Perovskite LEDs with an external quantum efficiency exceeding 25%

We design and synthesize a bifunctional molecular additive to fabricate reduced-dimensional perovskites with a more monodispersed quantum well thickness distribution and passivated surfaces. We report as a result bright perovskite LEDs with narrowband emission and a high EQE of 25.6%.
Published in Materials
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Dongxin Ma, Postdoctoral fellow, University of Toronto

Written by Dongxin Ma, Edward H. Sargent.

Metal-halide perovskites hold promise in solar cell, laser, photodetector, and light emission applications. They combine structural diversity, tunable bandgap, saturated emission colours, superior luminescence efficiencies, low-cost solution-processing, and high charge mobilities. Reduced-dimensional perovskites (RDPs) consisting of quantum wells (QWs) separated by organic intercalating cations exhibit high exciton binding energies and have the potential to increase stability and photoluminescence quantum yield (PLQY); however, until now, RDP-based LEDs have exhibited external quantum efficiency (EQE) < 22% and inferior colour purities.

 We posited that the presence of variably-confined QWs may contribute to non-radiative recombination losses and broadened emission.

 We explored therefore the goal of more monodispersed populations of QWs. We found that using a bifunctional molecular additive simultaneously controlled RDP polydispersity while passivating perovskite QW surfaces. We report particularly bright and narrowband-emitting materials achieved using this new strategy.

 We developed the molecular additive tris(4-fluorophenyl)phosphine oxide (TFPPO), and studied for comparison its non-fluorinated cousin, triphenylphosphine oxide (TPPO). We found that the phosphine oxide moiety in both TFPPO and TPPO passivated perovskite grain boundaries via coordination bonding with unsaturated sites, suppressing defect formation; and that the fluorine atom in TFPPO further hydrogen-bonded with the organic cations, controlling their diffusion during RDP film deposition. This suppresses the formation of low-thickness QWs (Figure 1).

Figure 1. Distribution control strategy. During antisolvent-induced crystallization, [PbBr6]4- nuclei, MA, and Cs+ cations assemble to form the perovskite flakes, allowing the PEA organic cations to diffuse and enter the extended perovskite lattice, thereby forming RDPs. In the control case, rapid diffusion of PEA leads to QW polydispersity. In the case of crystallization with additives in the antisolvent, the P=O groups of TPPO and TFPPO bind to the perovskite flakes via P=O:Pb2+, limiting their diffusion and passivating the perovskite grain boundaries. In the case of TFPPO-assisted crystallization, the fluorine atoms bind to PEA via hydrogen bonds, limiting their diffusion and promoting the formation of RDPs with monodispersed QWs.

The resultant RDP thin films show narrowband emission and high PLQY. This enables us to fabricate LEDs exhibiting a high EQE of 25.6% with an average of (22.1±1.2)% over 40 devices, as well an operating half-life of two hours at an initial luminance of 7,200 cd m-2 (Figure 2). Taken together, the results provide tenfold-enhanced operating stability relative to the best prior perovskite LEDs having EQE exceeding 20%.

Figure 2. LED performance. a, Device structure, cross-sectional TEM image and energy band diagram. b, Current density versus voltage, luminance versus voltage, and EQE versus luminance curves; c, EQE histogram of 40 devices based on control, TPPO-treated and TFPPO-treated RDPs. d, Current density versus voltage curves of hole- and electron-only devices based on control, TPPO-treated and TFPPO-treated RDPs. e, Operating stability of LEDs based on TFPPO-treated RDPs.

For more details, please check out our paper “Distribution control enables efficient reduced-dimensional perovskite LEDs” in Nature (https://www.nature.com/articles/s41586-021-03997-z).

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