**1. Introduction**

#### **1.1 Lead Free Perovskite Materials**

The continual development in efficiency of lead-halide-based perovskites has yielded phenomenal success with efficiencies above 25% [1]. To date, however, there have been few reports of scalable solution or vapor processing techniques being applied to the deposition of perovskite films with nontoxicity and stability. It is

apparent that the path toward commercialization of solution-processed perovskite solar cells requires the development of fabrication protocols compatible with high-volume roll-to-roll or sheet-fed processing techniques. Although the use of lead in perovskite is expected to have many human and environmental issues in the future, these factors are not currently being considered due to the battle for solar cell efficiency. This research trends make the future of perovskite solar cell darker. Therefore, the most important key to the commercialization of perovskite solar cells is the development of lead-free structured materials with competitive solar cell efficiency.

#### **1.2 Tin-based perovskite**

Tin of a group 14 element with comparable ionic radii is currently considered as one of the best candidates to replace lead compounds due to its meaningful electrical and optical properties [2]. Completely lead-free perovskite solar cells based on CH3NH3SnI3 were reported in 2014 yielding efficiencies of 5.23% [3] and 6.4% [4] with the suppression of Sn4+ accomplished through ultrahigh purity starting materials, fastidious synthesis, and glove box device fabrication protocols. In terms of optical bandgap calculated by Shockley-Queisser for the highest efficiency, CH3NH3SnI3 (Eg 1.3 eV) is considered the ideal materials (Eg, ideal 1.35 eV). However, relatively lower efficiency, the poor long-term stability, and low reproducibility of these films caused by the tendency for Sn to get oxidized are insufficient to replace this material with lead-based perovskite [4]. To improve power conversion efficiencies (PCEs) as inhibiting the oxidation of Sn2+ to Sn4+, *CH3NH3SnxPb1–xI3*, which is mixture compounds of Sn and Pb, is reported [5]. Mixtures containing about x = 0.25 of Sn (*CH3NH3Sn0.25Pb0.75I3*) showed 7.4% of the best efficiency [6]. Further improvement (PCE 10.1%) was demonstrated as adding Cl, which show a better film coverage, effective exciton dissociation, and charge transport [7].
