Extracting evolution of recombination zone position
in sandwiched solid-state light-emitting electrochemical cells
by employing microcavity effect
Ting-Wei Wang, Hai-Ching Su
⇑Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan
a r t i c l e
i n f o
Article history: Received 6 March 2013 Accepted 29 April 2013 Available online 18 May 2013 Keywords:
Light-emitting electrochemical cells Recombination zone
Microcavity effect
a b s t r a c t
Techniques of probing for time-dependent evolution of recombination zone position in sandwiched light-emitting electrochemical cells (LECs) would be highly desired since they can provide direct experimental evidence to confirm altered carrier balance when device parameters are adjusted. However, direct imaging of recombination zones in thin emissive layers of sandwiched LECs could not be obtained easily. In this work, we propose an alter-native way to extract evolution of recombination zone position in sandwiched LECs by uti-lizing microcavity effect. Recombination zone positions can be estimated by fitting the measured electroluminescence spectra to simulated output spectra based on microcavity effect and properly adjusted emissive zone positions. With this tool, effects of modified car-rier transport and carcar-rier injection on performance of LECs are studied and significantly altered carrier balance can be measured, revealing that microcavity effect is useful in trac-ing evolution of recombination zone position in sandwiched LECs.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Recently, organic light-emitting diodes (OLEDs) have attracted intense attention due to their potential applica-tions in flat-panel displays and solid-state lighting [1,2]. Compared with conventional OLEDs, solid-state light-emitting electrochemical cells (LECs)[3,4]possess several promising advantages. LECs possesses electrochemically doped regions, i.e. p-type doping near the anode and n-type doping near the cathode, which are induced by spatially separated ions under a bias. Such doped regions significantly reduce carrier injection barriers at electrodes, giving balanced carrier injection, low operating voltages, and consequently high power efficiencies. As a result, LECs generally require only a single emissive layer, which can be easily deposited by solution processes and can conve-niently utilize air-stable electrodes. The emissive materials
of LECs can be roughly divided into two categories: fluores-cent polymers[3]and phosphorescent cationic transition metal complexes (CTMCs) [5]. Compared with polymer LECs, LECs based on CTMCs show several further advanta-ges and have drawn much research interests in recent years[5–34]. In such devices, no ion-conducting material is needed since these CTMCs are intrinsically ionic. Fur-thermore, higher electroluminescent (EL) efficiencies are expected due to the phosphorescent nature of CTMCs. Green[18]and white LECs[24,34]based on Ir (III) com-plexes have been shown to exhibit high device efficiencies up to 40 and 15 lm/W, respectively.
To realize highly efficient LECs, spatial control of the recombination zone for reducing exciton quenching in close proximity to electrodes[35,36]would be an impor-tant issue. However, it is difficult to directly observe the recombination zone in sandwiched LECs due to their thin emissive layers (<1
l
m). A feasible alternative way is direct optical probing of the recombination zone in planar LECs with much larger interelectrode gaps (up to mm) [37– 43]. With this approach, several important issues affecting 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.04.052
⇑Corresponding author. Tel.: +886 6 3032121x57792; fax: +886 6 3032535.
E-mail address:[email protected](H.-C. Su).
