Myung Jong Ju

N-Doped graphene nanoplatelets as superior metal-free counter electrodes for organic dye-sensitized solar cells.

Highly efficient counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) were developed using thin films of scalable and high-quality, nitrogen-doped graphene nanoplatelets (NGnP), which was synthesized by a simple two-step reaction sequence. The resultant NGnP was deposited on fluorine-doped SnO2 (FTO)/glass substrates by using electrospray (e-spray) coating, and their electrocatalytic activities were systematically evaluated for Co(bpy)3(3+/2+) redox couple in DSSCs with an organic sensitizer. The e-sprayed NGnP thin films exhibited outstanding performances as CEs for DSSCs. The optimized NGnP electrode showed better electrochemical stability under prolonged cycling potential, and its Rct at the interface of the CE/electrolyte decreased down to 1.73 Ω cm(2), a value much lower than that of the Pt electrode (3.15 Ω cm(2)). The DSSC with the optimized NGnP-CE had a higher fill factor (FF, 74.2%) and a cell efficiency (9.05%), whereas those of the DSSC using Pt-CE were only 70.6% and 8.43%, respectively. To the best of our knowledge, the extraordinarily better current-voltage characteristics of the DSSC-NGnP outperforming the DSSC-Pt for the Co(bpy)3(3+/2+) redox couple (in paticular, FF and short circuit current, Jsc) is highlighted for the first time.

Direct nitrogen fixation at the edges of graphene nanoplatelets as efficient electrocatalysts for energy conversion

Nitrogen fixation is essential for the synthesis of many important chemicals (e.g., fertilizers, explosives) and basic building blocks for all forms of life (e.g., nucleotides for DNA and RNA, amino acids for proteins). However, direct nitrogen fixation is challenging as nitrogen (N2) does not easily react with other chemicals. By dry ball-milling graphite with N2, we have discovered a simple, but versatile, scalable and eco-friendly, approach to direct fixation of N2 at the edges of graphene nanoplatelets (GnPs). The mechanochemical cracking of graphitic C−C bonds generated active carbon species that react directly with N2 to form five- and six-membered aromatic rings at the broken edges, leading to solution-processable edge-nitrogenated graphene nanoplatelets (NGnPs) with superb catalytic performance in both dye-sensitized solar cells and fuel cells to replace conventional Pt-based catalysts for energy conversion.

A Desirable Hole‐Conducting Coadsorbent for Highly Efficient Dye‐Sensitized Solar Cells through an Organic Redox Cascade Strategy

Over the past decade, dye-sensitized solar cells (DSSCs) based on nanocrystalline mesoporous metal oxides (typically TiO2) have been intensively studied and developed as a promising, low-cost alternative to conventional silicon photovoltaic devices. The components of such DSSCs are a dye sensitizer, a TiO2 metal oxide coated on conductive glass, a redox electrolyte couple, and a counter electrode. As illustrated in Scheme 1, the power-conversion efficiency of DSSCs is strongly dependent upon the minimization of charge recombination losses at the TiO2/dye/electrolyte interface (pathways 3 and 4). There are two recombination pathways of importance in DSSCs, in which electrons photoinjected into the TiO2 electrode can recombine with either dye cations or with redox electrolytes. Moreover, such charge recombination leads to losses in both the short-circuit photocurrent (Jsc) and the open-circuit photovoltage (Voc), resulting in a decrease in overall energy conversion efficiency (h). To reduce the possible charge recombination pathways occurring at the TiO2/dye/electrolyte interface, several kinds of additives, such as decylphosophonic acid (DPA), dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DINHOP), and chenodeoxycholic acid (CDCA) have been introduced to adsorb onto the TiO2 surface. [3] Among them, for example, cholic acid (CA) derivatives as coadsorbents in DSSCs, based on a Ru–pyridyl complex, courmarin, porphyrin, phthalocyanine, and naphthalocyanine dye, have been investigated well. Deoxycholic acid (DCA) is often used as a coadsorbent to break up organic and Ru–dye aggregates and hence to significantly improve Voc and Jsc. [5,9] Moreover, alternative approaches directed toward the minimization of recombination losses have been extensively studied by not only the insertion of a metal oxide blocking layer, but also by energetic cascades for multistep hole conductors (HC), as well as superior molecular sensitizer dyes and the insertion of a barrier layer between the sensitizer dye and the HC. To date, however, the desired redox intermediate mediators between the dye and I /I3 redox electro-

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%).

