3 research outputs found
n‑Type Naphthalene Diimide–Biselenophene Copolymer for All-Polymer Bulk Heterojunction Solar Cells
A new solution processable n-type polymer semiconductor
is synthesized
and characterized for use as an electron acceptor material in all-polymer
bulk heterojunction solar cells. The new crystalline copolymer, polyÂ(naphthalene
diimide-<i>alt</i>-biselenophene) (PNDIBS), has a high field-effect
electron mobility (0.07 cm<sup>2</sup>/(V s)) and broad visible-near-infrared
absorption band with an optical band gap of 1.4 eV. All-polymer bulk
heterojunction solar cells comprised of PNDIBS acceptor and polyÂ(3-hexylthiophene)
donor have a photovoltaic power conversion efficiency of 0.9%. The
external quantum efficiency spectrum of the all-polymer solar cells
shows that about 19% of the photocurrent comes from the near-infrared
(700–900 nm) light harvesting by the new n-type polymer semiconductor
All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor
The lack of suitable acceptor (n-type)
polymers has limited the
photocurrent and efficiency of polymer/polymer bulk heterojunction
(BHJ) solar cells. Here, we report an evaluation of three naphthalene
diimide (NDI) copolymers as electron acceptors in BHJ solar cells
which finds that all-polymer solar cells based on an NDI-selenophene
copolymer (PNDIS-HD) acceptor and a thiazolothiazole copolymer (PSEHTT)
donor exhibit a record 3.3% power conversion efficiency. The observed
short circuit current density of 7.78 mA/cm<sup>2</sup> and external
quantum efficiency of 47% are also the best such photovoltaic parameters
seen in all-polymer solar cells so far. This efficiency is comparable
to the performance of similarly evaluated [6,6]-Phenyl-C<sub>61</sub>-butyric acid methyl ester (PC<sub>60</sub>BM)/PSEHTT devices. The
lamellar crystalline morphology of PNDIS-HD, leading to balanced electron
and hole transport in the polymer/polymer blend solar cells accounts
for its good photovoltaic properties
Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics
New
electron-acceptor materials are long sought to overcome the
small photovoltage, high-cost, poor photochemical stability, and other
limitations of fullerene-based organic photovoltaics. However, all
known nonfullerene acceptors have so far shown inferior photovoltaic
properties compared to fullerene benchmark [6,6]-phenyl-C<sub>60</sub>-butyric acid methyl ester (PC<sub>60</sub>BM), and there are as
yet no established design principles for realizing improved materials.
Herein we report a design strategy that has produced a novel multichromophoric,
large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T,
whose π-conjugated framework occupies space comparable to an
aggregate of 9 [C<sub>60</sub>]-fullerene molecules. Comparative studies
of DBFI-T with its planar monomeric analogue (BFI-P2) and PC<sub>60</sub>BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration
and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a
high short-circuit current (10.14 mA/cm<sup>2</sup>) with a high open-circuit
voltage (0.86 V) to give a power conversion efficiency of 5.0%. The
external quantum efficiency spectrum of PSEHTT:DBFI-T devices had
peaks of 60–65% in the 380–620 nm range, demonstrating
that both hole transfer from photoexcited DBFI-T to PSEHTT and electron
transfer from photoexcited PSEHTT to DBFI-T contribute substantially
to charge photogeneration. The superior charge photogeneration and
electron-accepting properties of DBFI-T were further confirmed by
independent Xenon-flash time-resolved microwave conductivity measurements,
which correctly predict the relative magnitudes of the conversion
efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC<sub>60</sub>BM > PSEHTT:BFI-P2. The results demonstrate that the large
size, multichromophoric, nonplanar 3D molecular design is a promising
approach to more efficient organic photovoltaic materials