Journal
CHEMSUSCHEM
Volume 9, Issue 18, Pages 2656-2665Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/cssc.201600879
Keywords
additives; perovskite; phase transition; thin films; x-ray diffraction
Funding
- ANSER Center, an Energy Frontier Research Center - U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0001059]
- MRSEC program of the National Science Foundation at the Materials Research Center of Northwestern University [DMR-1121262]
- International Institute for Nanotechnology
- Northwestern University MRSEC [NSF DMR-1121262]
- Keck Foundation
- State of Illinois
- Northwestern University
- U.S. DOE [DE-AC02-06CH11357]
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Although reactive additives have been employed in perovskite solar cells to enhance film morphology and significantly increase device performance, little is known about the effect of these additives on perovskite structural and optical properties. Here we report a systematic study of how the properties of methylammonium lead iodide perovskite (CH3NH3PbI3) are influenced by hydrohalic acid additives (HX; X=I, Br, Cl) in the precursor solution. Detailed structural and optical spectroscopic analysis reveals that all three acids affect the optical properties and alter the unit cell lattice parameters. Depending on the identity and concentration of HX, optical bandgaps widen or compress: addition of HBr yields a wider bandgap, whereas HI compresses the gap at high concentrations; HCl, on the other hand, has no significant effect on the bandgap. These changes can be understood by correlating them with the types of defects present in polycrystalline perovskite thin films in combination with the structural strain induced in very small crystallites. The presence of extra halides from HX in the precursor solution enables filling of the lattice vacancies in the perovskite, thereby altering metal-halogen-metal bond connectivity and consequently cell volumes and optical bandgaps. Remarkably, a room temperature tetragonalcubic phase transition is observed for CH3NH3PbI3 films treated with high HX concentrations. Further insights into this anomalous phase transformation are obtained from insitu variable-temperature X-ray diffraction in the 25-55 degrees C (298-328K) range, revealing a monotonic fall in transition temperature with increasing precursor solution HX concentration.
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