Dominik Kubicki

Amplifying Dynamic Nuclear Polarization of Frozen Solutions by Incorporating Dielectric Particles

There is currently great interest in understanding the limits on NMR signal enhancements provided by dynamic nuclear polarization (DNP), and in particular if the theoretical maximum enhancements can be achieved. We show that over a 2-fold improvement in cross-effect DNP enhancements can be achieved in MAS experiments on frozen solutions by simply incorporating solid particles into the sample. At 9.4 T and ∼105 K, enhancements up to εH = 515 are obtained in this way, corresponding to 78% of the theoretical maximum. We also underline that degassing of the sample is important to achieve highest enhancements. We link the amplification effect to the dielectric properties of the solid material, which probably gives rise to scattering, diffraction, and amplification of the microwave field in the sample. This is substantiated by simulations of microwave propagation. A reduction in sample heating at a given microwave power also likely occurs due to reduced dielectric loss. Simulations indicate that the microwave field (and thus the DNP enhancement) is inhomogeneous in the sample, and we deduce that in these experiments between 5 and 10% of the solution actually yields the theoretical maximum signal enhancement of 658. The effect is demonstrated for a variety of particles added to both aqueous and organic biradical solutions.

Cation Dynamics in Mixed-Cation (MA)x(FA)1–xPbI3 Hybrid Perovskites from Solid-State NMR

Mixed-cation organic lead halide perovskites attract unfaltering attention owing to their excellent photovoltaic properties. Currently, the best performing perovskite materials contain multiple cations and provide power conversion efficiencies up to around 22%. Here, we report the first quantitative, cation-specific data on cation reorientation dynamics in hybrid mixed-cation formamidinium (FA)/methylammonium (MA) lead halide perovskites. We use 14N, 2H, 13C, and 1H solid-state MAS NMR to elucidate cation reorientation dynamics, microscopic phase composition, and the MA/FA ratio, in (MA)x(FA)1-xPbI3 between 100 and 330 K. The reorientation rates correlate in a striking manner with the carrier lifetimes previously reported for these materials and provide evidence of the polaronic nature of charge carriers in PV perovskites.

Phase Segregation in Cs-, Rb- and K-doped Mixed-Cation (MA)x(FA)1-xPbI3 Hybrid Perovskites from Solid-State NMR

Hybrid (organic–inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use 133Cs, 87Rb, 39K, 13C, and 14N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability.

Probing the Role of π Interactions in the Reactivity of Oxygen Species: A Case of Ethylzinc Aryloxides with Different Dispositions of Aromatic Rings toward the Metal Center

Ethylzinc derivatives of ortho-hydroxybiphenyl and 2,6-diphenylphenol that bear different nuclearity and dispositions of aromatic rings toward the metal center were synthesized and structurally characterized in the solid state and solution. This family of well-defined compounds was examined as a model system for the activation of dioxygen mediated by using complexes that feature lack of a redox-active metal center. Experimental and theoretical studies indicate an essential role in the oxygenation process of intramolecular interactions that involve aromatic subunits. Additionally, novel results for the oxygenation chemistry of alkylzinc compounds, including the isolation and structural characterization of the unique octanuclear aryloxide (hydroxide) compound Zn8 (OAr)8 (OH)6 (O2 ) with an encapsulated peroxide species, are presented.

Reactions of ZnR2 Compounds with Dibenzoyl: Characterisation of the Alkyl‐Transfer Products and a Striking Product‐Inhibition Effect

The first systematic theoretical and experimental studies of reaction systems involving ZnR(2) (R=Me, Et or tBu) with dibenzoyl (dbz) as a non-innocent ligand revealed that the character of the metal-bonded R group as well as the ratio of the reagents and the reaction temperature significantly modulate the reaction outcome. DFT calculations showed four stable minima for initial complexes formed between ZnR(2) and dbz and the most stable structure proved to be the 2:1 adduct; among the 1:1 adducts three structural isomers were found of which the most stable complex had the monodentate coordination mode and the chelate complex with the s-cis conformation of the dbz unit appeared to be the least stable form. Interestingly, the reaction involving ZnMe(2) did not lead to any alkylation product, whereas the employment of ZntBu(2) resulted in full conversion of dbz to the O-alkylated product [tBuZn{PhC(O)C(OtBu)Ph}] already at -20 °C. A more complicated system was revealed for the reaction of dbz with ZnEt(2). Treatment of a solution of dbz in toluene with one equivalent of ZnEt(2) at room temperature afforded a mixture of the O- and C-alkylated products [EtZn{PhC(O)C(OEt)Ph}] and [EtZn{OC(Ph)C(O)(Et)Ph}], respectively. The formation of the C-alkylated product was suppressed by decreasing the initial reaction temperature to -20 °C. Moreover, in the case of the dbz/ZnEt(2) system monitoring of the dbz conversion over the entire reaction course revealed a product inhibition effect, which highlights possible participation of multiple equilibria of different zinc alkoxide/ZnEt(2) aggregates. Diffusion NMR studies indicated that dbz forms an adduct with the O-alkylated product, which is a competent species for executing the inhibition of the alkylation event.

