Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Oxidized Nickel films as highly transparent HTLs for inverted planar perovskite solar cells Vishesh Manjunath, Santosh Bimli, Kaushal H. Parmar, Parasharam M. Shirage, Rupesh S. Devan ⁎ Discipline of Metallurgy Engineering & Materials Science, Indian Institute of Technology Indore, Simrol, Indore 453552, India ARTICLE INFO Keywords: NiO Islands morphology Perovskite solar cells Ambient process Thermal evaporation PL ABSTRACT Inverted planar perovskite solar cells (PSCs) with nickel oxide (NiO) as a hole transporting layer were fabricated in an ambient atmosphere. Nickel (Ni) film synthesized at optimized evaporation conditions using low-cost thermal evaporation were transformed from island-like structure to compact porous thin films of NiO after oxidation at 580 ℃. The formation of highly transparent NiO films without any impurity was confirmed from UV–visible spectroscopy and energy dispersive x-ray analysis. These optically tailored NiO films with island-like morphology conceived minimum absorption to the visible light than that of compact porous thin films. The NiO island-like films coated with single cationic CH 3 NH 3 PbI 3 perovskite overlayer in ambient conditions via a modified two-step method showed higher hole quenching than the compact porous NiO thin films. PSCs con- sisting of NiO island-like films showed 39.3% improvement in power conversion efficiency (PCE), and 41.4% enhancement in current density (J SC ) compared to the compact porous NiO thin films. Overall, the present approach of utilizing optically engineered island-like inorganic films with single cationic CH 3 NH 3 PbI 3 perovskite overlayer has opened up a novel approach toward the improvement in high-performance optoelectronic devices fabricated at an ambient atmosphere. 1. Introduction Recently, organic-inorganic hybrid perovskite solar cells (PSCs) have shown remarkable light-harvesting owing to its desired photo- voltaic properties. With long charge diffusion lengths, high absorption coefficient, ease of processing, etc. researchers have achieved effi- ciencies close to 23% (Jung et al., 2019). Miyasaka et al. reported an initial description of the light-harvesting capabilities of organic-in- organic perovskite in 2009 (Kojima et al., 2009). Then, these solar cells evolved through various modifications such as the use of solid elec- trolytes, tuning the bandgap of the light harvester, use of multiple or- ganic transport layers, utilization of different inorganic nanostructures as transport layers, and development of new device configurations, etc. (Ameen et al., 2016; Hao et al., 2014; Kim et al., 2012; Malinkiewicz et al., 2013; Maniarasu et al., 2018; Singh et al., 2019; Stranks et al., 2015). In the case of device architectures, PSCs are broadly classified as mesoporous, planar, and carbon-based architectures. Further, PSCs are categorized depending on the sequence of deposition and type of transport layer as n-i-p and p-i-n configuration (Mali and Hong, 2016). Owing to the fact that, PSCs are the evolution of dye-sensitized solar cells, the majority of all device architectures and configurations uses semiconducting transport layers (Arora et al., 2017; Didwal et al., 2016; Ganapathy et al., 2014; Karthikeyan et al., 2017; Mousavi-Kamazani et al., 2015a, 2015b; Ramasamy and Lee, 2011). Semiconducting transport layers are selected based on conduction band edge levels, mobility of charge carriers, and absorption range, because conduction band edge mismatch could lead to recombination of photo-generated charge carriers, the difference in mobility of the transport layer and light harvester will cause charge accumulation at their interface, and the absorption range decides the optical transparency to irradiate the light harvester respectively (Jung and Park, 2015). Therefore, many inorganic hole transport layers (HTL) are developed and checked for its suitability in PSCs. Additionally, a variety of thin film techniques are used to coat the HTL. Despite such progress in inorganic transport layers, high efficiency PSCs are realized by using expensive organic HTLs such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) or 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobi- fluorene (spiro-MeOTAD) which hinder their use in PSCs owing to the use of hygroscopic dopants that trigger degradation of the perovskite layer, and the requirement of an inert atmosphere for processing (Rong et al., 2015). Thus, to get through this problem, among all the inorganic transport layers, NiO is of prime importance and found more promising candidate for the optoelectronic and energy storage/conversion studies because of its chemical stability (Dalavi et al., 2013a, 2013b; Devan https://doi.org/10.1016/j.solener.2019.09.070 Received 22 June 2019; Received in revised form 5 September 2019; Accepted 19 September 2019 ⁎ Corresponding author. E-mail address: rupesh@iiti.ac.in (R.S. Devan). Solar Energy 193 (2019) 387–394 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T