18 Interfacial Materials for Organic Solar Cells
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gave improved PCE of 7.0 to 7.18% when used without or with LiF layer than that
of device with bare Al cathode or only LiF/Al (Li et al. 2016).
Gregg and co-workers reported a self-doping, O 2 -Stable water soluble dicationic
perylene bis(2-ethyltrimethylammonium hydroxide imide (PETMA
+ -OH
– ) salt as
CIL (Reilly III et al. 2012). The molecule doped by dehydration and de-doped by
hydration. It has been shown that the conductivity for the doped state could be
significantly increased from 4 × 10
−3 to 2 × 10
−3 S cm
−1 .
Zhang et al. reported a thickness insensitive room temperature processed perylene
derivatives comprising dimethylamino (PDIN) or dimethylamino-N-oxide (PDINO)
as terminal groups and explore their use for CIL (Wang et al. 2014b). PDINO was
synthesized from PDIN by treatment with H 2 O 2 . The large interfacial dipole formed
by the perylene derivatives induced the vacuum level shift and change the WF of metal
electrode. The LUMO energy levels of PDIN and PDINO are estimated to be –3.63 eV
and –3.72 eV, respectively and close to the PC 71 BM, while the HOMO levels are at
−6.05 and −6.21 eV lowered enough to block the holes from the donors. Ultraviolet
photoelectron spectroscopy (UPS) results revealed the shift of the WF of Al from
4.3 to 3.5 eV by deposition of a thin layer (5 nm) of either PDI derivatives. However,
the WF of Ag and Au strongly depend on the type of PDI layer. For both electrodes,
PDIN/PDINO treatment lowered the WF to ~3.93 eV/~3.60 eV allowing them to form
good ohmic contacts with the fullerene acceptor. This change in the WF is related
to the polar nature and dipole formation ability of the terminal amino or aminoN-oxide groups in PDIN and PDINO. The lowered WFs of PDI/metal cathodes
also increase the built-in field used to break the electrical symmetry inside of the
cells, beneficial for charge extraction and reduce recombination losses. Conventional
device using PDIN/Al or PDINO/Al as CIL and PEDOT:PSS as AIL gave excellent
PCE of 7.68–8.24% comparatively higher than that of Ca/Al CIL (PCE = 6.98%).
Moreover, the perylene derivatives also lowered the WF of Au and Ag metal to use as
cathode. Due to lowering of the WF, Ag or Au were also used as cathode generating
high photovoltaic performance above 7% and high FF close to 0.73.
Min et al. used ZnO/PDINO as CIL in small molecule based OSC devices (BDTTS-TR:PC 71 BM) and Ag as top electrode. The ZnO/PDINO bilayer not only served
as an effective cathode interlayer but also acts as a protective coating on top of the
active layer. The device gave a PCE of 8.2%. They further fabricate all solutionprocessed OSCs using highly transparent Ag nanowire as top electrode resulting
in a PCE of 3.62% (Min et al. 2016). When PDINO interlayer was implemented
in conventional fullerene-free devices PCEs up to 14.04% have been achieved for
PBDB-TF:IDIC-C4Ph blend after thermal annealing at 130 °C (Li et al. 2019).
Yu et al. reported a high performance fullerene-free OSC using PDIN as CIL
and B-DIPDI:PTB7-Th as photoactive layer. PDIN helps in interfacial doping of
B-DIPDI acceptor and facilitate the charge transport and extraction (Yu et al. 2016).
The use of PDIN interlayer as a surface modifier on ZnO resulted in a ≈ 14%
enhancement of the PCE than that of the pristine ZnO-based device. The grazing
incident wide angle X-ray scattering (GIWAXS) study revealed a more crystalline
face-on orientation of the BHJ film achieved due to compatibility of both PDI acceptor
and interlayer. Lin et al. observed a PCE of 6.31% using PDIN CIL PTB7Th:IEIC
389
gave improved PCE of 7.0 to 7.18% when used without or with LiF layer than that
of device with bare Al cathode or only LiF/Al (Li et al. 2016).
Gregg and co-workers reported a self-doping, O 2 -Stable water soluble dicationic
perylene bis(2-ethyltrimethylammonium hydroxide imide (PETMA
+ -OH
– ) salt as
CIL (Reilly III et al. 2012). The molecule doped by dehydration and de-doped by
hydration. It has been shown that the conductivity for the doped state could be
significantly increased from 4 × 10
−3 to 2 × 10
−3 S cm
−1 .
Zhang et al. reported a thickness insensitive room temperature processed perylene
derivatives comprising dimethylamino (PDIN) or dimethylamino-N-oxide (PDINO)
as terminal groups and explore their use for CIL (Wang et al. 2014b). PDINO was
synthesized from PDIN by treatment with H 2 O 2 . The large interfacial dipole formed
by the perylene derivatives induced the vacuum level shift and change the WF of metal
electrode. The LUMO energy levels of PDIN and PDINO are estimated to be –3.63 eV
and –3.72 eV, respectively and close to the PC 71 BM, while the HOMO levels are at
−6.05 and −6.21 eV lowered enough to block the holes from the donors. Ultraviolet
photoelectron spectroscopy (UPS) results revealed the shift of the WF of Al from
4.3 to 3.5 eV by deposition of a thin layer (5 nm) of either PDI derivatives. However,
the WF of Ag and Au strongly depend on the type of PDI layer. For both electrodes,
PDIN/PDINO treatment lowered the WF to ~3.93 eV/~3.60 eV allowing them to form
good ohmic contacts with the fullerene acceptor. This change in the WF is related
to the polar nature and dipole formation ability of the terminal amino or aminoN-oxide groups in PDIN and PDINO. The lowered WFs of PDI/metal cathodes
also increase the built-in field used to break the electrical symmetry inside of the
cells, beneficial for charge extraction and reduce recombination losses. Conventional
device using PDIN/Al or PDINO/Al as CIL and PEDOT:PSS as AIL gave excellent
PCE of 7.68–8.24% comparatively higher than that of Ca/Al CIL (PCE = 6.98%).
Moreover, the perylene derivatives also lowered the WF of Au and Ag metal to use as
cathode. Due to lowering of the WF, Ag or Au were also used as cathode generating
high photovoltaic performance above 7% and high FF close to 0.73.
Min et al. used ZnO/PDINO as CIL in small molecule based OSC devices (BDTTS-TR:PC 71 BM) and Ag as top electrode. The ZnO/PDINO bilayer not only served
as an effective cathode interlayer but also acts as a protective coating on top of the
active layer. The device gave a PCE of 8.2%. They further fabricate all solutionprocessed OSCs using highly transparent Ag nanowire as top electrode resulting
in a PCE of 3.62% (Min et al. 2016). When PDINO interlayer was implemented
in conventional fullerene-free devices PCEs up to 14.04% have been achieved for
PBDB-TF:IDIC-C4Ph blend after thermal annealing at 130 °C (Li et al. 2019).
Yu et al. reported a high performance fullerene-free OSC using PDIN as CIL
and B-DIPDI:PTB7-Th as photoactive layer. PDIN helps in interfacial doping of
B-DIPDI acceptor and facilitate the charge transport and extraction (Yu et al. 2016).
The use of PDIN interlayer as a surface modifier on ZnO resulted in a ≈ 14%
enhancement of the PCE than that of the pristine ZnO-based device. The grazing
incident wide angle X-ray scattering (GIWAXS) study revealed a more crystalline
face-on orientation of the BHJ film achieved due to compatibility of both PDI acceptor
and interlayer. Lin et al. observed a PCE of 6.31% using PDIN CIL PTB7Th:IEIC