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.

Advances of optoelectronic devices based on methylammonium lead halide perovskites depend on understanding the role of excitons, whether it is marginal as in inorganic semiconductors, or crucial, like in organics. However, a consensus on the exciton binding energy and its temperature dependence is still lacking, even for widely studied methylammonium lead iodide and bromide materials (MAPbI3, MAPbBr3). Here we determine the exciton binding energy based on an f-sum rule for integrated UV-vis absorption spectra, circumventing the pitfalls of least-squares fitting procedures. In the temperature range 80-300 K, we find that the exciton binding energy in MAPbBr3 is EB = (60 ± 3) meV, independent of temperature; for MAPbI3, in the orthorhombic phase (below 140 K) EB = (34 ± 3) meV, while in the tetragonal phase the binding energy softens to 29 meV at 170 K and stays constant up to 300 K. Implications of binding energy values on solar cell and LED workings are discussed.

Metal-halide perovskite solar cells rival the best inorganic solar cells in power conversion efficiency, providing the outlook for efficient, cheap devices. In order for the technology to mature and approach the ideal Shockley-Queissier efficiency, experimental tools are needed to diagnose what processes limit performances, beyond simply measuring electrical characteristics often affected by parasitic effects and difficult to interpret. Here we study the microscopic origin of recombination currents causing photoconversion losses with an all-optical technique, measuring the electron-hole free energy as a function of the exciting light intensity. Our method allows assessing the ideality factor and breaks down the electron-hole recombination current into bulk defect and interface contributions, providing an estimate of the limit photoconversion efficiency, without any real charge current flowing through the device. We identify Shockley-Read-Hall recombination as the main decay process in insulated perovskite layers and quantify the additional performance degradation due to interface recombination in heterojunctions.

Excitons in lead halide perovskites often go unnoticed as minority species, yet they account for almost all of light emission.

COMMUNICATIONRecent studies have shown amplifi ed spontaneous emission (ASE) under short pulse excitation conditions, with lasing threshold densities comparable to or even lower than those observed in state-of-the-art organic materials. [ 18,19,[30][31][32] Short pulse excitation is a very favorable regime for light amplifi cation, as carrier densities well above the threshold required for lasing can be easily injected. Yet light amplifi cation disappears just a few picoseconds after excitation, a transient regime that is far away from the working conditions of interest for applications. How long light amplifi cation can last in perovskite materials is the open question we address in this Communication.Here, we demonstrate ASE sustained over transients two orders of magnitude longer than the excited state lifetime. Through optical spectroscopy, we measure threshold densities for ASE as a function of the temperature of the environment and the duration of the exciting laser pulse. Particularly, we employ an optical thermometry technique to track the dynamics of the temperature of the electron-hole plasma and identify the runaway heating mechanism limiting the maximum achievable duration of light amplifi cation. We then discuss the conditions needed for true continuous wave operation of light amplifi cation.Light emission in methylammonium lead iodide (MAPbI 3 ) and methylammonium lead bromide (MAPbBr 3 ) thin fi lms has fi rstly been analyzed under 130 fs pulsed laser excitation (392 nm in wavelength), using a streak camera to detect timeresolved photoluminescence and a cooled charge-coupled device (CCD) camera for the time-integrated spectra. The resulting photoluminescence decays, shown in Figure 1 a, had characteristic decay times much longer than the pulse duration, few nanoseconds for MAPbBr 3 , tens of nanoseconds for MAPbI 3 , meaning that such an excitation regime can be considered as impulsive. ASE was demonstrated by a sharp peak (Figure 1 b,c) appearing in the low energy side of the emission spectrum for both fi lms once the excitation laser fl uence reached a threshold value; such a value turned out to be 26 µJ cm −2 per pulse for MAPbI 3 , a factor of two lower than for MAPbBr 3 (54 µJ cm −2 per pulse). The corresponding average excited population densities, as calculated by averaging the laser fl uence times the fi lm absorption coeffi cient over the fi lm thickness, were 4 × 10 18 cm −3 and 7 × 10 18 cm −3 for MAPbI 3 and MAPbBr 3 , respectively (see absorption coeffi cients in the Supporting Information paragraph). Films realized from different solution batches and with different age showed variations in ASE threshold fl uence, mainly due to the different optical losses occurring as a result of different morphology. All samples were therefore stored in vacuum and typically measured within the

