All-Solution-Processed Organic Light-Emitting Diodes Based on

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All-Solution-Processed Organic Light-Emitting Diodes Based on Photostable Photocrosslinkable Fluorescent Small Molecules Lionel Derue, Simon Olivier, Denis Tondelier, Tony Maindron, Bernard Geffroy, and Eléna Ishow ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05197 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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ACS Applied Materials & Interfaces

All-Solution-Processed Organic Light-Emitting Diodes Based on Photostable Photocrosslinkable Fluorescent Small Molecules Lionel Derue,† Simon Olivier,‡§ Denis Tondelier,≠ Tony Maindron,§ Bernard Geffroy,*,† Eléna Ishow*,‡ †

LICSEN, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif-sur-Yvette

Cedex, France. ‡CEISAM–UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, 44322 Nantes, France. §Université Grenoble Alpes, CEA, LETI, MINATEC Campus, Département Optique et Photonique, Laboratoire des Composants pour la Visualisation, 38054 Grenoble Cedex 9, France. ≠LPICM, CNRS, Ecole Polytechnique, Université Paris Saclay, 91128, Palaiseau, France. KEYWORDS.

Solution-processed

OLEDs;

small

fluorophores;

cross-linking;

photopolymerization; acrylate monomers.

ABSTRACT. We demonstrate herein the fabrication of small molecule-based OLEDs where four organic layers from the hole- to the electron-transporting layers have successively been deposited by using an all-solution process. The key feature of the device relies on a novel photopolymerizable red-emitting material, made of small fluorophores substituted with two 1 ACS Paragon Plus Environment


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acrylate units, and displaying high-quality film-forming properties as well as high emission quantum yield as non-doped thin films. Insoluble emissive layers were obtained upon UV irradiation using low illumination doses, with no further need of post-curing. Very low photodegradation was noticed, giving rise to bright layers with a remarkable surface quality, characterized by a mean RMS roughness as low as 0.7 nm after development. Comparative experiments between solution-processed OLEDs and vacuum-processed OLEDs made of fluorophores with close architectures show external quantum efficiencies in the same range while displaying distinct behaviors in terms of current and power efficiencies. They validate the proof of concept of non-doped solution-processable emissive layers exclusively made of photopolymerized fluorophores, thereby reducing the amount of components and opening the way toward cost-effective fabrication of solution-processed OLED multilayer architectures.

1.

INTRODUCTION Organic light-emitting diodes (OLEDs) are commonly fabricated by thermal evaporation

under high vacuum or solution-based deposition.1 Thermal evaporation provides easy access to stacks of multiple layers such as charge transport or carrier blocking layers providing highperformance devices. However, this technology is essentially restricted to mass production of small- and middle-size OLED displays.2 By contrast, it is widely accepted that solution processes allow for low-cost production of large-area devices. However, due to solubilization limitations, the fabrication of multilayered devices following the latter technique remains very challenging. To address this issue, several approaches have been adopted such as the polyelectrolyte deposition route,3 the use of “orthogonal solvents”,4-7 the introduction of a buffer layer8 or the use of organic semiconductors with crosslinkable moieties. Among all these techniques, the 2 ACS Paragon Plus Environment

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crosslinking approach appears particularly attractive since it provides insoluble layers with no solvent restrictions. It implies polymerizable units like siloxane, styrene, trifluorovinyl ether, benzocyclo-butene, cinnamate, oxetane or acrylate that have largely been used for various purposes ranging from organic electronics9-10 to highly resolved structure photofabrication11-12 and optical data storage.13 Depending on the reactive monomers, crosslinking can be initiated thermally or photochemically. Thermal polymerization can be efficient but usually requires heating times ranging from minutes to hours at high annealing temperatures (> 100 °C), which is often the case for trifluorovinyl-ethers,14-15 benzocyclo-butenes16 or styrenes.17-18 Unfortunately, at such high temperatures, the underlying organic active layers and flexible substrates constituting the devices may undergo severe thermal damages. In this respect, photocrosslinking represents a very attractive alternative method, overcoming these constraints and enabling layer patterning using standard photolithography. Various photopolymerizable moieties such as cinnamates, oxetanes or acrylates have thus been incorporated into functional materials, essentially as OLED hole-transporting layers. In order to solve the issue of weak performance19-20 and undesirable UV photodamage21 encountered with cinnamate units that have recently been improved,22 the group of K. Meerholz et al. has extensively studied oxetane-based materials used as insoluble hole-injection layers,23-24 electroluminescent polymers layers,25-26 and more recently phosphorescent layers.27 This family of crosslinkable molecules however requests a two-step procedure: i- a first curing step at around 100 °C to achieve full crosslinking since the photopolymerized films remain still soluble after UV irradiation; ii- a second post-curing step at 180 °C after development to destroy the radical cations left after the photoinitiation and propagation steps. In the case of photocrosslinkable phosphorescent complexes, it turns out that the use of a host matrix and large photodegradation were mentioned, conducting to reduced performances.27 In contrast with oxetane units, the use of 3 ACS Paragon Plus Environment


