238 



RADIATION BIOLOGY 



EVAPORATED 

 FRONT ELECTRODE- 



CONTACT 

 RING^^ 



CRYSTALLINE 

 SELENIUM 



IRON 



requires no external source of power, as do the photoemission and photo- 

 conductive cells. The first photovoltaic cells were of the wet type and 

 consisted of two electrodes immersed in an electrolyte. These cells have 

 been displaced by the more stable and convenient dry, or barrier-layer, 



cell (Zworykin and Ramberg, 1949), 

 which consists of a semiconductor 

 layer between a metal backing plate 

 as one electrode and a thin trans- 

 parent film of metal as the other elec- 

 trode (Fig. 3-36). The electromotive 

 force is generated at a barrier layer 

 that is in the region of the interface 

 between the semiconductor and one 

 electrode. The most efficient cells 

 are of the front-wall type, in which the barrier layer is between the thin 

 transparent film on the front surface of the cell and the semiconductor. 

 Most of the manufactured cells consist of selenium deposited as a thin 



^ 



P'iG. 3-36. Diagram of the photo 

 voltaic selenium barrier-layer cell. 



120 



I I 



100 



90 



8 



> 



> 



7 



6 



50 



40 



30 



2 



I 



• BRADLEY "LUXTRON"! NO COVER) x WESTON "PHOTRONIC" 

 O VICKERS SELF-GENERATING TYPE 3 (GLASS COVER) 



PHOTOCELL (LACOUER-COATED) ^ WESTON "PHOTRONIC" 



o GENERAL ELECTRIC (GLASS COVER) TYPE 1 ( GLASS COVER 1 



«^^ 



200 



300 



800 



900 



400 500 600 700 

 WAVE LENGTH, m^< 



Fig. 3-37. Spectral responses of various selenium barrier-layer cells. {From Zworykin 

 and Ramberg, 1949.) 



