324
S. Peiris et al.
since contemporary tandem DSCs are limited by the power conversion efficiency
of p-DSCs. This could be achieved by focusing on to two main strategies, namely,
(1) maximizing the light harvesting, and (2) minimizing electron losses (Gong et al.
2017). Novel practices based on these strategies are elaborately discussed in this
review, in Sects. 16.8 and 16.9, respectively.
Stability of the device is another aspect identified in the critical triangle that
has been studied. A major hurdle in extending the lifetime of DSCs has been the
use of liquid electrolytes, which can evaporate. A strategy to avoid this problem is
to substitute liquid electrolytes with solid electrolytes, yielding in solid state dyesensitized solar cells (ssDSCs). These devices are briefly discussed in Sect. 16.10.
16.8 Maximizing Light Harvesting
The light harvesting efficiency is predominantly determined by the performance of
the photosensitizer. Photon capture is followed by excitation of the dye, causing injection of positive charge carriers into the valence band of the p-type semiconductor
material. An efficient photo sensitizer should capture a maximum amount of sunlight, usually from photons with energy higher than the band gap; from wavelengths
corresponding to UV and visible spectral region and reaching the near infrared (NIR)
region of the electromagnetic spectrum. Since each absorption event can generate
and push a pair of charges into the external circuit, enhancing the light harvesting
capacity can increase the photocurrent density. An efficient sensitizer should also be
strongly adsorbed on to the semiconductor surface so that the charge injection into
the semiconductor is efficient. In addition, rapid regeneration of the sensitizer by the
redox mediator is essential to minimize the recombination process, and importantly,
the dye should be stable in both its excited and ground states. There are few novel sensitizers that are being used in p-DSCs: donor-π-acceptor systems, push-pull dyes,
cyclometalated dyes, quaraines, and ku Quinones (Lyu et al. 2016; Ji et al. 2013;
Jiang et al. 2014; Bonomo et al. 2017b).
Donor-(π linker)-acceptor systems have common structural motif including the
integrants arranged in the order as donor–π–acceptor (see Fig. 16.9). The absorption and redox properties of the dye can be fine-tuned by changing the individual
components. For example, arylamines are common donors while, cyanoacrylates
are common acceptors, and the pi-bridge, which is the linker, may contain one or
more thiophene units. Scientists have developed novel dyes by modifying these
donor, linker and acceptor units. According to one investigation, when carboxylic
acid groups are used as the acceptor, surface protonation of NiO occurs, causing a
negative shift of the valence band edge. This leads to reduce the hole-injection and
the photovoltage (Cui et al. 2014). Jin Cui et al. have developed a novel donor-πacceptor organic sensitizer incorporating pyridine as the anchoring group that could
overcome this problem. Under full sunlight (AM 1.5G,100 mW cm
−2 ) the reported
conversion efficiency was ~0.16%, while V OC is 117.9 mV, J SC is 4.05 mA cm
−2 and
FF is 0.34. This causes positive shifting of VB edge of NiO, minimizing interfacial
S. Peiris et al.
since contemporary tandem DSCs are limited by the power conversion efficiency
of p-DSCs. This could be achieved by focusing on to two main strategies, namely,
(1) maximizing the light harvesting, and (2) minimizing electron losses (Gong et al.
2017). Novel practices based on these strategies are elaborately discussed in this
review, in Sects. 16.8 and 16.9, respectively.
Stability of the device is another aspect identified in the critical triangle that
has been studied. A major hurdle in extending the lifetime of DSCs has been the
use of liquid electrolytes, which can evaporate. A strategy to avoid this problem is
to substitute liquid electrolytes with solid electrolytes, yielding in solid state dyesensitized solar cells (ssDSCs). These devices are briefly discussed in Sect. 16.10.
16.8 Maximizing Light Harvesting
The light harvesting efficiency is predominantly determined by the performance of
the photosensitizer. Photon capture is followed by excitation of the dye, causing injection of positive charge carriers into the valence band of the p-type semiconductor
material. An efficient photo sensitizer should capture a maximum amount of sunlight, usually from photons with energy higher than the band gap; from wavelengths
corresponding to UV and visible spectral region and reaching the near infrared (NIR)
region of the electromagnetic spectrum. Since each absorption event can generate
and push a pair of charges into the external circuit, enhancing the light harvesting
capacity can increase the photocurrent density. An efficient sensitizer should also be
strongly adsorbed on to the semiconductor surface so that the charge injection into
the semiconductor is efficient. In addition, rapid regeneration of the sensitizer by the
redox mediator is essential to minimize the recombination process, and importantly,
the dye should be stable in both its excited and ground states. There are few novel sensitizers that are being used in p-DSCs: donor-π-acceptor systems, push-pull dyes,
cyclometalated dyes, quaraines, and ku Quinones (Lyu et al. 2016; Ji et al. 2013;
Jiang et al. 2014; Bonomo et al. 2017b).
Donor-(π linker)-acceptor systems have common structural motif including the
integrants arranged in the order as donor–π–acceptor (see Fig. 16.9). The absorption and redox properties of the dye can be fine-tuned by changing the individual
components. For example, arylamines are common donors while, cyanoacrylates
are common acceptors, and the pi-bridge, which is the linker, may contain one or
more thiophene units. Scientists have developed novel dyes by modifying these
donor, linker and acceptor units. According to one investigation, when carboxylic
acid groups are used as the acceptor, surface protonation of NiO occurs, causing a
negative shift of the valence band edge. This leads to reduce the hole-injection and
the photovoltage (Cui et al. 2014). Jin Cui et al. have developed a novel donor-πacceptor organic sensitizer incorporating pyridine as the anchoring group that could
overcome this problem. Under full sunlight (AM 1.5G,100 mW cm
−2 ) the reported
conversion efficiency was ~0.16%, while V OC is 117.9 mV, J SC is 4.05 mA cm
−2 and
FF is 0.34. This causes positive shifting of VB edge of NiO, minimizing interfacial