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Bulk-heterojunction Hybrid Solar Cells Based on
Colloidal CdSe Quantum Dots and Conjugated Polymers
DISSERTATION
ZURERLANGUNG DES AKADEMISCHEN GRADES EINES
DOKTOR-INGENIEUR
DERTECHNISCHEN FAKULTT
DERALBERT-LUDWIGS-UNIVERSTT FREIBURG IM BREISGAU
YUNFEI ZHOUFREIBURG IM BREISGAU, 2011
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DEKAN: Prof. Dr. Hans Zappe
REFERENT: Prof. Dr. Gerald A. Urban (University of Freiburg)
KOREFERENT: Prof. Dr. Klaus Meerholz (Unviersity of Cologne)
DATUM DERDISPUTATION: 21.02.2011
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I
Abstract
Emerging alternative photovoltaic technologies such as dye sensitized solar cells
(DSSCs) and organic solar cells (OSCs) have recently gained much attention and are
on the step of being commercialized. Bulk-heterojunction hybrid solar cells
containing inorganic nanoparticles and semiconducting polymers are still lagging
behind the DSSCs and fullerene derivative-based OSCs in respect of device
performance. Nevertheless, hybrid solar cells have the potential to exceed better
performance while still retaining the benefits such as low-cost, thin and flexible, and
easy to produce, because NCs have the features of tunable bandgap, high absorption
coefficient, and high intrinsic charge carrier mobility. In addition, it is possible to
synthesize stable elongated or even branched- nanostructures on the length scale of
2-100 nm with desirable exciton dissociation and charge transport properties.
In this dissertation, the results of a research aiming at the development of
bulk-heterojunction hybrid solar cells based on colloidal CdSe quantum dot (QDs)
and conjugated polymers are presented. Both the materials and device structures are
investigated and optimized systematically in respect of QD synthesis and
post-synthetic modification, hybrid nanocomposites formation, and device fabrication,
leading to an improvement of hybrid solar cells power conversion efficiency (PCE).
This dissertation begins with a general introduction of solar cells and organic/hybrid
solar cells. The state-of-the-art development of bulk heterojunction hybrid solar cells
is reviewed. Critical factors limiting the solar cell device performance are highlighted
and strategies for further device improvement are demonstrated by giving recent
examples from literature.
Highly reproducible synthesis methods for CdSe QDs are applied, leading to a narrow
size distribution and excellent photophysical properties. Pre-heating of the
hexadecylamine (HDA) ligand and aging of the Se-TOP precursor are proven as two
critical parameters for synthesizing high quality QDs. The influence of the QD
characteristics such as diameter, photoluminescence (PL) peak wavelength, and PL
intensity on the performance of hybrid solar cells is studied, revealing that the
synthesis conditions have a crucial impact on the QD surface quality, which can be
partially detected by the PL intensity. As a result, high quality QDs are desirable for
achieving efficient photovoltaic devices.
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II
An effective post-synthetic hexanoic acid treatment on HDA-capped CdSe QDs
before their integration into photovoltaic devices is demonstrated. Solar cells with
optimized ratios of QDs to poly(3-hexylthiophene) (P3HT) exhibit PCEs of about
2.0%. A simple ligand sphere model is derived from PL quenching, TEM and dynamic
light scattering results. The results indicate that an effective reduction of the
immobilized ligand sphere is a crucial factor to enhance the device performance.
Furthermore, extended investigations on applying the hexanoic acid treatment to
different ligand-capped (i.e. mixture of trioctylphosphine (TOP) and oleic acid (OA))
CdSe QDs are presented. The comparable performance of devices based on P3HT and
different ligand capped QDs indicates that the acid treatment is generally applicable to
QDs with TOP/OA ligands for improving device performance. In addition, lowerbandgap polymer
Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,
1,3-benzothiadiazole)] (PCPDTBT) has been used instead of P3HT as polymer part
for the formation of the photoactive hybrid film. Here, optimized devices exhibit
PCEs of 2.7% after spectral mismatch correction. This value is the highest reported
one for spherical QD based hybrid solar cells. Comparison studies of P3HT and
PCPDTBT based devices revealed that the improved PCEs in PCPDTBT:CdSe device
can be mainly attributed to the increased short-circuit current density (Jsc) as a result
of the improved match of the blend absorption with the solar emission spectrum, as
supported by UV-Vis absorption and external quantum efficiency (EQE)
measurements. In addition, it is demonstrated that low bandgap polymers which can
harvest photons at longer wavelength region and have adequate energy levels are
promising to be incorporated into hybrid solar cells.
Finally, a summary of the results is presented and an outlook for further investigations
is given. In the additional appendix part, the pre-evaluation setup and procedure for
solar cells fabrication and measurement in our lab are demonstrated.
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III
Deutsche Zusammenfassung
Aufstrebende alternative Photovoltaik Technologien wie die Farbstoffsolarzellen und
die organischen Solarzellen haben in letzter Zeit viel Aufmerksamkeit auf sich
gezogen und sind auf dem Wege kommerzialisiert zu werden.
Bulk-Heterojunction Hybridsolarzellen enthalten anorganische Nanopartikel und
halbleitende Polymere und hinken in der Entwicklung noch hinter den
Farbstoffsolarzellen und den Fulleren basierten organischen Solarzellen in puncto
Solarzelleneffizienz hinterher. Dennoch habenHybridsolarzellen das Potential ihre
Effizienz zu steigern unter Beibehaltung ihrer Vorteile wie Kostengnstigkeit,
Flexibilitt, geringe Dicke und leichte Herstellung. Desweiteren haben Nanokristalle
den Vorteil, dass sich ihre Bandlcke einstellen lsst, sie einen groen
Absorptionskoeffizienten besitzen und eine hohe intrinsische Mobilitt der
Ladungstrger vorweisen. Zustzlich ist es mglich, elongierte oder sogar verzweigte
Nanostrukturen auf einer Lngenskala zwischen 2-100 nm herzustellen, die eine
verbesserte Trennung der Ladungstrger sowie einen besseren Ladungstransport
ermglichen.
In dieser Dissertation werden Resultate ber die Entwicklung von
Bulk-Heterojunction Hybridsolarzellen basierend auf kolloidalen CdSe Quantum
Dots (QDs) und konjugierten Polymeren prsentiert. Beides, die Materialien und der
Solarzellenaufbau sind untersucht und systematisch optimiert worden. Es werden die
QD Synthese, die post-synthetische Modifikation der QDs, die Herstellung der
hybriden Nanokomposite und die Herstellung der Solarzellen beschrieben, die zu
einer Verbesserung der Effizienz der Hybridsolarzellen fhrt.
Die Dissertation beginnt mit einer generellen Einfhrung ber Solarzellen und
organischen- wie auch Hybridsolarzellen. Der Stand der Technik in der Entwicklung
von Hybridsolarzellen wird zusammengefasst. Entscheidende Faktoren, die die
Solarzelleneffizienz limitieren werden aufgezeigt und Strategien zu ihrer
Verbesserung werden anhand von neueren Beispielen aus der Literatur gegeben.
Eine hoch reproduzierbare Synthesemethode fr CdSe QDs wurde angewandt, die zu
QDs mit einer geringen Grssenverteilung und exzellenten photophysikalischen
Eigenschaften fhrte. Das Vorheizen des Hexadecylamin (HDA) Liganden sowie das
Altern der Se-TOP Ausgangsverbindung haben sich dabei als notwendige Faktoren
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IV
zur Synthese hochqualitativer QDs gezeigt. Der Einfluss der QD Gre und des
resultierenden Emissionssignals einschlielich der Signalintensitt auf die Effizienz
entsprechender Hybridsolarzellen wurde untersucht. Es hat sich herausgestellt, dass
die Synthesebedingungen einen groen Einfluss auf die Qualitt der QD-Oberflche
hat, welche sich zum Teil in der Intensitt der Photolumineszenz widerspiegelt, mit
dem Resultat, dass hochqualitative QDs notwendig sind, um effiziente
photovoltaische Zellen herzustellen.
Eine effektive postsynthetische Behandlung der HDA bedeckten CdSe QDs mit
Hexansure und die Integration der behandelten QDs in Solarzellen wird aufgezeigt.
Solarzellen mit optimierten Mischungsverhltnissen aus QDs und
Poly-3-hexylthiophen (P3HT) fhren zu Zellen mit Effizienzen von 2,0%. Eineinfaches Liganden-Sphren Modell wurde zur Erklrung herangezogen, basierend
auf Photolumineszenz Auslschungsexperimenten, Ergebnisen der dynamischen
Lichtstreuung und elektronenmikroskopischen Untersuchungen. Diese Experimente
zeigten, dass die effektive Reduzierung der immobilisierten Ligandensphre ein
entscheidender Faktor fr die Effizienzsteigerung der Solarzellen ist.
