KCN Chemical Etching of van der Waals SbSe Thin Films Synthesized at Low Temperature Leads to Inverted Surface Polarity and Improved Solar Cell Efficiency.

Recent developments in SbSe van der Waals material as an absorber candidate for thin film photovoltaic applications have demonstrated the importance of surface management for improving the conversion efficiency of this technology. SbSe thin films' versatility in delivering good efficiencies in both superstrate and substrate configurations, coupled with a compatibility with various low-temperature deposition techniques (below 500 °C and often below 350 °C), makes them highly attractive for advanced photovoltaic applications. This study presents a comparative analysis of the most effective chemical etchings developed for related thin film chalcogenide technologies to identify and understand the most appropriate surface chemical treatments for SbSe in substrate configuration, synthesized using a sequential process at very low temperatures (320 °C).

Polar Polymorphism: A New Intermediate Structure toward the Thin-Film Phase in Asymmetric Benzothieno[3,2-][1]-benzothiophene Derivatives.

Understanding structure and polymorphism is relevant for any organic device optimization, and it is of particular relevance in 7-decyl-2-phenyl[1]benzothieno[3,2-][1]benzothiophene (Ph-BTBT-10) since high carrier mobility in Ph-BTBT-10 thin films has been linked to the structural transformation from the metastable thin-film phase to the thermodynamically stable bilayer structure via thermal annealing. We combine here a systematic nanoscale morphological analysis with local Kelvin probe force microcopy (KPFM) that demonstrates the formation of a polar polymorph in thin films as an intermediate structure for thicknesses lower than 20 nm. The polar structure develops with thickness a variable amount of structural defects in the form of individual flipped molecules (point defects) or sizable polar domains, and evolves toward the reported nonpolar thin-film phase.

Monodentate versus Bidentate Anchoring Groups in Self-Assembling Molecules (SAMs) for Robust p-i-n Perovskite Solar Cells.

Current improvement in perovskite solar cells (PSCs) has been achieved by interface engineering and fine-tuning of charge-selective contacts. In this work, we report three novel molecules that can form self-assembled layers (SAMs) as an alternative to the most commonly used p-type contact material, PTAA. Two of these molecules have bidentate anchoring groups (MC-54 and MC-55), while the last one is monodentate (MC-45).

SbSeI and SbSeBr micro-columnar solar cells by a novel high pressure-based synthesis process.

Van der Waals chalcogenides and chalcohalides have the potential to become the next thin film PV breakthrough, owing to the earth-abundancy and non-toxicity of their components, and their stability, high absorption coefficient and quasi-1D structure, which leads to enhanced electrical anisotropic properties when the material is oriented in a specific crystalline direction. However, quasi-1D semiconductors beyond Sb(S,Se), such as SbSeX chalcohalides, have been scarcely investigated for energy generation applications, and rarely synthesised by physical vapor deposition methodologies, despite holding the promise of widening the bandgap range (opening the door to tandem or semi-transparent devices), and showing enticing new properties such as ferroelectric behaviour and defect-tolerant nature. In this work, SbSeI and SbSeBr micro-columnar solar cells have been obtained for the first time by an innovative methodology based on the selective halogenation of SbSe thin films at pressure above 1 atm.

Synergistic Effect of Solvent Vapor Annealing and Chemical Doping for Achieving High-Performance Organic Field-Effect Transistors with Ideal Electrical Characteristics.

Contact resistance and charge trapping are two key obstacles, often intertwined, that negatively impact on the performance of organic field-effect transistors (OFETs) by reducing the overall device mobility and provoking a nonideal behavior. Here, we expose organic semiconductor (OSC) thin films based on blends of 2,7-dioctyl[1]benzothieno[3,2-][1]benzothiophene (C8-BTBT-C8) with polystyrene (PS) to (i) a CHCN vapor annealing process, (ii) a doping I/water procedure, and (iii) vapors of I/CHCN to simultaneously dope and anneal the films. After careful analysis of the OFET electrical characteristics and by performing local Kelvin probe force microscopy studies, we found that the vapor annealing process predominantly reduces interfacial shallow traps, while the chemical doping of the OSC film is responsible for the diminishment of deeper traps and promoting a significant reduction of the contact resistance.

Dielectric behavior of curcuminoid polymorphs on different substrates by direct soft vacuum deposition.

Our work examines the structural-electronic correlation of a new curcuminoid, AlkCCMoid, as a dielectric material on different substrates. For this purpose, we show a homemade sublimation method that allows the direct deposition of molecules on any type of matrix. The electronic properties of AlkCCMoid have been evaluated by measurements on single crystals, microcrystalline powder, and sublimated samples, respectively.

Temperature-induced polymorphism of a benzothiophene derivative: reversibility and impact on the thin film morphology.

