Low-Band-Gap Small Molecule for Efficient Organic Solar Cells with a Low Energy Loss below 0.6 eV and a High Open-Circuit Voltage of over 0.9 V

Small molecule organic solar cells (SMOSCs) have received considerable attention in recent years. However, one of the key factors limiting the performance of SMOSCs is their large energy loss (<i>E</i>_loss), which is typically between 0.6 and 1.0 eV, and therefore significantly higher than those of perovskite solar cells and inorganic solar cells (<i>E</i>_loss < 0.5 eV). Herein, we successfully report a new acceptor–donor–acceptor (A–D–A) type dimeric squaraine electron donor (<b>D-IDTT-SQ</b>) with a low optical band gap of 1.49 eV and deep HOMO energy level of −5.20 eV. Consequently, a high open-circuit voltage (<i>V</i>_oc) of 0.93 V with an impressive power conversion efficiency (PCE) of 7.05% is achieved for solution-processed bulk heterojunction SMOSCs, showing an <i>E</i>_loss of only 0.56 eV. This is the first report wherein SMOSCs result in such a low <i>E</i>_loss, while simultaneously exhibiting a considerably high <i>V</i>_oc over 0.9 V and an excellent PCE above 7.0%

A <i>m</i>-Terphenyl-Modifed Sulfone Derivative as a Host Material for High-Efficiency Blue and Green Phosphorescent OLEDs

A <i>m</i>-Terphenyl-Modifed Sulfone Derivative as a Host Material for High-Efficiency Blue and Green Phosphorescent OLED

Two-Dimensional Ca₂Nb₃O₁₀ Perovskite Nanosheets for Electron Injection Layers in Organic Light-Emitting Devices

We report in this article the application of calcium niobate (CNO) perovskite nanosheets for electron injection layers (EILs) in organic light-emitting devices (OLEDs). Four kinds of tetraalkylammonium hydroxides having different alkyl lengths were utilized as the exfoliation agents of a layered compound precursor HCa₂Nb₃O₁₀ to synthesize CNO nanosheets, including tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide (TPAOH), and tetrabutylammonium hydroxide. CNO nanosheet EILs were applied in fluorescent poly­[(9,9-di-<i>n</i>-octylfluorenyl-2,7-diyl)-<i>alt</i>-(benzo­[2,1,3]­thiadiazol-4,8-diyl)] (F8BT) organic light-emitting polymer-based devices. The effects of dispersion concentrations and alkyl chain length on the devices’ performances were investigated. The results demonstrated that OLEDs’ performances were related to the coverage ratio of the CNO nanosheets, their thicknesses, and their work function values. Among the four exfoliation agents, the device with CNO nanosheets exfoliated by TPAOH showed the lowest driving voltage. The OLEDs with the CNO nanosheet EILs showed lower driving voltages compared with the devices with conventional EIL material lithium 8-quinolate

A Solution-Processed Heteropoly Acid Containing MoO₃ Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices

We report hole-injection layers (HILs) comprising a heteropoly acid containing MoO₃ units, phosphomolybdic acid (PMA), in organic light-emitting devices (OLEDs). PMA possesses outstanding properties, such as high solubility in organic solvents, very low surface roughness in the film state, high transparency in the visible region, and an appropriate work function (WF), that make it suitable for HILs. We also found that these properties were dependent on the postbaking atmosphere and temperature after film formation. When the PMA film was baked in N₂, the Mo in the PMA was reduced to Mo­(V), whereas baking in air had no influence on the Mo valence state. Consequently, different baking atmospheres yielded different WF values. OLEDs with PMA HILs were fabricated and evaluated. OLEDs with PMA baked under appropriate conditions exhibited comparably low driving voltages and higher driving stability compared with OLEDs employing conventional hole-injection materials (HIMs), poly­(3,4-ethylene­dioxy­thiophene):poly­(4-styrene­sulfonate), and evaporated MoO₃, which clearly shows the high suitability of PMA HILs for OLEDs. PMA is also a commercially available and very cheap material, leading to the widespread use of PMA as a standard HIM

Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics

Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission

The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m^–2, most likely because of the long delayed fluorescent lifetime (τ_d). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τ_d. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac–<i>X</i>MHPMs (<i>X</i> = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (<i>E</i>_S) and triplet state (<i>E</i>_T) energies. Among them, Ac–3MHPM, with a high <i>E</i>_T of 2.95 eV, exhibited a high external quantum efficiency (η_ext,max) of 18% and an η_ext of 10% at 100 cd m^–2 with Commission Internationale de l′Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters

Solution-Processed Inorganic–Organic Hybrid Electron Injection Layer for Polymer Light-Emitting Devices

A lithium quinolate complex (Liq) has high solubility in polar solvents such as alcohols and can be spin-coated onto emitting polymers, resulting in a smooth surface morphology. A polymer light-emitting device fabricated with spin-coated Liq as an electron injection layer (EIL) exhibited a lower turn-on voltage and a higher efficiency than a device with spin-coated Cs₂CO₃ and a device with thermally evaporated Ca. The mixture of ZnO nanoparticles and Liq served as an efficient EIL, resulting in a lower driving voltage even in thick films (∼10 nm), and it did not require a high-temperature annealing process

Natural Photosynthetic Carotenoids for Solution-Processed Organic Bulk-Heterojunction Solar Cells

In this work, we demonstrate utilization of natural carotenoids (Cars), namely, fucoxanthin, β-carotene, and lycopene, as electron-donor molecules together with the electron-acceptor fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) in organic solar cells (OSCs). Unlike fucoxanthin and β-carotene, which form amorphous films, lycopene readily forms aggregates through a simple spin coating process. A high carrier mobility of up to 2.1 × 10^–2 cm²/(V s) was observed for lycopene, which is three orders of magnitude greater than those of fucoxanthin and β-carotene, with values of (8.1 and 1.8) × 10^–5 cm²/(V s), respectively. OSCs with different Car:PCBM blend ratios were optimized for these Cars. The highest photovoltaic performance was obtained for lycopene with a blend ratio of 1:1, at which the film morphology and charge transport were optimized. Replacement of the acceptor molecule PCBM with a high-lowest-unoccupied-molecular-orbital fullerene derivative indene-C₆₀ bisadduct improved the overall conversion efficiency of lycopene-based OSCs by enhancing the open-circuit current (<i>V</i>_oc). Interestingly, further investigation on charge-separation dynamics revealed that photocurrent is generated only from the S₂ (1B_u⁺) state, and the others underwent ultrafast excitation relaxation through S₂ → S₁ (2A_g^–) → S₀ (ground state), leaving much room for further improvement

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices

Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode

Influence of Fullerene Multiadducts on the Morphology and Charge Photogeneration of Their Photovoltaic Blends with Poly(3-hexylthiophene)

A series of benzene octyl ether adducted fullerene derivatives (PCBOEs) of higher LUMO levels with reference to PCBM were synthesized and were blended with P3HT to assemble polymer solar cells (PSCs). Upon increasing the adduct order, the PCBOE-based PSCs exhibited higher open-circuit voltage (<i>V</i>_OC) but increasingly lower short-circuit current density (<i>J</i>_SC) with respect to the P3HT:PCBM device. Morphological analyses of the P3HT:fullerene blends reveal different extents of fullerene aggregation, which are shown to impact intimately on the charge photogeneration dynamics. An increase of adduct order is found to accelerate geminate charge recombination and to induce nanosecond nongeminate charge recombination that is absent in the cases of monoadducted PCBOE and PCBM. The severer charge recombination found for higher order fullerene adducts is ascribed to the deterioration of electron mobility owing to the discontinuity of the fullerene phase. The results suggest that, upon increasing the adduct order, the electron and hole transport in photoactive layers turn from ambipolarly balanced to space-charge limited states, a crucial issue for the application of multiadducted fullerenes as the electron accepting materials of PCSs