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ARTICLES PUBLISHED ONLINE: 21 JUNE 2009 | DOI: 10.1038/NMAT2478
The dynamic organic p–n junction Piotr Matyba1, Klara Maturova2, Martijn Kemerink2, Nathaniel D. Robinson3 and Ludvig Edman1* Static p–n junctions in inorganic semiconductors are exploited in a wide range of today’s electronic appliances. Here, we demonstrate the in situ formation of a dynamic p–n junction structure within an organic semiconductor through electrochemistry. Specifically, we use scanning kelvin probe microscopy and optical probing on planar light-emitting electrochemical cells (LECs) with a mixture of a conjugated polymer and an electrolyte connecting two electrodes separated by 120µm. We find that a significant portion of the potential drop between the electrodes coincides with the location of a thin and distinct light-emission zone positioned >30µm away from the negative electrode. These results are relevant in the context of a long-standing scientific debate, as they prove that electrochemical doping can take place in LECs. Moreover, a study on the doping formation and dissipation kinetics provides interesting detail regarding the electronic structure and stability of the dynamic organic p–n junction, which may be useful in future dynamic p–n junction-based devices.
Organic semiconductors are heralded for their simpleprocessing and extraordinary chemical customizability, andemerging low-cost and flexible devices based on these materials are expected to revolutionize the role electronics have in our everyday lives1,2. Some devices are already commercially available—a notable example being the organic light-emitting diode in television and cell-phone displays—but it is clear that further opportunities exist beyond the current state-of-the-art. For instance, the soft nature of organic semiconductors can enable in situ electrochemical tuning of important material properties3–13. One device that exploits this opportunity in an attractive manner is the light-emitting electrochemical cell14–22 (LEC). The nominal difference between an LEC and an organic light-emitting diode is that the former contains mobile ions in the active material23–30. These ions rearrange during operation, which in turn allows for a range of attractive device properties, including low-voltage operationwith thick active layers and stable electrodematerials31–36.
However, the further development of LECs is currently hampered by an inadequate understanding of the device operation. In fact, an active debate regarding the fundamental nature of LEC operation has continued for more than a decade, and two distinct models are competing for acceptance: the electrochemical doping model18,32,37–40 and the electrodynamic model36,41–44. To distinguish them, it is appropriate to establish the electrostatic potential profile in a device during steady-state operation, as the models predict distinctly differing profiles. The electrodynamic model predicts that the entire applied potential will drop over thin electric double layers (EDLs) at the electrode/active material interfaces, whereas the electrochemical doping model predicts that a significant fraction of the applied potential will drop over a light-emitting p–n junction. Thus, to discriminate between the two models, it is essential to record the potential profile within an LEC device where the light-emission zone is positioned away from the electrode/active material interfaces.
Two attempts to measure the electrostatic potential within an LEC device during operation have been published. Slinker et al.43 used electric force microscopy on a planar LEC, with an ionic organometallic semiconductor as the active material. Although their measurements have been questioned, primarily because light
1The Organic Photonics and Electronics Group, Department of Physics, Umeå University, SE-901 87 Umeå, Sweden, 2Department of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB, Eindhoven, The Netherlands, 3The Transport and Separations Group, Department of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden. *e-mail: email@example.com.
K+O¬ S C F
Figure 1 | Schematic diagram illustrating the probing of a planar LEC device with SKPM. The solid line marks the topographic scan and the dashed line indicates the SKPM scan in lift mode. The active material components are included in the inset to the right.
emission and the potential profile were observed under rather differing experimental conditions37, they demonstrated that such direct potential measurements could be carried out in planar devices. In amore recent report, Pingree et al.45 used scanning kelvin probe microscopy (SKPM) on planar LECs containing a mixture of a conjugated polymer and a solid-state electrolyte as the activemate- rial. They did not report any light-emission data, and the presented potential profiles were unfortunately inconclusive in that the poten- tial drop was positioned at the negative electrode interface, and as such in effect consistentwith both proposedmodels of operation.