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polymerizable acrylate moieties advantageously eliminates any post-curing step after photopolymerization. Whereas acrylate monomers are widely used in the industry for construction adhesives, only a few OLED architectures fabricated out of them have been published in the literature to the best of our knowledge. In 1996, P. Schuhmacher et al. reported the synthesis of photocrosslinked acrylate-functionalized triphenylenes as charge transport materials, coupled to tris(8-hydroxyquinolinato)aluminium (Alq3) to provide a two-layer OLED with reasonable performances.28 More recently, a crosslinkable derivative of Alq3 was published by M. Lu et al. to inhibit Alq3 crystallization observed upon thermal deposition under vacuum.29 Surprisingly, no particular attention has been paid so far on the fabrication of photopolymerizable layers containing acrylate units, serving as emitting layers despite the high challenges linked to the competitive photoinitiation and photoluminescence processes. Moreover, there is a strong lack of systematic comparisons of the performances between solution-processed and vacuumprocessed emitting layers in OLEDs. This implies the need for novel compounds with similar structures amenable to form neat thin films upon photopolymerization or thermal evaporation. In the past, we reported on a fluorescent red-emitting starburst triarylamine derivative (called FVIN),30-31 yielding upon vacuum evaporation bright amorphous thin films that were successfully incorporated as emitting layers in OLEDs. Following these lines, we want to show that the derivation of FVIN with two acrylate groups provides novel non-doped emitting layers, conducting to insoluble fluorescent materials after wet deposition and photoirradiation. In this way, a solution of electron-transporting material could be spread without causing dissolution of the lower fluorescent emitting layer. Such an approach opens the way for the first time to our knowledge toward all-solution processed multilayered OLEDs made of small fluorescent molecules with no solvent restriction. Comparative studies between both vacuum and solution

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deposition processes have been performed to carry out conclusive cross-linking effects on the device performance.

2.

RESULTS AND DISCUSSION

2.1. Structural description The structure of the photopolymerizable compound FVIN-A presents strong similarities with the FVIN one, namely: i- strong push-pull charge transfer from the triphenylamino to the dicyanovinylidene units that is responsible for the emission in the visible range, ii- bulky groups to limit intermolecular π-π aggregation and subsequent emission quenching upon dark state formation (Figure 1).

Figure 1. Chemical structures of the red emitters FVIN-A and FVIN for solution and vacuumprocessed emitting layers respectively.

In addition, FVIN-A incorporates two flexible chains terminated each with an acrylate unit, known to react readily under UV illumination. The length and chemical nature of the chain were 5 ACS Paragon Plus Environment


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carefully chosen such that the reactive acrylate units present enough mobility to facilitate the photopolymerization process in the solid state. Longer alkyl chains have been avoided to limit the risks of poor charge migration as already reported.32-33 The additional introduction of chiral centers in a racemic ratio imparts the material with remarkable amorphous and film-forming properties, characterized by a glass transition temperature Tg of only 11.9 °C (Table 1). This low Tg value, well below room temperature, ensures efficient photoreaction in neat films without requiring the addition of liquid diluent or distinct multi-acrylate monomers. More details on the FVIN-A synthesis could be found in previous studies.34

2.2. Photophysical properties in thin films The photophysical properties of FVIN and FVIN-A processed as thin films display strong similarities. Their solution properties have been described extensively elsewhere.35 A first absorption band was observed in the UV range at 346 nm (3.58 eV) and 319 nm (3.87 eV) for FVIN-A and FVIN respectively (Figure 2a – Table 1). a)

Figure 2. (a) Normalized absorption and emission spectra of solution-processed FVIN-A (red curves) and evaporated FVIN (black curves) 50 nm-thin films. (b) Electronic density of the HOMO, LUMO and LUMO+1 of an alkyl model of FVIN-A (FVIN-Am) and FVIN, and 6 ACS Paragon Plus Environment

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oscillator strengths of the HOMO→LUMO and HOMO→LUMO+1 transitions following TDDFT computations (B3LYP, 6-31G+(d,p)) in the gas phase.

Such band could be mainly ascribed to the S0→S2 transition owing to its larger oscillator strength as determined from time-dependent density functional theory (TD-DFT) using B3LYP as the exchange-correlation functional and 6-31+G(p,d) as a basis set. The calculations, performed in the gas phase, reveal that the transition involves charge transfer from the HOMO, spread on the triphenylamino core and to a lesser extent to the dicyanovinylidene moiety, to the LUMO+1 distributed on the bulky peripheral biphenyl units (Figure 2b). The slightly lower electronic transition S0→S2 found at 3.52 eV for the alkyl model of FVIN-A (FVIN-Am) against 3.54 eV for FVIN match the absorption band order of the experimental spectra. This order could be explained as a result of additional LUMO+1 stabilization brought by the more electronwithdrawing benzyloxy groups for FVIN-A compared to the tert-butyl ones in FVIN. The discrepancy found between the experimental and theoretical data comes from the contribution of weaker higher-order transitions (S0→Sn=3,4,5), lying at close energy, that should also be taken into account.

