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Light‐harvesting layers in organic bulk‐heterojunction solar cells are commonly fabricated from aromatic solvents which are often toxic or hazardous. To fully omit harmful solvents, (surfactant‐free) nanoparticle dispersions of organic semiconductor blends are investigated, but only a very limited choice of materials form blend nanoparticles. This work is a systematic study of the nanoparticle formation by nanoprecipitation of electrostatically stabilized polymer: fullerene blends. It turns out that miscibility of both components is crucial for the incorporation of the fullerenes into the nanoparticles. This finding demystifies why certain polymer:fullerene combinations seemingly randomly form stable nanoparticle dispersion while others do not. The need of miscibility constitutes an important design criterion for processing future semiconductor blends along the nanoparticle route. Some mitigation to the unwanted release of larger amounts of fullerenes can be achieved by using ternary semiconductor blends. [ABSTRACT FROM AUTHOR]

High‐throughput (HT) synthesis and HT characterization techniques are becoming increasingly important due to the ever‐increasing complexity of materials and applications of advanced functional compounds. This work reports on the high‐throughput compilation of material libraries of high‐entropy oxides with fluorite crystal structure and tunable band gaps to be used as, e.g., semiconductors for photovoltaic applications. The material libraries cover the high‐entropy range of rare‐earth oxides with 5, 6, and 7 different cations (Ce, La, Sm, Pr, Tb, Y, and Zr) in near equimolar concentrations, but also the medium entropy range with 4 cations. The atmosphere used during or after synthesis is found to have a large effect on the band gap of these materials. Multivalent rare‐earth cations such as Ce/Pr/Tb enable reversible tuning of the band gap between 2.0 and 3.5 eV upon calcination under various oxidizing and reducing atmospheres. The high‐entropy fluorite oxides with smaller band gaps exhibit high electron mobility and transport energy levels compatible with common solar cell architectures. [ABSTRACT FROM AUTHOR]

High‐performance organic solar cells are deposited from eco‐friendly semiconductor dispersions by applying reversible electrostatic stabilization while omitting the need for stabilizing surfactants. The addition of iodine fosters the oxidation (p‐doping) of the light‐harvesting polymer, effectively promoting the electrostatic repulsion of the nanoparticles and hence the colloidal stability of the respective dispersions. The oxidation of polymers with iodine is reversible: after thin‐film deposition and after thermal evaporation of the iodine, the corresponding polymer:non‐fullerene solar cells yield power conversion efficiencies of up to 10.6%. [ABSTRACT FROM AUTHOR]

After the discovery of ferroelectricity in archetypal methylammonium lead iodide (MAPbI3), the discussion arose, if more advanced derivatives thereof are also ferroelectric and to what extent the polar domains mitigate detrimental charge carrier recombination in perovskite solar cells. Herein, the A‐site cation methylammonium is gradually substituted with formamidinium and cesium. The domain contrast measured by piezoresponse force microscopy is correlated with the distortion of the crystal structure measured by X‐ray diffraction. By Rietveld analysis, a detailed structural model together with texture information is determinded, which reveals an intriguing interplay between lead iodide and the perovskite phases. Finally, the bearing of ferroelectric domains in mixed‐cation perovskites on the solar cell performance is discussed. [ABSTRACT FROM AUTHOR]

State‐of‐the‐art solvents for the fabrication of organic solar cells are mostly toxic or hazardous. First attempts to deposit light‐harvesting layers from aqueous or alcoholic nanoparticle dispersions instead have been successful on laboratory scale, enabling future eco‐friendly production of organic solar cells. In this work, a scalable high‐throughput continuous‐flow microfluidic system is employed to synthesize surfactant‐free organic semiconductor dispersions by nanoprecipitation. By adjusting the differential speed of the syringe pumps, the concentration of the initial solute and the irradiation of the microfluidic chip, the synthesis can be controlled for tailored dispersion concentrations and nanoparticle sizes. The resulting dispersions are highly reproducible, and the semiconductor inks are stable for at least one year. The synthesis of the dispersions is exemplified on a polymer/fullerene combination with large‐scale availability. [ABSTRACT FROM AUTHOR]

