Enhanced Electroluminescence Efficiency in Metal Halide Nanocluster

Jul 30, 2018 - Metal halide nanoclusters represent an attractive class of molecular ... in biomedical imaging, sensors, telecommunications, and night ...
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Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Enhanced Electroluminescence Efficiency in Metal Halide Nanocluster Based Light Emitting Diodes through Apical Halide Exchange Padmanaban S. Kuttipillai,† Chenchen Yang,† Pei Chen,† Lili Wang,† Matthew Bates,† Sophia Y. Lunt,†,‡ and Richard R. Lunt*,†,§ †

Department of Chemical Engineering and Materials Science, ‡Department of Biochemistry and Molecular Biology, and Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, United States

§

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 178.57.65.115 on 08/15/18. For personal use only.

S Supporting Information *

ABSTRACT: Metal halide nanoclusters represent an attractive class of molecular building blocks for the design of functional materials with superior optical properties that can be utilized in a range of applications. Here, we demonstrate red and near-infrared light emitting diodes with a maximum external quantum efficiency >1%, utilizing phosphorescent octahedral molybdenum iodide nanoclusters. Efficiency improvement in these devices is realized by substituting heavier ligands in the apical nanocluster position that lead to the improvement in photoluminescence and exciton formation efficiencies in the nanoclusters. These results highlight how modulation of nanocluster salts with key terminal ligands has a profound effect on photoluminescence as well as electrical injection. KEYWORDS: nanoclusters, inorganic-emitters, phosphorescence, metal halide, near-infrared

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development of emitters for near-infrared (NIR) emissions are still major challenges.7 NIR luminescence has important applications in biomedical imaging, sensors, telecommunications, and night vision. To date, very few examples of phosphorescent emitters have been demonstrated for NIR emission from organometallic complexes.8 While various fluorescent (ns) based emitters have been reported for NIR light emitting diodes, all typically suffer from the lower overall potential of intrinsically limited internal quantum efficiencies.9 To expand the catalog of high efficiency phosphorescent emitters, we have demonstrated a platform based on metal halide nanoclusters (NCs) with high abundance, low cost, and high quantum yield with a potential to span the visible as well as the infrared spectral range.10 Nanoclusters are species with precisely defined chemical composition and structure (i.e., molecules). In contrast, nanocrystals are nanoscale crystalline ensembles of bulk semiconductors with a particle size distribution and less precise structural characterization.11 Previous reports on fluorescent metal nanoclusters, based on metals such as Pt, Au, Ag, and Cu, have also been studied for applications in sensors, bioimaging, and optoelectronic devices.12 Nanocluster

LEDs (organic light emitting diodes) are now a key component in displays, such as mobile electronics, wearable electronics, and televisions, and are being considered as a solid-state lighting technology along with light emitting diodes (LEDs) to reduce worldwide energy demand. Developing phosphorescent materials that utilize both the singlet and triplet excitons for light generation is of great importance as they help in achieving 100% internal quantum efficiency (IQE) in devices.1 The presence of strong spin− orbital coupling in these emitters mixes the singlet and triplet excited states and allows radiative relaxation of the forbidden triplet state to the ground state.2,3 However, typical phosphorescent emitters for OLED applications consist of costly organometallic complexes of Ru, Rh, Os, Pt, and Ir.4 To reduce the cost associated with the expensive phosphorescent emitters, organic thermally activated delayed florescence (TADF) emitters have recently emerged.5,6 The triplet excitons formed in TADF emitters undergo reverse intersystem crossing (RISC) to the singlet manifold due to the small energy gap between the singlet and triplet energy levels and perform spin allowed transitions to the ground state leading to 100% utilization of excitons without the use of heavy metals. Although TADF based emitters offer a cheaper alternative to the expensive phosphorescent emitters with emission spanning the entire visible spectrum, material longevity and the © XXXX American Chemical Society

