Plasmonic Organic Solar Cells - Charge Generation and Recombination - Bo Wu Nripan Mathews Tze-Chien Sum - eBooks
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SPRINGER BRIEFS IN APPLIED SCIENCES AND
TECHNOLOGY NANOSCIENCE AND NANOTECHNOLOGY
Bo Wu
Nripan Mathews
Tze-Chien Sum
Plasmonic
Organic Solar
Cells
Charge
Generation and
Recombination
123
SpringerBriefs in Applied Sciences and Technology Nanoscience and Nanotechnology Series editor Hilmi Volkan Demir, Nanyang Technological University, Singapore, Singapore
Nanoscience and nanotechnology offer means to assemble and study superstructures, composed of nanocomponents such as nanocrystals and biomolecules, exhibiting interesting unique properties. Also, nanoscience and nanotechnology enable ways to make and explore design-based artificial structures that do not exist in nature such as metamaterials and metasurfaces. Furthermore, nanoscience and nanotechnology allow us to make and understand tightly confined quasi-zero-dimensional to two-dimensional quantum structures such as nanoplatelets and graphene with unique electronic structures. For example, today by using a biomolecular linker, one can assemble crystalline nanoparticles and nanowires into complex surfaces or composite structures with new electronic and optical properties. The unique properties of these superstructures result from the chemical composition and physical arrangement of such nanocomponents (e.g., semiconductor nanocrystals, metal nanoparticles, and biomolecules). Interactions between these elements (donor and acceptor) may further enhance such properties of the resulting hybrid superstructures. One of the important mechanisms is excitonics (enabled through energy transfer of exciton-exciton coupling) and another one is plasmonics (enabled by plasmon-exciton coupling). Also, in such nanoengineered structures, the light-material interactions at the nanoscale can be modified and enhanced, giving rise to nanophotonic effects. These emerging topics of energy transfer, plasmonics, metastructuring and the like have now reached a level of wide-scale use and popularity that they are no longer the topics of a specialist, but now span the interests of all “end-users” of the new findings in these topics including those parties in biology, medicine, materials science and engineerings. Many technical books and reports have been published on individual topics in the specialized fields, and the existing literature have been typically written in a specialized manner for those in the field of interest (e.g., for only the physicists, only the chemists, etc.). However, currently there is no brief series available, which covers these topics in a way uniting all fields of interest including physics, chemistry, material science, biology, medicine, engineering, and the others. The proposed new series in “Nanoscience and Nanotechnology” uniquely supports this cross-sectional platform spanning all of these fields. The proposed briefs series is intended to target a diverse readership and to serve as an important reference for both the specialized and general audience. This is not possible to achieve under the series of an engineering field (for example, electrical engineering) or under the series of a technical field (for example, physics and applied physics), which would have been very intimidating for biologists, medical doctors, materials scientists, etc. The Briefs in NANOSCIENCE AND NANOTECHNOLOGY thus offers a great potential by itself, which will be interesting both for the specialists and the non-specialists. More information about this series at http://www.springer.com/series/11713
Bo Wu Nripan Mathews
•
Tze-Chien Sum
Plasmonic Organic Solar
Cells
Charge Generation and Recombination
123Bo Wu Tze-Chien Sum Division of Physics and Applied Physics, Division of Physics and Applied Physics, School of Physical and Mathematical School of Physical and Mathematical Sciences Sciences Nanyang Technological University Nanyang Technological University Singapore Singapore Singapore Singapore Nripan Mathews School of Materials Science and Engineering Nanyang Technological University Singapore Singapore ISSN 2191-530X ISSN 2191-5318 (electronic) SpringerBriefs in Applied Sciences and Technology ISSN 2196-1670 ISSN 2196-1689 (electronic) Nanoscience and Nanotechnology ISBN 978-981-10-2019-3 ISBN 978-981-10-2021-6 (eBook) DOI 10.1007/978-981-10-2021-6 Library of Congress Control Number: 2016948627 © The Author(s) 2017 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #22-06/08 Gateway East, Singapore 189721, Singapore
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 1
1.1 Organic Photovoltaics: Background . . . . . . . . . . . . . . . . . . . . .... 1
1.2 Materials: Conjugated Polymers . . . . . . . . . . . . . . . . . . . . . . . .... 2
1.3 Operation Principles and Physical Insights in Organic Solar
Cells (OSCs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Organic Solar Cell Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 State-of-the-Art, Challenges and Opportunities in OSCs . . . . . . . . . 9
1.6 Surface Plasmons for Improving Light Harvesting Efficiency . . . . . 10
1.7 Other Contributions to Organic Photovoltaic Performance
Improvement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 15
1.8 State-of-the-Art and Challenges in Plasmonic Organic
Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 17
2 Surface Plasmon Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Surface Plasmon Polariton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 Localized Surface Plasmon Resonance . . . . . . . . . . . . . . . . . . . . . . 28
