Enhanced
Air Stability of Polymer Solar Cells with
a Nanofibril-Based Photoactive Layer
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Abstract
In
spite of the rapid increase in the power conversion efficiency
(PCE) of polymer solar cells (PSCs), the poor stability of the photoactive
layer in air under sunlight is a critical problem blocking commercialization
of PSCs. This study investigates the photo-oxidation behavior of a
bulk-heterojunction (BHJ) photoactive film made of single-crystalline
poly(3-hexlythiophene) (P3HT) nanofibrils and fullerene derivatives
[phenyl-C<sub>61</sub>-butyric methyl ester (PCBM), indene-C 60 bisadduct
(ICBA)]. Because the single-crystalline P3HT nanofibrils had tightly
packed π–π stacking, the permeation of oxygen and
water into the nanofibrils was significantly reduced. Chemical changes
in P3HT were not apparent in the nanofibrils, and hence the air stability
of the nanofibril-based BHJ film was considerably enhanced as compared
with conventional BHJ films. The chemical changes were monitored by
Fourier-transform infrared (FT-IR) spectroscopy, Raman spectroscopy,
and UV–vis absorbance. Inverted PSCs made of the nanofibril-based
BHJ layer also showed significantly enhanced air stability under sunlight.
The nanofibril-based solar cell maintained more than 80% of its initial
PCE after 30 days of continuous exposure to sunlight (AM 1.5G, 100
mW/cm<sup>2</sup>), whereas the PCE of the conventional BHJ solar
cell decreased to 20% of its initial PCE under the same experimental
conditions