15786
J. Phys. Chem. C 2010, 114, 15786–15790
Effect of Thermal Annealing on Polymer Light-Emitting Diodes Utilizing Cationic Conjugated Polyelectrolytes as Electron Injection Layers Chi-Yen Lin,† Andres Garcia,‡ Peter Zalar,‡ Jacek Z. Brzezinski,‡ and Thuc-Quyen Nguyen*,‡ Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids, UniVersity of California, Santa Barbara, California 93106 ReceiVed: April 8, 2010; ReVised Manuscript ReceiVed: August 9, 2010
The effect of thermal annealing on the performance of polymer light-emitting diodes (PLEDs) with a cationic conjugated polyelectrolyte as an electron injection layer is investigated. Thermal annealing at 180 °C leads to the loss of ionic content via Hofmann elimination and, hence, increases the device turn-on voltage. The ability to reduce the ionic charge density opens opportunities to design experiments for disentangling the operating mechanism for the reduction of electron injection barriers in PLEDs. Introduction Conjugated polyelectrolytes (CPEs) are a class of organic semiconducting materials composed of a π-conjugated backbone with pendant ionic functional groups.1 Unlike neutral conjugated polymers, the ionic content makes CPEs soluble in water and other polar organic solvents. This unique solubility enables their incorporation as efficient electron injection/transport layers (EILs/ETLs) in solution-processed multilayer polymer lightemitting diodes (PLEDs)2 and organic field effect transistors (OFETs)3 via an orthogonal solvent deposition approach. Moreover, the ionic functionalities have been shown to play an important role in reducing the barriers to electron injection in PLEDs and in OFETs. The improved electron injection mechanism in PLEDs can be explained by the following two models. One involves the formation of permanent interfacial dipole layers between the cathode and the organic semiconductor.4 Interfacial dipole has been shown theoretically and experimentally to alter work functions of metal surfaces.5 Another mechanism requires ion migration by the applied electric field and concomitant redistribution of the internal electric field.6 Thermal annealing is a processing technique that can be used to improve the performance of PLEDs.7 Additionally, the film morphologies including the orientation of dipole moments and the degree of interchain interactions can be altered by heat treatments.7f Despite the successful device improvements by thermal annealing, its influence on CPEs-based PLED devices has not been explored. Indeed, the electronic characteristics of CPEs are strongly associated with their thermal transition.8 It is thus of significance to consider whether the thermal treatment can be a useful protocol for optimizing the performance of devices that contain CPE layers. In this contribution, we provide a detailed study of how thermal treatments influence the EIL function of a cationic CPE, poly[9,9′-bis[6′′-(N,N,N-trimethylammonium)-hexyl]fluorenealt-co-phenylene] with fluoride counteranions (PFP-NF), in a standard PLED test structure. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is used as the emissive layer (EL). It is found that heating the device at 180 °C * To whom correspondence should be addressed. E-mail: quyen@ chem.ucsb.edu. † National Taiwan University. ‡ University of California.
can lead to degradation of the CPE structure and an undesirable increase of the device turn-on voltage from 2.5 to 5.7 V. In addition, annealing experiments of thick and thin CPE films were carried out to gain insight into how the ionic functionalities work in PLEDs. Experimental Section MEH-PPV9 and PFP-NF10 were synthesized according to previous reports. Films were prepared on indium tin oxide (ITO)-coated glass substrates (Thin Film Devices) for electrical measurements. The ITO-coated glass substrate was first cleaned with detergent, then ultrasonicated in acetone and isopropyl, and subsequently dried in an oven overnight. The substrates were treated with UV/O3 (UVO Cleaner 42, Jelight Co. Inc.) for an hour prior to polymer deposition. For PLED fabrication, 0.5% w/v solutions of MEH-PPV in toluene and 0.5% or 0.05% w/v solutions of PFP-NF in methanol were prepared and stirred at 40 °C overnight prior to use. ITOcoated glass substrates were used as the anode to which a solution of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS (Baytron 4083, H. C. Stark), was spincoated at 1500 rpm for 70 s, yielding a ∼60 nm thick film. The PEDOT:PSS layer was dried at 150 °C for 1 h. The MEH-PPV solution was spin-coated at ∼1000 rpm for 60 s atop of the PEDOT:PSS layer to yield a ∼80 nm thick film. Subsequently, the PFP-NF solutions were spin-coated atop the MEH-PPV layer at 3000 rpm for 60 s. The PFP-NF thickness was controlled by varying the polymer concentration, ∼20 nm for 0.5% w/v solution and ∼3 nm for 