Desweiteren wurden intensive Untersuchungen des Hexansure Behandlungsschrittes
auch an CdSe QDs die die Oberflchenligandenmischung lsure/Trioctylphosphinenthalten durchgefhrt. Es wurden vergleichbare Solarzelleneffizienzen bei
Verwendung dieser QDs mit P3HT gefunden, was die generelle Anwendbarkeit des
Surebehandlungsschrittes zeigt. Zustzlich wurde das P3HT durch das Polymer
Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,
1,3-benzothiadiazol)] (PCPDTBT) zur Bildung des photoaktiven Filmes ersetzt,
welches eine kleinere Bandlcke besitzt. Hier zeigten optimierte Testsolarzellen
Effizienzen von 2.7% nach Bercksichtigung des spektralen Korrekturfaktors
(spectral mismatch correction). Dieser Wert ist der hchste bisher erzielte fr
Hybridsolarzellen basierend auf sphrischen QDs. Vergleichende Studien zwischen
P3HT und PCPDTBT basierten Hybridsolarzellen zeigen, dass die
Effizienzsteigerungen von PCPDTBT:CdSe Solarzellen auf eine hhere
Kurzschlussstromdichte zurckzufhren ist, die aufgrund der verbesserten Absorption
der Energie des solaren Spektrums resultiert, was durch UV-Vis
Absorptionsspektroskopie sowie durch Bestimmung der externen Quanteneffizienz
besttigt wird. Somit ergibt sich, dass Polymere mit einer kleinen Bandlcke, die
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V
zustzlich Photonen mit greren Wellenlngen absorbieren knnen und geeignete
HOMO-LUMO Energienieveaus besitzen, vielversprechend fr die Integration in
Hybridsolarzellen sind.
Am Ende werden die Ergebnisse zusammengefasst und ein Ausblick fr weitereUntersuchungen wird gegeben. Im Anhang werden der Messaufbau, der fr die
Vorevaluationen der Hybridsolarzellen verwendet wurde, beschrieben, ebenso wie die
Fabrikation der Solarzellen und die Details zu den Solarzellenmessungen.
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VI
Table of Contents
Abstract I
Deutsche Zusammenfassung III
1. Introduction 11.1.Introduction to solar cells 11.2.Organic/hybrid solar cells 91.3.Motivation and context 221.4.
Outline 22
2. Bulk-heterojunction hybrid solar cells 252.1.Colloidal semiconductor nanocrystals (NCs) 252.2.Devices based on CdSe NCs 282.3.Strategies for efficiency improvement 312.4.Hybrid solar cells based on other semiconductor NCs 40
3. Synthesis of CdSe quantum dots (QDs) 443.1.CdSe QD synthesis 463.2.Characterization of CdSe QDs 473.3.Critical parameters for high quality CdSe QDs synthesis 50
4. Surface modification of CdSe QD 554.1.Hexanoic acid treatment 554.2.P3HT:CdSe composites 574.3.Solar cell performances 604.4.Ligand sphere model 664.5.Influence of the QD synthesis on the device performance 69
5. Hybrid solar cells based on acid treated CdSe QDs and low bandgap polymerPCPDTBT 74
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VII
5.1.Hexanoic acid treatment on TOP/OA-capped CdSe QDs 745.2.Solar cells based on low bandgap polymer PCPDTBT 80
6. Summay and outlook 896.1.Summary 896.2.Outlook 91
Appendix 98
A1. Hybrid solar cells fabrication 98
A2. Solar cell performance test program based on LabVIEW 102
Abbreviation 110
References 112
Curriculum vitae 122
List of publications 123
Acknowledgements 126
Erklrung 129
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1.1 Introduction to solar cells
1
Chapter 1
Introduction
1.1 Introduction to solar cells
Renewable energy resources from natural sources, like sunlight, wind, rain, tides, and
geothermal heat, will neither run out nor have any significant harmful effect on our
environment. Photovoltaic (PV) energy conversion is attracting more and more
attention as the need for renewable energy sources replacing current environmental
critical technologies becomes more urgent. PV is an attractive way of producing
electrical energy directly from sunlight, without producing noise, toxic substances and
greenhouse gas emission, while requiring very little maintenance. PV technologies
have found markets in various fields, ranging from consumer electronics and small
scale distributed power systems to megawatt scale power plants.
The discovery of the PV effect can be traced back to 1839 when a French physicist A.
E. Becquerels performed pioneering studies in liquid electrolytes1. Until 1883,
Charles Fritts built the first solar cell by coating the semiconductor selenium with an
extremely thin layer of gold to form the junctions. Albert Einstein explained the
photoelectric effect in 1905, and he received the Nobel Prize in Physics in 1921 for it.
In the modern era, Chapin et al. 2 reported on a silicon based single p-n junction
device with a solar power conversion efficiency (PCE, defined as the percentage of
maximum output of electrical power to the incident light power) of 6%, which is
believed to be the tipping point that transformed photovoltaic into practical
technology to convert solar energy into electricity.
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Chapter 1. Introduction
2
1.1.1 Development of solar cells
1) First generation solar cells
Currently, the PV market is still dominated by crystalline silicon wafer based solar
cells, so-called first generation solar cells. The best single crystalline silicon solar
cells exhibited PCEs of about 25%3, which is approaching the theoretical
Shockley-Queisser limit efficiency of 31.0% for single junction solar cells without
light concentrating4. Commercial products typically achieve module efficiencies of
about 15-18%5. However, silicon wafers are fragile, making the manufacturing
processes complex limiting potential applications. In addition, high purity crystalline
silicon wafers are very expensive, resulting in inherently high-cost and it may take
several years to gain the payback for their purchasing and installation costs.
2) Second generation solar cells
In order to simplify manufacturing and reduce costs, second generation solar cells,
which are mostly associated with thin film solar cells, were developed. Second
generation solar cells are much cheaper to produce than first generation cells since
they are fabricated by depositing thin films of photoactive materials on substrates,using less amount of materials and cheaper manufacturing processes. Copper Indium
Gallium Selenide (CIGS), CdTe, amorphous silicon (a-Si), and nanocrystalline silicon
(nc-Si) are the most commonly used materials6. As direct band gap semiconductors,
the thin film semiconductor materials have much higher absorption coefficients than
silicon, therefore much thinner semiconductor layers (< 1 um), which is 100-1000
times less than for Si, are required. The highest confirmed efficiencies by certified test
centers for CIGS cell, CdTe cell, a-Si cell, and nc-Si are 20.1% a, 16.70.5%7,
9.50.3%8, and 10.10.2%9, respectively. However, the PCEs remain lower than for
the first generation solar cells, and the promise of low cost power has not been
realized yet.
a Press release 05/2010, Zentrum fr Sonnenenergie- und Wasserstoff-Forschung Baden-Wrttemberg (ZSW),Germany.
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1.1 Introduction to solar cells
3
3) Third generation solar cells
Third generation solar cells are the cutting edge of solar technology and still in the
research phase. They are expected to achieve reasonable efficiencies at lower costs
than first and second generation technologies4
and contain a wide range of promisingtechnologies including multijunction tandem cells10, dye-sensitized solar cells
(DSSCs)11, organic solar cells based on small-molecule12, 13 and conjugated
polymer14-22, and organic/inorganic hybrid solar cells21, 23-26.
For example, multijunction inorganic solar cells are currently the most efficient solar
cells. It consists of multiple photoactive thin films to absorb nearly the entire solar
spectrum, thus generating electric power from as much of the solar energy as possible.
In single band gap solar cells, efficiency is limited due to the inability to convert abroad range of photons of the solar emission into electricity. Photons with lower
energy than the band gap of photoactive materials are lost, since they are not able to
excite electron over the band gap. This part of photons are either not absorbed or
converted into heat within the materials. Energy in the photons above the band gap
energy is also lost, since only the energy necessary to generate hole-electron pair is
used, and the remaining energy is converted into heat. In multijunction solar cells, the
band gap of each layer can be tuned to absorb a specific range of the solar spectrumby using different alloys of III-V semiconductors. Recently, a research team from
Boeing Spectrolab claims commercial availability of cells at 40.72.4% efficiency in
GaInP/GaAs/Ge (2-terminal) device structure under an intensity of concentrated sun
illumination (240 suns)10. Fraunhofer Institute for Solar Energy Systems (FhG-ISE) in
Germany and National Renewable Energy Laboratory (NREL) in United States also
reported 40.82.4%27 and 41.1%2.5%b efficiencies of multijunction solar cells,
respectively. With such high efficiencies, multijunction solar cells are well-suited for
space application and have a good development potential. However, the cost is too
high to allow a large scale use of it due to the complex device structure and the high
price of materials. Figure 1.1 summarises the progress of the best research-cell PCEs
based on various materials and technologies updated till June, 2010.
bhttp://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-ise
http://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-isehttp://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-isehttp://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-isehttp://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-isehttp://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-isehttp://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-ise7/27/2019 Bulk Heterojunction With Quantum Dots
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Chapter 1. Introduction
4
Figure 1.1Overview of the best research-cell efficiencies of various PV technologies (This
graph was created and prepared by the National Renewable Energy Laboratory (NREL) forthe U.S. Department of Energy).