Contents lists available atSciVerse ScienceDirect
Organic Electronics
device performance of LECs such as cationic effects of salts
[39], work functions of electrodes[41], doping processes
[38,40] and recombination zone positions [42,43] were
studied with clear experimental evidence. Nevertheless, these experiments were performed on planar devices using interdigitated electrodes with spacings much larger than the interelectrode spacings of sandwiched devices and the electric fields would be significantly different in magni-tudes for planar and sandwiched LECs. Since ion mobility, carrier injection efficiency and carrier mobility, which af-fect the recombination zone position, would be field dependent, measured EL characteristics of planar LECs would not necessarily match those of sandwiched LECs. To study device physics and to further improve device per-formance of sandwiched LECs based on CTMCs, for which promising device efficiencies have been achieved
[11,13,15,18,20,24,26,34], feasible techniques to acquire evolution of recombination zone position of sandwiched LECs under driving are highly desired. In this work, we pro-pose a novel technique to dynamically probe recombina-tion zone posirecombina-tion of sandwiched LECs by utilizing microcavity effect. Microcavity structures of sandwiched LEC devices modify wavelength dependent optical outcou-pling efficiencies and thus lead to tailored EL spectra when the recombination zone is moving. Hence, the recombina-tion zone posirecombina-tions of sandwiched LECs can be estimated by fitting measured EL spectra to simulated EL spectra with proper emitting zone positions. With this technique, ef-fects of carrier trapping[26,29]and carrier injection effi-ciency[33]on carrier balance of sandwiched LECs can be studied with clear experimental evidence. It would be use-ful in optimizing device efficiencies of sandwiched LECs. 2. Experiment
2.1. Materials
The emissive complex used in this study is Ru(dtb-bpy)3(PF6)2 (where dtb-bpy is 4,40
-ditertbutyl-2,20-bipyridine) [6]. The hole transporting material
N,N0-dicarbazolyl-3,5-benzene (mCP) with a high
ioniza-tion potential was utilized to impede hole injecioniza-tion into Ru(dtb-bpy)3(PF6)2 emissive layer. Both Ru(dtb-bpy)3
(PF6)2 and mCP were purchased from Luminescence
Technology Corp. and were used as received. The low-gap cationic fluorescent near-infrared (NIR) laser dye 3,30
-diethylthiatricarbocyanine iodide (DTTCI)[25]was utilized as the carrier trapper in Ru(dtb-bpy)3(PF6)2emissive layer.
DTTCI was purchased from Sigma–Aldrich Co. and was used as received.
2.2. LEC device fabrication and characterization
Indium tin oxide (ITO)-coated glass substrates were cleaned and treated with UV/ozone prior to use. A poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer was spin-coated at 4000 rpm onto the ITO substrate in air and baked at 150 °C for 30 min. For De-vice III, an mCP layer (20 nm) was spin-coated at 5000 rpm from chlorobenzene solutions on the PEDOT:PSS
layer under ambient conditions and baked at 60 °C for 6 h in a nitrogen glove box (oxygen and moisture levels below 1 ppm) while this step was skipped for Devices I and II. The emissive layers (450 nm, as measured by profilome-try) were then spin-coated at 3000 rpm from the acetoni-trile solutions of complex 1 (Devices I and III) and complex 1 containing 0.1 wt.% DTTCI (Device II) under ambient conditions. The concentration of the solutions used for spin coating of the emissive layers is 250 mg/ mL. After spin coating, the samples were then baked at 70 °C for 10 h in a nitrogen glove box, followed by thermal evaporation of a 100 nm Ag top contact in a vacuum chamber (106torr). The electrical and emission
characteristics of LEC devices were measured using a source-measurement unit and a Si photodiode calibrated with the Photo Research PR-650 spectroradiometer. All device measurements were performed under a constant bias voltage (2.5 V) in a nitrogen glove box. The EL spectra were taken with a calibrated CCD spectrograph.
