Elucidating Deviating Temperature Behavior of Organic Light‐Emitting Diodes and Light‐Emitting Electrochemical Cells

Organic light‐emitting diodes (OLEDs) and light‐emitting electrochemical cells (LECs) exhibit different operational modes that render them attractive for complementary applications, but their dependency on the device temperature has not been systematically compared. Here, the effects of a carefully controlled device temperature on the performance of OLEDs and LECs based on two common emissive organic semiconductors are investigated. It is found that the peak luminance and current efficacy of the two OLEDs are relatively temperature independent, whereas, the corresponding LECs exhibit a significant increase by ≈85% when the temperature is changed from 20 to 80 °C. A combination of simulations and measurements reveal that this deviating behavior is consistent with a shift of the emission zone from closer to the transparent anode toward the center of the active material for both the OLEDs and the LECs, which in turn can be induced by a stronger positive temperature dependence of the mobility of the holes than the electrons.


Introduction
Electroluminescent devices based on organic semiconductors (OSCs), notably the organic light-emitting diode (OLED) and the light-emitting electrochemical cell (LEC), can be flexible and light-weight, [1] feature large-area emission, [2] and operate with high efficiency at high brightness. [3] The OLED can, in fast turn-on time, [11a,11b] and that recent studies established the strong effects that self-heating can have on the operation of free-standing LEC devices. [11c,12] However, we also note that a systematic study aimed at establishing the influence of temperature on LEC operation is lacking. Moreover, a direct comparison between the effects of temperature on OLED and LEC devices based on the same OSC is, to the best of our understanding, nonexistent in the scientific literature.
It is therefore the goal of the present study to address these issues through the systematic investigation of two OLEDs and LECs based on the same two emissive OSCs, and with their device temperature accurately controlled by a carefully designed temperature setup. Quite unexpectedly, we find that the peak luminance and current efficacy of the two LEC devices increase by ≈85% when the device temperature increases from 20 to 80 °C, whereas the luminance and current efficacy of the two OLEDs are essentially invariant to the same temperature change. Using a combination of optical modeling and experiments, we demonstrate that this behavior can be rationalized by a shift of the peak of the exciton distribution from closer to the transparent anode toward the center of the active material, and we note that such a shift could be induced by a stronger increase of the hole mobility than the electron mobility with increasing temperature. We further find that the operational stability of the LECs is more sensitive to an increase in temperature than the OLEDs, and that the turn-on time of the LECs drops significantly with temperature because of a thermally activated ion mobility within the active material. Figure 1a presents the electron-energy structure of the two OLED devices, which are distinguished by the selection of the polymeric OSC emitter, being either a yellow emitter termed Super Yellow [13] (SY) or a blue emitter termed Polymer Blue [14] (PB). The corresponding two OLEDs are accordingly termed "Yellow-OLED" and "Blue-OLED." A relatively balanced hole and electron injection is achieved by employing high work-function poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) for the positive anode and low work-function Ca for the negative cathode. In order to attain a sufficiently high anode conductivity, a transparent film of indium-tin-oxide (ITO) was included in between the transparent PEDOT:PSS anode and the glass substrate, whereas a layer of Al was deposited on top of the reflective Ca cathode to improve the device stability. Figure 1b displays the corresponding electron-energy structure of the two LEC devices, the "Yellow-LEC" and the "Blue-LEC," as identified by the selection of the polymeric OSC emitter. The active material of an LEC comprises an electrolyte (i.e., mobile ions) in addition to the OSC, and we employed a KCF 3 SO 3 salt dissolved in a hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH) ion-transporter for this end since LEC devices based on this electrolyte and the SY [11c,13] and PB [15] emitters have demonstrated good device performance. For the LEC, we employed reflective Al for the top cathode and transparent ITO for the bottom anode on top of the glass substrate, since this electrode combination has been reported to deliver a good performance in similar LEC devices. [16] Figure 1c-f presents the temporal evolution of the voltage (upper panels) and the luminance (lower panels) for the four different devices, as identified in the upper panels, during driving by a constant current density of 50 mA cm −2 (Yellow-OLED and Yellow-LEC) or 25 mA cm −2 (Blue-OLED and Blue-LEC). The device data were recorded at a set of externally controlled temperatures of 20, 40, 60, and 80 °C, as identified in the lower left insets of Figure 2c,e. The temperature was accurately controlled by positioning the device under study on a temperature stage, comprising a Peltier element as a heater, and by sandwiching a conformal and high thermal-conductivity three-layer structure, consisting of a 3-mm-thick Al plate, a soft thermal pad, and thermal paste, between the device and the temperature stage. The merit of this approach was verified in a spatially resolved temperature measurement using a thermal camera, which demonstrated that the temperature difference between the center emission area and the non-emitting substrate edges was < 1 °C at the maximum measurement temperature of 80 °C. In addition, we found that this measured device temperature during electrical driving corresponded well with the input temperature of the temperature stage.

Results and Discussion
The Yellow-OLED ( Figure 1c) and the Blue-OLED (Figure 1e) both display a decrease of the initial voltage with increasing temperature (upper panels). This observation is primarily assigned to an increase of the electron and hole mobility with temperature, which is a characteristic general feature of OSCs. [17] The initial luminance response to the change in temperature differs somewhat between the two devices (lower panels). The Yellow-OLED exhibits a minor decrease of the initial luminance with temperature, which we assign to the commonly observed temperature-induced lowering of the photoluminescence quantum yield (PLQY) of OSCs. [17a] The initial luminance of the Blue-OLED is, in contrast, essentially independent of temperature within the investigated range. We tentatively explain this observation by that the lowering of the PLQY is compensated by an improved balance of the electron and hole injection with increasing temperature for the Blue-OLED (note the big difference between the electron-and hole-injection barriers for the Blue-OLED in comparison to the Yellow-OLED in Figure 1a). The long-term stability of the voltage and luminance is, as expected, dropping with increasing temperature for both OLED devices (note the logarithmic x-axis in Figure 1c-f), and Figure S1, Supporting Information, reveals that the time to half-peak luminance (LT 50 ) exhibits an Arrhenius dependence, that is, , within the probed temperature range, with an activation energy (E a ) of E a, Yellow-OLED = 0.36 eV and E a, Blue-OLED = 0.20 eV.
The temporal behavior of the Yellow-LEC and the Blue-LEC at different temperatures is depicted in Figure 1d,f, respectively. All characterized LEC devices exhibited a decreasing voltage and an increasing luminance during the early operation, which is due to the formation of the EDLs at the electrode interfaces and the subsequent formation of a p-n junction doping structure within the bulk of the active material. [9b,c] The observation of these characteristic LEC transients thus verifies that the investigated devices are well functioning LECs. The characteristic LEC operation is further manifested in that the two LEC devices feature a significantly lower minimum voltage by 1-3 V than the corresponding OLEDs, despite that the active-material thickness (≈100 nm) and the OSC are the same. Figure S2a,b, Supporting Information, summarizes the time to peak luminance ( Lpeak t ) and the time to minimum voltage ( Vmin t ) as a function of temperature for the Yellow-LEC and the Blue-LEC, respectively. It was possible to fit the Arrhenius equation to these turn-on data (t / a B e E k T ∝ − ; see dashed lines in Figure S2a,b, Supporting Information), which is in agreement with that the ion motion within the active material is thermally activated.
[11d] E a is the activation energy in the Arrhenius equation, and in the context of ion motion it can be thought of as the effective energy barrier height between the initial and final ion state. We find that the activation energy for the turn-on time to peak luminance is 0.81 eV for the Yellow-LEC and 0.61 eV for the Blue-LEC. We note that Burnett and coworkers report a significantly higher activation energy for "the light intensity growth rate" of 1.6 eV for a slightly different LEC system, [11d] which is in agreement with that the active-material morphology plays a critical role for the transient behavior of LEC devices. From an application viewpoint, we also wish to call attention to that the Yellow-LEC exhibits a fast sub-second turn-on time to a high luminance of >1000 cd m −2 already at 20 °C (see Figure 1d).
