TY - JOUR
T1 - Tetragonal CH3NH3Pbi3 is ferroelectric
AU - Rakita, Yevgeny
AU - Bar-Elli, Omri
AU - Meirzadeh, Elena
AU - Kaslasi, Hadar
AU - Peleg, Yagel
AU - Hodes, Gary
AU - Lubomirsky, Igor
AU - Oron, Dan
AU - Ehre, David
AU - Cahen, David
N1 - Y.R. thanks Dr. Isai Feldman for guidance with X-ray diffraction measurements, and Dr. Omer Yaffe, Igal Levin, and Arava Zohar (all from the Weizmann Institute) for fruitful discussions. We thank Prof. V. M. Fridkin (Shubnikov Institute of Crystallography) for suggesting the low-temperature approach to find a ferroelectric loop. We are grateful to Dana and Yossie Hollander via the Weizmann Institute’s Alternative Sustainable Energy Research Initiative, the Israel Science Foundation (I.L.); to the Israel Ministry of Science’s Tashtiot, Israel–China, and India–Israel programs; and to the Israel National Nano-initiative, for partial support; and acknowledge the historic generosity of the Harold Perlman family. D.C. holds the Sylvia and Rowland Schaefer Chair in Energy Research. Author contributions: Y.R., O.B.E., I.L., D.O., D.E., and D.C. designed research; Y.R., O.B.E., and D.E. performed research; Y.R., O.B.E., E.M., H.K., Y.P., and D.E. contributed new reagents/analytic tools; Y.R., O.B.E., E.M., G.H., I.L., D.O., D.E., and D.C. analyzed data; and Y.R., O.B.E., H.K., G.H., D.O., D.E., and D.C. wrote the paper.
PY - 2017/7/11
Y1 - 2017/7/11
N2 - Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that thematerial's ferroelectric nature, can, but need not be important in a PV cell at room temperature.
AB - Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that thematerial's ferroelectric nature, can, but need not be important in a PV cell at room temperature.
KW - Ferroelectricity
KW - Halide perovskites
KW - Photovoltaics
KW - Pyroelectricity
KW - Semiconductors
UR - http://www.scopus.com/inward/record.url?scp=85023208043&partnerID=8YFLogxK
U2 - https://doi.org/10.1073/pnas.1702429114
DO - https://doi.org/10.1073/pnas.1702429114
M3 - Article
C2 - 28588141
SN - 0027-8424
VL - 114
SP - E5504-E5512
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 28
ER -