Band-gap tuning of lead halide perovskite using a single step spin-coating deposition process
In this paper, we present a study of the structural and optical properties of organic-inorganic halide-based perovskite semiconductors with band gaps varying from NIR to visible at room temperature. Thin films of nanocrystalline CH3NH3PbI3 with different concentrations of methylammonium iodide (MAI) have been successfully deposited onto glass substrates using single step spin-coating technique. The prepared films have been characterized by X-ray diffraction, optical absorption spectroscopy and photoluminescence measurements. X-ray diffraction scans revealed that even for stoichiometric atomic cOncentrations of MA:Pb:I of 1:1:3, the PbI2 phase was also present. The PbI2 phase showed a highly textured along the (001) direction in all the prepared films, with a crystallite size in the range of approximately 30-40 nm. The optical absorbance edge of CH3NH3PbI3 thin films is described in terms of direct transition model proposed by Tauc in the strong absorption region. The band gap of the pure PbI2 film was calculated to be 2.40 eV, whereas the band gap of the pervoskite film with stoichiometric ionic ratio of 1:1 PhI2 and CH3NH3I was calculated to be 1.46 eV, while absorption coefficient is similar to 10(5) cm(-1). Photoluminescence measurements showed a red shift in the perovskite emission with increasing the MAI concentration, confirming the correct placement of the MA ions in the perovskite crystalline structure showing the PbI2 octahedra. Our results confirm the existence of PbI2 phase in the highly efficient perovskite solar cells, as demonstrated in the recent publication. (C) 2015 Elsevier B.V. All rights reserved.