Biomedical Engineering Reference
In-Depth Information
as a means of generating free
charges from otherwise energetically bound photoexcited electron-
hole pairs (excitons). Unfortunately, excitons in many interesting
materials (including many organic molecular systems) diffuse only
a short distance (of the order 10 nm) before recombination of
the charge pair. As such, only excitons generated within 10 nm of
the heterojunction interface have the possibility to contribute to
photocurrent in the cell. Since a viable solar cell must also be thick
enough to absorb most of the incident sun light, in planar organic
films a significant fraction of excitons decay prior to reaching the
hetrojunction where they are capable of ionising and generating free
charges. The idea of a
acceptor interface or
heterojunction
is to distribute the donor-
acceptor interface throughout the active layer, while simultaneously
maintaining free charge extraction pathways outwards through
both material phases. This concept calls for an intimate vertical
interdigitation of two materials on the 10 nm length scale.
A great number of donor-acceptor material combinations,
including organic, inorganic, and hybrid composites, are now being
explored as the basis for solar cells. Often simple mixing of the two
components, and relying on a degree of spontaneous macrophase
separation during processing, is enough to achieve a fairly efficient
bulk heterojunction architecture. However, optimizing an intimate
and distributed interdigitation on the right length scale in a film in
this way is extremely difficult and varies enormously from system
to system and batch to batch. Hence the attraction of strategies in
which an idealized morphology can be templated into a great many
donor-acceptor combinations which can then be chosen for their
interesting intrinsic material properties such as light absorption,
molecular energy levels, and ease of charge transport.
A concerted exploration of the concept of patterning
semiconductor composite films using BCP templates (replicated
electrochemically or otherwise) should be possible now that suitable
copolymers are more widely available, and the required processing
is better understood. Evaluation and comparison of the relative
effectiveness of each microphase morphology has already started,
using for example the dye-sensitized solar cell.
bulk heterojunction
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