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3 Characterization Plasmonic Organic Photovoltaic Devices . . . . . . . . . 33
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Optical Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.1 Steady-State and Transient Absorption Spectroscopies . . . . 33
3.2.2 Time-Integrated Photoluminescence
and Time-Resolved Photoluminescence . . . . . . . . . . . . . . . . 37
3.2.3 Spatially Resolved Spectroscopy . . . . . . . . . . . . . . . . . . . . . 39
3.3 Electrical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.1 Current Voltage (I-V) Measurement . . . . . . . . . . . . . . . . . . 39
3.3.2 Internal Photon to Current Efficiency (IPCE)
Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 40
vvi Contents
3.4 Numerical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.4.1 Optical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4.2 Electrical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4 Plasmonic Entities within the Charge Transporting Layer . . . . . . . . 47
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 Case Study (1): Silver Nano-Triangle Arrays in PEDOT:PSS . . . . 49
4.3 Case Study (2): Gold Nanowire Network in PEDOT:PSS . . . . . . . 58
4.4 Case Study (3): Single Silver Nanowire in PEDOT:PSS . . . . . . . . 67
4.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5 Plasmonic Entities within the Active Layer . . . . . . . . . . . . . . . . . . . . . 81
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.4 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . ............. 101
6.1 Implications for the Design of Hybrid Plasmonic
OPV Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............. 101
6.2 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . ............. 102
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............. 105About the Authors
Dr. Bo Wu is currently a research fellow at the
School of Physical and Mathematical Sciences
(SPMS), Nanyang Technological University (NTU).
He obtained his bachelor degree in 2009 at Beijing
Normal University (BNU). After that, he pursued his
Ph.D. degree at Nanyang Technological University
(NTU) under the supervision of Prof. Tze-Chien Sum,
working on the device fabrication and photophysics of
hybrid plasmonic organic photovoltaics (2010–2014).
Currently, his research focuses on the photophysics of
new types of PV materials, such as organic–
inorganic halide perovskites, organic polymers, and
small molecules, etc. E-mail: Wu.bo@ntu.edu.sg;
wubobnu@gmail.com
Dr. Nripan Mathews is an assistant professor at the
School of Materials Science and Engineering in
Nanyang Technological University. He pursued his
Ph.D. at a joint Commissariat à l’énergie atomique
(CEA)—Centre national de la recherche scientifique
(CNRS)—Universite de Pierre et Marie Curie (Paris
VI University) laboratory in the area of molecular
crystals, studying the signatures of optical excitations
within them (2008). He was also a visiting scientist at
Prof. Michael Graetzel’s laboratory at École
Polytechnique Fédérale de Lausanne (EPFL), working
on a Pan-European project on photoelectrochemical
hydrogen production. His research focuses on a wide variety of novel materials
(metal oxides, organic semiconductors, graphene, carbon nanotubes, sulfides,
selenides) and novel morphologies (one-dimensional structures such as nanowires
viiviii About the Authors
and nanotubes, thin films, and two-dimensional nanosheets) produced through a
range of fabrication procedures. He has focused primarily on the electronic and
optical properties of these materials and how they can be adapted for practical
applications. E-mail: Nripan@ntu.edu.sg
Dr. Tze-Chien Sum is an associate professor at the
School of Physical and Mathematical Sciences
(SPMS), Nanyang Technological University (NTU),
where he leads the Femtosecond Dynamics
Laboratory. He received his Ph.D. in 2005 from the
National University of Singapore, where he worked on
the development of proton beam writing for photonic
applications. Upon joining NTU in 2005 as a lecturer,
he switched to the rapidly expanding field of fem-
tosecond time-resolved spectroscopy and established
the xC-Lab research group—a laboratory for the
investigation of exCited-state phenomena. His research
focuses on investigating light matter interactions; energy and charge transfer
mechanisms; and probing carrier and quasi-particle dynamics in a broad range of
emergent nanoscale and light harvesting systems. E-mail: Tzechien@ntu.edu.sg;
http://www3.ntu.edu.sg/home/tzechien/spms/Abstract
The incorporation of plasmonic nanostructures into organic solar cells offers an
attractive light trapping and absorption approach to enhance the power conversion
efficiencies. However, there has been much controversy over the effects of such
integration—where both enhancement and detraction in performance have been
reported. Here, we review the current progress in the field and examine our work on
characterizing these hybrid devices using a combination of optical and electrical
probes. Transient optical spectroscopy techniques such as transient absorp-
tion spectroscopy and transient photoluminescence spectroscopy are powerful
probes of charge carrier dynamics in plasmonic organic solar cells. In conjunction
with device electrical characterization techniques, they provide unambiguous proof
of the effect of plasmonic nanostructures on the solar cell performance. Importantly,
the new insights into the photophysics and charge dynamics of plasmonic organic
solar cells uncovered by these probes would resolve the existing controversies and
provide clear guidelines for device design and fabrication.
