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KDMiller_presentationFinal

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  1. 1. Replacing lead in hybrid perovskite for more sustainable optoelectronics Kyle Miller Home Institution: University of Puget Sound MRSEC Faculty Advisor: Prof. Russell Holmes Graduate Student Advisor: Catherine Clark
  2. 2. What is perovskite? Green, Martin A, et al. “The Emergence of Perovskite Solar Cells.” Nat Photon 8.7 (2014): 506–514. Web.
  3. 3. What is hybrid perovskite? Organic Inorganic Green, Martin A, et al. “The Emergence of Perovskite Solar Cells.” Nat Photon 8.7 (2014): 506–514. Web.
  4. 4. Why perovskite? National Center for Photovoltaics. "Best Research-Cell Efficiencies." National Renewable Energy Laboratory (2016)
  5. 5. Issues with Perovskite Dominant Device Architecture: Methylammonium lead triiodide (CH3NH3PbI3) Environmental Health and Safety Hazards of Lead Lead Mining and Processing Chemical Instability Commercialization Inhibitors Negative PR for lead content
  6. 6. Potential Solution Pb CH3NH3 I Green, Martin A, et al. “The Emergence of Perovskite Solar Cells.” Nat Photon 8.7 (2014): 506–514. Web.
  7. 7. Experimental Foundation CH3NH3BaI3 characteristics - Direct bandgap - 4.0 eV (simulated) - 3.87 eV (measured) - λ ≈ 320 nm - Chemical stability (measured) Kumar, Akash et al. “Crystal Structure, Stability and Optoelectronic Properties...” Manuscript. (2016). Web.
  8. 8. Pb  Ba CH3NH3 I Project Goal CH3NH3BaI3 Green, Martin A, et al. “The Emergence of Perovskite Solar Cells.” Nat Photon 8.7 (2014): 506–514. Web.
  9. 9. Synthesis: Solution Process BaI2 powder MAI DMF 12 hours - stir at 1000 rpm - heat to 80ºC 1 week - Let sit MAI = methylammonium iodide, CH3NH3 +I- DMF = N,N-dimethylformamide, HCON(CH3)2 Kumar, Akash et al. “Crystal Structure, Stability and Optoelectronic Properties...” Manuscript. (2016). Web.
  10. 10. Spin Coating BaI2 MAI in DMF Quartz substrate at 2000 rpm DMF Quartz substrate at 2000 rpm CH3NH3BaI3 (Perovskite) +
  11. 11. Unstable Films
  12. 12. Sealing Films between Substrates Spin-coated film sealed between two quartz substrates with epoxy Cross section of side view UV-cured Epoxy Spin-coated film Quartz substrate
  13. 13. Ultraviolet-Visible Absorption Spectroscopy (UV-Vis) Marcq, Emmanuel and Loic Rossi. “Radiative Processes.” SESP (2016). Web.
  14. 14. Photoluminescence (PL) Spectroscopy “Fluorescence, Phosphorescence, and Photoluminescence Spectroscopy.” Princeton Instruments. Web.
  15. 15. UV-Vis: Atmosphere-controlled 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 200 250 300 350 400 450 500 550 600 650 700 Absorbance(a.u.) Wavelength (nm) 320 nm
  16. 16. UV-Vis: Atmosphere-controlled 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 200 250 300 350 400 450 500 550 600 650 700 Absorbance(a.u.) Wavelength (nm) Strong DMF absorption 320 nm
  17. 17. 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 200 250 300 350 400 450 500 550 600 650 700 Absorbance(a.u.) Wavelength (nm) UV-Vis: Atmosphere-controlled (DMF background subtracted) 320 nm
