Yu Kyung Eom

Novel D–π–A structured porphyrin dyes with diphenylamine derived electron-donating substituents for highly efficient dye-sensitized solar cells

A series of push–pull structured (D–π–A) porphyrin dyes with an electron-donating group (EDG) attached at the meso-position (HP, EHOP and HOP) were designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSCs). The nature of the EDG exerts a significant influence on the spectral, electrochemical and photovoltaic properties of these sensitizers. The absorption bands of these porphyrin dyes are broadened and red-shifted upon introduction of alkoxy chains to the electron-donating groups at the meso-position opposite to the anchoring cyanoacrylic acid group. Electrochemical studies showed that the first oxidation occurred at a potential greater than that of the I−/I3− redox couple. Attachment of alkoxy chains to the electron-donating group at the meso-position in the porphyrin sensitizer could prevent electron recombination and induce easy electron injection, resulting in enhanced efficiency of the DSSC. Among the sensitizers, the highest performance of the DSSC fabricated with HOP and PTZ1 as the co-absorbent was 7.6%, which was higher than that of the DSSC with HOP only by a factor of 2.9.

Edge-carboxylated graphene nanoplatelets as oxygen-rich metal-free cathodes for organic dye-sensitized solar cells

Edge-carboxylated graphene nanoplatelets (ECGnPs) were synthesized by the simple, efficient and eco-friendly ball-milling of graphite in the presence of dry ice and used as oxygen-rich metal-free counter electrodes (CEs) in organic dye-sensitized solar cells (DSSCs), for the first time. The resultant ECGnPs are soluble in many polar solvents including 2-propanol due to the polar nature of numerous carboxylic acids at edges, allowing an electrostatic spray (e-spray) to be deposited on fluorine-doped SnO2 (FTO)/glass substrates. The ECGnP-CE exhibited profound improvements in the electrochemical stability for the Co(bpy)32+/3+ redox couple compared to the platinum (Pt)-CE. The charge transfer resistance (RCT), related to the interface between an electrolyte and a CE, was significantly reduced to 0.87 Ω cm2, much lower than those of (Pt)-CE (2.19 Ω cm2), PEDOT:PSS-CE (2.63 Ω cm2) and reduced graphene oxide (rGO)-CE (1.21 Ω cm2). The DSSC based on the JK-303-sensitizer and ECGnP-CE displayed a higher photovoltaic performance (FF, Jsc, and η, 74.4%, 14.07 mA cm−2 and 9.31%) than those with the Pt-CE (71.6%, 13.69 mA cm−2 and 8.67%), PEDOT:PSS (68.7%, 13.68 mA cm−2 and 8.25%) and rGO-CE (72.9%, 13.88 mA cm−2 and 8.94%).

A simple triaryl amine-based dual functioned co-adsorbent for highly efficient dye-sensitized solar cells

We have developed 4-(bis(9,9-dimethyl-9H-flouren-2-yl)amino)benzoic acid (HC-Acid) to use as an alternative co-adsorbent to deoxycholic acid (DCA) in organic dye-sensitized solar cells (DSSCs). It has dual functioned effects, that is, the prevention effect of the π–π stacking of organic dye molecules like DCA and the light harvesting effect at shorter-wavelength regions. When the HC-Acid was used as the co-adsorbent onto the TiO2 surface with the organic dye NKX2677 sensitized solar cell, an extremely high conversion efficiency of 9.09% was achieved under 100 mW cm−2 AM 1.5G simulated light (Jsc = 18.01 mA cm−2, Voc = 0.663 V, and FF = 76.16). As we herein demonstrate, an increase in short-circuit photocurrent density (Jsc) of 20%, open-circuit photovoltage (Voc) of 11% and overall conversion efficiency (η) of 38% occurred in comparison to the NKX2677 based solar cell. As a consequence, this new class of a low molecular weight organic co-adsorbent (HC-Acid) is a promising candidate as an alternative co-adsorbent with a new function not seen in DCA or CA derivatives for highly efficient dye-sensitized solar cells.

Biphenylene-bridged mesostructured organosilica as a novel hybrid host material for LnIII (Ln = Eu, Gd, Tb, Er, Yb) ions in the presence of 2-thenoyltrifluoroacetone

A hierarchically ordered 4,4′-biphenylene-bridged mesoporous organosilica framework (Bp-PMO) has been synthesized with the aim of using it as a new hybrid host for LnIII ions. A series of novel organic–inorganic luminescent hybrid materials have been obtained by covalently linking LnIII complexes (Ln = Eu, Gd, Tb, Er, and Yb) to Bp-PMO functionalized with 2-thenoyltrifluoroacetone (HTTA). The visible and NIR photoluminescence studies of the hybrid materials show that Bp-PMO acts both as an inorganic host and an organic co-sensitizer for LnIII ions. The intrinsic quantum yield and lifetime data obtained for the hybrid material containing EuIII (54.5% and 615 μs) are among the highest values reported for hybrid materials containing EuIIItetrakis(β-diketonate) complexes. The combination of Bp-PMO and HTTA also provides sizable sensitization for TbIII, ErIII and YbIII ions. The overall quantum yields and lifetime measurements for mixed metallic hybrid materials show that Ln → Ln′ energy transfer occurs in the mixed lanthanide systems. In addition, controlling the Ln/Ln′ ratio allows one to tune the emission colour.

