Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 52

and Al electrode to function as a buffer layer for improving the performance of PSC.
An enhancement of the PCE of the PSCs from 3.05 % to 3.56 % on average, and the
highest PCE of 3.86 % were achieved by inserting a PDMS-b-PMMA into the
P3HT:PC 61 BM solar cells due to the reduction of charge carrier recombination at
the organic/metal interface [ 59 ].

3.2.1.6 Ferroelectric Polymer as Cathode Interlayer

It is well-known that a sufficient intrinsic internal electric field is required to effi-
ciently dissociate the charge transfer excitons generated in active layer [ 60 ].
However, most widely used electrode materials deliver a work function offset
of\2 eV, affording a much smaller internal electric field for efficient charge transfer
excitons dissociation in BHJ-PSCs [ 36 ]. Therefore, an external bias voltage is
generally needed to efficiently dissociate the electrons and holes. Most recently,
Yuan et al. demonstrated another innovative method to provide sufficient intrinsic
internal electric field in BHJ-PSCs by inserting a ultra-thin ferroelectric polymer
layer of vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)) between
organic active layer and Al electrode [ 61 ]. After poling, an induced net polarization
electric field is generated, which is ten times-higher than that achieved by the use of
electrodes with different work functions. As a consequence, an enhanced PCE from
1-2 % without the ferroelectric film to 4-5 % was demonstrated for BHJ-PSCs
based on several different active layers. Also note that these improved PCEs are
higher than those achieved by other methods, including active layer morphology and
electrode work-function optimization [ 61 ].

3.2.2 Anode Interlayer

3.2.2.1 PEDOT: PSS as Anode Interlayer

Interface engineering at the hole-collecting electrode is also of paramount
importance for the improvement of the performance of BHJ-PSCs. The summary
of performance of conventional PSCs using different anode interlayer designs is
listed in Table 3.2 and the chemical structures of organic materials used as anode
interlayer in conventional PSCs are shown in Scheme 3.3 . In conventional device
configuration using ITO as the anode, the polymer complex of poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been commonly
used as the anode interlayer to improve the contact property between ITO and
organic active layer, and to increase the work function of ITO for effective hole
collection and transporting [ 62 , 63 ]. However, it was revealed by Rutherford
backscattering (RBS) studies and XPS that the acidic nature of PEDOT:PSS etches
the ITO and results in the poor chemical stability at the ITO/PEDOT:PSS interface
[ 64 - 66 ].

Moreover, the electrical inhomogeneities of PEDOT:PSS limit its