3. Results and discussions
Since the thickness of the organic layer of the LECs (450 nm) is comparable to the visible optical wavelength and a highly reflective metal (Ag) is used as the cathode, the emission properties of the emissive layer can be mod-ified in such a microcavity structure, which alters the opti-cal mode density within it and spectrally redistributes the EL spectrum. The output EL spectrum of a bottom emitting OLED device can be calculated approximately by using the following equation[44]: jEextðkÞj2¼ T21NPNi¼1 1 þ R1þ 2 ffiffiffiffiffiR1 p cos 4pzi k þ
u
1 h i 1 þ R1R2 2 ffiffiffiffiffiffiffiffiffiffiR1R2 p cos 4pL k þu
1þu
2 jEintðkÞj2where R1and R2are the reflectance from the cathode and
from the glass substrate, respectively,
u
1andu
2are thephase changes on reflection from the cathode and from the glass substrate, respectively, T2 is the transmittance
from the glass substrate, L is the total optical thickness of the cavity layers, jEintðkÞj2is the emission spectrum of the
organic materials without alternation of the microcavity effect, jEextðkÞj2is the output emission spectrum from the
glass substrate, ziis the optical distance between the
emit-ting sublayer i and the cathode. The emitemit-ting layer is di-vided into N sublayers and their contributions are summed up. Since the width of p-n junction estimated by capacitance measurements when p- and n-type layers were fully established was shown to be ca. 10% of the thickness of the active layer of LECs[45], the emitting layer width should not broader than tenth of the active layer thickness and thus an emitting layer width of 45 nm and N = 45 were estimated in optical simulation. The PL spec-trum of a thin film (450 nm) of complex 1 coated on a quartz substrate was used as the emission spectrum with-out alternation of the microcavity effect since no highly reflective metal layer is present in this sample. Based on this simulation method, the time-dependent recombina-tion zone locarecombina-tion of LECs can be estimated by fitting
measured EL spectra to simulated EL spectra with proper emitting layer position.
To demonstrate extracting evolution of recombination zone location in LECs under operation by employing micro-cavity effect, time-dependent EL spectra of LEC devices in three different configurations were measured and were fit-ted to simulafit-ted results. EL characteristics of these LEC de-vices were measured and are summarized inTable 1. These three types of LECs have the structures shown below. De-vice I: ITO (120 nm)/PEDOT:PSS (30 nm)/complex 1 (450 nm)/Ag (100 nm), Device II: ITO (120 nm)/PEDOT:PSS (30 nm)/complex 1 doped with 0.1 wt.% DTTCI (450 nm)/ Ag (100 nm) and Device III: ITO (120 nm)/PEDOT:PSS (30 nm)/mCP (20 nm)/complex 1 (450 nm)/Ag (100 nm). Device I is a standard LEC type that is commonly reported in literatures[5–34]. In Device II, a low-gap cationic fluo-rescent NIR laser dye DTTCI was incorporated in the emis-sive layer to serve as a carrier trapper, resulting in tailored carrier balance [26]. In Device III, a hole transporting material with a high ionization potential was used to im-pede hole injection and thus affected carrier balance as well[33]. Energy levels alignments[25,46–48]of Devices I, II and III are shown inFig. 1. Energy levels of complex 1 were estimated by cyclic voltammetry. Time-dependent current density and brightness of Devices I, II and III at 2.5 V are shown inFig. 2a–c, respectively. All LECs exhib-ited similar trends in time-dependent current and bright-ness characteristics under constant-bias operation. After the bias was applied, device current and brightness in-creased due to enhanced carrier injection induced by grad-ually formed p- and n-type doped layers near electrodes.
When the doped layers were well established, both device current and brightness approach steady-state values. These results are similar to those reported for LECs based on Ru complexes[6,8]. Estimating recombination zone po-sition by comparing measured and fitted simulated EL spectra and related discussions of each type of LEC devices are detailed in the following subsections.
3.1. LECs in a standard configuration
A standard LEC configuration consists of an emissive layer sandwiched between a PEDOT:PSS layer and Ag elec-trode. The energy levels of a standard LEC is depicted in
Fig. 1(Device I). Measured EL spectra of Device I at 8, 12, 18 and 58 min after a bias of 2.5 V was applied are shown inFig. 3a–d, respectively. Time-dependent evolution of EL spectra would not be attributed to degradation of the emis-sive material since devices with thinner emisemis-sive layers (ca. 100 nm) exhibited relatively stable EL spectra during 10 h operation (data not shown). Such phenomena would result from modified output EL spectra in a microcavity structure
[44]. In thicker devices (450 nm), enhanced output EL intensity due to constructive interference occurs at the red to NIR region and thus broadened (Fig. 3b and c) or even dual-peak (Fig. 3d) EL spectra were measured when the recombination zone moved accordingly. However, in thin-ner devices (ca. 100 nm), such constructive interference would take place at regions outside the emission spectrum of complex 1, leading to relatively stable EL spectra even when the recombination zone is moving. Hence, no signifi-cant change in EL spectra during operation was observed in Table 1
Summary of the LEC device characteristics. Devicea
Bias (V) tmax(min)b Lmax(cd m2)c gext, max(%)d gp, max(lm W1)e gext, steady(%)f
I 2.5 115 0.66 2.69 3.07 1.8
II 2.5 >600g 2.85 2.20 2.18 1.1
III 2.5 >600g 2.90 2.03 2.21 1.2
a
Device I: ITO (120 nm)/PEDOT:PSS (30 nm)/complex 1 (450 nm)/Ag (100 nm), Device II: ITO (120 nm)/PEDOT:PSS (30 nm)/complex 1 doped with 0.1 wt.% DTTCI (450 nm)/Ag (100 nm) and Device III: ITO (120 nm)/PEDOT:PSS (30 nm)/mCP (20 nm)/complex 1 (450 nm)/ Ag (100 nm).
