Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 11794−11800
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Nanostructured Light-Emitting Polymer Thin Films and Devices Fabricated by the Environment-Friendly Push-Coating Technique Varun Vohra,*,† Francesco Galeotti,‡ Umberto Giovanella,‡ Wojciech Mróz,‡ Mariacecilia Pasini,‡ and Chiara Botta‡ †
Department of Engineering Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan Istituto per lo Studio delle Macromolecole, CNR-ISMAC, Via Corti 12, 20133 Milano, Italy
‡
S Supporting Information *
ABSTRACT: Push-coating is a green and extremely low-cost process in which only few microliters of conjugated polymer solutions are used to produce thin films using capillary forces. Here, we adapt this fabrication technique to replicate selfassembled nanoporous structures on green and red lightemitting conjugated polymer thin films. These films display ring-like photoluminescence and are successfully integrated into polymer light-emitting devices as emitting layers. At low applied voltages, the green-emitting devices exhibit electroluminescence (EL) from hexagonally arranged nanopixel arrays resulting from a stronger electric field in the thinner areas inside the pores. By gradually increasing the voltage up to 10 V, the emission extends to the areas around the pores. At voltages higher than 10 V, a nonreversible nanopixel to nanoringlike switching of the EL can be observed. After filling the pores with a second blue-emitting conjugated polymer, voltagedependent reversible color tuning of the EL is achieved in the nanostructured light-emitting bilayers. KEYWORDS: push-coating, PLED, F8BT, MEH-PPV, nanofabrication, self-assembly
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tures.30−35 Nonetheless, even these advanced device architecture strategies would highly benefit from a decrease in production cost through novel nanopatterning processes which could be applied to a large number of solution-processed emitting polymers. Straightforward nanopatterning of the electrodes or the emitting layer in light-emitting devices can be achieved with high-cost techniques such as laser-assisted lithography or photolithography25−27,29,36 or the low-cost nanoimprinting technology.37 However, most imprinting methods require heating the polymer layer over its glass-transition temperature (e.g., hot embossing), and consequently, a time-consuming optimization becomes necessary when switching to a new material.38 Furthermore, the stamps or molds used for imprinting are commonly patterned via photolithography and etching, which increases the overall fabrication cost.38,39 Studies on low-cost fabrication of structured conjugated polymers have demonstrated that micropixel PLEDs can be successfully produced.24,38,39 The transition to sub-micrometer dimensions could likely be achieved using nanoporous templates produced from polymer blend solutions of poly(3hexylthiophene-2,5-diyl) (P3HT) with polystyrene (PS) and/
INTRODUCTION Recent technological advances have demonstrated the potential of light-emitting conjugated molecules and polymers when applied to fields such as lighting and display technologies.1−8 Compared to the state-of-the-art semiconductors, conjugated polymers exhibit good mechanical properties (elasticity, flexibility, and filmability), which facilitate the development of high-productivity roll-to-roll compatible processes.9−12 To achieve high performances, polymer light-emitting devices (PLEDs) commonly adopt a multilayer structure in which additional hole and electron injection layers are deposited between the emitting layer and the anode or cathode, respectively.13−16 Because of the demand for dimension reduction of portable devices, finding methods to fabricate nanostructured conjugated polymer films and light-emitting devices with color-switchable properties has been raising the interest of both academics and industrials over the past couple of decades.17−27 In fact, advances in deposition technologies have played a major role to fabricate small-dimension colorselective micrometric pixels for high-resolution full-color displays, which are composed of laterally arranged single color-emitting PLEDs.28,29 An alternative strategy for colortunability was introduced through the concepts of vertically stacked PLEDs or combinations of vertically and horizontally connected PLEDs, which rely either on semitransparent electrodes or on relatively complicated device architec© 2018 American Chemical Society
Received: January 4, 2018 Accepted: March 16, 2018 Published: March 16, 2018 11794
DOI: 10.1021/acsami.8b00137 ACS Appl. Mater. Interfaces 2018, 10, 11794−11800
Research Article
ACS Applied Materials & Interfaces
Figure 1. Three-dimensional AFM images of the nanoporous templates and their PDMS negatives along with the schematic representation of the push-coating process to form replica nanoporous conjugated polymer films.
