Chapter 4. Conjugated polyelectrolytes bearing various ion densities: spontaneous dipole
4.3 Results and discussion
Figure 4.1 shows the device schematics of iPLEDs and iPSCs and the chemical structures of a series
of multiply charged CPEs (MPBs) containing cationic quaternary ammonium side-chains with tetrakis (1-imidazolyl) borate (BIm4) counter-ions. Each device consists of indium tin oxide (ITO) as a transparent electrode, ZnO as an electron-transport layer, a CPE as an interfacial layer, molybdenum oxide (MoO3) as a hole-transport layer, and an anode, which is composed of gold (Au) or silver (Ag) in an iPLED or iPSC, respectively. Super yellow (SY, Merck Co. Ltd) as a light-emissive material and a mixture of poly((4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-(3-fluoro-2-((2- ethylhexyl)carbonyl)thieno[3,4-b]thiophenediyl)) (PTB7, 1-Materials Inc.) and [6,6]-phenyl-C71- butyric acid methyl ester (PC71BM) as light-absorbing materials were used in the iPLEDs and iPSCs, respectively.
Figure 4.1b depicts the chemical structures of the MPB-series polymers,where the ion density of the CPE was systematically controlled from 2 (MPB2) to 6 (MPB6) by modulating the number of ionic side-chains per RU. The synthesis of neutral precursor polymers (number-average molecular weight, Mn = 10–16 KDa) was reported in our previous paper5. The neutral polymers were quaternized in ~90%
yield by reaction with trimethylamine in a mixture of tetrahydrofuran and methanol to produce a series of cationic CPEs with quaternary alkylammonium bromide side-chains. Finally, bromide counter- anions were exchanged by BIm4 using excess sodium tetrakis(1-imidazolyl) borate through dialysis to yield MPB-series CPEs (see the detailed procedures in the experimental section). All of the MPB polymers had the same fluorene-phenylene-based conjugate backbone. The different numbers of ionic side-chains on their peripheries negligibly influenced their electronic structures, because the ionic terminal groups were not conjugated to their main chains with insulating alkyl spacers. As shown in Figure 4.2, the MPBs exhibited almost identical UV-vis absorption (with absorption maxima at λabs = 380 nm) and fluorescence spectra (with emission maxima at λem = 420 nm) in water. The molecular weights and optical properties of the MPBs are summarized in Table 4.1.
Figure 4.1. Schematics of iPLEDs and iPSCs and chemical structures of MPBs. (a) Device architectures of (i) iPLEDs and (ii) iPSCs with MPBs. (b) Chemical structures of MPBs bearing different ion densities.
Figure 4.2. UV-vis absorption (solid lines) and PL (dash lines) spectra of CPEs dissolved in water.
Table 4.1. Summary of photophysical properties of CPEs.
CPEs Mn λabs λPL
MPB2 11.9 380.0 422.0
MPB3 15.6 382.0 420.0
MPB4 12.3 378.5 418.4
MPB5 16.6 380.0 414.8
MPB6 15.5 378.5 412.8
First, modulation of the interfacial energy levels was studied with a series of multi-charged CPEs by measuring VOC for a sandwich-type device with the structure ITO/ZnO/CPE/SY/MoO3/Au. A schematic of the ionic group distribution within the CPE layer and the energy level diagrams of ZnO/CPE/SY before and after electric poling are provided in Figure 4.3a-c. The hydrophilic BIm4 anions are located near the surface of ZnO, and the hydrophobic polymer backbone bearing cationic pendant groups is
located next to the SY layer without electric poling (Figure 4.3a), leading to spontaneous negative dipole formation. The conduction band (CB) of ZnO shifted toward the vacuum level due to the spontaneous negative dipole formation, which can be confirmed by measuring VOC based on the difference between the CB of ZnO and the highest occupied molecular orbital (HOMO) of SY. To confirm the dipole generation by spontaneous polarization of CPE on ZnO, we measured the contact angles of ZnO/Si with and without MPBs. The contact angle measurements showed the surfaces of MPBs/ZnO (~45°) are more hydrophobic than that of pristine ZnO surface (25°) (Figure 4.4), which indicates that the hydrophilic BIm4 ions are located on the surface of ZnO and the hydrophobic CPE polymer backbone is located at the top surface. The negative interfacial dipole strength increases with increasing CPE ion density (when the CPE is 2 nm thick), thereby enhancing VOC (Figure 4.3b). VOC
was measured to be 1.1, 1.25, 1.35, 1.45, and 1.50 V for the devices with MPB2, MPB3, MPB4, MPB5, and MPB6, respectively. To directly characterize the change of work function (WF), we measured ultraviolet photoelectron spectra of ZnO with and without MPBs. As the ion density of CPEs increased from MPB2 to MPB6, the WF of CPE deposited ZnO layers gradually up-shifted toward the vacuum level, showing WF = 3.68, 3.48, 3.36, 3.27 and 3.16 eV (see WF of ZnO: 4.01 eV) because of the enhanced spontaneous dipole generation (Figure 4.5). Fine tuning of the interfacial energy barrier was successfully achieved by utilizing the spontaneous polarization in the CPE layer.
Figure 4.3. Schematic energy diagrams and VOC measurements of iPLEDs with CPEs. (a) Schematic energy diagrams of iPLEDs with CPEs. (b) VOC measurements of iPLEDs with and without MPBs (2 nm). (c) Schematic energy diagrams of iPLEDs with CPEs after (i) negative poling and (ii) positive poling.
Figure 4.4. Contact angles of ZnO/Si and MPBs/ZnO/Si.
Figure 4.5. (a) Ultraviolet photoelectron spectra of ZnO with and without MPB series in the range from 17 eV to 18.5 eV. (b) Work function of ZnO with and without MPB series.
Next, we attempted to control the redistribution of ionic moieties by applying external electric field (poling). Negative bias (-3 V) or positive bias (3 V) was applied to the SY side to achieve negative or positive poling, respectively, and VOC was subsequently measured to investigate the interfacial energy level alignments (Figure 4.3c). The poling effect was tested with 2- and 10-nm-thick CPE interfacial
layers, and the poling effect was clearly observable when the CPE interlayer was 10 nm thick. Negative poling caused VOC to increase, indicating the redistribution of ionic groups with more BIm4 anions on the ZnO side, thereby enhancing the negative interfacial dipole moment (Figure 4.6). After negative poling, VOC increased by 0.1–0.15 V for the devices with MPB interlayers. The VOC variation for the devices with MPB6 was slightly larger than it was for the other devices, but the poling effect did not depend significantly on the ion density in the case of negative poling since negative dipoles had already formed spontaneously.
We expect that this feature may result from the mobile BIm4 mainly being redistributed by poling, unlike alkylammonium ions that are chemically connected to the polymer backbone. On the contrary, positive poling decreased VOC significantly due to the reorientation of the BIm4 anions toward the SY side, decreasing the negative dipole moment at the interface. The significant VOC difference between before and after positive poling occurred for two main reasons (Figure 4.7): 1) negative dipoles were already formed by the preferred orientation even before poling and 2) hole accumulation at the CPE/SY interface under positive bias facilitated ion migration by creating an additional electric field.[4,9] The VOC values after positive poling were decreased by 0.2, 0.2, 0.4, 0.5, and 0.6 V compared to those before poling for the devices with MPB2, MPB3, MPB4, MPB5, and MPB6, respectively. The effect of positive poling noticeably increased with increasing ion density in the MPB. When the CPE interlayer was thin (2 nm), no difference in VOC could be discerned following either negative or positive poling (Figure 4.8), probably due to the very limited space being insufficient for charge redistribution.
