Strategies for optimizing organic solar cells [Elektronische Ressource] : correlation between morphology and performance in DCV6T - C60 heterojunctions / vorgelegt von David Wynands

Sujets

• Physik

Informations

• Publié par: technische_universitat_dresden
• Publié le: 01 janvier 2010
• Nombre de lectures: 58
• Langue: English
• Poids de l'ouvrage: 15 Mo

Extrait

Institut fur¨ Angewandte Photophysik
Fachrichtung Physik
Fakultat¨ Mathematik und Naturwissenschaften
Technische Universitat¨ Dresden
Strategies for Optimizing Organic
Solar Cells:
Correlation Between Morphology and
Performance in DCV6T - C60 Heterojunctions
Dissertation
zur Erlangung des akademischen Grades
Doctor rerum naturalium,
(Dr. rer. nat.)
vorgelegt von
David Wynands
geboren am 01.04.1982 in Dresden
Dresden 2010
LEingereicht am 20.08.2010
1. Gutachter: Prof. Dr. K. Leo
2. Prof. Dr. C. Brabec
Verteidigt am 04.02.2011Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Physical Properties of Organic Semiconductors . . . . . . . . . . . . . . . . . . . . 11
2.1 Organic Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Molecules with Conjugated-Electron Systems . . . . . . . . . . . . . 12
2.2.1 Energy Splitting in Molecular Orbital Theory . . . . . . . . . . 12
2.2.2 Extended-Conjugated Systems . . . . . . . . . . . . . . . . . 14
2.3 Optical Excitations in Organic Molecules . . . . . . . . . . . . . . . . 16
2.4 From Molecules to Solids . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4.1 Self-Polarization in Organic Solids . . . . . . . . . . . . . . . 19
2.4.2 Excitations in Organic Solids . . . . . . . . . . . . . . . . . . . 21
2.4.3 Charge Carriers and Transport . . . . . . . . . . . . . . . . . . 23
3 Organic Photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.1 Solar Cell Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.1.1 Conversion of Radiation into Chemical Energy . . . . . . . . . 28
3.1.2 Conv of Chemical Energy into Electrical Energy . . . . . 32
3.1.3 Conventional pn-Junction as Photodiode . . . . . . . . . . . . . 35
3.1.4 Simple Equivalent Circuit . . . . . . . . . . . . . . . . . . . . 39
3.2 Organic Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2.1 Donor-Acceptor Heterojunction . . . . . . . . . . . . . . . . . 40
3.2.2 Recombination Processes . . . . . . . . . . . . . . . . . . . . . 46
3.2.3 Transport Layers – The p-i-n Concept . . . . . . . . . . . . . . 48
4 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.1 C60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.2 Transport Materials . . . . . . . . . . . . . . . . . . . . . . . . 50
4.2 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.3 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.3.1 X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . 54
4.3.2 Optical Characterization . . . . . . . . . . . . . . . . . . . . . 55
4.3.3 Topography . . . . . . . . . . . . . . . . . . . 554 Contents
4.3.4 Mobility Measurements . . . . . . . . . . . . . . . . . . . . . 56
4.3.5 Electrical Characterization of Solar Cells . . . . . . . . . . . . 59
4.3.6 Optical Simulation . . . . . . . . . . . . . . . . . . . . . . . . 60
4.3.7 Ultraviolet Photoelectron Spectroscopy . . . . . . . . . . . . . 60
4.4 Standard Reporting Conditions and Mismatch . . . . . . . . . . . . . . 62
5 The Material System DCV6T - C60. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.1 Oligothiophenes as Donors in Heterojunctions with C60 . . . . . . . . 67
5.2 Basic Material Properties of DCV6T . . . . . . . . . . . . . . . . . . . 71
5.2.1 Optical . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.2.2 Electronic Properties . . . . . . . . . . . . . . . . . . . . . . . 74
5.3 Effect of Substrate Heating on Layer Morphology . . . . . . . . . . . . 75
5.3.1 Neat DCV6T Layers . . . . . . . . . . . . . . . . . . . . . . . 76
5.3.2 Mixed : C60 Layers . . . . . . . . . . . . . . . . . . . 86
5.4 Effect of Substrate Heating on Mobility . . . . . . . . . . . . . . . . . 92
6 DCV6T - C60 Solar Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.1 Effect of Substrate Heating in DCV6T - C60 Solar Cells . . . . . . . . . 99
6.1.1 Flat Heterojunction Solar Cells . . . . . . . . . . . . . . . . . . 100
6.1.2 Mixed Solar Cells . . . . . . . . . . . . . . . . 103
6.2 Influence of the Mixing Ratio . . . . . . . . . . . . . . . . . . . . . . . 117