1.1.2 Photovoltaic market
Nowadays, solar cells have found markets in various application fields ranging from
consumer electronics and small scale distributed power systems to megawatt scale
power plants. According to a Solarbuzz reportc, global PV market installations
reached a record high of 6.43 gigawatt in 2009 (Fig. 1.2), representing a growth of 6%
over the previous year. The top three European countries are Germany, Italy, and
Czech Republic. The PV industry generated US$ 38 billion in global in 2009, while
successfully raising over US$13.5 billion in equity and debt up 8% on 2008. In 2010and also over the next 5 years, the PV industry is expected to return to high growth,
and the global market will be 2.5 times its current size by 2014, and the annual
industry revenues will approach US$ 100 billion then.
c http://www.solarbuzz.com/marketbuzz2010-intro.htm
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1.1 Introduction to solar cells
5
Figure 1.2 Photovoltaic market in 2009 (Data taken from Solarbuzz,http://www.solarbuzz.com/marketbuzz2010-intro.htm)
1.1.3 Renewable energy
As a renewable energy technology, PV technology has to be compared with other
technologies based on certain indicators, such as energy payback time, CO2 emissions,
and the recycling management at their end of life.
Energy payback time is defined as the recovery time required for generating the
energy input during the whole life cycle, which includes the energy requirement for
manufacturing, installation, energy use during operation, and energy needed for
decommissioning. Depending on the solar cells module and installation location, the
typical energy payback times are ranging between 1 to 4 years28. With a typical
lifetime of 20 to 30 years, modern solar cells generate significantly more energy over
their lifetime then energy spent for their production, thus they are net energy
producers.
Opposite to fossil energy sources, the operation of solar cell systems are CO2-free,
and the greenhouse gas emission occurs almost entirely during their manufacturing. In
this term, the CO2 emissions (g/KWh) of the present grid connected roof-top systems
have been estimated to be significantly lower than those of fossil fuel power plants.
Another important issue is the recycling of solar cells after its lifetime. Depending on
the solar cells technology, they might contain small amounts of hazardous and toxicmaterials, such as Cd, Pb, and Se. However, there are already existing technologies
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Chapter 1. Introduction
6
for recycling solar cells and it can be therefore considered economically feasible.
1.1.4 Solar cells characterization
1) Power Conversion Efficiency
Figure 1.3Current density-voltage (J-V) characteristic of a typical solar cell in the dark(dashed line) and under illumination (solid line). Typical solar cell parameters such asshort-circuit current density Jsc, open-circuit voltage Voc, and the maximum power point Pmare illustrated on the graph. (Image taken from Ref.29)
Power conversion efficiency (PCE) is one of the most import parameter tocharacterize solar cell performances. It is defined as the percentage of maximum
output of electrical power to the incident light power. Fig.1.3 shows the current
density-voltage (J-V) characteristic for a typical hybrid solar cell in the dark and
under illumination. The PCE can be described as
=
=
(1.1)
where Pm is maximum power point, Pin is the incident light intensity, Jsc is the
short-circuit current density, and Voc is the open-circuit voltage, and FF is the fill
factor, which is defined as the ratio of Pm to the product of Jsc and Voc
=
(1.2)
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1.1 Introduction to solar cells
7
2) Quantum efficiency
External quantum efficiency (EQE) is another important parameter for solar cell
characterization. It is calculated by the number of electrons extracted in an external
circuit divided by the number of incident photons at a certain wavelength undershort-circuit condition
() =
=
()/
()/(
)
=()
() (1.3)
where is the wavelength, e is the elementary charge, h is the Planck constant, and cis the speed of light in vacuum. Note that the EQE represents the external quantum
efficiency, meaning that the losses due to reflection at the surface, and/or the
transmission through the device are also included in EQE. Considering the fraction of
the actually absorbed photons by the photoactive layer, EQE can be converted into the
internal quantum efficiency (IQE)
() =()
1()()(1.4)
where Ref() is the fraction of reflected light and Tran() is the fraction of the
transmitted light. The IQE is also very helpful in organic solar cells to investigate the
physical processes occurring in the organic semiconductor materials.
3) Spectral mismatch correction
A sun simulator which can simulate natural sunlight is usually used as a light source
for repeatable and accurate indoor testing solar cells. In order to compare results from
various devices, solar cells are characterized based on international accepted standard
reporting conditions (SRC), which are referred to a cell temperature of 25C under air
mass 1.5 global (AM1.5 G) illumination spectrum at an intensity of 1000 W/m2. This
AM1.5 G condition corresponds to the spectrum and irradiance of sunlight incident
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Chapter 1. Introduction
8
upon an inclined plane at 37o tilt towards the equator with an elevation of 41.81o
above the horizond.
A Si reference solar cell is mostly used as the reference cell for calibrating a sun
simulator. Usually there is a spectral mismatch in the measured short-circuit current ofthe solar cell with respect to a AM1.5 G reference spectrum. The reasons for this
spectral mismatch are: 1) the spectrum mismatch between the light source and the
AM1.5G reference spectrum, and 2) the difference on spectral response between Si
reference cell and testing cell. The mismatch value can be calculated by using the
spectral responsively data for the testing cell and reference cell combinations. For
most organic solar cell, the short-circuit current density shows a linear relationship
with the incident light intensity
30
. The open-circuit voltage and fill factor are muchweaker dependent on the light intensity31, 32. Therefore, once a mismatch factor is
known, the Jsc and the PCE of the testing cell can be calibrated. The spectral mismatch
correction especially for organic solar cells are described in detail elsewhere33.
d ASTM Standard G173, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal andHemispherical on 37 Tilted Surface, American Society for Testing and Materials, West Conshocken, PA, USA.
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1.2 Organic/Hybrid solar cells
9
1.2 Organic/Hybrid solar cells
Generally, PV technologies are currently dominated by inorganic semiconductor
based solar cells. However, the manufacturing processes of traditional inorganic solar
cells often involve elevated temperature, high vacuum, and numerous lithographicsteps, resulting in high production costs and energy consumption. Alternatively, solar
cells based on organic materials such as small molecules and conjugated polymers
offer a cost-effective way to convert solar energy into electricity. In contrast to their
inorganic counterparts, organic solar cells (OSCs) have several advantages. They are
able to be manufactured by low-temperature processing: either from small molecules
from the vapor phase, or polymers from solution. The organic materials are relatively
inexpensive, and only 100-200nm thick films are required due to the high opticalabsorption capabilities of organic semiconductors. Additionally, roll-to-roll processing
like low cost printing techniques34 can be used for manufacturing. In Fig. 1.4a a
photograph is shown illustrating a roll-to-roll manufacturing machine for fabricating
flexible polymer solar cells35. Therefore, OSCs have the promising potential to be
applied in consumer products with the features of thin, flexible, light weight and low
cost. Additionally, the properties of small molecules or polymers can be tailored by
modifying their chemical composition, resulting in greater customization than
traditional inorganic solar cells. Since Tangs pioneering work on the bilayer
heterojunction OSC reported in 198636, intensive investigation have been performed
and the PCEs of devices have been improved substantially. Currently, OSCs with
small active area have reached a PCE level 8% (announcement by Konarka with a
certified PCE of 8.3%e), which are close to the requirement for entering the business
market37. Several companies such as Konarka, Heliatek, Solarmer, and Plextronics
have already started or are preparing to make commercial available products to offer
alternative inexpensive solar power modules. It has to be mentioned that the PCEs of
solar power modules based on OSCs are currently in the range of 2-4%. For example,
solar bags with a polymer based OSC panel on the surface are already commercially
available on the market based on the technology developed by Konarka. They are able
to provide electricity to charge mobile phones and other handheld devices from solar
e www.konarka.com, accessed on December 3, 2010.
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Chapter 1. Introduction
10
power (shown in Fig. 1.4b). The light-weight, thin-film photovoltaic material is much
more versatile for various applications than traditional solar panels.
Figure 1.4(a) Photograph of a roll-to-roll manufacturing facility for fabricating flexiblepolymer solar cells35
.(b) Solar bag with an integrated polymer based OSC panel on the bagflap (http://www.energy-sunbags.de).
1.2.1 Device structure
OSCs are typically thin film devices consisting of photoactive layer(s) between two
electrodes of different work functions (see Fig. 1.5).
Figure 1.5Schematic structure of a typical Organic solar cell. (Modified image according to
ref.29)
High work function, conductive and transparent indium tin oxide (ITO) on a flexible
plastic or glass substrate is often used as anode. The conducting polymer
poly(3,4-alklenedioxythiophenes):poly(styrenesulfonate) (PEDOT:PSS) is usually
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used as anode buffer material for smoothing the ITO surface, enhancing the adhesion
to the upper light absorbing layer, better energy level matching, and improving the
device stability by hindering oxygen and indium diffusion through the anode38-40. The
photoactive layer can be either deposited thermally (small molecules) in vacuum or
spin-coated (polymer) onto the ITO substrate to form a film thickness of around
100-200 nm. Finally, a top metal electrode (e.g. Al, LiF/Al, Ca/Al) is vacuum
deposited onto the photoactive layer.
