Enhanced Electroluminescence from a Thiophene-Based Insulated

Letter pubs.acs.org/macroletters

Enhanced Electroluminescence from a Thiophene-Based Insulated Molecular Wire Gábor Méhes,†,‡,# Chengjun Pan,§,¶ Fatima Bencheikh,‡ Li Zhao,‡ Kazunori Sugiyasu,*,§ Masayuki Takeuchi,§ Jean-Charles Ribierre,*,‡,∥ and Chihaya Adachi*,†,‡,∥,⊥ †

Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA), Fukuoka 819-0388, Japan Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka 819-0395, Japan § Organic Materials Group, Polymer Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ∥ Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, Fukuoka 819-0395, Japan ⊥ International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan ‡

S Supporting Information *

ABSTRACT: We report on the realization of polymer light-emitting diodes (PLEDs) based on fluorescent polythiophene (PT)-based insulated molecular wires (IMWs). PLEDs using PT emitting layers traditionally have low external quantum efficiencies (ηeqe) below 0.1%. Moreover, IMWs lack a thorough exploitation for electroluminescent applications due to concerns about reduced charge transport between their chains. We constructed multilayer PLEDs containing PT IMW emitting layers that show the maximum ηeqe close to 1.4%, luminance at 3700 cd/m2, and low turn on voltage at 2.5 V. We also show a strong influence of the thickness of electron transport layer on ηeqe through device optimization and optical simulations. variety of molecular building blocks such as carbazole, fluorene, benzothiadiazole, and thiophene derivatives.7 In particular, poly(thiophene) (PT) derivatives are widely used in PLEDs, especially in their doped form with p-type charge transport behavior in hole transport layers (HTLs).8 On the other hand, undoped PTs can be used as emitter layers (EMLs) in PLEDs thanks to their electrical transitions in the visible range. Easy tunability of optoelectric properties, such as emission color, energy gap, and molecular orbital energy levels, through versatile synthetic methods and functionalization, generated high interest in PT research, such as poly(3-alkylthiophenes).9 Efficient EL is however a difficult task to achieve in PLEDs based on a neat PT EML. A major problem is the low photoluminescence quantum yield (Φ PL) of homopolymer PTs, dropping typically from 30−40% in solutions to less than 1−4% in films.9 Interchain interactions leading to exciton quenching and enhanced nonradiative deactivation caused by the heavy metal effect of a sulfur atom in the thiophene ring were shown to be the main causes behind this large drop in Φ PL.10,11 Due to low Φ PL, EL efficiency (external quantum efficiency, ηeqe) values of PT EML-based PLEDs used to be well below 0.1%,9 a substantial obstacle for commercial applications. In that

C

onjugated polymers (CPs) have been intensively studied in the last decades due to their applications in optoelectronics including organic light-emitting diodes, organic lasers, organic solar cells, and organic field-effect transistors.1−3 CPs provide several advantages compared to inorganic semiconductors in terms of low cost, lightweight, and mechanical flexibility. These macromolecular systems, which generally show a good solubility in organic solvents, are suitable for a rapid industrial-scale device fabrication by solutionprocessing techniques including spin-coating, inkjet printing, and roll-to-roll deposition. In addition, a large variety of CPs with various functional moieties and molecular architectures can be designed and synthesized in order to meet the requirements needed for the realization of high performance optoelectronic devices. Efficient electroluminescence (EL) in polymer light-emitting diodes (PLEDs) is a key property for realistic deployment in displays and lighting technologies.4−6 PLEDs prepared at low temperatures via printing techniques represent indeed an economically affordable way for high-scale production of light-emitting panels compared to inorganic LEDs. Additional product-enabling factors such as processability on large area and flexible substrates give PLEDs distinct advantages in the application domain over LEDs. Since the first demonstration of PLEDs based on poly(p-phenylenevinylene) (PPV),4 a large number of light-emitting CPs have been developed using a © XXXX American Chemical Society

