Ion transfer into solution-processed electrodes can significantly shift the p–n junction and emission efficiency of light-emitting electrochemical cells

A light-emitting electrochemical cell (LEC) comprises mobile ions in its active material, which enable for in situ formation of a p–n junction by electrochemical doping. The position of this emissive p–n junction in the interelectrode gap is important, because it determines whether the emission is affected by constructive or destructive interference. An appealing LEC feature is that the entire device can be fabricated by low-cost solution-based printing and coating. Here, we show, somewhat unexpectedly, that the replacement of conventional vacuum-deposited indium-tin-oxide (ITO) for the positive anode with solution-processed poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) can result in an increase in the peak light-emission output by 75%. We demonstrate that this emission increase is due to that the p–n junction shifts from a position of destructive interference in the center of the interelectrode gap with ITO to a position of constructive interference closer to the anode with PEDOT:PSS. We rationalize the anodic p–n junction shift by significant anion transfer into the soft and porous PEDOT:PSS electrode during LEC operation, which is prohibited for the ITO electrode because of its compact and hard nature. Our study, thus, contributes with important design criteria for the attainment of efficient light emission from solution-processed LEC devices.

A light-emitting electrochemical cell (LEC) is a thin-film emissive device, which can feature a notably simple device structure in the form of a single-layer active material sandwiched between a reflective electrode and a transparent electrode.
1–41. Q. B. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger, Science 269(5227), 1086 (1995). https://doi.org/10.1126/science.269.5227.10862. J. Gao, ChemPlusChem 83(4), 183 (2018). https://doi.org/10.1002/cplu.2017004613. J. M. Leger, Adv. Mater. 20(4), 837 (2008). https://doi.org/10.1002/adma.2007018744. X. C. Pang, K. Zhang, Y. J. Song, Y. Xiu, R. Y. Yu, and L. He, Chem. Eng. J. 450, 137987 (2022). https://doi.org/10.1016/j.cej.2022.137987 The active material essentially comprises a blend of a luminescent organic semiconductor (OSC) and mobile ions, and during the initial LEC operation, the mobile ions redistribute in a characteristic manner.55. W. S. Tseng, C. S. Hsieh, M. C. Chan, and H. C. Su, Opt. Express 30(16), 28817 (2022). https://doi.org/10.1364/OE.463352 First, they drift to the electrode interfaces to form thin injection-facilitating electric double layers (EDLs),6–86. T. Ouisse, O. Stephan, M. Armand, and J. C. Lepretre, J. Appl. Phys. 92(5), 2795 (2002). https://doi.org/10.1063/1.14992017. M. H. Bowler, A. Mishra, A. C. Adams, C. L. D. Blangy, and J. D. Slinker, Adv. Funct. Mater. 30, 1906715 (2020). https://doi.org/10.1002/adfm.2019067158. D. Gets, M. Alahbakhshi, A. Mishra, R. Haroldson, A. Papadimitratos, A. Ishteev, D. Saranin, S. Anoshkin, A. Pushkarev, E. Danilovskiy, S. Makarov, J. D. Slinker, and A. A. Zakhidov, Adv. Opt. Mater. 9(3), 2001715 (2021). https://doi.org/10.1002/adom.202001715 and thereafter, they enable for electrochemical doping of the OSC by electrostatic compensation of the injected electronic charge carriers.99. S. Jenatsch, L. Wang, M. Bulloni, A. C. Veron, B. Ruhstaller, S. Altazin, F. Nuesch, and R. Hany, ACS Appl. Mater. Interfaces 8(10), 6554 (2016). https://doi.org/10.1021/acsami.5b12055 This doping is termed p-type at the positive anode (injected positive holes compensated by negative anions) and n-type at the negative cathode (injected electrons compensated by cations); with time, the doped regions grow in size to make contact in the form of a p–n junction.1,10,111. Q. B. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger, Science 269(5227), 1086 (1995). https://doi.org/10.1126/science.269.5227.108610. S. B. Meier, S. van Reenen, B. Lefevre, D. Hartmann, H. J. Bolink, A. Winnacker, W. Sarfert, and M. Kemerink, Adv. Funct. Mater. 23(28), 3531 (2013). https://doi.org/10.1002/adfm.20120268911. P. Matyba, K. Maturova, M. Kemerink, N. D. Robinson, and L. Edman, Nat. Mater. 