polymer surface without damaging the nanofeatures of the films. A nanoporous template with average pore diameters of approximately 810 nm was successfully replicated on two different conjugated polymers, namely, F8BT and MEH-PPV. After characterizing the PL properties of these films, we integrated these polymer layers into working PLEDs whose emission properties can be tuned by adjusting the local electric fields inside and outside the pores. For instance, this enabled the possibility to fabricate devices which exhibit EL from nanopixels at low operating voltages in simple device architectures that do not require additional insulating layers. This could be achieved because, for the same voltage applied to the electrodes, the electric field in the thinner zones inside the pores becomes much stronger than that outside the pores. Finally, the pores can be filled with a second light-emitting polymer to produce nanostructured PLEDs with voltagedependent color-switchable EL properties.
or poly(methyl methacrylate) (PMMA), which self-assemble over large areas during spin-coating.40 Additionally, one should find a versatile process to easily replicate these nanostructures on virtually any conjugated polymer material (e.g., blue-, green-, and red-emitting). Push-coating is such an adaptable ecofriendly process, which we have recently applied to PLED fabrication.41−43 During push-coating, a millimeter thick poly(dimethylsiloxane) (PDMS) film is placed on a small volume of conjugated polymer solution in a chlorinated solvent dropped on the substrate. The solution spreads between the substrate and the PDMS film through capillary forces, and the solvent slowly diffuses into PDMS to form uniform thin films, which can be used as emitting layers in PLEDs with similar or higher performances compared to spin-coated polymer films. As the amount of the chlorinated solvent employed for pushcoating is 20 times lower than that used for spin-coating, this process becomes much more environment-friendly. Additionally, push-coating requires 50 times lower amounts of active materials than spin-coating and is therefore an extremely lowcost solution for PLED fabrication. Here, we combine two low-cost approaches, namely, polymer self-assembled template fabrication and push-coating, and propose a method to deposit nanostructured thin films made from any polymer materials soluble in solvents that diffuses in PDMS (e.g., chlorinated aromatic solvents, chloroform, toluene, tetrahydrofuran, and dimethoxyethane) (Figure 1). We successfully replicated self-assembled templates on PS, poly(9-vinylcarbazole), and fluorescent polymers such as poly(9,9-di-n-dodecylfluorenyl-2,7-diyl), poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT), and poly[2-methoxy-5-(2′ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) (data shown for F8BT and MEH-PPV only). PDMS negative films can be fabricated on the nanoporous templates formed by blending P3HT, PS, and PMMA in a 2:1:1 ratio after selective removal of PS and PMMA using acetone. The cleaned nanostructured PDMS films are carefully deposited on top of small volumes of conjugated polymer solutions to generate replica nanoporous thin films with thickness-dependent photoluminescence (PL) and electroluminescence (EL) properties. We verified that a single template can be used for repetitive fabrication of structured PDMS without degradation. As the chlorinated solvent diffuses into PDMS during the short drying period, the solvent-swollen PDMS can be easily removed from the dried conjugated
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RESULTS AND DISCUSSION
Nanoporous Structure Formation on Green- and RedEmitting Conjugated Polymers. The fabrication process described in Figure 1 was applied to low concentration chlorobenzene (CB) solutions (2 mg/mL) of F8BT and MEH-PPV deposited by push-coating with nanostructured PDMS (nanoP-C) on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-covered indium tin oxide (ITO) substrates. Note that self-assembled templates with pore diameters down to 150 nm may be similarly employed for nanoP-C.44,45 However, observing EL features smaller than 600 nm is not within the resolution of our optical microscope, and consequently, we selected the 800 nm diameter self-assembled films as templates for this study. After a single self-assembled template is used to produce 50 PDMS negatives, no major damage can be seen in terms of thin film morphology of the template. As can be observed from the PL spectra in Figure 2 and the atomic force microscopy (AFM) images in Figure S1, thin films prepared using nanoP-C display the same nanoporous morphologies (pore diameters) as the templates. However, topographic characterizations reveal that the average film thicknesses at the center of the pores and between the pores of the replica films are approximately 30 and 80 nm, respectively (Figure S1). This correlates well with the darker areas observed at the center of the pores in the fluorescence micrographs, which 11795
DOI: 10.1021/acsami.8b00137 ACS Appl. Mater. Interfaces 2018, 10, 11794−11800
Research Article
ACS Applied Materials & Interfaces
