An alpha voltaic battery utilizes a radioactive substance that emits energetic alpha particles and is coupled to a semiconductor p/n junction diode. Alpha voltaics have not been technologically successful to date primarily because the alpha particles damage the semiconductor material, thus degrading the electrical output of the solar cell in just a matter of hours. The key to future development resides in the ability to limit this degradation.
Several approaches to solving this problem have been investigated. One approach uses photovoltaic devices which have good radiation tolerance such as InGaP. Another involves the use of non-conventional cell designs, such as a lateral junction n-type/intrinsic/p-type/intrinsic cell, which minimizes the effect of radiation damage on the overall cell performance. A third approach uses an intermediate absorber which converts the alpha energy into light which can be converted by the photovoltaic junction. The intermediate absorbers used in this approach are inherently radiation-hard semiconducting quantum dots. The first results of such an indirect device are shown below.
A conservative estimate of only 20% energy conversion efficiency of _-particle energy into useful photons (e.g. one 5.44 MeV _-particle results in 105 2.2 eV photons) and a monochromatic conversion efficiency of only 30% could result in the generation of up to 5 mW by a device whose area is less than 1 cm2 and which would weigh approximately 3 grams assuming 2.5 Ci of Am241 is used (i.e., 2 _W/mCi).
The half-life of the Am241, the alpha particle source, is over 400 years. Even with a conservative estimate on the efficiency of the quantum dot emitters due to radiation degradation, the usable lifetime of the device is orders of magnitude greater than a comparable rechargeable battery system.
The quantum dot alpha-voltaic devices would be capable of operating at low temperatures at which current battery systems would be rendered useless. They can be easily fabricated in microscale sizes which are extremely difficult for power sources such as conventional batteries. This attribute makes them extremely attractive for microsystem applications such as biomedical or MEMS sensors.
These small devices would be capable of providing low levels of power for an extremely long period of time (i.e., >100 years) and would be capable of operating over a wide range of operational environments with little if any loss of performance, most notably at extremely low temperatures (i.e., < 100 K), but also in harsh biological environments.
This work began under a Director’s Discretionary Fund award with collaboration with Dr. Ryne Raffaelle at Rochester Institute of Technology and Dr. Stephanie Castro at OAI. In house cell development has been done by Dave Wilt.
Photovoltaics and Power Technologies Branch
Space Environment & Experiments Branch