Understanding the 2D/3D Heterostructure Interface in Highly Efficient Green PeLEDs

Light emitting diodes (LEDs) based on metal halide perovskite semiconductors have attracted considerable research interest due to their potential for low cost, high efficiency LEDs with excellent colour purity and bandwidth. In particular, two dimensional (2D) - three dimensional (3D) hybrid perovskite heterostructures (often described as quasi-2D structures) have been subject to considerable investigation within the perovskite LED community as one route through which to produce highly efficient devices.^1,2 It has been shown that in these heterostructures the emission wavelength and device performance can be informed by the organic cation used to form the 2D perovskite structure.³ To ensure the optimal continued development of optoelectronic devices based on these structures it is essential that we have a comprehensive understanding of the 2D/3D interface structure and the charge transport across this interface.<br/><br/>To probe this interface we develop a range of quasi-2D perovskite emitting layers, based on a (BABr)_x(CsPbBr₃)_1-x (where BA = butylammonium) structure with varying amounts of the 2D perovskite. We implement these emitters in PeLED devices and demonstrate excellent electroluminescence external quantum efficiencies &gt; 16 %. We observe a strong relationship between the 2D content of these heterostructures and the efficiency of the corresponding PeLEDs. We investigate the structure of the 2D-3D heterostructures using X-ray diffraction and observe a mixture of n=1 and n=2 2D-perovskite structures in addition to the 3D perovskite. We comprehensively study the charge carrier dynamics of a range of (BABr)_x(CsPbBr₃)_1-xstructures using absolute and time-resolved photoluminescence studies. We find that the 2D perovskite only influences the emission profile of our LEDs in specific cases and that<br/>the predominant role of the 2D perovskite structure is to confine the size of the 3D perovskite domains and therefore boost the absolute electro- and photo-luminescence of the 3D perovskite domains.<br/><br/>To understand the charge transport across the interface between the 2D and 3D perovskite structures we investigate the energetic band alignment using a combination of ultraviolet photoelectron spectroscopy studies (UPS) and density functional theory (DFT) modelling. We demonstrate that charge transfer between the 2D and 3D perovskite structures is determined by the type of 2D structure (whether the structure is n=1 or n=2) and we suggest a route to predicting whether a 2D structure will contribute to the emission of an LED. We show that there is a critical 2D/3D compositional limit for achieving high efficiency devices. Whilst we do not find a mixture of 2D perovskite structures impacts LED efficiency, we show that there is a tight tolerance window for the n=1 perovskite structure and once this window is exceeded it has a large detrimental effect on device performance.<br/><br/>Our work highlights the importance of understanding the 2D/3D heterostructure interface in engineering highly efficient devices and we show our initial results exploring the impact of phase pure 2D/3D perovskite heterostructures on PeLED performance.<br/><br/>1. Liu, Y. <i>et al.</i> Boosting the efficiency of quasi-2D perovskites light-emitting diodes by using encapsulation growth method. <i>Nano Energy</i> <b>80</b>, 105511–105511 (2021).<br/>2. Fakharuddin, A. <i>et al.</i> Reduced Efficiency Roll Off and Improved Stability of Mixed 2D/3D Perovskite Light Emitting Diodes by Balancing Charge Injection. <i>Advanced Functional Materials</i> <b>29</b>, 1904101 (2019).<br/>3. Warby, J. H. <i>et al.</i> Revealing Factors Influencing the Operational Stability of Perovskite Light-Emitting Diodes. <i>ACS Nano</i> <b>14</b>, 8855–8865 (2020).