Metal-Halide Perovskites: Emerging Light-Emitting Materials

Perovskite LEDs are an emerging display technology with potential for low-cost manufacturing (from abundant materials), high light quality, and energy efficiency.  These features make perovskites competitive with light-emitting materials such as organic molecules and quantum dots.

by Lianfeng Zhao and Barry P. Rand

THE desire for high color-quality, low-cost, and long-lasting lighting and display technology has driven intense research and development of better light-emitting materials and devices. Among the materials under investigation, metal-halide perovskites are emerging as a promising option.

Metal-halide perovskites are direct bandgap, defect-tolerant semiconductors, making them promising materials for optical devices such as light-emitting diodes (LEDs), lasers, solar cells, and photodetectors. A schematic for a metal-halide perovskite appears in Fig. 1. Given their direct bandgap, perovskites emit nearly monochromatic light, either under optical excitation (photoluminescence) or when electrically excited (electroluminescence), with the color of the emitted light determined by the materials’ bandgap energy. Due to their defect-tolerant properties, metal-halide perovskites can be extremely efficient in electroluminescence and photoluminescence; thus, they can be used as efficient light emitters or downconverting phosphors.

Fig. 1:  Metal-halide perovskites are a class of materials with an ABX₃ stoichiometry and crystal structure, where A is a monovalent cation (e.g., Cs⁺ or CH₃NH₃⁺), B a divalent metal cation (e.g., Pb²⁺ or Sn²⁺), and X a halide (Cl^–, Br^–, I^–). This diagram depicts a perovskite crystal structure in schematic format.

Correspondingly, perovskites can be used in two different configurations, as shown in Fig. 2. In the first (Fig. 2a), they are configured as red, green, blue (RGB) systems, in which multiple monochromatic-perovskite light-emitting diodes (e.g., blue, green, and red) are used as individual pixels for displays or mixed to generate white light for lighting. In Fig. 2b, the perovskites are configured for wavelength conversion through excitation by an external light source (e.g., a blue LED). The perovskites then downconvert incident light to other colors such as green and red. For the RGB systems, either perovskite thin films or colloidal perovskite nanocrystals can be used. Currently, thin-film perovskites show better performance in general compared to their colloidal counterparts. However, for the purpose of optical downconversion, colloidal perovskite nanocrystals continue to be more applicable in comparison to thin films. Preparation techniques for colloidal perovskite nanocrystals and thin films will be discussed in the next section.

Fig. 2:  In these two example configurations of perovskites in LEDs, perovskites operate as active monochromatic light-emitting layers in (a) and as optical downconversion phosphors in (b).

Solution Processing of Perovskites

One important potential advantage of perovskite LEDs over other inorganic or organic LEDs is their low cost. Perovskites can be synthesized and processed from solution at relatively low temperatures. This means perovskites are suitable for large-area deposition on a wide range of substrates such as glass or flexible plastic substrates. Perovskite preparation techniques, in general, can be divided into two categories, leading to two different product formats: colloidal perovskite nanocrystals or perovskite thin films. See Fig. 3 for a summary of several common methods for perovskite colloidal nanocrystals synthesis and thin-film deposition.

Fig. 3:  Examples of various methods applied to perovskite processing include (a) representative colloidal perovskite-nanocrystal synthesis methods (reprinted with permission,⁷ copyright 2018, American Chemical Society) and (b) representative perovskite thin-film deposition methods (reprinted with permission,⁸ copyright 2011, Royal Society of Chemistry).

Direct synthesis of colloidal perovskite nanocrystals typically occurs via ligand-assisted reprecipitation or hot injection.¹ For ligand-assisted reprecipitation, perovskite precursors (e.g., lead halide and organic halide salts) are dissolved in strong polar solvents and subsequently added to nonpolar solvents in the presence of ligands, often in large excess. The different solubility in polar and nonpolar solvents triggers the recrystallization of perovskite nanocrystals and the ligands prevent them from aggregating in bulky forms that would inhibit their ability to be suspended in a solution. In the hot-injection method, organic or cesium cations and metal cations are dissolved in nonpolar solvents and heated up to a desirable temperature (typically below 200 °C). Then halide precursors are injected to trigger the nucleation and growth of perovskite nanocrystals.

The process of ligand-assisted reprecipitation is simpler and often performed at lower temperatures in air, whereas the hot-injection method requires air-free conditions and a fine control of the reaction temperatures due to the sensitivity of the colloidal crystal size to reaction temperature. However, one major advantage of the hot-injection approach is an absence of polar solvents, which prevents any possible dissolution or decomposition of the formed perovskite nanocrystals. The hot-injection method is currently the most popular strategy, as it provides effective control over the size of the nanocrystals and their distribution.

