FRET-Assisted Deep-Blue Electroluminescence in Intercalated

Article pubs.acs.org/cm

FRET-Assisted Deep-Blue Electroluminescence in Intercalated Polymer Hybrids Umberto Giovanella,*,† Giuseppe Leone,† Francesco Galeotti,† Wojciech Mróz,† Francesco Meinardi,‡ and Chiara Botta† †

CNR, Istituto per lo Studio delle Macromolecole (ISMAC), via E. Bassini 15, 20133 Milano, Italy Dipartimento di Scienze dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milano, Italy

‡

S Supporting Information *

ABSTRACT: Polymer/layered silicate nanocomposites (PSNs), obtained through a bottom-up strategy with both free and intercalated poly(styrene) carrying fluorescent terfluorene side-chains, allow for the fabrication of optically transparent films with controlled morphology and fine-tuning of the organic/inorganic interface properties. The successful assessment of the hybrid material as an efficient emitter in a light-emitting device is reported, and the complex mechanism responsible for its emission is elucidated. The approach allows planarization and chemical and photostabilization of short intercalated oligo(fluorene)s whose emissive properties are enhanced thanks to exciton localization. Both photoluminescence and electroluminescence are assisted by a sensitization mechanism exploiting resonant energy transfer from nonplanar, nonintercalated polymer pendant conformers wrapping the silicates. A single-layer, solution-processable, hybrid light emitting diode (LED) displays deep-blue electroluminescence with a record external quantum efficiency, for this class of materials, of 1.2%, maximum luminance of 860 cd/m2, Commission Internationale de l’Eclairage (CIE) 1931 (x, y) chromaticity coordinates of (0.158, 0.114), and low efficiency roll-off thanks to the separation of the emissive region from the charge transport one.

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INTRODUCTION The demand for polymeric materials with superior chemical and physical properties has motivated vigorous research about organic−inorganic hybrids.1,2 Research efforts are now focused into the possibility of fabricate nanoassembling superstructures by functionalizing layered inorganics with conjugated dyes in order to develop fluorescent host−guest materials with specific and tunable features for functional applications and for the development of nanotechnologies.3 This interest is further motivated by the need of new concepts for durable and efficient emitters, mainly for the blue, as a necessary key step for the development of the next generation of consumer displays and solid-state lighting systems. In fact, one critical issue regarding the commercialization of organic light-emitting devices (OLEDs) is overcoming the instability of organic molecules in the presence of oxygen and moisture.4,5 Moreover, the short operational lifetime and poor color quality of blue emitting devices based on conjugated emitters is one of the reasons why current OLED displays are not as popular as LED type. The fabrication of integrated hybrid materials combines the chemical and thermal stability of the inorganic counterparts with the organic conjugated moieties functions optimizing and maximizing complementary properties (e.g., density, permeability, mechanics). Hence this strategy is leading to breakthrough results in sensors, functional coatings and applications in electronics and energy sectors. In this respect, layered silicates are particularly appealing for fabricating such functional hybrids6,7 because of their adsorptive properties, cation© 2014 American Chemical Society

exchange ability and high specific surface area, which permit to easily tune the interaction between the emitting centers by surface chemistry and a sandwich-type intercalation,8−13 and to improve the barrier to the oxygen and moisture14 thus protecting the emitter against degradation.15,16 Recently, layered silicate have been successfully proposed as a strategy to disaggregate blue-emitting polyfluorenes17 and prevent the formation of keto-defects with red-shifted emission.18−20 Nonetheless, the application of such hybrids in LEDs is still limited to few examples with poor electro-optical properties.15,16,18,19,21,22 More specifically, all the PSN-based hybrid OLEDs (HyPLEDs) reported so far are based on materials obtained by conventional solution blending between the inorganic and the preformed polymer,23 often with an uncontrollable polymer chain intercalation.19 The best HyPLEDs date back to 10 years ago and despite the huge efforts of various groups to tackle this challenge, the expectations have not been fulfilled yet. The highest external quantum efficiency (EQE) of 1% has been reported by Kim and co-workers,19 for a blue emitting polyfluorene, but spoiled by the contribution of photo-oxidizable macromolecules not intercalated in the 2D lamellar structure. Lee et al.21,22 first reported the fabrication of silicate hybrids including intercalated emitting poly[2-methoxt-5-(2′-ethylhexyloxy)-1,4-phenylevinyReceived: May 23, 2014 Revised: July 18, 2014 Published: July 20, 2014 4572

dx.doi.org/10.1021/cm501870e | Chem. Mater. 2014, 26, 4572−4578

Chemistry of Materials

Article

Scheme 1. Simplified Illustration of PF22 under Investigationa

a

A schematic view of the FRET mechanism occurring in the hybrid upon photoexcitation is shown. The structure of the poly(styrene), carrying terfluorene sidechains, polymer is reported on the right.

