Article pubs.acs.org/JPCC
Polymer Light-Emitting Electrochemical Cell Blends Based on Selection of Lithium Salts, LiX [X = Trifluoromethanesulfonate, Hexafluorophosphate, and Bis(trifluoromethylsulfonyl)imide] with Low Turn-On Voltage Kenji Jianzhi Chee,†,‡ Vipin Kumar,† Cuong Viet Nguyen,† Jiangxin Wang,† and Pooi See Lee*,†,‡ †
School of Materials Science and Engineering, and ‡Institute for Sports Research, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore S Supporting Information *
ABSTRACT: Light-emitting electrochemical cell (LEEC) performance is drastically affected by the selection of suitable electrolyte. With the limited n-type doping in MEH-PPV near the cathodic interface, we hypothesized that anions in the electrolyte are critical to attain a higher conductivity p-doped area. Poly(ethylene oxide)-lithium salts electrolyte systems have been investigated in the LEEC devices to determine the influence of anions on device turn-on voltage and operation. The TFSI− anions showed higher resonance states, low turn-on voltages near the optical bandgap of the emissive conjugated polymer, high ionic conductivity in solid state (1.05 × 10−4 S cm−1), and larger electrochemical stability window compared to conventional CF3SO3− anion. Device maximal brightness (∼100 cd/m2) can be achieved. Modulated differential scanning calorimetry (mDSC) studies of the polymer blend films correlate the thermal stability and doping effectiveness during operation and delineate the onset of degradation that entails burnout of the devices.
1. INTRODUCTION Since the discovery of polymer light-emitting diodes (PLEDs) in 1990, organic light-emitting diodes (OLEDs) have already been realized in a multitude of everyday electronic devices ranging from wearable technology to displays and future lighting applications due to many benefits over conventional lighting technology. Their advantages include: (1) large surface area lighting for easy scalability; (2) high theoretical luminous efficiency which reduces the need for heat sinks; (3) wider viewing angles which reduces glare; and (4) turn-on voltages that adapt well with small low-powered devices. However, consumer driven factors such as the need for larger displays have pushed OLED manufacturing toward its limits. For example, the active matrix organic light-emitting diodes (AMOLEDs) require multilayer processing steps which are expensive to scale up, thus having high fixed costs. Efforts to tackle this problem have led to the emergence of polymer lightemitting electrochemical cells (PLEECs) first reported by Pei et al.,1 potentially circumvent much of these issues. A LEEC requires an active layer consisting of emissive conjugated polymer, electrolyte salt, and an ion-transporting matrix. These ingredients are blended together and spin coated onto transparent conducting electrodes (TCEs) before application of the top electrode. The difference between that of OLEDs and LEECs is that the emissive polymer in LEECs undergoes oxidation and reduction in the presence of salts, creating midgap energy states between the HOMO and LUMO, enhancing conductivity and thus reducing the charge © XXXX American Chemical Society
injection barriers. In Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) based LEECs studies, maximal brightness values have been hovering around 102−103 cd/ m2,2−7 while the highest brightness around ∼104 cd/m2 is championed by SuperYellow (SY)8,9 based LEECs due to its high quantum efficiency and air-stability. Moreover, the active layer thickness is relatively larger than most OLEDs which made the fabrication possible with low-cost processing methods such as screen-printing, lamination, inkjet printing and spray coating. Thus, the optimization of the active layer’s constituents directly impacts the performance of the device in terms of the lifetime, brightness, and turn-on voltages. More often than not, the selection of electrolyte salt and its weight ratio to the light-emitting conjugated polymer are of utmost concern because of the limited understanding on its impact on the device physics, albeit with some inference from simulations or studies from planar light-emitting electrochemical cells. Planar light-emitting electrochemical cells (PLEECs) have been fabricated to study the effects of electrolytes,5,10−16 salts,3,17−22 annealing temperature,23−25 and electrodes,26−28 with the assumption that the underlying fundamental operation mechanism holds true for the practical sandwich type LEEC devices. One of these observations points toward an always noncentered emission zone which enhances Received: January 29, 2016 Revised: April 13, 2016
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Figure 1. (a) Device configuration of the typical sandwich LEECs. (b−d) Schematic diagram of the in situ formation of p-i-n junction. (b) After application of an external bias, electric double layers depicted by the gray regions are formed at the polymer/electrode interface. (c) When V > Eg/e, holes and electrons are injected across the barrier. These charges electrostatically attract compensating ions which establish p- and n-doped regions. (d) These doped regions grow with time and form a graded p-i-n structure with i being an intrinsically nondoped region. These doped regions facilitate electron and hole columbic capture and form excitons which decay radiatively.
