5 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
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
Low-Vapor-Pressure Solvent Additives Function as Polymer Swelling Agents in Bulk Heterojunction Organic Photovoltaics
Bulk
heterojunction (BHJ) photovoltaics based on blends of conjugated
polymers and fullerenes require an optimized nanoscale morphology.
Casting BHJ films using solvent additives such as 1,8-diiodooctane
(DIO), 1,8-octanedithiol (ODT), chloronapthalene (CN), or diphenyl
ether (DPE) often helps achieve this proper morphology: adding just
a few volume percent of additive to the casting solution can improve
polymer/fullerene mixing or phase separation, so that solvent additives
have become staples in producing high-efficiency BHJ solar cells.
The mechanism by which these additives improve BHJ morphology, however,
is poorly understood. Here, we investigate how these additives control
polymer/fullerene mixing by taking advantage of sequential processing
(SqP), in which the polymer is deposited first and then the fullerene
is intercalated into the polymer underlayer in a second processing
step using a quasi-orthogonal solvent. In this way, SqP isolates the
role of the additives’ interactions with the polymer and the
fullerene. We find using ellipsometry-based swelling measurements
that when adding small amounts of low-vapor-pressure solvent additives
such as DIO and ODT to solutions of polyÂ(3-hexylthiophene-2,5-diyl)
(P3HT), polyÂ[(4,4′-bisÂ(3-(2-ethyl-hexyl)ÂdithienoÂ[3,2-b:″,3′-d]Âsilole)-2,6-diyl-<i>alt</i>-(2,5-bisÂ(3-(2-ethyl-hexyl)Âthiophen-2yl)ÂthiazoloÂ[5,4-<i>d</i>]Âthiazole)] (PSEHTT), or polyÂ[4,8-bisÂ(2-ethylhexyloxy)-benzolÂ[1,2-b:4,5-b′]Âdithiophene-2,6-diyl-<i>alt</i>-4-(2-ethylhexyloxy-1-one)ÂthienoÂ[3,4-<i>b</i>]Âthiophene-2,6-diyl] (PBDTTT-C), the additives remain in the polymer
film, leading to significant swelling. Two-dimensional grazing-incidence
wide-angle X-ray scattering measurements show that the swelling is
extensive, directly affecting the polymer crystallinity. When we then
use SqP and cast phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM)
onto DIO-swollen polymer films, X-ray photoelectron spectroscopy and
neutron reflectometry measurements demonstrate that vertical mixing
of the PCBM in additive-swollen polymer films is significantly improved
compared with films cast without the additive. Thus, low-vapor-pressure
solvent additives function as cosolvent swelling agents or secondary
plasticizers, allowing fullerene to mix better into the swollen polymer
and enhancing the performance of devices produced by SqP, even when
the additive is present only in the polymer layer. DIO and ODT have
significantly different fullerene solubilities but swell polymers
to a similar extent, demonstrating that swelling, not fullerene solubility,
is the key to how such additives improve BHJ morphology. In contrast,
higher-vapor-pressure additives such as CN and DPE, which have generally
high polymer solubilities, function by a different mechanism, improving
polymer crystallinity