Characterization of mixed films Marko vaelma 30.3.2016

Mixed thin films Thin films with inorganic and organic parts Perovskite structure (ABX3 , A = organic cation (green), B = metal ion (blue) and X = halide (Cl, Br, I…) (red)) Methylammonium Lead Iodide ( 𝐶𝐻 3 𝑁 𝐻 3 𝑃𝑏 𝐼 3 ) or mixed halide structures New materials & better properties Organic materials can be easy and cheap to synthezise Fabrication method Spin coating (Evaporation) Liang, K. et al. Synthesis and Characterization of Organic-Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chemistry of Materials. 1998. vol. 10. pp. 403-411. Zhang, W. et al. Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells. Nature communications. 2015. Picture from: https://en.wikipedia.org/wiki/File:Perovskite.jpg

Perovskite organic-inorganic films Properties Electrical conductivity Magnetic properties Optical properties Applications Two dimensional semiconductors Electroluminescent devices Solar cells Compositions can differ, for example in 𝐶𝐻 3 𝑁 𝐻 3 𝑃𝑏 𝐼 3 or 𝐶𝐻 3 𝑁 𝐻 3 𝑃𝑏 𝐼 3−𝑥 𝐶𝑙 𝑥 𝐶𝐻 3 𝑁 𝐻 3 𝑃𝑏 𝐼 3 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒: 𝐶𝐻 3 𝑁 𝐻 3 + ion surrounded by 𝑃𝑏 𝐼 6 octahedra Picture from: Eames, C. Ionic transport in hybrid lead iodide perovskite solar cells. Nature commications. 2015.

Characterization of thin films Goal is to optimize the thin film fabrication process to achieve the wanted properties The structure of the thin film Crystalline or not Information about the functional groups Influence to the crystal structure Optimizing the application specific properties Absorbance for photovoltaics Picture from: http://image.slidesharecdn.com/patrickstarcharacterization-130828153424-phpapp01/95/patrick-star-characterization-2-638.jpg%3Fcb%3D1377722093

XRD (X-Ray Diffraction) X-rays are shot towards the sample Reflection of x-rays from atom planes Constructive interference according to Bragg’s Law Intensity of the peaks in function of the angel is measured Structural analysis Lattice parameters: Interatomic distances & Bond angles Phase identity Phase purity Crystallinity Crystal structure Schematic picture of XRD University of South Carolina: http://www.chem.sc.edu/faculty/zurloye/xrdtutorial_2013.pdf Picture from: http://chemwiki.ucdavis.edu/Core/Analytical_Chemistry/Instrumental_Analysis/Diffraction/Powder_X-ray_Diffraction

XRD for perovskite Detection of crystal structures Perovskite structure Crystallinity The effect of lead source to the structure Lattice parameters Lattice parameters in perovskite structure: a = b = 8.82 Å, c = 12.64 Å Important in optimizing the fabrication process The XRD spectrum of perovskite strucutre obtained by different lead sources are similar, thus the crystal structure is the same Zhang, W. et al. Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells. Nature communications. 2015.

The XRD spectrum of mixed halide perovskite ( 𝐶𝐻 3 𝑁 𝐻 3 𝑃𝑏 𝐼 3−𝑥 𝐶𝑙 𝑥 ) powder heat treated in different temperatures M = methyl ( 𝐶𝐻 3 ) A = ammonia ( 𝑁𝐻 3 ) The purest perovskite structure can be obtained by heat treatment in 150˚C Impurity peaks of PbCl2 and MACl can be seen in lower temperatures (eliminated in higher temperatures)  not completely reacted Impurity peak of PBI2 can be seen in higher temperature (175˚C) due to the sublimation of MAI or MACl Song, D. et al. Reproducible formation of uniform CH3NH3PbI3xClx mixed halide perovskite film by separation of the powder formation and spincoating process. Journal of Power Sources. 2016. vol. 310. pp. 130-136.

