**3.4 Working of QDSSC**

When the light is incident on the solar cell through a transparent conducting oxide, photons are absorbed to generate excitons. As the band positions between the Conduction Band (CB) of the Photoelectrode and the CB of QD is greater than the binding energy of the exciton, then it gets separated at the Metal Oxide (MO)/QD interface into electrons and holes. The electron moves into the CB of MO to reach the transparent Conducting Oxide (TCO) surface before it reaches the band. In the other way, the electrolyte reduces the reductant species (Re), i.e., hole created in the QD. The Re turns into Oxidant species (Ox) after losing electron to the hole, which diffuses toward the Counter Electrode (CE) to receive an electron coming from the external circuit. With the hole reduction, the QD is ready to absorb another photon for the creation of an exciton.

Thus, a photovoltaic effect converting light into electricity does useful work in the external load. The constant photoconversion process is continual till the light is incident on the active area of the solar cell. The photovoltaic processes discussed above are summarized as,

**Figure 9.** *Schematic of quantum dot sensitized solar cells. Source: Ref. [16].*

( ) <sup>∗</sup> *QD hv QD* + → formationof exciton to absorptionof photon *due* (1)

( ) ∗ +− *QD MO QD MO* +→ + Electron tranfer from to *QD MO* (2)

( ) <sup>+</sup> *QD QD Ox QD* +→ + Re charged getting reduced to neutral*QD* (3)

$$\text{Ox} + \text{e}^- \rightarrow \text{Re} \quad \text{(Reduction of oxidation from - supplied by CE}) \tag{4}$$

Components of QDSSC:

Photoelectrode: Electronic structure of metal oxide and its effect on electron injection.

The wide-band-gap semiconductors are needed for transmitting maximum solar spectrum (visible and infrared region) for effective power conversion efficiency. The conduction band of Photoanode should be more positive with respect to the conduction band of the sensitizer. That creates a band counterbalance between the QD-MO and has electron injection from QD to MO.

In addition, electrical band gap, the electronic mobility, and electronic structure are taken into consideration. For smooth passage of the carriers through the metal oxide semiconductors SnO2 and ZnO are preferred materials as photoelectrodes. In spite of that, TiO2 has appeared as a model photoanode semiconducting system, which demonstrated the best of the efficiencies in both DSSC and QDSSC.

The electron transfer from sensitizer to MO in both DSSC and QDSSC is directed by Marcus theory [18, 19]. According to this theory, the rate of electron injection is directly proportional to Density of States (DOS) in the conduction band of metal oxide. But, the DOS depends on the effective mass of the electron (me\*). me\* of CB electrons in titania is around 5–10 me (me is the mass of the electron) and about 0.3 me in ZnO and SnO2 [20]. TiO2 has two orders of magnitude densities of vacant states

**Figure 10.** *Band positions of metal oxides with respect to vacuum. Source: Ref. [21].*

greater than in ZnO and SnO2. Some of the MO and their band positions with respect to vacuum level are shown in **Figure 10** [21].

The effective area offered by the photoelectrode or photoanode surface is many times greater than its geometrical area. This assists the wider coverage of sensitizer for better light harvesting capability in the electrode. Various studies have proven the influence of particle size of photoanode on electron transport properties such as charge transport resistance (Rt) and charge recombination resistance Rr. Improved charge transport was observed in particles with greater sizes leading to longer diffusion length and minimal collisions at grain boundaries.

(\*The grain boundaries defect (the interface between two grains) reduces the electronic transport through a material)

With a porosity of 60%, the calculated electron diffusion length in a mesoporous titania photo electrode is 15–20 μm. The use of one-dimensional nanostructure improves electron transport properties such structures includes nano tubes, nano rods, nano wires, and nano fibers. Some of the metal oxides are listed below:

## *3.4.1 Titanium dioxide (TiO2)*

Titanium dioxide is in general n-type a wide direct band gap semiconductor. It is inexpensive and nontoxic. This metal oxide has three morphologies, namely Anatase, Rutile, and Brookite having band gap 3.2 eV, 3.05 eV, and 3.26 eV, respectively.

Rutile is the most stable morphology of the titania, it is used in device application when light scattering is important. The Fermi level of anatase is 100 mV, which is higher than rutile; due to this anatase is attaining higher open-circuit voltages when used as electrode in solar cell applications.

In addition, anatase has greater surface area and hence large amount of sensitizer loads on it, rather than rutile, which results in to greater photocurrents when used in DSSC/QDSSC.

#### *3.4.2 Zinc oxide (ZnO)*

Zinc Oxide (ZnO) is wide-band-gap semiconducting material II–VI having direct band gap 3.2 eV, and bond energy is 60 meV at room temperature. ZnO is frequently used as the domain of solar energy conversion due to its stability against photo

corrosion and photochemical property and alternative material to TiO2 owing huge surface area and high catalytic activity. It has a great benefit to be applied in a catalytic reaction process. Zinc oxide crystallizes in three main forms, hexagonal wurtzite, cubic zinc blende, and rocksalt. The wurtzite structure is most stable at environmental conditions, and thus, it is most common. ZnO is used in gas sensing, photocatalyst, solar cell, ultraviolet light emitting materials, field effect transistors, and transparent conductors.

#### *3.4.3 Zirconium dioxide ZrO2*

The white crystalline oxide ZrO2 adopts monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The band gap of ZrO2 depends on the phase (cubic, tetragonal, monoclinic, or amorphous) and synthesis method, due to trap states, its band gap decreases up to 2.8–3.7 eV.

