Frequency-Selective Photobleaching as a Route to Chromatic Control

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Frequency-Selective Photobleaching as a Route to Chromatic Control in Supramolecular OLED Devices Yu-Tang Tsai, Hsiang-Fang Liu, Bo-Ji Peng, Kuo-Pi Tseng, Ming-Cheng Kuo, Ken-Tsung Wong, Guillaume Wantz, Lionel Hirsch, Guillaume Raffy, André Del Guerzo, and Dario M. Bassani ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06640 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

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ACS Applied Materials & Interfaces

Frequency-Selective Photobleaching as a Route to Chromatic Control in Supramolecular OLED Devices Yu-Tang Tsai,†,§ Hsiang-Fang Liu, ‡ Bo-Ji Peng, ‡ Kuo-Pi Tseng,‡ Ming-Cheng Kuo, ‡ KenTsung Wong*,‡ Guillaume Wantz,§ Lionel Hirsch,§ Guillaume Raffy,† Andre Del Guerzo,† and Dario M. Bassani*,† †

Inst. of Molecular Science, CNRS UMR 5255 and Univ. Bordeaux, F-33405 Talence, France

§

IMS, Univ. Bordeaux, Bordeaux INP, ENSCBP, CNRS UMR 5218, F-33400 Talence, France

‡

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

KEYWORDS. Self-assembly, energy transfer, patterning, electroluminescence, organic electronics, fluorenes, biuret.

ABSTRACT

We report a series of molecules that spontaneously self-organize into small

electroluminescent domains of sub-micrometer dimensions when dissolved in tetrahydrofuran. The self-assembled spherical aggregates have an average size of 300 nm in diameter and exhibit effient energy transfer from the blue to the green or red component. The aggregates can be chromatically addressed or patterned by selective bleaching the energy acceptor component

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using a laser source. This allows the fabrication of electroluminescence devices by directly photopatterning the active layer without the need of additional steps. Sub-micron features (700 nm) can be achieved using a collimated light source.

INTRODUCTION Supramolecular self-assembly is a promising strategy for attaining resolutions much beyond those available by top-down methodologies. In this approach, reversible interactions between molecular components are used to direct the formation of desired nano- or micrometer sized architectures that are trapped either energetically or kinetically. Such architectures may possess a high degree of molecular order that is interesting for applications in molecular electronics or where very small features are desirable, as in the fabrication of single point emitters.

A

significant drawback of this approach is that it is difficult to control the spatial arrangement of the individual assemblies over large areas. This can limit practical applications where patterning of different properties with high resolution is required such as, for example, in the fabrication of multifunctional devices. In the case of organic light-emitting diode (OLED) displays, much effort has been devoted to develop lithographic processes for precise patterning of the emissive layer that would supplant or complement the currently used industrial processes based on evaporation the individual colors through a fine metal mask. In these and other organic electronic devices, the sensitivity of the active layer's organic components towards exposure to the harsh conditions of traditional photolithography is a serious limitation.1 Possible routes that have been explored are based on the photopatterning of an intermediate resistive layer2, 3 or electrode.4, 5 6 Other techniques such


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as microcontact printing,7 laser transfer ablation, stamp-transfer,8 and aligned microfibers9 have also been investigated. To date, however, few examples of direct patterning of a solutionprocessable active layer exist, even though this would be the most cost-effective route for largearea, low-cost devices. For example, the introduction of a light-sensitive oxetane functionality in the emissive compound can be used to induce cross-linking reactions leading to the insolubilization of the material.10-12 Using this technique, Gather et al. showed that features down to 2 µm are obtained by UV irradiation through a mask.13 Other polymerization reactions can be used14-17 and resolutions of ca. 600 nm were achieved using an acrylate precursor.18 A different strategy based on the introduction of a photochromic diarylethene moiety led to materials in which emission could be photocontrolled by the opening and closing of the photochrome. The latter is an efficient hole trap in its closed ring form, thus gating charge transport in the device.19, 20

Direct writing of color information onto an emissive material is a form of chromatic addressing and patterning that has been achieved using e-beam lithography to destroy a polymer matrix in an OLED device.21 Once burned by the electron beam, the emission of the active layer was shifted from blue to green through degradation and cross-linking, and lines with a resolution of 2 µm were obtained by raster scanning of the device. It occurred to us that a similar result could be more easily obtained by exploiting the very efficient energy transfer processes that are present in self-assembled aggregates. In these assemblies, it is common for Förster resonance energy transfer (FRET) to proceed efficiently over long distances and even a small quantity of a lower band-gap dopant can be sufficient to completely quench the emission of the higher energy donor material.22-24 We and others have taken advantage of this to construct emissive aggregates whose color can be tuned over a wide range of the visible spectrum by mixing a small number of


