Correction to "Performance Improvement with an Ultrathin p-Type Interfacial Layer in n-Type Vertical Organic Field-Effect Transistors Based on Reduced Graphene Oxide Electrode".

[This corrects the article DOI: 10.1021/acsomega.2c02085.].

Synthesis and Characterization of Dibenzothieno[a,f]pentalenes Enabling Large Antiaromaticity and Moderate Open-Shell Character through a Small Energy Barrier for Bond-Shift Valence Tautomerization.

Experimental and theoretical rationalization of bond-shift valence tautomerization, characterized by double-well potential surfaces, is one of the most challenging topics of study among the rich electronic properties of antiaromatic molecules. Although the pseudo-Jahn-Teller effect (PJTE) is an essential effect to provide attractive characteristics of 4nπ systems, an understanding of the structure-property relationship derived from the PJTE for planar 4nπ electron systems is still in its infancy. Herein, we describe the synthesis and characterization of two regioisomers of the thiophene-fused diareno[a,f]pentalenes 6 and 7. The magnetic and optoelectronic properties characterize these sulfur-doped diareno[a,f]pentalenes as open-shell antiaromatic molecules, in sharp contrast to the closed-shell antiaromatic systems of 3 and 5, in which these main cores consist of the same number of π electrons as 6 and 7. Notably, thiophene-fused 6b and 7b showed pronounced antiaromaticity, the strongest among the previous systems, as well as moderate open-shell characteristics. Our experimental and theoretical investigations concluded that these properties of 6b and 7b are derived from the small energy barrier Ea‡ for the bond-shift valence tautomerization. The energy profile of the single crystal of 6b showed the temperature-dependent structural variations assigned to the dynamic mutual exchange between the two Cs-symmetric structures, which was also supported by changes in the chemical shifts of variable-temperature 1H NMR spectra in the solution phase. Both experimental and computational results revealed the importance of introducing heteroaromatic rings into 4nπ systems for controlling the PJTE and manifesting the antiaromatic and open-shell natures originating from the high-symmetric structure. The findings of this study advance the understanding of antiaromaticity characterized by the PJTE by controlling the energy barrier for bond-shift valence tautomerizations, potentially leading to the rational design of optoelectronic devices based on novel antiaromatic molecules possessing the strong contributions of their high-symmetric geometries.

Nonpolymer Organic Solar Cells: Microscopic Phonon Control to Suppress Nonradiative Voltage Loss via Charge-Separated State.

Recent remarkable developments on nonfullerene solar cells have reached a photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this regard, understanding the impact of small donor molecules on nonpolymer solar cells is essential. Here, we systematically investigated mechanisms of solar cell performance using diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP) conjugates of C4-DPP-H2BP and C4-DPP-ZnBP, where C4 represents the butyl group substituted at the DPP unit as small p-type molecules, while an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester is employed. We clarified the microscopic origins of the photocarrier caused by phonon-assisted one-dimensional (1D) electron-hole dissociations at the donor-acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking. This ensures carrier transport through stacking molecular conformations to suppress nonradiative voltage loss capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. We show that, while disordered lattice motions by the π-π stackings via zinc ligation are essential to enhance the entropy for charge dissociations at the interface, too much ordered crystallinity causes the backscattering phonon to reduce the open-circuit voltage by geminate charge-recombination.

Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks.

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node-linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations.

Exploration of Alkyl Group Effects on the Molecular Packing of 5,15-Disubstituted Tetrabenzoporphyrins toward Efficient Charge-Carrier Transport.

