Kurt Schenk

Efficient luminescent solar cells based on tailored mixed-cation perovskites.

Researchers developed a perovskite solar cell with high power-conversion efficiency (>20%) and intense electroluminescence yield (0.5%). We report on a new metal halide perovskite photovoltaic cell that exhibits both very high solar-to-electric power-conversion efficiency and intense electroluminescence. We produce the perovskite films in a single step from a solution containing a mixture of FAI, PbI2, MABr, and PbBr2 (where FA stands for formamidinium cations and MA stands for methylammonium cations). Using mesoporous TiO2 and Spiro-OMeTAD as electron- and hole-specific contacts, respectively, we fabricate perovskite solar cells that achieve a maximum power-conversion efficiency of 20.8% for a PbI2/FAI molar ratio of 1.05 in the precursor solution. Rietveld analysis of x-ray diffraction data reveals that the excess PbI2 content incorporated into such a film is about 3 weight percent. Time-resolved photoluminescence decay measurements show that the small excess of PbI2 suppresses nonradiative charge carrier recombination. This in turn augments the external electroluminescence quantum efficiency to values of about 0.5%, a record for perovskite photovoltaics approaching that of the best silicon solar cells. Correspondingly, the open-circuit photovoltage reaches 1.18 V under AM 1.5 sunlight.

Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells

Lead halide perovskites have attracted considerable interest as photoabsorbers in PV-applications over the last few years. The most studied perovskite material achieving high photovoltaic performance has been methyl ammonium lead iodide, CH3NH3PbI3. Recently the highest solar cell efficiencies have, however, been achieved with mixed perovskites where iodide and methyl ammonium partially have been replaced by bromide and formamidinium. In this work, the mixed perovskites were explored in a systematic way by manufacturing devices where both iodide and methyl ammonium were gradually replaced by bromide and formamidinium. The absorption and the emission behavior as well as the crystallographic properties were explored for the perovskites in this compositional space. The band gaps as well as the crystallographic structures were extracted. Small changes in the composition of the perovskite were found to have a large impact on the properties of the materials and the device performance. In the investigated compositional space, cell efficiencies, for example, vary from a few percent up to 20.7%. From the perspective of applications, exchanging iodide with bromide is especially interesting as it allows tuning of the band gap from 1.5 to 2.3 eV. This is highly beneficial for tandem applications, and an empirical expression for the band gap as a function of composition was determined. Exchanging a small amount of iodide with bromide is found to be highly beneficial, whereas a larger amount of bromide in the perovskite was found to cause intense sub band gap photoemission with detrimental results for the device performance. This could be caused by the formation of a small amount of an iodide rich phase with a lower band gap, even though such a phase was not observed in diffraction experiments. This shows that stabilizing the mixed perovskites will be an important task in order to get the bromide rich perovskites, which has a higher band gap, to reach the same high performance obtained with the best compositions.

Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells

Lead halide perovskites have over the past few years attracted considerable interest as photo absorbers in PV applications with record efficiencies now reaching 22%. It has recently been found that not only the composition but also the precise stoichiometry is important for the device performance. Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3)x(CH(NH2)2)(1-x)PbBryI(3-y). In this work a broad range of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photo electron spectroscopy (PES), transient absorption spectroscopy (TAS), UV-vis, electroluminescence (EL), photoluminescence (PL), and confocal PL mapping have been used to further understand the importance of remnant PbI2 in perovskite solar cells. Our best devices were over 18% efficient, and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent dropped, which could be attributed to accumulation of organic species at the grain boundaries, low charge carrier mobility, and decreased electron injection into the TiO2. The PbI2-deficient compositions did, however, also have advantages. The record Voc was as high as 1.20 V and was found in PbI2-deficient samples. This was correlated with high crystal quality, longer charge carrier lifetimes, and high PL yields and was rationalized as a consequence of the dynamics of the perovskite formation. We further found the ion migration to be obstructed in the PbI2-deficient samples, which decreased the JV hysteresis and increased the photostability. PbI2-deficient synthesis conditions can thus be used to deposit perovskites with excellent crystal quality but with the downside of grain boundaries enriched in organic species, which act as a barrier toward current transport. Exploring ways to tune the synthesis conditions to give the high crystal quality obtained under PbI2-poor condition while maintaining the favorable grain boundary characteristics obtained under PbI2-rich conditions would thus be a strategy toward more efficiency devices.

A simple spiro-type hole transporting material for efficient perovskite solar cells

We developed a cost-effective spiro-type 4,4′,4′′,4′′′\-(2H,2′H,4H,4′H-3,3′-spiro-bi[thieno[3,4-b][1,4]dioxepine]-6,6′,8,8′-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) hole transporting material (PST1) for perovskite solar cells (PSCs) that works efficiently even without a cobalt dopant. The PST1 is obtained by employing facile synthetic routes and tends to crystallize in the solid state. An X-ray diffraction study of PST1 revealed a unique quasi-spiro molecular configuration and found multiple CH/π and π–π intermolecular contacts. For the first time, the crystal structure of 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is also studied for comparison. The device based on PST1 exhibited a PCE of 13.44%, and a comparable 12.74% PCE was achieved for its undoped form, which paves the way for developing new low cost hole transporting materials and final industrialization of perovskite solar cells.

