6 research outputs found
Charge Separation Pathways in a Highly Efficient Polymer: Fullerene Solar Cell Material
PBDTTPD is one of the best conjugated
polymers for solar cell applications
(up to 8.5% efficiency). We have investigated the dynamics of charge
generation in the blend with fullerene (PCBM) and addressed highly
relevant topics such as the role of bulk heterojunction structure,
fullerene excitation, and excess energy. We show that there are multiple
charge separation pathways. These include electron transfer from photoexcited
polymer, hole transfer from photoexcited PCBM, prompt (<100 fs)
charge generation in intimately mixed polymer:fullerene regions (which
can occur from hot states), as well as slower electron and hole transfer
from excitons formed in pure PBDTTPD or PCBM domains (diffusion to
an interface is necessary). Very interestingly, all the charge separation
pathways are highly efficient. For example, the yield of long-lived
carriers is not significantly affected by the excitation wavelength,
although this changes the fraction of photons absorbed by PCBM and
the amount of excess energy brought to the system. Overall, the favorable
properties of the PBDTTPD:PCBM blend in terms of morphology and exciton
delocalization allow excellent charge generation in all circumstances
and strongly contribute to the high photovoltaic performance of the
blend
The Role of Excitons and Free Charges in the Excited-State Dynamics of Solution-Processed Few-Layer MoS<sub>2</sub> Nanoflakes
Solution-processed semiconducting
transition metal dichalcogenides
are emerging as promising two-dimensional materials for photovoltaic
and optoelectronic applications. Here, we have used transient absorption
spectroscopy to provide unambiguous evidence and distinct signatures
of photogenerated excitons and charges in solution-processed few-layer
MoS<sub>2</sub> nanoflakes (10â20 layers). We find that photoexcitation
above the direct energy gap results in the ultrafast generation of
a mixture of free charges in direct band states and of excitons. While
the excitons are rapidly trapped, the free charges are long-lived
with nanosecond recombination times. The different signatures observed
for these species enable the experimental extraction of the exciton
binding energy, which we find to be âŒ80 meV in the nanoflakes,
in agreement with reported values in the bulk material. Carrier-density-dependent
measurements bring new insights about the many-body interactions between
free charges resulting in band gap renormalization effects in the
few-layer MoS<sub>2</sub> nanoflakes
Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material
PBDTTPD is a conjugated polymer with high power conversion
efficiency
if used in organic solar cells together with fullerene derivatives.
We have investigated for the first time the excited state dynamics
of pristine PBDTTPD thin film as well as the ultrafast evolution of
charge carriers in PBDTTPD:PCBM bulk heterojunction blend using femtosecond
transient absorption spectroscopy. In the latter, charges appear within
the time resolution of the experiment (<100 fs), but clean spectral
signatures allowed to directly follow slower âŒ1 ps charge separation.
Only the slower quenching component competes with excitonâexciton
and excitonâcharge annihilation, leading to a reduced yield
of charge carriers at high laser fluence. Our excellent measuring
sensitivity made it possible to reduce pump power to a point where
annihilation is quasi suppressed. In this case >80% of charges
survive
after 1 ns; the rest recombines (most probably geminately) on the
200 ps time scale
Femtosecond Dynamics of Photoexcited C<sub>60</sub> Films
The well known organic
semiconductor C<sub>60</sub> is attracting
renewed attention due to its centimeter-long electron diffusion length
and high performance of solar cells containing 95% fullerene, yet
its photophysical properties remain poorly understood. We elucidate
the dynamics of Frenkel and intermolecular (inter-C<sub>60</sub>)
charge-transfer (CT) excitons in neat and diluted C<sub>60</sub> films
from high-quality femtosecond transient absorption (TA) measurements
performed at low fluences and free from oxygen or pump-induced photodimerization.
We find from preferential excitation of either species that the CT
excitons give rise to a strong electro-absorption (EA) signal but
are extremely short-lived. The Frenkel exciton relaxation and triplet
yield strongly depend on the C<sub>60</sub> aggregation. Finally,
TA measurements on full devices with applied electric field allow
us to optically monitor the dissociation of CT excitons into free
charges for the first time and to demonstrate the influence of cluster
size on the spectral signature of the C<sub>60</sub> anion
Origin of the Enhanced Photoluminescence Quantum Yield in MAPbBr<sub>3</sub> Perovskite with Reduced Crystal Size
Methylammonium
lead bromide perovskite (MAPbBr<sub>3</sub>) has
been widely investigated for applications in visible perovskite light-emitting
diodes (LEDs). Fine-tuning of the morphology and of the crystal size,
from the microscale down to the quantum confinement regime, has been
used to increase the photoluminescence quantum yield (PLQY). However,
the physical processes underlying the PL emission of this perovskite
remain unclear. Here, we elucidate the origin of the PL emission of
polycrystalline MAPbBr<sub>3</sub> thin films by different spectroscopic
techniques. We estimate the exciton binding energy, the reduced exciton
effective mass, and the trap density. Moreover, we confirm the coexistence
of free carriers and excitons, quantifying their relative population
and mutual interaction over a broad range of excitation densities.
Finally, the enhanced PLQY upon crystal size reduction to the micro-
and nanometer scale in the presence of additives is attributed to
favored excitonic recombination together with reduced surface trapping
thanks to efficient passivation by the additives
Breaking Down the Problem: Optical Transitions, Electronic Structure, and Photoconductivity in Conjugated Polymer PCDTBT and in Its Separate Building Blocks
Conjugated polymers with alternating electron-withdrawing
and electron-donating
groups along their backbone (donorâacceptor copolymers) have
recently attracted attention due to high power conversion efficiency
in bulk heterojunction solar cells. In an effort to understand how
the bandgap in a typical donorâacceptor copolymer is reduced
by internal charge transfer character and what the implications of
this charge transfer are, we have synthesized the isolated repeat
unit (CDTBT) of the photovoltaically highly successful PCDTBT polymer.
We compare here the spectroscopic and electrochemical properties of
the polymer, the repeat unit, and the separate carbazole donor and
dithienylbenzothiadiazole acceptor moieties (CB and dTBT, respectively)
in the solid state and in solutions of various polarity. The results
are interpreted with the help of time-dependent density functional
theory (TD-DFT) calculations. We identify the dominant electronic
transitions responsible for the first two absorption bands in the
âcamel backâ spectrum of PCDTBT as partial charge transfer
transitions with significant delocalization in the directly excited
states. The low bandgap, overall shape, and partial charge transfer
character of the PCDTBT absorption spectrum originate from transitions
in the dTBT unit. The attached CB moiety extends the conjugation length
in CDTBT, rather than acting as a localized donor. Further electronic
delocalization, leading to a relatively small reduction in bandgap,
occurs upon polymerization. We use our finding of higher delocalization
following excitation in the second absorption band to explain the
increased yield of photogenerated charges from this band in PCDTBT
solid thin films. Moreover, we point out the importance of initial
delocalization in the functioning of bulk heterojunction solar cells.
The results presented here are therefore not only highly important
for a better understanding of donorâacceptor copolymers in
general but can also potentially guide the strategic development of
future photovoltaic materials