Received: March 14, 2016 Accepted: June 3, 2016

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DOI: 10.1021/acsmacrolett.6b00205 ACS Macro Lett. 2016, 5, 781−785

Letter

ACS Macro Letters context, reducing aggregation and interchain interactions in PT PLEDs is a promising way to achieve reasonably high Φ PL and ηeqe. Functional approaches of molecular engineering that may lead to relatively high EL efficiencies using these strategies include substituted PTs,12 blending PTs with other types of polymers13 and perovskite materials,14 copolymerization,9 and incorporation of PT oligomers into dendrimers as lightemitting cores15 (Table S1). Relatively successful examples among these approaches are Φ PL = 16% in films of 3substituted PTs16 and Φ PL ∼ 30% in phenylenethiophene copolymer films17 (Table S1). It is worthy to mention that a modification of the thiophene ring into thiophene-S,S-dioxide was shown to lead to high Φ PL = 37%18 (high in the context of PTs) and high luminance values in an advanced p-i-n device but with rather low ηeqe = 0.8%.19 Although the “hetero” approaches presented above may provide improved Φ PL, they mostly introduce another active component into the EML or a substantial modification of the thiophene backbone. In consequence, these changes often alter the original characteristics of the device, such as emission color, operating voltage, and charge balance. On the other hand, homopolymers of PTs can also be modified by optoelectrically nonactive components. For example, attaching long alkyl chains to thiophene units increases the interchain distance and can lead to a several-fold increase in the PL and EL efficiencies.20 In a different approach, Φ PL = 11 ± 0.1% (film) was achieved by increasing the conformational distortion between thiophene units that in turn reduced the interchain quenching.21 In the latter case, although a large distortion of the thiophene units may reduce interchain interactions, it also causes an overall (film and solution) drop in Φ PL.9 Therefore, molecular engineering done on homopolymer PTs does not usually lead to a sufficient increase in Φ PL, while heteropolymeric approaches mostly introduce different optoelectrically active components that might influence some key properties of PLEDs. In this letter we report on the fabrication of PLEDs using thiophene-based light-emitting insulated molecular wires (IMWs) as emitters. As shown in Figure 1a, the repeating unit of the IMW used here possesses cyclic side chain, allowing a substantial reduction of the interchain quenching in neat films of this PT derivative. In a previous work, several IMWs with different light-emitting cores based on this molecular design were found to exhibit reduced concentration quenching in films with a fluorescence spectrum tunable across the entire visible range. 22 In addition, these IMWs were found to be thermoformable and well miscible. It should also be mentioned that reports on efficient EL in IMWs and in similar cyclodextrin-based polyrotaxenes are quite rare and represented in the literature only by the work of a few groups.23−27 The latter fact is perhaps due to questionable charge transport abilities of IMWs resulting from the insulating feature of the ring structures.28 In addition, most of the IMW PLEDs demonstrated so far reach their highest ηeqe values below 1%, regardless of the light-emitting core.28 The approach described here led to enhanced EL in multilayer stacks of PT-based IMW PLEDs with maximum ηeqe ∼ 1.4% and maximum luminance levels L ∼ 3700 cd/m2, without using any host layers. This study provides a rare addition to the well-studied PT PLEDs and should generate strong interest in the efficient exploitation of EL from common fluorescent emitting cores molecularly engineered into IMWs. Synthetic procedures and PL properties of PC12C6, Mn = 19.6K, Mw = 36.5K, were already detailed in a previous

Figure 1. (a) Molecular structure of PC12C6. (b) Absorption and PL spectra of PC12C6 in THF, 10−3 mg/mL (green line) and spin-coated film (blue line).