is 0.5 V higher than their turn-on voltages. At these voltages, the F8BT and MEH-PPV devices emit approximately 109 photons/s. The two devices display similar nanopixel-like EL corresponding to the exact opposite emission pattern of those in the PL micrographs. The maximum external quantum efficiencies (EQEs) of F8BT and MEH-PPV nanoPLEDs are 0.033 and 0.018%, respectively. For comparison, 50 nm thick flat F8BT devices prepared by conventional push-coating without pattern produce maximum EQEs of 0.18%. To fully understand the voltage-dependent behavior of nanoPLEDs, we employ the F8BT-based devices that can switch on at lower applied voltage compared to MEH-PPV ones. Applied Voltage-Dependent EL Properties of nanoPLEDs. Unlike the PL micrographs, in which a stronger emission can be observed from the thicker areas of the film, which contain larger amounts of emitting material, EL mainly originates from inside the pores when the devices are driven at relatively low voltages. As the interelectrode distance is much shorter in the areas at the center of the pores, a stronger electric field is generated there. The electric field is inversely proportional to the interelectrode distance, and consequently, the field at the center of the F8BT pores (30 nm thick) can be estimated to be approximately 2.6 times stronger than that generated outside the pores (80 nm thick) when the same potential difference is applied to the electrodes. Stronger electric fields in thinner areas decrease energy barriers at the electrodes more effectively than in thicker ones. Therefore, current will preferentially flow through the center of the pores, resulting in a nanopixel-like emission from the F8BT devices at low operating voltages. This assumption also strongly suggests that the emission area can be controlled through the voltage applied to the device. The influence of the built-in potential should be negligible when compared to the impact of the thickness variation. Consequently, as a first approximation, we neglect the built-in potential and simply define the electric field as the ratio between the potential difference at the electrodes over the interelectrode distance at the center of the pores or in the area between two adjacent pores. For instance, at 4 V, the electric fields at the center of the pores and in the thicker walls between the pores have values of 133 and 50 MV/m, respectively. In fact, similar to Figure 2c, the EL micrograph collected at 4 V (Figure 3a) exhibits an intense emission originating from the center of the pores in the nanoporous structure. Applying a similar electric field (125 MV/m) to the walls between the pores requires increasing the voltage to 10 V. Note that at 10 V,
Figure 2. (a) Molecular structures of F8BT and MEH-PPV, (b) PL (excitation at 407 nm) and (c) EL micrographs (collected at 3 and 6 V, respectively) of F8BT and MEH-PPV nanoporous films. The scale bar corresponds to 10 μm.
contrast with the bright emission from the walls between adjacent pores corresponding to the thicker zones of the films. Note that the original templates have an average height difference between the center of the pore and the walls of approximately 240 nm. This large decrease in height difference can be attributed to the film formation process during nanoP-C. Upon deposition of the nanostructured PDMS film on the polymer solution, the solution spreads over the substrate to form the bottom continuous layer and some solution infiltrates into the spaces between adjacent nanodomes present on the surface of the PDMS film (Figure S1). As the solvent diffuses inside the PDMS layer, a continuous 30 nm thick film is formed, which is covered by a 50 nm thick top nanoporous layer produced from the polymer solution present between the PDMS nanodomes. Nanoporous morphologies with similar dimensions are produced on films obtained with F8BT and MEH-PPV. Note that this fabrication method was also successfully employed to replicate nanostructures on PS, poly(9-vinylcarbazole), and poly(9,9-di-n-dodecylfluorenyl-2,7diyl) thin films (results not shown), suggesting that nanoP-C can be virtually applied to any polymer soluble in solvents that can diffuse into PDMS. The nanoporous replica films of F8BT and MEH-PPV deposited on ITO/PEDOT:PSS substrates are then sequentially covered with evaporated barium (Ba) and aluminum (Al), which act as the cathode in regular architecture PLEDs. The turn-on voltages necessary to produce 5 × 108 photons/ s from the F8BT and MEH-PPV devices were found to be 2.5 and 5.5 V, respectively (Figure S2a). The EL micrographs in Figure 2c were collected by applying a voltage to the device that
Figure 3. (a) Normalized EL spectra from F8BT nanoPLEDs and their corresponding micrographs obtained at 4 and 10 V, respectively. The scale bar corresponds to 5 μm. (b) Schematic representations of the devices operating at 4 and 10 V. The green arrows symbolize the recombination zones, and dark green areas show the damaged active layer areas. 11796
DOI: 10.1021/acsami.8b00137 ACS Appl. Mater. Interfaces 2018, 10, 11794−11800
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Figure 4. (a) Molecular structure of PFNBr; (b) confocal micrograph (10 × 10 μm2) of the PL from the F8BT nanostructured network filled with PFNBr; (c) normalized EL spectra of the bilayer nanoPLEDs displaying intensified blue emission with increasing voltage; (d) EL micrographs of bilayer nanoPLEDs operating at 3 and 5 V; (e) schematic representation of the energy levels of F8BT and PFNBr along with hole and electron injections, respectively, from PEDOT:PSS and Ba; and (f) schematic representations of bilayer nanoPLEDs operating at 3 and 5 V.
