340
S. Peiris et al.
Energy losses associated with the redox process of I
− /I 3
− , which is the typically
used electrolyte, have been overcome by discovering/designing novel redox mediators such as Co (II/II) and Fe (II/II) systems. These materials have reached high
VOC, even though the slow diffusion of ions is still a significant issue for most of
the studied mediators. The developed Fe (II/III) systems have seemingly overcome
this limitation by exhibiting diffusion rates near the theoretical limit, but the light
absorption profiles of these colored redox mediators compete for light with the sensitizer, lowering the overall device performance. Hence, further development of redox
mediators with high diffusion rates and low light absorption remains a challenge in
p-DSC research. Fine-tuning existing coordination complexes has been identified as
one possible pathway to overcome this challenge.
The second major issue is replacing the widely used p-semiconductor material,
NiO. It has been discussed here, and elsewhere, that the coloration of NiO due to the
presence of Ni
0 is a major issue, again due to competing for light absorption with the
sensitizer. Further, it has been shown that the electrochromic nature of NiO reduces
the light harvesting efficiency of the p-DSC near V OC . Moreover, the existence of
trap states and the low conductivity of bulk NiO both contribute to unfavourable
charge recombination reactions. However, NiO is still under investigation due to its
capability of generating high photocurrents and the low-lying VB edge (−5.1 eV vs.
vacuum) compared to other p-type semiconductors. Other p-type semiconductors
with a VB edge between −5 and −6 eV (vs. Vacuum) are yet to be discovered. Thus,
extensive research on alternative p-type semiconductor materials may be required to
outperform NiO.
Further, in order to realize high light harvesting efficiencies it is important to
develop sensitizers with broader absorption wavelength rage that extends from visible
to near-IR region of the electromagnetic spectrum. Even though a lot of research has
been done developing p-type sensitizers, the main focus has been the improvement of
charge separation but not the light harvesting efficiencies. Since the charge separation
challenge has been overcome with novel sensitizers, shifting focus to enhancing
light harvesting efficiencies, while maintaining proper charge separation, for p-type
sensitizers.
As elaborated throughout this chapter, remarkable development has occurred
in the field of DSCs since the introduction of high-performance n-DSCs in 1999.
Although n-DSCs are far ahead of p-DSCs in terms of efficiency at present, the
historical improvements of n-DSC performance bode well for the advancement of
p-DSCs too, particularly because there is no fundamental limitation to achieve high
efficiencies. Although contemporary p-DSCs are limiting the overall performance of
tandem-DSCs, the opportunities and avenues to upgrade these devices are clear and
promising.
Acknowledgements Authors would like to express their gratitude to Mr. Dushan Wijewardena for
the extended support.
S. Peiris et al.
Energy losses associated with the redox process of I
− /I 3
− , which is the typically
used electrolyte, have been overcome by discovering/designing novel redox mediators such as Co (II/II) and Fe (II/II) systems. These materials have reached high
VOC, even though the slow diffusion of ions is still a significant issue for most of
the studied mediators. The developed Fe (II/III) systems have seemingly overcome
this limitation by exhibiting diffusion rates near the theoretical limit, but the light
absorption profiles of these colored redox mediators compete for light with the sensitizer, lowering the overall device performance. Hence, further development of redox
mediators with high diffusion rates and low light absorption remains a challenge in
p-DSC research. Fine-tuning existing coordination complexes has been identified as
one possible pathway to overcome this challenge.
The second major issue is replacing the widely used p-semiconductor material,
NiO. It has been discussed here, and elsewhere, that the coloration of NiO due to the
presence of Ni
0 is a major issue, again due to competing for light absorption with the
sensitizer. Further, it has been shown that the electrochromic nature of NiO reduces
the light harvesting efficiency of the p-DSC near V OC . Moreover, the existence of
trap states and the low conductivity of bulk NiO both contribute to unfavourable
charge recombination reactions. However, NiO is still under investigation due to its
capability of generating high photocurrents and the low-lying VB edge (−5.1 eV vs.
vacuum) compared to other p-type semiconductors. Other p-type semiconductors
with a VB edge between −5 and −6 eV (vs. Vacuum) are yet to be discovered. Thus,
extensive research on alternative p-type semiconductor materials may be required to
outperform NiO.
Further, in order to realize high light harvesting efficiencies it is important to
develop sensitizers with broader absorption wavelength rage that extends from visible
to near-IR region of the electromagnetic spectrum. Even though a lot of research has
been done developing p-type sensitizers, the main focus has been the improvement of
charge separation but not the light harvesting efficiencies. Since the charge separation
challenge has been overcome with novel sensitizers, shifting focus to enhancing
light harvesting efficiencies, while maintaining proper charge separation, for p-type
sensitizers.
As elaborated throughout this chapter, remarkable development has occurred
in the field of DSCs since the introduction of high-performance n-DSCs in 1999.
Although n-DSCs are far ahead of p-DSCs in terms of efficiency at present, the
historical improvements of n-DSC performance bode well for the advancement of
p-DSCs too, particularly because there is no fundamental limitation to achieve high
efficiencies. Although contemporary p-DSCs are limiting the overall performance of
tandem-DSCs, the opportunities and avenues to upgrade these devices are clear and
promising.
Acknowledgements Authors would like to express their gratitude to Mr. Dushan Wijewardena for
the extended support.