Chapter 1. Introduction
1.3 Interfacial Engineering
1.3.2 Defect Control
Figure 1.17. Schematics of dangling bonds as defects on metal oxide surface with and without passivating materials. Defect passivation can be proved by measuring photoluminescence of metal oxide.
Zhou et al. have investigated the effect of polar solvent treatment on device performance of polymer solar cells30. Methanol treatment on polymer solar cells enhanced JSC, VOC, and FF, which leads to significant improvement of PCEs from 7.1% to 7.9%, as shown in Figure 1.18. Methanol treatment increased internal field (built-in potential) by passivation of surface traps, which induced a lowered series resistance, an accelerated charge extraction and a reduced bimolecular recombination caused by defects, as shown in Figure 1.19.
Figure 1.18. (a) Current density versus voltage characteristics of polymer solar cells with and without methanol treatment under 100 mW m-2 AM 1.5G illumination. (b) Current density versus voltage characteristics of polymer solar cells with and without methanol treatment in the dark.
Figure 1.19. Impedance spectra of polymer solar cells with and without methanol treatment. (C/A)-2 versus voltage (a) with and (b) without methanol treatment. Nyquist plots of polymer solar cells (c) with and (d) without methanol treatment.
Lee et al. have demonstrated efficient inverted polymer LEDs by introducing amine-based polar solvent between ZnO as ETL and F8BT as emitting layer31. In this work, light extraction of waveguiding mode was enhanced by fabricating spontaneously nanostructured ZnO layer. Optimized devices exhibited a remarkably enhanced luminous efficiency of 61.6 cd A-1 (from 1.28 cd A-1) and an external quantum efficiency of 17.8% (from 0.36%) compared with control devices, as shown in Figure 1.20.
Solution-processed ZnO layers contained unavoidably defect sites that caused considerable exciton quenching. Moreover, nanostructured ZnO effectively extracted light of waveguiding mode, but the increased roughness of ZnO film induced more exciton quenching. Defect passivation of ethanolamine as interfacial materials is much effective to reduce exciton quenching at interface between ZnO and F8BT. Time-resolved PL spectra and summarized PL lifetime are shown in Figure 1.21.
Figure 1.20. (a) Current density versus voltage, (b) luminance versus voltage, (c) luminous efficiency versus current density, (d) power efficiency versus current density, (e) external quantum efficiency versus current density characteristics and (f) normalized EL spectra of inverted-structured polymer LEDs.
Figure 1.21. Time-resolved PL spectra of (a) Quartz/ZnO-F/F8BT, (b) Quartz/ZnO-R1/F8BT and (c) Quartz/ZnO-R2/F8BT with and without amine-based polar solvent. (d) Summarized PL lifetime of F8BT on the different ZnO layers.
1.3.3 Compatibility Control
The formation of smooth thin film is crucial for efficient organic optoelectronic devices. Favorable compatibility should be required to form the neat thin films with fewer defects. To deposit the smooth thin films, bottom substrates should be compatible with precursor solutions. If precursor solutions are not compatible with bottom substrates, it is difficult to fabricate the smooth and full-coverage thin film, as shown in Figure 1.22. Interfacial engineering can adjust surface characteristics of bottom substrates by aligning hydrophilic or hydrophobic functional groups on bottom substrates in a certain direction.
The compatibility between precursors and bottom substrate can be confirmed by measuring contact angle, as shown in Figure 1.23. Improved compatibility can not only enhance film formation quality but also reduce series resistance at interface, which enhances device performance of OLEDs and OSCs.
Figure 1.22. (a) Hydrophilic and hydrophobic characteristics of various functional groups. (b) Hydrophilic and hydrophobic surface on a substrate. (c) Schematic of compatibility between precursors and substrate.
Figure 1.23. Compatibility can be proved by measuring contact angle.
Choi et al. have reported that CPE layer enabled the TiOx layer to be smooth and hydrophobic, which
improved the compatibility between hydrophilic TiOx layer and hydrophobic organic active layer28. The CPE layer reduced the contact resistance between electron transport layer and active layer, as confirmed by electrical impedance spectroscopy (Figure 1.24). The series resistance of device with interfacial engineering was reduced from 1.72 Ω cm2 to 1.44 Ω cm2 and the shunt resistance increased from 1210 Ω cm2 to 2270 Ω cm2. Summarized device performance of iPSCs with and without the CPE layer are shown in Table 1.3.
Figure 1.24. Electrical impedance for inverted-structured polymer solar cells with and without the CPE layer.