We also measured the current density–voltage (J-V) characteristics. Neither VOC nor J was changed by electric poling in the iPLEDs with 2-nm-thick MPB2 (Figure 4.8a,b), which indicates that the ions do not appear to migrate when bias is applied. When the CPE film is thin, the permanent dipole model is suitable for explaining the interfacial energy level adjustment and electron injection barrier reduction (Figure 4.7a) 36,37.The energy barrier is determined by the energy level difference between the CB of ZnO and lowest unoccupied molecular orbital (LUMO) of SY. The shift of the CB of ZnO toward the vacuum level due to the spontaneous negative dipole creation of the CPE reduces the electron injection barrier. In this case, the energy barrier can be gradually decreased with increasing ion density in the MPB, due to the formation of stronger spontaneous negative dipoles. As the MPB ion density increases, the electron current densities of electron-only devices (ITO/ZnO/CPE/SY/LiF/Al) gradually increase (Figure 4.9).
Figure 4.6. VOC measurements of iPLEDs with MPBs (10 nm) before and after poling. VOC
measurements of iPLEDs with (a) MPB2, (b) MPB3, (c) MPB4, (d) MPB5, and (e) MPB6. (f) VOC of iPLEDs with MPBs before and after poling.
Figure 4.7. (a) Schematic energy diagram for flat band conditions at the ZnO/SY interface with thin CPE film. (b) Schematic energy diagrams at the ZnO/SY interface with thick CPE film under positive voltage (i) without ion redistribution and hole accumulation, (ii) with ion redistribution and without hole accumulation, and (iii) with ion redistribution and hole accumulation.
Figure 4.8. VOC measurements and J-V characteristics of iPLEDs with different thicknesses of MPB2.
(a) VOC measurements and (b) J-V characteristics of iPLEDs with 2-nm-thick MPB2 before and after poling. (c) VOC measurements and (d) J-V characteristics of iPLEDs with 10-nm-thick MPB2 before and after poling.
Figure 4.9. (a) Schematic energy diagram of iPLEDs with 2-nm thick MPBs. (b) electron-only current densities of ITO/ZnO/SY/LiF/Al with and without 2-nm-thick MPBs.
When the CPE interlayer is thick, under a poling field, the ion redistribution should be considered to determine the interfacial energy levels and the resulting energy barrier for charge injection in iPLEDs.
The energy barrier is determined by the energy level difference between the CB of ZnO and LUMO of the CPE because the electrons cannot tunnel directly from ZnO to SY through thick CPE film. The redistribution of the internal field by the combined effect of ion reorientation and hole accumulation under positive bias reduces the electron tunneling distance (Figure 4.7b), which reduces the effective energy barrier4,7,9.J increased after positive poling and decreased after negative poling in the iPLEDs with 10-nm-thick MPB2 because the effective energy barrier was adjusted by the redistribution of the internal field (Figure 4.8d). When the CPE is thick, the energy barrier can be further precisely controlled with MPBs bearing different ion densities by electric poling. J increased more after positive poling and decreased after negative poling in the devices with MPBs bearing higher ion densities (Figure 4.10).
Furthermore, the J-V curves of the devices with MPBs bearing higher ion densities exhibited larger hysteresis during forward and reverse scans, also supporting ion migration in thick CPE film. Current density in the reverse scan is higher than that in the forward scan because the devices are positively poled during the forward scan. At the point where the maximum difference in the current density was measured in the forward and reverse scans, the current density in the reverse scan shows 1.14, 1.17 and 1.22 times larger than that in the forward scan for devices with MPB2, MPB4 and MPB6.
Figure 4.10. J-V characteristics of iPLEDs with 10-nm-thick (a) MPB2, (b) MPB4, and (c) MPB6 before and after poling. J-V characteristics of iPLEDs with 10-nm-thick (d) MPB2, (e) MPB4, and (f) MPB6, measured successively in the forward and reverse directions.
