Sjoerd A. Veldhuis

Perovskite Materials for Light-Emitting Diodes and Lasers

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Giant five-photon absorption from multidimensional core-shell halide perovskite colloidal nanocrystals

Multiphoton absorption processes enable many technologically important applications, such as in vivo imaging, photodynamic therapy and optical limiting, and so on. Specifically, higher-order nonlinear absorption such as five-photon absorption offers significant advantages of greater spatial confinement, increased penetration depth, reduced autofluorescence, enhanced sensitivity and improved resolution over lower orders in bioimaging. Organic chromophores and conventional semiconductor nanocrystals are leaders in two-/three-photon absorption applications, but face considerable challenges from their small five-photon action cross-sections. Herein, we reveal that the family of halide perovskite colloidal nanocrystals transcend these constraints with highly efficient five-photon-excited upconversion fluorescence—unprecedented for semiconductor nanocrystals. Amazingly, their multidimensional type I (both conduction and valence band edges of core lie within bandgap of shell) core–shell (three-dimensional methylammonium lead bromide/two-dimensional octylammonium lead bromide) perovskite nanocrystals exhibit five-photon action cross-sections that are at least 9 orders larger than state-of-the-art specially designed organic molecules. Importantly, this family of halide perovskite nanocrystals may enable fresh approaches for next-generation multiphoton imaging applications.

Self-assembled hierarchical nanostructured perovskites enable highly efficient LEDs via an energy cascade

Metal halide perovskites have established themselves as extraordinary optoelectronic materials, exhibiting promise for applications in large area illumination and displays. However, low luminescence, low efficiencies of the light-emitting diodes (LEDs), and complex preparation methods currently limit further progress towards applications. Here, we report on a new and unique mesoscopic film architecture featuring the self-assembly of 3D formamidinium lead bromide (FAPbBr3) nanocrystals of graded size, coupled with microplatelets of octylammonium lead bromide perovskites, which enables an energy cascade, yielding very high-performance light-emitting diodes with emission in the green spectral region. These hierarchically structured perovskite films exhibit photoluminescence quantum yields of over 80% and LEDs associated with record high efficiencies in excess of 57.6 cd A−1 with an external quantum efficiency above 13%. Additionally, due to low turn-on voltages (∼2.2 V) the LEDs have power efficiencies exceeding 58 lumens per watt, obtained without any light-outcoupling structures.

Atomically Altered Hematite for Highly Efficient Perovskite Tandem Water-Splitting Devices

Photoelectrochemical (PEC) cells are attractive for storing solar energy in chemical bonds through cleaving of water into oxygen and hydrogen. Although hematite (α-Fe2 O3 ) is a promising photoanode material owing to its chemical stability, suitable band gap, low cost, and environmental friendliness, its performance is limited by short carrier lifetimes, poor conductivity, and sluggish kinetics leading to low (solar-to-hydrogen) STH efficiency. Herein, we combine solution-based hydrothermal growth and a post-growth surface exposure through atomic layer deposition (ALD) to show a dramatic enhancement of the efficiency for water photolysis. These modified photoanodes show a high photocurrent of 3.12 mA cm-2 at 1.23 V versus RHE, (>5 times higher than Fe2 O3 ) and a plateau photocurrent of 4.5 mA cm-2 at 1.5 V versus RHE. We demonstrate that these photoanodes in tandem with a CH3 NH3 PbI3 perovskite solar cell achieves overall unassisted water splitting with an STH conversion efficiency of 3.4 %, constituting a new benchmark for hematite-based tandem systems.

Benzyl Alcohol-Treated CH3NH3PbBr3 Nanocrystals Exhibiting High Luminescence, Stability, and Ultralow Amplified Spontaneous Emission Thresholds

We report the high yield synthesis of about 11 nm sized CH3NH3PbBr3 nanocrystals with near-unity photoluminescence quantum yield. The nanocrystals are formed in the presence of surface-binding ligands through their direct precipitation in a benzyl alcohol/toluene phase. The benzyl alcohol plays a pivotal role in steering the surface ligands binding motifs on the NC surface, resulting in enhanced surface-trap passivation and near-unity PLQY values. We further demonstrate that thin films from purified CH3NH3PbBr3 nanocrystals are stable >4 months in air, exhibit high optical gain (about 520 cm-1), and display stable, ultralow amplified spontaneous emission thresholds of 13.9 ± 1.3 and 569.7 ± 6 μJ cm-2 at one-photon (400 nm) and two-photon (800 nm) absorption, respectively. To the best of our knowledge, the latter signifies a 5-fold reduction of the lowest reported threshold value for halide perovskite nanocrystals to date, which makes them ideal candidates for light-emitting and low-threshold lasing applications.

Low threshold and efficient multiple exciton generation in halide perovskite nanocrystals

Multiple exciton generation (MEG) or carrier multiplication, a process that spawns two or more electron–hole pairs from an absorbed high-energy photon (larger than two times bandgap energy Eg), is a promising way to augment the photocurrent and overcome the Shockley–Queisser limit. Conventional semiconductor nanocrystals, the forerunners, face severe challenges from fast hot-carrier cooling. Perovskite nanocrystals possess an intrinsic phonon bottleneck that prolongs slow hot-carrier cooling, transcending these limitations. Herein, we demonstrate enhanced MEG with 2.25Eg threshold and 75% slope efficiency in intermediate-confined colloidal formamidinium lead iodide nanocrystals, surpassing those in strongly confined lead sulfide or lead selenide incumbents. Efficient MEG occurs via inverse Auger process within 90 fs, afforded by the slow cooling of energetic hot carriers. These nanocrystals circumvent the conundrum over enhanced Coulombic coupling and reduced density of states in strongly confined nanocrystals. These insights may lead to the realization of next generation of solar cells and efficient optoelectronic devices.The hot carriers in halide perovskite nanocrystals cool much slower than those in conventional semiconductor nanocrystals due to the phonon bottleneck. Here, Li et al. demonstrate enhanced multiple exciton generation with lower threshold in intermediate-confined perovskite nanocrystals based on this effect.