Effect of the Presence of Iodide on the Electron Injection Dynamics of Dye-Sensitized TiO_2-Based Solar Cells

The electron injection dynamics of dye-sensitized TiO_2-based solar cells have been investigated to determine the effects of replacing the I_3^−/I^− redox system by non-redox-active supporting electrolytes. TiO-2 films were sensitized with Ru(dcbpy)_2(NCS)_2, where dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine (the “N3” dye), and placed in contact with either M(ClO_4) or M(I_3−/I−) solutions (M = Li^+ or (n-C_4H_9)_4N^+); cells that contained I_3−/I− were fully functional solar cells whose steady-state photocurrents were directly measured. In (n-C_4H_9)_4N^+-containing solutions, significant differences were observed between the measured kinetics when ClO_4^− was replaced by the redox-active I3^−/I^− system. In particular, a ps time scale loss of the metal-to-ligand charge-transfer excited-state of the N3 dye, associated with electron injection, that was observed in cells containing either LiClO_4 or [(n-C_4H_9)4N]ClO_4 was absent in fully functional solar cells that contained [(n-C_4H_9)_4N]I/I_2. These results underscore the importance of performing kinetics measurements on this class of solar cells under operational conditions if one is to obtain reliable correlations between the dynamics data and the steady-state performance metrics of the solar cell devices

Recent advances and future directions to optimize the performances of p-type dye-sensitized solar cells

This review provides a summary of the most important developments in the field of solar cells based on the sensitization of p-type semiconductors, such as NiO, and identifies the future challenges and opportunities to enhance their overall performance. In particular, the main factors responsible for the low open-circuit voltage, short circuit photocurrent and fill factor are discussed in detail

Recent advances and future directions to optimize the performances of p-type dye-sensitized solar cells

This review provides a summary of the most important developments in the field of solar cells based on the sensitization of p-type semiconductors, such as NiO, and identifies the future challenges and opportunities to enhance their overall performance. In particular, the main factors responsible for the low open-circuit voltage, short circuit photocurrent and fill factor are discussed in detail. (C) 2012 Elsevier BM. All rights reserved

Ultrafast recombination for NiO sensitized with a series of perylene imide sensitizers exhibiting Marcus normal behaviour

Ultrafast recombination observed from several perylene imide sensitizers bound to NiO appears to align with Marcus normal region behaviour; this indicates recombination to intra-bandgap states

Cobalt Polypyridyl-Based Electrolytes for p-Type Dye-Sensitized Solar Cells

Polypyridyl Co complexes with different substituents were applied as redox mediators in p-type dye-sensitized solar cells (p-DSCs), consisting of mesoporous NiO sensitized with a perylenemonoimide-naphthalenediimide (PMI-NDI) dyad. The photocurrent and photovoltages of the devices were found to depend on the steric bulk of the redox species rather than their electrochem. potential. Bulky substituents were found to slow the detrimental charge recombination reactions between holes in the NiO semiconductor and the reduced form of the redox couple. The open-circuit potential (VOC) of each of the devices was superior to the equiv. PMI-NDI-sensitized p-DSCs contg. the triiodide/iodide redox couple

Cobalt Polypyridyl-Based Electrolytes for p-Type Dye-Sensitized Solar Cells

A series of polypyridyl cobalt complexes with different substituents was applied as redox mediators in p-type dye-sensitized solar cells (p-DSCs), consisting of mesoporous NiO sensitized with a perylenemonoimide naphthalenediimide (PMI-NDI) dyad. The photocurrent and photovoltages of the devices were found to depend on the steric bulk of the redox species rather than their electrochemical potential. Bulky substituents were found to slow the detrimental charge recombination reactions between holes in the NiO semiconductor and the reduced form of the redox couple. The open-circuit potential (V(OC)) of each of the devices was superior to the equivalent PMI-NDIsensitized p-DSCs containing the triiodide/iodide redox couple

Synthesis, photophysical and photovoltaic investigations of acceptor-functionalized perylene monoimide dyes for nickel oxide p-type dye-sensitized solar cells

We report on the synthesis, electrochemical, photophysical, and photovoltaic properties of a series of three organic dyads comprising a perylene monoimide (PMI) dye connected to a naphthalene diimide (NDI) or a fullerene (C(60)) for application in dye-sensitized solar cells (DSCs) with nanocrystalline NiO electrodes. It was found that the secondary electron acceptor (NDI or C(60)) in all the three dyads extends the charge separated state lifetime by about five orders of magnitude compared to the respective parent PMI dye. Nanosecond pump-probe experiments of the NiO/dyads in the presence of the electrolyte show that the reduction of triiodide by the secondary electron acceptor is slow in all the dyads, which we ascribe to a weak driving force for this reaction. This reaction is significantly faster with the cobalt electrolyte (tris(4,4'-di-tert-butyl-2,2'-bipyridine)cobalt(II/III)), whose driving force is larger; however, its reaction with the reduced dyads is still rather slow. We demonstrate that the larger photovoltage observed with the cobalt electrolyte (V(OC) = 285 mV) relative to the iodide electrolyte (V(OC) = 120 mV) is due to a decrease in the dark current for the former owing to slower interfacial electron transfer of the reduced mediator with the injected holes into the NiO electrode. In terms of photovoltaic performances, the most efficient dyad is the system in which the NDI is directly connected to the PMI (eta = 0.14% under AM 1.5 with the cobalt electrolyte), but the dyad containing the fullerene acceptor exhibits the highest IPCE and the highest short circuit current density (IPCE = 57%, J(SC) = 1.88 mA cm(-2)) with the iodide electrolyte. The latter performances are attributed to the slightly stronger reducing power of C(60) relative to NDI, which favours the reduction of the mediator in the electrolyte

