Synthesis and characterization of CH3NH3Pb(I1‑xBrx)3 Solid Solution Perovskite Semiconductors

Procedure modified by Megan Young and George Lisensky, Beloit College, from "Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells", J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, and S. I. Seok, Nano Lett. 2013, 13, 1764-1769.

For solid solutions with the same arrangement of atoms, the size of a unit cell depends on the size of the atoms in the unit cell. Consider samples of the CH3NH3Pb(I1‑xBrx)3 semiconductor. Since I- and Br- are different sizes, changing the value of x between 0 and 1 should change the unit cell size. Since the distance between nuclei helps determine how strongly electrons are held, changing the value of x should also change the band gap energy. In this experiment you will study CH3NH3Pb(I1‑xBrx)3 for an assigned value of x, measure the unit cell size by powder x-ray diffraction, and estimate the band gap energy by visible absorption spectroscopy. Combining class values will let us see the overall relationship between composition, atomic size, and electron energy levels.

Caution: these samples contain lead. Care should be taken not to eat them. Clean up all spills. Dispose of the material in an appropriate waste container.

Procedure

Wear eye protection

Chemical gloves
required
Fumehood required

Synthesis

Align the end of an aluminum XRD holder with the end of a glass microscope slide. Use a marker to draw the XRD sample position on the microscope slide. Write the value of x in the CH3NH3Pb(I1‑xBrx)3 formula on the slide.

Heat a hotplate to 120°C in a hood. Place the microscope slide with the drawn side down onto the hotplate. Repeatedly dip a pasteur pipet (no bulb) into your assigned solution and tap the pipet onto the hot microscope slide to transfer solution and cover the sample area.

Heat 5-10 minutes. If a sample phase-separates (a color change occurs on cooling) reheat the sample.

Use a straightedge and a diamond pencil to score the center of a 22x40mm cover slip. Draw the line only once. Use about the same amount of pressure that you would use to write on paper with a regular pencil. Fold away along the line to produce two pieces of glass that will fit in the visible spectrometer.

Prepare a sample for visible spectroscopy measurements by using a micropipet to add 10µL of mixture to half a cover slip on a 120° hotplate. Heat for 5-10 minutes.

Cleanup

Do not rinse with water or you will get an insoluble precipitate. Use DMF to rinse the spot plate into the lead waste container. When finished with the spectroscopy below, return the labeled glass pieces.

Characterization

Record the visible absorption spectrum of the sample on the cover slip.

Visible spectrum extrapolation
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Extrapolate the two linear portions of the visible spectra to find their intersection. This wavelength corresponds to the minimum energy needed to excite an electron from the conductance band to the valence band which is called the band gap energy, E=hc/λ. If two steps are observed, reheat the sample.

Record the XRD spectrum of the sample on the glass slide. Use a second blank slide to give a thicker piece of glass in the instrument holder. Scan from 27.5° to 31° (step width 0.02° and a 1 sec count gives a 3 minute scan).

XRD patterns for 3 samples
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Measure the peak position, 2θ, and use Bragg's Law, λ = 2d sinθ, to determine the spacing between planes, d. (To do the calculation in Jade, click on .) This is the (200) peak so double the d-spacing to get the unit cell size.

(The crystal structure of CH3NH3PbI3 has lower symmetry than the crystal structure of CH3NH3PbBr3 but can be analyzed as a pseudocubic unit cell. Ignore the shoulder for low values of x. See original reference.)

Change in XRD with heating
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If a shoulder is apparent in the XRD of the solid solutions, reheat the sample and obtain the XRD pattern again. The figure shows the XRD change upon additional heating (thick line) for a phase-separated sample. (An option is to heat the sample while waiting for the XRD instrument.)

Conclusions

  1. What observations did you record for any step that changes color?
  2. Plot unit cell size as a function of x. What happens to the unit cell size when x gets bigger? Why? Explain using periodic properties.
  3. Plot band gap as a function of x. What happens to the band gap when x gets bigger? Why? Explain using periodic properties and your knowledge of band gap energy.

Materials

Stock Solutions

Preparation of solids used to make the stock solutions


This page created by George Lisensky, Beloit College.  Last modified April 13, 2017 .