Non-doped deep blue light-emitting electrochemical cells from charged organic small molecules

Abstract

Blue emitters are still elusive for solid-state light-emitting electrochemical cells, and limit the development of white light emitting devices for display applications. We report the photophysical, electrochemical, thermal and electroluminescence properties of two charged organic deep blue-emitters. The synthesized materials showed intense blue fluorescence with high quantum efficiencies and good thermal stabilities. Single-layered non-doped LEC devices were fabricated from solution. The fabricated non-doped LEC devices exhibited deep blue electroluminescence centered at 432 and 434 nm with the CIE coordinates of (0.15, 0.09) and (0.16, 0.10), respectively for compounds 1 and 2, which are quite close to the National Television System Committee (NTSC) standard for blue color coordinates. Electroluminescent devices operated at very low turn-on voltages reveal maximum luminance of 118 cd m⁻² for compound 1 and 136 cd m⁻² for compound 2. These promising results are highly desirable for the development of low cost white light-emitting devices.

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Introduction

Optoelectronic devices based on organic semiconducting materials have emerged as an attractive alternative to their inorganic counterparts during the last few decades. Even though the devices employing inorganic emitters show highly desirable performance,^1–3 the low abundance and high cost of these emitters inhibit their large-scale use in lighting technology. The fascinating performances of organic light-emitting diodes (OLEDs) have successfully reached commercialization, but still having adversity over the work function of the electrodes and manufacturing cost of the devices.⁴ In this perspective, researchers have focused on the efficient alternative for next generation lighting technologies called light-emitting electrochemical cells (LECs). In LECs, a single emitting layer enables both ionic and electronic conduction which performs the charge injection, transport and recombination of charges.^5,6 This unique nature of LECs simplifies its preparation and manufacturing cost. To date, luminescent materials used in LECs can be divided into two main classes such as ionic transition-metal complexes (iTMCs)⁷-based LECs and polymer⁵-based LECs. In iTMC-LECs, cationic iridium(III) complexes are the most explored active material used in LEC device fabrication.^8,9 The highest performing LECs are a class of phosphorescent iridium iTMCs emitters when compared to other reported emitters.^10–12

LEC with high stabilities and efficiencies were reported for longer wavelength devices by employing cationic biscyclometalated iridium(III) complexes as the active material.^13–18 Despite all these significant features, several challenges still remains, such as low turn on voltage and suitable materials for blue and deep blue emitting LECs, which is highly desired for white-light applications.^19,20 Ir-complexes based blue emitters are challenging to make as a result of the intrinsically narrow energy gaps and involving high energy barriers.^21,22 A number of blue LECs based on ionic iridium complexes have been reported but only to provide low efficiency, life time and luminance.^23–26 Polymers have intrinsic drawbacks such as laboriousness in the synthesis and purifications and structure defects which hinders their device performance.²⁷ To overcome these barriers, alternative approaches for the discovery of blue emitters are on high demand for the development of energy saving full color displays and solid state lightings.

In recent time, the commercialized optoelectronic devices employed organic small molecules for large scale applications with high efficiency and lifetime.^28,29 For the first time, Hill et al.³⁰ reported light-emitting electrochemical cells with perylene-based organic small molecule as active material mixed with additional ions for electrochemical doping, while Edman et al.³¹ demonstrate that non-ionic small molecules are emissive compounds in light-emitting electrochemical cells which were similar to polymer based devices. To resolve charge transport in single layered LEC device, small molecule must be charged to simplify film architecture and to avoid tricomponent blend. Recently, Bolink et al. reported an LEC device based solely on charged cyanine dyes as the active components and achieved promising performances with near-infrared electroluminescence.³² Recently, our research group has reported an ionic small molecules³³ that shows better performance than non-ionic small molecules.³⁴ Bolink et al. demonstrates that, ionic derivatives are potential hosts for charge-transport in host-guest LECs and achieved blue electroluminescence centered at 474 nm with maximum luminance of 420 cd m⁻².²¹ Currently, Zysman-Colman et al. shows that solution processed electroluminescence devices for the development of low cost LEC devices from charged organic thermally activated delayed fluorescence (TADF) emitters.³⁵ However, those reported blue LECs employing charged organic small molecules are ranging from 470–490 nm.^21,33

