ttTPD:PC₇₁BM-based OPDs show promising photodetecting properties having a low dark current of 3.72 × 10^-9 A cm^-2 and high responsivity of 0.39 A W^-1 because of the well-controlled morphology, high molar absorption coefficient, and excellent carrier mobility of the PBDTT-8ttTPD:PC₇₁BM layer. Consequently, the specific detectivity of the PBDTT-8ttTPD-based OPD devices was 1.13 × 10¹³ Jones at -2 V on irradiation with a light-emitting diode (530 nm wavelength) with a power density of 55.6 μW cm^-2.