The fine modulation of interfacial energy levels could be very useful for further optimization of multi- layered organic electronic devices by improving their charge injection and extraction properties. First,
we fabricated iPLEDs with 10-nm-thick CPE interlayers (MPB2–MPB6). To investigate the effect of the ion density on the device performance before and after poling, the device characteristics of the iPLEDs were investigated, and the results are presented in Figure 4.11. The luminance (L) and efficiencies of the iPLEDs with MPBs are remarkably higher than those without CPEs, which is attributed to the reduction of the energy barrier for electron injection with blocking holes41. Moreover, the L and efficiencies of the iPLEDs with MPBs gradually increase with increasing MPB ion density (Figure 4.11a,b). The maximum L and external quantum efficiency (EQE) are remarkably improved (42,100–62,600 cd m-2 and 2.91%–5.29%, respectively) in the devices with MPBs compared to those in the device without MPB interlayers (5,020 cd m-2 and 0.22%, respectively). The luminous and power efficiencies (LE and PE) increase from (7.11 cd A-1 and 4.25 lm W-1) with MPB2 to (12.8 cd A-1 and 6.96 lm W-1) with MPB6. The resulting EQE exhibits a strong dependence on the ion density of the MPB interlayers and was found to be 2.91%, 3.45%, 4.28%, 4.67%, and 5.29% with MPB2, MPB3, MPB4, MPB5, and MPB6, respectively. The best iPLED device with MPB6 exhibited a maximum L of 62,600 cd m-2, a LE of 12.80 cd A-1, and an EQE of 5.29% (Table 4.2). A similar tendency was also measured for the devices with 2-nm-thick MPB interlayers (Figure 4.12 and Table 4.3), which is attributed to spontaneous negative dipole formation. However, the devices with 10-nm-thick MPBs showed higher device performances than those with 2-nm-thick MPBs, which may have occurred for several reasons. A thick interlayer effectively blocks holes, improving the recombination yield and reducing exciton quenching at the interface between ZnO and SY. In addition, thick MPBs can be poled under external applied bias, which further enhances the electron injection and device efficiencies.
However, the effects of poling (negative or positive) on the performances of the iPLEDs with 10-nm- thick MPBs could not be measured because the devices were positively poled spontaneously during measurement (It takes a long time to measure device performance due to limitation of scan rate, which was the limit of the spectroradiometer (Konica Minolta Co., CS-2000)). Instead, we measured the time responses of the J and L of the iPLEDs with 10-nm-thick MPBs at a constant voltage of 6 V for 3 min.
(Figure 4.13). As the MPB ion density increases, J and the L increase more significantly over time under positive bias. This characteristic indicates that the effective energy barrier gradually decreased with increasing MPB ion density due to enhanced ion migration. The J-V characteristics of the iPLEDs with 2- and 10-nm-thick MPBs are shown in Figure 4.11c and Figure 4.12. J was measured to be smaller for the devices with MPBs than for those without MPBs in the low voltage regime because the leakage current was reduced by passivating defect sites of ZnO with CPEs. The passivation of defects on the ZnO surface was confirmed by performing time-resolved and steady-state photoluminescence (PL) measurements (Figure 4.14 and Table 4.4). Quartz/ZnO/MPB6/SY yielded both a higher PL intensity and longer exciton lifetime (0.41–0.44 ns) than quartz/ZnO/SY (0.35 ns), confirming that MPB6 clearly reduces the exciton quenching caused by ZnO defect sites. The passivation effect also
contributes to the enhanced L and efficiencies observed when MPB interlayers were present.
Figure 4.11. Performances of iPLEDs with MPBs and J-V characteristics of iPSCs with MPB2, MPB4, and MPB6 before and after poling. (a) L-V characteristics, (b) LE-J characteristics and (c) J-V characteristics of iPLEDs with MPBs (10 nm). J-V characteristics of iPSCs with 5-nm-thick (d) MPB2, (e) MPB4, and (f) MPB6 before and after poling.
Table 4.2. Summary of device performances of iPLEDs with MPBs and iPSCs with MPB2, MPB4, and MPB6 before and after poling.
