Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping

In recent years, halide perovskite materials have been used to make high-performance solar cells and light-emitting devices. However, material defects still limit device performance and stability. Here, synchrotron-based Bragg coherent diffraction imaging is used to visualize nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. Significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals is found in spite of their high optoelectronic quality, and both 〈100〉 and 〈110〉 edge dislocations are identified through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, dramatic light-induced dislocation migration across hundreds of nanometers is uncovered. Further, by selectively studying crystals that are damaged by the X-ray beam, large dislocation densities and increased nanoscale strains are correlated with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. These results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability.

Lecithin Capping Ligands Enable Ultrastable Perovskite-Phase CsPbI3 Quantum Dots for Rec. 2020 Bright-Red Light-Emitting Diodes

Bright-red light-emitting diodes (LEDs) with a narrow emission line width that emit between 620 and 635 nm are needed to meet the latest industry color standard for wide color gamut displays, Rec. 2020. CsPbI₃ perovskite quantum dots (QDs) are one of the few known materials that are ideally suited to meet these criteria. Unfortunately, CsPbI₃ perovskite QDs are prone to transform into a non-red-emitting phase and are subject to further degradation mechanisms when their luminescence wavelength is tuned to match that of the Rec. 2020 standard. Here, we show that zwitterionic lecithin ligands can stabilize the perovskite phase of CsPbI₃ QDs for long periods in air for at least 6 months compared to a few days for control samples. LEDs fabricated with our ultrastable lecithin-capped CsPbI₃ QDs exhibit an external quantum efficiency (EQE) of 7.1% for electroluminescence centered at 634 nm─a record for all-inorganic perovskite nanocrystals in Rec. 2020 red. Our devices achieve a maximum luminance of 1391 cd/m² at 7.5 V, and their operational half-life is 33 min (T₅₀) at 200 cd/m²─a 10-fold enhancement compared to control samples. Density functional theory results suggest that the surface strain in CsPbI₃ QDs capped with the conventional ligands, oleic acid and oleylamine, contributes to the instability of the perovskite structural phase. On the other hand, lecithin binding induces virtually no surface strain and shows a stronger binding tendency for the CsPbI₃ surface. Our study highlights the tremendous potential of zwitterionic ligands in stabilizing the perovskite phase and particle size of CsPbI₃ QDs for various optoelectronic applications.

Size-controlled CdSe quantum dots to boost light harvesting capability and stability of perovskite photovoltaic cells

Here, we report that incorporation of size-controlled CdSe quantum dots (QDs) into perovskite photovoltaic cells (PSCs) boosts their light harvesting capability. X-ray photoemission and optical absorption spectroscopy analyses also show that the electronic structure of CdSe QDs makes them efficient charge transfer mediators between perovskite and Spiro-MeOTAD layers. In addition, electrochemical impedance spectroscopy experiments demonstrate that QDs help to decrease charge transfer resistance at the interfaces. Additionally, time-correlated single photon counting measurements show that small (2 nm) QDs enhance visible light collection of PSCs in the short wavelength region via Förster resonance energy transfer while large (4 nm) QDs improve light collection of PSCs in the long wavelength region via enhanced light backscattering at the perovskite/QD interface. Moreover, the photocurrent density in the PSCs with QDs retained over 95% of the initial value in a 100 h stability test, thus supporting that the perovskite layer that has been encapsulated with QDs acts to prevent penetration of water molecules through the perovskite layer. Consequently, these results support that utilization of size-controlled hybrid QDs could open up a new route to realize high-performance PSCs even under humid conditions.