Bulk Heterojunction With Quantum Dots

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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


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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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-ise


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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
http://www.solarbuzz.com/marketbuzz2010-intro.htmhttp://www.solarbuzz.com/marketbuzz2010-intro.htmhttp://www.solarbuzz.com/marketbuzz2010-intro.htm


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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
http://www.solarbuzz.com/marketbuzz2010-intro.htmhttp://www.solarbuzz.com/marketbuzz2010-intro.htmhttp://www.solarbuzz.com/marketbuzz2010-intro.htm


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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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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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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


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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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
http://www.energy-sunbags.de/http://www.energy-sunbags.de/http://www.energy-sunbags.de/http://www.energy-sunbags.de/


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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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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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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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1.4 Outline

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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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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.)


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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Chapter 2. Bulk-heterojunction hybrid solar cells

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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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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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 efficiency improvement

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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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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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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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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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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/
http://rredc.nrel.gov/solar/spectra/am1.5/http://rredc.nrel.gov/solar/spectra/am1.5/http://rredc.nrel.gov/solar/spectra/am1.5/


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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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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

Bulk Heterojunction With Quantum Dots

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