146
S. Nair and J. V. Gohel
has surpassed to more than 20% with the advent of new measures to control stability
and increase the power conversion efficiency of perovskite solar cells. Currently, the
highest reported efficiency is at 22.7% (NREL 2018). This spurge in efficiency was
in a matter of few years. Contrastingly, silicon-based solar cells took almost 60 years
to achieve the same level of efficiency (NREL 2018; Economist 2018). Talks are also
underway to bring the first commercial perovskite solar cells to the market in a year
(Economist 2018).
This development in perovskite research is not free from pitfalls. It is still plagued
with many problems surrounding stability of the different layers involved (HTM,
Perovskite layer, and ETL).
To compete with the silicon-based solar cells which dominate the PV market, there
are certain issues that are of paramount concerns. The stability of perovskite-based
solar cells is very low compared to silicon-based cells. There have been countless
research studies pertaining to the stability of the different layers involved in the
perovskite cells, and there have also been many attempts to improve the efficiency
of perovskite cells by using a tandem configuration with silicon.
The major instability issues for PSC can be attributed to moisture, temperature,
oxygen, and UV Light (Conings et al. 2015; Misra et al. 2015; Aristidou et al. 2015;
Han et al. 2015; Senocrate et al. 2018). HTM plays an important role in the longterm stability of the solar cells. It plays a vital role in transferring holes from the
perovskite layer to the back contact metal electrode. Additionally, it acts as a barrier
between perovskite and metal electrode; this in turn increases the device stability
and effectively blocks the transfer of the electron from perovskite layer to the anode.
Consequently, HTM should ensure higher stability and efficiency. HTM would also
require higher thickness to avoid leakage currents and pinholes.
The most commonly used HTM is a solid-state HTM called Spiro-OMeTAD. It
was discovered by Park and his co-workers (Kim et al. 2012) as an alternative to
conventional liquid-based electrolyte HTM. This gave an efficiency of 9.7%. Since
then, researchers have focussed on different types of HTM using organic, inorganic,
and polymeric materials.
2 Organic-Based HTM
2.1 Spiro-Based HTM
After the first use of spiro-based HTM by Kim et al. (2012), there is a great progress
in the use of spiro-based HTM using different dopants. The molecular configuration
of Spiro-OMeTAD is such that it offers a good optical stability and proper thermal
stability. Moreover, methoxy substituent located at the end of triphenylamine in the
spiro configuration influences the optical and electrical properties. Consequently,
many derivatives are synthesized by changing the location of the methoxy groups.
The spiro structure (Fig. 1) suffers from poor hole mobility (~10
−4 cm
2 V
−1 s
−1 )
S. Nair and J. V. Gohel
has surpassed to more than 20% with the advent of new measures to control stability
and increase the power conversion efficiency of perovskite solar cells. Currently, the
highest reported efficiency is at 22.7% (NREL 2018). This spurge in efficiency was
in a matter of few years. Contrastingly, silicon-based solar cells took almost 60 years
to achieve the same level of efficiency (NREL 2018; Economist 2018). Talks are also
underway to bring the first commercial perovskite solar cells to the market in a year
(Economist 2018).
This development in perovskite research is not free from pitfalls. It is still plagued
with many problems surrounding stability of the different layers involved (HTM,
Perovskite layer, and ETL).
To compete with the silicon-based solar cells which dominate the PV market, there
are certain issues that are of paramount concerns. The stability of perovskite-based
solar cells is very low compared to silicon-based cells. There have been countless
research studies pertaining to the stability of the different layers involved in the
perovskite cells, and there have also been many attempts to improve the efficiency
of perovskite cells by using a tandem configuration with silicon.
The major instability issues for PSC can be attributed to moisture, temperature,
oxygen, and UV Light (Conings et al. 2015; Misra et al. 2015; Aristidou et al. 2015;
Han et al. 2015; Senocrate et al. 2018). HTM plays an important role in the longterm stability of the solar cells. It plays a vital role in transferring holes from the
perovskite layer to the back contact metal electrode. Additionally, it acts as a barrier
between perovskite and metal electrode; this in turn increases the device stability
and effectively blocks the transfer of the electron from perovskite layer to the anode.
Consequently, HTM should ensure higher stability and efficiency. HTM would also
require higher thickness to avoid leakage currents and pinholes.
The most commonly used HTM is a solid-state HTM called Spiro-OMeTAD. It
was discovered by Park and his co-workers (Kim et al. 2012) as an alternative to
conventional liquid-based electrolyte HTM. This gave an efficiency of 9.7%. Since
then, researchers have focussed on different types of HTM using organic, inorganic,
and polymeric materials.
2 Organic-Based HTM
2.1 Spiro-Based HTM
After the first use of spiro-based HTM by Kim et al. (2012), there is a great progress
in the use of spiro-based HTM using different dopants. The molecular configuration
of Spiro-OMeTAD is such that it offers a good optical stability and proper thermal
stability. Moreover, methoxy substituent located at the end of triphenylamine in the
spiro configuration influences the optical and electrical properties. Consequently,
many derivatives are synthesized by changing the location of the methoxy groups.
The spiro structure (Fig. 1) suffers from poor hole mobility (~10
−4 cm
2 V
−1 s
−1 )