Nanophotonics 2016; 5 (1):74–95 Special Issue Article Open Access Smart nanophotonics for renewable energy and sustainability Talha Erdem and Hilmi Volkan Demir* Colloidal nanocrystals for quality lighting and displays: milestones and recent developments DOI 10.1515/nanoph-2016-0009 Received October 1, 2015; accepted October 1, 2015 1 Introduction Abstract: Recent advances in colloidal synthesis of Since the end of the 20th century, we have been witnessing nanocrystals have enabled high-quality high-efficiency the advance of electronics and photonics in a mutual re- light-emitting diodes, displays with significantly broader lation. The developments in one field clearly contribute to color gamut, and optically-pumped lasers spanning the further developments in the other. An important binding whole visible regime. Here we review these colloidal plat- force of this relation has been the research on emerging forms covering the milestone studies together with re- materials including in all disciplines of materials science, cent developments. In the review, we focus on the devices chemistry, physics, and electrical engineering. Thanks to made of colloidal quantum dots (nanocrystals), colloidal these multidisciplinary efforts at a global scale, today we quantum rods (nanorods), and colloidal quantum wells have very sophisticated optoelectronic devices, for exam- (nanoplatelets) as well as those of solution processed per- ple, luminaries, displays, sensors, imaging tools, etc. Es- ovskites and phosphor nanocrystals. The review starts pecially within the last two decades, significant contribu- with an introduction to colloidal nanocrystal photonics tions to photonics have come from the science and tech- emphasizing the importance of colloidal materials for nology of semiconductor colloids [1–6] leading to today’s light-emitting devices. Subsequently, we continue with the commercial devices [7, 8] made of colloidal semiconduc- summary of important reports on light-emitting diodes, tor nanoparticles. Some of the attractive features of these in which colloids are used as the color converters and colloidal materials can be listed as their high quantum ef- then as the emissive layers in electroluminescent devices. ficiencies, precisely controllable emission colors, narrow Also, we review the developments in color enrichment and emission bandwidths, large absorption cross sections, im- electroluminescent displays. Next, we present a summary proved stabilities, cost-effectiveness, and abundance [9]. of important reports on the lasing of colloidal semicon- Among the devices involving colloidal materials, in this ductors. Finally, we summarize and conclude the review review we focus on the light-emitting devices, in partic- presenting a future outlook. ular, light-emitting diodes (LEDs), displays, and lasers in which colloidal semiconductor quantum dots (QDs), rods (also known as nanorods), and wells (also known as nanoplatelets, NPLs) along with perovskites and phos- phor nanocrystals are utilized. Here, we summarize early milestone works and recent important advancements on these topics for each device. Talha Erdem: Department of Electrical and Electronics Engineer- LEDs were one of the first devices utilizing colloidal ing, Department of Physics, Institute of Materials Science and Nan- semiconductor nanoparticles, which offer spectral tuning otechnology, and UNAM-National Nanotechnology Research Center, in white light generation along with high efficiencies and Bilkent, Ankara Turkey 06800 high quality lighting for indoor [10] and outdoor [11] light- *Corresponding Author: Hilmi Volkan Demir: Department of ing applications. In LEDs, colloidal nanoparticles were Electrical and Electronics Engineering, Department of Physics, Insti- tute of Materials Science and Nanotechnology, and UNAM-National employed as both color converters typically on epitaxi- Nanotechnology Research Center, Bilkent, Ankara Turkey 06800 ally grown LEDs and as emissive layers of electrolumines- and Luminous! Center of Excellence for Semiconductor Lighting and cent devices. In the LEDs employing colloidal nanopar- Displays, School of Electrical and Electronic Engineering, School of ticles as color converters, the color-conversion layer is, Physical and Mathematical Sciences, School of Materials Science in general, prepared by blending the colloidal material and Engineering, Nanyang Technological University, Singapore within a polymeric encapsulation matrix and placed on 639798, E-mail: [email protected] © 2016 Talha Erdem and Hilmi Volkan Demir, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Unauthenticated Download Date | 6/12/16 12:09 AM Colloidal nanocrystals for quality lighting and displays Ë 75 top of a near-ultraviolet (near-UV) or blue-emitting LED colloids can be developed. However, in contrast to LEDs chip that subsequently excites the colloidal semiconduc- and displays, the problems that need to be overcome to tors in the film. In the electroluminescent devices, the realize colloidal lasers are more serious. Since the lasers colloidal materials were usually sandwiched between lay- operate in the nonlinear regime in which the population ers of hole injection and transport, and layers of electron inversion of electrons and holes is required, suppression injection and transport. Electroluminescence is obtained of all the loss mechanisms including Auger recombination through the radiative recombination of injected electrons and surface traps and increasing the stability of the emit- and holes within the colloidal semiconductor material. A ters are of significant importance [17]. Therefore, develop- major advantage of using colloidal materials in both color- ing lasers involving colloidal emitters takes more effort but converting and electroluminescent LEDs comes from their it is also an open field for further improvements. To date, narrow-band emission spectra allowing for quality white- no electrically driven laser of colloidal materials could be light-emitting devices. As a result, high color rendition per- presented; however, a wide variety of lasers with optical formance along with high photometric efficiency can be excitation have already been reported for QDs, nanorods, realized at the same time [12]. In addition, due to the ris- NPLs, and perovskites. ing concerns regarding the supply of rare-earth ion based phosphors [13], colloidal semiconductor materials have stepped forward in recent years [14] since they can be syn- 2 Color-converting colloidal thesized from abundant materials having supply chains more immune to political tensions. materials for lighting Similar to lighting, colloidal materials have been in- vestigated in displays as color converters and also as ac- High-quality lighting requires the optimization of various tive electroluminescence layers. Their use in displays for performance metrics including the ability to render the lighting inherits significant similarities; however, the use real colors of the objects along with a strong overlap of of colloidal materials in displays poses additional advan- the emission spectrum with the eye sensitivity function tages. First of all, purer colors (in other words, more sat- and a warm white shade [15]. The color rendition capa- urated colors in color science terms) can be realized as a bility of the light sources is, in general, evaluated using result of significantly narrow band edge emission of the color rendering index (CRI). The worst color rendition per- colloidal materials. These pure colors obtained from col- formance is quantified with −100 while the perfect per- loids increase the range of colors that can be defined by formance is 100. In addition to the color rendition per- the display [15], which is also known as the color gamut. formance of the light source, it is also important to max- When these materials are utilized as color converters, they imize the fraction of the light produced by the light source are in general integrated on a blue LED chip as in the case that can be perceived by the human eye. Even if the emit- of lighting applications or as a remote color converter in ter is quantum mechanically very efficient, it might not a glass tube on the periphery of the optical back plane have any meaning as a light source for general lighting ap- or in a plastic film on the back plane of the display away plications in the case that the optical power is wasted in from LEDs. The generated light then passes through polar- the spectral regime where the human eye is not sensitive. izers, color filters, and liquid crystals to obtain the desired Therefore, overlap of the emission spectrum with the hu- color controlled at individual pixel level [16]. The use of man eye sensitivity function is very crucial for efficient il- the colloidal materials as active emissive layers in electro- lumination. This overlap is quantified using luminous ef- luminescent devices, on the other hand, requires pixelated ficiency (LE) of the optical radiation (LER), which takes formation of the LEDs. In these displays, there is no need values >350 lm/Wopt for high-efficiency light sources. The to use color filters or polarizers, which block a significant electrical efficiency of the light source is also an important amount of light leading to decreased efficiencies and heat- parameter for the performance of a light source. This met- ing, if single colored LEDs controlled at individual pixel ric is typically evaluated using the