Chapter 5. Amine-based passivating materials for enhanced optical properties and
5.3 Results and Discussion
Figure 5.1a,b presents a schematic illustration of the device design and a cross-sectional scanning electron microscopy (SEM) image of a PeLEDs. An energy level diagram of a PeLEDs and the chemical structures of the two APMs showing their different molecular sizes are depicted in Figure 5.1c,d. The APMs contain amine groups, including lone pairs, which can passivate under-coordinated lead ions by donating electrons. Schematic illustrations of MAPbBr3 with and without APMs are helpful for understanding the passivation mechanism (Figure 5.2). There are many under-coordinated lead ions in MAPbBr3 because MAPbBr3 is crystallized through a solution process, and bromide and methylammonium ions can be lost from the MAPbBr3 during post-deposition thermal annealing process.
Bromide ion vacancies result in positively charged under-coordinated lead ions; these unintentional defects can be passivated through coordinate bonding with the nitrogen atoms of amine groups, resulting in charge neutralization and a consequent reduction in the number of electronic trap sites.
Figure 5.1. Device schematic, cross-sectional image and energy level diagram of a PeLEDs as well as the chemical structures of the APMs. (a) Schematic illustration of the device structure for the PeLEDs with APMs. (b) Cross-sectional scanning electron micrograph of a PeLEDs. (c) Energy levels of the components of a PeLEDs. (d) Chemical structures of the APMs: (i) EDA and (ii) PEI.
Figure 5.2. Schematic illustrations of MAPbBr3 with and without APMs. (a) Schematic of bromide ion vacancies on the surface of MAPbBr3 without APMs. (b) Schematic of APMs on the surface of MAPbBr3 passivated with APMs.
To investigate the number of existing amine groups on the surface of the MAPbBr3 passivated with APMs and the penetration depth of the APMs into the MAPbBr3, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed (Figure 5.3a,b). The number of existing amine groups on the surface of PEI-passivated MAPbBr3 was larger than that on EDA-passivated MAPbBr3. However, the effective passivation of the amine groups of EDA on the MAPbBr3 surface and the passivation depth of EDA in the MAPbBr3 film were superior to those of PEI because the relatively small molecules could more effectively passivate the top surface of the MAPbBr3 and penetrate more deeply; schematic illustrations of the penetration of the two APMs into MAPbBr3 are shown in Figure 5.3c,d.
Figure 5.3. Negative ion TOF-SIMS spectra, depth profiles, and schematic illustrations of the depth penetration of APMs deposited on MAPbBr3. (a) TOF-SIMS spectra of CN- ions in MAPbBr3 with and without APMs. (b) Depth profiles of CN- ions in MAPbBr3 with and without APMs. (c) Schematic illustration of the depthwise penetration of PEI into MAPbBr3. (d) Schematic illustration of the depthwise penetration of EDA into MAPbBr3.
Figure 5.4 show the improvement of optical properties of MAPbBr3 with APMs, which are strong evidence for the defect passivation of MAPbBr3. MAPbBr3 materials passivated with APMs showed long PL lifetimes and high steady-state PL intensities compared with MAPbBr3 without APMs (Figure 5.4a,b and Table 5.1), indicating the suppression of non-radiative recombination through a reduction in the number of electronic trap sites. In particular, EDA-passivated MAPbBr3 showed a longer PL lifetime and a higher PL intensity compared with PEI-passivated MAPbBr3, which suggests that EDA passivates the defect sites in MAPbBr3 more effectively than PEI does. We also measured the amplified spontaneous emission (ASE) of the MAPbBr3 materials with and without EDA to confirm the improvement of the optical properties of the MAPbBr3 upon treatment with EDA. The MAPbBr3 with EDA showed a lower threshold for ASE (10.6 J cm-2) and a smaller full width at half maximum (FWHM) of its ASE (3.60 nm) than those (28.6 J cm-2, 4.63 nm) of the MAPbBr3 without EDA; these findings indicate a reduction in optical loss caused by electronic trap sites with EDA treatment. The normalized PL spectra of the MAPbBr3 with APMs are slightly blue-shifted compared with that of MAPbBr3 without APMs (Figure 5.5), and they show sharp band-edge emission because of the passivation of shallow trapping levels44. However, these slight peak shifts cannot be attributed to any structural change in the MAPbBr3 with APMs because the X-ray diffraction (XRD) patterns of MAPbBr3 with and without APMs are nearly identical (Figure 5.6).