the probability of doping-induced short circuits29 and metalinduced exciton quenching30,31 which leads to short device lifetime. This also suggests that there is limited n-doping capability of cations.32 Other studies have attributed it to electrode work-function and HOMO−LUMO charge injection barriers28 affecting doping propagation and thin degradation layer of oxidized Aluminum electrode that created a thin thermodynamically favorable degradation and blocking layer causing a hindrance to electron injection.33 Interestingly enough, there is a higher doping capability of anions compared to cations in these LEECs where the conductivity of p-doped regions is relatively higher.34 The question to whether the anion or cation doping is dominant in the device performance has yet to be investigated. Salts in the LEEC (PEO-Salt) electrolyte systems studied include cations such as Li+, K+, Rb+; anions of interest include TFSI− and CF3SO3−.17 It is clear that ionic conductivity and probable phase miscibility of the electrolyte affects the device characteristics in terms of turnon duration, doping propagation and overall brightness of the device. LEEC operational simulations35 also provided a deeper understanding of salt selection where it was proposed that size of anion/cation does not play a dominant role but rather a lesser salt dissociation energy allows for ease of transport and eventual doping. Here, we present a systematic study on how the selection of anion in salt affects the electrolyte ionic conductivity, electrochemical stability window and subsequent doped regions, correlating it with the device performance in terms of brightness and turn-on voltages. We report results based on sandwiched LEECs active layers containing poly[2-methoxy-5(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly(ethylene oxide) (PEO), and LiX salt, [X = bis(trifluoromethylsulfonyl)imide (TFSI−) and hexafluorophosphate (PF6−)] in comparison with an optimized active layer reference devices based on the typical lithium trifluoromethanesulfonate (LiCF3SO3) salts. We reveal that TFSI− anion blend delivers high ionic conductivity and better electrochemical stability, resulting in a lower turn-on voltage and
higher device brightness, related to the enhancement in anion doping.
2. EXPERIMENTAL SECTION 2.1. Chemical Used. 1,2-Dichlorobenzene (99%) was purchased from Alfa Aesar. Lithium bis(tifluoromethanesulfonyl)imide (LiTFSI) (>98%) was purchased from Tokyo Chemical Industry. Poly[2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) (Mn = 40 000−70 000), lithium trifluoromethanesulfonate (LiCF3SO3) (96%), poly(ethylene oxide) (Mn = 5 × 106 g/ mol), anhydrous inhibitor-free tetrahydrofuran (THF) (≥99%), ACS reagent grade cyclohexanone (≥99%), and lithium hexafluorophosphate (LiPF6) (98%) were purchased from Sigma-Aldrich. 2.2. Device Fabrication. Salts were dried overnight at a temperature of 373 K to remove residual moisture. Master solutions of 10 mg/mL concentration were prepared: MEHPPV dissolved in 1,2-dichlorobenzene, PEO dissolved in cyclohexanone and salts were dissolved in tetrahydrofuran separately and left to stir on a magnetic hot plate at T = 323 K overnight. Three types of salt (LiCF3SO3, LiPF6, LiTFSI) were used, respectively. Constituents were admixed by blending the master solutions in appropriate ratios and subsequently left to stir on a magnetic hot plate under the same conditions above. The indium tin oxide (ITO) glass substrates used has a transmittance 83% and sheet resistance 10 Ω/□. The substrates were etched with Aqua-Regia in the volume ratio of (HCl (37% weight in water):HNO3 (69% weight in water):H2O (deionized) = 3:1:1) under T = 333 K in a fumehood setup for 2 min to form 5 mm width finger electrodes. Consequently, it was cleaned using ultrasonic treatments in detergent, deionizied water, acetone, and isopropanol. The substrates were then treated by a plasma chamber for 1 min to remove any organic residue on the ITO surface and as a hydrophilic treatment. Active layer blends were spin coated onto the cleaned ITO surface, and the resulted film thickness (∼200 nm) was measured by a step profilometer B
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Figure 2. (a) Current density and brightness as a function of voltage of LEEC devices with PEO optimizations. Brightness and current values were taken after it was stabilized. (b) Luminous current efficiency vs voltage as a function of PEO/Li+ ratios with LiCF3SO3 salt.