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lene] (MEH-PPV) by solution blending. OLED fabricated from such composite exhibited a high EQE (0.38%) which was enhanced by 100 times compared with that of the pure MEHPPV device. Marchese and his group intercalated bifunctional polyhedral oligomeric silsesquioxanes bearing a cyanine moiety and an amino group, reducing both the dye’s mobility and aggregation.24 Frey and co-workers achieved a stabilized blue electroluminescence from polyoctylfluorene confined in a layered metal dichalcogenide envisaging new opportunities for the design of long-lived blue LED, but efficiency of the device was not reported.16 In this context, we recently showed an alternative, facile and robust bottom-up approach for fabricating blue-luminescent poly(styrene) (PS) with oligo(fluorene) side-chains/fluoromica-type clay hybrids.25 This nanofabrication process involves the hybrid’s construction and the polymer growth using noncovalently fluoromica grafted precursors that become organized mainly through chemical processes.26,27 The key advantages of this strategy are the superior intercalation ability of the (co)monomers with respect to the preformed polymer, the controlled polymerization propagation behavior, and hence, the possibility to tune the polymer microstructure and morphology. The experimental results indicated that the strategy was rational and efficacious: the materials, with a controlled interlayer arrangement of the organic moieties, exhibited tunable optical properties, easy processability and deep-blue photoluminescence (PL). A PL quantum yield (PLQY) as high as 0.95 in the solid-state with a specific content of fluorescent oligo(fluorene) (22% by weight with respect to PS, hereafter PF22) was achieved, claiming for further development of such materials. In this work we demonstrate the incorporation of the PF22 material into a HyPLED, and we shed light onto the intriguing mechanism responsible for the light emission (Scheme 1). We find out that the lying-flat intercalated oligo(fluorene) pendants exhibit enhanced photostability, and are sensitized through a Förster resonant energy transfer (FRET) mechanism.28 The HyPLED shows the highest deep-blue EL efficiency reported so far for a true solution processed single layer device based on a PSNs, with an EQE of 1.2%, low efficiency roll-off, maximum luminance of 860 cd/m2 and Commission Internationale de l’Eclairage CIE 1931 (x, y) chromaticity coordinates (0.158, 0.114).

EXPERIMENTAL SECTION

The PSNs were synthesized as recently reported by the authors25 and filtered and centrifuged prior to the assessment in device. Glass substrates coated with indium tin oxide (ITO) were cleaned ultrasonically in distilled water, acetone and 2-propanol. Subsequently, a 50 nm layer of poly(3,4-ethylene-dioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) (H.C. Starck CleviosP VP.AI 4083) was spin-coated from a water solution filtered through nylon filter (pore size 0.45 mm), and annealed at 100 °C for 10 min under a N2 atmosphere. PSNs were dissolved in degassed toluene with concentration 15 mg/mL. An 80 μL portion of such solution was deposited on glass/ITO/PEDOT:PSS substrate. Finally 5 nm of barium and 100 nm of aluminum were evaporated at 10−6 mbar. PL and EL spectra were recorded using Spex 270 M monochromator combined with CCD. PLQY was measured with an integrating sphere.29 Time resolved measurements have been recorded by exciting the samples with the second harmonic of a Ti:Shappire laser (pulse duration 150 fs, repetition rate 76 MHz) and collecting the signal with a Hamamatsu streak camera coupled with a Cromex spectrograph (overall system time response shorter than 4 ps). Electrical characterization of devices was performed with Keithley 2602 source-meter combined with a calibrated photodiode. EQE was calculated using Lambertian source assumption.30 Diodes were fabricated and measured in nitrogen atmosphere. A photostability test was performed in air using Hamamatsu Lightning Cure LC5 Xenon−Mercury 100 mW/cm2 UV lamp at 365 nm with optical fiber. EL-PL images were obtained using a Nikon Eclipse TE2000-U inverted confocal microscope. Atomic Force Microscopy investigations were performed using a NT-MDT NTEGRA instrument in contact mode in ambient conditions.

