Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 58

3.2.2.5 Self-Assembled Monolayers as Anode Interlayer
SAMs with different end groups (-CH 3 , -NH 2 , -CF 3 ) were also employed to
modify the work function of ITO, and thereby to improve the performance of
P3HT:PC 61 BM solar cell. The work function of ITO increased initially from 4.7 to
5.16 eV after the insertion of a -CF 3 substituted SAM molecules, affording a much
better match with the HOMO of P3HT for efficient hole extraction at anode
junction. Moreover, it was found that the surface properties of the inserted SAMs
can greatly influence the morphology of the upper organic active layer. Active
layer spin coated onto the hydrophobic surface of -CF 3 SAM exhibited little
undesired phase separation, leading to a high PCE of 3.15 % for the corresponding
solar cell [ 91 ].
3.3 Interface Engineering for Inverted Solar Cell Devices
The conventional device structure of BHJ-PSCs has some inherent device stability
problems. The transparent conducting ITO used as the hole-collecting contact can
be etched over time by the acidic PEDOT:PSS hole-transporting layer [ 64 ]. The
thermo-deposition of low work-function cathode usually requires high vacuum,
thus leading to increases in fabrication costs. Moreover, to avoid the exposure of
low work-function cathode to air, encapsulation technologies have to be applied
which further complicate the fabrication process. Therefore, device architectures
that can remove the need of PEDOT:PSS (or at least separate it from ITO) and use
nonvacuum-deposited air-stable high work-function metal electrodes at the top
interface are desired. Based on these considerations, inverted solar cell device
architecture was proposed. The device structure is shown in Fig. 3.1 . This archi-
tecture has recently attracted considerable attention due to the device stability and
processing advantages compared to the conventional architecture. In an inverted
device, the polarity of charge collection is the opposite of the conventional
architecture, allowing the use of higher work function and air-stable materials
(Au, Ag, and Cu) as the top anode which is exposed to air. The use of higher work-
function metals offer better ambient device stability and the possibility for using
low-cost solution-processed techniques such as spray coating [ 92 ] or screen
printing [ 93 ] to deposit the top anode. The focus of current research for the
inverted device architectures is to understand how to improve the device efficiency
and stability through the design and processing of the different interfacial layers in
the device structure. In this section, we will review the recent progress of interface
engineering for inverted solar cells in two aspects: the progress in anode interlayer
design and the progress in cathode interlayer design.