High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 169

Fig. 4 Effects of the growth rates of the photoactive blends. a J-V characteristics under
illumination at 100 mW cm -2 for devices prepared at solvent evaporation times (t evp ) of 20 min
(No. 1), 3 min (No. 5), 40 s (No. 6), and 20 s (No. 7). b Absorption spectra for slowly (No. 1) and
rapidly (No. 7) grown films [ 15 ]
cosolvents to slow down the evaporation rate and control the polymer morphology
may be called ''cosolvent annealing.''
Another interesting, noncontact annealing approach is microwave heating,
which might be suitable for efficient industrial production of OPV devices [ 20 ].
Similar to thermal and solvent annealing, microwave annealing can increase the
crystallinity of the polymer layer and, thereby, improve the device efficiency.
Moreover, this method can selectively anneal the material, minimizing energy
loss, during the period of heating treatment. For example, we have demonstrated
that microwave annealing can selectively heat the semiconducting layer and metal
electrodes. Because the energy can be focused on specific layers, this rapid and
energy-saving approach should be attractive for the future development of low-
carbon products [ 20 ].
3 Electrode Modification and Engineering
The nature of the interfaces between the polymer active layer and the electrodes
directly affects the efficiency of charge collection. Ideally, both the anode and
cathode contacts should be ohmic (i.e., the contact resistances are negligible rel-
ative to the bulk resistance of the polymer materials). To achieve ohmic contacts,
many functional interlayers have been inserted into the interface between the
metals and the polymer films; for example, poly(3,4-ethylenedioxythio-
phene):polystyrenesulfonate (PEDOT:PSS) [ 21 ], metal oxides (V 2 O 5 and MoO 3 )
[ 21 ], and graphene oxide [ 22 ] are efficient buffer layers at the anodic interface for
collecting holes, while alkali metal complexes (LiF [ 23 ], CsF [ 24 ], and Cs 2 CO 3
[ 25 ] ), ZnO modified with self-assembled monolayers (SAMs) [ 26 ], and
poly(ethylene oxide) (PEO) [ 27 ] are effective cathode interlayers for collecting