Polycations Stabilised by Borosulfates: [Au3 Cl4 ][B(S2 O7 )2 ] and the One-Dimensional Metal [Au2 Cl4 ][B(S2 O7 )2 ](SO3 ).

(SO4 )-rich silicate analogue borosulfates are able to stabilise cationic cluster-like and chain-like aggregates. Single crystals of [Au3 Cl4 ][B(S2 O7 )2 ] and [Au2 Cl4 ][B(S2 O7 )2 ](SO3 ) were obtained by solvothermal reaction with SO3 , and the electronic properties were investigated by means of density functional theory-based calculations. [Au3 Cl4 ][B(S2 O7 )2 ] exhibits a cluster-like cation, and the cationic gold-chloride strands in [Au2 Cl4 ][B(S2 O7 )2 ](SO3 ) are found to resemble one-dimensional metallic wires. This is confirmed by polarisation microscopy.

Modulating the Optical Characteristics of Spiropyran@Metal-Organic Framework Composites as a Function of Spiropyran Substitution.

Understanding the interactions between the single components of hybrid systems is essential to drive the development of advanced functional materials. A prerequisite for this is the systematic variation of the building blocks of such compounds. Focusing on spiropyran@metal-organic framework (MOF) composite materials with noncovalently attached spiropyran dyes, both the host scaffold and the dye molecules can be systematically tuned. In this work, a broad substitution pattern was applied to systematically elucidate the characteristics of the resulting hybrid materials as a function of the supplemental substitution on spiropyran. The newly developed 12 composites exhibit substitution and host-dependent optical characteristics, which are particularly affected by the substitution of the 6'-position on the chromene ring. Through the favorable combination of the MOF host's polarity and an adequate strength of the spiropyran's indolinedonor-chromeneacceptor pair, reversible conversion between photoisomers is efficiently accomplished, especially for nitro-substituted spiropyrans inside MIL-68(In).

Doped but Stable: Spirobisacridine Hole Transporting Materials for Hysteresis-Free and Stable Perovskite Solar Cells.

Four spirobisacridine (SBA) hole-transporting materials were synthesized and employed in perovskite solar cells (PSCs). The molecules bear electronically inert alkyl chains of different length and bulkiness, attached to in-plane N atoms of nearly orthogonal spiro-connected acridines. Di-p-methoxyphenylamine (DMPA) substituents tailored to the central SBA-platform define electronic properties of the materials mimicking the structure of the benchmark 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), while the alkyl pending groups affect molecular packing in thin films and affect the long-term performance of PSCs. Devices with SBA-based hole transporting layers (HTL) attain efficiencies on par with spiro-MeOTAD. More importantly, solar cells with the new HTMs are hysteresis-free and demonstrate good operational stability, despite being doped as spiro-MeOTAD. The best performing MeSBA-DMPA retained 88% of the initial efficiency after a 1000 h aging test under constant illumination. The results clearly demonstrate that SBA-based compounds are potent candidates for a design of new HTMs for PSCs with improved longevity.

Ni[B2 (SO4 )4 ] and Co[B2 (SO4 )4 ]: Unveiling Systematic Trends in Phyllosilicate Analogue Borosulfates.

Borosulfates are compounds analogous to silicates, with heteropolyanionic subunits of vertex-linked (SO4 )- and (BO4 )-tetrahedra. In contrast to the immense structural diversity of silicates, the number of borosulfates is yet very limited and the extent of their properties is still unknown. This is particularly true for representatives with phyllosilicate and tectosilicate analogue anionic substructures. Herein, we present Ni[B2 (SO4 )4 ] and Co[B2 (SO4 )4 ], two new borosulfates with phyllosilicate analogue topology. While the anionic subunits of both structures are homeotypic, the positions of the charge compensating cations differ significantly: NiII is located between the borosulfate layers, while CoII -in contrast-is embedded within the layer. Detailed analysis of these two structures based on single-crystal X-ray diffraction, magnetochemical investigations, X-ray photoelectron spectroscopy, and quantum chemical calculations, unveiled the reasons for this finding. By in silico comparison with other divalent borosulfates, we uncovered systematic trends for phyllosilicate analogues leading to the prediction of new species.

High fatigue resistance of a photochromic dithienylethene embedded into the pores of a metal-organic framework (MOF).