The high design flexibility of organic semiconductors should lead to diverse and complex electronic functions. However, currently available high-performance organic semiconductors are limited in variety; most of p-type materials are based on thienoacenes or related one-dimensionally (1D) extended π-conjugated systems. In an effort to expand the diversity of organic semiconductors, we are working on the development of tetrabenzoporphyrin (BP) derivatives as active-layer components of organic electronic devices. BP is characterized by its large, rigid two-dimensionally (2D) extended π-framework with high light absorptivity and therefore is promising as a core building unit of organic semiconductors for optoelectronic applications. Herein, we demonstrate that BP derivatives can afford field-effect hole mobilities of >4 cm2 V-1 s-1 upon careful tuning of substituents. Comparative analysis of a series of 5,15-bis(n-alkyldimethylsilylethynyl)tetrabenzoporphyrins reveals that linear alkyl substituents disrupt the π-π stacking of BP cores, unlike the widely observed "fastener effect" for 1D extended π-systems. The n-octyl and n-dodecyl groups have the best balance between high solution processability and minimal π-π stacking disruption, leading to superior hole mobilities in solution-processed thin films. The resulting thin films show high thermal stability wherein the field-effect hole mobility stays above 1 cm2 V-1 s-1 even after heating at 160 °C in air, reflecting the tight packing of large BP units. These findings will serve as a good basis for extracting the full potential of 2D extended π-frameworks and thus for increasing the structural or functional diversities of high-performance organic semiconductors.

Performance Improvement with an Ultrathin p-Type Interfacial Layer in n-Type Vertical Organic Field-Effect Transistors Based on Reduced Graphene Oxide Electrode.

Vertical organic field-effect transistors (VOFETs) with a large current on/off ratio and easy fabrication process are highly desirable for future organic electronics. In this paper, we proposed an ultrathin p-type copper (II) phthalocyanine (CuPc) interfacial layer in reduced graphene oxide (rGO)-based VOFETs. The CuPc interfacial layer was sandwiched between the rGO electrode and the N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) organic layer. The introduced CuPc interfacial layer not only decreased the off-current density of the device but also slightly enhanced the on-current density. The threshold voltage of the device was also effectively improved and stabilized at around 0 V. The obtained device exhibited a current on/off ratio exceeding 106, which is the largest value reported for rGO-based VOFETs. The vertical electron mobility of the PTCDI-C8 layer estimated by the space-charge-limited current technique was 1.14 × 10-3 cm2/(V s). However, it was not the main limiting factor for the current density in this device. We totally fabricated 48 devices, and more than 75% could work. Besides, the device was stable with little performance degradation after 1 month. The use of low-cost, solution-processable rGO as work-function-tunable electrode and the application of an ultrathin CuPc interfacial layer in VOFETs may open up opportunities for future organic electronics.

A Windmill-Shaped Molecule with Anthryl Blades to Form Smooth Hole-Transport Layers via a Photoprecursor Approach.

Preparation of high-performance organic semiconductor devices requires precise control over the active-layer structure. To this end, we are working on the controlled deposition of small-molecule semiconductors through a photoprecursor approach wherein a soluble precursor compound is processed into a thin-film form and then converted to a target semiconductor by light irradiation. This approach can be applied to layer-by-layer solution deposition, enabling the preparation of p-i-n-type photovoltaic active layers by wet processing. However, molecular design principles are yet to be established toward obtaining desirable thin-film morphology via this unconventional method. Herein, we evaluate a new windmill-shaped molecule with anthryl blades, 1,3,5-tris(5-(anthracen-2-yl)thiophen-2-yl)benzene, which is designed to deposit via the photoprecursor approach for use as the p-sublayer in p-i-n-type organic photovoltaic devices (OPVs). The new compound is superior to the corresponding precedent p-sublayer materials in terms of forming smooth and homogeneous films, thereby leading to improved performance of p-i-n OPVs. Overall, this work demonstrates the effectiveness of the windmill-type architecture in preparing high-quality semiconducting thin films through the photoprecursor approach.

High Vertical Carrier Mobilities of Organic Semiconductors Due to a Deposited Laid-Down Herringbone Structure Induced by a Reduced Graphene Oxide Template.

