In recent years, highly flexible solar cells have been gathering great interest as a power source for operating wearable and/or on-skin electronic devices which are necessary technologies for the Internet of things (IoT) society. By adapting such flexible solar cells, wearable and on-skin devices can be free from the troubles of replacement of batteries and contact problems of external wirings. In this talk, our approaches for ultra-thin organic solar cells will be introduced with three important keywords, namely 1. high power conversion efficiency (PCE) , 2. strethchability/flexibility, and 3. robustness against the environmental conditions. We succeeded to fabricate organic solar cells with total thickness of 3 μm and achieved excellent PCE up to 10.5% for free-standing ultra-thin solar cells. There are trade-off relations between ultimate thinness and environmental stabilities such as air, water, and heating. We tackled to solve these problem and developed technologies to improve thermal stability and water stability of ultra-thin organic solar cells. Especially, high thermal stability of more than 100 degrees Celsius allows hot-melt adhesion process onto textiles, which enabled wearable power source systems. Additionally, we developed self-powered on-skin sensor systems by integrating our solar cells with ultra-thin organic electrochemical transistors and monitored heart-beat rate with such integrated devices.
We have successfully manufactured rubber-like large-area stretchable integrated circuits comprising printed elastic
conductors, organic transistor-based circuits, and silicon transistor-based circuits. Employing the first direct
integration of organic and silicon (Si) integrated circuits, we have realized to develop a stretchable electromagnetic
interference (EMI) measurement sheet that can detect EMI distribution on the surface of electronic devices by
wrapping the devices in the sheet. The stretchable devices can spread over arbitrary surfaces including free-formed
curvatures and movable parts, thereby significantly increasing the applications of electrical circuits.
Stretchability significantly expands the scope of electronic applications-particularly large-area electronics such as
displays, sensors, and actuators-because stretchable electronics can cover arbitrary surfaces and movable parts, which is
impossible with conventional electronics. However, the realization of stretchable electronics for the manufacturing of
electrical wiring with high conductivity, high stretchability, and large-area compatibility is a major hurdle. We
manufactured printable elastic conductors comprising single-walled carbon nanotubes (SWNTs) uniformly dispersed in
fluorinated rubber. Using ionic liquid and jet milling, we produced longer and finer SWNT bundles that formed
well-developed conducting networks in rubber. A conductivity and stretchability greater than 100 S/cm and 100%,
respectively, were obtained. In order to demonstrate the feasibility of the elastic conductors for electrical wiring, we
manufactured a rubber-like large-area organic transistor active matrix comprising printed organic transistors and elastic
conductors. The effective area of the matrix was 20 × 20 cm2. The active matrix sheet was uniaxially and biaxially
stretched to 70% without incurring mechanical or electrical damage. Furthermore, we constructed a rubber-like
stretchable active matrix display comprising integrated printed elastic conductors, organic transistors, and organic
light-emitting diodes. The display could stretch by 30-50% and spread over a hemisphere without being mechanically or
electrically damaged.
A technique to control growth area of self-assembled quantum dots is required to avoid optical absorption in fabricating quantum dot lasers with optical passive devices such as photonic crystals in the same growth plane as the active layer. We propose and demonstrate a scheme (Area-Controlled Growth) for controlling growth area of self-assembled InAs quantum dots using selective metalorganic chemical vapor deposition (MOCVD). Using this growth technique, the amount of material deposited within mask windows is controlled by varying the width of the mask. In the growth of self-assembled quantum dots, the density of quantum dots largely depends on the quantities of group III sources used. Therefore, by optimizing the growth conditions and mask pattern, quantum dots can be formed in only selected areas of a growth plane. However, in the regions where dots are formed there is variation of dot density and size along the mask stripe direction because of the diffusion of species in the vapor phase, which is peculiar to selective MOCVD. We achieve more uniform distributions of dot density and size by improving the mask pattern. This growth technique can be also applied to fabricate integrated devices for optical communication system containing an external modulator such as photonic crystals together with quantum dot lasers lasing at 1.3 micrometers .
Conference Committee Involvement (2)
Organic Field-Effect Transistors VI
26 August 2007 | San Diego, California, United States
Organic Field-Effect Transistors V
13 August 2006 | San Diego, California, United States
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