In Taek Choi

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-

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%).

Unassisted photoelectrochemical water splitting exceeding 7% solar-to-hydrogen conversion efficiency using photon recycling

Various tandem cell configurations have been reported for highly efficient and spontaneous hydrogen production from photoelectrochemical solar water splitting. However, there is a contradiction between two main requirements of a front photoelectrode in a tandem cell configuration, namely, high transparency and high photocurrent density. Here we demonstrate a simple yet highly effective method to overcome this contradiction by incorporating a hybrid conductive distributed Bragg reflector on the back side of the transparent conducting substrate for the front photoelectrochemical electrode, which functions as both an optical filter and a conductive counter-electrode of the rear dye-sensitized solar cell. The hybrid conductive distributed Bragg reflectors were designed to be transparent to the long-wavelength part of the incident solar spectrum (λ>500 nm) for the rear solar cell, while reflecting the short-wavelength photons (λ<500 nm) which can then be absorbed by the front photoelectrochemical electrode for enhanced photocurrent generation.

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.

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.

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.