Session Chair: Mohammad Khaja Nazeeruddin, École Polytechnique Fédérale de Lausanne Session Chair: Hiroshi Segawa, The University of Tokyo
10:30am - 11:00am Invited
Time-Resolved Optical Studies of Perovskite Polycrystalline Films, Single Crystals and Their Surfaces
Matthew C. BEARD, Ye YANG, Kai ZHU, Joseph LUTHER
National Renewable Energy Laboratory, United States
Lead-iodide perovskites are emerging as promising light absorbers for solution-processed thin-film photovoltaic applications. In addition, these materials are also intensively studied for light-emitting diodes, photodetectors, and lasers. Understanding their photophysical properties is crucial for the better design and utilization of these applications. We have used time-resolved transient spectroscopies to better understand both bulk carrier dynamical processes as well as the surface carrier dynamics. We employed transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite single crystals and polycrystalline films. We find that the surface recombination velocity (SRV) in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, likely due to unintended surface passivation of the films during synthesis. In spite of the low SRV, surface recombination limits the total carrier lifetime in polycrystalline thin films, meaning that recombination inside grains and at grain boundaries is less important than top and bottom surface recombination. The suppressed SRV in the polycrystalline films appears to be related to an excess of methylammonium compared to the single crystals surfaces, determined by X-ray photoelectron spectroscopy analysis.
11:00am - 11:30am Invited
Next Generation LEDs: Organometal Perovskite Light-Emitting Diodes
1Seoul National University, South Korea; 2Pohang University of Science and Technology (POSTECH), South Korea
Organometal halide perovskites are emerging high color-purity emitters with low material cost. However, the low electroluminescence (EL) efficiency at room temperature is a challenge that should be overcome. Here, we present efficient perovskite light-emitting diodes (PeLEDs) using various strategies to overcome the EL efficiency limitations. First, we introduced a self-organized buffer hole injection layer to reduce the hole injection barrier and block the exciton quenching at the interface. Furthermore, we found that the formation of metallic lead atoms causes strong exciton quenching, and it was prevented by finely increasing the molar proportion of methylammonium bromide (MABr) in MAPbBr3 solution. Also, we suggest that the efficiency in PeLEDs can be increased by decreasing MAPbBr3 grain sizes and consequently improving uniformity and coverage of MAPbBr3 layers. Using these strategies, a high-efficiency PeLEDs was realized (current efficiency = 42.9 cd/A). Furthermore, quasi-2D perovskites were studied because of the advantages of quasi-2D perovskites such as the enhancement of film quality, exciton confinement and reduced trap density, and quasi-2D PeLEDs with high efficiency and brightness were demonstrated.
11:30am - 11:45am Oral
Organic Semiconductor-MAPbBr3 Blends for Light Emission
Giulia LONGO, Laura MARTINEZ-SARTI, Maria-Grazia LA-PLACA, Pablo P. BOIX, Michele SESSOLO, Henk J. BOLINK
University of Valencia, Spain
The unique properties of hybrid (organic-inorganic) perovskites like high absorption coefficient, high carrier mobility and long range carrier diffusion make them one of the most promising material for photovoltaic applications. The efficiency of perovskite solar cells improved over the last years, reaching values comparable to silicon photovoltaics. Recently, hybrid perovskites such as methylammonium lead bromide (MAPbBr3 ) have emerged as potential candidate for future electroluminescent devices. In general, for light emitting diodes, high efficiency are accompanied with high photoluminescence quantum yield (PLQY) of the emitting layer. Unfortunately, when the pure MAPbBr3 is prepared by simple solution methods (single step spin-coating of the precursor solution), its PLQY is rather limited, at least if measured at low excitation intensity. In analogy to inorganic semiconductors, the most promising strategy to promote radiative recombination is the spatial confinement of the charge carriers. In this work we present how the addition of organic semiconductor small molecules, acting as templating materials, can improve the emissive capability of methylammonium lead bromide perovskite. The structural and optical properties, together with the application of such materials in LEDs, will be discussed.