Chemistry Reference
In-Depth Information
electron vacancies or holes. To produce electric current, the electron-hole pairs must migrate to the
interface between the electron donor and electron acceptor materials. Upon reaching the interface, the
electron-hole pairs splits into separate mobile charges. The charges then diffuse to their respective
electrodes. The electrons are transported by the electron-accepting material to the cathode and the
holes by the hole-accepting material (electron donor) to the anode. To put it in other words, the
Coulomb-correlated electron-hole pair, the excitons, diffuse to the donor-acceptor interface where
exciton dissociation occurs via an electron-transfer process to the n-type layer. With the aid of an
internal electric field, the n-type layer then carriers the electrons in the opposite direction. The electric
field in turn generates the photocurrent and the photo voltage. Such devices are known as
planar
heterojunction
cells. Such an arrangement, however, is not very efficient, because the excitons can
decay back to the ground level before they diffuse into to the n-type layer. To overcome the difficulty,
the concept of a
bulk heterojunction
was introduced [
263
]. By blending donor and acceptor
materials together an interpenetrating bicontinuous network of junctions, large donor-acceptor
interfacial areas can be achieved. This results in an enhanced quantum efficiency of charge separation
and in efficient charge collection. Gaudiana [
264
] likened the morphology of a bulk heterojunction
active layer to a sponge. The solid part represents the nano-sized interconnected bits of acceptors. The
polymer is represented by the holes that are intimately connected to other holes throughout the sponge
and never far from a solid region. Blending the phases on that scale, in effect, distributes small regions
of interface throughout the photoactive layer. As a result, excitons need only to diffuse only a short
distance before quickly reaching a donor-acceptor interface where they can dissociate into separate
charges.
An advancement in efficiency of polymeric solar cells, from 3 to 5%, came in 2009 when it was
observed that promising efficient charge transfer materials can be prepared from combinations of poly
(alkyl-thiophenes) donors with 1-(3-methoxycarbonyl)propyl-1phenyl-[6,6]-methanofullere acceptors
[
265
]. Mild heating disperses the acceptor molecules among the donor molecules:
This led to exploration of many other combinations of various other polymers with different
derivatives of fullerine and with various chromophores. In an attempt to lower highest occupied
molecular orbitals (HMO) of the polymer with stronger electron-withdrawing groups, new polymers
were developed. The results were summarized in a review [
265
].
