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"Recombination Dynamics in Organic Solar Cells"

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Recombination Dynamics in Organic Solar Cells Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Julius-Maximilians-Universität Würzburg vorgelegt von Alexander Förtig aus Beuchen im Odenwald Würzburg 2013 Eingereicht am: 31.05.2013 bei der Fakultät für Physik und Astronomie 1. Gutachter: Prof. Dr. Vladimir Dyakonov 2. Gutachter: Prof. Dr. Jean Geurts der Dissertation. 1. Prüfer: Prof. Dr. Vladimir Dyakonov 2. Prüfer: Prof. Dr. Jean Geurts 3. Prüfer: Prof. Dr. Giorgio Sangiovanni im Promotionskolloquium. Tag des Promotionskolloqiums: 28.10.2013 Doktorurkunde ausgehändigt am: » Es ist nicht das Wissen, sondern das Lernen, nicht das Besitzen, sondern das Erwerben, nicht das Dasein, sondern das Hinkommen, was den grössten Genuss gewährt. « Carl Friedrich Gauß (1777-1855) I Abstract Besides established, conventional inorganic photovoltaics—mainly based on silicon—organic photovoltaics (OPV) are well on the way to represent a low- cost, environment friendly, complementary technology in near future. Produc- tion costs, solar cell lifetime and performance are the relevant factors which need to be optimized to enable a market launch of OPV. In this work, the efficiency of organic solar cells and their limitation due to charge carrier recombination are investigated. To analyze solar cells under operating conditions, time-resolved techniques such as transient photovoltage (TPV), transient photocurrent (TPC) and charge extraction (CE) are app- lied in combination with time delayed collection field (TDCF) measurements. Solution processed and evaporated samples of different material composition and varying device architectures are studied. The standard OPV reference system, P3HT:PC61BM, is analyzed for va- rious temperatures in terms of charge carrier lifetime and charge carrier den- sity for a range of illumination intensities. The applicability of the Shockley Equation for organic solar cells is validated in case of field-independent charge photogeneration. In addition, a consistent model is presented, directly relating the ideality factor to the recombination of free with trapped charge carriers in an exponential density of states. An approach known as j=V reconstruction enables to identify the per- formance limiting loss mechanism of as-prepared and thermally treated P3HT:PC61BM solar cells. This procedure, involving TPV, CE and TDCF measurements, is extended to samples based on the rather new, low-band gap polymer PTB7 in combination with PC71BM. While in the devices processed from pure chlorobenzene solution considerable geminate and nongeminate los- ses are observed, the use of a solvent additive facilitates efficient polaron pair dissociation minimizing geminate recombination. Finally, in collaboration with the IMEC institute in Leuven, the two main organic solar cell device architectures, planar and bulk heterojunction—both based on CuPc and C60—are directly compared in terms of nongeminate re- combination and charge carrier distribution. Two experimental techniques, TPV and CE, as well as a macroscopic device simulation are applied to reveal the origin of different Voc vs. light intensity dependence. II Zusammenfassung Neben herkömmlichen, konventionellen anorganischen Solarzellen— hauptsächlich auf Silizium basierend—ist die Organische Photovoltaik (OPV) auf dem besten Wege in naher Zukunft eine kostengünstige, umwelt- freundliche, komplementäre Technolgie darzustellen. Die Produktionskosten, die Lebenszeit der Solarzellen sowie deren Wirkungsgrad müssen dabei weiter optimiert werden, um einen Markteintritt der OPV zu ermöglichen. Die vorliegende Arbeit befasst sich mit der Effizienz organischer Solar- zellen und deren Limitierung durch die Rekombination von Ladungsträgern. Um funktionsfähige Zellen zu untersuchen, werden zeitaufgelöste Experimen- te wie die Messung der transienten Photospannung (TPV), des transienten Photostroms (TPC), die Ladungsextraktion (CE) sowie die time delayed col- lection field (TDCF) Methode angewandt. Untersucht werden sowohl flüssig prozessierte als auch aufgedampfte Proben, unterschiedliche Materialzusam- mensetzungen und verschiedene Probengeometrien. Das Standardmaterialsystem der OPV, P3HT:PC61BM, wird bei verschie- denen Temperaturen und Beleuchtungsstärken auf die Lebenszeit und Dichte der photogenerierten Ladungsträger überprüft. Für den Fall spannungsunab- hängiger Generation von Ladungsträgern zeigt sich die Anwendbarkeit der Shockley-Gleichung auf organische Solarzellen. Des Weiteren wird ein konsis- tentes Modell erläutert, welches den Idealtitätsfaktor direkt mit der Rekom- bination von freien mit gefangenen, exponentiell verteilten Ladungsträgern verknüpft. Ein Ansatz, bekannt unter der Bezeichung j=V Rekonstruktion, ermög- licht