**3. Quantum dot-sensitized solar cells (QDSSCs)**

As the research on DSSCs progressed, the idea of replacing dyes with QDs emerged. QDs are nano-dimensional structures with a narrow band gap suitable for absorbing light in the visible region. Therefore, when deposited over the mesoporous TiO2 layer, the excited electrons in the QDs can be transferred to the mesoporous TiO2. Research on sensitization of a wide band gap semiconductor by using a narrow band gap material such as dye started during the 1960s, but QDs was used for wide band gap semiconductor sensitization for the first time in 1986 by Gerischer et al. [34]. Advancement in research on sensitization led to DSSCs. Based on the highly porous TiO2 DSSCs introduced by O'Regan and Grätzel [6], QDs were introduced to replace the dye [35–37]. Until now, a lot of research has been geared towards improving QDSSCs performance. The highest efficiency recorded is now around 9% [38, 39].

There are several advantages of inorganic QDs over organic dyes. This is because inorganic QDs are easy to produce and durable [40]. Moreover, the optical band gap of QDs is tuneable [41]. Another special property of QDs is the production of at least two electron-hole pairs per photon with hot electrons. This is due to the impact of ionization in the QD nano-sized semiconducting material [42]. QDs can also reduce dark current and in doing so improve working of the photovoltaic system. This is because the extinction coefficient of QDs is high [43]. The theoretical efficiency for QDSSCs calculated by considering carrier multiplication due to impact of ionization was 44.4% [44].

QDSSCs and DSSCs have a lot of similarities and some differences. The major difference between these two is the sensitizer. QDSSCs utilize nano-sized semiconductor QDs and DSSCs utilize light absorbing dye. Another difference is material conformity. Some materials that worked effectively in DSSCs are not compatible with QDSSCs and could give a bad impact on the performance of the cells. **Table 1** compares the components for DSSCs and QDSSCs.


**Table 1.** A straightforward comparison between QDSSCs and DSSCs.

#### **3.1. QDSSC structure**

In general, organic dyes can be grouped as neutral and ionic organic dyes. Examples of neutral organic dyes are coumarins, triphenylamine, phenothiazine and indoline. Examples of ionic

Tian et al. [31] have synthesized organic dyes with phenothiazine (PTZ) as the electron donor and rhodamine-3-acetic acid or cyanoacrylic acid as the electron acceptor. The DSSC utilizing the dye with cyanoacrylic acid as the anchoring acceptor exhibited 5.5% efficiency. Marszalek et al. [32] reported two novel organic dyes. The dyes comprised of electron donating 10-butyl- (2-methylthio)-10*H*-phenothiazine with and without the vinyl thiophene group (VTP) as the *π*-bridge. The acceptor used is cyanoacrylic acid. With VTP, the IPCE value observed was up to 80% in the wavelength range between 380 and 750 nm, whereas without VTP, the range was between 380 and 650 nm. This results in higher *J*sc and efficiency for the DSSC using the VTP

Coumarin-based dye is a promising sensitizer for DSSC because it has good photoelectric conversion properties [33]. Wang et al. [33] reported that a DSSC using coumarin dye, 2 cyano-3-(5-{2-[5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-*1H, 4H, 10H*-11-oxa-3a-azabenzo[*de*] anthracen-9-yl)-thiophen-2-yl]-vinyl}, -thiophen-2-yl)-acrylic acid exhibited an

As the research on DSSCs progressed, the idea of replacing dyes with QDs emerged. QDs are nano-dimensional structures with a narrow band gap suitable for absorbing light in the visible region. Therefore, when deposited over the mesoporous TiO2 layer, the excited electrons in the QDs can be transferred to the mesoporous TiO2. Research on sensitization of a wide band gap semiconductor by using a narrow band gap material such as dye started during the 1960s, but QDs was used for wide band gap semiconductor sensitization for the first time in 1986 by Gerischer et al. [34]. Advancement in research on sensitization led to DSSCs. Based on the highly porous TiO2 DSSCs introduced by O'Regan and Grätzel [6], QDs were introduced to replace the dye [35–37]. Until now, a lot of research has been geared towards improving

There are several advantages of inorganic QDs over organic dyes. This is because inorganic QDs are easy to produce and durable [40]. Moreover, the optical band gap of QDs is tuneable [41]. Another special property of QDs is the production of at least two electron-hole pairs per photon with hot electrons. This is due to the impact of ionization in the QD nano-sized semiconducting material [42]. QDs can also reduce dark current and in doing so improve working of the photovoltaic system. This is because the extinction coefficient of QDs is high [43]. The theoretical efficiency for QDSSCs calculated by considering carrier multiplication due

