Jan Behrends

On an Easy Way To Prepare Metal–Nitrogen Doped Carbon with Exclusive Presence of MeN4-type Sites Active for the ORR

Today, most metal and nitrogen doped carbon catalysts for ORR reveal a heterogeneous composition. This can be reasoned by a nonoptimized precursor composition and various steps in the preparation process to get the required active material. The significant presence of inorganic metal species interferes with the assignment of descriptors related to the ORR activity and stability. In this work we present a simple and feasible way to reduce the contribution of inorganic metal species in some cases even down to zero. Such catalysts reveal the desired homogeneous composition of MeN4 (Me = metal) sites in the carbon that is accompanied by a significant enhancement in ORR activity. Among the work of other international groups, our iron-based catalyst comprises the highest density of FeN4 sites ever reported without interference of inorganic metal sites.

Direct detection of photoinduced charge transfer complexes in polymer fullerene blends

We report transient electron paramagnetic resonance (trEPR) measurements with sub-microsecond time resolution performed on a P3HT:PCBM blend at low temperature. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (positive polarons in P3HT and negative polarons in PCBM) can be identified by their characteristic g-values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e. originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes which form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.

THE POST-MERGER MAGNETIZED EVOLUTION OF WHITE DWARF BINARIES: THE DOUBLE-DEGENERATE CHANNEL OF SUB-CHANDRASEKHAR TYPE Ia SUPERNOVAE AND THE FORMATION OF MAGNETIZED WHITE DWARFS

Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the two white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly rotating white dwarf merger surrounded by a hot corona and a thick, differentially rotating disk. The disk is strongly susceptible to the magnetorotational instability (MRI), and we demonstrate that this leads to the rapid growth of an initially dynamically weak magnetic field in the disk, the spin-down of the white dwarf merger, and to the subsequent central ignition of the white dwarf merger. Additionally, these magnetized models exhibit new features not present in prior hydrodynamic studies of white dwarf mergers, including the development of MRI turbulence in the hot disk, magnetized outflows carrying a significant fraction of the disk mass, and the magnetization of the white dwarf merger to field strengths ~2 × 108 G. We discuss the impact of our findings on the origins, circumstellar media, and observed properties of SNe Ia and magnetized white dwarfs.

Pulsed electrically detected magnetic resonance for thin film silicon and organic solar cells

In thin film solar cells based on non-crystalline thin film silicon or organic semiconductors structural disorder leads to localized states that induce device limiting charge recombination and trapping. Both processes frequently involve paramagnetic states and become spin-dependent. In the present perspectives article we report on advanced pulsed electrically detected magnetic resonance (pEDMR) experiments for the study of spin dependent transport processes in fully processed thin film solar cells. We reflect on recent advances in pEDMR spectroscopy and demonstrate its capabilities on two different state of the art thin film solar cell concepts based on microcrystalline silicon and organic MEH-PPV:PCBM blends, recently studied at HZB. Benefiting from the increased capabilities of novel pEDMR detection schemes we were able to ascertain spin-dependent transport processes and microscopically identify paramagnetic states and their role in the charge collection mechanism of solar cells.

Lock-in detection for pulsed electrically detected magnetic resonance

We show that in pulsed electrically detected magnetic resonance (pEDMR) signal modulation in combination with a lock-in detection scheme can reduce the low-frequency noise level by one order of magnitude and in addition removes the microwave-induced non-resonant background. This is exemplarily demonstrated for spin-echo measurements in phosphorus-doped silicon. The modulation of the signal is achieved by cycling the phase of the projection pulse used in pEDMR for the readout of the spin state.

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.

Multi-frequency EDMR applied to microcrystalline thin-film silicon solar cells

Pulsed multi-frequency electrically detected magnetic resonance (EDMR) at X-, Q- and W-Band (9.7, 34, and 94GHz) was applied to investigate paramagnetic centers in microcrystalline silicon thin-film solar cells under illumination. The EDMR spectra are decomposed into resonances of conduction band tail states (e states) and phosphorus donor states (P states) from the amorphous layer and localized states near the conduction band (CE states) in the microcrystalline layer. The e resonance has a symmetric profile at all three frequencies, whereas the CE resonance reveals an asymmetry especially at W-band. This is suggested to be due to a size distribution of Si crystallites in the microcrystalline material. A gain in spectral resolution for the e and CE resonances at high fields and frequencies demonstrates the advantages of high-field EDMR for investigating devices of disordered Si. The microwave frequency independence of the EDMR spectra indicates that a spin-dependent process independent of thermal spin-polarization is responsible for the EDMR signals observed at X-, Q- and W-band.