report.29 Experimental procedures are described in the Supporting Information, SI. In this polymer, enhanced Φ PL is obtained due to the tetra-aryl benzene scaffold that determines a 3D propeller-like structure both acting as a sheath and introducing a twist into the molecule (Figure 1a). These features ensure the confinement of excitons to a smaller space.22 A sign for the resulting reduced inter- and intramolecular energy transfer and quenching processes is the very small red-shift of the PL spectrum in the film state compared to that in solution (Figure 1b). In addition, and most importantly, the PL efficiencies of spin-coated films exhibit a rather small decrease with Φ PL = 22.8 ± 0.2% compared to that diluted in tetrahydrofuran (THF, 1.8 × 10−3 mg/mL), Φ PL = 35.4 ± 0.1%. Although 22.8% is not considered as a high value for applications in PLEDs, this value is still significantly higher than for most other PTs and much higher compared to the 1−4% of pure PT homopolymers. Calculating with 20% of photon outcoupling efficiency (ηphot) for planar PLEDs, charge balance factor of 1 (γ), and 25% radiative exciton generation efficiency (ηrad) for fluorescent PLEDs, we can get a theoretical maximum ηeqe ∼ 1.14% out of the 22.8%,30 a value in line with the highest efficiency values for PT PLEDs demonstrated so far (Table S1). We fabricated the following multilayer device stacks for achieving good charge injection and transport abilities, where PC12C6 was the EML: indium tin oxide (ITO; 110 nm)/ poly(3,4-ethylenedioxy-2,4-thiophene)-polystyrenesulfonate (PEDOT:PSS; 40 nm)/PC12C6 (20 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI; 60 nm)/lithium-fluoride (LiF; 0.8 nm)/aluminum (Al; 100 nm), as shown in Figure 2a. The two well-known charge transport compounds, PEDOT:PSS and TPBI, served as the HTL and ETL, respectively; ITO was the anode, LiF the electron injection layer, and Al the cathode. The EL spectrum of PC12C6 (Figure 2b) was identical with that of PL in film with the main EL and PL peaks at 529 and 532 nm and lower intensity peaks at 572 and 566 nm, respectively. The 1931 Commission Internationale de 782


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Figure 2. (a) Device structure and energy diagram of PLEDs. (b) EL spectra of PC12C6 at varying ETL thicknesses measured at 10 mA/ cm2. The inset picture in (b) shows EL from PC12C6.

l’Eclairage (CIE) color coordinates of the yellowish green EL measured at 10 mA/cm2 were x = 0.360 and y = 0.611. Figure 3 shows the main electrical characteristics of the device. The results show a rather high luminance level at L = 3712 ± 48 cd/m2 and a low turn-on voltage Von = 2.5 V (at 0.05 cd/m2, Figure 3b), which can be considered as high performance for a PT PLED and IMW in general. The latter fact is due to the already mentioned intrinsically low charge transport ability of the insulated molecular structures. As shown in Figure S2 and described in SI, the charge carrier mobility in PC12C6 is significantly lower than those measured in poly(3hexylthiophene-2,5-diyl) (P3HT)31 and P3HT:poly(methyl methacrylate) (PMMA) blend films,29 which confirms that the IMW architecture is excellent to reduce interchain quenching but can affect the charge transport properties of the film. Note that a low Von in the PC12C6 OLED is the consequence of a relatively low energy barrier for holes (0.3 eV) and no barrier for electrons transported from the Fermi level of PEDOT:PSS to the highest occupied molecular orbital (HOMO) of PC12C6 and lowest unoccupied molecular orbital (LUMO) of TPBI to that of the EML, respectively (Figures 2a and S1). The relationship between ηeqe from the injected current density is shown in Figure 3a. The device exhibited a maximum ηeqe = 1.35 ± 0.03% at 0.17 mA/cm2, one of the highest values for PT PLEDs with neat EML reported so far (Table S1). To note, at 10 mA/cm2 the EL efficiency was still 1.0%. The slight difference between ηeqe values calculated from Φ PL (1.14%) and experimentally determined (1.35%) can be ascribed to an enhancement of ηphot in PLEDs resulting from a horizontal orientation of the polymer chains in the spin-coated IMW EML.32−38 Indeed, variable-angle spectroscopic ellipsometry analysis revealed an anisotropy in the optical constants related to a horizontal orientation of the absorption dipoles in spin-coated PC12C6 films (see Figure S3 in SI).