The photoactive layers can be constructed in a variety of ways, leading to single layer,
bilayer, or bulk-heterojunction devices.
1) Single layer device
Single layer cells are the oldest and simplest OSCs. In 1959 Kallman and Pope
discovered that anthracene can be used to make OSCs, exhibiting an efficiency of
only 210-6%41. In such single layer device, the internal electric field which arises
from the work function difference between two electrodes is much too weak to
overcome the exciton binding energy to dissociate the excitons into free electrons and
holes. For instance, only 10% of the excitons dissociate into free carriers in a pure
poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) film, while
the remaining excitons decay via radiative or nonradiative recombination42. Thus the
efficiencies of single-layer polymer solar cells exhibit rather low efficiency in the
order of 0.1%43, 44
2) Bilayer device
A major advancement in organic solar cells was realized in 1986 by Tang 36, who
firstly introduced the bilayer heterojunction structure by stacking two materials with
suitable energy levels offsets. Solar cells with a planar junction of CuPC as donor and
a perylene tetracarboxylic derivative as acceptor reached an efficiency of about 1%
under AM2 75 mW/cm2 illumination. This dramatically improvement on efficiency is
mainly due to the exciton dissociation at the heterojunction which is much more
efficient than in the bulk organic layer or in the organic/metal interface of a single
layer device. A schematic energy diagram of a device with a bilayer heterojunction
structure consisting of a donor and an acceptor is shown in Fig. 1.6, where HOMO is
the highest occupied molecular orbital, and LUMO is the lowest unoccupied
molecular orbital.
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Figure 1.6Schematic energy level diagram of a device with a bilayer heterojunction
consisting of a donor and an acceptor material. The process of electron charge transfer fromthe donor to the acceptor is shown in this case. (Modified image according to ref.
29)
In the heterojunction structure, an electron donor material (D) and electron acceptor
material (A) are used. Here, the HOMO and the LUMO of the donor material is
higher than that of the acceptor material. Photons are absorbed by the donor materials,
thus the energy from incident photons excite electrons from HOMO to LUMO to
generate excitons. When they diffuse close to the D/A interface, photo-generated
excitons can be efficiently dissociated into free carriers by charge transfer. The charge
transfer process, i.e. electron transfer from the LUMO of the donor to the LUMO of
the acceptor is energetically favorable. The exciton dissociation by charge transfer in
bilayer heterojunction device is intrinsically more efficient than that in the single bulk
organic semiconductor. Furthermore, during the charge transport process after exciton
dissociation, the possibility for recombination losses is significantly reduced since
electrons or holes transport to their respective electrodes in pure n-type or p-type
layers.
However, the exciton dissociation efficiency is limited due to the limiting interfacial
area in bilayer heterojunction device. Since the exciton diffusion lengths in conjugated
polymers are typically around 10-20 nm45-47, the optimum distance of the exciton to
the donor/acceptor (D/A) interface, where charge transfer takes place and excitons
dissociate into free charge carriers, should be in the same length range. This
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requirement limits the part of the active layer which has contribution to photocurrent
to a very thin region near the D/A interface. In other words, excitons generated in the
remaining area of the device are lost. Therefore, for efficient exciton dissociation at
the heterojunction, the donor and the acceptor materials have to be in a suitable
distance.
3) Bulk-heterojunction device
In order to overcome the problem that not all excitons are able to reach the D/A
interface, a so-called bulk-heterojunction structure48, 49was introduced by Yu et al.48
and Halls et al.49 independently. The bulk-heterojunction composite is made by
mixing both the electron donor and acceptor intimately together, thus the interfacial
area is dramatically increased and the distance that excitons have to travel to reach theinterface is reduced. After exciton dissociation into free charge carriers, holes and
electrons are transported via polymer and NCs percolation pathways towards the
respective electrodes. Fig. 1.7 shows the schematic illustration of device structure and
energy levels of a bulk-heterojunction solar cell. Compared to the bilayer
heterojunction structure where donor and acceptor phase contact the respective anode
and cathode selectively, the bulk-heterojunction requires percolated pathways for the
charge carrier transporting phases to the respective electrodes. Thus the donor andacceptor phases should form a bicontinuous and interpenetrating network for efficient
charge transport after exciton dissociation. Therefore, nanoscale morphology control
in the blend is very important for a bulk-heterojunction device.
Figure 1.7Schematic illustration of a) device structure, and b) energy level diagram of abulk-heterojunction solar cell. The photoactive layer is made up of blend solution consistingout of donor and acceptor materials. (Modified image according to ref
29)
The bulk-heterojunction device structure has been used extensively since its
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introduction and state-of-the-art polymer based solar cells are primarily using this
bulk-heterojunction device structure50-57. Recent progress reported on bulk
heterojunction solar cells with internal quantum efficiency approaching 100%50by
using
poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadia
zole) (PCDTBT) as donor the fullerene derivative [6,6]-phenyl C-70-butyric acid
methyl ester (PC70BM) as acceptor. It implies that essentially every absorbed photon
results in a separated pair of charge carriers and that all photo-generated carriers are
traveling to the respective electrodes. This result also indicates that the exciton
dissociation and charge carrier transport can be very efficient in such
bulk-heterojunction structure after optimization of the materials and device
manufacturing procedures.
1.2.2 Working principle
As shown in Fig. 1.8 photocurrent generation is a multistep process in OSCs for a
typical (a) bilayer heterojunction device and (b) bulk-heterojunction device, where
photons are mainly absorbed in the donor material. The physics of organic solar cells
is reviewed in detail in dedicated review articles15, 25
. In short, there are following fourmain steps which have to be considered: photon absorption, exciton diffusion, charge
transfer, charge carrier transport and collection.
Figure 1.8 Schematic diagram of the photocurrent generation mechanism in typical (a)bilayer heterojunction and (b) schematic of bulk-heterojunction organic solar cells, where
photons are mainly absorbed in the donor material. Photogeneration process: excitongeneration (1), exciton diffusion (2), charge transfer (3), charge carrier transport andcollection (4). (Image taken from ref.
29)
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1) Photon absorption
First, incident photons with an energy h are absorbed mainly by the donor material
and excite the electrons from the HOMO to LUMO level, creating excitons with a
certain binding energy (typically 200-500 meV43, 58
).
2) Exciton diffusion
In order to generate separated negative and positive charges, the excitons need to
diffuse to the D/A interface. Since excitons are neutral species, their motion is not
affected by any electric field and they diffuse via random hops driven by the
concentration gradient. Note that the exciton diffusion lengths are typically around 10
nm45-47for most conjugated polymers before recombination takes place. Excitons that
do not reach the D/A interface are lost for the energy conversion and have no
contribution to the photocurrent.
3) Charge transfer
Excitons dissociate at the D/A interface by charge transfer which is energetically
favored: the LUMO level of the NCs is lower than that of the polymer, and the
HOMO level of the polymer is higher than that of the NCs. The offsets in both
HOMO and LUMO levels must be larger than the exciton binding energy minus thecolumbic binding energy of the charge-separated state59.
4) Charge carrier transport and collection
In the final step, once charge transfer has occurred at the D/A interface, separated
holes and electrons are distributed within the donor and acceptor phases, respectively.
Holes and electrons are then transport towards their respective electrode driven by an
internal electric field deriving from the Fermi level difference of the electrodes with
efficiencies depending on their mobilities during the hopping process, and
consequently being collected at the respective electrodes.
1.2.3 Organic solar cells vs. inorganic solar cells
Compared to the classical crystalline inorganic semiconductors (e.g. silicon), organic
semiconductors are different in some fundamental aspects. The differences between
organic and inorganic solar cells have significantly consequences for understandingthe fundamental mechanisms of photoconversion in the two different systems.
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First of all, the absorption of a photon from the incident sunlight in organic solar cells
does not create free charge carriers, but strongly bound electron-hole pairs so- called
excitons60. The binding energy is typically in the range of 200-500 meV43, 58, which is
much higher than kBT at room temperature (kBT(300K)=26 meV). Due to their low
dielectric constants and weaker non-covalent electronic interactions between organic
molecules, organic solar cells are also classified as excitonic solar cells61-63.
Second, the charge carrier mobilities in organic semiconductors are much lower than
those in crystalline inorganic semiconductor. For example, hole mobilities for
conjugated polymers range from 10-1 to 10-7 cm2/(Vs)64-67, while electron mobilities
are typically even lower (10-410-9 cm2/(Vs))65, 68. By contrast, the hole and electron
mobilities in crystalline silicon are 475 and 1500 cm
2
/(Vs)
69
. The disordered structureof organic semiconductors causes the transport of carriers through hopping
mechanism rather than through band-like transport. Therefore, these low mobilities of
organic semiconductors limit the feasible thicknesses of the active layer to only few
hundred nanometers. Fortunately, organic semiconductors have relatively high optical
absorption coefficients (usually>105/cm) in the UV-Vis regime, thus only ca. 100-200
nm thick organic layers are needed for effective absorption.
