In an alpha-voltaic power source, a radioactive substance that emits energetic alpha particles is coupled to a semiconductor p/n junction diode or solar cell. An alpha-emitting radioistopeis used instead of a beta-emitting one to ensure a simple design that needs minimum shielding to contain the radiation.Americium-241 (Am241), with a half-life of 432.7 years and an activity of 3.5 curies per gram (Ci/g), emits a 5.5-MeV alpha particle. As the alpha particles penetrate into the p/n junction, they decelerate and give up their energy by creating electron-hole pairs in the semiconductor. These electron-hole pairs are collected by the p/n junction and are converted into useful electricity much like a solar cell. The primary reason alpha voltaics have not been technologically successful to date is that the alpha particles damage the semiconductor material so as to degrade the electrical output of the solar cell in just a matter of hours. At the NASA Glenn Research Center, several alpha-voltaic particle source designs were investigated that address this problem.
They key to future development resides in the ability to limit the radiation degradation in the photovoltaic portion of these devices. One approach to solving this problem is to use semiconductor materials that are more radiation tolerant. An indium gallium phosphide (InGaP) cell was fabricated and tested under alpha emission. The device degraded substantially but was found to recover significantly with an anneal at 200 °C for 1 hr.
"Nipi" structured alpha-voltaic battery.
Long description of figure 1.
A second approach involves the use of nonconventional device designs such as a lateral junction "nipi" device. This design spatially separates the n and p layers of a cell, which permits charge separation and transport to occur within two separate orthogonal planes, thus ensuring a higher radiation tolerance (see the preceding schematic).
The third approach uses an intermediate material to absorb the incident alpha energy and reemit it as light, without undergoing significant radiation damage. This material could be incorporated in a device structure to convert the alpha radiation indirectly. A possible candidate for this application would be a semiconducting quantum dot. In this design, seen in the following diagram, you would choose a phosphor material whose emissions were tailored to the bandgap of the photovoltaic device to ensure a high conversion efficiency while the phosphor protects the p/n junction of the device.
Alpha-voltaic battery using an intermediate quantum dot absorber layer.
Long description of figure 2.
The following graph shows the current-versus-voltage output of an alpha-voltaic battery that used an InGaP cell coated with zinc sulfide:gold (ZnS:Ag) intermediate absorber quantum dots after it was placed in contact with Am241.
Current-versus-voltage output of an alpha-voltaic battery. Operating temperature, Ė135 #&176;C.
A single alpha-voltaic stack would consist of an electrochemically deposited Am241 film on a thin metal foil alpha-particle source sandwiched between layers of quantum dots, with the quantum dots sandwiched between the bifacial p/n junction photovoltaic devices seen in the final figure.
Alpha-voltaic power stack.
Long description of figure 4.
Such a device, whose area would be less than 1 cm2 and whose weight would be approximately 3 g, could generate up to 5 mW, if 2.5 Ci of Am241 was used (i.e., 2 mW/mCi).
The quantum-dot alpha-voltaic devices could operate at low temperatures at which current battery systems would be rendered useless. In addition, they could be easily fabricated in microscale sizes, which is extremely difficult with conventional battery chemistries. This attribute makes them extremely attractive for microsystem applications such as biomedical or microelectromechanical systems (MEMS) sensors. These small devices could provide low levels of power for an extremely long period of time (i.e., >100 years) and could operate over a wide range of environments with little, if any, loss of performance, most notably at extremely low temperatures (i.e., <100 K) but also in harsh biological environments.
Find out more about the research of Glennís Photovoltaic and Space Environments Branch: http://www.grc.nasa.gov/WWW/5000/pep/photo-space/
Dr. Sheila G. Bailey, 216-433-2228, Sheila.G.Bailey@nasa.gov
Rochester Institute of Technology contact: Dr. Ryne P. Raffaelle, 585-475-5149, email@example.com
Authors: Dr. Sheila G. Bailey, David M. Wilt, Dr. Ryne P. Raffaelle, and Dr. Stephanie L. Castro
Programs/Projects: Glennís IR&D
Last updated: October 16, 2006
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