Chapter 6. Growth of Nano-Sized Single Crystals for Efficient Perovskite Light-Emitting
6.3 Results and Discussion
The PeLED device architecture and corresponding cross-sectional scanning electron microscopy (SEM) image are shown in Figure 6.1a,c. MAPbBr3 films were fabricated by the anti-solvent dropping method. In this process, the PMA-containing chlorobenzene (CB) anti-solvent is dropped onto the precursor-coated substrate during spin-coating (Figure 6.1d). The anti-solvent immediately washes out the “good” solvent, a dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) mixture in which the precursors are highly soluble; this leads to the fast crystallization of MAPbBr3, which hampers the growth of highly crystalline films (Figure S1a). However, the bulky PMA ligand added to the CB lowers the critical free energy of nucleation and critical nucleus size by effectively reducing the nuclei surface energies, which enhances the nucleation of MAPbBr3. Moreover, PMA retards crystal growth by capping the MAPbBr3 nuclei (Figure 6.1b and Figure 6.2b) and leads to the growth of highly crystalline and small crystals. In contrast to the case where surface passivation is applied after film growth, our method not only grows highly crystalline crystals with few defects but also passivates the surface defects of the crystals.
Although heterogeneous nucleation may be a more reasonable description of our system, we use the equations for homogenous nucleation for simplicity. The free energy change of nucleation as a function of the particle radius r is given by
∆𝐺𝑟 = −4
3𝜋𝑟3∆𝐺𝑣+ 4𝜋𝑟2𝛾𝑆𝐿 (6.1)
Where ∆𝐺𝑣 and 𝛾𝑆𝐿 describe the free energy change associated with the liquid-to-solid transformation and the solid/liquid interfacial energy, respectively.
From this equation, the critical free energy of nucleation and critical nucleus size can be obtained.
The critical free energy of nucleation is given by
∆G∗ = 16𝜋𝛾𝑆𝐿3
3(∆𝐺𝑣)2 (6.2) The critical nucleus size is given by
r∗ =2𝛾𝑆𝐿
∆𝐺𝑣 (6.3)
Decreasing the surface energy of the nuclei means that the critical free energy of nucleation and critical nucleus size decrease. Nuclei can hence be more easily generated by reducing their surface energies.
The addition of PMA decreases the critical free energy of nucleation and critical nucleus size by reducing the nucleus surface energy. Furthermore, PMA retards crystal growth by capping the nuclei.
This combined effect not only effectively reduces the MAPbBr3 crystal size but also enhances its crystallinity; more cubic-shaped crystals were grown because the precursors had enough time to react with the favorable lattice points.
Figure 6.1. Schematic of the device structure, the chemical structure of PMA, cross-sectional SEM image, and the schematic of the anti-solvent dropping process with PMA. (a) The structure of the PeLED devices. (b) Chemical structure of PMA. (c) Cross-sectional SEM image of a PeLED device.
(d) Schematic of the MAPbBr3 film formation by the anti-solvent dropping process with PMA added to the chlorobenzene anti-solvent.
Figure 6.2. Schematic illustrations of the MAPbBr3 film formation with and without PMA in the anti- solvent at different temperatures. Film formation (a) without PMA at high temperatures, (b) with PMA at high temperatures, and (c) with PMA at low temperatures.
The morphologies of the MAPbBr3 films fabricated with various concentrations of PMA at different temperatures were observed using SEM (Figure 6.3a). The MAPbBr3 film fabricated without PMA showed large round polycrystalline grains. The MAPbBr3 film prepared with 0.25 vol.% PMA showed a better crystallinity, with smaller and more cubic-shaped crystals. For the optimized PMA content of 0.5 vol.%, the MAPbBr3 crystals were almost perfectly cubic shape with a further reduced size.