b
Time required to reach the maximal brightness.
c Maximal brightness achieved at a constant bias voltage.
d Maximal external quantum efficiency achieved at a constant bias voltage. e Maximal power efficiency achieved at a constant bias voltage.
f
Steady-state external quantum efficiency achieved at a constant bias voltage.
g
Maximal brightness has not reached after 10 h operation.
LECs based on thin films (ca. 100 nm) of similar Ru com-plexes[6–8]. Simulated EL spectra were fitted to measured EL spectra by adjusting the recombination zone position (zi)
in the equation shown above. Fitted simulated EL spectra and consequently obtained recombination zone position (zi) are separately shown in each subfigure ofFig. 3.
Ini-tially, the recombination zone was located near the cathode (zi= 100 nm,Fig. 3a) and then gradually moved to the
cen-ter of the active layer (zi= 250 nm, Fig. 3d) after ca. 1 h
operation. After 1 h, EL spectra remained relatively un-changed and thus the recombination zone position would be fixed. The trend of evolution in the recombination zone position could be understood by energy level alignments of Device I shown inFig. 1. The injection barrier for hole (0.42 eV) is much lower than that for electron (1.03 eV). After a bias was applied, the required amount of accumu-lated ions near the anode to achieve ohmic contact for hole is smaller than that required to achieve ohmic contact for electron at the cathode. Therefore, the hole injection
efficiency was higher at the early stage of formation of elec-trochemically doped layers and the recombination zone would locate near the cathode consequently. When both p- and n-type layers were getting well established, bal-anced carrier injection could be achieved and the recombi-nation zone would move toward the center of the active layer. Further driving of LEC devices (after 1 h) results in continuous extension of the doped layers and the intrinsic layer between the doped layers shrinks [27]. The device current was thus enhanced continuously due to increased electric field in the intrinsic layer with gradually reduced thickness (Fig. 2a). With relatively stable carrier injection efficiency and spatial confinement of carrier recombination in a reduced intrinsic layer, the recombination zone posi-tion would be relatively fixed after 1 h operaposi-tion.
