12 research outputs found
Co(III) Complexes as pāDopants in Solid-State Dye-Sensitized Solar Cells
Following our recent work on the
use of CoĀ(III) complexes as p-type
dopants for triarylamine-based organic hole-conductors, we herein
report on two new CoĀ(III) complexes for doping applications. With
the aim of ameliorating the dopantās suitability for its use
in solid-state dye-sensitized solar cells, we show how the properties
of the dopant can be easily adjusted by a slight modification of the
molecular structure. We prove the eligibility of the two new dopants
by characterizing their optical and electrochemical properties and
give evidence that both of them can be used to oxidize the molecular
hole-transporter spiro-MeOTAD. Finally, we fabricate high-performance
solid-state dye-sensitized solar cells using a state-of-the-art metal-free
organic sensitizer in order to elucidate the influence of the type
of dopant on device performance
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-Ļ-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<sup>ā2</sup>). We expect these promising new dopants to find widespread applications in organic electronics in general and photovoltaics in particular
Subnanometer Ga<sub>2</sub>O<sub>3</sub> Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells
Herein, we present the first use of a gallium oxide tunnelling
layer to significantly reduce electron recombination in dye-sensitized
solar cells (DSC). The subnanometer coating is achieved using atomic
layer deposition (ALD) and leading to a new DSC record open-circuit
potential of 1.1 V with state-of-the-art organic D-Ļ-A sensitizer
and cobalt redox mediator. After ALD of only a few angstroms of Ga<sub>2</sub>O<sub>3</sub>, the electron back reaction is reduced by more
than an order of magnitude, while charge collection efficiency and
fill factor are increased by 30% and 15%, respectively. The photogenerated
exciton separation processes of electron injection into the TiO<sub>2</sub> conduction band and the hole injection into the electrolyte
are characterized in detail
Influence of Donor Groups of Organic DāĻāA 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 2ā3 Ī¼m. 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 DāĻāA dye, coded Y123, and 2,2ā²,7,7ā²-tetrakis(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9ā²-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<sup>ā2</sup>), which is a significant improvement compared to the analogue dyes C218, C220, and JK2 (<i>V</i><sub>oc</sub> 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 TiO<sub>2</sub> surface for the design of ssDSCs
Acid-Induced Degradation of Phosphorescent Dopants for OLEDs and Its Application to the Synthesis of Tris-heteroleptic Iridium(III) Bis-cyclometalated Complexes
Investigations of blue phosphorescent organic light emitting
diodes
(OLEDs) based on [IrĀ(2-(2,4-difluorophenyl)Āpyridine)<sub>2</sub>(picolinate)]
(FIrPic) have pointed to the cleavage of the picolinate as a possible
reason for device instability. We reproduced the loss of picolinate
and acetylacetonate ancillary ligands in solution by the addition
of BrĆønsted or Lewis acids. When hydrochloric acid is added to
a solution of a [IrĀ(C<sup>ā§</sup>N)<sub>2</sub>(X<sup>ā§</sup>O)] complex (C<sup>ā§</sup>N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)Āpyridine
(diFppy) and X<sup>ā§</sup>O = picolinate (pic) or acetylacetonate
(acac)), the cleavage of the ancillary ligand results in the direct
formation of the chloro-bridged iridiumĀ(III) dimer [{IrĀ(C<sup>ā§</sup>N)<sub>2</sub>(Ī¼-Cl)}<sub>2</sub>]. When triflic acid or boron
trifluoride are used, a source of chloride (here tetrabutylammonium
chloride) is added to obtain the same chloro-bridged iridiumĀ(III)
dimer. Then, we advantageously used this degradation reaction for
the efficient synthesis of tris-heteroleptic cyclometalated iridiumĀ(III)
complexes [IrĀ(C<sup>ā§</sup>N<sup>1</sup>)Ā(C<sup>ā§</sup>N<sup>2</sup>)Ā(L)], a family of cyclometalated complexes otherwise
challenging to prepare. We used an iridiumĀ(I) complex, [{IrĀ(COD)Ā(Ī¼-Cl)}<sub>2</sub>], and a stoichiometric amount of two different C<sup>ā§</sup>N ligands (C<sup>ā§</sup>N<sup>1</sup> = ppy; C<sup>ā§</sup>N<sup>2</sup> = diFppy) as starting materials for the swift preparation
of the chloro-bridged iridiumĀ(III) dimers. After reacting the mixture
with acetylacetonate and subsequent purification, the tris-heteroleptic
complex [IrĀ(ppy)Ā(diFppy)Ā(acac)] could be isolated with good yield
from the crude containing as well the bis-heteroleptic complexes [IrĀ(ppy)<sub>2</sub>(acac)] and [IrĀ(diFppy)<sub>2</sub>(acac)]. Reaction of the
tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic
chloro-bridged iridium dimer [{IrĀ(ppy)Ā(diFppy)Ā(Ī¼-Cl)}<sub>2</sub>], which can be used as starting material for the preparation of
a new tris-heteroleptic iridiumĀ(III) complex based on these two C<sup>ā§</sup>N ligands. Finally, we use DFT/LR-TDDFT to rationalize