ZrO2 is used as a photoelectrode for the fabrication of solar cell, due to its high refractive index, wide band gap, low absorption, and dispersion in the visible and near infrared spectral region. Along with these materials, tin oxide, cerium oxide are also used for fabrication of photoanodes.

#### *3.4.4 Photoelectrode sensitization with QDs*

The photoelectrode (mesoporous nanocrystalline) is the means of electron transport in solar cells, after the dissociation of exciton at the MO/QD interface. But it stands unworthy unless it is sensitized by the QDs and the photovoltaic process cannot be initiated. The QDs need to absorb maximum part of solar spectrum to create better exciton density at the interface. In order to have better charge transportation, the mere absorption is not sufficient for power conversion, but this demands proper band positioning between MO and QD, i.e., band offset should be greater than the exciton binding energy. Different types of QDs are involved in QDSSC; these includes CdS, CdSe, CdTe, ZnSe, PbS, PbSe, Bi2S3, Bi2Se3, Sb2S3, Sb2Se3, and InAs ternary compounds such as CuInS2, CuInSe2.

The band position, for example, lead chalcogenide QD system is shown in **Figure 11** with favorable band edges with respect to MO [16].

Multiple exciton generation: Under the illumination condition, a high-energy photon excites the electron from the valence band (VB) and jumps to conduction band (CB), and it generates one electron-hole pair, i.e., exciton.

As the energy of light (Photon) is twice the band gap of QD, the exciton has a very high kinetic energy, which can be used to excite another electron hopping from the VB to CB by impact ionization. Hence, one photon creates two excitons permitting the internal quantum efficiency to drive beyond 100%. Indeed, more excitons are produced, if the energy of the photon is sufficiently high. Multiple exciton generation is observed in PbS PbSe and CdSe/ZnS QDs. This property of QDs is useful to improve the efficiency of QDSSC and light-emitting devices. In PbS-based QDSSC, it is observed that an incident photon to current conversion efficiency (IPCE) is over 100%, which leads to the Multiple Exciton Generation.

#### *3.4.5 Surface passivation of quantum dots: Influence on QDSSC performance*

QDs have defect states, due to high surface to volume ratio. This results in low photocurrents obtained in QDSSC. Passivation of these surface or defect states using organic or inorganic passivating agents, which include ZnS, ZnSe, Cu-ZnS, and

halides, is seen to have immensely enhanced the performance of QDSSC. The original trapped states on QDs are reduced by such treatments and also reduce the recombination of charge carriers. ZnS is a very often used as surface passivator in QDSSC. It is observed that ZnS coating of MO besides QD has further improved the cell efficiency by improving the electrode/electrolyte interface [22]. Combined treatment of ZnS/ SiO2 on photoelectrode sensitized with CdSexTe1-x yielded a relatively very high efficiency in QDSSC, of around 8.2% [23]. Thus, a surface treatment of QDs as well as photoanodes improves the performance of QDSSC.

## *3.4.6 Electrolyte*

Electrolyte (redox) consists of species of which is an electron donor called reductant and an electron acceptor called oxidant. Dye/Quantum dot sensitized solar cell shows a photovoltaic action due to the circulation of charge carriers through an electrolyte. Polyiodide is the most favorable electrolyte with respect to DSSC, due to its electron transfer kinetics, which justifies their high photoconversion efficiencies. Though, the I-/I3- (Iodide/polyiodide) electrolyte displays amiable charge transfer kinetics due to its better hole evacuation capability, it is corrosive in case of QDSSC where the photocurrent degrades continuously.

Sulfide/polysulfide (S2− /Sn2−) is used as the electrolyte in QDSSC due to its stability with the semiconductor QDs and solar cell performance. The other redox couples such as Fe3+/Fe2+, Co2+/Co3+ and Fe(CN)6 3−/Fe(CN)6 4− [24–27] are employed. Polysulfide (PS) has a high redox potential, which supports in faster and better hole evacuation, but it reduces the open-circuit voltage and has poor fill factor (FF).

The stability of QDSSC is a question as long as liquid electrolytes are employed. Electrolytes volatilization, permeation of oxygen and water vapor from the atmosphere lead to quick degradation of the device performance. Rather than all disadvantages of the PS as an electrolyte, it has reported the best of efficiencies for QDSSC so far.

#### *3.4.7 Counter electrode (CE)*

The collection of the electrons from the external circuit and the reduction of the oxidant species in the electrolyte are done with the help of a component known as counter electrode. This completes the circuit and with the electrolyte, it makes the device ready for a constant photovoltaic action. Thus, a counter electrode reduces the oxidant species.

Pt and Au are the most popular CE in DSSC, and they are highly efficient and compatible with Iodide/Polyiodide redox species [28–31]. In QDSSC, Pt results in a poor device performance due to the chemical activity, which reduces chemisorption of sulfide species. The charge transfer at the CE/Electrolyte is also affected due to poisoning effect of electrolyte on the surface of CE leading to reduction in FF.

With their very good electronic mobility and corrosion resistance for electrolyte species, carbonaceous materials such as graphene are considered as CE with Graphene/CoS, graphene/PbS, Carbon nanofiber/CuS, Carbon black/PbS, multiwall carbon nanotubes (MWCNT)/Cu2ZnSnSe(CZTSe), etc.

### **4. Conclusion**

The chapter focused on different types of nanomaterial synthesis techniques such as physical, chemical, etc. In the latter part, it covers the synthesis of PbS quantum dots, by hazardous-chemical-free simple technique using ultrasonication and without capping agent. This technique produces quantum dots of 6 nm size, and it is confirmed from UV spectra, HRTEM images, and SAED pattern. In order to understand the structure of solar cells, the QDSSC is discussed in detail with its essential components and materials used in the architecture.
