Rational Design of Solar Cells for Efficient Solar Energy Conversion E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 682

Veröffentlichungsjahr: 2018

Beschreibung

An interdisciplinary guide to the newest solar cell technology for efficient renewable energy Rational Design of Solar Cells for Efficient Solar Energy Conversion explores the development of the most recent solar technology and materials used to manufacture solar cells in order to achieve higher solar energy conversion efficiency. The text offers an interdisciplinary approach and combines information on dye-sensitized solar cells, organic solar cells, polymer solar cells, perovskite solar cells, and quantum dot solar cells. The text contains contributions from noted experts in the fields of chemistry, physics, materials science, and engineering. The authors review the development of components such as photoanodes, sensitizers, electrolytes, and photocathodes for high performance dye-sensitized solar cells. In addition, the text puts the focus on the design of material assemblies to achieve higher solar energy conversion. This important resource: * Offers a comprehensive review of recent developments in solar cell technology * Includes information on a variety of solar cell materials and devices, focusing on dye-sensitized solar cells * Contains a thorough approach beginning with the fundamental material characterization and concluding with real-world device application. * Presents content from researchers in multiple fields of study such as physicists, engineers, and material scientists Written for researchers, scientists, and engineers in university and industry laboratories, Rational Design of Solar Cells for Efficient Solar Energy Conversion offers a comprehensive review of the newest developments and applications of solar cells with contributions from a range of experts in various disciplines.

Leseprobe

Table of Contents

Cover

Preface

1 Metal Nanoparticle Decorated ZnO Nanostructure Based Dye‐Sensitized Solar Cells

1.1 Introduction

1.2 Metal Dressed ZnO Nanostructures as Photoanodes

1.3 Conclusions and Outlook

References

2 Cosensitization Strategies for Dye‐Sensitized Solar Cells

2.1 Introduction

2.2 Cosensitization

2.3 Conclusions

Acknowledgements

References

3 Natural Dye‐Sensitized Solar Cells – Strategies and Measures

3.1 Introduction

3.2 Components of Dye‐sensitized Solar Cell

3.3 Fabrication of Natural DSSCs

3.4 Efficiency and Stability Enhancement in Natural Dye‐Sensitized Solar Cells

3.5 Other Strategies and Measures taken in DSSCs Using Natural Dyes

3.6 Conclusions

References

4 Advantages of Polymer Electrolytes for Dye‐Sensitized Solar Cells

4.1 Why Solar Cells?

4.2 Structure and Working Principle of DSSCs with Gel Polymer Electrolytes (GPEs)

4.3 Gel Polymer Electrolytes (GPEs)

4.4 Summary and Outlook

Acknowledgements

References

5 Advantages of Polymer Electrolytes Towards Dye‐sensitized Solar Cells

5.1 Introduction

5.2 Polymer Electrolytes

5.3 Dye‐sensitized Solar Cells

5.4 Quantum Dot Sensitized Solar Cells (QDSSC)

5.5 Perovskite‐Sensitized Solar Cells (PSSC)

5.6 Conclusion

Acknowledgements

References

6 Rational Screening Strategies for Counter Electrode Nanocomposite Materials for Efficient Solar Energy Conversion

6.1 Introduction

6.2 Principles of Next Generation Solar Cells

6.3 Platinum‐free Counterelectrode Materials

6.4 Summary and Outlook

References

7 Design and Fabrication of Carbon‐based Nanostructured Counter Electrode Materials for Dye‐sensitized Solar Cells

7.1 Photovoltaic Solar Cells – An Overview

7.2 Dye‐sensitized Solar Cells

7.3 Carbon‐based Nanostructured CE Materials for DSSCs

7.4 Conclusions

References

8 Highly Stable Inverted Organic Solar Cells Based on Novel Interfacial Layers

8.1 Introduction

8.2 Research Areas in Organic Solar Cells

8.3 An Overview of Inverted Organic Solar Cells

8.4 Issues in Inverted Organic Solar Cells and Respective Solutions

8.5 Overcoming the Wettability Issue and Light‐soaking Issue in Inverted Organic Solar Cells

8.6 Conclusions and Outlook

Acknowledgements

References

9 Fabrication of Metal Top Electrode via Solution‐based Printing Technique for Efficient Inverted Organic Solar Cells

9.1 Introduction

9.2 Organic Photovoltaic Cells

9.3 Working Principle

9.4 Device Architecture

9.5 Fabrication Process

9.6 Fabrication of Inverted Organic Solar Cells

9.7 Device Morphology

9.8 Device Performance

9.9 Conclusion

Acknowledgements

References

10 Polymer Solar Cells – An Energy Technology for the Future

10.1 Introduction

10.2 Materials Developments for Bulk Heterojunction Solar Cells

10.3 Materials Developments for Molecular Heterojunction Solar Cells

10.4 Developments in Device Structures

10.5 Conclusions

Acknowledgements

References

11 Rational Strategies for Large‐area Perovskite Solar Cells

11.1 Introduction

11.2 Perovskite

11.3 Perovskite Solar Cells

11.4 Device Processing

11.5 Enhancing the Stability of Devices

11.6 Summary

Acknowledgement

References

12 Hot Electrons Role in Biomolecule‐based Quantum Dot Hybrid Solar Cells

12.1 Introduction

12.2 Classifications of Solar Cells

12.3 Main Losses in Solar Cells

12.4 Hot Electron Concept in Materials

12.5 Methodology

12.6 Material Synthesis

12.7 Identification of Hot Electrons

12.8 Quantum Dot Sensitized Solar Cells

12.9 Conclusion

References

Index

End User License Agreement

List of Tables

Chapter 01

Table 1.1 Comparison of various metal nanoparticle dressed ZnO nanostructure DSSCs.

Chapter 02

Table 2.1 Photovoltaic properties of metal complexes cosensitized with organic sensitizers.

Table 2.2 Photovoltaic properties of organic–organic sensitizer cosensitization.

Chapter 03

Table 3.1 Photovoltaic parameters of natural DSSCs using solvents citric acid and acidified ethanol.

Table 3.2 Photovoltaic parameters of natural DSSCs with algal buffer layers using citric acid as solvent.

Table 3.3 Photovoltaic parameters of natural DSSCs with sodium‐doped nanorods as photoelectrode (with algal buffer layers and citric acid as solvent).

Table 3.4 Enhanced reported efficiencies of natural DSSCs with various strategies and measures.

Table 3.5 The enhanced photovoltaic performance of

C. Pulcherrima

and rose dye‐sensitized DSSCs.

Chapter 04

Table 4.1 Photovoltaic performance of DSSCs using chitosan‐based GPEs with their ambient conductivities.

Table 4.2 Photovoltaic performance of DSSCs using PhCh‐based GPEs with their ambient conductivities.

Table 4.3 Photovoltaic performance of DSSCs using PVA‐based GPEs with their ambient conductivities.

Table 4.4 Photovoltaic performance of DSSCs using PAN‐based GPEs with their ambient conductivities.

Table 4.5 Photovoltaic parameters of DSSCs using PVdF‐based GPEs with their ambient conductivities.

Chapter 07

Table 7.1 Photovoltaic characteristics of DSSCs using different carbon‐based nanostructured CEs.

Chapter 08

Table 8.1 Summary of recent development in device efficiency for inverted organic solar cells using various photoactive layer materials.

Table 8.2 IOSC device performance with various hole transport layer materials and deposition methods.

Table 8.3 IOSC device performance with various metal oxides and coating methods.

Table 8.4 List of chemical bath deposited oxide films on various substrates.

Table 8.5 Device performance of inverted organic solar cells with device architecture ITO/TiO

x

/P3HT:PC

61

BM/(PEDOT:PSS + additives)/Ag using various types of additives at TiO

x

thickness of about 100 nm.