Structural effect of carbazole-based coadsorbents on the photovoltaic performance of organic dye-sensitized solar cells

Two types of coadsorbents based on a carbazole unit containing a carboxylic acid acceptor linked by extended π-conjugated linkers, e.g., biphenyl (HC-A′) and tetramethylbiphenyl (HC-A′′), were synthesized. They were used as coadsorbents in dye-sensitized solar cells (DSSCs) based on the porphyrin dye (2Flu–ZnP–CN–COOH:FP). For comparison, the π-conjugated phenyl linker (HC-A1) previously developed by our group was used as a coadsorbent. As a result, the DSSCs based on HC-A′ and HC-A′′ displayed power conversion efficiencies (PCEs) of 6.47 and 5.85%, respectively, while the HC-A1-based DSSC achieved a PCE of 7.46%. The HC-A′′-based DSSC exhibited lower short-circuit current (Jsc) and Voc compared to the HC-A′-based DSSC, due to the fact that the dihedral angle of the π-conjugated linkers is too high for electron injection into the TiO2 CB, and has less preventing effect on the π–π stacking of dye molecules due to its lower adsorbed amount, resulting in lower Jsc and Voc values. Therefore, it is important for coadsorbents to have a smaller dihedral angle of the π-conjugated linkers for efficient electron injection and a compact blocking layer on the TiO2 surface for preventing the π–π stacking of dye molecules, simultaneously.

Significant light absorption enhancement by a single heterocyclic unit change in the π-bridge moiety from thieno[3,2-b]benzothiophene to thieno[3,2-b]indole for high performance dye-sensitized and tandem solar cells

The molecular design of organic sensitizers is one of the fundamental factors for high-efficiency dye-sensitized solar cells (DSSCs). In this study, we first utilize the alkylated thieno[3,2-b]indole (TI) moiety as the π-bridge unit to enhance the π-bridge capability of the thieno[3,2-b]benzothiophene (TBT) used in organic sensitizers. To improve the spectral response of the SGT-130 reference dye, we strategically designed and synthesized two novel TI-based organic sensitizers, SGT-136 and SGT-137, through a simple change of the π-bridge unit. By replacing the TBT with the alkylated TI moiety, SGT-136 and SGT-137 could have a red-shifted absorption band and upshifted highest occupied molecular orbital (HOMO) energy level. As a result, the SGT-137-based DSSC exhibits a higher PCE (12.45%) than that based on SGT-130 (9.83%) owing to the improvement of current density and retardation of charge recombination by the hexyl substituted TI unit. These results indicate that the TI moiety is a good candidate for remarkable π-electronic mediators in D–π–A organic sensitizers with the characteristic of facile synthesis compared to other complicated π-bridges. Furthermore, the parallel-connected tandem device with SGT-137 and SGT-021 porphyrin-based DSSCs shows a significantly improved current density (22.06 mA cm−2) and PCE (14.64%), which is the highest value reported for organic-based tandem solar cells to date.

Ln(III)-cored complexes based on boron dipyrromethene (Bodipy) ligands for NIR emission

Two Bodipy-based chromophores have been synthesized, which bear two methoxyphenyl substituents in the 1,7 positions and either hydrogen (L1) or bromine (L2) atoms in the 2,6 positions, with the aim of testing their ability to sensitize the luminescence of near-infrared emitting LnIII ions. Despite the presence of the two halogen substituents in L2, the two chromophores display very similar absorption and emission properties, the only difference being a drop in the quantum yield from 31% for L1 to 21% for L2. In addition, no triplet state emission could be evidenced for both chromophores as well as for their tris complexes with GdIII ions. This is traced back to unfavorable conformation of the molecule, preventing efficient intersystem crossing. Complexes with stoichiometry [Ln(Li)3(tpy)] have been isolated for Ln = Gd, Er, Yb and fully characterized. Ligand emission is partially quenched (20–40%) in the ErIII and YbIII complexes and typical metal-centred luminescence is detected at 1530 (ErIII) and 978 (YbIII) nm upon excitation in the orange portion of the visible spectrum (583 nm). In these complexes, the main energy transfer pathway involves the singlet state(s) of the ligands: the lowest 1ππ state emission overlaps the Er(4F9/2 ← 4I15/2) absorption band and the 1ππ–Yb(2F5/2) gap amounts to only 5700 cm−1. This study demonstrates that the Bodipy framework is adequate for sensitizing the luminescence of NIR-emitting lanthanide ions, allowing excitation wavelengths extending into the orange portion of the visible spectrum and yielding complexes with large molar absorption coefficients (log e ≈ 5.0–5.2). Additionally it also points to the importance of the conformation of the chromophores on the yield of intersystem crossing.