active blend layer will create a p-i-n junction (Figure 1b). Injected electrons and holes are electrostatically compensated by the salt ions (e.g., Li+ and CF3SO3−) giving rise to electrochemical doping p-type doping at anodic and n-type doping at cathodic interface (Figure 1c). These doped regions continue to grow in size with more injected charges and gradually meet to form a p−n junction where the electrons and holes recombine through Coulombic capture forming excitons which relax via fluorescence (Figure 1d) with a theoretical emission efficiency limit of 25%. This in situ doping structure does not require physical charge injection/transport layers for inducing optimized charge carrier injection and transport, allowing less stringent work function requirements of electrodes and simplified fabrication process. The LEEC active layer was first prepared using the standard LiCF3SO3 salt by adjusting the PEO/Li ratio varying from 7.6 to 32 wt %. The mass ratios (MEH-PPV:PEO:LiCF3SO3) of active layer blends with PEO/Li+ ratios 10, 30, and 60 are 1:0.085:0.03, 1:0.25:0.03, and 1:0.50:0.03, respectively. In Figure 2a, it shows the values of the current density and brightness with respect to the voltage step increment. The PEO/Li+ ratio of ∼30 shows the lowest turn-on and highest brightness when compared to the other blends. A low PEO content (PEO/Li+ ratio ∼10) would reduce the transport of ions and enhance electrolyte crystallinity;36 too high an amount of PEO (6 times the amount) would reduce the overall active layer conductivity and enhance the phase separation between PEO and MEH-PPV. Evidently, the current density and brightness values of the active blend with too high a PEO content also shows lower current density and brightness compared to the lower PEO/Li+ ratio of ∼10. The luminous current efficiency is defined by the device luminance at normalized by the current density. As shown in Figure 2b, the device with PEO/Li+ ratio of ∼30 gives the best performance in terms of turn-on voltage and maximal brightness. Studies have shown that the PEO/Li+ ratio >15 provides maximal ionic conductivity and allows better charge transport.36,37 The Nyquist plot of the sample with PEO/Li+ ratio of ∼30 is shown in Figure S1 (Supporting Information). With this as a reference and with the assumption that all the cations and anions participated in the electrochemical doping, we substituted LiCF3SO3 with two other type of salt LiPF6 and LiTFSI with the same PEO to Li+ ratio of ∼30 with the
(Alpha-step 200, Tencor Instruments). The active layer was dried under a hot plate at T = 323 K overnight. Al electrodes were deposited by thermal evaporation at pressure Eg/e; with Eg being the semiconductor bandgap of the emissive polymer), the C
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blend and improved dual phase polymer blend formation with LiTFSI indicates a potential improvement in device performance. The polymer blends with LiPF6 shows an almost exact PL curve (not shown in Figure 3a) with LiTFSI but does not show a bicontinuous structure in AFM phase images (Figure 3d). This could be due to the limited salt dissociation and therefore low ionic conductivity, in agreement with Table 1. In Figure 4a, it is evident that the TFSI− anion based devices show the lowest turn-on voltages (defined as the onset voltage at which brightness starts to rise) of 2.4 V. This value (Von) is approximately equal to Eg/e (Eg is the band gap of MEH-PPV and e is the electronic charge constant), which shows the effective electrochemical doping and charge recombination. The low turn-on voltage is two times lower than the PF6− anion based devices. This is directly correlated to the high ionic conductivity of the TFSI− based devices at 1.05 × 10−4 S cm−1 (Table 1). This ionic conductivity is close to that of CF3SO3 anion based devices with ionic conductivity at 1.29 × 10−4 S cm−1. High ionic conductivity generally results in facile electrochemical doping and lower turn-on voltage. LiPF6 based LEECs suffer the lowest ionic conductivity of around 5 × 10−5 S cm−1, resulting in inefficient doping and low redox currents during p− n junction formation. On the contrary, the TFSI− anions having a larger charge delocalization that leads to relatively weak binding energies with cations, which prompt cation transport and result in higher conductivity electrolyte solutions. The TFSI− anion has five resonance states compared to CF3SO3− (four resonance states) and one in PF6− in Figure S2 (Supporting Information),41 it allows higher TFSI− anionic migration along the PEO chains, leading to efficient doping and faster turn-on. It is important to note that, a larger volume of the anion, (PF6− = 73.0 Å3; CF3SO3− = 86.9 Å3; TFSI− = 163
