Search

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.

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.

Realizing efficient near-infrared to visible photon upconversion in the solid state is pivotal for commercial applications in various fields. We previously reported a solid-state upconversion device which imitated the photovoltaic conversion mechanisms of organic solar cells. This leads to a significant improvement of up to 2.3% in the external quantum efficiency, which is two orders of magnitude higher than that of conventional devices. Here, we investigate the upconversion mechanism of this device. We examine exciton and charge dynamics using transient absorption spectroscopy and find that approximately 67% of incident photons are utilized owing to fast singlet exciton diffusion in the nonfullerene acceptor layer. Strikingly, triplet excitons are accumulated near the donor/acceptor interface, enabling accelerated triplet–triplet annihilation by a factor of more than 10.

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 ...

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.

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.

Clarifying critical differences in free charge generation and recombination processes between inorganic and organic semiconductors is important for developing efficient organic photoconversion devices such as solar cells (SCs) and photodetector. In this study, we analyzed the dependence of doping concentration on the photoconversion process at the organic pn-homojunction interface in a single organic semiconductor using the temperature dependence of J–V characteristics and energy structure measurements. Even though the organic pn-homojunction SC devices were fabricated using a single host material and the doping technique resembling an inorganic pn-homojunction, the charge generation and recombination mechanisms are similar to that of conventional donor/acceptor (D/A) type organic SCs; that is, the charge separation happens from localized exciton and charge transfer (CT) state being separated by the energy offset between adjacent molecules, and the recombination happens from localized charge carrier at two adjacent molecules. The determining factor for photoconversion processes is the localized nature of charges in organic semiconductors. The results demonstrated that controlling the delocalization of the charges is important to realize efficient organic photoconversion devices.

C60:Li@C60 hybrid n-type semiconducting films were first fabricated. The Fermi level of 1% Li@C60-added C60 films was determined to be −4.52 eV, which was 0.12 eV higher than that of pristine C60 films. A fraction of Li@C60 is distributed uniformly within the C60 film. Its application in PSCs was demonstrated, in which the addition of Li@C60 into a C60 film improved the device performance.

Afterglow materials, which are also known as persistent emitters, have a long emission lifetime that varies from seconds to tens of minutes. These materials have shown potential applications in photoenergy conservation devices such as evacuation signage and emitting lines in a motorway surface. In this study, we synthesized afterglow materials, Eu2+, Dy3+@SrAl2O4, wherein the emitting agents (Eu2+ and Dy3+) are co-doped in a SrAl2O4 framework, which was confirmed by X-ray diffraction. The liquid-state reaction process plays a crucial role in the synthesis of materials. When the surfactant-template method is used for synthesis, reverse micelles, which control the morphology of afterglow product, hold the raw reactant materials, thereby yielding characteristic morphologies of approximately 40–80 nm. The reverse micelles have been detected by small-angle X-ray scattering in this study. In contrast, the hydrothermal method produces morphologies up to approximately 2–3 μm. Results suggest that both the synthetic routes yield similar emission spectra and mean afterglow time of ~ 10 s.

In this study, the photovoltaic behavior of centimeter-long lateral organic junctions, reaching 1.8 cm, is reported. The organic junctions are formed using organic semiconductor films with high mobilities of holes and electrons. The lateral diffusion lengths of photogenerated electrons and holes are 4.7 and 5.5 mm, respectively. The photovoltaic behavior in the centimeter-long lateral junctions is controlled by the trap-assisted recombination between the electrons and holes.

To develop the high-performance filters and duplexers required for recent long-term evolution frequency bands in mobile handsets, a surface acoustic wave (SAW) resonator is needed that has a higher quality (Q) and a lower temperature coefficient of frequency (TCF). To achieve this, the authors focused on acoustic energy confinement in the depth direction for a rotated Y-X LiTaO3 (LT) substrate. Characteristics of multilayered substrates with low-impedance and high-impedance layers under LT layer were studied numerically in terms of acoustic energy distribution, phase velocity, coupling coefficient, and temperature characteristics employing a finite-element method simulation. After several calculations, a novel multilayered structure was developed that uses SiO2 for a low-impedance layer and AlN for a high-impedance layer under the thin LT layer. A one-port resonator using the new substrate was fabricated, and its experimental results showed that the developed resonator had a Bode-Q over 4000 and TCF of −8 ppm/°C, which are four times higher than and one-fifth as small as those of a conventional 4° YX-LT SAW resonator, respectively. By applying this technology, a band 25 duplexer with very narrow duplex gap was successfully developed, which shows extremely low insertion loss, steep cutoff characteristics, and stable temperature characteristics.

The operation of single crystal organic photovoltaic cells using lateral electron transport was demonstrated. The spacing between the collection electrodes, which is determined by the lateral range of the electrons, was estimated to be 30 μm. The possibility of electron ranges of the millimeter order was indicated.

C60:Li@C60 hybrid n-type semiconducting films were first fabricated. The Fermi level of 1% Li@C60-added C60 films was determined to be -4.52 eV, which was 0.12 eV higher than that of pristine C60 films. A fraction of Li@C60 is distributed uniformly within the C60 film. Its application in PSCs was demonstrated, in which the addition of Li@C60 into a C60 film improved the device performance.

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 LiTaO

The rare-earth complexes [Eu(hfa)3(phen)] and [Eu(hfa)3(TPPO)2] were dispersed in the fibrous network structure of an organogel of phenol and surfactant AOT. Steady-state and time-resolved emission properties arising from the complex–organogel interactions of these two complexes were investigated. The results suggest that [Eu(hfa)3(phen)] localizes around the fibrous structure of the organogel architecture, whereas [Eu(hfa)3(TPPO)2] favors the solvent domain even when dispersed in the gel environment. These properties characterize the steady state and time-resolved behavior in the emission processes of the [Eu(hfa)3(phen)] and [Eu(hfa)3(TPPO)2] complexes, which can be regarded as “gelaphilic” and “solvophilic,” respectively.

The effects of doping at concentrations at the ppm level in organic photovoltaic cells were clarified using simple n + p -homojunctions. With doping from 0 to 10 ppm, the fill factor increased due to the appearance of majority carriers. From 10 to 100 ppm, the photocurrent density increased due to an increase in the built-in potential, i.e., the formation of an n + p -homojunction. The photocurrent was increased by a factor of 1.3 by directly doping the photoactive co-deposited layer with acceptor molecules at a concentration of 100 ppm.

Open-circuit voltage ( VOC) is a key parameter governing the power conversion efficiency of organic solar cells. We clarified the effect of band bending in organic semiconductor films on the VOC of phthalocyanine (H2Pc)/fullerene (C60) planar heterojunction (PHJ) OSCs. The VOC was significantly affected by the H2Pc layer thickness, varying from 0.20 V at 2 nm to 0.49 V at 100 nm thickness. The large VOC change is due to the band bending in the H2Pc layer, where the difference in H2Pc layer thickness led to significant changes in the degree of the vacuum-level shift in the C60 layer because of the Fermi level ( EF) difference between the H2Pc and C60 layers. This result demonstrated that a new influential factor affecting the VOC is the position of the EF that is dependent on band bending in organic semiconductor films.

Recently developed organic photovoltaics (OPVs) show distinct advantages over their inorganic counterparts due to their lighter weight, flexible shape, versatile materials synthesis and device fabr ...