Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials

Abstract

Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. Despite the remarkable progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. High on the list of unexplained phenomena are very modest mobilities despite low charge carrier effective masses. Moreover, experiments elucidate unique degradation of photocurrent affecting stable operation of perovskite solar cells. These puzzles suggest that, while ionic hybrid perovskite devices may have efficiencies on par with conventional Si and GaAs devices, they exhibit more complicated charge transport phenomena. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using both periodic boundary conditions and isolated structures. Using the hybrid density functional theory, we found that volumetric strain in a CsPbI3 cluster creates a polaron with binding energy of around 300 and 900 meV for holes and electrons, respectively. In the MAPbI3 (MA = CH3NH3) cluster, both volumetric strain and MA reorientation effects lead to larger binding energies at around 600 and 1300 meV for holes and electrons, respectively. Such large reorganization energies suggest appearance of small polarons in organometallic perovskite materials. The fact that both volumetric lattice strain and MA molecular rotational degrees of freedom can cooperate to create and stabilize polarons indicates that in order to mitigate this problem, formamidinium (FA = HC(NH2)2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications.

UV Degradation and Recovery of Perovskite Solar Cells

“…According to the literature, continuous 1-sun illumination will generate a quasi-static charge state by lattice distortion and phase separation. These states will be accumulated and coexist with photo-generated carriers, leading to current degradation during further 1-sun light soaking22626364.…”

“…This suggest longer UV exposure can change UV degradation/recovery mechanism. Light-induced meta-stable trap states226263 and holes accumulation65 can have rules in UV degradation/recovery.…”

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

“…There is an on-going debate about electron and hole mobilities in organic-inorganic perovskites, 15,16,18,19 while only one theoretical study exists for inorganic ones 79 . As far as the organic-inorganic ones are concerned, Zhu and Podzorov 18 have suggested that charge carriers may be protected by the formation of a large (heavy) polaron, which could also explain the modest charge carrier mobilities.…”

“…These considerations concern the organic-inorganic perovskites but carrier mobilities appear to be significantly higher in inorganic ones, which hints to different charge transport properties 7,80,81 . Using the hybrid density functional theory, Neukirch et al 79 found that in a CsPbI 3 cluster electron and hole polarons are formed, with a larger BE for the electrons. This seems to fit with the conclusion of a weakly localized exciton in organic-inorganic perovskites proposed by He et al ., 30 which they attributed to band tail states, mainly due to electrons at the bottom of the CB.…”

A New Perspective on the Role of A‐Site Cations in Perovskite Solar Cells

“…We used norm-conserving PBE pseudopotential including d core states of Pb and sampled an unshifted (8 × 8 × 8) grid in BZ for geometry optimization and for electronic calculation at energy cutoff of 80 Ry for PSCs containing organic cations and 100 Ry for CsPbI 3 . Because lattice distortion is not significantly affected by types of functional and spin-orbit coupling, [42] we used PBE exchange functional. We used DFPT for phonon dispersions and considered the non-analytic term of dynamical matrix at Γ of BZ to account for LO-TO splitting.…”