Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br)${}_3$ and indirect band gap Cs${}_4$Pb(Cl,Br)${}_6$ phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br)${}_3$ grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 10${}^6$ V·m${}^{-1}$ electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m${}^{-2}$, and turn-on bias of 2.5 V.

Perovskite light-emitting diodes (pero-LEDs) is a rapidly developing technology that is supposed to supersede existing ones in the near future. In comparison with organic and A ^III B ^V analogues, pero-LEDs possess the following advantages: very narrow spectral linewidth of electroluminescence (EL), spectral tunability in the whole visible range and the possibility of a cost-effective large-scale fabrication by means of wet chemistry techniques. CsPbX₃ (X = Cl, Br, I) are the most robust perovskites suitable for LEDs production due to their excellent optical properties. There are numerous reports describing green and red electroluminescence of such tribromide and triiodide materials, respectively, whereas a blue color is not easy to achieve. The main obstacles in the way of development of blue pero-LEDs based on chlorine salts are poor solubility of perovskite precursors in the same organic solvents as well as light- and an electric field-induced phase instability of mixed-halide (CsPbBr_3−x Cl _x ) materials. The latter leads to red-shift of EL spectrum with the increase in applied voltage. In this work, we present a design of a single-layer sky-blue pero-LED based on CsPbBr₂ Cl-poly(ethylene oxide) (PEO) thin film, study the morphology of the emissive layer, its phase instability under UV illumination and in the electric field.