Here, we use the SKPM technique in parallel with light- emission detection on planar LEC devices, comprising an ac- tive material mixture of poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH–PPV)+poly(ethylene oxide)(PEO) + KCF3SO3 positioned between two Au electrodes, where the electrodes define an inter-electrode gap of 120 µm. Figure 1 shows a schematic diagram illustrating the SKPMprobing of a planar device, and the active material constituents are shown in the inset on the
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NATURE MATERIALS DOI: 10.1038/NMAT2478 ARTICLES
¬40 0 40 Position (µm)
l ( V
Figure 2 | Light-emission and potential profiles in a planar LEC device during operation. a, Micrograph showing the light emission from a planar LEC device during steady-state operation at V= 5 V; the positive and negative electrodes are indicated by the+ and− sign, respectively. b, 2D topographic image of an identical planar LEC device. c, Electrostatic potential profile recorded on a pristine device under open-circuit conditions. d, Temporal evolution of the potential profile during the initial operation at V= 5 V; the arrow indicates increasing time. The time delay between successive potential profiles was∼20 s. e, The subsequent steady-state potential profile recorded at V= 5 V.
right. As we used a perpendicular tip–electrode geometry, which may lead to artefacts when interpreting SKPM data, we also carried out a deconvolution (see Supplementary Information) that demon- strated that the recorded potential profiles are at least qualitatively correct. Importantly, we carried out SKPM measurements and light-emission probing under similar conditions on separate sets of LEC devices (see the Methods section for details), in which the light-emission zone is positioned far away from the electrode/active material interfaces35,39. As mentioned above, measuring the electro- static potential within the device is critical to distinguish between the two models. It is made possible in these experiments by the use of an appropriate organic semiconductor/electrolyte activematerial driven with a relatively high voltage, so that undesired electro- chemical side reactions at the electrode/activematerial interfaces are effectively suppressed46,47.
Figure 2a is an optical micrograph of an LEC device taken at t = 300 s after an external voltage of V = 5V was applied. A distinct light-emission zone is apparent in the bulk of the active material at a distance d∼35 µm away from the negative Au electrode interface. Figure 2b,c shows the two-dimensional (2D) topography image and the potential profile at open circuit, respectively, recorded on an identical device. The structure in the potential profile is due to a minor charge transfer between the Au electrodes and the MEH-PPV polymer. The two interfaces between the electrodes and the active material are clearly distinguishable at d = 0 and 120 µm and indicated by the vertical dashed lines. Atomic force microscopy (AFM) data demonstrating the sharpness of this interface are included in Supplementary Information.
Figure 2d,e shows the temporal evolution of the potential profile up to t = 180 s (with increasing time indicated by the arrow) and between t = 180 s and t = 600 s, respectively, for the same device during operation at V = 5V. The potential profile changes from
l ( V
) ¬40 0 40
Position (µm) 80 120
Figure 3 | Experimental data illustrating the formation and reformation of an organic p–n junction. a, 2D topography image of a planar LEC device. b, Electrostatic potential profile during steady-state operation at V=+5 V. c, Transient potential profile measured with the device disconnected (open circuit), directly after long-term operation at V=+5 V. d, Temporal evolution of the potential profile after a subsequent switch to V=−5 V; the arrow indicates increasing time. e, Steady-state potential profile at V=−5 V. f, Micrograph showing the light emission during steady-state operation at V=+5 V. g, Subsequent micrograph showing the light emission from the same device during steady-state operation at V=−5 V.
dropping essentially linearly between the two electrodes at t ∼ 10 s (first line scan in Fig. 2d) to reach steady state (potential profile changes very little over several minutes) at t ≥ 180 s (last line scan in Fig. 2d,e), where the steepest potential drop is localized over a limited spatial region centred at d ∼ 35 µm. It is notable that the spatial position of this potential drop coincides very well with the location of the light-emission zone in Fig. 2a. The corresponding currentmeasurement is included in Supplementary Fig. S2.
