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Solar & Alternative Energy

Flexible electronics developing for diverse applications

New flexible electronics technologies are becoming more viable and durable, with some ready for market as early as next year.
21 August 2009, SPIE Newsroom. DOI: 10.1117/2.2200908.01

Many research groups have recently reported developments in flexible electronics technology, for applications ranging from photovoltaics to imaging without a lens.

Julian Perrenoud, Stephan Buecheler, and Ayodhya N. Tiwari of EMPA (Switzerland) recently reported on their development of lightweight and flexible CdTe thin film solar cells on polymer film with a low temperature process suitable for roll-to-roll manufacturing, some with reported efficiencies of over 10%.

The company Solarmer has also created a flexible solar panel. Based on patents licensed from the laboratory of Dr. Yang Yang at UCLA, the polymer bulk-head junction solar cells are formed on a transparent flexible substrate, making the entire device flexible. The lightweight, flexible, plastic solar panels that Solarmer is developing are expected to cost a fraction of what silicon solar panels cost, according to the company. So far they have received a certification from Newport for 3.87% efficiency for 6x6 inch modules.

"We're looking to start with smaller applications that include portable electronics and small portable applications," says Vishal Shrotriya, Solarmer's director of product and business development. "Things like cell phones, MP3s, integrating into their bags and backpacks."

Solarmer's plastic solar panels for smart fabrics applications will be available for potential customers and partners interested in testing beginning mid-2010.

Figure 1. A possible future application of Solarmer's solar fabric. Courtesy of Solarmer.

Researchers at the University of California, Berkeley, have made a new kind of bendable solar cell by growing an array of upright nanoscale pillars on aluminum sheets.

"We used anodized alumina membranes as the template for the single-crystalline n-CdS nanopillar synthesis by using the vapor-solid-liquid (VLS) process," says Ali Javey, co-developer of the cell and SPIE Carbon Nanotube Program Committee member for Optics + Photonics. The solar cells are made of uniform 500-nanometer-high pillars of cadmium sulfide embedded in a thin film of cadmium telluride. The nanopillars allow the developers to use cheaper, lower-quality materials than those used in conventional silicon and thin-film technologies. The cells can be bent to large radii, 3 cm, without noticeable degradation in performance or life-time over multiple bendings.

Eventually, says Javey, "the process could potentially be made compatible with a roll-to-roll fabrication scheme, therefore making the process potentially very cost effective." However, they still need to be made more efficient. "The first simple approach for improving our efficiency is to optimize and/or replace our current top contact material."

Outside of the world of photovoltaics, a collaborative team of researchers at TNO (Netherlands), Agfa (Belgium), and Phillips (Netherlands) recently demonstrated a large-area (12x12cm2) flexible OLED without using ITO, and with printed metal shunting lines. This process eliminates the need for lithography, and means OLEDs can be created much cheaper than before.

Yasuyuki Watanabe and Kazuhiro Kudo, Chiba University (Japan), have been working on vertical-type organic light emitting transistors, and specifically static induction transistors, which can be combined with OLEDs to create large flexible displays.

Another application for flexible electronics that absorb light is to incorporate light sensors into polymer fibers and create a flexible fabric that can detect the angle, intensity, phase, and wavelength of light hitting it, information that can be used to re-create a picture of an object without a lens. Yoel Fink and his team at MIT discussed their success at doing just that at SPIE Defense, Security, and Sensing 2009.

Fink and his team wove multi-strand fibers together with eight sensors grouped in pairs consisting of an inner and outer sensor. These thin fibers were then woven into a 10 sq cm section of fabric. When light hits the sensors, it creates an electrical current which transmits the information to a computer. The careful positioning of the light-sensitive sensors means that the team knew which signals were being sent by which sensors.

Figure 2. The composition of Fink's light-sensitive fibers, and how they function. From an article that appeared in Nano Letters by Sorin, Fink, et al. (2009). Courtesy of Yoel Fink.

"The ability to image, or extract images, from fabric without needing a lens, really relies on the fact that the fibers in the fabric are light-detecting, and not only that but they're able to do it with different wavelengths," says Fink.

Technology using the optical fiber has already been used in biomedical optics for almost six years, including non-invasive surgery, hearing restoration, brain and spine surgery. This new advance in the fiber technology has already found another application, with line-of-sight optical communication. The U.S. Army is working to create a laser-based communication system that would allow soldiers to communicate silently over long distances by shining lasers onto each other's uniforms or helmets.

These breakthroughs in mass-producible flexible electronics indicate the near-term potential for development of other flexible, light technologies.

SPIE articles featuring work by flexible electronic researchers:

"Flexible CdTe solar cells and modules: challenges and prospects," by Julian Perrenoud, Stephan Buecheler, Ayodhya N. Tiwari, Proc. of SPIE 7407, (2009).

"Multimaterial photosensitive fiber constructs enable large-area optical sensing and imaging" by Ayman Abouraddy and Yoel Fink, Proc. SPIE, Vol. 7314, 73140H (2009).

"Measurement issues of organic solar cell," by Gang Li, Vishal Shrotriya, Jinsong Huang, and Yang Yang, Proc. SPIE 7052, 70520E (2008).

"Vertical type organic transistor for flexible sheet display," by Yasuyuki Watanabe, Kazuhiro Kudo, Proc. of SPIE 7415, (2009).

Vertical-type organic transistor advances flexible sheet displays, SPIE Newsroom (2009)