Steering perovskite precursor solutions for multijunction photovoltaics.

Multijunction photovoltaics (PVs) are gaining prominence owing to their superior capability of achieving power conversion efficiencies (PCEs) beyond the radiative limit of single-junction cells, where improving narrow bandgap tin-lead perovskites is critical for thin-film devices. With a focus on understanding the chemistry of tin-lead perovskite precursor solutions, we herein find that Sn(II) species dominate interactions with precursors and additives and uncover the exclusive role of carboxylic acid in regulating solution colloidal properties and film crystallisation, and ammonium in improving film optoelectronic properties. Materials that combine these two function groups, amino acid salts, considerably improve the semiconducting quality and homogeneity of perovskite films, surpassing the effect of the individual functional groups when introduced as part of separate molecules.

Dependence of Exciton Binding Energy on Bandgap of Organic Semiconductors.

Strongly bound excitons crucially affect the operation of organic optoelectronic devices. Nevertheless, precise experimental data on the exciton binding energy of organic semiconductors are lacking. In this study, we determine the exciton binding energy as the difference between the optical and transport bandgaps with a precision of 0.

Synthesis of 3,3'-dihydroxy-2,2'-diindan-1,1'-dione derivatives for tautomeric organic semiconductors exhibiting intramolecular double proton transfer.

To investigate potential applications of the 3,3'-dihydroxy-2,2'-biindan-1,1'-dione (BIT) structure as an organic semiconductor with intramolecular hydrogen bonds, a new synthetic route under mild conditions is developed based on the addition reaction of 1,3-dione to ninhydrin and the subsequent hydrogenation of the hydroxyl group. This route affords several new BIT derivatives, including asymmetrically substituted structures that are difficult to access by conventional high-temperature synthesis. The BIT derivatives exhibit rapid tautomerization by intramolecular double proton transfer in solution.

Tin Halide Perovskite Solar Cells with Open-Circuit Voltages Approaching the Shockley-Queisser Limit.

The power conversion efficiency of tin-based halide perovskite solar cells is limited by large photovoltage losses arising from the significant energy-level offset between the perovskite and the conventional electron transport material, fullerene C. The fullerene derivative indene-C bisadduct (ICBA) is a promising alternative to mitigate this drawback, owing to its superior energy level matching with most tin-based perovskites. However, the less finely controlled energy disorder of the ICBA films leads to the extension of its band tails that limits the photovoltage of the resultant devices and reduces the power conversion efficiency.

Synthesis of 2,5,8-Tris(1-phenyl-1-benzo[]imidazol-2-yl)benzo[1,2-:3,4-':5,6-″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films.

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1-benzo[]imidazol-2-yl)benzo[1,2-:3,4-':5,6-″] trithiophene (TPBTT) and its ethyl derivative (-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and -ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1--benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.

Synergistic Surface Modification of Tin-Lead Perovskite Solar Cells.

Interfaces in thin-film photovoltaics play a pivotal role in determining device efficiency and longevity. In this work, the top surface treatment of mixed tin-lead (≈1.26 eV) halide perovskite films for p-i-n solar cells is studied.

Impact of Narrowing Density of States in Semiconducting Polymers on Performance of Organic Field-Effect Transistors.

To improve the performance of organic field-effect transistors (OFETs) employing π-conjugated polymers, a basic understanding of the relationships between the material properties and device characteristics is crucial. Although the density of states (DOS) distribution is one of the essential material properties of semiconducting polymers, insights into how the DOS shape affects the mobility (µ), subthreshold swing (S), and contact resistance (R ) in OFETs remain lacking. In this study, by combining sensitive DOS measurements and multilayered OFET structures, it is experimentally demonstrated that narrower DOS widths in the polymer channels lead to higher µ, smaller S, and lower R .

Tuning Molecular Conformations to Enhance Spontaneous Orientation Polarization in Organic Thin Films.

Three isomeric derivatives of 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1--benzimidazole) (TPBi) bearing ethyl groups on the -phenyl moieties were synthesized to elucidate the effects of intramolecular interactions on spontaneous orientation polarization (SOP) in thin films. The films of the TPBi derivatives displayed enhanced SOP with a surface potential change of up to 1.8 times that for TPBi, and the -substituted derivative exhibited the largest potential change reported to date (+141.