Table 8.6 Summary of light‐soaking time (τ

soak

) of P3HT:PC

61

BM, PTB‐7:PC

71

BM and SMDPPEH:PC

61

BM IOSC using (a) sol‐gel TiO

x

and (b) F‐TiO

x

as electron transport layer.

Chapter 09

Table 9.1 Photovoltaic parameters of inverted organic solar cells with respect to the spray time of silver top electrode.

Chapter 10

Table 10.1 The maximum efficiency attained for each type of polymer solar cell.

Chapter 12

Table 12.1 Fitting parameters for the fluorescent decay of pure and βC capped CdSe QDs.

List of Illustrations

Chapter 01

Figure 1.1 Schematic representation of photoinduced charge separation and charge distribution in ZnO/metal nanocomposites. E

F

and E′

F

represent Fermi levels attained before and after charge distribution.

Figure 1.2 Various types of metal dressed ZnO nanostructure used as photoanodes in DSSCs.

Figure 1.3 (a) TEM image of Au@ZnO nanoparticles and (b)

I–V

curves obtained for the modified photoanode‐based DSSCs using N719 dye‐sensitized Au@ZnO nanoparticles. Inset:

I–V

curves obtained for the N719 dye‐sensitized ZnO nanoparticles based DSSCs.

Figure 1.4 Energy level diagram and mechanism of photocurrent generation in the DSSC with FTO/ZnO/Au/PCA/polymer electrolyte.

Figure 1.5 (a) TEM image of the Ag NPs/ZnO NRs composite arrays prepared by 10 adsorption–reduction cycles and (b)

J–V

characteristics of the solar cells prepared with the bare ZnO NR arrays and the Ag NPs/ZnO NR composite arrays (free dye). Energy level diagram and mechanism of the photocurrent generation in the photoelectrochemical cell prepared with (c) the Ag NPs/ZnO NR composite arrays and (d) the bare ZnO NRs array.

Figure 1.6 (a) SEM image of ZnO nanoflowers with gold nanoparticles and (b) HRTEM image of ZnO nanoflower petal surface loaded with gold nanoparticles. Inset (a) a single ZnO nanoflower with gold nanoparticles and (b) gold nanoparticles loaded ZnO nanoflower petals. (c)

J–V

curve obtained for N

3

dye‐sensitized ZnO nanoflowers films with and without gold nanoparticles.

Figure 1.7 (a) and (b) SEM image of

sheaf‐like

‐(ZnO@Ag)

NCM

. (c)

J–V

curves recorded for ZnO NRs/eosin‐Y,

sheaf‐like

‐(ZnO@Ag)

NCM

/eosin‐Y, ZnO NRs/N719, and

sheaf‐like

(ZnO@Ag)

NCM

/N719 photoanodes with TiO

2

NPs modified PEO electrolyte based solid‐state DSSCs under simulated AM 1.5 G solar irradiation of 100 mW/cm

2

. Area of the cell was 1 cm

2

. (d) Mechanism of photocurrent generation at

sheaf‐like

‐(ZnO@Ag)

NCM

modified photoelectrode based dye‐sensitized solar cells. Where E

fs

and E

fm

are Fermi level of semiconductor and metal, respectively. ɸ

s

and ɸ

m

are work function of semiconductor and metal, respectively.

Chapter 02

Figure 2.1 The composition of a dye‐sensitized solar cell.

Figure 2.2 IPCE spectrum and

I–V

curve of a JK2/TT1/TiO

2

DSSC with an active area of 0.2 cm

2

[35].

Figure 2.3 Molecular structures of cosensitized Zn–Phthalocyanine and organic sensitizers.

Figure 2.4 Molecular structures of porphyrin sensitizers.

Figure 2.5 Molecular structures of cosensitized organic sensitizers with porphyrin dyes.

Figure 2.6 Molecular structures of porphyrin sensitizers.

Figure 2.7 Molecular structures of cosensitized organic sensitizers.

Figure 2.8 Molecular structures of N3 and cosensitized organic sensitizers.

Figure 2.9 The fabrication process of DSC‐2‐TiO and image of light‐harvesting with a wide range of wavelength. A: glass; B: F‐doped SnO

2

; BD: black dye and CD process: pressurized CO

2

condition [65].

Figure 2.10 IPCE spectrum of the selectively positioned three‐dye‐sensitized solar cell and those of the single‐dye cell with P5 on the bottom, N719 in the middle, and N749 on the top of the TiO

2

film [66].

Figure 2.11 Molecular structures of black dye and cosensitized organic sensitizers.

Figure 2.12 Molecular structures of N719 and cosensitized organic sensitizers.

Figure 2.13 Cosensitization of CCC and CTC dye molecules with N719 dye [85].

Figure 2.14 Molecular structures of ruthenium and cosensitized organic sensitizers.

Figure 2.15 Molecular structures of cosensitized organic dyes.

Figure 2.16 Molecular structures of the JD1, 5C, JD‐10, D35, Y1, HSQ3, HSQ4, T4BTD‐A, and HSQ5 organic dyes.

Figure 2.17 Molecular structures of the D131, XS41, TC2, WS‐26, S1, IQ21, S2, WS‐1, VG1‐C8, WS‐39, and WS‐2 organic dyes.

Figure 2.18 Molecular structures of the TA‐St‐CA, D, C239, C259, C258, FNE46, FNE53, WS64, TP3, YR‐6, LI‐102, SFD‐5, ADEKA‐1, and LEG4 organic dyes.

Chapter 03

Figure 3.1 Comparison of the Z–scheme of photosynthesis and the dye‐sensitized solar cell.

Figure 3.2 (a), (b), (c) SEM image of TiO

2

film prepared at 160 °C, 180 °C, and 190 °C, respectively; (d) XRD spectra of TiO

2

film (180 °C); (e) UV‐VIS spectra of TiO

2

film (180 °C); (f), (g), (h) TEM image of TiO

2

nanorods

Figure 3.3 Graphical abstract on the effect of PCA in a DSSC [3].

Figure 3.4 Extraction of natural dye from the flowers rose (top) and

C.Pulcherrima

(bottom)using citric acid as solvent and sensitizing the TiO

2

film with the natural extract.

Figure 3.5 J‐V characteristics of (a)

C.Pulcherrima

and (b) rose sensitized DSSCs using different solvents respectively.

Figure 3.6 Fabrication of natural DSSC with algal buffer layers [5].

Figure 3.7 J‐V characteristics of DSSCs with algal buffer layers sensitized by (a) rose and (b)

C.Pulcherrima

dye (citric acid as solvent).

Figure 3.8 (a), (b), (c) FESEM image of TiO

2

nanorods prepared by the hydrothermal method; (d), (e), (f) FESEM images of sodium‐doped TiO

2

nanorods prepared by the hydrothermal method (6% dopant concentration).

Figure 3.9 J‐V characteristics of rose and

C.Pulcherrima

sensitized DSSCs with sodium‐doped nanorods as photoelectrode (with algal buffer layers and citric acid as solvent).

Chapter 04

Figure 4.1 Schematic diagram of the DSSC configuration.

Figure 4.2

J‐V

characteristics of best performing N3 DSSC using PhCh‐PEO‐EC‐DMF‐TPAI(+I

2

)‐BMII GPE.

Chapter 05

Figure 5.1 (a) Various generations of solar cell; (b) types of electrolytes used in third generation solar cells.

Figure 5.2 Mechanism of iodide ion transport in polymer electrolyte [P(MMA‐co‐EMA)] [51].

Figure 5.3 (a) Components of dye‐sensitized solar cells; (b) operational principle of dye‐sensitized solar cells.

Figure 5.4 Ruthenium polypyridyl complexes as metal‐based sensitizers.

