Unsymmetrical Squaraine Dye-Based Organic Photodetector Exhibiting Enhanced Near-Infrared Sensitivity

Abstract

An unsymmetrical squaraine (SQ) derivative containing anthracene and phenyl hydrazine (ANPHSQ) units linked to the central SQ was synthesized and used as electron donors in bulk heterojunction photovoltaic mode organic photodetectors (OPD), containing PCBM as the electron acceptor. Although the ANPHSQ exhibited a strong narrow band in the near infrared peaking at ∼760 nm in solution, the OPD constructed with this dye exhibited a broad spectral response extending to 950 nm. The enhanced sensitivity in the long wavelength region could be attributed to formation of ANPHSQ aggregates within the devices. A photocurrent of 1.3 mA/cm2, almost 3–5 orders of magnitude larger than the dark current (∼50 nA/cm2), was observed in the presence of light at −1 V bias condition. Devices showed an increase in external quantum efficiency from ∼4 to ∼12% as the bias varied from 0 to −1 V. At a reverse bias of −1 V, the device with a ANPHSQ exhibited a maximum shot-noise-limited specific detectivity of 6 × 1011 cm Hz1/2 W–1 (Jones) @ 840 nm with an ultrafast photoresponse in the range of ∼15 ns. The study of relaxation dynamics of the ANPHSQ excited state using femtosecond pump–probe spectroscopy and time-correlated single-photon counting indicated efficient charge transfer between the excited state of ANPHSQ and PCBM.