Environmental Engineering Reference
In-Depth Information
The photohole in the dye, residing at 4.9 eV below vacuum, has no chance of
entering the valence band of the TiO
x
(which would require an excitation of 8.1 eV
4.9 eV), but it easily enters the hole-conducting PEDOT layer at 5eV. Similarly, in
the back junction (3D from right) the absorption energy is 5.1 eV
3.2 eV
¼
1.9 eV,
corresponding to wavelength 1240/1.9
653 nm, closer to the visible. The subse-
quent movements of exciton, photoelectron, and photohole are the same as discussed
for the front junction. The maximum raising of energy across the back junction is
4.3 eV
¼
3.2 eV
¼
1.1 eV.
7.1.3.2 Performance of the Advanced Polymer Tandem Cell
The experimenters have measured the I
-
V characteristics of the tandem device, and
also of each of the separate types of junctions, and
find that the open-circuit voltage of
the tandemdevice is close to the sumof the open-circuit voltages of the separate cells.
Tandem cell operating properties are clearly shown in Figure 7.11.
It appears that these efficiency values are the best ever obtained for polymeric
organic cells of any type. The authors [92] discuss briefly that the tandem cell
properties are reasonably stable over storage and running times, but note that any
major application would require close attention to mechanisms of degradation.
7.1.4
Low-Cost Tandem Technology: Amorphous Silicon:H-Based Solar Cells
We have spoken earlier about the optical absorption of crystalline Si as being relatively
small because of the indirect bandgap. This was shown in Figure 6.3 and led to the
with irradiation intensity of 100mW/cm
2
(about
one sun). The tandem cell shows short-circuit
current density 7.8mA/cm
2
, open-circuit
voltage 1.2 V, and power efficiency 6.5%.
Figure 7.11 J
-
V characteristics [92] of single
cells and tandem cell with PCPDTBT:PCBM and
P3HT:PC
70
BM composites under AM 1.5 G
illumination from a calibrated solar simulator