8(8), 672 (2009). https://doi.org/10.1038/nmat2478 At this p–n junction, subsequently injected electrons and holes recombine into excitons, which can decay radiatively by the emission of photons.The photons generated in the p–n junction are either emitted toward the transparent electrode where they can escape the device directly or reflected at the reflective electrode and thereafter able to leave the device structure at the transparent electrode. Since the thickness of the active material typically is of the same size as the emission wavelength (i.e., a few hundred nm) and the effective width of the p–n junction is commonly rather thin (of the order of a few ten nm),12,1312. E. M. Lindh, P. Lundberg, T. Lanz, and L. Edman, Sci. Rep. 9(1), 10433 (2019). https://doi.org/10.1038/s41598-019-46860-y13. A. Munar, A. Sandstrom, S. Tang, and L. Edman, Adv. Funct. Mater. 22(7), 1511 (2012). https://doi.org/10.1002/adfm.201102687 this implies that the LEC should exhibit strong cavity effects. In other words, the generated light is strongly affected by constructive or destructive interference depending on the distance between the emissive p–n junction and the reflective electrode. Several studies have verified that cavity effects are important in LECs by a systematic modulation of the thickness of the active material12,14–1612. E. M. Lindh, P. Lundberg, T. Lanz, and L. Edman, Sci. Rep. 9(1), 10433 (2019). https://doi.org/10.1038/s41598-019-46860-y14. E. M. Lindh, P. Lundberg, T. Lanz, J. Mindemark, and L. Edman, Sci. Rep. 8(1), 6970 (2018). https://doi.org/10.1038/s41598-018-25287-x15. M. Diethelm, A. Schiller, M. Kawecki, A. Devižis, B. Blülle, S. Jenatsch, E. Knapp, Q. Grossmann, B. Ruhstaller, F. Nüesch, and R. Hany, Adv. Funct. Mater. 30(33), 1906803 (2020). https://doi.org/10.1002/adfm.20190680316. G.-R. Lin, H.-F. Chen, H.-C. Shih, J.-H. Hsu, Y. Chang, C.-H. Chiu, C.-Y. Cheng, Y.-S. Yeh, H.-C. Su, and K.-T. Wong, Phys. Chem. Chem. Phys. 17, 6956 (2015). https://doi.org/10.1039/C4CP05380J or by the inclusion of mobility-modifying additives into the active material.17–1917. T. W. Wang and H. C. Su, Org. Electron. 14(9), 2269 (2013). https://doi.org/10.1016/j.orgel.2013.04.05218. J. Ràfols-Ribé, X. Zhang, C. Larsen, P. Lundberg, E. M. Lindh, C. T. Mai, J. Mindemark, E. Gracia-Espino, and L. Edman, Adv. Mater. 34(8), 2107849 (2022). https://doi.org/10.1002/adma.20210784919. H.-C. Su, ChemPlusChem 83(4), 197 (2018). https://doi.org/10.1002/cplu.201700455The LEC-characteristic in situ formation of a p–n junction doping structure is further attractive, since it enables for low-cost fabrication2020. A. Sandström and L. Edman, Energy Technol. 3(4), 329 (2015). https://doi.org/10.1002/ente.201402201 of the entire LEC structure, including both electrodes, by solution-based printing and coating under ambient air.21–2521. G. Mauthner, K. Landfester, A. Kock, H. Bruckl, M. Kast, C. Stepper, and E. J. List, Org. Electron. 9(2), 164 (2008). https://doi.org/10.1016/j.orgel.2007.10.00722. G. Hernandez-Sosa, S. Tekoglu, S. Stolz, R. Eckstein, C. Teusch, J. Trapp, U. Lemmer, M. Hamburger, and N. Mechau, Adv. Mater. 26, 3235 (2014). https://doi.org/10.1002/adma.20130554123. A. Sandström, A. Asadpoordarvish, J. Enevold, and L. Edman, Adv. Mater. 26(29), 4975 (2014). https://doi.org/10.1002/adma.20140128624. A. Sandström, H. F. Dam, F. C. Krebs, and L. Edman, Nat. Commun. 3, 1002 (2012). https://doi.org/10.1038/ncomms200225. E. Auroux, A. Sandström, C. Larsen, P. Lundberg, T. Wågberg, and L. Edman, Org. Electron. 84, 105812 (2020). https://doi.org/10.1016/j.orgel.2020.105812 However, it was recently demonstrated that the employment of a solution-processed “soft” electrode (instead of the conventional vacuum-deposited “hard” electrode) can result in an electric-field driven significant ion transfer over the “open” soft-electrode/active-material interface during LEC operation.2626. E. Auroux, A. Sandström, C. Larsen, E. Zäll, P. Lundberg, T. Wågberg, and L. Edman, Adv. Electron. Mater. 7, 2100253 (2021). https://doi.org/10.1002/aelm.202100253 As each ion in the active material can effectuate one doping event of the OSC,27–2927. J. Fang, P. Matyba, and L. Edman, Adv. Funct. Mater. 19(16), 2671 (2009). https://doi.org/10.1002/adfm.20090047928. J. F. Fang, Y. L. Yang, and L. Edman, Appl. Phys. Lett. 93(6), 063503 (2008). https://doi.org/10.1063/1.296903429. M. Diethelm, Q. Grossmann, A. Schiller, E. Knapp, S. Jenatsch, M. Kawecki, F. Nüesch, and R. Hany, Adv. Opt. Mater. 