Owing to its unique molecular structure, fluorene has excellent optical and electroluminescence properties and has emerged as an attractive material for display techniques. In addition, very favorable functionalization along with tunable color emission of fluorene ring brought the focus of flat-panel display application to scientific interest. Fluorene derivatives are well-known pure blue emitters for electroluminescent devices. Single layered LEC devices were reported for blue electroluminescence ranging from 388–455 nm by employing ionic fluorene derivative as active component without additional ions in its active layer.^36–38

Here we report, fluorene–naphthalene based non-doped deep blue-emitting electrochemical cells processed from solution. The organic emitters are charged to carry out the charge transport in single-layered electroluminescence devices. The device gives off deep blue emission, with the CIE coordinates of (0.15, 0.09) and (0.16, 0.10) respectively, for active materials 1 and 2, quite close to the NTSC standard for blue coordinates of (0.14, 0.08). Therefore, we envisage that these deep-blue-emitting compounds shall pave the way towards the development of white LECs in very near future.

Experimental section

General information

All the reagents and solvents used for synthesis were purchased from commercial suppliers and used without any further purification. The reactions were carried out under an inert atmosphere. ¹H NMR spectra of compounds were recorded at room temperature using a Varian unity Inova 500 MHz spectrometer. Purification of materials and spin coating process were carried out under ambient condition. UV-visible absorption and photoluminescence emission spectra were measured at room temperature using UV-vis spectrometer, Lamda-20, PerkinElmer and Hitachi F-7000 FL spectrophotometer in 10⁻⁵ M dichloromethane solution. Photoluminescence quantum yields (PLQYs) of both the compounds were measured in dichloromethane solution using 9,10-diphenylanthracene as a standard. The optical band gap (E_g) energy level was obtained from the absorption onset potential of materials. Electrochemical properties of compounds 1 and 2 were measured in dichloromethane solution (10⁻³ M) using cyclic voltammetry (CV) model of potentiostat/galvanostat (Iviumstat) voltammetric analyzer with a scan rate 100 mV s⁻¹. An electrochemical cell consists of platinum as the working electrode, platinum wire as the counter electrode and Ag/AgCl as the reference electrode. 0.1 M dichloromethane solution of tetra-n-butylammonium hexafluorophosphate (TBAPF₆) was used as a supporting electrolyte. All the potentials were recorded against ferrocenium/ferrocene (Fc⁺/Fc) was used as an internal standard. The HOMO energy level of the molecule calculated from the onset of oxidation potentials using the formula E_HOMO = −4.4 − E_onset(ox) and the LUMO was obtained by the adding the E_g to the calculated HOMO energy level.

Synthesis of 4

Suzuki cross-coupling between 2,7-dibromo-9,9-bis-(6-bromo hexyl)fluorene (3) (0.40 g, 0.62 mmol) and 6-methoxy-2-naphthalene boronicacid (0.27 g, 1.35 mmol) in the presence of Pd(PPh₃)₄ (46 mg, 0.04 mmol), tetra-n-butylammonium bromide (TBAB) (32 mg, 0.1 mmol) and K₂CO₃ (0.69 g, 5.00 mmol) in toluene/water (2 : 1, v/v) was degassed for 30 min and then refluxed for 16 h under argon atmosphere. The reaction mass was cooled to room temperature, extracted with dichloromethane. Then the organic layer washed with water and brine solution, dried over sodium sulfate and solvent was evaporated. Finally, the residue was purified by silica-gel column chromatography using hexane and ethyl acetate as the eluent. Yield (70%). ¹H NMR (500 MHz, CDCl₃, ppm) δ 8.02–8.12 (m, 2H), 7.77–7.92 (m, 8H), 7.64–7.76 (m, 4H), 7.14–7.20 (d, 4H), 3.90–4.10 (s, 6H), 3.33 (t, 4H), 2.15 (m, 4H), 1.70 (m, 4H), 1.05–1.20 (m, 8H), 0.70–0.85 (m, 4H).

