Results and Discussion

CHAPTER 2. Manipulated Interface for Enhanced Energy Cascade in Quasi-2D Blue Perovskite

2.3. Results and Discussion

In Figure 2.1a, absorption spectra of the perovskite films on pristine PEDOT:PSS (denoted as p- PEDOT:PSS) and modified PEDOT:PSS (L-phenylalanine doped, denoted as m-PEDOT:PSS) show common bands with peaks at 419 and 446 nm and a shoulder at approximately 463 nm corresponding to 2D phases with n= 2, 3, and ≥ 4, respectively.^40,42The relative absorbance among different bands were similar for the two films indicating that modifying the characteristics of the interface with the additive did not significantly affect the relative distribution of the 2D phases or their primary absorption bands. Indeed, the PLQY of the perovskite film with m-PEDOT:PSS with the optimized weight of 0.89 wt% L-phenylalanine (Figure 2.2) was improved by 4-fold to 26.37% compared to that with p- PEDOT:PSS (6.73%) as shown in Figure 2.1band Figure 2.2b. These results clearly show that ) the engineered interface induced excellent growth of perovskite films and ) at the interface between the adjacent layers and/or within the perovskite film, exciton quenching was greatly suppressed by the presence of L-phenylalanine. This may be explained by the stronger coordination and passivation of Pb²⁺by the bidentate L-phenylalanine ligand (Figure 2.1c).^50,51

The interfacial characteristics between the underlying HIL and perovskite films were investigated using XPS and GIWAXS. Thin perovskite films (10 nm) were prepared on PEDOT:PSS, representing interface-like property.⁵²In Figure 2.1d, perovskite films on p-PEDOT:PSS showed XPS spectrum with two prominent peaks at 137.6 and 142.4 eV assigned to Pb 4f7/2 and Pb 4f5/2, respectively.⁵³ In contrast, XPS spectra for the perovskite films on m-PEDOT:PSS showed a significant shift of the Pb 4f peaks to lower binding energies of 137.0 and 141.8 eV. The change of the binding energy indicates modified interaction between Pb²⁺ of perovskite with m-PEDOT:PSS resulting from a change in the local energy state of Pb²⁺.^54,55 Such shifts of the characteristic Pb 4f peaks were also observed when PbBr2 films were treated with L-phenylalanine (Figure 2.3a). The Pb 4f7/2and Pb 4f5/2 peaks in the PbBr2 films located at 137.9 and 142.8 eV, respectively, are shifted to 137.4 and 142.3 eV on the incorporation of L-phenylalanine. The observed peak shift towards lower binding energy corroborates the interaction between the carboxylate and ammonium groups of L-phenylalanine with Pb²⁺and the change in the local chemical bonding environment of Pb²⁺, as depicted in Figure 2.1c.^56-58 Further evidence of the Lewis acid-base interaction between L-phenylalanine and Pb²⁺ presumably in the perovskites was obtained through Fourier transform infrared (FTIR) absorption measurements (Figure 2.3b). Both the carbonyl and primary ammonium peaks in the FTIR spectrum of L-phenylalanine showed small shifts to higher energies upon the addition of PbBr2.

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Figure 2.1.Optical and geometrical properties of quasi-2D perovskite films deposited on p- and m- PEDOT:PSS. (a) UV absorption spectra and (b) PL spectra. (excitation wavelength was 375 nm) (c) Schematic illustration of coordination bonding between L-phenylalanine and a quasi-2D perovskite. (d) XPS spectra corresponding to Pb 4f of 10 nm-thick perovskite films, and (e, f) GIWAXS patterns of 10 nm-thick perovskite films deposited on p- and m-PEDOT:PSS HILs.

Figure 2.2. Optimization of PLQY of deposited quasi-2D perovskite films. (a) PL spectra and (b) PLQYs for perovskite films deposited on HILs with increasing L-phenylalanine concentration.