The incorporation of photochromic dyes into porous metal-organic frameworks (MOFs) is an attractive way to transfer the photochromic properties of the dye to a solid crystalline material. In this work, the well-known P-type chromophore 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene (DTE) is embedded in three different MOFs, namely MOF-5, MIL-68(In), and MIL-68(Ga). The successful filling of the MOF pores with the DTE guest was proven by X-ray powder diffraction, while the amount of the embedded guest molecules was investigated by X-ray photoelectron spectroscopy (XPS), liquid-state NMR and thermal analysis (DSC/TGA). The measurements reveal an unexpectedly low filling of the MOF pores with the DTE guest (e.g. in MOF-5 only every fifth MOF pore is filled with a guest molecule) as well as an inhomogeneous loading throughout the material. Reflection spectra clearly show the transitions of the colourless open-ring and the coloured closed-ring forms of the DTE guest upon UV (λ = 365 nm), blue (λ = 405 nm) and green (λ = 535 nm) light exposure, where the latter is usually suppressed in crystalline DTE. Remarkably, no fatigue after ten switching cycles was observed and a high thermal stability of the coloured closed-ring form (at 50 °C for 1 h) was achieved.

Highly Crystalline and Semiconducting Imine-Based Two-Dimensional Polymers Enabled by Interfacial Synthesis.

Single-layer and multi-layer 2D polyimine films have been achieved through interfacial synthesis methods. However, it remains a great challenge to achieve the maximum degree of crystallinity in the 2D polyimines, which largely limits the long-range transport properties. Here we employ a surfactant-monolayer-assisted interfacial synthesis (SMAIS) method for the successful preparation of porphyrin and triazine containing polyimine-based 2D polymer (PI-2DP) films with square and hexagonal lattices, respectively. The synthetic PI-2DP films are featured with polycrystalline multilayers with tunable thickness from 6 to 200 nm and large crystalline domains (100-150 nm in size). Intrigued by high crystallinity and the presence of electroactive porphyrin moieties, the optoelectronic properties of PI-2DP are investigated by time-resolved terahertz spectroscopy. Typically, the porphyrin-based PI-2DP 1 film exhibits a p-type semiconductor behavior with a band gap of 1.38 eV and hole mobility as high as 0.01 cm2 V-1 s-1 , superior to the previously reported polyimine based materials.

Planar Perovskite Solar Cells with High Open-Circuit Voltage Containing a Supramolecular Iron Complex as Hole Transport Material Dopant.

In perovskite solar cells (PSCs), the most commonly used hole transport material (HTM) is spiro-OMeTAD, which is typically doped by metalorganic complexes, for example, based on Co, to improve charge transport properties and thereby enhance the photovoltaic performance of the device. In this study, we report a new hemicage-structured iron complex, 1,3,5-tris(5'-methyl-2,2'-bipyridin-5-yl)ethylbenzene Fe(III)-tris(bis(trifluoromethylsulfonyl)imide), as a p-type dopant for spiro-OMeTAD. The formal redox potential of this compound was measured as 1.29 V vs. the standard hydrogen electrode, which is slightly (20 mV) more positive than that of the commercial cobalt dopant FK209. Photoelectron spectroscopy measurements confirm that the iron complex acts as an efficient p-dopant, as evidenced in an increase of the spiro-OMeTAD work function. When fabricating planar PSCs with the HTM spiro-OMeTAD doped by 5 mol % of the iron complex, a power conversion efficiency of 19.5 % (AM 1.5G, 100 mW cm-2 ) is achieved, compared to 19.3 % for reference devices with FK209. Open circuit voltages exceeding 1.2 V at 1 sun and reaching 1.27 V at 3 suns indicate that recombination at the perovskite/HTM interface is low when employing this iron complex. This work contributes to recent endeavors to reduce recombination losses in perovskite solar cells.

Solution-Like Behavior of Photoswitchable Spiropyrans Embedded in Metal-Organic Frameworks.

