Silver Zeolite Composite-Based LEDs: Origin of ... - ACS Publications

Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Silver Zeolite Composite-Based LEDs: Origin of Electroluminescence and Charge Transport Koen Kennes,*,† Cristina Martin,†,‡ Wouter Baekelant,† Eduardo Coutino-Gonzalez,§ Eduard Fron,† Maarten B. J. Roeffaers,∥ Johan Hofkens,† and Mark Van der Auweraer† †

Chem&Tech−Molecular Imaging and Photonics, Katholieke Universiteit Leuven, Celestijnenlaan 200F, Leuven B-3001, Belgium Departamento de Química Física, Facultad de Farmacia, Universidad de Castilla-La Mancha, Campus Universitario, Albacete 02071, Spain § CONACYT − Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Querétaro C.P.76703, Mexico ∥ Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, Heverlee 3001, Belgium

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‡

S Supporting Information *

ABSTRACT: In this contribution, we report on the first time use of silver-exchanged zeolites embedded in the nonconductive polystyrene (PS) and their use as hybrid emitters in light-emitting diodes (ZEOLEDs). The turn on voltage and EL intensity are strongly dependent on the concentration of metal clusters. It is shown that the key to optimize this technology is improving the zeolite anode contact. Such an optimized device based on cheap abundant materials could provide an alternative for the commercial phosphor converted LEDs. A ZEOLED with a voltage polarity dependent color is demonstrated.

KEYWORDS: LED, zeolite, silver cluster, defect, electroluminescence, charge transport

F

nonconductive polystyrene-based devices not only allow us to study the electroluminescent properties of the specific ZEOLED without possible polymer based emission they also generate fundamental insights into the charge transport through the AgCL-zeolites. Hence, we could separate the polymer versus the silver-zeolite contribution in these ZEOLEDs and gain useful insight for optimizing these promising devices. The use of a series of electrodes with different work functions allowed to gain further insight in the transport properties and injection mechanism in these ZEOLEDs. For more details about the synthesis of the used zeolite samples we refer to the material section in the Supporting Information. For the sake of consistency, we used the same zeolite samples as in our previous work.15 In a first step, we have characterized the photoluminescent properties of the AgCLzeolites by recording the 2D excitation−emission spectra shown in Figure S1. The details of the photophysics and the origin of the large Stokes shifts are discussed elsewhere.8,12,15,30 These materials were used to construct LEDs using the nonconductive polymer polystyrene containing 10 w% (weight)

ew-atom silver clusters have been shown to possess welldefined and intense photoluminescence characterized with high external quantum efficiencies.1−11 Different strategies have been utilized to solve the problem of stabilization and geometry control, of which template-mediated strategies, such as zeolite incorporation, are perhaps the most successful strategies.5,6 By tuning the framework topology, chemical composition, and the nature and mixing ratio of the counterions, it is possible to vary the geometry and nuclearity of metal clusters. Optimization of these parameters has recently led to the generation of silver clusters with external quantum efficiencies close to 100%.12 Making these silver-loaded zeolites attractive as next-generation secondary emitters in fluorescent lamps and remote LEDs13 and as wavelength converting materials in solar cells.14 In a previous contribution,15 we reported on the use of Agexchanged zeolites as emitting material in a polymer light emitting diode using polyvinylcarbazole (PVK) as charge-carrier transport matrix. The experimental results suggested that the electroluminescence generated in these devices was related to defects or impurities in the zeolite framework rather than to the luminescent silver cluster. These experiments also showed that the presence of the exchanged zeolites increased the conductivity of the PVK matrix significantly suggesting that a conductive matrix is not necessarily required to fabricate AgCL zeolite based LEDs. Compared to our previous work, new © XXXX American Chemical Society

Received: November 29, 2018 Accepted: March 18, 2019 Published: March 18, 2019 A

DOI: 10.1021/acsami.8b20534 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (A) EL spectra of a ZEOLED containing LTA[Na]-Ag6 dispersed in PS at different applied voltages. (B) orresponding IV curve.