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To check the crystallinity and preferential orientation of the perovskite films, GIWAXS measurements were performed for 10 nm-thick films (Figures 2.1e,f, and Figure 2.4a). Preferential face-on orientation was observed in the GIWAXS patterns (Figure 2.1f) of the perovskite films deposited on m-PEDOT:PSS, while relatively weak diffraction signals along the z-direction were obtained for the perovskite films on p-PEDOT:PSS (Figure 2.1e). Figure 2.1f shows predominant crystal diffractions (k00) at 0.81 Å^–1 originating from the lower-n 2D phases and diffractions (100) and (200) at 1.05 Å^–1 and 2.09 Å^–1, respectively, from the bulk phase. These results suggest that 2D phases preferentially self-assemble in a neat face-on orientation relative to the substrate in the films with m- PEDOT:PSS which results in the distinct crystallinity of the perovskite films as revealed in the X-ray diffraction patterns (Figure 2.4b).

Figure 2.3. (a) XPS spectra corresponding to Pb 4f peaks of PbBr2 and L-phenylalanine-treated PbBr2

films. (b) FTIR spectra of L-phenylalanine and L-phenylalanine in presence of PbBr2.

Figure 2.4. (a) Line-cut profiles for the GIWAXS patterns of quasi-2D perovskite films deposited on p- and m-PEDOTL:PSS HILs. (b) XRD patterns of quasi-2D perovskite films deposited on p- and m- PEDOT:PSS HILs.

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Figure 2.5. Influence of HIL acidity on the exciton quenching process. (a, b) PL kinetic profiles of quasi-2D perovskite films with p- and m-PEDOT:PSS with different substrates on (a) bare glass and (b) ITO. (c) XPS spectra corresponding to In 3d for p- and m-PEDOT:PSS on ITO substrates. (d) Schematic illustrations of the exciton quenching process caused by diffused In species.

Figure 2.6. Perovskite film stability. Photographs of blue emissive quasi-2D perovskite films deposited on p- and m-PEDOT:PSS using different substrates including bare glass (left) and ITO (right) under λ

= 365 nm UV excitation.

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TRPL measurements were conducted for perovskite films with p- and m-PEDOT:PSS (Figure 2.5).

For perovskite films with m-PEDOT:PSS, a longer average lifetime (τavg) of 5.34 ns was observed compared to that for p-PEDOT:PSS (τavg= 2.61 ns) as shown in Figure 2.5aand Table 2.1. Using an ITO substrate (ITO/HIL/perovskite), perovskite films with m-PEDOT:PSS exhibited an even longer average lifetime of 5.06 ns than with p-PEDOT:PSS (τavg= 1.93 ns), as shown in Figure 2.5band Table 2.1. In general, the PL lifetime of perovskite films on ITO substrates shortens due to quenching caused by indium (In) species which diffuse from the ITO substrate; this effect has been widely reported as a non-radiative exciton quenching pathway.^59,60The high acidity of p-PEDOT:PSS has a tendency to etch ITO substrates and promote the diffusion of In species into the perovskite film, resulting in defect states therein.^48,49The emergence of In 3d5/2and 3d3/2peaks in the XPS spectra (Figure 2.5c) directly reveals the diffusion of In species into p-PEDOT:PSS, uniquely for ITO/p-PEDOT:PSS. To further confirm the diffusion of In from the ITO substrate, the pH values were measured and found to be lower (more acidic) for p-PEDOT:PSS as summarized in Table 2.2. In contrast, the perovskite films with m-PEDOT:PSS showed similar average lifetimes regardless of the substrate used, indicating that the presence of L- phenylalanine substantially suppressed exciton quenching caused by In species. The stability of the PL of the perovskite film with m-PEDOT:PSS under ambient conditions was observed to be superior to that with p-PEDOT:PSS, demonstrating that the detrimental effect of the acidity of HIL on the perovskite layer was suppressed by adding L-phenylalanine (Figure 2.6).

Not only the PL lifetime (and PLQY), but the long-term PL stability was greatly improved with m- PEDOT:PSS. While the PL of the perovskite films with p-PEDOT:PSS was as strong as that with m- PEDOT:PSS on bare glasses for at least 3 hours, on ITO substrates the PL of films on p-PEDOT:PSS severely degraded, in contrast to the film with m-PEDOT:PSS, which did not decrease over the same period. The introduction of L-phenylalanine greatly reduced the degradation of the perovskite layer and exciton quenching pathways minimizing the detrimental diffusion of In species from ITO substrates caused by the acidity of PEDOT:PSS. Thus, the presence of L-phenylalanine leads to stronger PL emission and long-lived excitons in the bulk phases as illustrated in Figure 2.5d.

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Table 2.1.Triexponential fitting parameters for PL lifetimes of quasi-2D perovskite films with p- and m-PEDOT:PSS on bare glass and ITO substrates.

Bare glass τ₁(ns) f₁(%) τ₂(ns) f₂(%) τ₃(ns) f₃(%) τ_avg(ns) χ²

p-PEDOT:PSS 1.05 70.38 4.09 25.54 20.28 4.08 2.61 1.15

m-PEDOT:PSS 3.24 77.55 9.01 18.34 28.66 4.10 5.34 1.07

ITO τ₁(ns) f₁(%) τ₂(ns) f₂(%) τ₃(ns) f₃(%) τ_avg(ns) χ²

p-PEDOT:PSS 0.61 75.73 3.21 20.22 20.22 4.05 1.93 1.27

m-PEDOT:PSS 2.99 73.19 7.83 22.50 25.74 4.31 5.06 1.07

Table 2.2.pH values of p- and m-PEDOT:PSS solutions.

Condition Acidity [pH]

p-PEDOT:PSS 1.23

m-PEDOT:PSS 2.98

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Efficient energy transfer from low-nphases to high-nphases in perovskite PeLEDs is directly related to device performance.^61,62 To investigate the dynamics of exciton transfer, we performed TA spectroscopy. Figures 2.7a,bshow the TA spectra of the perovskite films with p- and m-PEDOT:PSS at variable delay times following photoexcitation at 375 nm. The spectra are characterized by photoinduced bleaching (PIB) bands that correspond well to the steady-state absorption peaks of the perovskite films. Bands centered at 419 and 446 nm can be assigned to absorption by the n= 2 and n= 3 phases, respectively. The hint of a shoulder at approximately 463 nm and a tail extending to 480 nm can be assigned to absorption by bulk (n≥ 4) phases. A distinct PIB centered at approximately 492 nm observed for the perovskite film with p-PEDOT:PSS (Figure 2.7a) but not with m-PEDOT:PSS (Figure 2.7b) can be assigned to the population of a dark, defect state, as it appears at an energy below the bulk- phase PL.^62-64

Figure 2.7cshows the kinetic profiles extracted for the phases of n = 2, 3, and bulk for both the perovskite films. The profiles were fitted to multi-exponential functions characterized by rise and decays with timescales extending over several orders of magnitude. The relevant fit parameters are summarized in Table 2.3.Although the dynamics of both n= 2 and n= 3 phases were largely unaffected after treating the underlying HIL with L-phenylalanine, the nature of the subsequent energy cascade to the bulk phase (n ≥ 4) was substantially different. For the perovskite film with p-PEDOT:PSS, the dynamics in the bulk phase are characterized by the complete relaxation of excitons within several picoseconds. In contrast, the excitons are long-lived in the bulk phase with m-PEDOT:PSS, as reflected in the contribution of the nanosecond-long component. These effects are illustrated in Figure 2.7d.

Suppression of the detrimental defect states that act as quenching centers facilitates the survival of more excitons in the bulk phase, extending their lifetimes to span several nanoseconds with a concomitant increase in PLQY.

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Figure 2.7.Ultrafast dynamics of photogenerated charge carriers in quasi-2D perovskite. TA spectra of quasi-2D perovskite films deposited on (a) p- and (b) m-PEDOT:PSS at variable time delays as noted in the panels. The corresponding normalized, steady-state UV-vis absorption (black, solid) and PL (black, dotted) spectra are also presented in each panel for comparison. Also included are magnified portions of the PL spectra (gray, dashed) obtained from independent measurements depicting the PL peak features attributed to n= 2 and n= 3 2D phases. (c) Kinetic profiles obtained from TA spectra at selected wavelengths representing the temporal behavior of bleached excitonic states corresponding to different phases. The profiles at 419 nm, 446 nm, 463 nm, and 492 nm correspond to the phases of n= 2, n= 3, bulk, and the defect states, respectively. The signal intensities have been scaled for comparison relative to the peak intensities of the n= 2 phase. (d) Schematic energy transfer mechanism in quasi-2D perovskites on m-PEDOT:PSS.

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Table 2.3.Multi-exponential fit^aparameters for the kinetic profiles representative of various phases for perovskite films deposited on pristine and modified PEDOT:PSS substrates.

