Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance.

The aristotype and the two ettotypes of the complex perovskite PbFe 0.5 Nb 0.5 O 3 have been refined by the Rietveld method from neutron and x-ray powder diffraction data. To avoid errors due to sample off-stoichiometry or inhomogeneity, only powders obtained by grinding single crystals were analysed. At 523 K the cubic phase (space group P m 3m), which is known to be stable for T > 376 K, has the lattice parameter a c = 4.010(1) Å and is characterized by disorder of the lead and oxygen atoms, as in most Pb-based complex perovskites. At 363 K the intermediate phase, stable for 355 K < T < 376 K, is confirmed to be tetragonal, space group P 4mm, with lattice parameters a t = 4.007(1) Å and c t = 4.013(1) Å. The best refinement for the low-temperature structure (T < 355 K) is obtained, both at 250 and 80 K, in monoclinic symmetry, space group Cm. The monoclinic cell at 250 K has parameters a m = 5.674(1) Å, b m = 5.663(1) Å, c m = 4.013(1) Å and β = 89.84 • (2).

spectrum featuring a marked excitonic resonance, the majority optical excitation in prototypical HP materials for photovoltaics are not bound excitons, but unbound charge carriers. Therefore, electrons and holes excited by solar light can be directed to the electrodes at a negligible energy cost, without the need to split tightly bound excitons as in organics. Taking advantage of the flexibility of the materials class, layered 2D HPs are obtained by inserting bulky organic cations into the formulation, leading to materials inherently more stable than their 3D counterparts against degradation. [8][9][10] However, in 2D HPs the exciton binding energy can be as large as 400 meV, [8] so that it is commonly assumed that their excited states are mostly excitons.A second peculiar characteristic of the excited states in perovskites is the formation of large polarons, that is, charge carriers coupled to lattice deformations and delocalized over many crystal lattice sites. [11][12][13][14][15][16][17][18][19][20] Unlike small polarons in organics, localized in a single molecule, large polarons are compatible with band transport, but are also able to screen the excited states from scattering with defects and reduce non-radiative recombination through trap states, resulting in large mobilities and long lifetimes. Large polarons are also believed to reduce scattering with phonons and have been proposed as an explanation for hot carriers persisting for several nanoseconds at temperatures significantly higher than the lattice one. [12,16,17,[21][22][23][24][25][26] Large polarons may therefore be the enabling microscopic mechanism for efficient solar cells, including innovative architectures that exploit photoconversion with hot carriers. [27,28] Theoretical estimations forecast that the energy associated with polaron formation is comparable with the binding energy gained by forming an exciton, maybe even larger in some materials. [12,14,24,[29][30][31][32] When do polarons form and whether excitons or polarons are the lowest-energy optical excitations is still an open question. The issue is particularly relevant for layered 2D HPs, where polaronic effects have been demonstrated, although it is not clear if small or large polarons are formed. [33][34][35][36][37] In spite of the large exciton binding energy, unbound charge carriers have been reported, so that it is not clear yet how much energy needs to be spent in solar cells to split bound excitons.

The ferroelastic symmetry changes of the lead-based complex perovskite PbFe 0.5 Ta 0.5 O 3 are investigated by means of single crystal x-ray diffraction and observations in polarized light microscopy at different temperatures. The position of the 400 and 222 cubic reflections is followed between 80 K and 300 K. Results indicate the presence of two structural transitions at about 270 K and 220 K. Such temperatures define the stability range of three different phases. The high-temperature phase (T > 270 K) is the optically anomalous paraelectric prototype, the optical symmetry of which is uniaxial (tetragonal) in spite of the diffractometric (pseudo)cubic symmetry. The intermediate derived phase (270 K > T > 220 K) is tetragonal. This tetragonal phase coexists with a monoclinic one for 220 K > T > 200 K. Below 200 K only the monoclinic phase is stable. The (pseudo)cubic-to-tetragonal transition seems to be of the second order. The tetragonal-to-monoclinic one is of the first order.

The high symmetry parent phase and the two derived low symmetry phases of the complex perovskite PbFe0.5 Ta0.5 O3 have been refined by the Rietveld method from neutron powder diffraction data. The analysed powders were obtained by grinding single crystals. Owing to the very small distortions from the cubic structure, the lattice symmetry of the derived phases was determined by means of synchrotron radiation powder diffraction. At 350 K the cubic phase (which is known to be stable for T > 270 K) is characterized by positional disorder or anharmonic thermal motion of lead atoms, as happens in most Pb-based complex perovskites. It was refined in space group Pm m , with strongly anisotropic thermal motion of oxygen atoms. The synchrotron powder diffraction pattern of the intermediate phase (stable for 220 K < T < 270 K), collected at 230 K, agrees with a small tetragonal distortion. Neutron data at 230 K were refined in symmetry P 4mm . Only oxygen atoms are significantly displaced from the cubic positions. The analysis of line broadening and splitting in the synchrotron radiation patterns collected at 130 K and 15 K indicate the low temperature symmetry to be monoclinic. Neutron data at 15 K were refined in space group Cm .