Probing Reaction Dynamics of Transition-Metal Complexes in Solution via Time-Resolved X-ray Spectroscopy

We report measurements of the photo-induced Fe(II) spin crossover reaction dynamics in solution via time-resolved x-ray absorption spectroscopy. EXAFS measurements reveal that the iron?nitrogen bond lengthens by 0.21+-0.03 Angstrom in the high-spin transient excited state relative to the ground state. XANES measurements at the Fe L-edge show directly the influence of the structural change on the ligand-field splitting of the Fe(II) 3d orbitals associated with the spin transition

Synthesis, photophysical and photovoltaic investigations of acceptor-functionalized perylene monoimide dyes for nickel oxide p-type dye-sensitized solar cells

We report on the synthesis, electrochem., photophys., and photovoltaic properties of a series of three org. dyads comprising a perylene monoimide (PMI) dye connected to a naphthalene diimide (NDI) or a fullerene (C60) for application in dye-sensitized solar cells (DSCs) with nanocryst. NiO electrodes. It was found that the secondary electron acceptor (NDI or C60) in all the three dyads extends the charge sepd. state lifetime by about five orders of magnitude compared to the resp. parent PMI dye. Nanosecond pump-probe expts. of the NiO/dyads in the presence of the electrolyte show that the redn. of triiodide by the secondary electron acceptor is slow in all the dyads, which we ascribe to a weak driving force for this reaction. This reaction is significantly faster with the cobalt electrolyte (tris(4,4'-di-tert-butyl-2,2'-bipyridine)cobalt(II/III)), whose driving force is larger; however, its reaction with the reduced dyads is still rather slow. We demonstrate that the larger photovoltage obsd. with the cobalt electrolyte (VOC = 285 mV) relative to the iodide electrolyte (VOC = 120 mV) is due to a decrease in the dark current for the former owing to slower interfacial electron transfer of the reduced mediator with the injected holes into the NiO electrode. In terms of photovoltaic performances, the most efficient dyad is the system in which the NDI is directly connected to the PMI (η = 0.14% under AM 1.5 with the cobalt electrolyte), but the dyad contg. the fullerene acceptor exhibits the highest IPCE and the highest short circuit c.d. (IPCE = 57%, JSC = 1.88 mA cm-2) with the iodide electrolyte. The latter performances are attributed to the slightly stronger reducing power of C60 relative to NDI, which favors the redn. of the mediator in the electrolyte

Electron Transfer within Self-Assembling Cyclic Tetramers Using Chlorophyll-Based Donor–Acceptor Building Blocks

The synthesis and photoinduced charge transfer properties of a series of Chl-based donor–acceptor triad building blocks that self-assemble into cyclic tetramers are reported. Chlorophyll <i>a</i> was converted into zinc methyl 3-ethylpyrochlorophyllide <i>a</i> (Chl) and then further modified at its 20-position to covalently attach a pyromellitimide (PI) acceptor bearing a pyridine ligand and one or two naphthalene-1,8:4,5-bis­(dicarboximide) (NDI) secondary electron acceptors to give Chl–PI–NDI and Chl–PI–NDI₂. The pyridine ligand within each ambident triad enables intermolecular Chl metal–ligand coordination in dry toluene, which results in the formation of cyclic tetramers in solution, as determined using small- and wide-angle X-ray scattering at a synchrotron source. Femtosecond and nanosecond transient absorption spectroscopy of the monomers in toluene–1% pyridine and the cyclic tetramers in toluene shows that the selective photoexcitation of Chl results in intramolecular electron transfer from ^1*Chl to PI to form Chl^+•–PI^–•–NDI and Chl^+•–PI^–•–NDI₂. This initial charge separation is followed by a rapid charge shift from PI^–• to NDI and subsequent charge recombination of Chl^+•–PI–NDI^–• and Chl^+•–PI–(NDI)­NDI^–• on a 5–30 ns time scale. Charge recombination in the Chl–PI–NDI₂ cyclic tetramer (τ_CR = 30 ± 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (τ_CR = 10 ± 1 ns in toluene–1% pyridine). This indicates that the self-assembly of these building blocks into the cyclic tetramers alters their structures in a way that lengthens their charge separation lifetimes, which is an advantageous strategy for artificial photosynthetic systems