No abstract

One merit of organic-inorganic hybrid perovskites is their tunable bandgap by adjusting the halide stoichiometry, an aspect critical to their application in tandem solar cells, wavelength-tunable light emitting diodes (LEDs), and lasers. However, the phase separation of mixed-halide perovskites caused by light or applied bias results in undesirable recombination at iodide-rich domains, meaning open-circuit voltage (V) pinning in solar cells and infrared emission in LEDs. Here, we report an approach to suppress halide redistribution by self-assembled long-chain organic ammonium capping layers at nanometer-sized grain surfaces. Using the stable mixed-halide perovskite films, we are able to fabricate efficient and wavelength-tunable perovskite LEDs from infrared to green with high external quantum efficiencies of up to 5%, as well as linearly tuned V from 1.05 to 1.45 V in solar cells.

Hybrid organic-inorganic halide perovskite semiconductors are attractive candidates for optoelectronic applications, such as photovoltaics, light-emitting diodes, and lasers. Perovskite nanocrystals are of particular interest, where electrons and holes can be confined spatially, promoting radiative recombination. However, nanocrystalline films based on traditional colloidal nanocrystal synthesis strategies suffer from the use of long insulating ligands, low colloidal nanocrystal concentration, and significant aggregation during film formation. Here, we demonstrate a facile method for preparing perovskite nanocrystal films in situ and that the electroluminescence of light-emitting devices can be enhanced up to 40-fold through this nanocrystal film formation strategy. Briefly, the method involves the use of bulky organoammonium halides as additives to confine crystal growth of perovskites during film formation, achieving CHNHPbI and CHNHPbBr perovskite nanocrystals with an average crystal size of 5.4 ± 0.8 nm and 6.4 ± 1.3 nm, respectively, as confirmed through transmission electron microscopy measurements. Additive-confined perovskite nanocrystals show significantly improved photoluminescence quantum yield and decay lifetime. Finally, we demonstrate highly efficient CHNHPbI red/near-infrared LEDs and CHNHPbBr green LEDs based on this strategy, achieving an external quantum efficiency of 7.9% and 7.0%, respectively, which represent a 40-fold and 23-fold improvement over control devices fabricated without the additives.

Hybrid organic-inorganic metal halide perovskites are particularly promising for light-emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide-based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5-10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole-free films. As a result of time-resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.photodetectors, [7,8] memories, [9] etc. Perovskite light-emitting diodes (PeLEDs) are particularly interesting due to certain advantages over both organic LEDs (OLEDs) and quantum dot LEDs (QLEDs). [6] For example, full width at half-maximum of PeLEDs reaches 20 nm at an emission wavelength around 520 nm, smaller than that of either OLEDs (>40 nm) or QLEDs (≈30 nm). [5] Therefore, the emission color is more pure. The emission wavelength of PeLEDs can be tuned by component engineering such as by alloying the halide component. [10,11] In addition, the carrier mobility of hybrid perovskites can exceed 100 cm 2 V −1 s −1 , an order of magnitude higher than organic semiconductors and quantum dots (QDs). [3] Therefore, the current density through PeLEDs and thus the highest brightness has the potential to be higher than that of OLEDs and QLEDs. [6] The performance of PeLEDs has experienced dramatic improvement over the last 4 years. [12][13][14][15][16][17][18][19] Since the first report of PeLEDs using 3D perovskite phases, CH 3 NH 3 PbI 3 (MAPbI 3 ) and MAPbBr 3 , with an external quantum efficiency (EQE) of 0.76% and 0.1%, respectively, in 2014, [20] various approaches have been developed to improve the performance of PeLEDs. For example, Cho et al. incorporated a 5% molar excess of MABr to passivate surfaces and

Perovskite light-emitting diodes (LEDs) require small grain sizes to spatially confine charge carriers for efficient radiative recombination. As grain size decreases, passivation of surface defects becomes increasingly important. Additionally, polycrystalline perovskite films are highly brittle and mechanically fragile, limiting their practical applications in flexible electronics. In this work, the introduction of properly chosen bulky organo-ammonium halide additives is shown to be able to improve both optoelectronic and mechanical properties of perovskites, yielding highly efficient, robust, and flexible perovskite LEDs with external quantum efficiency of up to 13% and no degradation after bending for 10 000 cycles at a radius of 2 mm. Furthermore, insight of the improvements regarding molecular structure, size, and polarity at the atomic level is obtained with first-principles calculations, and design principles are provided to overcome trade-offs between optoelectronic and mechanical properties, thus increasing the scope for future highly efficient, robust, and flexible perovskite electronic device development.