Polymer light-emitting electrochemical cells with ultralow salt content: performance enhancement through synergetic chemical and electrochemical doping actions

Shiyu Hu , Hung-Wei Yeh and Jun Gao *
Department of Physics, Engineering Physics and Astronomy, Queen's University, Kingston, Ontario, K7L 3N6, Canada. E-mail: jungao@queensu.ca

Received 12th November 2020 , Accepted 18th December 2020

First published on 21st December 2020


Polymer light-emitting electrochemical cells (PLECs) employing a silver trifluoromethanesulfonate (Ag triflate) salt are demonstrated. The red-emitting PLECs contained 0.2–1 wt% salt, but exhibited a peak luminance of 6000 cd m−2, with high efficiency and minimal efficiency roll-off. Notably, the Ag triflate cells activate more than 100 times faster than an optimized reference cell containing a potassium triflate salt. The high performance of the Ag triflate PLECs can be attributed to the partial chemical doping of the light-emitting polymer by the silver cations. The synergetic action of chemical and electrochemical doping provides a new avenue to design high-performance mixed conductor devices.


Light-emitting electrochemical cells (LECs) are solid-state electroluminescent (EL) devices with a mixed ionic/electronic conductor active layer.1–4 LECs are attractive candidates for low-cost lighting and display applications due to their simple structure and solution processability.5–7 Among various types of LECs, the prototypical polymer LEC (PLEC) continues to improve in performance and serve as an important model for mechanistic studies.8–11 A PLEC contains a luminescent conjugated polymer (CP) mixed with an ionic conductor. The latter is typically a solid polymer electrolyte (SPE) such as polyethylene oxide (PEO):salt complexes. Under bias, the mobile ions in the SPE redistribute to compensate the injected electrons and holes, leading to electrochemical p- and n-doping of the CP. An activated PLEC is a dynamic, forward-biased light-emitting p–n or p–i–n junction.12

As the source of dopant ions, the salt and its concentration in a PLEC play a central role in its operation.13 Alkali triflate salts such as LiOTf and KOTf are frequently used due to their excellent complex-forming abilities with PEO.14,15 Some PLECs also employ ionic liquids as the electrolyte, thus negating the need for an ionic solvent.16 In the first report of PLECs, the active material consisted of CP:PEO:LiOTf in a weight ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]0.9.2,17 The high electrolyte content ensures high ionic conductivity and is responsible for the characteristic bipolar/symmetric current and EL of these PLECs despite their dissimilar electrodes. The drawback is increased phase separation between the incompatible CPs and SPEs, which can be alleviated by adding a bifunctional additive.18 Alternatively, the electrolyte content has been reduced, which led to a record operating lifetime and efficiency in sandwich PLECs with a salt content of only 2–3 wt% of the CP.5,19–23

Low-salt PLECs, however, become sensitive to the electrode work function and the active layer thickness.24–26 Moreover, the cells can be very slow (hours) to reach maximum EL due to diminished ionic conductivity.22 To overcome these trade-offs without resorting to a multi-layer construction or a pre-biasing scheme, a new salt is needed.

Silver salts are unlikely candidates for LEC applications due to the propensity of Ag+ ions to quench the fluorescence of organic compounds.27,28 The fluorescence quenching by Ag+ has been used to detect their presence in trace amounts.29,30 AgOTf heavily quenches the photoluminescence (PL) of MEH-PPV, a red-emitting CP commonly used in PLECs. Remarkably, extra-wide planar MEH-PPV:PEO:AgOTf PLECs containing 32 wt% AgOTf could be activated to emit strongly despite a PL-quenched active layer.31 The effect of the AgOTf concentration and AgOTf sandwich cells, however, have not been reported. In this study, we report MEH-PPV:PEO:AgOTf sandwich LECs with ultralow salt concentrations. Unexpectedly, the AgOTf cells greatly outperformed the control KOTf cells, reaching a peak luminance in excess of 6000 cd m−2 and a peak current efficiency of 0.91 cd A−1 with minimal efficiency roll-off. A most unusual result is the ultrafast turn-on response of these cells even at an AgOTf concentration of 0.2 wt%–0.5 wt%. These results demonstrate the importance of the salt cations in LECs, and raise questions as to how these cells operate with so little salt.