Domain (probe wavelength)

Pristine Modified

Time component (ps)

Amplitude (fraction)

Time component (ps)

Amplitude (fraction)

n= 2 (419 ± 3 nm)

0.10 ± 0.02 ^b 0.18 ± 0.02

3.2 ± 0.2 25.0 ± 1.0

166 ± 11 3860 ± 180

1.00 ± 0.92 0.75 ± 0.36 0.08 ± 0.00 0.10 ± 0.00 0.04 ± 0.00 0.03 ± 0.00

0.06 ± 0.01 ^b 0.21 ± 0.01

2.7 ± 0.1 25.6 ± 0.5

158 ± 5 3190 ± 90

1.00 ± 0.59 0.49 ± 0.06 0.18 ± 0.00 0.20 ± 0.00 0.09 ± 0.00 0.04 ± 0.00

n= 3 (446 ± 3 nm)

0.13 ± 0.02 0.19 ± 0.02

3.9 ± 0.1 20.2 ± 1.1

141 ± 12 3920 ± 290

1.00 ± 0.73 ^c 0.83 ± 0.46 0.08 ± 0.00 0.06 ± 0.00 0.02 ± 0.00 0.01 ± 0.00

0.14 ± 0.03 0.19 ± 0.04

3.8 ± 0.1 22.8 ± 0.7

153 ± 7 4520 ± 210

1.00 ± 0.74 ^c 0.72 ± 0.80 0.12 ± 0.00 0.10 ± 0.00 0.04 ± 0.00 0.02 ± 0.00 n≥ 4

(463 ± 2 nm)

0.19 ± 0.12 0.33 ± 0.21

6.4 ± 0.3

1.00 ± 1.33 ^c 0.67 ± 1.44 0.33 ± 0.01

0.24 ± 0.01 12.9 ± 0.6

100 ± 8 13130 ± 2830

1.00 ± 0.61 ^c 0.58 ± 0.02 0.27 ± 0.02 0.15 ± 0.00 Defect

(492 ± 2 nm)

0.36 ± 0.02 ^b 1.7 ± 0.7 ^b 39.4 ± 1.5 255 ± 13 6170 ± 640

0.93 ± 0.11 ^c 0.07 ± 0.03 ^c 0.47 ± 0.01 0.37 ± 0.01 0.16 ± 0.01

a ∆A(t)=∑ⁿ_i=1A_i exp (−t/τ_i)⁄∑ⁿ_i=1A_i.

bThe ultrafast bleach component is attributed to the cooling of the “hot” excitons immediately following photoexcitation at 375 nm. The excess energy is rapidly dissipated through an intraband transition that is also featured as an ultrafast Stokes’ shift of the PIB bands in Figures 2.7a,b.

c Bleach component. The fastest recovery time for the n= 2 phases does not perfectly match the fastest bleach component for the n= 3 phases. This is because the energy transfer is ultrafast and was not fully resolved owing to the instrumental response function (IRF) of our setup (approximately 200 fs). Further, a fraction of n= 3 phases can be directly photoexcited bypassing the energy-transfer channel.

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Figure 2.8. Characteristics of PeLEDs. (a) Schematic flat-band energy diagram of the optimized PeLEDs, (b) J-V-L plot, (c) EQE-J curves, (d) EL spectra (inset shows a photograph of an operating PeLED) (e) Performance reproducibility and (f) J-V hysteresis of PeLEDs based on p- and m- PEDOT:PSS.

Figure 2.9. UPS spectra of p- and m-PEDOT:PSS. (a) Secondary edge region and (b) valence band edge plotted relative to a Au reference.

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In order to evaluate device performance, PeLEDs were fabricated having a configuration of ITO/HIL/perovskite/TPBi/LiF/Al. PeLEDs devices with p- and m-PEDOT:PSS are denoted as p- and m-PeLEDs, respectively (Figure 2.8a). In Figure 2.8b, it can be seen that the leakage current is reduced for m-PeLEDs and the turn-on bias (VT, corresponding to the luminance of 0.1 cd m^–2) is improved due to the reduced energy barrier by an interfacial dipole induced by L-phenylalanine (Figure 2.9).

