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Solution-processed CuInS – based white QDLEDs with mixed active layer architecture Svenja Wepfer, Julia Frohleiks, A-Ra Hong, Ho Seong Jang, Gerd Bacher, and Ekaterina Nannen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15660 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017
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ACS Applied Materials & Interfaces
Solution-processed CuInS2 – based white QD-LEDs with mixed active layer architecture Svenja Wepfer
1,2
, Julia Frohleiks
1,2
, A-Ra Hong 3, Ho Seong Jang 3, Gerd Bacher, 2,
Ekaterina Nannen* 1,2 1: Research Group „Solid State Lighting“, NanoEnergieTechnikZentrum, University Duisburg-Essen, 47057 Duisburg, Germany 2: Werkstoffe der Elektrotechnik and CENIDE, University Duisburg-Essen, 47057 Duisburg, Germany 3: Materials Architecturing Research Center, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
KEYWORDS quantum dots, white, light emitting devices, mixed layer, core/shell
ABSTRACT Colloidal quantum dots (QDs) are attractive candidates for future lighting technology. However, in contrast to display applications, the realization of balanced white lighting devices remains conceptually challenging. Here we demonstrate two-component white light-emitting QD-LEDs with high color rendering indices (CRI) up to 78. The implementation of orange CuInS2/ZnS (CIS/ZnS) QDs with a broad emission and high quantum yield together with blue ZnCdSe/ZnS QDs in a mixed approach allowed white light emission with low blue QD content. The devices reveal only a small color drift in a wide operation voltage range. The correlated color temperature (CCT) could be adjusted between 2200 and 7200°K (from warm
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white to cold white) by changing the volume ratio between orange and blue QDs (1:0.5 and 1:2). INTRODUCTION Colloidal semiconductor quantum dots (QDs) are considered as promising candidates for the next generation of surface light-emitting devices due to their high photoluminescence quantum yield (PL-QY), good stability and tunable emission wavelength over the entire visible spectrum range by controlling the size and composition.1–6 Their typically narrow emission profile (FWHM ca. 20-40 nm) is preferred for high color gamut in display applications, however, it is less favorable when approaching solid state lighting applications.7– 11
In contrast to converter based devices that benefit from the excellent performance of the
underlying GaN-based LED chip as optical pumping source12,13, QD light-emitting devices (QD-LEDs) are excited by direct electrical carrier injection into the QDs.4,14 In case of QDLEDs, it is possible to fabricate the whole device (electron and hole supporting layers as well as light-emitting QD layer) from material dispersions which enables printing or roll-to-roll technologies for future lighting applications.4,15 In the field of lighting applications, getting the best possible color rendering (quantified by color rendering index, CRI) is important, in addition to good color stability and adjustable correlated color temperature (CCT). Ideally, a white light source should replicate the natural sun spectrum or the one of a black body radiator, resulting in the best possible CRI of 100.
Typically, white light-emitting QD-LEDs are fabricated by a combination of multiple QD colors (e.g. red, green, blue; RGB) in one device whereby several device architectures have been introduced in literature yielding promising external quantum efficiencies (EQE) between 0.1 – 1.6 %.16–20 The stacking approach, where different colored QD layers are stacked on top of each other, implies careful control of the QD layer thickness and sequence to realize 2 ACS Paragon Plus Environment
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emission from all QD species for balanced and stable white color.19,21,22 Otherwise, unbalanced charge accumulation and/or unbalanced quenching of QD luminance due to high current may lead to a significant color shift during device operation.19
Favorable for fabrication and upscaling is the mixing approach, where a single active layer contains a mixture of RGB QDs that enables the adjustment of the emission color from “warm white” to “cold white” by the R:G:B ratio of corresponding QD loads.16,17,20,23 Typically, an unbalanced QD load with very high blue content (R
