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Replacing lead in hybrid perovskite for more sustainable optoelectronic devices Kyle Miller , Catherine Clark, and Russell Holmes Home Institution: University of Puget Sound Summer Sponsor: MRSEC REU This work was supported partially by the Research Experiences for Undergraduates (REU) Program of the National Science Foundation under Award Number DMR-1559833 and DMR-1420013 Introduction What is perovskite? Perovskites are crystals of the formula ABX 3 where A and B are large and small cations, respectively, and X is an anion bound to both cations. Hybrid perovskites have the usual inorganic B and X ions but have an organic A cation, typically methylammonium (CH 3 NH 3 ). Why is perovskite interesting? 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 CIGS (cadmium indium gallium (di)selenide) and CdTe 1,2 . Despite their excellent optical absorption and low rates of non-radiative recombination, perovskites have yet to see widespread commercial adoption in photovoltaic devices. The presence of lead in the most efficient and well-studied hybrid perovskite, methylammonium lead triiodide (CH 3 NH 3 PbI 3 ) is a major inhibitor of commercial feasibility. What is the purpose of this study? To address health and environmental safety concerns surrounding the manufacture and disposal of devices containing lead, this investigation attempts to replace lead with a safer alternative. Barium was selected as the lead replacement candidate after finding a manuscript that claims to have synthesized a stable perovskite with a 3.87 eV direct band gap (CH 3 NH 3 BaI 3 ), which could serve as an excellent transparent conducting layer in solar cells as well as things like touchscreens and LEDs 3 . References 1 Green, Martin A, et al. “The Emergence of Perovskite Solar Cells.” Nat Photon 8.7 (2014): 506–514. Web. 2 National Center for Photovoltaics. “Best Research-Cell Efficiencies.” National Renewable Energy Lab (2016). Web. 3 Kumar, Akash et al. “Crystal Structure, Stability and Optoelectronic Properties...” Manuscript. (2016). Web. 4 Harvey, David. “10.6: Photoluminescence Spectroscopy”. LibreTexts (2016). Web. 5 Clark, Catherine. “14 day spin coated BaI 2 and MAI film.” UMN CharFac* (2016). Unpublished data. The cubic crystal phase of perovskite is useful for visual clarity but CH 3 NH 3 BaI 3 at room temperature has a slightly different, tetragonal phase according to Density Functional Theory calculations. 3 (Graphic by Green et al. 1 ) Many perovskites have bandgaps suitable for an absorbing layer as in this diagram but the CH 3 NH 3 BaI 3 described by Kumar et al. would be better-suited as a transparent conductor, taking the place of FTO. (Graphic by Green et al. 1 ) Synthesis BaI 2 powder 12 hours - stir at 1000 rpm - heat to 80ºC 1 week - Let sit in N 2 at ambient temp. MAI DMF MAI = methylammonium iodide, CH 3 NH 3 + I - DMF = N,N-dimethylformamide, HCON(CH 3 ) 2 BaI 2 MAI in DMF Quartz substrate at 2000 rpm + DMF Quartz substrate at 2000 rpm CH 3 NH 3 BaI 3 ? (Perovskite?) Solution Process Characterization Spin Coating Photon absorption and emission diagram where S 0 is the valence band, S 2 is the conduction band, and vr is vibrational relaxation, a process which lowers excited electrons to the lowest-energy edge of the conduction band immediately after promotion. (Graphic by Harvey et al. 4 ) Results Continue solid-phase (crystalline films) characterization Re-attempt Kumar experiment when the paper is published with more details Explore alternatives: Barium precursors (BaI 2 Ba(OAc) 2 , Ba(NO 3 ) 2 ) Halogens (CH 3 NH 3 BaI 3 CH 3 NH 3 BaCl3, CH 3 NH 3 BaBr 3 ) Synthesis methods (solutions process vapor deposition) Solvents (DMF DMSO, GBL, DMAc) Conclusion Further Research UV-Vis absorption spectroscopy UV-Vis spectroscopy, or UV-Vis, measures photon absorption of a material across the UV and visible spectra, generating an absorption profile. Photoluminescence spectroscopy Photoluminescence, also called PL and fluorescence, is the phenomenon of photon re-emission using energy from an electron that was previously excited into the conduction band by a photon. By