of AgCL-zeolites using ITO (φ, 4.7 eV) as transparent bottom electrode and Ytterbium (φ, 2.6 eV) as a top electrode. Figure 1 shows the IV curve and electroluminescence (EL) spectra of the fabricated LTA[Na]-Ag6 ZEOLED. The electroluminescence (EL) spectrum of the ZEOLED, shown in Figure 1A, consists of a single broad band ranging from 400 to more than 800 nm, with its maximum centered at around 675 nm. This is a large shift compared to the PL emission and will be addressed later in the manuscript. The JV curve shows a turn on voltage of ∼7 V. Unfortunately, the efficiency was too low to be accurately measured. In Figure S12, a plot of the EL versus time of this ZEOLED, operating at 11 V, shows that even in ambient conditions after 30 min, 33% of the initial EL intensity remains, showing that these materials have a reasonable stability considering the fact that these are proof of concept devices without further optimization. The EL spectra of the ZEOLEDs with zeolites having different silver loading (LTA[Na]-Ag1, LTA[Na]-Ag3 and LTA[Na]Ag12) can be found in the Supporting Information (Figure S2). The main results obtained for these devices are summarized in Table 1 and Figure 2 (which shows the CIE color coordinates of

Figure 2. CIE coordinates measured at 14 V of the different ZEOLEDs in function of the silver−sodium exchange ratio.

Table 1. Summary of the ZEOLED Characteristics silver exchange ratio 1 3 6 12

current density at 10 EL max at 10 V V (mA) (nm) 0.31 25.00 68.75 218.75

600 640 675 750

Ag1) or Li-zeolites (LTA[Na]-Ag1). As shown in Figure 3A, the nonexchanged ZEOLEDs are also characterized by a very low current density (6 × 10−5 A/cm2 at 10 V, compared to more than 2 × 10−1 A/cm2 in LTA[Na]-Ag6), whereas again no EL was observed. Furthermore, the current densities for the LTA[Li]-Ag1 and LTA[Na]-Ag1 are nearly identical excluding a significant contribution trough the alkali ions.16 This clearly shows that conduction observed here mainly occurs via the silver clusters/ions rather than through the zeolite framework or the initial counterions. Figure 3B shows the IV curves along with their turn on voltage, obtained for ZEOLEDs where different fractions of the sodium ions are exchanged. It is clearly shown that the silver species are responsible for the observed current density. Furthermore, increasing the amount of silver species decreases the turn on voltage from 7.5 to 2.5 V for LTA[Na]-Ag12. To know whether silver ions or silver clusters were involved in the current and EL, we attempted to prepare devices using nonactivated AgCL-zeolite samples where the formation of luminescent AgCLs has not been observed (i.e., all silver atoms remain as cations). Nevertheless, these attempts were unsuccessful due to the sonochemical reduction of the silver ions inside the zeolite framework during the fabrication of the devices. To check whether the EL was due to the silver clusters or to defects or impurities in the zeolite matrix, we prepared new

relative EL intensity (counts/s) 45 181 360 500

the ZEOLEDs). Compared to what was observed for the ZEOLEDs using a PVK matrix,15 the maximum of the EL spectrum shifts to longer wavelengths with increasing silver content. The features and maximum of the observed EL spectra seem to be uncorrelated with the photoluminescence (PL) spectra (Figure S1). At the same applied voltage, the current density and the EL intensity increased with the silver content. A pure PS-OLED with a neat PS layer (∼150 nm) was fabricated with ITO and Yb electrodes. At an applied voltage of 10 V the current amounted to 1 × 10−6 A/cm2 or 4 orders of magnitude lower than that observed for samples with 10 wt % Ag-zeolites embedded in the PS layer (Figure 3A). As expected, this PS-OLED device showed no EL properties. To compare the contribution of the zeolite framework, its counterions and the silver clusters/ions ratio to the current, reference OLEDs were also made with a polystyrene matrix and nonexchanged Nazeolites (LTA[Na]) or with partially exchanged Na- (LTA[Na]B


Letter


Figure 3. (A) IV curves of ZEOLEDs with a PS matrix containing nonexchanged zeolites (black diamond), an exchanged zeolite with low silver loading LTA[Na]-Ag1 (blue triangle), and the same zeolite where the exchange of the sodium ions by lithium ions preceded the exchange with silver ions, LTA[Li]-Ag1 (red circle). (B) IV curves of ZEOLEDs: with a different exchange ratio for the silver ions LTA[Na]-Ag1 (red triangle), LTA[Na]-Ag3 (green triangle), LTA[Na]-Ag6 (blue triangle) and LTA[Na]-Ag12 (black circle).

