26 research outputs found
Quantitative Bimolecular Recombination in Organic Photovoltaics through Triplet Exciton Formation
The nanoscale morphology and high
charge densities in organic photovoltaics
(OPVs) lead to a high rate of bimolecular encounters between spin-uncorrelated
electrons and holes. This process can lead to the formation of low-energy
triplet excitons on the donor polymer that decay nonradiatively and
limit the device performance. We use time-resolved optical spectroscopy
to characterize the effect of morphology through the use of solvent
additives such as 1,8-octanedithiol (ODT) on triplet dynamics
and charge recombination in blends of poly[2,6-(4,4-bis(2-ethylhexyl)-4<i>H</i>-cyclopenta[2,1-<i>b</i>;3,4-<i>b</i>′]-dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole)]
and [6,6]-phenyl-C<sub>71</sub>-butyric acid methyl ester. This is
an attractive OPV system since the extended absorption of the polymer
into the near-infrared gives good coverage of the solar spectrum,
but nevertheless, the internal quantum efficiency (IQE) has not been
reported to be higher than ∼65% under short circuit conditions.
We find that, without ODT, the IQE is 48% and 16% of excitations decay
via bimolecular triplet formation. With ODT treatment, which improves
crystallinity and carrier mobility, the IQE increases to 65%, but
bimolecular triplet formation significantly increases and now accounts
for all of the recombination (35% of charges)
Compatibilization of All-Conjugated Polymer Blends for Organic Photovoltaics
Compatibilization
of an immiscible binary blend comprising a conjugated
electron donor and a conjugated electron acceptor polymer with suitable
electronic properties upon addition of a block copolymer (BCP) composed
of the same building blocks is demonstrated. Efficient compatibilization
during melt-annealing is feasible when the two polymers are immiscible
in the melt, i.e. above the melting point of ∼250 °C of
the semicrystalline donor polymer P3HT. To generate immiscibility
at these high temperatures, the acceptor polymer PCDTBT is equipped
with fluorinated side chains leading to an increased Flory–Huggins
interaction parameter. Compatibilization in bulk and thin films is
demonstrated, showing that the photovoltaic performance of pristine
microphase separated and nanostructured BCPs can also be obtained
for compatibilized blend films containing low contents of 10–20
wt % BCP. Thermodynamically stable domain sizes range between several
tens of microns for pure blends and ∼10 nm for pure block copolymers.
In addition to controlling domain size, the amount of block copolymer
added dictates the ratio of edge-on and face-on P3HT crystals, with
compatibilized films showing an increasing amount of face-on P3HT
crystals with increasing amount of compatibilizer. This study demonstrates
the prerequisites and benefits of compatibilizing all-conjugated semicrystalline
polymer blends for organic photovoltaics
Aqueous Self-Assembly of an Electroluminescent Double-Helical Metallopolymer
A new type of water-soluble copper-containing polymer
has been
synthesized using the technique of subcomponent self-assembly. Copper(I)-directed
imine bond formation between triethylene glycol functionalized 1,2-phenylenediamine
and 2,9-diformylphenanthroline subcomponents resulted in the formation
of a chain in which two conjugated helical ligand strands wrap around
a linear array of metal ions. Characterization data from a variety
of analytical methods are consistent with our formulation of this
material. After purification by dialysis, the polymer was shown to
possess several properties of conceptual and practical interest. (1)
Individual double-helical strands appear to further aggregate through
entanglement of their side chains to form well-defined superstructures
such as nanoscale bow ties and macrocycles, which can be imaged on
a surface. (2) The material’s copper(I) ions underwent reversible
electrochemical oxidation in solution, whereas analogous model compounds
were observed to decompose upon oxidation: the polymer’s greater
length appeared to stabilize oxidized states through delocalization
or entrapment. (3) Photophysical measurements reveal this material
to be photo- and electroluminescent. It has been successfully used
for the fabrication of electroluminescent devices and shows a weak
emission of white-blue light with CIE coordinates of (0.337, 0.359).
