4 research outputs found
Reducing Exciton Binding Energy by Increasing Thin Film Permittivity: An Effective Approach To Enhance Exciton Separation Efficiency in Organic Solar Cells
Photocurrent generation in organic
solar cells requires that excitons,
which are formed upon light absorption, dissociate into free carriers
at the interface of electron acceptor and donor materials. The high
exciton binding energy, arising from the low permittivity of organic
semiconductor films, generally causes low exciton separation efficiency
and subsequently low power conversion efficiency. We demonstrate here,
for the first time, that the exciton binding energy in B,O-chelated
azadipyrromethene (BO-ADPM) donor films is reduced by increasing the
film permittivity by blending the BO-ADPM donor with a high dielectric
constant small molecule, camphoric anhydride (CA). Various spectroscopic
techniques, including impedance spectroscopy, photon absorption and
emission spectroscopies, as well as X-ray spectroscopies, are applied
to characterize the thin film electronic and photophysical properties.
Planar heterojunction solar cells are fabricated with a BO-ADPM:CA
film as the electron donor and C<sub>60</sub> as the acceptor. With
an increase in the dielectric constant of the donor film from ∼4.5
to ∼11, the exciton binding energy is reduced and the internal
quantum efficiency of the photovoltaic cells improves across the entire
spectrum, with an ∼30% improvement in the BO-ADPM photoactive
region
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Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics
Recent
efforts in organic photovoltaics (OPVs) have been devoted
to obtaining low-bandgap non-fullerene acceptors (NFAs) for high photocurrent
generation. However, the low-lying lowest unoccupied molecular orbital
(LUMO) level in narrow bandgap NFAs typically results in a small energy
difference (Δ<i>E</i><sub>DA</sub>) between the LUMO
of the acceptor and the highest occupied molecular orbital (HOMO)
of the donor, leading to low open-circuit voltage (<i>V</i><sub>OC</sub>). The trade-off between Δ<i>E</i><sub>DA</sub> and photocurrent generation significantly limits the simultaneous
enhancement of both <i>V</i><sub>OC</sub> and short-circuit
current density (<i>J</i><sub>SC</sub>). Here, we report
a new medium-bandgap NFA, IDTT-T, containing a weakly electron-withdrawing <i>N</i>-ethyl thiabarbituric acid terminal group on each end of
the indacenodithienothiophene (IDTT) core. When paired with a benchmark
low-bandgap PTB7-th polymer donor, simultaneous enhancement of both
Δ<i>E</i><sub>DA</sub> and absorption spectral coverage
was realized. The OPV devices yield a <i>V</i><sub>OC</sub> of 1.01 V, corresponding to a low energy loss of 0.57 eV in around
10% efficiency single-junction NFA OPVs. The design demonstrates a
working principle to concurrently increase Δ<i>E</i><sub>DA</sub> and photocurrent generation for high <i>V</i><sub>OC</sub> and PCE in bulk fullerene-free heterojunction OPVs
Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay-Annulated Indigo (BAI) Acceptor as Stable Electrochromic Materials
The donor–acceptor (D–A)
approach, which is to incorporate
alternating electron-rich (donor) and electron-deficient (acceptor)
units along the conjugated polymer mainchain, has become an effective
method to provide an informed search for high performance electrochromic
low bandgap polymers. Herein a potent electron acceptor, namely, a
much more soluble version of the nature-inspired bay-annulated indigo
(BAI), was employed in the synthesis of two solution-processable donor–acceptor
polymers for efficient electrochromic devices (ECDs). The devices
fabricated from spin-coated polymer thin films can switch reversibly
between deep blue and transmissive light green hues, with high optical contrasts
in the visible and near-infrared (NIR) regions, good coloration efficiency
and promising ambient stability. In particular, electrochromic devices
based on the copolymer containing a carbazole donor unit exhibit optical
contrasts of 41% and 59% in the visible and NIR regions, respectively,
and a long-term stability of more than 7500 cycles under ambient conditions
with limited reduction in optical contrasts. Such longer term ambient
stability underlines the great potential of BAI-derived electron acceptors
for the development of practical EC materials
Electronic and Morphological Studies of Conjugated Polymers Incorporating a Disk-Shaped Polycyclic Aromatic Hydrocarbon Unit
As
more research findings have shown the correlation between ordering
in organic semiconductor thin films and device performance, it is becoming more essential to exercise
control of the ordering through structural tuning. Many recent studies
have focused on the influence of side chain engineering on polymer
packing orientation in thin films. However, the impact of the size
and conformation of aromatic surfaces on thin film ordering has not
been investigated in great detail. Here we introduce a disk-shaped
polycyclic aromatic hydrocarbon building block with a large π
surface, namely, thienoazacoronenes (TACs), as a donor monomer for
conjugated polymers. A series of medium bandgap conjugated polymers
have been synthesized by copolymerizing TAC with electron donating
monomers of varying size. The incorporation of the TAC unit in such
semiconducting polymers allows a systematic investigation, both experimentally
and theoretically, of the relationships between polymer conformation,
electronic structure, thin film morphology, and charge transport properties.
Field effect transistors based on these polymers have shown good hole
mobilities and photoresponses, proving that TAC is a promising building
block for high performance optoelectronic materials