Contents

Cover

Title page

Copyright page

Dedication

Foreword

Preface

Chapter 1: Our Universe and the Sun

1.1 Formation of the Universe
1.2 Formation of Stars
1.3 Summary
Reference

Chapter 2: Solar Energy and Its Applications

2.1 Introduction to a Semiconductor
2.2 Formation of a Compound
2.3 Quantum Theory Approach to Explain the Effect of Doping
2.4 Types of Carriers in a Semiconductor
2.5 Nature of Band Gaps in Semiconductors
2.6 Can the Band Gap of a Semiconductor Be Changed?
2.7 Summary
Further Reading

Chapter 3: Theory of Junction Formation

3.1 Flow of Carriers across the Junction
3.2 Representing Energy Levels Graphically
3.3 Depth of Charge Separation at the Interface of n- and p-Type Semiconductors
3.4 Nature of Potential at the Interface
3.5 Expression for Saturation (or Exchange) Current I₀
3.6 Contact Potential θ
3.7 Width of the Space Charge Region
3.8 Metal–Schottky Junction
3.9 Effect of Light on p:n Junctions
3.10 Factors to Be Considered in Illuminating the p:n Junction
3.11 Types of p:n Junctions
3.12 A Photoelectrochemical Cell
3.13 Summary
Further Reading

Chapter 4: Effect of Illumination of a PEC Cell

4.1 Effect of Light on the Depletion Layer of the Semiconductor—Electrolyte Junction
4.2 The Fate of Photogenerated Carriers
4.3 Magnitude of the Photocurrent
4.4 Gartner Model for Photocurrent
4.5 Carrier Recombination
4.6 A Mathematical Treatment for the Lifetime of Carriers
4.7 Effect of Illumination on Fermi Level-Quasi Fermi Level
4.8 Solar Cell Performance
4.9 Current—Voltage Characteristics of a Solar Cell
4.10 The Equivalent Circuit of a Solar Cell
4.11 Solar Cell Efficiency
4.12 Ohmic Contact
4.13 Defects in Solids
4.14 Summary
Further Reading
References

Chapter 5: Electrochemistry of the Metal–Electrolyte Interface

5.1 What Is a Metal?
5.2 What Is the Structure of Electrolyte and Water Molecules in an Aqueous Solution?
5.3 What Happens When a Metal Is Immersed in Solution?
5.4 Existence of a Double Layer Near the Metal–Electrolyte Interface
5.5 Influence of Concentration of Electrolyte on Helmholtz and Diffusion Potentials
5.6 Impact of Charge Accumulation at Various Regions
5.7 Electron Transfer and Its Impact on Potential Barrier
5.8 Butler–Volmer Approach to Electrochemical Reaction
5.9 Significance of Symmetry Factor β
5.10 Electrochemical Corrosion at the Metal–Electrolyte Interface
5.11 Summary
Further Reading
References

Chapter 6: Electrochemistry of the Semiconductor–Electrolyte Interface

6.1 Difference between Metal and Semiconductor
6.2 Gaussian Distribution of the Potential Energy of Electrolytes
6.3 Capacitance at the Semiconductor–Electrolyte Interface
6.4 Stability of the Semiconductor
6.5 Modifying the Surface of Low Band Gap Materials
6.6 Summary
References

Chapter 7: Impedance Studies

7.1 Types of AC Circuits
7.2 Significance of Vector Analysis
7.3 Impedance Measurement Techniques
7.4 AC Impedance Plots and Data Analysis
7.5 Equivalent Circuit Representation of a Simple System
7.6 Equivalent Circuit Representation for Electro-chemical Systems
7.7 Procedure for Running an Experiment
7.8 Semiconductor Interface
7.9 Summary
Further Reading
References

Chapter 8: Photoelectrochemical Solar Cell

8.1 Classification of Photoelectrochemical Cells Based on the Energetics of the Reactions
8.2 Solar Chargeable Battery
8.3 Electrolyte-(Ohmic)-Semiconductor-Electrolyte (Schottky) Junction
8.4 Synthesis of Value-Added Products
8.5 Summary
References

Chapter 9: Photoelectrochromism

9.1 Photochromic Glasses
9.2 Electrochromism
9.3 Electrochromic Devices and Their Applications
9.4 Imaging Employing a Semiconductor Photo-electrode
9.5 Summary
References

Chapter 10: Dye-Sensitized Solar Cells

10.1 The Dye-Sensitized Cell
10.2 Flexible Polymer Solar Cell
10.3 Summary
References

Index

End User License Agreement

List of Illustrations

Chapter 1

Figure 1.1 Variation of binding energy per nucleon as a function of mass number.
Figure 1.2 A schematic of the various regions of the sun.
Figure 1.3 A schematic spectrum of solar radiation reaching the surface of the earth approximate.
Figure 1.4 A schematic showing the various angles and positions of the sun vis-à-vis the earth’s orbit.
Figure 1.5 A schematic for calculating the true north of the earth.

