Kimin Lim

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.

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%).

New efficient ruthenium sensitizers with unsymmetrical indeno[1,2-b]thiophene or a fused dithiophene ligand for dye-sensitized solar cells.

Two novel ruthenium sensitizers containing unsymmetrical indeno[1,2-b]thiophene or a fused dithiophene unit in the ancillary ligand have been designed and synthesized. The photovoltaic performance of JK-188 using an electrolyte consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M I(2), 0.1 M LiI, 0.05 M guanidinium thiocyanate, and 0.5 M tert-butylpyridine in acetonitrile revealed a short-circuit photocurrent density of 18.60 mA/cm(2), an open-circuit voltage of 0.72 V, and a fill factor of 0.71, yielding an overall conversion efficiency of 9.54%. The cell exhibits a remarkable stability under 1000 h of light soaking at 60 °C using a quasi-solid-state electrolyte consisting of 5 wt % poly(vinylidenefluoride-co-hexafluoropropylene), 0.6 M 1-propyl-2,3-dimethylimidazolium iodide, 0.5 M N-methylbenzimidazole, and 0.1 M I(2) in 3-methoxypropionitrile, retaining 97% of the initial efficiency (7.38%).

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.

Copolymer-templated nitrogen-enriched nanocarbons as a low charge-transfer resistance and highly stable alternative to platinum cathodes in dye-sensitized solar cells

In this report, we demonstrate the superior performance of dye-sensitized solar cells (DSSCs) with novel, metal-free counter electrodes (CEs) comprised of copolymer-templated nitrogen-enriched nanocarbons (CTNCs) with well-controlled morphology, nanoporosity, and nitrogen content. This superior performance is due to the high catalytic activity of CTNCs toward the reduction of Co(bpy)32+/3+, as evidenced by unusually low charge transfer resistance (RCT) at the CE–electrolyte interface. The observed activity is attributed to the combination of the high surface area of CTNCs afforded by a three-dimensional, hierarchical pore structure, and to their unique electronic properties stemming from the presence of nitrogen heteroatoms located on the edges of nanographitic domains. Altogether, the use of CTNC CEs enhanced the efficiency and fill factor (FF) of JK-306 dye, Co(bpy)32+/3+ redox couple based DSSCs at one sun illumination up to 10.32% and 73.5%, respectively, suggesting the considerable promise of these materials as an attractive alternative to costly Pt-based CEs. Interestingly, the use of CTNCs did not lead to the analogous beneficial lowering of RCT in I−/I3− redox couple based N719-sensitized DSSCs, limiting their FF and short circuit current density (JSC). This chemical specificity indicates that the type of nitrogen bonding configurations, rather than the total N-content, is the key factor determining the catalytic activity.

Molecular engineering of organic sensitizers for highly efficient gel-state dye-sensitized solar cells

Three novel organic sensitizers incorporating a planar 4,4-dimethyl-4H-indeno[1,2-b]thiophene or 4,4-dimethyl-4H-indeno[1,2-b]thienothiophene in the bridged group have been designed and synthesized for use in gel-state dye-sensitized solar cells. The photovoltaic performance is quite sensitive to the bridged unit. The optimized cell in JK-276 using a gel electrolyte gave a short circuit photocurrent density of 16.61 mA cm−2, an open circuit voltage of 0.69 V and a fill factor of 0.72, affording an overall conversion efficiency of 8.31% under standard global AM 1.5 solar cell conditions. The efficiency is close to the value of the liquid-state cell (8.40%). The gel-electrolyte cell of JK-276 showed an excellent long-term stability, which remained almost unchanged during the 300 h light soaking test at 70 °C.

Synthesis and properties of low bandgap star molecules TPA-[DTS-PyBTTh3]3 and DMM-TPA[DTS-PyBTTh3]3 for solution-processed bulk heterojunction organic solar cells

Correction for ‘Synthesis and properties of low bandgap star molecules TPA-[DTS-PyBTTh3]3 and DMM-TPA[DTS-PyBTTh3]3 for solution-processed bulk heterojunction organic solar cells’ by Kimin Lim et al., J. Mater. Chem. C, 2014, 2, 8412–8422.