The
molecular weights and structural properties of polymers play key roles
in the efficiency of gelators in polymer gel electrolytes (PGEs) for
quasi-solid-state dye-sensitized solar cells (QSS-DSSCs). To find
an appropriate gelator, we synthesized well-defined poly(acrylonitrile-co-N,N-dimethylacrylamide)-block-poly(ethylene glycol)-block-poly(acrylonitrile-co-N,N-dimethylacrylamide)
ABA triblock copolymers with various molecular weights and copolymer
compositions by reversible addition–fragmentation chain-transfer
polymerization. The ratio of acrylonitrile (AN)/N,N-dimethylacrylamide (DMAA) in the triblock copolymers
influences their solubility in liquid electrolytes (LEs) and thermal
stability. The highest thermal stability was up to 360 °C, and
this was achieved by the polymer with an AN/DMAA ratio of ≤4.
The thermal stability was related to excessive randomness in the P(AN-co-DMAA) block that hinders cyclization among nitrile groups.
Both the molecular weights and the AN/DMAA ratios enabled gel formation
by controlling the amount of the polymer, and hence, they influence
the ionic conductivity and diffusion as well. Based on the electrochemical
properties, polymers with molecular weights above 100 kg/mole were
efficient as PGEs in QSS-DSSCs. The overall power conversion efficiency
(PCE) of 14 wt % SGT-626 PGE-based QSS-DSSCs was 9.72%
under AM 1.5G solar illumination, comparable with an overall PCE of
9.79% for LE DSSCs. The overall PCE of the QSS-DSSCs further increased
to 10.02% by incorporating 3 wt % TiO2 nanoparticles in
the 14 wt % SGT-626 PGE. The SGT-626 PGE-based
QSS-DSSC was also tested under indoor light conditions, and the best
PCE of 21.26% was achieved under a white LED light of 1000 lux, which
is higher than the PCE of 19.94% for the LE DSSC. The long-term device
stability test under adverse conditions (50 °C and 1 sun illumination)
reveals the improved stability of PGE-based QSS-DSSCs over LE DSSCs.
In terms of PCE and long-term device stability, our PGE QSS-DSSCs
have great potential over LE DSSCs for future indoor and outdoor applications.
A redox electrolyte
is a crucial part of dye-sensitized solar cells
(DSSCs), which plays a significant role in the photovoltage and photocurrent
of the DSSCs through efficient dye regeneration and minimization of
charge recombination. An I–/I3
– redox shuttle has been mostly utilized, but it limits the open-circuit
voltage (V
oc) to 0.7–0.8 V. To
improve the V
oc value, an alternative
redox shuttle with more positive redox potential is required. Thus,
by utilizing cobalt complexes with polypyridyl ligands, a significant
power conversion efficiency (PCE) of above 14% with a high V
oc of up to 1 V under 1-sun illumination was
achieved. Recently, the V
oc of a DSSC
has exceeded 1 V with a PCE of around 15% by using Cu-complex-based
redox shuttles. The PCE of over 34% in DSSCs under ambient light by
using these Cu-complex-based redox shuttles also proves the potential
for the commercialization of DSSCs in indoor applications. However,
most of the developed highly efficient porphyrin and organic dyes
cannot be used for the Cu-complex-based redox shuttles due to their
higher positive redox potentials. Therefore, the replacement of suitable
ligands in Cu complexes or an alternative redox shuttle with a redox
potential of 0.45–0.65 V has been required to utilize the highly
efficient porphyrin and organic dyes. As a consequence, for the first
time, the proposed strategy for a PCE enhancement of over 16% in DSSCs
with a suitable redox shuttle is made by finding a superior counter
electrode to enhance the fill factor and a suitable near-infrared
(NIR)-absorbing dye for cosensitization with the existing dyes to
further broaden the light absorption and enhance the short-circuit
current density (J
sc) value. This review
comprehensively analyzes the redox shuttles and redox-shuttle-based
liquid electrolytes for DSSCs and gives recent progress and perspectives.
For
efficient polymer gel electrolytes (PGEs) in quasi-solid-state
dye-sensitized solar cells (QSS-DSSCs), six ABA triblock copolymers
based on (poly(acrylonitrile-co-N-(isobutoxymethyl)acrylamide)-block-poly(ethylene glycol)-poly(acrylonitrile-co-N-(isobuto-xymethyl)acrylamide)) (P(AN-co-BMAAm)-b-PEG-b-P(AN-co-BMAAm))
with various copolymer compositions and molecular weights, coded as SGT-605, SGT-606, SGT-608, SGT-609, SGT-611, and SGT-612, have
been synthesized by reversible addition–fragmentation chain
transfer (RAFT) polymerization using PEG-functionalized macro-RAFT
agents. The effects of copolymer compositions and molecular weights
in P(AN-co-BMAAm)-b-PEG-b-P(AN-co-BMAAm) triblock copolymers were
investigated in terms of electrochemical properties and photovoltaic
performance as PGEs. The ionic conductivity was increased with N-(isobutoxymethyl)acrylamide
(BMAAm) composition of these triblock copolymers, which is attributed
to the availability of free iodide ions by complex formation among
acrylamide groups with Li+ ions. However, polymer gel electrolytes
with high molecular weights enhance ionic conductivity due to the
lower amount of polymers required for the gel formation. Thus, the
photovoltaic performances of PGE-based QSS-DSSCs improved along with
the increase in the molecular weight of the triblock copolymer. The
addition of 7 wt % TiO2 nanofiller into PGEs produced a
higher ionic conductivity and diffusion of I3
– than the corresponding PGEs. The resulting power conversion efficiency
(PCE) of QSS-DSSCs using SGT-612/TiO2 composite
PGEs under simulated 1-sun condition was 9.83% (V
oc, 792.8 mV; J
sc, 16.65 mA/cm2; FF, 74.52%), which was higher than that of liquid electrolyte
DSSCs (PCE 9.53%; V
oc, 743.8 mV; J
sc, 17.16 mA/cm2; FF, 74.64%). The
long-term device stability of PGE-based QSS-DSSCs was better than
the liquid-state DSSCs.
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