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Chemical stabilization of doping in conjugated polymers
Organic Electronics 11 (2010) 1079–1087
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Chemical stabilization of doping in conjugated polymers Shi Tang a, Knut Irgum b, Ludvig Edman a,* a b
The Organic Photonics and Electronics Group, Department of Physics, Umeå University, S-901 87 Umeå, Sweden Department of Chemistry, Umeå University, S-901 87 Umeå, Sweden
a r t i c l e
i n f o
Article history: Received 25 January 2010 Received in revised form 15 March 2010 Accepted 16 March 2010 Available online 20 March 2010 Keywords: Doping p–n Junction Chemical stabilization Light-emitting electrochemical cell Ion-pair monomer Superyellow
a b s t r a c t Conjugated polymers can be electrochemically doped to high-conductivity states under applied voltage, but such in situ formed doping structures are dynamic and dissipate when the formation voltage is removed. For some applications it is highly desirable to permanently stabilize the doping after its formation. Here, we report and compare results on four different approaches to chemical stabilization of an emissive and rectifying p–n junction doping structure in a light-emitting electrochemical cell (LEC) application: (i) polymerization of the dopant counter-ions, (ii) polymerization of the counterions utilizing a radical-initiator compound, (iii) polymerization of the ion-transport material, and (iv) polymerization of both the counter-ions and the ion-transport material utilizing a radical-initiator compound. We found that approach (i) resulted in LEC devices with poor stability, current rectification and light-emission, and that approach (ii) solely yielded a notable improvement in the light-emission. Approach (iii) resulted in good current rectification and stability, but the overall best results were clearly attained with approach (iv) as such stabilized LEC devices exhibited a respectable current rectification ratio of 2000, as well as a decent light-emission efficiency and longterm stability under idle conditions. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polymers (CPs) are semiconductors that can be doped into various highly conducting states via chemical or electrochemical means, and this Nobel-prize winning discovery has paved the way for a wide range of emerging ‘‘plastic electronic” devices [1–6]. The general mechanism of electrochemical doping of CPs involves an initial electronic charge injection from a metal electrode, and a subsequent insertion of a charge-compensating counter-ion into close proximity of the electronic charge carrier from a nearby electrolyte [7,8]. One plastic electronic device that exploits the opportunity of electrochemical doping in a particularly attractive manner is the light-emitting electrochemical cell (LEC) [8–15]. It
* Corresponding author. Tel.: +46 90 7865732; fax: +46 90 7866673. E-mail address: [email protected] (L. Edman). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.03.009
comprises a mixture of a CP and an electrolyte positioned between two electrodes. When a sufficiently high voltage (exceeding the band-gap potential of the CP) is applied between the electrodes, the CP is p-type doped at the anode and n-type at the cathode, respectively [16,17]. These doped regions grow in size with time and eventually make contact to form a light-emitting p–n junction [18– 20]. The p–n junction doping structure is a well-known and highly exploited feature in inorganic solid-state electronic devices, notably based on crystalline silicon and III/V semiconductors, and has enabled the realization of a wide variety of ubiquitous solid-state electronic devices, such as rectifying diodes, transistors, lasers, and solar cells, to name a few. The inorganic p–n junction, and some more recent manifestations of organic p–n junctions [21,22], are, however, static structures with compositions that are defined during the fabrication of the devices; and as such, these p–n junctions are distinctly different from
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those in LEC devices, which are dynamic structures that form, readjust, and dissipate in response to changes in the applied voltage [20]. Although the dynamic nature of the electrochemical doping of CPs offers many important advantages, it would for several applications be of interest to permanently stabilize a desired doping structure that is formed during the doping process. For instance, a significant time lag between the application of voltage and attainment of a stable response (e.g., current rectification and/or light emission), during which the doping structure is built via a redistribution of mobile ions, is an obvious drawback in applications where a fast and consistent behavior is important. Immobilization of ions is the key to the stabilization of an electrochemically induced doping structure in CPs in general and LECs in particular. More specifically, ion mobility is an obvious prerequisite during the doping process, but the subsequent stabilization of a desired doping structure requires that the mobility of the ions is eliminated. Gao and co-workers were the first to demonstrate LEC devices with stabilized doping structures, when they utilized the temperature dependence of the ion mobility of lithium-salts in a poly(ethylene oxide) (PEO) matrix, thereby establishing the so-called ‘‘frozen-junction” concept [23,24]. They formed the p–n junction at room temperature, at which the lithium-salt ions exhibit a reasonable mobility in the PEO matrix. When the device is subsequently cooled to a low temperature with the bias maintained, the ion mobility drops to essentially zero [23,24]. A number of subsequent studies on the topic of frozen-junction operation have appeared in the scientific literature, but the concept currently suffers from an impractical sub-ambient operation temperature and/or limited long-term stability [25–30]. An alternative novel approach for doping structures with room temperature stability, conceptualized by Cheng and Lonergan, includes the employment of a bi-layer structure comprising charge-complementary ionic CPs and a subsequent solvent-induced removal of the mobile ions [31]. However, the plausibly most practical and generic route towards permanently stabilized doping structures, as introduced by Leger et al., involves a ‘‘chemical stabilization” route [32–35]. In the pioneering paper [33], it was the dopant counter-ions (being polymerizable ion-pair monomers) that were chemically modified and effectively locked in place via a polymerization reaction. In a later study, Yu et al. have demonstrated that it is possible to attain stabilized doping via chemical cross-linking of the ion-transport material (in the form of a polymerizable small molecule) [36]. In this article, we report results on an expanded study of the merits of the chemical stabilization concept. We evaluate and compare the performance of LEC devices comprising light-emitting p–n junctions built and stabilized in accordance with the approaches of Leger et al. and Yu et al., as well as by two new approaches. We find that the best performing LEC devices, from the viewpoints of light-emission, current rectification, and long-term stability, are those that utilize a combination of polymerizable ions, a polymerizable ion-transport material, and a radicalinitiator compound.
