**1. Introduction**

Zinc oxide is an inorganic compound having a chemical formula ZnO. It is a white powder which is nearly insoluble in water. It crystallizes in two main forms, the hexagonal wurtzite and cubic zinc blende. The wurtzite structure with lattice parameters *a* = 0*.*3296 and *c* = 0*.*52065 nm is found to be more stable than the zinc blende structure, and hence it is more widely used [1]. The ZnO structure is commonly described as consisting of a number of alternating planes composed of tetrahedrally coordinated O2− and Zn2+ ions, stacked alternately along the *c*-axis without a central symmetry as illustrated in **Figure 1** [1, 2]. It is a group II–VI semiconductor with a wide band gap of about 3.33 eV. Due to its direct and wide band gap in the near-UV spectral region [3–5] and a large free exciton binding energy, it has become a promising functional semiconductor material, which possesses a wide range of novel applications. ZnO has been identified with many unique properties

### **Figure 1.** *Hexagonal Wurtzite crystal structure of ZnO [1].*

such as excitonic emission at or even above room temperature, optical transparency in the visible range, high surface-to-volume ratio and quantum confinement effect [6], amongst others, which have motivated intensive study of the semiconductor during the last two decades. ZnO is mostly known to crystalize as an n-type semiconductor, whereas synthesis of the p-type is not generally easy [1, 7].

ZnO is simple to synthesize; both chemical and physical techniques are used to produce excellent epitaxial films. The most commonly used techniques to grow epitaxial films of ZnO include electrodeposition, spray pyrolysis, sol–gel process, successive ionic layer adsorption and reaction (SILAR), RF sputtering, chemical bath deposition (CBD), spin coating, electron beam epitaxy, laser evaporation and ion beam sputtering, amongst others [7, 8]. **Figure 2** illustrates the various synthetic techniques (chemical as well as physical) that are generally used to grow compound and alloys of ZnO. The choice of a particular technique would be guided by some factors such as the application intended for the synthesis, effectiveness of the technique and cost implication [10, 11]. ZnO has been identified as one of the semiconductors with the largest number of novel nanostructures such as nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires, nanorods, nanotubes, nanocages, etc., with a wide range of technological applications [12–15]. Novel applications of ZnO nanostructures include optical modulator waveguide, photonic crystals, surface acoustic wave filters, varistors, photodetectors, gas sensors, lightemitting diode, photodiodes and solar cells, amongst others [12].

Photovoltaic (PV) application of ZnO nanostructures requires large internal surface area with porous and high surface roughness to support good penetration of electrolyte [13, 14]. Chemical techniques are very simple, much reliable and cost-effective for the synthesis of high-quality electrodes for PV application. Most especially, chemical bath deposition technique is very suitable for growing large area films of ZnO with fascinating properties for photoelectrochemical solar cells [15, 16]. This technique is suitable for growing ZnO nanostructures on many substrates including microscope glass and stainless steel [6].

In several applications such as optoelectronics, ZnO can be used as a complement or alternative to some semiconductors such as GaN, and many researches are ongoing globally to further improve the properties of the semiconductor [10]. Trying to control the unintentional n-type conductivity and to achieve p-type

**59**

known [18].

**Figure 2.**

impurities (dopants) (down to 10–14 cm<sup>−</sup><sup>3</sup>

*Broad classification of thin film deposition techniques [9].*

thus enhancing its performance [1].

*Doped Zinc Oxide Nanostructures for Photovoltaic Solar Cells Application*

conductivity are such famous research themes. Other approaches such as firstprinciples calculations based on density functional theory (DFT) are theoretical and, however, are useful to provide in-depth understanding of the role of native point defects and impurities on the unintentional n-type conductivity in ZnO [14, 17]. Acceptor doping in ZnO which will lead to stable p-type is not yet well

It has been noted that wide application of ZnO in electronic devices has been limited by the lack of inadequate control over its electrical conductivity [10, 19]. Controlling the conductivity in ZnO can be achieved by means of band-gap engineering [10]. Introducing small concentrations of native point defects and

electrical, structural, optical and morphological properties of the semiconductors [14, 18]. Therefore, understanding the role of native point defects (i.e. vacancies, interstitials and antisites) and the incorporation of impurities (doping) is the key towards controlling the conductivity in ZnO, which in effect alters the band gap,

Band-gap engineering of ZnO can also be achieved by alloying with MgO or CdO. The band gap of ZnO is increased with the addition of Mg, whereas the addition of Cd decreases the band gap, which is similar to the effects of Al and In in GaN [1, 18]. It is well known that MgO and CdO crystallize in the rock salt structure;

or 0.01 ppm) can significantly affect the

*DOI: http://dx.doi.org/10.5772/intechopen.86254*

*Doped Zinc Oxide Nanostructures for Photovoltaic Solar Cells Application DOI: http://dx.doi.org/10.5772/intechopen.86254*