QDSSCs and DSSCs have a lot of similarities and some differences. The major difference between these two is the sensitizer. QDSSCs utilize nano-sized semiconductor QDs and DSSCs

QDSSCs performance. The highest efficiency recorded is now around 9% [38, 39].

and the efficiency

organic dyes are squarylium, cyanine, hemicyanine and merocyanine.

attached dye. The photocurrent density enhanced from 11.2 to 15.2 mA/cm2

**3. Quantum dot-sensitized solar cells (QDSSCs)**

to impact of ionization was 44.4% [44].

reached 7.4%.

16 Nanostructured Solar Cells

efficiency of 8.2%.

Although progress has been made, the efficiency value of QDSSCs has not surpassed that of DSSCs, which is 13% [26]. There is still a lot of improvement to be done in obtaining a better material for QDSSCs. **Figure 8** illustrates schematically the QDSSC device and its components.

**Figure 8.** An illustration of QDSSCs with its three main components: photoanode, electrolyte and counter electrode.

#### *3.1.1. Photoanode*

In works concerning QDSSCs, very frequently TiO2 was utilized as the wide band gap semiconductor compared to other oxides. Out of the many QDs chalcogenides, cadmium chalcogenides (CdS, CdSe and CdTe) are most popularly used in QDSSCs [45–47]. Another important component in QDSSC photoanodes is the passivation layer. The passivation layer prevents electron recombination that can improve performance of QDSSCs since the short circuit current density will not be reduced.

Chalcogenides of cadmium can easily be fabricated and have a tuneable band gap that can be achieved by controlling their size [45, 48–50]. CdS, CdSe and CdTe chalcogenide QDs have a band gap 2.3, 1.7 and 1.4 eV, respectively. Hence, incident light in the visible wavelength can be absorbed up to ~540 nm for CdS, ~731 nm for CdSe and ~887 nm for CdTe. **Figure 9** shows the valence band (VB) and conduction bands of cadmium chalcogenide QDs and TiO2.

**Figure 9.** Energy levels of cadmium chalcogenide QDs (CdS, CdSe and CdTe) and TiO2.

The use of two species of QDs in a single QDSSC has proven to enhance the efficiency, for example, CdS/CdSe, CdTe/CdSe and CdTe/CdS combinations were used as sensitizers [43, 51, 52]. When CdS and CdSe make contact with each other, electron redistribution will occur resulting in the CdS and CdSe band edge to shift to more or less positive potentials, respectively. The shifting of the band edge is referred to Fermi level alignment [43]. This process affects electron injection. The same process also happens in the combinations of CdTe/CdSe and CdTe/CdS. **Figure 10** shows how CdTe/CdSe and CdS/CdSe combinations produce an effective electron injection. Application of co-sensitizing QDs in QDSSCs has shown excellent performance compared to QDSSCs fabricated with a single QD sensitizer [43, 51, 52].

**Figure 10.** Changing of the band edge level of QDs after electron redistribution of: (a) CdTe/CdSe and (b) CdS/CdSe. This arrangement is necessary for electron injection from CdSe to CB of TiO2 due to the alignment of the Fermi level.

Although tuning band gap with the size of the QDs is promising in enhancing performance of QDSSCs, this may give rise to stability problem [53]. To avoid this, alloyed cadmium chalcogenide QDs (AB*x*C1*-x*, A = Cd, B and C = S or Se or Te) were used to tailor the band gap of the QDSSCs without having to change the particle size [53, 54]. An example of alloyed cadmium chalcogenide is CdTe*x*S1*-x*. The band gap of the CdTe*x*S1*-x* alloyed QD can be adjusted to the range of visible light by changing the tellurium molar ratio and make it exhibit a high potential in photovoltaic application [55]. Another excellent alloyed cadmium chalcogenide used in QDSSCs is CdSe*x*Te1*-x*. CdSe*x*Te1*-x*has been utilized in QDSSCs by Ren et al. [38] and Yang et al. [39]. Photon-to-electricity efficiency obtained was 9 and 9.4% respectively. Employment of alloyed cadmium chalcogenide in QDSSCs have a very promising future since it will give a better efficiency value and high stability towards performance of QDSSCs.

