Chloride Incorporation Process in CH₃NH₃PbI_3-xCl_x Perovskites via Nanoscale Bandgap Maps

Authors

Jungseok Chae, National Institute of Standards and Technology
Qingfeng Dong, University of Nebraska-LincolnFollow
Jinsong Huang, University of Nebraska-LincolnFollow
Andrea Centrone, National Institute of Standards and Technology

Date of this Version

2015

Citation

Nano Lett. 2015 December 9; 15(12): 8114–8121.

Comments

Copyright 2015 American Chemical Society. Used by permission.

Abstract

CH₃NH₃PbI_3-xCl_x perovskites enable fabrication of highly efficient solar cells. Chloride ions benefit the morphology, carrier diffusion length and stability of perovskite films; however, whether those benefits stem from the presence of Cl⁻ in the precursor solution or from their incorporation in annealed films is debated. In this work, the photothermal induced resonance (PTIR), an in situ technique with nanoscale resolution, is leveraged to measure the bandgap of CH₃NH₃PbI_3-xCl_x films obtained by a multicycle coating process that produces high efficiency (≈16 %) solar cells. Because chloride ions modify the perovskite lattice, thereby widening the bandgap, measuring the bandgap locally yields the local chloride content. After a mild annealing (60 min, 60°C) the films consist of Cl-rich (x < 0.3) and Cl-poor phases that, upon further annealing (110 °C), evolve into a homogenous Cl-poorer (x < 0.06) phase, suggesting that methylammonium-chrloride is progressively expelled from the film. Despite the small chloride content, CH₃NH₃PbI_3-xCl_x films show better thermal stability up to 140 °C with respect CH₃NH₃PbI₃ films fabricated with the same methodology.