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.

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 ...

Hybrid lead halide perovskites are unique solution‐processed semiconductors with very large optical absorption coefficients in the visible spectrum, large diffusion length of photoexcited charge carriers, and long excited state lifetimes, properties that have been exploited for the realization of solar cells and LEDs. However, one of the most fundamental properties of hybrid perovskites, whether the optical bandgap is direct or indirect, is actively debated. While perovskites have been considered direct bandgap semiconductors in most published literature, recent studies have proposed that the Rashba spin–orbit coupling gives rise to an indirect gap, few tens of meV lower in energy than the direct one. Here, the radiative recombination rates in hybrid perovskites are measured as a function of temperature, extracting their values from the instantaneous intensity of photoluminescence under pulsed excitation. Experimental data show that radiative recombination becomes faster with decreasing temperature, as in all direct bandgap materials and contrary to what expected for 3D Rashba semiconductors. The technique has been applied to CH3NH3PbI3 and CH3NH3PbBr3, both in polycrystalline and single crystal samples, as well as to GaAs for validation purposes.

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Metal halide perovskites are maturing as materials for efficient, yet low cost solar cells and light‐emitting diodes, with improving operational stability and reliability. To date however, most perovskite‐based devices contain Pb, which poses environmental concerns due to its toxicity; lead‐free alternatives are of importance to facilitate the development of perovskite‐based devices. Here, the germanium‐based Ruddledsen–Popper series (CH3(CH2)3NH3)2(CH3NH3)n−1GenBr3n+1 is investigated, derived from the parent 3D (n = ∞) CH3NH3GeBr3 perovskite. Divalent germanium is a promising, nontoxic alternative to Pb2+ and the layered, 2D structure appears promising to bolster light emission, long‐term durability, and moisture tolerance. The work, which combines experiments and first principle calculations, highlights that in germanium bromide perovskites the optical bandgap is weakly affected by 2D confinement and the highly stereochemically active 4s2 lone pair preludes to possible ferroelectricity, a topic still debated in Pb‐containing compounds.

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Hybrid metal halide perovskite materials are produced with facile routes, but their morphology is sensitive to water, oxygen, temperature, and exposure to light. While phase separation and self-assembly of perovskite nanostructures have been demonstrated, the realization of controlled perovskite–perovskite heterostructures has been limited up to now. We demonstrate here the growth of stable CH3NH3PbI3–xBrx nanocrystals in a CH3NH3PbBr3 matrix. Optical emission from the nanocrystals can be reversibly activated upon illumination through a photobrightening process. Optical microscopy images show that nanocrystals are stable in time, through several illumination cycles. Ultrafast photoluminescence measurements imply that optical excitations are funneled from the matrix into the lower bandgap nanocrystals. Because the nanocrystals represent <2% of the materials volume, the local carrier concentration is higher in the nanocrystals than in the matrix, leading to an increase in the photoluminescence quantum yield, highlighting the promise of such self-assembled heterostructures for efficient light-emitting device

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