The widespread adoption of solution-processed halide perovskites LEDs demands to surpass the luminous efficacies of conventional LEDs while having full control over the emission properties.
In this talk, we report directional and enhanced emission in a green perovskite LED through hybrid photonic-plasmonic modes. We employ advanced optical modelling powered by genetic algorithms to find the best combination of materials and structures compatible with the fabrication of efficient LEDs. The experimental realisation of the optimum designs allows us to show low-temperature processed devices with enhanced emission and fine control over the emission angle. This methodology is versatile and easily integrable in cost-effective LEDs across the whole visible spectrum, opening avenues for their application in displays and light sources where the angular dispersion of light is crucial.
Tiarnan Doherty, Sam Stranks, Andrew Winchester, Stuart Macpherson, Sofiia Kosar, Duncan Johnstone, Felix Kosasih, Aron Walsh, Paul Midgley, Keshav Dani
Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices. Though widely considered defect tolerant materials, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states, which create local variations in photoluminescence that fundamentally limit device performance. These trap states have also been associated with light-induced halide segregation in mixed halide perovskite compositions and local strain, both of which can detrimentally impact device stability5. Understanding the nature of these traps will be critical to ultimately eliminate losses and yield devices operating at their theoretical performance limits with optimal stability.
In this talk we outline the distribution and compositional and structural origins of non-radiative recombination sites in (Cs0.05FA0.78MA0.17)Pb(I0.83Br0.17)3 thin films (Doherty, Winchester, et al., Nature, 2020). By combining scanning electron and synchrotron X-Ray microscopy techniques with photoemission electron microscopy (PEEM) measurements we reveal that nanoscale trap clusters are distributed non-homogenously across the surface of high performing perovskite films and that there are distinct structural and compositional fingerprints associated with the generation of these detrimental sites. Finally, we will show how combining our scanning electron diffraction with convolutional neural networks can enable low-dose (~6 e/Å2), high-resolution (4nm) automated structural phase identification in beam sensitive thin-film perovskites. This nanoscale insight will help answer ongoing open questions in the field such as “What are the nanoscale origins of instability in perovskite devices?”, “how important is phase purity for performance?”
Hybrid perovskites have emerged as exceptional semiconductors for optoelectronic applications. Here, we control the cation alloying to push optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photo-excited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge-carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale-mapping of the optical bandgap and elemental composition, electrically-gated transport measurements and first-principles calculations, we demonstrate that spatially-varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photo-doped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and LEDs.
Organic-inorganic perovskites such as CH3NH3PbI3 are highly promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21% and promising applications in light-emitting diodes, lasers and photodetectors also emerging. A key enabling property of the perovskites is their high photoluminescence quantum efficiency, suggesting that these materials could in principle approach the thermodynamic device efficiency limits in which all recombination is radiative. However, non-radiative recombination sites are present which vary heterogeneously from grain to grain and limit device performance.
Here, I will present results where we probe the local photophysics of neat CH3NH3PbI3 perovskite films using confocal photoluminescence (PL) measurements and correlate the observations with the local chemistry of the grains using energy-dispersive X-ray spectroscopy (EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We investigate the connection between grains that are bright or dark in emission and the local Pb:I ratios at the surface and through the grains. We also examine how the photophysics, local chemistry and non-radiative decay pathways change slowly over time under illumination. Our results reveal a “photo-induced cleaning” arising from a redistribution of iodide content in the films, giving strong evidence for photo-induced ion migration. These slow transient effects appear to be related to anomalous hysteresis phenomena observed in full solar cells. I will discuss how immobilizing ions, reducing trap densities and achieving homogenous stoichiometries could suppress hysteresis effects and lead to devices approaching the efficiency limits.
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