Spin-correlated doublet pairs as intermediate states in charge separation processes

ABSTRACT Spin-correlated charge-carrier pairs play a crucial role as intermediate states in charge separation both in natural photosynthesis as well as in solar cells. Using transient electron paramagnetic resonance (trEPR) spectroscopy in combination with spectral simulations, we study spin-correlated polaron pairs in polymer:fullerene blends as organic solar cells materials. The semi-analytical simulations presented here are based on the well-established theoretical description of spin-correlated radical pairs in biological systems, however, explicitly considering the disordered nature of polymer:fullerene blends. The large degree of disorder leads to the fact that many different relative orientations between both polarons forming the spin-correlated pairs have to be taken into account. This has important implications for the spectra, which differ significantly from those of spin-correlated radical pairs with a fixed relative orientation. We systematically study the influence of exchange and dipolar couplings on the trEPR spectra and compare the simulation results to measured X- and Q-band trEPR spectra. Our results demonstrate that assuming dipolar couplings alone does not allow us to reproduce the experimental spectra. Due to the rather delocalised nature of polarons in conjugated organic semiconductors, a significant isotropic exchange coupling needs to be included to achieve good agreement between experiments and simulations.

Higher triplet state of fullerene C70 revealed by electron spin relaxation

Spin-lattice relaxation times T1 of photoexcited triplets (3)C70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T1 was fitted by the Arrhenius law with an activation energy of 172 cm(-1). This indicates that the dominant relaxation process of (3)C70 is described by an Orbach-Aminov mechanism involving the higher triplet state t2 which lies 172 cm(-1) above the lowest triplet state t1. Chemical modification of C70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in (3)C70 and its absence in triplet C70 derivatives. The presence of the higher triplet state in C70 is in good agreement with the previous results from phosphorescence spectroscopy.