0.05% w/v solution. Devices were completed after they were dried under a 10-4 Torr vacuum overnight by thermal evaporation of Al electrodes at a pressure of 10-8 Torr. All fabrication, annealing, and testing were carried out inside a nitrogen atmosphere drybox. The film thicknesses were determined by atomic force microscopy (AFM) measurements. AFM was done under nitrogen environment using a commercial scanning probe microscope (MultiMode and Nanoscope Controller IIIa, Veeco Inc.). All AFM images were collected in tapping mode using sillion probes with a spring constant of 5 N/m and a resonant frequency of ∼75 kHz (Budget Sensors). X-ray photoelectron spectroscopy (XPS) measurements were performed on Kratos Axis Ultra XPS system with a base pressure of 1 × 10-10 mbar (UHV), using a monochromated
10.1021/jp103184z 2010 American Chemical Society Published on Web 08/26/2010
Effect of Thermal Annealing on PLEDs
J. Phys. Chem. C, Vol. 114, No. 37, 2010 15787
Figure 1. (a) Chemical structures of MEH-PPV and PFP-NF used as emissive and EILs, respectively, and a schematic drawing of the multilayer PLED structure used in these studies. (b) The effect of thermal annealing on the current density vs bias (J-V) and luminance vs bias (L-V) curves of the ITO/PEDOT:PSS/MEH-PPV/PFP-NF/Al multilayer devices.
Al KR X-ray source at hν ) 1486 eV. High-resolution scans of the N 1s signals were taken at 0.05 eV steps. A thermo gravimetric analyzer (TGA) was coupled to a 300 AMU Balzers ThermoStar Mass Spectrometer (MS) (Mettler Toledo, Inc.). The TGA can go up to 1100 °C, and its balance resolves to 1 µg. The MS can measure the gas evolved from the thermal decomposition of the sample up to 300 AMU with the detection limit in the ppb range. Fifteen milligrams of CPE powder was used for this experiment. Results and Discussion Figure 1a shows the molecular structures of the materials used in these studies, together with the test device configuration. Multilayer device structures were fabricated by taking advantage of the orthogonal solubility of MEH-PPV in toluene and of PFPNF in methanol. Current density versus bias (J-V) and luminance versus bias (L-V) curves of annealed and as-cast devices are shown in Figure 1b. The current density drops over 2 orders of magnitude after heating the devices at 180 °C for 30 min in a nitrogen atmosphere. More importantly, we note that the turn-on voltage (Vturn-on ) V when luminance, L ) 1 Cd/m2) shifts remarkably from 2.5 to 5.7 V. This effect is also observed for PFP-NBIm4 (Figure S1 in the Supporting Information). PEDOT:PSS and MEH-PPV do not undergo chemical changes at this annealing temperature when heating under nitrogen atmosphere; therefore, we excluded the possible degradation of these materials. It is worth pointing out that the turn-on voltage in these devices is influenced primarily by
electron injection, since the energy difference between the work function of ITO/PEDOT:PSS (∼5.2 eV) and MEH-PPV (∼5.1 eV) is on the order of 0.1 eV and is anticipated a negligible hole-injection barrier.6b These considerations lead us to examine possible thermal degradation of PFP-NF via a process whereby the pendant ionic groups are removed, ultimately leading to conditions where electron injection is more difficult. To gain more insight into the effect of thermal annealing, we first investigated the morphology of the PFP-NF layers atop the ITO/PEDOT:PSS/MEH-PPV surfaces by using tappingmode AFM. The tip in this measurement mode vibrates at its resonance frequency while scanning over the sample surface to give surface topographic and phase images simultaneously. The topographic image provides surface feature roughness with subangstrom resolution, while the phase image provides information on nanoscale variations in the modulus, and thus mechanical properties, of the film. Phase contrast results when the tip and surface interaction results in a change in the phase of the cantilever as the energy of the vibrating tip is dissipated in the sample.11,12 Figure 2 shows the topographic and phase images of the as-cast and annealed PFP-NF surface collected from regions between Al electrodes. The topographic images of the PFP-NF surface before and after thermal annealing are featureless with the surface roughness increased slightly from ∼0.59 to ∼0.70 nm. However, the corresponding phase images show significant changes. Before annealing, there is no phase contrast on the PFP-NF layer, indicating a homogeneous film. After annealing at 180 °C for 30 min in nitrogen, strong phase
15788
J. Phys. Chem. C, Vol. 114, No. 37, 2010
Figure 2. AFM surface topographic and phase images of PFP-NF surfaces between the Al electrodes in ITO/PEDOT:PSS/MEH-PPV/ PFP-NF/Al devices before and after annealing at 180 °C for 30 min in nitrogen atmosphere. The scan size for all images is 2 µm × 2 µm.