Low-cost industrially available molybdenum boride and carbide as “platinum-like” catalysts for the hydrogen evolution reaction in biphasic liquid systems

Rarely reported low-cost molybdenum boride and carbide microparticles, both of which are available in abundant quantities due to their widespread use in industry, adsorb at aqueous acid-1,2-dichloroethane interfaces and efficiently catalyse the hydrogen evolution reaction in the presence of the organic electron donor - decamethylferrocene. Kinetic studies monitoring biphasic reactions by UV/vis spectroscopy, and further evidence provided by gas chromatography, highlight (a) their superior rates of catalysis relative to other industrially significant transition metal carbides and silicides, as well as a main group refractory compound, and (b) their highly comparable rates of catalysis to Pt microparticles of similar dimensions. Insight into the catalytic processes occurring for each adsorbed microparticle was obtained by voltammetry at the liquid-liquid interface.

Phase Transitions in Disordered Lead Iron Niobate - X-Ray and Synchrotron Radiation Diffraction Experiments

Diffraction above room temperature, on powders and crystals of Pb(Fe0.5Nb0.5)O-3, using X-rays and synchrotron radiation, led to a novel and consistent picture of the symmetry of the phases in this disordered perovskite. The compound is a monoclinic ferroelectric at 293 K, undergoes a structural transition to another ferroelectric, tetragonal state at about 355 K and finally transforms into a cubic form at approximately 376 K

Synthesis and spectroscopic characterization of [CoIII(salophen)(amine)2]ClO4 (amine=morpholine, pyrrolidine, and piperidine) complexes. The crystal structures of [CoIII(salophen)(morpholine)2]ClO4 and [CoIII(salophen)(pyrrolidine)2]ClO4

Abstract Three Co(III) complexes of the type [Co(salophen)(amine) 2 ]ClO 4 , salophen= N , N ′-disalicylidene-1,2-phenylendiamine dianion and amine=morpholine ( 1 ), pyrrolidine ( 2 ), and piperidine ( 3 ), have been synthesized and characterized by elemental analysis, IR, UV–Vis, 1 H, and 13 C NMR spectroscopy. [Co(salophen)(morpholine) 2 ]ClO 4 ( 1 ) and [Co(salophen)(pyrrolidine) 2 ]ClO 4 ( 2 ) have been studied by X-ray diffraction. Compound 1 crystallizes in ribbons of complexes and perchlorates held together by weak NH⋯O and CH⋯O hydrogen bonds between morpholines and perchlorates. The latter also interconnect the chains to a 3D network. Some minor π–π interactions exist. Compound 2 crystallizes as endless chains of complexes linked by weak CH⋯O hydrogen bonds to the disordered perchlorates. The pyrrolidine moiety is turned by 90° with respect to 1 and forms intramolecular NH⋯O hydrogen bonds. The coordination polyhedra of 1 and 2 possess C s symmetry, and the salophens are not planar in either of them.

A nanoporous oxygen evolution catalyst synthesized by selective electrochemical etching of perovskite hydroxide CoSn(OH)6 nanocubes

The development of efficient and Earth-abundant oxygen evolution reaction (OER) catalysts is essential for the generation of renewable hydrogen. The synthesis of nanoporous and high-surface-area catalysts remains challenging. Herein, we show that large nanocubes of perovskite hydroxide CoSn(OH)6 can be electrochemically etched to form a highly active OER catalyst, reaching 10 mA cm−2 at an overpotential (η) of only 274 mV. The good catalytic activity was attributed to the formation of hierarchical nanoporous CoOOH particles, which were created by selective dissolution of Sn hydroxides. Crystal defects of oxygen vacancies appear to be important for the formation of active catalysts.

Structural and photophysical properties of europium(III) mixed complexes with β-diketonates and o-phenanthroline

Abstract The luminescence of solid samples of europium mixed complexes with β-diketonates and o -phenanthroline (Phen), EuL 3 Phen, is investigated at 77 K in order to probe the structure of these compounds. A ligand-to-metal energy transfer process is evidenced. The quantum yields of 10 −5 M solutions in CH 2 Cl 2 are measured relative to 9,10-dichloroanthracene. Except for benzylacetonate, these yields are quite low, due to the presence of low-lying electronic states of the ligands. The crystal structure of Eu(NO 3 )(DBM) 2 (TBPO) 2 as well as its photophysical properties in CH 2 Cl 2 solution are presented.

Switchable reactivity: the site-selective functionalization of trifluoromethyl-substituted pyrazoles

Modern organometallic methods enable the regioflexible conversion of simple heterocyclic starting materials into families of isomers and congeners. Depending on the choice of the reagent, 1-methyl-5-(trifluoromethyl)pyrazole undergoes deprotonation and subsequent carboxylation mainly or exclusively at either the 4-position of the heterocycle or at the nitrogen-attached Me group. Similarly, 1-phenyl-5-(trifluoromethyl)pyrazole and 3-methyl-1-phenyl-5-(trifluoromethyl)pyrazole are selectively attacked by lithium diisopropylamide at the heterocyclic 4-position and by butyllithium concomitantly at the 4-position and the ortho position of the Ph ring. In contrast, metalation of 1-methyl-3-(trifluoromethyl)pyrazole occurs only at the 5-position, whatever the organometallic or metal amide base. Further sites become accessible to functionalization if bromine is introduced into the heterocyclic or arom. ring. This has been demonstrated with 4-bromo-1-methyl-5-(trifluoromethyl)pyrazole, 4-bromo-1-methyl-3-(trifluoromethyl)pyrazole, 4-bromo-1-methyl-5-(trifluoromethyl)pyrazole and 1-(2-bromophenyl)-5-(trifluoromethyl)pyrazole. [on SciFinder (R)]