Transient electrically detected magnetic resonance spectroscopy applied to organic solar cells

The influence of light-induced paramagnetic states on the photocurrent generated by polymer:fullerene solar cells is studied using spin-sensitive techniques in combination with laser-flash excitation. For this purpose, we developed a setup that allows for simultaneous detection of transient electron paramagnetic resonance as well as transient electrically detected magnetic resonance (trEDMR) signals from fully processed and encapsulated solar cells. Combining both techniques provides a direct link between photoinduced triplet excitons, charge transfer states, and free charge carriers as well as their influence on the photocurrent generated by organic photovoltaic devices. Our results obtained from solar cells based on poly(3-hexylthiophene) as electron donor and a fullerene-based electron acceptor show that the resonant signals observed in low-temperature (T = 80 K) trEDMR spectra can be attributed to positive polarons in the polymer as well as negative polarons in the fullerene phase, indicating that both centers are involved in spin-dependent processes that directly influence the photocurrent

Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer

We investigate the delocalization of holes in the semicrystalline conjugated polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by directly measuring the hyperfine coupling between photogenerated polarons and bound nuclear spins using electron nuclear double resonance spectroscopy. An extrapolation of the corresponding oligomer spectra reveals that charges tend to delocalize over 4.0–4.8 nm with delocalization strongly dependent on molecular order and crystallinity of the PBTTT polymer thin films. Density functional theory calculations of hyperfine couplings confirm that long-range corrected functionals appropriately describe the change in coupling strength with increasing oligomer size and agree well with the experimentally measured polymer limit. Our discussion presents general guidelines illustrating the various pitfalls and opportunities when deducing polaron localization lengths from hyperfine coupling spectra of conjugated polymers

Reaction of porphyrin-based surface-anchored metal-organic frameworks to prolonged illumination

Crystalline surface-anchored metal–organic framework (SURMOF) thin films made from porphyrin-based organic linkers have recently been used in both photon upconversion and photovoltaic applications. While these studies showed promising results, the question of photostability in this organic–inorganic hybrid material has to be investigated before applications can be considered. Here, we combine steady-state photoluminescence, transient absorption, and time-resolved electron paramagnetic resonance spectroscopy to examine the effects of prolonged illumination on a palladium-porphyrin based SURMOF thin film. We find that phototreatment leads to a change in the material\u27s photoresponse caused by the creation of stable products of photodecomposition – likely chlorin – inside the SURMOF structure. When the mobile triplet excitons encounter such a defect site, a short-lived (80 ns) cation–anion radical pair can be formed by electron transfer, wherein the charges are localized at a porphyrin and the photoproduct site, respectively

Strongly exchange-coupled triplet pairs in an organic semiconductor

From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 µs and a spin coherence time approaching 1 µs, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors.Gates-Cambridge Trust, Winton Programme for the Physics of Sustainability, Freie Universität Berlin within the Excellence Initiative of the German Research Foundation, Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from Nature Publishing Group at http://dx.doi.org/10.1038/nphys3908

Ladungstrennung und -transport in Organischen Solarzellen untersucht mit Elektronenspinresonanz-Spektroskopie

