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Wearable Solar Cells
Understand a groundbreaking new energy technology
Solar energy is one of the most important paths to a sustainable future. In recent years, extensive research and development has begun to produce wearable solar cells, whose novel planar and fiber format gives them enormous flexibility and a wide range of potential uses. The possibility of a solar energy source that can be fitted to the human body promises to become an extraordinary tool for meeting various kinds of personal energy needs.
Wearable Solar Cells: Mechanisms, Materials, and Devices serves as a comprehensive introduction to this cutting-edge technology and its applications. Recent research pointing towards fiber-format solar cells as a bold new frontier is summarized and explored. The result is an essential resource for both experienced researchers and newcomers to the field.
Wearable Solar Cells readers will also find:
- Close coverage of integrated energy harvesting and storage devices
- Detailed discussion of dye-sensitized solar cells, polymer solar cells, perovskite solar cells, and more
- An authorial team with decades of combined research experience
Wearable Solar Cells is ideal for materials scientists, polymer chemists, electrical engineers, solid-state physicists, and advanced students interested in these and related topics.
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Seitenzahl: 427
Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
About the Authors
Preface
1 Introduction
1.1 Development of Wearable Solar Cells
1.2 Characteristics of Fiber‐Shaped Solar Cells
1.3 Functionalization and Integration
1.4 Applications of Wearable Solar Cells
References
2 Working Mechanisms of Solar Cells
2.1 Introduction
2.2 Working Mechanism and Efficiency Limitation for Solar Cells
2.3 Working Mechanism of Dye‐Sensitized Solar Cells
2.4 Working Mechanism of Polymer Solar Cells
2.5 Working Mechanism of Perovskite Solar Cells
2.6 Comparison of Solar Cells for Flexible and Wearable Application
2.7 Conclusions
References
3 Flexible Film Solar Cells
3.1 Overview of Flexible Film Solar Cells
3.2 Flexible Transparent Conductive Electrodes
3.3 Fabrication Techniques of Flexible Film Solar Cells
3.4 Flexible Organic Solar Cells
3.5 Flexible Dye‐Sensitized Solar Cells
3.6 Flexible Perovskite Solar Cells
3.7 Summary and Perspective
References
4 Flexible Fiber Electrodes
4.1 Introduction
4.2 Metal Wires
4.3 Polymer Fibers
4.4 Carbon‐Based Fiber
4.5 Functional Fibers
4.6 Summary and Perspectives
References
5 Fiber‐Shaped Dye‐Sensitized Solar Cells
5.1 Introduction
5.2 Twisting Structure
5.3 Coaxial Structure
5.4 Functionalization and Integration
5.5 Summary and Prospects
References
6 Fiber Polymer Solar Cells
6.1 Overview of Polymer Solar Cells
6.2 Overview of Fiber‐Shaped Polymer Solar Cells
6.3 Polymer Solar Cell Textiles
6.4 Summary and Perspective
References
7 Fiber Perovskite Solar Cells
7.1 Introduction
7.2 Perovskite Photoactive Materials and Solar Cells
7.3 Deposition of Perovskite Layer on Fiber/Wire Surfaces
7.4 Charge Transport Layers for Fiber Perovskite Solar Cells
7.5 Electrode Materials for Fiber Perovskite Solar Cells
7.6 Stretchable and Wearable Fiber Perovskite Solar Cells
7.7 Summary and Perspectives: Challenges for Fiber Perovskite Solar Cells
References
8 Fiber‐Shaped Integrated Device
8.1 Overview of Integrated Device Based on Solar Cells
8.2 Overview of Fiber‐Integrated Device Based on Solar Cells
8.3 Summary and Perspective
References
9 Novel Interfaces in Wearable Solar Cells
9.1 Introduction
9.2 Interfacial Charge Transfer
9.3 Charge Separation
9.4 Interface and Device Performances
9.5 Interface of Functional Wearable Solar Cells
9.6 Interfaces for Encapsulation
9.7 Summary and Outlook
References
10 Textile Solar Cells
10.1 Introduction
10.2 Textile Solar Cell Structures
10.3 Weaving Patterns and Incidence Angle for Textile Solar Cells
10.4 Mechanics and Breathability of Textile Solar Cells
10.5 Encapsulation Structure and Materials for Textile Solar Cells
10.6 Application of Textile Solar Cells
10.7 Summary and Perspectives
References
11 Summary and Outlook
11.1 Introduction
11.2 Development of Planar Solar Cells for Wearable Applications
11.3 Performance Improvement of Fiber and Textile Solar Cells
11.4 Continuous Production of Fiber and Textile Solar Cells
11.5 Potential Applications of Fiber and Textile Solar Cells
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Properties of common metal materials.
Chapter 7
Table 7.1 Fiber‐shaped perovskite solar cell structure and corresponding pho...
List of Illustrations
Chapter 2
Figure 2.1 (a) The incident light from the Sun and the definition of air mas...
Figure 2.2 Schematic drawing showing the typical structure of silicon solar ...
Figure 2.3 (a) Schematic drawing showing the photogeneration of electron‐hol...
Figure 2.4 A typical current density–voltage (
J
–
V
) curves (solid line) and p...
Figure 2.5 Equivalent circuit model for solar cells by taking perovskite sol...
Figure 2.6 (a) The typical structure of dye‐sensitized solar cells. (b) The ...
Figure 2.7 Typical structure of bulk heterojunction polymer solar cells and ...
Figure 2.8 Polymer solar cells with a gradient bulk‐heterojunction structure...
Figure 2.9 Typical structures of perovskite solar cells. (a) The perovskite‐...
