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Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.

Spatial distribution of triplet excitons near the organic semiconductor interface is clarified. The result indicates that 90% of the triplet excitons are confined to less than 10 nm from the interface, where photoelectric conversion and charge recombination occur.

Li@PCBM was synthesized. It exists in a monomer-dimer equilibrium in solution but as a monomer in the PCBM matrix. The fully dispersed Li@PCBM served as an n-dopant for the surrounding empty PCBM, raising the Fermi level by 0.13 eV.

The energy of photons, that is, the wavelength of light, can be upgraded through interactions with materials in a process called photon upconversion1. Although upconversion in organic solids is important for various applications, such as photovoltaics and bioimaging, conventional upconversion systems, based on intersystem crossing (ISC), suffer from low efficiency2–6. Here we report a novel upconversion system with heterojunctions of organic semiconductors. The upconversion occurs through charge separation and recombination, which mediate charge transfer states at the interface. This process can efficiently convert the incident photons to triplets without relying on ISC, which is typically facilitated by the heavy-atom effect1. As a result, a solid-state upconversion system is achieved with an external efficiency that is two orders of magnitude higher than those demonstrated by conventional systems6. Using this result, efficient upconversion, from near-infrared to visible light, can be realized on flexible organic thin films under a weak light-emitting-diode-induced excitation, observable by naked eyes. Highly efficient upconversion of light by organic semiconductor heterojunction interfaces is demonstrated. This process is enabled by charge separation- and recombination-mediated charge transfer states at the interface.

Single-crystalline organic semiconductors exhibiting band transport have opened new possibilities for the utilization of efficient charge carrier conduction in organic electronic devices. The epitaxial growth of molecular materials is a promising route for the realization of well-crystallized organic semiconductor p–n junctions for optoelectronic applications enhanced by the improved charge carrier mobility. In this study, the formation of a high-quality crystalline interface upon “quasi-homoepitaxial” growth of bis(trifluoromethyl)dimethylrubrene (fmRub) on the single-crystal surface of rubrene was revealed by using out-of-plane and grazing-incidence X-ray diffraction techniques. Ultraviolet photoelectron spectroscopy results indicated abrupt electronic energy levels and the occurrence of band bending across this quasi-homoepitaxial interface. This study verifies that the minimization of the lattice mismatch enhances the crystalline qualities at the heterojunctions even for van der Waals molecular condensates, potentially opening an untested route for the realization of high-mobility organic semiconductor optoelectronics.

Curved perylene diimides fused with seven-membered rings have been synthesized using a regioselective bay-functionalization method and Pd-catalyzed intramolecular C-H/C-Br coupling reaction. X-Ray analysis and temperature-dependent NMR spectroscopy revealed the curved molecular structure with a certain degree of conformational flexibility. The curved and expanded π-conjugation altered the electronic properties while retaining the intrinsic properties of the parent perylene diimide. Despite the absence of solubilizing N-substituents, the curved perylene diimides showed sufficient solubility for application in solution-processed organic photovoltaic devices. The devices showed superior performance with a power conversion efficiency of up to 2.76% due to suppressed charge recombination. Our detailed investigations suggest that the introduction of a curved structure enables the removal of the bulky N-substituents, which is an effective way to achieve a thin-film morphology suitable for photoelectric conversion.

Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) is a p-type organic semiconductor that exhibits high charge-carrier mobility and atmospheric stability. Although it has been proposed that the ...

The development of an efficient synthetic protocol for multiply bay-functionalized perylenes and application of these products to photovoltaics are reported. Tetrabenzyl 1-(4-tert-butylphenoxy)perylene-3,4,9,10-tetracarboxylate underwent a regioselective bromination at the 7-position followed by a further bromination at the 12-position to provide a 7,12-dibromide in high 84% yield. This compound was transformed into a tetrasubstituted C2h -symmetric dibromide by a controlled monoetherification and a final bromination. The synthetic versatility of the new brominated precursors was demonstrated by their application to cross-coupling reactions, cyanation, and derivatization to the corresponding perylene diimide (PDI). Owing to the high solubility and the raised LUMO level, the obtained tetrasubstituted PDI showed significant advantages as an acceptor material for photovoltaic applications, exhibiting a remarkably high open-circuit voltage (VOC ) reaching 1.00 V. The observed high VOC could be understood by reduction of energy offset for charge separation and non-radiative recombination loss.