UV-Vis (Ultraviolet-Visible) spectroscopy White light is emitted towards the sample Characteristic portions of wavelengths are absorbed The absorbance is critical for photovoltaics! Optimization of composition and process temperatures Picture from: https://www.google.fi/search?q=uv-vis+spectroscopy&espv=2&biw=1517&bih=692&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiK7JLh5ebLAhUI3SwKHT6zDnEQ_AUIBigB&dpr=0.9#imgrc=dD6fTOGHCi8B2M%3A

UV-Vis for perovskite ( 𝐶𝐻 3 𝑁 𝐻 3 𝑃𝑏 𝐼 3 ) Absorbance of the different precursors used in perovskite fabrication The dependency of absorbance on temperature and composition Annealing time in each is 30 min Annealing at 190 ˚C causes MAPbI3 to decompose to MAI and PbI2 Song, Z. et al. Impact of Processing Temperature and Composition on the Formation of Methylammonium Lead Iodide Perovskites. Chemistry of Materials. 2015

Conclusions Perovskite is a mixed material with organic (methylammonium) and inorganic (lead halide) parts Due to its properties it is used in solar cells Optimization of the properties is crucial CHARACTERIZATION is needed XRD: Crystal structures UV-Vis spectroscopy: Absorbance

Thank you for your interest Questions?

Information slides

Fabrication of perovskites: Method #1 The metal film (MI2) is first deposited on the substrate by evaporation M is a metal ion (Pb or Sn) Thickness: 200-300 nm The metal coated substrate is then dipped to a solution containing organic ammonium (as a positive ion) Causes the reaction to form perovskite thin film The composition can be varied by By changing the carbon group of organic ammonium chain Butyl, pethyl etc. By changing the metal in the ion Pb, Sn etc. Liang, K. et al. Synthesis and Characterization of Organic-Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chemistry of Materials. 1998. vol. 10. pp. 403-411.

Fabrication of perovskites: Method #2 More used nowadays The solution containg the wanted substances is mixed ( 𝐶𝐻 3 𝑁 𝐻 3 𝐼 mixed with 𝑃𝑏𝑋 2 , where X is Cl, I, Ac) Spin coating of the solution on top of the substrate Annealing to evaporate the solvent and to crystallize the perovskite thin film Zhang, W. et al. Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells. Nature communications. 2015.

More about XRD X-rays are generated by bombarding a metal target (Cu or Mo) with electrons Ionizes the electrons in inner electron shells and they form a vacancy The vacancy is filled by electron dropping down from one of the outer shells X-rays are emitted Bragg’s Law: 𝑛𝜆=2𝑑 sin 𝜃 Where n is a integer number (1,2,3…), 𝜆 is the wavelength of the x-ray, d is the separation of lattice points and 𝜃 is the angle of the incident x-ray Indexing of the intensity peaks is done by pattern matching program Time consumption of XRD measurement depends on The range of angle (2θ) (for example 10 – 70˚) The step size Time used per step Bragg diffraction University of South Carolina: http://www.chem.sc.edu/faculty/zurloye/xrdtutorial_2013.pdf

More about UV-Vis spectroscopy Absorbed photon excites the molecule from its ground state From highest-energy occupied molecular orbital (HOMO) to lowest-energy unoccupied molecular orbital (LUMO) The excitation (absorption) of the photon depends on the HOMO-LUMO gap Smaller the gap is, smaller the energy that can excite the molecule  wider wavelength spectrum can be absorbed Energy (E) of the photon is dependent on the wavelength: 𝐸=ℎ𝑓=ℎ c 𝜆 where h is the Planck constant, f is the frequency, c is the speed of light and 𝜆 is the wavelength The absorbance A of material: 𝐴= log 𝐼 0 𝐼 where I0 is the intensity of the incident light and I is the intensity of the light transmitting the sample University of Colorado: http://orgchem.colorado.edu/Labs/Handbook/UV-Vis.pdf