384
A. Mishra
(Bob et al. 2013). The PCE of bulk- or extended-SnO 2 as CILs reduced to 0.7%
and 1.45%, respectively due to dramatic reduction in the V OC and FF. The lower
performance for bulk and extended SnO 2 were due to rough thin film formation
(relatively large particles) that prevents electron transport and increased the shunting
pathways.
Insulating low WF Al 2 O 3 or ZrO 2 were applied to the TiO 2 layer by ALD method
to passivate the surface trap states followed by a downward shift of the conduction
band minimum. A range of polymer donors were tested and highest PCE of 6.9 and
7.1% were achieved for PTB7:PC 71 BM device using Al 2 O 3 and ZrO 2 nanolayers due
to significant suppression of charge recombination and enhanced electron extraction
at the TiO 2 /Al 2 O 3 or ZrO 2 /organic interface (Vasilopoulou et al. 2014).
By intercalating cesium into the V 2 O 5 or MoO 3 , the WF of metal oxides can be
tuned over a wide range of 1.1 eV thus making them as both ETL and HTL (Li
et al. 2014c). Using MoO 3 /V 2 O 5 and Cs 0.5 MoO 3 /CsV 2 O 5 as HTL and ETL, the
PBDTDTTT-S-T:PC 71 BM-based devices exhibited PCEs of 6.0 and 6.08%, respectively. The best performance were achieved with the Cs:Mo and Cs:V mole ratios
of 0.5:1. By using with 0.5 wt% of Cs the WF of MoO 3 /V 2 O 5 were tuned from
5.32 eV/5.41 eV to 4.28 eV/4.19 eV, respectively.
Similar to Cs-doping, Al-doping also tuned the WF of MoO 3 to acts as CIL.
Liu et al. demonstrated an improvement in PCE for PCDTBT:PC 71 BM-based OSCs
using Al-doped MoO 3 as CIL and MoO 3 /Al as anode. A high FF of 0.66% and a PCE
of 6.28% was achieved. The MoO 3 -Al composite films are highly transparent and
exhibit a high WF of 4.09 eV with 55% Al doping compared to 5.5 eV for neat MoO 3 ,
thus able to form Ohmic contact with the LUMO of PC 71 BM. (Liu et al. 2012a) Lidoped ZnO (LZO) when used as CIL in PSEHTT:IC 60 BA generated a PCE of 6.59%
compared to 5.36% for ZnO. LZO layer was further used as interconnecting layer in
tandem and triple cell generating PCE up to 10.4 and 11.83% (Yusoff et al. 2014).
Using Mg-doped ZnO as CIL, Yin et al. obtained a PCE of 8.31% which was
significantly higher than those of device without CIL (3.5%) or with only ZnO
(7.1%) as CIL. (Yin et al. 2014) The WF can be tuned by changing the amount of Mg
doping and also the device stability improved significantly. The insertion of various
metal carbonates such as Li 2 CO 3 , K 2 CO 3 , Na 2 CO 3 , Cs 2 CO 3 , and (NH 4 ) 2 CO 3 as
gradient doping agent for ZnO layer can improve the electron extraction properties by
modifying the energy levels without making any damage to the ZnO nanostructures
(Nho et al. 2016). When 5 wt% Li-doped ZnO was used as CIL in PTB7-Th:PC 71 BM
based devices the PCE was improved from 8.59 to 10.05% (Soultati et al. 2019).
Fullerene-free solar cells based on the PTB7-Th:IT-4F blend exhibited PCEs up
to 8.96% under similar device structure. It has been proposed that the Li ions are
intercalated within the ZnO lattice as interstitial dopants and replace the defects
which acts as trap state in ZnO, improve the electron conductivity and alter the WF
of doped oxide.
The performance improvement was further visible with low temperature processed Al-doped ZnO (AZO) CIL. High PCE of 10.42% was achieved with PTB7Th:PC 71 BM blend device and was found to be insensitive to the thickness of AZO.
