5 research outputs found
Fluorination of Benzothiadiazole–Benzobisthiazole Copolymer Leads to Additive-Free Processing with Meliorated Solar Cell Performance
Processing
solvents and conditions have unique importance in the
performance of bulk heterojunction organic solar cells. In the present
work, we have investigated the role of a primary solvent and solvent
additive in the device performance of two benzobisthiazole (BBTz)-based
push–pull type polymers. In an inverted cell structure, the
BBTz-<i>co</i>-fluorinated benzothiadiazole polymer (PBBTzFT)
with a PC<sub>71</sub>BM acceptor showed additive-free enhanced performance
with a power conversion efficiency (PCE) of 6.4% from a 1,2-dichlorobenzene
solvent, while the BBTz-<i>co</i>-pyridylthiadiazole polymer
(PBBTzPT) showed maximum performance from a chlorobenzene (CB) solution
with a 1,8-diiodooctane (DIO) additive (PCE = 2.3%). The detailed
investigation by atomic force microscopy and two-dimensional grazing
incidence X-ray diffraction corroborates that the fluorination of
benzothiadiazole brought about optimal morphology without a solvent
additive, the PCE of which is comparable with the previous nonfluorinated
analogue (PCE = 6.5%) processed from CB with DIO
Light-Induced Ostwald Ripening of Organic Nanodots to Rods
Ostwald ripening allows the synthesis of 1D nanorods
of metal and
semiconductor nanoparticles. However, this phenomenon is unsuccessful
with organic π-systems due to their spontaneous self-assembly
to elongated fibers or tapes. Here we demonstrate the uses of light
as a versatile tool to control the ripening of amorphous organic nanodots
(ca. 15 nm) of an azobenzene-derived molecular assembly to micrometer-sized
supramolecular rods. A surface-confined dipole variation associated
with a low-yield (13–14%) <i>trans–cis</i> isomerization of the azobenzene moiety and the consequent dipole–dipole
interaction in a nonpolar solvent is believed to be the driving force
for the ripening of the nanodots to rods
Synthesis and Properties of Amphiphilic Photoresponsive Gelators for Aromatic Solvents
A sugar-based photoresponsive supergelator, <i>N</i>-glycosylazobenzene that shows selective gelation of aromatic solvents is described. The partial <i>trans–cis</i> isomerization of the azobenzene moiety allows photoinduced chopping of the entangled gel fibers to short fibers, resulting in controlled fiber length and gel–sol transition. The gelator is useful for the selective removal of toxic aromatic solvents from water
Exploring Alkyl Chains in Benzobisthiazole-Naphthobisthiadiazole Polymers: Impact on Solar-Cell Performance, Crystalline Structures, and Optoelectronics
The
shapes and lengths of the alkyl chains of conjugated polymers
greatly affect the efficiencies of organic photovoltaic devices. This
often results in a trade-off between solubility and self-organizing
behavior; however, each material has specific optimal chains. Here
we report on the effect of alkyl side chains on the film morphologies,
crystallinities, and optoelectronic properties of new benzobisthiazole-naphthobisthiadiazole
(PBBT-NTz) polymers. The power conversion efficiencies (PCEs) of linear-branched
and all-branched polymers range from 2.5% to 6.6%; the variations
in these PCEs are investigated by atomic force microscopy, two-dimensional
X-ray diffraction (2D-GIXRD), and transient photoconductivity techniques.
The best-performing linear-branched polymer, bearing dodecyl and decyltetradecyl
chains (C12-DT), exhibits nanometer-scale fibers along with the highest
crystallinity, comprising predominant edge-on and partial face-on
orientations. This morphology leads to the highest photoconductivity
and the longest carrier lifetime. These results highlight the importance
of long alkyl chains for inducing intermolecular stacking, which is
in contrast to observations made for analogous previously reported
polymers
Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure–Property Correlation of Thiophene Oligomers
Semiconducting
conjugated oligomers having same end group (<i>N</i>-ethylrhodanine)
but different central core (thiophene: <b>OT–T</b>, bithiophene: <b>OT–BT</b>, thienothiophene: <b>OT–TT</b>)
connected through thiophene pi-linker (alkylated terthiophene) were
synthesized for solution processable bulk-heterojunction solar cells.
The effect of the incorporation of an extra thiophene to the central
thiophene unit either through C–C bond linkage to form bithiophene
or by fusing two thiophenes together to form thienothiophene on the
optoelectronic properties and photovoltaic performances of the oligomers
were studied in detail. Flash photolysis time-resolved microwave conductivity
(FP–TRMC) technique shows <b>OT–TT</b> has significantly
higher photoconductivity than <b>OT–T</b> and <b>OT–BT</b> implying that the former can outperform the latter two derivatives
by a wide margin under identical conditions in a bulk-heterojunction
solar cell device. However, the initial photovoltaic devices fabricated
from all three oligomers (with PC<sub>71</sub>BM as the acceptor)
gave power conversion efficiencies (PCEs) of about 0.7%, which was
counterintuitive to the TRMC observation. By using TRMC results as
a guiding tool, solution engineering was carried out; no remarkable
changes were seen in the PCE of <b>OT–T</b> and <b>OT–BT</b>. On the other hand, 5-fold enhancement in the
device efficiency was achieved in <b>OT–TT</b> (PCE:
3.52%, <i>V</i><sub>OC</sub>: 0.80 V, <i>J</i><sub>SC</sub>: 8.74 mA cm<sup>–2</sup>, FF: 0.50), which was
in correlation with the TRMC results. The structure–property
correlation and the fundamental reasons for the improvement in device
performance upon solvent engineering were deduced through UV–vis
absorption, atomic force microscopy, bright-field transmission electron
microscopy, photoluminescence quenching analysis and two-dimensional
grazing incidence X-ray diffraction studies