Device configuration (iPLEDs)
LMax
[cd/m2]
@ bias
LEMax
[cd/A]
@ bias
PEMax
[lm/W]
@ bias
EQE Max
[%]
@ bias
Turn- on voltage
[V]
@ 0.1 cd/m2 ITO / ZnO / SY / MoO3 / Au
(Ref.) 5,020 (10.2 V) 0.59 (10.2 V) 0.18 (10.2 V) 0.22 (10.2 V) 5.4
ITO / ZnO / MPB2 (10 nm) / SY
/ MoO3 / Au 42,100 (11.0 V) 7.11 (6.2 V) 4.25 (4.2 V) 2.91 (6.2 V) 2.6 ITO / ZnO / MPB3 (10 nm) / SY
/ MoO3 / Au 54,500 (10.8 V) 8.06 (6.6 V) 4.30 (4.8 V) 3.45 (6.6 V) 2.6 ITO / ZnO / MPB4 (10 nm) / SY
/ MoO3 / Au 57,400 (11.0 V) 9.89 (6.6 V) 5.14 (4.8 V) 4.28 (6.6 V) 2.6 ITO / ZnO / MPB5 (10 nm) / SY
/ MoO3 / Au 53,500 (11.2 V) 10.46 (7.4 V) 5.48 (5.0 V) 4.67 (6.4 V) 2.6 ITO / ZnO / MPB6 (10 nm) / SY
/ MoO3 / Au 62,600 (11.2 V) 12.80 (6.2 V) 6.96 (4.8 V) 5.29 (6.2 V) 2.6
Device configuration (iPSCs) Poling JSC
(mA/cm2)
VOC
(V) FF η
(%) ITO / ZnO / PTB7:PC71BM / MoO3
/ Ag No 15.88 0.69 0.65 7.12
ITO / ZnO / MPB2 (5 nm) / PTB7:PC71BM / MoO3 / Ag
No 16.78 0.73 0.70 8.57
Negative 16.81 0.73 0.70 8.59
Positive 16.69 0.72 0.70 8.40
ITO / ZnO / MPB4 (5 nm) / PTB7:PC71BM / MoO3 / Ag
No 16.89 0.74 0.70 8.75
Negative 16.97 0.74 0.71 8.92
Positive 16.74 0.72 0.69 8.32
ITO / ZnO / MPB6 (5 nm) / PTB7:PC71BM / MoO3 / Ag
No 16.97 0.74 0.70 8.79
Negative 17.04 0.74 0.71 8.95
Positive 16.74 0.71 0.70 8.32
Figure 4.12. Performances of iPLEDs with 2-nm-thick MPBs. (a) J-V characteristics, (b) L-V characteristics, and (c) LE-J characteristics of iPLEDs with 2-nm-thick MPBs.
Table 4.3. Summary of device performances of iPLEDs with 2-nm-thick MPBs.
Device configuration (iPLEDs)
LMax [cd/m2]
@ bias
LEMax
[cd/A]
@ bias
PEMax
[lm/W]
@ bias
EQEMax
[%]
@ bias
Turn- on voltage
[V]
@ 0.1 cd/m2 ITO / ZnO / MPB2 (2 nm) / SY /
MoO3 / Au 39,500 (10.4 V) 6.14 (7.8 V) 2.70 (6.6 V) 2.45 (7.6 V) 2.6 ITO / ZnO / MPB3 (2 nm) / SY /
MoO3 / Au 42,500 (10.2 V) 6.32 (7.0 V) 3.17 (6.2 V) 2.57 (6.8 V) 2.6 ITO / ZnO / MPB4 (2 nm) / SY /
MoO3 / Au 42,500 (10.0 V) 6.50 (7.0 V) 3.23 (6.2 V) 2.64 (6.8 V) 2.6 ITO / ZnO / MPB5 (2 nm) / SY /
MoO3 / Au 46,200 (10.2 V) 6.76 (7.8 V) 3.08 (6.2 V) 2.73 (7.4 V) 2.6 ITO / ZnO / MPB6 (2 nm) / SY /
MoO3 / Au 53,600 (10.0 V) 7.09 (7.6 V) 3.39 (6.4 V) 2.84 (7.0 V) 2.6
Figure 4.13. Time responses of (a) J and (b) L of iPLEDs with 10-nm-thick MPBs.