2. Material and methods 2.1. Ion-pair monomer synthesis The ion-pair monomer (i.e., the polymerizable salt) 2(methacryloyloxy)ethyl trimethylammonium 2-acrylamido-2-methyl-1-propane sulfonate (METMA/AMPS) was synthesized in accordance with a previously published procedure [37,38]. About 0.250 g (1.47 mmol) silver nitrate and 0.060 g (1.5 mmol) sodium hydroxide were mixed together in 10 ml of water in darkness, and the resulting silver hydroxide precipitate was filtered over a frit. The recovered silver hydroxide was added to 0.20 g (0.95 mmol) AMPS-H (Aldrich), which had been dissolved in 2 ml water and treated twice with activated carbon. The mixture was stirred in an ice bath for 4 h to yield AMPS-Ag. 0.21 g (1.0 mmol) METMA-Cl (Polysciences) dissolved in 5 ml water was added drop wise to the AMPS-Ag mixture, during which precipitation of silver chloride was observed. This mixture was stirred for 3 h in an ice bath, where after the silver chloride was filtered away and the water allowed to evaporate under vacuum at room temperature. A glassy solid material was extracted using acetone, and this material was subsequently recrystallized to yield a white solid METMA/AMPS compound in a yield of 60%. 1H NMR (D2O): d 5.60 and 6.06 (m, 5H, vinyl-H), d 4.53 (s, 2H), d 3.69 (s, 2H), d 3.32 (s, 2H), d 3.13 (s, 9H), d 1.81 (s, 3H), d 1.38 (s, 6H). 2.2. Materials The CP used throughout this study was a decyloxyphenyl substituted poly(1,4-phenylene vinylene) termed superyellow (SY, Merck, Darmstadt, FRG), and it was used as received. The ion-transport materials, poly(ethylene oxide) (PEO; Mw = 5 106 g/mol), poly(ethylene glycol) diacrylate (poly-PEO; Mw = 575 g/mol) and trimethylolpropane trimethacrylate (TMPTMA), were all purchased from Aldrich (Steinheim, FRG) and used as received. The salt lithium trifluoromethanesulfonate (LiCF3SO3; Alfa Aesar, Karlsruhe, FRG) was dried at T = 473 K under vacuum before use. The radical-initiator 2,20 -azobis(2-methylpropionitrile) (AIBN) was purchased from Serva (Heidelberg, FRG) and used as received. Fig. 1 presents the chemical structures of the compounds used for the fabrication of the LEC devices. 2.3. Device preparation and measurements The solutions for the fabrication of the presented LEC devices (see Table 1) were prepared by mixing the below listed master solutions in appropriate amounts. System (A): 5 mg/ml SY in chlorobenzene (CB), 10 mg/ml PEO in CB, and 5 mg/ml METMA/AMPS in CB. System (B): 5 mg/ ml SY in toluene, 10 mg/ml PEO in cyclohexanone (CH), 5 mg/ml METMA/AMPS in CH, and 10 mg/ml AIBN in CH. System (C): 5 mg/ml SY in tetrahydrofuran (THF), 10 mg/ml TMPTMA in THF, and 10 mg/ml LiCF3SO3 in THF. System (D): 5 mg/ml SY in toluene, 10 mg/ml PEO in CH, 5 mg/ml METMA/AMPS in CH, and 10 mg/ml AIBN
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Fig. 1. Chemical structures of the constituent materials in the active layer of the LEC devices.
Table 1 Key devices properties for the four different types of stabilized doping systems. System group active material constituents (mass ratio)
Current rectification ratio at V = ±10 Va
Initial brightness at V = +10 V (cd/m2)a
Retained brightness after 12 h under open-circuit
Current efficacy at V = +10 V (cd/A)a
(A)
NAb [22]c
0 [36]d
NA (15%)d
0 (0.77)d
NAb
15
2%
1.1
700
16
86%
0.06
1900
50
87%
1.1
SY:PEO:METMA/AMPS (1:0.4:0.1) (B) SY:PEO:METMA/AMPS:AIBN (1:0.17:0.06:0.003) (C) SY:TMPTMA:LiCF3SO3 (1:0.5:0.1) (D) SY:poly-PEO:METMA/ AMPS:AIBN (1:0.17:0.06:0.003) a b c d
As measured directly after the removal of the pre-bias. It was not possible to measure a stable rectification ratio at V = ±10 V. The value in parenthesis is the rectification ratio at V = ±15 V. The value in parenthesis is measured at V = +15 V, as the brightness at V = +10 V was <1 cd/m2.
in CH. The solutions were stirred on a magnetic hot plate at T = 323 K for at least 5 h immediately before the deposition.
The indium tin oxide (ITO) coated glass substrates (1.5 1.5 cm2, 20 X/square; Thin Film Devices, Anaheim, CA) were cleaned by sequential ultrasonic treatments at
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room temperature in detergent (Extran MA 01, Merck), acetone, and isopropanol. The LEC devices were fabricated in a sandwich cell architecture by spin-coating solution onto an ITO-coated glass substrate for 60 s at 800 rpm (for active materials of systems (A), (B) and (D)) or 2000 rpm (for active materials of system (C)). The resulting active material film was thereafter dried at T = 333 K for at least 5 h. The thicknesses of the active material films, as established by
atomic force microscopy, were: 200 nm (system (A)), 120 nm (system (B)), 100 nm (system (C)), and 120 nm (system (D)). Aluminium electrodes were deposited on top of the film by thermal evaporation at p < 2 104 Pa. All of the above device preparation procedures, except the cleaning of the substrates, were carried out in two interconnected N2-filled glove boxes ([O2] < 3 ppm, [H2O] < 0.5 ppm). Device measurements were performed under
Fig. 2. Schematic illustration of the doping and chemical stabilization processes in the four types of baseline LEC systems (A–D) under study. Note that the dopant radical-initiators depicted in the second column of (A) and (C) correspond to injected holes and electrons, and are as such in reality chargecompensated by counter-ions in close proximity. For clarity, we have chosen to omit these ions from the schematic.
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vacuum (p < 103 Pa) in an optical-access cryostat (Advanced Research Systems). A computer-controlled sourcemeasure unit (Keithley 2400) in combination with a calibrated photo-diode with an eye response filter (Hamamatsu, S9219–01) connected through a current-to-voltage amplifier to a HP 34401A digital multimeter were employed for the optoelectronic characterization of the devices. The scan rate for the I–V measurements presented in Figs. 3–6 was 0.5 V/s.