Chapter 2

Figure 2.1 Schematic showing the splitting of energy levels when 1–1, 2–2, or N-N numbers of electrons interact, forming two types of energy levels, or bands.
Figure 2.2 A schematic showing the relationship between the wave number k and full period λ.
Figure 2.3 A, Schematic graph showing the variation of P(sin αa/αa)+cos αa versus αa. B, Schematic graph showing discontinuity in the graph P(sin αa/αa) + cos αa versus αa.
Figure 2.4 A schematic showing the variation in the forbidden energy gap with different energies versus ka axis.
Figure 2.5 A schematic showing the limits of valence and conduction bands for integration purposes.
Figure 2.6 Schematic showing the lattice arrangement of pure and doped Ge crystal lattices.
Figure 2.7 A schematic showing the band gap and the Fermi level for an intrinsic material.
Figure 2.8 A schematic showing the circular movement of electrons of doped phosphorous atoms in a Si lattice.
Figure 2.9 A schematic showing the various energy levels present in an extrinsic semiconductor.
Figure 2.10 A schematic showing the direction of electron and hole movements under unbiased conditions.
Figure 2.11 A schematic showing the direction of flow of majority carriers when n-type material is brought in contact with p-type material.
Figure 2.12 A schematic of a semiconductor with, A, direct band gap, where maxima and minima of Ev and Ec fall at the origin of axis, and, B, indirect band gap, where these two positions are situated differently.
Figure 2.13 Schematic band positions of two semiconductors (Sem-1 and Sem-2). Their band positions are such that the difference between the valence band of Sem-1 (i.e., 1Ev) and the conduction band of Sem-2 (i.e., 2Ec) is 1.4 eV. Values on the y-scale are negative and are not drawn as per a scale; they are shown only to give an idea of the band positions.

Chapter 3

Figure 3.1 A schematic showing the formation of band bending when p- and n-type semiconductors are joined together. For sake of convenience, in this diagram no changes in bands pEc and pEv are shown from its original values. Their changes and that of nEc and nEv are added in n-semiconductor as *nEc and n*Ev. nEF_equ or pEF_equ is the equilibrium Fermi level after the junction is formed. Band bending is shown with respect to a p-type semiconductor. Vacuum level is shown. Change in energy and concentration variation of different carriers in a semiconductor are also shown. A, width of space charge (x ₌₀ – w_p) in p-type and (x _{= 0} – w_n) in n-type and, C, width of diffusion regions (w_n – L_n) in n-type, and (w_p – L_p) in p-type are also shown. B, two charges accumulated at the interface and separated by a distance of one atom are also shown in the figure. Negative sign represent negative direction of x-axis.
Figure 3.2 A schematic showing the flow of majority and minority carriers across the n- and p-type semiconductors when p- and n-type semiconductors are joined. Pp_o and Nn_o are the equilibrium concentrations of holes in p-type and electrons in n-type, respectively, eg and hg are generating current due to electrons and holes, respectively, em and hm are majority carriers of electrons and holes, respectively, and θ is the contact potential.
Figure 3.3 Schematic of a p:n junction when p-type is connected with the positive terminal of an external battery. V₀ is the applied potential. Other nomenclature is the same as in earlier figures.
Figure 3.4 A schematic showing the position of energy levels when the p:n junction is under a reverse bias. Nomenclature is as in earlier figures.
Figure 3.5 A, A schematic showing variation in I-V of a p:n junction. Io is the saturation current measured under negative potential axis. B, A plot of ln I versus potential. Intercept gives the magnitude of In Io, and the slope gives the value of ideality factor n if temperature T and Boltzmann constant K are known.
Figure 3.6 A schematic showing the concentration gradient established under the forward bias of a p:n junction. A(a), Movement of minority carrier into p- and n-type semiconductor from either side of the interface. B(b), Same movement of the same carrier from the the interface to the bulk. C(c), Variation of concentration of minority carrier in the reverse direction, that is, toward the depletion region from the bulk. Symbols have similar significance as given in previous figures.
Figure 3.7 A schematic showing metal–Schottky junctions with p- and n-type semi-conductors and a metal. A, No junction is formed when the work function of metal and that of semiconductor matches. B, Junction is formed with an n-type semiconductor when its Fermi level is less negative than the work function of the metal. C, Junction is formed when a p-type semiconductor’s Fermi level is more negative than the work function of metal.
Figure 3.8 A schematic showing the distribution of carriers in metal–Schottky junctions.
Figure 3.9 A schematic of a p:n junction cell under the illuminated condition. Each junction is illuminated, and the direction of flow of carriers is shown by the arrow.
Figure 3.10 A schematic showing the design of a metal grid normally printed over the illuminated side of the p:n junction. (A) shows the direction of movement of carriers in p:n junction where n-type is illuminated with light and (B,C) shows two types of grids.
Figure 3.11 A schematic showing the construction of a typical photoelectrochemical photovoltaic cell.