At this stage, it is relevant to consider the key steps within the electrochemical doping model. (1) Following the application of an external voltage, thin EDLs form at the electrode/active material interfaces; (2) if the applied voltage is sufficiently large (V ≥ Eg/e, where Eg is the bandgap of the organic semiconductor and e is the elementary charge), holes and electrons are injected into the organic semiconductor through the EDLs at the positive and negative electrodes, respectively; (3) the injected electronic charge carriers attract electrostatically compensating ions, which establish doped regionswith high conductivity at the two electrode interfaces; (4) a p-type doping front (comprising holes and compensating anions) and an n-type doping front (comprising electrons and cations) grow towards each other and, after a turn-on time, make contact, forming a p–n junction; (5) subsequently injected holes and electrons migrate through the doped regions and recombine within, or in close proximity of, the undoped p–n junction, causing the emission of light32,38.
The electrodynamic model includes the first two steps above, but, notably, does not include any electrochemical doping, and
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ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2478
V = 5 V
Metal electrodes p¬n junction
Figure 4 | Schematic diagrams illustrating the electrostatic profile and the electronic and ionic charge distribution within a p–n junction structure established at V = +5 V. The net charge motion is indicated by the white arrows. a, The transient charge distribution and initial ionic motion directly after a shift to the open-circuit condition, where Vbi indicates the built-in potential over the junction. b, The steady-state charge distribution, electronic motion and radiative recombination at V=+5 V.
hence no conductivity increase or p–n junction formation. Instead, it claims that the electronic charge carriers are driven by diffusion in the undoped bulk of the active material, and importantly that the potential drops at steady state are localized at the electrode interfaces41,42.
However, our observation of a significant localized potential drop positioned in the bulk of the active material, far away from the electrode interfaces, at steady state (see Figs 2e and 3) directly contradicts the electrodynamic model. In contrast, the observation of a localized potential drop in the bulk that spatially coincides with a distinct light-emitting zone is consistent with the existence of a forward-biased p–n junction (as schematically illustrated in Fig. 4b). Thus, the presented data provide strong evidence that LEC devices can operate in a manner consistent with the electrochemical doping model.
We note that we do not detect the formation of EDLs at themetal electrode interfaces during the initial operation of these devices (as predicted by both models). We propose, on the basis of X-ray photoemission spectroscopy data and the temporal evolution of recorded AFM phase data (see Supplementary Figs S3 and S4 and the corresponding discussion in the Supplementary Information) as well as the absence of an interfacial potential drop between the negative Au electrode and the n-type-doped material at steady state, that this is due to the existence of a thin layer of ion- containing material on top of the metal electrodes that screens some of the potential.
We also wish to mention a number of interesting features of the probed dynamic organic p–n junction structure. First, both the light-emission data and the potential profiles indicate that the effective p–n junction is broad, with a width of the order of 10 µm. Second, the slope in the potential profile at steady state is notably steeper on the n-type side of the junction than on the p-type side (see Fig. 2e). We believe that the latter is an indication of the conductivity difference between the n-doped and the p-doped MEH-PPV. The n-doped material seems to be less conducting, so that a larger potential gradient is necessary on the n-type side to provide a constant current density across the device at steady state.
To obtain more detailed information on the structure and stability of the dynamic p–n junction and the kinetics and reversibility of the doping process, we have investigated the effects of shifting the voltage applied to a device operating at steady state. Figure 3a shows a 2D topography image that, together with potential images at open circuit (not shown) and micrographs, identifies the positions of the electrode interfaces as indicated by the vertical dashed lines. Figure 3b,f shows the potential profile and a micrograph recorded at steady state at a ‘forward bias’ of V = +5V. A direct correlation between a significant potential
drop in the bulk of the active material and the location of the emission zone is once again observed. The device was thereafter left disconnected at open circuit (impedance> 100M) for a brief period of time (∼30 s), and the first recorded potential profile after disconnection is shown in Fig. 3c. Finally, a ‘reverse bias’ of V =−5V was applied. Figure 3d shows the temporal evolution of the potential between t = 0 and 1,800 s following the application of the reverse bias (the arrow indicates increasing time). Figure 3e,g presents the subsequent steady-state potential profile at t ≥ 1,800 s and amicrograph recorded at t =1,920 s, respectively.