The influence of N-substituents on the photovoltaic properties of singly bay-linked perylene diimides (diPDIs) was systematically investigated to understand the aromatic-aliphatic balance, which is beneficial for achieving high device performance in organic photovoltaic (OPV) systems. The synthesis of various N-substituted diPDIs was successfully achieved using a newly developed one-step procedure, resulting in sufficiently high yields. Detailed investigations of seven variants of diPDIs demonstrated that the primary alkyl substituents, particularly the 2-ethylhexyl group, induce the self-organized growth of thin films with high crystallinity. This is beneficial for enhancing the device performance of bulk heterojunction (BHJ) systems. The results presented herein reveal the important roles of alkyl side chains as hydrophobic solubilizing auxiliaries or primary determinants in the control of the active layer nanomorphology. This offers a valuable guideline that is essential for developing high-performance organic semiconductor materials for future practical applications.

Charge transfer (CT) state is a key intermediate to understand recombination processes in organic solar cells (OSCs). In this study, we measured the recombination emission from the CT state under different applied voltages in OSCs and a photocurrent density flowing on the circuit simultaneously. We proposed a “photoluminescence (PL)–voltage (V) plot” that is the voltage dependence of PL intensity of the CT state. The PL–V plot includes information only from the CT state recombination at the donor/acceptor interface and is complementary to the “current density (J)–V plot” that is the most important information for evaluating OSCs. The results demonstrated that the fill factor (FF) of the PL–V plot is higher than that of the J–V plot, predicting the ideal FF of the device. Our result demonstrated that the simultaneous measurement of photocurrent and recombination emission could be a strong tool for evaluating photoconversion characteristics in OSCs.

Reducing operating voltage is the remaining frontier for organic light-emitting diodes (OLEDs) because their quantum efficiency (QE) of electroluminescence has been already maximized. Herein, we report an efficient OLED in which a blight emission equivalent to a luminance of a display is achieved by only a 1.5 V battery. The OLED is based on upconversion (UC) emission utilizing triplet-triplet annihilation occurring near donor/acceptor (D/A) interface. We found that a character of a charge transfer state that is key intermediate for the UC emission could be controlled by D/A interfacial interaction. As a result, parasitic loss processes for UC were greatly suppressed from over 90% to about 10%, and two order of magnitude higher QE than the previous UC-OLED was achieved. Our result demonstrated that the efficient UC could be realized by the management of the energy transfer steps at the D/A interface and utilizing UC emission can be one of the possible candidate for efficient OLED with extremely low driving voltage.

In response to the increased mobile data traffic, there is a growing need for more low-loss RF band filters with steep frequency characteristics, and high-quality ( $Q$ )-factor and low-temperature coefficient of frequency (TCF) resonators are required to achieve this. We previously reported that for a surface acoustic wave (SAW) resonator on a three-layer structure, which is composed of a thin LiTaO3 (LT) plate whose orientation is 50° rotated YX propagation, SiO2 layer, and AlN layer on a Si substrate, a Q-factor several times higher than that of an SAW resonator on a standard 42° rotated YX propagation LiTaO3 (42YX-LT) substrate could be obtained. In this study, we investigated this layer structure and found that a two-layer structure, in which the AlN layer is removed, achieves a high $Q$ -factor. Numerical analyses using a finite element method showed that the acoustic wave energy can be confined to the surface of the two-layer substrate, and the TCF and electromechanical coupling coefficient ( $k^{2}$ ) were improved by optimizing the thickness of each layer. We fabricated and evaluated prototype one-port resonators with the two-layer structure and the standard 42YX-LT SAW substrate with resonant frequencies from 0.95 to 3.6 GHz. An improvement of the Q-factor of 3 to 4 times compared with that of the resonator with standard 42YX-LT substrate was observed for the two-layer structure, which means that a reduction of complexity of the layer structure could be obtained without performance loss. The two-layer structure was applied to a 2.4-GHz band Wi-Fi filter to achieve high performances such as low-loss, better steepness, and high attenuation.