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3. Results and discussion We report results on four different types of baseline systems, for which the permanent stabilization of the doping profile was attempted in accordance with the following basic principles: (A) Chemical cross-linking of ion-pair monomers (i.e., a polymerizable salt); (B) chemical crosslinking of ion-pair monomers utilizing an added radicalinitiator compound; (C) chemical cross-linking of the
Fig. 3. Optoelectronic data for an LEC device of type (A), which had been pre-biased at V = +10 V at room temperature for 1.5 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the removal of the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Optoelectronic data for an LEC device of type (B), which had been pre-biased at V = +10 V and T = 360 K for 1.5 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Optoelectronic data for an LEC device of type (C), which had been pre-biased at V = +12 V at room temperature for 3.8 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Optoelectronic data for an LEC device of type (D), which had been pre-biased at V = +5 V at room temperature for 2 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ion-transport material; and (D) chemical cross-linking of ion-pair monomers and the ion-transport material utilizing an added radical-initiator compound. Fig. 2 presents a schematic of our view on how the initial doping and the subsequent stabilization of the doping should be attained. For clarity, we have chosen to present data (and experimental detail) for only one device structure for each system group. However, it is appropriate to point out that we have tested a large number of different device structures under different biasing conditions, and that the presented data represent the ‘‘best” performing device in each system group. Moreover, in order to accommodate a straightforward comparison and evaluation of the different system groups from a stabilized doping and functional device perspective, we have chosen to focus on a number of distinct device properties, specifically the current rectification ratio (RR), the initial brightness following the pre-bias step, the retained brightness after a 12 h period of storage under open-circuit condition, and the current efficacy for light-emission. These values for the best performing device structures in each group are summarized in Table 1 above, and the performance of each device structure is discussed in detail in the following four sections. 3.1. System (A): cross-linking of ions Leger and co-workers [33] introduced an innovative concept for the permanent stabilization of a p–n junction doping profile in LEC devices, when they replaced the conventionally used ‘‘inert” salt (typically an alkaline metal salt or an ionic liquid) with an ion-pair monomer that can be polymerized via radical initiation. Following a prebias period, such LEC devices exhibit uni-polar light-emission and a high current rectification ratio of RR > 100 at high operating voltage, which are both indicators of stabilized doping. The authors proposed that the polymerization of the ion-pair monomers could be initiated at the site of the electrochemical doping of the conjugated polymer, since the hole and electron dopants in effect are highly reactive radicals that presumably can initiate a polymerization reaction of the ion-pair monomers in accordance with the process depicted in Fig. 2A. We have utilized METMA/AMPS as the ion-pair monomer and fabricated sandwich cells with an active material
blend, comprising SY, PEO, and METMA/AMPS in a (1:0.4:0.1) mass ratio, sandwiched between an ITO anode and an Al cathode. The devices were initially ‘‘pre-biased” at V = +10 V at room temperature for an extended period of 1.5 h (data not shown). During the pre-bias period the current is observed to increase significantly with time in a continuous fashion, and at the end of the pre-bias period the device begins to emit light. These are well-known fingerprints of a functional LEC device, and it is accordingly clear that the employed METMA/AMPS ion-pair monomer: (i) exhibits ionic mobility in the active material, and (ii) can function as counter-ions to accommodate electrochemical doping of SY. The data displayed in Fig. 3 (and summarized in Table 1) for the device of system (A) were recorded after the pre-bias had been removed. In agreement with the results presented by Leger et al., we found that a pre-biased device exhibited uni-polar light-emission, i.e., it only emitted light when biased with the same voltage polarity as it had been pre-biased. In contrast, conventional LECs comprising mobile ions typically exhibit bi-polar light-emission, as the doping is reversible, and the light-emitting p–n junction accordingly can form in both bias directions [20]. However, a device based on a stabilized p–n junction doping profile is also expected to exhibit a high current rectification, but as we found a large and rather erratic current at reverse voltages up to V < 15 V (see Fig. 3a), the current rectification ratio was very low (and also difficult to quantify). Moreover, a practical stabilized doping profile should be stabile over time under open-circuit conditions, but we found that the brightness at V = +15 V had dropped to 15% of its initial value after 12 h at open-circuit (see Fig. 3b). A further drawback seen with the pre-biased devices of system (A) was that the brightness was invariably very low. This was also manifested in the turn-on voltage for light-emission (defined to be the voltage at which the device emits 1 cd/m2), which for the pre-biased device depicted in Fig. 3 was very high at Vturn-on = 11 V. 3.2. System (B): cross-linking of ions with added initiator The poor long-term stability of the pre-biased devices of system (A) under open-circuit conditions (see Fig. 3 and Table 1) implies that a significant fraction of the
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ion-pair monomers were not polymerized during the prebias step. Moreover, the low brightness indicates that the assumed polymerization mechanism with onset via dopant radical initiation might not be preferable, as this process plausibly damages the fluorescent and/or electronictransport capacity of the conjugated polymer (here, SY). In order to rectify these problems, and to attain a higher degree of polymerization and a more stable doping profile as well as higher brightness, we chose to investigate the effects of adding AIBN as a radical-initiator to the active material blend. The expectation was that the AIBN initiator would assist in the formation of an extended and stabile cross-linked chemical network between the different ion-pair monomers without including – and potentially damaging the electroluminescent properties of – the conjugated polymer, as illustrated in Fig. 2B. A sandwich cell, comprising an active material blend of SY, PEO, METMA/AMPS, and AIBN in a (1:0.17:0.06:0.003) mass ratio sandwiched between an ITO anode and an Al cathode, was pre-biased at V = +10 V for 1.5 h at T = 360 K. The employment of the elevated temperature was motivated by that the decomposition of AIBN into two free-radical fragments and the related rate of the subsequent radical-initiated polymerization will both increase with increasing temperature. During the pre-bias period the current and brightness were found to increase with time, in a similar fashion as for the device of system (A). The turn-on time for light-emission was, however, notably faster, which can be attributed to a significantly higher ion mobility of the METMA/AMPS ion-pair monomer in an amorphous PEO matrix (at T = 360 K), as compared to a semi-crystalline PEO matrix (at room temperature) [16]. The typical performance of a pre-biased device of system (B) is shown in Fig. 4 and summarized in Table 1. Following the removal of the pre-bias, it exhibited uni-polar light-emission, a relatively high current efficacy for a conjugated-polymer-based LEC of 1.1 cd/A at V = +10 V, and a notably increased brightness. (Note the expanded voltage range in Fig. 3.) However, although the overall performance of pre-biased devices of system (B) was improved in comparison to devices of system (A), it was still not satisfying. The latter is exemplified by the large and rather erratic current that was once again observed at reverse voltages, which precluded a meaningful establishment of a current rectification ratio. The long-term stability was furthermore not impressive, as manifested in the brightness that dropped to 2% of its initial value after 12 h storage under open-circuit conditions. 3.3. System (C): cross-linking of the ion-transport material Yu and co-workers [36] recently demonstrated an alternative approach for the attainment of chemically stabilized doping structures in LEC devices, when they employed a polymerizable ‘‘small molecule” ion-transport material in combination with a non-polymerizable alkaline salt as the electrolyte. In the seminal study, the liquid trivinylic monomer TMPTMA was utilized as ion-transport material instead of the commonly employed PEO. The chemical structure of TMPTMA is presented in Fig. 1. Its three methacrylate groups can readily be polymerized into a rigid,