Figure 5.4. Time-resolved, steady-state PL spectra and ASE measurement of MAPbBr3 with and without APMs. (a) Time-resolved PL spectra for MAPbBr3 with and without APMs. (b) Steady-state PL spectra of MAPbBr3 with and without APMs. (c) PL intensities and FWHM of MAPbBr3 without APMs at various excitation intensities. (d) PL spectra of MAPbBr3 without APMs at various excitation intensities illustrating the transition from spontaneous emission to ASE. (e) PL intensities and FWHM
of MAPbBr3 with EDA at various excitation intensities. (f) PL spectra of MAPbBr3 with APMs under at various excitation intensities illustrating the transition from spontaneous emission to ASE.
Table 5.1. Summarized PL lifetime of MAPbBr3 with and without APMs.
Film configuration avr [ns] 2
Glass / PEDOT:PSS / MAPbBr3 13.8 1.112
Glass / PEDOT:PSS / MAPbBr3 / PEI 26.7 1.078
Glass / PEDOT:PSS / MAPbBr3 / EDA 62.5 1.001
Figure 5.5. Normalized PL spectra of MAPbBr3 with and without APMs.
Figure 5.6. Normalized XRD patterns of MAPbBr3 with and without APMs.
Many scientists have reported a blinking phenomenon in hybrid organic-inorganic perovskites, which limits the applicability of PeOEDs. Although several mechanisms to explain blinking phenomenon have been proposed, but the details are not yet fully understood. Several researchers have suggested that the blinking phenomenon can be attributed to non-radiative Auger recombination between photogenerated electron-hole pairs and charges captured by electronic trap sites (Figure 5.7a) 20,21,45. Other researchers have reported a mechanism such that the blinking phenomenon is attributed to the photo-induced activation and deactivation of electronic trap sites associated with defect sites acting as PL quenchers (Figure 5.7b) 23,25,46. Both mechanisms suggest that the blinking phenomenon is closely related to electronic trap sites; therefore, the reduction of electronic trap sites through defect passivation is crucial for blinking suppression. To investigate the passivation effect of the studied APMs on the PL blinking of MAPbBr3, we acquired confocal PL images and PL intensity trajectories of the MAPbBr3 materials with and without APMs over time (Figure 5.8). The confocal PL images of the MAPbBr3 without APMs revealed large dark regions dominated by non-radiative recombination, whereas the confocal PL images of the MAPbBr3 materials passivated with APMs showed a reduction of the dark regions and
an increase in brightness, with almost full coverage in the case of EDA. (Figure 5.8a-c). Moreover, the MAPbBr3 without APMs showed a noticeable deviation in its PL intensity as a function of time, whereas the MAPbBr3 materials passivated with APMs showed reduced (PEI) or no (EDA) deviation of their PL intensities (Figure 5.8d-f). Vigorous PL blinking can be seen in the MAPbBr3 without APMs as a result of non-radiative recombination in the presence of electronic trap sites, whereas no PL blinking can be clearly observed in the EDA-passivated MAPbBr3, which unequivocally confirms the effective passivation of electronic trap sites due to EDA treatment.