Figure 2.10 Normal structure and inverted structure perovskite solar cells a...
Figure 2.11 The model and energy level diagram for the simulation of perovsk...
Chapter 3
Figure 3.1 (a) Room temperature sheet resistances of conductive PET/ITO, PEN...
Figure 3.2 (a) Comparison of square resistance and electrical conductivity o...
Figure 3.3 Major fabricated approaches of graphene synthesis.
Figure 3.4 (a) Sheet resistance versus transparency curves of graphene films...
Figure 3.5 (a) Sheet resistance versus transmittance at 550 nm before and af...
Figure 3.6 Relationship between optical transmittance and surface resistivit...
Figure 3.7 Single substrate‐based fabrication technologies. (a) Casting. (b)...
Figure 3.8 (a) SEM images of perovskite films prepared with different solven...
Figure 3.9 (a) Schematic illustration of the key steps involved in slot‐die ...
Figure 3.10 (a) Roll‐to‐roll‐processed organic solar modules with screen‐pri...
Figure 3.11 General configuration of a roll‐to‐roll printing and coating sys...
Figure 3.12 (a) A schematic illustration presented thickness for flexible OS...
Figure 3.13 (a) Chemical structures of PM6, Y6, and PC71BM. (b) The schemati...
Figure 3.14 (a) Schematic diagram displaying the Ag reduction and welding pr...
Figure 3.15 (a) Typical structure and (b) operational principle of FDSSCs.
Figure 3.16 (a) TiO2 NRs/Er
3+
and Yb
3+
co‐doped TiO
2
NPs structure (...
Figure 3.17 (a) Schematic of the failure model for the Flexible DSSCs under ...
Figure 3.18 Variety of device architectures for perovskite solar cells. (a) ...
Figure 3.19 (a) Schematic of meniscus‐coating for perovskite film. (b) Biomi...
Figure 3.20 (a) Schematic illustration of the functionalization process of p...
Chapter 4
Figure 4.1 (a) Morphological comparison before and after anodic oxidization ...
Figure 4.2 (a) Top schematic view of fiber‐shaped solar cell. SEM images of ...
Figure 4.3 (a) The cross‐section SEM images of the flexible conductive cotto...
Figure 4.4 (a, b) Photographs of the stretchable electrodes with different s...
Figure 4.5 (a) Schematic illustration of the wet‐spinning process. The singl...
Figure 4.6 (a–c) SEM images of NiCo
2
S
4
‐CF in different magnifications.(d...
Figure 4.7 (a) Schematic illustration of the experimental setup for fabricat...
Figure 4.8 (a) Cartoon of SWCNTs with van der Waals contact. (b) Formed char...
Figure 4.9 (a) TEM image of SWCNTs in CSA. (b) The wet‐spinning process of f...
Figure 4.10 (a) SEM images of the twist‐drawing spinning process. (b) SEM im...
Figure 4.11 (a) Pulling a MWCNT film through ethanol into a tight fiber. (b)...
Figure 4.12 SEM images of nanotubes before (a) and after (b) incorporation o...
Figure 4.13 Top view (a) and working mechanism (b) of the all carbon‐based D...
Figure 4.14 (a) Schematic illustration of the fiber‐shaped polymer solar cel...
Figure 4.15 Schematic illustration (a) and
J
–
V
curve (b) of the double‐twist...
Figure 4.16 (a) Optical photograph of the thin graphene oxide film. (b) SEM ...
Figure 4.17 (a) Continuous wet‐spinning of graphene oxide fibers from aqueou...
Figure 4.18 (a) Schematic illustration of the DSSC using the graphene fiber ...
Figure 4.19 (a) Schematic illustration for fabrication of stretchable conduc...
Chapter 5
Figure 5.1 Schematic illustration of the working mechanism of a DSSC.
Figure 5.2 Schematic illustration of a fiber‐shaped DSSC fabricated from two...
Figure 5.3 (a) Cross‐sectional structure of the fiber‐shaped DSSC. (b) The o...
Figure 5.4 (a) Schematic illustration of a fiber‐based DSSC. (b) Current–vol...
Figure 5.5 Schematic illustration of the novel fiber‐shaped DSSC from the (a...
Figure 5.6 (a) PEOx‐induced permanent dipole moment binding at TiO
2
/N719 dye...
Figure 5.7 (a) Schematic illustration of the transmission path of electrons ...
Figure 5.8 (a, b) PCEs of the fiber‐shaped DSSC as a function of bending ang...
Figure 5.9 (a, b) A graphene fiber at different magnifications. (c) Cross‐se...
Figure 5.10 (a) Schematic illustration of a single‐wire DSSC with a coaxial ...
Figure 5.11 Scanning electron microscopy (SEM) images of the coaxial fiber‐s...
Figure 5.12 Magnetic responses of the superparamagnetic nanoparticle‐based c...
Figure 5.13 (a) Schematic illustration of a stretchable fiber‐shaped DSSC. (...
Figure 5.14 (a) Schematic illustration of the structure of a stretchable fib...
Figure 5.15 (a) Schematic illustration of the integrated wire‐shaped device ...
Figure 5.16 (a) Schematic of a fiber‐based multi‐energy device comprising a ...
Figure 5.17 (a) Structure of the wire‐shaped energy device. (b) Working mech...
Figure 5.18 (a) Photocharging and discharging curves with increasing PC unit...
Chapter 6
Figure 6.1 Schematic illustration (a) and working principle (b) of a typical...
Figure 6.2 Schematic illustration of the first PSC based on a twisting struc...
Figure 6.3 Schematic illustration of a twisting structural fiber‐shaped PSC ...
