1. Characterization of mixed films
Marko vaelma

2. 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 vol. 10. pp
Zhang, W. et al. Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells. Nature communications
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3. 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

4. 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
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5. 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:
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6. 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 = 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

7. 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 vol pp

8. 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
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9. 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

10. 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

11. Thank you for your interest
Questions?

12. Information slides

13. 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: 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 vol. 10. pp

14. 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

15. 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:

16. 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: