Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 56

3.2.2.3 Organic/Polymer Materials as Anode Interlayer
Despite that p-type transition metal oxides exhibited outstanding anode interface
modification functions, the fabrication of thin films based on these oxides usually
involves high-cost thermal evaporation, which limits their application in large-area
devices. Organic hole-collecting/transporting materials were thus greatly desired
due to their solution processabilities and their facile tunable properties. A cross-
linkable blend of poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenyl-
amine] (TFB) and 4,4 0 -bis[(p-trichlorosiylpropylphenyl)phenylamino]biphenyl
(TPDSi 2 ) was spin coated onto a ITO substrate to form a robust, optical trans-
parent, homogeneous film after thermal annealing, which can be used as an
effective PEDOT:PSS alternative. TFB:TPDSi 2 possesses HOMO and LUMO
energy levels of -5.3 and -2.3 eV, respectively, which endow the buffer layer
good hole-collecting and electron-blocking ability. As a result, BHJ-PSCs of
poly(2-methoxyl-5-((3 0 ,7 0 -dimethyloctyl)oxyl)-1,4-phenylenevinylene) (MDMO-
PPV):PCBM with TFB:TPDSi 2 anodic buffer layer delivered much better photo-
voltaic performance and thermal stability than those of PEDOT:PSS control
devices [ 78 ]. Interestingly, the use of a double interfacial layer of PE-
DOT:PSS ? TFB:TPDSi 2 could decrease the FF of the solar cells, but increase
electron blocking to suppress charge leakage and thereby enhance V oc more than
using either PEDOT:PPS or TFB:TPDSi 2 independently [ 79 ]. After that, Subbiah
et al. demonstrated that a double interlayer of MoO 3 /TFB could also improve the
photovoltaic performance of the BHJ-PSC with respect to solar cells with a sole
PEDOT:PSS or MoO 3 anodic buffer layer, due to the enhanced electron blocking
and hole collecting from organic active layer to the anode [ 80 ].
Li et al. demonstrated the use of a novel self-doped polymer of sulfonated
poly(diphenylamine) (SPDPA) as the anodic buffer layer to replace PEDOT:PSS
in P3HT:PC 61 BM solar cell. The polar surface of SPDPA film induces the oriented
arrangement of P3HT in the active layer during the spin-coating and film-growing
processes, which is beneficial for enhancing the hole mobility, producing a better
Ohmic contact at the anode junction and thereby gives rise to an enhanced PCE
[ 81 ]. The implementation of a thin layer of polytetrafluoroethylene (PTFE)
between ITO and P3HT:PC 61 BM was found to form an dipole layer at the anode
junction, and thereby facilitate the hole extraction. Compared to the PEDOT:PSS
control device, solar cells based on PTFE showed obviously enhanced photovol-
taic performance. Note that such an insulated PTFE thin layer was deposited by
thermally evaporation, and it thereby can be prepared at low substrate tempera-
tures, which is compatible to the flexible polymer substrates for BHJ-PSCs
applications [ 82 ].
In addition to the modification of PEDOT:PSS or development of PEDOT:PSS
alternatives, the implementation of hole selective transporting materials below or
above PEDOT:PSS layers is another effective approach to facilitate hole-collecting
and electron-blocking at anode junction, and thereby to improve the performance
of BHJ-PSCs. It is well-known that there is a vertical composition gradient in
organic blend layer with a profile of PC 61 BM- or PC 71 BM-rich blend adjacent to