Table 1. UV-vis photophysical properties of FVIN-A and FVIN thin films and TD-DFT calculations. Tg (°C)a µ (D)b λmax(abs) (nm)c λcalc (nm)b λmax(em) (nm)c Φfd FVIN-A 11.9 9.68 458, 346 452.89, 351.89 635 0.37 FVIN 86.0 11.70 479, 319 465.11, 350.00 631 0.34 a -1 b DSC measurements using a 30 °C.min thermal gradient. From theoretical TD-DFT calculations (B3LYP/6-31+G(d,p)). cMeasurements in thin films. dDetermined using a 405 nm excitation and an integrating sphere.



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Conversely, the order of the absorption bands in the visible, ascribed to the S0→S1 electronic transition, was reversed for FVIN-A absorbing at higher energy (458 nm, 2.71 eV) than FVIN (479 nm, 2.59 eV). TD-DFT calculations confirm such a trend with the energy of the electronic transitions computed at 2.74 eV and 2.67 eV for FVIN-A and FVIN respectively. This evolution originates from the slightly larger HOMO stabilization for FVIN-A compared to FVIN (Table 2).

Table 2. Electrochemical properties and frontier orbital energies of FVIN-A and FVIN derived from cyclic voltammetry and TD-DFT calculations. Reductiona HOMO LUMO LUMO+1 Oxidationa red onset Eox Eox Ered Ered E E E E E b c b c (V) (V) (V) (V) (eV) (eV) (eV) (eV) (eV)c FVIN-A 0.67 0.58 -1.52 -1.45 -5.68 -5.85 -3.65 -2.87 -1.80 FVIN 0.65 0.58 -1.50 -1.45 -5.68 -5.63 -3.65 -2.75 -1.58 a + From electrochemical measurements with Fc /Fc used as a reference (Eox = 0.39 V vs ECS) in 0.1 M nBu4PF6 acetonitrile on glassy carbon electrode. bCalculated from the equation EHOMO/LUMO = -[Eox/redonset(vs Fc+/Fc) + 5.1]36. cFrom theoretical TD-DFT calculations (B3LYP /631+G(d,p)).

The LUMOs for FVIN-A and FVIN, specifically located on the dicyanovinylidene moiety, stay closer in energy accordingly to the quite remote and thus more electronically decoupled peripheral substituents. The emission spectra also showed very small discrepancies with a large Stokes shift (> 5000 cm-1) and emission maxima peaking at 635 and 631 nm for FVIN-A and FVIN respectively. Such remarkable Stokes shift originates from the large geometry distortion encountered in the excited state in polar surroundings as investigated upon femtosecond transient absorption spectroscopy.37 Indeed, FVIN-A and FVIN are strongly polar molecules with computed dipole moments of 9.68 and 11.70 D respectively, conducting to strong stabilization of the charge transfer excited state in the solid state. Finally, the emission quantum yields Φf, measured using an integrating sphere after excitation at 405 nm, were found equal to 0.37 and 8 ACS Paragon Plus Environment

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0.34 for FVIN-A and FVIN respectively. These values are quite remarkable for red-emitting fluorophores whose first excited state S1 is usually subject to strong internal conversion to the ground state S0 (so called “energy gap law”). Despite the large discrepancy in terms of rigidity (FVIN material exhibits a glass transition temperature Tg at 86.0 °C, namely much higher than that measured for FVIN-A at 11.9 °C), no large difference in the emission signal occurs. Rigidity effects thus play no role in pristine layers, contrarily to polarity effects. Indeed, the alkyl chains in FVIN-A, surrounding the backbone that supports the radiative center, tend to decrease the local polarity, hence less distortion may operate in the singlet excited state and a slightly larger value for Φf is obtained. From spectroscopic comparisons of both fluorophores, we can positively conclude that the functionalization of the red emitter with two acrylate moieties exerted no deleterious effect on the photophysical properties of the molecule in the solid state.

2.3. Comparative electrochemical properties of the emitters The redox potentials were measured in acetonitrile at a scan rate of 50 mV.s-1 using saturated calomel electrode (SCE) and glassy carbon electrodes as the reference and working electrodes respectively. Cyclic voltammetry (CV) was employed to determine experimentally the HOMO and LUMO energy levels of FVIN and FVIN-A compounds from the onset oxidation and reduction potentials (Figure 3).



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Figure 3. Cyclic voltammograms of FVIN-A (red) and FVIN (black) in 0.1 M nBu4PF6 acetonitrile solution, using a glassy carbon working electrode and a SCE reference electrode at a 50 mV.s-1 scan rate.

The first oxidation potential was observed at 0.67 V vs Fc+/Fc for FVIN-A against 0.65 V for FVIN, and could be ascribed to the formation of the triarylammonium radical monocation (Table 2). Also a quasi-reversible reduction potential could be observed at -1.52 V vs Fc+/Fc for FVINA against -1.50 V for FVIN, which can be attributed to the reduction of the dicyanovinylidene moiety. The presence of both oxidation and reduction potentials thus shows the ambipolar character of FVIN-A and FVIN. The electrochemical measurements allowed us to experimentally calculate the HOMO and LUMO levels of these two compounds from the empirical law EHOMO/LUMO = -[Eox/redonset (vs Fc+/Fc) + 5.1]36 with Fc+/Fc referred to SCE. We thus found the HOMO levels at -5.68 eV for both FVIN-A and FVIN, taking into account the solvation energy before and after oxidation in solution. In the same way, the LUMO levels calculated from the onset reduction potentials were found similar at -3.65 eV for FVIN-A and FVIN. Once more, both molecules behave electrochemically in a very similar manner, showing that the oxidation and reduction processes in solution are not influenced by the photopolymerizable alkyl chains. It 10 ACS Paragon Plus Environment

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is worth noting that the energies of the LUMO levels inferred from electrochemical measurements differ significantly from those computed by TD-DFT while no such difference occurred for the HOMO levels. As computations were carried out in the gas phase, this marked difference, especially for anionic species, is not surprising given the local electric contribution of the solvent and electrolyte to the energy stabilization of the reduced species in solution.

