The field of solution-processed semiconductors has made great strides; however, it has yet to enable electrically-driven lasers. To achieve this goal, improved materials are required that combine efficient (> 50% quantum yield) radiative recombination under high injection, large and balanced charge carrier mobilities in excess of 1 cm 2 V-1 s-1 , and high absorption and gain coefficients exceeding 10 4 cm-1. Solid-state perovskites are-in addition to galvanizing the field of solar electricity-showing great promise in photonic sources, and may be the answer to realizing solution-cast laser diodes. Here, we discuss the properties of perovskites that benefit light emission, review recent progress in perovskite electroluminescent diodes and optically pumped lasers, and examine the remaining challenges in achieving continuous-wave and electrically-driven lasing. 2 Trihalide perovskite semiconductors are an emerging class of materials that have led to great strides in the performance of solution-processed optoelectronic devices. 1-4 The bandgap is tunable via chemical composition, allowing it to be varied across visible and near-infrared wavelengths. 5 High-quality perovskite thin films are formed at low temperature using inexpensive earth-abundant reagents. Their low Urbach energy (as small as 15 meV), 6 minimal Stokes shift (less than 20 meV), 7 high charge carrier mobility (in excess of 10 cm 2 V-1 s-1), 8 long diffusion length (up to 1 μm and beyond), 7,9-11 and low trap density (less than 10 16 cm-3) 12 have enabled rapid advances in the field of solution-processed solar cell technology. Interest in perovskites soared once photovoltaic power conversion efficiencies ascended from 6.5% to 9.7% in 2012. 13,14 These impressive efficiencies quickly put perovskites in contention with leading third-generation solar harvesting materials including dyes, 15 organic polymers, 16 colloidal quantum dots, 17 and thin-film compound chalcogenides. 18 By building upon pioneering advances in mesoscopic dye-sensitized solar cells, researchers increased the efficiency of perovskite photovoltaics dramatically over a span of less than three years. Breakthroughs in perovskite film formation, composition, and electrodes progressed efficiency to 10.9%, 19 12%, 20,21 15%, 22 18.4%, 23 20.1%, 24 and now to 21% certified. 25 Recently, researchers have explored new applications of this material, including in light-emitting diodes and in semiconductor optical amplifiers and lasers. We offer a critical evaluation of the prospects of perovskites in these important areas of application. Perovskite materials for light emission Perovskites (Figure 1a) possess the crystal structure ABX3, where A and B are cations jointly bound to X, an anion. Methylammonium (MA) lead trihalide perovskites have been the most

The low toxicity and a near-ideal choice of bandgap make tin perovskite an attractive alternative to lead perovskite in low cost solar cells. However, the development of Sn perovskite solar cells has been impeded by their extremely poor stability when exposed to oxygen. We report low-dimensional Sn perovskites that exhibit markedly enhanced air stability in comparison with their 3D counterparts. The reduced degradation under air exposure is attributed to the improved thermodynamic stability after dimensional reduction, the encapsulating organic ligands, and the compact perovskite film preventing oxygen ingress. We then explore these highly oriented low-dimensional Sn perovskite films in solar cells. The perpendicular growth of the perovskite domains between electrodes allows efficient charge carrier transport, leading to power conversion efficiencies of 5.94% without the requirement of further device structure engineering. We tracked the performance of unencapsulated devices over 100 h and found no appreciable decay in efficiency. These findings raise the prospects of pure Sn perovskites for solar cells application.

Colloidal quantum dots (CQDs) offer promise in flexible electronics, light sensing and energy conversion. These applications rely on rectifying junctions that require the creation of high-quality CQD solids that are controllably n-type (electron-rich) or p-type (hole-rich). Unfortunately, n-type semiconductors made using soft matter are notoriously prone to oxidation within minutes of air exposure. Here we report high-performance, air-stable n-type CQD solids. Using density functional theory we identify inorganic passivants that bind strongly to the CQD surface and repel oxidative attack. A materials processing strategy that wards off strong protic attack by polar solvents enabled the synthesis of an air-stable n-type PbS CQD solid. This material was used to build an air-processed inverted quantum junction device, which shows the highest current density from any CQD solar cell and a solar power conversion efficiency as high as 8%. We also feature the n-type CQD solid in the rapid, sensitive, and specific detection of atmospheric NO2. This work paves the way for new families of electronic devices that leverage air-stable quantum-tuned materials.

Inorganic perovskites such as CsPbX3 (X=Cl, Br, I) have attracted attention due to their excellent thermal stability and high photoluminescence quantum efficiency. However, the electroluminescence quantum efficiency of their light-emitting diodes was <1%. We posited that this low efficiency was a result of high leakage current caused by poor perovskite morphology, high non-radiative recombination at interfaces and perovskite grain boundaries, and also charge injection imbalance. Here, we incorporated a small amount of methylammonium organic cation into the CsPbBr3 lattice and by depositing a hydrophilic and insulating polyvinyl pyrrolidine polymer atop the ZnO electron-injection layer to overcome these issues. As a result, we obtained light-emitting diodes exhibiting a high brightness of 91,000 cd m−2 and a high external quantum efficiency of 10.4% using a mixed-cation perovskite Cs0.87MA0.13PbBr3 as the emitting layer. To the best of our knowledge, this is the brightest and most-efficient green perovskite light-emitting diodes reported to date.

Organometallic trihalide perovskites are solution-processed semiconductors that have made great strides in third-generation thin film light-harvesting and light-emitting optoelectronic devices. Recently, it has been demonstrated that large, high-purity single crystals of these perovskites can be synthesized from the solution phase. These crystals' large dimensions, clean bandgap, and solid-state order have provided us with a suitable medium to observe and quantify two-photon absorption in perovskites. When CH3NH3PbBr3 single crystals are pumped with intense 800 nm light, we observe band-to-band photoluminescence at 572 nm, indicative of two-photon absorption. We report the nonlinear absorption coefficient of CH3NH3PbBr3 perovskites to be 8.6 cm GW(-1) at 800 nm, comparable to epitaxial single-crystal semiconductors of similar bandgap. We have leveraged this nonlinear process to electrically autocorrelate a 100 fs pulsed laser using a two-photon perovskite photodetector. This work demonstrates the viability of organometallic trihalide perovskites as a convenient and low-cost nonlinear absorber for applications in ultrafast photonics.