This study further demonstrates the utility of the technique of subcomponent
self-assembly for the straightforward generation of materials with
useful properties
Enhancing Phase Separation and Photovoltaic Performance of All-Conjugated Donor–Acceptor Block Copolymers with Semifluorinated Alkyl Side Chains
Phase separation of all-conjugated
donor–acceptor block
copolymers is more difficult to achieve compared to classical coil–coil
systems owing the intrinsic similarity of the two blocks having both
rigid conjugated backbones and alkyl side chains and their generally
low degrees of polymerization. Here we demonstrate that side chain
fluorination of a poly(carbazole-<i>alt</i>-dithienylbenzothiadiazole)
segment (SF-PCDTBT), to be used as electron acceptor block in combination
with poly(3-hexylthiophene) P3HT as donor block in all-conjugated
donor–acceptor block copolymers of type SF-PCDTBT-<i>b</i>-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT
leading to phase separation for already moderate molar masses. Key
to the successful synthesis of a new TBT-monomer with semifluorinated
side chains is a direct arylation step that elegantly bypasses classical
cross-coupling reactions in which the semifluorinated side chain causes
low yields. Suzuki polycondensation of the semifluorinated TBT monomer
with a suitable carbazole comonomer and in situ termination by P3HT-Br
is optimized extensively with respect to the yield of the end-capping
efficiency and molar mass control of the PCDTBT segment. When the
fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by <i>n</i>-hexyl chains (hex-PCDTBT), the tendency for phase separation
with covalently connected P3HT is much reduced as shown by differential
scanning calorimetry and grazing incidence small-angle scattering
measurements on thin films. Favorably, of all the block copolymers
made only SF-PCDTBT-<i>b</i>-P3HT is microphase separated,
exhibits face-on orientation of P3HT domains, and additionally displays
surface segregation of the SF-PCDTBT segment at the polymer/air interface.
All of these properties are beneficial for single layer single component
solar cells. SF-PCDTBT-<i>b</i>-P3HT exhibits the best solar
cells performance with a high open-circuit voltage of 1.1 V and a
power conversion efficiency of ∼1% which largely outperforms
devices based on the analogous H-PCDTBT-<i>b</i>-P3HT and
hex-PCDTBT-<i>b</i>-P3HT
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In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics
We
present a novel experimental approach which allows extraction of both
spatial and temporal information on charge dynamics in organic solar
cells. Using the wavelength dependence of the photonic structure in
these devices, we monitor the change in spatial overlap between the
photogenerated hole distribution and the optical probe profile as
a function of time. In a model system we find evidence for a buildup
of the photogenerated hole population close to the hole-extracting
electrode on a nanosecond time scale and show that this can limit
charge transport through space-charge effects under operating conditions
In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics
We
present a novel experimental approach which allows extraction of both
spatial and temporal information on charge dynamics in organic solar
cells. Using the wavelength dependence of the photonic structure in
these devices, we monitor the change in spatial overlap between the
photogenerated hole distribution and the optical probe profile as
a function of time. In a model system we find evidence for a buildup
of the photogenerated hole population close to the hole-extracting
electrode on a nanosecond time scale and show that this can limit
charge transport through space-charge effects under operating conditions
Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification
Photoinduced
charge separation at hybrid organic–inorganic
interfaces is poorly understood and challenging to control. We investigate
charge separation at a model system of ZnO/poly(3-hexylthiophene)
(P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA)
self-assembled modification to study and enhance the charge separation
efficiency. We find that doping alone lowers the efficiency of charge
separation due to the introduction of defect states at the oxide surface.
