Solution-Processed Bio-OLEDs with a Vitamin-Derived Riboflavin

Research Article pubs.acs.org/journal/ascecg

Solution-Processed Bio-OLEDs with a Vitamin-Derived Riboflavin Tetrabutyrate Emission Layer Nils Jürgensen,†,‡ Maximilian Ackermann,‡,§ Tomasz Marszalek,§,∥ Johannes Zimmermann,†,‡ Anthony J. Morfa,†,‡ Wojciech Pisula,∥,⊥ Uwe H. F. Bunz,§,# Felix Hinkel,§,# and Gerardo Hernandez-Sosa*,†,‡ †

Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131 Karlsruhe, Germany InnovationLab, Speyererstr. 4, 69115 Heidelberg, Germany § Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, 5128 Mainz, Germany ⊥ Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland # Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany ‡

S Supporting Information *

ABSTRACT: Solution processed biomaterials are required for the active component to develop printed biodegradable and biocompatible optoelectronic devices. Ideal film formation is crucial for the fabrication of multilayer thin film sandwich devices. We report on the characterization of thin films of the riboflavin-derived biomaterial riboflavin tetrabutyrate and its utilization in an organic light-emitting diode. We show that the nonsolution processable precursor can form homogeneous and smooth films with the addition of tailored side groups that change its solubility. We demonstrate by grazing incidence wide-angle X-ray scattering that this chemical derivative reduces the crystallinity and enhances emission, likely by suppressing π−π stacking interactions. Organic light-emitting diodes with a poly(9-vinylcarbazole)−emissive riboflavin tetrabutyrate bilayer structure yield a maximum luminance of 10 cd/m2 and external quantum efficiency of 0.02% with a 640 nm peak orange exciplex emission. External quantum efficiency measurements of a photodiode affirm the exciplex formation. KEYWORDS: Green electronics, Organic semiconductor, Biomaterial, Organic light-emitting diode, Thin film, Exciplex

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INTRODUCTION The development of electronic devices based on biosourced and biodegradable materials will open a spectrum of novel applications in transient electronics, bioelectronic biointerfacing, or healthcare.1,2 Combined with solution processing, it will enable the sustainable and low-cost production of electronics through industrially relevant printing technologies.3 In recent years, naturally sourced materials have been extensively investigated for the fabrication of fully biodegradable organic field-effect transistors (OFETs). The semiconducting properties of natural dyes such as indigo or tyran purple have been demonstrated and have shown promising results in comparison to state-of-the-art organic semiconductors.4 Moreover, biomaterials have been investigated for different functionalities in the fabrication of electronic devices such as substrate materials, planarization layers, or interlayers for charge injection/blocking in OFETs or organic lightemitting diodes (OLEDs).5−8 However, the use of naturally sourced or biodegradable materials as optoelectronic emitter materials has rarely been investigated. Few examples exist in © 2017 American Chemical Society

which OLEDs based on metalloporphyrins such as cytochrome c, myoglobin, hemin, and chlorophyll have been fabricated.9,10 However, these studies have mostly focused on the pure demonstration of device fabrication and not much on the processing or thin film characterization. In this work, we investigate the film-forming properties and fabrication of solution-processed OLEDs using a derivative of the vitamin riboflavin (RFL), riboflavin tetrabutyrate (RFLT). Ester groups of butyric acid are responsible for the characteristic smell of certain fruits such as pineapple or apricot. Therefore, the vitamin and the functional groups are biodegradable, although there are no direct studies on the biodegradation of RFLT in the literature. A different derivative of RFL, flavin mononucleotide, has already been used as a gain medium in a biolaser from a liquid state to reduce fluorescence self-quenching in the solid state.11 RFLT has been studied for Received: March 3, 2017 Revised: April 6, 2017 Published: April 19, 2017 5368