Results and discussion

Fig. 1 compares an AgOTf cell with a KOTf cell operated under the same constant current density of 83.3 mA cm−2. The two cells had the same amount of MEH-PPV and PEO, but the AgOTf cell had a salt content that was 10 times lower by weight than the KOTf cell. The cells are denoted as the Ag+(0.005) cell and K+(0.05) cell, respectively. The number inside the parentheses indicates the weight ratio of the salt relative to MEH-PPV. The weight ratio of PEO to MEH-PPV is fixed at 0.1 for all cells. The K+(0.05) cell had an optimized PEO and salt content, and behaved similarly to previously reported K+ or Li+ cells of comparable compositions.5,32 It took the K+(0.05) cell nearly 37 hours to reach a maximum luminance of 352 cd m−2. The Ag+(0.005) cell, by comparison, reached a luminance of 825 cd m−2 in 145 seconds. For much of the 80 hours shown, the Ag+(0.005) cell maintained a slowly increasing EL at nearly twice the peak luminance of the K+(0.05) cell. The device images shown in the inset confirmed the measured trend in EL.
image file: d0qm00937g-f1.tif
Fig. 1 Temporal evolution of the luminance and driving voltage of (a) a K+(0.05) cell with the composition MEH-PPV[thin space (1/6-em)]:[thin space (1/6-em)]PEO[thin space (1/6-em)]:[thin space (1/6-em)]KOTf = 1[thin space (1/6-em)]:[thin space (1/6-em)]0.1[thin space (1/6-em)]:[thin space (1/6-em)]0.05, and (b) an Ag+(0.005) cell with the composition MEH-PPV[thin space (1/6-em)]:[thin space (1/6-em)]PEO[thin space (1/6-em)]:[thin space (1/6-em)]AgOTf = 1[thin space (1/6-em)]:[thin space (1/6-em)]0.1[thin space (1/6-em)]:[thin space (1/6-em)]0.005. The cells were driven at a constant current density of 83.3 mA cm−2. The insets show EL images taken at the time shown. The exposure times were 1/100 s and 1/200 s for the KOTf and AgOTf cells, respectively.

The AgOTf cells display unusual dependence on the Ag salt concentration, as shown in Fig. 2 for the first hour of operation. The five AgOTf cells had a salt content ranging from 0.002 to 0.05. The luminance peaked in all five cells within the first hour, as shown in Fig. 2a. The Ag+(0.005) cell had a maximum luminance of 825 cd m−2. Even the Ag+(0.002) cell outperformed the K+(0.05) cell. As shown in Fig. 2b, the driving voltage in all cells decreased during the first hour, signifying an ongoing doping process. The maximum EL luminance vs. salt content is shown in Fig. 2c. A puzzling trend is observed in the response time of the cells, as shown in Fig. 2d. The AgOTf cells with the lowest salt contents were the quickest to reach maximum luminance. The Ag+(0.005) cell was about 900 times faster to reach maximum EL than the K+(0.05) cell.

image file: d0qm00937g-f2.tif
Fig. 2 Temporal evolution of the (a) luminance and (b) driving voltage of AgOTf cells with the compositions shown. (c) Maximum EL luminance and (d) time to maximum EL of the cells as a function of salt content. The cells were driven at a constant current density of 83.3 mA cm−2.

Since the Ag+ ions are easily reduced, the cells were subjected to current vs. voltage (IV) scans to evaluate their stability at high driving voltages and currents. Fig. 3a shows the IV curves of the cells, taken after the cells had been biased at 83.3 mA cm−2 for the durations indicated in the figure caption. All cells reached a current density of 500 mA cm−2 or higher. Fig. 3b plots luminance against current density. The turn-on voltages (defined as the voltage to reach 1 cd m−2) were close to 2.4 V for all cells. The Ag+(0.005) and Ag+(0.01) cells reached a peak luminance of 6000 cd m−2. Fig. 3c shows the current efficiency as a function of the current density of the same cells. The Ag+(0.005) cell had the highest efficiency of 0.91 cd A−1 at 69 mA cm−2. The Ag+(0.01) cell had a slightly lower efficiency and very little efficiency roll-off at high current densities. Repeated IV scans of the Ag+(0.01) cell yielded negligible change in the cell current density and EL, as shown in Fig. S1 (ESI). In comparison, the K+(0.05) cell only reached a peak current efficiency of 0.32 cd A−1 and suffered from greater efficiency roll-off.