objective to evaluate the effect of anions of the same ratio in the polymer blends in the LEECs. The ionic conductivities of all the electrolytes or the same PEO/Li+ ratio of ∼30 have been measured and tabulated in Table 1. The polymer blends with PEO/Li+ ratio of 30 for the LiCF3SO3 salt gives the highest ionic conductivity compared to that of ratio of 10 or 60. Table 1. Calculated Ionic Conductivities of the Different Electrolytes Used in This Study salt/ionic liquid Li Li Li Li Li
CF3SO3 (PEO/Li ratio ∼10) CF3SO3 (PEO/Li+ ratio ∼30) CF3SO3 (PEO/Li+ ratio ∼60) TFSI (PEO/Li+ ratio ∼30) PF6 (PEO/Li+ ratio ∼30) +
ionic conductivity (S cm−1) 5.23 1.29 7.64 1.05 4.68
× × × × ×
10−5 10−4 10−5 10−4 10−5
In Figure 3a, it shows the photoluminescence of the different spin-coated films. Pristine MEH-PPV films exhibit two observable peaks belonging to single-chain excitons (intrachain excitons) with a tail that shoulders at longer wavelengths associated with interchain interactions.38,39 After the formation of polymer blend with the addition of LiCF3SO3 salt (PEO/Li+ ratio of 30), there was an observable red-shift of the MEH-PPV intrachain exciton peak emission wavelength of about 10 nm. From the AFM phase images analyses; there is an observable lesser phase separation of the LiTFSI polymer blend in Figure 3b with smaller domain sizes and phase angles when compared to LiCF3SO3 polymer blend in Figure 3c. The phase separated domains could lead to charge trapping and surface states which could also act as intermediate energy levels in the MEH-PPV chain where excitation energy was lost through nonradiative means.40 The reduced PL red-shift in the LiTFSI polymer
Figure 3. (a) Photoluminescence of the as-spin coated pristine MEH-PPV films and MEH-PPV and PEO polymer blends with LiTFSI or LiCF3SO3 salt. (b) Atomic force microscopy phase images of LiTFSI based polymer blend with ratio PEO/Li+ ∼ 30, (c) LiCF3SO3 based polymer blends with PEO/Li+ ratio of ∼30, and (d) LiPF6 based polymer blends with PEO/Li+ ratio of ∼30. D
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Figure 4. (a) Current density and brightness as a function of voltage of LEEC devices based on lithium salts with differing anions (TFSI−, CF3SO3− and PF6−) where brightness and current values were taken after it was stabilized. (b) Luminous current efficiency as a function of voltage for the LiTFSI and LiCF3SO3 based devices. The active layer (MEH-PPV:PEO:LiX) where X = TFSI−, CF3SO3−, and PF6− were prepared in mass ratios of (1:0.25:Y) where Y = 0.055, 0.030, and 0.029, respectively, to maintain PEO/Li+ ratio of ∼30.
Figure 4a also shows overall low current densities of the PF6− anion devices at about ∼10 mA/cm2. This is in agreement with other reports that only showed 0.2 cd/m2 when driven at substantially higher voltages of 30 V.5 It requires voltage >5 V for light-emission of PF6− anion devices to attain reasonable brightness (∼10−100 cd/m2), this would be far beyond the electrochemical stable potential windows of the active layer blend, causing over-reduction and overoxidation of MEH-PPV, leading to unwanted electrochemical side reactions and overall decrease in luminance. Similar findings are also derived from the electrochromic studies of MEH-PPV, which hinted at the lower doping ability of PF6− anions compared to CF3SO3−, where it showed lower optical contrast and electrochromic performance with less reversibility.46 LiPF6 is also well-known to be thermally unstable even at slightly elevated temperatures of 60 °C47−49 and decomposes into LiF and PF5 that can trigger detrimental reactions on the electrode surfaces. In addition, LiPF6 and PF5 react with residual water to form unwanted by products. The formation of the highly acidic HF would also most likely lead to degradation according to density functional theory (DFT) calculations.50
Å3) could enhance the self-dissociating property due to the ease of detaching bigger anion.42 Quantum mechanical calculations also show a larger dissociation energy of PF6− anion vs TFSI− anion when paired with BMIM+ (80.7 vs 76.38 kcal mol−1).43 This is in agreement with a recent study by Stephan et al. that concluded that the binding energy between ions has a bigger role to play in the turn-on of LEECs rather than the ion mobility as the latter was uncorrelated.35 The low voltage regime of 2.0−3.2 V is favorable for TFSI− doping, leading to a higher luminous current efficiency as seen in Figure 4b. Although there is a lack of information on the LiCF3SO3 and LiTFSI binding energies in the same PEO polymer host, it is anticipated that LiTFSI would have a lower binding energy due to its larger anion volume. This, coupled with the fact that they have almost similar ionic conductivities, explains the small difference between the turn-on voltages (∼0.3 V) with that of LiCF3SO3 based LEECs. The low voltage regime (2.0−3.2 