Scheme 1. (A) Schematic Representation of the Work Functions of the Metals Used in This Study, the Zeolite HOMO and LUMO, and Literature Values of the Fermi Level/Ionization Energies of the Zeolite Framework, Silver Clusters and Ions within the Zeolite Cavities;7,29,30 (B) Schematic Representation of a Working Devicea

Electrons e− flow through the metallic clusters toward the anode where they recombine with trapped holes h+ in the zeolite framework.

a

discussion, we refer to the Supporting Information; here, we will limit ourselves to the main conclusion of the results: (1) Electrons are the majority charge carriers, with nearmetallic mobility, and can be injected with all cathodes used. (2) Except for the MoO3/Ag cathode, the electron current is mainly bulk-limited28 and injected through an Ohmic contact within the LUMO of the silver clusters. (3) For MoO3, a smaller current and different features of the current voltage plot suggest limitation of the current by a barrier for the injection (and perhaps also for the extraction) of the electrons. (4) Although the observation of EL indicates that injection of holes also occurs for all electrode combinations used when the work function of the anode is 4.3 eV or larger, the hole current is always significantly smaller than the electron current and probably absent when the work function of the anode is below 4.3 eV. Considering the energy of the HOMO of the silver clusters and the zeolite framework,29 the holes are injected in defect sites of the zeolite framework. (5) The EL is due to radiative recombination of electrons trapped in defect levels of the AgCLs or the zeolite framework and holes trapped in defect levels of the zeolite. These results lead to the model shown in Scheme 1B. Holes are trapped in the zeolite vacancies near the anode. Electrons are hopping from silver zeolite to silver zeolite with the most

batches of zeolites, where the zeolite cations were exchanged with gold (LTA[Na]-Au6), lead (LTA[Na]-Pb3), or copper (LTA[Na]-Cu3) instead of silver. For this particular experiment, we used zeolites of a larger size (∼2 μm). Although the gold and copper clusters (obtained after reduction) do not show any PL, their ZEOLEDs counterparts showed very similar EL and similar IV curves (Figure S4) as those with AgCL-zeolites. The lead exchanged zeolites show a blue PL (Figure S5), but their respective ZEOLEDs show the same EL and similar IV curves (Figure S4) as the other devices. This means that although the conductivity of the exchanged LTA zeolite samples is due to the exchanged metal clusters or ions, the emission must originate from defects within the zeolite framework itself. This is in strong contrast to the PL, which is attributed to emission from the respective metal clusters.15,17 Hence, the large difference between the observed PL and EL spectra. This also means that silver can be replaced by a cheaper metal such as copper. Because the majority of the data in the manuscript is gathered using Ag, we continue to use silver for the sake of consistency. To further elucidate the charge injection and transport mechanism, a series of ZEOLEDs consisting of PS with 10 w% zeolite (LTA[Na]-Ag6) with different electrodes were prepared.19 We used a series of electrodes with increasing work function (φ): ytterbium (Yb, φ: 2.6 eV), thulium (Tm, φ: 3.12 eV), silver (Ag, φ: 4.3 eV), nickel (Ni, φ: 5.1 eV) gold (Au, φ: 5.2 eV), and molybdenum oxide protected with a silver layer (MoO3/Ag, φ: 6.7 eV)18 (Scheme 1A). Three models were attempted to be applied: Richardson Schottky,19−21 Fowler− Nordheim.22−26 and space charge limited currents.27 For the full C


Letter


Figure 4. EL Micrograph of a “Flat”-Ag/PS:LTA[Na]-Ag6/Ag-”Rough” device at 10 V. (A) Hole injection via the “flat” Ag-anode, (B) hole injection via the “rough” Ag anode. (C) SEM image of “rough” electrode.