scanning for fluorescence across the spectrum, we obtain an emission profile that, in conjunction with the absorption profile from the UV-Vis, can serve as both a material signature and as information on the nature of the band gap, which is directly connected to a material’s ability to absorb and emit certain wavelengths of light. Band gap The band gap of a material is the energy difference between the top of the valence band and the bottom of the conduction band. Photons of energy equal to or greater than the band gap can promote an electron from the valence band to the conduction band. This photon will serve as a charge carrier for some time and then decay back to the valence band, thereby releasing a photon of energy equal to the band gap. A direct band gap material will emit light with energy about equal to the light that it absorbs. Characteristics to corroborate Kumar et al. manuscript Stable in ambient conditions Direct band gap of 3.87 eV (or fluorescence at λ ≈ 320 nm) Since it is difficult to probe some crystal characteristics in liquid phase, thin films were spin-coated onto quartz substrates. The solution was dropped onto spinning substrates and as the centrifugal effect pushes the solution off the substrate, a thin film of solute is left. This film is what would ideally be our target perovskite. Unfortunately, the UV-Vis and PL film data was not ready in time for this poster. This investigation did not the confirm findings of the Kumar et al. manuscript. Kumar et al. results 3 Our results Air-stable films Films liquefied in air MAI intercalation into BaI 2 No significant interaction between BaI 2 and MAI XRD patterns matched simulated perovskite patterns None of the predicted peaks were present 5 10 15 20 25 30 35 40 45 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 * peaks observed in Kumar et al. * * Intensity (a.u.) 2 14 day film from BaI2 + MAI solution Simulated CH3NH3BaI3 based on Kumar et al. CH3NH3I powder Orthorhombic BaI2 2D XRD 14 day spin coated BaI 2 and MAI film * 250 300 350 400 450 500 BaI 2 only 0 1000 2000 3000 4000 5000 6000 7000 8000 300 400 500 600 PL Intensity (counts) 300 400 500 600 700 0 10000 20000 30000 40000 50000 300 325 350 375 400 425 450 PL Intensity (counts) Wavelength (nm) 300 325 350 375 400 425 450 475 Wavelength (nm) Air-sensitivity of solutions After it was discovered that the solutions gradually turned yellow when exposed to air, we decided to gather data at various intervals over the course of the transformation since the Kumar manuscript seemed to suggest that there could be perovskite formation in solution. *Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. 0 0.5 1 1.5 2 2.5 3 3.5 4 250 300 350 400 450 Absorbance (a.u.) BaI 2 + MAI 0 hrs 2 hrs 4 hrs 6 hrs 21 hrs 28 hrs X-Ray Diffractometry 5 To the right, the x-ray diffraction pattern of a film spin-coated from two week-old solution displays none of the peaks reported by the Kumar et al. manuscript. Instead, the pattern displays two peaks that look like they come from the BaI 2 precursor and one peak that doesn’t match any of the predicted peak locations. UV-Vis absorption scan It is clear that over time, the overall absorbance of the material is increasing, with a peak rising at 370 nm. It is important to note that the peak turnover at 300 nm is due to detector saturation. It is also clear that the transformation occurring is mostly independent of the MAI in the solution. PL emission scan (pump λ = 275 nm) The material’s emission displays the opposite trend as its absorbance, decreasing over time. The peak indicates that it emits most efficiently at 480 nm. We also see once again that the MAI has little effect on the solution’s photonic profile. PL excitation scan (read λ = 500 nm) This peak at 420 nm, meaning that our material emits most effectively when it absorbs 420 nm light, is inconsistent with the absorption peak at 370 nm and has yet to be explained by our research. However, the consistent pattern of similarity between the solutions with and without MAI indicate that there is likely no reaction occurring between the two precursors of our target perovskite. background subtracted