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displays. [1,2] In particular, perovskite-based light-emitting diodes (PeLEDs) with superior external quantum efficiency (EQE) and luminance have been demonstrated in the green, red, and near-infrared emission regions. [3][4][5][6] As display technologies continue to improve, the requirements for the emitters in the display become more stringent. As of August 2012, the Rec. 2020 standard defines the display color gamut for ultrahigh definition television (UHDTV): it requires each of the primary red, green, blue (RGB) emitters to have a precisely defined wavelength (red: 630 nm, green: 532 nm, and blue: 467 nm) and a narrow emission linewidth (<20 nm). [7] In state-of-art liquid crystal display backlights, crosstalk between color filters reduces the color purity of the RGB primaries. [8] LED displays do not require color filters and as such are promising candidate for UHDTV. Organic molecules have had success in commercial LED displays, but the wide linewidths (>50 nm) of organic emitters limit the attainable color gamut. [9] Developing emitters with precisely defined emission profiles for displays is an active Perovskite-based light-emitting diodes (PeLEDs) are now approaching the upper limits of external quantum efficiency (EQE); however, their application is currently limited by reliance on lead and by inadequate color purity. The Rec. 2020 requires Commission Internationale de l'Eclairage coordinates of (0.708, 0.292) for red emitters, but present-day perovskite devices only achieve (0.71, 0.28). Here, lead-free PeLEDs are reported with color coordinates of (0.706, 0.294)-the highest purity reported among red PeLEDs. The variation of the emission spectrum is also evaluated as a function of temperature and applied potential, finding that emission redshifts by <3 nm under low temperature and by <0.3 nm V −1 with operating voltage. The prominent oxidation pathway of Sn is identified and this is suppressed with the aid of H 3 PO 2 . This strategy prevents the oxidation of the constituent precursors, through both its moderate reducing properties and through its forming complexes with the perovskite that increase the energetic barrier toward Sn oxidation. The H 3 PO 2 additionally seeds crystal growth during film formation, improving film quality. PeLEDs are reported with an EQE of 0.3% and a brightness of 70 cd m −2 ; this is the record among reported red-emitting, lead-free PeLEDs.

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Charge carrier transport in colloidal quantum dot (CQD) solids is strongly influenced by coupling among CQDs. The shape of as‐synthesized CQDs results in random orientational relationships among facets in CQD solids, and this limits the CQD coupling strength and the resultant performance of optoelectronic devices. Here, colloidal‐phase reconstruction of CQD surfaces, which improves facet alignment in CQD solids, is reported. This strategy enables control over CQD faceting and allows demonstration of enhanced coupling in CQD solids. The approach utilizes post‐synthetic resurfacing and unites surface passivation and colloidal stability with a propensity for dots to couple via (100):(100) facets, enabling increased hole mobility. Experimentally, the CQD solids exhibit a 10× increase in measured hole mobility compared to control CQD solids, and enable photodiodes (PDs) exhibiting 70% external quantum efficiency (vs 45% for control devices) and specific detectivity, D* > 1012 Jones, each at 1550 nm. The photodetectors feature a 7 ns response time for a 0.01 mm2 area—the fastest reported for solution‐processed short‐wavelength infrared PDs.

Metal halide perovskites have emerged as promising candidates for solution‐processed laser gain materials, with impressive performance in the green and red spectral regions. Despite exciting progress, deep‐blue—an important wavelength for laser applications—remains underexplored; indeed, cavity integration and single‐mode lasing from large‐bandgap perovskites have yet to be achieved. Here, a vapor‐assisted chlorination strategy that enables synthesis of low‐dimensional CsPbCl3 thin films exhibiting deep‐blue emission is reported. Using this approach,  high‐quality perovskite thin films having a low surface roughness (RMS ≈ 1.3 nm) and efficient charge transfer properties are achieved. These enable us to document low‐threshold amplified spontaneous emission. Levering the high quality of the gain medium,  vertical‐cavity surface‐emitting lasers with a low lasing threshold of 6.5 µJ cm−2 are fabricated. This report of deep‐blue perovskite single‐mode lasing showcases the prospect of increasing the range of deep‐blue laser sources.