To facilitate a discussion of the recorded data in Fig. 3, two schematic diagrams are shown in Fig. 4. First, we note that the steady-state potential profile and the position of the light-emission zone in the inter-electrode gap at V =−5V (recorded at the end of the measurement) are essentially mirror images of those at V =+5V. This implies that all of the relevant processes are highly reversible under the conditions and the timescale of this study, and that the effects of chemical and electrochemical side reactions can be excluded from the coming discussion.
We note with interest that Fig. 3c reveals a potential drop of ∼1.5V at the location of the p–n junction, immediately after the long-term, steady-state operation voltage of V = +5V is disconnected (the circuit is opened). This observation is consistent with the existence of a built-in potential of Vbi ∼ 1.5V (which is slightly lower than the bandgap potential, VBG = Eg/e, of the semiconductor, MEH-PPV; ref. 48) over the p–n junction, with a polarity opposite that of the applied voltage that built the p–n junction. We propose, in analogy with the well-established physics of p–n junctions in inorganic crystalline semiconductors, that Vbi can be attributed to an equilibration of electronic charge carriers over the junction region. More specifically, one can predict the establishment of a diffusion–drift balance for the electrons over a p–n junction at open circuit, resulting in a uniform electrochemical potential for the electrons (or Fermi level) throughout the device, as follows: ‘fast’ electrons diffuse from the n-type side to the p-type side leaving ‘slow’ cations behind; this process establishes a potential drop Vbi over the junction, which causes an equal and opposite drift of electrons from the p-type side to the n-type side. (An equivalent set of events involving ‘fast’ holes and ‘slow’ anions will simultaneously take place.)
Figure 4a shows the electrostatic potential (top part) and a schematic diagram of the doping structure and the charge separation over a p–n junction at open circuit (bottom part), which rationalizes the recorded potential profile shown in Fig. 3c. Figure 4a also indicates the onset of net ionic motion over the junction, which motivates the transient character of the potential profile. This relaxation process, which is a manifestation of the
674 NATUREMATERIALS | VOL 8 | AUGUST 2009 | www.nature.com/naturematerials © 2009 Macmillan Publishers Limited. All rights reserved.
NATURE MATERIALS DOI: 10.1038/NMAT2478 ARTICLES non-equilibrated electrochemical potential for the ions over the junction, is further discussed below.
The existence of Vbi over the junction region also explains why the probed potential Vtot over the p–n junction under steady-state conditions is smaller than the applied potentialV (see, for example, Fig. 3b). The potential probed with SKPM corresponds to the total electrostatic potential within the device, which here is a combination of the externally applied potential and the built-in potential, where the latter at steady state has a sign opposing the former; that is, Vtot = V −Vbi (see top part in Fig. 4b). The net charge separation over the junction region, as well as the charge transport and radiative recombination, in a planar LEC device at steady state is shown in Fig. 4b. The existence of Vbi also explains why the probed potential is larger than the applied voltage following a fast switch in the polarity of the applied voltage (from+5 to−5V in Fig. 3d). Here, the applied voltage and the built-in potential are temporarily oriented in the same direction, and the total probed potential is accordinglyVtot=V +Vbi >V .
The p–n junctions discussed here are distinctly different from conventional p–n junctions in, for example, Si, in that the compen- sating, and dopant-stabilizing, counter ions are mobile. Thus, these novel doping structures are appropriately termed dynamic p–n junctions, whereas conventional p–n junctions are static structures. The mobility of the ions enables the dynamic p–n junction to be formed in situ, but also means that the doping and junction will dissipate and/or reform in response to a shift in the external applied voltage, as demonstrated in Fig. 3. Such redistribution processes involve a complicated interplay of electronic and ionic motion, as exemplified in the complex evolution of the potential profiles in Fig. 3d following a shift in the applied voltage.
