TY - JOUR
T1 - Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
AU - Beecher, Alexander N.
AU - Semonin, Octavi E.
AU - Skelton, Jonathan M.
AU - Frost, Jarvist M.
AU - Terban, Maxwell W.
AU - Zhai, Haowei
AU - Alatas, Ahmet
AU - Owen, Jonathan S.
AU - Walsh, Aron
AU - Billinge, Simon J.L.
N1 - Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/10/14
Y1 - 2016/10/14
N2 - Lead halide perovskites such as methylammonium lead triiodide (CH3NH3PbI3) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution-processable material works so well is currently missing. Previous research has revealed that CH3NH3PbI3 possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in CH3NH3PbI3 and find direct evidence for another form of disorder in single crystals: large-amplitude anharmonic zone edge rotational instabilities of the PbI6 octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium (CH3NH3 +) couple strongly and cooperatively to these modes. The result is a noncentrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction but can explain key material properties such as the structural phase sequence, ultralow thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility. From the PDF we estimate the size of the fluctuating symmetry broken domains to be between 1 and 3 nm in diameter.
AB - Lead halide perovskites such as methylammonium lead triiodide (CH3NH3PbI3) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution-processable material works so well is currently missing. Previous research has revealed that CH3NH3PbI3 possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in CH3NH3PbI3 and find direct evidence for another form of disorder in single crystals: large-amplitude anharmonic zone edge rotational instabilities of the PbI6 octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium (CH3NH3 +) couple strongly and cooperatively to these modes. The result is a noncentrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction but can explain key material properties such as the structural phase sequence, ultralow thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility. From the PDF we estimate the size of the fluctuating symmetry broken domains to be between 1 and 3 nm in diameter.
UR - http://www.scopus.com/inward/record.url?scp=85016010571&partnerID=8YFLogxK
U2 - 10.1021/acsenergylett.6b00381
DO - 10.1021/acsenergylett.6b00381
M3 - Article
AN - SCOPUS:85016010571
SN - 2380-8195
VL - 1
SP - 880
EP - 887
JO - ACS Energy Letters
JF - ACS Energy Letters
IS - 4
ER -