Figure 1.9schematic diagrams of the differences of band gap structures and the distributionof photo-generated carriers between (a) a conventional solar cell and (b) an organic solar
cell. In the case of an organic solar cell, the concentration gradients are the driving force forcarriers transport. (Images drawn according to Ref.60)
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Fig. 1.9 shows the schematic diagram of the differences on band gap structure and the
distribution of photo-generated carriers between conventional solar cell and organic
solar cell. In conventional inorganic solar cells, the charge carriers are separated from
each other by the built-in electric field of the device and travel to their respective
electrode. The photo-generation is distributed throughout the active layers and
photo-generated carrier concentration gradients are negligible. Therefore,
photocurrent is dominantly driven by drift of minority carriers in the built-in electric
field (Fig. 1.9a). In organic solar cells, the charge carriers are tightly bound to each
other in the form of excitons. The excitons diffuse to the donor/acceptor (D/A)
interface where they dissociate into free carriers and create a majority of carriers,
leading to large concentration gradients. These high interfacial concentration gradients
promote the separation of carriers. The diffusion and drift driving forces act in the
same direction to separate the charge carriers (Fig. 1.9b).
1.2.4 Photoactive material
Based on the organic semiconductor components used for photoactive layers, organic
solar cells can be divided into three main types: small molecule solar cells, polymer
solar cells, and organic-inorganic hybrid solar cells.
1) Small molecule solar cells
In 1986, Tangs36firstly demonstrated the bilayer OSCs with a planar heterojunction
between copper phthalocyanine (CuPc) as electron donor and perylene derivative as
acceptor. Later on, in order to reduce the exciton lost due to quenching at the cathode
metal contact, a wide bandgap electron blocking layer (EBL) for excitons was
introduced between the acceptor materials and the cathode70, 71. Excitons are blocked
at the interface between the acceptor and the EBL because of the offset in energy
levels. Besides, the EBL prevents damages of the active layer due to the cathode
deposition. Bathocuproine (PCB) and 4,7-dipheny1-1,10-phenanthroline (BPhen) are
two commonly used materials as EBLs. Fig. 1.10 shows the chemical structures of
commonly used materials for small molecule solar cells.
A major improvement on PCEs of small molecule solar cells was the introduction of
a so-calledpin structure, where p, i and n refer to the type of the three different layers
as predominately p-type (donor), intrinsic absorber, and n-type (acceptor). Maennig et
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al.72have reported the first OSCs based on a pin structure where the Fermi levels in
the transport layers are controlled by molecular doping. In such pin devices, the
contact between the transport layer and the electrode is typically ohmic due to the
doping of the transport layers73. Additionally, the doped transport layer only permits
either electron or hole transport, thus blocks the opposite charge carriers and excitons.
Furthermore, the position of absorber can be optimized to the place where the
absorption in the optical interference pattern forming due to the reflecting back
contact, since the doped transport layers do not significantly absorb light from the sun
spectrum above 400 nm72. Recently, a record PCE was announced by Heliateck
GmbH, Germany for small molecules solar cells based on apin tandem structure with
certified efficiency of 8.3% and an active area of 1.1 cm2f.
Figure 1.10Chemical structures of commonly used materials for small molecule solar cells.Shown are copper phthalocyanine (CuPc), zinc phthalocyanine (ZnPC), fullerene (C60),bathocuproine (BCP), and 4,7-dipheny1-1,10-phenanthroline (BPhen).(Images taken from
http://www.sigmaaldrich.com)
2) Polymer solar cells
Polymer based solar cells have several attractive features. Because the active
materials used for device fabrication are soluble in most of the common organic
solvents, it has the potentials to be flexible and to be manufactured in a roll-to-roll
processing like low cost printing techniques34. Conjugated polymers generally have
relatively high hole mobilities but low electron mobilities. This intrinsic imbalance in
carrier mobility can be overcome by the incorporation of an n-type semiconductor
f http://www.heliatek.com, accessed on December 3, 2010.
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material as electron acceptor to provide a pathway for electron transport.
Figur e 1.11Chemical structures of commonly used materials for polymer solar cells. Shownare Poly(3-hexylthiophene-2,5-diyl) (P3HT), [6,6]-Phenyl C61 butyric acid methyl ester(PCBM), [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM),Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV), andPoly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene](MDMO-PPV). (Images
taken fromhttp://www.sigmaaldrich.com)
Since the first report of photoinduced charge transfer from a conjugated polymer to a
buckminsterfullerene (C60) in 1992 by Sariciftci et al.74, the field of polymer based
solar cells has been through an explosive development. After the bucky ball C60showed a strong tendency to crystallize in the polymer matrix, a new fullerene
derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) with increased
solubility was developed as electron acceptor material48. A successful method to
dissociate excitons and generate free charge carriers in conjugated polymer based
solar cells was reported by Yu et al. in 199548. MEH-PPV as the electron donor and
PCBM as the electron acceptor were blended to form a bulk-heterojunction
photoactive layer. The solar cells based on MEH-PPV:PCBM composite showed an
efficiency of about 1%, which was a major step for polymer based solar cells. In 2001,
the device with poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene]
(MDMO-PPV):PCBM blend eventually reached a benchmark PCE of 2.5%75. During
the last 6 years, research efforts have focused on poly(alkyl-thiophenes), in particular
on regioregular poly(3-hexylthiophene) (P3HT) as electron donor, because of its
higher hole mobility and its light absorption ability at longer wavelength compared to
PPV derivatives. In 2002, the first encouraging results for P3HT:PCBM solar cells
were published76. Due to the efforts of several groups worldwide50, 53-55, 77-84, PCE
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Chapter 1. Introduction
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records up to 4-5% was reported in 200553, 54, 76 and to around 6% as reported recently
in 200950, 84. Fig. 1.11 shows the chemical structures of commonly used materials for
polymer solar cells.
The main development over the last years consisted in understanding and optimizingthe processing of the active layer. It was found that the optimum P3HT:PCBM weight
ratio is between 1:0.8 and 1:1, and the best solvents to dissolve the polymer and
PCBM are chlorobenzene (CB) and ortho-dichlorobenzene (oDCB). Moreover, the
device annealing conditions appeared to be mandatory to achieve high efficiency30,
85-91. In order to ensure maximum exciton dissociation at the D/A interface, as well as
an efficient charge carriers transport to respective electrodes, morphology control on
the nano-scale of the bulk-heterojunction composite was found to be the keyparameter to reach high performance devices. After device annealing, the open-circuit
voltage (Voc) usually slightly decreased, while both the short-circuit current density
(Jsc) and the fill factor (FF) increased significantly. The organization of the
P3HT:PCBM is modified upon annealing54, 56, 92, and fibrillar-like P3HT crystals
embedded in a matrix believed to comprise mostly PCBM and amorphous P3HT56.
Later on, it was noticed that the molecular weight (Mw) of the polymer part has some
influence on the solar cell device performance. The ideal morphology was observedfor P3HT with an average Mw in the range of 30 000-70 000
93. Too low Mw P3HT has
inferior mobility, most likely because of the main-chain defects66, 94, 95. On the other
hand, too high Mw P3HT is less soluble in DCB93 and produces highly entangled
polymer networks, requiring higher temperatures and/or longer annealing times for
crystallization96.
Figur e 1.12Chemical structures of polythiophenes with head-to-tail (left) and head-to-head
(right) monomer configurations. (Images taken fromwww.wikipedia.org)
In addition, the influence of the polymers regioregularity (RR), which is defined as
the percentage of monomers with a head-to-tail configuration rather than head-to-head
(Fig. 1.12), is found to be critical as well. A specific threshold of about 95% RR
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seems to be necessary to give best performance because of the better transport
properties of highly RR P3HT97, 98
However, the further improvement space on P3HT:PCBM devices seems to be limited,
because the internal IQE for the >5% efficient devices is already approaching 100%99,
100. Two reasons are considered as the limiting factors of P3HT:PCBM system. First,
the open circuit voltage reaches only 0.7 V, which is quite small compared to the
bandgap of P3HT (1.9 eV). A large amount of energy is lost when the photoexcited
electron transfer from the LUMO of P3HT (-3 eV) to the LUMO of PCBM (-3.8 eV).
This mismatch of LUMO offset can be overcame either by increasing the PCBM
LUMO level to about 3.3 eV or by lowering both LUMO and HOMO level of the
polymer to be better aligned the relative energy levels of the PCBM
86, 91, 101
.The second limiting factor is the narrow absorption range of P3HT. It can absorb light
within the visible spectrum up to about 650 nm, which means that most of the low
energy photons of the sun emission cannot be harvested. Efforts have been taken to
increase the absorption range by synthesizing novel low-bandgap polymers. For
instance,
poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,
3-benzothiadiazole)] (PCPDTBT) is one promising low-bandgap polymer (see Fig.1.11). Muehlbacher et al. reported on devices based on this polymer reaching an
efficiency of 3.2%57. Recently, devices based on PCPDTBT:PC70BM system achieved
PCEs up to 6.1%102. In addition, solar cells based on thieno[3,4-b]thiophene and
benzodithiophene polymer (PTBs) family exhibited record efficiency of 7.4%103,
showing the bright potential of bulk-heterojunction polymer solar cells reaching the
threshold for commercialization.