Received: May 25, 2018 Accepted: July 30, 2018 Published: July 30, 2018 A

DOI: 10.1021/acsaem.8b00837 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials light emitting diodes (NCLEDs) consisting of Ag or Au nanoclusters have been reported with a maximum external quantum efficiency (EQE) of 0.1%.13 The lower efficiency in these devices is likely due to the poor exciton confinement and the fluorescent nature of these emitters.14 Recently, Cu based organic clusters with dual emissive characteristics have been reported with a peak EQE of 0.74%.15 We have reported the integration of phosphorescent metal halide nanoclusters and nanocluster salts into optically and electrically pumped NCLEDs.10 Subsequently, Mn based nanocluster salts have been demonstrated in green electrically driven LEDs with a maximum EQE as high as 9.6%.16 Despite these demonstrations, emitters that have potential application for electroluminescence beyond 650 nm are still rare.15,17,18 Molybdenum based nanoclusters consist of octahedral Mo6 units bearing eight face bridging inner halides and six terminal halides in the apical positions.19 These apical halides are labile and can be readily exchanged to other ligands to significantly improve the photochemical properties and make them attractive candidates for applications in catalysis,20,21 radiochemistry,22 sensors,23 intercalation chemistry,24 and solar concentrators.25 In addition, switching the inner halides from chloride to iodide has also led to improvement in the photoluminescence properties of these nanoclusters. For example, tetrabutylammonium (TBA) substituted nanoclusters such as (TBA)2[(Mo6I8)(C3F7COO)6] and (TBA)2[(Mo6I8)(CF3COO)6] were shown to have phosphorescence quantum yields (PLQY) of 1.0 in oxygen free dry solvents,26,27 while the parent nanoclusters Mo6Cl12 and Mo6I12 have PLQY < 0.05, indicating the major role of apical ligands on the photoluminescence properties and nonradiative rates of these nanoclusters. In addition, there are interesting similarities and differences with halide perovskites. Halide perovskites have the chemical formula of ABX3 (A and B are cations and X is a halide anion) and form a structure containing halide octahedra (X−X) surrounding a B cation. Although both exhibit octahedral structures (Mo−Mo octahedra vs X−X octahedra), perovskites generally are fluorescent in nature whereas metal halide nanoclusters are phosphorescent, making them more suitable for achieving the highest efficiencies in light emitting diode applications. Here we report the application of molybdenum iodide nanocluster derivatives in highly efficient NCLEDs. Tunable emission in the devices is demonstrated by changing the apical ligands in the nanoclusters with a maximum external quantum efficiency >1%. Improved electroluminescence efficiency is shown to result from the combination of enhanced photoluminescence, carrier injection, and recombination efficiencies in the heavier apical ligand substituted nanoclusters when paired with optimal hosts. These results highlight the promising potential of [Mo6I14]2− based nanoclusters for LED applications in the visible and near-infrared. Figure 1a−c displays the molecular structure of nanoclusters used in this study, and Figure 1d shows the absorption and photoluminescence spectra of nanoclusters in dry acetonitrile solution. The inset illustrates the emission from a solution of Cs2Mo6I8(C3F5OO)6 positioned over a UV lamp (λ = 365 nm). We note that the absorption onsets of and Cs2Mo6I14 and Cs2Mo6I8(C3F5OO)6 are blue-shifted compared with (TBA)2Mo6I14. While the starting clusters Cs2Mo6I14 and (TBA)2Mo6I14 show similar PL peaks at λ = 720 nm, Cs2Mo6I8(C3F5OO)6 has a blue-shifted PL peak at λ = 676 nm highlighting the effect of ligand substitution on the optical

Figure 1. Molecular structures of (a) Cs2Mo6I14, (b) (TBA)2Mo6I14, and (c) Cs2Mo6I8(C3F5OO)6. (d) Normalized absorption and photoluminescence spectra of nanoclusters in acetonitrile for Cs2Mo6I14 (− − −), (TBA)2Mo6I14 (---), and Cs2Mo6I8(C3F5OO)6 (). The inset shows the photoluminescence of Cs2Mo6I8(C3F5OO)6 solution illuminated with a UV lamp.