Synthesis of 5

Compound 5 was synthesized by following the procedure described above for compound 4, using 2,7-dibromo-9,9-bis-(6-bromo hexyl)fluorene (3) (0.40 g, 0.62 mmol) and 6-ethoxy-2-naphthalene boronicacid (0.29 g, 1.35 mmol) in the presence of Pd(PPh₃)₄ (46 mg, 0.04 mmol), TBAB (32 mg, 0.1 mmol) and K₂CO₃ (0.69 g, 5.00 mmol). Yield (65%). ¹H NMR (500 MHz, CDCl₃, ppm) δ 8.02–8.12 (m, 2H), 7.76–7.94 (m, 8H), 7.63–7.76 (m, 4H), 7.14–7.24 (d, 4H), 4.12–4.35 (m, 4H), 3.33 (t, 4H), 2.15 (m, 4H), 1.45–1.62 (m, 10H), 1.05–1.40 (m, 8H), 0.70–0.90 (m, 4H).

Synthesis of 1

A solution of (4) (0.35 g, 0.43 mmol) in 5 mL toluene was refluxed with 1-methylimidazole (1 mL) for 24 h and cooled to room temperature. Reaction mass was concentrated, then dissolved in 3 mL methanol and followed by anion metathesis with a saturated aqueous NH₄PF₆ solution. Then the mass was filtered, washed with excess water and dried. Yield (79%). ¹H NMR (500 MHz, d₆-DMSO, ppm) δ 8.90 (s, 2H), 8.02–8.12 (m, 2H), 7.77–7.92 (m, 12H), 7.64–7.76 (m, 4H), 7.14–7.20 (d, 4H), 4.10 (s, 6H), 3.95 (t, 4H), 3.70–3.80 (s, 6H), 2.15 (m, 4H), 1.70 (m, 4H), 1.05–1.20 (m, 8H), 0.70–0.85 (m, 4H).

Synthesis of 2

It was prepared by above described procedure for 1, using compound (5) (0.33 g, 0.39 mmol) with excess 1-methylimidazole. Yield (77%). ¹H NMR (500 MHz, d₆-DMSO, ppm) δ 8.85–8.95 (s, 2H), 8.18–8.30 (m, 2H), 7.75–8.05 (m, 12H), 7.50–7.62 (m, 4H), 7.18–7.40 (m, 4H), 4.10–4.30 (m, 4H), 3.90–4.05 (t, 4H), 3.65–3.80 (s, 6H), 2.10–2.30 (m, 4H), 1.38–1.62 (m, 10H), 0.90–1.20 (m, 8H), 0.60–0.70 (m, 4H).

LEC fabrication and characterization

The LEC devices were fabricated on pre-cleaned ITO coated glass substrate with the structure of ITO/PEDOT:PSS/emissive layer/Al. Prior to spin-coating PEDOT:PSS layer onto ITO substrate, the cleaned substrate were UV-ozone treated for 30 min. The 80 nm thickness of PEDOT:PSS were spin coated at 2000 rpm for 20 s and dried at 120 °C for 30 min in vacuum. The active layer of both the compounds were spin-coated onto the ITO/PEDOT:PSS layer with the 2000 rpm for 20 s from cyclohexanone solution and annealed at 120 °C for 1 h. All the process was performed under ambient environment. Subsequentially, aluminium was thermally evaporated onto the emissive layer in a closed vacuum chamber under high pressure. Electroluminescence characteristics of the LEC devices were measured using a Keithley 2400 source meter and calibrated with a silicon photodiode. The EL spectrum and CIE coordinates were measured by using an Avantes luminance spectrometer.

Results and discussion

Synthesis of ionic organic emitters 1 and 2 is outlined in Scheme 1. Suzuki coupling of 2,7-dibromo-9,9-bis(6-bromo hexyl)fluorene (3)³⁸ with 6-methoxy, 6-ethoxy-2-naphthalene boronicacids gave the coupled product (4) and (5). The final products 1 and 2 were synthesized by refluxing (4) and (5) with 1-methylimidazole, followed by an ion exchange with ammonium hexafluorophosphate.

Scheme 1 (a) 6-Methoxy, 6-ethoxy-2-naphthalene boronicacids, Pd(PPh₃)₄, TBAB, K₂CO₃, toluene/H₂O; (b) 1-methylimidazole, toluene.

Photophysical properties

The UV-visible absorption and photoluminescence examined in dichloromethane solution (10⁻⁵ M) are shown in Fig. 1. Both the compounds exhibited similar absorption peaks at 284, 312 and 347 nm which are characteristic of transition π–π* transition of the compounds. From the onset of absorption spectrum, calculated optical energy gaps were found to be 3.24 and 3.23 eV, for compound 1 and 2, respectively.

Fig. 1 UV-visible absorption (solid lines) and photoluminescence spectra (dotted lines). Compound 1 (black) and compound 2 (red) in DCM (λ_exc at 347 nm).

The photophysical properties of synthesized compounds are detailed in Table 1. Solution photoluminescence of compound 1 exhibited deep blue emission peak at 389 nm with shoulder peak at 406 nm and peak at 390 nm with shoulder peak at 408 nm was observed for compound 2 and shows high photoluminescence quantum yield of 0.95 and 0.98 for compound 1 and 2. Annealed thin film emissions (Fig. S1†) of both the compounds were observed at 399 nm which is slightly red-shifted around 10 nm compared to their solution PL emission which arises due to the close packing of the molecules in the amorphous thin film states.

Table 1 Photophysical properties

Compounds	λUV,max^a (nm)	λPL,max^b (nm)	λPL,max^c (nm)	Φf^d	Eg^e	Td^f (°C)
a Absorption in DCM (10^−5 M).b Photoluminescence in DCM (10^−5 M).c Solid photoluminescence.d Photoluminescence quantum yield (PLQY) in DCM against DPA as standard.e Optical band gap calculated from onset of absorption spectrum.f Thermal decomposition (Td) temperature.
1	347	389	411	0.95	3.24	379
2	347	390	407	0.98	3.23	339

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Fig. 2 shows the solid-state emission spectra of both the compounds in blue region with emission maximum at 411 nm for compound 1 and 407 nm for compound 2, respectively. In comparison to their solution photoluminescence, red shifted emission was observed which may have occurred due to its intermolecular interaction in the solid state.

Fig. 2 Photoluminescence spectra of solids of compound 1 (black) and compound 2 (red) (λ_exc at 347 nm).

Thermal properties

Thermal properties of the compounds were evaluated by thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere.

The decomposition temperature (T_d, corresponding to 5 wt% loss) of 379 and 339 °C was observed for compound 1 and 2, respectively (Fig. 3). All the materials show excellent thermal stabilities upon TGA scan which indicates that all the materials are highly appreciable for the electroluminescent applications. From DSC scan upon heating from 0 to 300 °C the glass transition (T_g) at 86 and 76 °C and endothermic melting transition (T_m) at 149 and 199 °C were observed for compound 1 and 2, respectively shown as in (Fig. S2 and S3†). Moreover, no distinct crystallization was observed in this series upon heating at a temperature beyond T_g. The high T_g values implies that it is possible to get stable glasses from these materials.

Fig. 3 TGA curves for compound 1 and compound 2.

Electrochemical properties

Electrochemical behavior of both the compounds was probed by cyclic voltammetry (CV). The oxidation process of compounds 1 and 2 in dichloromethane solution were shown in Fig. 4. The highest occupied molecular orbital (HOMO) energy was estimated from the electrochemical onset potentials.

Fig. 4 Cyclic voltammograms of compound 1 (black) and compound 2 (red).

The calculated HOMO energy level of the molecules was found to be −5.58 and −5.55 eV for compounds 1 and 2, respectively. From the edge of absorption spectra, the lowest unoccupied molecular orbital (LUMO) energy levels were found to be −2.34 and −2.31 eV for compounds 1 and 2, respectively. The photophysical results indicate that both the molecules are good for blue electroluminescent devices.

