Improved performance of ﬂexible perovskite light-emitting diodes with modiﬁed PEDOT:PSS hole transport layer Haesook Kim a , Ha Na Ra a , Ji Sun Kim a , Sang-Hyon Paek b , Jongwook Park b , Young Chul Kim a,b, * a Regional Innovation Center for Components and Materials for Information Display, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea b Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea A R T I C L E I N F O Article history: Received 23 January 2020 Received in revised form 2 June 2020 Accepted 5 July 2020 Available online 9 July 2020 Keywords: Perovskite light-emitting diodes Modiﬁed PEDOT:PSS Flexible substrates A B S T R A C T We demonstrated the improved device performance of ﬂexible perovskite light-emitting diodes (PeLEDs) on indium-tin oxide coated poly(ethylene terephthalate) (ITO/PET) substrates by using modiﬁed poly (3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL). Solvent modiﬁcation and dopant modiﬁcation of the PEDOT:PSS solution were carried out by adding isopropyl alcohol (IPA) and poly(sodium 4-styrenesulfonate) (PSS-Na), respectively, to the pristine solution. Devices with the modiﬁed HTLs, PEDOT:PSS + IPA and PEDOT:PSS + PSS-Na, exhibited a signiﬁcant enhancement in both luminance and efﬁciency. The work function of HTL was improved to reduce the energy barrier against the perovskite layer and facilitate hole transport. The perovskite ﬁlms deposited on the modiﬁed PEDOT:PSS layers showed a uniformly covered surface morphology without any defects, resulting in an increased PL intensity. In particular, the perovskite ﬁlm coated on the dopant modiﬁed HTL showed a better crystallinity and reduced luminescence quenching. The PeLED device with the PEDOT:PSS + PSS-Na HTL exhibited a maximum luminance of 3900 cd/m 2 , a maximum current efﬁciency of 25.1 cd/A, and a maximum external quantum efﬁciency of 5.9%. © 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Introduction Metal halide perovskites-based light-emitting diodes (PeLEDs) have emerged as one of promising candidates to replace conventional LEDs, which is because of their excellent character- istics such as band gap tunability, high color purity, cost effectiveness, and easy solution processability [1]. The perfor- mance of PeLEDs has enhanced signiﬁcantly in recent years: red and green PeLED have been reported to have the external quantum efﬁciency (EQE) exceeding 20% [2,3]. However, outstanding works about PeLEDs have been mostly based on the devices on rigid glass substrates and therefore it is quite necessary to design and fabricate PeLED devices on ﬂexible substrates in order to make the most of their unique features such as thin thickness, light weight, design ﬂexibility, and suitability for roll-to-roll processing [4]. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been commonly employed as a hole transport layer (HTL) in organic light-emitting diodes (OLEDs). However, it has been reported that its acidic and hygroscopic nature could severely deteriorate the long-term stability of the devices [5]. PEDOT:PSS may also degrade the device performance by limiting the radiative recombination of charge carriers due to the huge hole injection barrier against the perovskite emitting layer [6]. Therefore, as a substitute for PEDOT:PSS, various metal oxides, including vanadium oxide (VO x ), tungsten oxide (WO x ), molybde- num oxide (MoO x ), and nickel oxide (NiO x ) etc., have been investigated [7–11]. These metal oxides are more stable against the environmental moisture/oxygen and heat, and also exhibited better interfacial adhesion with the perovskite layer [5,12]. However, a high annealing temperature of 200500 ℃ is generally required for crystallization of metal oxides, although it tends to decrease gradually in recent works. This can be a signiﬁcant limitation in fabricating PeLED devices on ﬂexible plastic substrates by a large roll-to-roll process [8]. Simplicity of the one-step spin-coating process makes it preferable to other methods for PeLED fabrication [13]. The perovskite ﬁlms spin-coated on pristine PEDOT:PSS layers, * Corresponding author at: Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea. E-mail address: kimyc@khu.ac.kr (Y.C. Kim). https://doi.org/10.1016/j.jiec.2020.07.003 1226-086X/© 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Journal of Industrial and Engineering Chemistry 90 (2020) 117–121 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal homepa ge: www.elsev ier.com/locate/jie c