Moving of the recombination zone also shows physical correlations with the time-dependent device efficiency. Time-dependent external quantum efficiency (EQE) of Device I under 2.5 V during 10 h operation is shown in the inset of Fig. 4. Compared with previously reported time-dependent device efficiencies of LECs based on thinner films (<100 nm) of similar Ru complexes[6,8], it is interesting to note that thicker Device I abnormally exhibited a sharp transient peak at the beginning. To clar-ify the physical origin of this transient peak, time-depen-dent EQE of Device I under 2.5 V for the initial 70 min of operation is shown inFig. 4and the corresponding evolu-tion of recombinaevolu-tion zone posievolu-tion is also shown for com-parison. It is obvious that decrease of device efficiency was accompanied by moving of recombination zone and both recombination zone position and device efficiency ap-proached steady-state values after 1 h operation. Accord-ing to the proposed operatAccord-ing mechanism of sandwiched LECs[27], after the carrier injection barrier has been over-come, the intrinsic layer is still narrowing due to continu-ously extended doped layers. The excitons formed in the narrowed intrinsic layer adjacent to doped layers suffer quenching and thus device efficiency deteriorates. When the recombination zone was still moving, the doped layer was extending and continuous narrowing of the intrinsic layer resulted in decreasing of device efficiency over time (t < 1 h, inset ofFig. 4). As long as the recombination zone was relatively fixed, which implied the p-doped/intrinsic (undoped)/n-doped (p-i-n) structure was getting stable, the degree of exciton quenching would remain relatively unchanged and the device efficiency was consequently al-most the same after 1 h operation (t > 1 h, inset ofFig. 4). In LECs based on thinner films (<100 nm) of similar Ru com-plexes [6,8], significant exciton quenching would take place at the early stage of operation since the intrinsic layer would reduce rapidly due to fast extension of doped layers. Hence, sharp transient efficiency peaks shortly after applying biases were rarely measured for thinner LEC de-vices. Comparison of time-dependent evolution of device efficiency and recombination zone position provides an-other confirmation of previously reported operating model of sandwiched LECs [27]. Furthermore, time-dependent evolution of recombination zone position would be useful in revealing effects of doping of a carrier trapper and tailor-ing carrier injection efficiency on carrier balance of LEC devices.
Fig. 2. Time-dependent current density (open symbol) and brightness (solid symbol) of: (a) Device I, (b) Device II and (c) Device III under 2.5 V.
3.2. LECs doped with a carrier trapper
Doping of a carrier trapper in the emissive layer of LECs has been shown to be effective in affecting device effi-ciency due to significantly tailored carrier balance [26]. However, experimental evidence of modified recombina-tion zone posirecombina-tion resulting in affected device efficiency was not reported. In this study, a low-gap NIR carrier trap-per DTTCI was doped in the LECs based on complex 1 (De-vice II). The energy level alignments of De(De-vice II are
depicted inFig. 1. Both the host complex 1 and the guest DTTCI possess similar lowest unoccupied molecular orbital (LUMO) level energies while they show significantly differ-ent energies in highest occupied molecular orbital (HOMO) levels. Hole trapping on the guest rather than direct hole injection onto the host would be preferred. Therefore, the guest would serve as a hole trapper in the emissive layer. Similar procedures as treated for Device I were performed to estimate time-dependent evolution of recombination zone position for Device II. As shown inFig. 5a–d, the esti-mated recombination zone positions at 13, 50, 125 and 250 min after a bias of 2.5 V was applied were 390, 377, 338 and 295 nm away from the cathode. The recombina-tion zone moved from the proximity of the anode toward the center of the emissive layer during operation. The time-dependent moving direction of the recombination zone of LECs doped with a hole trapper (Device II) is con-trary to that of LECs based on neat films (Device I) (cf.
Figs. 3 and 5). As a bias was just applied, the doped layers have not well formed yet and thus the recombination zone position was determined mainly by hole trapping induced by the low-gap guest. As the doping processes continued, enhancement in injection efficiency of hole would be faster than that of electron due to a lower hole injection barrier (Device II,Fig. 1). Hence, the recombination zone was then shifted toward the center of the emissive layer. When the doped layers were well established (t > 250 min), the injec-tion efficiencies of hole and electron remained relatively unchanged and the recombination zone position was stabilized.
Fig. 3. Simulated (solid symbol) and measured (open symbol) EL spectra of Device I at (a) 8, (b) 12, (c) 18 and (d) 58 min after a bias of 2.5 V was applied. The recombination zone position (zi) estimated from fitting of simulated and measured EL spectra is shown in each subfigure.
Fig. 4. Time-dependent external quantum efficiency of Device I under 2.5 V for the initial 70 min of operation. Time-dependent evolution of recombination zone position is shown for comparison. Inset: complete time-dependent external quantum efficiency of Device I for 10 h operation.
