nanoGe - HOPV15 - Comparison of Transmission Electron Microscopy elemental mapping and photoluminescence emission of organic

Comparison of Transmission Electron Microscopy elemental mapping and photoluminescence emission of organic - inorganic halide perovskites

Caterina Ducati^a, Stefania Cacovich^a, Giorgio Divitini^a, Milan Vrućinić^b, Henning Sirringhaus^b, Felix Deschler^b, Richard H. Friend^b, Mete Atatüre^b, Sian E. Dutton^b, Aditya Sadhanala^b, Clemens Matthiesen^b, Henry J. Snaith^c

^aUniversity of Cambridge, Department of Materials Science and Metallurgy, UK, Cambridge, United Kingdom

^bCavendish Laboratory, University of Cambridge - UK, JJ Thomson Avenue, 9, Cambridge, United Kingdom

^cUniversity of Oxford, Department of Physics, Clarendon Laboratory, UK, Parks Road, United Kingdom

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Stefania Cacovich, 412
Publication date: 5th February 2015

Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Over the last few years organic – inorganic halide perovskite-based solar cells exhibited a rapid evolution^[1], reaching certified power conversion efficiencies now surpassing 20%. The understanding of the optical and electronic properties of such systems on the nanoscale is still an open problem. In this work we investigate two model perovskite systems (based on iodine - CH₃NH₃PbI₃ and bromine - CH₃NH₃PbBr₃), analysing the local elemental composition and crystallinity, identifying inhomogeneities and correlating the results with PL emission measurements carried out using scanning near-field optical microscopy (SNOM). A SNOM / atomic force microscope (AFM) setup was used to acquire spatially and spectrally-resolved PL emission maps simultaneously with topography maps. We observed the presence of localised hot-spots, that might be ascribed to recombination centres, in addition to a large spatial variation in the PL intensity. No correlations have been found between PL and topography maps, excluding an effect of the morphology of the film on the emission spots. High resolution transmission electron microscopy (HRTEM), in combination with Scanning Transmission Electron Microscopy (STEM) and EDX (Energy dispersed X-Ray spectroscopy) were used to characterise morphology, crystallinity and chemical composition of the perovskite films. HRTEM images show that both CH3NH3PbBr3 and CH3NH3PbI3 perovskite films present polycrystalline domains with different crystal orientation on a length scale of tens of nanometers. STEM-EDX analysis was carried out in order to assess a correlation between the previously observed PL intensity variations and the relative differences in the elemental composition. Spectral datasets were analysed using machine learning techniques to optimise the signal-to-noise ratio, obtaining EDX elemental maps with nm-level spatial resolution [Fig 1]. The iodine and bromine EDX maps resulted homogeneous, with variation of ± 5% with respect to the average elemental weight ratio. In conclusion, we interpret these areas of high PL intensity as regions characterised by higher crystallinity and enhanced material quality, since chemical homogeneity has been demonstrated by electron microscopy measurement.
Figure 1) STEM images of CH3NH3PbBr3 (a) and CH3NH3PbI3 (d). Elemental maps of Br (b), Pb (c,f) and I (e) are reported for the two samples. All images are 4µm x 4µm. From [2].
[1] H. Zhou, Q. Chen, G. Li, S. Luo, T. Song, H. Duan, Z. Hong, J. You, Y. Liu, Y. Yang. Interface engineering of highly efficient perovskite solar cells. Science 345, 542- 546, (2014) [2] M. Vrućinić, C. Matthiesen, A. Sadhanala, G.Divitini, S. Cacovich, S. E. Dutton, C. Ducati, M. Atatüre, H. J. Snaith, R. H. Friend, H. Sirringhaus, and F.Deschler - Local versus long-range diffusion effects of photoexcited states on radiative recombination in organic-inorganic lead halide perovskites - submitted

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