1536-125X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNANO.2019.2907529, IEEE Transactions on Nanotechnology Copyright (c) 2019 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained by sending a request to pubs-permissions@ieee.org Abstract— In this paper, we have reported the effect of optical resonance on the spectrum selectivity, external quantum efficiency (EQE), and time response of Metal (Pd, Au)/CdSe quantum dots (QDs)/ZnO QDs optical cavity based Schottky photodiodes on the ITO coated glass substrates. The CdSe QDs layer acts as the active layer while the ZnO QDs layer act as the electron transport layer (ETL) as well as a filter layer. The optical illumination is coupled into the device through the substrate side to enhance the optical absorption in the active layer due to multiple reflections from both metal (Pd, Au)/ CdSe QDs and CdSe QDs/ZnO QDs interfaces. The measured photoresponse over the 350-650 nm gives ~61 nm (190 nm), 2.21% (0.87%) and 17.15 ms (28.9 ms) values for the full width at half maximum (FWHM), EQE and response time of Pd (Au)/CdSe QDs based Schottky photodiodes respectively. The improvements in the spectrum selectivity, EQE and response time due to the change in the Schottky electrode from Au to Pd are attributed to higher reflection characteristic of the Pd/CdSe QDs interface than that of the Au/CdSe QDs interface which are duly justified by measured results and theoretical models presented in this paper. An improved responsivity with reduced FWHM obtained by merely replacing the Au by Pd is possibly reported for the first time for the spectrum selective photodetectors. Index terms: - Solution-processed, CdSe QDs, Spectrum Selective Photodetector, Schottky Diode, Quantum dots. I. INTRODUCTION The spectrum selectivity of a photodetector defines its ability to detect light with desired energy photons. Jansen Van Vuuren et al. [1] reported that photodetectors with a full width at half maximum (FWHM) below 100 nm could be explored for independent detection of the photons of different energy. Researchers have reported spectrum selective photodetectors for operation in the ultraviolet (UV) [2], [3], Visible [4], [5] and near infrared/ infrared (NIR/IR) [6] regions using additional filter layer engineering [3], [6] and active layer engineering [2], [4], [5] techniques. In recent years, solution processed photodetectors have gained popularity for their low-cost and large area fabrication process. Among various solution-based techniques, the solution processed Colloidal Quantum Dots (QDs) are widely used for low-cost and large- area photodetectors due to their particle size-dependent emission, absorption and charge carrier transport properties [7]. The QDs based photodiodes have a low thermal power loss, the capability of multiple exciton generations, and the ability for photon-up and down conversion features [8]. It is also interesting to mention that the operating wavelength of QDs based photodetectors can be varied over a wide range by simply optimizing the size of the QDs by controlling the environment (i.e., temperature, pressure and time) of the low- cost solution based synthesis process of the QDs [2]. In other words, the QDs based photodetectors can be designed for operating in the UV [2], visible [9] and NIR [6] regions to achieve spectrum selectivity. Deng et al. [10] have demonstrated a narrowband HgSe QDs based photodetector with a detectivity of 8.5×10 8 Jones without using any additional spectral filter layer. Qiao et al. [6] have reported a narrowband photodetector with an FWHM of ~100 nm and a responsivity < 1 A/W at a very high voltage of 10 V. In their work, [6] they have used PbS QDs as the active layer and a perovskite layer to act as a filter for short wavelength photons. Qin et al. [3] have reported a wavelength selective p- GaN/ZnO photodiode by using additional composites as a filter layer from the back side. A Fabry–Perot resonant cavity based organic photodetector (OPD) has been reported by An et al. [11] to achieve tunable photoresponse of the device. Furchi et al. [12] have reported a microcavity based graphene photodetector by exploring the wide bandgap non-absorbing material (at a wavelength of interest) as Bragg’s mirror in the device to increase the absorption of the graphene layer by 21 times. In the above-mentioned works, the active layer engineering [2], [4], [5] and spectral filter engineering[3], [6] techniques have been explored for achieving the spectrum selectivity whereas the resonant cavity phenomenon has been used for improving the responsivity and tuneability of the detectors across the desired spectrum [11], [12]. However, in general, the spectrum selectivity has been achieved at the cost of reduced responsivity of the photodetectors. In this paper, we have investigated the effect of the Schottky metal electrode on the photoresponse characteristics of metal (Pd, Au)/CdSe QDs/ZnO QDs optical cavity based spectrum selective photodiodes possibly for the first time. The device is illuminated from the ITO-substrate side so that the incident light can easily pass through the ZnO QDs layer placed to the CdSe QDs layer and then it can be reflected from the metal/CdSe QDs interface into the active CdSe QDs layer. Since the ZnO has larger bandgap energy than the CdSe, the reflected light from the metal/CdSe QDs interface are also reflected from the CdSe/ZnO interface into an active CdSe layer of the device. As a result, the light entered into the CdSe QDs layer is confined within this layer for a long time due to multiple reflections from both the metal/CdSe and CdSe/ZnO interfaces which can enhance the absorption as well as responsivity of the device. The effect of reflectance characteristics of light from the metal (Pd, Au)/ CdSe QDs interface on the optical response of the device has been analyzed by both experiment and theory in the paper. Effects of Optical Resonance on the Performance of Metal (Pd, Au)/CdSe Quantum Dots (QDs)/ ZnO QDs Optical Cavity Based Spectrum Selective Photodiodes Hemant Kumar 1,4 , Yogesh Kumar 1,4 , Bratindranath Mukherjee 2 , Gopal Rawat 1,4 , Chandan Kumar 1 , Bhola N. Pal 3 and Satyabrata Jit 1 , Senior Member, IEEE Authors are with the 1 Department of Electronics Engineering, 2 Department of Metallurgical Engineering, and 3 School of Material Science and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, and 4 Department of Electronics Engineering, Jaypee Institute of Information Technology-Noida (e-mail: hkumar.rs.ece13@iitbhu.ac.in, sjit.ece@iitbhu.ac.in).