V) is favorable for TFSI− doping due to a higher luminous current efficiency as seen in Figure 4b. LiCF3SO3 devices consistently show considerable current densities (10−600 mA/cm2) which would normally be related to high device temperatures resulting in enhanced ionic conductivity and doping compared to Li TFSI devices.4,44 However, the efficiency of the LiCF3CO3 based device did not match up to that of the LiTFSI based device. As shown in Figure 4b, LiCF3SO3 based LEECs showed a rise in the overall higher luminous current efficiency between 3.2 V but quickly deteriorated from 4.4 V onward. Heeger and co-workers reported that a LEEC device operating at 400 mA/ cm2 would experience considerable joule heating,45 which would heat up the device to 60−80 °C.4 This would naturally speed up thermodynamically unwanted side reactions or induce microshorts.2,3,29 In the case of the LiCF3SO3, the extent of joule heating might have led to enhanced LEEC device operation from 3.2 to 4.4 V, but with a steady and almost linear increase of current density leading up to its eventual burnout and degradative side reactions. LiTFSI devices were more robust in high voltage regime (>4.4 V) owing to a larger electrochemical stability window as evident in Figures 4a and S3 (Supporting Information). The CIE 1931 plot of the LEEC devices operating at 4.2 V is summarized in Figure S4 (Supporting Information).
Li PF6(s) ↔ LiF(s) + PF5(g) Li PF6(s) + H 2O(g) → LiF(s) + OPF3(g) + 2HF(g)
To understand the thermal behavior of the polymer blends with different types of salt, mDSC was performed on the as prepared films with the same processing history. Figure 5a,b show a lower melting point and melting enthalpy of LiTFSI based polymer films compared to the LiCF3SO3 polymer blend. The melting point in the mDSC would directly refer to the melting point of the electrolyte in the blend as there is no observable MEH-PPV melting peak due to it being amorphous. It succinctly validates that a lower energy is required for p-i-n junction formation in the LiTFSI polymer blends. Comparatively, the first LEEC anion study by Edman et al. showed that PEO10-LiTFSI electrolytes have a lower melting point of around 41−47 °C compared to PEO10-LiCF3SO3 which is 1.5−3 times higher at 60−120 °C.19 In Figure 5c, LiPF6 films have two distinct melting peaks which could be linked to the detrimental reactions mentioned earlier. Minute amounts of LiF might have been irreversibly produced from the E
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CF3SO3−, and PF6−) are shown. Cyclic voltammetry of MEH-PPV in PEO + LiX; where X = CF3SO3− and TFSI−. CIE coordinates of LEEC devices at 4.2 V. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (+65) 6790-6661. Fax: (+65) 6790-9081. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.J.C., V.K., C.V.N., and J.W. acknowledge the research scholarship provided by Nanyang Technological University in Singapore. This work is supported by the Institute for Sports Research and the NRF Investigatorship, Award No. NRFNRFI2016-05.
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Figure 5. Nonreversible heat flow modulated DSC thermographs of the different MEH-PPV films containing PEO and (a) LiTFSI, (b) LiCF3SO3, and (c) LiPF6.
thermally unstable LiPF6 reactions causing an additional melting point at 166 °C. LiF is a high melting point insulator, making it reasonable to assign the second melting point in the DSC thermograph.
4. CONCLUSIONS Due to in situ electrochemical doping, the optimization and selection of salt in the polymer electrolyte directly affect the LEEC device performance in terms of turn-on voltage, maximal brightness and the electrochemical stability window. We establish that MEH-PPV admixed with PEO polymer electrolytes of adequate ionic conductivities of 10−4 S cm−1, larger saltanion size and more resonance states of the LiTFSI salt devices show better turn-on voltages close to the optical bandgap of MEH-PPV (∼2.4 V). LiTFSI based polymer blend also proved better than the conventional LiCF3SO3 devices with nonexcessive current densities at the high voltage regime leading to better electrochemical stability and reduced likelihood of burnout. On the other hand, LiPF6, shows lowered doping ability with resultant poor device brightness due to low ionic conductivity and probable formation of unwanted species such as LiF and HF.
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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/acs.jpcc.6b00989. Electrochemical impedances spectrum of the electrolyte blends where PEO/Li+ ratio ∼30 are shown. Chemical resonance states of the different anions (TFSI−, F
DOI: 10.1021/acs.jpcc.6b00989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.6b00989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
