Environmental Engineering Reference
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Fig. 6.16 Schematic
structure of the inverted
device. The first layer
deposited on the substrate is
the metallic electron contact,
followed by the P3HT:PCBM
blend and the PEDOT:PSS
hole contact. The current is
collected by a metal grid [
89
].
(Reprinted from [
89
], with
permission from Elsevier)
the devices consist of two different electron contact materials, titanium (Ti) and
chromium (Cr), and two different thicknesses of P3HT:PCBM layers. In order to
get the information of chemical composition distribution at the interface of active
layer and metallic electron contact, Auger electron spectroscopy (AES) has been
employed to show that electron contact is partially oxidized during the fabrication
of inverted devices. It is concluded that oxygen is chemically bound at the organic/
metal interface as well for Ti as for Cr as a contact layer. Moreover, both electrode
material and active layer thickness have influences on the long-term stability of
P3HT:PCBM BHJ inverted OPVs, and their results indicate that the devices with
Cr as electrode are much more stable than those with Ti as electrode and a thinner
active layer also has a positive effect. Two possibilities are to explain the effect of
the layer thickness: one is that lower photocurrent caused by thinner active layers
induces lower voltage drop due to the increase in series resistance caused by the
decreased conductivity of PEDOT:PSS during aging [
89
,
90
]; the other one is that
defects in active layer affect and reduce the FF, especially for thick layers [
89
,
91
].
After 1500 h of continuous illumination under a sulfur plasma lamp (100 mW/
cm
2
)at50C, the most stable devices maintain 90 of their initial efficiency of
more than 2.5 %. The results indicate that inverted structure is beneficial to the
device stability.
We have presented inverted OPVs with LWF metals as interfacial layer
modifying ITO as cathode [
92
-
94
]. First, we have investigated the impact of an
ultrathin Ca deposited by thermal evaporation in inverted cell using a structure of
ITO/Ca/P3HT:PCBM/MoO
3
/Ag (Fig.
6.17
(left)) [
94
]. With the use of Ca low-
ering the work function of ITO and MoO
3
modifying the Ag anode, the efficiency
of inverted devices is significantly improved compared with those without either of
these two interfacial layers. Since Ca modifying the ITO is easily oxidized, we
fabricated a CaO modified ITO on purpose to check if the corresponding device
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