High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 166

Fig. 1 a Device architecture of a typical BHJ OPV. b Schematic representation of the working
principle of an OPV device: 1 light absorption 2 exciton generation 3 exciton diffusion 4 exciton
dissociation 5 charge transport, and 6 charge collection [ 10 ]

materials can dissociate into free carriers only when they diffuse to the donor-
acceptor junctions. In a so-called BHJ structure, the large area of the donor-
acceptor interface ensures efficient exciton dissociation to produce a high density
of free charge carriers. After exciton dissociation, the free charges are transported
through the donor and acceptor materials, respectively, to the electrodes. The
electrodes then collect the carriers, resulting in a photocurrent.
Based on the six-step mechanism, the external quantum efficiency (EQE) of an
OPV can be expressed using the equation [ 10 ]

EQE ð k Þ ¼g A ð k Þ g G ð k Þ g C ð l Þ ð 1 Þ

where g A ð k Þ , g G ð k Þ , and g C ð l Þ are the absorption efficiency, carrier generation
efficiency, and charge collection efficiency, respectively. For an OPV prepared
using the BHJ concept, the large area of the donor-acceptor interface throughout
the photoactive layer suggests that the carrier generation efficiency could, in
theory, be increased to close to 100 %. On the other hand, the charge collection
efficiency (g C ð l Þ ) strongly depends on the morphology of the photoactive layer
because this morphology will affect the nature of the conducting channel. Fur-
thermore, the quality of the interface between the metallic electrodes and the
organic materials also plays an essential role in determining the performance of the
device. In other words, the interfaces strongly affect the charge collection effi-
ciency. The other key issue is sufficient harvesting of sunlight. Although the use of
a thicker active layer is a straightforward approach toward obtaining high
absorption efficiency (g A ð k Þ ), this method inevitably results in increased device
resistance, due to the low carrier motilities of organic materials. Therefore, in real
devices, it is difficult to decouple the relationship between these factors; simul-
taneous improvement of both g A ð k Þ and g C ð l Þ is hard to achieve. In the following
section, we outline strategies for improving the efficiencies of OPVs and review
some recent progress in this area.