During "ideal" constant-current operation, it is reasonable that Lpeak t represents the shorter time for the formation of efficient EDLs, whereas Vmin t estimates the longer time to the establishment of the steady-state p-n junction doping structure (when all ions are locked up in the EDLs and the doping regions). As expected, we find that V  We tentatively attribute this unexpected behavior of the Blue-LEC to that undesired side reactions are starting to take place already during the initial operation of the Blue-LEC, and that these side reactions have a stronger impact on the charge transport than the emission, presumably since they are localized at one (or both) of the electrode interfaces, far away from the light-emitting p-n junction. [18] The LT 50 at 20 °C for the Yellow-LEC is essentially the same as for the Yellow-OLED ( Figure S1a, Supporting Information), while it is significantly shorter for the Blue-LEC than the Blue-OLED (Figure S1b, Supporting Information). This observation brings further support to the notion that side reactions take place in parallel to the electrochemical doping during the early operation of the Blue-LEC.
We also find that the operational stability of the two LECs drops faster with increasing temperature than the corresponding OLEDs (see Figure S1, Supporting Information), as manifested in higher E a values of E a,Yellow-LEC = 0.88 eV and E a,Blue-LEC = 0.56 eV and that the final decay behavior is more drastic for the LECs (see transients in Figure 1). [19] We further observe that the drastic final luminance decay of the two LEC devices is accompanied by a very fast increase of the driving voltage (and note that a similar observation has been made by other authors [20] ); we speculate that this drastic failure could be due to the formation of an electrically isolating layer of electrochemical side-reaction products within the active material. [18] An interesting observation is that the temperature dependence of the peak luminance (L peak ) and the efficiency are Figure 2. a,b) The peak luminance (left y-axis), the current efficacy (right y-axis), and c,d) the power efficacy as a function of temperature for the devices identified in the insets. e-g) A schematic presentation of the distribution of excitons (yellow shading), p-type doping (red shading), and n-type doping (blue shading) in an OLED (left) and in an LEC (right) for three exciton-distribution scenarios. h) The simulated normalized peak luminance as a function of the exciton peak position for the Yellow-OLED and the Yellow-LEC. The positive anode is located at 0 and the negative cathode at 1 in the interelectrode gap. The active-layer thickness is indicated in the legend (d AL ).
markedly different for the OLEDs and LECs. Figure 2a,b reveals that L peak (left y-axis) and the current efficacy (right y-axis) are essentially independent of temperature for the two OLEDs between 20 and 80 °C, whereas they increase significantly for the two LEC devices. Specifically, for the Yellow-LEC L peak increases by 83% between 20 and 60 °C, and for the Blue-LEC it increases by 85 % between 20 and 80°C. We mention in passing that the peak current efficacy of the Yellow-OLED of 8.6 cd A −1 (at 20 °C) corresponds to an external quantum efficiency (EQE) of 3.1%, while the peak current efficacy of the Yellow-LEC of 6.0 cd A −1 (at 60 °C) is equivalent to an EQE of 2.2%. Figure 2c,d presents the power efficacy as a function of temperature, a property for which the LEC is more competitive with the OLED because of its lower drive voltage. The cause is the LEC-characteristic operation, which renders the electron and hole injection ohmic and which improves the charge-transport capacity of the bulk of the active material. The temperature dependence for the power efficacy is also distinctly stronger for the LEC devices than the corresponding OLEDs.