Figure 3. (a) External quantum efficiency as a function of the injected current density and (b) current density and luminance vs voltage of PC12C6 PLEDs.

The highest efficiencies in PLEDs in principle are achieved under good charge balance condition. This condition is characterized, among other factors, by EL bearing purely the spectral signature of the EML. In contrary, we observed a low intensity emission band typical of TPBI at ca. 375 nm (Figure S4)39 in addition to the EL of PC12C6 at high current densities above 10 mA/cm2. This weak emission from the TPBI layer implies a slightly extended recombination zone into the ETL, while the maximum charge balance is not achieved. To shift the recombination zone away from the ETL into the EML, we prepared devices with the same structure as above, but with thicknesses of TPBI tTPBI = 50 nm and tTPBI = 40 nm (Figure 2a). As expected, the low intensity emission band from TPBI somewhat decreased with decreasing ETL thickness, especially in the case of tTPBI = 40 nm (Figure S4). However, we observed a clear decrease in both ηeqe and luminance at the same current densities with decreasing tTPBI (Figures 3a and S5). The maximum ηeqe decreased to 1.15 ± 0.04% and 0.90 ± 0.02% for devices with tTPBI = 50 and 40 nm, respectively (Table S1). An additional negative shift of the JV curve on the voltage axis and a lower Von (Figures 3b and S6, Table S1) with lower tTPBI can be easily explained by a lower voltage bias required to maintain the same electric field for operation. To explain the decrease in efficiency with decreasing ETL thickness we considered several possibilities. A poor electron mobility compared to that of the holes of the EML could lead to an accumulation of electrons at the EML/ETL interface, resulting in exciton−polaron annihilation processes leading to a 783


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ACS Macro Letters decrease in ηeqe.40 This is a very plausible scenario since PT homopolymers are generally used as unipolar p-type semiconductors in organic electronic devices.41,9 However, such an annihilation process is expected to also affect the efficiency rolloff at high current densities which is not confirmed by our observations. On the other hand, changes in the thickness of the transport layers can bring shifts in the recombination zone. To examine this aspect, optical simulations were carried out to investigate the influence of the ETL thickness on the EL spectrum. For this purpose, as explained in the SI section, the profile and the position of the recombination zone in the devices were extracted by fitting the measured EL spectra to the simulated ones.42 As displayed in Figure S7, the results show a widening recombination zone and gradual shifting of the zone from the HTL/EML to the EML/ETL interface with increasing tTPBI. Based on this finding, we can attribute the increase in ηeqe with tTPBI to the formation of a wider recombination zone resulting in a better charge balance. In conclusion, we demonstrated EL from PT−core IMW PLEDs, with maximum ηeqe = 1.35% and the maximum luminance L ∼ 3700 cd/m2 in a multilayer device. Our results prove that EL in thiophene-based polymers can be achieved by an intricate molecular engineering approach where the CP backbone is sheathed to control the interchain interchromophore interactions and to increase the PL in thin films. We also found that a thicker ETL layer in the PLEDs can lead to a significant improvement of charge balance through widening the recombination zone. At ETL thickness of 60 nm, ηeqe improves by as much as 50% and the maximum luminance by 41% compared to the usual ETL thickness of 40 nm. Further improvements in ηeqe of PT IMW PLEDs should come from an enhancement of Φ PL through, for example, blending the IMW with another compound to further minimize the interchain quenching.29 Due to the possibility of tuning the emission spectrum of the IMWs across the entire visible spectrum by the appropriate selection of light-emitting cores, we anticipate that the present study will serve as a strong basis for the future development of high performance IMW-based PLEDs.

