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

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 similar to 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/cm(2), almost 3-5 orders of magnitude larger than the dark current (similar to 50 nA/cm(2)), was observed in the presence of light at -1 V bias condition. Devices showed an increase in external quantum efficiency from similar to 4 to similar 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 x 10(11) cm Hz1/2 W-1 (Jones) @ 840 nm with an ultrafast photoresponse in the range of similar to 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.