We further note that a dynamic p–n junction can be stable only above a critical applied voltage exceeding the built-in potential, V > Vbi, because the ionic concentration gradients and the accompanying doping profiles will dissipate at a lower voltage. A simple example involving the stability of the cationic concentration gradient can illustrate this point (but the same argument holds for the anionic gradient as well): the cations on the n-type side of the p–n junction exhibit a net diffusive motion towards the p-type side (owing to the cationic concentration gradient over the junction), and this diffusive motion can be compensated by a drift motion of cations in the opposite direction only if the total electrostatic potential over the junction (the sum of V and Vbi) drives the cations in the direction opposing the concentration gradient. Thus, because Vbi opposes V under steady- state conditions, it follows directly that V >Vbi is the prerequisite for a stable dynamic p–n junction structure. One scenario under which the ionic concentration gradients and the junction structure are unstable (that is, V < Vbi) is illustrated in Fig. 4a, whereas Fig. 4b shows a situation (V = 5V > Vbi) at which the p–n junction structure is stable.
Finally, it is appropriate to mention that the primary differences between the experiments presented here and those of our predecessors is the chemical system used and the conditions under which the measurements were made. We have previously demonstrated that the use of small Li+ cations and/or the application of low voltages in conjugated polymer-based LECs can negatively affect the placement of the p–n junction, leaving it very near the negative metal electrode35,46. This is plausibly the reason why Pingree et al.45 observed a different potential profile than we do. The ruthenium-based ionic transition-metal complex used by Slinker et al.43 in their light-emitting device differs chemically from theMEH-PPV conjugated polymer presented here. It is not clear, for example, that this type of transition-metal complex can be doped into a highly conductive state, which would result in an operational mechanism unlike that observed in conjugated-polymer LECs.
Summary We have used a combination of SKPM and light-emission probing to establish that a p–n junction can form in situ in the bulk of the active material of an LEC device during operation. This observation provides evidence that the so-called electrochemical doping model can describe the operation of LECs, which is relevant in the context of an ongoing debate in the scientific literature. Moreover, the availability of a dynamic organic p–n junction opens the possibility for interesting fundamental physics. For example, we demonstrate the existence of a built-in potential over the junction, which must be compensated by an external voltage to stabilize the junction structure. Finally, we note that the tantalizing subject of dynamic organic p–n junctions is largely unexplored, and that further studies in this field could open a wide range of novel and useful applications.
Methods The conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV,Mw=150.000 gmol−1, Aldrich) and poly(ethylene oxide) (PEO, Mw = 5×106 gmol−1, Aldrich) were used as received. The salt KCF3SO3 (98%, Alfa Aesar) was dried at a temperature (T ) of 473K under N2 atmosphere before use. Master solutions were prepared at a concentration of 10mgml−1 by dissolving MEH-PPV in chloroform (>99%, anhydrous, Aldrich), and by dissolving PEO and KCF3SO3 separately in cyclohexanone (99%, Merck). A blend solution was prepared by mixing the master solutions together in a mass ratio of MEH-PPV/PEO/KCF3SO3 = 1:1.35:0.25, followed by stirring on a magnetic hot plate at T = 323K for 12 h. The glass substrates (1×1 cm2) were cleaned by subsequent ultrasonic treatment in detergent, acetone and isopropanol.
The glass substrates were spin-coated with the blend solution (at 800 r.p.m. for 60 s, followed by 1,000 r.p.m. for 10 s), and thereafter dried on a hot plate at T = 320K for 12 h. The resulting active material film had a thickness of ∼300 nm, as established with AFM. Au electrodes were deposited on top of the active material by thermal evaporation of Au under high vacuum (p∼ 1×10−6 mbar); the inter-electrode gap of 120 µm was established with an Al shadow mask positioned in close proximity to the active material. The sharpness of the Au electrode interfaces has been confirmed by AFM, as described in the Supplementary Information. All fabrication steps, excluding the cleaning of substrates, were done in two interconnected glove boxes filled with N2 ([O2]< 1 ppm and [H2O]< 1 ppm).