2.4. Impact of photocrosslinking on the density and emission properties of thin films Acrylate monomers are commonly polymerized through a radical mechanism. Therefore, to initiate the polymerization process by light, the commercial photoinitiator diphenyl(2,4,6 trimethylbenzoyl)phosphine oxide (TPO) was added. Upon UV irradiation at 365 nm, TPO decomposes into free radicals that initiate the polymerization of FVIN-A. Advantageously, the absorption spectrum of FVIN-A displays a minimum at 365 nm, enabling low TPO concentrations and low irradiation doses to initiate the polymerization (see ESI - Figure S1). Previous investigations have shown that an amount of 3 mol. % TPO corresponds to an optimal value yielding insoluble polymerized FVIN-A layers.34 However, achieving sufficient crosslinking without photodamage to the active materials can be challenging in some cases. Free radicals generated by the TPO decomposition or UV irradiation can produce emission quenching if the fluorophore undergoes deleterious reactions. Evolutions of the fluorescence quantum yield (commonly called photoluminescence quantum efficiency PLQE) of FVIN-A and FVIN layers as a function of the TPO concentration and UV dose are presented in Figure 4. The procedure used for these measurements and PLQE calculations closely follow the steps proposed by Friend et al (see ESI - Figure S2).33 The fluorescence quantum yield Φf of a pristine evaporated FVIN thin film was valued to be 34 % as shown in Figure 4a. Upon UV illumination, Φf remains constant, 11 ACS Paragon Plus Environment


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meaning that FVIN is photostable in the solid state. Incorporating acrylate moieties in the FVIN red emitting structure to generate FVIN-A has no impact on Φf, even after photopolymerization. In the same way, Φf remains almost unchanged whatever the UV dose (for a given TPO ratio fixed at 3 mol. %) (Figure 4a) or the TPO ratio up to 5 mol. % (for a given UV exposure dose fixed at 6.3 J.cm-2) (Figure 4b). Finally, no change in the absorption spectrum could be detected after photopolymerization (Figure 4c). All these observations indicate clean photopolymerization with no visible formation of by-products that would negatively interfere with the emitting layer. After removing unpolymerized material, Φf surprisingly drops down to 0.28. Since photochemical degradation was ruled out from the above analyses, we suspected a significant change in the fluorophore surroundings after development. Fluorescence intensity measurements of the films before and after irradiation, as well as after irradiation and development were thus performed to gain better insight into the origin of the photoluminescence decrease (see ESI – Figure S3). Given the complex multiexponential decay, we adopted an intensity-averaged time constant , so that the decay dynamics and fluorescence quantum yield could be expressed as follows (Eq. 1). Φf = kr τ

with τ =

kr kr + knr

(Eq. 1)

where kr and knr designate the radiative (fluorescence) and non-radiative rate constants respectively. The values of kr were found unchanged at 1.02-1.05×108 s-1, meaning that the electronic transitions, namely the energy levels and the molecular structure of the radiative species, are not impacted by the photoirradiation process. By contrast, the non-radiative component knr increased significantly from 1.98×108 s-1 before development or irradiation, to 2.72×108 s-1 after development. This notable change can reasonably be assigned to enhanced free volume around 12 ACS Paragon Plus Environment

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the fluorophores due to removal of the non-crosslinked material. This imparts the fluorophores, akin to pendant moieties of the crosslinked network after polymerization, with larger structural mobility and torsion, leading to increased radiationless deactivation at the expense of intrinsic fluorescence. Thin films with distinct initial thickness (from 23 nm to 47 nm) were fabricated by varying the material concentration while keeping the TPO:FVIN-A molar ratio and the spin-coating parameters constant. After irradiation as above with a 6.3 J.cm-2 dose, the thickness of the developed layer was found to be halved (see ESI - Figure S4) compared to its initial value before development, whatever the film thickness. The corresponding absorbance in the visible was reduced by three times and not halved as expected from the proportionality relationship between the absorbance and film thickness (see ESI - Figure S5c). Since the absorption spectra display the same shape, the pronounced decrease in absorbance can reasonably be ascribed to less dense thin films after development and not to a change in the molar absorption coefficient of the fluorophores.



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Figure 4. (a) Photoluminescence quantum efficiencies (PLQE) of FVIN-A (3 mol. % TPO with regard to FVIN-A) and FVIN thin films as a function of the UV exposure dose (λirr = 365 nm). (b) PLQE of FVIN-A layers as a function of the TPO:FVIN-A molar ratio before and after polymerization (6.3 J.cm-2 at 365 nm). (c) Normalized absorption spectra of FVIN-A thin films after various UV exposure doses (λirr = 365 nm).