Figure 6.4 (a) Schematic illustration of the fabrication process of a stretc...
Figure 6.5 (a) Schematic illustration of a fiber‐shaped PSC with active mate...
Figure 6.6 Schematic illustration of a fiber‐shaped PSC's structure and work...
Figure 6.7 Schematic illustration of fabrication process of fiber‐shaped PSC...
Figure 6.8 Schematic illustration of the fabrication process of a fiber‐shap...
Figure 6.9 (a) and (b), A stretchable PSC textile being integrated into clot...
Figure 6.10 (a) Schematic illustration of fabrication process of PSC textile...
Figure 6.11 (a) Schematic illustration of the PSC textile in different degre...
Chapter 7
Figure 7.1 Schematic drawing shows the typical structure of perovskite mater...
Figure 7.2 The performance of planar‐shaped and fiber‐shaped perovskite sola...
Figure 7.3 The schematic structure of fiber‐shaped perovskite solar cell wit...
Figure 7.4 The optical modeling of a planar‐shaped and a fiber‐shaped perovs...
Figure 7.5 Typical nucleation and crystal growth process for perovskite thin...
Figure 7.6 The coating methods for perovskite photoactive layers.
(
a) dip‐co...
Figure 7.7 Representative fiber‐shaped perovskite solar cell structures. (a)...
Figure 7.8 The uniform coating of perovskite layer around a curved surface w...
Figure 7.9 The ETL structures that are used in fiber‐shaped perovskite solar...
Figure 7.10 Stretchable fiber‐shaped perovskite solar cells, structure, and ...
Figure 7.11 Energy textiles based on fiber‐shaped perovskite solar cells and...
Figure 7.12 The strain properties of perovskite layers on a curved surface a...
Chapter 8
Figure 8.1 Two terminal integrated devices of a n‐Cd(Se, Te)/Cs
2
S
x
/SnS photo...
Figure 8.2 A three‐terminal integrated device based on a DSSC and a supercap...
Figure 8.3 (a–c) Schematic illustration and SEM images of the fiber‐integrat...
Figure 8.4 (a) and (b), Schematic illustration of the structure of a self‐po...
Figure 8.5 (a) Schematic illustration of the all‐solid‐state fiber integrate...
Figure 8.6 (a–f) Schematic illustration of the fabrication process of a fibe...
Figure 8.7 (a) Schematic of the multi‐energy device containing a nanogenerat...
Figure 8.8 Design and principle of a 3D optical fiber‐based integrated devic...
Figure 8.9 The fabrication of TENG cloth. (a) Schematic illustration of the ...
Figure 8.10 (a) Schematic illustration of the structure of an all‐solid inte...
Figure 8.11 (a) Schematic illustration of transportable lightweight cloth wi...
Chapter 9
Figure 9.1 (a) Schematic illustration of the general structure of a planar p...
Figure 9.2 Fitting of the conductivity data of our MWCNT fibers with two dif...
Figure 9.3 (a) Highly conductive carbon‐based fibers prepared by modified me...
Figure 9.4 (a) Schematic structural diagrams of a fiber‐shaped dye‐sensitize...
Figure 9.5 Photographs of the TiO
2
coating before (a, c) and after five cycl...
Figure 9.6 (a) Schematic of the fiber‐shaped dye‐sensitized solar cell struc...
Figure 9.7 (a) SEM images of different TiO
2
morphologies constructed in fibe...
Figure 9.8 Electrolyte infiltration into dye‐loaded TiO
2
nanotubes differs i...
Figure 9.9 The illustration of bare TiO
2
(a) and DNA‐modified TiO
2
surfaces ...
Figure 9.10 Schematic illustration of fiber‐shaped dye‐sensitized solar cell...
Figure 9.11 Schematic diagrams of typical fiber‐shaped solar cells with (a) ...
Figure 9.12 Surface morphology of perovskite film after (a) dip‐coating once...
Figure 9.13 Bending properties of fiber‐shaped solar cells with twisting str...
Figure 9.14 (a) Dependence of the PCE on the bending‐cycle number.(b) De...
Figure 9.15 (a) Schematic illustration of the fabrication of elastic conduct...
Figure 9.16 (a) Schematic of fiber‐based multi‐energy device comprising a na...
Figure 9.17 (a) Optical image of the flexible fiber‐shaped solar cell. (b) D...
Figure 9.18 (a) Schematic illustration of the structure of the fiber‐shaped ...
Figure 9.19 (a) The spiral‐shaped photoanode with gel electrode covered. The...
Figure 9.20 (a) SEM images of the twisted CH
3
NH
3
PbI
3−
x
Cl
x
/MWCNT photoa...
Chapter 10
Figure 10.1 The development requirement of wearable textile solar cells are ...
Figure 10.2 Textile solar cells based on various construction substrates and...
Figure 10.3 Wearable solar cells fabricated based on textile substrate. (a) ...
Figure 10.4 Strip‐shaped planar‐type solar cell weaved with cotton yarn to f...
Figure 10.5 Textile solar cells based on stacking textile electrodes. (a) Te...
Figure 10.6 Fiber‐shaped solar cells weaving into a flexible textile solar c...
Figure 10.7 Interlaced fiber electrode structured textile solar cells. (a) D...
Figure 10.8 Weaving pattern and effect of shading on the illumination area. ...
Figure 10.9 The effect of illumination light incidence angle on the device p...
Figure 10.10 Mechanical properties of textile solar cells. (a) Bending defor...
Figure 10.11 Encapsulation structure and washing stability. (a) Encapsulatio...
Figure 10.12 Wearable application of textile solar cells by integrating with...