  18. 18. Air-exposed Solution Air exposure time
  19. 19. UV-Vis: Time-resolved air exposure 0 0.5 1 1.5 2 2.5 3 3.5 4 200 250 300 350 400 450 500 550 600 650 700 Absorbance(a.u.) Wavelength (nm) 1 hr 3 hrs 6 hrs 7 hrs 11 hrs 28 hrs 320 nm
  20. 20. UV-Vis: Time-resolved air exposure 0 0.5 1 1.5 2 2.5 3 3.5 4 200 250 300 350 400 450 500 550 600 650 700 Absorbance(a.u.) Wavelength (nm) 1 hr 3 hrs 6 hrs 7 hrs 11 hrs 28 hrs (DMF background subtracted) 320 nm
  21. 21. UV-Vis: Time-resolved air exposure 0 0.5 1 1.5 2 2.5 3 3.5 4 250 275 300 325 350 375 400 425 450 Absorbance(a.u.) Wavelength (nm) 1 hr 3 hrs 6 hrs 7 hrs 11 hrs 28 hrs (DMF background subtracted) 320 nm
  22. 22. 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 300 350 400 450 500 550 600 650 700 Intensity(counts) Wavelength (nm) 0 hrs 2 hrs 4 hrs 6 hrs 21 hrs 28 hrs PL emission scan: Time-resolved air exposure Excitation λ = 275 nm 320 nm
  23. 23. 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 300 350 400 450 500 550 600 650 700 Intensity(counts) Wavelength (nm) 0 hrs 2 hrs 4 hrs 6 hrs 21 hrs 28 hrs PL emission scan: Time-resolved air exposure Excitation λ = 275 nm 320 nm
  24. 24. 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 300 325 350 375 400 425 450 475 Intensity(counts) Wavelength (nm) 0 hrs 2 hrs 4 hrs 6 hrs 21 hrs 28 hrs Scanned λ = 500 nm PL excitation scan: Time-resolved air exposure
  25. 25. 0 1000 2000 3000 4000 5000 6000 7000 8000 300 350 400 450 500 550 600 650 700 Intensity(counts) Wavelength (nm) Excitation λ = 275 nm PL emission scans BaI2 + MAI BaI2 only 300 350 400 450 500 550 600 650 700 Wavelength (nm)
  26. 26. 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 300 325 350 375 400 425 450 475 Intensity(counts) Wavelength (nm) PL excitation scans 300 325 350 375 400 425 450 475 Wavelength (nm) BaI2 + MAI BaI2 only Scanned λ = 500 nm
  27. 27. UV-Vis: Raw data BaI2 + MAI BaI2 only 0 0.5 1 1.5 2 2.5 3 3.5 4 250 300 350 400 450 Absorbance(a.u.) Wavelength (nm) 250 300 350 400 450 Wavelength (nm)
  28. 28. 0 0.5 1 1.5 2 2.5 3 3.5 4 250 300 350 400 450 Absorbance(a.u.) Wavelength (nm) 250 300 350 400 450 Wavelength (nm) UV-Vis: DMF background subtracted BaI2 + MAI BaI2 only
  29. 29. Conclusion • Did not confirm Kumar findings • Unstable films • PL and UV-Vis data showed no significant difference between the BaI2 + MAI and BaI2 only solutions • XRD patterns* did not show perovskite peaks *XRD was performed by Catherine Clark.
  30. 30. Further Research • Continue solid-phase (crystalline films) characterization • Re-attempt Kumar experiment with published paper • Explore alternatives: • Barium precursors (BaI2 Ba(OAc)2, Ba(NO3)2) • Halogens (CH3NH3BaI3  CH3NH3BaCl3, CH3NH3BaBr3) • Synthesis methods (solutions process  vapor deposition) • Solvents (DMF  DMSO, GBL, DMAc)
  31. 31. Acknowledgements • Catherine Clark • Professor Russell Holmes • Holmes Group • This work was supported primarily by the National Science Foundation MRSEC and REU programs under Award Numbers DMR-1263062 & DMR-1420013