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mutually compatible constituents.25-28 We therefore reasoned that an energy acceptor that could be efficiently photo-converted to a non-acceptor by a high-intensity laser and whose degradation products do not interfere with the emission of the higher energy donor component would allow photo-patterning of the colors in an OLED device. With this in mind, we now report the use of frequency-selective photobleaching as a means to achieve spatial resolution that is limited by the diffraction of the incident irradiation. Beyond the direct control of emission color, the system described also offers an example of an organic electronic device responding to an external stimuli (light) to change its operation. Such behavior is not trivially introduced in inorganic semiconductor devices and may offer significant opportunities for future molecule-based electronic components.

Methods: Fabrication and measurements of layered nano-aggregate EL device. Electroluminescent devices having the device structure of Figure 1 were fabricated using commercially available ITO coverslips as an anode and LiF/Al as a cathode. The structure of the device included a layer of indium tin oxide (ITO) with a sheet resistance of 10 - 20 Ω/square. This substrate underwent a wet cleaning procedure of successive 30 min ultrasonic bath treatment in acetone, ethanol, and deionized water at room temperature. The substrates subsequently underwent a UV-ozone cleaning treatment and a layer of PEDOT:PSS was spin coated at 4000 rpm for 60s. The PEDOT:PSS was then baked at 180 °C under nitrogen for 1 h. Then, the emitting layer (EML) was deposited by spin-coating on top of the PEDOT:PSS layer. Solutions (10-4 M) of blended 1 / 3 (10:1) and 2 /3 (10:1) were prepared by dissolving the solid materials in THF (dried over Na/ benzophenone and distilled prior to use). Dissolution was achieved by gentle shaking of the vial.


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The solutions were spin-cast onto the substrates at 1000 rpm for 60sec using a syringe to deposit approximately 50 µL of solution, and the solvent was allowed to evaporate in air. After the aggregates were spin-cast, the electron transport layer, TPBi, was thermally evaporated under vacuum (ca. 10-6 mbar) at a low deposition rate of 0.3 Å s-1. The deposited thickness (50 nm) was monitored using a thickness monitor inside the vacuum chamber close to the substrate holder. Thicknesses were checked with a Tencor AS-IQ profilometer. Bilayer cathodes of lithium fluoride LiF (1 nm thick) and aluminium Al (120 nm thick) were then evaporated through a shadow mask. All devices had an active area of 10 mm2. Samples were then stored and characterized under inert atmosphere in a nitrogen glovebox (O2 and H2O < 0.1 ppm). Currentvoltage-luminance (I-V-L) curves were measured using a Keithley 4200 semiconductor analyser coupled to a Si-photodetector (Hamamatsu, S2281-01) with a preamplifier (Hamamatsu, C9329). The setup was calibrated with a Minolta CS-100 luminance meter. External quantum efficiency measurement system coupled to an integration sphere.

Optical image measurements. Measurements were performed on a Picoquant Microtime 200 inverted confocal fluorescence microscope (CFM), using a PicoHarp 300 multichannel single-photon counter and two MPD SPADs. The excitation originated from a diode laser at 375nm (PicoQuant LDH-D-C-375) operated in pulsed mode (40–300ps at 5MHz repetition rate) or in continuous mode. The laser beam was coupled into a polarization maintaining single-mode fiber optic, collimated and finally injected by 90° reflection on a 80%T/20%R spectrally flat beam splitter into the microscope oil immersion objective (100 X UPLSAPO, N.A. 1.4). Emitted light was collected by the same


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objective and, after the tube-lens and the pinhole, was diverted into an intensity-corrected spectrometer (Andor SR300i) equipped with a Newton EMCCD for emission microspectroscopy measurements, or to a beam-splitter and the two MPD SPADs for Fluorescence Lifetime Imaging Microscopy (FLIM), where a band-pass filter [420-480nm] and a long-pass filter [>570nm] were placed before each detector to select the donor and the acceptor emission, respectively. In FLIM images, a fast-FLIM algorithm was used to calculate the average lifetime of each pixel, defined as the first time of photon arrival minus the time of steepest increase in the onset of the decay. Lifetime fittings were performed by tail-fitting as the emission was accumulated on a broad spectral range and the instrument response function (IRF) is wavelength dependent. The fit was done according to = ∑ ⁄ and the average decay times were defined as ∑ ⁄ . The EL devices were placed flat on the sample tray and EL light was analyzed either through the CFM setup for spectroscopy measurement, or was alternatively diverted to a tube-lens and an imaging EMCCD (512×512 pixels, Hamamatsu ImagEM).