Chapter 4

Figure 4.1 Graph showing the shift in current-voltage characteristics from the dark condition to the illuminated condition. The shift in potential is the magnitude of the photopotential.
Figure 4.2 A, Schematic of the n-semiconductor–electrolyte junction showing the photoelectrochemical reactions of a semiconductor with electrolyte occurring only at the interface. B, p-type semiconductor in contact with redox electrolyte showing reduction of electrolyte at the surface. C, The same system as in B, where a photogenerated electron is able to reduce the semiconductor. C⁺ⁿ/C^+(n–1) is the redox electrolyte.
Figure 4.3 Three typical plots of – ln(1 – ϕ) versus (V)^1/2. The numbers on the axis are arbitrary units. To show the effect of the wavelength of light on the flat band potential, three wavelengths of light are shown.
Figure 4.4 A, Typical transmittance, absorption, and reflection of spectra obtained with a silicon wafer. B, By applying Equation 4.19, variation in absorption coefficient α with wavelength of light calculated using results shown in Figure 4.4A and shown in Figure 4.4B. The sharp increase in α at 1.1 eV gives the value of the band gap of silicon.
Figure 4.5 A typical graph showing linear plots for both values of n (i.e., for n = 1 and 4). The graph is plotted between [– ln(1 – ϕ)]^n/2 versus hv for a semiconductor–electrolyte junction under a constant forward bias. In this figure, the semiconductor seems to possess both a direct band gap of larger magnitude and an indirect band gap of smaller magnitude.
Figure 4.6 Schematic explaining the existence of the surface states. Carbon atoms present at the surface are not electrically satisfied as much as the carbon atoms present below the first layer of carbon. Surface carbon thus can be charged either positively or negatively, depending on whether it lost electrons or gained electrons.
Figure 4.7 Schematic showing the two possible ways to represent the Fermi levels (X and Y) when a junction is formed with metal and n-semiconductor or p-semiconductor. Both suggest that the difference between the E_c and E_f should be the same at the interface as in the bulk. If this happens, then there is a difference between the Fermi level of metal and that of the semiconductor by an amount δF.
Figure 4.8 A schematic showing the variation in Fermi levels for majority carriers and minority carriers in a semiconductor.
Figure 4.9 Typical current–voltage characteristics of a solar cell. A, Equivalent electrical circuit of the cell. I_L is photon flux incident on the cell. I_D(V) is the dark current flowing through the diode. R_sh is shunt resistance. R_s is the series resistance. I is photocurrent, and V is the photopotential passing through the load. B, Under illuminated condition. C, Under dark condition. D, Current–voltage characteristics represented in the first quadrant of the axis with ideal condition represented by dotted lines and actual drawable current/potential under a load represented by the shaded square. I_sc and V_oc are short-circuit current and open circuit potential, respectively. I_max and V_max are maximum current and maximum potential that can be obtained from this cell, and P_max is maximum power.
Figure 4.10 C-V of a typical solar cell showing the variation in current versus potential of the cell. The dotted line shows when the measurement is done at equilibrium condition, and the solid line shows when measurement is done without achieving equilibrium. This latter shows a hysteresis in J-V with two different V_oc.
Figure 4.11 Current–voltage characteristics of a solar cell. (A) Effect of shunt resistance on the J-V. (B) Effect of series resistance on the J-V.
Figure 4.12 A schematic of theoretical maximum efficiency achievable under ideal condition versus the band gap of the material.
Figure 4.13 A sketch of a Sharon ohmic Schottky solar cell. A, n-Fe₂O₃ deposited over metal iron dipped in an electrolyte. Band bending is shown to illustrate the formation of a junction with the electrolyte. Iron metal acts like an ohmic contact. When light falls on the iron oxide surface, photogenerated holes oxidize the electrolyte, while photogenerated electrons move via a load to the counter metal electrode to reduce the electrolyte that was oxidized at the surface of the n-type semiconductor. In this way, a full cell is formed. B, Two cells of a type as described in A are joined such that each one acts like a full cell. Illumination is done from the top onto each electrode. Photogenerated holes move in one direction and electrons move in the opposite direction. Electrons move through the load to the metallic counter electrode and reduce the oxidized electrolyte. This is an example of metal acting as an ohmic contact while the semiconductor forms a Schottky-type junction with electrolyte.

