Optical Materials 109 (2020) 110456 Available online 25 September 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved. Invited Article Effect of acetic acid concentration on optical properties of lead acetate based methylammonium lead iodide perovskite thin flm Kindness A. Uyanga a, b, * , Sabastine C. Ezike c , Amadi T. Onyedika b , Abdulazeez B. Kareem b , Timothy M. Chiroma b a Energy Harvesting and Wearable Energy Technologies, School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b Department of Chemical Engineering, School of Engineering and Engineering Technology, Modibbo Adama University of Technology, Yola, Nigeria c Department of Physics, School of Physical Sciences, Modibbo Adama University of Technology, Yola, Nigeria A R T I C L E INFO Keywords: Tauc’s plot Dielectric constants Extinction coeffcient Refractive index Photoconductivity Two-step spin-coating method ABSTRACT High absorption of perovskite thin flms is crucial for effcient metal halide perovskite solar cells. In this study, methylammonium lead iodide (CH 3 NH 3 PbI 3 ) perovskite thin flms are prepared using lead (II) acetate Pb(Ac) 2 , acetic acid (HAc) and CH 3 NH 3 I through solution method. The effect of HAc volume (0 ml–100 ml at step size of 25 ml HAc) on optical properties and parameters of the CH 3 NH 3 PbI 3 perovskite thin flms developed are investigated using UV–Vis spectrophotometry and mathematical correlations. Interestingly, the increase of HAc amount to 75 ml leads to an increase in absorbance, refractive index, extinction co-effcient, dielectric properties and optical conductivity and a decrease in transmittance and optical band gaps of the CH 3 NH 3 PbI 3 perovskite thin flms. The optimized flm is obtained at 75 ml HAc at which refractive index, flm thickness and optical band gap is 1.45 at 485 nm, 355 nm and 1.47 eV, respectively. The enhanced absorption of the optimized thin flm is attributable to an increase in structure ordering at HAc amount equals 75 ml. The fabricated CH 3 NH 3 PbI 3 perovskite thin flms have potential as ideal antirefection coatings for solar cells and optoelectronic applications. 1. Introduction Hybrid perovskites have been the subject of much systematic investigation. Their unique and wide range of optoelectronic applica- tions [1,2] and distinguishing properties such as high defect tolerance, high electron/hole mobilities [3,4], direct and tunable band gap [5,6], strong light absorption coeffcients [7] and easy processability into thin flms using low-cost techniques [1,8,9] have stimulated the observed high research interest. Hybrid perovskite solar cells (PSCs) assume chemical structure ABX 3 (where the A, B and X sites are occupied by an organic cation (CH 3 NH 3 + ), metal cation (Pb 2+ ) and halide anion (I  ), respectively) and contain active/light-absorbing materials [10–12]. Among hybrid perovskite active materials, methylammonium lead iodide (CH 3 NH 3 PbI 3 ) is widely selected due to the intense absorption at the entire visible spectrum [9,13]. CH 3 NH 3 PbI 3 has a direct band gap of 1.55 eV corresponding to an absorption offset at 800 nm. When deployed in planar form, the fabricated PSCs have advantages of enhanced power conversion effciency (PCE), low-cost, lightweight, and compatibility with solution-based roll-to-roll technique [14–16]. How- ever, with no mesoporous metal oxide skeleton in the planar structure owing to differences in surface energy between the substrate and the perovskite [15,16], there is diffculty of forming lead halide-based perovskite flm without pinholes, having full coverage on the conduc- tive substrate through solution process [9]. Liu et al. [17] compared the quality of perovskite (CH 3 NH 3 PbI 3-x Cl x ) flms and cells deposited using solution-deposited and vapour-deposited methods. They observed that the vapour-deposited flm has uniform thickness of approximately 330 nm while the solution-based flm has non-uniform thickness varying between 50 and 410 nm. The undulating nature of the solution-processed flm thickness made the cell to perform below the vapour-deposited cell following either insuffcient absorption of sun- light by perovskite or inadequate collection of charge carriers by transport layers. Abdy et al. [18] deposited CH 3 NH 3 PbI 3 layer using three different techniques – two-step solution deposition, solvent annealing and electrodeposition. Among these methods, electrodeposi- tion of perovskite yielded best flm coverage, uniformity and largest * Corresponding author. Energy Harvesting and Wearable Energy Technologies, School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail address: mkauyanga2-c@my.cityu.edu.hk (K.A. Uyanga). Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2020.110456 Received 7 August 2020; Received in revised form 18 September 2020; Accepted 18 September 2020