The <strong>efficiency of collecting the injected charge at the back contact</strong> 
is exactly as it sounds - the ratio of the number of electrons that
reach the back contact (the counterelectrode) to the total number of 
electrons that were injected into the semiconductor.  This expresses
the efficiency of the actual circuit - that is, the chance that a charge,
one injected into the system, will pass through the circuit and reach the
counterelectrode.