6.3 Optimizing the Layer Stack . . . . . . . . . . . . . . . . . . . . . . . . 124
6.3.1 Influence of the Transport Layer Thickness . . . . . . . . . . . 126
6.3.2 of the Mixed Layer . . . . . . . . . . . . . 129
6.3.3 Discussion of Quantum Efficiency and Loss Mechanisms . . . . 135
6.4 Thermal Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
7 Conclusions and Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .145
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
7.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .151
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .161
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179Abstract
This work investigates organic solar cells made of small molecules. Using the material
system,!-bis(dicyanovinylene)-sexithiophene (DCV6T) - C60 as model, the
correlation between the photovoltaic active layer morphology and performance of the solar
cell is studied. The chosen method for controlling the layer morphology is applying
different substrate temperatures (T ) during the deposition of the layer.sub
In neat DCV6T layers, substrate heating induces higher crystallinity as is shown by
X-ray diffraction and atomic force microscopy (AFM). The absorption spectrum
displays a more distinct fine structure, a redshift of the absorption peaks by up to 11 nm
and a significant increase of the low energy absorption band atT = 120 C comparedsub
toT = 30 C. Contrary to general expectations, the hole mobility as measured in fieldsub
effect transistors and with the method of charge extraction by linearly increasing
voltage (CELIV) does not increase in samples with higher crystallinity. In mixed layers,
investigations by AFM and UV-Vis spectroscopy reveal a stronger phase separation
induced by substrate heating, leading to larger domains of DCV6T. This is indicated by
an increased grain size and roughness of the topography, the increase of the DCV6T
luminescence signal, and the more distinct fine structure of the DCV6T related
absorption.
Based on the results of the morphology analysis, the effect of different substrate
temperatures on the performance of solar cells with flat and mixed DCV6T - C60
heterojunctions is investigated. In flat heterojunction solar cells, a slight increase of the
photocurrent by about 10 % is observed upon substrate heating, attributed to the increase
of DCV6T absorption. In mixed DCV6T : C60 heterojunction solar cells, much more
pronounced enhancements are achieved. By varying the substrate temperature from
-7 C to 120 C, it is shown that the stronger phase separation upon substrate heating
facilitates the charge transport, leading to a significant increase of the internal
quantum efficiency (IQE), photocurrent, and fill factor. Consequently, the power conversion
efficiency (PCE) increases from 0.5 % atT = -7 C to about 3.0 % at T 77 C.sub sub
Subsequent optimization of the DCV6T : C60 mixing ratio and the stack design of the
solar cell lead to devices with PCE of 4.90.2 %. Using optical simulations, the IQE
of these devices is studied in more detail to identify major remaining loss mechanisms.
The evaluation of the absorption pattern in the wavelength range from 300 to 750 nm
shows that only 77 % of the absorbed photons contribute to the exciton generation in
photovoltaic active layers, while the rest is lost in passive layers. Furthermore, the
IQE of the photovoltaic active layers, consisting of an intrinsic C60 layer and a mixed
DCV6T : C60 layer, exhibits a lower exciton diffusion efficiency for C60 excitons
compared to DCV6T excitons, attributed to exciton migration into the adjacent electron
transport layer.Kurzfassung
Diese Arbeit befasst sich mit organischen Solarzellen aus kleinen Molekulen.¨
Anhand des Materialsystems,!-bis(Dicyanovinylen)-Sexithiophen (DCV6T) - C60 wird
der Zusammenhang zwischen Morphologie der photovoltaisch aktiven Schicht und dem
Leistungverhalten der Solarzellen untersucht. Zur Beeinflussung der Morphologie
werden verschiedene Substrattemperaturen (T ) wahrend¨ des Schichtwachstums der ak-sub
tiven Schicht eingestellt.
Beim Heizen des Substrates weisen DCV6T Einzelschichten eine erhohte¨ Kristallinitat¨
auf, die mittels Rontgenbeugung¨ und Rasterkraftmikroskopie (AFM) erkennbar ist.
Zudem bewirkt die Erhohung¨ der Substrattemperatur von 30 C auf 120 C eine
ausgepragtere¨ Feinstrukturierung des Absorptionsspektrums, eine Rotve