Fine-tuning the Electronic Structure of Organic Dyes for Dye-Sensitized Solar Cells

A series of metal-free organic dyes exploiting different combinations of (hetero)cyclic linkers (benzene, thiophene, and thiazole) and bridges (4<i>H</i>-cyclopenta[2,1-<i>b</i>:3,4-<i>b′</i>]dithiophene (CPDT) and benzodithiophene (BDT)) as the central π-spacers were synthesized and characterized. Among them, the sensitizer containing the thiophene and CPDT showed the most broad incident photon-to-current conversion efficiency spectra, resulting in a solar energy conversion efficiency (η) of 6.6%

Analysis of Electron Transfer Properties of ZnO and TiO₂ Photoanodes for Dye-Sensitized Solar Cells

Mesoporous TiO₂ nanoparticle films are used as photoanodes for high-efficiency dye-sensitized solar cells (DSCs). In spite of excellent photovoltaic power conversion efficiencies (PCEs) displayed by titanium dioxide nanoparticle structures, the transport rate of electrons is known to be low due to low electron mobility. So the alternate oxides, including ZnO, that possesses high electron mobility are being investigated as potential candidates for photoanodes. However, the PCE with ZnO is still lower than with TiO₂, and this is typically attributed to the low internal surface area. In this work, we attempt to make a one-to-one comparison of the photovoltaic performance and the electron transfer dynamics involved in DSCs, with ZnO and TiO₂ as photoanodes. Previously such comparative investigations were hampered due to the morphological differences (internal surface area, pore diameter, porosity) that exist between zinc oxide and titanium dioxide films. We circumvent this issue by depositing different thicknesses of these oxides, by atomic layer deposition (ALD), on an arbitrary mesoporous insulating template and subsequently using them as photoanodes. Our results reveal that at an optimal thickness ZnO exhibits photovoltaic performances similar to TiO₂, but the internal electron transfer properties differ. The higher photogenerated electron transport rate contributed to the performances of ZnO, but in the case of TiO₂, it is the low recombination rate, higher dye loading, and fast electron injection

Blue Phosphorescence of Trifluoromethyl- and Trifluoromethoxy-Substituted Cationic Iridium(III) Isocyanide Complexes

We report the first comprehensive comparative synthetic, structural, electrochemical, and spectroscopic study of an extended series of fluorocarbon-modified iridium­(III) complexes. We prepared seven new cationic Ir­(III) complexes with <i>tert</i>-butyl isocyanide and trifluoromethyl- or trifluoromethoxy-substituted cyclometalating 2-phenylpyridines, [(C^∧N)₂Ir­(CN<i>t</i>Bu)₂]­(CF₃SO₃), and characterized five of them by crystal structure analysis. The redox potentials and photophysical properties of Ir­(III) complexes are determined by the type, position, and number of fluorocarbon groups in the cyclometalating ligand. The complexes exhibit pale blue to yellow-green phosphorescence at room temperature with quantum yields and excited-state lifetimes up to 73% and 84 μs in solution (under argon) and 7.5% and 4.3 μs in neat solid (under air). The structured and solvent-independent phosphorescence spectra, with 0–0 emission transition at 445–467 nm, and the long calculated radiative lifetimes, 43–160 μs, indicate that the complexes emit from a cyclometalating-ligand-centered triplet excited state. Bulky fluorocarbon groups prevent intermolecular interaction (aggregation) of the complexes, thereby minimizing red-shift of phosphorescence color in going from solution to neat solid

Molecular Engineering of Phthalocyanine Sensitizers for Dye-Sensitized Solar Cells

A series of novel near-infrared-absorbing zinc phthalocyanines bearing donor–chromophore–acceptor/anchoring groups have been synthesized to investigate their influence on solar cell performance. With the aim of extending the absorption spectra while maintaining the minimization of aggregation by using 2,6-diphenylphenoxy bulky groups, the benzodithiadazole was used as an electron acceptor moiety and the carboxylic acid and the anhydride unit were adopted as an anchoring group. These dyes have been used as sensitizers in dye-sensitized solar cells, and their performance has been compared with that of phthalocyanine (Pc) sensitizer TT40, which has an ethynyl bridge between the anchoring carboxy group and the macrocycle. The new ZnPcs show an extended π-conjugated system which produces red shift in the absorption maximum of ca. 10 nm in comparison to that of TT40. Despite the red-shifted spectral response, these cells gave modest power conversion efficiencies of ca. 3% because of the lower lowest unoccupied molecular orbital level of the Pcs, which limits the electron injection from the dyes to the TiO₂