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covalently cross-linked network that effectively impedes diffusion of other molecules. The authors demonstrated pre-biased LEC devices with rather impressive stability and relatively high light-emission efficiency following this concept, and it was proposed, in analogy with the physical ‘‘frozen-junction” approach [13,24,39], that the stability resulted from an effective elimination (freeze-in) of the ion-transport capacity of TMPTMA when it was spatially immobilized via polymerization [36]. We have fabricated sandwich cells with an active material blend, comprising SY, TMPTMA and LiCF3SO3 in a (1:0.5:0.1) mass ratio, sandwiched between an ITO anode and an Al cathode. The devices were pre-biased at V = +12 V at room temperature for almost 4 h. At the end of the pre-bias period, the light-emission intensity and the current exhibited a drastic rapid increase to high values, in agreement with the observation for a similar system in Ref. [36], but it is notable that the elapsed time up to this point was markedly longer in our experiments. We attribute this difference to a thicker active material in the herein studied devices, which is concomitant with a lower ionic conductance and a slower turn-on time [40]. The data for a typical pre-biased device of system (C) are displayed in Fig. 5 and summarized in Table 1. The sandwich cell device showed promising uni-polar current and emission characteristics, and the current rectification ratio was rather high (RR = 700 at V = ±10 V). These data are in good qualitative agreement with those reported by Yu et al. on a similar system, but the quantitative values for the brightness and current efficacy were lower in the experiments reported here. Furthermore, the p–n junction doping profiles in system (C) are considerably more stabile over time in comparison to systems (A) and (B), as evidenced by the brightness drop by only 14% from its initial value after 12 h under open-circuit conditions. 3.4. System (D): cross-linking of ions and ion-transport material with added initiator Inspired by the promising results for devices of system (C), we attempted to combine the approaches taken in systems (B) and (C). In other words, we simultaneously employed a polymerizable ion-pair monomer (METMA/AMPS), a polymerizable ion-transport compound (poly-PEO), and a radical-initiator compound (AIBN) in addition to the conjugated polymer (SY) as the active material. The goal was to induce a doping profile by the pre-bias, which would at the end of the pre-bias period include both the ions and the ion-transport material as integral parts of a large and interconnected immobile ionic network. We further hoped that the addition of the radical-initiator compound would facilitate an efficient and complete polymerization and eliminate the need for polymerization initiation via dopant radicals generated on the conjugated polymer SY, as we suspected that a dopant-initiation could lead to quenching of the electroluminescence and result in low brightness and current efficacy values. An idealized view of the operation and stabilization of such an LEC is illustrated in Fig. 2D. Sandwich cells, with (SY:poly-PEO:METMA/AMPS:AIBN) in a (1:0.17:0.06:0.003) mass ratio as the active material positioned between an ITO anode an Al cathode,
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were pre-biased at V = +5 V for 2 h at room temperature. The current and brightness increased in a continuous manner during the initial part of the pre-bias period, but at the end of the period they stabilized at essentially constant values, which were taken as an indicator of stabilized doping. The data displayed in Fig. 6 and Table 1 were recorded after the pre-bias had been removed. Such pre-biased devices exhibited notably well-behaved uni-polar current and emission characteristics, as for instance quantified by an excellent current rectification of RR = 1900 at V = ±10 V. Moreover, the brightness and current efficacy for the pre-biased devices of system (D) were clearly higher than in the other investigated systems, and the stability over time was relatively impressive as the drop in brightness after 12 h under open-circuit was a mere 13%, with the major drop taking place during the first 2 h following the removal of the pre-bias. Thus, the overall performance of stabilized devices of system (D) was superior to the other investigated systems (see Table 1), although it is clear that further improvement in the long-term stability of the doping structure is clearly needed for practical applications. It should also be noted that the optoelectronic performance of these stabilized devices are not yet on par with stateof-the-art dynamic LEC devices, as demonstrated by that turn-on voltages of the order of the band-gap potential and current efficacies of the order of 3–9 cd/A have recently been reported for various manifestations of dynamic-junction LECs based on the same conjugated polymer SY [41– 43].
4. Conclusions We have systematically tested, evaluated, and compared a number of different ‘‘old” and new approaches for the attainment of a chemically stabilized doping structure in LEC devices. We found that the pioneering concept of chemically connecting the dopant counterions via utilization of polymerizable ion-pair monomers alone resulted in light-emitting devices with poor stability and low brightness. The addition of a radical-initiator compound to this type of active material blend improved the brightness, presumably because the emissive conjugated polymer is relieved of the role of acting as radical initiation site for polymerization; however, the stability remained poor. The alternative concept of employing a small molecule ion-transport material that can be cross-linked into large and immobile polymeric structures during the initial pre-bias operation yielded distinctly improved results in the form of a high current rectification ratio and a decent stability under open-circuit conditions. The best performance was, however, attained from a device that combined all of the above concepts, as its active material comprised polymerizable ion-pair monomers, a cross-linkable ion-transport material, and a radical-initiator in addition to the conjugated polymer. Such chemically stabilized devices exhibited a highly respectable current rectification ratio of almost 2000, a good light-emission efficiency, and a decent long-term stability under idle conditions.
Acknowledgments The authors wish to thank Jeroen Verhage and Ryan Chadwick for valuable preliminary experiments in the synthesis of ion-pair monomers, and Andreas Sandström for assistance with the LEC characterization. The authors are grateful to Kempestiftelserna, Carl Tryggers Stiftelse, and the Swedish Research Council (Vetenskapsrådet) for financial support. L.E. is a ‘‘Royal Swedish Academy of Sciences Research Fellow” supported by a grant from the Knut and Alice Wallenberg Foundation. References [1] nobelprize.org/nobel_prizes/chemistry/laureates/2000/public.html. [2] H. Shirakawa, E.J. Louis, A.G. Macdiarmid, C.K. Chiang, A.J. Heeger, J. Chem. Soc.-Chem. Commun. (16) (1977) 578–580. [3] C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. Macdiarmid, Phys. Rev. Lett. 39 (17) (1977) 1098–1101. [4] M. Gross, D.C. Muller, H.G. Nothofer, U. Scherf, D. Neher, C. Brauchle, K. Meerholz, Nature 405 (6787) (2000) 661–665. [5] M.H. Bolin, K. Svennersten, D. Nilsson, A. Sawatdee, E.W.H. Jager, A. Richter-Dahlfors, M. Berggren, Adv. Mater. 21 (43) (2009) 4379. [6] J.H. Zhao, R.G. Pillai, M. Pilapi, M.S. Freund, D.J. Thomson, Polypyrrole Nanoionic Composites for Solid State Electronics, CRC Press-Taylor & Francis Group, Boca Raton, 2008. [7] J.M. Leger, Adv. Mater. 20 (4) (2008) 837–841. [8] L. Edman, Electrochim. Acta 50 (19) (2005) 3878–3885. [9] Q.B. Pei, Y. Yang, G. Yu, C. Zhang, A.J. Heeger, J. Am. Chem. Soc. 118 (16) (1996) 3922–3929. [10] .Q.B. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (5227) (1995) 1086–1088. [11] Q.J. Sun, Y.F. Li, Q.B. Pei, J. Dispersion Sci. Technol. 3 (2) (2007) 211– 224. [12] Z.B. Yu, L.B. Hu, Z.T. Liu, M.L. Sun, M.L. Wang, G. Gruner, Q.B. Pei, Appl. Phys. Lett. 95 (20) (2009). [13] D.H. Lee, J.H. Park, H. Chae, S.M. Cho, Org. Electron. 10 (7) (2009) 1345–1351. [14] J. Fang, P. Matyba, N.D. Robinson, L. Edman, J. Am. Chem. Soc. 130 (13) (2008) 4562–4568. [15] J.F. Fang, P. Matyba, L. Edman, Adv. Funct. Mater. 19 (16) (2009) 2671–2676. [16] J.H. Shin, A. Dzwilewski, A. Iwasiewicz, S. Xiao, A. Fransson, G.N. Ankah, L. Edman, Appl. Phys. Lett. 89 (1) (2006) 013509. [17] J.H. Shin, L. Edman, J Am. Chem. Soc. 128 (49) (2006) 15568–15569. [18] J. Gao, J. Dane, Appl. Phys. Lett. 83 (15) (2003) 3027–3029. [19] J. Gao, J. Dane, J. Appl. Phys. 98 (6) (2005). [20] P. Matyba, K. Maturova, M. Kemerink, N.D. Robinson, L. Edman, Nat. Mater. 8 (8) (2009) 672–676. [21] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 459 (7244) (2009) 234. U116. [22] S.G. Robinson, D.H. Johnston, C.D. Weber, M.C. Lonergan, Chem. Mater. 22 (1) (2009) 241–246. [23] J. Gao, Y.F. Li, G. Yu, A.J. Heeger, J. Appl. Phys. 86 (8) (1999) 4594– 4599. [24] J. Gao, G. Yu, A.J. Heeger, Appl. Phys. Lett. 71 (10) (1997) 1293–1295. [25] J.H. Shin, S. Xiao, A. Fransson, L. Edman, Appl. Phys. Lett. 87 (4) (2005) 043506. [26] L. Edman, M. Pauchard, D. Moses, A.J. Heeger, J. Appl. Phys. 95 (8) (2004) 4357–4361. [27] Y. Cao, Q.B. Pei, M.R. Andersson, G. Yu, A.J. Heeger, J. Electrochem. Soc. 144 (12) (1997) L317–L320. [28] J. Dane, C. Tracy, J. Gao, Appl. Phys. Lett. 86 (15) (2005). [29] C.H. Yang, Q.J. Sun, J. Qiao, Y.F. Li, J. Phys. Chem. B 107 (47) (2003) 12981–12988. [30] G. Yu, Y. Cao, M. Andersson, J. Gao, A.J. Heeger, Adv. Mater. 10 (5) (1998) 385–388. [31] C.H.W. Cheng, M.C. Lonergan, J. Am. Chem. Soc. 126 (34) (2004) 10536–10537. [32] J.M. Leger, D.G. Patel, D.B. Rodovsky, G.P. Bartholomew, Adv. Funct. Mater. 18 (8) (2008) 1212–1219. [33] J.M. Leger, D.B. Rodovsky, G.R. Bartholomew, Adv. Mater. 18 (23) (2006) 3130. [34] L.S.C. Pingree, O.G. Reid, D.S. Ginger, Adv. Mater. 21 (1) (2009) 19–28.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Chemical stabilization of doping in conjugated polymers Shi Tang a, Knut Irgum b, Ludvig Edman a,* a b
The Organic Photonics and Electronics Group, Department of Physics, Umeå University, S-901 87 Umeå, Sweden Department of Chemistry, Umeå University, S-901 87 Umeå, Sweden
a r t i c l e
i n f o
Article history: Received 25 January 2010 Received in revised form 15 March 2010 Accepted 16 March 2010 Available online 20 March 2010 Keywords: Doping p–n Junction Chemical stabilization Light-emitting electrochemical cell Ion-pair monomer Superyellow
a b s t r a c t Conjugated polymers can be electrochemically doped to high-conductivity states under applied voltage, but such in situ formed doping structures are dynamic and dissipate when the formation voltage is removed. For some applications it is highly desirable to permanently stabilize the doping after its formation. Here, we report and compare results on four different approaches to chemical stabilization of an emissive and rectifying p–n junction doping structure in a light-emitting electrochemical cell (LEC) application: (i) polymerization of the dopant counter-ions, (ii) polymerization of the counterions utilizing a radical-initiator compound, (iii) polymerization of the ion-transport material, and (iv) polymerization of both the counter-ions and the ion-transport material utilizing a radical-initiator compound. We found that approach (i) resulted in LEC devices with poor stability, current rectification and light-emission, and that approach (ii) solely yielded a notable improvement in the light-emission. Approach (iii) resulted in good current rectification and stability, but the overall best results were clearly attained with approach (iv) as such stabilized LEC devices exhibited a respectable current rectification ratio of 2000, as well as a decent light-emission efficiency and longterm stability under idle conditions. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polymers (CPs) are semiconductors that can be doped into various highly conducting states via chemical or electrochemical means, and this Nobel-prize winning discovery has paved the way for a wide range of emerging ‘‘plastic electronic” devices [1–6]. The general mechanism of electrochemical doping of CPs involves an initial electronic charge injection from a metal electrode, and a subsequent insertion of a charge-compensating counter-ion into close proximity of the electronic charge carrier from a nearby electrolyte [7,8]. One plastic electronic device that exploits the opportunity of electrochemical doping in a particularly attractive manner is the light-emitting electrochemical cell (LEC) [8–15]. It
* Corresponding author. Tel.: +46 90 7865732; fax: +46 90 7866673. E-mail address: [email protected] (L. Edman). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.03.009
comprises a mixture of a CP and an electrolyte positioned between two electrodes. When a sufficiently high voltage (exceeding the band-gap potential of the CP) is applied between the electrodes, the CP is p-type doped at the anode and n-type at the cathode, respectively [16,17]. These doped regions grow in size with time and eventually make contact to form a light-emitting p–n junction [18– 20]. The p–n junction doping structure is a well-known and highly exploited feature in inorganic solid-state electronic devices, notably based on crystalline silicon and III/V semiconductors, and has enabled the realization of a wide variety of ubiquitous solid-state electronic devices, such as rectifying diodes, transistors, lasers, and solar cells, to name a few. The inorganic p–n junction, and some more recent manifestations of organic p–n junctions [21,22], are, however, static structures with compositions that are defined during the fabrication of the devices; and as such, these p–n junctions are distinctly different from
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those in LEC devices, which are dynamic structures that form, readjust, and dissipate in response to changes in the applied voltage [20]. Although the dynamic nature of the electrochemical doping of CPs offers many important advantages, it would for several applications be of interest to permanently stabilize a desired doping structure that is formed during the doping process. For instance, a significant time lag between the application of voltage and attainment of a stable response (e.g., current rectification and/or light emission), during which the doping structure is built via a redistribution of mobile ions, is an obvious drawback in applications where a fast and consistent behavior is important. Immobilization of ions is the key to the stabilization of an electrochemically induced doping structure in CPs in general and LECs in particular. More specifically, ion mobility is an obvious prerequisite during the doping process, but the subsequent stabilization of a desired doping structure requires that the mobility of the ions is eliminated. Gao and co-workers were the first to demonstrate LEC devices with stabilized doping structures, when they utilized the temperature dependence of the ion mobility of lithium-salts in a poly(ethylene oxide) (PEO) matrix, thereby establishing the so-called ‘‘frozen-junction” concept [23,24]. They formed the p–n junction at room temperature, at which the lithium-salt ions exhibit a reasonable mobility in the PEO matrix. When the device is subsequently cooled to a low temperature with the bias maintained, the ion mobility drops to essentially zero [23,24]. A number of subsequent studies on the topic of frozen-junction operation have appeared in the scientific literature, but the concept currently suffers from an impractical sub-ambient operation temperature and/or limited long-term stability [25–30]. An alternative novel approach for doping structures with room temperature stability, conceptualized by Cheng and Lonergan, includes the employment of a bi-layer structure comprising charge-complementary ionic CPs and a subsequent solvent-induced removal of the mobile ions [31]. However, the plausibly most practical and generic route towards permanently stabilized doping structures, as introduced by Leger et al., involves a ‘‘chemical stabilization” route [32–35]. In the pioneering paper [33], it was the dopant counter-ions (being polymerizable ion-pair monomers) that were chemically modified and effectively locked in place via a polymerization reaction. In a later study, Yu et al. have demonstrated that it is possible to attain stabilized doping via chemical cross-linking of the ion-transport material (in the form of a polymerizable small molecule) [36]. In this article, we report results on an expanded study of the merits of the chemical stabilization concept. We evaluate and compare the performance of LEC devices comprising light-emitting p–n junctions built and stabilized in accordance with the approaches of Leger et al. and Yu et al., as well as by two new approaches. We find that the best performing LEC devices, from the viewpoints of light-emission, current rectification, and long-term stability, are those that utilize a combination of polymerizable ions, a polymerizable ion-transport material, and a radicalinitiator compound.