However, the MAPbBr3 film fabricated with 1.0 vol.% PMA showed additional crystal shapes, such as plates and rods. As the concentration of PMA increased further, the plate-like crystals gradually became dominant until the MAPbBr3 film was composed only of such crystals at 4.0 vol.% PMA. This is attributed to the formation of a 2D layered perovskite structure: the high concentration of the bulky amine meant that the 3D perovskite structure was limited to a 2D layered perovskite structure by steric hindrance (Figure 6.4). The 3D to 2D layered perovskite structural change was confirmed by X-ray diffraction (XRD) and absorption measurements (Figure 6.3b,c). The XRD peak at 14.9° and the band edge absorption at 520 nm correspond to 3D perovskite, and the XRD peaks at 5.28° and 10.56° and the absorption peak at 399 nm correspond to 2D layered perovskite. The MAPbBr3 films fabricated with
0 vol.% to ≤0.5 vol.% PMA showed only the XRD peak at 14.9° and the band edge absorption at 520 nm. In contrast, those fabricated with >0.5 vol.% PMA showed additional XRD peaks at 5.28° and 10.56° and a second absorption peak at 399 nm. As the concentration of PMA increases, the XRD peaks at 5.28° and 10.56° and absorption peak at 399 nm increase in intensity while the XRD peak at 14.9°
and absorption peak at 520 nm decrease, which indicates the structural change from the 3D to 2D layered perovskite. Finally, MAPbBr3 grown with 4.0 vol.% PMA showed only the XRD peaks at 5.28°
and 10.56° and absorption peak at 399 nm, which indicates the presence of only the 2D layered perovskite. In agreement with the XRD and absorption results, this 4.0 vol.% PMA MAPbBr3 showed a 404 nm photoluminescence (PL) emission due to the large Bandgap of the 2D perovskite (Figure 6.3d). Moreover, the morphologies of the MAPbBr3 films fabricated with different PMA concentrations at a low temperature (10~15°C) were also observed (Figure 6.5). These films possessed smaller crystals because crystal growth was further slowed at the low temperature (Figure 6.3a and Figure 6.2c), while their morphologies showed a similar trend according to PMA concentration to that of the high- temperature cases. The average crystal size of the MAPbBr3 film fabricated with 0.5 vol.% PMA reduced from 81.6 nm to 31.7 nm on decreasing the temperature (Figure 6.6).
Figure 6.3. SEM images, XRD patterns, and absorption and PL spectra of the MAPbBr3 films fabricated
with and without PMA. (a) SEM images of the MAPbBr3 films grown without PMA (Ref.) and with various PMA concentrations at high temperatures (20~25oC), and that grown with 0.5 vol.% PMA at a low temperature (10~15oC). (b) Normalized XRD patterns, (c) absorption spectra, and (d) normalized PL spectra of the MAPbBr3 films fabricated with 0–4.0 vol.% PMA at high temperatures.
Figure 6.4. Schematic illustrations of the different MAPbBr3 crystal shapes. Schematics of (a) the 3D cubic structure, (b) the 2D layered structure, and (c) the 3D rod-like structure.
Figure 6.5. SEM images of the MAPbBr3 films fabricated without PMA (Ref.) and with various PMA concentrations at low temperatures.
Figure 6.5. Variations in the grain size distributions of the MAPbBr3 films prepared with 0.5 vol.%
PMA at different temperatures. (a,b) SEM image and grain size distribution of the film fabricated at a high temperature. (c,d) SEM image and grain size distribution of the film fabricated at a low temperature.
The 0.5 vol.% MAPbBr3 crystals also were investigated by transmission electron microscopy (TEM).
These crystals were prepared by sonicating the MAPbBr3 film in CB and dispersing them in CB. The MAPbBr3 crystals had a width of approximately 100 nm and a cubic shape (Figure 6.6a). The electron diffraction (ED) pattern of a MAPbBr3 crystal confirmed the presence of a single crystal of high crystallinity (Figure 6.6b), which gave intense diffraction peaks from the {100} planes. Fourier transform infrared (FT-IR) spectroscopy was performed to investigate the existence of PMA after the MAPbBr3 film growth (Figure 6.7). The presence of PMA was confirmed by the apparent benzene ring absorption peaks, which include the peak at 693, 750 cm−1 from C-H out-of-plane deformation, 1400–
1500 cm−1 from C-C ring stretching, and 3000–3100 cm−1 from aromatic C-H stretching.30,31 The FT- IR spectrum of PMA-free MAPbBr3 did not show any absorption peaks related to the benzene ring
modes, which indicates that the CB anti-solvent was vaporized during spin-coating. In contrast, the FT- IR spectrum of PMA-containing MAPbBr3 showed benzene ring absorption peaks, which indicate that PMA remains on the surface of the MAPbBr3 crystals due to their strong mutual interaction. Therefore, for the optimized PMA concentration of 0.5 vol.%, PMA ligands not only grow nano-sized single crystals with few defects but also passivate the surface defects of the crystals. To investigate the MAPbBr3 trap density with and without PMA, the dark current of hole-only devices was measured under applied bias (Figure 6.8). The MAPbBr3 with PMA has lower trap density compared with the MAPbBr3 without PMA. Figure S6 showed the current-voltage characteristics for the hole-only device with and without PMA. At low voltages (< VTFL), the linear J-V relation (red line) indicates an ohmic response. A trap-filling region (blue line) was identified by abruptly increase of the current injection at a voltage (> VTFL) where all the traps are filled. From this region, the trap density was calculated using following relation.