Keywords Plasmonics Organic solar cells Photophysics Charge dynamics
Device characterization
ixChapter 1
Introduction
1.1 Organic Photovoltaics: Background
Over the past two decades, numerous efforts have been devoted to the development of
renewable energy resources due to the limited supply of fossil fuel reserves. Solar
energy is a rich, inexhaustible, environmental-friendly energy resource that can be
harvested to satisfy all our energy needs. However, the use of solar power to date is
still very much limited—contributing only a very tiny fraction to the world’s overall
energy supply [1]. Presently, the solar cell market is dominated by conventional
inorganic solar cells with power conversion efficiencies in the range of *18–20 %.
One factor limiting the proliferation of these cells is the cost of solar energy compared
to those of fossil fuels with a recent report placing the former to be at $396.1 per MWh
compared to only $100.4 per MWh for the latter [2]. To reduce the energy cost,
researchers have been exploring alternative light harvesting materials and technolo-
gies such as dye sensitized solar cells and organic photovoltaics (OPV) or organic
solar cells (OSCs). In particular, OPV or OSCs has gained much attention in the last
decade due to their advantages such as low-cost, ease-of-fabrication, flexibility,
light-weight as well as tunability of their properties. The origins of OPV can be traced
back to devices in the 1970s using tetracene as the active layer, yielding less than
10−4 % in power conversion efficiencies (PCEs) [3]. Since then, the PCEs of newer
generations of OSCs have improved dramatically over the years [4–8]. More recently,
the incorporation of plasmonic nanostructures in OSCs to further improve their
performances have been explored. The purpose of the plasmonic nanoparticles in
these cells is to trap light and increase the light absorption in the organic active layer.
In this chapter, a review of organic solar cells will first be presented. These
include: the materials, working principles, and the device architectures. The
state-of-the-art in OSCs as well as the challenges facing them and the motivation for
utilizing plasmonics in OSCs will also be discussed. Following which, a brief
introduction of surface plasmon resonance will also be given. Lastly, we will
© The Author(s) 2017 1
B. Wu et al., Plasmonic Organic Solar Cells, Nanoscience and Nanotechnology,
DOI 10.1007/978-981-10-2021-6_12 1 Introduction
review plasmonic OSCs and detail the mechanisms of using plasmonics to improve
the performance of photovoltaic devices.
1.2 Materials: Conjugated Polymers
Organic solar cells are typically fabricated using either solution-processed semi-
conducting polymer/molecules or vacuum-processed small organic molecules. The
latter requires careful management of evaporation processes and vacuum formation
that can limit its cost-effectiveness, while the low temperature processability of the
former is a major factor to its huge popularity. The semiconducting behavior of
polymers arises from conjugation through the alternation of single and double
bonds between carbon atoms [9]. The atomic configuration of an isolated carbon
atom is 1s22s22p2. In a conjugated polymer, the hybridization of s and p orbitals
forms 3 sp2 orbitals (r-bonds), while the remaining fourth orbital pz overlaps with
those from the neighboring carbon atoms to form the delocalized p-bonds. The
Peierls instability results in the formation of bonding (p) and anti-bonding (p*)
orbitals. The bonding orbital is also known as the highest occupied molecular
orbital (HOMO), while the anti-bonding orbital is called the lowest unoccupied
molecular orbital (LUMO). The energy difference between LUMO and HOMO
yields the bandgap of the polymer and therefore gives rise to its semiconductor
properties (Fig. 1.1).
For OPV applications, specific polymers have been synthesized with their
absorption properties that are better matched with the solar irradiation spectrum and
π*
LUMO
π
HOMO
Fig. 1.1 The bonding and antibonding orbitals in conjugated polymer. Adapted from reference
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