Formation of pn homojunction in a single semiconductor material through doping is key for photocurrent generation in inorganic solar cells (SCs). In contrast, a donor/acceptor heterojunction is generally utilized in organic SCs in order to split coulombically bound exciton forming in organic semiconductors. Here, an organic pn homojunction SC showing high internal quantum efficiency of about 30% is reported. pn Homojunction was created by co-deposition of p or n type dopants and an organic semiconductor. An increase of the photocurrent density by more than 8 times, from the undoped device to pn homojunction device with 5% doping, was observed. The increase of photocurrent due to charge separation was assisted by the built-in potential near the pn homojunction interface that was controlled by doping concentration. The result demonstrated that the pn homojunction interface in a single organic semiconductor layer formed by doping can achieve efficient charge separation and provide a suitable alternative to the donor/acceptor interface that has been considered necessary for photoconversion in organic SCs.

Blended junctions are indispensable for organic solar cells; however, the fabrication of electron and hole transport routes in blended cells remains quite challenging. Herein, a lateral alternating multilayered junction using a high-mobility organic semiconductor is proposed and demonstrated. A total of 93% of the photogenerated electrons and holes are laterally collected over a long distance (0.14 mm). The exciton-collection efficiency reaches 75% in a lateral alternating multilayered junction with a layer thickness of 10 nm. A lateral organic alternating multilayered junction that completely collects both excitons and carriers can be an alternative blended junction for organic solar cells.

Doped rubrene single-crystal photovoltaic cells were fabricated. Whole photocurrent generated in the pn-homojunction with the macroscopic area of 2 mm × 1 mm was confirmed to be collected through the p-doped homoepitaxial layer. A single-crystal substrate collected excitons to the pn-homojunction with the collection efficiency reaching 46%, owing to the long exciton diffusion length of 2.7 μm.

In the field of organic solar cells, lateral junction is a new concept. Moreover, the photovoltaic behavior is firstly observed for surprising long organic lateral junctions reaching 1.8 cm and the new operation mechanism dominated by trap-assisted recombination is proposed.

In the field of organic solar cells, lateral junction is a new concept. Moreover, the photovoltaic behavior is firstly observed for surprising long organic lateral junctions reaching 1.8 cm and the new operation mechanism dominated by trap-assisted recombination is proposed.

Controlling the pn-type behavior of a semiconductor such as silicon by adding an extremely small quantity of an impurity (doping) has been a central part of inorganic semiconductor electronics since the 20th century. Recent progress in the doping of organic semiconductors strongly suggests the advent of a new era of doped organic semiconductors. In this chapter, the principles and effects of doping at the level of parts per million (ppm) in organic semiconductor films and in single crystals are summarized. This chapter includes descriptions of complete pn-control, doping sensitization, ppm-doping effects in organic single crystals measured by the Hall effect. The Wannier excitonic doping of organic single crystals possessing band conduction and the defect science of organic single crystals related to carrier trapping and scattering are introduced as a new scientific field. In addition, organic junctions made by ppm-doping technology are summarized. The pn-homojunctions in the single films as well as the D/A co-deposited films and the ohmic junctions at organic/metal and organic/organic interfaces, which are indispensable to fabricate novel organic solar cells, are described.

In the field of organic solar cells, nanostructure fabrication with electron and hole transport routes in the blended donor (D) and acceptor (A) molecules based on percolation remains a problem. The author believes that to overcome this difficulty, novel methods for the fabrication of intentionally designed nanostructures that do not rely on random luck assisted by percolation should be introduced. Based on this consideration, we have proposed several new principles of donor/acceptor junctions that can act as alternative blended junctions. First, we proposed a vertical superlattice junction (vertical alternating multilayered junction), by utilizing cross sections of vacuum-deposited multilayers with angstrom-order precision. We confirmed that the vertical superlattice has the ability to collect both excitons and carriers, although the area of the vertical superlattice junction is very small. We also proposed an advanced lateral multilayered junction using high-mobility organic semiconductors. An essential point is that the photogenerated holes and electrons are laterally transported and extracted to the respective electrodes. A total of 93% of the photogenerated electrons and holes were laterally collected over a millimeter-scale distance of 0.14 mm. The exciton-collection efficiency reached 75% in the lateral junction with a layer thickness of 10 nm. The lateral junctions have the ability to collect both excitons and carriers, have sufficient cell area, and can be regarded as an alternative blended junction for organic solar cells.