Chapter 5

Figure 5.1 A, The structure of hydrated cations containing various layers of water molecules shows a single water molecule with an angle between two hydrogen atoms of 104.45°. B, Single water molecule behaving like a dipole. C, Two-dimensional structure of water molecules. D, Formation of a water molecule attached with a cation such that the dipole is directed toward the cation. E, Formation of a second layer of water molecules surrounding the first layer of water molecules with their dipoles randomly arranged.
Figure 5.2 A schematic of the double layer. A, Helmholtz model. B, Gouy-Chapman point charge model. C, Stern model for finite distribution. D, General description including all three conditions. E, Potential energy distribution for Helmholtz region and diffusion region.
Figure 5.3 A typical representation of double layer capacity profiles’ dependence on the concentration of electrolyte with applied potential to a Pt electrolyte system with respect to a reference electrode.
Figure 5.4 A schematic showing the symmetrical and unsymmetrical barriers formed at the interface of metal and electrolyte.
Figure 5.5 A schematic showing the position of the electrode assembly to measure the potential by a digital voltmeter of high impedance value.
Figure 5.6 A schematic showing, A, how the electron transfer reactions occur between two types of redox reactions in solution and, B, how the same reaction, if it were separated by a membrane, could generate electrical power.
Figure 5.7 A schematic showing the hydrogen, calomel, and vacuum scales.
Figure 5.8 A, A schematic showing the arrangement of an electrochemical cell. B, Sketch of the formation of a double layer at both electrodes. C, A typical current–potential curve obtained for a single type of electrolyte.
Figure 5.9 A schematic showing, A, the potential barrier formation between the metals surface and the Helmholtz plane and, B, current-potential effect on electronation current.
Figure 5.10 A current-voltage relationship for conditions, A, when the barrier is asymmetrical in nature and, B, when the barrier is symmetrical.
Figure 5.11 X, A plot of potential energy with the distance between atoms leaving the metal electrode and arriving at the Helmholtz plane and, Y, a similar plot when the potential difference across the interface is changed from its equilibrium potential to a condition when some overpotential is applied, causing a shift in the initial or final states energy level. The potential is drawn linearly because it is occurring within a distance equivalent to the diameter of a water molecule.
Figure 5.12 X, A typical current–voltage graph for anodic and cathodic reactions. The cathodic current (R) increases from a less negative value to a more negative value, whereas the anodic current increases from a less positive value to a more positive quantity (A). The cathodic graph is plotted on the negative potential axis and the anodic current is plotted on the positive potential axis. When the current (both anodic and cathodic) becomes almost independent of potential, a tangent is drawn, as shown by a dotted line. The point at which anodic and cathodic current becomes almost the same (K) is considered as the corrosion potential. A point of intersection of anodic current with a mirror image of cathodic current (R’) is considered as corrosion current (L) (Figure 5.12Y). A point of intersection of plots of log (current) for both anodic and cathodic current versus potential (by showing both in the positive quadrant of the axis) is also used for calculating corrosion current (L) and corrosion potential (K).
Figure 5.13 A typical current–potential graph to show the effect of potential drop (IR) in the solution on the determination of corrosion current and potential. If IR drop is not corrected, the graphically obtained Icorr would have an incorrect value; that is, it would appear wrongly as Icor max. Under such conditions, corrosion cathodic potential (δϕr) would be different from anodic corrosion potential (δϕ_o,).