DOI: 10.1021/acssuschemeng.7b00675 ACS Sustainable Chem. Eng. 2017, 5, 5368−5372

Research Article

ACS Sustainable Chemistry & Engineering

active area. For the photodiode, a 10 nm Ca layer followed by a 100 nm Al electrode layer replaced the Ag electrode. For OLED characterization, a calibrated Botest LIV functionality test system was used inside the glovebox. Luminance−current−voltage (LIV) characterizations were measured with a 200 mV/s sweeping rate. Film thicknesses were determined on a Veeco Dektak 150 profilometer. External Quantum Efficiency and JV Characteristics. The optical part of the EQE setup contained a LOT Quantumdesign 450 W halogen light source, a Princeton Instruments monochromator (Acton SP 2150), a Thorlabs mechanical chopper (MC2000), and collimating optics. The electrical current measurement system involved a Stanford Research (SR830) lock-in amplifier and a Keithley source measurement unit for applying bias voltages (2636A). The calibration of the system was done using a Thorlabs silicon photodiode (FDS100). Purpose-built LabView software controlled the measurement and calculated the SR and the EQE spectra. JV characteristics were recorded using a LOT Quantumdesign 400 W solar simulator, calibrated with a dedicated LOT Quantumdesign Si photodiode and a Keithley (2601B) source measurement unit.

enzymology and its medical use as an antioxidant but not for any kind of electronic devices to date.12,13 Here, we show that the butyrate ester groups improve the material’s film-forming properties, while suppressing emission quenching induced by aggregates in the solid state. Via the orthogonal solvent approach, we produced a bilayer of poly(9-vinylcarbazole) (PVK) as the hole-injecting layer and RFLT as the emission layer in an orange OLED. We demonstrate OLEDs yielding a maximum luminance of 10 cd/m2 with turn-on voltages of ∼11 V.

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EXPERIMENTAL SECTION

Materials. All the materials were purchased and used as received. Riboflavin (RFL, 98%), poly(9-vinylcarbazole) (PVK, 25−50 k Mn; orbital energies as indicated by supplier) were purchased from SigmaAldrich, toluene (99.9%) from Merck, and acetone (99.8%) from Carl Roth. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, P VP AI 4083, work function as indicated by supplier) was obtained from Heraeus. Prestructured indium tin oxide (ITO)-coated electronic grade glass (180 nm, 10 Ω/□, work function as indicated by supplier) was acquired from Kintec. Synthesis. Riboflavin tetrabutyrate (RFLT) was synthesized via a literature-known procedure and purified by column chromatography. Purity was confirmed by 1H NMR spectroscopy and elemental analysis that are in agreement with the literature.12,14 Sample Preparation for Microscopy. Glass substrates were cleaned in an ultrasonic bath for 10 min in acetone and isopropanol, consecutively, followed by 5 min oxygen plasma treatment. RFL was drop cast from a saturated solution in deionized water. RFLT was dissolved at 10 g/L in acetone and spin-cast at 2000 rpm for 20 s. Microscopy. Optical micrographs were measured with a Nikon Eclipse 80i microscope with LU Plan Fluor 50× objectives and an Intensilight C HGFI Precentered Fiber Illuminator with a 380−600 nm mercury lamp and an EPI-FL B 2E/C filter for photoluminescence microscopy. Atomic force microscopy (AFM) was done with a Semilab DME Compact Granite DS 95 SPM Head. Photoluminescence Quantum Yield. For the photoluminescence quantum yield measurements, the samples were excited with an excitation wavelength of 444 nm with Quantum Master 40 with Ulbricht Sphere (PTI). GIWAXS. The sample preparation was the same as in Microscopy. GIWAXS measurements were performed by means of a solid anode Xray tube (Siemens Kristalloflex X-ray source, copper anode X-ray tube operated at 35 kV and 40 mA), Osmic confocal MaxFlux optics, X-ray beam with pinhole collimation, and a MAR345 image plate detector. The beam size was 0.5 mm × 0.5 mm, and samples were irradiated just below the critical angle for total external reflection for the X-ray beam (∼0.18°). Data analysis was carried out with Datasqueeze software. Spectroscopy. Absorbance of pure films on glass was measured on an AvaLight-DH-S-BAL spectrometer from Avantes. Photoluminescence spectra (PL) were recorded on a Jasco FP6500 spectrometer. The EL spectrum was measured on an USB2000+UV−vis spectrometer from Ocean Optics. OLED Fabrication and Characterization. RFLT was dissolved to a concentration of 10 g/L in acetone. PVK was dissolved to a concentration of 5 g/L in toluene. ITO substrates were cleaned in an ultrasonic bath for 10 min in acetone and isopropanol, consecutively, followed by 5 min oxygen plasma treatment. PEDOT:PSS was filtered with a 0.45 μm PVDF filter and spin-cast at 3800 rpm for 30 s with 3 min annealing at 200 °C to obtain 25 nm films. The PVK solution was spin-cast with the same parameters to obtain 10 nm films and annealed at 150 °C for 5 min. The RFLT solution was spin-cast at 2000 rpm for 20 s to obtain 70 nm films. The samples were transferred into a nitrogen glovebox where a 100 nm layer of Ag was thermally evaporated through a shadow mask at 10−6 mbar yielding a 24 mm2