In order to understand this deviating temperature behavior of the OLEDs and LECs, we have formulated and analyzed a simple model in which the hole and electron injection is considered balanced, the thickness of the active material (d AL ) is 100 nm, and the key free parameter is the exciton distribution. The latter is described by a Gaussian with a full width at half maximum (FWHM) of 53 nm for the OLED and 12 nm for the LEC. The motivation for the thinner exciton distribution in the LEC devices is that the two in situ formed doping regions confine the exciton formation to the thin p-n junction region, [21] which is supported by, for instance, direct observations of the light-emitting p-n junction in planar surface cells [9c,22] and by impedance measurements. [23] Figure 2e-g presents three scenarios for the simulated steady-state exciton distribution (yellow shading) in the interelectrode gap for the Yellow-OLED (left panels) and the Yellow-LEC (right panels), with the exciton peak position located: i) Closer to the positive anode (Figure 2e), ii) in the center of the interelectrode gap (Figure 2f), and iii) closer to the negative cathode ( Figure 2g). The anodic interface is positioned at 0 and the cathodic interface at 1. The LEC-defining doping regions are modeled with constant gradients, and the p-type (n-type) doping at the anode (cathode) is indicated by red (blue) shading. More details on the simulation can be found in Section 4 and in refs. [9e,24]. Figure 2h presents the modeled forward luminance for the Yellow-OLED and the Yellow-LEC as a function of the exciton peak position, and the simulation data imply a distinctly different behavior of the two devices. Specifically, the ideal value for the exciton peak position is close to the anode at 0.28 for the Yellow-OLED, while it is essentially centered in the interelectrode gap at 0.52 for the Yellow-LEC. This is in agreement with that the metallic Al cathode (positioned at 1) is a more significant exciton quencher than the transparent ITO electrode (positioned at 0), but also demonstrates that the two doping regions in the LEC devices are highly effective quenching sites. More specifically, the essentially centered ideal exciton distribution for the Yellow-LEC is caused by a stronger exciton quenching capacity of the p-type doped Super Yellow region (next to the ITO anode) than the n-type doped Super Yellow region (next to the Al cathode), [25] which effectively compensates for Al being a stronger exciton quencher than ITO.
A comparison between the measured temperature-independent luminance data for the Yellow-OLED in Figure 2a and the simulated luminance data in Figure 2h implies that the exciton peak position for the OLED (open symbols) either is relatively invariant with increasing temperature or confined to migrate within the range of 0.2-0.5, where the peak luminance is relatively constant (and close to its maximum). Figure S3, Supporting Information, presents the measured and simulated forward electroluminescence (EL) spectrum of the Yellow-OLED as a function of temperature, and these data strongly imply that the exciton peak position is migrating toward the cathode with increasing temperature. Thus, we conclude that the exciton distribution in the OLED is migrating from closer to the anode toward the center of the interelectrode gap with increasing temperature.
A corresponding comparison of the temperature-dependent measured luminance of the Yellow-LEC in Figure 2a with the simulated luminance data in Figure 2h demonstrates the exciton peak position for the Yellow-LEC (filled symbols) either must be shifting from closer to the anode toward the center of the interelectrode gap or from closer to the cathode toward the center, with increasing temperature. In this context, we mention that a recent study on a similar Yellow-LEC device revealed that the room-temperature emission zone was positioned at 0.3, that is, closer to the positive anode. [9e,24a] Moreover, preliminary data on the evolution of the measured and simulated forward EL spectrum as a function of temperature suggest that the exciton distribution in the Yellow-LEC is migrating from a region closer to the anode toward the center of the interelectrode gap. Thus, our conclusion is that both the OLED and LEC devices exhibit similar qualitative behavior with a steady-state exciton distribution that is migrating from closer to the anode at room temperature toward the center of the interelectrode gap at elevated temperatures. At this point, we wish to mention that other temperature-induced effects, such as a changing effective width of the exciton distribution or a shifting exciton environment, [12,26] also can influence the device performance, but that an investigation on these effects are outside the scope of this study.