the impact of the two different C<sup>ā§</sup>N ligands on the
observed photophysical and electrochemical properties
Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study
We report a combined experimental and computational investigation
to understand the nature of the interactions between cobalt redox
mediators and TiO<sub>2</sub> surfaces sensitized by ruthenium and
organic dyes, and their impact on the performance of the corresponding
dye-sensitized solar cells (DSSCs). We focus on different ruthenium
dyes and fully organic dyes, to understand the dramatic loss of efficiency
observed for the prototype RuĀ(II) N719 dye in conjunction with cobalt
electrolytes. Both N719- and Z907-based DSSCs showed an increased
lifetime in iodine-based electrolyte compared to the cobalt-based
redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with
long alkoxy chains, show no significant change in the electron lifetime
regardless of employed electrolyte and deliver a high photovoltaic
efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular
dynamics simulations show the formation of a complex between the cobalt
electrolyte and the surface-adsorbed ruthenium dye, which brings the
[CoĀ(bpy)<sub>3</sub>]<sup>3+</sup> species into contact with the TiO<sub>2</sub> surface. This translates into a high probability of intercepting
TiO<sub>2</sub>-injected electrons by the oxidized [CoĀ(bpy)<sub>3</sub>]<sup>3+</sup> species, lying close to the N719-sensitized TiO<sub>2</sub> surface. Investigation of the dye regeneration mechanism
by the cobalt electrolyte in the Marcus theory framework led to substantially
different reorganization energies for the high-spin (HS) and low-spin
(LS) reaction pathways. Our calculated reorganization energies for
the LS pathways are in excellent agreement with recent data for a
series of cobalt complexes, lending support to the proposed regeneration
pathway. Finally, we systematically investigate a series of CoĀ(II)/CoĀ(III)
complexes to gauge the impact of ligand substitution and of metal
coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy
difference and reorganization energies. Our results allow us to trace
structure/property relations required for further development of cobalt
electrolytes for DSSCs
Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study
We report a combined experimental and computational investigation
to understand the nature of the interactions between cobalt redox
mediators and TiO<sub>2</sub> surfaces sensitized by ruthenium and
organic dyes, and their impact on the performance of the corresponding
dye-sensitized solar cells (DSSCs). We focus on different ruthenium
dyes and fully organic dyes, to understand the dramatic loss of efficiency
observed for the prototype RuĀ(II) N719 dye in conjunction with cobalt
electrolytes. Both N719- and Z907-based DSSCs showed an increased
lifetime in iodine-based electrolyte compared to the cobalt-based
redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with
long alkoxy chains, show no significant change in the electron lifetime
regardless of employed electrolyte and deliver a high photovoltaic
efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular
dynamics simulations show the formation of a complex between the cobalt
electrolyte and the surface-adsorbed ruthenium dye, which brings the
[CoĀ(bpy)<sub>3</sub>]<sup>3+</sup> species into contact with the TiO<sub>2</sub> surface. This translates into a high probability of intercepting
TiO<sub>2</sub>-injected electrons by the oxidized [CoĀ(bpy)<sub>3</sub>]<sup>3+</sup> species, lying close to the N719-sensitized TiO<sub>2</sub> surface. Investigation of the dye regeneration mechanism
by the cobalt electrolyte in the Marcus theory framework led to substantially
different reorganization energies for the high-spin (HS) and low-spin
(LS) reaction pathways. Our calculated reorganization energies for
the LS pathways are in excellent agreement with recent data for a
series of cobalt complexes, lending support to the proposed regeneration
pathway. Finally, we systematically investigate a series of CoĀ(II)/CoĀ(III)
complexes to gauge the impact of ligand substitution and of metal
coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy
difference and reorganization energies. Our results allow us to trace
structure/property relations required for further development of cobalt
electrolytes for DSSCs
Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands
Charged
cyclometalated (C<sup>ā§</sup>N) iridiumĀ(III) complexes
with carbene-based ancillary ligands are a promising family of deep-blue
phosphorescent compounds. Their emission properties are controlled
primarily by the main C<sup>ā§</sup>N ligands, in contrast to
the classical design of charged complexes where N<sup>ā§</sup>N ancillary ligands with low-energy Ļ* orbitals, such as 2,2'-bipyridine,
are generally used for this purpose. Herein we report two series of
charged iridium complexes with various carbene-based ancillary ligands.