Table 1.3. Device characteristics of iPSCs with and without the CPE layer.
Device configuration JSC
[mAcm-2] VOC
[V] FF η
[%]
RS
[Ω∙cm2]
RSh
[Ω∙cm2]
TiOx / P3HT:PCBM 7.23 0.57 0.64 2.65 1.72 1210
TiOx / CPE / P3HT:PCBM 8.85 0.58 0.70 3.55 1.44 2270
Lee et al. have demonstrated highly efficient inverted polymer solar cells by introducing amine-based polar solvent32. Ethanolamine reduced the contact barrier between ZnO as ETL and active layer, which leaded to reduction of the series resistance and enhancement of the shunt resistance. Hydrophilic amine and hydroxyl groups was bonded with ZnO surface and hydrophobic aliphatic groups oriented upward,
which enabled the ZnO surface to be hydrophobic. The compatibility was confirmed by measuring contact angle (Figure 1.25) The improved compatibility between ZnO as ETL and active layer reduced the series resistance from 1.08 Ω cm2to 0.69 Ω cm2, which enhanced JSC, FF and PCE. Device performance of inverted-structured polymer solar cells are shown in Figure 1.26.
Figure 1.25. Contact angle on ZnO-R with (a) methanol, ethanol (b) ethanolamine and (c) ethylene diamine different concentrations.
Figure 1.26. (a) Current density versus voltage characteristics and (b) EQEs of inverted-structured polymer solar cells under 100 mW cm-2 AM 1.5G illumination.
1.4 Organic-Inorganic Hybrid Perovskite Devices
Organic-inorganic hybrid perovskites (OIPs) have the general formula of ABX3, where A is monovalent cations (CH3NH3+, NH2CHNH2+, Cs+), B is divalent metal cations (Pb2+, Sn2+, Ge2+) and X is halide anions (Cl-, Br-, I-), as shown in Figure 1.2732. The idea structure is a cubic, where the B cation is coordinated by X halide anions in an octahedral configuration. These perovskites have many advantages such as band gap tunability from the ultraviolet to infrared region through compositional modulation, high absorption coefficient, high mobility, ambipolar transport, direct band gap, and strong defect tolerance34-39. Owing to these properties, the OIPs were intensely investigated as a promising semiconductor material for solar cells, photodetector, LED, and laser applications.
Figure 1.27. Crystal structure of the metal halide perovskite.
1.4.1 Perovskite Light-Emitting Diodes
The OIPs have attracted considerable interest due to wide range of color tunability, narrow full-width at half maximum (FWHM) and high photoluminescence quantum yields (PLQYs)40-42. However, the low exciton binding energy of OIPs causes that the photogenerated excitons are thermally dissociated at room temperature, resulting in generation of free charges43-46. Figure 1.28 shows advantages and limitations of organic-inorganic hybrid perovskite as light emitting materials for light-emitting diodes.
Subsequent free charges can be rapidly trapped in trap sites caused by defects of perovskite, leading to trap-assisted non-radiative recombination. This leads to a low exciton binding rate for radiative decay and a PLQY dependence on the photoexcitation density.
Figure 1.28. (a) Advantages of organic-inorganic hybrid perovskite as light emitting materials for light- emitting diodes. (b) Limitations of organic-inorganic hybrid perovskite such as low exciton binding energy and ion migration.
Therefore, high PLQY of OIPs can be realized when photoexcitation density is much higher than trap density. To increase bimolecular recombination for radiative decay, perovskite film should be formed with minimized defects, and free charges should recombine for radiative decay faster than they are trapped in defect-states. In this way, several strategies have been adopted to facilitate the radiative bimolecular recombination rate to enhance the PLQY and EQE values of PeLEDs.
Yuan et al. and Wang et al. reported that free charges were successfully concentrated in the smaller band gap emitters through energy transfer from the larger band gap emitters using multi-phased quasi- 2D perovskites, as shown in Figure 1.2942,47. Multiple quantum wells of perovskites effectively remarkably enhance device performance by maximizing radiative bimolecular recombination.
Figure 1.29. Multi-phase perovskite channel energy across inhomogeneous energy landscape,
concentrating carriers to small band gap emitters.
Cho et al. boosted the current efficiency and external quantum efficiency of green emissive PeLEDs through nanocrystal pinning. Reducing the perovskite grain size increases the probability of bimolecular recombination by spatially confining the charges within the small grains, as shown in Figure 1.3048.