Ultrathin and Efficient Organic Photovoltaics with Enhanced Air Stability by Suppression of Zinc Element Diffusion.

Ultrathin (thickness less than 10 µm) organic photovoltaics (OPVs) can be applied to power soft robotics and wearable electronics. In addition to high power conversion efficiency, stability under various environmental stresses is crucial for the application of ultrathin OPVs. In this study, the authors realize highly air-stable and ultrathin (≈3 µm) OPVs that possess high efficiency (15.

Reduction of Electric Current Loss by Aggregation-Induced Molecular Alignment of a Non-Fullerene Acceptor in Organic Photovoltaics.

Y6 is a recently developed non-fullerene electron acceptor (NFA) with dithienothiophen[3.2-]-pyrrolobenzothiadiazole as the central unit and improves the performance of organic photovoltaics (OPVs) in combination with many electron-donor polymers. Although Y6 has desirable electronic properties for OPVs, the origin of its superiority as an acceptor is unclear.

Triphenyleno[1,2-:7,8-']bis([1,2,5]thiadiazole) as a V-Shaped Electron-Deficient Unit to Construct Wide-Bandgap Amorphous Polymers for Efficient Organic Solar Cells.

The backbone shape of semiconducting polymers strongly affects their electronic properties and morphologies in films, yet the conventional design principle for building blocks focuses on using linear main chains to maintain high crystallinity. Here, we developed a V-shaped unit, triphenyleno[1,2-:7,8-']bis([1,2,5]thiadiazole) (TPTz), featuring two 1,2,5-thiadiazole rings fused to a triphenylene core with strong electron-withdrawing properties and an extended conjugation plane. We used TPTz to prepare a highly soluble copolymer, PTPTz-indacenodithiophene (IDT), which exhibited a wide bandgap of 1.

Mechanism of charge accumulation of poly(heptazine imide) gel.

Photo-stimuli response in materials is a fascinating feature with many potential applications. A photoresponsive gel of poly(heptazine imide), PHI, termed PHIG, exhibits photochromism, photoconductivity, and photo-induced charge accumulation, and is generated using ionic liquids and PHI. Although there are several examples of ionic liquid gels that exhibit photochromism and photoconductivity, this is the first report of an ionic liquid gel that exhibits both these properties as well as charge accumulation.

Highly Sensitive Evaluation of Density of States in Molecular Semiconductors by Photoelectron Yield Spectroscopy in Air.

Despite the critical importance of the density of states (DOS) to the electrical properties of molecular semiconductors, there are few reliable measurement methods for the DOS, especially for the edge regions with low DOS that determine the charge conduction. Here, we evaluate the DOS for occupied states with high sensitivity (down to 10 relative to the peak maxima) using a commercially available photoelectron yield spectroscopy (PYS) system operated in air. The conduction edges of the DOS for both the semiconducting polymers and molecules are expressed as a single Gaussian with no gap states with one exception.

Inversion of Circular Dichroism Signals in Chiral Polythiophene Films Induced by End-On-Oriented Surface-Segregated Monolayers.

A chiral polythiophene surfactant based on poly(3-()-2-methylbutylthiophene) (()-P3MBT) with a semifluoroalkyl group at one end of the main chain was synthesized and used to form surface-segregated monolayers (SSMs). Films of pure ()-P3MBT mainly adopted the edge-on orientation, whereas ()-P3MBT films with a SSM of the polymer surfactant ()-P3MBT-F contained a large proportion of end-on-oriented polythiophene, both at the surface and inside the films. The thin films with the SSM showed circular dichroism signals, with the sign opposite to those observed in ()-P3MBT films.

Immobilization of Ethynyl-π-Extended Electron Acceptors with Amino-Terminated SAMs by Catalyst-Free Click Reaction.

Surface modification of SiO using a catalyst-free quantitative reaction between an amine and an ethynyl-π-extended naphthalenediimide was investigated. A post-reaction method, in which the catalyst-free reaction was performed at the surface after the formation of amino-terminated self-assembled monolayers (SAMs), resulted in dense, uniform modification of the SiO surface with the naphthalenediimide molecules. Both X-ray reflectivity and angle-resolved X-ray photoemission spectroscopy showed consistent results for the layer thickness and density.