New thieno[3,2-b][1]benzothiophene-based organic sensitizers containing π-extended thiophene spacers for efficient dye-sensitized solar cells

Three new thieno[3,2-b][1]benzothiophene (TBT)-based D–π–A organic sensitizers containing the thiophene π-spacer (SGT-121, 123 and 125) have been synthesized for the application of dye-sensitized solar cell (DSSC), where TBT was employed as a new fused π-bridge unit using the advantages of good co-planarity with the linkage between the thiophene unit and the phenyl unit of the triphenylamine group. Specifically, the combination of a dihexyloxyphenyl-substituted biphenylamine donor and the TBT π-bridge plays multifunctional roles, e.g., the enhanced ability of the π-bridge and donor, slow charge recombination and prevention of dye aggregation in the D–π–A sensitizer. The photophysical, electrochemical and photovoltaic properties of the SGT sensitizers were systematically investigated. As a strategy for the improvement of absorption abilities, the various thiophene derivatives, e.g., those with thiophene (T, SGT-121), bithiophene (BT, SGT-123) and thienothio[3,2-b]thiophene (TT, SGT-125) moieties, were incorporated as π-spacers between the TBT π-bridge and the acceptor unit. The introduction of thiophene π-spacers significantly improved the photovoltaic performance (in particular, in terms of the photocurrent Jsc and open-circuit voltage Voc) compared to SGT-127 without the thiophene unit. The SGT sensitizers were systematically evaluated for DSSCs based on the Co(bpy)32+/3+ (bpy = 2,2′-bipyridine) redox couple. Among the four SGT sensitizers, SGT-123-based DSSC including the BT moiety exhibited the highest power conversion efficiency of 9.69%, Jsc of 16.16 mA cm−2, Voc of 830 mV and fill factor of 0.72. These results present the impact of thiophene π-spacers for enhancing the photovoltaic performances of a D–π–A organic sensitizer.

Lanthanide(III) dendrimer complexes based on diphenylquinoxaline derivatives for photonic amplification

AbstractA series of novel lanthanide(III) complexes (Ln=Gd, Er, Yb) based on dendritic diphenylquinoxaline (DPQ) ligands was designed and synthesized with the aim of enhancing the luminescence intensity of Er3+ and Yb3+ ions for photonic applications. The diphenyl-quinoxaline ligand was introduced as a photon antenna for efficient light harvesting and subsequent energy transfer onto the Ln3+ ions. The dendritic complexes showed strong near-IR emission at 981 (Yb3+) and 1,530 (Er3+) nm, which was sensitized through energy transfer from the excited states of the diphenyl-quinoxaline ligands. The near-IR emission intensity of the lanthanide ions in second-generation [Ln(G2-DPQ-COO)3(terpy)] complexes was significantly enhanced, due to the light-harvesting effect, with respect to [Ln(G1-DPQ-COO)3(terpy)]. However, increasing the size of the dendron in [Ln(G3-DPQ-COO)3(terpy)] was found to be detrimental to the emission efficiency. This may be attributed to the twisted structure of the dendritic ligand and suggests that conformational effects should be taken into consideration when designing ligands for photonic amplification.

Luminescent Lanthanide Complexes for Advanced Photonics Applications

Luminescent lanthanide complexes have been overviewed for advanced photonics applications. Lanthanide(III) ions () were encapsulated by the luminescent ligands such as metalloporphyrins, naphthalenes, anthracene, push-pull diketone derivatives and boron dipyrromethene(bodipy). The energy levels of the luminescent ligands were tailored to maintain the effective energy transfer process from luminescent ligands to ions for getting a higher optical amplification gain. Also, key parameters for emission enhancement and efficient energy transfer pathways for the sensitization of ions by luminescent ligands were investigated. Furthermore, to enhance the optophysical properties of novel luminescent complexes, aryl ether-functionalized dendrons as photon antennas have been incorporated into luminescent complexes, yielding novel -cored dendrimer complex such as metalloporphyrins, naphthalenes, and anthracenes bearing the Frchet aryl-ether dendrons, namely, ( (terpy), (terpy) and (terpy)). These complexs showed much stronger near-IR emission bands at 1530 nm, originated from the 4f-4f electronic transition of the first excited state () to the ground state () of the partially filled 4f shell. A significant decrease in the fluorescence of metalloporphyrins, naphthalenes and anthracene ligand were accompanied by a strong increase in the near IR emission of the ions. The near IR emission intensities of ions in the lanthanide(III)-encapsulated dendrimer complexes were dramatically enhanced with increasing the generation number (n) of dendrons, due to the site-isolation and the light-harvesting(LH) effects. Furthermore, it was first attempted to distinguish between the site-isolation and the light-harvesting effects in the present complexes. In this review, synthesis and photophysical studies of inert and stable luminescent complexes will be dealt for the advanced photonics applications. Also, the review will include the exploratory investigation of the key parameters for emission enhancement and the effective energy transfer pathways from luminescent ligands to ions with -chelated prototype complexes.