1,3,3-Trimethylindolino-6'-nitrobenzopyrylospiran (SP-1) as an example of a photoswitchable spiropyran was loaded into the pores of different prototypical metal-organic frameworks, namely MOF-5, MIL-68(In), and MIL-68(Ga), by a vapor-phase process. The successful incorporation in the pores of the MOF was proven by X-ray powder diffraction, and the amount of the embedded photoswitchable guest was determined by X-ray photoelectron spectroscopy and elemental analysis. In contrast to the sterically hindered crystalline state, SP-1 embedded in solid MOF hosts shows photoswitching under irradiation with UV light from the spiropyran to its merocyanine form with a nearly complete photoisomerization. Switching can be reversed by heat treatment. These switching properties were confirmed by means of UV/vis and IR spectroscopy. Remarkably, the embedded guest molecules show photoswitching and absorption properties similar to those in the dissolved state, so that MOFs might be considered as "solid solvents" for photoswitchable spiropyrans. In contrast to that, embedment of SP-1 in the smaller pores of MIL-53(Al) was not successful. SP-1 is mainly adsorbed on the surfaces of the MIL-53(Al) particles, which also leads to photoswitching properties.

Impact of mesoscale order on open-circuit voltage in organic solar cells.

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

Probing the Reactivity of ZnO with Perovskite Precursors.

To achieve more stable and efficient metal halide perovskite devices, optimization of charge transport materials and their interfaces with perovskites is crucial. ZnO on paper would make an ideal electron transport layer in perovskite devices. This metal oxide has a large bandgap, making it transparent to visible light; it can be easily n-type doped, has a decent electron mobility, and is thought to be chemically relatively inert. However, in combination with perovskites, ZnO has turned out to be a source of instability, rapidly degrading the performance of devices. In this work, we provide a comprehensive experimental and computational study of the interaction between the most common organic perovskite precursors and the surface of ZnO, with the aim of understanding the observed instability. Using X-ray photoelectron spectroscopy, we find a complete degradation of the precursors in contact with ZnO and the formation of volatile species as well as new surface bonds. Our computational work reveals that different pristine and defected surface terminations of ZnO facilitate the decomposition of the perovskite precursor molecules, mainly through deprotonation, making the deposition of the latter on those surfaces impossible without the use of passivation.

K3Mo2O5.6F3.4 and K3V2O3.3F5.7 - exploring transition metal cation valence and anion distribution in oxyfluorides.

Oxyfluorides come in many different structures and are highly adaptable in composition, not least because of their mixed-anionic nature. Slight changes, unless specifically looked for, can easily go unnoticed. In this paper, we present two oxyfluorides, K3Mo2O5.6F3.4 and K3V2O3.3F5.7, synthesized under high-pressure/high-temperature conditions, and demonstrate the importance of careful analysis of composition, oxidation state and O/F anion distribution for an accurate description of oxyfluorides. Their crystal structures were determined by single-crystal X-ray diffraction and the transition metal cation valences analyzed by X-ray photoelectron spectroscopy (XPS). The O/F anion ratio was calculated using the principle of charge neutrality and the local distribution within the crystallographic framework was studied using bond valence (BV) and charge distribution (CHARDI) calculations. Madelung Part of Lattice Energy (MAPLE) calculations and magnetic measurements provide insight into phase stability and corroborate the mixed-valent nature of the compounds.

Polarity profiling of porous architectures: solvatochromic dye encapsulation in metal-organic frameworks.

Metal-organic frameworks (MOFs) have gathered significant interest due to their tunable porosity leading to diverse potential applications. In this study, we investigate the incorporation of the fluorosolvatochromic dye 2-butyl-5,6-dimethoxyisoindoline-1,3-dione ([double bond, length as m-dash]Phth) into various MOF structures as a means to assess the polarity of these porous materials. As a purely inorganic compound, zeolite Y was tested for comparison. The fluorosolvatochromic behavior of Phth, which manifests as changes in its emission spectra in response to solvent polarity, provides a sensitive probe for characterizing the local environment within the MOF pores. Through systematic variation of the MOF frameworks, we demonstrate the feasibility of using (fluoro-)solvatochromic dyes as probes for assessing the polarity gradients within MOF structures. Additionally, the fluorosolvatochromic response was studied as a function of loading amount. Our findings not only offer insights into the interplay between MOF architecture and guest molecule interactions but also present a promising approach for the rational design and classification of porous materials based on their polarity properties.

Mechanistic study on the solution-phase n-doping of 1,3-dimethyl-2-aryl-2,3-dihydro-1H-benzoimidazole derivatives.

