Sec. 5-1] RADIATION TRANSDUCERS 269 



Detectors," in "Advances in Electronics," p. 63, Academic Press, Inc., New 

 York, 1953. For an extensive list of references, see also P. Gorlich, Arch. tech. 

 Messen, J 394-1, September, 1958. 



5-13. Photovoltaic Cells 



The photovoltaic, or barrier-layer, cell, also Photronic cell (trade 

 name, Weston Instruments Division of Daystrom, Inc.), consists 

 basically of three solid layers, Fig. (5-1)30, a base plate B, a semicon- 

 ducting layer S, and a transparent metal electrode T . Proper proc- 

 essing produces a thin insulating barrier layer between the semi- 

 conductor and the transparent metal layer. If light strikes the 

 barrier layer, a voltage is generated Light 



between the output terminals, the r x 

 base plate being positive, the s- 

 transparent electrode negative. b 



Crystalline selenium is mainly 



V/////////////, 



used as a semiconducting material. Fig. (5-1)30. Photogalvanic system: 



Cells With Silicon and germanium B - base plate; S, layer forming barrier 

 n . , 1-1111 layer at interface with transparent 



layers have been described lately. n / etal layer T 

 The cuprous oxide employed for- 

 merly is rarely used today. Cuprous oxide lends itself to the produc- 

 tion of two different cell types : one where the barrier layer is formed 

 between the base plate and the cuprous oxide layer (back-wall cells) 

 and another where the barrier layer is formed between the cuprous 

 oxide and the transparent metal layer (front- wall cells). 



For references concerning processing of photogalvanic cells or semiconduct- 

 ing substances other than those mentioned above, see the review articles by 

 Zworykin and Ramberg, op. cit., chap. 11 and references, pp. 214-215; D. 

 Geist, Arch. tech. Messen, J 392-2, December, 1954; A. Kellermann, Arch, 

 tech. Messen, J 392-3, February, 1955. 



The physical mechanism which initiates the generation of an emf 

 in photogalvanic cells is the absorption of an incident quantum 

 which leads to the formation of an electron and hole pair. The 

 electric field in the barrier layer causes a separation of the carriers 

 and, in further sequence, the generation of a charge between the 

 semiconductor and the metal layer. The charges can either flow 

 back through the barrier layer (internal current) or can cause a 

 current in an external circuit connected to the terminals. 



The open-circuit voltage is independent of the area of the photo- 

 galvanic cells. It increases in a near-logarithmic fashion with 

 increasing illumination, as shown in Fig. (5-1)31 . The short-circuit 

 current is proportional to the area of the cell and increases linearly 