**Figure 2.**

*Zinc Oxide Based Nano Materials and Devices*

*Hexagonal Wurtzite crystal structure of ZnO [1].*

such as excitonic emission at or even above room temperature, optical transparency in the visible range, high surface-to-volume ratio and quantum confinement effect [6], amongst others, which have motivated intensive study of the semiconductor during the last two decades. ZnO is mostly known to crystalize as an n-type semi-

ZnO is simple to synthesize; both chemical and physical techniques are used to produce excellent epitaxial films. The most commonly used techniques to grow epitaxial films of ZnO include electrodeposition, spray pyrolysis, sol–gel process, successive ionic layer adsorption and reaction (SILAR), RF sputtering, chemical bath deposition (CBD), spin coating, electron beam epitaxy, laser evaporation and ion beam sputtering, amongst others [7, 8]. **Figure 2** illustrates the various synthetic techniques (chemical as well as physical) that are generally used to grow compound and alloys of ZnO. The choice of a particular technique would be guided by some factors such as the application intended for the synthesis, effectiveness of the technique and cost implication [10, 11]. ZnO has been identified as one of the semiconductors with the largest number of novel nanostructures such as nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires, nanorods, nanotubes, nanocages, etc., with a wide range of technological applications [12–15]. Novel applications of ZnO nanostructures include optical modulator waveguide, photonic crystals, surface acoustic wave filters, varistors, photodetectors, gas sensors, light-

Photovoltaic (PV) application of ZnO nanostructures requires large internal surface area with porous and high surface roughness to support good penetration of electrolyte [13, 14]. Chemical techniques are very simple, much reliable and cost-effective for the synthesis of high-quality electrodes for PV application. Most especially, chemical bath deposition technique is very suitable for growing large area films of ZnO with fascinating properties for photoelectrochemical solar cells [15, 16]. This technique is suitable for growing ZnO nanostructures on many

In several applications such as optoelectronics, ZnO can be used as a complement or alternative to some semiconductors such as GaN, and many researches are ongoing globally to further improve the properties of the semiconductor [10]. Trying to control the unintentional n-type conductivity and to achieve p-type

conductor, whereas synthesis of the p-type is not generally easy [1, 7].

emitting diode, photodiodes and solar cells, amongst others [12].

substrates including microscope glass and stainless steel [6].

**58**

**Figure 1.**

*Broad classification of thin film deposition techniques [9].*

conductivity are such famous research themes. Other approaches such as firstprinciples calculations based on density functional theory (DFT) are theoretical and, however, are useful to provide in-depth understanding of the role of native point defects and impurities on the unintentional n-type conductivity in ZnO [14, 17]. Acceptor doping in ZnO which will lead to stable p-type is not yet well known [18].

It has been noted that wide application of ZnO in electronic devices has been limited by the lack of inadequate control over its electrical conductivity [10, 19]. Controlling the conductivity in ZnO can be achieved by means of band-gap engineering [10]. Introducing small concentrations of native point defects and impurities (dopants) (down to 10–14 cm<sup>−</sup><sup>3</sup> or 0.01 ppm) can significantly affect the electrical, structural, optical and morphological properties of the semiconductors [14, 18]. Therefore, understanding the role of native point defects (i.e. vacancies, interstitials and antisites) and the incorporation of impurities (doping) is the key towards controlling the conductivity in ZnO, which in effect alters the band gap, thus enhancing its performance [1].

Band-gap engineering of ZnO can also be achieved by alloying with MgO or CdO. The band gap of ZnO is increased with the addition of Mg, whereas the addition of Cd decreases the band gap, which is similar to the effects of Al and In in GaN [1, 18]. It is well known that MgO and CdO crystallize in the rock salt structure;

however, alloys of Mg1 <sup>−</sup> *x*Zn*x*O and Cd1 <sup>−</sup> *x*Zn*x*O with moderate concentrations will assume the wurtzite structure of the parent compound with significant band-gap variation [1]. This chapter will carry out a detailed review of the doping effects of Cu, Al and In on ZnO and the influence of the doped electrodes on the PEC solar cell performance.

## **2. Doping as a technique for engineering structural, optical and morphological properties of ZnO**

Doping implies the deliberate inclusion of impurities into the crystal structure of a semiconductor in order to improve its conductivity and modify some of its characteristics [19]. For elemental semiconductors such as silicon and germanium, the commonly used dopants include boron, aluminum and indium (trivalent elements) and phosphorus, arsenic and antimony (pentavalent element) [20]. In the process of doping, the dopant is integrated into the lattice structure of the semiconductor crystal. The number of valence electrons of the dopant defines the type of doping that would be achieved [20]. Doping a semiconductor with a trivalent element results into p-type doping, whereas using a pentavalent element produces an n-type doping as illustrated in **Figure 3**. For an n-doping, electrons are the majority charge carriers, while holes are the majority carriers in p-doping. The conductivity of a silicon crystal which is properly doped can be increased by a factor of 106 [1].

Compound semiconductors such as ZnO can also be doped with the same or similar dopants like copper and indium. There are reports in the literature on the modification of structural, morphological and optical properties of ZnO by doping with Al, Cu or In. Cu and In dopants have been confirmed to lower the band gap of ZnO appreciably [10, 18].

