To perform its function, a microelectromechanical system (MEMS) needs its own power source. To date, magnetic-field-induced power and voltage supplies, rechargeable lithium microbatteries, miniature fuel cells and even silicon photovoltaic (PV) sources have been used. Among these methods, solar cells offer an excellent size-to-power-density ratio compared to other power sources, are self-contained and permit on-chip integration with other MEMS circuits.

Recognizing that organic solar cells offer certain advantages over silicon solar cells, including mechanical flexibility and fabrication on a variety of substrates via spin-coating, spraying, or inkjet printing, researchers at the University of South Florida are the first to use an organic solar cell to power a tiny MEMS device - a solar cell that has the potential to be as small as 0.01 mm².

The advantages of organic solar cells over silicon-based devices include easy isolation of the organic cell from the MEMS device, considering that the organic cell can be fabricated on any substrate, including plastic. Also, organic cells have a much higher absorption coefficient than silicon and can be fabricated with active layers as thin as 100 nm.

In this work the researchers used a four-step process to build a 2.2 cm² miniature organic solar module consisting of 20 solar cells in series. In the first step, a one-inch-square piece of indium tin oxide (ITO) was spin-coated with resist, exposed with a photomask, and then etched to create the anode pattern. Next, the photoactive layer solution, consisting of P3HT and PCBM organic materials, was then spin-coated onto the anode layer at a thickness of 200 nm. Third, a shadow mask was created acting as a pattern for the evaporation of aluminum contacts in the required pattern on top of the photoactive layer. Finally, the aluminum cathode was deposited under heavy vacuum at a thickness of 100 nm.

Fabrication of several arrays that consisted of  9 and 18 solar cells in series, with thicknesses ranging 202…232 nm, produced open-circuit voltage values between 5.2…7.8 V and short-circuit current values between 13…55 µA.

 Twenty miniature organic solar cells are connected in series to create a 2.2 cm² solar array that is close to creating enough voltage to power a tiny microelectromechanical system (MEMS) device. The overall device is one square inch in size, and each of the solar cells has an active area of 1 mm². 

These arrays were designed to act as a power source for running a microscopic sensor for detecting chemicals and toxins. The MEMS-based sensor is built using carbon nanotubes and has been tested using ordinary DC power supplied by batteries. When fully powered and hooked into a circuit, the carbon nanotubes detect particular chemicals by measuring the electrical changes that occur when chemicals enter the tubes. So far, the device requires a 15 V power source to work, so the researchers are planning to optimize their solar array to meet this requirement.

Vasil Sidorov 15.07. 2009. E-mail: sidorovvasil@gmail.com

from Renewable and Sustainable Energy