2. Material and methods 2.1. Ion-pair monomer synthesis The ion-pair monomer (i.e., the polymerizable salt) 2(methacryloyloxy)ethyl trimethylammonium 2-acrylamido-2-methyl-1-propane sulfonate (METMA/AMPS) was synthesized in accordance with a previously published procedure [37,38]. About 0.250 g (1.47 mmol) silver nitrate and 0.060 g (1.5 mmol) sodium hydroxide were mixed together in 10 ml of water in darkness, and the resulting silver hydroxide precipitate was filtered over a frit. The recovered silver hydroxide was added to 0.20 g (0.95 mmol) AMPS-H (Aldrich), which had been dissolved in 2 ml water and treated twice with activated carbon. The mixture was stirred in an ice bath for 4 h to yield AMPS-Ag. 0.21 g (1.0 mmol) METMA-Cl (Polysciences) dissolved in 5 ml water was added drop wise to the AMPS-Ag mixture, during which precipitation of silver chloride was observed. This mixture was stirred for 3 h in an ice bath, where after the silver chloride was filtered away and the water allowed to evaporate under vacuum at room temperature. A glassy solid material was extracted using acetone, and this material was subsequently recrystallized to yield a white solid METMA/AMPS compound in a yield of 60%. 1H NMR (D2O): d 5.60 and 6.06 (m, 5H, vinyl-H), d 4.53 (s, 2H), d 3.69 (s, 2H), d 3.32 (s, 2H), d 3.13 (s, 9H), d 1.81 (s, 3H), d 1.38 (s, 6H). 2.2. Materials The CP used throughout this study was a decyloxyphenyl substituted poly(1,4-phenylene vinylene) termed superyellow (SY, Merck, Darmstadt, FRG), and it was used as received. The ion-transport materials, poly(ethylene oxide) (PEO; Mw = 5 106 g/mol), poly(ethylene glycol) diacrylate (poly-PEO; Mw = 575 g/mol) and trimethylolpropane trimethacrylate (TMPTMA), were all purchased from Aldrich (Steinheim, FRG) and used as received. The salt lithium trifluoromethanesulfonate (LiCF3SO3; Alfa Aesar, Karlsruhe, FRG) was dried at T = 473 K under vacuum before use. The radical-initiator 2,20 -azobis(2-methylpropionitrile) (AIBN) was purchased from Serva (Heidelberg, FRG) and used as received. Fig. 1 presents the chemical structures of the compounds used for the fabrication of the LEC devices. 2.3. Device preparation and measurements The solutions for the fabrication of the presented LEC devices (see Table 1) were prepared by mixing the below listed master solutions in appropriate amounts. System (A): 5 mg/ml SY in chlorobenzene (CB), 10 mg/ml PEO in CB, and 5 mg/ml METMA/AMPS in CB. System (B): 5 mg/ ml SY in toluene, 10 mg/ml PEO in cyclohexanone (CH), 5 mg/ml METMA/AMPS in CH, and 10 mg/ml AIBN in CH. System (C): 5 mg/ml SY in tetrahydrofuran (THF), 10 mg/ml TMPTMA in THF, and 10 mg/ml LiCF3SO3 in THF. System (D): 5 mg/ml SY in toluene, 10 mg/ml PEO in CH, 5 mg/ml METMA/AMPS in CH, and 10 mg/ml AIBN
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Fig. 1. Chemical structures of the constituent materials in the active layer of the LEC devices.
Table 1 Key devices properties for the four different types of stabilized doping systems. System group active material constituents (mass ratio)
Current rectification ratio at V = ±10 Va
Initial brightness at V = +10 V (cd/m2)a
Retained brightness after 12 h under open-circuit
Current efficacy at V = +10 V (cd/A)a
(A)
NAb [22]c
0 [36]d
NA (15%)d
0 (0.77)d
NAb
15
2%
1.1
700
16
86%
0.06
1900
50
87%
1.1
SY:PEO:METMA/AMPS (1:0.4:0.1) (B) SY:PEO:METMA/AMPS:AIBN (1:0.17:0.06:0.003) (C) SY:TMPTMA:LiCF3SO3 (1:0.5:0.1) (D) SY:poly-PEO:METMA/ AMPS:AIBN (1:0.17:0.06:0.003) a b c d
As measured directly after the removal of the pre-bias. It was not possible to measure a stable rectification ratio at V = ±10 V. The value in parenthesis is the rectification ratio at V = ±15 V. The value in parenthesis is measured at V = +15 V, as the brightness at V = +10 V was <1 cd/m2.
in CH. The solutions were stirred on a magnetic hot plate at T = 323 K for at least 5 h immediately before the deposition.
The indium tin oxide (ITO) coated glass substrates (1.5 1.5 cm2, 20 X/square; Thin Film Devices, Anaheim, CA) were cleaned by sequential ultrasonic treatments at
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room temperature in detergent (Extran MA 01, Merck), acetone, and isopropanol. The LEC devices were fabricated in a sandwich cell architecture by spin-coating solution onto an ITO-coated glass substrate for 60 s at 800 rpm (for active materials of systems (A), (B) and (D)) or 2000 rpm (for active materials of system (C)). The resulting active material film was thereafter dried at T = 333 K for at least 5 h. The thicknesses of the active material films, as established by
atomic force microscopy, were: 200 nm (system (A)), 120 nm (system (B)), 100 nm (system (C)), and 120 nm (system (D)). Aluminium electrodes were deposited on top of the film by thermal evaporation at p < 2 104 Pa. All of the above device preparation procedures, except the cleaning of the substrates, were carried out in two interconnected N2-filled glove boxes ([O2] < 3 ppm, [H2O] < 0.5 ppm). Device measurements were performed under
Fig. 2. Schematic illustration of the doping and chemical stabilization processes in the four types of baseline LEC systems (A–D) under study. Note that the dopant radical-initiators depicted in the second column of (A) and (C) correspond to injected holes and electrons, and are as such in reality chargecompensated by counter-ions in close proximity. For clarity, we have chosen to omit these ions from the schematic.