3) Hybrid solar cells
Replacing the fullerenes as organic nanoparticles for polymer solar cells by inorganic
nanocrystals (NCs) such as colloidal inorganic semiconductor NCs as electron
acceptors is an alternative approach leading to so-called hybrid solar cells24-26, 104, 105.
NCs based on CdS, CdSe, CdTe, ZnO, SnO2, TiO2, Si, PbS, and PbSe have been used
so far as electron acceptors. Colloidal NCs synthesized in organic media are usually
soluble in common organic solvents thus they are able to be incorporated into
conjugated polymers which are soluble in the same solvents. By tuning the diameter
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Chapter 1. Introduction
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of the NCs, their band gap as well as their energy level can be varied based on the
quantum size effect106. Furthermore, quantum confinement leads to an enhancement
of the absorption coefficient compared to that of the bulk materials107. As a result, in
the NCs/polymer system, both components have the ability to absorb incident light,
unlike the typical polymer/fullerene system where the fullerene contributes very little
to the photocurrent generation108, 109. In addition, NCs can provide stable elongated
structures on the length scale of 2-100 nm with desirable exciton dissociation and
charge transport properties110. The overview of the development of
bulk-heterojunction hybrid solar cells and strategies for device performance
improvement will be discussed in detail in Chapter 2.
1.3 Motivation and context
This PhD work was performed within the PhD program of the German Research
Foundation (DFG) graduate school GRK 1322 Micro Energy Harvesting. This
graduate school is supported by the Department of Microsystems Engineering
(IMTEK) and the Freiburg Materials Research Center (FMF), the
Albert-Ludwigs-University Freiburg, and the Fraunhofer Institute for Solar Energy
Systems (FhG-ISE) which is an associate partner of the graduate school. Three
research areas are included in this graduate school: A. Conversion Mechanism, B.
Materials and Storage Mechanism, and C. Energy and System Management.
In the frame of the research area B: materials and storage mechanism, this PhD work
is aiming to develop bulk-heterojunction nanocomposites for photovoltaics. These
nanocomposites consist out of conjugated polymers as electron donors and inorganic
NCs (e.g. CdSe) as electron acceptors. Both materials and device structure are
investigated and optimized. The intention is to enhance the carrier mobility and
balance the electron and hole transport within the system. The increase of the solar
cell efficiency and the investigation of fundamental principles are targeted.
1.4 Outline
Chapter 1 presents a general introduction of solar cells and organic/hybrid solar cells,
their history and development.
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Chapter 2 reviews the state-of-the-art development of bulk heterojunction hybrid solar
cells based on colloidal nanocrystals and conjugated polymers. Critical factors
limiting the solar cell device performance are highlighted and strategies for further
device improvement are demonstrated by giving recent examples from literature.
Chapter 3 presents highly reproducible synthesis methods for CdSe QDs, leading to a
narrow size distribution and excellent photophysical properties. Pre-heating of the
hexadecylamine (HDA) ligand and aging of the Se-TOP precursor are proven as two
critical parameters for synthesizing high quality QDs. The influence of the QD
characteristics such as diameter, photoluminescence (PL) peak wavelength, and PL
intensity on the performance of hybrid solar cells is studied, revealing that the
synthesis condition has a crucial impact on the QD surface quality, which can bepartially detected by the PL intensity.
Chapter 4 presents an effective post-synthetic hexanoic acid treatment on
HDA-capped CdSe QDs before their integration into photovoltaic devices is
demonstrated. Solar cells with optimized ratios of QDs to P3HT exhibit PCEs of
about 2.1%. A simple ligand sphere model is derived from PL quenching, TEM and
dynamic light scattering (DLS) results to explain the improved PCEs. The results
indicate that an effective reduction of the immobilized ligand sphere is a crucial factorto enhance the solar cell performance.
Chapter 5 describes extended investigations on applying the hexanoic acid treatment
to trioctylphosphine (TOP)/oleic acid (OA capped CdSe QDs. The comparable
performance of devices based on P3HT and different ligand capped QDs indicates that
the acid treatment is generally applicable to QDs with TOP/OA ligands for improving
device performance. In addition, by using the low bandgap polymer
Poly[2,6-(4,4-bis-(2-ethylhexy)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) as electron donor, optimized devices exhibited
PCEs of 2.7% after spectral mismatch correction. This value is the highest reported
one for spherical CdSe QDs based hybrid solar cells. Comparison studies on devices
using P3HT and PCPDTBT as donor material reveal that the polymer has an essential
impact on the absorption properties of the blend film as well as the device
performance consequently.
Finally, chapter 6 summaries the results presented in this dissertation, and describes
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Chapter 1. Introduction
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an outlook for further investigation and potential device improvement. In an
additional appendix, the pre-evaluation procedure for solar cells fabrication (A1) and
measurement setup (A2) in our lab are presented.
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2.1 Colloidal semiconductor nanocrystals
25
Chapter 2
Bulk-heterojunction hybrid solar cells
(The main content of this chapter was published in Energy & Environmental Science 3,
1815 (2010) entitled Bulk-heterojunction hybrid solar cells based on colloidal
nanocrystals and conjugated polymers as a review article.)
2.1 Colloidal semiconductor nanocrystals
Due to the decreased size of semiconductor nanocrystals (NCs) down to the
nanometer scale, quantum effects become dominant thus a number of physical (e.g.
mechanical, electrical, optical, etc.) properties change when compared to those of bulk
materials. For example, the quantum confinement effect106can be observed once the
diameter of the material is in the same magnitude as the wavelength of the electronwave function. Along with the decreasing size of NCs, the energy levels of NCs turn
from continuous states to discrete ones, resulting in a widening of the band gap
apparent as a blue shift in optical properties.
Figure 2.1Schematic illustration of the energy level distribution of a bulk semiconductor anda semiconductor NC. (Image taken from ref.111
)
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26
As shown in Fig. 2.1, a bulk semiconductor has continuous conduction and valence
energy bands separated by a fixed energy gap, whereas a semiconductor NC is
characterized by discrete atomic-like states and a size dependent energy gap. In
general, there are two distinct routes to produce NCs: by physical approach where
they can be grown by lithographic methods, ion implantation, and molecular beam
deposition; or by chemical approach where they are synthesized by the method of
colloidal chemistry in a solvent medium. Due to their scalability and the relative
simplicity of the process involved, colloidal synthetic methods are widely used and
are promising for large batch production and commercial applications. Due to their
unique optical and electrical properties, colloidal semiconductor NCs have attracted
numerous interests and have been explored in various applications like light-emitting
diodes (LEDs)112, 113, fluorescent biological labeling114, lasers115, and solar cells110.
Colloidal semiconductor NCs with suitable bandgap and energy levels can be
incorporated into conjugated polymers to form so-called bulk-heterojunction hybrid
solar cells. Fig. 2.2 shows the energy levels (in eV) of commonly used conjugated
polymers as donors and NCs as acceptors for bulk-heterojunction hybrid solar cells.
The Fermi levels of the electrodes and the energy levels of PCBM are shown as well.
The variation of the values for the energy levels derived from Ref.57, 116-126 is due to
different applied measurement methods for extracting the respective values of the
HOMO-LUMO levels such as cyclic voltammetry (CV), X-ray photoelectron
spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS), and due to
different experimental boundary conditions. In this chapter, recent state-of-the-art
results will be summarized and concepts for improving the device performance are
highlighted.
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2.1 Colloidal semiconductor nanocrystals
27
Figure 2.2 Fig. 6Energy levels (in eV) of commonly used conjugated polymers andsemiconductor NCs, as well as the electrode Fermi levels (in eV). Values for CdSe are for 4.8 nmQD and 4.6 nm QD, respectively. ZnO values correspond to NRs with diameters of 55 nm and 100
nm, respectively. In the literature the published values for the energy levels of the donor andacceptor materials differ slightly depend on e.g. different applied experimental methods to extractthe HOMO-LUMO levels such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy
(XPS), ultra-violet photoelectron spectroscopy (UPS) and due to different experimental boundaryconditions. (Image taken from Ref.
29)
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2.2 Devices based on CdSe NCs
CdSe NCs were the first NCs being incorporated into solar cells which still exhibit the
highest PCEs compared to devices with NCs from other materials, and are under
extensive studies for utilization in hybrid solar cells. CdSe NCs have some advantages:they absorb at a useful spectral range for harvesting solar emission from 300 nm to
650 nm (shown in Fig. 2.3a), they are good electron acceptors in combination with
conjugated polymers, and the synthetic methods for their synthesis are
well-established. In 1996, Greenham et al.127 firstly reported on the incorporation of
CdSe spherical quantum dots into MEH-PPV. At a high concentration of NCs of
around 90% by weight (wt%), external quantum efficiencies (EQE) up to 10% were
achieved, indicating an efficient exciton dissociation at the polymer/NCs interface.Although the phase separation, between the polymer and the NCs was observed to be
in the range of 10-200 nm, the PCEs of devices were very low of about 0.1%. This
was attributed to an inefficient electron transport via hopping from NCs to NCs. After
the breakthrough synthetic work of Peng et al.128, shape control of CdSe NCs was
introduced and different elongated CdSe structures were obtainable. Fig. 2.3b
illustrates different shapes of NCs used in hybrid solar cells as electron acceptor
materials such as spherical quantum dots (QDs), nanorods (NRs) and tetrapods (TPs).
