Deok-Ho Roh

G ENERAL I NTRODUCTION

In a π-conjugated system, the electrons in the π-orbitals are delocalized across the molecule, resulting in a resonance structure (Figure 1.1). In particular, organic materials containing π-conjugated systems can behave as semiconductors and exhibit optoelectronic properties.3,4.

Applications of π-Conjugated Organic Materials

The DSC image is taken from the Swiss Tech Convention Center, Copyright © RDR/Fernando Guerra (actu.epfl.ch/news/participation-au-15eme-congres-photovoltaique-suis/). The electric vehicle diagram was taken from the Argonne National Laboratory (www.flickr.com/photos/argonne.

Molecular Engineering of π-Conjugated Organic Materials

Energy level scheme and ICT are reproduced with permission from ref 30, Copyright © 2017 Springer Nature. Control image of alkyl side chains in photosensitizers reproduced with permission from ref 40, Copyright © 2013 American Chemical Society.

Figure 1.3. Molecular engineering strategies of π-conjugated organic materials. The schematic of energy levels and ICT are reproduced with permission from ref 30 , Copyright © 2017 Springer Nature

E LECTRONIC C OUPLING

Electronic Coupling and Marcus Theory
Marcus–Hush Theory – Adiabatic Free-Energy Surfaces
Derivation of a Two-State Model
Derivation of a Three-State Model
Description of a Charge-Transfer Band Shape
Mulliken–Hush Theory – Evaluation of Electronic Coupling

The local minima of the adiabatic free energy surfaces in the ground state represent the initial and final states. Detailed derivation and equations of the diabatic and adiabatic free energy surfaces are as follows.

Figure 1.5. Diabatic free-energy surfaces of a donor–acceptor material. Black solid lines indicate the diabatic free-energy surfaces

E NERGY C ONVERSION S YSTEM

Dye-Sensitized Solar Cells
Photosensitizers

In general, organic photosensitizers have been designed with a donor–π-bridge–acceptor configuration to maximize ICT properties (Figure 1.16a). However, as shown in Figure 1.18, photosensitizers with a low band gap of 1.3–1.6 eV show frustrated photovoltaic parameters.

Figure 1.12. Working principle of DSCs. Reproduced with permission from ref 62 , Copyright © 2012 Nature Publishing Group

E NERGY S TORAGE S YSTEM

Supercapacitors
Conjugated Microporous Polymers

Therefore, the capacity of EDLC is directly related to the surface area and pore size of the active materials. Therefore, the energy storage performance of supercapacitors is mainly determined by the physical and electrochemical properties of the active materials.

Figure 1.20. Working principle and device structure of a supercapacitor. The device structure of supercapacitor comprised of two porous active material-coated electrodes, a separator, and an electrolyte

R EFERENCES

Electronic coupling of BT-based sensitizers also significantly affects the light-harvesting efficiency (LHE) of TiO2 films (Figure 3.5d). Optical properties of BT-based sensitizers. a) Electronic absorption and (b) photoluminescence spectra of BT-based sensitizers in solution [20 µM in CHCl3:EtOH (v/v = 7:3)]. The electronic coupling of the BT-based sensitizers was controlled by the conformational effect of the π-spacers.

Optical properties of DPP-based sensitizers. a) Electronic absorption profiles in solutions (20 mM in tetrahydrofuran). CV measurements (Figure 4.5) were performed to evaluate the electrochemical properties of DPP-based sensitizers. DPP-based fs-TA DSC profiles with TiO2 films are shown in Figure 4.19.

M ATERIALS AND M ETHODS

Photosensitizers

The ground state oxidation potentials (ES/S+) of the BT-based sensitizers were measured using CV curves (Figure 3.6a). The current-voltage (J-V) results of the BT-based sensitizers under TiO2 thin films (1.8 μm) are shown in Figure 3.7c. Adjust parameters for the fs-TA kinetic decay of the DPP-based DSCs with TiO2 films.

The improved JSC of the DPP-based DSCs was clearly shown in the IPCE spectra (Figure 4.27b).

Figure 2.1. Temperature profile for the sintering process of the photoanode. The ramp rate and sintering time for each step are indicated in the panel

Monomers of Conjugated Microporous Polymers

M ETHODS

Nuclear Magnetic Resonance
High-Resolution Mass Spectrometry
Fourier-Transform Infrared Spectroscopy
Electrochemical Measurement
Device Fabrication
Desorption of Dyes from the Photoanode

FTO current collector glasses were cleaned with deionized (DI) water, ethanol, acetone, and water for 15.0 min consecutively under continuous sonication. Cleaned FTO glasses were treated with a UV-ozone system for 20.0 min to make their surface hydrophilic. Compact TiO2-coated FTO glasses were fabricated on UV-ozone-treated FTO glass by immersing the substrate in 40.0 mM TiCl4 (aq) solution in an oven at 70.0 °C for 30.0 min and then washed sequentially with DI water and ethyl alcohol.