properties of the clusters. Mass spectrometry data confirming the Mo6I142− and Mo6I8(C3F5OO)62− products are provided in Figure S1 and further emphasize their molecular nature compared to nanocrystals. On the basis of the luminescence and transient absorption, previous studies revealed that the emission in these types of nanoclusters is attributed to the radiative relaxation of triplet states with a long natural lifetime of >60 μs.28 Photoluminescence quantum yields of 68 ± 7% and 17 ± 4% are measured for synthesized Cs2Mo6I8(C3F5OO)6 and (TBA)2Mo6I14, respectively, in dry, air-free acetonitrile, while the starting cluster Cs2Mo6I14 showed a PLQY of 6 ± 2%. The PL intensity of these nanoclusters significantly decreases in air due to the energy transfer to molecular oxygen and the formation of singlet oxygen species.29 Consequently, the PLQY values of Cs2Mo6I8(C3F5OO)6 and (TBA)2Mo6I14 are reduced in air to 25% and 8%, respectively. In Figure S2, we show the reversible nature of this PL reduction after oxygen is introduced and then purged from the system, along with transmission data that show there is no change in absorption after exposure to oxygen and light. Figure 2a shows the flatband energy level diagram of the NCLED devices. Cs2Mo6I8(C3F5OO)6 and (TBA)2Mo6I14 nanoclusters exhibit hole-injection barriers of 2.0 and 2.6 eV, respectively. We fabricated 12 NCLED devices with Cs2Mo6I8(C3F5OO)6 and (TBA)2Mo6I1, and Figure 2b shows the J−V− brightness (B) characteristics of the devices. Ultraviolet photoelectron spectroscopy (UPS) spectra of the nanocluster films as a function of binding energy are included in Figure S3. While the devices with Cs2Mo6I8(C3F5OO)6 have typical diode characteristics and exhibit an optical turn-on voltage of B

DOI: 10.1021/acsaem.8b00837 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 2. (a) Energy level diagram of the NCLED devices. HOMO levels of the NCs were obtained from UPS measurement, and LUMO levels were estimated by adding the optical bandgap to the HOMO for both the singlet and triplet (---).31 Energy levels for the transport layers were obtained from the literature.32−35 (b) J−V−B characterization of the representative NCLED devices made with (TBA)2Mo6I14 and Cs2Mo6I8(C3F5OO)6. (c) Comparison of electroluminescence () and photoluminescence (---) spectra of (TBA)2Mo6I14 and Cs2Mo6I8(C3F5OO)6. (d) Comparison of the quantum efficiency (EQE) vs current density of (TBA)2Mo6I14 and Cs2Mo6I8(C3F5OO)6 devices. The inset shows a photograph of the Cs2Mo6I8(C3F5OO)6 nanocluster NCLED. The legend in part b applies to part d.

Figure 3. Emissive layer thickness dependent optimization. (a) Comparison of current density vs voltage. (b) Brightness vs voltage. (c) External quantum efficiency vs current density comparison of the devices with Cs2Mo6I8(C3F5OO)6. Legend for part a applies to parts b and c.

that the charge carriers might be injected into a different triplet state as temperature dependent emission studies have confirmed multiple triplet energy levels in these types of nanoclusters.30 Various attempts were made to fabricate devices with Cs2Mo6I14, but the devices showed no nanocluster emission. Figure 2d shows the external quantum efficiency (EQE) versus current density comparison of the optimized devices. Cs2Mo6I8(C3F5OO)6 devices show a maximum EQE of 1.1 ± 0.2%, and (TBA)2Mo6I14 devices show a maximum EQE of (3.4 ± 0.7) × 10−4%. Optimization of the Cs2Mo6I8(C3F5OO)6 devices was performed by changing the emissive layer thickness and

