High-efficiency sky blue perovskite light-emitting diodes with ammonium thiocyanate additive

Metal halide perovskite light-emitting diodes have attracted much attention due to their excellent characteristics such as low-cost solution-processing, high luminous efficiency and excellent color purity. However, low luminous efficiency and spectrum stability of blue perovskite light-emitting device restrict the further development of perovskite materials in the field of displays and lighting. Here in this work, we study the effects of ammonium thiocyanate (NH4SCN) addition on the morphology, crystal structure, photo-physics, charge transport and electroluminescence properties of quasi-two-dimensional mixed-halide perovskite films by measuring scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectrum, steady-state photoluminescence (PL), and transient PL and analyzing the current density-voltage characteristics of hole-dominated device and current density-voltage-luminance plots of light-emitting device. The results indicate that ammonium thiocyanate (NH4SCN) can effectively passivate the defects, improve the crystallinity, and modulate the phase distribution of quasi-two-dimensional mixed-halide perovskite film, thereby increasing charge transport and luminescent efficiency. Notably, PL intensity of the 20%-NH4SCN sample is 1.7 times higher than that of the control sample, which is attributed to the defect passivation effect of NH4SCN probably due to the Lewis acid-base interaction with Pb2+. Meanwhile, the hole mobility of the 20%-NH4SCN sample is measured to be 1.31 x 10(-5) cm(2)/(V.s), which is much higher than that of the control sample (3.58 x 10(-6) cm(2)/(V.s)). As a result, sky-blue quasi-two-dimensional mixed-halide perovskite light-emitting diode with 20%-NH4SCN possesses an EL maximum at 486 nm and a maximum external quantum efficiency (EQE) of 5.83% and a luminance of 1258 cd/m(2), which are 6.7 and 3.6 times higher than those of the control device without NH4SCN, respectively. At the same time, the EL spectra of the 20%-NH4SCN device are barely changed under different operating voltages, whereas the EL spectra of the control device show a 7-10 nm red-shift under the same condition, indicating that the NH4SCN addition inhibits halide phase separation and improves the EL spectrum stability. In addition, the T-50 operational life-time of the 20%-NH4SCN device is measured to be about 110 s, which is superior to that of the control device (39 s) due to improved film quality of NH4SCN-modified sample. This research provides a simple and effective method to improve the performances of quasi-two-dimensional mixed-halide perovskite blue-emitting diodes.