𝑛t = 2𝑉TFL𝜀𝜀0
𝑒𝐿2 (6.4)
where VTFL is the trap-filled limit voltage, is relative dielectric constant (25.5 for MAPbBr3), 0 is the vacuum permittivity, e is the electron charge, and L is the thickness of the MAPbBr3 (~ 200 nm). VTFL
of MAPbBr3 with PMA (0.65 V) is lower than that without PMA (1.05 V) (Figure 6.8), which indicates reduction in trap density of perovskite with PMA treatment. The trap densities were calculated to be 7.4
× 1016 cm-3 for MAPbBr3 without PMA and 4.58 × 1016 cm-3 for MAPbBr3 with PMA.
Figure 6.6. TEM image and ED pattern of the MAPbBr3 crystals grown with 0.5 vol.% PMA. (a) Bright-field TEM image of the MAPbBr3 crystals grown under the optimized condition of 0.5 vol.%
PMA. (b) The corresponding ED pattern of the central MAPbBr3 crystal.
Figure 6.7. FT-IR spectra of PMA and the MAPbBr3 films prepared with and without PMA. FT-IR spectra over ranges of (a) 650–1000 cm−1, (b) 1400–1550 cm−1, and (c) 3000–3100 cm−1.
Figure 6.8. J-V characteristics of hole-only device (ITO/Perovskite/Au). J-V characteristic of hole only device (a) without PMA and (b) with PMA.
To confirm the improvement of optical properties of PMA-containing MAPbBr3 films, time-resolved PL decay and steady-state PL measurements were performed. The lifetime of MAPbBr3 was enhanced from 18.2 ns (without PMA) to 114.2 ns by the addition of 0.5 vol.% PMA, indicating a strong reduction in the non-radiative decay pathway (Figure 6.9a and Table 6.1). The 2D layered perovskite films formed with higher PMA contents showed shorter lifetimes, which reduced to 0.34 ns for 4.0 vol.%
PMA. The MAPbBr3 films prepared with PMA treatment showed much higher PLQYs compared with the pristine film (Figure 6.9b) and the MAPbBr3 with optimized PMA concentration of 0.50 vol.%
showed much enhanced external PLQY (from 10.7% to 57.8%). PeLEDs were optimized by testing different electron transport layer (ETL) thicknesses to increase the recombination rate by balancing the charge carriers (Figure 6.10 and Table 6.2). The charge injection-balance was confirmed by measuring single carrier current densities of single carrier devices with different thicknesses of 2,2′,2"-(1,3,5- Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (Figure 6.11). Perovskite films were also fabricated at an optimized low temperature to reduce the grain size, which increases the recombination
rate by confining the charges within the small grains. The MAPbBr3 film fabricated at the optimized temperatures showed significantly enhanced PLQY compared with the MAPbBr3 fabricated at other temperatures. Along with enhancement of PLQY, PeLEDs fabricated at the optimized temperature showed increased maximum luminance and device efficiencies (Figure 6.12 and Table 6.3). The voltage-dependent current density, luminance, and device efficiency characteristics were measured for PeLEDs fabricated with the optimized temperature and ETL thickness and different PMA concentrations; the results are shown in Figure 6.9 and Table 6.4. The PMA-treated devices showed lower leakage current densities than the reference device due to the reduced number of perovskite defect sites (Figure 6.9c). Moreover, the devices with a PMA content of <2 vol.% showed substantially increased maximum luminance and device efficiency values and lower turn-on voltages than the control device by reducing defects with small grain size (Figure 6.9d-f). The optimized device with 0.5 vol.%
PMA exhibited a maximum luminance of 55,440 cd m−2 at 135.1 mA cm−2, CE of 55.2 cd A−1, an external quantum efficiency of 12.1%, and high color purity with a FWHM of 19.5 nm. The EQE of device are higher than best EQEs of a previously reported PeLEDs. We consider these enhanced properties to be enabled by the fabrication of nano-sized defect-free crystals in the MAPbBr3 film.