In the first series the C<sup>ā§</sup>N ligand is 2-phenylpyridine,
whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One
bis-carbene (:C<sup>ā§</sup>C:) and four different pyridineācarbene
(N<sup>ā§</sup>C:) chelators are used as bidentate ancillary
ligands in each series. Synthesis, X-ray crystal structures, and photophysical
and electrochemical properties of the two series of complexes are
described. At room temperature, the :C<sup>ā§</sup>C: complexes
show much larger photoluminescence quantum yields (Ī¦<sub>PL</sub>) of ca. 30%, compared to the N<sup>ā§</sup>C: analogues (around
1%). On the contrary, all of the investigated complexes are bright
emitters in the solid state both at room temperature (1% polyĀ(methyl
methacrylate) matrix, Ī¦<sub>PL</sub> 30ā60%) and at 77
K. Density functional theory calculations are used to rationalize
the differences in the photophysical behavior observed upon change
of the ancillary ligands. The N<sup>ā§</sup>C:-type complexes
possess a low-lying triplet metal-centered (<sup>3</sup>MC) state
mainly deactivating the excited state through nonradiative processes;
in contrast, no such state is present for the :C<sup>ā§</sup>C: analogues. This finding is supported by temperature-dependent
excited-state lifetime measurements made on representative N<sup>ā§</sup>C: and :C<sup>ā§</sup>C: complexes
Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands
Charged
cyclometalated (C<sup>ā§</sup>N) iridiumĀ(III) complexes
with carbene-based ancillary ligands are a promising family of deep-blue
phosphorescent compounds. Their emission properties are controlled
primarily by the main C<sup>ā§</sup>N ligands, in contrast to
the classical design of charged complexes where N<sup>ā§</sup>N ancillary ligands with low-energy Ļ* orbitals, such as 2,2'-bipyridine,
are generally used for this purpose. Herein we report two series of
charged iridium complexes with various carbene-based ancillary ligands.
In the first series the C<sup>ā§</sup>N ligand is 2-phenylpyridine,
whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One
bis-carbene (:C<sup>ā§</sup>C:) and four different pyridineācarbene
(N<sup>ā§</sup>C:) chelators are used as bidentate ancillary
ligands in each series. Synthesis, X-ray crystal structures, and photophysical
and electrochemical properties of the two series of complexes are
described. At room temperature, the :C<sup>ā§</sup>C: complexes
show much larger photoluminescence quantum yields (Ī¦<sub>PL</sub>) of ca. 30%, compared to the N<sup>ā§</sup>C: analogues (around
1%). On the contrary, all of the investigated complexes are bright
emitters in the solid state both at room temperature (1% polyĀ(methyl
methacrylate) matrix, Ī¦<sub>PL</sub> 30ā60%) and at 77
K. Density functional theory calculations are used to rationalize
the differences in the photophysical behavior observed upon change
of the ancillary ligands. The N<sup>ā§</sup>C:-type complexes
possess a low-lying triplet metal-centered (<sup>3</sup>MC) state
mainly deactivating the excited state through nonradiative processes;
in contrast, no such state is present for the :C<sup>ā§</sup>C: analogues. This finding is supported by temperature-dependent
excited-state lifetime measurements made on representative N<sup>ā§</sup>C: and :C<sup>ā§</sup>C: complexes
Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands
Charged
cyclometalated (C<sup>ā§</sup>N) iridiumĀ(III) complexes
with carbene-based ancillary ligands are a promising family of deep-blue
phosphorescent compounds. Their emission properties are controlled
primarily by the main C<sup>ā§</sup>N ligands, in contrast to
the classical design of charged complexes where N<sup>ā§</sup>N ancillary ligands with low-energy Ļ* orbitals, such as 2,2'-bipyridine,
are generally used for this purpose. Herein we report two series of
charged iridium complexes with various carbene-based ancillary ligands.
In the first series the C<sup>ā§</sup>N ligand is 2-phenylpyridine,
whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One
bis-carbene (:C<sup>ā§</sup>C:) and four different pyridineācarbene
(N<sup>ā§</sup>C:) chelators are used as bidentate ancillary
ligands in each series. Synthesis, X-ray crystal structures, and photophysical
and electrochemical properties of the two series of complexes are
described. At room temperature, the :C<sup>ā§</sup>C: complexes
show much larger photoluminescence quantum yields (Ī¦<sub>PL</sub>) of ca. 30%, compared to the N<sup>ā§</sup>C: analogues (around
1%). On the contrary, all of the investigated complexes are bright
emitters in the solid state both at room temperature (1% polyĀ(methyl
methacrylate) matrix, Ī¦<sub>PL</sub> 30ā60%) and at 77
K. Density functional theory calculations are used to rationalize
the differences in the photophysical behavior observed upon change
of the ancillary ligands. The N<sup>ā§</sup>C:-type complexes
possess a low-lying triplet metal-centered (<sup>3</sup>MC) state
mainly deactivating the excited state through nonradiative processes;
in contrast, no such state is present for the :C<sup>ā§</sup>C: analogues. This finding is supported by temperature-dependent
excited-state lifetime measurements made on representative N<sup>ā§</sup>C: and :C<sup>ā§</sup>C: complexes