Figure 5.5 General donor‐π‐bridge‐acceptor (D‐π‐A) structure of organic dyes and some examples of metal free dyes.

Figure 5.6 Schematic illustration of the dye‐sensitized photovoltaic cell [FTO/TiO2/dye/E/CCC/FTO] where E is HPA impregnated PVDF polymer electrolyte with TiO

2

nanofiller [118].

Figure 5.7 Electrostatic interactions of amino groups of (a) acetamide and (b) urea with PEO ether groups [142,143].

Figure 5.8 UV‐Visible spectrum of 0.3% solution of GPEs in acetonitrile. A represents thiourea‐free GPE. B, C, and D represent GPEs containing 1, 2, and 3 wt‐% of thiourea, respectively (Inset: Dimerization reaction of thiourea (TU) with iodine molecule) [163].

Figure 5.9 (a) ABX

3

perovskite structure showing BX

6

octahedral and larger A cation occupied in cubooctahedral site. (b) The unit cell of cubic CH

3

NH

3

PbI

3

perovskite. Original figure in (b) was reprinted from Reference [187].

Chapter 06

Figure 6.1 Representative arrangement of a DSSC.

Figure 6.2 Basic working processes and constituent components of a DSSC.

Figure 6.3 Working processes of a QDSSCs, with energy band levels, flow of charge carriers.

Figure 6.4 Band diagram and main processes for a PSC: 1 absorption of photon and free charge generation; 2 charge transport; 3 charge extraction [62].

Figure 6.5 Schematic diagram of DSSCs developed with graphene‐based counterelectrode [90].

Figure 6.6 Graphic diagram of the fabrication steps of all‐carbon flexible CE based HTM‐free PSCs [98].

Figure 6.7 The schematic illustration of DSSCs based on MoS

2

/CNF CE in visible‐light illumination [122].

Figure 6.8 Photocurrent–voltage curves of DSSCs fabricated with MoS

2

, MoS

2

/AB, MoS

2

/VC, MoS

2

/CNT, MoS

2

/CNF, and MoS

2

/RHA CEs [122].

Chapter 07

Figure 7.1 Typical schematic structure of a DSSC [15].

Figure 7.2 The chemical structures of N3, N719, and N749 (black) dyes.

Figure 7.3 (a–c) FE‐SEM images and (d) TEM images of the carbonized HPC sample under inert gas atmosphere [35].

Figure 7.4

J‐V

curves of fabricated DSSCs based on the platinum, PPy, MWCNT, and PPy/MWCNT CEs [38].

Figure 7.5 Schematic illustration of the formation mechanism for p‐GN@CuInS

2

/C composite nanofibers [42].

Figure 7.6 CV curves of Cr‐Pt

3

Ni/CNs, Pt

3

Ni/CNs, Pt, and CNs CEs towards the I

−

/I

3

−

redox couple [47].

Figure 7.7 The schematic representation of the synthesis of the NS‐GN [52].

Figure 7.8 Schematic illustration of device process: (a) synthesis of GMWNTs on SiO

2

/Si, (b) lift‐off process of GMNTs, (c) transplant to FTO glass, and (d) half cell without TiO

2

and dye. [61].

Figure 7.9 Top view SEM images of (a) a 11.2 µm thick carbon/TiO

2

composite layer and (b) a pure TiO

2

nanoparticle layer on a FTO substrate. (c) Cross‐section SEM image of a carbon/TiO2 composite layer [71].

Figure 7.10 SEM morphological images of various carbon materials such as Ca – activated carbon, Cb – carbon black, Cc – conductive carbon, Cd – carbon dye, Cf – carbon fiber, Cn – carbon nanotube, Com – mesoporous carbon, Cp – discarded toner of a printer, C60 – fullerene [81]

Chapter 08

Figure 8.1 Research areas for organic solar cells.

Figure 8.2 Comparison of the lifetime of conventional and inverted P3HT:PC

61

BM OSC under exposure to ambient condition without encapsulation.

Figure 8.3 Schematic representation for (a) conventional OSC cell structure and (b) an inverted OSC. Note the direction of charge collections between the two types of devices.

Figure 8.4 Schematic showing the necessity of UV ozone treatment on photoactive layers prior to coating PEDOT:PSS with conventional surfactant modification.

Figure 8.5 (a) Typical solar irradiance as a function of time for a typical sunny day in Singapore; (b) light‐soaking calculation with assumption of 10 minutes under the solar simulator; (c) the corresponding light‐soaking estimation under the sun by equating the energy densities (area under the curves in (b) and (c)).

Figure 8.6 Device performance showing (a) V

oc

, (b) j

sc

, (c) fill factor, and (d) efficiency of ITO/TiO

x

/P3HT:PC

61

BM/PEDOT:PSS:CFS‐31/Ag IOSC with various CFS‐31 concentrations (ranging from 0 to 15 v/v%).

Figure 8.7 Optical microscopy images showing P3HT:PC

61

BM/PEDOT:CFS‐31 with (i–viii) 0, 0.5, 1.0, 3.0, 5.5, 8.0, 10.0, and 15.0%, respectively. The dark orange region is PEDOT:PSS, while the light yellow region is P3HT:PC

61

BM. (a) Illuminated

J‐V

characteristics of representative devices with various CFS‐31 surfactant concentration; (b) shunt and series resistance of corresponding devices extracted from (a), the lines connecting the data points only serves as guidance.

Figure 8.8 Normalized V

oc

, J

sc

, fill factor, and efficiency of ITO/TiO

x

/P3HT:PC

61

BM/PEDOT:PSS:CFS‐31/Ag with 5.5 v/v% CFS‐31 concentration under exposure in N

2

(first 24 hours) and in air (subsequent exposure time) without encapsulation. The parameters were normalized by their respective maximum values. The pictures on the left show the freshly exposed IOSC while the ones on the right shows the same device exposed to air after 600 hours.

Figure 8.9 Current‐voltage (

J‐V

) characteristics of inverted organic solar cells with (a) sol‐gel TiO

x

and (b) F‐TiO

x

as electron transport layer when subjected to light‐soaking treatments; the illumination time are shown (guided by arrows). (c) Light‐soaking effect on the efficiency of sol‐gelTiO

x

layers upon AM 1.5G illumination in P3HT:PC

61

BM, PTB7:PC

71

BM and SMDPPEH:PC

61

BM devices. (d) Light‐soaking effect on the efficiency of F‐TiO

x

layers upon AM 1.5G illumination in P3HT:PC

61

BM, PTB7:PC

71

BM and SMDPPEH:PC

61

BM devices The symbols represent experimental data while the solid lines are fitted data. The figures were extracted from [32] and [34].

Figure 8.10 Schematic showing various combinations of TiO

x

used for studying the contribution of different interfaces to the light‐soaking effect.

Figure 8.11 Light‐soaking control experiment of fresh inverted organic solar cells with sol‐gel TiO

x

(80 nm), F‐TiO

x

(80 nm), sol‐gel TiO

x

(40 nm)/F‐TiO

x

(40 nm), and F‐TiO

x

(40 nm)/sol‐gel TiO

x

(40 nm) as electron transport layers upon AM 1.5G illumination. The symbols represent experimental data while the solid lines are fitted data by logistic function. The indicated time represents the corresponding light‐soaking time (τ

soak

) for each sample as defined in the main text.

