Tetragonal CH ₃ NH ₃ PbI ₃ is ferroelectric

Abstract

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI 3 ). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI 3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI 3 (unlike MAPbBr 3 ) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI 3 , we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.halide perovskites | photovoltaics | semiconductors | ferroelectricity | pyroelectricity N ew optoelectronic materials are of interest for producing solar cells with higher power and voltage efficiencies, lower costs, and improved long-term reliability. A very recent entry is the family of halide perovskites (HaPs), in particular those based on methylammonium (MA) lead iodide (MAPbI 3 ), MAPbBr 3 , and its inorganic analog CsPbBr 3 . Devices based on these perform remarkably well as solar cells (1, 2), as well as in other optoelectronic applications, such as LEDs and electromagnetic radiation detectors (3-5). Understanding possible unique characteristics of HaPs may show the way to other materials with similar key features.The ABX 3 (X = I, Br, Cl) HaP semiconductors (SCs), that is, with perovskite or perovskite-like structures, reach, via a steep absorption edge, a high optical absorption coefficient (∼10 5 cm −1 ) (6, 7), long charge carrier lifetimes (∼0.1-1 μs) (8), and reasonable carrier mobilities (less than or equal to ∼100 cm 2 ·V −1 ·s −1 ) (9), and have a low exciton binding energy (10). With these characteristics, the thickness of the optical absorber layer can be ≤0.5 μm, which allows the charge carriers (separated electrons and holes) to diffuse/d...

Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites

“…Our observation of the increased lifetime in the cubic phase of HC(NH 2 ) 2 PbI 3 and CH 3 NH 3 PbI 3 with higher rotational entropy of organic cations and larger thermal energy suggests that ferroelectric domain formation is not likely to be responsible. It is because, even if ferroelectric domains are formed in the lower symmetry phases at lower temperatures (8,37), such domains are expected to disappear in the high-symmetry cubic phase (37). Among other mechanisms that have been proposed to explain the long lifetime, our results support the scenario of formation of large polarons, which is consistent with the long carrier lifetime but modest carrier mobilities observed in HOIPs (14)(15)(16).…”

Inversion symmetry and bulk Rashba effect in methylammonium lead iodide perovskite single crystals

“…The PbI framework can statically distort from a set of ideal corner-sharing octahedra, forming a non-centrosymmetric phase as has been recently suggested 23 , and resulting in a static bulk Rashba effect. Alternatively, inversion symmetry can be broken dynamically by the MA ion rotations in the absence of a static distortion of the PbI framework.…”

“…The rotations of the methylammonium (MA) ions 19,21 and their interactions with the lead iodide (PbI) framework further complicate the structure. The space group of the room temperature tetragonal phase—the most relevant for practical applications—has been the subject of extensive debate, with some works reporting a centrosymmetric space group of I 4/ mcm (point group 4/ mmm ) 19,20 and others a non-centrosymmetric space group of I 4 cm (point group 4 mm ) 22,23 .…”

“…Recent work proposed that a bulk Rashba effect—i.e., not due to the surface or interfaces—may be the cause of long carrier lifetimes and diffusion lengths in MAPbI 3 26–28 . This idea has received surprising attention in spite of the fact that, with the exception of a few reports on the tetragonal phase 22,23 , most diffraction experiments attribute centrosymmetric space groups to all three crystal phases 19,20 .…”

Absence of ferroelectricity in methylammonium lead iodide perovskite

“…It has been recently shown that CH 3 NH 3 PbI 3 does not have a polar crystal structure at room temperature down to 77 K. 35 None polar centrosymmetric crystal structure implies absence of ferroelectricity. On the other hand there are reports that based on dielectric constant measurements, 36 or piezoelectric force microscopy (PFM) 37 claim ferroelectricity in CH 3 NH 3 PbI 3 .…”