High vertical carrier mobilities in organic semiconductor films are a challenging issue for fundamentally improving the performance of vertical devices. To achieve improvement in the vertical direction, a reduced graphene oxide (rGO) template is used with pentacene and DNTT having a herringbone structure enabling two-dimensional (2D) transport in comparison with CuPc having a slipped-stack structure. A thin-film structure and the optoelectrical properties of the oriented films are investigated with respect to molecular structures and packing modes. The rGO template induces a "laid-down" herringbone structure for pentacene and DNTT with a face-on orientation. Our results reveal that intermolecular dispersion energy is an additional important factor to form face-on states of molecules and influences face-on ratios in the films on rGO. Vertical charge mobilities of the films are significantly enhanced by the rGO template. Particularly, the DNTT film with a laid-down herringbone structure produces a vertical mobility as high as 0.27 cm2 V-1 s-1, one of the highest values for ordinary thin films with several hundred nanometer thickness. These findings suggest that 2D transport is advantageous for vertical carrier transport also.

Orbital-Energy Modulation of Tetrabenzoporphyrin-Derived Non-Fullerene Acceptors for Improved Open-Circuit Voltage in Organic Solar Cells.

Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage (VOC). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels-Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high VOC of up to 0.94 V when used as a donor or can even work as a new class of nonfullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.

Open-circuit-voltage shift of over 0.5 V in organic photovoltaic cells induced by a minor structural difference in alkyl substituents.

The recent surge in the efficiency of organic photovoltaic devices (OPVs) largely hinges on the reduction of energy loss (E loss) that leads to improvements in open-circuit voltage (V OC). However, there are still many unclarified factors regarding the relationship between the molecular structure and V OC, hampering the establishment of widely applicable, effective principles for the design of active-layer materials. In this contribution, we examine the origin of the large V OC shifts induced by minor structural differences in end-alkyl substituents on a series of anthracene-based p-type compounds. The examined p-type compounds are all highly crystalline, thereby enabling detailed investigation of the molecular packing with X-ray diffraction analysis. At the same time, they are strongly aggregating and hardly soluble; therefore, they are deposited with the aid of a photoprecursor approach which we have been employing for controlled deposition of insoluble acene-based organic semiconductors. The resultant OPVs afford the highest V OC of 0.966 V when the end-alkyl groups are 2-ethylbutyl, and the lowest of 0.419 V when n-butyl is used in replacement of 2-ethylbutyl. X-ray diffraction analyses and density-functional-theory calculations indicate a critical impact of the non-slipped herringbone arrangement on the observed large loss in V OC. This type of molecular arrangement is prohibited when branched alkyl chains are introduced at the ends of linear π-systems, which we consider an important factor contributing to the relatively high V OC obtained with the 2-ethylbutyl derivative. These results may serve as a basis of useful molecular-design rules to avoid unnecessary losses in V OC.

Synthesis of Anthracene Derivatives with Azaacene-Containing Iptycene Wings and the Utilization as a Dopant for Solution-Processed Organic Light-Emitting Diodes.

Substituted acene derivatives are regarded as promising materials for organic electronic devices such as organic light-emitting diodes (OLEDs). In particular, anthracene derivatives are known to exhibit good fluorescence property, with the air stability and solubility in common organic solvents expected to give advantages for solution-processed device fabrication. In this study, a series of bistriisopropylsilyl(TIPS)ethynyl anthracene derivatives with azaacene-containing iptycene wings have been synthesized by using condensation reactions. Effects of size of azaacenes on optical properties and packing structures were investigated. UV/Vis absorption and fluorescence spectra indicate that the π-elongation of iptycene units has small effects on the overall π-system, which is also supported by electrochemical measurements. Secondly, single-crystal X-ray analysis implies that the molecules likely have interactions with the iptycene units of adjacent molecules, while the iptycene wings and TIPSethynyl groups can prevent the central anthracene unit from undesirable non-radiative energy loss. Finally, the most emissive derivative was used as a dopant for solution-processed OLEDs, showing obvious electroluminescence with a luminance of over 920 cd m-2 .

A remarkably strained cyclopyrenylene trimer that undergoes metal-free direct oxygen insertion into the biaryl C-C σ-bond.