Figure 8.12 (a) Ultraviolet photoelectron spectroscopy (UPS) secondary cut‐off spectra of ITO, ITO/sol‐gel TiO

x

, and ITO/F‐TiO

x

before light‐soaking. (b) Normalized photocurrent of sol‐gel TiO

x

(O at%) /Ag (∎) and F‐TiO

x

(1.3 F at%)/Ag (•) under UV illumination; the step (—) indicates UV light ON/OFF state. (c) A proposed model for trap‐filling phenomenon of sol‐gel TiO

x

and F‐TiO

x

before and after light‐soaking treatment. (d) Resulting energy band diagrams at ITO/TiO

x

interface before light‐soaking treatment. All values were obtained from UPS measurements, the bandgaps were estimated by their respective absorption spectra and the arrows at the ITO/TiO

x

interface represent the resulting barrier width due to the Fermi level realignment. The evolution behavior of light‐soaking effect was adopted from [91].

Chapter 09

Figure 9.1 Renewable electricity capacity growth (GW).

Figure 9.2 India sets targets to reach ambitious 2021 solar goal.

Figure 9.3 Organic conducting materials (donor and acceptor) for OPVs devices.

Figure 9.4 Mechanism of photocurrent generation in an OPV device.

Figure 9.5 OPV device architectures: (a) single layer, (b) planar heterojunction, (c) bulk heterojunction, and (d) ordered bulk heterojunction.

Figure 9.6 Schematic illustration of an OPV device: (a) conventional structure and (b) inverted structure.

Figure 9.7 Schematic illustration of the Hybrid‐EHDA set‐up.

Figure 9.8(a–b) Influence of applied potential on spray or atomization mode of silver ink at constant pneumatic pressure: (a) 0.0 bar and (b) 0.3 bar.

Figure 9.8(c–d) Influence of applied potential on spray or atomization mode of silver ink at constant pneumatic pressure: (c) 0.5 bar and (d) 0.7 bar.

Figure 9.8(e–f) (e) Influence of applied potential on spray or atomization mode of silver ink at constant pneumatic pressure of 1 bar and (f) influence of pneumatic pressure on spray at constant applied potential (15 kV)

Figure 9.9 The cross‐sectional view of the organic device.

Figure 9.10 (a) Thickness of silver thin films with respect to spray times; (b) sheet resistance of silver films with respect to film thickness (in both figures, the error bars represents the standard deviation).

Figure 9.11 Energy level diagram of inverted organic solar cells.

Figure 9.12 The current density–voltage characteristics of inverted organic solar cells produced with silver top contact via the Hybrid‐EHDA process.

Chapter 10

Figure 10.1 General operating principle of a polymer solar cell.

Figure 10.2 Structures of commonly used polymer donors and fullerene acceptors.

Figure 10.3 Structures of widely investigated conjugated polymers.

Figure 10.4 Chemical structures of heteroatoms such as Se, Si and Ge present in the core structure of donor polymers.

Figure 10.5 Chemical structures of non‐fullerene electron acceptors ITIC and PMI‐F‐PMI.

Figure 10.6 Structures of non‐fullerene polymer electron acceptor materials.

Figure 10.7 Structures of the charm‐bracelet type double‐cable polymers COP2, D3, and PPV‐C

60

.

Figure 10.8 (a) Layer stacks of the triple‐junction tandem solar cell in the inverted architecture. (b) Optical parameters n and k for P3HT:ICBA, PTB:PC

71

BM, and LBG:PC

71

BM structures used in the simulations. (c) Energy levels of the materials investigated in this study.

Figure 10.9 (a) Solar cell device stack and chemical structures for PTB7‐Th, PC

71

BM, and PEOz. (b) Flat energy band diagram for PTB7‐Th:PC

71

BM BHJ solar cells with ZnO/PEOz ECBL/interlayer. The blue arrow denotes the shift in ZnO work function caused by the PEOz. (c) Absorption coefficient (

α

) spectra for pristine films of PTB7‐Th and PC71BM, and for a BHJ PTB7‐Th:PC

71

BM blend film.

Chapter 11

Figure 11.1 ABX

3

crystal structure [14].

Figure 11.2 Evolution of PSCs [29].

Figure 11.3 A schematic representation of (a) and (b) n‐i‐p mesoporous and planar, (c) and (d) p‐i‐n n‐i‐p mesoporous and planar structures [36].

Figure 11.4 (a) Schematic representation of interfacial engineering using DMSO as solvent [59]. (b) First principle studies on 2D/3D interface using simulation [79].

Figure 11.5 (a) Series connected five subcells and cross‐section FESEM images with the variation of effect in solvent [90]. (b) pictorial representation of four subcells with active area of 4 cm

2

[91].

Figure 11.6 (a) Pictorial representation of doctor blade instrument with its growth of grain size [94]. (b) Image of large area module of 100 cm

2

fabricated using doctor blade method [95].

Figure 11.6 (c) Schematic representation of the facile one‐step processing of perovskite film using a slot die coater [99]. (d) Kinetic growth study of perovskite crystal. Figure adapted from [101].

Figure 11.7 (a) Schematic representation of the screen printing method. (b) Pictorial representation of the spray coating method for PSCs. Figure adapted from [105].

Figure 11.7 (c) Schematic representation of lateral crystal growth fabricated using R‐R. Figure adapted from [119].

Figure 11.8 (a) Schematic representation of the CVD setup. Figure adapted from [121]. (b) Pictorial representation of smaller module with 1 cm

2

to larger module of 8.8 cm

2

grown using CVD [122].

Figure 11.8 (c) Schematic representation of the SCD method for fabricating PSCs [125].

Chapter 12

Figure 12.1 Classification of solar cells.

Figure 12.2 Energy conversion loss process in standard solar cells.

Figure 12.3 (a) The bulk band structure of a semiconductor. Here the photoexcited higher energy state of electrons relaxes through the continuous states of CB and reaches CB edge state. (b) Energy band structure of QD. Here the photoexcited electrons relax through the defect levels (hence in both the systems, slowing down of cooling process is challenging one).

Figure 12.4 Hot injection method experimental setup and QD nucleation.

Figure 12.5 Emission spectra of CdSe QD with different βC concentration in comparison with bare CdSe QD.

Figure 12.6 Schematic representation of arrested nonradiative relaxation of CdSe QD.

Figure 12.7 Three dimensional plot showing the excitation energy dependent PL behavior for a bare CdSe QD sample.

Figure 12.8 Three dimensional plot showing the excitation energy dependent PL behavior for a CdSe QD‐βC hybrid sample.

Figure 12.9 The calculated quantum yield as a function of excitation energy for the bare (curve a) and CdSe QD‐βC hybrid (curve b) samples.

Figure 12.10 Interfacial energy level diagram between a CdSe QD and beta‐carotene explaining various possible quenching mechanisms.

Figure 12.11 PL decay curves for bare and CdSe QD‐βC hybrid samples recorded at 620 nm and 500 nm.

Figure 12.12 Transient absorption spectra recorded from a βC‐CdSe QD hybrid sample.

Figure 12.13 Schematic representation of the electron transfer process.

Figure 12.14 Schematic representation of the back transfer process.

Figure 12.15 Time dependent absorption profiles of CdSe QDs at 620 nm (a) and beta‐carotene at 450 nm (b).

Figure 12.16 General schematic structure of the QD sensitized solar cell showing all the components and a process of excited electron transfer under illumination.

Figure 12.17 Schematic representation of a QD sensitized solar cell device structure.

Guide

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Rational Design of Solar Cells for Efficient Solar Energy Conversion

Edited by

Alagarsamy Pandikumar

CSIR‐Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India

and

Ramasamy Ramaraj

Madurai Kamaraj University, Madurai, Tamil Nadu, India

This edition first published 2018© 2018 John Wiley & Sons, Inc.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.

The right of Alagarsamy Pandikumar and Ramasamy Ramaraj to be identified as the editors of the editorial material in this work has been asserted in accordance with law.