This assumption was confirmed by further refractive index measurements using ellipsometry. Refractive index increases with molecular polarizability and the density of π-electrons in the case of π-conjugated molecules and molecular thin films.38-39 The values of the refractive index n were found systematically lower for the developed thin films (n = 1.61 at 650 nm for instance) compared to those determined before development (n = 1.80 at 650 nm) (see ESI - Figure S5a). Such a considerable diminution in the refractive index after development interestingly illustrates 14 ACS Paragon Plus Environment

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the fact that photopolymerized layers loose compactness and gain free volume after development. This impacts directly fluorescence as discussed above. The microscopic surroundings of the photocrosslinked fluorophores after development become less impaired by the polyacrylate network, allowing for more flexibility and vibrational relaxation at the expense of radiative relaxation. Since any surface defect negatively affects the performances of multilayered systems, in particular for optoelectronics applications, we finally investigated the topography of thin films (5 mol. % TPO:FVIN-A ratio; initial thickness 90 nm) photoirradiated at 365 nm. Using low UV irradiation power (30 mW.cm-2 for 5 min), the thin film surface profile, recorded with a profilometer, displays excellent smoothness with a root-mean-square (RMS) surface roughness Rq of hardly 0.2 nm before development and 0.7 nm after development (Figure 5a and see ESI Figure S6).

Figure 5. Surface profile of a layer of FVIN-A doped with 5 mol. % TPO after UV irradiation at λirr = 365 nm with: (a) low irradiation dose (9 J.cm-2 corresponding to a 30 mW.cm-2 power for 5 min); (b) high irradiation dose (31 J.cm-2 corresponding to a 520 mW.cm-2 power for 1 min).

On the contrary, high-power UV irradiation (520 mW.cm-2 for 1 min) generates significant irregularities as high as 100 nm, as already reported in previous studies (Figure 5b).24 Photocrosslinking of acrylate monomers is known to induce large mechanical stress, causing 15 ACS Paragon Plus Environment


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undesirable shrinkage of the resulting photoirradiated thin films.40 Replacement of intermolecular van der Waals interactions with short covalent bonds bridging the carbon atoms of different monomers after polymerization has been proposed as the main structural explanation for such morphological changes.41 In conclusion, we have been able to generate photocrosslinked FVIN-A layers with a remarkable surface quality using a low illumination dose, which allowed us to envision their incorporation as emitting layers in OLED following an all-solution process.

2.5. Comparative solution-processed and vacuum-evaporated OLED performance In order to investigate and compare the performance of solution-processed and vacuumevaporated OLEDs, two kinds of bottom-emission OLEDs were fabricated along the following structures A and B (Figure 6). The structures differ by their emitting layer and the way the ETL is subsequently processed. The first two layers preceding the EML deposition were processed in solution using spin-coating: PEDOT:PSS, a conductive polymer blend composed of poly(3,4ethylenedioxythiophene) doped with poly(styrenesulfonic acid), was first deposited in aqueous solution and annealed at 150 °C for 10 min to remove residual water. It widely serves as a hole injecting layer thanks to its good electrical, optical and structural properties but also helps the cationic

polymerization

of

the

HTL,

made

out

of

bis(4-[6-{(3-ethyloxetan-3-

yl)methoxy}hexyloxy]phenyl)-N,N'-bis(4-methoxyphenyl)biphenyl-4,4'-diamine (QUPD) and spin-cast on PEDOT:PSS. Recent works have indeed reported proton assistance of PEDOT:PSS for oxetane ring-opening and cross-linking thanks to the interdiffusion of PEDOT:PSS and QUPD molecules at the layer interface after spin-coating.42 For the solution-processed device A, the EML, made of FVIN-A, was first spin-cast on the hole-transporting layer QUPD, photocrosslinked at room temperature at 365 nm using a 6.3 J.cm-2 irradiation dose, and directly 16 ACS Paragon Plus Environment

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developed with chloroform. Resorting to EML photopolymerisation with no need of further heating presents the great advantage of easy and cost-effective reactions conditions (no contamination, no implementation of an additional curing protocol) as well as limited risks of phase change that could occur during the heating-cooling steps. Then, 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene (TBPI) constituting the ETL was spin-cast to further extend the concept of an all-solution fabrication process. As for the device B, both the FVIN-based EML and TBPI-based ETL were vacuum-evaporated.

Figure 6. (a) Multilayered structures of FVIN-A-based OLED (device A) and FVIN-based OLED (device B), and schematic energy-level diagram in eV. Solid and hatch patterns correspond to solution-processed and vacuum-evaporated layers respectively. (b) Molecular structures used for the multilayered OLED devices A and B.

The compositions of devices A and B are as follows: 17 ACS Paragon Plus Environment


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•

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device A: ITO//PEDOT:PSS (40 nm – solution-processed)//QUPD (35 nm – solutionprocessed)//FVIN-A (25 nm – solution processed)//TPBI (50 nm – solution-processed//LiF (1 nm)//Al (100 nm).