The optical probing of the light-emission zone was carried out at room temperature in two different set-ups: (1) in an optical-access cryostat under high vacuum (p< 10−5 mbar), using a single-lens reflex camera (Canon EOS20) equipped with a macro lens (focal length: 65mm) and a teleconverter (×2), (2) in a glove box under the same conditions as the SKPM measurements (see below), using an optical microscope equipped with a video camera (Hengtech). We find that the device data, notably the position of the light-emission zone in the inter-electrode gap, are effectively the same in both set-ups, and we chose to present the data acquired in the cryostat owing to image quality. In parallel with the optical probing, the current was measured with a computer-controlled source-measure unit (Keithley 2400).
All SKPM images were recorded in a glove box under N2 atmosphere ([O2]< 1 ppm and [H2O]< 1 ppm) with a commercial AFM system (Veeco Instruments MultiMode AFM with Nanoscope IV controller) operating in lift mode; that is, each line is scanned twice, first to measure the topography in tapping mode (oscillation amplitude setpoint over free amplitude A/A0 ≈ 1.4/1.8) and second to measure the electrostatic potential (using amplitude modulation at the first resonance of the cantilever, with Va.c. = 8V and Vd.c. applied to the cantilever) at a predefined lift height. We used zero lift height, which results in a typical tip–sample gap of about 15 nm. We used sharp silicon tips with a Pt coating (Olympus OMCL-AC240TM-B2, apex radius< 15 nm, k ∼ 2Nm−1, cone angle< 25◦, resonant frequency∼ 70 kHz). The scan rate was 0.2Hz, so each line takes 2×5 s to scan. Tip coarse positioning and in situ electroluminescence detection were carried out using a CCD (charge-coupled device) camera attached to a long-working-distance microscope inside the glove box. The parallel electrical characterization was carried out with a Keithley 4200 unit connected to the SKPM through an electrical port in the glove box. All measurements were carried out at room temperature.
Received 10 December 2008; accepted 15 May 2009; published online 21 June 2009
References 1. Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances
on plastic. Nature 428, 911–918 (2004). 2. Malliaras, G. & Friend, R. An organic electronics primer. Phys. Today 58,
NATUREMATERIALS | VOL 8 | AUGUST 2009 | www.nature.com/naturematerials 675 © 2009 Macmillan Publishers Limited. All rights reserved.
ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2478
3. Moller, S., Perlov, C., Jackson, W., Taussig, C. & Forrest, S. R. A polymer/semiconductor write-once read-many-times memory. Nature 426, 166–169 (2003).
4. Leger, J. M. Organic electronics: The ions have it. Adv. Mater. 20, 837–841 (2008).
5. Yazaki, S., Funahashi, M. & Kato, T. An electrochromic nanostructured liquid crystal consisting of pi-conjugated and ionic moieties. J. Am. Chem. Soc. 130, 13206–13207 (2008).
6. Gao, L., Johnston, D. & Lonergan, M. C. Synthesis and self-limited electrochemical doping of polyacetylene ionomers. Macromolecules 41, 4071–4080 (2008).
7. Wei, D. & Amaratunga, G. Photoelectrochemical cell and its applications in optoelectronics. Int. J. Electrochem. Sci. 2, 897–912 (2007).
8. Leger, J. M., Patel, D. G., Rodovsky, D. B. & Bartholomew, G. P. Polymer photovoltaic devices employing a chemically fixed p-i-n junction. Adv. Funct. Mater. 18, 1212–1219 (2008).
9. Edman, L., Swensen, J., Moses, D. & Heeger, A. J. Toward improved and tunable polymer field-effect transistors. Appl. Phys. Lett. 84, 3744–3746 (2004).
10. Lin, F. D. & Lonergan, M. C. Gate electrode processes in an electrolyte-gated transistor: Non-faradaically versus faradaically coupled conductivity modulation of a polyacetylene ionomer. Appl. Phys. Lett. 88, 133507 (2006).
11. Lu, W. et al. Use of ionic liquids for pi-conjugated polymer electrochemical devices. Science 297, 983–987 (2002).
12. Beaujuge, P. M., Ellinger, S. & Reynolds, J. R. The donor–acceptor approach allows a black-to-transmissive switching polymeric electrochrome. Nature Mater. 7, 795–799 (2008).
13. Kaihovirta, N. J., Tobjork, D., Makela, T. & Osterbacka, R. European Congress and Exhibition on Advanced Materials and Processes (Euromat 2007) 640–643 (Wiley–VCH, 2007).