RESULTS AND DISCUSSION The compounds used in this study (compounds 1 – 3, Figure 1, see Scheme S1 and S2 in supplementary information for details on synthesis and characterization) are related to a family of bis-biuret-containing rigid luminophores we have previously investigated. Similarly to their para isomers, 1 - 3 spontaneously form hollow spheres (vesicles) when solubilized in an anhydrous aprotic solvent such as THF. This behavior is unusual in that vesicles are meta-stable entities whose formation generally requires the input of energy through e.g. sonication, oscillating electric fields, or the rapid dispersion of a homogeneous solution in an aqueous environment. A salient point of this system is that the anhydrous solution of spontaneously


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generated vesicles can be directly used for the fabrication of an OLED device using solutionbased techniques such as spin coating. Following solvent evaporation, the vesicles collapse to thin discs that act as electroluminescent point light sources with a diameter of ca. 250 nm. The aggregates formed can be easily visualized by scanning electron microscope (Figure 1 and supplementary information), and we have previously shown that the localization of the emitting active areas into aggregates can limit cross-contamination.28

Figure 1. Structure of compounds 1 – 3 and scanning electron microscope images of the spontaneously-formed spheres obtained by spin-coating from anhydrous THF solution (0.1 mM) onto SiO2 / Si substrates (scale bar is 2 µm).


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The chromophores are based on a bifluorene core into which is incorporated a benzothiadiazole or thieno-[3,4-b]-pyrazine moiety to induce a bathochromic shift of the emission. The thienopyrazine group was previously used to fabricate efficient dopant-free redlight emitting LEDs29, 30 and, more recently, as an infra-red active sensitizer in solar cells.31 The photophysical properties (fluorescence lifetimes and quantum yields, absorption and emission maxima) of compound 1 – 3 in solution (THF, 1 µM) or as thin films are summarized in Table S1 and their absorption and fluorescence spectra in solution are shown in Figure 2. All the compounds are highly emissive in solution (ΦF = 0.85, 0.98, and 0.63 for 1 – 3 in 1µM deaerated THF solution, respectively).


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Figure 2. Absorption (top left) and normalized emission (top right) spectra of compounds 1-3 in THF (10-6 M); the photo below shows the perceived color of the compounds in solution under illumination by a UV lamp. Principle of selective energy transfer bleaching (bottom left): Efficient FRET between a donor and an acceptor within a single self-assembled aggregate results in emission from the acceptor (PLAcceptor).

Upon prolonged intense excitation (IBleach), the

acceptor is progressively bleached to levels such that FRET is no longer competitive with emission by the donor, thereby resulting in a progressive blue shift of the emission (PLDonor).

The emission from the individual aggregates can be tuned over the entire region of the visible spectrum by blending the compounds in solution before deposition. This relies on efficient energy transfer from the higher energy emitter (donor) component to the acceptor through FRET, yielding objects whose emission covers the gamut between the donor (when no acceptor is present) to the pure acceptor when the FRET process is quantitative. For this to work, the compounds must be freely miscible within the aggregates, without affecting their spherical morphology. To test this, we proceeded to add increasing amounts of 2 or 3 into a solution of 1 in THF (10-4 M). The aggregates obtained possessed emission spectra (as determined using an intensity corrected spectrometer coupled to the CFM) that were a combination of the emissions of the pure compounds. With this approach, it is therefore possible to target a specific color and to determine a combination of primary emitters 1–3 that will combine into a single aggregate with the desired emission profile (including D65-white light). Although the electroluminescence emission from compound 3 is relatively stable, we found that prolonged irradiation using a 375 nm laser results in the bleaching of the chromophore. Interestingly, the products of the degradation (not identified) do not affect the emission of the donor molecules in the same


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aggregate, therefore allowing tuning, on the fly, of the relative proportions of acceptor and donor components. Because the electroluminescence color of an aggregate depends closely on the acceptor / donor ratio, it is possible to take advantage of this process to selectively control the emission of the active layer using a photomask or by raster-scanning a laser beam.

Figure 3. Time-resolved emission spectra from single vesicles composed of (A) 1 and 3 (10:1, λex = 375 nm, excitation power = 20nW) and (B) 2 / 3 (10:1, λex = 375 nm, excitation power = 92nW).

Upon continued irradiation, a rise in the emission of the donor is observed

concomitantly with a decrease of the emission of the acceptor.

To investigate the color variation in the donor – acceptor systems composed of compounds 1 (blue) or 2 (green) as the donor, and 3 (red) as the acceptor, we employed confocal fluorescence microscopy as described in the methods section.