Limiting the number of defects suppresses trap-assisted non-radiative decay and a high recombination probability is achieved by confining the charges within the small grains. In contrast, the device with 2 vol.% PMA showed relatively low luminance and device efficiency values with a reduced current density, which indicate that the bulky amine ligand in the 2D layered structure interrupted charge transport in the perovskite layers.
Figure 6.9. Time-resolved PL spectra, PLQYs and device performance curves of the PeLEDs fabricated with and without PMA at low temperatures. (a) Time-resolved PL spectra and (b) PLQYs of the MAPbBr3 films fabricated with 0–4.00 vol.% PMA. (c) Current density versus voltage (J–V), (d) luminance versus voltage (L–V), (e) current efficiency versus current density (CE–J), and (f) external
quantum efficiency versus current density (EQE–J) characteristics, and (g) normalized EL spectra of the PeLEDs according to PMA concentration. (h) A photograph showing the green EL emission of a PeLED device fabricated with 0.5 vol.% PMA.
Figure 6.10. Performance of the PeLED devices prepared under the optimized conditions with different thicknesses of the ETL. (a) Current density versus voltage (J–V), (b) luminance versus voltage (L–V), (c) current efficiency versus current density (CE–J), and (d) external quantum efficiency versus current density (EQE–J) characteristics of PeLEDs fabricated with 0.5 vol.% PMA at low temperatures with different ETL thicknesses.
Figure 6.11. J-V characteristics of hole-only device (ITO/PEDOT:PSS/MAPbBr3/Poly-TPD/MoO3/Au) and electron-only device (ITO/ZnO/MAPbBr3/TPBi/LiF/Al) prepared under the optimized conditions with different thicknesses of TPBi.
Figure 6.12. Differences in the performance of PeLED devices and PLQYs of perovskite films prepared at different temperatures. (a) J–V, (b) L–V, (c) CE–J, and (d) EQE–J characteristics of PeLEDs and (e) PLQYs of perovskite films fabricated with 0.5 vol.% PMA at different temperatures.
Table 6.1. Summarized PL lifetime of MAPbBr3 films with and without different concentration of PMA.
Film configuration avr [ns] 2
Glass / perovskite (Ref.) 18.2 1.261
Glass / perovskite (PMA 0.25 vol. %) 36.5 1.170
Glass / perovskite (PMA 0.50 vol. %) 114.2 1.218
Glass / perovskite (PMA 1.00 vol. %) 61.6 1.176
Glass / perovskite (PMA 2.00 vol. %) 28.4 1.142
Glass / perovskite (PMA 4.00 vol. %) 0.34 1.536
Table 6.2. Summarized device performance of PeLEDs with different thickness of TPBi.
Device configuration (PeLEDs)
Luminance
max [cd/m2]
@ bias
CE max [cd/A]
@ bias
EQE max [%]
@ bias
Turn-on voltage [V]
@ 0.1 cd/m2 ITO / PEDOT:PSS / MAPbBr
3 (PMA 0.50 vol. % in CB) / TPBi (50 nm) / LiF / Al
48,700 @ 5.6
V 42.5 @ 4.8 V 9.30 @ 4.8 V 2.8 ITO / PEDOT:PSS / MAPbBr
3 (PMA 0.50 vol. % in CB) / TPBi (55 nm) / LiF / Al
55,400 @ 5.6
V 55.2 @ 5.0 V 12.1 @ 5.0 V 2.8 ITO / PEDOT:PSS / MAPbBr
3 (PMA 0.50 vol. % in CB) / TPBi (60 nm) / LiF / Al
36,200 @ 6.2
V 39.5 @ 5.4 V 8.63 @ 5.4 V 2.8 ITO / PEDOT:PSS / MAPbBr3 (PMA 0.50 vol. %
in CB) / TPBi (70 nm) / LiF / Al
24,000 @ 7.4
V 24.7 @ 6.2 V 5.39 @ 6.2 V 3.0
Table 6.3. Summarized device performance of PeLEDs with different temperature.
