Shallow trapping vs. deep polarons in a hybrid lead halide perovskite, CH₃NH₃PbI₃

Abstract

There has been considerable speculation over the nature of charge carriers in organic-inorganic hybrid perovskites, i.e., whether they are free and band-like, or they are prone to self-trapping via short range deformation potentials. Unusually long minority-carrier diffusion lengths and moderate-to-low mobilities, together with relatively few deep defects add to their intrigue. Here we implement density functional methods to investigate the room-temperature, tetragonal phase of CHNHPbI. We compare charge localization behavior at shallow levels and associated lattice relaxation versus those at deep polaronic states. The shallow level originates from screened Coulomb interaction between the perturbed host and an excited electron or hole. The host lattice has a tendency towards forming these shallow traps where the electron or hole is localized not too far from the band edge. In contrast, there is a considerable potential barrier that must be overcome in order to initiate polaronic hole trapping. The formation of a hole polaron (I center) involves strong lattice relaxation, including large off-center displacement of the organic cation, CHNH. This type of deep polaron is energetically unfavorable, and active shallow traps are expected to shape the carrier dynamics in this material.

Perovskite-quantum dots interface: Deciphering its ultrafast charge carrier dynamics

“…Such modifications were observed after passivation by the QDs due to the lattice mismatch between the two materials [18,21,44,45]. The interpretation of the blue shift also agrees with reported band gap expansion of MAPbI3 related to structural deformation [46].…”

“…Their presence in the perovskite domain might by due to their low mobility (e.g. trapped in shallow states of perovskite band gap) or lack of interaction with QDs [34,46,59]. However, the comparable features of TA and THz signals suggest that these charges keep their mobility during the gating time (up to 1.5 ns).…”

Static and Dynamic Disorder in Triple-Cation Hybrid Perovskites

“…Moreover, in contrast to III-V semiconductors, [72][73][74] the Stokes shift of the main PL peak begins to increase again at higher temperatures. The absolute value of the Stokes shift increases by a factor of two from 15 meV at 50 K to 30 meV at 150 K. This non-vanishing and increasing Stokes shift at high temperatures may be the hallmark of the domination of dynamic disorder and the formation of large polarons: 25,27,30,77 in case of fully static disorder, we would expect a progressive decrease of the Stokes shift. The transmission measurement provides information about the energy of the nascent, bare exciton transition, while the emission originates from the electron and holes dressed by the polarization of organic cations or inorganic lattice deformation.…”

“…16,23,24 Moreover, a strong coupling between the lattice and free carriers/excitons can lead to local screening of carriers and the formation of polarons. 13,22,[25][26][27][28][29][30] In the high temperature phase (T > 160 K for MA and T > 120 K for FA), the organic cations are free to rotate, and the induced disorder has a dynamic nature. 17,19,20,24,31,32 At lower temperatures, the motion of organic cations becomes progressively more restricted.…”

Excited-state vibrational dynamics toward the polaron in methylammonium lead iodide perovskite

“…This inhomogeneity is surprising because most other inorganic solar cell materials require pure crystalline composition for high PCEs 21 , 22 . Altogether these observations suggest that the inhomogeneity in the polycrystalline HOIP does not greatly influence the performance by protecting free charge carriers from ultrafast thermal relaxation, although these defects do accumulate in humidity-degraded materials 23 – 25 . Associated with the high PCE, polycrystalline HOIP exhibits a long lifetime ( τ ~1−3 μs) and a long diffusion length ( L D ~ 1−3 μm), as well as modest mobility ( μ ~30−100 cm 2 V −1 S −1 ) for the free charge-carriers 26 – 28 .…”