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pISSN: 0256-1115 eISSN: 1975-7220

INVITED REVIEW PAPER

INVITED REVIEW PAPER

ZnO/conducting polymer bilayer via sequential spin-coating for enhanced UV sensing Taehyun Park*, Hyung Wook Choi**, and Jaehyun Hur*,† *Department of Chemical and Biological Engineering, Gachon University, Seongnam, Gyeonggi 13120, Korea **Department of Electrical Engineering, Gachon University, 1342 Seongnam Daero, Sujeong-gu, Seongnam-si, Gyeonggi-do 13120, Korea (Received 28 August 2019 • Revised 18 March 2020 • Accepted 30 April 2020)

AbstractZinc oxide (ZnO) has been widely investigated as an important ultraviolet (UV) sensing material in view of its wide band gap (~3.4 eV). However, the fabrication of continuous thin films of ZnO generally requires complex, time-consuming, and expensive processes, such as sputtering and atomic layer deposition. Herein, we demonstrate a bilayer film consisting of a conducting polymer and ZnO nanoparticles sequentially deposited using a simple, rapid, and inexpensive two-step spin-coating process. In this approach, it is not necessary to have a continuous ZnO nanoparticle film as the active layer, because the conducting polymer deposited under the ZnO nanoparticles acts as a conductive and continuous supporting layer for the particles. Poly(3,4-ethylenedioxythiophene) : polystyrene sulfonate (PEDOT : PSS) is used as the auxiliary layer to promote the efficient transport of photo-carriers generated from ZnO nanoparticles under UV light. As a result, under UV light (365 nm), photocurrents obtained from a ZnO/PEDOT : PSS bilayer film are significantly higher (~20 times) than that from a ZnO layer for a given voltage bias. The photoelectric performance can be further tuned by controlling the speed of spin-coating in the deposition of ZnO nanoparticles. The stability and photo response (rise and decay time) of the ZnO/PEDOT : PSS bilayer film under the repeated on-off condition are also reported. Keywords: Zinc Oxide Nanoparticles, PEDOT : PSS, Bilayer, Spin Coating, UV Sensor

sensor [14-16]. Fundamentally, this concept is based on including a continuous layer of a conductive material in the proximity of ZnO NPs to effectively reduce the distance travelled by photocarriers between electrodes; this approach also serves to provide continuous connectivity between the ZnO NPs. However, previously mentioned studies used mostly graphene-based materials as auxiliary conducting layers which cannot be easily formed using a simple spin-coating process [14-16]. In this study, we introduce poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) as an auxiliary conductive layer in ZnO NP-based UV sensors (a bilayer film consisting of ZnO NP and PEDOT: PSS) using a simple and facile sequential spin-coating process. PEDOT : PSS acts as a continuous conductive layer under the ZnO NPs, which effectively reduces the distance travelled by photocarriers, thus enhancing carrier transport and improving the performance (both sensitivity and photo-responsivity) of UV sensors. Furthermore, the performance of UV se