Organic semiconductor nanoparticle dispersions are electrostatically stabilized with the p‐doping agent 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ), omitting the need for surfactants. Smallest amounts of F4TCNQ stabilize poly(3‐hexylthiophene) dispersions and reduce the size of the nanoparticles significantly. The concept is then readily transferred to synthesize dispersions from a choice of light‐harvesting benzodithiophene‐based copolymers. Dispersions from the corresponding polymer:fullerene blends are used to fabricate organic solar cells (OSCs). In contrast to the widely used stabilizing surfactants, small amounts of F4TCNQ show no detrimental effect on the device performance. This concept paves the way for the eco‐friendly fabrication of OSCs from nanoparticle dispersions of high‐efficiency light‐harvesting semiconductors by eliminating environmentally hazardous solvents from the deposition process. [ABSTRACT FROM AUTHOR]

Combining light emission and charge carrier transport into one molecule simplifies the manufacturing of organic light-emitting diodes (OLEDs). In this work, phosphorescent iridium(III) complex-emitters are protected by conjugated dendrons to reduce intermolecular interchromophore interactions that lead to concentration quenching of the luminescence in host-free emission layers. When two dendrons are added to each ligand (doubly dendronized) of an iridium(III) complex at the core of the dendrimers (versus one), the core is found to have improved protection, leading to an enhancement of the external quantum efficiency of the OLEDs from 4% to more than 10%. Likewise, OLEDs comprising the doubly dendronized emitters exhibit improved device lifetimes. A detailed study of the degradation mechanisms reveals that exciton-induced ligand and/or dendron detachment promotes interchromophore interactions (aggregation) that lead to quenching sites and hence red electromer emission inside the OLED emissive layer, with the effects being less pronounced for the doubly dendronized emitter. [ABSTRACT FROM AUTHOR]

Due to their high transparency, electrodes fabricated from conductive polymers are often implemented in semitransparent organic solar cells. Opaque solar cells usually employ metal back electrodes with high reflectivity for best photon confinement in the light‐harvesting layer. Herein, a bilayer back electrode comprising conductive polymers and nanofoamed poly(methyl methacrylate) (PMMA) is investigated, the latter of which creates diffuse reflection of the incoming light. By tuning the thickness of the nanofoamed PMMA layer, absorption and transmission of the solar cells can be tailored from opaque to vastly transparent. Due to its diffusive character, this versatile electrode enhances the light absorption in the wavelength regimes with lower absorption coefficient. The solar cells are particularly suited for deployment in frosted window applications. [ABSTRACT FROM AUTHOR]

While more and more experimental evidence for the ferroelectricity of methylammonium lead iodide (MAPbI3) is being reported in the literature, the scientific community still controversially discusses whether or not the ferroelectric polarization has any influence on the performance of perovskite solar cells. In this work, the evolution of ferroelectric domains and their polarization orientation in MAPbI3 thin films during thermal annealing are investigated using piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). Right after deposition and annealing for 5 s, small grains with non-uniform crystal orientation and polarization are formed. During the next 30–60 s, these small grains increase in diameter and large ferroelectric domains with out-of-plane polarization appear. In the annealing regime of several minutes to one hour, these large grains produce uniform domains with alternating in-plane polarization and (110) texture. The corresponding MAPbI3 solar cells show a distinct performance enhancement and improved operational stability if the ferroelectric polarization is oriented in-plane. In contrast, solar cells with out-of-plane-polarized MAPbI3 exhibit only moderate fill factors and reduced open-circuit voltages. [ABSTRACT FROM AUTHOR]

The power conversion efficiencies of organic solar cells delicately depend on the morphology of the light‐harvesting bulk heterojunctions (BHJ). Upon deposition from solution, the formation of tailored bicontinuous networks of polymers and fullerenes is often achieved using combinations of solvents and solvent additives. Common wisdom infers that best solar cell performances are achieved when the solvent additives exhibit excellent fullerene solubility. Herein, this concept is revisited based on the investigation of a series of structurally similar, substituted benzaldehydes. It is concluded that the solvent additives do not only have to feature the commonly accepted good fullerene solubility, but must also exhibit lowest polymer solubility to suppress liquid–liquid demixing and hence achieve best solar cell performance. Thus, this study adds an important item to the list of selection criteria of solvent additives toward the production of polymer:fullerene solar cells with optimized power conversion efficiencies. The microscopic picture of the resulting domain configurations within the light‐harvesting layers is developed around comprehensive multiscale investigations of the BHJ morphology, using atomic force microscopy, scanning transmission electron microscopy, and nano‐infrared microscopy. The latter is operated in two complementary modes, one of which is more bulk sensitive, whereas the other mode is surface sensitive. [ABSTRACT FROM AUTHOR]