Recently, deQuilettes et al. and Mosconi et al. have reported that perovskites with higher trap densities exhibit more photo-induced PL enhancement because of the reduction in the trap density caused by halide migration47,48. To observe the electronic trap densities of the MAPbBr3 materials with and without APMs, we measured the variations in their PL intensities over time both under illumination and in the dark (Figure 5.9a,b). The MAPbBr3 without APMs showed a significant increase in PL intensity compared with those of the MAPbBr3 materials with APMs, indicating that the MAPbBr3
materials passivated with APMs had a lower electronic trap density than did the MAPbBr3 without APMs. Moreover, the higher initial PL intensity of the MAPbBr3 passivated with APMs compared with that of the photo-stabilized MAPbBr3 without APMs demonstrates the effective passivation capability of the APMs. When the illumination was removed, the photo-enhanced PL intensity of the MAPbBr3
without APMs rapidly relaxed within hours (Figure 5.9b), which indicates that the photo-induced enhancement is reversible. By contrast, the MAPbBr3 materials passivated with APMs showed almost no change in their PL intensities over time, as confirmed by the confocal microscopy images (Figure 5.9c and Figure 5.10); these findings indicate that the defect passivation induced by the APMs was maintained over a long duration. We believe that the sustainability of the APMs effects can be attributed to a strong interaction between the nitrogen atoms and the under-coordinated lead ions, such as coordinate bonding. Therefore, this method of passivating the defect sites in MAPbBr3 using EDA is very simple, effective and long lasting.
Figure 5.7. PL blinking mechanism. (a) Non-radiative Auger recombination. (b) Trap-assisted non- radiative recombination.
Figure 5.8. Confocal PL images and variations in PL intensities observed over time for MAPbBr3
materials with and without APMs. (a) Confocal PL image of MAPbBr3 without APMs. (b) Confocal PL image of MAPbBr3 with PEI. (c) Confocal PL image of MAPbBr3 with EDA. (d) Variations in the PL intensities of MAPbBr3 without APMs. (e) Variations in the PL intensities of MAPbBr3 with PEI.
f) Variations in the PL intensities of MAPbBr3 with EDA.
Figure 5.9. Variations in PL over time under illumination, in the dark and after photo-stabilization. (a) PL intensities of MAPbBr3 with and without APMs over time under initial illumination. (b) PL intensities of MAPbBr3 with and without APMs in the dark after removing illumination. (c) PL intensities of MAPbBr3 with and without APMs over time after photo-stabilization.
Figure 5.10. Confocal PL images observed over time for MAPbBr3 materials with and without APMs after photo-stabilization. (a) Confocal PL image of MAPbBr3 with PEI at 0 min just after photo- stabilization. (b) Confocal PL image of MAPbBr3 with PEI at 296 min just after photo-stabilization. (c) Confocal PL image of MAPbBr3 with EDA at 0 min just after photo-stabilization. (d) Confocal PL image of MAPbBr3 with EDA at 296 min just after photo-stabilization.
Figure 5.11 and Table 5.2 show the current density and luminance versus voltage (J-V-L) characteristics and the device efficiencies of PeLEDs with and without optimized concentrations of APMs. The PeLEDs were optimized by testing different APMs concentrations. The optimized weight concentration for PEI was found to be 0.3 wt. %. A PEI layer with such a high concentration effectively passivates the defect sites in MAPbBr3, whereas a thicker insulating layer reduces electron injection
from the electron transport layer into the emissive layer, leading to unbalanced charge transport.
Moreover, the J-V-L characteristics and device efficiencies of PeLEDs with various concentrations of EDA are shown in Figure 5.12 and Table 5.3. A high-concentration EDA layer effectively passivates the defect sites in MAPbBr3, but a high concentration of EDA also results in partial melting of the MAPbBr3 crystal (Figure 5.13), which causes the crystal quality to deteriorate. Thus, we optimized the weight concentration of EDA to be 0.10 wt. %. At low voltages, the current densities of the PeLEDs with EDA were substantially reduced compared with that of the control device, indicating that EDA caused the leakage current to decrease as a result of the defect passivation of the MAPbBr3, as shown in Figure 5.11a. The luminance and device efficiencies of the PeLEDs with EDA were much higher than those of the control device, indicating effective defect passivation. The morphology of the MAPbBr3 passivated with APMs with the optimized APMs concentrations did not change compared with that of the MAPbBr3 without APMs, as shown in Figure 5.11d-f. The optimized EDA-passivated device achieved a luminance of 22,800 cd m-2 and an EQE of 6.2 %. The histogram of EQE for 15 devices with EDA are shown in Figure 5.14.