• device B: ITO//PEDOT:PSS (40 nm – solution-processed)//QUPD (35 nm – solutionprocessed)//FVIN (25 nm – vacuum-evaporated)//TPBI (50nm – vacuum-evaporated)//LiF (1 nm)// Al (100 nm). The electroluminescence (EL) characteristics of both devices A et B are reported in Figure 7. The EL spectrum of the evaporated FVIN OLED showed a maximum emission at around 625 nm at 30 mA.cm-2 (Figure 7a). By comparison, the EL spectra of the all-solution-processed FVIN-A OLED appeared at a slightly higher energy at 600 nm at the same current density, leading to a variation of the CIE color coordinates from a red deep emission (0.61, 0.38) to an orange-red emission (0.54, 0.45).


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Figure 7. a) EL spectra and the CIE color coordinates of the solution-processed OLED comprising photopolymerized FVIN-A (red triangles) and vacuum-evaporated OLED containing FVIN (black squares). b) Corresponding current-voltage-luminescence J-V-L characteristics of device A (red symbols) and device B (black symbols). Open symbols display the current density, closed symbols display the brightness vs applied voltage. c) Current efficiency (open symbols) and power efficiency (filled symbols) vs current density calculated from the J-V-L curves in Figure 7b. d) External quantum efficiency vs current density. Red triangle and black square symbols correspond respectively to FVIN-A and FVIN based OLEDs.

This difference, already noticed in previous studies related to solution-processed OLEDs,29,26 has not to deal with crystalline aggregation since the deposited molecules form fully amorphous thin films before and after photopolymerization. We rather ascribe the blue shift of the crosslinked EML to local polarity change upon reticulation and reinforcement of the apolar contribution of the alkyl linkers since the fluorophore excited state displays positive solvatochromism.37 The current density-voltage-brightness (J-V-L) characteristics of FVIN-A and FVIN-based devices are shown in Figures 7b-c. The evaporated device turned on at a rather low voltage (4.2 V) while the solution-processed device required a higher threshold voltage of 8.7 V to obtain the same current density (Figure 7b). This observation could at first sight be explained by the reduced conductivity of the FVIN-A emissive layer despite the chain shortness and the presence of polarizing oxygen atoms,32-33, 43 affecting the intermolecular hopping process. This assumption is further supported by the lower slope (higher electrical resistance) of the J-V curve for device A compared to that for device B (Figure 7b). As for the power and current efficiencies, the solutionprocessed device A, compared to device B, remarkably displayed a higher current efficiency at 19 ACS Paragon Plus Environment


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lower injected current density (Figure 7c). This unexpected feature would prove better charge migration or injection within the crosslinked FVIN-A layer, which is contradictory with the argument of reduced conductivity presented above. To elucidate such apparent contradiction, another phenomenon has to be considered. Recently, Kido et al compared the performance of solution and evaporation-processed phosphorescent EML in OLEDs and emphasized the importance of molecular interdiffusion at the HTL-EML interface by using neutron reflectometry where HTL was made of the linear polymer poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (PTPD).44 In our case, since the HTL was made of highly cross-linked QUPD material instead of PTPD, only unilateral diffusion of FVIN-A into the HTL could occur after solution deposition. We may thus consider that hole injection becomes highly favored at low current density for solution-processed FVIN-A due to a possible intermixing at the interface. However, at larger current densities, the charge recombination zone may move far from the interface. In device A, FVIN-A molecules are logically less packed than the homologous evaporated FVIN molecules in device B. Indeed, the refractive indices n, giving insight into the material density, were determined to be equal to n = 1.61 for the photocrosslinked FVIN-A layer after development, and n = 1.82 for the evaporated FVIN layer at 650 nm.45 Therefore, charge migration upon intermolecular hopping is far less efficient in solution-processed FVIN-A OLEDs, leading to a higher threshold voltage. Although the turn-on voltage of device A is high compared to those of doped photocrosslinked phosphorescent OLEDs,18, 27, 46, its value for nondoped crosslinked fluorescent layer is the lowest measured so far.29, 47 Moreover, the brightness, valued at around 150 cd.m-2 for the spin-coated device A, represents also the highest value reported up to now for non-doped photocrosslinked fluorescent materials involving small organic fluorophores. This value was found similar to that of device B, proving that the emissive performance of the fluorescent material is not affected by the photopolymerization process. Since 20 ACS Paragon Plus Environment

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the solution-processed OLED required a higher threshold voltage compared to that for the evaporated OLED, its resulting power efficiency appeared logically lower. The values for the maximum external quantum efficiency (EQE) of devices A and B were calculated to be almost similar, namely 0.30 % and 0.37 % respectively (Table S1 – entries 1 and 2). This very slight difference comes from the more favorable photopic response for FVIN-A since close maximum current efficiencies were measured at around 0.37-0.40 cd.A-1 for both devices A and B. Given the main sources of errors and the modest electroluminescence performance, we can thus consider that both devices A and B exhibit close EQE values as depicted on Figure 7d. These results could contrast at first sight with various reports concluding on the superiority of solution-processed OLEDs over vacuum-deposited ones due to more compact layers obtained for the former upon solution spin-coating.