Chapter 6

Figure 6.1 A schematic showing the Gaussian distribution of the energy levels around a characteristic mean value for each component of a redox system of energy levels for the oxidized levels and the reduced levels of a redox electrolyte. D_ox(E) and D_red(E) are the distribution functions for the oxidized and reduced levels. E⁰_ox and E⁰_red are the maximum values of energy for the oxidation and reduction processes, respectively. E_Fredox and λ are the standard redox potential and reorientation energy, respectively.
Figure 6.2 A schematic of possible overlaps between energy levels of redox electrolytes and the conduction and valence bands of a semiconductor with redox electrolyte. (a), Overlap of E_red with E_c. (b) Overlap of E_red with both E_c and E_v. (c) Overlap of E_red with E_v. (d) Overlap of E_ox with E_c. (e) Overlap of E_ox with E_v. (f) Overlap of E_ox with both E_c and E_v of the semiconductor.
Figure 6.3 A schematic of semiconductor and redox electrolyte to show how some specific reactions can be performed only with some specific semiconductor.
Figure 6.4 A schematic showing the variation of capacitance versus the applied potential to a semiconductor–electrolyte interface. It shows the presence of two parabolas.
Figure 6.5 A graph showing the variation of (1/C_SC)² versus the applied potential to get two linear plots: depletion region and shallow region.
Figure 6.6 A graph showing the variation of (1/C_SC)² versus applied potential for n-type and p-type semiconductors.
Figure 6.7 A schematic showing the decomposition potential and bands of the semiconductor for four different conditions. (a) and (d) Stable anodically and cathodically. (b) and (e) Unstable anodically and cathodically. (c) Unstable cathodically but stable anodically. (f) Unstable anodically and stable cathodically. nE_{decomposition} refers to cathodic decomposition potential and pE_{decomposition} refers to anodic decomposition potential, E_c, E_v, E_F, nE_F*, and pE_F* are conduction band, valence band, majority Fermi levels, minority quasi Fermi level for electron, and minority quasi Fermi level for hole, respectively.
Figure 6.8 Same as Figure 6.7, except that the Fermi levels are instead shown as horizontal dotted lines, and two Fermi levels are shown separately. Because Fermi level depends on the carrier concentration, its position from the conduction band (for n-type) or from the valence band (for p-type) should remain the same throughout the material, from the bulk to the interface. This behavior is shown by a dotted line. The second Fermi level position depends on the concentration of photogenerated carrier. This concentration would be different at different places in the semiconductor, with a maximum where the photogenerated carrier would be highest. This Fermi level is denoted as the quasi Fermi level. Like Figure 6.7, the various positions of decomposing levels and bands of the semiconductor for four different conditions are also shown. (a) and (d) Stable anodically and cathodically. (b) and (e) Both stable cathodically and unstable anodically. (c) and (f) Stable cathodically but unstable anodically. nE_{decomposition} refers to cathodic decomposition potential and pE_{decomposition} refers to anodic decomposition potential. E_c, E_v, E_F, nE_F*, and pE_F* are conduction band, valence band, majority Fermi level, minority quasi Fermi level for electron, and minority quasi Fermi level for hole, respectively.
Figure 6.9 Schematic of the direct binding of ligands to the crystal edge sites in WSe₂. A, Diphose molecule of bis(1,2-diphenylphospinol)ethane. B, Phenyl isocyanide (C) chelating with dimethyldithiocarbamate.
Figure 6.10 Schematic of the interaction of t-butyl pyridine with edges of a WSe₂ crystal, which acts like an umbrella to protect the tungsten atom.

Chapter 7

Figure 7.1 Variation in the shape of output current with respect to input current, depending on the magnitude of resistance and inductance/capacitance used in the circuit. A, Square pulse with its characteristics. B, Square pulse height reduced at the output due to the resistance. C, Fall time of output square pulse decreased due to capacitance. D, Output pulse that has completely lost its square pulse nature due to large capacitance. E, Output sin wave pulse that has shifted its peak position due to the presence of capacitance.
Figure 7.2 A, When a sine wave enters a resistor, the output signal is also a sine wave with its higher amplitude. There is no phase shift. B, When a sine wave enters a circuit containing a capacitor and a resistor (inductor), its amplitude is increased, and there is a shift in its phase.
Figure 7.3 A, Projection of sine wave in the form of a circle. B, Magnitude and phase shift of a sine wave.
Figure 7.4 (a) Explains the wave behavior. If magnitude of a wave (i.e., wave height) is measured by keeping the reference point always fixed at a convenient time, the alternation of pulse height would follow a circle with the center being the reference time. At the x-axis, the nodes of pulse appear repeatedly at 0, π, and maxima would appear repeatedly at π/2, i.e., π/2, 3π/2. The radius of this circle is equal to λ/4, where λ is the wavelength. (b) A sinusoidal wave showing the variation in potential (E) and current (I) with nodes at 0, π, 2π, 3π, …, and maxima at π/2, 3π/2,….
Figure 7.5 A schematic of an audio frequency bridge. Here X and Y are the amplifier, and DVM is digital voltmeter. V_A is output voltage of input voltage V_in. R₁ and R₂ are standard resistances. Z_x is a sample whose impedance is to be determined. V_B is voltage coming out of the Wheatstone bridge, which gives a signal V_out through the amplifier. Filter and amplifier are needed to measure the signal after filtering out noise and amplifying the signal. Z_s is standard variable resistance. One gets the real part of impedance (Z¹_x), and while it is standard variable capacitance, one gets the imaginary part of the impedance (Z_x¹¹).
Figure 7.6 Schematic diagram of the transformer ratio arms bridge circuit.
Figure 7.7 A schematic circuit diagram for a Berberian-Cole bridge technique.
Figure 7.8 A typical circuit used for the potentiostatic method to measure the impedance of the cell.
Figure 7.9 A typical circuit for measuring impedance by the oscilloscope technique.
Figure 7.10 A schematic showing that the nature of potential |eR| is the peak-to-peak output voltage across the cell with reference to reference electrode, |e| is the peak-to-peak output voltage across the cell with reference to working electode, φ is the phase shift between two sinusoidal waves, and b is λ/2 of a sinusoidal wave.
Figure 7.11 Nyquist plot (Cole–Cole plot). The imaginary component of impedance (Z¹¹) is plotted against the real component of the impedance (Z¹) for each excitation frequency (e. g., f_n, f_n–1, f_n–2, …, f₃, f₂f). For high frequency, Z¹¹ → 0, Z¹ → R, and for low frequency, Z¹¹ → 0, Z¹→R_e+R_p.
Figure 7.12 (a) Circuit diagram used for a Bode plot and the nature of various types of plots obtained from the measurements: (b) Plot of imaginary impedance Z¹¹ versus real impedance Z¹; (c) plot of log |Z| versus log(ɷ); (d) plot of θ versus log(ɷ); and (e) plot of |Z| versus log(ɷ) to calculate the value of C_dl, R_p + R_e, and the frequency at which phase shift of the response is maximum. R_s or R_e is uncompensated resistance, R_p is polarization resistance, and C_dl is the double-layer capacitance.
Figure 7.13 (a) Nature of Nyquist plot and, (b) its equivalent circuit. (c) Idealized Randles plot of Z¹ versus ω^1/2.
Figure 7.14 Corresponding variation in real and imaginary parts of the signal for different frequencies when a sinusoidal wave is applied to a circuit. (a) When resistance and capacitance are in series. No dispersion of frequency, i.e., all lying in one plane. (b) When resistance and capacitance are in parallel. Frequency dispersion occurs, i.e., each frequency shows a different ω value.
Figure 7.15 A self-explained typical arrangement for impedance spectroscopy.
Figure 7.16 A typical Mott–Schottky plot of 1/C² versus potential of the working electrode measured with respect to a reference electrode.
Figure 7.17 Result shown in Table 7.1 was used to explain these types of plots. (a) Nyquist plot (sometimes known as a Cole–Cole plot) of the imaginary components (Z¹¹) against the real component of the impedance (Z¹). (b) Bode plot impedance modulus log of |Z| versus log of frequency and, (c) the equivalent circuit diagram.

