Ambient atmospheric temperature processed lead halide perovskites Vani Pawar 1 • Manish Kumar 1 • Priyanka A. Jha 1 • S. K. Gupta 2 • A. S. K. Sinha 3 • Pardeep K. Jha 1 • Prabhakar Singh 1 Received: 27 November 2018 / Accepted: 7 August 2019 Ó Akade´miai Kiado´, Budapest, Hungary 2019 Abstract Perovskite, ABX 3 , materials are usually synthesized in inert atmosphere. Their properties are severely affected by the humidity and temperature of the surroundings which is very far from real-life applications. Hence, in the present work, bulk systems of CH 3 NH 3 PbI 3 and CsPbI 3 processed at ambient temperature and pressure have been investigated for their thermodynamics parameters, structural, optical and electronic features. Simultaneously, an attempt has been made to understand the role of methylammonium ion (CH 3 NH 3 ? ) in lead halide perovskite. Keywords Solar cell materials  Raman  XPS  Thermodynamics Introduction Hybrid lead halide perovskite solar cells have gained enormous attention in recent years due to their high power conversion efficiency up to * 21% [1]. Perovskite, ABX 3 , materials exhibit exceptional properties such as long-range charge carrier diffusion length [2], suitable band gap [3], low exciton binding energy and remarkably high absorp- tion [4–7]. The A-site organic cations such as methylam- monium (MA) ion (CH 3 NH 3 ? ) and formamidinium ion (CH(NH 2 ) 2 ? ) play an important role to decide the opto- electronic properties of perovskite materials [6]. However, these cations are highly sensitive to atmospheric conditions such as moisture, oxygen, heat, light and thermal stability [5]. Moreover, the samples or thin films of such materials are not only being prepared but also characterized in the inert atmosphere [8–10]. Despite the intense study, origin of instability in ambient conditions could not be under- stood. Unfortunately, it is very far from the real objective for which these materials got attention. Moreover, corre- lation between thermodynamics and optical band gap to study stability is not explored. Currently, none of the papers report the experimental thermodynamic property of halides. In the present work, we have tried to address some of these major issues with special emphasis to synthesize and characterize the hybrid perovskite material in ambient atmosphere and study the thermodynamic parameters of CH 3 NH 3 PbI 3 (MAPbI 3 ). Further, in order to understand the role of MA, we have synthesized another material by replacing organic MA with inorganic cesium, i.e., cesium lead iodide (CsPbI 3 ). In the present work, we report ther- modynamic parameters of ambient atmospheric tempera- ture processed bulk MAPbI 3 and CsPbI 3 systems along with the correlation of free energy and band gap. Experimental Synthesis The initial precursor of methylammonium iodide (CH 3- NH 3 I) and cesium iodide (CsI) was procured from Sigma- Aldrich. PbI 2 (lead iodide) was synthesized by mixing Pb(NO 3 ) 2 and KI in a stoichiometric proportion. The & Pardeep K. Jha pardeepjha.jiit@gmail.com & Prabhakar Singh psingh.app@iitbhu.ac.in 1 Department of Physics, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221005, India 2 Department of Physics, Banasthali Vidyapeeth, Banasthali, Rajasthan 304022, India 3 Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221005, India 123 Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973-019-08676-w