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vacuum (p < 103 Pa) in an optical-access cryostat (Advanced Research Systems). A computer-controlled sourcemeasure unit (Keithley 2400) in combination with a calibrated photo-diode with an eye response filter (Hamamatsu, S9219–01) connected through a current-to-voltage amplifier to a HP 34401A digital multimeter were employed for the optoelectronic characterization of the devices. The scan rate for the I–V measurements presented in Figs. 3–6 was 0.5 V/s.
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3. Results and discussion We report results on four different types of baseline systems, for which the permanent stabilization of the doping profile was attempted in accordance with the following basic principles: (A) Chemical cross-linking of ion-pair monomers (i.e., a polymerizable salt); (B) chemical crosslinking of ion-pair monomers utilizing an added radicalinitiator compound; (C) chemical cross-linking of the
Fig. 3. Optoelectronic data for an LEC device of type (A), which had been pre-biased at V = +10 V at room temperature for 1.5 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the removal of the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Optoelectronic data for an LEC device of type (B), which had been pre-biased at V = +10 V and T = 360 K for 1.5 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Optoelectronic data for an LEC device of type (C), which had been pre-biased at V = +12 V at room temperature for 3.8 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Optoelectronic data for an LEC device of type (D), which had been pre-biased at V = +5 V at room temperature for 2 h. (a) The current (black solid squares) and the brightness (red open circles) as a function of voltage, as measured directly following the removal of the pre-bias. (b) The brightness-voltage dependence measured at different times following the pre-bias, as specified in the inset. The device was left at open-circuit between the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ion-transport material; and (D) chemical cross-linking of ion-pair monomers and the ion-transport material utilizing an added radical-initiator compound. Fig. 2 presents a schematic of our view on how the initial doping and the subsequent stabilization of the doping should be attained. For clarity, we have chosen to present data (and experimental detail) for only one device structure for each system group. However, it is appropriate to point out that we have tested a large number of different device structures under different biasing conditions, and that the presented data represent the ‘‘best” performing device in each system group. Moreover, in order to accommodate a straightforward comparison and evaluation of the different system groups from a stabilized doping and functional device perspective, we have chosen to focus on a number of distinct device properties, specifically the current rectification ratio (RR), the initial brightness following the pre-bias step, the retained brightness after a 12 h period of storage under open-circuit condition, and the current efficacy for light-emission. These values for the best performing device structures in each group are summarized in Table 1 above, and the performance of each device structure is discussed in detail in the following four sections. 3.1. System (A): cross-linking of ions Leger and co-workers [33] introduced an innovative concept for the permanent stabilization of a p–n junction doping profile in LEC devices, when they replaced the conventionally used ‘‘inert” salt (typically an alkaline metal salt or an ionic liquid) with an ion-pair monomer that can be polymerized via radical initiation. Following a prebias period, such LEC devices exhibit uni-polar light-emission and a high current rectification ratio of RR > 100 at high operating voltage, which are both indicators of stabilized doping. The authors proposed that the polymerization of the ion-pair monomers could be initiated at the site of the electrochemical doping of the conjugated polymer, since the hole and electron dopants in effect are highly reactive radicals that presumably can initiate a polymerization reaction of the ion-pair monomers in accordance with the process depicted in Fig. 2A. We have utilized METMA/AMPS as the ion-pair monomer and fabricated sandwich cells with an active material
blend, comprising SY, PEO, and METMA/AMPS in a (1:0.4:0.1) mass ratio, sandwiched between an ITO anode and an Al cathode. The devices were initially ‘‘pre-biased” at V = +10 V at room temperature for an extended period of 1.5 h (data not shown). During the pre-bias period the current is observed to increase significantly with time in a continuous fashion, and at the end of the pre-bias period the device begins to emit light. These are well-known fingerprints of a functional LEC device, and it is accordingly clear that the employed METMA/AMPS ion-pair monomer: (i) exhibits ionic mobility in the active material, and (ii) can function as counter-ions to accommodate electrochemical doping of SY. The data displayed in Fig. 3 (and summarized in Table 1) for the device of system (A) were recorded after the pre-bias had been removed. In agreement with the results presented by Leger et al., we found that a pre-biased device exhibited uni-polar light-emission, i.e., it only emitted light when biased with the same voltage polarity as it had been pre-biased. In contrast, conventional LECs comprising mobile ions typically exhibit bi-polar light-emission, as the doping is reversible, and the light-emitting p–n junction accordingly can form in both bias directions [20]. However, a device based on a stabilized p–n junction doping profile is also expected to exhibit a high current rectification, but as we found a large and rather erratic current at reverse voltages up to V < 15 V (see Fig. 3a), the current rectification ratio was very low (and also difficult to quantify). Moreover, a practical stabilized doping profile should be stabile over time under open-circuit conditions, but we found that the brightness at V = +15 V had dropped to 15% of its initial value after 12 h at open-circuit (see Fig. 3b). A further drawback seen with the pre-biased devices of system (A) was that the brightness was invariably very low. This was also manifested in the turn-on voltage for light-emission (defined to be the voltage at which the device emits 1 cd/m2), which for the pre-biased device depicted in Fig. 3 was very high at Vturn-on = 11 V. 3.2. System (B): cross-linking of ions with added initiator The poor long-term stability of the pre-biased devices of system (A) under open-circuit conditions (see Fig. 3 and Table 1) implies that a significant fraction of the
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ion-pair monomers were not polymerized during the prebias step. Moreover, the low brightness indicates that the assumed polymerization mechanism with onset via dopant radical initiation might not be preferable, as this process plausibly damages the fluorescent and/or electronictransport capacity of the conjugated polymer (here, SY). In order to rectify these problems, and to attain a higher degree of polymerization and a more stable doping profile as well as higher brightness, we chose to investigate the effects of adding AIBN as a radical-initiator to the active material blend. The expectation was that the AIBN initiator would assist in the formation of an extended and stabile cross-linked chemical network between the different ion-pair monomers without including – and potentially damaging the electroluminescent properties of – the conjugated polymer, as illustrated in Fig. 2B. A sandwich cell, comprising an active material blend of SY, PEO, METMA/AMPS, and AIBN in a (1:0.17:0.06:0.003) mass ratio sandwiched between an ITO anode and an Al cathode, was pre-biased at V = +10 V for 1.5 h at T = 360 K. The employment of the elevated temperature was motivated by that the decomposition of AIBN into two free-radical fragments and the related rate of the subsequent radical-initiated polymerization will both increase with increasing temperature. During the pre-bias period the current and brightness were found to increase with time, in a similar fashion as for the device of system (A). The turn-on time for light-emission was, however, notably faster, which can be attributed to a significantly higher ion mobility of the METMA/AMPS ion-pair monomer in an amorphous PEO matrix (at T = 360 K), as compared to a semi-crystalline PEO matrix (at room temperature) [16]. The typical performance of a pre-biased device of system (B) is shown in Fig. 4 and summarized in Table 1. Following the removal of the pre-bias, it exhibited uni-polar light-emission, a relatively high current efficacy for a conjugated-polymer-based LEC of 1.1 cd/A at V = +10 V, and a notably increased brightness. (Note the expanded voltage range in Fig. 3.) However, although the overall performance of pre-biased devices of system (B) was improved in comparison to devices of system (A), it was still not satisfying. The latter is exemplified by the large and rather erratic current that was once again observed at reverse voltages, which precluded a meaningful establishment of a current rectification ratio. The long-term stability was furthermore not impressive, as manifested in the brightness that dropped to 2% of its initial value after 12 h storage under open-circuit conditions. 