24.1% External Quantum Efficiency of Flexible Quantum Dot Light‐Emitting Diodes by Light Extraction of Silver Nanowire Transparent Electrodes

Abstract

high photoluminescence (PL) quantum yield, narrow emission wavelength bandwidth, size-controllable emission wavelength, and easy material acquirement, as well as the solution-processed manufacture, make QLEDs a star, following the contemporary organic light-emitting diodes. [1][2][3] However, the development of high-performance FQLEDs is much slower, lagging behind their counterparts on glass substrates. For example, the reported highest current efficiency (CE) of red FQLEDs is below 17 cd A −1 and the maximum external quantum efficiency (EQE) is about 14%, [4,5] far behind 18% of the QLED on a glass substrate with similar inverted structure. [6] The main reasons can be attributed to the inferior mechanical performance of indium tin oxide (ITO) transparent conductive electrodes (TCEs) on plastic substrates and the refractive-index mismatch-induced severe total internal reflection (TIR) light loss in the QLED stack. [7,8] The brittle nature of ITO runs counter to good flexibility, and many efforts have been devoted to replace it with graphene, [9] silver nanowires (AgNWs), [10,11] and poly(3,4-ethylen edioxythiophene):poly(styrenesulfonate) [12,13] as flexible TCEs. On the other side, the refractive index of the commonly used plastic substrates (≈1.5) is much lower than ITO (1.8-2.1) in a standard substrate-emitting architecture, which goes against effective light extraction into the viewing domain, and it will consume ≈40% and ≈20% of the total-emitting light in the waveguide mode and substrate mode, respectively. [14,15] Numerous attempts have been made to enhance light outcoupling efficiency, such as using microlens arrays and scattering microspheres, to extract the trapped light in the substrate mode, [16,17] using photonic crystals or optical gratings, [18][19][20] high-index substrates, [21,22] low-index grids, [23] and structured TCEs, [7,24] to extract the waveguided light. However, all of these light extraction methods need complicated processing procedures.Embedding AgNWs network into polymer substrate is an ideal strategy for solving the aforementioned issues. The robust composite can afford repeated bending even folding, [25][26][27][28] and the AgNWs as light-scattering centers can enhance the light outcoupling efficiency. Furthermore, the reduced light Flexible quantum dot light-emitting diodes (FQLEDs) always suffer poor performance, and current efforts towards performance improvement need complicated procedures but still ending with limited progress. An extremely efficient and simply structured FQLED is demonstrated profited from the substantially enhanced light outcoupling efficiency by employing solutionprocessed flexible silver nanowires (AgNWs) transparent conductive electrodes (TCEs). As is uncovered by rigorous simulations, AgNWs TCEs extract enormous light trapped in the substrate mode and waveguide mode compared with indium tin oxide (ITO) TCEs, which greatly agrees with the experimental measurements in this work. As an ultimate achievement, the FQLED shows the record-breaking maximum exter...

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