Near-Infrared Organic Photodiodes with Improved Charge Transfer Absorption

Abstract

Near-infrared organic photodiodes (NIR OPDs) have recently been studied for the detection of vein-imaging-based identification or pulse oximetry data [1,2]. While vein-imaging and pulse oximeter used mainly the wavelength of 850 nm, interaction with biological components in the NIR window covers up to the wavelength of 1700 nm (e.g. skin color detection, cell function sensing, regulating in cancer theranostics, regenerative medicine, neuroscience research) [3,4]. In line with this trend, considerable efforts (e.g. conjugation length modulation, intermolecular charge transfer, coherent perfect absorption, etc) have been made to fabricate NIR OPDs absorbing the longer wavelength [5,6]. Among them, non-fullerene acceptors (NFA) were shown to have the best performance ; for example, 2-((Z)-2-((5-(6-(5-((Z)-(1-(dicyanomethylene)-5,6-difluoro-3-oxo-1H-inden-2(3H)-ylidene)methyl)-3-((2-ethylhexyl)oxy)thiophen-2-yl)-4,4-bis(2-ethylhexyl)-4H-S-8cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-4-(2-ethylhexyl)thiophen-2-yl)methylene)-5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (CO1-4F) has a small bandgap (λmax = 950 nm) using intramolecular charge transfer and high responsivity (R = 0.52 A/W) because it has good morphology with high planarity of the molecule [5]. To fully utilize the potential of NIR OPDs, however, NIR charge-transfer (CT) absorption dynamics between electron donor and electron acceptor needs to be further understood to fulfill the requirements for many of the applications. In this respect, this study aims to explore the NIR CT absorption dynamics between polymer (electron donor) and NFA (electron acceptor) through photophysical analysis and ellipsometric investigation. Our study confirms that the CT absorption onset wavelength is ca. 1000 nm, which is close to the CT state energy difference, estimated to be 1040 nm in wavelength. Furthermore, the microcavity effect is applied to compensate for a relatively low CT absorption in the NIR region to improve the responsivity. Upon optimization with optical simulation, we demonstrate enhanced NIR OPDs with EQE of 0.95 % in the wavelength of 1045 nm, illustrating that the proposed approach holds a great promise in detecting NIR photons with relatively long wavelengths at high efficiency.