A remarkably strained cyclopyrenylene trimer CP3 was synthesized and it underwent the first biaryl C-C σ-bond cleavage by direct oxygen insertion without the aid of any metal agents. A priori highly strained CP3 exhibits the longest wavelength emission among all pyrene-based fluorophores due to the intensive electronic interactions between pyrenes. The color of the emission drastically changes from orange to light blue upon oxidation. Theoretical studies revealed that the release of ring strain reasonably drives the reaction between two CP3 molecules and O2. This strain-induced transformation could be also applied for sulfur atom insertion into a biaryl σ-bond.

Torsional chirality generation based on cyclic oligomers constructed from an odd number of pyrenes.

A Ni(0)-mediated coupling reaction of a 1,8-dibromopyrene gave a series of cyclic pyrene oligomers CPn up to nonadecamer. CP5 and CP7 represent a unique class of asymmetric compounds with no stereogenic chiral centers in terms of their global chirality, which are achieved because of an odd number of pyrenes.

Photoconversion of 6,13-α-diketopentacene single crystals exhibiting light intensity-dependent morphological change.

Recently, we revealed that 6,13-dihydro-6,13-ethanopentacene-15,16-dione (PDK) could be quantitatively photoconverted into pentacene even in the crystal phase, accompanied by the destruction of the crystals. In this work, we investigated the relationship between the photoinduced morphological changes and the light intensity for the photoconversion at a single micrometre-sized crystal level. Photoirradiation with a strong intensity (over 100 kW cm-2) resulted in hole formation in a single crystal. When medium intensity (0.5-100 kW cm-2) was irradiated, destruction including separation and jumping of the crystal was observed. Absorption spectrum measurement of the single crystal revealed that when almost same number of pentacene was generated, the destruction was induced by the generated strain within crystal due to the stacking mismatch between the different molecules. Upon photoirradiation with a low intensity (below 0.5 kW cm-2), protruding pillar objects were observed on the crystal surface. This formation is a result of the surface movement of molecules through the relaxation of strain. Our results provide important insight into stimuli-responsive crystal materials and could contribute to the generation and application of remotely controllable smart materials.

A photochemical layer-by-layer solution process for preparing organic semiconducting thin films having the right material at the right place.

The synergistic action of properly integrated semiconducting materials can bring about sophisticated electronic processes and functions. However, it is often a great challenge to achieve optimal performance in organic devices because of the limited control over the distribution of different materials in active layers. Here, we employ a unique photoreaction-based layer-by-layer solution process for preparing ternary organic photovoltaic layers. This process is applicable to a variety of compounds from wide-band-gap small molecules to narrow-band-gap π-extended systems, and enables the preparation of multicomponent organic semiconducting thin films having the right compound at the right place. The resulting ternary photovoltaic devices afford high internal quantum efficiencies, leading to an approximately two times higher power-conversion efficiency as compared to the corresponding binary bulk-heterojunction system. This work opens up new possibilities in designing materials and active layers for solution-processed organic electronic devices.

An Ethynylene-Bridged Pentacene Dimer: Two-Step Synthesis and Charge-Transport Properties.

A rigid and planar ethynylene-bridged pentacene dimer (PenD) was synthesized from pentacenequinone in two steps, skipping the conventional stepwise approach. A brickwork motif in the single crystal shows two-dimensionally extended electronic interaction in the solid state. Highly crystalline dip-coated films exhibited average hole mobility of 0.24 cm2 V-1 s-1 , comparable to that of the single-crystal organic field-effect transistors. This discovery and understanding of the reaction for the facile synthesis of ethynylene-bridged π-conjugated systems enables to the synthesis of a wide range of organic semiconducting materials.

Semiconducting π-Extended Tetrathiafulvalene Derivatives.

Tetrathiafulvalene (TTF) has been one of the most studied compounds, since the discovery of electrical conductivity as a charge-transfer complex in combination with tetracyano-p-quinodimethane (TCNQ) in 1973. In 2004, TTF was realized in a new light as an innate semiconductor material, as well as conductor and superconductor material. Because of the ready modification of its core skeleton and better solubility than that of acene compounds, many TTF derivatives have been reported to attain better charge-carrier mobility for field-effect transistor devices. Considering the high charge-carrier mobility of acene and TTF derivatives, annulation of the acene and TTF moieties is expected to improve the charge-carrier mobilities. This Minireview focuses on the syntheses, crystal structures, and electronic properties of state-of-the-art π-extended TTF derivatives, based on the history of the development of TTF derivatives. The relationship between the packing structure and charge-carrier mobilities is also discussed.