es, den leistungslimitierenden Verlustmechanismus in unbehandelten und thermisch geheizten P3HT:PC61BM Solarzellen zu identifizieren. Dieses Ver- fahren, welches TPV, CE und TDCF Messungen beinhaltet, wird auf Proben basierend auf dem neuartigen, low-band gap Polymer PTB7 in Verbindung mit dem Fulleren PC71BM ausgeweitet. Während in der Zelle hergestellt aus reinem Chlorbenzol beträchtliche geminale wie nichtgeminale Verluste zu be- obachten sind, erleichtert die Zugabe eines Lösungsmittelzusatzes die Polaro- nenpaartrennung, was zu einer starken Reduktion geminaler Verluste führt. In einer Kooperation mit dem IMEC Institut in Leuven, werden abschlie- ßend die beiden bedeutensten Probenarchitekturen organischer Solarzellen, die planare und die Mischübergang Struktur, jeweils basierend auf CuPC und C60, bezüglich nichtgeminaler Rekombination und Ladungsträgerverteilung mitei- nander verglichen. Neben den beiden experimentellen Techniken um TPV und CE werden makroskopische Simulationen herangezogen, um den Ursprung un- terschiedlichen Voc vs. Lichtintensität–Verhaltens zu erklären. Contents 1 Introduction1 2 Organic Photovoltaics5 2.1 From Organic Semiconductors to Solar Cells – A Brief Overview5 2.2 Organic Semiconductors.....................7 2.3 Operating Principles in Organic Solar Cells...........8 2.3.1 Photon Absorption – Exciton Generation........9 2.3.2 Exciton Diffusion & Dissociation............ 10 2.3.3 Polaron Pair Dissociation................ 10 2.3.4 Charge Transport..................... 12 2.3.5 Charge Extraction.................... 14 2.4 Current–Voltage characteristics................. 15 3 Charge Carrier Recombination Dynamics 17 3.1 Geminate Recombination..................... 17 3.2 Nongeminate Recombination................... 19 3.3 Impact of Charge Recombination on the j=V Characteristics. 21 4 Experimental 25 4.1 Transient Photovoltage (TPV).................. 25 4.2 Transient Photocurrent (TPC).................. 28 4.3 Charge Extraction (CE)..................... 29 4.4 Time Delayed Collection Field (TDCF)............. 33 4.5 Organic Materials......................... 34 4.6 Device Preparation........................ 37 5 Shockley Equation Parameters by Transient Experiments 39 5.1 Introduction............................ 39 5.2 Theoretical Background..................... 40 5.3 Device Preparation & Measurements.............. 40 5.4 Results and Discussion...................... 41 5.4.1 SE parameters from Static and Transient Methods... 43 5.4.2 Loss Current vs. Photocurrent.............. 46 5.4.3 Contributions to Recombination Ideality Factor nR .. 47 5.4.4 Interpretation of Ideality Factors............ 48 5.5 Conclusion............................. 52 IV Contents 6 Impact of Charge Recombination on j=V response 53 6.1 Pristine vs. annealed P3HT:PC61BM Solar Cells........ 53 6.1.1 Introduction........................ 53 6.1.2 T-dependent nongeminate Recombination....... 54 6.1.3 j=V Reconstruction of P3HT:PC61BM......... 58 6.1.4 Conclusion......................... 62 6.2 Impact of Solvent Additive on PTB7:PC71BM Solar Cells... 64 6.2.1 Introduction........................ 64 6.2.2 Device Preparation & Measurements.......... 64 6.2.3 Results........................... 65 6.2.4 Discussion......................... 69 6.2.5 Conclusion......................... 74 7 Planar vs. Bulk heterojunction 75 7.1 Introduction............................ 75 7.2 Device Preparation & Measurements.............. 76 7.3 Results............................... 77 7.4 Discussion............................. 82 7.4.1 Recombination...................... 82 7.4.2 Open Circuit Voltage................... 83 7.5 Conclusion............................. 87 8 Conclusion 89 Bibliography 95 A Relation between charge carrier decay order and τ∆n 109 B Device simulation parameters 111 C List of Publications 113 D Danksagung 115 Chapter 1 Introduction The need for a sustainable, nonpolluting and affordable energy supply for a fast growing world-wide population has been one of the most addressed issues in recent years. The goal to establish renewables as a substantial energy source involves changes in landscape and human environment and will certainly re- quire changes in attitude. Mankind has exploited fossil energy resources such as coal and crude oil for decades and deployed nuclear power plants, without recognizing or considering the effects of atmosphere pollution, especially by carbon dioxide emission, or the dangers of nuclear power and its waste. Even though peak oil, the point in time of maximum rate of crude oil extraction has not been reached yet and uranium supply still suffices, one can observe an increased preference for green energies. In 2011 Fukushima brought the potential threats of nuclear power back to our mind. At least since then, politicians especially
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