As for perovskite thin-film deposition, common solution-processing methods are compatible with perovskite precursors, such as chemical bath, spincoating, dip coating, doctor blade coating, metering rod, slot-casting, spray coating, screen printing, inkjet printing, and aerosol jet printing.² Nonsolvent processing methods such as thermal evaporation are also available for perovskite thin-film deposition.³ For high-performance perovskite LEDs, ultrathin and pinhole-free perovskite films are desirable. Notably, even in the form of thin films, special techniques are required to make properly passivated small grains in perovskite films for highly efficient light-emitting devices.⁴ A well-accepted “solvent exchange” step (also often referred to as an antisolvent treatment) is performed during deposition, whereby dropping a nonsolvent (e.g., toluene, sometimes with other molecules dissolved in it) onto the sample during film formation extracts the solvent rapidly, which freezes perovskite crystallization and grain growth in situ, resulting in thin, smooth perovskite films with relatively small grain sizes.⁵ Introducing bulky organo-ammonium halide salts as additives in the perovskite precursor solution then leads to ultra-smooth perovskite thin films with a roughness of about 1 nm, and grain sizes ~10 nm.⁶ The implementation of these techniques has proven useful to increasing perovskite-LED efficiency.

Color Tunability

Perovskites can be tuned to emit light within a very wide range of wavelengths, from violet to near infrared. This is made possible by changing composition, as shown in Fig. 4. For example, by controlling the stoichiometry of chloride, bromide, and iodide in lead-based perovskites, bandgaps can be tuned across blue, green, and red regions. This can be further extended to the infrared by replacing lead with tin. Furthermore, if the relatively small organic cations are replaced with bulkier organic cations, the perovskite crystal structure can be altered into two-dimensional Ruddlesden–Popper phases, which provide more opportunities for color tuning, into even the deep blue or violet spectral regions. Color can also be tuned by quantum-confinement effects when the perovskite nanocrystals are small (e.g., 1–3 nm), analogous to the color tunability of quantum dots. However, such principles applied to perovskites are only in their infancy and are thus not addressed in this article.

Fig. 4:  Wavelength-tunable light emission from various perovskite compositions.

Although halide mixing provides an opportunity to tune emitting colors, the emitting wavelength is not stable due to considerable halide-phase separation, which has been the central challenge that limits the practical use of mixed halides to control color. Recently, a promising way to overcome this obstacle has been demonstrated, which involves using bulky organic additives to stabilize the fine-tuned halide composition and avoid halide separation.⁹ Color purity is an important indicator for display quality. Regardless of preparation techniques and grain sizes, luminescence spectra of perovskites are generally narrow (typical spectral widths of 0.07–0.09 eV, or 12 nm for blue-light emission, 20 nm for green, and 40 nm for red/near infrared), which makes perovskites suitable for high-end display applications. This represents an advantage over quantum-dot LEDs, because quantum dots require a narrow particle-size distribution to achieve narrow spectral width. This inevitably increases the fabrication complexity and cost, whereas narrow spectral widths can be manufactured easily for perovskites without dedicated processing techniques.

High-Efficiency Perovskite LEDs

Perovskite LEDs have achieved rapid and remarkable progress over the past several years. Figure 5 compares the highest external quantum efficiency (EQE) and luminance of organic LEDs, quantum-dot LEDs, and perovskite LEDs fabricated to date.¹⁰ Although the highest EQE of perovskite LEDs is still lower than that of OLEDs and QLEDs, the maximum luminance of perovskite LEDs is similar to that of state-of-the-art OLEDs. Further improvements in the EQE of perovskite LEDs are expected in the near future.

Fig. 5:  Timelines show (a) maximum EQE and (b) maximum luminance for organic LEDs (OLEDs), quantum-dot LEDs (QLEDs) and perovskite LEDs (PeLEDs).^10–13

Notably, the highest EQE achieved for perovskite LEDs varies considerably in terms of emitting color. Table 1 summarizes representative perovskite LEDs of different colors. Green and red perovskite LEDs currently lead in performance, while blue and infrared perovskite LEDs require further attention.

