Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 50

3.2.1.3 Water/Alcohol Soluble Polymers as Cathode Interlayer
Although the above-mentioned inorganic materials exhibited outstanding cathode
modification functions, the thermal evaporation of most of them requires high
vacuum, which leads to increases in fabrication costs and thereby limits their
applications. Organic interfacial materials were thus widely developed to improve
the interface property of metal electrode, because of their solution processability,
facile modification of chemical structures, and in turn electrical and electronic
properties. The commonly used organic interfacial materials for cathode modifi-
cation are summarized in Scheme 3.2 . Zhang et al. had reported the use of
environment friendly aqueous solution processable poly(ethylene oxide) (PEO) as
interlayer to modify the cathode in BHJ-PSCs, which showed similar function as
LiF. The PSCs with PEO modified cathode showed not only enhanced V oc , but also
J sc and FF [ 45 ].
Water/alcohol soluble conjugated polyelectrolytes (CPEs) and related neutral
polymers, which bear pendant hydrophilic polar groups, were widely used as
promising interfacial materials in organic optoelectronic devices such as polymer
light-emitting diodes, field effect transistors (FETs), and solar cells. Their special
solubility in environment friendly solvents such as alcohol and water provide the
convenience of fabricating multilayer organic devices by avoiding the problem of
corrupting the low-lying neutral organic semiconducting layers which are usually
dissolved in nonpolar solvents. It was generally considered that the interfacial
dipoles were formed by inserting a CPE layer, and thereby the reduction of
electron extraction barrier to electron collection and transporting at cathode. In
addition, UV photoelectron spectroscopy has shown that this kind of polymers can
effectively influence the work function of adjacent electrodes. Moreover, a large
amount of water/alcohol soluble fluorene-based CPEs and related neutral polymers
were successfully used as electron injection/transporting materials in PLEDs [ 46 ,
47 ]. Luo et al. reported the use of a thin layer of alcohol-soluble fluorene-based
CPE PFNBr-DBT15 between the active layer and metal electrode, and the V oc of
the resulted PSCs can be enhanced by 0.3 V [ 48 ]. Based on Luo's work, He et al.
has insightfully investigated five different alcohol-soluble fluorene-based conju-
gated polymers or CPEs as cathode interlayers for PSCs based different donor
materials system. He's results revealed that the V oc enhancement is only observed
in devices that contain a polyfluorene copolymer as the donor material, while no
significant enhancement in V oc was found for P3HT and poly(2-methoxyl-5-((2 0 -
ethylhexyl)oxyl)-1,4-phenylenevinylene) (MEH-PPV, see Scheme 3.1 ) devices
[ 49 ]. Similar works of using other fluorene-based alcohol polymers as interfacial
materials for cathode modifications were also demonstrated by Na et al. and Zhao
et al. almost at the same time [ 50 , 51 ]. Notably, BHJ-PSCs with high work-
function metals such as Ag, Au, and Cu also showed encouraging device perfor-
mances with PCE above 3 % with the implementation of fluorene-based CPE
WPF-6-oxy-F [ 52 ]. After that, the use of alcohol-soluble conjugated polymer as
cathode interlayer gradually become a commonly applied approach to further
improve the performance of PSCs [ 53 , 54 ], and the encouraging PCE of 6.07 %