48-49

In these studies, no

photopolymerization was performed, and the emitting or transporting layers were still soluble, precluding further solution deposition for the upper layers. Despite the apparently modest EQE value, it is worth noting that the EL performance of the solution-processed device A actually overpasses those reported for OLEDs incorporating a nondoped photocrosslinkable fluorescent material as an EML (see ESI – Table S1). In the latter case, the EML was based on photocrosslinked methacrylate-functionalized Alq3 (entry 3)29 or oxetanefunctionalized triphenylene derivatives (entry 4).47 We could notice a maximum brightness ELmax two to three times lower than that for device A (120 cd.m-1 @ 15 V) at similar or higher voltage. As for the EQE, the only available value was for entry 3 mentioning an EQE equal to 0.1 %, namely three times lower again than that for device A. Regarding the other devices comprising either a semi-conductive polymer (entry 5)26 or phosphorescent iridium complexes (entries 68),27,

46, 50

their performances were found higher than those of device A in terms of turn-on

voltage Von (2-6 V against 8.7 V for device A) and maximum luminous efficiency LEmax (2-34 21 ACS Paragon Plus Environment


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cd.A-1 @ 5-7.7 V). Nevertheless, for all these devices, the emitters were incorporated as dopants in hole-transporting layers and not used as pure substances as the device A reported in these studies. Moreover, phosphorescent complexes display higher brightness due to triplet exciton recombination, making such comparisons between device A and the series of doped and phosphorescent devices not fully accurate In our case, the surface quality of the molecular thin layer, the absence of aggregation permitted by the glass-forming property of the fluorophores, the adoption of short oxygenated chains instead of long alkyl chains as photopolymerizable linkers, and finally the high photostability of the retained fluorophores are undoubtedly at the origin of the improved performance compared to the state-of-the-art in the field of photocrosslinkable small fluorophores. Further experiments to transfer this concept of an all solution-processed OLED to acrylate-based small fluorophores and especially those displaying thermally activated delayed fluorescence, with optimized luminance and above all improved charge transport properties, are currently under progress to achieve higher performance.

3.

CONCLUSION

In this study, we have fabricated a multilayered OLED architecture incorporating a non-doped red-emitting layer made exclusively of small fluorophores by using an all-solution process. Such an unprecedented approach has been made possible by grafting two photopolymerizable acrylate units on the fluorophores. Insoluble thin layers were obtained after UV illumination and subsequent development at room temperature without requiring a post-annealing step. Comparative photophysical and electrochemical studies with a model compound FVIN with no photocrosslinkable units showed that the acrylate moieties did not impact the electronic properties of the fluorophore in solution or in thin films. Remarkably, low thin-film shrinkage 22 ACS Paragon Plus Environment

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after photopolymerization was observed contrary to most common reports in the literature. Excellent surface properties with a mean RMS roughness of hardly 0.7 nm were obtained after development. Finally, OLEDs with four solution-processed layers from the HIL to the ETL were successfully fabricated thanks to the photocrosslinkable emitting FVIN-A material. They exhibit superior luminous efficiency at lower injected current density than that of OLED incorporating a vacuum-evaporated EML based on the homologous FVIN fluorophore. Upon solution deposition, molecular diffusion at the HTL-EML interface may occur, thereby favoring charge migration and recombination. This novel approach allowed us to obtain an EQE of about 0.3 % whose modest value was attributed to low-density EML after photopolymerization and development, and reduced charge transporting properties. Removal of non-polymerized material makes the active material more diluted, hence exciton diffusion upon hoping becomes impaired due to the lack of efficient intermolecular π molecular orbital overlap. Nevertheless, this EQE value appears among the best ones reported so far in the state-of-the-art of all solution-processed OLEDs incorporating non-doped fluorescent molecules.

4.

EXPERIMENTAL

4.1. Materials PEDOT:PSS (Clevios AI4380) was purchased from Osilla). ITO-glass substrates were purchased from Xin Yan Ltd with a sheet resistivity of 20 Ω.m-1. The cross-linkable QUPD hole transporting material was purchased from Lumtec. The FVIN and FVIN-A compounds were synthesized according to literature procedures.51

4.2. Theoretical calculations



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The molecular geometries were fully optimized by using the hybrid Becke-3 parameter exchange52 functional and the Lee-Yang-Parr non-local correlation functional52-53 (B3LYP), as well as the 6-31G+(d,p) basis set as implemented in the GAUSSIAN 09 package.54 Vertical optical transitions were computed through a time-dependent density functional approach (TDDFT) using the same functional and basis sets.

4.3. Electrochemical and photophysical measurements Electrochemical measurements were performed with a three-electrode cell consisting of a glassy carbon working electrode (1.5 mm diameter), a counter electrode made of a Pt wire coil and a saturated calomel reference electrode. The measured redox potentials were expressed vs ferrocene used herein as an internal reference. All solutions were prepared in acetonitrile with 0.1 mol.L-1 tetrabutylammonium hexafluorophosphate (nBu4NPF6) as a supporting electrolyte (Sigma-Aldrich). The solutions were deoxygenated by argon bubbling prior to each experiment. Cyclic voltammograms were performed with VSP potentiostat controlled by EC-Lab 10.02 software. Both potentiostat and software were purchased from Bio-Logic Instruments-France. Absorption spectra were recorded with a JENWAY 6800 UV/VIS spectrometer. Measurements of photoluminescence quantum yield (PLQE) in thin films were performed with an integrating sphere from LABSPHERE according to a procedure described in the ESI – Figure S2. Timeresolved fluorescence measurements in thin films were measured using the fully automated spectrofluorimeter (model Fluotime 300, PicoQuant) following the time-correlated photon counting method. Excitation was performed using a pulsed laser diode (LDH-D-C-450B) working at 450 ± 10 nm with a 70 ps full width at half maximum. The excitation and emission polarizers were set in the vertical position and at the magic angle (54.7°) respectively to get polarization-independent fluorescence signals Fluorescence decays were recorded using a 24 ACS Paragon Plus Environment

