CONVERTERS OF SOLAR ENERGY — ^HOTTEL 159 



the thin metal cover of the cuprous oxide, it passes through that and 

 through the cuprous oxide layer, penetrating to some point in the 

 structure where the composition lies between that of cuprous oxide 

 and copper (the so-called blocking layer) ; and there the quantum — 

 the bullet of energy — succeeds in knocking out an electron from the 

 crystal lattice. The electron, being liberated in territory where the 

 view depends on which way it looks, finds, in general, that the going 

 is easier when it migrates toward the copper rather than through the 

 cuprous oxide to the other metal film. This preferential movement 

 of the electrons in one direction constitutes an electric current. 



How important is this phenomenon for power generation from 

 sunlight? Tests on copper oxide photocells indicate that of the 

 visible light quanta falling on such a cell only about 5 percent suc- 

 ceed in causing an electron to show up in the external electric circuit, 

 that, furthermore, the voltage efficiency of the system is only about 

 10 percent, with a consequent over-all efficiency of conversion of 

 luminous energy to power of one-half of 1 percent. Preliminary 

 calculations indicate that a tenfold increase in this efficiency would 

 make copper oxide cells interesting for solar power production ; and 

 there is no present reason to believe such an accomplishment im- 

 possible. It is not easy, however, for the physicist doesn't really 

 know just what goes on in the blocking layer of the photocell. 

 Clearly the problem is one which demands a fundamental study com- 

 pletely divorced from any present considerations of a practical nature. 

 Such a project has been initiated in our Electrical Engineering Depart- 

 ment in connection with a broad program of study of insulators and 

 semiconductors — the cuprous oxide of our photocell is such — from the 

 atomphysical viewpoint. The problem is really one of studying the 

 laws of motion of electrons in semiconductors; the effect of crystal 

 versus amorphous structure; of crystal structures in which there is 

 strong ionic binding, such as sodium chloride versus crystal struc- 

 tures in which the bonding is atomic, as in sulfur; the effect of tem- 

 perature on conduction and break-down in insulators; the effect of 

 an excess of one of the components of a crystalline compound present 

 in the crystal. When the nature of the migration of electrons in 

 semiconductors is better understood, when their interaction with the 

 lattice structure is able to be formulated quantitatively, then one can 

 attack with some hope of success the difficult barrier-layer photocell 

 problem. Whether such an attack succeeds or not, the knowledge 

 acquired in the course of the problem is certain to be of enormous 

 value in a field of great practical importance, insulation research. 



I come now to the last of the M. I. T. solar-energy projects, one 

 which like the previous one depends on the special properties of 

 sunlight rather than its over-all energy content. Dr. Thimann 