3.3. System (C): cross-linking of the ion-transport material Yu and co-workers [36] recently demonstrated an alternative approach for the attainment of chemically stabilized doping structures in LEC devices, when they employed a polymerizable ‘‘small molecule” ion-transport material in combination with a non-polymerizable alkaline salt as the electrolyte. In the seminal study, the liquid trivinylic monomer TMPTMA was utilized as ion-transport material instead of the commonly employed PEO. The chemical structure of TMPTMA is presented in Fig. 1. Its three methacrylate groups can readily be polymerized into a rigid,
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covalently cross-linked network that effectively impedes diffusion of other molecules. The authors demonstrated pre-biased LEC devices with rather impressive stability and relatively high light-emission efficiency following this concept, and it was proposed, in analogy with the physical ‘‘frozen-junction” approach [13,24,39], that the stability resulted from an effective elimination (freeze-in) of the ion-transport capacity of TMPTMA when it was spatially immobilized via polymerization [36]. We have fabricated sandwich cells with an active material blend, comprising SY, TMPTMA and LiCF3SO3 in a (1:0.5:0.1) mass ratio, sandwiched between an ITO anode and an Al cathode. The devices were pre-biased at V = +12 V at room temperature for almost 4 h. At the end of the pre-bias period, the light-emission intensity and the current exhibited a drastic rapid increase to high values, in agreement with the observation for a similar system in Ref. [36], but it is notable that the elapsed time up to this point was markedly longer in our experiments. We attribute this difference to a thicker active material in the herein studied devices, which is concomitant with a lower ionic conductance and a slower turn-on time [40]. The data for a typical pre-biased device of system (C) are displayed in Fig. 5 and summarized in Table 1. The sandwich cell device showed promising uni-polar current and emission characteristics, and the current rectification ratio was rather high (RR = 700 at V = ±10 V). These data are in good qualitative agreement with those reported by Yu et al. on a similar system, but the quantitative values for the brightness and current efficacy were lower in the experiments reported here. Furthermore, the p–n junction doping profiles in system (C) are considerably more stabile over time in comparison to systems (A) and (B), as evidenced by the brightness drop by only 14% from its initial value after 12 h under open-circuit conditions. 3.4. System (D): cross-linking of ions and ion-transport material with added initiator Inspired by the promising results for devices of system (C), we attempted to combine the approaches taken in systems (B) and (C). In other words, we simultaneously employed a polymerizable ion-pair monomer (METMA/AMPS), a polymerizable ion-transport compound (poly-PEO), and a radical-initiator compound (AIBN) in addition to the conjugated polymer (SY) as the active material. The goal was to induce a doping profile by the pre-bias, which would at the end of the pre-bias period include both the ions and the ion-transport material as integral parts of a large and interconnected immobile ionic network. We further hoped that the addition of the radical-initiator compound would facilitate an efficient and complete polymerization and eliminate the need for polymerization initiation via dopant radicals generated on the conjugated polymer SY, as we suspected that a dopant-initiation could lead to quenching of the electroluminescence and result in low brightness and current efficacy values. An idealized view of the operation and stabilization of such an LEC is illustrated in Fig. 2D. Sandwich cells, with (SY:poly-PEO:METMA/AMPS:AIBN) in a (1:0.17:0.06:0.003) mass ratio as the active material positioned between an ITO anode an Al cathode,
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were pre-biased at V = +5 V for 2 h at room temperature. The current and brightness increased in a continuous manner during the initial part of the pre-bias period, but at the end of the period they stabilized at essentially constant values, which were taken as an indicator of stabilized doping. The data displayed in Fig. 6 and Table 1 were recorded after the pre-bias had been removed. Such pre-biased devices exhibited notably well-behaved uni-polar current and emission characteristics, as for instance quantified by an excellent current rectification of RR = 1900 at V = ±10 V. Moreover, the brightness and current efficacy for the pre-biased devices of system (D) were clearly higher than in the other investigated systems, and the stability over time was relatively impressive as the drop in brightness after 12 h under open-circuit was a mere 13%, with the major drop taking place during the first 2 h following the removal of the pre-bias. Thus, the overall performance of stabilized devices of system (D) was superior to the other investigated systems (see Table 1), although it is clear that further improvement in the long-term stability of the doping structure is clearly needed for practical applications. It should also be noted that the optoelectronic performance of these stabilized devices are not yet on par with stateof-the-art dynamic LEC devices, as demonstrated by that turn-on voltages of the order of the band-gap potential and current efficacies of the order of 3–9 cd/A have recently been reported for various manifestations of dynamic-junction LECs based on the same conjugated polymer SY [41– 43].
4. Conclusions We have systematically tested, evaluated, and compared a number of different ‘‘old” and new approaches for the attainment of a chemically stabilized doping structure in LEC devices. We found that the pioneering concept of chemically connecting the dopant counterions via utilization of polymerizable ion-pair monomers alone resulted in light-emitting devices with poor stability and low brightness. The addition of a radical-initiator compound to this type of active material blend improved the brightness, presumably because the emissive conjugated polymer is relieved of the role of acting as radical initiation site for polymerization; however, the stability remained poor. The alternative concept of employing a small molecule ion-transport material that can be cross-linked into large and immobile polymeric structures during the initial pre-bias operation yielded distinctly improved results in the form of a high current rectification ratio and a decent stability under open-circuit conditions. The best performance was, however, attained from a device that combined all of the above concepts, as its active material comprised polymerizable ion-pair monomers, a cross-linkable ion-transport material, and a radical-initiator in addition to the conjugated polymer. Such chemically stabilized devices exhibited a highly respectable current rectification ratio of almost 2000, a good light-emission efficiency, and a decent long-term stability under idle conditions.
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