Figure 4.14. Exciton lifetime and PL intensity. (a) Time-resolved PL spectra of quartz/ZnO/SY with and without MPB6 (2- and 10-nm-thick), as measured at an emission wavelength of 545 nm with 450 nm excitation. (b) PL spectra of quartz/ZnO/SY with and without MPB6 (2- and 10-nm-thick) at 470 nm excitation.
Table 4.4. Summary of exciton lifetimes of SY on ZnO with and without different thicknesses of MPB6.
Film configuration τAvg [ns] χ2
Quartz / ZnO / SY 0.348 1.222
Quartz / ZnO / MPB6 (2 nm) / SY 0.410 1.294
Quartz / ZnO / MPB6 (10 nm) / SY 0.442 1.252
The device characteristics of the iPSCs containing 2-nm-thick CPEs with various ion densities were reported in our previous publication5. The power conversion efficiencies (PCEs) with CPEs are substantially improved compared to those without CPEs, and the PCE gradually increases with increasing CPE ion density due to stronger spontaneous negative dipole formation. To investigate the poling effect of the MPB ion density on the device performances of the iPSCs, we measured the J-V characteristics of PTB7:PC71BM-based iPSCs using 5-nm-thick CPE interlayers (ITO/ZnO/CPE (5 nm)/PTB7:PC71BM/MoO3/Ag) before and after electric poling (Figure 4.11d–f). The device performances of the iPSCs with MPBs were higher than those without MPBs due to spontaneous negative dipole formation (even before poling). For all of the devices with MPB interlayers (MPB2, MPB4, and MPB6), the device performance was clearly improved by negative poling (compared to positive poling) with concomitant VOC and short-circuit current density (JSC) enhancements, resulting in increased PCE. The additional shift of the CB of ZnO toward the vacuum level due to stronger negative dipole formation (by negative poling) enhanced the built-in potentials of the iPSCs with MPBs, facilitating the charge extraction and increasing VOC. The JSC and VOC values measured following
negative poling are substantially higher than those measured following positive poling (Table 4.2). The PCEs increased to 8.59%, 8.92%, and 8.95% after negative poling and 8.40%, 8.32%, and 8.32% after positive poling for the devices with MPB2, MPB4, and MPB6, respectively. However, the poling effect is not large because of their thinness. To clarify the poling effect on the performances of the iPSCs, devices with 8-nm-thick CPE layers were also prepared. The PCEs of the iPSCs with 8-nm-thick MPBs are lower than those with 5-nm-thick MPBs due to poor electron extraction (due to the difficultly of charge tunneling when the CPE interlayer is thick), whereas the poling effect is more significant than it is for the 5-nm-thick MPBs (Figure 4.15 and Table 4.5). Negative poling yielded JSC, VOC, and fill factors clearly improved relative to those measured following positive poling. The PCEsafter negative (and positive) poling were determined to be 6.36% (5.74%), 6.56% (5.86%), and 6.92% (5.78%) for the devices with MPB2, MPB4, and MPB6, which clearly indicates that the poling effect increased with increasing CPE ion density. Moreover, the time responses of the J of the iPSCs with 10-nm-thick MPBs were measured over time at a constant bias of -3 V to confirm the effect of poling on the charge extraction capability (Figure 4.16). J was measured to increase and saturate over time, suggesting the ionic redistribution in CPEs under applied electric field. The J value increased with increasing CPE ion density and was the highest with MPB6, indicating that the charge extraction became more efficient as the charge extraction barrier decreased via efficient ionic redistribution with increasing ionic density.