3.7 V, the devices consisting of (TBA)2Mo6I14 show large leakage currents with an optical turn-on voltage of 6 V (see Figure S4). The optical turn-on voltage is defined as the voltage at which luminescence is detected above the noise floor of the detector. Figure 2c shows the comparison of the photoluminescence and electroluminescence spectra of the nanoclusters and confirms that the electroluminescence predominantly arises from the nanoclusters along with (in some cases) a small component of host/exciplex emission (λ = 500 nm). We note that the EL spectrum is blue-shifted compared with the optical PL emission in these nanoclusters. While we do not have a clear explanation for this, we expect C

DOI: 10.1021/acsaem.8b00837 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 4. (a) Electroluminescence spectra recorded at 6 V for Cs2Mo6I8(C3F5OO)6 devices made with different doping concentration in the emissive layer. (b) Photoluminescence excitation scans of host/nanocluster films recorded at the nanocluster emission wavelength along with excitation of the neat host film recorded at the host emission wavelength. (c) AFM images (5 μm × 5 μm) showing the morphology of spin-coated ITO/PEDOT/Cs2Mo6I8(C3F5OO)6:Pyd-2Cz (10 wt %) and (d) ITO/PEDOT/ (TBA)2Mo6I14:Pyd-2Cz (10 wt %) blended films with a thickness of 30 nm.

due to the improved PLQY in this heavy ligand substituted cluster. Temperature dependent emission studies on [Mo6X8(nC3F7COO)6]2− (X = Cl, Br, I) have revealed that the radiative rate constant for triplet state relaxation improved significantly due to the increased spin−orbital coupling in the nanocluster with the heavier inner halides combined with a bulkier apical ligand.30 Our calculations in [MoCl14]2− revealed that the excited state geometry is distorted due to the Jahn− Teller effect. While similar distortions are observed in [Mo6I14]2− nanoclusters, the substitution of heavier ligands stabilizes the triplet state compared to the halide counterparts resulting in longer natural lifetimes.28,30 To explore the mechanism of electroluminescence, excitation scans were recorded on 30 nm thick films of 10 wt % nanocluster doped in Pyd-2Cz at the PL emission wavelength (λ = 675 and 720 nm) for Cs2Mo6I8(C3F5OO)6 and (TBA)2Mo6I14 nanoclusters, respectively. Excitation scans are plotted in Figure 4b along with the excitation of Pyd-2Cz at its peak PL wavelength (λ = 375 nm). The data clearly show the excitation signature of Pyd-2Cz while measuring the emission of the nanoclusters. This suggests that there is energy transfer between the host and the nanoclusters likely stemming from Förster mediated transfer from the host singlet to the nanocluster singlet states but this could also stem from Dexter mediated transfer from the host triplet to the nanocluster triplet state. Due to the presence of this energy transfer for both nanoclusters, it then suggests that while energy transfer can be present in these systems, it is not the dominant mechanism for nanocluster excitation given the significant performance variations seen in the EQE. On the basis of the HOMO energy level measurement of these nanoclusters, we find key differences in the electrical injection efficiency between the two nanoclusters that stem from the different ligands. A steric effect was previously indicated with chloride NCs based on [Mo6Cl14]2−, where bulkier anions (TBA versus hydronium cations) led to increases in PLQY but much lower quantum efficiency. Such a steric effect could be a reinforcing effect here that reduces injection tunneling probability. To clarify the role of the host, we also fabricated devices using the chloride based derivative demonstrated previously (i.e., (TBA)2Mo6Cl14), where the best devices still showed an EQE of 0.008%. This further indicates that the variation in the

