2 research outputs found
Green-Solvent-Processable, Dopant-Free Hole-Transporting Materials for Robust and Efficient Perovskite Solar Cells
In addition to having
proper energy levels and high hole mobility
(μ<sub>h</sub>) without the use of dopants, hole-transporting
materials (HTMs) used in n-i-p-type perovskite solar cells (PSCs)
should be processed using green solvents to enable environmentally
friendly device fabrication. Although many HTMs have been assessed,
due to the limited solubility of HTMs in green solvents, no green-solvent-processable
HTM has been reported to date. Here, we report on a green-solvent-processable
HTM, an asymmetric D–A polymer (asy-PBTBDT) that exhibits superior
solubility even in the green solvent, 2-methylanisole, which is a
known food additive. The new HTM is well matched with perovskites
in terms of energy levels and attains a high μ<sub>h</sub> (1.13
× 10<sup>–3</sup> cm<sup>2</sup>/(V s)) even without the
use of dopants. Using the HTM, we produced robust PSCs with 18.3%
efficiency (91% retention after 30 days without encapsulation under
50%–75% relative humidity) without dopants; with dopants (bisÂ(trifluoromethanesulfonyl)
imide and <i>tert</i>-butylpyridine, a 20.0% efficiency
was achieved. Therefore, it is a first report for a green-solvent-processable
hole-transporting polymer, exhibiting the highest efficiencies reported
so far for n-i-p devices with and without the dopants
High-Field-Effect Mobility of Low-Crystallinity Conjugated Polymers with Localized Aggregates
Charge carriers typically
move faster in crystalline regions than
in amorphous regions in conjugated polymers because polymer chains
adopt a regular arrangement resulting in a high degree of π–π
stacking in crystalline regions. In contrast, the random polymer chain
orientation in amorphous regions hinders connectivity between conjugated
backbones; thus, it hinders charge carrier delocalization. Various
studies have attempted to enhance charge carrier transport by increasing
crystallinity. However, these approaches are inevitably limited by
the semicrystalline nature of conjugated polymers. Moreover, high-crystallinity
conjugated polymers have proven inadequate for soft electronics applications
because of their poor mechanical resilience. Increasing the polymer
chain connectivity by forming localized aggregates via π-orbital
overlap among several conjugated backbones in amorphous regions provides
a more effective approach to efficient charge carrier transport. A
simple strategy relying on the density of random copolymer alkyl side
chains was developed to generate these localized aggregates. In this
strategy, steric hindrance caused by these side chains was modulated
to change their density. Interestingly, a random polymer exhibiting
low alkyl side chain density and crystallinity displayed greatly enhanced
field-effect mobility (1.37 cm<sup>2</sup>/(V·s)) compared with
highly crystalline polyÂ(3-hexylthiophene)