White PeLED employing a mixed emission layer composed of a small organic molecule and an organic-inorganic hybrid perovskite

Abstract

Organic-inorganic hybrid perovskites are classes of direct-bandgap semiconductors that enjoy outstanding optoelectric properties and feasible solution processing. Besides being demonstrated as a new class of photovoltaic materials for next-generation (solar cell with efficiency>20%), the organic-inorganic hybrid perovskites are showing great potential in lightings and displays because of their high brightness and tunable color across the entire visible range. Since Chondroudis and Mitzi first reported the room-temperature electroluminescence from organic-inorganic hybrid perovskites, remarkable progresses have been made in organic-inorganic hybrid perovskite electroluminescent devices (PeLED), including reducing the grain size of polycrystalline perovskite, improving the morphologies of perovskite film and bipolar charge injection into the emissive layer. Notably, electroluminescence (EL) efficiencies of organic-inorganic hybrid perovskites have been recently boosted to 42.9 cd/A by Cho et al . via a nanocrystal pinning method, getting close to the level of organic light-emitting diodes (OLED). Despite these advantages and the potential described above, the studies were mostly concerned in “green” PeLED based on CH3NH3PbBr3 or CsPbBr3 and there are few reports on white PeLED. Because white light plays most important role in daily life’s lightings and displays, the developments of white PeLED should have significant values in both scientific research and commercial applications. In this work, a white PeLED employing a mixed emission layer composed of a small organic molecule of 1,3-bis(9-carbazolyl) benzene (mCP) and an organic-inorganic hybrid perovskite of CH3NH3PbBr3 was newly reported. Thus, the merits of good processability of organic small molecules and the excellent charge transportation properties of organic-inorganic hybrid perovskites can be well combined. The white PeLED were fabricated as follows: firstly, uniform and dense mCP:CH3NH3PbBr3 crystal film was prepared through one-step spin-coating techniques, resulting in relatively higher film coverage than that of pure CH3NH3PbBr3 film. Secondly, white PeLED composed of such mCP:CH3NH3PbBr3 mixed film was fabricated with the structure of ITO/PEDOT:PSS/mCP:CH3NH3PbBr3/tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBI)/LiF/Al through thermal evaporation, where the ITO was functionalized as the anode, the PEDOT:PSS was used as the hole injection layer, mCP:CH3NH3PbBr3 was emissive layer, TPBI was employed as electron transporting layer and hole blocking layer, and LiF/Al was the electron injection layer/cathode. Under electrical excitation, EL spectra show that the emissions of white PeLED were composed of RGB lights, i.e., deep-blue light (peak around 380 nm) coming from mCP molecules, green light (peak around 520 nm) coming from CH3NH3PbBr3 perovskites, and deep-red light (peak around 780 nm) coming from exciplex between mCP molecules and CH3NH3PbBr3 perovskites. The turn-on voltage of white PeLED was ~3.2 V, the maximum luminance was~170 cd/m2, and the EL efficiency was ~0.15 cd/A. Moreover, the recombination zone can be effectively broadened with increasing the driving voltage, which tunes the color of white PeLED. At a driving voltage of 7 V, we got white emission with CIE coordinated at (0.32, 0.31 ), indicating the white light was ideal. The results indicate that our white PeLED has already met the basical requirements of industry, and may provide a route for more efficient and stable white PeLED in the future.