Chapter 11
Figure 11.1 Wearable solar cell applications based on flexible planar‐type s...
Figure 11.2 Implantable photovoltaic solar cells for wearable applications....
Figure 11.3 The mechanical properties of typical functional layer and photoa...
Figure 11.4 Wearable sensor systems requiring wearable energy supply systems...
Guide
Cover
Table of Contents
Title Page
Copyright
About the Authors
Preface
Begin Reading
Index
End User License Agreement
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Wearable Solar Cells
Mechanisms, Materials, and Devices
Hao SunZhibin YangLongbin QiuHuisheng Peng
Authors
Dr. Hao SunFrontiers Science Center for Transformative MoleculesSchool of Chemistry and Chemical EngineeringZhangjiang Institute for Advanced StudyKey Laboratory of Green and High‐End Utilization of Salt Lake Resources (Chinese Academy of Sciences)Shanghai Jiao Tong UniversityShanghai 200240China
Dr. Zhibin YangSchool of Chemistry and Chemical EngineeringShanghai Jiao Tong UniversityShanghai 200240China
Dr. Longbin QiuSUSTech Energy Institute for Carbon NeutralityDepartment of Mechanical and Energy EngineeringSouthern University of Science and TechnologyShenzhen 518055China
Prof. Huisheng PengState Key Laboratory of Molecular Engineering of PolymersDepartment of Macromolecular Science and Laboratory of Advanced MaterialsFudan UniversityShanghai 200438China
Cover Image: © Andrey Suslov/Getty Images
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2024 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐35055‐1ePDF ISBN: 978‐3‐527‐83822‐6ePub ISBN: 978‐3‐527‐83823‐3oBook ISBN: 978‐3‐527‐83824‐0
About the Authors
Dr. Hao Sun is currently a tenure‐track associate professor at Shanghai Jiao Tong University. He received his BE in Polymer Materials and Engineering from Sun Yat‐Sen University in 2012 and his PhD degree in Polymer Chemistry and Physics from Fudan University in 2017 and then worked at Stanford University as a Postdoc Fellow. His research interests include fiber electronics and energy storage devices. He has published over 60 papers and authorized ten invention patents. He also contributes to five book chapters. He received the Materials Research Society (MRS) Graduate Student Gold Award and the Geneva International Exhibition of Inventions Silver Medal.
Dr. Zhibin Yang is currently a tenure‐track associate professor at Shanghai Jiao Tong University. He received his bachelor's degree from East China University of Science and Technology in 2009 and his PhD from Fudan University in 2014. And then he successively worked as a postdoc at the University of Washington and the University of North Carolina at Chapel Hill between 2014 and 2019. His research interests focus on photovoltaics. Till now, he has published over 80 papers and authorized 11 invention patents. He was also awarded the IUPAC‐SOLVAY International Award for Young Chemists, the MRS Graduate Student Silver Award, etc.
Dr. Longbin Qiu is currently a tenure‐track assistant professor at the Southern University of Science and Technology. He received his BE in Polymer Materials and Engineering from Sun Yat‐Sen University in 2011 and his PhD in Polymer Chemistry and Physics from Fudan University in 2016 and then worked at Okinawa Institute of Science and Technology as a Postdoc Fellow. His current interests focus on flexible and wearable electronics and scalable and stable photovoltaic devices. He has published over 80 papers and authorized three invention patents.
Dr. Huisheng Peng is currently a university professor at Fudan University. He received his BE in Polymer Materials from Donghua University in China in 1999, his MS in Macromolecular Chemistry and Physics from Fudan University in China in 2003, and his PhD in Chemical and Biomolecular Engineering from Tulane University in the USA in 2006. He then worked at Los Alamos National Laboratory in the USA between 2006 and 2008 and joined the Laboratory of Advanced Materials and Department of Macromolecular Science at Fudan University in 2008. He focuses on fiber electronics.
Preface
The ever‐growing demands for wearable devices have facilitated the development of the corresponding power sources in both flexible energy harvesting and storage devices. In particular, flexible solar cells in a wearable format can be ideal candidates to ease the frequent charging process, thus promoting the operation duration of wearable devices. However, conventional solar cells that are bulky and rigid cannot fully meet wearable requirements such as portability, flexibility, and breathability. To this end, a variety of wearable solar cells in a novel fiber format have attracted extensive attention attributed to their unique and promising features. For instance, their high flexibility ensures stable performances under complex deformations such as twisting and stretching, so they can adapt to irregular substrates effectively in various applications. More importantly, fiber solar cells can be woven into textiles that breathe freely or integrated into multifarious structures to fit the human body.
This book provides an overview of the basic mechanisms and state‐of‐the‐art development of different types of wearable solar cells, including but not limited to dye‐sensitized solar cells, polymer solar cells, and perovskite solar cells. In particular, wearable solar cells in a novel fiber format are highlighted as they represent an emerging and attractive field in the past decade. In addition, the integration and functionalization of energy harvesting and storage devices in a wearable format will be covered with a variety of promising applications. We also summarize the current challenges and future opportunities for wearable solar cells, particularly for the continuous production of long‐fiber solar cells and the weaving techniques for wearable applications. It covers both fundamental and application aspects, which are helpful for people with various backgrounds. For example, what are the unique features of fiber solar cells compared to conventional planar ones? At the highly curved interfaces of fiber electrodes, how can charge transport and separation occur? The readers may find the answers in this book.
This book can be a useful resource for scientists and engineers in industry, which provides insights for emerging fields. It may bridge the academic and industrial communities, inspiring professors and students in material science, energy, electrochemistry, applied physics, nanoscience and nanotechnology, and polymer science and engineering.
