14:00 - 14:15ID: 155
TRANSIENT ABSORPTION AND ELECTROLUMINESCENCE REVEAL SPATIAL CARRIER DISTRIBUTION IN QUASI TWO-DIMENSIONAL PEROVSKITE LEDS.
1Center for Physical Sciences and Technology, Vilnius, Lithuania; 2IMEC, Leuven, Belgium; 3Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine
Metal halide perovskites initially emerged as effective materials for solar cells. Currently, they are also attempted to use for other devices such as photodetectors and light emitting diodes, lasers. However, despite many unique advantages such as high luminescence yield, large mobilities, long carrier diffusion lengths, etc. perovskites also have some undesirable features which still limit their development in high efficiency LEDs or lasers. The main critical factors of bulk perovskites is their low exciton binding energy, poor stability under continuous illumination, the presence of carrier traps. The charge trapping influence has been demonstrated to be reduced in perovskite nanocrystals, or by employing hybrid two/three-dimensional (2D/3D) perovskite materials due to the spatial charge carrier confinement. Careful balancing of the electron and hole injection currents was also demonstrated to play important role, particularly at high current densities. However, physical mechanisms responsible for these improvements still remain vague.
Here, by using femtosecond transient absorption and transient electroluminescence (EL) techniques, we address charge carrier dynamics in perovskite LEDs based on quasi two-dimensional (2D) perovskites. In particular we demonstrate that 3D domains present in 2D perovskite rapidly trap carriers of one type, preventing trap-assisted nonradiative SRH recombination in host 2D material. Transient EL investigations revealed complex EL dynamics under device pumping by short electrical pulses. Data interpretation and modelling show that electron and hole spatial distributions within perovskite layer play an important role determining EL efficiency. Weak overlap of electron and hole distributions diminish radiative carrier recombination rate, and are partly responsible for the efficiency roll-off of perovskite LEDs at high current densities. Strong EL pulse was observed after voltage switch-off. This pulse, known as EL overshoot, is an interesting effect demonstrating importance of the spatial carrier distribution.
14:15 - 14:30ID: 173
BRINGING NONLINEAR STIMULATED EMISSION TO THE INFRARED - FROM SAPPHIRE AND FUSED SILICA TO PEROVSKITES
1Department of Physics, University of Cambridge, United Kingdom; 2Department of Physics and Astronomy, Aarhus University, Denmark; 3Institute for Physics and CINSaT, University of Kassel, Germany
While the stimulated emission of light was postulated by Albert Einstein over 100 years ago and has found its way into nearly every laboratory, its nonlinear counterpart has only been observed in a handful of experiments so far. Therefore, it was very surprising, when we recently discovered the nonlinear coherent amplification of an ultraviolet femtosecond laser pulse in a piece of optically excited sapphire (LADIE effect ). The nonlinear (i.e. two-photon) stimulated emission holds high promises for laser technologies, microscopy or laser-spectroscopy as it is inherently nonlinear (temporal and spatial focusing) and provides a different set of selection rules. Here, we present extended studies showing the possibility of switching between two nonlinear amplification processes in fused silica, one being related to the LADIE effect, whereas the other is related to the characteristic and long-living self-trapped exciton states. Temporal and energetic dependencies are measured in an ultrafast pump-probe and spectral-interference microscope.
Furthermore, we discuss our recent experimental studies to expand the nonlinear stimulated emission from the ultraviolet into the infrared regime. To that extent we utilize novel 2D and 3D perovskite materials, which showed promising properties for optoelectronic devices [2,3]. Having band gaps in the visible spectrum around 500 nm and long carrier lifetimes, they are an ideal sample system to probe the two-photon stimulated emission in an ultrafast pump-probe experiment.
 Thomas Winkler et al. Nature Physics volume 14, pages 74–79 (2018)
 Michael B. Price et al. Nature Communications volume 6, 8420 (2015)
 Johannes M. Richter et al. Nature Communications volume 8, 376 (2017)
14:30 - 14:45ID: 163
LIGHT EMISSION FORM STOICHIOMETRICALLY TUNED QUASI TWO-DIMENSIONAL PEROVSKITES
1IMEC, Leuven, Belgium; 2Center for Physical Sciences and Technology, Vilnius, Lithuania; 3Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine
Quasi two-dimensional or layered perovskites are among the benchmark materials for perovskite light emitting diodes (PeLEDs). They demonstrate external quantum efficiency (EQE) at par with the state-of-the-art organic and quantum-dot based LEDs without additional light outcoupling. Despite their remarkable performance, the EQE in PeLEDs drops at modest current densities (J) and their lifetime currently is only a few hours at J ≥ 10 mA/cm2. Various mechanisms such as Auger recombination and Joule heating combined with unbalanced charge injection are suggested as the key factors leading to the EQE drop.[1, 2] In this talk, we provide insights on how the stoichiometry tuning of the perovskite emissive layer together with charge injection balance lead to reduced EQE roll-off and improved device lifetime. We note that the EQE of PeLEDs doubles when the stoichiometry of the perovskite emissive layer is tuned from three-dimensional (3D) to mixed 2D/3D. This also reduces the EQE roll-off: at balanced charge injection conditions, a higher EQE drop is observed for 3D PeLEDs than in 2D/3D PeLEDs. We also demonstrate the importance of charge injection balance to maintain EQE at modest J = 100 mA/cm2 and beyond. Finally, we report that the lifetime of PeLEDs heavily depends on the excess charge carrier concentrations, which we improve from only a few minutes for PeLEDs with dominating electron or hole current, to a remarkable half lifetime beyond 40 h at J = 10 mA/cm2, in optimized PeLEDs with balanced charge injection.