Nanosheets of two mixed linker (anilate/carboxylate)/Yb III -based two-dimensional (2D) layered coordination polymers (CPs) are herein reported. Bulky sized CPs, formulated as [Yb 4 (ClCNAn) 5 (DOBDC) 1 (DMSO) 10 ] n •(DMSO) 2 (1) and [Yb 2 (ClCNAn) 2 (F 4 BDC)(DMSO) 6 ] n (2), are formed by twodimensional neutral polymeric networks based on Yb III ions, heterosubstituted ClCNAn 2− anilate, and dicarboxylate linkers (DOBDC 2− in 1; F 4 BDC 2− in 2): in 1, neighbor layers are eclipsed and form six-membered rings showing either hexagonal cavities, formed by ClCNAn 2− and Yb III ions or rectangular cavities formed by ClCNAn 2− , DOBDC 2− , and Yb III while distorted square-like cavities formed by ClCNAn 2− , F 4 BDC 2− , and Yb III are obtained in 2. The 1 and 2 nanosheets, that is, 1-NS and 2-NS, are produced by exfoliation of bulk CPs by the wellknown top-down approach and characterized by atomic force microscopy and high resolution transmission electron microscopy.The 1-NS and 2-NS show thickness in the range of 1−5 nm, whereas the lateral dimensions are in the micrometer scale and the presence of well-defined lattice fringes, key evidence of their robustness. Remarkably the powder X-ray diffraction (PXRD) patterns highlight that the crystallinity is retained at the nanoscale level. Nanosheets photophysics are investigated by an innovative Multiprobe Approach where lifetime, photoluminescence (PL) and integrated PL are simultaneously measured in order to study how these parameters can be affected by the presence of the different ligands. These advanced photophysical studies pave the way to challenging luminescent sensing applications and to contribute to the ongoing research on fabricating robust and crystalline hybrid (organic/inorganic) lanthanide sheetlike 2D nanomaterials.

opposite charges toward opposite electrodes. On the other hand, light-emitting diodes (LEDs) require high rates of radiative recombination at low injection levels, in order to operate efficiently. This condition is best satisfied if the emitting species are excitons, as their radiative recombination rate is proportional to the exciton density. Conversely, light emission from free charges is quite inefficient at low carrier concentrations, since radiative decay is a second order process in the carrier density. Understanding exciton formation processes is therefore of importance to design perovskite materials that bolster light emission.However, exciton formation in hybrid perovskites has been elusive so far. The mass-action law, known as Saha equation, [12] foresees an equilibrium with predominance of free carriers for excited state densities relevant to devices, despite the fact that the measured exciton binding energy is larger than the ambient thermal energy, as for CH 3 NH 3 PbBr 3 (E b = 60 meV). [13] Saha equation also predicts the formation of a majority population of excitons at large excitation densities, but such crossover has not been observed yet, hinting that thermodynamic equilibrium between bound and unbound states may not be assumed. In order to identify the exciton formation processes, the determination of the photoexcitation kinetics becomes the central issue. The task has been pursued using an array of time-resolved optical spectroscopy techniques. Many reports investigated the photoexcitation dynamics by differential femtosecond transmission or reflection spectroscopy. [5,[14][15][16][17][18][19][20][21][22][23][24] This technique detects small changes in the linear absorption/reflection spectrum near the optical gap induced by a population of photoexcitations, but it is not very selective to the nature of photoexcitations. Terahertz (THz) pump-probe spectroscopy overcomes this limitation and directly measures the absorption spectrum due the internal quantum transitions of the exciton population (1s→2p,3p, and so on) and free electronhole plasma. In hybrid perovskites, the THz absorption spectrum also contains interfering contributions from phonon modes. Most experiments reported to date agree that free carriers are the only photoexcitations in hybrid perovskites; [3,[25][26][27] however, two recent studies suggest that transitions between excitonic Rydberg states are also observed in CH 3 NH 3 PbI 3 ; yet, the ensuing estimates of Understanding exciton formation is of fundamental importance for emerging optoelectronic materials, like hybrid organic-inorganic perovskites, as excitons are the lowest-energy photoexcitations in semiconductors, are electrically neutral, and do not directly contribute to charge transport, but can emit light more efficiently than free carriers. However, despite the increasing attention toward these materials, experimental results on the processes of formation of an exciton population in perovskites are still elusive. Here, an ultrafast differential photoluminescence ...

The synthesis, structural characterization, photophysical studies, and exfoliation of two-dimensional (2D) layered coordination polymers, formulated as {[Ln2(ClCNAn)3(DMF)6]·(DCM) x } n (Ln­(III) = Yb­(x = 0), Nd, and Er (x = 2)) based on the heterosubstituted chlorocyananilate ligand, are reported. These compounds consist of neutral polymeric 2D networks of the chlorocyananilate ligand alternating with Ln­(III) ions. They form six-membered rings with rectangular cavities, where neighbor layers are eclipsed along the a axis (Yb), and a regular honeycomb-like structure, with hexagonal cavities filled by dichloromethane solvent molecules (Nd and Er), where neighbor layers alternate along the c axis. Several interlayer interactions between lanthanide centers and dimethylformamide molecules, facing the cavities, are present in all compounds. Free-standing nanosheets, obtained by a top-down strategy involving sonication-assisted solution synthesis and characterized by atomic force microscopy and high-resolution transmission electron microscopy, show lateral dimensions on the micrometer scale, thicknesses down to the monolayer, and the presence of lattice fringes. Time-resolved photoluminescence studies performed on both the bulk and nanosheets clearly demonstrate that the chlorocyananilate ligand acts as an efficient antenna toward Ln­(III) ions and that emission sensitization occurs as a multistep relaxation process involving, in sequence, intersystem crossing and energy transfer from ligand triplet states to the Ln­(III) ions. Effects induced by the exfoliation process on the photophysical properties of the nanosheets are also discussed.