Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers

4 years ago

Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers

Brahim Dkhil, Juras Banys, Doru C. Lupascu, Mehmet Sanlialp, Sergejus Balčiūnas, Irina Anusca, Maksim Ivanov, Šarūnas Svirskas, Pascale Gemeiner, Christian Fettkenhauer, Gerhard Lackner, Vladimir V. Shvartsman, Vytautas Samulionis, Jaroslavas Belovickis

Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic–inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found. Charge carrier screening in the methylammonium lead halides is the major mechanism assuring long charge carrier lifetime. The high dielectric response is shown to stem from lattice as well as dipole orientation contributions both acting independently at different frequencies. The resulting screening is more effective than a single mechanism and acts on electronic carriers as well as charged defects.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/aenm.201700600