Furthermore, the CE and EQE of m-PeLEDs increased by 3-fold (CE = 11.73 cd A^–1 and EQE: 10.98%) compared to those for p-PeLEDs (CE = 3.59 cd A^–1 and EQE: 3.03%) (Figure 2.8c and Table 2.4).

Well-balanced charge carrier injection was achieved in devices using m-PEDOT:PSS as a consequence of effective hole injection due to the improved energy band structure of m-PeLEDs. To further investigate the effect of L-phenylalanine on defect density and charge carrier behavior, the space- charge-limited current (SCLC) model was used to analyze the behavior of p-type charge carriers in hole-only diodes (Figure 2.10). The trap-filled limiting voltage (VTFL) was reduced from 1.33 to 0.91 V in the m-PEDOT:PSS-based devices, consistent with a lower hole injection barrier. Consequently, the trap state density in the perovskite films decreased from 2.37 × 10¹⁸ cm^–3 with p-PEDOT:PSS to 1.62 × 10¹⁸ cm^–3 with m-PEDOT:PSS.

The EL spectra for p- and m-PeLEDs were centered at 485 and 480 nm, respectively (Figure 2.8d).

The red-shift of the EL spectrum in p-PeLEDs compared to m-PeLEDs originates from the defect- mediated halide migration caused by thermal annealing treatment of the TPBi film. Moreover, m- PeLEDs exhibited narrower EL spectra with FWHM values of 23 nm, indicating higher color purity and corresponding to CIE 1931 chromatic coordinates of (0.106, 0.138). The EQE statistics were obtained for 20 PeLED devices of each type and demonstrated good reproducibility (Figure 2.8e) and a very clear improvement in the efficiency of m-PeLEDs.

Figure 2.10. SCLC J-Vcurves of quasi-2D perovskite hole-only-diodes deposited on (a) p- and (b) m- PEDOT:PSS for the device architecture of ITO/p- and m-PEDOT:PSS/perovskite/MoO3/Ag.

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Table 2.4.Summary of device characteristics of optimized PeLEDs using p- and m-PEDOT:PSS HILs.^a L_max[cd m^–2]

@ bias

CE_max[cd A^–1]

@ bias

EQE_max[%]

@ bias

Tun-on Voltage [V]

@ 0.1 cd m^–2

p-PEDOT:PSS 343.70@8.5 3.59@6.5 3.03@6.5 4.5

m-PEDOT:PSS 548.65@6.0 11.73@4.5 10.98@4.5 3.0

aDevice structure: ITO/HIL/perovskite/TPBi/LiF/Al.

To investigate the photocurrent hysteresis in PeLEDs, we measured current density-voltage curves in forward and reverse scanning directions. As shown in Figure 2.8f, m-PeLEDs exhibited relatively low hysteresis, which indicates that coordination bonding completely passivated interfacial trap- assisted deactivation channels.⁶⁴ Furthermore, the spectral and operational stability of PeLEDs were characterized to check the stability of the perovskite phase in devices. The EL peak of p-PeLEDs indeed shifted from 485 to 491 nm as current density increased, accompanied by a broadening of the EL spectrum, whereas the EL spectrum of the m-PeLEDs remained constant (Figures 2.11a,b). Defects in PeLEDs can serve as pathways for electrically-driven ion migration during the device operation. The introduction of L-phenylalanine reduces such damaging channels in the case of m-PeLEDs and ensures greater stability of the EL spectrum. Also, the luminance decay measured under a constant current density of 1.5 mA cm^–2shows improved operational stability with a T50value of 7.68 min in the case of m-PeLEDs, compared to 2.81 min for p-PeLEDs (Figure 2.12). The peak of the EL spectrum for p- PeLEDs underwent a small bathochromic shift from 485 to 488 nm during the operational stability test (~3 min). In contrast, the peak position of the EL spectrum for m-PeLEDs remained unchanged at 480 nm, over ~7 min, indicating superior color stability (Figures 2.11c,d).

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Figure 2.11. EL spectra at different injected current densities for PeLEDs using (a) p- and (b) m- PEDOT:PSS HILs. EL spectra at different time intervals for PeLEDs using (c) p- and (d) m- PEDOT:PSS HILs.

Figure 2.12. Operational stability of PeLEDs using p- and m-PEDOT:PSS under a continuous constant current of 1.5 mA cm^–2.

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