Notas do Editor

  • Introduce self
    I had the pleasure this summer of working with Catherine Clark and Professor Russ Holmes
    Today I’ll be talking to you
  • A perovskite is a crystal with the formula ABX3 where A and B are large…
  • In this study we look specifically at a hybrid perovskite which has an organic A cation and inorganic B and X ions.
  • In terms of power conversion efficiency, perovskite solar cells are currently the fastest-improving architecture in modern photovoltaics. Since 2006, perovskite photovoltaic cells have experienced an unprecedented increase from 2.2% to 22.1% power conversion efficiency, now rivaling more established architectures such as CdTe (NREL 2016)
  • However, there are some serious issues

    Our society is phasing out lead due to its environmental health and safety concerns, so it is a serious inhibitor to commercialization.

    - Moreover these films are moisture-sensitive and fall apart in humid air, making it more difficult to construct robust devices fit for consumer use.
  • Considering these difficulties, it would be very useful to find a replacement for lead that would prove less problematic for large-scale, consumer-oriented devices.
  • There has been a lot of talk in literature about the potential of barium

    Furthermore the manuscript claims that the perovskite synthesized has a wide bandgap (which makes it transparent to visible light) and is stable in ambient conditions.

    Ultimately, the manuscript describes a material that would serve as an excellent transparent conducting layer in not just solar cells, but things like touchscreens and LEDs as well.
  • Our project seeks to replicate the Kumar manuscript and replace lead with barium, to obtain what could be a more stable, environmentally friendly perovskite.
  • Our main synthesis method is the solution process. Here we mix iodide salts of the B cation and methylammonium and then dissolve in a common organic solvent, dimethylformamide, or DMF. This solution is stirred and heated for 12 hours for complete dissolution. Interestingly, the Kumar paper also noted that the solution had to sit in a vial for a full week before any of the perovskite features showed up in their characterization efforts.
  • Since some characteristics, namely crystal structure, are most easily probed when a material is in a solid phase, we prepare thin films of our material by spin coating on quartz substrates. Our spin coater spins the substrate at a few thousand rpms and then I drop the solution onto the substrate as it spins. As the centrifugal effect pushes the solution toward the outside and off the substrate, a layer of solute is left behind. That layer is what we would hope to be a film of perovskite.
  • However, issues quickly surfaced after we started spin