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Library of Congress Cataloging‐in‐Publication Data

Names: Pandikumar, Alagarsamy, editor. | Ramaraj, Ramasamy, editor.Title: Rational design of solar cells for efficient solar energy conversion / edited by Alagarsamy Pandikumar, Ramasamy Ramaraj.Description: First edition. | Hoboken, NJ : John Wiley & Sons, 2018. | Includes bibliographical references and index. |Identifiers: LCCN 2018008636 (print) | LCCN 2018013539 (ebook) | ISBN 9781119437468 (pdf) | ISBN 9781119437451 (epub) | ISBN 9781119437406 (cloth)Subjects: LCSH: Solar cells–Design and construction. | Direct energy conversion.Classification: LCC TK2960 (ebook) | LCC TK2960 .R325 2018 (print) | DDC 621.31/244–dc23LC record available at https://lccn.loc.gov/2018008636

Cover Design: WileyCover Image: © EzumeImages/iStockphoto

Biographies

Dr. Alagarsamy Pandikumar

Dr. Alagarsamy Pandikumar is currently working as a Scientist at the CSIR‐Central Electrochemical Research Institute, Karaikudi, India. He obtained his PhD in chemistry (2014) from the Madurai Kamaraj University, Madurai, India. His doctoral research focused on the synthesis and characterization of plasmonic nanocomposite materials and the application of their photophysical/chemical properties in photocatalysis, photoelectrocatalysis, and dye‐sensitized solar cells. He successfully completed his postdoctoral fellowship tenure (2014–2016) at the University of Malaya, Malaysia, under a high Impact Research Grant. During his postdoctoral research, he developed several graphene‐based metal, metal oxide, polymeric nanocomposites for electrochemical sensors and dye‐sensitized solar cell applications. He was appointed as a Research Assistant Professor in the Department of Chemistry & Research Institute at SRM University, India. His current research involves development of novel materials with graphene, graphitic carbon nitrides, transition metal chalcogenides in combination to metals, metal oxides, polymers, and carbon nanotubes for photocatalysis, photoelectrocatalysis dye‐sensitized solar cells, and electrochemical sensor applications. His results outcomes have been documented in 95 peer‐reviewed journals, including eight review articles; among them seven of the key articles have been cited more than 59 times and also have 1940 citations with the h − index of 26. On other side, he has served as Guest Editor for a special issue of the Materials Focus journal and edited four books for TransTech Publishers, Switzerland.

Professor Ramasamy Ramaraj

Professor Ramasamy Ramaraj is currently working as an Emeritus Scientist in the School of Chemistry and Centre for Photoelectrochemistry, Madurai Kamaraj University, Madurai, India. He obtained his PhD degree in chemistry from the Department of Inorganic Chemistry, University of Madras, Chennai, India. He had postdoctoral research experience at the Institute of Physical & Chemical Research (RIKEN), Japan, and the Max‐Planck Institute for Radiation Chemistry, Mülheim an der Ruhr, Germany. His research interests are nanomaterials and their applications in catalysis, sensors and solar energy conversion, chemically modified electrodes, photoelectrochemistry, photoelectrocatalysis, artificial photosynthesis, and electron transfer reactions in organized assemblies. He was awarded a Doctor of Science (DSc) by Ibaraki University, Japan, in 2001 and a Doctor of Science (DSc) by Madurai Kamaraj University, Madurai, in 2004. He has published more than 160 research papers in peer‐reviewed journals.

List of Contributors

Sambandam AnandanNanomaterials and Solar Energy Conversion LaboratoryNational Institute of TechnologyIndia

A.K. ArofCentre for Ionics University of MalayaPhysics DepartmentFaculty of ScienceUniversity of Malaya50603 Kuala LumpurMalaysia

Arunachalam ArulrajUniversity College of Engineering‐Bharathidasan Institute of Technology (BIT) campusAnna UniversityIndia

Prabhakarn ArunachalamElectrochemistry Research GroupKing Saud UniversitySaudi Arabia

Muthupandian AshokkumarSchool of ChemistryUniversity of MelbourneAustralia

R. BalasundaraprabhuCentre for Surface SciencePSG College of TechnologyIndia

G. BharathiDepartment of PhysicsBharathiyar UniversityIndia

Kyung‐Hyun ChoiDepartment of Mechatronics EngineeringJeju National UniversityRepublic of Korea

Navaneethan DuraisamyDepartment of ChemistryPeriyar UniversityIndia

Dhanaraj GopiDepartment of ChemistryPeriyar UniversityIndia

Gregory Thien Soon HowDepartment of PhysicsUniversity of MalayaMalaysia

Kandasamy JothivenkatachalamDepartment of ChemistryAnna University‐ BIT CampusTiruchirappalli‐ 620024TamilnaduIndia

Kavitha KandiahDepartment of MicrobiologyPeriyar UniversityIndia

Ananthanarayanan KrishanamoorthySRM Research Institute and Department of ChemistrySRM UniversityIndia

Giovanni LandiDepartment of Industrial EngineeringUniversity of SalernoItalyInstitute for Polymers, Composites and Biomaterials (IPCB),National Research Council of Italy (CNR), Italy

Fang Jeng LimSolar Energy Research Institute of Singapore (SERIS)National University of SingaporeSingapore

Jagannathan MadhavanSolar Energy LaboratoryThiruvalluvar UniversityIndia

Nay Ming HuangFaculty of EngineeringUniversity Xiamen MalaysiaMalaysia

D. NatarajDepartment of PhysicsBharathiyar UniversityIndia

D. NavaneethanDepartment of ChemistryPeriyar UniversityIndia

Maadeswaran PalanisamyDepartment of Energy StudiesPeriyar UniversityIndia

Alagarsamy PandikumarFunctional Materials DivisionCSIR‐Central Electrochemical Research InstituteKaraikudi‐630006India

Nagaraj PavithraNanomaterials and Solar Energy Conversion LaboratoryNational Institute of TechnologyIndia

T. PazhanivelDepartment of PhysicsPeriyar UniversityIndia

N. PrabavathyCentre for Surface SciencePSG College of TechnologyIndia

Ramesh RajendranDepartment of PhysicsPeriyar UniversityIndia

Alagar RamarGraduate Institute of Applied Science and TechnologyNational Taiwan University of Science and TechnologyTaiwan, R.O.C.

Mohan RameshCSIR‐Central Electrochemical Research InstituteIndia

R. RameshDepartment of ChemistryPeriyar UniversityIndia

Gachumale SarithaNanomaterials and Solar Energy Conversion LaboratoryNational Institute of TechnologyIndia

Raja Arumugam SenthilSolar Energy LaboratoryThiruvalluvar UniversityIndia

Andrea SorrentinoInstitute for Polymers, Composites and Biomaterials (IPCB)National Research Council of Italy (CNR), Italy

Jayaraman TheerthagiriCentre of Excellence for Energy ResearchSathyabama Institute of Science and TechnologyIndiaSolar Energy LaboratoryThiruvalluvar UniversityIndia

L.P. TeoCentre for Ionics University of MalayaPhysics DepartmentFaculty of ScienceUniversity of Malaya50603 Kuala LumpurMalaysia

Pazhanivel ThangaveluDepartment of PhysicsPeriyar UniversityIndia

Dhayalan VelauthapillaiFaculty of EngineeringWestern Norway University of Applied SciencesNorway

Fu‐Ming WangGraduate Institute of Applied Science and TechnologyNational Taiwan University of Science and TechnologyTaiwan, R.O.C.