Figure 2.3 (a) Absorption spectrum of CdSe QDs with different sizes. Inset:
Photoluminescence (PL) of differently sized QDs (3 nm - 6 nm) under UV irradiation. (b)Schematic illustration of different shapes of NCs, from left to right: quantum dot (QD),nanorod (NR), and tetrapod (TP). (Images taken from Ref.
29)
Numerous approaches were published regarding synthesizing various morphologies
and structures of CdSe NCs and their application in hybrid solar cells. A significant
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2.2 Devices based on CdSe NCs
29
advance was reported in 2002 by Huynh et al.110, who demonstrated efficient hybrid
solar cells based on elongated CdSe nanorods and P3HT. Elongated nanorods were
used which naturally provide directed pathways for effective electron transport.
Additionally, P3HT was used as donor material since it has a comparatively high hole
mobility and absorbs at a longer wavelength range compared to PPV derivatives76. By
increasing the nanorod length, improved electron transport properties were
demonstrated resulting in an improvement of the EQE (Fig. 2.4a). The optimized
devices consisting out of 90wt% pyridine treated nanorods (7 nm in diameter and
60 nm in length) and P3HT exhibited an EQE over 54% and a PCE of 1.7%. Fig. 2.4b
shows a J-V characteristic of an optimized device under AM1.5 G illumination. This
work highlights the importance of shape control of materials on the nanometer scale,
which opens new opportunities for the development of future generation solar cells.
Later on, numerous literatures have been published for hybrid solar cells based on
conjugated polymers and QDs52, 129-131 NRs110, 132-135, TPs51, 130, 136-138 and
hyperbranched139CdSe NCs, exhibiting the highest PCEs of 2.7%140, 2.6%132, 3.2%51,
and 2.2%139, respectively. The selected performance parameters of hybrid solar cells
based on CdSe NCs and conjugated polymers are summarized in Table 2.1.
Figure 2.4(a) Dependence of EQE on the NR lengths. (b) Current density (J)-voltage (V)characteristic of a CdSe NR (7 nm in diameter, 60 nm in length) containing device under
AM1.5 G illumination. (Images taken from Ref.110
)
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Chapter 2. Bulk-heterojunction hybrid solar cells
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Table 1. Selected performance parameters of hybrid solar cells based on CdSe NCs and conjugated polymer. All measurements were performed
under AM1.5 G at intensities of 90-100 mW/cm2 illumination.
Abbreviations: TP: Tetropods, NR: Nanorods, Hybrch: Hyperbranched, QD: Quantum Dots, MEH-PPV: poly(2-methoxy-5-(2`-ethyl)-hexyloxy-p-phenylene vinylene),
OC1C10-PPV: poly(2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylenevinylene), APFO-3: poly(2,7-(9,9-dioctylfluorene)-alt-5,5-
(4,7-di-2-thienyl-2,1,3,-benzothiadiazole)), pyr: pyridine, dithiol: benzene1,3-dithiol, hex acid: hexanoic acid, btylmn: butylamine, tBOC: tert-buthoxycarbonyl.a functionalized P3HT, b volume percent
CdSe
NCsPolymer d(nm)
Aspect
ratio
treatment
on NCSolvent
NC
wt%Cathode
Mismatch
CorrectionVoc(V) Jsc(mA/cm2) FF PCE(%) Ref
TP PCPDTBT 5 6-10 pyr CHCl3/Pyr/TCB 90 LiF/Al yes 0.678 10.1 0.51 3.19 51TP OC1C10-PPV 5 10 pyr CHCl3/Pyr/TCB 86 LiF/Al yes 0.76 9.1 0.44 2.8 136
QD PCPDTBT 4.7 1 hex acid CB 87.5 Al yes 0.59 8.3 0.56 2.7 140NR P3HT 15 2-6.7 pyr/dithiol CHCl3/TCB Al no 0.55 9.7 0.49 2.65 135
NR P3HT 5 13 pyr CHCl3/Pyr/TCB 90 Al yes 0.62 8.79 0.5 2.6 132
TP APFO-3 5 6-14 pyr CHCl3/Pyr/p-xylene 86 Al yes 0.95 7.23 0.38 2.4 137Hybrch P3HT pyr CHCl3/Pyr/TCB 86 Al no 0.6 7.1 0.51 2.2 139
QD P3HT 5.5 1 hex acid DCB 87 Al yes 0.623 5.8 0.56 2 52TP OC1C10-PPV 5 10 pyr CHCl3/Pyr/DCB 86 Al no 0.65 7.3 0.35 1.8 138QD P3HT 1 pyr/btylmn CHCl3/ btylmn 92 Al no 0.55 6.9 0.47 1.8 131NR P3HT 7 8.6 pyr CHCl3/Pyr 90 Al no 0.7 5.7 0.4 1.7 110NR P3HTa 7 4.3 pyr CHCl3/Pyr 40
b Al no 0.6 6.2 0.4 1.5 133TP MEH-PPV pyr CHCl3/Pyr/CB 90 Al no 0.69 2.86 0.46 1.13 130TP P3HT 5 6-10 pyr CHCl3/Pyr/TCB 90 Al yes 0.633 4.83 0.52 1.49 141QD MEH-PPV 4.3 1 pyr CB/Pyr 89 Ba/Al no 0.9 2.03 0.47 0.85 129QD P3HT 3.3 1 pyr CB/Pyr 89 Al no 0.67 2.6 0.36 0.65 142
NR P3HT 6 2-3 tBOC 90 Al no 0.76 1.43 0.37 0.44 134
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2.3 Strategies for eff iciency improvement
2.3.1 NC surface modificationIn principle, polymer/NC hybrid solar cells should perform better compared to
polymer/fullerene systems due to the additional higher absorption coefficient of
inorganic semiconductor NCs and potential higher intrinsic electron mobility
compared to that of PCBM (10-3 cm2/V-1s-1 143). Nevertheless, there has been no higher
PCEs reported in hybrid solar cells compared to fullerene based OSCs so far. One
important reason is that despite the relatively high intrinsic conductivity within the
individual NCs, the electron mobility transport through the NC network in hybrid
solar cells is quite low, which could be mainly attributed to the electrical insulating
organic ligands on the NC surface52. In most cases, the ligands used for preventing
aggregation during the growth of the NCs contain long alkyl chains, such as oleic acid
(OA) or trioctylphosphine oxide (TOPO), which form electrically insulating layers
thus impedes an efficient charge transfer between NCs and polymer, as well as
electron transport between NCs127, 144. Ginger et al.145 have investigated charge
injection and charge transfer in thin films of spherical CdSe NCs covered with TOPO
ligand sandwiched between two metal electrodes. Very low electron mobilities in the
order of 10-5 cm2V-1s-1 were measured, whereas the electron mobility of bulk CdSe is
in the order of 102 cm2V-1s-1 146. In order to overcome this problem, extensive
investigations on the surface modification of NCs have been reported based on ligand
exchange approaches using various shorter capping ligands. The interparticle distance
is expected to be reduced, thus facilitating the electron transport through the NC
domain phases. For example, post-synthetic pyridine treatment of the NCs is the most
commonly used and effective procedure, leading to the state-of-the-art efficiencies for
hybrid solar cells51, 110, 129, 130, 132, 133, 136-139, 142
. Generally, as-synthesized NCs arewashed by methanol several times and consequently refluxed in pure pyridine at the
boiling point of pyridine around 115oC under inert atmosphere overnight. Afterwards,
the NCs are precipitated with hexanes, recovered by centrifugation, and then
dispersed into a mixture of chloroform/pyridine (90:10, vol/vol). This pyridine
treatment is believed to replace the synthetic insulating ligand with shorter and more
conductive pyridine molecules. Treatments with other materials such as chloride147,
amine
131
, and thiols
148, 149
were also investigated. Aldakov et al.
149
systematicallyinvestigated CdSe NCs modified by various small ligand molecules with nuclear
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Chapter 2. Bulk-heterojunction hybrid solar cells
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magnetic resonance (NMR), optical spectroscopy and electrochemistry, although their
hybrid devices exhibited low efficiencies. Olson et al.131 reported on CdSe/P3HT
blended devices exhibiting PCEs up to 1.77% when butylamine was used as a shorter
capping ligand for the NCs. In an alternative approach, shortening of the insulating
ligands by thermal decomposition was demonstrated and led to a relative
improvement of the PCEs of the CdSe/P3HT-based solar cells134. However, NCs after
ligand exchange with small molecules tend to aggregate and precipitate out from the
organic solvent once the long alky chain ligands are replaced110, 150, resulting in
difficulties to obtain stable mixtures of NCs and polymer. Recently, we have
demonstrated a novel post-synthetic treatment on spherical CdSe QDs using a
non-ligand-exchange approach52, where the NCs were treated by a simple and fast
hexanoic acid-assisted washing procedure.