Persistent spin coherence and bipolarons

To the Editor — In their Commentary Boehme and Lupton1 discuss the current challenges for organic spintronics, and correctly highlight the importance of spectroscopic methods that directly detect the participating spin species. We have previously observed room-temperature coherent electron spin Rabi oscillations persisting beyond 500 ns by pulsed electrically detected magnetic resonance (pEDMR) from a conventional bulk heterojunction organic solar cell2, comprising a blend of 20% conjugated polymer (MEH-PPV) and 80% PCBM (C60). We also observed spinlocking with high-microwave magnetic field amplitudes and the results proved unambiguously that the two spin species responsible were S = 1/2. This observation eliminated models for the spin-dependent transport that provide sensitivity to magnetic fields, such as quenching of triplet excitons by polarons or mutual annihilation of two triplet excitons. Considering both the spectroscopic evidence and the device physics we concluded that the spin-dependent transport mechanism involved hopping via doubly occupied polymer segments, that is, bipolaron states. Boehme and Lupton discuss the importance of distinguishing between spin-dependent transport resulting from bipolaron-mediated hopping and polaron-pair recombination (P+P−) and in so doing question our conclusions. It is indeed worthwhile to review the evidence supporting these models. The overlap of pEDMR spectra complicates the interpretation, so careful consideration of the device structures and operating conditions used for these experiments is necessary. It is important to note that our measurements were made on blends, whereas Boehme and Lupton studied neat films. Our pEDMR measurements on blend devices observed a spectrum at the g-value (2.0028(3)) characteristic of polarons in MEH-PPV, enabling us to conclude that these were the responsible spin species. Two contributions to the spectrum could be resolved, both with the same g-value but with linewidths of 0.6(1) mT and 1.5(1) mT, respectively2. The pEDMR spectrum from a pure MEH-PPV device observed by Boehme, Lupton and co-workers3,4, also shows a spectrum at g = 2.003 comprising two components, but with linewidths of approximately 1.3 mT and 3 mT. Boehme and Lupton1 show a new room-temperature Rabi oscillation spectra from a similar pure MEH-PPV device (Fig. 1b of ref. 1) and compare these results with those from our blend device (Fig. 1a of ref. 1). The only unambiguous conclusion that follows from the similarity shown is that in both types of device two S = 1/2 entities are responsible for the spin-dependent transport. The onset of spin locking depends on the degree of spectral overlap for the two contributing species, and the linewidth of the narrower spectral component. The apparent similarity of the onset microwave B1 value (Fig. 1a,b of ref. 1) is puzzling, given the differences in linewidths for the two pEDMR spectra from the blend and pure devices. Boehme and Lupton proceed to show a new pEDMR spectrum for a pure PCBM film at room temperature (Fig. 1c in ref. 1). It comprises two components, one narrow the other broad (~3.5 mT), centred at a g-value of ~2.002, and is similar to a previous result from a pure C60 film that exhibited coincident components with linewidths of ~0.3 mT and ~3 mT (ref. 5). The electron paramagnetic resonance spectrum of the radical anion, P−, on PCBM has a g-value of 1.9995 and a linewidth of ~0.3 mT at 100 K (refs 6,7). Furthermore, it has been shown that the signature of the PCBM anion localized at the heterojunction interface is similar7. However, the identity of the spin partners responsible for the observed pEDMR spectra from fullerene thin films has yet to be established. It is also important to note that no spin locking was observed in the study of pure C60 films5, indicating this signal is different in nature from that we observed in the blend device. Boehme and Lupton propose that our pEDMR spectrum from the blend comprises a superposition of two independent P+P− recombination processes, one exclusively in MEH-PPV, the other exclusively in PCBM. We disagree, there is no spectroscopic evidence for the involvement of spin entities within the PCBM component of the blend. The spectrum we observed showed a different g-value and is noticeably narrower than that shown in Fig. 1c in ref. 1. The absence of spin locking in pEDMR signals from the pure fullerene films provides evidence against a contribution from a recombination process exclusive to the PCBM component. The lack of involvement of spin entities from the PCBM was perhaps surprising, as an obvious spin-dependent process would be a P+P− recombination process occurring at the heterojunction interface between MEH-PPV P+ and PCBM P−. We agree that P+P− recombination in MEH-PPV is responsible for the spindependent contribution, comprising on the order of 1 in 104 of the transport current4, in the pure MEH-PPV devices. This is supported by evidence that the g-value for P− is similar to that for the positive polaron8. However, the situation is different in the blend devices; although the pEDMR spectra again show two contributions at the polaron g-value the linewidths are different from those in the pure devices, and importantly the device physics are markedly altered by the inclusion of PCBM. The presence of a high density of heterojunction interfaces with PCBM, at which the band offsets present a ~1 eV energy gain for P− transfer to PCBM, means that the probability of negative polarons being present in the conjugated polymer component is negligible. Further, our pEDMR measurements on the blend devices were performed with low bias (U = 1 V) inhibiting electron injection to MEH-PPV. By contrast, the pEDMR spectra from the pure MEH-PPV devices used medium to high applied bias values (U ~4–15 V). Under these conditions the electrical injection of P+ and P− is possible so P+P− recombination is highly plausible. In consequence, we maintain our interpretation that the data suggest that the most probable mechanism explaining the pEDMR in the MEH-PPV:PCBM blend device is the percolation transport of P+ mediated by transient spin-dependent bipolaron formation from weakly coupled P+P+ precursor states. Boehme and Lupton claim the bipolaron model describes an energetically unfavourable ‘new particle’. The suggestion that bipolarons in conjugated polymers are ‘new’ is wrong: they were proposed more than 30 years ago9 and are supported by experimental evidence10. The key importance of magnetic fieldsensitive transport in organic semiconductors requires that we fully explore and test the models for spin-dependent transport. This requires careful examination of spectroscopic evidence and the device physics, and should result in a cohesive and predictive framework. The key result of our paper, namely longlived spin coherence in a polymer:fullerene blend at room temperature is unchallenged and, together with work on the neat materials, suggests the possibility of coherent spin manipulation on the microsecond timescale at room temperature2,4,5. The Persistent spin coherence and bipolarons