Hideo Hosono, Tokyo Institute of Technology
Transparent Oxide Semiconductors for Contact in OLEDs and Halide Perovskite LEDs
Written by Vignesh Murugadoss, Korea University
One of the key issues in light-emitting diodes (LEDs) is electron injection from the electrode to the active layer because electron affinity of luminous organic materials and halide perovskites are smaller than the work function of conventional electrode materials such as indium-doped tin oxide (ITO), aluminum (Al), and zinc oxide (ZnO). Hence it is critical to reducing the electron injection layer to enhance the performance of LEDs. The widely used combination of LiF and Al used to reduce the electron injection barrier limits the device structure to normal stacking. Other challenges are to achieve high electron mobility, strong color and chemical stability. Low electron mobility of materials results in a thin layer which leads to the low yield by the current leakage. Hideo Hosono and his team developed different strategies to overcome these challenges, which are highlighted in this talk. To solve the problem with work function and electron mobility, two new materials, amorphous C12A7 electride (12 CaO.7Al2O3:e-) and amorphous zinc silicate (ZSO) [ZnO:SiO2 (8:2)] are developed as electron injection layers. As the processing temperature of amorphous C12A7 is greater than 1000oC, it is deposited using a sputtering target. The sputtered amorphous C12A7 exhibited work function of 3.1eV (comparable to Li and Ca) along with striking properties that are optically transparent and chemically stable. Another advantage of this material is its amorphous nature so that its top layer is flat. This feature is very favorable for its application as an electron injection layer in organic light-emitting diodes (OLEDs). The sputtered thin film of amorphous ZSO exhibited a work function of 3.5 eV which is comparatively smaller than convention transparent electrodes and metals. The addition of SiO2 induces the amorphous nature in ZnO. Furthermore, the electron mobility of amorphous ZSO (~1 cm2/Vs) is comparatively higher than that of Alq3 (10-6 cm2/Vs). In amorphous ZSO, ZnO is confined by the thin zinc silicate outer layer and ZnO is connected by electron hopping. In other words, they are optically confined but electrically connected. The unique feature of amorphous ZSO is that it forms an ohmic contact with metal irrespective of work function. Amorphous ZSO established ohmic contact in inverted type OLED devices whose electron injection barrier is found to be lower than the devices fabricated with LiF and Al. Tandem OLED fabricated with amorphous ZSO exhibited comparatively lesser voltage loss of 0.4 V at 100 mA/cm2 than other devices. Furthermore, to obtain high efficient luminescence, a strategy to confine electrons and holes using the electron transport layer and hole transport layer is adopted. The (CsPbBr3) perovskite-based LED demonstrated that employing amorphous ZSO improved the luminance tremendously compared to the devices employing ZnO. Thus better electron injection/transport layers are needed for improved device performance.