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RESULTS AND DISCUSSION A synthetic bottom-up procedure, optimized in our laboratory in the last three years,11,12,25 was followed to fabricate PSNs with different ratio of fluorescent terfluorene side groups of 0.6, 1.2, and 22 wt % (PF0.6, PF1.2 and PF22 respectively) with respect to PS. Our bottom-up procedure consists of two steps. The first one involves the fluoromica silicate intercalation of a 2,2,6,6-tetramethyl-1-piperidyloxy (TEMPO) bearing an ammonium functional moiety by cation-exchange in a water/ alcohol solution. The second step consists of the in situ copolymerization of styrene (St) with a St para-substituted with (oligo)fluorene moieties (St-Fl3) by surface-initiated TEMPOmediated polymerization. In this way, an intimate mixing of the “soft” polymer with the “hard” inorganic component was achieved, thus preventing fluoromica nanoparticle aggregation and making the materials easily processable from colloidal solution. All the materials of each synthetic step were 4573



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Figure 1. (a) Normalized steady-state PL spectra of PF22 diluted toluene solution and film obtained by exciting at 350 nm (vertically shifted for clarity). (inset) PLQY of films. (b) Time-resolved PL spectra of PF22 film collected at different delays after the excitation pulse. (inset) Corresponding normalized PL intensity decays recorded at selected wavelengths. Chromatic stability of steady-state PL spectra upon UV exposure for 0 (solid line), 5 (dotted line), and 60 s (dashed line) of PF22 film (c), blend22 (d), or neat St-Fl3 (e).

corresponding blend22 (0.59), PF0.6 and PF1.2 (0.71 and 0.82, respectively). These results rule out the hypothesis that the redshift of the PF22 emission originates from aggregation phenomena as in the case of neat St-Fl3 film and blend22. Interestingly, the PLQY of PF22 is even higher than that of StFl3 diluted solution (0.81) and would be inconsistent with selfabsorption effects in the film.32 We tentatively explain the intriguing photophysical behavior of PF22 in terms of a planarization of fluorenyl−fluorenyl backbone. This interpretation is motivated by the similar batochromic shift of PL band earlier observed in oligo(fluorene)s once fluorenes are linearly fused in a ladder-type conformation.33,34 The planarization of fluorenyl−fluorenyl backbone is typically accompanied by extension of the conjugation length and thus by a red-shift of emission band.35,36 The planar morphology can be generated spontaneously through different methods in long oligo(fluorene)s39 or polyfluorenes,37,38 but it has never been observed in short isolated oligo(fluorene)s. Indeed, the backbone must exceed the minimum length of five monomer units to be stabilized,39 and ladder-type oligomers have represented up to now the simplest strategy for achieving planar blue-emitting chromophores. We suggest that the planarization of the terfluorene moieties is achieved in PF22 by the oligomer nanoconstriction in between the fluoromica lamellae. The more planar arrangements of fluorene rings explains the observed red-shift in PF22 film emission. The appearance of a well resolved vibronic progression spaced by ∼0.16 eV, which is a characteristic energy of the vibrational modes in conjugated compounds,40 is a footprint of the narrow energy distribution of emitting species, consistently with the high conformational rigidity of the intercalated fluorene oligomers. Indeed, the

characterized by XRD, TGA, NMR, and the molecular weight are in the range 6−20 × 104 g/mol as determined by SEC.19 A blend of PS and St-Fl3 with the same ratio PS:St-Fl3 as in PF22, but without inorganic content, is prepared for comparison (hereafter called blend22). The steady-state PL spectra of PF22 solution (Figure 1a), and of PF0.6, PPF1.2, and St-Fl3 (Figure S1), show an evident vibronic progression, as usual for all the fluorene-based oligomers, with the pure electronic 0−0 transition peaked at about 403 nm. When solid-state is considered, the PL of samples with a low content of fluorene moiety, PF0.6 and PPF1.2, does not change significantly (Figure S2), while the PF22 emission presents an overall batochromic shift of 25 nm, with 0−0, 0−1, and 0−2 well resolved vibronic replica at 428, 453, and 481 nm respectively, and a weak shoulder at about 400 nm (probably related to the residual 0−0 transition observed in the PF22 solution, see later). This red-shift might not be ascribable to inner filter effect (self-absorption) since timeresolved PL measurements clearly show the copresence of two distinct contributions to the emission (Figure 1b). The first component, corresponding to the origin of the PF22 vibronic progression in solution, displays a quite fast intensity decay and almost disappears after 100 ps. The second component, significantly slower, dominates the emission beyond 150 ps resembling the steady-state PL spectrum of the hybrid films. To investigate whether aggregation phenomena are responsible for the red-shifted component, a PLQY study is performed, since in fluorene-derivatives a PL quenching accompanies the red-shift of the PL band upon aggregation.31 The PLQY measurements of thin films deposited on quartz substrates placed in an integrating sphere show a higher value for PF22 (0.95) with respect to neat St-Fl3 film (0.43), the 4574