Preface

Solar cell technology is a potential alternative to overcome the issues related to energy demand and environmental pollution caused by fossil fuels. Dye‐sensitized solar cells, organic solar cells, polymer solar cells, perovskite solar cells, and quantum dot solar cells are promising next‐generation alternative renewable energy technology to substitute for fossil fuels and other energy sources due to their high performance, ease of fabrication, long‐term stability, and low manufacturing cost. This new book gathers and surveys a variety of novel ideas that have emerged in the fields of dye‐sensitized solar cells, organic solar cells, polymer solar cells, perovskite solar cells, and quantum dot solar cells from over forty experts in the interdisciplinary areas of chemistry, physics, materials science, and engineering and widely explores the materials development and device fabrication in the field of solar cells to achieve higher solar energy conversion efficiency. This book presents a collection of twelve chapters written by researchers who are the leading experts in their fields of research and they explain the strategies needed to overcome the challenges in solar cell fabrication. The first chapter of this book is a succinct summary of the state of the art of the fabrication of plasmonic nanoparticles incorporated into photoanodes for dye‐sensitized solar cells. Chapters 2 and 3 focus more on the aspects of sensitization processes with cosensitizer and natural dyes, and their impact in dye‐sensitized solar cells. Chapters 4 and 5 explore the durability, stability, and performance enhancement strategies needed to adapt polymer and gel electrolytes for use in in dye‐sensitized solar cells. Chapters 6 and 7 discuss the details of replacing the expensive platinum counterelectrode with alternative electrocatalysts to minimize the fabrication cost of dye‐sensitized solar cells. Chapters 8–10 address the key challenges in the fabrication and possible strategies to improve the efficiency of the polymer solar cells with different approaches. Chapter 11 summarizes the possible methodologies to fabricate perovskite solar cells from laboratory scale to industrial scale. Chapter 12 presents the possible role of biomolecules and their charge transfer dynamics in quantum dot solar cells. Finally, we would like to express our sincere thanks to the all contributing authors for sharing their knowledge on solar cells; this has made it possible to prepare this book for the benefit of those parties interested in light harvesting assemblies and applications.

Dr. Alagarsamy PandikumarProfessor Ramasamy Ramaraj

1Metal Nanoparticle Decorated ZnO Nanostructure Based Dye‐Sensitized Solar Cells

Gregory Thien Soon How1, Kandasamy Jothivenkatachalam2, Alagarsamy Pandikumar3, and Nay Ming Huang4

1 Department of Physics, University of Malaya, Malaysia

2 Department of Chemistry, Anna University‐BIT Campus, Tiruchirappalli‐620024, Tamilnadu, India

3 Functional Materials Division, CSIR‐Central Electrochemical Research Institute, Karaikudi‐630006, India

4 Faculty of Engineering, University Xiamen Malaysia, Malaysia

1.1 Introduction

Solar energy has always been an ideal renewable energy source that is clean, abundant, inexpensive, and widely distributed regionally in the world [1–3]. Understanding this, the emergence of dye‐sensitized solar cells (DSSCs) for converting solar energy to electricity has been very promising due to the ease of the manufacturing process, the low fabrication cost, the fact that it is nonpolluting, and the relatively high efficiency [1, 4–6]. It is known that a typical DSSC consists of various subsections, includinng a nanocrystalline semiconductor oxide photoanode, dye sensitizer, redox couple electrolyte, and counterelectrode [3, 4]. The main idea behind the operating principle of DSSCs is based on the optical excitation of a dye that results in the injection of an electron into the conduction band of a wide band gap semiconductor oxide. The oxidized dye molecule is regenerated afterwards when it is reduced to its ground state by gaining one electron from a redox couple that is found in the electrolyte around the sensitized semiconductor oxide nanostructured film [3–5]. Since the first outstanding research work on DSSC was demonstrated by O’Regan and Gratzel in 1991 [5], each of its components has been extensively investigated and optimized, with the aim to maximize the power conversion efficiency (PCE) of DSSCs [4, 7, 8]. Recently, a PCE of 12.3% has been achieved by using the cosensitization of two dyes and a Co(II/III) tris(bipyridyl)‐based redox electrolyte [9]. Hence, study to find a suitable and high performance DSSC output has greatly increased over the years.

Amongst all the materials studied for use in DSSCs, nanocrystalline TiO2 has been most commonly employed as the metal oxide semiconductor material in high efficiency DSSCs [4–6]. Several methods were used for the preparation of the TiO2 nanoparticles in DSSCs, such as sol‐gel [10, 11], gas‐phase pyrolysis [12], or the commonly used hydrothermal synthesis method [13, 14]. However, hydrothermal methods are not ideal because both synthesis and purification processes take a prolonged time to achieve well‐formed and highly crystalline TiO2 particles [12]. To minimize the costs of metal oxide semiconductor materials for DSSCs, simple preparation methods are essential to control the formation of crystal structure, crystallization, and particle size [15]. Besides TiO2, there are reports of other alternative metal oxides, such as SnO2, Nb2O5, and ZnO, being used as porous semiconductor materials for DSSC photoelectrodes [16–20].

ZnO is an another attractive and alternative photoanode to replace TiO2 as an electron conductor owing to its higher bulk electron mobility and easily tunable morphology, which allows the rational design and development of hierarchical ZnO nanostructures able to simultaneously optimize charge carrier path and dye loading [19, 20]. Hence, ZnO is considered an excellent backbone to produce high‐efficiency DSSCs. The ZnO characteristic of higher electron mobility (∼205–1000 cm2 V−1 s−1) than TiO2 (∼0.1 − 4 cm2 V−1 s−1), enables the rapid diffusion transport of photoinjected electrons when it is employed as a photoanode material in DSSCs. In addition, ZnO is a suitable material for the fabrication of mesoporous photoanodes in DSSCs; it has a band gap of 3.2 eV and a conduction band edge position of −4.3 eV, both of which are similar to TiO2 [15–17]. Moreover, ZnO can be easily prepared into tunable nanostructures, such as nanoparticles, nanowires, nanotubes, nanorods, nanosheets, and tetrapods, providing numerous alternatives for optimizing photoanode morphology so as to improve the charge collection. However, the conversion efficiency of ZnO‐based DSSCs reported so far still remains lower than those fabricated from TiO2, leaving plenty of room to improve the efficiency through structural and morphology modifications of the ZnO nanostructures. Previous review articles [16, 17] have explored recent developments in ZnO nanostructures for application in DSSCs and suggest that the nanostructured ZnO can significantly enhance solar cell performance due to the large surface area for dye adsorption, direct transport pathways for photoexcited electrons, and efficient scattering centers for enhanced light‐harvesting efficiency. Furthermore, the limitations of ZnO‐based DSSCs are also discussed and a few suggestions are also given for the conversion efficiency improvement.

1.2 Metal Dressed ZnO Nanostructures as Photoanodes

One of the major challenges in the development of high efficiency DSSCs is the competition between the generation and recombination of photoexcited carriers. The use of low‐dimensional nanostructures is able to support a direct pathway for the rapid collection of photogenerated electrons and, hence, reduce the charge recombination [21, 22]. Thus, the possible alternative way to improve the charge separation in DSSCs is to introduce a barrier layer at the semiconductor/electrolyte interface to block the back electron transfer from the semiconductor to the redox electrolyte. Doping of metals on ZnO nanostructures significantly reduces the charge recombination, which is another way to improve the charge separation in DSSCs. Rapid charge transfer and improved charge separation upon incorporation of metal nanoparticles on ZnO, leading to enhanced DSSC performance, have been demonstrated [23–33]. Moreover, the metal nanoparticles (namely silver and gold) that possess surface plasmon resonance can couple to visible light, which increases the optical absorption of the photoelectrode in the visible region.

Metal nanoparticles doped on ZnO exhibit unusual redox activity by readily accepting electrons either from a dye molecule or an electrode. Such metal nanoparticles, when in contact with a ZnO nanostructure, can equilibrate and undergo Fermi‐level equilibration, thus forming a Schottky barrier at the metal/ZnO nanocomposite interfaces (Figure 1.1) [22].