image file: d0qm00937g-f3.tif
Fig. 3 (a) Current density vs. voltage, (b) luminance vs. current density, and (c) current efficiency vs. current density characteristics of Ag+ and K+ cells. The Ag+(0.002), Ag+(0.005), Ag+(0.01) and K+(0.05) cells were pre-biased for 373 s, 331 s, 492 s, and 301 s, respectively, before multiple current vs. voltage vs. luminance scans were performed to extract the above data.

The two most efficient AgOTf cells were also subjected to long-term, continuous stress tests as shown in Fig. 4. The Ag+(0.01) cell sustained a luminance of over 100 cd m−2 for 584 hours, the longest for a MEH-PPV LEC without a hole-injection layer. By contrast, the K+(0.05) cell only lasted ≈220 hours at above 100 cd m−2. The Ag+(0.005) cell maintained a stable luminance of over 550 cd m−2 for about 300 hours before a sudden spike in the driving voltage occurred. Fig. S2 (ESI) compares the degradation behavior of all Ag+ cells. The most efficient Ag+ cells are also the longest lasting ones.

image file: d0qm00937g-f4.tif
Fig. 4 Stress tests of AgOTf PLECs with weight ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]0.1[thin space (1/6-em)]:[thin space (1/6-em)]0.01 and 1[thin space (1/6-em)]:[thin space (1/6-em)]0.1[thin space (1/6-em)]:[thin space (1/6-em)]0.005. The cells were driven at a constant current of 83.3 mA cm−2.

From the results presented, the ultralow salt content AgOTf cells are brighter, faster, more efficient and longer lasting than the K+(0.05) cell. The AgOTf cells are LECs despite their fast response and tolerance to high currents. When the activated AgOTf cells were shorted, a lasting discharge current was detected as the polymer film slowly de-doped. Fig. S3 (ESI) shows the discharge curves. The integrated de-doping charge allowed for an estimate of the overall doping concentration. For example, the Ag+(0.002) cell had a total de-doping charge of 1.89 × 10−7 C measured in 800 s. This is very close to the total ionic charge of 4.08 × 10−7 C stored in the material assuming complete ionization. Assuming a density of 1 g cm−3 for the active material and knowing the size and thickness of the active layer, the doping concentration of the cell is 2.17 × 1018 cm−3 or 9.95 × 10−4 dopant per CP unit. This is about 1 to 2 orders of magnitude lower than that of planar PLECs containing 24 wt% of salt.33 The doping concentration of these AgOTf sandwich cells is closer to that of the original LiOTf sandwich cells, despite the latter's much higher (18 wt%) salt content.17

The unexpected results suggest that AgOTf is a highly effective LEC salt. This is especially true when the LECs had the lowest salt content. By contrast, a reference ultralow salt content K+(0.0073) cell was barely emitting (≈2 cd m−2) and lasted minutes, as shown in Fig. S4 (ESI). Like alkali cations, Ag+ can be coordinated by ether oxygen, and silver salt-based SPEs have been widely reported and found applications in electrochemical devices and facilitated transport membranes.34–39 However, density functional theory studies showed that AgOTf had larger bond dissociation and lattice energies than KOTf.40 This is in part due to the larger ionic radius of K+ (r = 1.37–1.64 Å) than that of Ag+ (r = 1.00–1.28 Å). In addition, based on the Hard–Soft Acid–Base (HSAB) theory,41 Ag+ is a soft cation (soft acid), which is coordinated less strongly than the hard cation K+ (hard acid) by the ether oxygens (hard bases) of PEO. As such, AgOTf releases less energy upon solvation. These facts suggest that KOTf is more easily solvated and forms stronger complexes than AgOTf. On the other hand, the solvated Ag+ ions should be more mobile due to the weaker cation–ligand interaction. This inverse relationship between complex formation and ion mobility has been exploited to develop more conductive SPEs.42,43 The higher mobility of Ag+ ions agrees with the much faster turn-on response of the AgOTf cells than the KOTf cells.