Even though QDs have many advantages as a sensitizer compared to organic dyes, the efficiency recorded for QDSSCs is still lower compared to DSSCs. Excited electrons in QDs can take one of three possible routes which are: (1) jump into the TiO2 conduction band which will be beneficial to the performance of the QDSSCs, (2) relax into the valence band by emitting energy and finally (3) combine with redox mediator ions (recombination process) in the electrolyte which are routes detrimental to the QDSSC performance. To overcome recombination, researchers have QDs coated on the surface with ZnS, SiO2 and amorphous TiO2 (am-TiO2) [38, 56, 57]. Ren et al. [38] have introduced a novel strategy to overcome recombination by implementing three passivation layers am-TiO2/ZnS/SiO2 resulting in 9% efficiency. Yang et al. [39] utilized the CdS layer as a passivation layer to the CdSeTe QDs and achieved 9.4% efficiency.

#### *3.1.2. Electrolyte*

*3.1.1. Photoanode*

18 Nanostructured Solar Cells

circuit current density will not be reduced.

In works concerning QDSSCs, very frequently TiO2 was utilized as the wide band gap semiconductor compared to other oxides. Out of the many QDs chalcogenides, cadmium chalcogenides (CdS, CdSe and CdTe) are most popularly used in QDSSCs [45–47]. Another important component in QDSSC photoanodes is the passivation layer. The passivation layer prevents electron recombination that can improve performance of QDSSCs since the short

Chalcogenides of cadmium can easily be fabricated and have a tuneable band gap that can be achieved by controlling their size [45, 48–50]. CdS, CdSe and CdTe chalcogenide QDs have a band gap 2.3, 1.7 and 1.4 eV, respectively. Hence, incident light in the visible wavelength can be absorbed up to ~540 nm for CdS, ~731 nm for CdSe and ~887 nm for CdTe. **Figure 9** shows the valence band (VB) and conduction bands of cadmium chalcogenide QDs and TiO2.

**Figure 9.** Energy levels of cadmium chalcogenide QDs (CdS, CdSe and CdTe) and TiO2.

The use of two species of QDs in a single QDSSC has proven to enhance the efficiency, for example, CdS/CdSe, CdTe/CdSe and CdTe/CdS combinations were used as sensitizers [43, 51, 52]. When CdS and CdSe make contact with each other, electron redistribution will occur resulting in the CdS and CdSe band edge to shift to more or less positive potentials, respectively. The shifting of the band edge is referred to Fermi level alignment [43]. This process affects electron injection. The same process also happens in the combinations of CdTe/CdSe and CdTe/CdS. **Figure 10** shows how CdTe/CdSe and CdS/CdSe combinations produce an effective electron injection. Application of co-sensitizing QDs in QDSSCs has shown excellent

performance compared to QDSSCs fabricated with a single QD sensitizer [43, 51, 52].

Another important component in QDSSCs is the electrolyte. The electrolyte in QDSSCs functions as a charge carrier transporter between the photoanode and the counter electrode

done via the redox mediators. The redox species in the electrolyte are also responsible for turning the oxidized QD species by donating an electron to the QDs. In QDSSCs, polysulphide electrolytes with <sup>2</sup> −/ 2 − are widely utilized by researchers since they can give good performance and stability [58–60]. Performance of QDSSCs can also be improved by utilization of chemical additives in the polysulphide electrolyte. Park et al. [61] reported that by introducing sodium hydroxide (NaOH) into the polysulphide electrolyte of QDSSCs, *V*oc and *FF* can be increased.

Due to problems that arise from utilization of liquid electrolytes such as leakage and easy vaporization, researchers have begun to use polymer electrolytes. However, the performance of QDSSCs based on the solid polymer electrolyte [62, 63] is low compared to QDSSCs fabricated with liquid electrolytes. This is because solid state electrolytes suffer from low ionic conductivity. Another alternative to the liquid electrolyte is to use gel polymer electrolytes (GPEs). GPE is very competitive since GPE based QDSSC performance is comparable with QDSSCs fabricated with the liquid electrolyte [64–66]. Kim et al. [65] successfully fabricated CdSe/CdS GPE based QDSSCs with 5.45% efficiency, which is comparable with QDSSCs based on the liquid electrolyte. As the GPE based QDSSCs is comparable with QDSSCs fabricated with the liquid electrolyte, utilization of GPE in QDSSCs will be an advantage in terms of providing stability and overcoming problems that arise from liquid electrolytes.