Formation of Stable Mixed Guanidinium–Methylammonium Phases with Exceptionally Long Carrier Lifetimes for High-Efficiency Lead Iodide-Based Perovskite Photovoltaics

Methylammonium (MA)- and formamidinium (FA)-based organic-inorganic lead halide perovskites provide outstanding performance as photovoltaic materials, due to their versatility of fabrication and their power conversion efficiencies reaching over 22%. The proposition of guanidinium (GUA)-doped perovskite materials generated considerable interest due to their potential to increase carrier lifetimes and open-circuit voltages as compared to pure MAPbI3. However, simple size considerations based on the Goldschmidt tolerance factor suggest that guanidinium is too big to completely replace methylammonium as an A cation in the APbI3 perovskite lattice, and its effect was thus ascribed to passivation of surface trap states at grain boundaries. As guanidinium was not thought to incorporate into the MAPbI3 lattice, interest waned since it appeared unlikely that it could be used to modify the intrinsic perovskite properties. Here, using solid-state NMR, we provide for the first time atomic-level evidence that GUA is directly incorporated into the MAPbI3 and FAPbI3 lattices, forming pure GUA xMA1- xPbI3 or GUA xFA1- xPbI3 phases, and that it reorients on the picosecond time scale within the perovskite lattice, which explains its superior charge carrier stabilization capacity. Our findings establish a fundamental link between charge carrier lifetimes observed in photovoltaic perovskites and the A cation structure in ABX3-type metal halide perovskites.

Experimental and Computational Insights into Carbon Dioxide Fixation by RZnOH Species

Organozinc hydroxides, RZnOH, possessing the proton-reactive alkylzinc group and the CO2 -reactive Zn-OH group, represent an intriguing group of organometallic precursors for the synthesis of novel zinc carbonates. Comprehensive experimental and computational investigations on 1) solution and solid-state behavior of tBuZnOH (1) species in the presence of Lewis bases, namely, THF and 4-methylpyridine; 2) step-by-step sequence of the reaction between 1 and CO2; and 3) the effect of a donor ligand and/or an excess of tBu2Zn as an external proton acceptor on the reaction course are reported. DFT calculations for the insertion of carbon dioxide into the dinuclear alkylzinc hydroxide 12 are fully consistent with (1)H NMR spectroscopy studies and indicate that this process is a multistep reaction, in which the insertion of CO2 seems to be the rate-determining step. Moreover, DFT studies show that the mechanism of the rearrangement between key intermediates, that is, the primary alkylzinc bicarbonate with a proximal position of hydrogen and the secondary alkylzinc bicarbonate with a distal position of hydrogen, most likely proceeds through internal rotation of the dinuclear bicarbonate.

Phase Segregation in Potassium-Doped Lead Halide Perovskites from 39K Solid-State NMR at 21.1 T

Organic-inorganic lead halide perovskites are a promising family of light absorbers for a new generation of solar cells, with reported efficiencies currently exceeding 22%. A common problem of solar cells fabricated using these materials is that their efficiency depends on their cycling history, an effect known as current-voltage ( J- V) hysteresis. Potassium doping has recently emerged as a universal way to overcome this adverse phenomenon. While the atomistic origins of J- V hysteresis are still not fully understood, it is essential to rationalize the atomic-level effect of protocols that lead to its suppression. Here, using 39K MAS NMR at 21.1 T we provide for the first time atomic-level characterization of the potassium-containing phases that are formed upon KI doping of multication and multianion lead halide perovskites. We find no evidence of potassium incorporation into 3D perovskite lattices of the recently reported materials. Instead, we observe formation of a mixture of potassium-rich phases and unreacted KI. In the case of Br-containing lead halide perovskites doped with KI, a mixture of KI and KBr ensues, leading to a change in the Br/I ratio in the perovskite phase with respect to the undoped perovskite. Simultaneous Cs and K doping leads to the formation of nonperovskite Cs/K lead iodide phases.

Multifunctional molecular modulators for perovskite solar cells with over 20% efficiency and high operational stability

Perovskite solar cells present one of the most prominent photovoltaic technologies, yet their stability, scalability, and engineering at the molecular level remain challenging. We demonstrate a concept of multifunctional molecular modulation of scalable and operationally stable perovskite solar cells that exhibit exceptional solar-to-electric power conversion efficiencies. The judiciously designed bifunctional molecular modulator SN links the mercapto-tetrazolium (S) and phenylammonium (N) moieties, which passivate the surface defects, while displaying a structure-directing function through interaction with the perovskite that induces the formation of large grain crystals of high electronic quality of the most thermally stable formamidinium cesium mixed lead iodide perovskite formulation. As a result, we achieve greatly enhanced solar cell performance with efficiencies exceeding 20% for active device areas above 1 cm2 without the use of antisolvents, accompanied by outstanding operational stability under ambient conditions.Engineering hybrid perovskites at the molecular level to solve the stability problem remains a challenge. Here Grätzel et al. design a multifunctional molecular modulator that interacts with the perovskite via modes elucidated by solid state NMR spectroscopy and show high efficiency and operational stability.

BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, α,γ-bisdiphenylene-β-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field 27Al and 29Si DNP surface enhanced NMR spectra of amorphous aluminosilicates and 17O NMR on silica nanoparticles.