Doped semiconductors are electrically neutral. The terms n- and p-type doped do only refer to the majority charge carriers. Each positive or negative charge carrier belongs to a fixed negative or positive charged dopant as illustrated in **Figure 4**.

**61**

*Doped Zinc Oxide Nanostructures for Photovoltaic Solar Cells Application*

**3. Effects of Cu dopant on ZnO and the PEC solar cell performance of** 

*Doped semiconductor showing energy levels of (a) n-type doping (b) p-type doping [20].*

Wide band-gap semiconductors such as ZnO and TiO2 (3.3 eV) are suitable for many semiconductor applications such as PEC solar cells due to their thermal, photo- and electrochemical stability and resistance against atmospheric corrosion [21]. However, the wide band gap in such semiconductors is a drawback on their light absorption capability because only photons below a threshold wavelength λg can be absorbed, since the solar spectrum has its maximum intensity at about 2.7 eV [19]. Previous investigations have confirmed that band gap in ZnO semiconductor can be controlled by doping with appropriate dopants [10]. This can also modify optical and structural properties of the semiconductor to meet pre-desired

Transitional metals are good dopants; however, Cu and Al are prevalently studied as dopants for ZnO [18]. Cu is a highly conducting metal with conductivity higher than that of Al and can enhance green luminescence band through creation of localized states in the band gap of ZnO. It is also known that due to the high ionization energy and low formation energy of Cu, it can rapidly substitute Zn in

Tyona et al. [10] investigated the effect of Cu doping on optoelectronic properties of chemically synthesized ZnO electrodes. These properties of Cu-doped ZnO nanostructures were influenced by various parameters such as growth conditions, Cu concentration and post-growth annealing. Cu concentration in ZnO was varied in the range of 1–5%. This quantity may be small; however, it produces significant physical changes in ZnO, and it is considered to be within a strict doping range of up to 10%. Beyond this range, such a reaction may be turning towards composite

Their experimental procedures showed that Zn(NO3)2.6H2O (SD Fine Chemicals) was used as the source of Zn2+ and CuCl2.2H2O (Chemco Fine, India) as the source of Cu2+, and NH3 solution (28%) (Thomas Baker) was the complexing agent. An aqueous solution of 0.1 M Zn(NO3)2.6H2O was prepared, and cupric chloride dihydrate (CuCl2.2H2O) was added. Aqueous NH3 solution (28%) was used as the complexing agent. The solution was maintained at a pH ≈ 11.5. Microscopic

*DOI: http://dx.doi.org/10.5772/intechopen.86254*

**Cu-doped ZnO (CZO) electrodes**

applications [19].

**Figure 4.**

ZnO lattice [22].

growth or alloys [10].

**Figure 3.** *(a) N-doping with phosphorus and (b) p-doping with boron [2].*

*Doped Zinc Oxide Nanostructures for Photovoltaic Solar Cells Application DOI: http://dx.doi.org/10.5772/intechopen.86254*

*Zinc Oxide Based Nano Materials and Devices*

**morphological properties of ZnO**

cell performance.

factor of 106

**Figure 4**.

[1].

ZnO appreciably [10, 18].

however, alloys of Mg1 <sup>−</sup> *x*Zn*x*O and Cd1 <sup>−</sup> *x*Zn*x*O with moderate concentrations will assume the wurtzite structure of the parent compound with significant band-gap variation [1]. This chapter will carry out a detailed review of the doping effects of Cu, Al and In on ZnO and the influence of the doped electrodes on the PEC solar

Doping implies the deliberate inclusion of impurities into the crystal structure of a semiconductor in order to improve its conductivity and modify some of its characteristics [19]. For elemental semiconductors such as silicon and germanium, the commonly used dopants include boron, aluminum and indium (trivalent elements) and phosphorus, arsenic and antimony (pentavalent element) [20]. In the process of doping, the dopant is integrated into the lattice structure of the semiconductor crystal. The number of valence electrons of the dopant defines the type of doping that would be achieved [20]. Doping a semiconductor with a trivalent element results into p-type doping, whereas using a pentavalent element produces an n-type doping as illustrated in **Figure 3**. For an n-doping, electrons are the majority charge carriers, while holes are the majority carriers in p-doping. The conductivity of a silicon crystal which is properly doped can be increased by a

Compound semiconductors such as ZnO can also be doped with the same or similar dopants like copper and indium. There are reports in the literature on the modification of structural, morphological and optical properties of ZnO by doping with Al, Cu or In. Cu and In dopants have been confirmed to lower the band gap of

Doped semiconductors are electrically neutral. The terms n- and p-type doped

do only refer to the majority charge carriers. Each positive or negative charge carrier belongs to a fixed negative or positive charged dopant as illustrated in

**2. Doping as a technique for engineering structural, optical and** 

**60**

**Figure 3.**

*(a) N-doping with phosphorus and (b) p-doping with boron [2].*

**Figure 4.** *Doped semiconductor showing energy levels of (a) n-type doping (b) p-type doping [20].*