Device configuration (PeLEDs)
Luminance
max [cd/m2]
@ bias
CE max [cd/A]
@ bias
EQE max [%]
@ bias
Turn-on voltage [V]
@ 0.1 cd/m2 ITO / PEDOT:PSS / MAPbBr3 (PMA 0.50 vol. %
in CB) / TPBi / LiF / Al (20~25oC)
23,502 @ 5.8
V 35.4 @ 5.0 V 7.8 @ 5.0 V 2.8 ITO / PEDOT:PSS / MAPbBr3 (PMA 0.50 vol. %
in CB) / TPBi / LiF / Al (15~20oC)
31,777 @ 5.8
V 41.3@ 5.0 V 9.0@ 5.0 V 2.8
ITO / PEDOT:PSS / MAPbBr3 (PMA 0.50 vol. % in CB) / TPBi / LiF / Al (10~15oC)
55,400 @ 5.6
V 55.2 @ 5.0 V 12.1 @ 5.0 V 2.8
Table 6.4. Summarized device performance of PeLEDs with and without different concentration of PMA.
Device configuration (PeLEDs)
Luminance
max. [cd/m2]
@ bias
CE max.
[cd/A]
@ bias
EQE max.
[%]
@ bias
EQE average
[%]
from 15 devices
Turn-on voltage [V]
@ 0.1 cd/m2 ITO / PEDOT:PSS / MAPbBr3 (Drop CB) /
TPBi / LiF / Al
4,100 @ 6.4 V
1.72 @ 6.2 V
0.38 @ 6.2 V
0.31 3.2
ITO / PEDOT:PSS / MAPbBr
3 (drop PMA 0.25 vol. % in CB) / TPBi / LiF / Al
31,100 @ 6.0 V
37.6 @ 5.4 V
8.31 @ 5.4 V
6.75 3.0
ITO / PEDOT:PSS / MAPbBr
3 (drop PMA 0.50 vol. % in CB) / TPBi / LiF / Al
55,400 @ 5.6 V
55.2 @ 5.0 V
12.1 @ 5.0 V
9.63 2.8
ITO / PEDOT:PSS / MAPbBr3 (drop PMA 1.00 vol. % in CB) / TPBi / LiF / Al
19,700 @ 6.8 V
26.5 @ 5.6 V
5.81 @ 5.6 V
4.25 3.0
ITO / PEDOT:PSS / MAPbBr
3 (drop PMA 2.00 vol. % in CB) / TPBi / LiF / Al
500 @ 7.6 V
1.30 @ 5.6 V
0.29 @ 5.6 V
0.22 3.2
The poor stability of PeLEDs remains a serious issue that should be resolved for commercialization.
Intrinsically, low interaction energy of perovskite allows ionic defects to be generated, which facilitates decomposition and interaction of perovskite with adjacent layers due to ion migration. To investigate the effect of the defect-minimizing PMA treatment on device stability, the operational stability of PMA- free and PMA-treated PeLEDs with encapsulation were measured at current density of 5 mA cm−2 under ambient conditions as function of operation time (Figure 6.13). The device with 0.5 vol.% PMA showed improved operational stability compared with the PMA-free device. PMA-free device exhibited a sharp drop to less than 50% of the initial luminance within 22 min, whereas PMA-treated device maintained over 50% of the initial luminance until 135 min. The operational stability is improved by fabricating nano-sized defect-free crystals. Moreover, many researchers have reported a blinking phenomenon in OIPs that must be avoided for PeLED applications.21-26 EL blinking is thought to arise from charge trapping and ion movement, which are attributed to defects. To investigate the effect of the defect- minimizing PMA treatment on EL blinking, EL images of PeLEDs were recorded over time by optical microscopy (Figure 6.14). The methylammonium bromide (MABr):lead bromide (PbBr2) = 1.05:1 PMA-free device also exhibited significant EL blinking, which indicates that a considerable number of defects remained despite the ability of excess MABr to passivate surface defects such as uncoordinated Pb atoms. However, the PeLEDs fabricated with the optimized conditions (0.5 vol.% PMA and low temperature) showed no EL blinking, confirming that our method substantially suppressed both inner and outer grain defects. To investigate the influence of defect inhibition on hysteresis behavior, we also
measured the current density versus voltage (J–V) characteristics of PeLEDs with different scan directions (Figure 6.15). The reference device demonstrated hysteresis while the optimized PeLEDs showed no such hysteresis behavior, in good agreement with the results of the EL blinking tests.
Figure 6.13. Operational stability of PMA-free and PMA-treated PeLEDs with encapsulation measured at current density of 5 mA cm-2 under ambient conditions as function of operation time.
Figure 6.14. Optical microscope EL images observed over time for PeLEDs fabricated with and without PMA.
Figure 6.15. PeLED hysteresis curves. J–V characteristics of PeLEDs prepared (a) without PMA and (b) with 0.5 vol.% PMA at a scan speed of 0.3 V s−1.