Triarylamine derivatives are widely used for hole transport in organic optoelectronic devices, but their excellent solubility in many common solvents limits their use for multi-layer device fabrication from solution. In this work, a novel process to the formation of conjugated triarylamine polymer thin-film networks by crosslinking of thiol-substituted and alkyne-substituted triarylamines is investigated. After deposition of the monomer blend and crosslinking under UV exposure, an insoluble thiol–yne polymer network is formed. The applicability of the thiol–yne polymer network as hole-transport layers is exemplified on organic light-emitting diodes. Its implementation reduces the device degradation as compared to OLEDs comprising hole-transport layers from PEDOT:PSS. [ABSTRACT FROM AUTHOR]

Organic solar cells have come a long way from fundamental considerations of charge carrier dynamics in organic semiconductors to devices with laboratory power conversion efficiencies exceeding 17% and first power harvesting installations. Despite this story of success, these days, the scientific community witnesses a shift of research effort to other solar concepts, leaving behind a high‐potential solar technology with better applicability forecasts than ever before. Very compelling reasons still exist why organic solar cells can become the solar technology of the future that offers design versatility and enables unprecedented applications while offering the lowest energy payback times and ecologic sustainability. This perspective article highlights why organic solar cells remain a research field of the highest socioeconomic relevance, which challenges remain to be overcome in the future, and how organic solar cells can make a difference in the future energy landscape. [ABSTRACT FROM AUTHOR]

We demonstrate modular modifications of the widely employed emitter 2,4,5,6‐tetra(9H‐carbazol‐9‐yl)isophthalonitrile (4CzIPN) by replacing one or both nitrile acceptors with oxadiazole groups via a tetrazole intermediate. This allows the introduction of various functional groups including halides, alkynes, alkenes, nitriles, esters, ethers and a protected amino acid while preserving the thermally activated delayed fluorescence (TADF) properties. The substituents control the emission maximum of the corresponding emitters, ranging between 472–527 nm, and show high solid‐state photoluminescence quantum yields up to 85 %. The TADF emission of two compounds, 4CzCNOXDtBu and 4CzdOXDtBu, a mono‐ and a bis‐oxadiazole substituted 4CzIPN is characterized in detail by time‐ and temperature‐dependent photoluminescence. Solution‐processed OLEDs comprising 4CzCNOXDtBu and 4CzdOXDtBu show a significant blue‐shift of the emission compared to the reference 4CzIPN, with external quantum efficiencies of 16 %, 5.9 % and 17 % at 100 cd m−2, respectively. [ABSTRACT FROM AUTHOR]

Imaging the phase distribution of amorphous or partially crystalline organic materials at the nanoscale and analyzing the local atomic structure of individual phases has been a long-time challenge. We propose a new approach for imaging the phase distribution and for analyzing the local structure of organic materials based on scanning transmission electron diffraction (4D-STEM) pair distribution function analysis (PDF). We show that electron diffraction based PDF analysis can be used to characterize the short- and medium-range order in aperiodically packed organic molecules. Moreover, we show that 4D-STEM-PDF does not only provide local structural information with a resolution of a few nanometers, but can also be used to image the phase distribution of organic composites. The distinct and thickness independent contrast of the phase image is generated by utilizing the structural difference between the different types of molecules and taking advantage of the dose efficiency due to use of the full scattering signal. Therefore, this approach is particularly interesting for imaging unstained organic or polymer composites without distinct valence states for electron energy loss spectroscopy. We explore the possibilities of this new approach using [6,6]-phenyl-C61- butyric acid methyl ester (PC61BM) and poly(3-hexylthiophene-2,5-diyl) (P3HT) as the archetypical and best-investigated semiconductor blend used in organic solar cells, compare our phase distribution with virtual dark-field analysis and validate our approach by electron energy loss spectroscopy. [ABSTRACT FROM AUTHOR]