Surface Analysis of Native Spider Draglines by FE-SEM and XPS.

Although the physical and biological functions of the skin layer of spider dragline have been studied and partially clarified, the morphology and elemental contents of the skin layer of silk fibers have not been investigated in detail to date. Here, the surface of spider dragline was evaluated by field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) to obtain clear surface morphological and molecular information. The FE-SEM images of the spider dragline indicate that the spider dragline forms a bundle of microfibrils.

Noncentrosymmetric Columnar Liquid Crystals with the Bulk Photovoltaic Effect for Organic Photodetectors.

The bulk photovoltaic effect (BPVE) has drawn intensive attention due to its unique features that cannot be accessed with the conventional photovoltaic effect. However, the BPVE is observed in noncentrosymmetric materials and has been studied mainly for inorganic materials. Here, we report a simple subphthalocyanine (SubPc) derivative that assembles into a noncentrosymmetric columnar liquid crystal with the help of a DC E-field.

Anatomy of the energetic driving force for charge generation in organic solar cells.

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In this study, we systematically investigate 16 combinations of four donor polymers and four acceptors in planar heterojunction. The charge generation efficiency and its electric field dependence correlate with the energy difference between the singlet excited state and the interfacial charge transfer state.

Intermolecular Arrangement of Fullerene Acceptors Proximal to Semiconducting Polymers in Mixed Bulk Heterojunctions.

Precise control of the molecular arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging, despite the correlation between structural characteristics and efficiency in organic photovoltaics (OPVs). This study reveals that the substitution patterns of linear and branched alkyl side chains on electron-donating/-accepting alternating copolymers can control the positions of an acceptor molecule (C ) around the π-conjugated main chains in mixed BHJs. Two-dimensional solid-state NMR demonstrates a marked difference in the location of C in the blend films.

Quantitative Evaluation of Molecular Diffusion in Organic Planar Heterojunctions by Time-of-Flight Secondary Ion Mass Spectroscopy.

Understanding molecular diffusion across the interfaces in planar heterojunctions is fundamentally important to improving the performance and stability of organic electronic devices. In this study, we quantitatively evaluated the diffusion of [6,6]-phenyl-C-butyric acid methyl ester (PCBM) across the interface of planar heterojunctions into the polymer layers by time-of-flight secondary ion mass spectroscopy. Careful calibration allowed the concentration of PCBM to be determined in the polymer layer at concentrations as low as 0.

Crystallization and Polymorphism of Organic Semiconductor in Thin Film Induced by Surface Segregated Monolayers.

Preparation of highly crystalline organic semiconductor films is vital to achieving high performance in electronic devices. Here we report that surface segregated monolayers (SSMs) on top of phenyl-C-butyric acid methyl ester (PCBM) thin films induce crystal growth in the bulk, resulting in a dramatic change in the structure to form a new crystal phase. Highly ordered crystalline films with large domain sizes of several hundreds of nanometers are formed with uniaxial orientation of the crystal structure perpendicular to the substrate.

High Sensitivity Tuning of Work Function of Self-Assembled Monolayers Modified Electrodes Using Vacuum Ultraviolet Treatment.

We demonstrate systematic work function tuning of thiol-based SAM-modified gold electrodes with high controllability and sensitivity as high as 0.05 eV using vacuum ultraviolet technique (VUV). Under different irradiation times, both work function and wettability of the metal surface is modified.

Organic Solar Cells with Controlled Nanostructures Based on Microphase Separation of Fullerene-Attached Thiophene-Selenophene Heteroblock Copolymers.

Heteroblock copolymers consisting of poly(3-hexylthiophene) and fullerene-attached poly(3-alkylselenophene) (T-b-Se-PCBP) were synthesized for organic photovoltaic applications by quasi-living catalyst transfer polycondensation and subsequent conversion reactions. Characterization of the polymers confirmed the formation of well-defined diblock structures with high loading of the fullerene at the side chain (∼40 wt %). Heteroblock copolymer cast as a thin film showed a clear microphase-separated nanostructure approximately 30 nm in repeating unit after thermal annealing, which is identical to the microphase-separated nanostructure of diblock copolymer consisting of poly(3-hexylthiophene) and fullerene-attached poly(3-alkylthiophene) (T-b-T-PCBP).