However, with the combination of doping and molecular modification,
charge separation efficiency is significantly enhanced due to the
passivation of interfacial traps and improved modifier coverage. This
demonstrates a complex noncumulative effect of doping and surface
modification and shows that with the correct choice of metal oxide
dopant and organic modifier, a poorly performing hybrid interface
can be turned into an efficient one
Morphology-Dependent Charge Photogeneration in Donor–Acceptor Block Copolymer Films Based on Poly(3-hexylthiophene)-<i>bloc<i>k</i></i>-Poly(perylene bisimide acrylate)
We have examined how the nanomorphology and crystallinity
of semiconducting double-crystalline block copolymers determine their
photophysical and photovoltaic responses. The block copolymers consist
of a poly(3-hexylthiophene) (P3HT) donor block coupled to a polymerized
perylene bisimide acrylate (PPerAcr) acceptor. Different molecular
weights and processing solvents allow the modification of the donor–acceptor
interface with regard to their morphology and crystallinity. Transient
absorption spectroscopy was used to resolve photoinduced charge transfer
seen on the ∼1 ps time scale, consistent with substantial photoluminescence
quenching caused by finely dispersed, disordered donor–acceptor
interfaces. For high molecular weight block copolymers, microphase
separation is enhanced by slow film formation, leading to slower charge
photogeneration. The crystallinity of the P3HT component is of particular
importance, which has been monitored spectroscopically. Crystalline
P3HT/PPerAcr interfaces lead to high levels of long-lived charge pairs
that are more easily extracted in an applied electric field. While
external quantum efficiencies of over 25% were obtained, the overall
power conversion efficiency of the best block copolymer device is
still limited. This is due to the unsuitable orientation of the block
copolymer nanomorphology, and the performance lies below that achieved
for a blend of equivalent homopolymers. This suggests that increasing
the molecular weight of the block copolymers to tune the microphase
separation could further improve the photovoltaic efficiency. Our
photophysical results give guidelines for future development of promising
block copolymer-derived devices, highlighting the importance of interfacial
crystallinity and sufficient phase separation
Red-shifted delayed fluorescence at the expense of photoluminescence quantum efficiency ‒ an intramolecular charge-transfer molecule based on a benzodithiophene-4,8-dione acceptor
Employing the thiophene based quinone, benzo[1,2-b:4,5-b′]dithiophene-4,8-dione, as the electron-accepting moiety alongside N-phenylcarbazole donors to produce a donor–π–acceptor–π–donor (D–π–A–π–D) molecule has yielded a new red emitter displaying delayed fluorescence. This new molecule shows strongly (over 100 nm) red-shifted emission when compared to an anthraquinone based analogue. Cyclic voltammetry complemented by computational insights prove that this red-shift is due to the significantly stronger electron-accepting ability of the thiophene quinone compared to anthraquinone. Photophysical and computational studies of this molecule have revealed that while the presence of the thiophene containing acceptor facilitates rapid intersystem crossing which is comparable to anthraquinone analogues, the reverse intersystem crossing rate is slow and non-radiative decay is rapid which we can attribute to low-lying locally excited states. This limits the total photoluminescence quantum efficiency to less than 10% in both solution and the solid state. These results provide a useful example of how very minor structural variations can have a defining impact on the photophysical properties of new molecular materials
Interface-Dependent Radiative and Nonradiative Recombination in Perovskite Solar Cells
Interfacial
engineering has shown to play an essential role to
optimize recombination losses in perovskite solar cells; however,
an in-depth understanding of the various loss mechanisms is still
underway. Herein, we study the charge transfer process and reveal
the primary recombination mechanism at inorganic electron-transporting
contacts such as TiO<sub>2</sub> and its modified organic rivals.
The modifiers are chemically ([6,6]-phenyl C<sub>61</sub> butyric
acid, PC<sub>60</sub>BA) or physically ([6,6]-phenyl C<sub>61</sub> butyric acid methyl ester, PC<sub>60</sub>BM and C<sub>60</sub>)
attached fullerene to the TiO<sub>2</sub> surface to passivate the
density of surface states. We do not observe any change in morphology,
crystallinity, and bulk defect density of halide perovskite (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> in this case) upon interface
modification. However, we observe compelling results via photoluminescence
and electroluminescence studies that the recombination dynamics at
both time scales (slow and fast) are largely influenced by the choice
of the selective contact. We note a strong correlation between the
hysteresis and the so-called slow charge dynamics, both significantly
influenced by the characteristics of the selective contact, for example,
the presence of surface traps at the selective contact not only shows
a larger hysteresis but also leads to higher charge accumulation at
the interface and distinguishable slow dynamics (a slower stabilization
of recombination dynamics at a time scale of several minutes)