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Hybrid-PMT detector combined with an acquisition temporal resolution up to 25 ps. A 473 nm Notch filter (BPL01-473R-25) purchased from Semrock was used to discard any possible contribution of excitation light scattering.

4.4. Ellipsometry and AFM measurements Refractive index measurements were performed by ellipsometry using a spectroscopic ellipsometer Jobin-Yvon-Horiba UVISEL-SE working from the UV to NIR range (250 to 1100 nm). AFM images were obtained by a Dimension 5000, with a Nanoscope IIIa controller. Images were realized in a tapping mode, with a silicon tip with a nominal frequency of 204-497 kHz.

4.5. OLED fabrication and measurements The first two layers preceding the EML deposition were processed in solution. PEDOT:PSS (Clevios Al4083), serving as a hole injecting layer, was spin-coated on pre-cleaned and ozonetreated ITO-glass substrates before further annealing at 150 °C during 10 min to remove residual water. The HTL was solution-processed and consisted of a triphenylene-diamine derivative, N,N‘-bis(4-[6-{(3-ethyloxetan-3-yl)methoxy}hexyloxy)]phenyl)-N,N'-bis(4methoxyphenyl)biphenyl-4,4'-diamine (QUPD) crosslinked via cationic-opening polymerization in the presence of 2 wt. % 4-octyloxydiphenyliodonium hexafluoroantimonate (OPPI) as a photoinitiator. This commercial photocrosslinkable HTL was first introduced by K. Meerholz et al.55 The three fabrication steps, affecting the HTL crosslinking process, were optimized:26 an UV illumination at 365 nm (210 mJ.cm-2) was carried out to generate protons initiating the polymerization. Subsequently, a soft curing step at 100 °C during 2 min was performed to promote sufficient rotational diffusion of the oxetane side groups. Finally, an additional postcuring step at high temperatures allowed for the destruction of the radical cations left after 25 ACS Paragon Plus Environment


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photoinduced electron transfer. A fully insoluble layer was successfully obtained with a hole mobility of around 10-5 cm2.V-1.s-1 measured by space charge limited currents (see ESI - Figure S7). These results were in good agreement with the mobility data from D. Hertel et al.56-57 At this step, the EML was fabricated either by spin-casting a 47 nm-thin film of FVIN-A together with 3 mol. % TPO photoinitiator to yield device A, or upon thermal vacuum evaporation of a 25 nmthin FVIN layer to yield device B. The FVIN-A crosslinking reaction was initiated by UV irradiation at 365 nm with an energy dose of 6.3 J.cm-2 and followed by a rinsing step with chloroform to remove the non-crosslinked molecules of the emitting layer. In order to test the insolubility quality of the underneath photocrosslinked emitting layer, the electron-transport layer (ETL) made of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TBPI) was deposited following a solution process in the case of device A, and a vacuum process in the case of devices B. In the former case, chloroform was used as a solvent and provided a 50 nm-thin layer with a low RMS roughness Rq value of 0.4 nm (see ESI - Figure S8). Finally, for both devices, the top cathode made of LiF/Al was thermally evaporated. The current-voltage-luminance (J-V-L) characteristics of the devices were measured with a regulated power supply (Laboratory Power Supply EA-PS 3032-10B) combined with a multimeter and a 1 cm2 area calibrated silicon photodiode (Hamamatsu). The spectral emission was recorded with a SpectraScan PR650 spectrophotometer. All the measurements were performed at room temperature and at ambient atmosphere with no further encapsulation of the devices. The luminous efficiencies were converted to EQE and power efficiencies assuming Lambertian emission.

ASSOCIATED CONTENT


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Supporting Information. Absorption characteristics of TPO, description of the procedures for PLQE and time-resolved fluorescence measurements, AFM topography of FVIN-A and FVINA/TBPI bilayers, hole-mobility measurements of QUPD thin films. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Phone :+33-1-6933-4382 * E-mail: [email protected]. Phone :+33-2-5112-5375 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Co-funding CNRS-CEA n° LS81085.

ACKNOWLEDGMENT

This work was supported by the Chimtronique program from Commissariat à l’énergie atomique et aux énergies alternatives (CEA). The authors thank J.-C. Vanel from PICM laboratory for the equipment software development and S. Jung, J. Tran, J. Farci for measuring instrument assistance. M. Boujtita and N. Szuwarski are deeply acknowledged for their kind advices regarding the electrochemical measurements. The CNRS is deeply acknowledged for its strong support in the PhD co-funding CNRS-CEA n° LS81085.



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TOC GRAPHIC


All-Solution-Processed Organic Light-Emitting Diodes Based on

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