High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 167

2 Morphological Control in Photoactive Layers
In BHJ OPVs, the light absorption, exciton generation, exciton dissociation, and
charge transport and collection processes are strongly influenced by the morphology
of the photoactive layer [ 11 - 20 ]. The most common way of controlling the mor-
phology of the active polymer blend has been through the application of annealing
processes. For example, in 2005 Erb et al. studied the effect of thermal annealing on
thin films containing poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycar-
bonyl)propyl-1-pheny[6,6]methanofullerene (PCBM) [ 13 ]. Their X-ray diffraction
(XRD) data (Fig. 2 a) indicated that the (100) peak had increased significantly after
thermal annealing, suggesting the formation of crystalline P3HT domains. The
authors inferred that thermal annealing led to changes in the morphology of the
P3HT:PCBM thin film, as illustrated in Fig. 2 b. Hence, the device performance had
improved as a result of increased crystallinity.
Meanwhile, Ma et al. also investigated the effects of post-annealing treatment on
device performance [ 14 ]. After thermal annealing at 150 C for 30 min, they
observed a higher degree of crystallinity of P3HT (Fig. 3 a). Furthermore, the
authors used atomic force microscopy (AFM) to investigate the surface morphology
after removal of the Al cathode (Fig. 3 b). The rougher surface indicated that
adhesion between the active layer and the Al cathode was also enhanced. In other
words, the quality of the polymer-Al contact improved after post-annealing, leading
to decreased contact resistance. Combining these effects (higher P3HT crystallinity
and lower contact resistance), the series resistance of the device decreased signifi-
cantly, from 113 to 7.9 X cm 2 , after post-annealing. Overall, the optimized post-
annealing conditions resulted in an excellent PCE, approaching 5 %.
In addition to thermal annealing, Li et al. reported another important approach:
''solvent annealing'' [ 15 , 16 ]. They controlled the growth rate of the active layer
(P3HT/PCBM) from solution to the solid state, resulting in a decrease in series
resistance (R s ) and an increase in optical absorption. The J-V characteristics of
devices prepared using various solvent evaporation times (t evp ) (Fig. 4 a) revealed
that the value of J sc increased upon increasing the solvent evaporation time.
Absorption spectra suggested that slower growth of the film led to increased
absorption and a red shift (Fig. 4 b). The pronounced vibronic shoulders for the
slowly grown layer indicated a higher degree of polymer ordering. The authors
concluded that self-organization originating from the slow growth of the active
layer plays an important role in improving device efficiencies [ 15 ].
From the discussion above, we find that one of the keys to high device per-
formance is improving the self-organization of the polymers. Other methods for
achieving higher degrees of polymer ordering have also been proposed. For
instance, Jin et al. found that using P3HT:PCBM blends dissolved in a cosolvent
during device fabrication could improve the device efficiency [ 17 ]. Moulé et al.
also revealed that the PCEs of devices could be improved after the addition of a
high-boiling-point solvent (e.g., nitrobenzene) to the base solvent, chlorobenzene
[ 18 ]. Furthermore, our group also developed a new solvent mixture system