1. Zou, W., et al., Minimising efficiency roll-off in high-brightness perovskite light-emitting diodes. Nat. Commun., 2018. 9(1): p. 608.
2. Kim, K., et al., Hybrid perovskite light emitting diodes under intense electrical excitation. Nat. Commun., 2018. 9(1): p. 4893.
14:45 - 15:00ID: 272
CARBAZOLE-BASED MONOLAYERS AS A HOLE-SELECTIVE CONTACT FOR PEROVSKITE SOLAR CELLS
1Kaunas University of Technology, Kaunas, Lithuania; 2Institute for Silicon Photovoltaics, Helmholtz-Zentrum Berlin, Germany
Perovskite solar cells (PSCs) is one of the fastest developing technology in the field of material science. While the significant advance in terms of power conversion efficiency (PCE) was achieved in the last years, further progress is needed in terms of price and stability. At the current stage of the research, one of the bottleneck materials is organic hole transporting materials (HTMs). Therefore the further improvements are strongly dependent on the development of the new functional materials.
One of the degradation pathways in PSCs is related to the chemical changes, happening in the doped HTM layer. One way to avoid doping is to make layer as thin as possible. Unfortunately, HTM films below 10 nm thickness, prepared by conventional spin-coating technique does not uniformly cover ITO substrate. In our work, we have proposed an alternative approach for the film formation, namely self-assembly of the organic phosphonic acids.
To achieve the desired hole-selectivity we have synthesized three novel carbazole-based materials, bearing with phosphonic acid moiety. These materials were further used for the formation of the hole-selective monolayer via simple dipping procedure.
One of the materials showed superior performance in PSCs, resulting in close to 21% power conversion efficiency. This is on par with the current record efficiency (20.9%; D. Luo et al. 2018), achieved for the inverted PSCs. As the method is dopant-free, initial stability assessment showed superior stability under operational conditions, making this technique promising for further optimization. In addition, our method of the selective layer formation provides several advantages over traditional spin-coated HTMs, namely: minimal material consumption; negligible parasitic absorption; possibility of the conformal coverage on rough substrates.
15:00 - 15:15ID: 113
IMPROVING THE PHOTOLUMINESCENCE QUANTUM YIELDS OF QUANTUM DOT FILMS THROUGH A DONOR/ACCEPTOR SYSTEM FOR NEAR-IR LEDS
1School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand; 2Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
Near-infrared light-emitting diodes (LEDs) show potential for telecommunication and medical applications. Quantum dot nanocrystals (QDs), specifically lead chalcogenides, are candidate LED materials since they exhibit tuneable luminescence across the whole near-infrared region, but their surface structure must be carefully controlled to achieve efficient emission. We demonstrate an efficient donor–acceptor QD system by embedding low-energy QDs with high photoluminescence quantum efficiency (PLQE) into a matrix of higher-energy QDs with lower PLQE. We find that the overall PLQE of densely packed cross-linked QD films can be improved by the incorporation of a relatively small fraction of well-passivated acceptor QDs, also leading to improved LED performance. Excitations are transferred into the isolated low-energy acceptor QDs, where they recombine with high radiative efficiency.
15:15 - 15:30ID: 283
CARRIER RECOMBINATION AND DIFFUSION IN WET-CAST TIN IODIDE PEROVSKITE LAYERS UNDER HIGH INTENSITY PHOTOEXCITATION
1Institute of Photonics and Nanotechnology, Physics Faculty, Vilnius University, Vilnius, Lithuania; 2Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi, Fukuoka, Japan; 3Adachi Molecular Exciton Engineering Project, Japan Science and Technology Agency (JST), ERATO, Nishi, Fukuoka, Japan; 4Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), Fukuoka Industry-Academia Symphonicity (FiaS), Nishi, Fukuoka, Japan; 5International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Nishi, Fukuoka, Japan
Tin iodide perovskite CH3NH3SnI3 is often considered as a replacement for toxic lead halide perovskites. Tin iodide is not only suitable for production of solar cells, but also it emits in the near infrared spectral region, which is unique among the metal halide perovskites. On the downside, the CH3NH3SnI3 layers tend to be of high unintentional p-type doping, which significantly limits the solar cell efficiency. On the other hand, it is little known how this doping could affect other optical and electrical properties, important for light-emitting applications.
Here, we present an optical study of carrier diffusion and recombination pathways by time-resolved photoluminescence, differential transmission, and light induced transient grating techniques at excitations close to the lasing regime. We investigate several CH3NH3SnI3 layers formed by solvent bathing method and using different anti-solvents, causing different structural quality and doping level of the layers.
We observe amplified spontaneous emission with a threshold excitation as low as 5 μJ/cm2; ; however, the threshold is sensitive to structural quality and increases significantly in the layers with larger surface roughness. We present an all-optical method to determine the equilibrium density of holes, which varies in the range of (0.7–5.0)x1018 cm-3, depending on the anti-solvent used for production of a particular layer. Finally, we observe band-like diffusion of carriers with high values of ambipolar diffusion coefficient: it grows from 0.5 to 1.5 cm2/s with excitation due to carrier degeneracy. High diffusivity, large quantum yield even at low densities, and low stimulated emission threshold allow us to argue that unintentional p-type doping can be beneficial for light emitting applications.