Figure 1.30. The grain size distribution of MAPbBr3 nanograin layers of (A) 1:1.05, (B) 1:1, (C) 1.05:1 with S-NCP, and (D) 1.05:1 with A-NCP.
More recently, mixed-cation perovskites were incorporated to reduced defects and improve the morphology, leading to a reduction in non-radiative recombination. Moreover, defect passivation has also been a potential approach to increase bimolecular recombination by reducing trap-mediated non-
radiative recombination. Various passivating materials were employed such as amine, hydroxyl, oxide derivatives, and ammonium halide compounds. These approaches successfully enhanced the EQE of PeLEDs up to 14.36% for green emissions49 and 12.7% for near-infrared emissions50.
1.4.2 Perovskite Solar Cells
Perovskite solar cells emerged using methylammonium lead halide (CH3NH3PbX3) perovskites as light absorbers in dye sensitized liquid electrolyte solar cells by Miyasaka in 2009, as shown in Figure 1.3151. Since then, OIPs have been intensely investigated by many researchers because of their advantages such as high absorption coefficient, high mobility and long charge diffusion length.
However, perovskite films were suffered from unfavorable morphological defects including pinhole and grain boundary, resulting in poor device performance, stability and reproducibility. To overcome these issues, several approaches have been introduced such as thermal evaporation, two-step deposition, anti-solvent dropping and additive-induced crystallization. These efforts have remarkably improved the PCE of perovskite solar cells52,53. However, device stability issue should be solved further for the commercialization of perovskite solar cells.
Figure 1.31. (a) Crystal structures of perovskite compounds. (b) SEM image of particle of nanocrystalline CH3NH3PbBr3 deposited on the TiO2 surface. (c) IPCE spectra for perovskite solar cells with CH3NH3PbBr3 (solid line) and CH3NH3PbI3 (dashed line).
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Chapter 2. Combination effect of polar solvent treatment on ZnO and polyfluorene-based polymer blends for highly efficient blue-based hybrid organic-inorganic polymer light-emitting diodes
Chapter 2 is reproduced in part with permission. Copyright 2014, The Royal Society of Chemistry.
2.1 Research background
Great interest for polymeric light-emitting diodes (PLEDs) has been increasing because of low cost and large area fabrication by solution processing and mechanical flexibility for their potential use in solid-state lighting and flexible display applications1-3. The balanced Red-Green-Blue (RGB) based device efficiencies and stability are required to realize white PLEDs specially for solid-state lighting and full-color display. However, the efficiencies of blue-based PLEDs fall far behind, compared to those of the green and red PLEDs. Therefore, highly efficient blue-based PLEDs should be developed to realize RGB display application4.
Inverted-type polymer light-emitting diodes (IPLEDs) have been studied for the substitute of conventional-type PLEDs due to exceptionally good air-stability of devices using metal-oxide as charge transport layers and gold (Au) with high work function as an anode5-8,10-17. However, the efficiencies of IPLEDs without surface modifications on solution processed n-type metal-oxide layer are very low due to the unbalanced electron and hole carrier injection/transport without charge blocking behavior5-8. Previous studies of IPLEDs demonstrated that molybdenum oxide (MoO3) provides ohmic hole injection from Au anode to organic semiconductor and electron injection is more difficult due to larger energy barrier between n-type metal-oxide and active layer in PLEDs9. Therefore, efficient electron injection with blocking the hole carriers is required to maximize the recombination of charge carriers near the interface of metal-oxide and emissive layer and enhance the efficiency in IPLEDs5. For excellent hole blocking as well as electron injection efficiency, surface modifiers such as conjugated polymer electrolyte (CPE)12,13, barium hydroxide (Ba(OH)2)15, and cesium carbonate (Cs2CO3)8,17 on n- type metal-oxides have been utilized to balance charge carrier injection/transport and maximize the recombination of holes and electrons near the interface of metal-oxide and emissive polymer, leading to improved efficiency of IPLEDs. However, there are still room for enhancement of the efficiency of IPLEDs by reducing significant exciton quenching near the interface of metal-oxide and emissive polymer with excellent hole blocking and electron injection efficiency15. Moreover, addition of a trap material into emissive layer for highly efficient PLEDs has been demonstrated to balance charge- carriers injection and transport with blocking one of charge-carrier due to the difference energy levels between two materials, leading to improvement of device efficiency18-21.
In this study, we demonstrate the significant enhancement of blue-based IPLEDs efficiency by using