 

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Flexible Perovskite LEDs

Low-temperature processing makes perovskites compatible with the most commonly used flexible substrates. However, perovskite films are brittle, due to the salt-like perovskite crystal structure. This aspect has been recently overcome by introducing carefully selected bulky organoammonium halide salts as additives, which confine the perovskite grains during their formation to sizes of ~10 nm while surrounded by a flexible additive matrix.⁴

Flexible perovskite LEDs fabricated with this method show an EQE of 13 percent, similar to state-of-the-art perovskite LEDs on rigid substrates. Furthermore, no degradation is observed after bending for 10,000 cycles at a radius of 2 mm, showing the potential of perovskite LEDs for future highly efficient, robust, and flexible electronic-device applications (Fig. 6).

Fig. 6:  Flexible perovskite LED characterization and bending-test results include (a) a photo of a working flexible perovskite LED; (b) EQE vs. current density curves of perovskite LEDs with or without various additives; and (c) normalized EQE vs. bending cycles at bending radii of 1 and 2 mm. Adapted and reproduced with permission,⁴ copyright 2018, Wiley.

Device Stability

Although perovskite LEDs have improved rapidly in terms of initial performance, stability remains a major challenge. Perovskites are considerably redox-active²⁰ and moisture sensitive, resulting in rapid degradation under typical LED-driving conditions. As is the case for commercialized organic LEDs, sufficient encapsulation will be needed for perovskite LEDs to prevent extrinsic stability issues. In terms of intrinsic stability, operational lifetimes exceeding 46 hours have been demonstrated under relatively low current density,¹⁶ but perovskite LEDs degrade rapidly when a larger current is applied or after operation for an extended period. Ionic processes (interface reactions and/or ion migration) are believed to be primarily responsible.²¹ Due to halide migration, large concentrations of iodine have been found in adjacent layers in perovskite devices.²¹ Furthermore, halide loss has been revealed as a spontaneous process for perovskites,²² which calls for more attention to stabilize halide motion for stable perovskite device development. Better surface passivation, defect reduction, and perovskite composition engineering appear to be key to further improving device stability.

Nontoxic Element Alternatives

Although their semiconducting properties make perovskites promising for next-generation LEDs, the best-performing compositions contain lead and, as such, raise toxicity concerns. Therefore, the development of lead-free perovskites is important to increase the likelihood of commercial success. The three most promising elements being considered to replace lead in perovskites, due to their ability to retain the defect-tolerance properties, are: (1) tin, which is adjacent to lead in the same column of the periodic table; (2) bismuth, adjacent to lead in the same row of the periodic table; and (3) antimony, another less-toxic element in the same column with bismuth in the periodic table.⁷ However, studies on lead-free perovskite emissions are still in their infancy. Currently, the most efficient photoluminescence for lead-free perovskites is achieved with Cs₃Sb₂Br₉ nanocrystals, which show a photoluminescent quantum yield (PLQY) of 46 percent in the blue region (410 nm).²³ The performance from these initial trials is encouraging, but still behind that attained from lead perovskites, which exceeds 90 percent PLQY. At this point, no electroluminescence has been achieved in lead-free perovskites except for tin-based infrared perovskite LEDs.¹⁹ Nonetheless, replacing lead in perovskites represents a critical issue that deserves further research and development.

Outlook

The implications and applications of perovskite LEDs are exciting, with potential for low-cost manufacturing from abundant materials, high light quality, and energy efficiency.  These features make perovskites competitive in comparison with existing light-emitting materials such as organic molecules and quantum dots. In particular, the solution-processed, scalable fabrication technique makes this technology most suited to continuous emitting films for lighting. On the other hand, patterning techniques that are compatible with the solution-based fabrication process are required for display applications, and thus represent an area requiring more attention.

Before electrically driven perovskite LEDs can become a commercial reality, some challenges facing them need to be addressed; mainly device efficiency (most critically for blue) and stability. The fundamental operating mechanisms of perovskite LEDs represent an important area of further research and study, which can lead to an efficient and versatile technology. The commercialization of perovskite materials working as wavelength down-converters can be realized sooner than electrically driven perovskite LEDs, owing to fewer technological challenges. Other promising commercialization options for perovskites include large-area solar modules and x-ray detectors. In addition, given that optically pumped, continuous-wave lasing has been achieved based on metal-halide perovskites,²⁴ it is expected that after addressing the aforementioned issues regarding perovskite LEDs, the world’s first solution-processed, electrically pumped laser diode based on perovskites may be within reach, with additional prospects for display and lighting applications.

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Lianfeng Zhao is a doctoral student working with Prof. Barry P. Rand at Princeton University. Barry P. Rand is an assistant professor at Princeton University in electrical engineering and the Andlinger Center for Energy and the Environment. He can be reached at brand@princeton.edu.