7(3), 1801278 (2019). https://doi.org/10.1002/adom.201801278 it is plausible that this ion “loss” from the active material can have a significant influence on the steady-state p–n junction doping structure, notably the position of the emissive p–n junction.12,14,15,30–3312. E. M. Lindh, P. Lundberg, T. Lanz, and L. Edman, Sci. Rep. 9(1), 10433 (2019). https://doi.org/10.1038/s41598-019-46860-y14. E. M. Lindh, P. Lundberg, T. Lanz, J. Mindemark, and L. Edman, Sci. Rep. 8(1), 6970 (2018). https://doi.org/10.1038/s41598-018-25287-x15. M. Diethelm, A. Schiller, M. Kawecki, A. Devižis, B. Blülle, S. Jenatsch, E. Knapp, Q. Grossmann, B. Ruhstaller, F. Nüesch, and R. Hany, Adv. Funct. Mater. 30(33), 1906803 (2020). https://doi.org/10.1002/adfm.20190680330. S. Tang, A. Sandström, P. Lundberg, T. Lanz, C. Larsen, S. van Reenen, M. Kemerink, and L. Edman, Nat. Commun. 8(1), 1190 (2017). https://doi.org/10.1038/s41467-017-01339-031. F. AlTal and J. Gao, Electrochim. Acta 220, 529 (2016). https://doi.org/10.1016/j.electacta.2016.10.13632. J. Gao and J. Dane, Appl. Phys. Lett. 84(15), 2778 (2004). https://doi.org/10.1063/1.170212633. S. Y. Hu and J. Gao, Chemelectrochem 7(7), 1748 (2020). https://doi.org/10.1002/celc.202000153 It is, therefore, the aim of this study to investigate the effects of this ion transfer on the position of the emissive p–n junction and the device performance of solution-processed LECs.For this purpose, we have designed and fabricated two different LEC devices, which are structurally and energetically depicted in their pristine state in Figs. 1(a) and 1(b), respectively. The conventional “closed-electrode-LEC” features vacuum-deposited indium-tin-oxide (ITO) and Al for the two electrodes, while the “open-electrode-LEC” is distinguished by that the bottom ITO electrode is replaced by a solution-processed thin layer of poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Both devices feature the same 270-nm thin solution-processed active material, comprising a blend of the polymeric OSC “super yellow” and the ionic liquid tetrahexylammonium tetrafluoroborate (THABF4). The closed-electrode-LEC was named in consideration of that both the bottom ITO anode and the top Al cathode are effectively closed to ion transfer by the virtue of their hard and compact nature. The name of the open-electrode-LEC was motivated by that its bottom PEDOT:PSS anode, in contrast, is anticipated to be open for ion transfer because of its soft and porous nature. The devices were driven by a constant current density of 25 mA cm−2 with the Al top electrode biased as the negative cathode. More details on the device fabrication and characterization can be found in the supplementary material.Figures 1(c) presents the temporal evolution of the voltage for the closed-electrode-LEC (solid red squares) and the open-electrode-LEC (open blue circles). Both devices exhibit a drive voltage that is decreasing monotonously with time during the initial constant-current operation. This lowering of the effective device resistance is in line with that the voltage-driven ion redistribution first leads to EDL formation for efficient injection and thereafter enables for efficient transport by electrochemical doping (which in turn leads to the formation of the p–n junction).However, it is interesting that the performance of the two devices otherwise deviates markedly. Figure 1(c) shows that the open-electrode-LEC invariably exhibits higher drive voltage, and Fig. 1(d) reveals distinctly different luminance transients. It is particularly notable that the open-electrode-LEC delivers significantly stronger luminance throughout the evaluation period, and its peak luminance is much higher at 1050 vs 625 cd m−2. Moreover, the temporal evolution of the luminance is drastically different with the luminance of the open-electrode-LEC exhibiting an undulating variation with time, whereas the luminance of the closed-electrode-LEC decreases monotonously.In order to shed light on the cause of this deviating behavior, we determined the position of the emissive p–n junction in the interelectrode gap using a combined measurement and simulation procedure. In brief, the procedure constitutes the measurement and simulation of the electroluminescence spectrum as a function of the viewing angle with the “free parameter” in the simulation being the position of the emissive p–n junction. By identifying the value of the simulated p–n junction position that produces best agreement between the simulated and measured data, the position of the emissive p–n junction can be determined with high accuracy.12,1412. E. M. Lindh, P. Lundberg, T. Lanz, and L. Edman, Sci. Rep. 9(1), 10433 (2019). https://doi.org/10.1038/s41598-019-46860-y14. E. M. Lindh, P. Lundberg, T. Lanz, J. Mindemark, and L. Edman, Sci. Rep. 8(1), 6970 (2018). https://doi.org/10.1038/s41598-018-25287-x By executing this procedure at regular time intervals during the LEC operation, we can establish the temporal evolution of the position of the emissive p–n junction in the interelectrode gap.Figure 2(a) shows that the first detected position of the p–n junction for the closed-electrode-LEC is at 0.35 (i.e., closer to the bottom transparent anode, which is located at 0; the reflective cathode is located at 1), and it thereafter migrates toward the center of the active material to reach 0.42 at steady state, i.e., at minimum voltage. The corresponding transient for the open-electrode-LEC is notably different, since its p–n junction forms closer to the transparent anode at 0.27 and thereafter migrates even closer to the anode to reach 0.18 at steady state. For these LECs, which feature a 270-nm thick active material, this corresponds to that the steady-state p–n junction is ∼65 nm closer to the anode (∼50 nm vs ∼115 nm) in the open-electrode-LEC.Figure 2(b) presents the simulated luminance as a function of the position of the emissive p–n junction for the open-electrode-LEC (dashed gray line) and the closed-electrode-LEC (solid black line). The two graphs are very similar, which is expected since the two devices are solely distinguished by the selection of the transparent anode (being PEDOT:PSS or ITO). The simulation clearly visualizes the cavity effect, in that the luminance undulates in a periodic manner between high peaks (when the p–n junction is located at a point of constructive interference) and low valleys (when the p–n junction is shifted to a position of destructive interference). The practical importance of the cavity effect for the herein studied devices is manifested in that the simulated luminance increases by a factor of ∼16, when the p–n junction shifts from the first position of destructive interference at 0.49 to the first position of constructive interference at 0.22.Figure 2(b) also includes the data for the measured luminance as a function of the derived p–n junction position for the open-electrode-LEC (open blue circles) and the closed-electrode-LEC (solid red squares). The simulated and measured luminance are normalized for clarity. The observed excellent agreement between the measured and the simulated data is interesting and important since it implies that the difference in the light-emission performance between the two devices primarily originates in the shifting position of the emissive p–n junction.The critical question that then arises is: why is the p–n junction is positioned so differently in the two LECs? After all, they comprise the same active material with the same thickness, and the lowering of the hole injection barrier for the open-electrode-LEC [see Fig. 1(b)] should, if detectable, result in a cathodic shift of the p–n junction, i.e., the opposite to the anodic shift that is observed in Fig. 2(a).3434. J. Xu, A. Sandström, E. M. Lindh, W. Yang, S. Tang, and L. Edman, ACS Appl. Mater. Interfaces 10(39), 33380 (2018). https://doi.org/10.1021/acsami.8b13036 We propose that the anodic shift of the p–n junction in the open-electrode-LEC originates in anion transfer from the active material into the soft PEDOT:PSS anode during LEC operation, which is prohibited in the closed-electrode-LEC because of the hard and compact nature of its ITO anode. This hypothesis is supported by a recent study that showed that a significant number of anions indeed can migrate into a solution-processed PEDOT:PSS electrode during LEC operation.2626. E. Auroux, A. Sandström, C. Larsen, E. Zäll, P. Lundberg, T. Wågberg, and L. Edman, Adv. Electron. Mater. 