the electric field applied to the thinner parts at the center of the pores has a high value of 333 MV/m. When strong electric fields are applied to the thin layers at the center of the pores, high current flows through them, which heavily damages the active layer from which EL can no longer be observed. Consequently, the devices driven at 10 V display EL originating only from the thicker active layer areas (walls between the pores). These devices have a nonreversible nanopixel to nanoringlike switching behavior with nanopixel emission at voltages up to 6 V, a transition step between 6 and 10 V where the whole active layer emits (Figure S2b), and nanoring emission at voltages higher than 10 V. This nonreversible behavior is a direct consequence of the nonuniformity of the active layer thickness. Color-Switchable Properties of Bilayer nanoPLEDs. Filling the pores of the nanostructured F8BT with a second light-emitting polymer could therefore lead to PLEDs with reversible switching properties. This was realized in a two-step push-coating process in which F8BT nanoporous networks were first fabricated by nanoP-C with an additional pressure of approximately 0.5 kPa applied on top of the PDMS negative films. The additional pressure ensures that the nanodomes present on the PDMS surface sink through the whole polymer solution and that no continuous underlying F8BT film is formed (Figure S3). The resulting F8BT layers display ringlike network structures with film thicknesses of approximately 40 nm. We then filled the pores with a second emitting polymer using push-coating with a flat PDMS. We chose the alcoholsoluble blue emitting poly[(2,7-(9,9′-dioctyl)fluorene)-alt-(2,7(9,9′-bis(5″-trimethylammoniumbromide)hexyl)fluorene)])] (PFNBr, Figure 4a) as the second light-emitting polymer.46 Because PFNBr can be deposited from nonsolvents for F8BT, the underlying nanoporous network is not damaged
during the second layer deposition. However, it is worth mentioning here that the PL and EL quantum efficiencies of PFNBr are much lower than those of F8BT (weaker emission). PL quantum yields of PFNBr and F8BT are, respectively, of 4.7% and approximately 41%.46,47 Similarly, the EQEs of PFNBr and F8BT when employed as emitting layers in regular architecture PLEDs are 0.01% and around 0.3%, respectively.41,46 Consequently, to characterize the emission properties of the nanostructured multilayer films, we used a confocal microscope with a large blue to green gain ratio of 3:1. The resulting images (Figure 4b) and the corresponding topographic characterizations (Figure S3) clearly evidence that 25 nm thick PFNBr layers are successfully formed inside the pores of the 40 nm thick F8BT nanonetwork. The highest occupied molecular orbital/lowest unoccupied molecular orbital levels (Figure 4e) and the I−V characteristics of the hole-/electron-only devices (Figure S4) of the two emitting materials suggest that while hole-only devices show similar injection properties (the same voltage to switch from ohmic to exponential behavior), electrons should be preferentially injected from Ba to F8BT which is therefore expected to switch on at lower applied voltages.46,48 In fact, the nanostructured bilayer devices exhibit strong nanoring-like emission from F8BT at 3 V (Figure 4c,d). By increasing the voltage to 4 and 5 V, electrons are gradually injected into PFNBr, and consequently, its EL intensity increases. The EL micrographs from the bilayer nanoPLEDs collected at 5 V display a weak blue emission from the filled nanopores, which can be reversibly switched on and off by varying the voltage between 3 and 5 V. Energy transfer from PFNBr (donor) to F8BT (acceptor) most probably takes places, which further decreases the EL observed from PFNBr in the nanoPLEDs for lower voltage in favor of green F8BT emission. However, with increasing voltage, the relative EL from PFNBr with respect to 11797
DOI: 10.1021/acsami.8b00137 ACS Appl. Mater. Interfaces 2018, 10, 11794−11800
