18 Interfacial Materials for Organic Solar Cells
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18.2 Interfacial Design for Efficient Organic Solar Cells
The metal:organic interface plays a crucial role in overall device performance
improvement. Insertion of an interfacial layer between the metal and organics can
dramatically alter the interface properties. They can control the charge transfer and
recombination process and minimize the contact resistance in the device and lower
the series resistance thus improve the FF, thus overall PCE. The processing conditions and wettability between the components are also very important for device
performance improvement. Another important aspect is the charge selectivity at the
electrode interface. The interfacial layer should ensure selective charge extraction by
blocking the flow of unfavorable charge carrier. The interfacial layers can improve
the charge selectivity at the organic: electrode interface by selectively transporting
either electrons or holes, thus acting as an electron-transport layer (ETL) and block
holes to flow through, and a hole-transport layer (HTL) by blocking electron to flow
through (Irwin et al. 2008; Walzer et al. 2007). Till date numerous small molecules
and polymers have been developed and used as interfacial materials in OSCs and
successfully improved the device PCE and most importantly the stability. Below
we will discuss the design and development some efficient CIL and AIL and their
performance in high efficiency OSCs.
18.2.1 Electron Transport Materials as Cathode Interface
Layers
The prerequisite for a cathode interfacial layer is low WF to match with the LUMO
of the organic acceptor for efficient charge extraction, electron transport with hole
blocking properties, lower the energy loss and reduce interfacial defects. The CIL
should have transparent in nature for inverted devices in order for efficient transmission of light and good stability to prevent metal electrode diffusion in conventional
devices. Till date a variety of ETL, such as low WF metal oxide, composite materials,
organic molecules and polymers, have been used in high performance OSCs. Here
we will focus some of the recent advance in the use of CIL including their development and performance in OSC devices and deduce some structure-performance
relationships.
18.2.1.1 Metal Oxides as CIL
n-Type metal oxides such as TiO 2 , ZnO, SnO x , Al 2 O 3 , ZrO 2 were used as CIL
materials due to their low lying energy levels (~4.3 to 4.4 eV) (Waldauf et al. 2006;
Trost et al. 2012; Hau et al. 2008; Trost et al. 2015; Jheng et al. 2013). These
materials have good optical transparency in the visible region and efficiently transport
377
18.2 Interfacial Design for Efficient Organic Solar Cells
The metal:organic interface plays a crucial role in overall device performance
improvement. Insertion of an interfacial layer between the metal and organics can
dramatically alter the interface properties. They can control the charge transfer and
recombination process and minimize the contact resistance in the device and lower
the series resistance thus improve the FF, thus overall PCE. The processing conditions and wettability between the components are also very important for device
performance improvement. Another important aspect is the charge selectivity at the
electrode interface. The interfacial layer should ensure selective charge extraction by
blocking the flow of unfavorable charge carrier. The interfacial layers can improve
the charge selectivity at the organic: electrode interface by selectively transporting
either electrons or holes, thus acting as an electron-transport layer (ETL) and block
holes to flow through, and a hole-transport layer (HTL) by blocking electron to flow
through (Irwin et al. 2008; Walzer et al. 2007). Till date numerous small molecules
and polymers have been developed and used as interfacial materials in OSCs and
successfully improved the device PCE and most importantly the stability. Below
we will discuss the design and development some efficient CIL and AIL and their
performance in high efficiency OSCs.
18.2.1 Electron Transport Materials as Cathode Interface
Layers
The prerequisite for a cathode interfacial layer is low WF to match with the LUMO
of the organic acceptor for efficient charge extraction, electron transport with hole
blocking properties, lower the energy loss and reduce interfacial defects. The CIL
should have transparent in nature for inverted devices in order for efficient transmission of light and good stability to prevent metal electrode diffusion in conventional
devices. Till date a variety of ETL, such as low WF metal oxide, composite materials,
organic molecules and polymers, have been used in high performance OSCs. Here
we will focus some of the recent advance in the use of CIL including their development and performance in OSC devices and deduce some structure-performance
relationships.
18.2.1.1 Metal Oxides as CIL
n-Type metal oxides such as TiO 2 , ZnO, SnO x , Al 2 O 3 , ZrO 2 were used as CIL
materials due to their low lying energy levels (~4.3 to 4.4 eV) (Waldauf et al. 2006;
Trost et al. 2012; Hau et al. 2008; Trost et al. 2015; Jheng et al. 2013). These
materials have good optical transparency in the visible region and efficiently transport