ILLUMINATION FOOT CANDLES 

 TUNGSTEN LIGHT 2700° K 



Fig. 6. 



Open circuit voltage as a function 

 of illumination. 



The improved performance is "believed to be due 

 to the difference in the construction of the cells. 

 The photovoltaic junction in the old type cell is 

 bet-ween crystalline selenium and a thin, sputtered 

 film of cadmium. A top layer of gold is applied 

 over the cadmium to improve the conductivity of 

 the cells . In the new type cell a thin film of 

 cadmium oxide replaces the cadmium and gold films. 

 Sputtered cadmium oxide is more or less trans- 

 parent and electrically conducting, forming a. 

 highly photosensitive barrier with selenium. 4 ' 

 Selenium is a p-type semiconductor and cadmium 

 oxide is n-type. A complex junction is believed 

 to exist between the two types of semiconductors.-' 



The semiconducting properties of selenium and 

 cadmium oxide, and the manner of application of 

 these materials, determine very considerably the 

 photovoltaic properties of the resulting cell. 

 The crystalline nature and the type and amount 

 of impurities in selenium are extremely important 

 as well as the degree of non-stoichiometry and 

 the impurity concentration of cadmium oxide. The 

 optical and electrical properties of cadmium 

 oxide which is an oxygen deficient semiconduc- 

 tor, have been shown to be strongly dependent on 

 these parameters. 



The relative spectral response of the cell is 

 seen in Fig. 7. The cell sensitivity is moderate 

 in the blue, peaks in the green and falls off 

 rapidly towards the red. By using a visual cor- 

 rection filter with the cell the resultant 

 response is made to match that of the human eye. 



The time constant of the cell is a function 

 of the illumination and the load resistance. The 

 rise and decay times are usually different from 

 each other. For moderate illuminations and with 

 moderate values of load resistance, the rise and 

 decay times range between 0.1 and 5 milliseconds. 



All selenium cells show a small amount of 

 fatigue which is the gradual change in the out- 

 put for a brief interval immediately following 

 exposure to light. With the present cells such 



400 



650 



450 500 550 600 



WAVELENGTH MILLIMICRONS 



Fig. 7. Relative spectral response. 



EXT RES IN OHMS 



1000 



TIME IN MIN. 

 100 



Fig. 8. Cell fatigue. 



effects are small and are generally within 5$. 

 Fig. 8 shows this effect. 



Temperature effects are also a function of 

 the illumination and the load resistance. Fig. 9 

 shows the temperature dependence between 5 and 

 l4-5°C at two different light levels. The varia- 

 tion with temperature is due to the fact that the 

 internal resistance of the cell is strongly 

 dependent on temperature. 



A selenium photovoltaic cell can be repre- 

 sented electrically as a current generator, 

 shunted by a capacitance, C, and an internal 

 resistance, R^ in series with a small resis- 

 tance, R s . If the primary photoelectric current 

 is ip and the load resistance R, the current 

 through the latter is given by 



i R. 

 P i 



E i +E s +R 



(1) 



This picture is a rather naive representation 

 of the cell performance. The actual mechanism 

 of the cell is much more involved and can only 

 be explained in terms of semiconductor junction 

 theory. So far the exact junction parameters 

 have not been worked out . 



186 