Chapter 8

Figure 8.1 A description of a photoelectrochemical solar cell comprising a p-type semiconductor. The figure also explains the energetic condition of a p-semiconductor as an anode and Pt as a counter electrode.
Figure 8.2 A description of a PEC solar cell utilizing both a p-type semiconductor (as a cathode) and an n-type semiconductor (as an anode). The figure also indicates the energy levels of the two semiconductors.
Figure 8.3 Schematic of semiconductor electrolyte junctions for three types of electrochemical photocells: A, when ΔE > 0, B, when ΔE = 0, and C, when ΔE < 0.
Figure 8.4 Solar chargeable battery (Saur Viddyut Kosh) utilizing a p-type semiconductor.
Figure 8.5 A schematic of a Saur Vidduyt Kosh (solar chargeable battery) using a three-electrode system with a p-type semiconductor.
Figure 8.6 A schematic of a solar chargeable battery (Saur Vyddut Kosh) with a four-electrode system using n-type and p-type semiconductors.
Figure 8.7 A rechargeable photoelectrochemical storage battery. Compartment I contains a Ce^3+/4+ redox system with pH = 1.0. K1 and K2 are keys for measuring charging current and for discharging the cell, respectively.
Figure 8.8 The variation of current versus time for charging and discharging modes of the solar chargeable battery.
Figure 8.9 A solar chargeable battery of configuration n- Cd(Se, Te)/aqueous CsHS, CsOH/Sn/SnS. P, S, and L indicate the direction of electron flow through the photoelectrode, tin electrode, and external load, respectively.
Figure 8.10 A representation of a rechargeable PEC cell based on a four-electrode system.
Figure 8.11 A, A spherical particle floating in solution such that one side of the particle is positively charged and the other side is negatively charged. The side that is illuminated shows the accumulation of minority carrier, and the other side shows majority carrier. B, A cell is constituted of platinum electrodes separated by a semicondcutor. The central plate contains a pellet of iron oxide semiconductor, and two compartments contain two metal electrodes. C, A schematic of the Sharon–Schottky type cell showing the different redox couples in each compartment of the cell.
Figure 8.12 A schematic showing the necessary requirements of energy levels of the redox couples vis-à-vis the semiconductor to be used for making a Sharon–Schottky type photoelectrochemical solar cell.
Figure 8.13 A schematic of a solar charger of configuration [C/Pb/Pb₃O₄/K₄Fe(CN)₆/K₃Fe(CN)₆, pH 9.2]n/C. Its two terminals are connected with a lead acid battery for charging. Slanting electrodes = carbon/Pb/Pb₃O₄; Red = Fe²⁺(CN)₆^4-, Ox = Fe³⁺(CN)₆^3-.
Figure 8.14 A graph showing an increase in potential of a lead acid battery while it is being charged by the solar charger.
Figure 8.15 A septum solar cell for photoelectrolysis of water.
Figure 8.16 Energy levels (conduction and valence bands) of a few semiconductors, redox potential of water decomposition, and reduction of carbon dioxide. Vacuum scale as well as NHE scales are shown for comparison.
Figure 8.17 A schematic of semiconductor-electrolyte interfaces with a p-semiconductor to illustrate the energy levels and conditions required for photoelectrolysis of water.

