It is a remarkable advantage of organic-inorganic hybrid perovskite that their bandgap can be tuned by adjusting the stoichiometric ratio of halides, which is critical to their applications in tandem solar cells, wavelength-tunable light emitting diodes (LEDs) and laser. However, illumination or applied bias will cause the phase separation of mixed-halide perovskite, resulting in the formation of iodide-rich domains, the reduction of photovoltaic devices open-circuit voltage, and the emission wavelength of luminous LED being pinned to the infrared region. The phase separation is suppressed by covering self-assembled long-chain organic ammonium capping layers on nanometer-sized grain surfaces, and the LED with stable emission wavelength is obtained with external quantum efficiency being improved to 2%.