CHAPTER 3. A Multifunctional Self-Assembled Monolayer for Highly Luminescent Pure-Blue
3.3. Results and Discussion
The synthesis of 36ClCzEPA is shown in Figure 3.1. Carbazole was converted to 36ClCz using N- chlorosuccinimide in DMF at 60 �.83 36ClCz was reacted with 1,2-dichloroethane in a biphasic mixture containing toluene and 50% aqueous NaOH in the presence of tetrabutylammonium bromide as a phase- transfer catalyst to obtain 36ClCzECl. This compound was converted to its iodo-analogue, 36ClCzEI, through a reaction with potassium iodide in acetonitrile at 90 �.84 36ClCzEI was subjected to Michaelis–Arbuzov reaction by treatment with triethylphosphite at refluxing temperature to yield 36ClCzEPE. Lastly, 36ClCzEPE was changed into 36ClCzEPA by the use of bromotrimethylsilane in 1,4-dioxane at room temperature.78
Figure 3.1. Synthetic scheme for 36ClCzEPA.
XPS spectra of the 36ClCzEPA SAM on the ITO surface were conducted to investigate the presence of the SAM and its binding affinity. The C 1s spectrum in Figure 3.2a was deconvoluted by three main peaks, at 283.9, 284.8, and 285.5 eV, showing the structural signal from the 14 carbon atoms in 36ClCzEPA. Each peak corresponds to (�) the aromatic C–C carbons, (�) a combination of carbons in the C–Cl and C–N bonds from the carbazole unit, and (�) aliphatic carbons in the CH2–P and CH2–N structure, respectively.77 The intensity ratio of the three peaks was 8.0:5.2:1.6, in good agreement with the expected area ratio of 8:4:2. From the O 1s spectrum in Figure 3.2b, four main peaks were found.
The peak at 530.3 eV originated from the bulk oxygen (O2–) from ITO, 531.4 eV from the oxygen in the In-O-In on the ITO surface, 532.4 eV from the oxygens in the –OH on the ITO surface, P=O, and In–O–P, and 533.4 eV from the oxygen in the unbonded P–O–H.85 The relative peak area of the four peaks was 55.9:30.4:11.7:2.0, respectively. The presence of In–O–P bonding peaks indicated that 36ClCzEPA formed a SAM structure on the surface of ITO with the successful anchoring of phosphonic
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acid onto the ITO substrate. Further information regarding the binding of 36ClCzEPA onto the ITO was revealed in the P 2p spectrum in Figure 3.2c. 2 pairs of peaks having the spin-orbit splitting with 0.9 eV binding energy shift between 2p3/2 and 2p1/2were resolved. One pair is located at 133.1 and 134.0 eV, originated from PO32–, corresponding to the bidentate binding of the phosphonic acid unit onto the ITO surface. Another pair of peaks at 134.2 eV and 135.1 eV were assigned to phosphorus in the PO2(OH)–, corresponding to the unbonded hydroxyl group of the phosphonic acid anchoring group.86 From the relative peak area ratio of 95.7:4.3 for PO32– 2p3/2 and PO2(OH)– 2p3/2, we concluded that almost all the 36ClCzEPA molecules were self-assembled in the bidentate state on the surface of ITO.
In the case of the Cl 2p spectrum (Figure 3.2d), one pair of peaks were found at 199.6 and 201.2 eV with the spin-orbit splitting of 1.6 eV binding energy shift, originating from the chlorines of 36ClCzEPA.
The absence of any other shifted chlorine peaks reflects that the 36ClCzEPA molecules form a SAM layer on the ITO uniformly and stand on the ITO surface without any direct interaction between the ITO surface and the chlorine atoms. Lastly, the surface coverage was estimated to be 3.8 × 1014 molecules/cm2based on the P 2p and In 3d5/2 signals from the 36ClCzEPA SAM ITO.[14]Conclusively, all the XPS signals support that the 36ClCzEPA SAM covered the ITO surface well.
Figure 3.2.XPS spectra corresponding to (a) C 1s, (b) O 1s, (d) P 2p, and (d) Cl 2p of the 36ClCzEPA SAM on ITO substrates.
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Figure 3.3. (a) UPS spectra of the bare ITO substrates (black) and ITO/36ClCzEPA (blue). (b) Schematic energy level diagram of the ITO/36ClCzEPA. Electrostatic potential profiles by DFT-PBE for (c) bare ITO (111) and (d) ITO with a 36ClCzEPA SAM. The side view of each model structure is shown in the inset (In, pink; Sn, gray; O, red; C, brown; N, blue; Cl, green-yellow; P, violet; H, light pink).