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Figure 2. 5 μm × 5 μm topography (a), lateral force (b), and force modulation images (c) of PF22 film. The yellow circles drive the eye to the fluoromica tactoids lying on the film surface detected by probing lateral force and embedded in the polymer layer located by force modulation technique. Film section profile along the yellow dashed line (d). 150 μm × 150 μm PL (e) and EL images (f) of the hybrid device obtained by confocal microscope at the edge of cathode electrode.

PF22 solution, despite the higher content of fluorene moieties, the average donor−acceptor distance is out of dipole−dipole coupling range while in the solid-state the compactness and wrapping of the free polymer chains on the polymerintercalated fluoromica tactoids makes the FRET process efficient.25 Only a weak residual emission at 403 nm in PF22 film suggests an incomplete FRET, hence the ratio between free and intercalated polymer chains can be further optimized. This picture is supported by the lifetime data collected at different wavelengths, in agreement with a classical FRET process in the presence of a distribution of donor-to-acceptor distances.28 In fact, the decay of the 403 nm donor emission is strongly nonexponential, being as fast as 20 ps in the first part, then showing a progressive lengthening (inset of Figure 1b). On the contrary, the PL emission of the acceptors (428−516 nm band) is a slow nearly single-exponential (lifetime around 365 ps) as expected for noninteracting dyes on which the excitation is localized after the FRET. This scenario well explains the extremely high PL efficiency of the PF22 film, related to the energy transfer to planar, isolated and chemically stable chromophores, acting as efficient traps for the excitation and preventing the population of lower energy weakly emissive states.17 The PLQY of PF1.2 and PF0.6 films, lower with respect to PF22 and comparable to the PLQY of St-Fl3 diluted solution, is consistent with a less efficient energy transfer from free polymer to highly efficient intercalated chromophores. A valuable property of the PF22 compound, which makes it suitable for real-world applications, emerges from the photostability test. In fact, the incorporation of polymer chains into the interlayer galleries of the inorganic scaffold has improved dramatically their photophysical stability, a critical issue to extend operational lifetime of blue emitting devices.18 PF22 shows good chromatic stability when irradiated by 100 mW/ cm2 UV lamp at 365 nm (Figure 1c), with respect to both corresponding blend22 (Figure 1d) or neat St-Fl3 film (Figure 1e). The photodegradation of fluorene-based compounds is characterized by the appearance of the keto-defects broad green emission, peaked at around 530 nm, at the expense of the blue

fluoromica’s interlayer spacing (d001 = 1.59 nm) observed in PF2225 forces the oligomers to adopt a planar conformation lying flat between the silicate planes (dfree = 0.63 nm),41 hence producing quasi-isolated emitting moieties and preventing πstaking among conjugated backbones.18 Yet, the high solid-state PLQY observed in PF22 is not simply ascribable to the oligofluorene backbone rigidity and planarity (PLQY of the ladder-type oligo(fluorene)s is the same as parent oligomers33,42). The chemical characterization of the PSN compounds has shown the presence of free macromolecular chains25 with terfluorene moieties exhibiting a higher degree of torsion between the fluorene rings (with the term “free” we mean polymer chains bearing oligo(fluorene) groups that are not intercalated) and responsible for the emission in solution. Since the emission band of nonplanar terfluorene conformers overlaps with the red-shifted absorption of planar ladder-terfluorene33,42 and similarly, the absorption of the planar intercalated polymer in PF22 overlaps with the PL of free chains (Figure S3), we attribute the spectral behavior of PFs to the cooperative contribution of both the nonplanar and planar conformers of terfluorene side groups,25 assisted by a FRET mechanism.28 FRET approach has been largely exploited to modulate the optical properties of multichromophoric systems and, in particular, to deliver energy from chromophores outside the inorganic hosts to conjugated systems confined within the hosts43,44 or vice versa.45 The emission properties of PF22 film can be explained through a FRET process between the terfluorenes with the two different conformations, in a similar way to that observed for the poly(9,9-dioctylfluorene).37 The excitons, created on the free polymer chains bearing nonplanar fluorenyl groups (acting as donor), transfer their energy to the planar conjugated moieties (acting as acceptor) at the intercalated polymer chains. Consistently with a lower content of emitting species, in PF0.6 and PF1.2 films FRET is prevented due to the high dilution of the St-Fl3 side-chains along the PS backbone and consequently their optical properties are typical of nonplanar oligomers outside the silicates and similar to those of St-Fl3 oligomers in solution. In 4575