The discovery of air-stable n-dopants for organic semiconductor materials has been hindered by the necessity of high-energy HOMOs and the air sensitivity of compounds that satisfy this requirement. One strategy for circumventing this problem is to utilize stable precursor molecules that form the active doping complex in situ during the doping process or in a postdeposition thermal- or photo-activation step. Some of us have reported on the use of 1H-benzimidazole (DMBI) and benzimidazolium (DMBI-I) salts as solution- and vacuum-processable n-type dopant precursors, respectively. It was initially suggested that DMBI dopants function as single-electron radical donors wherein the active doping species, the imidazoline radical, is generated in a postdeposition thermal annealing step. Herein we report the results of extensive mechanistic studies on DMBI-doped fullerenes, the results of which suggest a more complicated doping mechanism is operative. Specifically, a reaction between the dopant and host that begins with either hydride or hydrogen atom transfer and which ultimately leads to the formation of host radical anions is responsible for the doping effect. The results of this research will be useful for identifying applications of current organic n-doping technology and will drive the design of next-generation n-type dopants that are air stable and capable of doping low-electron-affinity host materials in organic devices.

Chiral self-organized single 2D-layers of tetramers from a functional donor-acceptor molecule by the surface template effect.

The ability to control the structural properties of molecular layers is a key for the design and preparation of organic electronic devices. While microscopic growth studies of planar, rigid and symmetric π-conjugated molecules have been performed to a larger extent, this is less the case for elongated donor-acceptor molecules with flexible functional groups, which are particularly interesting due to their high dipole moments. Prototypical molecules of this type are merocyanines (MCs), which have been widely studied for the use as efficient absorbers in organic photodetectors. For maximized light absorption and optimized electronic properties the molecular arrangement which is affected by the initial assembly of the films at the supporting substrate interface is decisive. The situation deserves special attention, when the surface nucleation leads to so far not known and bulk-unlike aggregates. Here, we report on the growth of a typical MC (HB238) on the Ag(100) surface, serving as the substrate. In the energetically preferred phase, the molecules adsorb in a face-on geometry and organize in tetramers with a circular dipole arrangement. The tetramers further self-order in large, enantiopure domains with a periodicity that is commensurate to the Ag(100) surface, likely due to a specific bonding of the thiophene and thiazol rings to the Ag surface. Using scanning tunneling microscopy (STM) in combination with low energy electron diffraction we derive the detailed structure of the tetramers. The center of the tetramer, which is most prominent in STM images, consists of four upward pointing tert-butyl groups from four molecules. It is encircled by a ring of four hydrogen bonds between terminal CN-groups and thiophene rings on neighboring molecules. In parallel, the surface interaction modifies the intramolecular dipole, which is revealed from photoemission spectroscopy. Hence, this example shows how the surface template effect leads to an unforeseen molecular organization which is considerably more complex compared to that in the bulk phases of HB238, which feature paired dipoles.

Revealing the Role of Tin Fluoride Additive in Narrow Bandgap Pb-Sn Perovskites for Highly Efficient Flexible All-Perovskite Tandem Cells.

Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of the SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface due to its rapid oxidation. In addition, at the buried perovskite interface, we detected and visualized the accumulation of F- ions. We found that the photoluminescence quantum yield of Pb-Sn perovskite reached the highest value with 10 mol % SnF2 in the precursor solution. When integrating the optimized absorber in flexible devices, we obtained the flexible Pb-Sn perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and demonstrated 23.1% efficient flexible four-terminal all-perovskite tandem cells.

Ultralow doping in organic semiconductors: evidence of trap filling.

Tail states in organic semiconductors have a significant influence on device performances by acting as traps in charge transport. We present a study of the controlled passivation of acceptor tail states in fullerene C(60) by the addition of electrons introduced by molecular n doping. Using ultralow doping, we are able to successively fill the traps with charges and examine the changes in conductivity, activation energy, mobility, and Fermi-level position. Passivation of the traps leads to an increase of the electron mobility in C(60) by more than 3 orders of magnitude, to reach 0.21 cm(2)/(V s).

Decomposition of Organic Perovskite Precursors on MoO3: Role of Halogen and Surface Defects.