We would like to acknowledge our graduate students who participated in writing this book: Chapter 1 was drafted by Zhaofeng Ouyang, Chapter 3 by Jinglin Sun, Chapter 4 by Shuo Wang, Chapter 5 by Qiuchen Xu, Chapter 8 by Jie Zhou, Tianyu Wen, Yang Shi, and Yue Wu, and Chapter 9 by Bin Yuan.
November 12, 2023Shanghai
Hao SunZhibin YangLongbin QiuHuisheng Peng
1Introduction
1.1 Development of Wearable Solar Cells
Wearable devices can be worn close to the human body, have a good fit on irregular body surfaces, and perform ideal functions when worn. They are considered to be the next‐generation electronic devices with broad applications. They thus enable a variety of desirable and unfulfilled functions, including but not limited to energy harvesting, storage, sensing, display, and actuation in a wearable format, which can reshape our traditional lifestyle [1]. Wearable solar cells, an important branch of wearable devices, can convert light into electric energy, namely the photovoltaic conversion process, in a wearable format and can be readily integrated with other wearable devices such as supercapacitors, batteries, sensors, displays, and actuators [2, 3]. These appealing features and potential applications have greatly facilitated the development of wearable solar cells in the past decades.
Although thin‐film solar cells have been investigated as potential candidates in wearable applications, their limited flexibility generally results in inferior interfacial stability that severely degrades their photovoltaic conversion performance under large deformation in wearable applications [4]. More importantly, the two‐dimensional configuration sacrifices breathability and cannot meet the requirements for breathability and comfortability. Therefore, the emergence of fiber‐shaped solar cells has offered a promising approach to realizing fully wearable solar cells [5]. In 2002, a pioneering work reported a fiber‐shaped dye‐sensitized solar cell (DSSC), in which the photoactive materials were layer‐by‐layer coated on a metal wire, shedding light on the feasibility of fiber‐shaped solar cells with promising wearability [6]. This also inspired the production of a fiber‐shaped polymer solar cell five years later, which consisted of photoactive materials coaxially incorporated into an optical fiber [7]. In 2014, the first perovskite solar cell in a fiber format was developed with all‐solid‐state characteristics, which were highly appealing for the requirement for practical applications [8]. Over the past twenty years, a variety of exciting progresses has been achieved in device structure, fiber electrode, photoactive material, and electrolyte, as well as the functionalization, integration, weaving, and application of the obtained fiber‐shaped and textile solar cells. These will be mainly introduced in the other chapters of this monograph with more details.
1.2 Characteristics of Fiber‐Shaped Solar Cells
As mentioned above, wearable solar cells in the fiber format can afford a variety of attractive characteristics that benefit wearable applications. For instance, fiber‐shaped solar cells generally demonstrate excellent flexibility owing to their unique one‐dimensional configuration, which can tolerate multiple deformations such as bending, twisting, and even stretching with specific device design [9]. This overcomes the long‐standing bottleneck for traditional silicon‐based solar cells, which are highly rigid and cannot fully meet the requirement of flexibility for wearable applications [10].
For instance, fiber‐shaped solar cells can absorb light from all directions, recognized as the three‐dimensional light harvesting capability originating from the unique one‐dimensional configuration of the device. For traditional solar cells, such as Si‐based solar cells, the power conversion efficiencies (PCE) are highly dependent on the incident light angle. In contrast, fiber‐shaped solar cells can harvest light from all directions without significant performance fluctuation. This is an attractive feature for practical indoor applications in which scattered light is highly abundant. This unique feature allows rational device design to further improve the photovoltaic conversion performance toward practical applications.
The weavability of fiber‐shaped solar cells represents another key advantage relevant to their production and application. Ideally, fiber‐shaped solar cells can be woven into various forms, such as clothing, curtains, and tents, either alone or with other common fibers through proven textile manufacturing techniques such as knitting and weaving [11]. The weavability is highly dependent on the strength and flexibility of the fiber‐shaped solar cells as the basic building blocks, which play a key role in the continuous weaving process. The resulting fabrics and textiles can not only inherit the flexibility of the fiber devices but, more importantly, show breathability different from all their bulky and planar counterparts, which provides the best wearability for the resulting devices. This allows for producing wearable solar cells with the best flexibility and breathability that cannot be easily achieved for state‐of‐the‐art solar cells.
Despite the above advantages, fiber‐shaped solar cells still suffer from several disadvantages that hinder their practical production and application [12]. For instance, the electrical resistance of fiber‐shaped solar cells generally rises when the fiber length increases, which requires the use of highly conductive fiber electrodes. In addition, the light shielding issue based on either twisting or coaxial structure remains a general challenge for fiber‐shaped solar cells. These should be taken into serious consideration and will be introduced in more detail in the corresponding chapters.
1.3 Functionalization and Integration
Functionalization and integration have been widely recognized as an important branch for fiber‐shaped solar cells, which accommodate complex and harsh applications. For instance, fiber‐shaped devices are required to be remotely controlled when a physical detachment is not available in aerospace, which gives birth to a novel magnetic response fiber‐shaped solar cell [13]. The Fe3O4 nanoparticles were incorporated into multi‐walled carbon nanotube (MWCNT) fiber via a dry‐spinning method, offering a high saturation magnetization of 17.9 emu/g. With the above Fe3O4/MWCNT hybrid fiber as the counter electrode and a modified Ti wire as the working electrode, the obtained fiber‐shaped solar cells could be reversibly attached to and detached from a substrate controlled by a magnet. This work also sheds light on smart wearable solar cells through rational material design for practical applications.