14. Shin, J. H., Xiao, S., Fransson, A. & Edman, L. Polymer light-emitting electrochemical cells: Frozen-junction operation of an ‘ionic liquid’ device. Appl. Phys. Lett. 87, 043506 (2005).
15. Latini, G. et al. Cyclodextrin-threaded conjugated polyrotaxanes for organic electronics: The influence of the counter cations. Adv. Funct. Mater. 18, 2419–2427 (2008).
16. Hohertz, D. & Gao, J. How electrode work function affects doping and electroluminescence of polymer light-emitting electrochemical cells. Adv. Mater. 20, 3298–3302 (2008).
17. Alem, S. & Gao, J. The effect of annealing/quenching on the performance of polymer light-emitting electrochemical cells. Org. Electron. 9, 347–354 (2008).
18. Pei, Q. B., Yu, G., Zhang, C., Yang, Y. & Heeger, A. J. Polymer light-emitting electrochemical-cells. Science 269, 1086–1088 (1995).
19. Gao, J. & Dane, J. Planar polymer light-emitting electrochemical cells with extremely large interelectrode spacing. Appl. Phys. Lett. 83, 3027–3029 (2003).
20. Gao, J. & Dane, J. Visualization of electrochemical doping and light-emitting junction formation in conjugated polymer films. Appl. Phys. Lett. 84, 2778–2780 (2004).
21. Graber, S. et al. A supramolecularly-caged ionic iridium(III) complex yielding bright and very stable solid-state light-emitting electrochemical cells. J. Am. Chem. Soc. 130, 14944–14945 (2008).
22. Shao, Y., Bazan, G. C. & Heeger, A. J. Long-lifetime polymer light-emitting electrochemical cells. Adv. Mater. 19, 365–370 (2007).
23. Jin, Y., Bazan, G. C., Heeger, A. J., Kim, J. Y. & Lee, K. Improved electron injection in polymer light-emitting diodes using anionic conjugated polyelectrolyte. Appl. Phys. Lett. 93, 123304 (2008).
24. Yu, M. X., Kang, J. H. & Cheng, C. H. Synthesis of diarylamino-benzo[de]anthracen-7-ones and their light emitting property. Chin. J. Org. Chem. 28, 1393–1397 (2008).
25. Jin, Y. et al. Novel green-light-emitting polymers based on cyclopenta[def]phenanthrene.Macromolecules 41, 5548–5554 (2008).
26. Oh, S. H., Vak, D., Na, S. I., Lee, T. W. & Kim, D. Y. Water-soluble polyfluorenes as an electron injecting layer in PLEDs for extremely high quantum efficiency. Adv. Mater. 20, 1624–1629 (2008).
27. Sun, J. et al. Pi-conjugated poly(anthracene-alt-fluorene)s with X-shaped repeating units: New blue-light emitting polymers. Polymer 49, 2282–2287 (2008).
28. Ortony, J. H. et al. Thermophysical properties of conjugated polyelectrolytes. Adv. Mater. 20, 298–302 (2008).
29. Shao, Y., Bazan, G. C. & Heeger, A. J. LED to LEC transition behavior in polymer light-emitting devices. Adv. Mater. 20, 1191–1193 (2008).
30. Hoven, C. et al. Ion motion in conjugated polyelectrolyte electron transporting layers. J. Am. Chem. Soc. 129, 10976–10977 (2007).
31. Sun, Q. J., Li, Y. F. & Pei, Q. B. Polymer light-emitting electrochemical cells for high-efficiency low-voltage electroluminescent devices. J. Disp. Technol. 3, 211–224 (2007).
32. Edman, L. Bringing light to solid-state electrolytes: The polymer light-emitting electrochemical cell. Electrochim. Acta 50, 3878–3885 (2005).
33. Shin, J. H. et al. Light emission at 5 V from a polymer device with a millimeter-sized interelectrode gap. Appl. Phys. Lett. 89, 013509 (2006).
