5 research outputs found
Cross-Linkable Molecular Hole-Transporting Semiconductor for Solid-State Dye-Sensitized Solar Cells
In this study, we investigate the
use of a cross-linkable organosilane semiconductor, 4,4â˛-bisÂ[(<i>p</i>-trichlorosilylpropylphenyl)Âphenylamino]Âbiphenyl (TPDSi<sub>2</sub>), as a hole-transporting material (HTM) for solid-state dye-sensitized
solar cells (ssDSSCs) using the standard amphiphilic Z907 dye which
is compatible with organic HTM deposition. The properties and performance
of the resulting cells are then compared and contrasted with the ones
based on polyÂ(3-hexylthiophene) (P3HT), a conventional polymeric HTM,
but with rather limited pore-filling capacity. When processed under
N<sub>2</sub>, TPDSi<sub>2</sub> exhibits excellent infiltration into
the mesoporous TiO<sub>2</sub> layer and thus enables the fabrication
of relatively thick devices (âź5 Îźm) for efficient photon
harvesting. When exposed to ambient atmosphere (RH<sub>amb</sub> âź
20%), TPDSi<sub>2</sub> readily undergoes cross-linking to afford
a rigid, thermally stable hole-transporting layer. In addition, the
effect of <i>tert</i>-butylpyridine (TBP) and lithium bisÂ(trifluoromethylsulfonyl)Âimide
salt (Li-TFSI) additives on the electrochemical properties of these
HTMs is studied via a combination of cyclic voltammetry (CV) and ultraviolet
photoemission spectroscopy (UPS) measurements. The results demonstrate
that the additives significantly enhance the space charge limited
current (SCLC) mobilities for both the P3HT and TPDSi<sub>2</sub> HTMs
and induce a shift in the TPDSi<sub>2</sub> Fermi level, likely a
p-doping effect. These combined effects of improved charge transport
characteristics for the TPDSi<sub>2</sub> devices enhance the power
conversion efficiency (PCE) by more than 2-fold for ssDSSCs
Buta-1,3-diyne-Based ĎâConjugated Polymers for Organic Transistors and Solar Cells
We
report the synthesis and characterization of new alkyl-substituted
1,4-diÂ(thiophen-2-yl)Âbuta-1,3-diyne (R-DTB) donor building blocks,
based on the âCîźCâCîźCâ conjugative
pathway, and their incorporation with thienyl-diketopyrroloÂpyrrole
(Râ˛-TDPP) acceptor units into Ď-conjugated PTDPP-DTB
polymers (<b>P1</b>â<b>P4</b>). The solubility
of the new polymers strongly depends on the DTB and DPP solubilizing
(R and Râ˛, respectively) substituents. Thus, solution processable
and high molecular weight PDPP-DTB polymers are achieved for <b>P3</b> (R = <i>n</i>-C<sub>12</sub>H<sub>25</sub>, Râ˛
= 2-butyloctyl) and <b>P4</b> (R = 2-ethylhexyl, RⲠ=
2-butyloctyl). Systematic studies of <b>P3</b> and <b>P4</b> physicochemical properties are carried using optical spectroscopy,
cyclic voltammetry, and thermal analysis, revealing characteristic
features of the dialkynyl motif. For the first time, optoelectronic
devices (OFETs, OPVs) are fabricated with 1,3-butadiyne containing
organic semiconductors. OFET hole mobilities and record OPV power
conversion efficiencies for acetylenic organic materials approach
0.1 cm<sup>2</sup>/(V s) and 4%, respectively, which can be understood
from detailed thin-film morphology and microstructural characterization
using AFM, TEM, XRD, and GIWAXS methodologies. Importantly, DTB-based
polymers (<b>P3</b> and <b>P4</b>) exhibit, in addition
to stabilization of frontier molecular orbitals and to âCîźCâCîźCâ
relief of steric torsions, discrete morphological pliability through
thermal annealing and processing additives. The advantageous materials
properties and preliminary device performance reported here demonstrate
the promise of 1,3-butadiyne-based semiconducting polymers
Slip-Stacked Perylenediimides as an Alternative Strategy for High Efficiency Nonfullerene Acceptors in Organic Photovoltaics
Perylenediimide (PDI)-based acceptors offer a potential replacement
for fullerenes in bulk-heterojunction (BHJ) organic photovoltaic cells
(OPVs). The most promising efforts have focused on creating twisted
PDI dimers to disrupt aggregation and thereby suppress excimer formation.
Here, we present an alternative strategy for developing high-performance
OPVs based on PDI acceptors that promote slip-stacking in the solid
state, thus preventing the coupling necessary for rapid excimer formation.
This packing structure is accomplished by substitution at the PDI
2,5,8,11-positions (âheadland positionsâ). Using this
design principle, three PDI acceptors, <i>N</i>,<i>N</i>-bisÂ(n-octyl)-2,5,8,11-tetraÂ(<i>n</i>-hexyl)-PDI
(<b>Hexyl-PDI</b>), <i>N</i>,<i>N</i>-bisÂ(n-octyl)-2,5,8,11-tetraphenethyl-PDI
(<b>Phenethyl-PDI</b>), and <i>N</i>,<i>N</i>-bisÂ(n-octyl)-2,5,8,11-tetraphenyl-PDI (<b>Phenyl-PDI</b>),
were synthesized, and their molecular and electronic structures were
characterized. They were then blended with the donor polymer <b>PBTI3T</b>, and inverted OPVs of the structure ITO/ZnO/Active
Layer/MoO<sub>3</sub>/Ag were fabricated and characterized. Of these,
1:1 <b>PBTI3T</b>:<b>Phenyl-PDI</b> proved to have the
best performance with <i>J</i><sub>sc</sub> = 6.56 mA/cm<sup>2</sup>, <i>V</i><sub>oc</sub> = 1.024 V, FF = 54.59%,
and power conversion efficiency (PCE) = 3.67%. Devices fabricated
with <b>Phenethyl-PDI</b> and <b>Hexyl-PDI</b> have significantly
lower performance. The thin film morphology and the electronic and
photophysical properties of the three materials are examined, and
although all three materials undergo efficient charge separation, <b>PBTI3T</b>:<b>Phenyl-PDI</b> is found to have the deepest
LUMO, intermediate crystallinity, and the most well-mixed domains.