Electroluminescent properties

The single-layer LEC devices were prepared on patterned indium tin oxide (ITO) coated glass substrate. The fabricated device consisting active materials with the structure of ITO/PEDOT:PSS/active layer/aluminium. Prior to fabrication of devices, ITO patterned glass substrate as cleaned by sonication. To increase the cells reproducibility, 80 nm thickness of PEDOT:PSS layer was spin coated. The active layer of 100 nm thickness was spin coated from cyclohexanone solution containing 2% of the active materials and finally aluminium cathode was thermally evaporated onto the active layer in a high vacuum chamber. The fabricated devices were subjected into Keithley 2400 source meter to determine LEC device performances. EL spectra were measured by using single voltage scan (current 10 mA) with sweep rate of 0.5 V s⁻¹. Fig. 5 shows the featureless electroluminescence (EL) spectra of compound 1 and 2 in deep blue region with the maximum peak at 432 and 434 nm, respectively.

Fig. 5 EL spectra of compound 1 (black) and compound 2 (red).

As shown in Fig. 5, the EL spectra were shifted to longer wavelength as compared to their photoluminescence spectrum due to their intermolecular interaction in solids.^39,40 However, both the compounds shows CIE coordinates of (0.15, 0.09) and (0.16, 0.10), which is almost equal to the coordinates for pure blue color. The time-independent brightness (L–V) and current density (J–V) were investigated by voltage scanning with sweep rate of 0.5 V s⁻¹.

Fig. 6 depicts the maximum brightness of 118 cd m⁻² and current density of 80 mA m⁻² was reached for compound 1 with low turn-on voltage of 5.1 V.

Fig. 6 Current density–voltage–luminescence (J–V–L) curves of compound 1.

For compound 2, maximum brightness of 136 cd m⁻² and current density of 100 mA m⁻² were observed and shown in Fig. 7. The results are summarized in Table 2.

Fig. 7 Current density–voltage–luminescence (J–V–L) curves of compound 2.

Table 2 Electroluminescence properties

Compounds	ELmax^a	Von^b (V)	Lmax^c (cd m^−2)	Effmax^d (cd A^−1)	CIE (x, y)^e
a Maximum luminescence.b Turn-on voltages at 1 cd m^−2.c Maximum efficiency (brightness in cd m^−2 at maximum efficiency is in parentheses).d Maximum luminescence (current efficiency in cd A^−1 at maximum luminescence is given in parentheses).e Commission International de I'Eclairage coordinates (CIE) measured at 50 mA.
1	432	5.1	118	0.15	(0.15, 0.09)
2	434	4.5	136	0.14	(0.16, 0.10)

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Current efficiency calculated for compounds 1 and 2 was found to be 0.15 and 0.14 cd A⁻¹, respectively. In comparison with brightness of both the compounds, the luminance can be seen increasing with decrease in driving voltages. And moreover, it shows that the brightness decreases with decreasing current density. The same behavior has been reported for both iTMC-based LECs^41–43 and charged organic small molecule based LECs,³⁵ which may be the reason of decreased quenching of excitons due to the reduction of charge carriers or excited states. Both the devices shows low brightness for deep blue devices which shall be attributed to the non-radiative decay processes such as quenching of excitons in the active layer.⁴⁴ However, these results highly indicate the potential of both the compounds for developing white light-emitting devices. Moreover, low turn-on voltage of 5.1 and 4.5 V and the deep blue chromaticity CIE coordinates of (0.15, 0.09) and (0.16, 0.10), were obtained respectively for both the compounds which is quite close to pure blue coordinates of (0.14, 0.08).

Conclusion

In this work, two new deep blue emitting charged organic materials containing fluorene–naphthalene segments were synthesized in good yield and characterized through various spectroscopic techniques. The synthesized compounds exhibit excellent thermal stabilities with high decomposition temperatures and glass transition temperatures. The single-layered LEC devices based on two active materials showed deep blue EL emissions with the CIE coordinates of (0.15, 0.09) and (0.16, 0.10), respectively, extremely close to NTSC color gamut for blue coordinates. These CIE values represent the pure blue CIE that has ever been obtained from LECs. The constructed deep blue LECs exhibited peak luminance of 118 and 136 cd m⁻², respectively. These results are of particular importance because they are essential to the development of energy-saving full-color displays and solid-state lighting.

Acknowledgements

This research work has been supported by Basic Science Research program through National Research Foundation of Korea (NRF) financial support by Ministry of Education, Science and Technology (NRF-2013R1A1A4A03009795) and Brain Korea 21 Plus project.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02156e

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