Nb-doped TiO2 nanoparticles for organic dye-sensitized solar cells

Nb-doped anatase TiO2 nanoparticles were prepared by the sol–gel process followed by a hydrothermal treatment and successfully used as the photoanodes in organic dye-sensitized solar cells (DSSCs). Phase identification of the TiO2 samples was confirmed by X-ray diffraction and Raman shift spectroscopy. The electronic structure and Nb doping in the TiO2 lattice were confirmed by X-ray photoelectron spectroscopy and energy-disperse X-ray spectroscopy, respectively. Also, the conduction band edge (CB) shift due to the Nb-doping in the TiO2 lattice by UV-vis diffuse reflectance spectroscopy and the effect of Nb doping on the charge transporting and recombination behaviors of the DSSCs by electrochemical impedance spectroscopy (EIS) analysis were investigated. The Nb-doped TiO2 exhibited a positive shift of the conduction band edge (CB) compared to the undoped TiO2. Consequently, the increased driving force for electron injection, that is, the difference between the CB of TiO2 and the lowest unoccupied molecular orbital (LUMO) energy level of the dye, could correspondingly contribute to the enhancement of the electron injection efficiency, but the Voc has the opposite behavior. The Voc drop in the DSSCs based on the Nb-doped TiO2 could be prevented using a multi-functional HC-A as a coadsorbent instead of DCA. As expected, a PCE of 7.41% was obtained for the NKX2677/HC-A-sensitized DSSC based on the 2.5 mol% Nb-doped TiO2, which was an improvement of 11% relative to that of the DSSC based on the undoped TiO2.

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.

Novel D–π–A structured Zn(II)–porphyrin dyes with bulky fluorenyl substituted electron donor moieties for dye-sensitized solar cells

Two kinds of push–pull structured (D–π–A) Zn(II)–porphyrin dyes with bulky fluorenyl substituted electron donor moieties were designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSCs). A high solar-to-electricity conversion efficiency of 8.46% was achieved with a short circuit current (Jsc) of 15.39 mA cm−2, an open circuit voltage (Voc) of 0.74 V and a fill factor (FF) of 0.74 for the 2,4-ZnP-CN-COOH dye with HC-A1, which is a multi-functional co-adsorbent previously developed by our group, under 100 mW cm−2 AM 1.5 G simulated light.

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.

Molecular Engineering of Organic Sensitizers with Planar Bridging Units for Efficient Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have received a great deal of attention as low-cost alternatives to conventional p– n junction solar cells. In these cells, the sensitizer is the key component. Although several Ru polypyridyl complexes exhibited high efficiencies above 10 % and long-term stability, they are quite expensive and hard to purify. Recently, the performance of solar cells based on organic sensitizers has been remarkably improved, resulting in impressive efficiencies in the range of 8–10 %. However, one of the drawbacks of organic sensitizers is the sharp and narrow absorption bands of their UV spectra in the blue region, impairing their light-absorption capabilities. Therefore, molecular engineering of organic sensitizers is required in order to broaden and redshift their absorption spectra. A successful approach was achieved through structural modification of the bridged unit. The introduction of a planar p-conjugated unit in the bridged framework is presumed to be the reason for the increase in the spectral response in the red region of the solar spectrum. Although organic-dye-based cells using an I /I3 electrolyte have afforded high power conversion efficiencies, recent studies on replacing the conventional I /I3 electrolyte with a Co/Co electrolyte have received renewed attention. Recently, Yella et al. reported an efficiency of 12.3 % by using a Co/Co electrolyte in conjunction with a porphyrin sensitizer. While high conversion efficiency of 9–10 % has been reached with organic sensitizers and porphyrin dyes using a liquid electrolyte, such as I /I3 or Co/Co redox couple, the stability issue still remains a major challenge due to leakage and evaporation. Accordingly, extensive studies have been conducted to substitute liquid electrolytes with quasi-solid-state or solid-state electrolytes. Herein, we report meticulously designed organic sensitizers incorporating a planar indenoACHTUNGTRENNUNG[1,2-b]thiophene or indenoACHTUNGTRENNUNG[1,2-b]thienoACHTUNGTRENNUNG[2,3-d]thiophene bridging unit to understand the structure–property relationship (Scheme 1). We also investigate the photovoltaic performance of dyes using I /I3 , Co/Co, polymer gel, and solid-state electrolytes.

Molecular design and synthesis of ruthenium(II) sensitizers for highly efficient dye-sensitized solar cells

We synthesized hydrophobic ruthenium(II) sensitizers (SY-04 and SY-05) with high molar extinction coefficient by extending the π-conjugation of 3,4- or 3-alkylthiophene-substituted bipyridine ligands. Both dyes displayed a remarkably high molar extinction coefficient of 21.7 × 103 M−1 cm−1 arising from red-shift of their metal-to-ligand charge transfer band when compared to a commonly used N3 sensitizer. The solar-to-electrical energy conversion efficiency (η) of the SY-04 based dye-sensitized solar cell (DSC) was 7.70%, which is 27% higher than that (6.05%) of the N3-based DSC under the same cell fabrication conditions. The increased η was attributed to the increase in life time and recombination half-life measured by electrochemical impedance and transition absorption spectroscopy, respectively. Density functional theory and time-dependent density functional theory calculations of two dyes in both gas phase and solution were performed. The calculated values of the highest occupied and the lowest unoccupied molecular orbitals and absorption spectra are in good agreement with the experimental results.