Extreme Tuning of Redox and Optical Properties of Cationic Cyclometalated Iridium(III) Isocyanide Complexes

We report seven heteroleptic cationic iridium­(III) complexes with cyclometalating N-arylazoles and alkyl/aryl isocyanides, [(C^∧N)₂Ir­(CNR)₂]­(CF₃SO₃), and characterize two of them by crystal structure analysis. The complexes are air- and moisture-stable white solids that have electronic transitions at very high energy with absorption onset at 320–380 nm. The complexes are difficult to reduce and oxidize; they exhibit irreversible electrochemical processes with peak potentials (against ferrocene) at −2.74 to −2.37 V (reduction) and 0.99–1.56 V (oxidation) and have a large redox gap of 3.49–4.26 V. The reduction potential of the complex is determined by the azole heterocycle (pyrazole or indazole) and by the isocyanide (<i>tert</i>-butyl or 2,6-dimethylphenyl) and the oxidation potential by the Ir–aryl fragment [aryl = 2′,4′-R₂-phenyl (R = H/F), 9′,9′-dihexyl-2′-fluorenyl]. Three of the complexes exhibit phosphorescence in argon-saturated dichloromethane and acetonitrile solutions at room temperature with 0–0 transitions at 473–478 nm (green color; the emission spectra are solvent-independent), quantum yields of 3–25%, and long excited-state lifetimes of 62–350 μs. All of the complexes are phosphorescent at 77 K with 0–0 transitions at 387–474 nm (blue to green color). The extremely long calculated radiative lifetimes, 0.5–3.5 ms, confirm that the complexes emit from a cyclometalating-ligand-centered excited state

Extreme Tuning of Redox and Optical Properties of Cationic Cyclometalated Iridium(III) Isocyanide Complexes

We report seven heteroleptic cationic iridium­(III) complexes with cyclometalating N-arylazoles and alkyl/aryl isocyanides, [(C^∧N)₂Ir­(CNR)₂]­(CF₃SO₃), and characterize two of them by crystal structure analysis. The complexes are air- and moisture-stable white solids that have electronic transitions at very high energy with absorption onset at 320–380 nm. The complexes are difficult to reduce and oxidize; they exhibit irreversible electrochemical processes with peak potentials (against ferrocene) at −2.74 to −2.37 V (reduction) and 0.99–1.56 V (oxidation) and have a large redox gap of 3.49–4.26 V. The reduction potential of the complex is determined by the azole heterocycle (pyrazole or indazole) and by the isocyanide (<i>tert</i>-butyl or 2,6-dimethylphenyl) and the oxidation potential by the Ir–aryl fragment [aryl = 2′,4′-R₂-phenyl (R = H/F), 9′,9′-dihexyl-2′-fluorenyl]. Three of the complexes exhibit phosphorescence in argon-saturated dichloromethane and acetonitrile solutions at room temperature with 0–0 transitions at 473–478 nm (green color; the emission spectra are solvent-independent), quantum yields of 3–25%, and long excited-state lifetimes of 62–350 μs. All of the complexes are phosphorescent at 77 K with 0–0 transitions at 387–474 nm (blue to green color). The extremely long calculated radiative lifetimes, 0.5–3.5 ms, confirm that the complexes emit from a cyclometalating-ligand-centered excited state

Tris(2-(1<i>H</i>-pyrazol-1-yl)pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors and Its Application in Highly Efficient Solid-State Dye-Sensitized Solar Cells