Time-dependent EQE of Device II under 2.5 V during 10 h operation is shown in the inset ofFig. 6. A transient EQE peak at the beginning (t < 300 min) was also ob-served in Device II and decrease of device efficiency was accompanied accordingly by moving of recombina-tion zone (Fig. 6), indicating exciton quenching in recom-bination zone due to extension of doped layers. However, as compared to neat-film Device I, it took more time for doped Device II to reach stable device efficiencies. The time required for the brightness of Device II to reach
the maximum value was also much longer than that of Device I (cf. Fig. 2a and b). Similar results were reported for LECs doped with low-gap guests, in which signifi-cantly lengthened turn-on times were measured
[25,26]. Doping of a low-gap guest would increase the
resistivity of the emissive layer due to carrier trapping. Since some voltage drop occurs in the doped layers, the effective voltage drop across the undoped layer would thus be lowered. It results in lower electric field available for driving mobile ions and slower device re-sponse consequently. Furthermore, the steady-state EQE of Device II (1.1%) achieved when the recombination zone was relatively fixed is significantly lower than that of Device I (1.8%) (cf. Figs. 4 and 6). It would be related to the discrepancy in stabilized recombination zone posi-tions for these two devices. For Device I, the stabilized recombination zone position was close to the center of the emissive layer (250 nm away from the cathode). It ensures larger distances between the recombination zone and the doped layers and thus a lesser degree of exciton quenching would be expected. As compared to Device I, the stabilized recombination zone position of Device II was pushed toward the anode by 45 nm due to hole trapping (Fig. 6). Since the recombination zone is closer to the p-type doped layer, severer exciton quenching would take place in Device II, rendering deteriorated de-vice efficiencies. These results confirm that carrier bal-ance can be tailored by doping a carrier trapper to adjust the degree of exciton quenching and to modify the device efficiency in consequence.
Fig. 5. Simulated (solid symbol) and measured (open symbol) EL spectra of Device II at (a) 13, (b) 50, (c) 125 and (d) 250 min after a bias of 2.5 V was applied. The recombination zone position (zi) estimated from fitting of simulated and measured EL spectra is shown in each subfigure.
Fig. 6. Time-dependent external quantum efficiency of Device II under 2.5 V for the initial 300 min of operation. Time-dependent evolution of recombination zone position is shown for comparison. Inset: complete time-dependent external quantum efficiency of Device II for 10 h operation.
3.3. LECs with a transporting layer impeding carrier injection Tailoring carrier injection efficiency by adding a trans-porting layer to facilitate or to impede carrier injection has been reported to be effective in modifying device effi-ciencies of LECs[33]. However, the explanation of mecha-nisms responsible for altered device efficiencies lacked direct experimental evidence[33]. To clarify the effects of modifying carrier injection efficiency on carrier balance of LECs, a thin hole-transporting layer (20 nm) made of mCP, which exhibits a high ionization potential, was in-serted between the PEDOT:PSS layer and the emissive layer to impede hole injection (Device III). The energy level alignments of Device III are shown inFig. 1. Similar proce-dures as mentioned above were performed for Device III to estimate time-dependent evolution of recombination zone position. As shown inFig. 7a–d, the estimated recombina-tion zone posirecombina-tions at 12, 23, 58 and 175 min after a bias of 2.5 V was applied were 365, 355, 290 and 273 nm away from the cathode. The time-dependent moving direction of the recombination zone of Device III is the same as that of Device II while is contrary to that of Device I. With a thin transporting layer to impede hole injection, the recombination zone moved from the proximity of the an-ode toward the center of the emissive layer during opera-tion. When the doped layers have not well formed yet, the amount of hole injected into the emissive layer would be significantly reduced due to a high injection barrier at the PEDOT:PSS/mCP interface (Device III, Fig. 1). Hence, as compared to Device I, the recombination zone of Device III at the early stage of operation was closer to the anode.
As the doping processes proceeded, anions in the emissive layer would drift into the intrinsic mCP layer and result in p-type doping consequently[49]. Since the hole injection barrier gradually reduced as the doped layer developed, the amount of injected holes increased significantly and the recombination zone was shifted toward the center of the emissive layer with time. Finally, after the doped layers were well established (t > 200 min), the injection efficien-cies of hole and electron remained relatively unchanged and the recombination zone position was stabilized.
