Hybrid perovskite solar cells: In situ investigation of solution-processed PbI 2 reveals metastable precursors and a pat
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peng Li and Detlef-M. Smilgies Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, NY 14853, USA
Aram Amassiana) KAUST Solar Center (KSC), and, Physical Science and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia (Received 27 February 2017; accepted 17 March 2017)
The successful and widely used two-step process of producing the hybrid organic-inorganic perovskite CH3NH3PbI3, consists of converting a solution deposited PbI2 film by reacting it with CH3NH3I. Here, we investigate the solidification of PbI2 films from a DMF solution by performing in situ grazing incidence wide angle X-ray scattering (GIWAXS) measurements. The measurements reveal an elaborate sol–gel process involving three PbI2DMF solvate complexes—including disordered and ordered ones—prior to PbI2 formation. The ordered solvates appear to be metastable as they transform into the PbI2 phase in air within minutes without annealing. Morphological analysis of air-dried and annealed films reveals that the air-dried PbI2 is substantially more porous when the coating process produces one of the intermediate solvates, making this more suitable for subsequent conversion into the perovskite phase. The observation of metastable solvates on the pathway to PbI2 formation open up new opportunities for influencing the two-step conversion of metal halides into efficient light harvesting or emitting perovskite semiconductors.
I. INTRODUCTION
Photovoltaic devices based on organic–inorganic hybrid perovskites have been attracting more and more attention lately especially as their power conversion efficiency (PCE) has recently surpassed 20%.1–3 In addition to their high performance, their potential low cost, earth abundant elements and solution processability have also been sources of excitement.4–7 The organic–inorganic hybrid perovskite compounds based on metal halides are represented as AMX3 perovskite structure, where A is an organic cation, most often methylammonium (CH3NH31) or formamidinium (HC(NH2)21), M is a metal cation (typically Sn21 or Pb21) and X is a monovalent anion (typically a halide ion Cl, Br, or I).5,8 Several deposition methods have been developed for perovskite film preparation such as: one-step solution deposition,2,9,10 two-step (sequential) solution deposition,3,4,11–13 and one-step or two-step vapor deposition.14,15 Contributing Editor: Moritz Riede a) Address all correspondence to this author. e-mail: [email protected] b) School of Nanoscience and Technology (SNST), Shivaji University, Kolhapur, 416 004, India DOI: 10.1557/jmr.2017.117
The two-step sequential deposition method is one of the earliest and most widely used protocols to be developed for the synthesis of hybrid perovskite thin films. It was first developed to overcome the challenges associated to the formation of pin-hole free films using the single step solution process.3,16,17 In the two-step solution process, the PbI2 powder is first dissolved in dimethylformamide (DMF) then spin
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