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RESULTS AND DISCUSSION Materials that, due to their optoelectronic properties, are aimed to be used in thin film devices must fulfill morphologic requirements. For OLEDs, homogeneous, smooth, and pinhole-free films are desirable to prevent shortcuts or inhomogeneous electrical fields due to thickness variations. The chemical structures of the investigated compounds RFL and RFLT are presented in Figure 1a and b, respectively. Figure 1c and d show digital photographs of RFL and RFLT films under UV illumination. Due to its low solubility in water, the RFL film was deposited by drop-casting from a saturated

Figure 1. Representative micrographs of dark field (e, g) and fluorescence (f, h) of RFL (shown schematically in panel a) and RFLT (shown schematically in panel b), respectively. Digital photographs show the film-forming quality of RFL and RFLT in panels c and d, respectively. 5369


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ACS Sustainable Chemistry & Engineering aqueous solution on glass in order to achieve films over 100 nm. Spin coating, even at slow spinning rates, only yielded patchy films with thicknesses about 10 nm. However, the deposition process resulted in inhomogeneous films with agglomerates on the μm scale. For this reason, RFL was determined to be nonideal for thin film sandwich devices. Butyrate ester groups were introduced to RFL to improve its solubility in solution, to suppress molecular aggregation, and to reduce luminescence quenching in the solid state (the improved luminescence is shown in Figure 1c, d). Under UV illumination, the deposited RFLT film (Figure 1d) showed homogeneous yellow fluorescence throughout the film in contrast to the RFL film, which only exhibited emission at the edges of the dried drops. The photoluminescence quantum yield of the RFLT films was 3.3%, and the photoluminescence decay time was 1 ns as shown in Figure S1 of the Supporting Information. Dark field microscopy of the RFL film (Figure 1e) contained more light-scattering centers than the RFLT film (Figure 1g), suggesting a lower degree of molecular aggregation in the case of RFLT. Due to the RFL aggregation and its selfquenching, the corresponding film did not exhibit as much fluorescence, as shown in the photoluminescence microscopy image (Figure 1f) compared to the homogeneous emission of the RFLT film (Figure 1h). The observed formation of fluorescent films from solvents other than water make RFLT preferable to RFL for OLED fabrication. In bilayer optoelectronic devices, it is important to understand the morphology and film-forming properties at the semiconductor−semiconductor interface as well as the semiconductor−metal interface. To determine the nature of supramolecular ordering of RFLT films from acetone solution, grazing incidence wide-angle X-ray scattering (GIWAXS) has been performed, while atomic force microscopy (AFM) was used to probe the surface morphology. The GIWAXS scattering pattern is shown in Figure 2a (inset) and shows two scattering bands. The corresponding integration of the GIWAXS pattern (Figure 2a) reveals a locally ordered structure as evident by two prominent, sharp, and intense scattering peaks that correspond to d-spacings of 2.1 and 1.1 nm, which has been reported previously.14 Interestingly, these similar spacings were only observed by Ebitani et al. from solvents which disturb the hydrogen bonding and π−π stacking interactions between RFLT molecules. Therefore, the same crystal structure found for the acetone-processed RFLT film suggests a sheet-like structure formation14 (continuous films are confirmed by AFM studies) and a reduction of the crystallinity in comparison to the more ordered RFL as shown in Figure S2 of the Supporting Information. Hence, acetone is an acceptable solvent for continuous thin-film formation, which suppresses the hydrogen bonding and improves the fluorescence of the material by the reduction of the π−π stacking. Figure 2b presents an AFM image of the solution-cast RFLT film, which shows a lack of observable sheets, and an RMS roughness of 0.6 nm for a film with a peak-to-valley height, Z0, of 5 nm. The film exhibited a smooth surface with no visible preferential order or grain boundaries below 5 μm. These values, along with the lack of strong GIWAXS scattering patterns, indicate that the introduction of butyrate ester groups enables solution processing of smooth fluorescent films without any observable aggregates or long-range order even from a nontypical solvent like acetone. We utilized an RFLT layer as the light-emitting layer in an OLED. Figure 3a presents the energy levels of the materials