To investigate the effect of the 36ClCzEPA on the ITO WF, UPS measurement was carried out on the ITO/36ClCzEPA SAM and UV/O3–treated bare ITO substrates. UPS spectrum of the ITO/36ClCzEPA SAM is shown in Figure 3.3a. The WF (Φ) of the ITO/36ClCzEPA was found to be 5.46 eV, which is significantly higher than the WF of bare ITO (Φ = 4.70 eV). In other words, the Fermi level of the ITO surface became much deeper than the bare ITO surface, as illustrated in Figure 3.3b. Furthermore, the WF was also calculated by using the DFT method for bare ITO surface, and for 36ClCzEPA functionalized ITO surface with a surface coverage of 5.6 × 1013molecules cm–2(Figures 3.3c,d). The evaluated WF for bare ITO was 4.56 eV and increased to 5.34 eV for 36ClCzEPA SAM functionalized ITO surface with a dipole moment of 3.97 D, confirming the experimentally measured WF trend.
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Figure 3.4. Optical properties of the perovskite films. (a) The normalized steady-state UV–vis absorption and (b) second derivative of the absorption spectra, (c) PL emission spectra of perovskite films with the PEDOT:PSS and 36ClCzEPA SAM as HIL. The inset shows a photograph of the corresponding perovskite films under λ = 365 nm UV excitation. DFT-PBE band structures of heterointerfaces of the layered 2D-CsPbBr3 with the (d) H- and (e) Cl-terminated SAMs. In the band structural plots, the contribution from the SAM molecule is shown by blue circles. Isosurfaces for the Bloch orbitals indicated by the arrow in each inset for the layered 2D-CsPbBr3 with the (f) H- and (g) Cl-terminated SAMs. The yellow and blue colors indicate positive and negative, respectively. (Cs, green;
Pb, dark gray; Br, brown; O, red; C, dark brown; N, blue; Cl, green-yellow; P, violet; H, light pink).
Figure 3.5. Photographs of water droplets in contact with (a) PEDOT:PSS and (b) 36ClCzEPA SAM.
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UV–vis absorption spectra of 2D perovskite films deposited on the PEDOT:PSS and 36ClCzEPA SAM HILs show three distinctive excitonic absorption features at 416, 443, and 459 nm corresponding to 2D phases with n = 2, 3, and ≥ 4, respectively (Figure 3.4a). Surprisingly, hypsochromic shift and absence of n= 1 2D phase at 389 nm were observed for the 2D perovskite film on the 36ClCzEPA SAM (Figure 3.4b). The absence of n= 1 phase is attributed to poor surface wettability of the 36ClCzEPA SAM layer to the perovskites (Figure 3.5). It is well-known that the hydrophobic surface guides the formation of large-sized perovskites i.e. higher –n 2D phases. The introduction of 36ClCzEPA SAM led to a significant increase in contact angle from 24.14° to 41.56° for the water droplet, thereby bringing about the absence of small-sized n= 1 phase. The presence of n = 1 phase can be translated into the aggravated luminescence property of quasi-2D perovskite emitter owing to the inefficient energy transfer and severe nonradiative recombination by strong exciton-phonon coupling.87,88 Accordingly, the PL intensity of the 2D perovskite film on the 36ClCzEPA SAM was improved by 4- fold compared to that of the 2D perovskite film on the PEDOT:PSS layer as shown in Figure 3.4c. It is important to note that the PL emission peaks are different, i.e., 473 and 480 nm for the 2D perovskite films on the 36ClCzEPA SAM and the PEDOT:PSS layer, respectively. Both the different UV–vis absorption features and the blue-shifted PL spectra might be originated from the strong interaction between the chlorine atoms of the 36ClCzEPA molecule and the 2D perovskite surface (see further discussions below).