In addition to the salt lattice energy and cation/PEO interaction, the cation/CP interaction also played an important role in determining the device performance. Ag+ ions solvated from AgOTf have been shown to spontaneously p-dope (oxidative doping) various poly(3-alkylthiophenes).44 The resulting spectral changes caused by Ag+ ion doping are identical to those observed in electrochemically doped P3HT.45 In addition, Ag+ doping caused an orders-of-magnitude increase in conductivity. Vastly enhanced (≈20 fold) initial conductivity had also been observed in the aforementioned planar MEH-PPV:PEO:AgOTf cells. The AgOTf planar cell reached maximum EL about 20 times faster than a control cell made with the same molar amount of LiOTf salt. This can be understood by an already chemically doped, partially conductive LEC film prior to the application of a bias voltage. The time it took to form a light-emitting junction was determined by the speed to reverse the chemically p-doped polymer near the cathode into an electrochemically n-doped polymer. This was a faster process than forming a p–n junction in a pristine polymer due to a much greater voltage drop at the cathode interface. The same mechanism could also be responsible for the fast response of the sandwich AgOTf cells in this study, although the AgOTf concentration was much lower. Thus, the AgOTf LECs operate through the synergetic actions of chemical and electrochemical doping.


In summary, we have demonstrated sandwich PLECs made with an AgOTf salt in ultralow concentrations. The PLECs exhibit record luminance, efficiency, lifetime and response speed. The study exemplifies the importance of salt selection in LECs. The synergetic chemical and electrochemical doping actions afforded by the AgOTf salt provide an additional degree of freedom to design high-performance LECs with a greatly reduced salt content.


Device preparation

The PLECs of this study were prepared and tested in nitrogen-filled gloveboxes (O2 < 1 ppm, H2O < 1 ppm). The luminescent conjugated polymer poly[5-(2′-ethylhexyloxy)-2-methoxy-1,4-phenylenevinylene] (MEH-PPV) was obtained from OLED King Optoelectronic Materials, China. The ion-transporting polymer polyethylene oxide (PEO) and salts silver triflate (AgOTf) and potassium triflate (KOTf) were purchased from Sigma Aldrich and used as received. Cyclohexanone solutions of MEH-PPV, PEO, AgOTf, and KOTf were prepared separately and mixed to create casting solutions with the desired weight ratios. Indium tin oxide (ITO) coated glass substrates were cleaned in ultrasonic baths that contained detergent, distilled water, acetone, and isopropanol, respectively. The casting solution was spun cast onto the substrates at 2000 rpm. The cast films were dried at 50 °C for 4 h to remove any residual solvent. The thickness of the films is approximately 100 nm, measured using a DektakXT stylus profiler. Aluminum driving electrodes of 100 nm were thermally evaporated on the polymer films through a shadow mask. The active area of each device is a rectangle 2 × 3 mm2 in size.

Device testing

The devices were tested inside a black box with a calibrated photodiode. A LabVIEW controlled Keithley 2400 was used to supply a constant current or voltage and to measure photodiode signals. The electroluminescence images were captured using a Nikon D610 camera through the glovebox window in the dark. The images were taken using the same camera settings, except the exposure time was 1/100 s for KOTf cells and 1/200 s for AgOTf cells.

Conflicts of interest

There are no conflicts to declare.


This study was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) (Discovery grant 2020-04026 and CREATE grant 511093).

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Electronic supplementary information (ESI) available: Fig. S1. Current vs. voltage vs. luminance (IVL) scans of an Ag+(0.01) cell. Fig. S2. Stress tests of all Ag cells. Fig. S3. Discharge curves and integrated charges. Fig. S4. Stress test of a K+(0.0073) cell. See DOI: 10.1039/d0qm00937g

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