For the Pt-counter electrode, the FTO glasses were drilled to make a hole and then cleaned by DI. After the electrodeposition process, the PEDOT counter electrode was washed with ethanol and then dried using N2 gas. The amount of photosensitizer adsorption on the photoanode was estimated using Beer Lambert's law (A = ε·l·c), where c is the amount of dye loading, l is the optical path length (1.0 cm), ε is the extinction coefficient of the photosensitizer, and A absorbance values of the desorption solution (0.1 M triethylamine in CH2Cl2) measured by UV-vis spectroscopy after separation of the sensitizers from the photoanode.

R EFERENCES

I NTRODUCTION

This result indicates that photosensitizers with weakened electronic coupling need a thick photoelectrode (>9 μm) to enhance their low light-harvesting ability. However, this strategy is limited in the commercialization of DSCs due to cost issues of using a large amount of sensitizing materials and is also unsuitable for flexible DSCs that require thin photoelectrodes. 17,18 Furthermore, a previous study reported that porphyrin photosensitizers showed lower photovoltaic performance (PCE = 5.6%) under a thin photoelectrode (2.4 μm) than a thick photoelectrode (PCE). Therefore, it is essential to develop electronic coupling engineering and device strategies to overcome the BET problem under thin photoelectrode DSC. we synthesized five BT-based photosensitizers and modified their electronic coupling using a π-spacer.

The electronic coupling of the photosensitizers was controlled by varying the torsion angles between the BT and the π-spacer units (Figure 3.1a). The photovoltaic results explain that the reduction of the electronic coupling of the photosensitizers inhibits the BET process (Figure 3.1b), while the light-harvesting ability was reduced. As a result, BT-T with a planar π-spacer exhibits the highest PCE of 6.5% among BT-based sensitizers under a thin photoelectrode (1.8 μm).

R ESULTS AND D ISCUSSION

Design and Synthesis
Computational Study and Electronic Coupling
Optical Properties
Electrochemical Properties
Photovoltaic Performance
Charge Collection Efficiency
Electrochemical Impedance Spectroscopy
Photostability

Schematic figure of the BT-based sensitizers. a) Molecular structures of the BT-based sensitizers with torsion angles. DFT calculation results of the BT-based sensitizers. a) Optimized ground-state geometries of the BT-based sensitizers. The excited-state oxidation potentials (ES*/S+) of the BT-based sensitizers were evaluated using the following equation: ES*/S+ = ES/S+ − optical band gap (E0–0).

The photovoltaic performances of the BT-based DSCs were evaluated under different TiO2 film thicknesses from 1.8 μm to 10 μm. We evaluated the charge collection efficiency of the BT-based DSCs under different TiO2 film thicknesses by the following equation:27. Overall, the BT-based DSCs exhibit improved charge collection efficiency as the TiO2 film thickness decreased.

Figure 3.2. Synthesis scheme of the BT-based sensitizers.

C ONCLUSION

The photostability of the BT-based sensitizers was evaluated by the change in absorption spectra of dye-absorbed TiO2 films without any regeneration process under 1 sun conditions.30 Figure 3.12 shows the change in absorption spectra of the BT-based sensitizers absorbed on TiO2 films. under light irradiation for 0–60 min. BT-T exhibits more robust photostability than other BT-based sensitizers because the high electronic coupling induces the effective ICT and thus could stabilize the oxidized state of sensitizers. The change in absorption spectra of the BT-based sensitizers on 1.8 μm thick TiO2 films before and after light irradiation for 20, 40 and 60 min.

Therefore, the twisted sensitizers with relatively low electronic coupling show inferior photovoltaic performance than the planar sensitizer, BT-T, under TiO2 thin films of 1.8 μm, even though the twisted structure suppresses the BET. BT-T maintains a high LHE of 90% under 1.8 μm-thick TiO2 thin films and high charge collection efficiency (≥97%) mitigates the influence of BET on DSC with BT-T, as the film thickness TiO2 decreases. Increasing the electronic coupling of photosensitizers is a suitable strategy for achieving high DSC performance because the light harvesting ability is the more important factor than the BET suppression in TiO2 thin films.

R EFERENCES

These electrochemical and photophysical results of DPP-based sensitizers are also consistent with the DFT calculation results (Figures 4.7 and 4.12). The presented electronic coupling engineering was confirmed by measuring the electronic coupling of the photosensitizers. Furthermore, electronic couplings are important parameters to understand the photophysics of DPP-based sensitizers.

Evaluation electronic coupling of the TPA-based monomers. a) Electronic absorption spectra changes of the TPA-based monomers. As shown in Figure 5.10, the TPA-based monomers exhibit two characteristic absorption bands at 300-550 nm. The energy storage performance of the TPA-based CMPs strongly depends on the degree of electronic coupling between the TPA and adjacent devices.

