Environmental Engineering Reference
In-Depth Information
PEDOT:PSS
-
ITO interface with electrons from the external circuit that
ow in
through the ITO layer.
7.1.3.1 Band-Edge Energies in the Multilayer Tandem Semiconductor Polymer
Structure
Some of this is clari
ed in an energy-level diagram for this complicated eight-layer
structure, shown in Figure 7.10.
The absorption energy in the semiconducting donor polymer PCPDTBT (front
cell, 3D from the left in Figure 7.10) is seen to be 4.9 eV
3.5 eV
¼
1.4 eV, corre-
sponding to wavelength
l¼
886 nm. This creates an exciton,
which migrates to a heterojunction interface with the acceptor. At such an interface,
the photoelectron (resulting from decay of the exciton, and residing at 3.5 eV below
vacuum level), falls across a heterojunction into the fullerene acceptor (PCBM)
component of the bulk distributed heterojunction layer (energy 4.3 eV down from
vacuum), where it can move freely to the conduction band of the TiO
x
(down 4.4 eV
from the vacuum energy). This can raise the conduction band energy of the fullerene
layer by as much as 4.3 eV
hc/E
¼
1240/1.4 nm
¼
3.5 eV
¼
0.8 eV, to contribute to the open-circuit
photovoltage of the cell.
Figure 7.10 Energy-level schematic [92] of
tandempolymer organic solar cell structurewith
efficiency over 6%. (Left to right in this diagram
corresponds to rising vertically from bottom to
top in Figure 7.9, right panel. So electrons flow
from left to right in this diagram, and holes flow
from right to left, under photoexcitation.)
Energies aremeasured positively down from the
vacuum level, so that 4.8 eV, on the left, is the
work function of the ITO layer and 4.3 eV on the
right is the work function of the aluminum film.
For TiO
x
layer (center) the valence band is at
8.1 eV (relative to free electrons in vacuum),
which is too deep to allowholes flowing from the
PEDOT layer to be transmitted, and these holes
annihilate with photoelectrons from the front
cell at the TiO
x
-
PEDOT interface, carrying
current across the device. For the dye
fullerene
bulk distributed heterojunction
charge-
separating layers (second from left, and second
from right), the solid lines are the upper LUMO
and lower HOMO energies of the donor
polymer dye and the dashed lines are the
conduction and valence band energies of the
fullerene acceptors.
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