Environmental Engineering Reference
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was achieved by inserting a thin layer of PFN (see Scheme
3.2
) for the BHJ-PSC
based on a newly developed low-band-gap donor material [
54
]. Recently, new
water-/alcohol-soluble conjugated polymers and ionic conjugated di-block
copolymer based on carbazole or thiophene were also developed to use as cathode
interfacial materials [
55
,
56
]. Compared to the device with sole Al as cathode, the
PCE of PSCs can be enhanced from 5 to 6.5 % by inserting an ultra-thin, ionic
conjugated, di-block copolymer poly[9,9-bis(2-ethylhexyl)-fluorene]-b-poly[3-(6-
trimethylammoniumhexyl)thiophene] (PF2/6-b-P3TMAHT) [
56
]. These encour-
aging results indicate that water-/alcohol-soluble conjugated polymers would be
good candidates of cathode interfacial materials for the future development of
printable large-scale solar cells.
3.2.1.4 n-Type Organic Semiconductors as Cathode Interlayer
n-Type organic semiconductors having low-lying LUMO levels, which are close to
the LUMO of PC
61
BM and the work function of Al cathode, are potentially
electron extraction and transporting materials for BHJ-PSCs. Recently, Zuo et al.
had reported the use of n-type phthalocyanine tin (IV) dichloride (SnCl
2
Pc) as
cathode interlayer to form SnCl
2
Pc/LiF/Al trilayer cathode for PSCs. The inte-
gration of SnCl
2
Pc into the solar cell not only enhances the electron transporting
and collection efficiencies due to the step-like electron injection barrier to cathode
after the implementation of SnCl
2
Pc interlayer, but also improves the exciton
dissociation efficiency because of the formation of additional MEH-PPV/SnCl
2
Pc
exciton dissociation junction. Consequently, the 15.2 % enhancement of PCE up to
2.49 % was achieved for PSCs with the thermally evaporated SnCl
2
Pc as cathode
interlayer [
57
].
3.2.1.5 Organic Cathode Interlayer Formed by Surface Self-Segregation
Compared to the thermal evaporation, the formation of a interfacial layer of n-type
organic semiconductor by controlling the surface segregation of the materials during
spin coating may be a much milder approach for the low-lying organic active layer.
Wei et al. had reported that when a small amount of a fullerene derivative with a
fluorinated alkyl chain (F-PCBM) is mixed with the blend of P3HT and PC
61
BM, the
F-PCBM spontaneously migrates to the surface of the organic layer during spin
coating due to the low surface energy of the fluorinated alkyl chain. The interfacial
dipole moment toward the Al cathode caused by the perfluoroalkyl chains is
favorable for the reduction of the work function of Al for better alignment with
PC
61
BM energy levels. The energy barrier for electron collection and the R
s
of the
modified device were hence reduced, and consequently the enhanced PCE of the
device was achieved [
58
]. The surface self-segregation approach was also applied to
incorporate a thin layer of poly(dimethylsiloxane)-b-poly(methylmethacrylate)
(PDMS-b-PMMA) di-block copolymer at the interface between organic active layer
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