7, 2100253 (2021). https://doi.org/10.1002/aelm.202100253Figures 2(c) and 2(d) schematically illustrate our hypothesis. For the closed-electrode-LEC [Fig. 2(c)], the ion motion, and the associated electrochemical doping, is restricted to the active material because of its ion-blocking interfaces with the hard and compact ITO and Al electrodes. In contrast, for the open-electrode-LEC [Fig. 2(d)], the negative anions can transfer over the “open” interface between the active material and the soft positive PEDOT:PSS anode. This anion transfer will enable for additional p-type doping and/or electrochemically induced overoxidation of PEDOT:PSS.2626. E. Auroux, A. Sandström, C. Larsen, E. Zäll, P. Lundberg, T. Wågberg, and L. Edman, Adv. Electron. Mater. 7, 2100253 (2021). https://doi.org/10.1002/aelm.202100253 Figure S1 and the associated text in the supplementary material show that an increase in the ion concentration by a factor of three results in an ion-induced overoxidation of the PEDOT:PSS electrode, as manifested in a fast voltage increase,3535. S. Hu and J. Gao, Adv. Mater. Technol. 3, 1800229 (2018). https://doi.org/10.1002/admt.201800229 which is detrimental for the device performance.An important consequence of this anion loss from the active material (regardless of whether it causes doping or degradation of the electrode) is that the number of anions available for p-type doping in the active material is correspondingly decreased. This selective lowering of the p-type doping concentration (and not the n-type doping concentration) in the active material can be anticipated to cause the anodic shift of the p–n junction, as observed in Fig. 2(a). The lowering of the (p-type) doping concentration in the active material also explains why the open-electrode-LEC invariably featured a higher drive voltage, and thereby a higher device resistance, during LEC operation in Fig. 1(c).At this stage, it is relevant to point out that the locations for destructive and constructive interference of the p–n junction in the interelectrode gap, as depicted in Fig. 2(b), are directly dependent on the active-material thickness, dAM. The consequence is that, while the anodic shift of the p–n junction by anion transfer into the soft anode for the herein investigated open-electrode-LEC (with dAM = 270 nm) resulted in significantly improved light-emission performance [Fig. 1(d)], the opposite can be true for open-electrode-LECs that exhibit different dAM or comprise different active-material constituents. This insight, thus, highlights the importance of appropriate systematic characterization and simulation of LEC devices in order to attain peak efficiency and before discarding new materials and device architectures.

In summary, we demonstrated that the performance of solution-processed LECs can be strongly affected by a shifting position of the emissive p–n junction, which is effectuated by ion transfer into the soft and solution-processed electrodes. We specifically show that the anion transfer from a common active material into a soft PEDOT:PSS anode can result in a shift of the emissive p–n junction from a position of destructive to constructive interference, which is manifested in an improvement of the emission efficiency by 75%. An important take-home message of our study is, thus, that a rational LEC design should consider the nature of the electrodes in order to enable for the attainment of optimum light-emission performance.

See the supplementary material for a detailed description of LEC fabrication and characterization and for the performance of open-electrode and closed-electrode LEC devices with higher ion concentration.

We acknowledge generous financial support from Kempestiftelserna, the Swedish Research Council, the Swedish Energy Agency, the Swedish Foundation for Strategic Research, Wenner-Gren Foundations, and Bertil & Britt Svenssons Stiftelse för Belysningsteknik.

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Etienne Auroux and So-Ra Park contributed equally to this work.

Etienne Auroux: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (lead); Writing – original draft (equal). So-Ra Park: Data curation (equal); Investigation (equal); Methodology (equal). Joan Rafols-Ribe: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (lead); Visualization (equal). Ludvig Edman: Conceptualization (lead); Formal analysis (equal); Funding acquisition (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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