Figure 2. Radial integration of the GIWAXS pattern of a RFLT film (a) with the corresponding GIWAXS pattern in the inset. AFM micrograph of an RFLT film demonstrating a maximum height difference, Z0, of 5 nm and a root mean squared surface roughness (RMS) of 0.6 nm (b).

comprising the OLED architecture, with the description of energy level determination described below. We introduced a 10 nm interlayer of PVK to assist with hole transport and electron blocking between PEDOT:PSS and RFLT as electron and hole only devices showed a low value of 2 × 10−9 S/m for electron and hole conductivity. The value was calculated from linear fits in the ohmic regime of the IV curves, as shown in Figure S3 of Supporting Information. The energy levels of RFLT were determined from cyclic voltammograms, as shown in Figure S4 of the Supporting Information. The difference between the reduction and oxidation potentials, ∼2.6 eV, was found to be comparable to the optical bandgap (2.4 eV) determined from a linear fit at the edge of the UV−vis absorbance onset shown in Figure 3b). The device luminance− current−voltage (LIV) characteristics are plotted in Figure 3c). The RFLT OLEDs showed a turn-on voltage (Von) of 11 V, which was defined as the voltage needed to achieve a luminance of 1 cd/m2. A maximum luminance of ∼10 cd/m2 was achieved with a maximum current efficiency of 0.02 Cd/A. The maximum external quantum efficiency (EQE) was 0.02%. The electroluminescence (EL) spectrum, shown in Figure 3b, is red-shifted in comparison to that of the photoluminescence (PL) and was not observed to depend on the applied voltage (Figures S5 and S6) as observed in Figure 3b. The device showed a stable red-orange emission, as shown in the inset of Figure 3c, with the peak emission at 640 nm and corresponding CIE color coordinates at x = 0.57 and y = 0.42. This wavelength corresponds to a photon energy of 1.9 eV, which can be assigned to the energetic difference between the RFLT lowest unoccupied molecular orbital (LUMO) at −3.9 eV and the PVK highest occupied molecular orbital (HOMO) at −5.8 eV. 5370


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electrode was used to reduce the dark current of the device. In Figure 4, the EQE is shown for different reverse bias voltages.

Figure 4. EQE of the RFLT−PVK bilayer photodiode for varying reverse biases. The EQE shows an increase for wavelengths lower than ∼530 nm. The peak around 660 nm can be assigned to the RFLT LUMO and PVK HOMO exciplex transition. The JV characteristics with and without illumination are shown in the inset.

By decreasing the bias voltage further, more charges are extracted, as indicated by higher values in the EQE. The photoresponse below 530 nm corresponds to the RFLT absorbance spectrum shown in Figure 3b, with the maximum of the EQE occurring around 440 nm, as presented in Figure S10 of the Supporting Information. Hence, RFLT generates charges in the visible spectrum, which are attributed to the measured photocurrent shown in the inset of Figure 4. The open circuit voltage of ∼0.9 V is obviously small compared to the optical bandgap of RFLT (2.4 eV) but was expected due to the relatively high potential barriers between the interlayers. Nonetheless, the device showed typical photodiode characteristics with a detectivity and responsivity maximum of 108 Jones and 0.6 mA/W, respectively, at 440 nm and −0.8 V, as shown in Figure S11 of the Supporting Information, and could reach a photocurrent/dark-current ratio of 51 at the same bias voltage. These values are far from state-of-the art organic photodetectors;17,18 however, biomaterial-based photodetectors are difficult to benchmark due to the difficulty of finding reports in literature. The EQE spectra show a small peak around 660 nm. This feature cannot be assigned to any peak in the absorbance or PL spectrum of RFLT in Figure 3b but can be matched to the peak shown in the EL around 640 nm. The fact that the wavelength of the EL and EQE peaks closely fit the transition energy between the PVK HOMO and RFLT LUMO and that this transition could not be detected in the RFLT absorbance or PL spectrum suggests that that this transition corresponds to an RFLT−PVK exciplex transition.