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min. ITO substrates are cleaned following the same procedure as described above. A 40 nm thick PEDOT:PSS layer (Clevios AI4083, Heraeus) is then spin-coated at 4000 rpm for 30 s on these substrates and annealed at 150 °C for 30 min. NanoP-C is typically performed by depositing 5 μL of F8BT (American Dye Source, ADS133YE) or MEH-PPV (American Dye Source, ADS100RE) solutions (2 mg/mL in CB) on the PEDOT:PSS-covered cleaned ITO substrates and by slowly letting the solvent diffuse into the nanostructured PDMS at 50 °C for 5 min. When no additional pressure is applied, the average thicknesses of the areas inside and around the pores are 30 and 80 nm (±10 nm), respectively. Applying an additional pressure of 0.5 kPa during nanoP-C produces nanostructured networks with average thicknesses inside and around the pores of 0 and 40 nm, respectively. PFNBr (Mw ≈ 30 000) was synthesized following a previously published procedure.49 Filling of the pores with PFNBr is achieved by push-coating using flat 3 mm thick PDMS stamps applied to 5 μL of low concentration (0.5 mg/mL) PFNBr solution in ethanol on the nanoporous F8BT network. Once the pores are filled with the solution, the flat PDMS layer is removed, and the films are dried at 50 °C for 10 min. Note that because of the underlying nanostructure, the solution does not spread as homogeneously as for the conventional push-coating process (applied on flat surfaces), and consequently, bicolor structure formation can only be observed over an area of approximately 0.5 cm2. Both single-layer and double-layer nanostructured films deposited on ITO/PEDOT:PSS substrates were used for PLED fabrication. The PLEDs were simply finalized by evaporating Ba/Al cathodes on top of the structured emitting layers prior to an encapsulation step to prevent photo-oxidation of the devices. The active area of the PLEDs is defined by the intersection of the patterned ITO and metal electrodes and corresponds to an area of 0.054 cm2. Characterization of Nanostructured Films and Devices. Surface morphologies of the thin films were measured using AFM from Keyence (VN-8000). The thicknesses inside the pores were measured using a Dektak XT (Bruker) profilometer across scratches produced on the films with a scalpel. The PL and laser scanning confocal microscopy images were collected using a Nikon Eclipse TE2000-U inverted confocal microscope with two laser excitation sources at 407 nm (DAPI diode) and 488 nm (Argon). For PLEDs, EL spectra were measured using a charge-coupled device combined with a monochromator (Spex 270M) and applying constant bias between 3 and 10 V using a sourcemeter (Keithley2401). EL micrographs were collected by fixing the devices on the sample holder of the Nikon Eclipse microscope and using a sourcemeter to apply constant bias between 3 and 10 V. The I−V characteristics of hole- and electron-only devices were acquired using the same Keithley2401. To evaluate the PLED performances (number of photons and charges per second), the emitted photons were collected using an integration sphere installed with a silicon photodiode calibrated with the EL spectra and the J−V characteristics were registered using Keithley2400.
F8BT gradually increases, suggesting that these energy-transfer processes, occurring only at the polymer−polymer interface, have a limited effect on the overall EL spectra and micrographs. The EL spectrum of PFNBr displays a main peak around 430 nm and a secondary EL peak around 510 nm.46 The presence of this secondary peak in the PFNBr EL spectrum correlates well with the shift observed in the main EL peak of the nanostructured bilayer devices, which is initially positioned at 540 nm at 3 V and gradually blue-shifts to 534 and 530 nm when increasing the voltage to 4 and 5 V, respectively. Although the efficiencies of the nanostructured F8BT and nanostructured bilayer devices are relatively low (Figure S5), we were able to successfully fabricate PLEDs with submicrometer-scale emission zones and either color or emission zone switching properties.
