Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide

Abstract

Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light emission, and detection applications. A hot-phonon bottleneck effect significantly extends the cooling time of hot charge carriers, which thermalize through carrier–optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. To understand these processes, we adjust the lattice dynamics independently of electronics by changing isotopes. We show that doubling the mass of hydrogen in methylammonium lead iodide by replacing protons with deuterons causes a large 20 to 50% softening of the longitudinal acoustic phonons near zone boundaries, reduces thermal conductivity by ~50%, and slows carrier relaxation kinetics. Phonon softening is attributed to anticrossing with the slowed libration modes of the deuterated molecules and the reduced thermal conductivity to lowered phonon velocities. Our results reveal how tuning the organic molecule dynamics enables control of phonons important to thermal conductivity and the hot-phonon bottleneck.

Dynamics & Spectroscopy with Neutrons—Recent Developments & Emerging Opportunities

“…The most-recent research on MAPbI 3 further addresses the isotope effect revealed earlier by Kong et al [ 77 ]. Manley et al [ 109 ] have extended their previous INS-TAS experiments on a per-deuterated MAPbI 3 [ 110 , 113 ] (see Figure 4 a), revealing that isotope substitution in MA + causes a large (20–50%) softening of the longitudinal acoustic (LA) phonons near zone boundaries, reduces thermal conductivity by nearly 50%, and slows carrier relaxation kinetics. These significant effects serve to emphasize once more out the importance of a coupling of LA phonons to the librational modes of the methylammonium molecule, which are strongly mass-dependent [ 109 ].…”

“…A number of neutron scattering instruments have been used so far for the study of OHPs. These instruments can be roughly classified as triple-axis spectrometers (TAS), operating at reactor sources (e.g., HB3, HFIR, ORNL, US [ 109 ]; BT4 and SPINS at NIST [ 110 , 111 , 112 ], US; IN12 and IN22 at ILL, FR. [ 112 , 113 ]; 4F1, 4F2 and 1T at LLB, FR [ 113 , 114 , 115 , 116 ]); and the time-of-flight (TOF) instruments available at spallation (e.g., MAPS and MARI at ISIS, UK [ 117 , 118 , 119 ]; 4SEASONS and AMATERAS at J-PARC, JP [ 120 , 121 ]; or ARCS at SNS, ORNL, US [ 120 ]) and reactor sources (IN4 and IN5 at ILL, FR [ 116 , 117 , 122 ]).…”

“…INS-TAS in combination with INS-TOF experiments also bring to the fore the role of optical phonons in the photophysics of these materials, as discussed in Refs. [ 109 , 127 , 128 , 129 , 130 ]. As noted by Mozur et al, optical phonons are responsible for over half of the polarizability in the dielectric response of MAPbI 3 [ 108 ].…”

“…The first high-resolution INS studies of MAPbI over a wide spectral range were reported by Drużbicki et al [ 144 ] using the previous incarnation of TOSCA (see the black curve in Figure 5 ). More recently, Manley et al have reported the INS spectrum recorded on VISION [ 109 ] (blue curve in the figure), with a considerably better signal-to-noise ratio. These latter data are of comparable quality to those of a more recent study by Drużbicki et al [ 75 ], obtained on an upgraded TOSCA, hereafter TOSCA+ [ 138 , 139 , 140 ] (red curve).…”

“… Phonon dispersion relations in the cubic phase of MAPbI 3 , measured on both perdeuterated and hydrogenous single-crystals using IXS (cyan circles [ 123 ]) and INS-TAS (orange squares [ 109 ] and green circles [ 113 ], respectively). Wave vectors on the abscissa are given in reduced lattice units.…”