Chapter 9

Figure 9.1 Various possible excitation processes that can occur in a semiconductor while irradiating it with photons of energy greater than its band gap.
Figure 9.2 Decay and growth of excited levels by either luminescence or phosphorescence.
Figure 9.3 Phosphorescence of a ruby (Al₂O₃ pure), doped with 1 percent Ti and with 1 percent Eu.
Figure 9.4 A reflecting electrochromic device consisting of several layers of a transparent electrochromic system. Through ITO glass a solar cell is connected, which provides potential to the photochromic materials when light falls on the photovoltaic cell.
Figure 9.5 A schematic of the electrochromic window, indicating the different materials and layers used.
Figure 9.6 The complementary tungsten trioxide–Prussian blue electrochromic cell assembly.

Chapter 10

Figure 10.1 Description of a dye-sensitized solar cell.
Figure 10.2 Schematic of SWNT-P30T flexible solar cells.
Figure 10.3 A schematic energy diagram of a polymer-metal flexible cell utilizing P30T, a semiconductor polymer with its conduction band EC at 2.85 eV and valance band EV at 5.25 eV, doped with SWCNT of work function 4.5 eV. P30T is sandwiched between ITO glass (work function 4.7 eV) and aluminum metal (work function 4.3 eV). On illumination, exactions (e–h pairs) are formed. The electron moves to ITO whereas the hole moves to Al. When these two electrodes are short-circuited, V_oc 0.4 V is produced.

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ISBN 978-1-119-27433-9

Foreword

The relation between light-driven semiconductors and electrochemistry remained hidden to the scientific community until 1972, when Fujishima (author of this foreword) and Honda discovered the photolysis of water using a semiconductor electrode of titanium dioxide (TiO₂) without the aid of any external bias potential. Although, in its initial stages, the scientific community had not been exposed to the scenario of water photolysis on a semiconductor surface, successive reports on the photolysis of water and the photoconversion of small organic molecules using semiconductor materials led to a giant step forward toward utilization of solar to fuel conversion. In the 1970s and beyond, the sudden increase in energy consumption, despite a shortage of crude oil, and the demand for hydrogen or other renewable energy sources, such as photovoltaics and wind energy, were the focus of research around the globe. Photoelectrochemical or photoelectrocatalytic water splitting to produce hydrogen using sunlight with reliable efficiency has thus been investigated in depth for clean energy production.

Despite the huge scope of such photoelectrochemical hydrogen production, the major factors that perturb its commercial application are efficiency and effective utilization of solar energy. Steps have been made toward efficient solar energy conversion or to overcome the hindrance of water oxidation kinetics, but this research is still in its primary stages. Therefore, achieving the commercialization of solar hydrogen production is a great challenge to modern society. Although the field began more than a half-century ago, enormous studies are still ongoing. The U.S. Department of Energy announced that an efficiency of greater than 10 percent is suitable for solar hydrogen energy production. In general, the efficiency of hydrogen production using modified TiO₂ or other semiconductor photoelectrodes remains far below the benchmark for long-term stability and reproducibility. Thus, realizing the physics and electrochemistry of semiconductors is essential in designing a more efficient photoelectrochemical cell that can serve for reliable production of hydrogen fuel as well as for environmental pollutant removal in the near future.

The basic principle of water photolysis is associated with the generation of electron–hole pairs in photoanodes and photocathodes, which must take part in the water-splitting reaction. TiO₂ is one energetic material the photoexcited holes of which are strong enough to oxidize water, and photoexcited electrons can be utilized in hydrogen production. Apart from fuel conversion, TiO₂ has been recognized for its use in multifunctional applications in the environmental cleanup and biomedical fields. Over my forty years of research experience, I have seen great progress in TiO₂-based materials owing to their simplicity in demonstration and their unique characteristics of high stability, biocomptiablity, abundance in earth, and low cost.

In addition to TiO₂, other semiconducting materials, such as TaON, Fe₂O₃, and InP, have been tested for hydrogen production. Depending on the energy position of the valence band maxima and conduction band minima of the respective semiconducting material, it can be used either as a photoanode (TiO₂, Fe₂O₃, etc.) or a photocathode (Si, Cu₂O, p-GaP, etc.) in photoelectrochemical solar cells or in tandem solar cells. The efficiency of such photoelectrochemical solar cells primarily depends on the thickness of the respective material thin films, the material’s properties (energetic structure, mobility of photoexcited carriers, stability), charge carrier transporting materials, and the nature of the electrolyte used and its pH.

