Solar Cells: Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers (Adv. Energy Mater. 19/2017)

Abstract

Charge carriers in methylammonium lead perovskite solar cell absorbers assume a combined (hyper‐) polaronic state combining a classical Fröhlich polaron with a micelle‐like arrangement of electric dipoles formed by the methylammonium ions. This “micellion” provides an extra contribution to the dielectric constant which screens defects but maintains mobility of the polaron. This is reported by Doru C. Lupascu and co‐workers in article number https://doi.org/10.1002/aenm.201700600. In the image, the structure of the hyperpolaron is shown.

Anomalous Dielectric Behavior of a Pb/Sn Perovskite: Effect of Trapped Charges on Complex Photoconductivity

“…Accordingly, Figure g illustrates the mechanism of anomalous signal generation in mixed MHPs, showing that excitation results in the generation of mobile charge carriers (holes and electrons) and thus gives rise to normal photoconductivity (Δσ′ > 0 and Δσ″ < 0). Trapping and charge recombination cause the decrease of normal signal intensity, and subsequently, the ordering of MA permanent dipoles (2.29 D) with respect to the trapped charges (formation of polaronic states) ,,, leads to the observation of anomalous signals. The orientation of MA dipoles is strongly supported by the results of previous studies, which showed that the MA rotation barrier in MAPbI 3 (10–20 meV) is approximately half of the thermal energy at room temperature, ,, with MA cations in octahedral Pb–Br cages being randomly oriented with a short relaxation time (∼7 ps) .…”

“…Although the optical and electronic properties of MHPs have been intensively studied, the dielectric properties of these compounds remain underexplored, ,, particularly in the high-frequency region. − Notably, the mostly free rotation of methylammonium dipoles in these species at room temperature contributes to their large dielectric constants under external electric fields ,, and induces the formation of ferroelectric-like domains that may aid charge separation and reduce recombination via charge carrier segregation . This is intuitively associated with the softness of the ionic lattice and liquid-like behavior of the permanent dipole of the A-site cation. , Giant photoinduced dielectric constants , and their polarity switching are notable examples of unique MHP properties. On the other hand, ion migration and charge accumulation at low frequency are thought to cause device hysteresis , not involving the ferroelectric nature of MHPs …”

Dielectric and ferroic properties of metal halide perovskites

“…Observation of Jonscher's law in spectroscopic measurements of MAPbI 3 therefore suggests that lossy processes, such as ion migration, dominate at low frequency. 114,122 Presenting an equivalent circuit model, Moia et al suggest that the accumulation of charged ionic species at the perovskite-electrical contact interfaces modulates the energetic barrier to charge injection and recombination. It is expected that this effect shall be significantly enhanced under illumination due to an increase in the concentration of free carriers and point defects due to photoexcitation.…”

Photodriven Dipole Reordering: Key to Carrier Separation in Metalorganic Halide Perovskites

“…Furthermore, understanding organometal halide perovskites in actual operating devices requires investigations under illumination. − Such studies are highly critical in view of the recent observation of a light-induced giant dielectric constant − and prediction of switching of (macroscopic) ferroelectric domains. , These effects may be related to a light-induced reordering of the MA organic molecule cations and thermalization of carriers, , but thus far there is no direct real-space microscopic experimental proof of any of these theories. Previous microscopic approaches by scanning tunneling microscopy lack a mapping of the potential landscape and, more importantly, provide no atomically resolved investigation under simultaneous illumination. , …”