Additionally, due to low temperature processing flexible devices on poly(ethylene
A. Mishra
(Bob et al. 2013). The PCE of bulk- or extended-SnO 2 as CILs reduced to 0.7%
and 1.45%, respectively due to dramatic reduction in the V OC and FF. The lower
performance for bulk and extended SnO 2 were due to rough thin film formation
(relatively large particles) that prevents electron transport and increased the shunting
pathways.
Insulating low WF Al 2 O 3 or ZrO 2 were applied to the TiO 2 layer by ALD method
to passivate the surface trap states followed by a downward shift of the conduction
band minimum. A range of polymer donors were tested and highest PCE of 6.9 and
7.1% were achieved for PTB7:PC 71 BM device using Al 2 O 3 and ZrO 2 nanolayers due
to significant suppression of charge recombination and enhanced electron extraction
at the TiO 2 /Al 2 O 3 or ZrO 2 /organic interface (Vasilopoulou et al. 2014).
By intercalating cesium into the V 2 O 5 or MoO 3 , the WF of metal oxides can be
tuned over a wide range of 1.1 eV thus making them as both ETL and HTL (Li
et al. 2014c). Using MoO 3 /V 2 O 5 and Cs 0.5 MoO 3 /CsV 2 O 5 as HTL and ETL, the
PBDTDTTT-S-T:PC 71 BM-based devices exhibited PCEs of 6.0 and 6.08%, respectively. The best performance were achieved with the Cs:Mo and Cs:V mole ratios
of 0.5:1. By using with 0.5 wt% of Cs the WF of MoO 3 /V 2 O 5 were tuned from
5.32 eV/5.41 eV to 4.28 eV/4.19 eV, respectively.
Similar to Cs-doping, Al-doping also tuned the WF of MoO 3 to acts as CIL.
Liu et al. demonstrated an improvement in PCE for PCDTBT:PC 71 BM-based OSCs
using Al-doped MoO 3 as CIL and MoO 3 /Al as anode. A high FF of 0.66% and a PCE
of 6.28% was achieved. The MoO 3 -Al composite films are highly transparent and
exhibit a high WF of 4.09 eV with 55% Al doping compared to 5.5 eV for neat MoO 3 ,
thus able to form Ohmic contact with the LUMO of PC 71 BM. (Liu et al. 2012a) Lidoped ZnO (LZO) when used as CIL in PSEHTT:IC 60 BA generated a PCE of 6.59%
compared to 5.36% for ZnO. LZO layer was further used as interconnecting layer in
tandem and triple cell generating PCE up to 10.4 and 11.83% (Yusoff et al. 2014).
Using Mg-doped ZnO as CIL, Yin et al. obtained a PCE of 8.31% which was
significantly higher than those of device without CIL (3.5%) or with only ZnO
(7.1%) as CIL. (Yin et al. 2014) The WF can be tuned by changing the amount of Mg
doping and also the device stability improved significantly. The insertion of various
metal carbonates such as Li 2 CO 3 , K 2 CO 3 , Na 2 CO 3 , Cs 2 CO 3 , and (NH 4 ) 2 CO 3 as
gradient doping agent for ZnO layer can improve the electron extraction properties by
modifying the energy levels without making any damage to the ZnO nanostructures
(Nho et al. 2016). When 5 wt% Li-doped ZnO was used as CIL in PTB7-Th:PC 71 BM
based devices the PCE was improved from 8.59 to 10.05% (Soultati et al. 2019).
Fullerene-free solar cells based on the PTB7-Th:IT-4F blend exhibited PCEs up
to 8.96% under similar device structure. It has been proposed that the Li ions are
intercalated within the ZnO lattice as interstitial dopants and replace the defects
which acts as trap state in ZnO, improve the electron conductivity and alter the WF
of doped oxide.
The performance improvement was further visible with low temperature processed Al-doped ZnO (AZO) CIL. High PCE of 10.42% was achieved with PTB7Th:PC 71 BM blend device and was found to be insensitive to the thickness of AZO.
Additionally, due to low temperature processing flexible devices on poly(ethylene