34. Shin, J. H. & Edman, L. Light-emitting electrochemical cells with millimeter-sized interelectrode gap: Low-voltage operation at room temperature. J. Am. Chem. Soc. 128, 15568–15569 (2006).
35. Shin, J. H., Robinson, N. D., Xiao, S. & Edman, L. Polymer light-emitting electrochemical cells: Doping concentration, emission-zone position, and turn-on time. Adv. Funct. Mater. 17, 1807–1813 (2007).
36. Slinker, J. D. et al. Electroluminescent devices from ionic transition metal complexes. J. Mater. Chem. 17, 2976–2988 (2007).
37. Pei, Q. & Heeger, A. J. Operating mechanism of light-emitting electrochemical cells. Nature Mater. 7, 167–167 (2008).
38. Pei, Q. B., Yang, Y., Yu, G., Zhang, C. & Heeger, A. J. Polymer light-emitting electrochemical cells: In situ formation of a light-emitting p–n junction. J. Am. Chem. Soc. 118, 3922–3929 (1996).
39. Robinson, N. D., Shin, J. H., Berggren, M. & Edman, L. Doping front propagation in light-emitting electrochemical cells. Phys. Rev. B 74, 155210 (2006).
40. Dick, D. J., Heeger, A. J., Yang, Y. & Pei, Q. B. Imaging the structure of the p–n junction in polymer light-emitting electrochemical cells. Adv. Mater. 8, 985–987 (1996).
41. deMello, J. C. Interfacial feedback dynamics in polymer light-emitting electrochemical cells. Phys. Rev. B 66, 235210 (2002).
42. deMello, J. C., Tessler, N., Graham, S. C. & Friend, R. H. Ionic space-charge effects in polymer light-emitting diodes. Phys. Rev. B 57, 12951–12963 (1998).
43. Slinker, J. D. et al. Direct measurement of the electric-field distribution in a light-emitting electrochemical cell. Nature Mater. 6, 894–899 (2007).
44. Malliaras, G. G. et al. Operating mechanism of light-emitting electrochemical cells—Authors’ response. Nature Mater. 7, 168–168 (2008).
45. Pingree, L. S. C., Rodovsky, D. B., Coffey, D. C., Bartholomew, G. P. & Ginger, D. S. Scanning kelvin probe imaging of the potential profiles in fixed and dynamic planar LECs. J. Am. Chem. Soc. 129, 15903–15910 (2007).
46. Fang, J., Matyba, P., Robinson, N. D. & Edman, L. Identifying and alleviating electrochemical side-reactions in light-emitting electrochemical cells. J. Am. Chem. Soc. 130, 4562–4568 (2008).
47. Matyba, P., Andersson, M. R. & Edman, L. On the desired properties of a conjugated polymer-electrolyte blend in a light-emitting electrochemical cell. Org. Electron. 9, 699–710 (2008).
48. Holt, A. L., Leger, J. M. & Carter, S. A. Electrochemical and optical characterization of p- and n-doped poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene]. J. Chem. Phys. 123, 044704 (2005).
Acknowledgements L.E. and P.M. acknowledge the Swedish Research Council (VR) and Wenner-Gren stiftelserna for scientific financial support. L.E. is a ‘Royal Swedish Academy of Sciences Research Fellow’ supported by a grant from the Knut and Alice Wallenberg Foundation. N.D.R. acknowledges VR and Norrköpings Kommun for financial support of part of this work. The work of K.M. is made possible by a NanoNed grant (NanoNed is the Dutch nanotechnology initiative by the Ministry of Economic Affairs). The authors acknowledge A. Shchukarev at Umeå University for help with the X-ray photoemission spectroscopy measurements.
Author contributions P.M., K.M. and N.D.R. carried out the experiments. L.E., N.D.R. and M.K. wrote the manuscript. P.M., K.M., M.K., N.D.R. and L.E. contributed to data analysis and project planning.
Additional information Supplementary information accompanies this paper on www.nature.com/naturematerials. Reprints and permissions information is available online at http://npg.nature.com/ reprintsandpermissions. Correspondence and requests for materials should be addressed to L.E.
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