Materials Science and Engineering B 296 (2023) 116645 Available online 13 June 2023 0921-5107/© 2023 Elsevier B.V. All rights reserved. Structural and optical properties of vacancy-ordered double halide perovskites, Cs 2 TiI 6 flms Sameen Aslam a, b , Sunila Bakhsh b , Yushamdan Yusof a , Mohd Yusri Abd Rahman c , Abdul Razak Ibrahim a , Siti Azrah Mohamad Samsuri a, * a Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia (USM), 11800 Pulau Pinang, Malaysia b Department of Physics, Balochistan University of Information Technology Engineering and Management Sciences, Quetta 87300, Pakistan c Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia A R T I C L E INFO Keywords: Cs 2 TiI 6 Lead-free perovskites Double halide perovskites Inverse temperature crystallization ABSTRACT Vacancy-ordered double halide perovskites (DHP) structures have received considerable attention in photovol- taics due to their role in overcoming the lead toxicity issue for conventional halide perovskites. These materials also have tunable bandgaps and excellent optoelectronic properties suitable for a wide range of applications. Herein, we report the synthesis of titanium-based vacancy-ordered DHP, Cs 2 TiI 6 using the Inverse Temperature Crystallization method for the frst time. The Cs 2 TiI 6 flm’s formation, structural and optical properties were also discussed. The flms have a single-phase cubic structure with a space group of Fm 3m (2 2 5) and a lattice parameter of 11.55 Å. The crystallite size, dislocation density, and micro-strain of Cs 2 TiI 6 flm calculated at (2 2 2) plane is 22 nm, 2.5 × 10 3 nm 2 , and 0.53 × 10 3 , respectively. Optical analysis reveals that Cs2TiI6 exhibits strong and wide optical absorption in the visible spectrum, with direct and indirect band gaps of 1.56 eV and 1.58 eV, respectively. The VESTA-calculated tolerance factor of 0.98 indicates excellent structural stability. These Titanium-based perovskites exhibit promising optoelectronic properties, opening the door for non-toxic perovskite devices with high performance and stability. 1. Introduction Halide perovskite-structured materials have gotten much attention regarding their potential application in optoelectronic devices, espe- cially photovoltaics [1]. Halide perovskite-structured materials have shown great potential as effcient light absorbers, particularly in photovoltaic devices. Halide perovskite solar cells’ current power con- version effciency (PCE) reached 25.5% in 2021 [2]. However, the ex- istence of methylammonium lead (Pb) iodide or bromide in the composition of halide perovskite has hindered the widespread use of these materials in perovskite solar cells [3,4]. Given the concern regarding the toxicity of Pb to humans and the environment on future utilization of perovskite solar cells, the effort to fnd a Pb replacement is necessary [4,5,6]. One of the alternate structures designed to replace Pb with other tetravalent cations is vacancy-ordered double halide perov- skites having a general formula of A 2 BX 6 (where A = organic/inorganic cation, B = metal cation and X = Iodide/Bromide/Chloride) [7,8,9]. The unit cell of ABX 3 is doubled along all three crystallographic axes, and every other B-site cation is removed to create a distinct structure from the typical perovskite, known as a vacancy-ordered double halide perovskite (DHP). The distribution of A and B ions across multiple crystallographic locations makes these vacancy-ordered double perov- skites fexible. This fexibility in their lattice ion positions over the crystal structure generates much interest in exploring these systems more deeply. To achieve the desired optical and electrical properties, vacancy-ordered DHP occupies a diverse range of compositional, structural, and dynamic phases, which can be accessed and modifed by altering the composition at all three sites [7]. The stable perovskite structure could be predicted when Gold- schmidt’s tolerance factor (t) ranges between 0.75 and 1.0. Several different metal elements with comparable ion radii (1.19 Å) can replace Pb [10,11]. In this regard, Tin (Sn) and Germanium (Ge), belonging to group 14, and the trivalent elements such as Bi +3 and Sb +3 from group 15, also depicted similar electronic confgurations as lead (Pb), are the most likely options [12]. Having a similar electrical structure with Pb, Sn-based vacancy- * Corresponding author. E-mail address: sitiazrah@usm.my (S.A.M. Samsuri). Contents lists available at ScienceDirect Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb https://doi.org/10.1016/j.mseb.2023.116645 Received 17 April 2023; Received in revised form 1 June 2023; Accepted 6 June 2023