Figure 5.11. Device performance of PeLEDs and SEM top-surface images of MAPbBr3 with and without APMs. (a) Current density versus voltage (J-V) characteristics. (b) Luminance versus voltage (L-V) characteristics. (c) Luminous efficiency versus voltage (LE-V) characteristics. (d) EQE versus luminance (EQE-V) characteristics. (e) SEM top-surface image of MAPbBr3 without APMs. (f) SEM top-surface image of MAPbBr3 with PEI. (g) SEM top-surface image of MAPbBr3 with EDA.
Table 5.2. Summarized device performance of PeLEDs with and without APMs.
Device configuration (PeLEDs)
L max
[cd/m2]
@ bias
LE max
[cd/A]
@ bias
EQE max
[%]
@ bias
Turn-on voltage [V]
@ 0.1 cd/m2 ITO / PEDOT:PSS / MAPbBr3 / SPW-111 /
LiF / Ag
7,080 (4.8 V)
0.559 (4.8
V) 0.12 (4.8 V) 2.2 V ITO / PEDOT:PSS / MAPbBr3 / PEI / SPW-
111 / LiF / Ag
10,700 (4.8
V) 2.72 (4.8 V) 0.58 (4.8 V) 2.4 V ITO / PEDOT:PSS / MAPbBr3 / EDA / SPW-
111 / LiF / Ag
22,800 (3.4
V) 28.9 (3.0 V) 6.19 (3.0 V) 2.4 V
Figure 5.12. Device performance of PeLEDs with various concentration of EDA. (a) Current density versus voltage (J-V) characteristics. (b) Luminance versus voltage (L-V) characteristics. (c) Luminous
efficiency versus voltage (LE-V) characteristics. (d) EQE versus luminance (EQE-V) characteristics.
Table 5.3. Summarized device performance of PeLEDs with various concentration of EDA.
Device configuration (PeLEDs) L max [cd/m2]
@ bias
LE max [cd/A]
@ bias
EQE max [%]
@ bias
Turn-on voltage [V]
@ 0.1 cd/m2 ITO / PEDOT:PSS / MAPbBr3 / EDA (0.07 wt. %)
/ SPW-111 / LiF / Ag 17,600 (3.6 V) 19.8 (3.0 V) 4.24 (3.2 V) 2.4 V ITO / PEDOT:PSS / MAPbBr
3 / EDA (0.10 wt. %)
/ SPW-111 / LiF / Ag 22,800 (3.4 V) 28.9 (3.0 V) 6.19 (3.0 V) 2.4 V ITO / PEDOT:PSS / MAPbBr3 / EDA (0.12 wt. %)
/ SPW-111 / LiF / Ag 21,800 (3.4 V) 28.7 (3.0 V) 6.13 (3.0 V) 2.4 V ITO / PEDOT:PSS / MAPbBr3 / EDA (0.15 wt. %)
/ SPW-111 / LiF / Ag 19,400 (3.4 V) 8.51 (3.0 V) 1.82 (3.0 V) 2.4 V
Figure 5.13. SEM top-surface images of MAPbBr3 with and without various concentration of EDA. (a) SEM top-surface image of MAPbBr3 with 0.07 wt. % of EDA. (b) SEM top-surface image of MAPbBr3
with 0.10 wt. % of EDA. (c) SEM top-surface image of MAPbBr3 with 0.12 wt. % of EDA. (d) SEM top-surface image of MAPbBr3 with 0.15 wt. % of EDA.
Figure 5.14. Histograms of peak EQEs obtained from 15 devices (a) without APMs and with (b) PEI and (c) EDA.