Figure 3 shows the variation of the

photoluminescence obtained from single vesicles deposited onto a glass substrate during the course of illumination with the laser source (λex = 375 nm). In Figure 3a, it can be seen that the emission from a single vesicle composed of 1 and 3 (10 : 1) is dominated by the red component


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at t = 0. As the irradiation proceeds, the emission of the red component decreases slowly, and it is gradually replaced by the emission of 1, resulting in a change of color from red to blue. An identical situation is observed in the case of single vesicles obtained from a blend of 2 and 3 (Figure 3b), where the color gradually changes from red to green. We assign this change in color to the reduction of the efficiency of FRET within the aggregate rather than on an internal filter effect. In both systems investigated, the incident radiation is absorbed predominantly by the donor component (> 99% and 85 % for the blue / red and green / red combinations, respectively). Therefore, efficient quenching of the excited donor initially results in exalted absorption by the acceptor through FRET. As the acceptor bleaches, the FRET process becomes less efficient, resulting in less emission by the acceptor. As a result, the fluorescence of the objects gradually shifts from red to either blue or green, as shown in Figure 4, where the chromatic coordinates of different single vesicles are tracked with increasing irradiation time. This process could be exploited in photolithography using a simple mask, which is also shown in Figure 4 as a proofof-concept.


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Figure 4. CIE colorimetry coordinates of single self-assembled vesicles illustrating the change in chromatic coordinate due to energy transfer bleaching. Square dot represent a single vesicle of 1 and 3 (10 / 1) and the filled squares a single vesicle of 2 and 3 (10 / 1). Both vesicles begin at the point indicated by the yellow square. The 375nm laser of the CFM was kept focused on the same single vesicle at a constant power of 20nW while a kinetic series of intensity corrected spectra was recorded with an integration time of 1 sec/spectrum. The CIE coordinates of each spectrum was then reported in the diagram Right: PL images of color patterning on a glass substrate onto which was deposited a layer of vesicles of 1 and 3 (10 / 1). The separate 5 × 5 µm2 square grid was produced using a UV lamp by projecting a 100 × 100 µm2 grid mask through a 20× objective.

The kinetics of FRET are readily probed through comparison of the excited-state lifetimes of the donor and acceptor in the blend vs. those of the pure compounds. Figure 5 shows the fluorescence lifetime images (FLIM) of the donor and the acceptor channels in vesicles composed of a blend of 1 and 3 (1 : 10, see Figure S4 for the corresponding FLIM image of a bled of 2 and 3). The intensity of the emission from the blue component (1) is strongly quenched, whereas that of the red component is intensified. In the absence of 3, the decay of 1 in the vesicles can be fit to a tri-exponential decay with two principal components (0.96 ns and 2.64 ns) accounting for ca. 90% of the emission (see supplementary information for a full description of the decay analysis). In the presence of 3, these decays are shortened to 0.41 and 1.57 ns, respectively, due to quenching by energy transfer. Concomitantly, the time-resolved emission of 3 in the blend (1 / 3 10 : 1) shows a rise component with 1 / λ = 0.38 ns, very close to the shorter decay component of 1 in the mixture. To further analyze the bleaching process in this donor-


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acceptor system, we traced the evolution of the pre-exponential (Ai) factors and decay parameters (λi) according to I(t) = ΣAie(–λit). The results (Figure S5 and S6) show that the decay parameters of 3 remain constant during the course of the irradiation, whereas those of 1 show an initial fast increase in 1 / λi as the FRET quenching channel is switched off, and then remain constant. At the same time, the pre-exponential factor corresponding to the grow-in of excited 3 gradually tends towards 0. Taken together, these results suggest that the bleaching of 3 eventually leads to a lowering of the concentration of the acceptor to levels such that FRET is no longer competitive with the emission from the donor, and that the photoproducts that are formed do not generate additional quenching pathways for the donor as no acceleration of the decay of 1 is observed.

Figure 5. Top : mFB / mFR (10/1) vesicle images under dual channel fluorescence-lifetime imaging microscopy (FLIM) using 375nm UV laser excitation. The FLIM image of the donor channel (λem = 420 – 480 nm) is shown on the left, whereas the FLIM image of the acceptor


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channel (λem > 570 nm) is shown on the right. (Image size is 40×40 µm).

