Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 49

potentially limit the device performance. To further improve the property of a
TiO x interlayer, Park et al. fabricated Cs-doped film TiO 2 by mixing Cs 2 CO 3
solution with a nanocrystalline TiO 2 solution prepared from a sol to gel process.
The work function of TiO 2 was successfully downshifted by Cs doping to yield a
better Ohmic contact between organic active layer and metal electrode. The
improved electron extraction combining the preserved hole-blocking ability of the
TiO x interlayer afforded a much better photovoltaic performance for the poly(3-
hexylthiophene) (P3HT, see Scheme 3.1 )/PC 61 BM solar cell than that of cell using
sole TiO x interlayer [ 38 ].
ZnO is another n-type metal oxide, which possesses a high electron mobility of
0.066 cm 2 V -1 s -1 [ 39 ] and a Fermi level of 4.4 eV [ 40 ]. The high electron
mobility, ideal Fermi level, and solution processability of ZnO nanoparticles (NPs)
indicate that ZnO would be a good interfacial material to facilitate not only the
efficient charge transfer from PC 61 BM (or PC 71 BM) to ZnO, but also the effective
electron collection at cathode. In addition, the wide band gap of ZnO (3.2 eV)
would endow it good hole blocking ability and good transparency. Moreover, Gilot
et al. had proved that ZnO can also serve as an effective optical spacer for thin
(\60 nm) active layers [ 41 ]. These advantages of ZnO render its widely appli-
cation in inverted BHJ-PSCs. However, the direct use of ZnO as cathode buffer
layer for conventional PSCs is scarce.

3.2.1.2 Self-Assembled Monolayer on Metal Oxide Surface

The contact property of ZnO/metal bilayer cathode and resulted device perfor-
mance could be further improved by spin coating the functional SAM on the ZnO
surface [ 42 , 43 ]. Modifying the ZnO NPs surface with a benzoic acid group con-
stituted with electron donating end groups (such as -OCH 3 , -CH 3 , -H), which form
a negative dipole (toward metal and away from ZnO), the band offset between the
conduction band of ZnO and the work function of metal can be decreased, and
thereby the Ohmic contact was formed. Compared to the unmodified devices, the
devices with SAMs-modified ZnO NPs interlayers showed significant improvement
in all the J-V characteristics including J sc , V oc , FF, and PCE. On the other hand,
opposite effect was observed when the ZnO NPs layer was modified with electron-
withdrawing end groups substituted SAM molecules [ 40 ]. PCE of 4.2 % was
achieved for device with SAM modification with respect to 3.2 % for the control
device with unmodified cathode [ 40 ]. A further enhanced PCE of 4.6 % was
obtained when the ZnO NPs layer was modified with captoundecanoic acid [ 44 ].
More importantly, the stable high work-function metals such as Ag and Au can also
be used as cathodes, and the PCE of 3.65 % and 3.22 % were achieved for
P3HT:PC 61 BM

devices with ZnO/SAM/Ag and ZnO/SAM/Au cathodes,

respectively [ 40 ].