Figure 2.5(a) J-V characteristic of a device containing 87 wt% CdSe QDs and P3HT asphotoactive layer under AM1.5G illumination, exhibiting a PCE of 2.1% after spectralmismatch correction (Inset: Photograph of the hybrid solar cell device structure). (b)Schematic illustration of the proposed QD sphere model: an outer insulating HDA ligandsphere is supposed to be responsible for the insulating organic layer in untreated QDs directlytaken out of the synthesis matrix and is effectively reduced in size by methanol washing andadditional acid treatment (Images taken from Ref.
52).
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2.3 Strategies for efficiency improvement
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Devices with optimized ratios of QDs to P3HT exhibited reproducible PCEs up to 2%
after spectral mismatch correction which could meanwhile be exceeded (Fig. 2.5a).
This is the highest reported value for a CdSe QDs based hybrid solar cell so far. It is
notable that the FF is relatively high up to 0.54, implying a good charge carrier
transport capability in the devices. A simple reduced ligand sphere model was
proposed to explain the possible reason for improved photovoltaic device efficiencies
after acid treatment as shown in Fig. 2.5b. By the assistance of hexanoic acid this
immobilized insulating spheres formed by HDA ligands are effectively reduced in
size due to the salt formation of HDA. This organic salt is also much more easily
dissolved in the supernatant solution than unprotonated HDA and can be separated
easily from the QDs by subsequent centrifugation. Another advantage of avoiding the
exchange of the synthesis capping ligands is that the QDs retain a good solubility after
acid treatment, resulting in reproducible performance as well as allowing a high
loading of the CdSe QDs in the blend, which is preferable for an efficient percolation
network formation during the annealing of the photoactive composite film.
2.3.2 Polymer functionalization
From the polymer side, modifications such as end-group functionalization have been
demonstrated as a route for improving the dispersion in solvents and electronic
interaction between polymer and NCs151. Liu et al.133 have shown that
pyridine-treated CdSe NRs can form well dispersed composite films with
amine-terminated regioregular P3HT, exhibiting a maximum PCE of 1.4%. The
end-group amine-functionalized P3HT is expected to provide intimate contact
between NCs and polymer through covalent interactions, and thereby enhancing the
NRs miscibility with P3HT, resulting in a favorable morphology improving the chargetransfer between them. Other approaches to end-group functionalization of P3HT such
as using H/(-SH), H/Br, and Br/allyl as terminating chemical groups have been
reported152. Hybrid solar cells with CdSe QDs and Br/allyl-terminated and
H/(-SH)-terminated P3HT led to efficiencies of 0.9% and 0.6%, respectively. Zhang
et al.153have demonstrated a route for directly attaching P3HT on CdSe NR surfaces,
by coupling vinyl-terminated P3HT to CdSe NRs with arylbromide-functionalized
phosphine oxides and thiols. Additionally, the direct synthesis of uncapped NCs insidethe polymer phase is expected as an ideal situation, since the effects of the capping
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Chapter 2. Bulk-heterojunction hybrid solar cells
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ligand on charge transfer and charge transport are eliminated and the step for
transferring NCs to the polymer solution can be bypassed. Dayal et al.154 have
reported on the direct synthesis of homogeneously dispersed CdSe QDs in a P3HT
solution. Photoinduced charge separation was observed, indicating that this
composite could be a promising material for hybrid solar cells.
2.3.3 Photon absorption
In order to harvest the maximum possible amount of the solar energy, absorption of a
large fraction of the incident photons is required. Generally incident photons are
mainly absorbed by the donor polymer materials and partially also from the inorganic
NCs. For example in blends containing 90 wt% CdSe nanoparticles in P3HT, about
60% of the total absorbed light energy can be attributed to P3HT due to the strong
absorption coefficient 141. Previous work mostly focused on systems based on CdSe
NCs and P3HT as a high hole mobility polymer. For examples, by using P3HT as
donor material, hybrid solar cells with spherical QDs, NRs, and hyperbranched CdSe
NCs exhibited the best efficiencies of 2.0%52, 2.6%132, 135, and 2.2%139, respectively.
However, due to the poor overlap between the P3HT absorption spectrum and the
solar emission spectrum124, further improvement on PCEs seems to be difficult to
obtain with this polymer system.
Figure 2.6AM1.5 G photon flux (black) and the integrated values (red) of photons as well asthe maximum theoretical achievable short circuit current densities as a function of themaximal absorbed wavelengths (which results from the band-gap of the respective material).Crystalline silicon, P3HT, and PCPDTBT are shown as examples for comparison. (Irradiancedata is taken from NREL, Image taken from Ref29).
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2.3 Strategies for efficiency improvement
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Fig. 2.6 shows the photons flux and its integral under AM1.5 G conditions (the AM1.5
G solar irradiance data is taken from NRELg). Two different scales are shown on the
right side: one axis shows the integral from 280-4000 nm of the number of photons
which can be absorbed by having a certain band gap. The second axis shows the
maximum theoretical Jsc calculated by assuming that all photons are absorbed up to
the band gap and converted into electrons without any losses (e.g. EQE is constant 1).
Crystalline silicon has a band gap of 1.1 eV and can absorb up to 64% of the photons
under AM1.5 G illumination, with a theoretical achievable current density Jsc of about
45 mA/cm2, while in the case of P3HT having a band gap of 1.85 eV, only 27%
photons can be absorbed, resulting in a maximal Jsc of 19 mA/cm2. By using the low
band gap polymer PCPDTBT with a band gap of about 1.4 eV, more photons can be
absorbed theoretically, leading to a maximum Jsc up to 32 mA/cm2.
Therefore, polymers with a smaller band gap absorbing at longer wavelengths are
promising donor materials to increase the PCE of devices. Dennler et al.12
demonstrated that for a minimum energy offset of 0.3 eV between the donor and
acceptor LUMO level, PCEs of >10% are practical available for a donor polymer with
an ideal optical band gap of ~1.4 eV. Most low band gap polymers are from the
material classes of thiophene, fluorene, carbazole, and cylopentadithiophene based
polymers, which are reviewed in detail in review articles12, 23, 124. Among those low
band gap polymers,
poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,
3-benzothiadiazole)] (PCPDTBT, chemical structure shown in Fig.3) with a band gap
of ~1.4 eV and a relatively high hole mobility up to 1.510-2 cm2V-1s-1155appears to
be an excellent candidate as a photon-absorbing and electron donating material156.
OSCs based on PCPDTBT:PC70BM system achieved already efficiencies up to 5.5%157 and 6.1%102. Recently, a bulk-heterojunction hybrid solar cell based on CdSe
tetrapods and PCPDTBT was reported by Dayal et al.51with a certified PCE of 3.13%
(measured by NREL). This is up to date the highest efficiency for colloidal NCs based
bulk-heterojunction hybrid solar cells. As shown in Fig. 2.7, devices out of PCPDTBT
and CdSe tetrapods, exhibited an EQE of >30% in a broad range from 350 nm to 800
g http://rredc.nrel.gov/solar/spectra/am1.5/
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Chapter 2. Bulk-heterojunction hybrid solar cells
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nm, which is the absorption band of the polymer. It is notable that the devices reached
very high Jsc values above 10 mA/cm2, indicating that the broad absorption ability of
the photoactive hybrid film consequently contributes to the photocurrent.
Figure 2.7EQE and absorption spectra of a device containing CdSe tetrapods andPCPDTBT exceeding a PCE of 3 % (Image taken from Ref.
51).
It should be noted that lowering the band gap of photo-absorbing materials below a
certain limit will lead to a decrease in efficiency, because the maximum Voc is limited
by the band gap. In addition, the energy of absorbed photons with a larger energy than
the band gap will be wasted as the electrons and holes relax to the band edges. Thislimitation might be overcome by multiple exciton generation in semiconductor NCs,
where a single high-energy absorbed photon can produce multiple electron-hole pairs.
This phenomenon has been studied using PbSe, CdSe, and PbS NCs158-160. Very high
efficiencies exceeding 42% for solar cells with NCs having a band gap of 0.45 eV are
theoretically possible, assuming that additional carriers from multiple exciton
generation can be extracted efficiently161. Nevertheless the contribution of multiple
exciton generation in inorganic NCs to the solar cell efficiency is quite controversial
in literature up to date162since initial results could not be verified and it needs further
more convincing experimental verification. So far there is no practical evidence that
the additional carriers can be extracted quickly enough to compete with the rapid
exciton recombination processes within the NCs.
2.3.4 Control of the nanomorphology
Generally, by controlling the morphology of the active layer, the performance of
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2.3 Strategies for efficiency improvement
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bulk-heterojunction solar cells can be increased, because the efficient charge transfer,
transport and collection strongly depend on the nanoscale morphology of the
composite film163, 164. For example, the crystallization of P3HT, induced during a
thermal annealing step during device preparation, improves the light absorption
property and