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emission.17 Even though the PS matrix partially prevents the fluorene oxidation and excimer formation in the blend with respect to the neat St-Fl3 film, only in the PF22 film the contribution of keto-defects emission is almost suppressed. The morphology of PF22 film, investigated by means of contact mode scanning probe microscopy, shows an homogeneous distribution of the fluorescent inorganic nanoparticles smaller than 70 nm (Figure 2a,b,c). The combination of contact mode lateral force and force modulation techniques allowed us to detect both the fluoromica tactoids lying on the film surface and those embedded in the polymer layer.11 The film surface is substantially flat with a root-mean-square roughness of 0.63 nm, peak-to-peak distance of 2 nm (Figure 2d) which are satisfactory values for a device active layer. The superior optical performance with respect to the similar polymer hybrids as well as polyfluorene derivatives, together with the excellent film forming ability, make PF22 a promising candidate as electroluminescent active layer in a solution processed HyPLEDs. This material is particularly appealing since through a FRET process emission occurs mainly from the nanoconfined oligomers lying flat within the interlayer silicate lamellae. However, nanoconfined oligomers cannot be electrically addressed because of the insulating feature of the nanocomposite.23 As a consequence, in a HyPLED device the emissive excitons are spatially confined within 2D lamellar structure,22 and separated from the region where charges are circulating (free polymer), therefore reducing charge induced exciton quenching processes. The high PLQY is clearly only one of the requirements for PLED application, in fact balanced charge injection and transport are also quite important. The presence of a significant amount of free polymer chains bearing the oligofluorene moieties is thus imperative for a successful assessment of PF22 as efficient emitter in the HyPLED. Previously, the intercalation by conventional solution blending between the inorganic and the preformed polymer has often led to a similar partial intercalation. Nonetheless, the energy transfer mechanism has never been exploited and discussed to explain emission properties of HyPLEDs, and the presence of amorphous luminescent polymer, although necessary to generate electroluminescence, has represented a detrimental issue in device operational lifetime.21 To demonstrate the crucial role of the FRET mechanism and the potentiality of PF22 as emitter, devices with the basic architecture ITO/PEDOT:PSS (50 nm)/PF22/Ba (5 nm)/Al (100 nm) were fabricated, electroluminescence (EL) has been measured together with the current density−voltage and luminance−voltage curves. The EL spectral shape of ITO/PEDOT:PSS/PF22/Ba/Al device (Figure 3a) closely resembles the corresponding PL spectrum, indicating the occurrence of the emission from the same excited state and the EL image obtained by confocal microscope shows a homogeneous emission comparable to the corresponding PL image (Figure 2e and f). We propose that the mechanism of emission observed in PF22 devices is not a straight EL from the intercalated dyes, but rather a two steps mechanism. Once electrons and holes are injected into the active layer, they travel the bulk thanks to the side group conjugated moieties of the free polymer chains and form the excitons. Successively energy is transferred toward the nanoconfined planar segments, not directly reachable by the charge carries due to the insulating property of lamellar silicate, through the same FRET mechanism discussed above. There-

Figure 3. EL spectrum of single layer ITO/PEDOT:PSS/PF22/Ba/Al HyPLED device (a); (inset) EQE of HyPLED devices. Best EQE vs luminance (b), a representative JLV characteristics (c), and luminous efficiency (LE) vs current density (J) (d) of ITO/PEDOT:PSS/PF22/ Ba/Al device.