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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 Authors acknowledge Fukuoka Industry, Science & Technology Foundation (FIST) for financial support.

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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/acsmacrolett.6b00205. Experimental methods, estimation of molecular orientation of neat PC12C6 films, estimation of hole mobility of PC12C6, P3HT, and P3HT:PMMA (70:30 wt %), optical modeling, Figures S1−S7, and Table S1 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K. S.). *E-mail: [email protected] (J.-C. R.). *E-mail: [email protected]. (C. A.). Present Addresses #

Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, SE-601 74 Norrköping, Sweden. ¶ Department of Polymer Science and Engineering, College of Materials Science and Engineering, Shenzhen University, Nanhai Ave 3688, Shenzhen, Guangdong, P. R. China, 518060. 784


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ACS Macro Letters (26) Frampton, M. J.; Sforazzini, G.; Brovelli, S.; Latini, G.; Townsend, E.; Williams, C. C.; Charas, A.; Zalewski, L.; Kaka, N. S.; Sirish, M.; Parrott, L. J.; Wilson, J. S.; Cacialli, F.; Anderson, H. L. Adv. Funct. Mater. 2008, 18, 3367−3376. (27) Fenwick, O.; Sprafke, J. K.; Binas, J.; Kondratuk, D. V.; Di Stasio, F. D.; Anderson, H. L.; Cacialli, F. Nano Lett. 2011, 11, 2451− 2456. (28) Brovelli, S.; Cacialli, F. Small 2010, 6, 2796−2820. (29) Pan, C.; Sugiyasu, K.; Takeuchi, M. Chem. Commun. 2014, 50, 11814−11817. (30) ηeqe = ηphot × ηrad × γ × ΦPL. (31) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Fréchet, J. M. J. Appl. Phys. Lett. 2005, 86, 122110−1−122110−3. (32) Yim, K.-H.; Friend, R. H.; Kim, J.-S. J. Chem. Phys. 2006, 124, 184706−1−184706−8. (33) Jurow, M. J.; Mayr, C.; Schmidt, T. D.; Lampe, T.; Djurovich, P. I.; Brütting, W.; Thompson, M. E. Nat. Mater. 2016, 15, 85−91. (34) Kim, K.-H.; Lee, S.; Moon, C.-K.; Kim, S.-Y.; Park, Y.-S.; Lee, J.H.; Lee, J. W.; Huh, J.; You, Y.; Kim, J.-J. Nat. Commun. 2014, 5, 4769. (35) Zhao, L.; Komino, T.; Inoue, M.; Kim, J. H.; Ribierre, J.-C.; Adachi, C. Appl. Phys. Lett. 2015, 106, 063301. (36) Yokoyama, D. J. Mater. Chem. 2011, 21, 19187−19202. (37) Flämmich, M.; Gather, M. C.; Danz, N.; Michaelis, D.; Bräuer, A. H.; Meerholz, K.; Tünnermann, A. Org. Electron. 2010, 11, 1039− 1046. (38) Kim, J.-S.; Ho, P. K. H.; Greenham, N. C.; Friend, R. H. J. Appl. Phys. 2000, 88, 1073−1081. (39) Tao, Y. T.; Balasubramaniam, E.; Danel, A.; Tomasik, P. Appl. Phys. Lett. 2000, 77, 933−935. (40) Murawski, C.; Leo, K.; Gather, M. C. Adv. Mater. 2013, 25, 6801−6827. (41) Sugiyasu, K.; Honsho, Y.; Harrison, R. M.; Sato, A.; Yasuda, T.; Seki, S.; Takeuchi, M. J. Am. Chem. Soc. 2010, 132, 14754−14756. (42) Granlund, T.; Pettersson, L. A. A.; Inganäs, O. J. Appl. Phys. 2001, 89, 5897−5902.

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Enhanced Electroluminescence from a Thiophene-Based Insulated

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