Chemical doping is an important strategy to alter the charge-transport properties of both molecular and polymeric organic semiconductors that find widespread application in organic electronic devices. We report on the use of a new class of Co(III) complexes as p-type dopants for triarylamine-based hole conductors such as spiro-MeOTAD and their application in solid-state dye-sensitized solar cells (ssDSCs). We show that the proposed compounds fulfill the requirements for this application and that the discussed strategy is promising for tuning the conductivity of spiro-MeOTAD in ssDSCs, without having to rely on the commonly employed photo-doping. By using a recently developed high molar extinction coefficient organic D-π-A sensitizer and p-doped spiro-MeOTAD as hole conductor, we achieved a record power conversion efficiency of 7.2%, measured under standard solar conditions (AM1.5G, 100 mW cm^–2). We expect these promising new dopants to find widespread applications in organic electronics in general and photovoltaics in particular

Influence of Donor Groups of Organic D−π–A Dyes on Open-Circuit Voltage in Solid-State Dye-Sensitized Solar Cells

In solid-state dye-sensitized solar cells (ssDSCs), the poor pore filling of the mesoporous semiconductor and the short diffusion length of charge carriers in the hole-transport material (HTM) have limited the mesoscopic titania layer to a thickness of 2–3 μm. To increase the amount of light harvested by ssDSCs, organic dyes with high molar extinction coefficients are of great importance and have been the focus of intensive research. Here we investigate ssDSCs using an organic D−π–A dye, coded Y123, and 2,2′,7,7′-tetrakis(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene as a hole-transport material, exhibiting 934 mV open-circuit potential and 6.9% efficiency at standard solar conditions (AM1.5G, 100 mW cm^–2), which is a significant improvement compared to the analogue dyes C218, C220, and JK2 (<i>V</i>_oc values of 795, 781, and 914 mV, respectively). An upward shift in the conduction band edge was observed from photovoltage transient decay and impedance spectroscopy measurements for devices sensitized with Y123 and JK2 dyes compared to the device using C220 as sensitizer, in agreement with the high photovoltage response of the corresponding ssDSCs. This work highlights the importance of the interaction between the HTM and the dye-sensitized TiO₂ surface for the design of ssDSCs

Stable Quasi-Solid-State Dye-Sensitized Solar Cells Using Novel Low Molecular Mass Organogelators and Room-Temperature Molten Salts

Stable quasi-solid-state dye-sensitized solar cells (DSCs) were fabricated by using room-temperature molten salts (1-methyl-3-hexyl-imidazolium iodide), and a series of diamine derivatives with different lengths of alkyl chain as low molecular mass organogelators (LMOGs). The number of methylene (−CH₂−) units between the two amide carbonyl groups in the gelator molecule has significant influence on the charge transport property of gel electrolyte, and the kinetic processes of the electron transport and recombination. Less compact networks of the ionic liquid gel electrolytes containing odd-numbered −CH₂– gelator facilitate the diffusion of I₃^– and I^–. Also, the odd-numbered −CH₂– gelators-based DSCs exhibit longer electron recombination lifetime and a higher open circuit potential (<i>V</i>_oc) compared with the DSCs based on even-numbered −CH₂– gelators; consequently, the photovoltaic performances of DSCs based on odd-numbered −CH₂– gelators are much better than those even-numbered −CH₂– gelators. Remarkably, the results of the accelerated aging tests showed that the ionic liquid gel electrolyte-based DSCs could retain 93%–99% of their initial photoelectric conversion efficiencies (η) under heat at 60 °C, and 100% of their initial photoelectric conversion efficiencies under one sun light soaking with UV cutoff filter at 50 °C for 1000 h. This excellent long-term stability of quasi-solid-state DSCs is very important for application and commercialization of DSCs

Bright Blue Phosphorescence from Cationic Bis-Cyclometalated Iridium(III) Isocyanide Complexes

We report new bis-cyclometalated cationic iridium­(III) complexes [(C^∧N)₂Ir­(CN-<i>tert</i>-Bu)₂]­(CF₃SO₃) that have <i>tert</i>-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4′-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-<i>tert</i>-butyl, or 5-trifluoromethyl) as C^∧N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58–3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440–459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46–82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky <i>tert</i>-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium­(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid