INTRODUCTION 



Photovoltaic cells, commonly called solar cells, have for many years served as a reliable 

 source of electric power for satellites. The power output of solar cells at that location is 

 maximized due to absence of the earth's atmosphere, which would otherwise absorb some of 

 the sun's radiation. As a result of solar cell technology developed for outer space applica- 

 tions, the cost of high-efficiency silicon solar cells has decreased progressively from approxi- 

 mately $100/W to less than $25/W. At this price, solar cells are rapidly becoming an attractive 

 source of electrical power for many terrestrial applications that require a steady supply of 

 direct current for charging storage batteries. The absence of moving parts and associated 

 maintenance, as well as total independence of fuel supplies, makes solar cells very cost effective 

 for supplying electric power to electronic instrumentation at remote locations. 



Solar cells have not yet been utilized as sources of electric power for electronic instru- 

 mentation mounted in submerged marine electronic devices. The reasons for this are many: 

 the high cost of the cells, the lack of data on the effect of submergence on power output, 

 and the detrimental effect of biofouling. This state of affairs will change in the near future 

 as (1) improved fabrication technology lowers the cost of solar cells, (2) performance data 

 for submerged cells become available and (3) effective approaches are developed to control 

 biofouling on solar cell surfaces submerged for long periods of time. This report summarizes 

 an exploratory study conducted in this technological area by the Naval Ocean Systems Center, 

 San Diego, California. 



BACKGROUND 



SOLAR CELL CONSTRUCTION 



Solar cells represent semiconductor devices capable of converting light directly into 

 electricity. The most efficient one developed to date for practical applications is a silicon 

 solar cell; its power output is in the 12- to 15-percent range of incident normal solar insolation. 

 Because pure silicon is a poor conductor of electricity, it is doped with other elements to 

 make it more conductive. The addition of phosphorus during the growth of the silicon 

 crystal develops a negative charge carrier; such silicon is designated n-type. The addition of 

 boron creates positive charge carriers; such silicon is designated p-type. 



The typical silicon solar cell consists of a thin slice of boron-doped silicon, which has 

 been diffused on one side under high temperature by phosphorus. The boundary between 

 the p-type and n-type materials constitutes the p-n junction. When the photon from the sun 

 strikes an electron near the p-n junction region, a negative-charge (electron) and a positive- 

 charge (hole) pair is created. The negative charge will now travel toward the n-type silicon 

 region, while the positive charge will move to the p-type silicon region. 



1. Kelly, B. P., Eckert, J. A., and Berman, E., "Investigation of Photovoltaic Applications," presented at 

 International Congress on The Sun in the Service of Mankind, July 1973, Paris, France. 