Figure 3. Energy levels (a) and spectroscopic measurements (b) including absorbance (Abs), photoluminescence (PL), and electroluminescence (EL) of RFLT films and LIV characteristics (c) of the asdescribed OLED stack with a micrograph of an operating 4 mm × 6 mm pixel in the inset of panel c.

This observation suggests that the EL spectra is composed of two contributions, a direct HOMO−LUMO transition and the recombination of a PVK−RFLT exciplex.15,16 The derivative of the PL and EL spectra (Figure S7, Supporting Information) shows the portions of the emission corresponding to each of the mechanism. Thermal annealing of the RFLT layer resulted in a decline of device characteristics and a blue shift of the EL toward the PL spectrum, as shown in Figure S8 of the Supporting Information. This shift suggests a change in the film morphology to enhance exciton recombination on the RFLT molecules. The fact that exciplex recombination takes place at PVK−RFLT renders it very sensitive to morphological changes in the film, which in addition to the irreversible oxidation mechanism of RFLT (Figure S4) can cause a reduced operational stability of the devices as shown in Figure S9. Nonetheless, it is of great importance to point out that the device characteristics with an EQE of 0.02% exceed, by far, previous results of bioderived emission layers which only reached EQEs on the order of 10−7 %.9,10 We measured a similar stack in reverse bias as a photodiode to determine whether the 640 nm peak in the EL is observable via charge photogeneration. Instead of an Ag electrode, a Ca/Al

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CONCLUSION In summary, we demonstrated the fabrication of a solutionprocessed OLED using a derivative of RFL, RFLT, as the active layer. We studied the positive effect of butyrate ester groups in the film formation, with decreased molecular aggregation, compared to the initial compound. The devices showed a maximum luminance of ∼10 cd/m2 and a broad spectral emission with a peak at 640 nm. The electroluminescence spectra were found to be composed of the direct HOMO− LUMO transition and an exciplex emission with the hole injection interlayer, PVK. Although, we demonstrated the use of RFLT for optoelectronics and achieved a remarkable 5371


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(7) Kim, S.-J.; Jeon, D.-B.; Park, J.-H.; Ryu, M.-K.; Yang, J.-H.; Hwang, C.-S.; Kim, G.-H.; Yoon, S.-M. Nonvolatile Memory ThinFilm Transistors Using Biodegradable Chicken Albumen Gate Insulator and Oxide Semiconductor Channel on Eco-Friendly Paper Substrate. ACS Appl. Mater. Interfaces 2015, 7, 4869−4874. (8) Zimmermann, J.; Jürgensen, N.; Morfa, A. J.; Wang, B.; Tekoglu, S.; Hernandez-Sosa, G. Poly (lactic-Co-Glycolic acid) (PLGA) as IonConducting Polymer for Biodegradable Light-Emitting Electrochemical Cells. ACS Sustainable Chem. Eng. 2016, 4, 7050. (9) Tajima, H.; Ikeda, S.; Shimatani, K.; Matsuda, M.; Ando, Y.; Oh, J.; Akiyama, H. Light-Emitting Diodes Fabricated from Cytochrome c and Myoglobin. Synth. Met. 2005, 153, 29−32. (10) Tajima, H.; Shimatani, K.; Komino, T.; Ikeda, S.; Matsuda, M.; Ando, Y.; Akiyama, H. Light-Emitting Diodes Fabricated from Biomolecular Compounds. Colloids Surf., A 2006, 284-285, 61−65. (11) Nizamoglu, S.; Gather, M. C.; Yun, S. H. All-Biomaterial Laser Using Vitamin and Biopolymers. Adv. Mater. 2013, 25, 5943−5947. (12) Okuda, J.; Horiguchi, N. Determination of Carboxylesterase in Rat Tissues and Blood Using Riboflavin-5′-Monobutyrate. Chem. Pharm. Bull. 1980, 28, 181−188. (13) Toyosaki, T.; Yamamoto, A.; Mimeshita, T. Antioxidant Effect of Riboflavin Tetrabutylate in Emulsions. J. Food Sci. 1987, 52, 1377− 1380. (14) Ebitani, M.; Kashiwagi, H.; Inoue, M.; Enomoto, S.; Ishida, T. X-Ray Diffraction Patterns and Crystal Structures of Riboflavin Tetrabutyrate. Chem. Pharm. Bull. 1989, 37, 2273−2275. (15) Kalinowski, J. Excimers and Exciplexes in Organic Electroluminescence. Mater. Sci. Pol 2009, 27, 735−756. (16) Palilis, L.; Mäkinen, A.; Uchida, M.; Kafafi, Z. Highly Efficient Molecular Organic Light-Emitting Diodes Based on Exciplex Emission. Appl. Phys. Lett. 2003, 82, 2209−2211. (17) Baeg, K.-J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y.-Y. Organic Light Detectors: Photodiodes and Phototransistors. Adv. Mater. 2013, 25, 4267−4295. (18) Eckstein, R.; Rödlmeier, T.; Glaser, T.; Valouch, S.; Mauer, R.; Lemmer, U.; Hernandez-Sosa, G. Aerosol-Jet Printed Flexible Organic Photodiodes: Semi-Transparent, Color Neutral, and Highly Efficient. Advanced Electronic Materials 2015, 1, 1500101.