Figure 1.1 Schematic representation of photoinduced charge separation and charge distribution in ZnO/metal nanocomposites. EF and E′F represent Fermi levels attained before and after charge distribution.

Source: Adapted from Subramanian 2003 [22]. Reprinted with permission of American Chemical Society.

The charge equilibration between the metal and ZnO nanocomposite interfaces in contact drives the Fermi level close to the conduction band edge of the semiconductor and, thus, influences the photovoltaic performance of DSSCs. So far, ZnO‐based DSSC performance has been reviewed but there is no summary of the metal dressed ZnO based DSSC performance. In this review, the recent progress on metal dressed ZnO based DSSC and the role of metal nanoparticles on various ZnO nanostructures in DSSCs (Figure 1.2) in improving the device performance (through improved charge separation introduced by the Schottky barrier formed at the metal/ZnO nanocomposite interface) are discussed. Furthermore, the influence of silver and gold nanoparticles leading to enhanced optical absorption on the performance of DSSCs is also discussed.

Figure 1.2 Various types of metal dressed ZnO nanostructure used as photoanodes in DSSCs.

1.2.1 Metal Dressed ZnO Nanoparticles as Photoanodes

There are few related works reported involving metal dressed ZnO nanoparticles as photoanodes in DSSC applications. Among them, Tripathi [23] and coworkers reported a bilayer TiO2:Ag/ZnO:Ag (TZO:Ag) oxide film using a sol‐gel process for DSSCs. They have investigated the effect of Eosin‐Y dye and a cocktail dye (C) (Rhodamine B, Rose Bengal, Fast Green, Acridine Orange, Fast Green) for DSSC application. In comparison to the undoped ZnO/C film, their TZO/Ag/C film exhibits higher Voc, Jsc, and PCE of 0.158%. This is due to the surface plasmon resonance effect of the silver nanoparticles in enhancing visible light absorption and also the Schottky barrier established at the semiconductor/metal interface. Sarkar et al. reported a nanocomposite consisting of gold and ZnO nanoparticles (NPs) for photocatalysis and DSSC applications [24]. Their ZnO–Au nanocomposite (NC) was synthesized based on the formation of gold NPs on the surface of ZnO NPs, using chloroauric acid ethanolic solution added into readily prepared ZnO NP colloid solution. To obtain the gold NPs, they added sodium borohydride (through a chemical reduction method) in order for gold chloride to undergo reduction. Interestingly, the ZnO–Au NC morphology (Figure 1.3) reveals the uniform distribution of gold on spherical ZnO nanoparticles, with average diameters of 6 nm and 8 nm, respectively. In their DSSC application, the ZnO–Au NC depicts a higher overall PCE of 0.8% than the pure ZnO (PCE = 0.012%). Such major difference in the PCE reported was claimed to be attributed to role of gold in increasing the electron mobility in ZnO–Au NC in addition to its plasmon resonance.

Figure 1.3 (a) TEM image of Au@ZnO nanoparticles and (b) I–V curves obtained for the modified photoanode‐based DSSCs using N719 dye‐sensitized Au@ZnO nanoparticles. Inset: I–V curves obtained for the N719 dye‐sensitized ZnO nanoparticles based DSSCs.

Adapted from [24] with permission of The Royal Society of Chemistry.

Mikroyannidis and coworkers also reported gold‐coated ZnO photoanodes for DSSC application using their own synthesized perylene monoimide derivative, PCA, as the sensitizer (Figure 1.4) [25]. Initially, they synthesized pure ZnO NPs using a sol‐gel method and then immerse the preformed ZnO NPs in chloroauric acid to obtain ZnO NPs coated with gold nanoparticles. Later, the photoanodes were immersed in their own prepared PCA sensitizer, which is a metal‐free dye. By comparing their DSSC obtained PCE, it was found that the PCE of their ZnO NPs coated with gold NPs was enhanced from 1.34 to 1.91%, which contributed to the fast transport of electrons from the gold NPs to the ZnO NPS through the Fermi‐level equilibration and the formation of a Schottky barrier at the ZnO/Au interface, which reduces the charge recombination rate, thus increasing the PCE as described earlier. The PCE was also improved by adding TiO2 nanofillers into the ZnO–Au DSSC, which again increased the PCE values to 2.44%. It was reported that the increasing PCE value in the presence of nanofillers (TiO2) is due to the enhancement of exchange charge density and diffusion coefficient of tri‐iodide, which depress the concentration of I3−, and the suppression of the dark reaction by the polymer gel electrolyte with TiO2. The improved PCE could be due to the incorporation ZnO into TiO2 DSSCs as a result of the inhibition of the injected electrons from the dye molecules in recombining back with the electrolyte species [34].

Figure 1.4 Energy level diagram and mechanism of photocurrent generation in the DSSC with FTO/ZnO/Au/PCA/polymer electrolyte.

Adapted from [25] with permission of The Royal Society of Chemistry

1.2.2 Metal Dressed ZnO Nanorods as Photoanodes

Recently, great interest was shown in a ZnO nanorod (NR) structure (compared to other ZnO morphologies) due to its high surface area and its capability of rapid charge collection that enhances the overall short circuit current [35, 36]. Yin and coworkers [26] have reported on their ZnO nanorods array deposited with silver nanoparticles (Ag NPs/ZnO NRs) for DSSC application. This composite was synthesized through a combination of liquid phase epitaxial growth process and the reduction of silver on the surface of the ZnO NRs (Figure 1.5). According to this report, the role of silver NPs was seen to improve the DSSC efficiency (PCE = 0.81%) compared to the pure ZnO NRs; this can be related to the surface plasmon resonance of the silver NPs and the Schottky barrier at the Ag/ZnO interface.

Figure 1.5 (a) TEM image of the Ag NPs/ZnO NRs composite arrays prepared by 10 adsorption–reduction cycles and (b) J–V characteristics of the solar cells prepared with the bare ZnO NR arrays and the Ag NPs/ZnO NR composite arrays (free dye). Energy level diagram and mechanism of the photocurrent generation in the photoelectrochemical cell prepared with (c) the Ag NPs/ZnO NR composite arrays and (d) the bare ZnO NRs array.

Reprinted by permission from Springer Nature Copyright 2012 [26].

Instead of using silver nanoparticles, a number of works related to dressing the ZnO nanorods with gold nanostructures have also appeared. Lou et al. reported the fabrication of gold NPs on hydrothermally prepared ZnO NR films using an ion sputtering technique [27]. Their work was mainly concerned with the effect of molecular modification on plasmon‐enchanced DSSCs using the dodecanethiol molecule to protect the gold NPs from the electrolyte. The ZnO/Au/N719/dodecanethiol device recorded the highest IPCE in the range of 440–470 nm, which can be attributed to the “shielding” effect of the dodecanethiol molecules on the exposed gold nanoparticle sites that reduces the charge recombination rate. Similarly, Chen et al. reported the fabrication of a DSSC using vertically aligned ZnO nanorod arrays sensitized with gold nanoparticles [28]. Based on a low‐temperature chemical seeding method, their composite was examined and compared with pure ZnO nanorod arrays, and also with and without the presence of ruthenium dye N719. Obviously, the composite photoanode in the presence of ruthenium dye exhibits the highest power conversion efficiency of 1.2%. They have claimed that the back electron transfer was significantly reduced by the interfacial ZnO–Au Schottky barrier that results in the increased efficiency.