Despite the rapid progress in perovskite solar cells, their commercialization is still hindered by issues regarding long-term stability, which can be strongly affected by metal oxide-based charge extraction layers next to the perovskite material. With MoO3 being one of the most successful hole transport layers in organic photovoltaics, the disastrous results of its combination with perovskite films came as a surprise but was soon attributed to severe chemical instability at the MoO3/perovskite interface. To discover the atomistic origin of this instability, we combine density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) measurements to investigate the interaction of MoO3 with the perovskite precursors MAI, MABr, FAI, and FABr. From DFT calculations we suggest a scenario that is based upon oxygen vacancies playing a key role in interface degradation reactions. Not only do these vacancies promote decomposition reactions of perovskite precursors, but they also constitute the reaction centers for redox reactions leading to oxidation of the halides and reduction of Mo. Specifically iodides are proposed to be reactive, while bromides do not significantly affect the oxide. XPS measurements reveal a severe reduction of Mo and a loss of the halide species when the oxide is interfaced with I-containing precursors, which is consistent with the proposed scenario. In line with the latter, experimentally observed effects are much less pronounced in case of Br-containing precursors. We further find that the reactivity of the MoO3 substrate can be moderated by reducing the number of oxygen vacancies through a UV/ozone treatment, though it cannot be fully eliminated.

Defects in CsPbX3 Perovskite: From Understanding to Effective Manipulation for High-Performance Solar Cells.

The rapid development of all inorganic metal perovskite (CsPbX3 , X represents halogen) materials holds great promise for top-cells in tandem junctions due to their glorious thermal stability and continuous adjustable band gap in a wide range. Due to the presence of defects, the power conversion efficiency (PCE) of CsPbX3 perovskite solar cells (PSCs) is still substantially below the Shockley-Queisser (SQ) limit. Therefore, it is imperative to have an in-depth understanding of the defects in PSCs, thus to evaluate their impact on device performances and to develop corresponding strategies to manipulate defects in PSCs for further promoting their photoelectric properties. In this review, the latest progress in defect passivation in the CsPbX3 PSCs field is summarized. Starting from the effect of non-radiative recombination on open circuit voltage (Voc ) losses, the defect physics, tolerance, self-healing, and the effect of defects on the photovoltaic properties are discussed. Some techniques to identify defects are compared based on quantitative and qualitative analysis. Then, passivation manipulation is discussed in detail, the defect passivation mechanisms are proposed, and the passivation agents in CsPbX3 thin films are classified. Finally, directions for future research about defect manipulation that will push the field to progress forward are outlined.

Band-Gap Tuning in All-Inorganic CsPbxSn1-xBr3 Perovskites.

We investigate all-inorganic perovskite CsPbxSn1-xBr3 thin films to determine the variations in the band gap and electronic structure associated with the Pb/Sn ratio. We observe that the band gap can be tuned between 1.86 eV (x = 0) and 2.37 eV (x = 1). Intriguingly, this change is nonlinear in x, with a bowing parameter of 0.9 eV; furthermore, a slight band gap narrowing is found for low Pb content (minimum x ∼ 0.3). The wide tunability of the band gap makes CsPbxSn1-xBr3 a promising material, e.g., for a wide-gap subcell in tandem applications or for color-tunable light-emitting diodes. Employing photoelectron spectroscopy, we show that the valence band varies with the Pb/Sn ratio, while the conduction band is barely affected.

Band Gap of Pb(Fe0.5Nb0.5)O3 Thin Films Prepared by Pulsed Laser Deposition.

Ferroelectric materials have gained high interest for photovoltaic applications due to their open-circuit voltage not being limited to the band gap of the material. In the past, different lead-based ferroelectric perovskite thin films such as Pb(Zr,Ti)O3 (Pb,La)(Zr,Ti)O3 and PbTiO3 were investigated with respect to their photovoltaic efficiency. Nevertheless, due to their high band gaps they only absorb photons in the UV spectral range. The well-known ferroelectric PbFe0.5Nb0.5O3 (PFN), which is in a structure similar to the other three, has not been considered as a possible candidate until now. We found that the band gap of PFN is around 2.75 eV and that the conductivity can be increased from 23 S/µm to 35 S/µm during illumination. The relatively low band gap value makes PFN a promising candidate as an absorber material.