I. Introduction
Near infrared (NIR) region is of particular relevance for many industrial applications, among which are plastic optical communications [1], biomedicine, night vision, imaging system [2] and remote monitoring. Organic photodiodes (OPDs) have been the subject of extensive research in the past due to several inherent advantages such as large-area detection, wide range of material selection and low-cost fabrication on flexible substrates. Infrared OPDs were realized mainly by using narrow-energy-gap materials, such as metal phthanocyanines [3], tin naphthalocyanine dichloride (SnNcCl2) [3], Zn-metallated porphyrin-dimer tape (Psub) [4], Ph2-benz-bodipy [5] and fused pyrene-diporphrin [6]. An exciplex is an excited charge-transfer complex formed by an electron donor molecule and an electron acceptor molecule [7]. Exciplex emission is widely used for tuning electroluminescence (EL) spectrum [8] and improving the efficiency [9] of organic light-emitting diodes (OLEDs). OPDs based on exciplex absorption in composite composed of rubrene and MoO3 were also reported [10]. Neodymium phthalocyanine (NdPc2) is an electron donating organic semiconductor material with high optical absorption in visible region. While C60 is an electron accepting organic semiconductor material with low lying LUMO level and widely used as electron acceptor in organic solar cells, organic photodiodes and photosensitive organic field-effect transistors [11], [12]. In this letter, we report on NIR OPDs based on exciplex absorption in the planar heterojunction (PHJ) consisted of NdPc2 and C60. For NIR light of 808 nm, a photo responsivity as high as 60 mA/W and an external quantum efficiency of 17%, were obtained for OPDs with NdPc2 layer thickness of 10 nm, and a specific detectivity of Jones was obtained with the optimized NdPc2 layer thickness of 30 nm.