Although the surface plasmon resonance effect is known to significantly enhance the DSSC efficiency, Peh and coworkers reported that the addition of gold nanoparticles on their ZnO nanorods has substantially reduced the overall conversion efficiency [29]. The decrease of conversion efficiency of the pure ZnO nanorod device from 5.2 to 2.5% for the Au–ZnO device is attributed to the aggregation of the gold nanoparticles that distorts the plasmonic effect, thus broadening the spectral enhancement of the device. In addition, this also decreases the surface area of ZnO nanorods adhering to the dye molecules, which dramatically reduces the sunlight conversion process. In terms of the effect of the DSSC’s active area, Bora et al. reported the presence of a highly efficient ZnO/Au Schottky barrier in their ZnO/Au nanocomposite DSSC [30]. They studied the effects of various sizes of the active area in the DSSC, and also the amounts of gold nanoparticles incorporated in their ZnO nanorod photoelectrodes. Their results are in good agreement with the previous work as an approximately60% decrease in PCE was reported when the amount of gold nanoparticles increased by varying the dipping time of the ZnO nanorod photoelectrodes in HAuCl4 · H2O solution (0.01 mM). Furthermore, the impact of DSSC active area size was also reported, as increment in the size of the DSSC active area reveals higher sheet resistance of the used substrates. Therefore, the PCE values decrease as the active area of the DSSC increases. Through this reported work, the importance of optimizing the amount of metal nanoparticles to avoid excess aggregation occurring on the semiconductor layer, in order to fully harness the enhanced plasmonic effect of the DSSCs, is well understood, as is the size of the active area of DSSC to reduce the series resistance occurring on the substrates.

1.2.3 Metal Dressed ZnO Nanoflowers as Photoanodes

ZnO nanoflowers have been of considerable interest to some researchers due to their unique structure. Through the application of the ZnO nanoflowers, it is possible to benefit from the increased dye loading and light harvesting capability whilst still maintaining satisfactory electron conductivity in the DSSC [37, 38]. Although they are plenty of works involving ZnO nanoflowers for DSSCs, there is only one known work reporting on metal dressed ZnO nanoflowers used as a photoanode. This work, by Dhas and co‐workers [31] involves the loading of ZnO nanoflowers on gold nanoparticles through a simple hydrothermal route. Initially, this one‐step method involves the dissolution of zinc acetate, in which the morphology is controlled later by the addition of NaOH, and then loading of gold NPs using HAuCl4. The wurzite structures of ZnO rod‐like petal structures were embedded with gold NPs that are uniformly distributed with an average size of approximately 10 nm, as shown in Figure 1.6. In terms of their DSSC performance, the role of gold nanoparticles on the ZnO nanoflowers was compared with the pure ZnO nanoflower device. As expected, their I–V curves reveal that the bifunctional gold NP‐loaded ZnO nanoflower shows enhanced properties (fill factor (FF) 0.33 and efficiency 2.5%) over the pure ZnO nanoflower device (FF 0.32 and efficiency 1.6%). Such an increase was attributed to the gold nanoparticles reducing the recombination spots due to the oxygen vacancies in ZnO in the surface layers and the surface plasmon resonance effect.

Figure 1.6 (a) SEM image of ZnO nanoflowers with gold nanoparticles and (b) HRTEM image of ZnO nanoflower petal surface loaded with gold nanoparticles. Inset (a) a single ZnO nanoflower with gold nanoparticles and (b) gold nanoparticles loaded ZnO nanoflower petals. (c) J–V curve obtained for N3 dye‐sensitized ZnO nanoflowers films with and without gold nanoparticles.

Reprinted from [31] with the permission of AIP Publishing.

1.2.4 Metal Dressed ZnO Nanowires as Photoanodes

Similar to ZnO nanorods, one‐dimensional ZnO nanowires also exhibits good photoconversion enhancement, which benefits from direct conduction paths for electrons to transport from the point of injection to the collection electrode, making them a much more preferable nanostructure in DSSC application [39, 40.] So far, they have only been reported once, by Lu and coworkers, in literature on metal dressed ZnO nanowires used in DSSC applications [32]. In their project, the synthesis was reported of vertically grown gold NPs on ZnO nanowire arrays (Au NPs @ZnO nanowire) through a two‐step method. Their structure reveals that the size of the gold nanoparticles ranges from 5 to 10 nm and the shell is around 15 nm. Under solar simulation, bare ZnO nanowire arrays and the Au NPs @ZnO nanowires have been compared, with the latter displaying increased PCE values from 0.154 to 0.191% and from 0.269 to 0.387% using different length ZnO nanowire arrays (3 and 7.5 µm, respectively). Through this research, they have concluded that a plasmonic‐enhanced DSSC requires a lower thickness (about 20–40%) to achieve the same efficiency as conventional DSSCs. However, more experimental investigation is still needed in order to verify their conclusion.

1.2.5 Less Common Metal Dressed ZnO Nanostructures as Photoanodes

Although the most commonly reported ZnO nanostructures, such as nanorods and nanoparticles, are widely used in plasmonic‐enhanced DSSCs, less common and unique ZnO nanostructures have also been reported during investigations into the use of other alternative rare nanostructures for enhancing DSSC properties. A sheaf‐like‐ZnO@Ag nanocomposite (sheaf‐like‐(ZnO@Ag)NCM) has been reported as the photoanode in a DSSC application [33]. The synthesis of sheaf‐like‐(ZnO@Ag)NCM was made through a two‐step method whereby ZnO NRs were synthesized using a microwave irradiation method at 150 W. Following this, the preformed ZnO NRs were mixed with AgNO3 before undergoing a reduction process with NaOH to form the sheaf‐like‐(ZnO@Ag)NCM, as shown in Figure 1.7a. The morphological structure obtained (Figure 1.7b) – the sheaf‐like‐(ZnO@Ag)NCM – was composed entirely of 1D closely‐packed nanorods with a typical diameter of about 150–200 nm. It was well understood that the unique sheaf‐like structure formation takes place during the conversion of Ag+ to Ag NPs with NaOH as the reducing agent.

Figure 1.7 (a) and (b) SEM image of sheaf‐like‐(ZnO@Ag)NCM. (c) J–V curves recorded for ZnO NRs/eosin‐Y, sheaf‐like‐(ZnO@Ag)NCM/eosin‐Y, ZnO NRs/N719, and sheaf‐like (ZnO@Ag)NCM/N719 photoanodes with TiO2 NPs modified PEO electrolyte based solid‐state DSSCs under simulated AM 1.5 G solar irradiation of 100 mW/cm2. Area of the cell was 1 cm2. (d) Mechanism of photocurrent generation at sheaf‐like‐(ZnO@Ag)NCM modified photoelectrode based dye‐sensitized solar cells. Where Efs and Efm are Fermi level of semiconductor and metal, respectively. ɸs and ɸm are work function of semiconductor and metal, respectively.

Reprinted from [33] with the permission of AIP Publishing.

In terms of DSSC use, the sheaf‐like‐(ZnO@Ag)NCM was pasted on fluorine‐doped tin oxide (ITO) through a doctor blade method and was dipped in two different sensitizers: eosin‐Y and N719 dye. The J–V graph (Figure 1.7c) shows various photoanodes used to compare the different sensitizers and sheaf‐like‐(ZnO@Ag)NCM with pure ZnO photoanodes. Obviously, the ZnO NR–Ag photoanodes outperformed the pure ZnO NR photoanodes; this was attributed to the effective charge separation and charge transfer and the formation of a Schottky barrier due to the introduction silver nanoparticles on ZnO. As for the different sensitizers, ZnO NR–Ag/N719 exhibits a better PCE value (1.80%) than ZnO NR–Ag/EY (1.39%). Based on this work, the N719 dye remains a more appropriate sensitizer than the eosin‐Y dye, as it has a broader absorption range in the entire visible region of the solar spectrum and the presence of silver NPs.

1.2.6 Comparison of the Performance of Metal Dressed ZnO Nanostructures in DSSCs

The performances of the various metal dressed ZnO nanostructures described in DSSCs are listed in Table 1.1