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We demonstrate a bicoloured metal halide perovskite (MHP) light emitting diode (LED) fabricated in two sequential inkjet printing steps. By adjusting the printing parameters, we selectively and deliberately redissolve and recrystallize the first printed emissive layer to add a pattern emitting in a different color. The red light emitting features (on a green light emitting background) have a minimum size of 100 μm and originate from iodide-rich domains in a phase-segregated, mixed MHP. This phase forms between the first layer, a bromide-based MHP, which is partially dissolved by printing, and the second layer, an iodide-containing MHP. With an optimised printing process we can retain the active layer integrity and fabricate bicolour, large area MHP-based LEDs with up to 1600 mm2active area. The two emission peaks at 535 nm and 710 nm are well separated and produce a strong visual contrast.

Solution-processable metal halide perovskites are increasingly implemented in perovskite-based light-emitting diodes (PeLEDs). Especially green PeLEDs based on methylammonium lead bromide (MAPbBr3) composites exhibit impressive optoelectronic properties, while allowing processing by low-cost and upscalable printing methods. In this study, we have investigated the influence of potassium chloride (KCl) blended into the common hole injection material poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to boost PeLED device performance. The inclusion of KCl firstly results in a change in morphology of the PEDOT:PSS layer, which then acts as a template during deposition of the perovskite layer. A MAPbBr3:polyethylene glycol (PEG) composite was used, which does not require the deposition of an anti-solvent droplet to induce preferential perovskite crystallization and is therefore suitable for spin coating and scalable inkjet printing processes. PeLEDs utilizing the KCl induced templating effect on a planar PEDOT:PSS/MAPbBr3:PEG architecture show improved performance, predominantly due to improved crystallization. PeLEDs incorporating spin-coated perovskite layers yield a 40-fold increase in luminance (8000 cd m−2) while the turn-on voltage decreases to 2.5 V. KCl-modified PEDOT:PSS contact layers enabled the realization of inkjet-printed PeLEDs with luminance increased by a factor of 20 at a maximum of 4000 cd m−2and a turn-on voltage of 2.5 V. This work paves the way for inkjet-printed perovskite light-emitting devices for a wide variety of low-cost and customizable applications.