Summary v Zusammenfassung vii List of Publications ix 1 Introduction 1.1 State of the Art of Organic Solar Cells 1.2 About this Thesis 2 Fundamentals 2.1 Organic Solar Cells 2.1.1 Exemplified Structure of an Organic Solar Cell 2.1.2 The Charge Separation within Organic Solar Cells 2.1.3 The Physics of Organic Solar Cells 2.2 Electron Paramagnetic Resonance (EPR) 2.2.1 Electron-Zeeman Splitting 2.2.2 g-factor and g-matrix 2.2.3 The EPR Spectrum of Triplet States 2.2.4 ISC Triplets versus BET Triplets 2.2.5 The EPR Spectrum of CT States 2.2.6 The EPR Spectrum of Free Polarons 2.3 Experimentals 2.3.1 Sample Preparation 2.3.2 Continuous Wave EPR (cwEPR) 2.3.3 Pulsed EPR (pEPR) 2.3.4 Transient EPR (trEPR) 2.3.5 EDMR versus EPR 2.3.6 Transient EDMR (trEDMR) 2.3.7 Pulsed EDMR (pEDMR) 3 Charge Separation from an EPR Point of View 3.1 Charge Transfer States in Organic Solar Cells 3.1.1 Spin-Correlated Doublet Pairs as Intermediate States in Charge-Separation Processes 3.1.2 Transient Electrically Detected Magnetic Resonance Spectroscopy Applied to Organic Solar Cells 3.2 Triplets in Organic Solar Cells 3.2.1 Intersystem Crossing Triplets in Pristine OPV Materials 3.2.2 Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective 3.2.3 Triplets in All-Polymer Solar Cells 4 Efficiency Enhancing Systems for OSCs studied by EPR 4.1 Singlet Fission and Tandem Solar Cells 4.1.1 Strongly Exchange-Coupled Triplet Pairs in an Organic Semiconductor 4.1.2 Transport-Related Triplet States and Hyperfine Couplings in Organic Tandem Solar Cells Probed by Pulsed Electrically Detected Magnetic Resonance Spectroscopy 4.2 Doping & Mobilities 4.2.1 P-Type Doping of Poly(3-Hexylthiophene) with the Strong Lewis Acid Tris(Pentafluorophenyl)Borane 5 Conclusion and Outlook A Experimental Details of EPR/EDMR Spectroscopy A.1 Transient EDMR and EPR Setup A.2 Other EPR Instrumentations B Publications B.1 B.2 B.3 B.4 B.5 B.6 B.7 B.8 B.9 Declaration of the Author’s Contribution within the Publications Bibliography List of Abbreviations List of Oral and Poster Presentations Danksagung ErklärungThis thesis presents a comprehensive study of organic solar cells (OSCs) by electron paramagnetic resonance (EPR) techniques. Magnetic resonance techniques use the spin of charged particles as a probe to measure interactions of the excitations rele- vant for organic solar cells such as spin- 12 polarons, charge-transfer (CT) states, spin-1 triplets and spin-2 quintet states. EPR spectroscopy is a powerful method to discover processes within solid state molecular materials. Time-resolved EPR techniques allow observation of dynamic molecular processes such as electron transfer from a donor to an acceptor molecule and spin-dependent processes such as hopping transport or charge-carrier recombination. This thesis focuses on the spin states generated during charge separation from an initial photo-excited exciton into separated (free) polarons. Intermediate states such as CT states and higher spin states including triplets or quin- tets are extensively studied. To observe and investigate these states, various organic semiconductors were explored in the form of thin films or bulk-heterojunction blends with time-resolved EPR and electrically detected magnetic resonance (EDMR). Firstly, in this thesis we develop a semi-analytical simulation code for multi- frequency EPR spectra of CT states. This code involves both dipolar and exchange coupling between the polarons. In combination with multi- frequency experiments the simulations show that it is generally necessary to include exchange as well as dipolar interactions for CT state simulations. We apply rate equation models in combination with EPR spectrum simulations to study the generation of triplet excitons in low- bandgap organic photovoltaic materials. We investigate interfacial triplets appearing in donor:acceptor blends based on the much sought after non-fullerene acceptors. Secondly, building on this insight on triplet signatures EPR is used on highly in- teresting singlet fission materials to prove the occurrence of weakly and strongly ex- change coupled triplets generated by singlet fission. Additionally, this work contains technique developments: We introduce a new method called transient EDMR (trEDMR), a combination of time resolved EPR and electrical detection known from EDMR. The method demonstrates how short-lived EPR signatures influence the macroscopic conductivity of a complete solar cell. A miniaturized encapsulated OSC design was developed that fits into commercial EPR resonators to enable EDMR measurements of oxygen sensitive OSCs. This setup was also applied to complex organic tandem solar cells which were investigated with pulsed EDMR methods. We visualize that a triplet- exciton quenching process occur- ring within the acceptor domains of a high- efficient tandem solar cell influences its photo-conductivity. Finally, in a combined optical, electrical and EPR study we show that the new p-type dopant, the Lewis acid BCF, reliably, stably, and efficiently dopes an organic semiconductor and thereby massively increases the conductivity of the doped layer. These studies open up new perspectives for subsequent investigations of OSCs by magnetic resonance techniques.Die vorgestellte Doktorarbeit ist eine umfassende Studie zu organischen Solar- zellen (OSZ) basierend auf Elektronspinresonanz Spektroskopie (ESR). Diese nutzt den Spin der geladenen Teilchen als Sonde, um Ladungsträgerinteraktionen von rele- vanten angeregten Zuständen zu untersuchen, wie z.B. Polaronen (Spin- 12 ), Ladungs- transferzustände (LTZ), Triplett-Exzitonen (Spin-1) oder Quintett-Exzitonen (Spin-2). ESR ist eine vielseitige Methode um molekulare Prozesse in Festkörpern und Mo- lekülen zu analysieren. Zeitaufgelöste ESR- Techniken ermöglichen hierbei auch die Beobachtung von dynamischen molekularen Prozessen, wie den Elektronentransfer von einem Donator- zu einem Akzeptormolekül oder spinabhängige Prozesse, wie Ladungsträger-Hüpftransport oder -Rekombination. Unser Fokus liegt auf Spin- Zuständen, die während des Ladungstrennungsprozesses von Exziton auftreten, beispielsweise LTZ, Triplett- und Quintett-Zustände. Um diese zu analysieren wird eine Vielzahl von organischen Halbleitern mittels zeitaufgelöster ESR und elektrisch detektierter magnetischer Resonanzspektroskopie (EDMR) untersucht. Der erste Teil dieser Arbeit behandelt die Entwicklung eines teilweise analyti- schen Simulationscodes um LTZ in mehreren ESR-Frequenzen spektral zu beschrei- ben. Die Simulation beinhaltet sowohl dipolare als auch Austauschkopplungen. Die Kombination von ESR-Messungen in zwei Frequenzen und den dazugehörigen LTZ- Simulationen konnte zeigen, dass es im Allgemeinen notwendig ist, beide Kopplun- gen zu berücksichtigen. Weiterhin untersuchen wir den Entstehungsmechanismus von Triplett-Exzitonen in organischen Mischsystemen, die vielversprechende Halb- leiter mit kleiner Bandlücke enthalten, indem wir spektrale Simulationen von Ex- perimenten mit einem Ratenmodell verknüpfen. Ein angegliedertes Projekt befasst sich mit Grenzflächen-Tripletts in Donator- Akzeptor-Mischzellen, die auf aktuell stark nachgefragten Fulleren-freien Akzeptoren basieren. Im zweiten Teil nutzen wir erlangtes Wissen über Tripletts in der ESR-Spektro- skopie, um viel diskutierte "Singlet-Fission" (SF) Materialien zu untersuchen. Wir weisen nach, dass SF in TIPS-Tetracen auftritt und sowohl schwach- als auch stark- gekoppelte Triplett-Exzitonen erzeugt. Wir beschreiben die Neuentwicklung einer transiente EDMR genannten Methode, die eine Kombination der transienten ESR und elektrischer Detektion ist. Hierdurch kann der Einfluss von kurzlebigen ESR- Signaturen auf die Leitfähigkeit von Solarzellen gezeigt werden. Ein miniaturisiertes Design für verkapselte OSZ wird vorgestellt, dass EDMR-Messungen an kompletten OSZ innerhalb eines ESR-Resonators ermöglicht. Dieses Design wird in der Folge auch für komplexere organische Tandemsolarzellen (OTSZ) verwendet. Gepulste EDMR zeigt den Einfluss von Triplett-Exzitonen-Vernichtungsprozessen auf den Pho- tostrom in effizienten OTSZ. Im dritten Teil wird eine Kooperationsstudie vorgestellt, in der optische, elektri- sche und ESR-Messungen kombiniert werden, um einen neuen p-Dotanten, die Lewis- Säure BCF, für organische Halbleiter zu charakterisieren. Schließlich zeigt die Arbeit neue Perspektiven für zukünftige Untersuchungen von organischen Solarzellen mit Elektronspinresonanz auf