1,3-Phenylene-bridged naphthalene wheels synthesized by one-pot Suzuki-Miyaura coupling and the complex of the hexamer with C60.

A large 1,3-phenylene-bridged hexameric naphthalene wheel N6 and a heptameric wheel N7 were synthesized simply by Suzuki-Miyaura coupling via one-pot reaction from monomers. We could control the distribution of N6 and N7via the reaction conditions. The hexameric wheel structure was revealed by X-ray diffraction analysis. The wheel N6 exhibited C60 encapsulation ability in the solid state, which was also confirmed by single crystal X-ray analysis.

Improvement in interlayer structure of p-i-n-type organic solar cells with the use of fullerene-linked tetrabenzoporphyrin as additive.

The additive effect on small-molecule-based p-i-n-type devices has been little investigated so far. We focus on the improvement of the miscibility of tetrabenzoporphyrin (BP) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blend film by addition of fullerene-linked tetrabenzoporphyrin (BP-C60) as an additive to the interlayer (i-layer). BP is one of the most promising p-type organic semiconductors, and BP films can be prepared readily by heating as-cast films of the precursor (a bicyclo[2.2.2]octadiene-fused porphyrin; CP), that results in changes from amorphous CP films to polycrystalline BP films. Because of the high crystallinity of BP, large BP grains on the scale of tens to hundreds of nanometers are generated in blend films of BP and PC61BM during film fabrication. We found that the addition of BP-C60 as an additive (3, 5, 7, and 10 wt%) to the i-layer composed of BP and PC61BM improves the miscibility of BP and PC61BM. The power conversion efficiency of p-i-n-type organic solar cells consisting of a blend film of BP and PC61BM (i-layer) sandwiched by BP (p-layer) and PC61BM (n-layer) improved by up to 50% as compared to that of a control device after the addition of BP-C60 to the i-layer. The film morphology was investigated using atomic force microscopy, fluorescence microspectroscopy, two-dimensional grazing-incident wide-angle X-ray diffraction measurements, and scanning electron microscopy. Interacting with both BP and PC61BM, the addition of BP-C60 led to changes in the grain size as well as an increase in the size of the BP/PC61BM interface and hence effective charge separation in the p-i-n device. This morphological improvement is attributable to the ability of BP-C60, which exhibits the characteristics of both BP and C60, to promote the compatibility of BP and PC61BM. This study is a significant step towards the development of high-performance p-i-n-type solar cells and should pave the way for the fabrication of high-performance bulk-heterojunction layers in solution-processed organic photovoltaic devices.

Dinaphthotetrathiafulvalene Bisimides: A New Member of the Family of π-Extended TTF Stable p-Type Semiconductors.

Air-stable organic semiconductors based on tetrathiafuluvalene (TTF) were developed by synthesising a series of dinaphthotetrathiafulvalene bisimides (DNTTF-Im) using electron-donating TTF, π-extended naphthalene, and electron-withdrawing imide. Electron-spin-resonance spectroscopy and X-ray single-crystal structure analysis of aryl-substituted DNTTF-Im radical cations confirmed that localisation of the spin resides on the electron-donating TTF moiety. The organic field-effect transistor properties derived from the use of highly crystalline n-butyl (C4) and n-hexyl(C6)-substituted DNTTF-Im were assessed. The hole carrier mobility of C6-DNTTF-Im was improved from 3.7×10-3  cm2 V-1 s-1 to 0.30 cm2 V-1 s-1 in ambient conditions. This is attributed to the raise of the substrate temperature from 25 °C to 200 °C during sublimation. The XRD and microscopy analysis suggested that increasing the substrate temperature accelerates the end-on packing resulting in larger grains suitable for hole charge transport parallel to the substrate.