To investigate the role of chlorine atoms in the 36ClCzEPA SAM on the interfacial coupling behavior between the layered 2D perovskite and SAM molecule, DFT-PBEcalculations were carried out for two model structures: a 2D layer of CsPbBr3(2D-CsPbBr3) contacted by a 36ClCzEPA SAM (Cl-terminated SAM) or a 2PACz SAM (H-terminated SAM). The optimized atomic structures of the layered 2D- CsPbBr3surface with H- and Cl-terminated SAM molecules are presented in Figure 3.6. The perovskite layer with the Cl-terminated SAM exhibited stronger binding energy of 1.09 eV/molecule, compared to 0.81 eV/molecule for that with the H-terminated SAM, indicating that 2D perovskite phases are more energetically favorable on the Cl-terminated SAM layer. Also, the electronic band structures for heterointerfaces of the layered 2D-CsPbBr3with H- and Cl-terminated SAM molecules were calculated (Figures 3.4d,e), and the SAM molecule-driven states are indicated by the blue circles. The comparison of the two cases indicates the lowered energy levels for the layered 2D-CsPbBr3/Cl-terminated SAM case by Cl adsorption. Moreover, close to an almost crossing point between a molecular flat band and a dispersive surface band (magnified in the inset), the Cl-terminated SAM exhibits enhanced level splitting (~0.035 eV) due to a significant interfacial orbital hybridization. Figures 3.4f,g show the isosurfaces of the associated Bloch orbitals indicated by the arrows in Figures 3.4d,einsets. The Bloch orbitals are almost completely localized in the molecular region for the H-terminated SAM (Figure 3.4f), whereas they are mixed with 2D perovskite states via the coupled p orbitals of Pb and Cl for the Cl-terminated SAM (Figure 3.4g). By the enhanced interfacial orbital coupling between the 2D
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perovskite layer and the Cl-terminated SAM, the energy gap of bare 2D-CsPbBr3 (2.25 eV) increased to 2.28 eV, whereas the energy gap decreased to a small degree (by 0.01 eV) for the H-terminated SAM.
This Pb-Cl orbital coupling effect was experimentally supported by the PL spectra of the 2D perovskite films deposited on 2PACz and 36ClCzEPA SAMs (Figure 3.7), revealing that the 2D perovskite film on the 36ClCzEPA SAM exhibited the blue-shifted PL spectra (λmax = 473 nm) compared to the perovskite film on the 2PACz SAM (λmax = 476 nm).
Figure 3.6. Side views of the optimized atomic structure of the layered 2D-CsPbBr3 surface with the (a) H- and (b) Cl-terminated SAM. The calculated binding energies are indicated. (Cs, green; Pb, dark gray; Br, brown; O, red; C, brown; N, blue; Cl, green-yellow; P, violet; H, light pink).
Figure 3.7. Normalized steady-state PL emission spectra of 2D perovskite films deposited on the 36ClCzEPA and 2PACz SAMs.
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Figure 3.8. Geometrical properties of perovskite films. AFM images of (a) ITO/PEDOT:PSS, (b) ITO/36ClCzEPA SAM, and 2D perovskite films deposited on (c) ITO/PEDOT:PSS and (d) ITO/36ClCzEPA SAM (The scale bars are 1 µm). GIWAXS patterns of 2D perovskite films deposited on the (e) PEDOT:PSS and (f) 36ClCzEPA SAM HILs.
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Figure 3.9. Line-cut profiles for the GIWAXS patterns of 2D perovskite films deposited on the PEDOT:PSS and 36ClCzEPA SAM HILs.
The morphological characteristics of the PEDOT:PSS, the 36ClCzEPA SAM, and the perovskite films deposited on both layers were investigated using AFM. The ITO/PEDOT:PSS film exhibited the film morphology with a root-mean-square (RMS) roughness of 1.09 nm, which is a smoother surface than the ITO/36ClCzEPA SAM (RMS of 1.48 nm) (Figures 3.8a,b). Interestingly, the 2D perovskite films on the ITO/36ClCzEPA SAM (RMS of 1.54 nm) were observed to be much smoother than that on the ITO/PEDOT:PSS (RMS of 2.30 nm) as shown in Figures 3.8c,d. In addition, GIWAXS measurements were performed to obtain further insight into the preferential orientation of the perovskite films deposited on the PEDOT:PSS and the 36ClCzEPA SAM. In Figure 3.8e, the perovskite films on the PEDOT:PSS showed bulk-like perovskite crystalline features. On the other hand, the perovskite films on the 36ClCzEPA SAM exhibited relatively strong diffraction signals along the z-direction accompanied by distinct crystallinity compared to that on the PEDOT:PSS (Figure 3.8f). From the line- cut profile, (k00) diffractions were found at 0.54, 0.71, and 0.81 �–1 corresponding to the low – n 2D phases, and (100) diffraction was observed at 1.05 �–1 arising from the bulk-like perovskites (Figure 3.9).42,89 This observation suggests that the face-on-oriented 2D perovskite films are formed preferentially on the 36ClCzEPA SAM with self-stacked 2D nanoplatelets structure.90,91 Accordingly, the highly oriented 2D perovskite nanoplatelets parallel to a substrate explain the much smoother surface of the 2D perovskite film on the 36ClCzEPA SAM (Figure 3.8d). We conjecture that the better morphological characteristics and the development of a highly crystalline 2D perovskite layer on the 36ClCzEPA SAM arise from the enhanced Pb-Cl electronic coupling found in the theoretical calculation.