improvement of thin biomaterial emission layers, a clear enhancement of the electrical properties is still necessary. This could be through engineering of the device architecture or additional chemical modification of the compound. Such compounds will help focus work on low-toxicity and biodegradable materials for applications such as compostable or bioresolvable optoelectronic devices.

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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/acssuschemeng.7b00675. Photoluminescence lifetime of RFLT; GIWAXS measurement of RFL; electron and hole only devices; cyclic voltammogram of RFLT; EL stability, spectral derivative of PL and EL of RFLT, LIV characteristics and spectra of annealed RFLT films, lifetime of OLEDs; EQE, detectivity, and spectral response of photodiodes. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wojciech Pisula: 0000-0002-5853-1889 Uwe H. F. Bunz: 0000-0002-9369-5387 Gerardo Hernandez-Sosa: 0000-0002-2871-6401 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to D. Jänsch and J. Freudenberg (OCI Heidelberg University) for fruitful discussions, Noah Strobel (LTI Karlsruhe Institute of Technology, InnovationLab) for help with the EQE and JV-characteristics measurements, and Irina Rörich (Max Planck Institute for Polymer Research) for the photoluminescence lifetime measurements. The authors acknowledge financial support of the Federal Ministry for Education and Research (BMBF) through Grant FKZ: 03X5526.

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REFERENCES

(1) Simon, D. T.; Gabrielsson, E. O.; Tybrandt, K.; Berggren, M. Organic Bioelectronics: Bridging the Signaling Gap between Biology and Technology. Chem. Rev. 2016, 116, 13009. (2) Irimia-Vladu, M. Green” Electronics: Biodegradable and Biocompatible Materials and Devices for Sustainable Future. Chem. Soc. Rev. 2014, 43, 588−610. (3) Søndergaard, R. R.; Hösel, M.; Krebs, F. C. Roll-to-Roll Fabrication of Large Area Functional Organic Materials. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 16−34. (4) Irimia-Vladu, M.; Glowacki, E. D.; Voss, G.; Bauer, S.; Sariciftci, N. S. Green and Biodegradable Electronics. Mater. Today 2012, 15, 340−346. (5) Hagen, J. A.; Li, W.; Steckl, A.; Grote, J. Enhanced Emission Efficiency in Organic Light-Emitting Diodes Using Deoxyribonucleic Acid Complex as an Electron Blocking Layer. Appl. Phys. Lett. 2006, 88, 171109. (6) Morfa, A.; Rödlmeier, T.; Jürgensen, N.; Stolz, S.; HernandezSosa, G. Comparison of Biodegradable Substrates for Printed Organic Electronic Devices. Cellulose 2016, 23, 3809. 5372


Solution-Processed Bio-OLEDs with a Vitamin-Derived Riboflavin

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