The identification of polymorphs in organic semiconductors allows for establishing structure-property relationships and gaining understanding of microscopic charge transport physics. Thin films of 2,7-bis(octyloxy)[1]benzothieno[3,2-]-benzothiophene (CO-BTBT-OC) exhibit a substrate-induced phase (SIP) that differs from the bulk structure, with important implications for the electrical performance in organic field effect transistors (OFETs). Here we combine grazing incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) to study how temperature affects the morphology and structure of CO-BTBT-OC films grown by physical vapor deposition on SiO.

Charge-Transfer Complexes in Organic Field-Effect Transistors: Superior Suitability for Surface Doping.

We demonstrate the key role of charge-transfer complexes in surface doping as a successful methodology for improving channel field-effect mobility and reducing the threshold voltage in organic field-effect transistors (OFETs), as well as raising the film conductivity. Demonstrated here for 2,7-dioctyl[1]benzothieno[3,2-][1]benzothiophene (C-BTBT) doped with 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (FTCNNQ), channel doping by sequential deposition is consistently rationalized by the development of a cocrystalline structure that forms and evolves from the surface of the organic semiconductor film without trading the thin-film structure integrity. This scenario brings higher benefits for the device operation than doping by codeposition, where a decrease in the field-effect mobility of the device, even for a dopant content of only 1 mol %, makes codeposition less suitable.

Real-Space Image of Charged Patches in Tunable-Size Nanocrystals.

The remarkable dual nature of faceted-charge patchy metal fluoride nanocrystals arises from the spontaneous selective coordination of anionic and cationic ligands on the different facets of the nanocrystals. In previous studies, the identification and origin of the charge at the patches were obtained by combining computer simulations with indirect experimental evidence. Taking a step further, we report herein the first direct real-space identification by Kelvin probe force microscopy of the predicted faceted-charge patchy behavior, allowing the image of the dual faceted-charge surfaces.

From high quality packing to disordered nucleation or phase separation in donor/acceptor interfaces: ClAlPc-C on Au(111).

The dramatic consequences that the orientation adopted by the molecular dipoles, in diverse arrays of chloroaluminum phthalocyanine (ClAlPc) on Au(111), have on the ulterior adsorption and growth of C60 are explored by means of an all scanning probe microscopy approach. The unidirectional downwards organization of the molecular dipoles at the first layer reduces charge transfer from the metal to C60. Imbalance between attractive and repulsive interactions of the fullerenes are crucial for their ordered supramolecular aggregation.

Effect of the Organic Semiconductor Side Groups on the Structural and Electronic Properties of Their Interface with Dopants.

Two derivatives of [1]benzothieno[3,2-b][1]benzothiophene (BTBT), namely, 2,7-dioctyl-BTBT (C8-BTBT) and 2,7-diphenyl-BTBT (DPh-BTBT), belonging to one of the best performing organic semiconductor (OSC) families, have been employed to investigate the influence of the substitutional side groups on the properties of the interface created when they are in contact with dopant molecules. As a molecular p-dopant, the fluorinated fullerene CF is used because of its adequate electronic levels and its bulky molecular structure. Despite the dissimilarity introduced by the OSC film termination, dopant thin films grown on top adopt the same (111)-oriented FCC crystalline structure in the two cases.

Surface specificity and mechanistic pathway of de-fluorination of CF on coinage metals.

We provide experimental and theoretical understanding on fundamental processes taking place at room temperature when a fluorinated fullerene dopant gets close to a metal surface. By employing scanning tunneling microscopy and photoelectron spectroscopies, we demonstrate that the on-surface integrity of CF depends on the interaction with the particular metal it approaches. Whereas on Au(111) the molecule preserves its chemical structure, on more reactive surfaces such as Cu(111) and Ni(111), molecules interacting with the bare metal surface lose the halogen atoms and transform to C.

Double Beneficial Role of Fluorinated Fullerene Dopants on Organic Thin-Film Transistors: Structural Stability and Improved Performance.

The present work assesses improved carrier injection in organic field-effect transistors by contact doping and provides fundamental insight into the multiple impacts that the dopant/semiconductor interface details have on the long-term and thermal stability of devices. We investigate donor [1]benzothieno[3,2-]-[1]benzothiophene (BTBT) derivatives with one and two octyl side chains attached to the core, therefore constituting asymmetric (BTBT-C8) and symmetric (C8-BTBT-C8) molecules, respectively. Our results reveal that films formed out of the asymmetric BTBT-C8 expose the same alkyl-terminated surface as the C8-BTBT-C8 films do.

Impact of Nanomorphology on Surface Doping of Organic Semiconductors: The Pentacene-CF Interface.

Establishing the rather complex correlation between the structure and the charge transfer in organic-organic heterostructures is of utmost importance for organic electronics and requires spatially resolved structural, chemical, and electronic details. Insight into this issue is provided here by combining atomic force microscopy, Kelvin probe force microscopy, photoemission electron microscopy, and low-energy electron microscopy for investigating a case study. We select the interface formed by pentacene (PEN), benchmark among the donor organic semiconductors, and a p-type dopant from the family of fluorinated fullerenes.