In other words, the uniformly distributed chlorine atoms on the top surface of the SAM function as the
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nucleation seeds for the crystal growth of the layered 2D perovskites.92
TRPL measurements were conducted for perovskite films deposited on the PEDOT:PSS and the 36ClCzEPA SAM to investigate the effect of underlying HILs on the PL dynamics of perovskite films (Figure 3.10a). For the perovskite films on the 36ClCzEPA SAM, a longer average lifetime (τavg) of 3.32 ns was observed compared to that for the perovskite films on the PEDOT:PSS (τavg= 1.55 ns). The fitted parameters for TRPL decay curves using a tri-exponential fitting model are summarized in Table 3.1, revealing that both the fast and slow decay lifetimes for the perovskite films on the 36ClCzEPA SAM are much longer than that of perovskite films on the PEDOT:PSS. Especially, the fast decay (τ1) lifetime which is related to trap-assisted nonradiative recombination at grain boundaries/interfaces, greatly prolong from 0.46 to 1.76 ns, meanwhile slight improvement in the slow decay component is observed. This can be attributed to the suppressed interfacial defect states and nonradiative quenching pathways for the 2D perovskite films on the 36ClCzEPA SAM.93 To further verify the effect of 36ClCzEPA SAM on the trap density of 2D perovskite films, hole-only devices were fabricated in a configuration of ITO/HIL/2D perovskite/MoO3/Ag and their electrical properties were analyzed based on the SCLC model (Figures 3.10b,c). The trap-filled limiting voltage (VTFL) was reduced from 1.42 to 1.16 V when using the 36ClCzEPA SAM. The trap density in the perovskite film was estimated according to the equation
Ntrap=(2VTFL r 0) ⁄(eL2) (3-2)
where Lis the thickness of the deposited perovskite film (L~ 25 nm), eis the electron charge, εris the relative dielectric constant (εr= 14.52, the relative dielectric constant of PEABr decorated perovskites94), and ε0is the vacuum permittivity. The trap density was reduced from 1.86 × 1018cm–3(for perovskite film on the PEDOT:PSS) to 1.51 × 1018cm–3for the perovskite film on the 36ClCzEPA SAM. These results clearly show that exciton quenching and formation of trap state at the interface between the adjacent layers and/or within the perovskite film were significantly suppressed by the introduction of the 36ClCzEPA SAM.
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Figure 3.10.Characteristics of trap densities and color stabilities. (a) PL kinetic profiles of perovskite films with the PEDOT:PSS and 36ClCzEPA SAM HILs on ITO substrates. (b, c) SCLC J–Vcurves of 2D perovskite hole-only-diodes deposited on the PEDOT:PSS and 36ClCzEPA SAM with the device structure of ITO/HIL/2D perovskite/MoO3/Ag. (d) XPS spectra displaying indium (In) 3d signals in the 2D perovskite film on the PEDOT:PSS and 36ClCzEPA. PL spectra of 2D perovskite films before and after (e) being kept under ambient conditions for 1 day and (f) thermal annealing at 100 °C for 20 min.
The inset shows a photograph of the corresponding perovskite films under λ = 365 nm UV excitation.
Table 3.1. Triexponential fitting parameters for PL lifetimes of perovskite film deposited on the PEDOT:PSS and 36ClCzEPA SAM on ITO substrates.
ITO τ1(ns) f1(%) τ2(ns) f2(%) τ3(ns) f3(%) τavg(ns) χ2
PEDOT:PSS 0.46 73.96 2.50 22.14 16.68 3.91 1.55 1.18
36ClCzEPA 1.76 72.65 5.27 23.64 21.48 3.70 3.32 1.10
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The trap formation can be attributed to the metallic indium species diffused from the ITO substrates.
It is well-known that the highly acidic PEDOT:PSS etches the ITO substrates easily and the released indium species can diffuse into the perovskite layer and then form trap states therein, which act as exciton quenching centers.48,59 To confirm this, the XPS spectra were taken for the ITO/HIL/2D perovskite films (Figure 3.10d). The 2D perovskite film on the 36ClCzEPA SAM showed no indium signals, whereas the perovskite film on the PEDOT:PSS clearly exhibited the emergence of indium species. This difference originated from the fact that the strong binding of the phosphonic acids of the 36ClCzEPA SAM to the ITO surface prevents the corrosion of ITO substrates. Moreover, the detrimental effect of acidic PEDOT:PSS appears to be directly correlated with the color stability of overlying 2D perovskite films. The PL of the perovskite film on the PEDOT:PSS after storage under ambient conditions for 1 day showed a significant bathochromic shift from 480 to 511 nm, while only a marginal shift in the PL for the perovskite film on the 36ClCzEPA SAM was observed (Figure 3.10e).
