Hybrid remote quantum dot/powder phosphor designs for display backlights

Quantum dots are ideally suited for color conversion in light emitting diodes owing to their spectral tunability, high conversion efficiency and narrow emission bands. These properties are particularly important for display backlights; the highly saturated colors generated by quantum dots justify their higher production cost. Here, we demonstrate the benefits of a hybrid remote phosphor approach that combines a green-emitting europium-doped phosphor with red-emitting CdSe/CdS core/shell quantum dots. Different stacking geometries, including mixed and separate layers of both materials, are studied at the macroscopic and microscopic levels to identify the configuration that achieves maximum device efficiency while minimizing material usage. The influence of reabsorption, optical outcoupling and refractive index-matching between the layers is evaluated in detail with respect to device efficiency and cost. From the findings of this study, general guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems. When reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial

Size and shape control in the synthesis of CIGS nanocrystals

During the last decade, the colloidal synthesis of especially Cd – and Pb chalcogenide nanocrystals has been well-developed. Monodisperse sols of nanocrystals over a large size range have become available and are widely used as building blocks for electronic and optical devices. However, the issue of toxicity remains a major obstacle towards large-scale integration of semiconductor nanocrystals in applications. I-III-VI materials (Cu(In,Ga)(S,Se)2 (CIGS) and related compounds) are interesting candidates towards greener chemistry and offer the possibility of band gap engineering both by using quantum confinement and altering material composition. Moreover, CIGS is a well-known absorber material for high efficiency thin-film photovoltaics, and the use of nanocrystals as precursor inks is an interesting route to decrease the production cost of CIGS solar cells. Up to now, the synthesis of CIGS nanocrystals is mostly developed on an empirical basis. In this work, our aim is to understand the relation between reaction parameters (temperature, concentration, reaction time,..) on one hand and nanocrystal properties (size, shape, composition) on the other hand. We demonstrate that by altering reaction parameters in a rational way (1), CuInS2 and CuGaS2 nanocrystals with sizes from 5 to 20 nm can be obtained which vary in shape from quasi-spheres to flat hexagonal prisms. The ability to control the size and shape of CIGS nanocrystals is of particular importance for the deposition of crack-free thin films by, e.g., inkjet printing that can be incorporated in thin film solar cells. (1) S. Abé, R. Čapek, B. De Geyter, Z. Hens (2012), "Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application", ACS Nano, 6, 1: 42-53

Hybrid remote phosphors for white LEDs : combining quantum dots and rare earth doped phosphors

Current white LEDs (wLEDs) are composed of a blue pumping LED and color convertors – phosphors – which are typically rare earth doped inorganic materials. As backlight in displays these wLEDs often have the disadvantage that a substantial fraction of the emitted light needs to be filtered out. In general lighting, efficiency losses are often generated by an emission extending into the infrared region, while also color rendering can be suboptimal. Recently, the use of quantum dots (QDs) as color convertor materials in wLEDs has gained attention as a solution to these problems. Here, we discuss our recent research results obtained to improve both performance and cost-efficiency of wLEDs for display and lighting applications. The focus lies on developing color convertor materials and implementing these in a remote phosphor configuration, thereby improving efficiency, (color) homogeneity and stability compared to conventional designs where the phosphor material is deposited directly on the LED chip. We explore the use of hybrid phosphor layers, consisting of a combination of rare earth doped phosphors with CdSe or InP-based QDs. We provide an overview of synthesis strategies to obtain the required QDs and configurations to implement them in a remote phosphor layer. Characterization of optical and scattering properties of these layers using e.g. emission decay measurements to asses self-absorption is discussed. Finally, we show relevant characteristics of a benchmark wLED device

Synthesis, characterization and emission tuning of I-III-VI2 semiconductor nanocrystals as color converting alternatives for white LEDs

Colloidal I-III-VI2 semiconductor nanocrystals form a versatile family of nanomaterials with similar properties as Cd-containing quantum dots (QDs), but with lower toxicity. Their interesting and tunable optical properties have resulted in their use in a variety of applications, ranging from biolabeling over solar cells to white LEDs. For an optimal performance each of these applications requires the QDs to show specific optical properties. The relatively narrow emission of Cd-based QDs can easily be tuned over the visible range at a high chemical yield by adjusting specific synthesis conditions[1,2]. However, the typical broad emission of Cu-In-Zn-S QDs has proven to be more challenging to tune and requires a different strategy. We propose a study on the one-pot synthesis and characterization of Cu-In-Zn-S QDs with efficient and broad emission. A careful preparation of the precursors and control of the reaction time is required to synthesize CuInS2 and Cu-In-Zn-S QDs reproducibly. We discuss the limited emission tuning obtained by changing reaction conditions (e.g. precursor concentrations). More successfully, we have expanded the emission range by composition tuning, where In can for instance be replaced by Ga or Al and Cu by Ag. Finally, we address the potential of the resulting materials as alternative color convertors for white LEDs[3]. 1) S. Abe et al., ACS Nano, 2012, 6, 42. 2) S. Abe et al., ACS Nano, 2013, 7, 943. 3) P.F. Smet et al, J. Electrochem. Soc., 2011, 158, R3

Chalcopyrite semiconductor nanocrystal phosphors with composition tunable emisison

The fact that lighting represents a significant share of all globally consumed electrical energy (20%) drives the current transition from incandescent light bulbs to less energy-consuming alternatives such as solid state lighting. Their high efficiency (lm/W) and long lifetime make white light emitting diodes (LEDs) a viable alternative(1). However, there has not yet been a true breakthrough of white LEDs in daily life, especially in lighting applications. An important reason is the observation that cold, unpleasant light is emitted by typical commercially available white LEDs. There is a great need to improve their light color (too high color temperature) and poor color rendering properties. Using quantum dots (QDs) as – red – phosphors can make the light emitted by white LEDs more pleasant while also increasing its efficiency. Chalcopyrite nanocrystals provide a solution to the toxicity of Cd in commonly used visibly emitting QDs

I-III-VI2 semiconductor nanocrystals as remote color converting alternatives for white LEDs

Colloidal I-III-VI2 semiconductor nanocrystals form a versatile family of nanomaterials with similar properties as Cd-containing quantum dots (QDs), but with lower toxicity. Their interesting and tunable optical properties have resulted in their use in a variety of applications, ranging from biolabeling over solar cells to white LEDs. For an optimal performance each of these applications requires the QDs to show specific optical properties. The relatively narrow emission of Cd-based QDs can easily be tuned over the visible range at a high chemical yield by adjusting specific synthesis conditions. However, the typical broad emission of chalcopyrite nanocrystals has proven to be more challenging to tune and requires a different strategy. We propose a study on the one-pot synthesis and characterization of colloidal chalcopyrite nanocrystals with efficient and broad emission. A careful preparation of the precursors and control of the reaction time is required to synthesize Cu-In-Zn-S nanocrystals reproducibly. We discuss the limited emission tuning obtained by changing reaction conditions and composition tuning, where In can for instance be replaced by Ga. Finally, we address the potential of the resulting materials as alternative color convertors for white LEDs in lighting and displays by means of a demonstration device. [1] Speranskaya et al., Langmuir, 2014, 30, 7567; [2] S. Abe et al., ACS Nano, 2012, 6, 42; [3] P.F. Smet et al., J. Electrochem. Soc., 2011, 158, R3