18 Interfacial Materials for Organic Solar Cells
409
Conventional solution-processed OSCs generally used PEDOT:PSS (WF ≈
5.1 eV) as AIL and many high performance devices have been reported. However,
PEDOT:PSS influences the device long term stability due to its acidic nature. To
improve the device performance and stability PEDOT:PSS has been modified with
metal nanoparticles, such as Ag and Au. The photovoltaic data of some representative
devices with various HTLs are presented in Table 18.4. Using colloidal Ag nanoprism
embedded with PEDOT:PSS as AIL and modified bis-C 60 as CIL, enhancement in
the PCE from 7.7 to 9.0% has been observed due to enhanced optical absorption.
(Yao et al. 2014) Au@Ag core–shell nanocube (NC) showing strong plasmonic light
scattering effect than Au nanoparticles when used as AIL with PEDOT:PSS the PCE
of PCDTBT:PC 71 BM solar cells improved from 5.3 to 6.3% and for PTB7:PC 71 BM
PCE improved from 7.9 to 9.2% (Baek et al. 2014). The Ag shell acted to enhance
the plasmonic scattering and improve the absorption of Au@Ag nanocube. Solutionprocessable MoO x as AIL have improved the photovoltaic performance and device
stability compared to PEDOT:PSS. (Tan et al. 2013) NiO nanoparticle when used as
AIL in PTB7:PC 71 BM blend achieved a high PCE of 9.16% which is higher than
that of PEDOT:PSS based devices (8.6%) (Jiang et al. 2015).
Solution-processed OSCs using dithieno[3,2-b:2
,3
-d]pyrrole (DTP)-based small
molecule donor and PC 61 BM acceptor as active layer and either PEDOT:PSS or V 2 O 5
as AIL were prepared. Devices with V 2 O 5 gave much better performance with very
high FF up to ~0.60 compared to PEDOT:PSS (~0.50) was ascribed to the improved
hole collection at the anode associated with a better match between the WF of V 2 O 5
and the HOMO of the donor as well different surface energy at the interface. When
polydimethylsiloxane (PDMS) was used as solvent additive the PCE with V 2 O 5
AIL was improved to 5.3% (Schulz et al. 2016). Small molecule OSCs employing
CuSCN as AIL obtained a PCE of 8.22% (V OC = 0.86, FF = 0.68) compared to
7.54% (V OC = 0.84, FF = 0.65) for PEDOT:PSS AIL (Mishra et al. 2016). Both
device gave nearly similar photocurrent values which could be ascribed to the similar
transparencies in the visible region. The improvement in the FF was related to the
formation of better ohmic contact between the active layer and CuSCN, because of
the high WF of CuSCN than PEDOT:PSS. The high V OC was due to better alignment
of HOMO of the donor and valance band of CuSCN (−5.35 eV) which also acts as
good electron blocking layer.
Various p-type small molecules and polymers have been prepared and used in
conventional and inverted devices. Heeger and co-workers used chlorobenzoic acid
(CBA) as surface modifier of ITO electrode which produce a high WF electrode
with better transparency as an alternative to PEDOT:PSS. The benzoic acid bound
to the surface and chlorine atom pointed towards the organic layer forming a dipole.
The PTB7:PC 71 BM based device achieved an improved PCE of 8.5% for CBA AIL
compared to 7.5% for PEDOT:PSS, ascribed improved light absorption and formation
of an Ohmic contact for hole transport (Choi et al. 2015). Huang et al. demonstrated
the enhancement of WF of ITO surface by self-assembling different benzoic acid and
sulfonic acids on the ITO surface (Huang et al. 2016). The WF of SAM-modified
ITO changed in the order CBSA (5.18 eV), CBA (4.91 eV), FBA (4.85 eV), and
BA (4.75 eV) when applied with same concentration. Among all derivatives CBSA
409
Conventional solution-processed OSCs generally used PEDOT:PSS (WF ≈
5.1 eV) as AIL and many high performance devices have been reported. However,
PEDOT:PSS influences the device long term stability due to its acidic nature. To
improve the device performance and stability PEDOT:PSS has been modified with
metal nanoparticles, such as Ag and Au. The photovoltaic data of some representative
devices with various HTLs are presented in Table 18.4. Using colloidal Ag nanoprism
embedded with PEDOT:PSS as AIL and modified bis-C 60 as CIL, enhancement in
the PCE from 7.7 to 9.0% has been observed due to enhanced optical absorption.
(Yao et al. 2014) Au@Ag core–shell nanocube (NC) showing strong plasmonic light
scattering effect than Au nanoparticles when used as AIL with PEDOT:PSS the PCE
of PCDTBT:PC 71 BM solar cells improved from 5.3 to 6.3% and for PTB7:PC 71 BM
PCE improved from 7.9 to 9.2% (Baek et al. 2014). The Ag shell acted to enhance
the plasmonic scattering and improve the absorption of Au@Ag nanocube. Solutionprocessable MoO x as AIL have improved the photovoltaic performance and device
stability compared to PEDOT:PSS. (Tan et al. 2013) NiO nanoparticle when used as
AIL in PTB7:PC 71 BM blend achieved a high PCE of 9.16% which is higher than
that of PEDOT:PSS based devices (8.6%) (Jiang et al. 2015).
Solution-processed OSCs using dithieno[3,2-b:2
,3
-d]pyrrole (DTP)-based small
molecule donor and PC 61 BM acceptor as active layer and either PEDOT:PSS or V 2 O 5
as AIL were prepared. Devices with V 2 O 5 gave much better performance with very
high FF up to ~0.60 compared to PEDOT:PSS (~0.50) was ascribed to the improved
hole collection at the anode associated with a better match between the WF of V 2 O 5
and the HOMO of the donor as well different surface energy at the interface. When
polydimethylsiloxane (PDMS) was used as solvent additive the PCE with V 2 O 5
AIL was improved to 5.3% (Schulz et al. 2016). Small molecule OSCs employing
CuSCN as AIL obtained a PCE of 8.22% (V OC = 0.86, FF = 0.68) compared to
7.54% (V OC = 0.84, FF = 0.65) for PEDOT:PSS AIL (Mishra et al. 2016). Both
device gave nearly similar photocurrent values which could be ascribed to the similar
transparencies in the visible region. The improvement in the FF was related to the
formation of better ohmic contact between the active layer and CuSCN, because of
the high WF of CuSCN than PEDOT:PSS. The high V OC was due to better alignment
of HOMO of the donor and valance band of CuSCN (−5.35 eV) which also acts as
good electron blocking layer.
Various p-type small molecules and polymers have been prepared and used in
conventional and inverted devices. Heeger and co-workers used chlorobenzoic acid
(CBA) as surface modifier of ITO electrode which produce a high WF electrode
with better transparency as an alternative to PEDOT:PSS. The benzoic acid bound
to the surface and chlorine atom pointed towards the organic layer forming a dipole.
The PTB7:PC 71 BM based device achieved an improved PCE of 8.5% for CBA AIL
compared to 7.5% for PEDOT:PSS, ascribed improved light absorption and formation
of an Ohmic contact for hole transport (Choi et al. 2015). Huang et al. demonstrated
the enhancement of WF of ITO surface by self-assembling different benzoic acid and
sulfonic acids on the ITO surface (Huang et al. 2016). The WF of SAM-modified
ITO changed in the order CBSA (5.18 eV), CBA (4.91 eV), FBA (4.85 eV), and
BA (4.75 eV) when applied with same concentration. Among all derivatives CBSA