The safety issue of fiber‐shaped solar cells is very important because they are closely attached on the human body. This inspires the development of safe materials to promote device safety. For instance, a polymer‐ionic liquid (IL) gel electrolyte was developed to replace the flammable carbonate and ether solvents, which significantly improved the safety and long‐term stability of the obtained fiber‐shaped solar cells [14]. Hydrophobic polyvinylidene fluoride‐hexafluoropropylene copolymer and 1‐butyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl) imide (BMImTFSI) IL served as the gelata and solvent, respectively, which could maintain the quasi‐solid state from room temperature to 300 °C. The resulting fiber‐shaped solar cells thus exhibited improved safety and long‐term stability and could retain 90% of the original PCE after 30 days.
Efficient integration of fiber‐shaped solar cells for enhancement of wearable applications also represents an important direction for real‐world applications of fiber‐shaped solar cells. To simultaneously realize energy harvesting and energy storage, Bae et al. integrated nanogenerator, supercapacitor, and DSSCs into a single fiber, which could convert the mechanical and optical energies into electric energy and store it in the supercapacitor [15]. Specifically, ZnO nanowires decorated with Kevlar fiber served as a common electrode, and the transparent graphene on Cu mesh was wound around the common electrode as another electrode. Although the total PCE of the integrated device was only 0.02%, this pioneering work showed the feasibility of rational integration of fiber‐shaped electronics, which opens a new avenue for further improvement.
1.4 Applications of Wearable Solar Cells
The unique characteristics of fiber‐shaped solar cells have made them promising candidates in a broad range of wearable applications, including but not limited to consumer electronics, the Internet of Things (IoT), and smart healthcare, which promise to innovate the conventional function paradigm. Their unrivaled flexibility enables the incorporation of irregular devices and substrates and provides a reliable power supply to diverse electronic devices with high performance.
For instance, wearable solar cells can power consumer electronics in a novel and fascinating way. Owing to their high flexibility and wearability, they can be easily integrated onto the surface of electronic devices without compromising their original flexibility and functionality. In addition, they can significantly extend the life span of consumer electronics as a secondary power source. They can form self‐sustaining electronics, for example, a self‐powered sports sensor that can generate electric power from outdoor or indoor light, which could overcome the bottleneck of wearable power supplies.
The IoT has been rapidly growing in the past few years, which can realize the connection of numerous electronic devices (e.g. wireless sensors, actuators, transmitters, and wearable devices) for efficient information interaction. All these devices require a stable power supply, which demands self‐sustainable power sources such as wearable solar cells [16]. Therefore, fiber‐shaped solar cells can be ideal candidates to support the IoT in a highly flexible and portable way. To achieve this goal, the power conversion efficiency, lifetime, and integration of fiber‐shaped solar cells should be taken into serious consideration. Overall, fiber‐shaped solar cells can significantly contribute to the development of IoT and open a new avenue to achieve the goal of “Internet of Everything” for sufficient information interaction and usage.
Another promising application of fiber‐shaped solar cells lies in smart healthcare, such as wearable medical devices. For example, a wearable health monitoring device equipped with a fiber‐shaped solar cell panel can continuously track the heart rate, blood pressure, and oxygen levels. It can potentially avoid replacing energy storage devices such as batteries to reduce the infection risk during surgery. Therefore, they can be potentially used to power implantable devices such as pacemakers, defibrillators, or neural stimulators [17]. Of course, there are still some concerns, such as toxicity, biocompatibility, and device stability for implementable applications. This interdisciplinary and innovative direction has attracted broad attention in recent years, which may create a brand new “wearing” way to extend the application of fiber‐shaped solar cells.
In summary, we introduce the development of wearable solar cells, particularly in a flexible fiber format, which demonstrates a variety of appealing advantages such as high flexibility, wearability, and weavability. The exploration of functionalization and integration has made them promising candidates in a variety of applications, including consumer electronics, IoT, and smart healthcare devices. We will introduce more details on the background and progress of wearable solar cells in the other chapters of this monograph, providing a comprehensive view of this emerging and highly promising field.
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2Working Mechanisms of Solar Cells
2.1 Introduction
Solar energy is the most abundant energy around the earth. All the energy on earth could be sourced back to the solar energy. The power that reaches the surface of the earth is in the magnitude of 1017 W [1, 2]. On the other hand, for the whole human society, the energy requirement is only around tens of 1012 W [3]. However, the solar energy cannot be used directly, and most of the energy is reflected and transferred to the heat in the environment [2]. Solar cells are a kind of device that can convert light energy directly to electricity, which makes great use of abundant solar energy and is especially applicable for portal and wearable electronics. The solar cells work based on the photovoltaic phenomenon, which was developed a hundred years ago. It was until the 1950s that the first practical silicon‐based solar cell was fabricated and showed potential practical applications for energy conversion, especially in a clean way and also providing electricity in outer space [4]. How does the solar cell work, and how much energy could be converted by the solar cell? It is essential to understand the basic principles and mechanisms of the photovoltaic process. In this chapter, we will briefly introduce the working mechanism of typical solar cells and the efficiency limitations of these cells. The readers could find more detailed information regarding the basic device physics in the textbook. The working mechanism for traditional silicon‐based solar cells is first summarized to elucidate the physical principle in photovoltaics [5, 6]. Aiming at the wearable and flexible applications, especially, the main efforts are then made to discuss the different mechanisms for different types of solar cells, i.e. dye‐sensitized solar cells [3], polymer solar cells [7], and perovskite solar cells [8]. The resulting advantages and disadvantages of the above solar cells are highlighted, particularly from a viewpoint of making wearable photovoltaic devices, to indicate their advancements and challenges in the past decade.