I have seen the interesting topics in this book explained with clear illustration and indispensable terminology. This book will guide the reader in understanding semiconductor photoelectrochemistry (band gap, electronic properties, mobility, redox potential or chemical potential, free energy change, reaction kinetics) and will foster new, innovative applications from a semiconductor photoelectrochemistry background. Importantly, this book covers both fundamental and advanced topics in photoelectrochemistry, and I believe that the content presented in this monograph will be a resource in the development of both academic and industrial research.

I congratulate the author of this book for elevating young minds by educating and training in a delightful way.

Professor Akira Fujishima
President, Tokyo University of Science, and Director,
Photocatalysis International Research Center,
Tokyo University of Science, Japan

Preface

Life has made me realize that the best way to learn is to teach. During my 30 years of teaching at IIT Bombay as a professor in chemistry to MTech energy students, especially topics such as photovoltaic solar cells and photoelectrochemical (PEC) solar cells, I realized the necessity of writing a specific book so as to expose the students to the physics and electrochemistry of semiconductors, an understanding of which is needed to develop wet-type photovoltaic solar cells.

Physics and Electrochemistry of the Semiconductor and Its Application was written with a view to helping researchers and students understand the basics of physics in developing a PEC solar cell, including the electrochemistry of metals and semiconductors, a knowledge required for understanding the basic principles of a PEC cell. While dealing with PEC cells, it is also necessary to understand some specific electronic and electrochemical methods needed to interpret the equivalent circuits of a PEC cell. Finally, it is also necessary to study the various applications of PEC cells, in addition to generating electrical power from solar rays, such as carrying out certain one-step syntheses that conventionally may comprise more than a few steps. Application of photochromism is another area needing development, in which the least effort has been made by present-day researchers. Dye-sensitized solar cells are also becoming very popular. This science has developed many highly efficient solar cells, which are also discussed.

My effort has been to highlight some specific questions that are normally in the mind of the purchaser, such as the following.

What is the theme and scope of this book? This book was written with the intention of including only relevant topics in the fundamentals of the physics of semiconductors and of electrochemistry needed for understanding the intricacy of the subject of PEC solar cells. Readers interested in carrying out either teaching or research in the electrochemistry of semiconductors will find this book extremely handy.

What prompted the undertaking of writing this book? I had been teaching MTech students and MSc students of physics and chemistry at IIT Bombay, giving them the basic concepts of semiconductors, p:n junctions, PEC solar cells, electrochemistry of semiconductors, and photochromism. Students in this course were coming from various disciplines. It was very difficult to prescribe for them a suitable textbook, not because there are no books on these topics, but because they are either too exhaustive or very elementary. Students wanted something that dealt with the topics needed in their own specific fields. Faced with such a demand, I started writing this book, and students in my class found it very useful. I was enthused by this to further compile my lecture notes into book form.

What special features are covered? Students carrying out research or engaged in teaching PEC cells or knowledge of our sun, its energy, and its distribution to the earth will find essential topics such as the physics of semiconductors, the electrochemistry of semiconductors, p:n junctions, Schottky junctions, the concept of Fermi energy, and photochromism and its industrial applications. It is very difficult to get all this information into one book, but this book covers all these topics; hence this book could be considered many books in one. This book does not need a teacher to explain the subjects. Some examples are humorous in style, which helps the student relate to the topic. Students will like such examples because they will help them to remember the intricacy of the subject.

What new approaches are taken in this book? Most of the books dealing with these topics assume some minimum knowledge from students. Because I was teaching this subject to multidisciplinary students, I had to begin by assuming that students did not have a common knowledge base. This book does not need much by the way of basic concepts, because it starts almost from the beginning. I have noticed that students of my class like this approach.

This book is suitable as a textbook for students interested in learning these special topics at various levels, undergraduate or postgraduate, such as MTech, BTech, or MSc (chemistry and/or physics), and also for teachers from colleges, universities, and IITs. It will also be suitable for R&D industries dealing with electrochemistry, the physics of semiconductors, the electrochemistry of semiconductors/metals, photovoltaic solar cells, and photchromism and its applications.

Last, I would request readers to point out any misconceptions that either need more explanation or should have been corrected in some other suitable form or any mistakes which should be corrected in the next edition. This information will help me to improve the quality of the book in its second edition.

Maheshwar Sharon
Retd Prof IIT Bombay, India
sharonmaheshwar@gmail.com
sharon@iitb.ac.in
July 2016