We describe the simple, scalable, single-step, and polar-solvent-free synthesis of high-quality colloidal CsPbX (X=Cl, Br, and I) perovskite nanocrystals (NCs) with tunable halide ion composition and thickness by direct ultrasonication of the corresponding precursor solutions in the presence of organic capping molecules. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed the cubic crystal structure and surface termination of the NCs with atomic resolution. The NCs exhibit high photoluminescence quantum yields, narrow emission line widths, and considerable air stability. Furthermore, we investigated the quantum size effects in CsPbBr and CsPbI nanoplatelets by tuning their thickness down to only three to six monolayers. The high quality of the prepared NCs (CsPbBr ) was confirmed by amplified spontaneous emission with low thresholds. The versatility of this synthesis approach was demonstrated by synthesizing different perovskite NCs.

Adding cesium (Cs) and rubidium (Rb) cations to FA0.83MA0.17Pb(I0.83Br0.17)3 hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, Time-of-Flight (ToF), Time-Resolved Microwave Conductivity (TRMC), and Thermally Stimulated Current (TSC) measurements were performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs-incorporation was shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open-circuit voltage and fill factor of Cscontaining devices. By comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current-voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs-Rb-FA-MA) perovskites, the advantages of both inorganic cations can be combined. Our study provides valuable insights into the role of these additives in multiple-cation perovskite solar cells, which are essential for the design of highperformance devices.

Rubidium and cesium cations have been recently identified as enhancers for perovskite solar cell performance. However, the impact of these inorganic cations on the stability of the (FA0.83MA0.17)­Pb­(I0.83Br0.17)3 perovskite crystal lattice has not been fully understood yet. Here, we show via in situ X-ray diffraction and energy-dispersive X-ray spectrometry measurements that the unsuitably small ionic radius of Rb+ can lead to several nonphotoactive side-products. During the perovskite film synthesis, RbPb­(I1–x Br x )3 is formed, while exposure to humid air leads to the rapid formation of another hitherto unreported side phase (RbPb2I4Br). The formation of the Rb-rich side phases not only results in a loss of light absorption but also extracts bromide ions from the photoactive perovskite phase, thereby reducing its band gap. In comparison, the moisture-assisted formation of a CsPb2I4Br phase upon Cs-addition occurs on a significantly longer time scale than its Rb analog. While the incorporation of Cs+ remains attractive for high-performance solar cells, the severe moisture-sensitivity of Rb-containing mixed-halide perovskites may create additional engineering challenges.

Methylammonium lead bromide (MAPbBr 3) perovskite nanoparticles (NPs) have been recently proposed as a new material for light-emitting diodes, as well as a new paradigm to elucidate the operational mechanism in perovskite solar cells. Here, we have expanded the synthesis concept to fabricate NPs based on formamidinium lead bromide (FAPbBr 3). Importantly, we have demonstrated that the photophysical features of this novel material can be easily tuned by exchanging the organic cation, achieving lower radiative bimolecular recombination rate for FAPbBr 3 NPs. Additionally, we report for the first time light-emitting electrochemical cells (LECs) based on perovskite NPs by an easily up-scalable spray-coating technique. Stable luminance of 1-2 cd/m 2 at low driving currents was achieved for both types of materials. Overall, this work opens a new avenue of research into the field of organic-inorganic metal halide nanoparticles bearing different alkyl ammonium groups and their application in the developing field of thin-film lighting devices. http://pubs.acs.org.

We tune the Fermi level alignment between the SnO electron transport layer (ETL) and Cs(FAMA)Pb(IBr) and highlight that this parameter is interlinked with current-voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL-perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs.