Halide perovskite composites enable next-generation fully printable LEDs

Organometal halide perovskites (‘peros’) are materials with an ABX₃ crystal structure resembling that commonly found in barium titanate (BaTiO₃). More specifically, in peros, A is a cesium (Cs⁺) or an aliphatic ammonium (RNH₃⁺) cation, B is a divalent cation of lead (Pb²⁺) or tin (Sn²⁺), and X is an anion such as chloride (Cl⁻), bromide (Br⁻), or iodide (I⁻). Peros have recently been discovered to have remarkable optoelectronic properties, eliciting research into their potential as photovoltaic and light-emitting devices.¹

Pero LEDs, in particular, have the potential to revolutionize the area of flexible lighting, heads-up displays, and wearable electronics because they may be directly printed on flexible substrates. In fact, peros can be processed like polymeric semiconductor materials, using solution processes to form large-area thin films on arbitrary substrates, while still crystallizing into three-dimensional periodic structures such as those of conventional inorganic semiconductors, with superior charge transportation properties.

In the past few years, research into peros has chiefly focused on high-efficiency solar cells that employ peros as light absorbers.^{2, 3} Power conversion efficiencies of more than 21% have been achieved,⁴ which is on par with the best thin-film solar cells based on copper indium gallium selenide and cadmium telluride, and approaches the efficiency of single-crystal silicon devices. Remarkably, the open-circuit voltage (V_oc) loss of the best-performing pero solar cells^{5, 6} is comparable to that of the best single-junction GaAs solar cells, indicating close to 100% internal photoluminescence efficiency in peros,⁷ showing their potential for LED applications. Green and red pero LEDs were reported in August 2014,⁸ and within a two-year period emission spectra were expanded to cover both the blue and near-IR regions. The maximum external quantum efficiency attained is about 8% for green pero LEDs⁹ and 3.5% for near-IR devices.¹⁰

Most reported pero LEDs have a multilayer device architecture comprising, at a minimum, a hole-injection layer (HIL) next to the anode, an electron-injection layer (EIL) next to the cathode, and the pero emissive layer. Such a multilayer approach is well established in all current LED technologies, including organic LEDs (OLEDs) based on organic small molecular semiconductors or conjugated polymers, and quantum-dot LEDs. The use of an HIL and EIL has been shown to lower the hole/electron injection barriers, resulting in low operating voltage and high electroluminescence efficiency in OLEDs and quantum-dot LEDs. However, it also greatly complicates the fabrication process, and as a result, none of those LEDs are fully printable.

Recently, we discovered that single-layer pero LEDs using a composite thin film of pero and poly(ethylene oxide) (PEO) can efficiently generate light without using an EIL or HIL.¹¹ Our first paper, published in August 2015, presented a device structure resembling that in Figure 1, comprising an indium tin oxide (ITO) bottom electrode, a pero/PEO composite, and an indium/gallium or gold top electrode. Despite this simple single-layer device structure, the green emission LEDs with methylammonium lead tribromide and PEO composite thin films exhibited good performance (a relatively low turn-on voltage of ∼2.8V, and a maximum luminance of 4064cd m⁻²). This is on par with many reported results at that time involving more complex multilayer device structures.

Figure 1. Single-layer organometal halide perovskite (pero) LEDs with a pero/poly(ethylene oxide) composite (0.5:1, 0.75:1, and 1:1 weight ratios) thin film as the emissive layer, indium tin oxide (ITO) as the anode, and a mixture of indium (In) and gallium (Ga) as the cathode.

It has been reported that pure pero films decompose in humid air and lose their structural integrity and luminous characteristics.^{12, 13} However, by forming a pero/polymer composite, we were able to obtain an emissive film that maintained the desired morphology and crystallinity even in high humidity environments. This makes the composite ideal for ambient air printing processes. We further demonstrated printed pero LEDs in ambient conditions with a humidity level at 70–85%.¹⁴ The fabricated devices showed good emission uniformity and promising efficiencies on both rigid glass and flexible plastic substrates (see Figure 2).

Figure 2. (a) Schematic of the pero-LED printing process carried out in ambient air. (b) A printed pero LED device on an ITO/glass substrate. The device had a current efficiency of ∼5cd A⁻¹ at 20,000cd m⁻². (c) The letters F, S, and U were formed using different stencil masks during top electrode printing. (d) A printed device on a flexible polyethylene terephthalate (PET) substrate. The device had 0.6cd A⁻¹ at 360cd m⁻².

In summary, we developed a novel fabrication process that allows pero LED devices, which can be printed on flexible substrates, to be easily manufactured in ambient conditions. In addition, our pero LEDs promise to be cheaper to produce than existing LED technologies, due to their extremely simple structure and undemanding processing conditions, while still maintaining high efficiency. We plan to follow up this work by further increasing the efficiency and brightness of the printed perovskite LEDs, and adopting a large-scale printing process to fabricate large-size LED panels.