So what could be the origin of a shift of the exciton peak position with increasing temperature? Previous studies on OLEDs and LECs have shown that the exciton peak position is dependent on the ratio of the hole and electron mobilities, μ p /μ n , and that an increase (decrease) in the μ p /μ n ratio will result in a shift of the exciton peak position toward the cathode (anode). [3d,27] We, therefore, suggest that our derived cathodic shift of the exciton peak position for the investigated OLED and LEC devices originates (at least partially) in an increasing μ p /μ n ratio with increasing temperature, or more specifically to that the hole mobility is increasing at a faster rate with temperature than the electron mobility.

Conclusions
To summarize, the investigated OLEDs and LECs, based on the same emissive OSCs, exhibit a distinctly different dependency on the device temperature. The peak luminance and current efficacy of the two OLEDs are relatively constant within the temperature interval of 20 to 80 °C, whereas, the two LECs exhibit a peak luminance and current efficacy increase of ≈85 %. Complementary simulations and measurements demonstrate that this deviating behavior is concomitant with a shift of the exciton peak position from closer to the positive anode at 20 °C to the center of the active material at 80 °C for both device types. We note that this shift can be provoked by a stronger increase of the hole mobility than the electron mobility with increasing temperature. We further find that the LEC turn-on is significantly shortened with temperature because of a thermally activated ion motion within the active material, whereas the operational lifetime of both the OLEDs and LECs is dropping, with the latter being more sensitive. These results thus highlight significant differences in the sensitivity to a changing temperature and emission zone position between OLED and LEC devices, and also reinforce the importance of controlling and reporting the temperature during device characterization.

Experimental Section
The electroluminescent OSCs are a yellow-emitting phenyl-substituted poly(paraphenylene vinylene) conjugated copolymer termed "Super Yellow" (Merck KGaA, Darmstadt, DE), and a blue-emitting conjugated polymer "Polymer Blue" (Livilux SPB 02T, Merck); their chemical structures are depicted in Figure S4, Supporting Information. The OLED inks comprised Super Yellow dissolved at 7 g L −1 in cyclohexanone (Yellow-OLED) and Polymer Blue dissolved at 10 g L −1 in cyclohexanone (Blue-OLED). The ITO coated glass substrate (145 nm, R s = 20 Ω □−1 , thin film devices) was cleaned by sequential ultrasonic treatment in detergent (Extran MA 01, Merck), deionized water, acetone, and isopropanol. The cleaned ITO-coated substrate was exposed to 10 min of UV-generated ozone (model 42-220, Jelight Company). Thereafter a poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS, Clevios P VP AI 4083, Heraeus) film was spin-coated at 4000 rpm for 60 s, and dried at 120 °C for 30 min. The dry thickness of the PEDOT:PSS film was 35 nm. The OLED ink was spin-coated on top of the PEDOT:PSS at 3000 rpm for 60 s, and thereafter dried at 70 °C for 2 h. The thickness of the dry active material was 100 nm. The reflective top electrode was deposited by thermal evaporation under vacuum (p < 8 × 10 −6 mbar) through a shadow mask, and it consisted of 20 nm Ca and 100 nm Al.
The LEC active material comprises a blend of the electroluminescent polymeric OSC, a KCF 3 SO 3 salt (Aldrich), and a hydroxyl-capped TMPE-OH (Aldrich; M w = 450 g mol −1 ) ion-transporter. The master solutions were prepared with the following solute concentrations in cyclohexanone (Aldrich): 8 g L −1 (Super Yellow) and 10 g L −1 (Polymer Blue, KCF 3 SO 3, and TMPE-OH). The LEC ink was prepared by mixing the master solutions in a solute mass ratio of OSC:TMPE-OH:KCF 3 SO 3 = 1:0.15:0.03. The LEC ink was spin-coated on the ITOcoated substrate for 60 s at either 3000 rpm (Yellow-LEC) or 2000 rpm (Blue-LEC), and the spin-coated active material was dried at 70 °C for 2 h. The dry thickness of the Yellow-LEC (Blue-LEC) active material was 100 nm (90 nm), as measured by a stylus profilometer (DektakXT, Bruker). The reflective Al top electrode was deposited on top of the active material by thermal evaporation.
