382
A. Mishra
(Yan et al. 2017). Further analysis suggested that the addition of TOPD into the
TiO 2 nanoparticle films reduced the WF from 4.43 to 4.23 eV (by Kelvin probe
measurements), which reduced the energy barrier for charge injection from active
layer to electrode. Similar effect was observed for UV and ethanolamine treatment
of TiO 2 .
The optical properties of ZnO CILs can be tuned by preparation method, composition, film morphology, thickness of the layer. The most widely used preparation
method of ZnO for OSC applications are chemical deposition (Park et al. 2013) solgel processing, (Kyaw et al. 2008; Yin et al. 2013) and nanoparticle approach. The
choice of precursor solution and annealing temperature strongly influence the device
performance of sol-gel prepared ZnO. Takanezawa et al. reported a PCE of 2.7% for
P3HT-PC 61 BM devices using ZnO nanorod array as ETL (Takanezawa et al. 2007).
White et al. introduced a solution-processed ZnO as CIL in P3HT-PC 61 BM devices
with Ag as hole extraction back contact exhibiting a PCE of 2.58%. (White et al.
2006) The authors suggested efficient electron transfer without loss of energy at the
PC 61 BM:ZnO interface due to near equal LUMO of PC 61 BM and conduction band
of ZnO. However, stability was the major issue in those devices due to desorption of
oxygen from ZnO in both air and inert atmosphere. Hau et al. revealed an excellent
device stability with PCE of 3.78% by incorporating PEDOT:PSS as AIL in inverted
device structure and ZnO nanoparticle as CIL processed by sol–gel method (Hau
et al. 2008). The PEDOT:PSS worked as an oxygen-diffusion barrier in inverted
devices. In addition, the WF with Ag was modified to −5.0 eV by oxidizing to Ag 2 O
in presence of air, thus matching well with the PEDOT:PSS HOMO level (−5.1 eV)
and improve electrical contact at the interface.
You et al. used TiO 2 , ZnO, or TiO 2 :Cs with similar WF ~4.2 eV as CILs in
inverted device using PTB7:PC 71 BM as active layer (You et al. 2012). PCE as high
as 7.3% with high stability have been achieved with ZnO nanoparticle. Chen et al.
reported a PCE of 8.1% using UV ozone treated ZnO nanoparticle as CIL (Chen
et al. 2012). The UV ozone treatment reduced the trap state and surface defects on
the nanoparticle surface.
Lee et al. demonstrated the reduction of energy barrier between the LUMO of
acceptor and conduction band of ZnO by treating the ZnO surface by polar solvents like 2-methoxyethanol (2-ME) + ethanolamine (EA) as co-solvent (Lee et al.
2014b). The polar solvent lowers the contact barrier for electron transport and extraction, reduce contact resistance thus series resistance and bimolecular recombination
resulting in dramatic improvement in PCE from 6.71 to 8.69% for PTB7:PC 71 BM
devices. The PCE improvement was due to increase in J SC and FF. The device with
Methanol treatment also improved the device PCE by about 1% compared to the
untreated ZnO. Using ethanolamine (EA) treated ZnO nanoparticle as optical spacer
and PEDOT:PSS as HTL in PTB7:PC 71 BM based conventional devices the PCE
could be increased from 5.8 to 7.6%. (Dkhil et al. 2014) The EA treatment lowered
the WF from ~4.3 to 4.1 eV (calculated from XPS study), which can be assigned
to dipolar polarization of the ZnO surface via the adsorption of EA. The enhancement was attributed to reduced contact barrier, reduced recombination and enhanced
electron extraction at the cathode.
A. Mishra
(Yan et al. 2017). Further analysis suggested that the addition of TOPD into the
TiO 2 nanoparticle films reduced the WF from 4.43 to 4.23 eV (by Kelvin probe
measurements), which reduced the energy barrier for charge injection from active
layer to electrode. Similar effect was observed for UV and ethanolamine treatment
of TiO 2 .
The optical properties of ZnO CILs can be tuned by preparation method, composition, film morphology, thickness of the layer. The most widely used preparation
method of ZnO for OSC applications are chemical deposition (Park et al. 2013) solgel processing, (Kyaw et al. 2008; Yin et al. 2013) and nanoparticle approach. The
choice of precursor solution and annealing temperature strongly influence the device
performance of sol-gel prepared ZnO. Takanezawa et al. reported a PCE of 2.7% for
P3HT-PC 61 BM devices using ZnO nanorod array as ETL (Takanezawa et al. 2007).
White et al. introduced a solution-processed ZnO as CIL in P3HT-PC 61 BM devices
with Ag as hole extraction back contact exhibiting a PCE of 2.58%. (White et al.
2006) The authors suggested efficient electron transfer without loss of energy at the
PC 61 BM:ZnO interface due to near equal LUMO of PC 61 BM and conduction band
of ZnO. However, stability was the major issue in those devices due to desorption of
oxygen from ZnO in both air and inert atmosphere. Hau et al. revealed an excellent
device stability with PCE of 3.78% by incorporating PEDOT:PSS as AIL in inverted
device structure and ZnO nanoparticle as CIL processed by sol–gel method (Hau
et al. 2008). The PEDOT:PSS worked as an oxygen-diffusion barrier in inverted
devices. In addition, the WF with Ag was modified to −5.0 eV by oxidizing to Ag 2 O
in presence of air, thus matching well with the PEDOT:PSS HOMO level (−5.1 eV)
and improve electrical contact at the interface.
You et al. used TiO 2 , ZnO, or TiO 2 :Cs with similar WF ~4.2 eV as CILs in
inverted device using PTB7:PC 71 BM as active layer (You et al. 2012). PCE as high
as 7.3% with high stability have been achieved with ZnO nanoparticle. Chen et al.
reported a PCE of 8.1% using UV ozone treated ZnO nanoparticle as CIL (Chen
et al. 2012). The UV ozone treatment reduced the trap state and surface defects on
the nanoparticle surface.
Lee et al. demonstrated the reduction of energy barrier between the LUMO of
acceptor and conduction band of ZnO by treating the ZnO surface by polar solvents like 2-methoxyethanol (2-ME) + ethanolamine (EA) as co-solvent (Lee et al.
2014b). The polar solvent lowers the contact barrier for electron transport and extraction, reduce contact resistance thus series resistance and bimolecular recombination
resulting in dramatic improvement in PCE from 6.71 to 8.69% for PTB7:PC 71 BM
devices. The PCE improvement was due to increase in J SC and FF. The device with
Methanol treatment also improved the device PCE by about 1% compared to the
untreated ZnO. Using ethanolamine (EA) treated ZnO nanoparticle as optical spacer
and PEDOT:PSS as HTL in PTB7:PC 71 BM based conventional devices the PCE
could be increased from 5.8 to 7.6%. (Dkhil et al. 2014) The EA treatment lowered
the WF from ~4.3 to 4.1 eV (calculated from XPS study), which can be assigned
to dipolar polarization of the ZnO surface via the adsorption of EA. The enhancement was attributed to reduced contact barrier, reduced recombination and enhanced
electron extraction at the cathode.