Ruddlesden−Popper perovskite films deposited with different methods show very diverse phase segregation and composition. When DMSO is used as solvent, the conventional method based on spin-coating and annealing produces very poor devices, whereas the vacuum-assisted method proposed here allows obtaining devices with efficiency up to 14.14%. The conventional method gives rise to a three-dimensional (3D)-like phase on the top of the film but dominant n = 2 phase with large domains (∼40 μm) at the bottom of the film. These n = 2 domains are oriented with their inorganic slabs parallel to the substrate and inhibit the charge transport in the vertical direction. Consequently, severe monomolecular and bimolecular charge recombination occurs in the solar cells. Conversely, the vacuum-assisted method yields films with a 3Dlike phase dominant throughout their entire thickness and only a small amount of n ≤ 2 domains of limited dimensions (∼3 μm) at the bottom, which facilitate charge transport and reduce charge recombination.

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.

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.

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.