Time-dependent EQE of Device III under 2.5 V during 10 h operation is shown in the inset ofFig. 8. A transient EQE peak at the beginning (t < 200 min) was also observed in Device III and decrease of device efficiency was accom-panied accordingly by moving of recombination zone
(Fig. 8). Since the recombination zone was moving while
the doped layers were extending, these results also indi-cate exciton quenching in recombination zone due to extension of doped layers. However, a dip followed by a slight recovery in EQE shortly after the bias was just ap-plied was observed for Device III (3–20 min,Fig. 8). This abnormal phenomenon may be attributed to altered dis-tance between the recombination zone and the frontier of p-type doped layer at the beginning of operation. Com-pared to non-ionic mCP layer, the ionic mobility would be higher in ionic emissive layer and the formation speed of doped layer would also be higher in the emissive layer. Thus, shortly after the bias was applied, the speed for the p-type doped layer to extend into the center of the emis-sive layer would be higher than the speed for the recombi-nation zone to move away from the anode, which would
Fig. 7. Simulated (solid symbol) and measured (open symbol) EL spectra of Device III at (a) 12, (b) 23, (c) 58 and (d) 175 min after a bias of 2.5 V was applied. The recombination zone position (zi) estimated from fitting of simulated and measured EL spectra is shown in each subfigure.
mainly result from enhanced hole injection efficiency due to formation of the p-type layer in mCP layer. The distance between the recombination zone and the frontier of p-type doped layer would reduce in consequence and significant exciton quenching would take place, leading to a rapid drop in EQE during the initial 3–6 min (Fig. 8). As the hole injection efficiency enhanced due to gradually formed p-type doped layer in mCP layer, the recombination zone would move away from the anode at an increased speed. It would increase the distance between the recombination zone and the frontier of p-type doped layer. Thus, it would result in mitigated exciton quenching and recovery in device efficiency at 6–20 min after a bias was applied. Sub-sequent decrease in device efficiency would be attributed to exciton quenching in recombination zone induce by fur-ther extension of doped layers. The steady-state EQE of De-vice III is 1.2%, which is lower than that of DeDe-vice I (1.8%). The reason is similar to that mentioned for Device II, which also exhibited a lower EQE than Device I. As shown in
Fig. 8, the stabilized recombination zone of Device III lo-cated at 273 nm away from the cathode. Compared to De-vice I, which showed a stabilized recombination zone at 250 nm away from the cathode, the recombination zone position for Device III was closer to the p-type doped layer and thus resulted in severer exciton quenching, rendering lowered device efficiencies. These results reveal that inserting a transporting layer to impede carrier injection is effective in altering carrier balance of LECs and device efficiencies can thus be modified.
4. Conclusions
In summary, we have demonstrated a novel method to dynamically monitor the evolution of recombination zone position of an operating sandwiched LEC by employing microcavity effect. With this tool, the recombination zone of CTMC-based LECs doped with a low-gap carrier trapper has been shown to be closer to the anode as compared to neat-film devices, leading to severer exciton quenching and deteriorated device efficiencies. Similarly, adding a
hole-transporting layer with a high ionization potential to impede hole injection also resulted in a recombination zone closer to the anode. Exciton quenching took place in the recombination zone in the proximity of the anode and lower device efficiencies were obtained as well. These results provide direct experimental evidence to confirm that carrier balance of LECs can be modified by adjusting carrier transport or carrier injection. Furthermore, micro-cavity effect has been shown to be useful in extracting time-dependent evolution of recombination zone of sand-wiched LECs, in which direct measurement of recombina-tion zone posirecombina-tion would be extremely difficult. The method proposed in this work would be a powerful tool for studying carrier balance of LECs.
Acknowledgement
The authors gratefully acknowledge the financial sup-port from the National Science Council of Taiwan. References
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