Raman Spectroscopy of Organic-Inorganic Halide Perovskites Martin Ledinsky ́ ,* ,†,‡ Philipp Lö per, † Bjoern Niesen, † Jakub Holovsky ́ , ‡ Soo-Jin Moon, § Jun-Ho Yum, § Stefaan De Wolf, † Antonín Fejfar, ‡ and Christophe Ballif †,§ † Photovoltaics and Thin-Film Electronics Laboratory, Institute of Microengineering (IMT), Ecole Polytechnique Fe ́ de ́ rale de Lausanne (EPFL), Neuchâ tel 2000, Switzerland ‡ Laboratory of Nanostructures and Nanomaterials, Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka ́ 10, 162 00 Prague, Czech Republic § CSEM PV-center, Neuchâ tel 2000, Switzerland ABSTRACT: Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri- iodide (CH 3 NH 3 PbI 3 ), tribromide (CH 3 NH 3 PbBr 3 ), and mixed iodide-bromide (CH 3 NH 3 PbI 3-x Br x ) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH 3 NH 3 PbI 3 layers and discuss the distinct Raman bands that develop during moisture- induced degradation. We also prove unambiguously that the final degradation products contain pure PbI 2 . Moreover, we describe CH 3 NH 3 PbI 3-x Br x Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to- bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio. T hin-film solar cells based on organic-inorganic halide perovskite absorber layers are emerging as a high- performance photovoltaic technology. Since the first report on perovskite-based solar cells by Kojima et al., 1 the conversion efficiency of these cells has increased rapidly to certified efficiencies up to 17.9%. 2,3 An important factor explaining this fast progress is found in the excellent semiconductor properties of CH 3 NH 3 PbI 3 perovskite films, displaying very high absorption coefficients in the visible part of the solar spectrum and a well-ordered microstructure of the deposited films, as evidenced by their sharp absorption edge. 4 An advantage of organic-inorganic halide perovskites, compared to many other thin-film solar cell materials, is the fact that their band gap is readily tunable by either changing the atom at the halogen site or by changing the alkyl substituent at the ammonium cation. 5 Depending on the iodide-to-bromide ratio in the CH 3 NH 3 PbI 3-x Br x perovskite structure, the band gap energy varies from 1.57 to 2.23 eV. 6 This property is especially important for tandem solar cell applications, where a perovskite top cell of sufficiently wide band gap is stacked on a crystalline silicon (c-Si), copper indium gallium selenide or other bottom cell with a lower band gap energy. 7-11 Unfortunately, perovskite solar cells typically degrade at moderate temperatures and upon moisture ingress. Insight into the fundamental principles underlying such degradation is of utmost importance to formulate mitigation strategies. For this, accurate characterization techniques are essential. Typically, the degradation process is nonhomogenous, often starting at the corner of a sample. One plausible degradation mechanism of CH 3 NH 3 PbI 3 thin films involves its decomposition in the presence of water vapor into CH 3 NH 3 , HI water solutions, and solid lead iodide PbI 2 . 12 Regardless of the exact mechanism, the degradation is irreversible and finally results in a layer consisting of yellow PbI 2 platelets. 12 Optically, the degradation is evidenced as a bleached optical absorption in the orange-red part of the spectra, 4 visible to the naked eye, or more quantitatively by appropriate methods such as spectropho- tometry, photothermal deflection spectroscopy (PDS), or Fourier-transform photocurrent spectroscopy (FTPS). 4 As the degradation induces a structural change, it can also be characterized with X-ray diffraction. 13 Unfortunately, all of these techniques have a low spatial resolution (typically in the mm to cm range) and do not permit locally resolved studies of the perovskite layer structural properties. However, precise mapping of microstructural information may be essential to unravel the fundamental degradation mechanisms as well as for probing of optoelectronic perovskite properties. In this Letter, we present micro-Raman spectroscopy as a well-suited measurement technique to probe organic-inorganic halide perovskite layers locally on the micrometer scale. By using very low excitation laser intensities, we are able to obtain Raman spectra of pristine CH 3 NH 3 PbI 3 layers. After repeatedly measuring the same sample, we observe structural changes, which we correlate with local moisture-induced degradation of the perovskite film. On the basis of these results, we discuss the Received: December 12, 2014 Accepted: January 13, 2015 Published: January 13, 2015 Letter pubs.acs.org/JPCL © 2015 American Chemical Society 401 DOI: 10.1021/jz5026323 J. Phys. Chem. Lett. 2015, 6, 401-406