contrast was observed. Because the phase detection provides information on the mechanical characteristics of the sample surface, the observed phase contrast may indicate that the physicochemical properties of the topmost PFP-NF layer are changed upon thermal treatment. Quaternary alkyl ammonium salts are well-known to undergo Hofmann elimination at high temperature.13 Thus, we anticipated that the same process could take place at the pendant alkyl quaternary ammonium salts of PFP-NF. Such a process leads to the formation of an olefin, HF, and neutral tertiary amines via an E2 elimination mechanism (Figure 3). Further evidence supporting this hypothesis is provided by XPS and TGA/MS measurements. MS analysis confirms that the gases evolved from the thermal decomposition of the PFP-NF and PFP-NBr are
Lin et al. water, trimethyl amine, and HF and HBr, respectively (Figure S2 in the Supporting Information). Figure 3b shows XPS plots of the PFP-NF N 1s signal before and after annealing. Before annealing, a peak between 400 to 398 eV was observed from the neat PFP-NF film. This peak indicates the presence of quaternary ammonium salts. After 30 min of heating at 180 °C, the ammonium peak intensity drops significantly from 100 to 15%, and an additional peak at binding energies between 397 to 395 eV begins to emerge. This newly formed peak at lower binding energy can be attributed to the presence of neutral amine14 and is consistent with the proposed degradation via Hofmann elimination. From the peak ratios of charged and neutral N functionalities, we can estimate that the conversion, at least at the topmost layers (∼3 nm) interrogated by XPS, is on the order of 85%. Therefore, we attribute the increase in turn-on bias to the severe reduction of ionic functionalities. Next, we investigated the effect of gases evolved from the thermal annealing process to the device performance by adding ∼2.0 µL of triethyl amine and ∼1.2 µL of HCl per mL of CPE solution. These volumes were calculated by finding the molarity of the CPE solution using the molecular mass of the repeat unit and adding two molar equivalents of either triethyl amine or HCl due to the presence of two side chains on each monomer. Triethyl amine is used instead of trimethyl amine since trimethyl amine is a gas and therefore is much more difficult to know the amount added. Adding HCl does not affect the turn-on voltage; however, the device performance reduces from 0.95 to 0.6 cd/A (Figure S3 in the Supporting Information). Adding triethyl amine results in a slight increase in the turn-on bias (3.9 V) and a larger drop in performance (0.16 cd/A). Therefore, the higher turn-on voltage observed for thermal annealed devices is a combination of both effects, the loss of the ions and the generation of trimethyl amine. Here, the assumption is that trimethyl amine and thimethyl amine affect the device similarly. Both thick and thin CPEs have been demonstrated as efficient EILs in PLEDs, although the operating mechanism as a function of layer thickness remains under debate.3,4 It is possible that for devices with a thick EIL (>8 nm) ion migration followed
Figure 3. (a) Proposed thermal degradation reaction of cationic PFP-NF via Hofmann elimination. (b) XPS plots of the N 1s signal in PFP-NF films before (black) and after (red) annealing.
Effect of Thermal Annealing on PLEDs
J. Phys. Chem. C, Vol. 114, No. 37, 2010 15789 motion in thin PFP-NF devices. Notably, the power efficiency for postannealed devices, where interfacial dipole is present, is about 1 order of magnitude larger than for preannealed devices with no interfacial dipole due to the loss of ionic functionality. These results imply that the removal of the interfacial dipole results in inefficient electron injection, which lowers the power efficiency dramatically. Therefore, it is anticipated that interfacial dipole plays a dominant role in mediating electron injection within thin PFP-NF devices. Conclusions
Figure 4. Luminance vs bias and power efficiency vs current density curves for ITO/PEDOT:PSS/MEH-PPV/PFP-NF/Al devices with thick (a) and thin (b) PFP-NF layers. The devices were thermal annealed before (red triangles) and after (blue diamonds) Al electrode evaporation. As cast device data (green circles) are also included for comparison.
by electric field concentrated at the interface is the predominant operating mechanism, whereas for devices with a thin EIL (