2.2 Working Mechanism and Efficiency Limitation for Solar Cells
Solar cells are fabricated based on semiconductors and, in the early days, were mainly based on Germanium (Ge), Silicon (Si), and GaAs from the development of semiconductor electronic devices [9]. Nowadays, the main product for commercialized solar cells is Si cells, which occupy over 90% of the market [10]. There are second‐generation thin‐film solar cells like cadmium telluride (CdTe) solar cells and copper indium gallium selenide (CIGS) solar cells, which are being developed and increasing market sharing [10]. The recently developed thin film solar cells, like dye‐sensitized solar cells, polymer solar cells, and perovskite solar cells, are promising for flexible and wearable applications. However, they are still in the lab proof‐of‐concept stages.
To understand how much energy is to be converted, the input power from the solar should be elucidated first (Figure 2.1) [11]. The sunlight incident from outer space to the surface of the ground, where the solar cells are mounted. The solar power is daytime‐ and season‐sensitive due to the varying incidence angle [12]. Due to the different incidence angles, the light paths are different (Figure 2.1a). The light spectrum that reaches the outer space of the earth is defined as air mass 0 (AM0). While with a direct angle incidence reaching the ground surface, the spectrum is defined as AM1.0. The other light incidence spectra are defined as the inverse of the cosine of the incidence angle [11]. The solar spectra measured with different air masses have been obtained, and it has been set that the standard solar spectrum for the solar simulator is AM1.5. Interestingly, it is based on the measured spectrum, the surface temperature of the Sun is supposed to be 6000 K when comparing a theoretical black body emission spectrum (Figure 2.1b). Due to the reflection and scattering of light in the cloud and also in the space, the real spectra on the ground surface are different from the ones in outer space. For a normal solar cell, the light spectrum that is to be considered is the AM 1.5G (global), which includes direct light and also reflective light. While for the application of concentrated solar cells, the spectra to be used are the AM 1.5D (direct), which only includes the direct incidence light [11]. Standard light intensity for AM 1.5G is 100 mW/cm2, while for most of the wearable and portable sensor systems, the energy requirement is only microwatt to several mW. The input energy from the Sun is beyond enough for application [13].
The most applied photovoltaic devices are silicon solar cells, with several typical high‐performance structures shown in Figure 2.2[5]. The silicon solar cells have a theoretical efficiency limit of 29.4% [5], and currently, the highest certified record efficiency is 26.8% [14], close to its limitation. The basic model for Si solar cells is a p–n junction, with a hole majority carriers (p‐type) and electron majority carriers (n‐type) silicon materials forming a junction, as shown in Figure 2.3a. By forming a p–n junction, the majority of electrons in n‐type semiconductor and majority of holes in p‐type semiconductor would diffuse through the junction due to the density gradient. And a voltage potential would be formed due to the movement of electrons and holes. With a balance of the drift and diffusion movement of electrons and holes, a built‐in potential (Vbi) would be obtained, with a higher electric potential in the n‐type side and a lower potential in the p‐type side. One should note that for electrons, the direction of the driven force is from the lower electric potential pointing to the higher potential [6].
Figure 2.1 (a) The incident light from the Sun and the definition of air mass (AM). (b) The spectral irradiance of the sunlight passing through different paths.
Source: Rühle [11], © 2016/Elsevier.
Figure 2.2 Schematic drawing showing the typical structure of silicon solar cells. (a) A top/rear contacted homojunction solar cell. (b) A top/rear contacted heterojunction solar cell. (c) A homojunction interdigitated back contact solar cell. (d) A heterojunction cell interdigitated back contact.
Source: Yoshikawa et al. [5], © 2017/Springer Nature.
Figure 2.3 (a) Schematic drawing showing the photogeneration of electron‐hole pairs in a semiconductor. (b) The typical energy level diagram shows the formation of p–n junction and built‐in potential under illumination.
Source: Islam and Saraswat [15], © 2018/Springer Nature/CC BY 4.0.
Under light illumination, the photons will be absorbed, and electron–hole pairs formed. With photons that have energy larger than the band gap, the semiconductors could absorb the photons, and the electrons in the valence band could be excited into the conduction band, forming an electron–hole pair [16]. Due to the driven force of the built‐in potential (Figure 2.3b), the electrons in the junction will move toward to the n‐type side and then transfer to the external circuit. Connecting to the variant load in the external circuit, there will be a potential drop which is referred to as V. And the effective driven force for the movement of electrons will be altered as q(Vbi − V) as shown in Figure 2.3b. The external circuit potential V is also equal to the difference of quasi‐fermi level for electrons and holes [(EFn − EFp)/q]. By varying the load from zero to infinite, short‐circuit and open‐circuit conditions are obtained. With a specific load, the maximum energy output could be achieved, and the typical I–V curve and power output‐voltage curve are shown in Figure 2.4. The output power could be calculated by multiplying the external circuit current I with voltage drop V:
Figure 2.4 A typical current density–voltage (J–V) curves (solid line) and power as a function of external circuit voltage (dash line) for solar cells.
Source: Saive [17], © 2019/IEEE/CC BY 4.0.
While the power input could be represented as:
where Φ is the incidence of light intensity.
Thus, the power conversion efficiency (PCE) could be calculated as:
where J is the representation of current density.
The most important efficiency and the solar cell efficiency are defined at the maximum power point (Jmax and Vmax). To correlate the solar cell efficiency with the intrinsic properties of photoactive materials, the current density at short circuit condition (JSC) and the output voltage at open circuit condition (VOC) are incorporated into the calculation of efficiency.
