Environmental Engineering Reference
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Fig. 4 Energy level diagram of PSC in normal architecture (left) and a schematic illustration of
bulk heterojunction (BHJ) morphophology. Superimposed in both diagrams are the four steps
involved in the current generation in a PSC: (1) Exciton generation (2) Exciton diffusion (not
shown in the band diagram) (3) Exciton dissociation (4) Charge transport
(1)
Exciton generation: Exciton generation occurs upon the absorption of inci-
dent light (photon) having an energy equal or higher than the bandgap of the
photoactive polymer. The band gap of the polymer is characterized by the
energy difference between the lowest unoccupied molecular orbital (LUMO)
and the highest occupied molecular orbital (HOMO) of the polymer. The
lower the band gap, the higher is the amount of exciton generated. Hence,
exhaustive research is being carried out in tailoring low-band gap-conjugated
polymers.
(2)
Exciton diffusion: The exciton thus generated in the photoactive polymer has
high binding energy which does not dissociate at room temperature unlike
inorganic solar cells such as silicon solar cell. An acceptor molecular pro-
vides the energy impetus for exciton dissociation. To achieve this, an exciton
must diffuse to a donor: acceptor interface. The optimum distance of the
exciton to a donor: acceptor interface must be similar to the exciton diffusion
length in conjugated polymers, which is in the order of 10-20 nm [
7
]. This is
realized by the intermixing of donor and acceptor materials during processing
that result in a bulk heterojunction (BHJ) morphology in the deposited film.
BHJ is characterized by interpenetrating network of donor and acceptor
domains. The difference in the HOMO of the donor polymer and the LUMO
of the acceptor molecule largely determines the open circuit voltage in a PSC.
Hence, conjugated polymers having deeper HOMO levels, apart from having
low-band gap, are preferable.
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