Yoshiharu Sato

Overcoming Short-Circuit in Lead-Free CH3NH3SnI3 Perovskite Solar Cells via Kinetically Controlled Gas–Solid Reaction Film Fabrication Process

The development of Sn-based perovskite solar cells has been challenging because devices often show short-circuit behavior due to poor morphologies and undesired electrical properties of the thin films. A low-temperature vapor-assisted solution process (LT-VASP) has been employed as a novel kinetically controlled gas-solid reaction film fabrication method to prepare lead-free CH3NH3SnI3 thin films. We show that the solid SnI2 substrate temperature is the key parameter in achieving perovskite films with high surface coverage and excellent uniformity. The resulting high-quality CH3NH3SnI3 films allow the successful fabrication of solar cells with drastically improved reproducibility, reaching an efficiency of 1.86%. Furthermore, our Kelvin probe studies show the VASP films have a doping level lower than that of films prepared from the conventional one-step method, effectively lowering the film conductivity. Above all, with (LT)-VASP, the short-circuit behavior often obtained from the conventional one-step-fabricated Sn-based perovskite devices has been overcome. This study facilitates the path to more successful Sn-perovskite photovoltaic research.

Effect of aromatic diamines as a cathode interface layer

Abstract The cathode interface layer (CIL) was investigated using aromatic diamine derivatives. The organic electroluminescent (EL) device with Ag cathode and CIL of Cz-TPD (4,4′-biscarbazolyl(9)-biphenyl) showed good EL characteristics comparable to a conventional MgAg device and also an improved storage stability. X-ray reflectivity measurement suggested that the rough structure of the Cz-TPD layer lowered the electron injection barrier at the Ag interface.

Characteristics of organic electroluminescent devices with new dopants

Abstract We conducted an investigation of electroluminescent (EL) characteristics of the organic EL devices with new dopants: 5,12-diphenyl-tetracene (DPT) and benzothioxanthene (BTX) derivatives. The organic EL cell doped with DPT showed a green emission with high color purity and also good operational stability. Concentration quenching was found out to be more severe for DPT, compared with rubrene. BTX derivatives exhibited an orange emission as a dopant both in the electron-transporting layer and in the hole-transporting layer. We demonstrated a white emission by the combination of an orange-emitting hole transport layer and a blue-emitting layer.

Chapter 4 Organic LED System Considerations

Publisher Summary This chapter discusses organic light-emitting diodes (OLEDs) system considerations. The advent of organic electroluminescent devices advanced from a long history of development of organic photocondoctors (OPCs) for xerographic copy machines. While the organic solar cell has a similar mechanism as the organic photoreceptor, the OLED or organic electroluminescent (EL) device has a different mechanism. In the application of organic EL devices to practical displays, the chapter discusses three fundamental issues: (1) stability (lifetime), (2) power efficiency, and (3) full-color capability. The chapter summarizes the present status of both molecular OLEDs and polymer LEDs in terms of panel performance and fabrication processes. Although efficiency is considered as a strong pro for molecular OLEDs, progress in the efficiency of polymer LEDs makes it comparable to that of molecular OLEDs.

Organic electroluminescent devices with polymer buffer layer

We have developed a new type of polymers having TPD unit combined with non-conjugated spacer group, poly(arylene ether sulfone)-containing and poly(arylene ether ketone)- containing tetraphenylbenzidine (PTPDES an PTPDEK) and also polymers with directly coupled triphenyl amine units (PPBA). When these polymers are mixed with strong acceptor such as TBPAH or DDQ, they indicated higher conductivity and facilitated hole injection from ITO to the hole transport layer. Spin-coating of such polymer from an organic solution on ITO was found to improve the surface roughness of ITO, resulting in reduced defects that cause electric short circuit between ITO and cathode. These buffer materials lowered the operation voltage and improved the thermal stability of the device. After storage of 1,000 hours at 85 degrees Celsius, the device with polymer buffer showed no degradation in luminance and small increase of operation voltage. In comparison with CuPc buffer, it is clear that the doped polymer is superior in terms of both efficiency and thermal stability.

Interface and material considerations of OLEDs

Three interfaces, anode interface, hole blocking layer and cathode interface were considered mainly from the viewpoint of materials. Vinyl polymers containing triphenylamine as a side group were investigated as an ITO buffer layer. When these polymers were doped with strong acceptor, they lowered operation voltage of OLED and also improved the thermal stability. Employment of high Tg hole transport material was also found effective for the thermally stable EL characteristics. Hole blocking material with a wider optical gap improved color purity of blue-emitting device. Various inorganic compounds were examined as a cathode interface layer to demonstrate that MgF2 was effective to improve operation lifetime of OLED.

Synthesis, properties, and LED performance of highly luminescent metal complexes containing indolizino[3,4,5-ab]isoindoles

We report the synthesis, X-ray structures, optical and electrochemical properties, as well as fabrication of light-emitting devices for complexes which have cyclometalated ligands, pin (2-pyridin-2-yl-indolizino[3,4,5-ab]isoindole) and qin (2-quinolin-2-yl-indolizino[3,4,5-ab]isoindole). The new complexes are [Ir(L1)3−x(L2)x], [Pt(L1)(L2)], and [Pd(L1)(L2)] (L1 = pin, qin; L2 = acac (acetylacenato), pic (2-pyridinecarboxylato); x = 0, 1). The Ir and Pt complexes are highly luminescent, even at room temperature, and the relative luminescence quantum efficiencies are as high as 61%. The Pd complexes exhibit luminescence in the solid state, but the luminescence was quenched in solution. The X-ray crystal structures of Pt(pin)(acac), Pd(qin)(acac) and Pd(qin)(acac) were also obtained. The brightness of the light-emitting device reached values as high as 1.4 × 104 cd/m2 and Ir(pin)3 emits beautiful green light.