This minimizes geminate recombination in <b>Phenyl-PDI</b> OPVs
and affords the highest PCE. Thus, slip-stacked PDI strategies represent
a promising approach to fullerene replacements in BHJ OPVs
Systematic Investigation of Organic Photovoltaic Cell Charge Injection/Performance Modulation by Dipolar Organosilane Interfacial Layers
With the goal of investigating and
enhancing anode performance
in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells, the
glass/tin-doped indium oxide (ITO) anodes are modified with a series
of robust silane-tethered bisÂ(fluoroaryl)Âamines to form self-assembled
interfacial layers (IFLs). The modified ITO anodes are characterized
by contact angle measurements, X-ray reflectivity, ultraviolet photoelectron
spectroscopy, X-ray photoelectron spectroscopy, grazing incidence
X-ray diffraction, atomic force microscopy, and cyclic voltammetry.
These techniques reveal the presence of hydrophobic amorphous monolayers
of 6.68 to 9.76 Ă
thickness, and modified anode work functions
ranging from 4.66 to 5.27 eV. Two series of glass/ITO/IFL/active layer/LiF/Al
BHJ OPVs are fabricated with the active layer = polyÂ(3-hexylthiophene):phenyl-C<sub>71</sub>-butyric acid methyl ester (P3HT:PC<sub>71</sub>BM) or polyÂ[[4,8-bisÂ[(2-ethylhexyl)Âoxy]ÂbenzoÂ[1,2-b:4,5-bâ]Âdithiophene-2,6-diyl]Â[3-fluoro-2-[(2-ethylhexyl)-carbonyl]Âthi-enoÂ[3,4-b]Âthiophenediyl]]:phenyl-C<sub>71</sub>-butyric acid methyl ester (PTB7:PC<sub>71</sub>BM). OPV
analysis under AM 1.5G conditions reveals significant performance
enhancement versus unmodified glass/ITO anodes. Strong positive correlations
between the electrochemically derived heterogeneous electron transport
rate constants (<i>k</i><sub>s</sub>) and the device open
circuit voltage (<i>V</i><sub>oc</sub>), short circuit current
(<i>J</i><sub>sc</sub>), hence OPV power conversion efficiency
(PCE), are observed for these modified anodes. Furthermore, the strong
functional dependence of the device response on <i>k</i><sub>s</sub> increases as greater densities of charge carriers are
generated in the BHJ OPV active layer, and is attributable to enhanced
anode carrier extraction in the case of high-<i>k</i><sub>s</sub> IFLs
Metal-Free Tetrathienoacene Sensitizers for High-Performance Dye-Sensitized Solar Cells
A new series of metal-free organic
chromophores (TPA-TTAR-A (<b>1</b>), TPA-T-TTAR-A (<b>2</b>), TPA-TTAR-T-A (<b>3</b>), and TPA-T-TTAR-T-A (<b>4</b>)) are synthesized for application
in dye-sensitized solar cells (DSSC) based on a donor-Ď-bridge-acceptor
(DâĎâA) design. Here a simple triphenylamine (TPA)
moiety serves as the electron donor, a cyanoacrylic acid as the electron
acceptor and anchoring group, and a novel tetrathienoacene (TTA) as
the Ď-bridge unit. Because of the extensively conjugated TTA
Ď-bridge, these dyes exhibit high extinction coefficients (4.5â5.2
Ă 10<sup>4</sup> M<sup>â1</sup> cm<sup>â1</sup>). By strategically inserting a thiophene spacer on the donor or
acceptor side of the molecules, the electronic structures of these
TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene
spacer has a significant influence on the dye orientation and self-assembly
modality on TiO<sub>2</sub> surfaces. The insertion of a thiophene
between the Ď-bridge and the cyanoacrylic acid anchoring group
in TPA-TTAR-T-A (dye <b>3</b>) promotes more vertical dye orientation
and denser packing on TiO<sub>2</sub> (molecular footprint = 79 Ă
<sup>2</sup>), thus enabling optimal dye loading. Using dye <b>3</b>, a DSSC power conversion efficiency (PCE) of 10.1% with <i>V</i><sub>oc</sub> = 0.833 V, <i>J</i><sub>sc</sub> = 16.5 mA/cm<sup>2</sup>, and FF = 70.0% is achieved, among the
highest reported to date for metal-free organic DSSC sensitizers using
an I<sup>â</sup>/I<sub>3</sub><sup>â</sup> redox shuttle.
Photophysical measurements on dye-grafted TiO<sub>2</sub> films reveal
that the additional thiophene unit in dye <b>3</b> enhances
the electron injection efficiency, in agreement with the high quantum
efficiency