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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.

type:Article, Triplet exciton formation at an organic semiconductor interface plays a crucial role in photophysical processes in electronic devices, such as organic light-emitting diodes (OLEDs) and organic photovoltaics, and in optical functions, such as photon upconversion (PUC) based on triplet-triplet annihilation (TTA), which produces a photon with high energy combining two photons with low energy. Herein, we report the spatial distribution of the triplet exciton generated after charge recombination at the organic semiconductor interface. The triplet distribution is assessed by examining the variation in TTA emission when the triplet quencher is doped near the interface in the OLED and PUC systems. The obtained result indicates that 90% of the triplet excitons are confined to less than 10 nm near the donor/acceptor interface, where charge recombination occurs and a charge transfer state forms, which is a precursor of the triplet exciton.The confinement of triplet excitons leads to efficient TTA emission, whose efficiency is triplet concentration-dependent, in OLED and PUC systems utilizing the mechanism of interfacial triplet formation., identifier:Journal of Materials Chemistry A, 2022, Advance Article

Triplet exciton formation at an organic semiconductor interface plays a crucial role in photophysical processes in electronic devices, such as organic light-emitting diodes (OLEDs) and organic photovoltaics, and in optical functions, such as photon upconversion (PUC) based on triplet-triplet annihilation (TTA), which produces a photon with high energy combining two photons with low energy. Herein, we report the spatial distribution of the triplet exciton generated after charge recombination at the organic semiconductor interface. The triplet distribution is assessed by examining the variation in TTA emission when the triplet quencher is doped near the interface in the OLED and PUC systems. The obtained result indicates that 90% of the triplet excitons are confined to less than 10 nm near the donor/acceptor interface, where charge recombination occurs and a charge transfer state forms, which is a precursor of the triplet exciton.The confinement of triplet excitons leads to efficient TTA emission, whose efficiency is triplet concentration-dependent, in OLED and PUC systems utilizing the mechanism of interfacial triplet formation., Article, Journal of Materials Chemistry A, 2022, Advance Article

Breakdown process of the Klebanoff mode and origin of turbulence are experimentally investigated in detail. An acoustic excitation using a 2D roughness attached on the wall are used for T-S wave generation. Piezo-ceramic actuators are used to introduce the Klebanoff mode fluctuation. It is found that another shear layer, in which the spike phenomenon takes place and the turbulence starts, exists between the Lambda structures aligned in rows. The spreading of the turbulence along the shear layer induced by the Lambda structures is newly found., JAXA Special Publication, 宇宙航空研究開発機構特別資料

Streamwise interfaces between laminar and turbulent regions in a flat-plate boundary layer are investigated using a rake of single hotwire probes. Two pieces of piezo-ceramic actuators are used to generate trapezoidal shaped turbulent regions, which have a wide leading edge in the spanwise direction. Its aim is to eliminate the contamination between the side interfaces and the downstream interface that are unavoidable in turbulent spot experiments. Experimental results show that a turbulent region has a sharp leading edge and a gradual trailing edge. It is shown that when a turbulent region passes by, the RMS (Root Mean Square) value remains high for a while although there is little disturbance remaining. It is also shown that inside this region, known as the calmed region, because the flow returns laminar away from the wall, the turbulent flow near the wall is accelerated, which may be causing the inverse-transition., JAXA Special Publication, 宇宙航空研究開発機構特別資料

A new method of vortical structure identification, named pi method, is proposed. The method focuses on the relation between the vorticity vector and the rotating condition of its neighboring fluid. The results are compared with the other methods, namely the Q method, the lambda(sub 2) method and the Delata method, using the DNS data of a forced isotropic homogeneous turbulence and a collision of four vortex rings. It is shown that pi method has the advantage of being not sensitive to the threshold., JAXA Special Publication, 宇宙航空研究開発機構特別資料

Means to explain the flow structure generated by thin piezo-ceramic actuators are discussed. In the previous experiments, it was shown that the oblique waves, observed in an initial stage of the transition, could be cancelled by generating intercepting-waves using thin piezo-ceramic actuators. The initial aim of this study is to explain the generation process of these intercepting-waves by modeling the flow structure using a large number of the vortex tubes. However, it is found that this approach is not useful in the near wall region, because in these areas the diffusion of vorticity pattern on the wall is dominant compared with the convection., JAXA Special Publication, 宇宙航空研究開発機構特別資料

The effect of outer disturbances on the boundary layer transition is investigated experimentally. The outer disturbances are introduced above a boundary layer by small jets that are ejected in the downstream direction through small holes opened in the circular pipe located outside the boundary layer. It is observed that the low-frequency component fluctuations of outer disturbances jump obliquely into the boundary layer in the spanwise and downward directions., 形態: カラー図版あり, 会議: 第42回「境界層遷移の解明と制御」研究会（2008年3月31日-4月1日）

The effect of a localized disturbance outside a boundary layer on the transition is investigated by a wind-tunnel experiment. The disturbance is introduced by a turbulence generator which consists of two vortex-ring-generating units arranged face to face along the spanwise direction in the freestream. Each vortex-ring-generating unit issues intermittent jets that soon roll up into vortex rings. The two rings each from a vortex-ring-generating unit collide outside the boundary layer generating a velocity fluctuation pattern of high-speed and low-speed regions, with the high-speed region on the downstream side. An opposite pattern appears inside the boundary layer near the wall. In the high-speed region inside the boundary layer, a generation and a growth of random velocity fluctuations are observed. The velocity fluctuation grows downstream and triggers the transition to turbulence., Original contains color illustrations, Meeting Information: 43rd JAXA Workshops on "Investigation and Control of Boundar-Layer Transition" (October 2-3, 2008) ; 44th JAXA Workshops on "Investigation and Control of Boundar-Layer Transition" (March 26-27, 2009), 形態: カラー図版あり, 会議: 第43回（2008年10月2日-10月3日）「境界層遷移の解明と制御」研究会; 第44回（2009年3月26日-3月27日）「境界層遷移の解明と制御」研究会

Hierarchical relation between the deformation fields and vortical structures in a homogeneous isotropic turbulence is investigated. The stretching rates of the vortical structures are evaluated by replacing the extracted vortical structures by the vortex blobs. As a result, it is found that the individual vortices are likely to be stretched along the principal strain axes of flow field of larger scales., 形態: カラー図版あり, 会議: 第42回「境界層遷移の解明と制御」研究会（2008年3月31日-4月1日）

type:text, The effect of thermal annealing on the energy levels of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films was investigated using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy inverse photoemission spectroscopy. We observed that thermal annealing at 150 °C induces reductions in both ionization potential (IP) and electron affinity (EA) with the narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and the reduction in the film thickness by 2.54%. Precise measurements of both IP and EA enabled us to evaluate the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.