Boosting Self-Assembly Diversity in the Solid-State by Chiral/Non-Chiral Zn -Porphyrin Crystallization.

A chiral Zn porphyrin derivative 1 and its achiral analogue 2 were studied in the solid state. Considering the rich molecular recognition of designed metalloporphyrins 1 and 2 and their tendency to crystallize, they were recrystallized from two solvent mixtures (CH Cl /CH OH and CH Cl /hexane). As a result, four different crystalline arrangements (1 a,b and 2 a,b, from 0D to 2D) were obtained.

Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties.

Disentangling the details of the vertical distribution of small semiconductor molecules blended with polystyrene (PS) and the contact properties are issues of fundamental value for designing strategies to optimize small-molecule:polymer blend organic transistors. These questions are addressed here for ultrathin blends of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and PS processed by a solution-shearing technique using three different blend composition ratios. We show that friction force microscopy (FFM) allows the determination of the lateral and vertical distribution of the two materials at the nanoscale.

Real Space Demonstration of Induced Crystalline 3D Nanostructuration of Organic Layers.

The controlled 3D nanostructuration of molecular layers of the semiconducting molecules CH (pentacene) and N,N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) is addressed. A tip-assisted method using atomic force microscopy (AFM) is developed for removing part of the organic material and relocating it in up to six layer thick nanostructures. Moreover, unconventional molecular scale imaging combining diverse friction force microscopy modes reveals the stacking sequence of the piled layers.

A molecular-scale portrait of domain imaging in organic surfaces.

Progress in the general understanding of structure-property relationships in organic devices requires experimental tools capable of imaging structural details, such as molecular packing or domain attributes, on ultra-thin films. An operation mode of scanning force microscopy, related to friction force microscopy (FFM) and known as transverse shear microscopy (TSM), has demonstrated the ability to reveal the orientation of crystalline domains in organic surfaces with nanometer resolution. In spite of these promising results, numerous questions remain about the physical origin of the TSM domain imaging mechanism.

Coming across a novel copper oxide 2D framework during the oxidation of Cu(111).

Two dimensional copper oxides obtained on Cu(111) by air-enriched argon sputtering plus annealing have been measured at room temperature by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) under ultra-high vacuum (UHV) conditions. Depending on the oxygen content different oxide frameworks and diverse stoichiometric metal/oxide interfaces exist. In particular, we report on a novel open honeycomb structure with a large unit cell which is modeled as a two dimensional network made out of CuO units.

Threshold-Voltage Shifts in Organic Transistors Due to Self-Assembled Monolayers at the Dielectric: Evidence for Electronic Coupling and Dipolar Effects.

The mechanisms behind the threshold-voltage shift in organic transistors due to functionalizing of the gate dielectric with self-assembled monolayers (SAMs) are still under debate. We address the mechanisms by which SAMs determine the threshold voltage, by analyzing whether the threshold voltage depends on the gate-dielectric capacitance. We have investigated transistors based on five oxide thicknesses and two SAMs with rather diverse chemical properties, using the benchmark organic semiconductor dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene.

Predicting supramolecular self-assembly on reconstructed metal surfaces.

The prediction of supramolecular self-assembly onto solid surfaces is still challenging in many situations of interest for nanoscience. In particular, no previous simulation approach has been capable to simulate large self-assembly patterns of organic molecules over reconstructed surfaces (which have periodicities over large distances) due to the large number of surface atoms and adsorbing molecules involved. Using a novel simulation technique, we report here large scale simulations of the self-assembly patterns of an organic molecule (DIP) over different reconstructions of the Au(111) surface.

Interplay between fullerene surface coverage and contact selectivity of cathode interfaces in organic solar cells.

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes.

Moving through the phase diagram: morphology formation in solution cast polymer-fullerene blend films for organic solar cells.

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry.

Growth mechanisms of phthalocyanine nanowires induced by Au nanoparticle templates.

We combine X-ray reflectivity and scanning electron microscopy measurements to investigate the mechanisms involved in the growth of vertical arrays of phthalocyanine nanowires directed by templates of Au nanoparticles. The study has been carried out for H(16)CuPc at different substrate temperatures. It is shown that three organic morphologies evolve during the growth: 1D nanostructures on top of the Au nanoparticles, a multilayer film on the substrate and a layer wetting the gold nanoparticles.

Tunable symmetry and periodicity in binary supramolecular nanostructures.

We present a route to change the "compositional" order of highly crystalline binary layers comprising diindenoperylene and copper-phthalocyanines from two- to one-dimensional periodicity. This is achieved by exchanging fluorine with hydrogen atoms in the phthalocyanines, thereby reducing the C-F···H-C interactions and allowing the interplay of long-range electrostatic interactions in mesoscopic phases. Linear patterns are thus obtained, whose periodicity can be additionally tuned by an appropriate stoichiometry of the components.