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Bottom :

corresponding time-correlated single photon counting (TCSPC) measurements. The inserted picture is a zoom of the onset of the curves that shows the time shift between excitation laser and PL from acceptor as a result of FRET

We have previously shown that the dispersions of vesicles obtained in anhydrous media such as THF were directly usable for the fabrication of electroluminescent (EL) devices in which the aggregate's size defines the dimensions of the EL points.28 The devices can be fabricated using a standard vertical (sandwich) architecture that is common in organic light-emitting diodes (Figure 6). The advantage of this approach is that solvent evaporation leads to the collapse of the aggregates which eventually form thin (ca. 30 nm thick) disks. These form the active matrix of the device, thereby facilitating device operation even for those materials with low charge carrier mobility. Importantly, the operational stability of devices prepared using similar compounds in which the biuret motifs were located in the para positions was found to be very good (less than 50% drop in intensity after 6 hrs of continuous operation) and degradation appears to be the result of delamination rather than degradation of the active layer. The device architecture is illustrated in Figure 6 and represents a true bottom-up approach to patterned EL devices. Device construction is simple and involves the spin-coating deposition of a dispersion of aggregates onto an ITO substrate covered with a hole-transport layer (PEDOT:PSS) deposited by spin-coating. An electron transport layer (TPBi) followed by a LiF interfacial layer and an Al top electrode were then thermally evaporated on top of the aggregates. Before evaporation of the top contacts, the device can be patterned using light of any wavelength that is absorbed by the active layer since FRET will populate the chromophore with the smallest energy gap. The devices were


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prepared using either solutions of compounds 1 and 3 (10 : 1, 100 µM in THF) to give devices that produce a blue and red combination, or a blend of compounds 2 and 3 (10 : 1, 100 µM in THF) to give devices that produce a green and red combination following photobleaching of the acceptor. Before patterning (Figure 6A), EL from the aggregates is uniform in intensity and originates almost exclusively from the acceptor regardless of the donor that is used. Irradiation using a conventional white light source through a mask projected onto the active layer resulted in photobleaching of the acceptor, thereby shifting the emission hypsochromically towards the donor component (Figure 6B). As expected, the emission collected in the acceptor and donor channels (Figures 6C and 6D) are mirror images, and it is interesting to note that the sum of the intensities of the two components (Figure 6E) results in a homogeneous EL image in which the pattern is lost. This signifies that the loss of EL emission in the acceptor channel is well compensated by the gain in emission from the donor. Sub-micron line widths (ca. 700 nm) that are very close to the size limit of the vesicles can be achieved using a collimated light source (Figure S7).


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Figure 6. (A) Device architecture for the construction of EL devices using a solution-based approach (HTL: PSS-PEDOT, ETL: TPBi) and (right) EL of a device prepared using a dispersion of vesicles made from a solution of 1 and 3 in THF (10 : 1, 100 µM) before photopatterning. B – E: EL images of devices prepared using a dispersion of vesicles made


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from a solution of 1 (left) or 2 (right) and 3 in THF (10 : 1, 100 µM) after photopatterning through a 5x5 µm2 metal grid showing the false color image obtained using a calibrated detector (B), the emission collected in the donor (C) and acceptor (D) channels, and total EL collected from the device across the visible spectrum (E). CONCLUSION By combining supramolecular self-assembly and frequency selective photobleaching, we have shown that it is possible to pattern EL devices with sub-micron resolution to obtain devices capable of emitting light of different colors. In our view, an important advantage of using selfassembled aggregates lies in the compartimentalization of the emitting chromophores, which limits blurring and contamination of nearby pixels. Using this technique, it is possible to address individual areas on an active layer following its deposition to draw or pattern areas in which energy transfer is operational or defeated. Blocking the FRET process in donor-acceptor pairs leads to a blue-shift the emission of each single nano-sized vesicle, which could be suited for single point emitters provided the spacing between objects is greater than the diffraction limit of the light that is used. More importantly, we have demonstrated that this technique is compatible with the fabrication of EL devices, making it suitable for future patterned OLED devices with sub-micron resolution.

ASSOCIATED CONTENT


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Supporting Information. Full details of synthesis and characterization of compounds 1 – 3, their photophysical properties, and additional electron microscopy images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *K.-T. Wong, E-mail: [email protected]; D. M. Bassani, E-mail: [email protected]

ACKNOWLEDGMENT The authors are grateful for financial support from LabEx AMADEus (ANR-10-LABX-0042AMADEUS through grant ANR-10-IDEX-0003-02) and the Agence Nationale de la Recherche Ministry of Science and Technology (MOST) Taiwan joint funding (grants ANR-13-IS07-0001 EVOLVE and 103-2923-M-002-001-MY3).

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Graphical Abstract


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Frequency-Selective Photobleaching as a Route to Chromatic Control

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