    Now this wouldn’t be a problem if we were synthesizing the perovskite described in the Kumar manuscript. But after our first few films ended up looking like this one which turned to liquid despite our sealing efforts, it became quite obvious that we had a different material on our hands.
  • We tried several methods of preventing air-exposure and this is one method that was sometimes effective but inconsistent. By sandwiching the film between two quartz substrates and sealing the edges with epoxy, we can place the films in the characterization machines usually without exposing the film to air and since quartz absorbs almost no light in the visible spectrum, our photons of interest are free of attenuation. However, our data on the films was not ready in time for this presentation. Thus, all of the following data is from solutions.
    MAKE NOTE OF THE POSSIBLITY OF IN-SOLUTION TRANSFORMATIONS – mention the time required in solution for perovskite to form
  • In this project, I performed 2 types of characterization. The first, ultraviolet-visible absorption spectroscopy, tells us the range of photon energies that the material most efficiently absorbs.
  • Photoluminescence spectroscopy tells us what the material does with the photons after they are absorbed. Both of these spectroscopies allow us to interrogate the bandgap of the material, which is the energy difference between the top of the valence band and the bottom of the conduction band *point out the band gap*. Photons with energy equal to or greater than the band gap can be absorbed by an electron in the valence band. This electron is then promoted to the conduction band, allowing for more efficient charge transfer. After some time, this electron will fall back down to the valence band and emit a photon with an energy about equal to the band gap. Knowing about a material’s band gap is useful on its own because it can help us determine potential applications of a material, but it will also help us determine if our data corroborates the findings of the Kumar paper, which found a direct bandgap of 3.87 eV.
  • Here is the UV-Vis spectrum for the MAI and barium iodide solution, to corroborate the Kumar manuscript, we would need to see an absorption peak at 320 nm. Instead, the spectrum is relatively featureless aside from the peak at low wavelengths.
  • This peak, however is simply the absorption of our solvent, DMF.
  • After subtracting the DMF background, we see that there is little to no absorption by our solution. So far it looks like we do not have any material that could be photonically interesting.
  • However, after inadvertently leaving some of the solution in a vial with a bad seal, we discovered that the solution turned yellow. While this was not described at all in the Kumar manuscript, we decided to further characterize the solutions transformation.
    - Don’t know if the paper’s solution had air-exposure and there was never a mention of solution color
  • This is a set of absorption spectra taken at intervals over the time it took for the solution to complete the majority of its color change. We can immediately see a gradual transformation occurring in solution, with our peak increasing over time, eventually maxing out the detector and merging with the DMF absorbance peak.
  • Here is the same set of spectra after the DMF background has once again been subtracted. Now we notice that there is what looks like another peak around 300nm. It is important to note that this peak is right on the edge of the DMF absorption peak so there is a significant chance that it might be the product of the saturation that occurs due to the DMF absorption. This makes it impossible to get an accurate sense of
  • Here is the same plot again with the x-axis zoomed in on the peaks. To make the graphical interpretation easier, I have highlighted the peaks that we see on this graph on all of the following graphs.
  • Taking a look at the emission spectrum, we immediately notice that we have a very broad, messy peak which over time, diminishes and thins on the left side to leave a peak at around 475nm. This emission decrease, in conjunction with the increase in absorbance we saw earlier indicate that the compound being formed in the transformation is an ineffective emitter, undergoing more non-radiative transitions to compensate for the decrease in emission.
  • Overlaying the absorption peak, we can see that the orange area does not overlap with the main peak at 475nm, indicating that our material is not emitting the same light that it had absorbed. This directly contradicts the claims of the Kumar et al. paper which reported a direct bandgap, which would require a material to emit light that was much closer to its absorption peak.
  • That possibility is on shaky ground with this photoluminescence excitation scan. With this type of scan, we hope to see our absorption peak line up with the peak on this graph, indicating that the photons being most effectively absorbed are also being most effectively emitted. Instead, this graph shows that only the very red edge of the absorption peak is being efficiently reemitted. This kind of deviation is quite abnormal and further investigation would be needed to find out why this occurs. Now at this point, we are quite eager to know what part of our solution is causing this transformation.
  • Well after we took enough data with the PL machine to be moderately confident that this shape is not an artifact of the machine itself, we can say that the removal of MAI does not appear to adversely affect the transformation we observed in the solution. The slightly inconsistent amplitudes can be attributed to small differences in concentrations of the solution.
  • Here with more comparison, this time with excitation scans, you’ll notice that there is a small difference at 340 nm, but the vast majority of the change seems to be independent of the MAI.
  • Our absorption data also confirmed this explanation.
  • As you can see, the solution made without MAI undergoes a very similar transformation. This means that the transformation observed in the air-exposed solution is likely just an interaction with an air-borne compound and the highly reactive barium iodide precursor, it could also be a reaction with the solvent or even a solvent complexation facilitated by something in the air but those are a bit far-fetched. Ultimately, since methylammonium is required for the formation of the target perovskite, we can be fairly certain that this is not that perovskite.
  • Ultimately, we did not confirm the findings of the Kumar et al. manuscript:
    Their films were air-stable and our films quickly liquefied in ambient conditions
    Our photoluminescence and uv-vis data showed only very small differences between the barium iodide solution with methylammonium iodide and the one without, indicating that there was likely no reaction occurring between the two and thus no perovskite formation
    Furthermore, our XRD data, which was not shown in this presentation and was taken by my mentor Catherine showed none of the peaks reported by the Kumar et al manuscript
  • Looking forward,
    Since we have not comprehensively characterized films of this solution, it would be valuable to continue pursuing that part of the project since some perovskites indeed do not form until they are brought into solid phase.
    Since the Kumar et al. work was still a manuscript and lacked some of the useful information describing their methods, it would make sense to reattempt to replicate their experiment once the full paper is published
    In the mean time, there are plenty of variables to explore which are well-documented for lead and tin perovskites such as our precursor and the halogen used, both of which were iodine in our experiment, our synthesis method, and our solvent.
  • I’d like to thank the Holmes research group and Catherine in particular for being a bottomless source of advice, friendliness, and patience.
    I’d like to thank Professor Holmes for always being ready to offer his expertise and guidance to not only this project but my life aspirations as well, and the NSF and MRSEC programs for providing me with this opportunity.

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