The overlap of the transparent ITO and the reflective top cathode defined four 2 × 2 mm 2 independent OLED/LEC devices on each substrate The devices were encapsulated by attaching a thin glass substrate on top of the reflective electrode with a single-component and UV-curable epoxy (Ossila) to allow for ambient-air characterization. [19] More details on the device fabrication are available in ref. [28].
The optoelectronic characterization was performed with the device under study positioned on a temperature stage, comprising a Peltier element as a heater, with the encapsulation glass facing downward toward the stage. A conformal and high thermal-conductivity three-layer structure, comprising a 3-mm-thick Al plate, a soft thermal pad, and thermal paste, was sandwiched between the device and the temperature stage in order to establish good thermal contact and accurately control the device temperature. The effectiveness of this approach was verified by a spatially resolved temperature measurement using a thermal camera (FLIR A35sc), which demonstrated that the temperature difference between the center emission area and the non-emitting substrate edges was <1 °C at the maximum measurement temperature of 80 °C. The device characterization started at the lowest temperature of 20 °C and finished at the highest temperature of 80 °C to minimize effects of thermal annealing.
The devices were driven by a constant current density of either 50 mA cm −2 (Yellow-OLED and Yellow-LEC) or 25 mA cm −2 (Blue-OLED and Blue-LEC), with the compliance voltage set to 21 V, and with the ITO biased as the positive anode. A source measure unit (Keithley 2400) supplied the current and recorded the corresponding voltage. The luminance was measured with a photodiode, equipped with an eye-response filter (BPW 21, Osram Semiconductors), which had been calibrated with a luminance meter (Konica Minolta LS-110). All measurements were performed on pristine devices. For the OLEDs, the measurement was stopped when the luminance reached 50% of its peak value, that is, at LT 50 ; while for the LECs, the measurement was ended when the luminance reached zero or the voltage reached the compliance. 1-3 independent OLED devices and 2-3 LECs were characterized at each temperature, and the presented data correspond to that of a typical device, with the exception being the lifetime measurement, for which all device data are presented.
The optical simulation was performed with a commercial software (Setfos 4.6.11, Fluxim), and a detailed description of the employed procedure can be found in refs. [9e,24a]. The configuration of the simulated OLED was: glass substrate (thickness = 0.75 mm), ITO (145 nm), PEDOT:PSS (35 nm), active material (100 nm), Ca (20 nm), and Al (100 nm). The exciton profile within the active material was simulated as a Gaussian distribution, with a standard deviation of 22.5 nm (corresponding to a FWHM of 53 nm). The simulation software dictated that the emissive region was transparent, and a thickness of the emissive region of 90 nm was opted, which implied that two 5-nm thin exciton-free and absorbing regions were positioned next to the electrode interfaces. The peak of the exciton profile was shifted from 10 to 90 nm away from the anodic PEDOT:PSS interface in the simulation.
The simulated configuration for the LEC device was: glass substrate (0.75 mm), ITO (145 nm), active material (100 nm), and Al (100 nm). The simulated steady-state doping structure comprised a 20 nm intrinsic region sandwiched between a p-type doped region and an n-type doped region. The doped regions featured a constant doping gradient, with the maximum doping next to the corresponding electrode interface. The exciton profile within the intrinsic region was estimated by a Gaussian distribution, with a standard deviation of 5 nm (corresponding to a FWHM of 12 nm). The position of the center of the intrinsic region was shifted from 30 to 70 nm away from the ITO anode in the simulation. The peak of the exciton profile within the intrinsic region displayed a corresponding relative shift to the center of the intrinsic region within the interelectrode gap.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.