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 that 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.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Stabilizing the electroluminescence of halide perovskites with potassium passivation

Halide perovskites are of great interest for light-emitting diodes (PeLEDs) in recent years due to their excellent photo- and electroluminescence properties. However, trap/defects and ion migration of devices under high external driving voltage/current are yet overcome. In this work, it is found that upon potassium (K) addition to a CsPbBr3/Cs4PbBr6 (3D:0D = 0.85:0.15) perovskite, a locally-disordered 0D Cs4-xKxPbBr6 phase is formed with nearly 0.35:0.65 admixture of 0D:3D, along with an unreacted KBr phase potentially passivating the surface and grain boundaries. The formation of CsPbBr3 nanocrystals (~10nm) confined within the Cs4-xKxPbBr6 matrix accompanied by larger CsPbBr3 grains (~50nm) is further confirmed by high-resolution transmission electron microscopy. In addition, the kinetics of ion migration were characterized with Auger electron spectroscopy and double-layer polarization using capacitive-frequency measurements, revealing significantly lower hysteresis, halide ion migration and accumulation for the K-incorporated samples during device operation, resulting in substantial improvements in LED performances and stability.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

One-pot synthesis and structural evolution of colloidal cesium lead halide-lead sulfide heterostructure nanocrystals for optoelectronic applications

Heterostructures, combining perovskite nanocrystals (PNC) and chalcogenide quantum dots, could pave a path to optoelectronic device applications by enabling absorption in the near-infrared region, tailorable electronic properties, and stable crystal structures. Ideally, the heterostructure host material requires a similar lattice constant as the guest which is also constrained by the synthesis protocol and materials selectivity. Herein, we present an efficient one-pot hot-injection method to synthesize colloidal all-inorganic cesium lead halide-lead sulfide (CsPbX3 (X = Cl, Br, I)-PbS) heterostructure nanocrystals (HNCs) via the epitaxial growth of the perovskite onto the presynthesized PbS nanocrystals (NCs). Optical and structural characterization evidenced the formation of heterostructures. The embedding of PbS NCs into CsPbX3 perovskite allows the tuning of the absorption and emission from 400 to 1100 nm by tuning the size and composition of perovskite HNCs. The CsPbI3-PbS HNCs show enhanced stability in ambient conditions. The stability, tunable optical properties, and variable band alignments accessible in this system would have implications in the design of novel optoelectronic applications such as light-emitting diodes, photodetectors, photocatalysis, and photovoltaics.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Accepted versionM.J. acknowledges the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award NRF-CRP14-2014-03). We acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy/X-ray facilities. M.J. acknowledge National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1A4A1019455). P.V. acknowledges a Presidential Postdoctoral Fellowship from Nanyang Technological University (NTU), Singapore, via Grant 04INS000581C150. We also acknowledge the NTU Centre of High Field NMR Spectroscopy and Imaging for the use of their NMR facilities. T.C.S. and J.W.M.L. acknowledge the financial support from the Singapore National Research Foundation through the NRF Investigatorship (NRF-NRFI-2018-04) and the Ministry of Education under its AcRF Tier 1 Grant RG91/19 and Tier 2 Grant MOE2019-T2-1-006

Enhanced Photoresponse of Inorganic Cesium Lead Halide Perovskite for Ultrasensitive Photodetector

We report on a simple way to enhance the photoresponse and efficiency of inorganic cesium lead halide (CsPbIBr2) perovskite for use as a light-absorbing layer in photodetectors integrated with a mechanoluminescent (ML) or triboluminescence (TL) materials for pressure sensing applications. Herein, we proposed to integrate a thermal and moisture stable inorganic cesium lead halide-based CsPbIBr2 perovskite with the TL materials to develop a novel pressure sensor for real-time and in-situ structural health monitoring (SHM) of aerospace vehicles’ fuselage, automobiles and structures. However, the inorganic CsPbIBr2 perovskite layer fabricated using a one-step spin-coating method is usually composed of small grain size with a large number of grain boundaries and compositional defects. Therefore, we employed a metal doping approach to enhance the CsPbIBr2 perovskite film quality. By introducing a small amount of silver iodide (AgI), the photoresponse, responsivity, and response time of the detector were enhanced. This work offers a promising approach for developing an integrated ML pressure sensor with high-quality polycrystalline perovskite for SHM