Tuning spin dynamics in crystalline tetracene

Tetracene is an archetypal material undergoing singlet fission—the generation of a pair of triplet excitons from one singlet exciton. Here, using time-resolved electron spin resonance, we show how the spin dynamics in tetracene crystals are influenced by temperature and morphology. Upon cooling from 300 to 200 K, we observe a switch between singlet fission and intersystem crossing generated triplets, manifesting as an inversion in transient spin polarization. We extract a spin dephasing time of approximately 40 ns for fission-generated triplets at room temperature, nearly 100 times shorter than the dephasing time that we measure for triplets localized on isolated tetracene molecules. These results highlight the importance of morphology and thermal activation in singlet fission systems

Tuning spin dynamics in crystalline tetracene

Tetracene is an archetypal material undergoing singlet fission—the generation of a pair of triplet excitons from one singlet exciton. Here, using time-resolved electron spin resonance, we show how the spin dynamics in tetracene crystals are influenced by temperature and morphology. Upon cooling from 300 to 200 K, we observe a switch between singlet fission and intersystem crossing generated triplets, manifesting as an inversion in transient spin polarization. We extract a spin dephasing time of approximately 40 ns for fission-generated triplets at room temperature, nearly 100 times shorter than the dephasing time that we measure for triplets localized on isolated tetracene molecules. These results highlight the importance of morphology and thermal activation in singlet fission systems