Figure 4.1. Electronic coupling engineering strategy inducing VET. Schematic free-energy surfaces for (a) the stongly and (b) weakly coupled donor-acceptor materials

C HAPTER 2. E LECTRONIC C OUPLING E NGINEERING FOR R EALIZING V ECTORIAL

R ESULTS AND D ISCUSSION

Electronic Coupling Engineering Strategy of the DPP-Sensitizers
Material Characterization
Electronic Coupling and Spin Density
Transient-Absorption Spectroscopic Study
Photovoltaic Performance

The dihedral angle changed the energy level and molecular orbital (MO) of DPP-sensitizers (Figure 4.7). Pump wavelengths were 560 nm for the solution and DSC samples for sensitizer excitation. fs-TA kinetic decays of DPP-based sensitizers. a) DPS based on DPP with TiO2. Fit parameters for fs-TA kinetic decays of DPP-based DSCs with Al2O3 films and DPP-based sensitizers in solution.

Fit parameters for fs-TA kinetic decay of the DSCs with DD-DPP-Ph and a TiO2. We performed ns-TA spectroscopy measurements of the DPP-based DSCs with iodine or redox-inactive inert electrolytes to investigate non-geminate BET and charge regeneration. The wavelength of the pump-pulse laser was 532 nm, which corresponds to the ICT range of the DPP-based sensitizers.

Figure 4.2. Ground-state optimized geometries and dihedral angles of the DPP-based sensitizers

C ONCLUSION

Photovoltaic performance of DSCs co-sensitized with cobalt electrolyte. a) Chemical structures of bTPA-DPP-DMP and D35.

R EFERENCES

The polymerization yields, surface areas, and electrochemical properties of the TPA-based CMPs are highly dependent on the electronic coupling values of the monomers. Scanning electron microscopy (SEM) measurements were performed to observe the porous structures of the TPA-based CMPs. The semiconducting properties and high surface area of the TPA-based CMPs are suitable for use in supercapacitors.

Therefore, the energy storage properties of TPA-based CMPs were investigated by measuring the CV in a three-electrode configuration. CV results of TPA-based CMPs at different scan rates. a–c) CV curves of gravimetric current density versus potential. Energy conservation performance of CMP-based ASC. a) Device structure of CMP-based ASC.

Figure 5.1. Chemical structures of the TPA-based monomers and CMPs. (a) Schematic of electronic coupling in the TPA-based monomers

C HAPTER 3. E LECTRONIC C OUPLING E NGINEERING OF C ONJUGATED

R ESULTS AND D ISCUSSION

Monomer Design Strategy and Electronic Coupling
Materials Characterization: Solid-State NMR
Materials Characterization: FTIR
Materials Characterization: UV–Vis Spectroscopy
Materials Characterization: Porosity
Electrochemical Analysis of The CMP Films
The LBL structured CMP/CNT supercapacitor
Asymmetric Supercapacitor: CMP-BT/CNT//RGO/CNT

DFT calculation results of the TPA-based monomers. a) Ground state optimized geometries of the TPA-based monomers and corresponding to their HOMO and LUMO. -BT/CNT supercapacitor by ultrasonic spray deposition method.15 (b) Electron microscopy image of the LBL structured CMP-BT/CNT supercapacitor. To further improve the charge storage performance of the TPA-based CMPs, an asymmetric supercapacitor (ASC) was fabricated using the LBL-structured CMP-BT/CNT electrode and the similar LBL-structured RGO/CNT electrode14, as described in Figure 5.19a.

Potential windows for LBL-structured RGO/CNT and LBL-structured CMP-BT/CNT are 0–1.0 and 0.9–1.7 V, respectively, as shown in Figure 5.19b. The working potential of the CMP-based ASC was optimized by measuring CV and GCD (Figures 5.19c and 5.19d). The energy storage performances of the CMP-based ASC were evaluated by GCD results (Figure 5.19f), and their values are summarized in Figure 5.19g.

Table 5.1. Electronic coupling values and distance between the molecular units.

C ONCLUSION

R EFERENCES

Molecular level control of the capacitance of two-dimensional covalent organic frameworks: The role of hydrogen bonding in energy storage materials. Conjugated microporous polymer based on star-shaped triphenylamine-benzene structure with improved electrochemical performance as the organic cathode material of Li-Ion batteries. Photosensitizers for dye-sensitized solar cells and photoelectrochemical cells - Conjugated microporous polymers for energy storage systems.

Deok-Ho Roh,† HyeonOh Shin,†Hyun-Tak Kim,†in Tae-Hyuk Kwon* (†Enakovreden parvisik) ACS Appl. Jun-Hyeok Park,† Un-Young Kim,† Byung-Man Kim, Wang-Hyo Kim, Deok-Ho Roh, Jeong Soo Kim, Tae-Hyuk Kwon*. HyeonOh Shin,† Byung-Man Kim,† Taehyung Jang, Kwang Min Kim, Deok-Ho Roh, Jung Seung Nam, Jeong Soo Kim, Un-Young Kim, Byunghong Lee, Yoonsoo Pang* in Tae-Hyuk Kwon*.