4 research outputs found
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
n‑Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend Morphology, and Compatibility Toward High-Performance All-Polymer Solar Cells
Knowledge
of the critical factors that determine compatibility, blend morphology,
and performance of bulk heterojunction (BHJ) solar cells composed
of an electron-accepting polymer and an electron-donating polymer
remains limited. To test the idea that bulk crystallinity is such
a critical factor, we have designed a series of new semiconducting
naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene
random copolymers, <i>x</i>PDI (10PDI, 30PDI, 50PDI), whose
crystallinity varies with composition, and investigated them as electron
acceptors in BHJ solar cells. Pairing of the reference crystalline
(crystalline domain size <i>L</i><sub>c</sub> = 10.22 nm)
NDI-selenophene copolymer (PNDIS-HD) with crystalline (<i>L</i><sub>c</sub> = 9.15 nm) benzodithiophene-thienoÂ[3,4-<i>b</i>]Âthiophene copolymer (PBDTTT-CT) donor yields incompatible blends,
whose BHJ solar cells have a power conversion efficiency (PCE) of
1.4%. However, pairing of the new 30PDI with optimal crystallinity
(<i>L</i><sub>c</sub> = 5.11 nm) as acceptor with the same
PBDTTT-CT donor yields compatible blends and all-polymer solar cells
with enhanced performance (PCE = 6.3%, <i>J</i><sub>sc</sub> = 18.6 mA/cm<sup>2</sup>, external quantum efficiency = 91%). These
photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the
best so far for all-polymer solar cells, while the short-circuit current
(<i>J</i><sub>sc</sub>) and external quantum efficiency
are even higher than reported values for [70]-fullerene:PBDTTT-CT
solar cells. The morphology and bulk carrier mobilities of the polymer/polymer
blends varied substantially with crystallinity of the acceptor polymer
component and thus with the NDI/PDI copolymer composition. These results
demonstrate that the crystallinity of a polymer component and thus
compatibility, blend morphology, and efficiency of polymer/polymer
blend solar cells can be controlled by molecular design
Polymer/Polymer Blend Solar Cells Using Tetraazabenzodifluoranthene Diimide Conjugated Polymers as Electron Acceptors
Two n-type semiconducting polymers
with alternating arylene (thiophene
or selenophene)–tetraazabenzodifluoranthene diimide (BFI) donor–acceptor
architecture have been investigated as new electron acceptors in polymer/polymer
blend solar cells. The new selenophene-linked polymer, PBFI-S, has
a significantly smaller optical band gap (1.13 eV) than the thiophene-linked
PBFI-T (1.38 eV); however, both polymers have similar HOMO/LUMO energy
levels determined from cyclic voltammetry. Blends of PBFI-T with the
thiazolothiazole–dithienylsilole donor polymer (PSEHTT) gave
a 2.60% power conversion efficiency (PCE) with a 7.34 mA/cm<sup>2</sup> short-circuit current. In contrast, PBFI-S:PSEHTT blends had a 0.75%
PCE with similarly reduced photocurrent and external quantum efficiency.
Reduced free energy for charge transfer and reduced bulk electron
mobility in PBFI-S:PSEHTT blends compared to PBFI-T:PSEHTT blends
as well as significant differences in bulk film morphology are among
the reasons for the large loss in efficiency in PBFI-S:PSEHTT blend
solar cells
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