This chapter provides a detailed summary of the invention of the blended junction. The author’s work on the blended junction was motivated by the surprising report by C. W. Tang regarding his two-layer organic semiconductor cell. The author then examined the photovoltaic behaviors as a function of interface fabrication and found that the molecular contact between perylene (PTC) and phthalocyanine (Pc) pigments is a key for the excellent photocarrier generation ability. This insight guided the author to develop the blended junctions with a vast number of perylene (PTC):phthalocyanine (Pc) molecular contacts that doubled the photocarrier generation efficiency. The three-layer organic solar cell was the first report of the blended junction. This report already contained the predictions for organic solar cells of the future, i.e., (i) the p-i-n energetic structure fabricated by doping, (ii) percolation as an inevitable issue of the blended junction, (iii) efficient carrier photogeneration via an exciplex (PTC−–Pc+)* and donor–acceptor sensitization that are related to the charge–transfer state. Blended junctions have become indispensable for today’s organic solar cells.

In this chapter, the proposals for future organic solar cells are discussed: the exciton dissociation using single bipolar band-conductive organic semiconductors, doped organic single-crystal substrates with a long exciton diffusion length, and lateral cells showing the efficiency beyond SQ-limit. A critical advantage of organic semiconductors is their suitability for a fine division of the solar spectrum. Suppression of non-radiative recombination dissipated to molecular vibration and trap-assisted non-radiative recombination is a cutting-edge issue for organic solar cells. Voc-loss due to non-radiative recombination would be suppressed to an extent close to the SQ-limit. The spatial and energetic nature of carrier traps should be clarified to suppress the trap-induced recombination. The research on organic solar cells has advanced gradually, on par with that of inorganic solar cells.

In 2018, solar cells supplied 2% of the global electricity demand. This must be increased over 20%; therefore, organic solar cells with inherent cost-reducing abilities are indispensable. In this chapter, the basic principles of modern organic solar cells are summarized. Exciton, donor/acceptor sensitization, exciton diffusion, blended junction, designed route formation, and \(\pi{-}\pi\) stacking orientation are discussed regarding the photocurrent, while HOMO–LUMO gap tuning and the non-radiative recombination loss are examined regarding the photovoltage. The tandem cells as well as junction fabrication by doping alone are considered for cell design. Historically, blend and tandem strategies have continuously enhanced efficiencies. The efficiency of organic solar cells, which continues to progressively increase, has risen steadily at the rate of 1% per year and has reached 17.3% in 2020. Organic solar cells, with their fascinating advantages, have sufficient potential to engender the next-generation solar cells.

Perylene has had a tremendous impact in the history of material research for the molecular semiconductors. Among numerous derivatives of this polyaromatic hydrocarbon, perylene diimide (PDI) represents a promising class of organic materials envisioned as non-fullerene acceptors (NFAs) for the practical organic photovoltaic (OPV) applications due to their enhanced photo- and thermal stability and remarkably high electron affinity, some of which realize band-like transport properties. The present review guides some of the representative achievements in the development of rationally designed PDI systems, highlighting synthetic methodologies based on bay-functionalization strategies for creating well-designed molecular nanostructures and structure-performance relationship of perylene-based small molecular acceptors (SMAs) for the photovoltaic outcomes.

In this study, the operation of donor/acceptor photovoltaic cells fabricated on homoepitaxially grown p-doped rubrene single-crystal substrates is demonstrated. The photocurrent density is dominated by the sheet conductivity (&sigma, □) of the p-type single-crystal layer doped to 100 ppm with an iron chloride (Fe2Cl6) acceptor. A 65 mm thick p-type rubrene single-crystal substrate is expected to be required for a photocurrent density of 20 mA&middot, cm&minus, 2. An entire bulk doping technique for rubrene single crystals is indispensable for the fabrication of practical organic single-crystal solar cells.