doping concentration in the devices. Figure 3 shows an example set of thickness dependent device data for current density versus voltage and luminescence current density versus voltage. As the thickness of the emissive layer increases, the turn-on voltage increases as expected. Figure 3c shows the EQE comparison of devices as a function of the host thickness leading to an optimum EML thickness of 42 nm. In phosphorescent OLEDs, emitters are typically doped in a host material to reduce the self-quenching of excited states, and device performance is optimized by changing the doping concentration of the emitter in the host materials. We therefore fabricated NCLEDs with different doping concentrations of nanoclusters in the host 2,6-di(9H-carbazol-9-yl) pyridine) (Pyd-2Cz) as shown in Figure 4a. Devices made with 2 and 5 wt % of the nanocluster show emission predominantly from the host. At concentrations ≥10 wt %, the host emission is significantly decreased and the EL is attributed to the nanoclusters. To understand the role of the host on the electroluminescence, we performed excitation studies on the host/emitter system to see if any energy transfer process occurs between the nanocluster and host material. In Figure 4b we show the excitation scans measured at the emission wavelength of the nanocluster. These scans clearly show the excitation signature of host materials, indicating that energy transfer can occur between the host and guest materials in the NCLEDs. Figure 4c,d shows atomic force microcopy (AFM) images of the surface of the blended host/nanocluster films Cs2Mo6I8(C3F5OO)6:Pyd-2Cz and (TBA)2Mo6I14:Pyd-2Cz. These images emphasize that both films have extremely smooth surface morphology with both having a roughness around 0.27 nm, indicating that the performance difference between these two nanocluster does not stem from the differences in morphologies. Previous work has highlighted that the cation substitution strongly affects the electroluminescence efficiency of [Mo6Cl14]2− based clusters.10 Despite higher PL quantum yield, devices made with (TBA)2Mo6Cl14 suffered poor exciton formation due to the cation arrangement in these nanoclusters. We have also seen a similar trend as the Cs cation showed drastically improved performance over the TBA cation substituted nanoclusters devices. Electroluminescence efficiency improvement in Cs2Mo6I8(C3F5OO)6 devices is partly D

DOI: 10.1021/acsaem.8b00837 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

bis(N-carbazolyl)benzene (mcp), 1,3-bis(triphenylsilyl)benzene (UGH-3), and 2,6-di(9H-carbazol-9-yl) pyridine) (Pyd-2Cz). While the devices made with mcp show a combination of nanocluster emission along with significant host emission, no nanocluster emission was observed from the UGH-3 devices. Pyd-2Cz devices were found to show nearly pure emission from the nanocluster with only small contributions from the host. Consequently, Pyd-2Cz was selected for further device optimization. An emissive layer (EML) consisting of 12−85 nm thick host matrix (Pyd-2Cz) doped with (2− 15 wt %) of nanoclusters was spin-cast at 3000 rpm for 30 s from a mixture of host and nanocluster dissolved separately in chlorobenzene and dry acetone. The EML was then heated at 70 °C for 10 min in the glovebox to remove the remaining solvent before substrates were loaded into an evaporation chamber. A 10 nm thick hole blocking layer (HBL) consisting of bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and an electron transport layer (ETL), 40 nm of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TpBi), were subsequently thermally evaporated on the emissive layer at 3 × 10−6 Torr. A LiF (0.8 nm)/Al (80 nm) cathode film was evaporated over the ETL. Control devices were fabricated without the nanocluster in the emissive layer for comparison. All devices were packaged using a UV curable epoxy prior to electrical characterization, which was performed using a Keithley 2420 source meter and Keithley 6487 picoammeter. Luminescence was measured using a large area Si photodetector (Hamamatsu), masked to only collect forward emitted light. Electroluminescence (EL) spectra of the devices were measured using a calibrated ocean optics spectrometer (USB-4000).

