AN IMPROVED SELENIUM PHOTOVOLTAIC CELL 



T. K. LAKSHMANAN 



Weston Instruments, Division of Daystrom, Inc. 



Newark, New Jersey 



ABSTRACT 



The selenium photovoltaic cell is frequently- 

 used in measurements of water transparency and 

 in underwater measurements in oceanographic 

 research. The photovoltaic cell described in 

 the present paper is an improved type of Weston 

 photronic cell. It consists of two semiconduc- 

 tors in intimate contact and the junction 

 between the semiconductors is the source of the 

 photo-emf . 



The cell has high sensitivity particularly at 

 low light levels. At very low illuminations the 

 new cell has been found to generate an emf sev- 

 eral times higher than the older types . The 

 various cell characteristics, such as dependence 

 of', output on illumination, spectral response, 

 time constant, fatigue and temperature dependence 

 are described. A brief description is also 

 given of the cell construction. It is presumed 

 that a photosensitive heterojunction exists 

 between the two types of semiconductors . 



shows excellent stability over a period of 

 several years. 



Cells of this type have been used in under- 

 water irradiance meters for attenuation measure- 

 ments of blue-green irradiance. Holmes and 

 Snodgrass have described an underwater irradiance 

 meter or submarine photometer designed to operate 

 in the upper 100 to 150 meters of water. It per- 

 mits direct measurement of the downward blue- 

 green irradiance. The detectors in the multi- 

 detector unit are cosine flux collectors equipped 

 with Weston photron cells. The use of the pho- 

 tronic cell in arctic oceanography has been 

 reported by LaFond.^ Water transparency is 

 measured with a hydrophotometer which consists 

 of a standard light source in a watertight 

 housing which transmits a focused light beam 

 through one meter of water to another housing 

 containing the photocell. The cell is also used 

 in an ambient light meter for measuring the 

 ambient light intensities at various depths under 

 water or ice. 



INTRODUCTION 



CELL CHARACTERISTICS 



Accurate measurement of light levels under 

 water is important in certain phases of oceano- 

 graphic research. Of the several types of opto- 

 electric transducers available the most commonly 

 used device is the selenium "barrier layer" cell. 

 This is a photovoltaic cell which generates a 

 potential difference between its terminals when 

 exposed to light. It is self-contained and can 

 operate a meter without the application of an 

 external source of power. On the other hand, 

 photoconductive cells and phototubes can only 

 be used when an external voltage supply is 

 inserted in the circuit. 



The selenium photovoltaic cell is rugged and 

 compact and can easily be encased in a water- 

 tight housing. Its output is high enough to 

 drive an ordinary meter even at low light levels . 

 Its spectral response is closer to that of the 

 human eye than that of any other photo-device. 

 With the use of a simple filter the resulting 

 response can be made to match that of the eye 

 so that the device can measure illumination as 

 opposed to radiant flux. Furthermore, the cell 



The new cell to be described is an improved 

 type of selenium photovoltaic cell. It differs 

 slightly in construction from the older type but 

 is considerably more sensitive than its predeces- 

 sor at low light levels (below 1 foot-candle). 

 Hence it is particularly suited for underwater 

 light measurements. The fabrication process is 

 more flexible and the cell has been made up in a 

 variety of shapes and sizes (Fig. l) . Standard 

 configurations are given in Fig. 2. 



Experimental cells have also been made up on 

 flexible substrates. These flexible cells, as 

 well as concave and convex cells, are useful in 

 special applications. The process also permits 

 the fabrication of large area cells. The output 

 characteristics of the new type and the old type 

 cells at moderately light levels are shown in 

 Fig. 3- The output is shown as a family of I-V 

 curves at various light levels ranging from 5 to 

 200 foot candles at a color temperature of 2870 K. 

 At each light level the external resistance is 

 changed from 3 to 10,000 ohms and the potential 

 difference across the resistor measured with a 



Superior numbers refer to similarly numbered references at the end of this paper. 



183 