The fill factor (FF) is defined as:
For a solar cell, the external circuit current is a function of the output voltage, which means the three parameters (JSC, VOC, and FF) are not completely independent. Obtaining the function between the J and V could be the key to the estimation of the theoretical efficiency limit based on photoactive material properties. There are three main models that describe the function describing the relation between J and V and will be discussed in detail, namely (i) detailed balance limitation, (ii) drift‐diffusion model, and (iii) equivalent circuit model. The realistic model considering recombination process, as shown in Figure 2.3b, will also be included. The mechanism and theoretical model are all applicable to the other solar cells, including dye‐sensitized solar cells, polymer solar cells, and perovskite solar cells, which will be discussed in the following sections.
2.2.1 Detailed Balance Limitation
The theoretical efficiency limit of solar cells is of interest to optimize the design of photoactive materials and device structures. The most popular model was the so‐called S–Q limit that based on thermodynamic law (detailed balance limit) proposed in 1961 [18]. The basic two assumptions for the detailed balance limit are (i) the photons with energy larger than the band gap will be absorbed, and one photon converts to one electron–hole pair and (ii) the excited electrons will be relaxed to the conduction band edge for all the electrons that has energy much higher than the band gap. The number of photons that could be absorbed depends on the incident light spectra. For comparable results from around the community, a standard incident light spectrum was introduced, which is AM 1.5G (Figure 2.1b). It could be easily understood that with a smaller band gap, a larger number of photons will be absorbed. However, in that case, the energy potential for each excited electron will also be smaller. The total energy output depends on the product of a number of photons (related to J) and energy potential (related to V).
To be specific, the photogenerated carriers that are excited by incident photons could be calculated by integrating the number of photons with energy higher than the band gap, as shown in Eq. (2.6).
Here the α(E) is the photon absorption coefficient as a step function of energy. When the photon energy is larger than bandgap Eg, the coefficient is 1. When the photon energy is smaller than the band gap, the coefficient is 0. And the Θ(E) is the photon number density in the energy range of E to E+dE.
Based on Kirchhoff's law, there will be light emission from the photoactive materials, and the emission spectrum is related to the temperature of the cell (black body). This light emission is assigned to the radiative recombination process, which is unavoidable. The number of photons emitted is called the reverse saturation current in a p–n junction [19]. Other than radiative recombination that would reduce the photogenerated electron–hole pairs, there is also non‐radiative recombination process. For a conserve of the particles (photons, electrons), the external circuit current density J at a steady state of the p–n junction could be described as:
where JSC is the maximum current density calculated based on the energy band gap. Jr(V) is the reverse current density is related to radiative recombination. Jr(0) is the carrier generation from environmental thermal energy in the dark condition. Jnr(0) and Jnr(V) are the electron–hole pair generation process and recombination process that are related to non‐radiative process, respectively. Based on the conventional semiconductor p–n junction current density–voltage relation:
where q is the elementary electron; k is the Boltzmann constant; T is temperature.
We can rewrite the above equation by defining a new factor fc, which is the faction of radiative recombination as: [18]
and
When fc equals to unity, there is only a radiative recombination process, and it shows the highest VOC. And Jr(0) is relatively small compared with JSC and could be neglected.
The output energy could be calculated by multiplying the external current density Jext and voltage potential across the load V.
By plotting the above equation, the maximum power output voltage Vmax and current density Jmax could be obtained. Thus, the PCE could be obtained from the above‐mentioned Eqs. (2.3) and (2.4). And the PCE could be described as VOC*JSC*FF/Pin. For AM 1.5G, the Pin is defined as 100 mW/cm2. The theoretical PCE is only a function of the energy band gap of a photoactive material based on a detailed balance limit [11].
However, the calculation is based on the assumption that the light absorbance coefficient is infinite and could absorb all the light. On the other hand, the mobility is also assumed to be infinite, that all the generated electron–hole pairs could be separated into corresponding electrodes. The above calculation is also only based on the spectrum and thermodynamic theory. Regarding the faction of radiative recombination fc, the calculation tells us no information about the origin of the non‐radiative recombination process. Based on the electric magnetic theory and the electronic properties of the functional materials, the device performance might be more realistic.
2.2.2 Drift‐Diffusion Model
For the drift‐diffusion model, the basic equations are the one‐dimensional continuity equation and the Poisson's equation [20, 21].
Here, Nn(p)(x, t) is the number density of electrons (holes) in the position of x and at time t. And Jn(p)(x, t) is the current vector that flows through position x at time t. G(x, t) and R(x, t) are the generation and recombination rate of electrons or holes in position x at time t. The continuity equation is the conservation of particles. Due to the planar structure of solar cells and the direction of the voltage potential, the electrons (holes) only move in the direction that is perpendicular to the solar cell plane. And one‐dimensional continuity equation is considered. The change of the electrons (holes) in an infinite thin space is equal to the change of the carrier density plus the change from the flow. When in a steady state, the change of current density is zero. The net change equals to the generation of carriers (G(x, t) – R(x, t), the difference between generation and recombination). While for the Poisson's equation, it is the correct electrical response of a device in a steady state following Maxwell's equation.
Other than the continuity equation and the Poisson's equation, the movement of carriers follows the drift‐diffusion equation. The drift of carriers describes the moving under the electric field, and the diffusion describes the moving force of the density gradient. Based on Einstein's relation, the diffusion coefficient (D) and carrier mobility (μ) are related as Dn(p) = μn(p)*k*T/q. The current flow from the movement of electrons and holes could be described as:
Solving the above equations gives the J–V
