August 21, 2016

One micron thick solar cells and three micron thick OLED displays

Scientists in South Korea have made ultra-thin photovoltaics flexible enough to wrap around the average pencil. The bendy solar cells could power wearable electronics like fitness trackers and smart glasses.

Thin materials flex more easily than thick ones - think a piece of paper versus a cardboard shipping box. The reason for the difference: The stress in a material while it's being bent increases farther out from the central plane. Because thick sheets have more material farther out they are harder to bend.

"Our photovoltaic is about 1 micrometer thick," said Jongho Lee, an engineer at the Gwangju Institute of Science and Technology in South Korea. One micrometer is much thinner than an average human hair. Standard photovoltaics are usually hundreds of times thicker, and even most other thin photovoltaics are 2 to 4 times thicker.

3 micron thick OLED displays
Previously Nextbigfuture has reported on Japanese work to develop OLED displays that are only 3 microns thick. The one micron thick solar power technology would match up well with the three micron thick displays.

Tokyo researchers demonstrated ultraflexible and conformable three-color, highly efficient polymer light-emitting diodes (PLEDs) and organic photodetectors (OPDs) to realize optoelectronic skins (oe-skins) that introduce multiple electronic functionalities such as sensing and displays on the surface of human skin. The total thickness of the devices, including the substrate and encapsulation layer, is only 3 μm, which is one order of magnitude thinner than the epidermal layer of human skin.

Smart e-skin system comprising health-monitoring sensors, displays, and ultraflexible PLEDs. (A) Schematic illustration of the optoelectronic skins (oe-skins) system. (B) Photograph of a finger with the ultraflexible organic optical sensor attached. (C) Photographs of a human face with a blue logo of the University of Tokyo and a two-color logo. The brightness can be changed by the operation voltage. (D) Photograph of a red seven-segment PLEDs displayed on a hand.

Applied Physics Letters - Ultra-thin flexible GaAs photovoltaics in vertical forms printed on metal surfaces without interlayer adhesives

Back the one micron thin solar cells

The researchers made the ultra-thin solar cells from the semiconductor gallium arsenide. They stamped the cells directly onto a flexible substrate without using an adhesive that would add to the material's thickness. The cells were then "cold welded" to the electrode on the substrate by applying pressure at 170 degrees Celcius and melting a top layer of material called photoresist that acted as a temporary adhesive. The photoresist was later peeled away, leaving the direct metal to metal bond.

The metal bottom layer also served as a reflector to direct stray photons back to the solar cells. The researchers tested the efficiency of the device at converting sunlight to electricity and found that it was comparable to similar thicker photovoltaics. They performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters.

The team also performed numerical analysis of the cells, finding that they experience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick.

"The thinner cells are less fragile under bending, but perform similarly or even slightly better," Lee said.

A few other groups have reported solar cells with thicknesses of around 1 micrometer, but have produced the cells in different ways, for example by removing the whole substract by etching.

By transfer printing instead of etching, the new method developed by Lee and his colleagues may be used to make very flexible photovoltaics with a smaller amount of materials.

The thin cells can be integrated onto glasses frames or fabric and might power the next wave of wearable electronics, Lee said


earable flexible electronics often require sustainable power sources that are also mechanically flexible to survive the extreme bending that accompanies their general use. In general, thinner microelectronic devices are under less strain when bent. This paper describes strategies to realize ultra-thin GaAs photovoltaics through the interlayer adhesiveless transfer-printing of vertical-type devices onto metal surfaces. The vertical-type GaAs photovoltaic devices recycle reflected photons by means of bottom electrodes. Systematic studies with four different types of solar microcells indicate that the vertical-type solar microcells, at only a quarter of the thickness of similarly designed lateral-type cells, generate a level of electric power similar to that of thicker cells. The experimental results along with the theoretical analysis conducted here show that the ultra-thin vertical-type solar microcells are durable under extreme bending and thus suitable for use in the manufacturing of wearable flexible electronics.

SOURCES - Eurekalert, Applied Physics Letters, Science Advances, University of Tokyo

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