The ionic defects that form in perovskite because of the low interaction energy lead to decomposition induced by moisture, light and heat as well as ion migration to adjacent layers or metal electrodes, resulting in poor stability of the material. In particular, the intrinsic instability of PeOEDs that is associated with ion migration from the perovskite layer cannot be eliminated by means of device encapsulation, and thus additional strategies are required for improving the intrinsic device stability, such as passivation of the perovskite to inhibit ion migration and modification of the components to enhance the interaction energy. We investigated the effect of the APMs on the corrosion resistance of Ag on MAPbBr3 and the device stability of PeLEDs under continuous operation. Figure 5.15a presents photographs of Ag on MAPbBr3 materials with and without APMs under ambient conditions over time without any encapsulation. Changes in the electrode colour indicate the corrosion of Ag. The Ag on MAPbBr3 without APMs showed a colour change within 1 day (24 h) and additional colour changes after 15 and 30 days, indicating that Ag on MAPbBr3 without APMs can be easily corroded. By contrast, the Ag on MAPbBr3 with APMs exhibited excellent stability, showing almost no colour change even after 30 days (720 h); these findings demonstrate that the APMs treatments effectively suppressed the corrosion of the electrodes. To investigate the specific origin of the corrosion of Ag on MAPbBr3, we performed X-ray photoelectron spectroscopy (XPS) and XRD analyses of the Ag on the MAPbBr3
materials with and without APMs after 30 days (Figure 5.15b,c). The Ag 3d spectra of pristine Ag and of the Ag on the MAPbBr3 materials with and without APMs show two peaks at 374.1 eV (3d3/2) and 368.1 eV (3d5/2) (Figure 5.15b). These two peaks can each be further divided into two additional peaks at 374.1 and 373.3 eV (3d3/2) and at 368.1 and 367.6 eV (3d5/2), respectively, where the 374.1 and 368.1
eV peaks are assigned to metal Ag0 and the 373.3 and 367.6 eV peaks are assigned to the Ag+ in AgBr49. The spectra of the Ag on the MAPbBr3 materials with APMs are nearly the same as that of the pristine Ag, whereas the spectrum of the Ag on the MAPbBr3 without APMs shows clear contributions from the peaks at 373.3 and 367.6 eV corresponding to the Ag+ in AgBr. These results are consistent with the XRD analysis (Figure 5.15c). The peaks at 38.1° and 31.0° correspond to the (111) crystal plane of Ag (JCPDS no. 04-0783) and the (200) crystal plane of AgBr (JCPDS no. 06-0438), respectively50. The XRD pattern of the Ag on the MAPbBr3 without APMs shows a clear signal from the (200) peak of AgBr in addition to the (111) peak of Ag, whereas the XRD patterns of the Ag on the MAPbBr3
materials with APMs show only the (111) peak of Ag, with no evidence of the (200) peak of AgBr. The XPS and XRD analyses indicate that the APMs treatments completely prevent electrode corrosion by inhibiting ion migration from the perovskite layer to the metal electrode because the APMs treatments effectively passivate the surface of the MAPbBr3 and block the ion migration paths, thereby improving the device stability. The luminances of encapsulated PeLEDs with and without APMs were measured at 20 mA cm-2 under ambient air conditions as functions of operation time (Figure 5.15d). The operational stability of the PeLEDs with APMs was found to be significantly better than that of the control device. The PeLEDs without APMs exhibited a sharp decrease to less than 10 % of the initial luminance after only 2,500 sec. By contrast, the PeLEDs with APMs retained over 70 % of their initial luminance after more than 14,000 sec.
Figure 5.15. Corrosion of Ag on MAPbBr3 and operational stability of PeLEDs with and without APMs.
(a) Photographs of Ag on MAPbBr3 materials with and without APMs under ambient conditions over time. (b) XPS spectra of Ag on MAPbBr3 materials with and without APMs after 30 days. (c) XRD patterns of Ag on MAPbBr3 materials with and without APMs after 30 days. (d) Normalized luminances of encapsulated PeLEDs with and without APMs under ambient conditions as functions of operation time.