Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article Improving the efficiency of perovskite light emitting diode using polyvinylpyrrolidone as an interlayer Yunho Ahn a , Seungjun Lee b , Do-Hyung Kwak b , Myeongseop Kim a , Dae Yeong Kim a , Jungwon Kim a , Yongsup Park b, ⁎ , Min Chul Suh a, ⁎⁎ a Department of Information Display, Kyung Hee University, Seoul 02447, Republic of Korea b Department of Physics and Institute of Basic Sciences, Kyung Hee University, Seoul 02447, Republic of Korea ARTICLE INFO Keywords: Interlayer analysis Perovskite materials Highly efficient green PeLED Improved device stability ABSTRACT A few nanometer-thick polyvinylpyrrolidone (PVP) interlayer between PEDOT:PSS anode and active layer was introduced in a perovskite light-emitting diode (PeLED) in conjunction with nanocrystal pinning (NCP) process. The device with the PVP interlayer exhibited dramatic improvements in performance resulting in current effi- ciency of 20.89 cd A -1 , external quantum efficiency (EQE) of 5.4%, the luminance of 39,023 cd m -2 , and FWHM of 18 nm, despite the fact that some degree of intermixing of the layers was expected because identical solvent was used for all three layers (PEDOT:PSS, PVP, and perovskite). The interface between PVP and perovskite layer was investigated by using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) sputter depth pro- filing with Ar gas cluster ion beam. Although obtaining a clear interface in XPS depth profile data was not possible because of partial interlayer mixing and the thinness of the PVP layer, an obvious difference across the interface was extracted in the UPS data. The observed energy level diagram was consistent with the char- acteristic of hole-only device (HOD) in which charge injection from PEDOT:PSS anode to the perovskite layer was suppressed due to the PVP layer. 1. Introduction A great deal of attention was focused on organic-inorganic hybrid perovskite (OIP) photovoltaic devices as the power conversion effi- ciency of 23% was recently achieved [1]. Other applications of the OIP appeared almost simultaneously including light-emitting diodes, optical sensors, and lasers [2–6]. The perovskite LEDs (PeLEDs) have shown great promise among these other applications and have spawned ra- pidly increasing research activities. This is partly due to the relative ease of controlling the nanometer-scale grain sizes of the material, which is a good way of maximizing the photoluminescence quantum yield (PLQY). Easy band gap control, narrow emission spectral peaks (high color purity), and low fabrication cost are some of the favorable characteristics of the PeLEDs [7–12]. Especially, in the case of red PeLED, the external quantum efficiency (EQE) is now more than 20.7% which is comparable to other commercialized LEDs [13]. Besides, green and blue PeLED have been reported to have high efficiencies of 20.3% and 9.5%, respectively [14–15]. Therefore, based on these achieve- ments, the key factors for achieving such a high efficiency have been established. It was identified that surface energy control of the layer on which perovskite is coated, morphology control of perovskite layer, surface passivation, nano-crystal pinning for grain size control are the key factors for the enhanced device performance [16–27]. However, controlling the surface energy poses difficulties compared to other strategies because a hydrophobic layer under the perovskite layer decreases the wettability of the perovskite layer leading to an undesirable surface morphology. Nevertheless, there are reports of highly efficient PeLEDs with the same polarity layers. For example, PEI or PVP was used between a ZnO layer and a perovskite layer to enhance the wettability in inverted PeLEDs [16,18]. The ZnO was first dissolved in ethanol and spin coated, on which PEI was spin coated and rinsed by DMF, leaving ultrathin PEI layer for the better wettability [16]. In PVP case, both PVP and perovskite were dissolved by DMSO [18]. These reports claim that the interlayer (PEI or PVP) is crucial to the high performance of PeLED, although the evidences are somewhat lacking. In particular, the presence of distinct interfaces in the fabricated de- vices, as originally planned, is a rarely confirmed. The reported thick- ness of the interlayer before and after DMSO washing is 8.4 and 5.0 nm, respectively [18]. Hence, it may be difficult to confirm the actual ex- istence of such a thin layer with cross-sectional electron microscope https://doi.org/10.1016/j.apsusc.2019.145071 Received 9 August 2019; Received in revised form 8 November 2019; Accepted 12 December 2019 ⁎ Corresponding author. ⁎⁎ Corresponding author. E-mail addresses: parky@khu.ac.kr (Y. Park), mcsuh@khu.ac.kr (M.C. Suh). Applied Surface Science 507 (2020) 145071 Available online 14 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved. T