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CONCLUSIONS In summary, we have demonstrated that self-assembled nanoporous structures obtained through a low-cost fabrication technique can be easily reproduced on a variety of lightemitting polymers using nanoP-C. Here, we focused on submicrometer porous structures with pore diameters of approximately 800 nm. Green and red emissions from nanofeatures can be observed for F8BT and MEH-PPV, respectively. While the PL micrographs display nanoring-like emission, EL is observed from sub-micrometer scale pixels when the films are integrated into PLED architectures operating at low voltages. This is due to the stronger electric field generated in the thinner areas of the nanoPLEDs. In F8BT nanoPLEDs, the thicker walls can be switched on at higher voltages. However, this results in a large current flowing through the center of the pores, which locally degrades the active layer, resulting in a nonreversible nanopixel- to nanoringlike switching of the EL. By filling the pores with a blueemitting polymer soluble in an orthogonal solvent prior to electrode deposition, a reversibly color-tunable device can be fabricated. Although the EQEs of the nanoPLEDs presented here are relatively low (around 0.05%), this study represents a proof-of-concept, which may open the path to extremely lowcost fabrication of color-switchable sub-micrometer nanoPLEDs to revolutionize the future of flexible display technology.
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EXPERIMENTAL SECTION
Nanoporous Templates and Nanostructured PDMS. The selfassembled nanoporous structures used as templates in this study were fabricated by blending P3HT (Lisicon SP001, EF431002) purchased from Merck with PS (Mw ≈ 35 000) and PMMA (Mw ≈ 15 000) acquired from Sigma-Aldrich in a 2:1:1 ratio in CB. The blend solution (30 mg/mL) was then spin-coated at 1000 rpm for 60 s on glass substrates cleaned by sequential ultrasonication in acetone, detergent, water, and hot isopropanol. PS and PMMA are dissolved by soaking the films into acetone to form nanoporous structures with mean pore diameters and depths of 810 and 240 nm, respectively. The nanostructured PDMS negatives are fabricated by depositing a mixed 10:1 ratio of PDMS base and curing agent (Sylgard 184, Dow Corning) on top of the template, followed by a curing step at 80 °C for 2 h. We have previously verified that thermal annealing of the nanoporous templates does not modify their surface morphology.45 The PDMS film thickness is controlled by the volume of the precursor mixture deposited on a defined area of the template and was maintained around 3 mm for the entire study. Fabrication of Nanostructured Films and Devices. After peeling off the PDMS negative from the nanoporous P3HT surface, they are cleaned with chloroform and dried in an oven at 80 °C for 30
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b00137. Schematic representation of the nanoP-C process and AFM image with cross section of the F8BT replica film fabricated using nanoP-C; voltage-dependent photon counts for F8BT and MEH-PPV nanoPLEDs and schematic representations and EL micrographs of F8BT nanoPLED at 4, 8, and 10 V; schematic representation and AFM images with their cross sections of the F8BT networks formed by applying pressure on top of the PDMS layer before and after filling with PFNBr; I−V characteristics of hole- and electron-only devices based on F8BT or PFNBr; preparation of the hole- and electron-only devices with ITO/PEDOT:PSS/F8BT or 11798
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PFNBr/PEDOT:PSS/Ag and ITO/ZnO/F8BT or PFNBr/Ba/Al architectures, respectively; and J−V and photons-V of F8BT nanoPLEDs and PFNBr-covered F8BT nanoPLEDs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Varun Vohra: 0000-0001-9663-4195 Francesco Galeotti: 0000-0003-4793-9827 Chiara Botta: 0000-0001-8722-0417 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the University of ElectroCommunications Financial Support for Researcher Exchange and by scientific cooperation agreement between CNR and RAS (project Giovanella/Khotina).
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REFERENCES
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DOI: 10.1021/acsami.8b00137 ACS Appl. Mater. Interfaces 2018, 10, 11794−11800
