18 Interfacial Materials for Organic Solar Cells
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dipole between the active layer and Al metal and demonstrated very similar OSC performance with PCEs around 7.7% for PBDTTT-C-T/PC 71 BM devices and showed
greater stability compared to Ca/Al or only Al.
The trade-off between stability due to oxidation and WF of electrode is a major
challenge in the field of OSCs. Page et al. used amine-functionalized C 60 -N and
sulfobetaine-substituted zwitterionic C 60 -SB as cathode independent buffer layer
in conventional device (Page et al. 2014). Specifically, using a thin layer of C 60 -
N the effective WF of Ag, Cu, and Au electrodes can be significantly reduced to
3.65 eV. C 60 -SB was prepared from C 60 -N by reaction with 1,3-propanesultone via
ring opening. The insertion of C 60 -N between active layer and metal electrodes
resulted in good Ohmic contact for electron injection, and a large built-in potential
difference for efficient charge extraction in OSCs. Devices with devices with C 60 -N or
C 60 -SB interlayers yielded PCE values 9.35% and 8.57%, respectively. Lower PCEs
were obtained for devices with Al (8.65%), Cu (8.67%) or Au (8.56%) cathodes,
relative to Ag (9.35%) cathodes, which can be credited to the lower reflectivity of
Cu and Au.
Russell and co-workers further implemented C 60 -SB as CIL in an inverted device,
which showed exceptional improvement of PCE to 9.23% for PTB7-Th:PC 71 BM
blend compared to 1.96% with bare ITO (Liu et al. 2015a). The CIL also acts as an
electron acceptor and modified the WF of ITO from 4.5 eV to ~ 4.0 eV as measured by
UPS. The highest performance was achieved with the CIL thickness of ~40 nm and
remain above 8.0% for the entire thickness range above 10 nm. The work function
modification of ITO by C 60 -SB may arised from orientation of the permanent dipole
at the interface, due to the preferential interactions of the sulfobetaine zwitterion
with the ITO surface.
Cao group developed an alcohol soluble phosphate-containing fullerene bisadducts, B-PCPO as CIL in inverted OSCs to improve the electron transport and
collection efficiency (Duan et al. 2012). It was observed that the B-PCPO interlayer
could effectively decrease the WF of ITO to 3.9 eV and thereby enhance the electron
collection at the ITO electrode. The device resulted in PCE increased from 4.83 to
6.20% by using a B-PCPO interlayer, compared to 5.31% for ZnO and 4.83% for
device without interlayer.
Chen et al. developed a CIL ZnO-C 60 by reacting zinc acetate with hydroxyl
containing fullerene derivative PCBE-OH (Liao et al. 2013). This ZnO-C 60 CIL provides dual functionalities for enhanced electron collection, producing a fullerenerich cathode surface and promote compatibility of the BHJ layer at the interface.
The C 60 -doped ZnO lower the LUMO level to −4.53 eV compared to pristine ZnO
(−4.1 eV) promoting better electron transport pathways. Inverted BHJ devices comprising PTB7-Th:PC 71 BM blend exhibited excellent PCEs of 9.35% higher than that
bare ZnO. Subbiah et al. reported a PCE of 9.4% using a ZnO−C 60 interlayer at the
cathode contact (Subbiah et al. 2015). The surface modification eliminates of oxygen vacancies on the ZnO surface that can act as electron traps and reduced carrier
recombination.
397
dipole between the active layer and Al metal and demonstrated very similar OSC performance with PCEs around 7.7% for PBDTTT-C-T/PC 71 BM devices and showed
greater stability compared to Ca/Al or only Al.
The trade-off between stability due to oxidation and WF of electrode is a major
challenge in the field of OSCs. Page et al. used amine-functionalized C 60 -N and
sulfobetaine-substituted zwitterionic C 60 -SB as cathode independent buffer layer
in conventional device (Page et al. 2014). Specifically, using a thin layer of C 60 -
N the effective WF of Ag, Cu, and Au electrodes can be significantly reduced to
3.65 eV. C 60 -SB was prepared from C 60 -N by reaction with 1,3-propanesultone via
ring opening. The insertion of C 60 -N between active layer and metal electrodes
resulted in good Ohmic contact for electron injection, and a large built-in potential
difference for efficient charge extraction in OSCs. Devices with devices with C 60 -N or
C 60 -SB interlayers yielded PCE values 9.35% and 8.57%, respectively. Lower PCEs
were obtained for devices with Al (8.65%), Cu (8.67%) or Au (8.56%) cathodes,
relative to Ag (9.35%) cathodes, which can be credited to the lower reflectivity of
Cu and Au.
Russell and co-workers further implemented C 60 -SB as CIL in an inverted device,
which showed exceptional improvement of PCE to 9.23% for PTB7-Th:PC 71 BM
blend compared to 1.96% with bare ITO (Liu et al. 2015a). The CIL also acts as an
electron acceptor and modified the WF of ITO from 4.5 eV to ~ 4.0 eV as measured by
UPS. The highest performance was achieved with the CIL thickness of ~40 nm and
remain above 8.0% for the entire thickness range above 10 nm. The work function
modification of ITO by C 60 -SB may arised from orientation of the permanent dipole
at the interface, due to the preferential interactions of the sulfobetaine zwitterion
with the ITO surface.
Cao group developed an alcohol soluble phosphate-containing fullerene bisadducts, B-PCPO as CIL in inverted OSCs to improve the electron transport and
collection efficiency (Duan et al. 2012). It was observed that the B-PCPO interlayer
could effectively decrease the WF of ITO to 3.9 eV and thereby enhance the electron
collection at the ITO electrode. The device resulted in PCE increased from 4.83 to
6.20% by using a B-PCPO interlayer, compared to 5.31% for ZnO and 4.83% for
device without interlayer.
Chen et al. developed a CIL ZnO-C 60 by reacting zinc acetate with hydroxyl
containing fullerene derivative PCBE-OH (Liao et al. 2013). This ZnO-C 60 CIL provides dual functionalities for enhanced electron collection, producing a fullerenerich cathode surface and promote compatibility of the BHJ layer at the interface.
The C 60 -doped ZnO lower the LUMO level to −4.53 eV compared to pristine ZnO
(−4.1 eV) promoting better electron transport pathways. Inverted BHJ devices comprising PTB7-Th:PC 71 BM blend exhibited excellent PCEs of 9.35% higher than that
bare ZnO. Subbiah et al. reported a PCE of 9.4% using a ZnO−C 60 interlayer at the
cathode contact (Subbiah et al. 2015). The surface modification eliminates of oxygen vacancies on the ZnO surface that can act as electron traps and reduced carrier
recombination.