376
A. Mishra
oxide such as TiO 2 , ZnO as cathode interfacial layer (CIL) to modify ITO surface
(Fig. 18.2). Subsequently, various organic and polymeric materials were synthesized
for their use as interlayer material in solution-processed OSCs. The interfacial layer
is very important to fine tune the device performance such as V OC , short-circuit current density (J SC ) and FF, therefore the overall PCE. Along with the photoactive
layers it is also highly important to optimize the charge extracting interfacial layer in
single-junction and charge recombination layer in tandem cell to achieve high performance OSCs. The interconnecting layer (ICL) in tandem cell is highly responsible
for extraction of holes and electrons from the adjacent subcells. Hence, the choice
of ICL plays a detrimental role in the device performance improvement. Mihailetchi
et al. observed a variation in the V OC of polymer:fullerene-based conventional solar
cells (~0.4 to 0.85 V) by using metal cathodes of variable WFs which is due to the
Fermi level pinning of the electrodes to the active layer which enables Ohmic contact
at the cathode (Mihailetchi et al. 2003). The low WF metals (Ag, Al, Ca etc.) generally forms Ohmic contact with the LUMO of the acceptor, while the high WF metals
(Au, Pd etc.) forms Ohmic contact with the HOMO of the donor material. Jen group
demonstrated the influence of metal anodes on the V OC of an poly-(3-hexylthiophene)
(P3HT): [6,6]-phenyl C 60 butyric acid methyl ester (PC 61 BM) based inverted solar
cell using ZnO-NPs/C 60 -self-assembled monolayer as interfacial material and found
that only Pd with high WF can form good Ohmic contact for hole extraction from
the donor (Hau et al. 2010).
In recent years major efforts have been devoted to develop interfacial layers for
single-junction and tandem OSCs. The interfacial layers are mostly used to modify
the energy level alignment between active layer and electrode, adjust the polarity
of the electrodes for charge selectivity, control the surface energy by modulating
active layer morphology, as well reduce carrier recombination and improve stability of photoactive layer and electrodes interface by preventing the metal ions from
diffusion into the organic layer (van Reenen et al. 2014; Yip and Jen 2012; Manders
et al. 2013; Bilby et al. 2014; Jørgensen et al. 2012; Ma et al. 2010). The interfacial materials have been designed in such a way that they can be processed from
orthogonal solvent relative to the active layer and thus, can be processed from solution. In the last few years, the number of publications on interfacial material design
and their implementation in various OSC devices have been considerably increased.
Recently, some of the reviews have comprehensively summarized the role of interfacial layers in OSCs (Lai et al. 2013; Chueh et al. 2015; Wang et al. 2015; Yin
et al. 2016; Yip and Jen 2012; Li et al. 2018a). Frey and co-workers discussed the
mechanistic studies on the driving force for interlayer formation and their influence
on device performance (Vinokur et al. 2016). In this chapter we systematically discuss some important progress on solution-processed interfacial materials including
organic-inorganic hybrids, transition metal oxide, composite materials, oligomeric
and polymeric molecules, and their implementation as cathode and anode interfacial layer for high efficiency single-junction OSCs. Finally, structure-property and
device performance relationship and challenges of the interfacial materials towards
high performance devices will be deduced.
A. Mishra
oxide such as TiO 2 , ZnO as cathode interfacial layer (CIL) to modify ITO surface
(Fig. 18.2). Subsequently, various organic and polymeric materials were synthesized
for their use as interlayer material in solution-processed OSCs. The interfacial layer
is very important to fine tune the device performance such as V OC , short-circuit current density (J SC ) and FF, therefore the overall PCE. Along with the photoactive
layers it is also highly important to optimize the charge extracting interfacial layer in
single-junction and charge recombination layer in tandem cell to achieve high performance OSCs. The interconnecting layer (ICL) in tandem cell is highly responsible
for extraction of holes and electrons from the adjacent subcells. Hence, the choice
of ICL plays a detrimental role in the device performance improvement. Mihailetchi
et al. observed a variation in the V OC of polymer:fullerene-based conventional solar
cells (~0.4 to 0.85 V) by using metal cathodes of variable WFs which is due to the
Fermi level pinning of the electrodes to the active layer which enables Ohmic contact
at the cathode (Mihailetchi et al. 2003). The low WF metals (Ag, Al, Ca etc.) generally forms Ohmic contact with the LUMO of the acceptor, while the high WF metals
(Au, Pd etc.) forms Ohmic contact with the HOMO of the donor material. Jen group
demonstrated the influence of metal anodes on the V OC of an poly-(3-hexylthiophene)
(P3HT): [6,6]-phenyl C 60 butyric acid methyl ester (PC 61 BM) based inverted solar
cell using ZnO-NPs/C 60 -self-assembled monolayer as interfacial material and found
that only Pd with high WF can form good Ohmic contact for hole extraction from
the donor (Hau et al. 2010).
In recent years major efforts have been devoted to develop interfacial layers for
single-junction and tandem OSCs. The interfacial layers are mostly used to modify
the energy level alignment between active layer and electrode, adjust the polarity
of the electrodes for charge selectivity, control the surface energy by modulating
active layer morphology, as well reduce carrier recombination and improve stability of photoactive layer and electrodes interface by preventing the metal ions from
diffusion into the organic layer (van Reenen et al. 2014; Yip and Jen 2012; Manders
et al. 2013; Bilby et al. 2014; Jørgensen et al. 2012; Ma et al. 2010). The interfacial materials have been designed in such a way that they can be processed from
orthogonal solvent relative to the active layer and thus, can be processed from solution. In the last few years, the number of publications on interfacial material design
and their implementation in various OSC devices have been considerably increased.
Recently, some of the reviews have comprehensively summarized the role of interfacial layers in OSCs (Lai et al. 2013; Chueh et al. 2015; Wang et al. 2015; Yin
et al. 2016; Yip and Jen 2012; Li et al. 2018a). Frey and co-workers discussed the
mechanistic studies on the driving force for interlayer formation and their influence
on device performance (Vinokur et al. 2016). In this chapter we systematically discuss some important progress on solution-processed interfacial materials including
organic-inorganic hybrids, transition metal oxide, composite materials, oligomeric
and polymeric molecules, and their implementation as cathode and anode interfacial layer for high efficiency single-junction OSCs. Finally, structure-property and
device performance relationship and challenges of the interfacial materials towards
high performance devices will be deduced.