amount of silver clusters within the zeolites. For a fully silver exchanged zeolite a value as low as 2.5 V could be achieved. For all cathodes with a work function of 5.2 eV or smaller this electron current seems to be injected through an ohmic contact, whereas for MoO3 (work function of 6.7 eV) a smaller current and different features of the current voltage plot suggest limitation by a barrier for the injection (and perhaps also for the extraction) of the electrons. These white light-emitting devices can be further optimized by improving the content of silver in the silver loaded zeolites, and the zeolite/anode contact. The potential of these devices demonstrated here together with the fact that they are based on abundant materials make them advantageous to current commercial phosphor-based LEDs.

efficient pathway involving metal cluster to metal cluster electron transfer until they reach holes within the zeolite vacancies where they recombine and emit a near white EL. The values for the different silver species used in Scheme 1A are a combination of literature values7,29 and our own calculations which are published elsewhere.30 Although we could not measure the hole mobility, in the Supporting Information, we use bilayer, dual-color devices to show that it must be indeed very low (Figure S13). During these experiments it was also observed that when ITO was used as a cathode, the EL intensity increased 10-fold for all devices with an anode work function >3.4 eV compared to the situation where they are used as cathode. Based on the proposed model this was attributed to the anode surface roughness. Therefore, a device with two silver electrodes was fabricated. The bottom electrode was evaporated on top of the ITO electrode and is considered “flat”, the top electrode is evaporated on top of the emissive layer which is considered to be “rough” as can be seen from the SEM image in Figure 4C. This is due to the zeolite clusters penetrating the surface and might give the impression that the zeolite density is quite low. Therefore, a cross-sectional SEM image is also shown in Figure S11, which shows the zeolites clusters to be in close contact. If the injection efficiency of electrons and holes would only depend upon the energetic aspects, we would expect a similar electroluminescence intensity for the bottom electrode acting as anode or acting as cathode. Nevertheless, we observe for the hole injection through the “flat” Ag anode a small electroluminescence intensity, analogous to that observed for the other devices with ITO as anode. However, injecting holes through the “rough” Ag anode leads to a much brighter device, as expected (Figure 4A, B). This opens a pathway for improving the ZEOLEDs performance, for example, when they can be directly grown on the transparent bottom electrode (anode) to ensure maximum contact. We showed that the possibility of construcingt light-emitting diodes from a dispersion of silver-loaded zeolites in a polymer matrix is not limited to a matrix of a charge transporting polymer as PVK;15 also polymers as polystyrene that under the experimental conditions do not transport current can be used. The observed electroluminescence could not be attributed directly to radiative LUMO−HOMO recombination in the metal clusters but rather to defects of the zeolite framework. A detailed study of the electrode properties suggests that the observed current is mainly bulk limited and is due to electrons transported through the silver clusters, where they have a nearly metallic mobility. The turn on voltage is dependent on the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20534. Materials and methods, 2D excitation−emission plots of different metal-exchanged zeolites, EL spectra, IV curves of ZEOLEDs with different types of metal clusters, a detailed description and analysis of the used chargetransport models (Fowler−Nordheim, Richardson Schottky, SCLC), EL-dependency on the used electrodes, cross-sectional SEM and stability plot of a ZEOLED (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Koen Kennes: 0000-0002-5987-2167 Cristina Martin: 0000-0002-7588-8759 Wouter Baekelant: 0000-0001-7541-2171 Eduardo Coutino-Gonzalez: 0000-0001-8296-0168 Eduard Fron: 0000-0003-2260-0798 Maarten B. J. Roeffaers: 0000-0001-6582-6514 Johan Hofkens: 0000-0002-9101-0567 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.K. was funded by a personal PhD grant from the Agency for Innovation, Science and Technology (IWT) Flanders. The authors are also indebted to Belspo program through IAP VI/27 and VII/05 and to the KULeuven Research Fund through GOA D


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2011/3 and C14/15/053. The authors gratefully acknowledge financial support from the European Union’s Seventh Framework Programme (FP7/2007-2013 under grant agreement 310651 SACS). The authors thank E. De Keyzer, Flip De Jong, and B. Dieu for their assistance in producing the Ag-zeolite samples and graphical material.

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Silver Zeolite Composite-Based LEDs: Origin of ... - ACS Publications

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