fore, through proper separation of electrically generated excitons on the free polymers from the photon emitting centers, we obtain in the hybrids the reduction of excitoncharge quenching processes. This substantially reduces efficiency roll-off drawbacks in the HyPLED (see later). While PS is essential both in dispersing fluoromica within the organic continuous phase and in environmental protection of the conjugated counterpart during device operation, the very large content of this insulating polymer in HyPLEDs could limit charge transport. Hence, the dependence of the device performances on the PF22 thickness has been evaluated (Supporting Information, Table S1) The device with an optimal 135 nm thickness of PF22 active layer shows an EQE higher than PF0.6, PF1.2 and blend22 based devices prepared in the same conditions (inset of Figure 3a and Table S2). EQE value of 1.2% is reached in PF22 HyPLED at a luminance of 60 cd/m2 decreasing to 1% at 100 cd/m2 (Figure 3b). The maximum luminance of 860 cd/m2 is higher than best reported results for this type of hybrid devices,19,22 and CIE chromaticity coordinates of (0.158, 0.114) place it in the highly desired deep blue region of the visible spectrum, without unpleasant green band. The diode characteristics show the onset for both current and light46 (Figure 3c) as low as 5.7 V despite the large content of both insulating silicate and PS chains. The low efficiency roll-off47 of PF22 based HyPLEDs achieved thanks to the reduced charge trapping at the emitting centers, with a critical current density (J0) of 160 mA/cm2 (Figure 3d), lowers the power consumption, and reduces the electrical stress. The success of our approach to fabricate efficient polymer hybrid emitters is further established if we consider that the current performance are obtained by a real single active layer device without the carrier regulating layers.48

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CONCLUSIONS In summary, we fabricate a new efficient HyPLED exploiting an indirect excitation mechanism of the protected emitting moieties. The device is based on an electroluminescent PSNs 4576



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with both free and intercalated poly(styrene) chains carrying fluorescent terfluorene side groups. The device shows the highest deep-blue electroluminesce efficiency reported so far for PSNs, with an EQE of 1.2%, low efficiency roll-off, maximum luminance of 860 cd/m2 and CIE chromaticity coordinates of (0.158, 0.114). According to the material design, the bottom-up approach allows us to achieve, at the same time, planarization and chemical and photostability of short oligo(fluorene)s. The photophysical characterization shows that thanks to a FRET mechanism taking place among polymer chains bearing terfluorene moieties in different conformations, the emission occurs mainly from intercalated planar fluorenyl-fluorenyl backbones possessing enhanced quantum efficiency and stability. This system provides an innovative strategy to optimize the radiative recombination process through the spatial separation of the active dyes from the charge transport region.

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ASSOCIATED CONTENT

S Supporting Information *

Tables including thickness dependence of PF22-based HyPLED performance and summary of all HyPLED performance; PL spectra of PF0.6, PF1.2, St-Fl3, and blend22 solutions and films; spectral overlap between absorption of intercalated planar chains and emission of not intercalated chains; EL spectra of PF0.6 and PF1.2 based HyPLED. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Fondazione Cariplo (Projects SOLCO 2010-0528 and EDONHIST 2012-0844). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors wish to thank Dr. Giovanni Ricci and Dr. William Porzio for the helpful discussion. REFERENCES

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(40) Schumacher, S.; Ruseckas, A.; Montgomery, N. A.; Skabara, P. J.; Kanibolotsky, A. L.; Paterson, M. J.; Galbraith, I.; Turnbull, G. A.; Samuel, I. D. W. J. Chem. Phys. 2009, 131, 159006−1. (41) dfree is calculated by subtracting the thickness of the fluoromica layer (0.96 nm) from the basal spacing determined from the (001) diffraction peak. (42) Li, Z. H.; Wong, M. S.; Tao, Y.; Lu, J. Chem.Eur. J. 2005, 11, 3285−3293. (43) Vohra, V.; Calzaferri, G.; Destri, S.; Pasini, M.; Porzio, W.; Botta, C. ACS Nano 2010, 4 (3), 1409−1416. (44) Bossart, O.; De Cola, L.; Welter, S.; Calzaferri, G. Chem.Eur. J. 2004, 10, 5771−5775. (45) Olivero, F.; Fabio Carniato, F.; Bisio, C.; Marchese, L. Chem. Asian J. 2014, 9, 158−165. (46) Current onset is the voltage at which the current density/ voltage behavior switch from ohmic to exponential regime. The light onset is defined as the voltage at which 1 cd/m2 is recorded. (47) The efficiency roll-off can be quantified by the critical current density J0, which represents the current density at which the EQE drops to half of its maximum value, see: Murawski, C.; Leo, K.; Gather, M. C. Adv. Mater. 2013, 25, 6801−6827. (48) Trattnig, R.; Pevzner, L.; Jäger, M.; Schlesinger, R.; Nardi, M. V.; Ligorio, G.; Christodoulou, C.; Koch, N.; Baumgarten, M.; Müllen, K.; List, E. J. W. Adv. Funct. Mater. 2013, 23, 4897−4905.

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FRET-Assisted Deep-Blue Electroluminescence in Intercalated

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