The etching of ITO substrate could be accelerated in the presence of water in the air which is a promoting source of proton generation from the PEDOT:PSS.95In addition to the humidity stability, the thermal stability was improved for the 2D perovskite film on the 36ClCzEPA SAM (Figure 3.10f). The mixed-halide (Br/Cl) perovskite typically has many defects such as halide vacancies or uncoordinated Pb2+sites and is vulnerable to the halide migration owing to the lattice mismatch, compared to the single halide (Br or Cl only) perovskite.96Accordingly, defect-mediated halide migration is prone to be induced by the thermal annealing treatment for the mixed-halide perovskite. This enhancement in thermal stability of the perovskite film can be concluded that the stronger interfacial interactions via the Pb-Cl orbital coupling prevent defect-mediated halide migration in the 2D perovskite layer with a reducing trap state formation.97,98
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Figure 3.11. Device characteristics of PeLEDs. (a) Cross-sectional view for the PeLEDs device with the 36ClCzEPA SAM HIL. (b) EL spectra (inset shows photographs of operating PeLEDs), (c) current density–voltage–luminance, (d) current density–EQE curves, and (e) histogram of PeLEDs using the PEDOT:PSS and 36ClCzEPA SAM HILs. (f) Current density–EQE curves of PeLEDs with the PEDOT:PSS and 36ClCzEPA SAM HILs before and after the aging process. The aging process involved storing the fabricated PeLEDs under nitrogen for 1 day.
Figure 3.12. Schematic illustrations of the hole-injection process in PeLEDs using the PEDOT:PSS and 36ClCzEPA SAM HILs.
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PeLEDs having a configuration of ITO/HIL/perovskite/TPBi/LiF/Al were fabricated, and their device performances were examined. Cross-sectional view of fabricated PeLEDs device with the 36ClCzEPA SAM HILs is shown in Figure 3.11a, respectively. The EL spectra are shown in Figure 3.11b, where the emission peaks are at 480 and 473 nm for the PeLEDs using the PEDOT:PSS and 36ClCzEPA as HILs, respectively. The EL emission peaks are in accord with the PL peaks (Figure 3.4c), indicating good confinement of injected holes and electrons within the 2D perovskite layer in PeLEDs.
PeLEDs using the 36ClCzEPA SAM exhibited a narrow EL spectrum with the FWHM of 22 nm, corresponding to CIE 1931 chromatic coordinates of (0.118, 0.087). Current density-voltage characteristics are shown in Figure 3.11c. The current density is significantly higher for PeLEDs using the 36ClCzEPA SAM than that of PeLEDs using PEDOT:PSS after turn-on. The turn-on voltage (VT, corresponding to the luminance of 0.1 cd m–2) decreased considerably from 4.5 to 3.0 V, which is attributed to the reduced hole injection barrier by the use of the ITO/36ClCzEPA SAM electrode as schematically illustrated in Figure 3.12. Also, PeLEDs using the 36ClCzEPA SAM exhibited the maximum luminance and EQE (Lmax= 944 cd m–2and EQE = 3.84%) with the pure-blue EL peak of 473 nm, both of which are significantly higher than those corresponding values for PeLEDs using the PEDOT:PSS HIL (Lmax = 466 cd m–2and EQE = 2.28%) (Figure 3.11d). Device characteristics are summarized in Table 3.2. The histograms of EQE statics obtained for 15 PeLED devices are displayed in Figure 3.11e, respectively, demonstrating a clear improvement in the device characteristics for PeLEDs using the 36ClCzEPA SAM. These enhancements can be attributed to the promoted radiative recombination as a consequence of the efficient hole injection, the interfacial trap passivation, and the suppressed luminescence quenching in the 2D perovskite layer by employing the 36ClCzEPA SAM. In addition, the device characteristics of PeLEDs were further investigated with and without the aging process (Figure 3.11fand Table 3.2). The aging process involved storing the fabricated PeLEDs under nitrogen conditions for 1 day. After aging, the maximum luminance and EQE of PeLEDs using the 36ClCzEPA SAM were substantially increased (Lmax = 1253 cd m–2and EQE = 4.80%), whereas the device characteristics of PeLEDs using the PEDOT:PSS remained almost the same. This observation implies that the trap passivation was reinforced by strong interaction between the chlorine atoms on the 36ClCzEPA SAM and interfacial uncoordinated Pb2+defect states during the aging process.52