host alone cannot explain the vastly improved quantum efficiency of the Cs2Mo6I8(C3F5OO)6 nanocluster. Factors including the charge injection and exciton formation appear to be the most critical factors for the enhanced efficiency. Large differences in EQE in the devices made with the two iodide based nanoclusters lead to the conclusion that the ability to inject charge directly into the nanocluster is the most important mechanism for electroluminescence in these NCLEDs and that this process is impacted collectively by the surrounding cations (smaller is better), terminal ligands (heavier is better), and host selection (for smallest injection barriers). In summary, we have demonstrated significantly enhanced electroluminescence from phosphorescent hexanuclear metal halide nanocluster salts. Devices made with heavier apical ligands show 4-fold improvement in performance compared to apical halides and nearly 3 orders of magnitude improvement in previous demonstrations utilizing Mo−Cl based nanoclusters, suggesting a potential pathway for further enhancing NCLEDs. The improvement in these devices is attributed, in part, to the high photoluminescence efficiency of the Cs2Mo6I8(C3F5OO)6 clusters with stronger spin−orbital coupling from the combination of heavier inner and apical ligands. In addition, we find that carrier injection efficiency and exciton formation efficiency into the nanocluster are the key limitations and can be overcome through combined pairings of host, cations, and apical ligands with aligned interactions from the host for efficient carrier injection directly into the nanocluster. Collectively, these improvements highlight the promising potential of phosphorescent, earth-abundant, and inexpensive metal halide nanocluster emitters.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00837. Mass spectrometry analysis of Cs2Mo6I8(C3F5OO)6 to confirm the composition of synthesized nanoclusters, photoluminescence (PL) intensity as a function of airexposure time, and UPS spectra of nanoclusters (PDF)

EXPERIMENTAL SECTION

Core Mo 6I 12 nanoclusters were synthesized by heating MoI 3 (Chempur-Germany) in a vacuum sealed quartz ampule (10 cm long, 1.4 cm diameter) at 650 °C with heating and cooling rates of 2 °C/min.36 To improve the solubility of the nanoclusters in common organic solvents, Mo6I12 was then converted to Cs2Mo6I14.37 Powder XRD patterns of Mo6I12 and Cs2Mo6I14 were collected to confirm the products.36 (TBA)2Mo6I14 was synthesized by reacting Cs2Mo6I14 and tetrabutylammonium iodide (TBA I), and Cs2Mo6I8(C3F5OO)6 was prepared by reacting Cs2Mo6I14 and Ag(C3F5OO)6 in the dark. Products were confirmed using high resolution mass spectrometry (Xevo G2-XS QTof), 19F NMR in acetone-d6, and energy dispersive X-ray spectroscopy (EDAX).28,38 All wet chemical reactions were carried out using dry, air-free solvents in a Schlenk line under Ar atmosphere. Absorption, photoluminescence (PL), excitation scans, and quantum yield (QY) of the nanoclusters prepared in air-free, dry acetonitrile were measured using a PerkinElmer spectrometer and PTI QuantaMaster 40 spectrofluorometer, respectively. A 500 nm long pass filter was used for the PL measurement to prevent wavelength doubling, and PL spectra were recorded with 375 nm excitation. The PL quantum yield of the nanocluster solutions were measured using 405 nm excitation under nitrogen atmosphere. Patterned indium tin oxide (ITO) (20 ohm sq) substrates were cleaned sequentially in soap, deionized water, and acetone for 3 min in a sonicating bath and then rinsed in boiling isopropanol for 3 min followed by UV−ozone treatment. A 40 nm thick layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spin-cast at 6000 rpm for 30 s onto the ITO as a hole-injection layer (HIL) and was annealed at 125 °C for 30 min. We adopted a guest/host architecture to fabricate the emissive layer in the NCLEDs. We directly mixed the host and the nanocluster solution in various volume ratios and spin-cast the emissive layer as dilutely doped amorphous films. A range of hosts were surveyed by testing their suitability in devices with 10% nanocluster doping concentration including 3-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Richard R. Lunt: 0000-0003-4248-6312 Funding

This work was supported by the U.S. Department of Energy (DOE) Office of Science, Basic Energy Sciences (BES), under Award DE-SC0010472. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Markus Ströebele and Arin-Daniel Fuhrmann from the Department of Chemistry at the University of Tüebingen for insightful discussions. We also would like to thank the MSU Mass Spectrometry Core for access to the mass spectrometry facilities.

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REFERENCES

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DOI: 10.1021/acsaem.8b00837 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX