2B.1. Silicon Cells.

At the outset it is important that in analyzing solar efficiency data one must determine whether the diffusion length is longer or shorter than the absorber layer thickness. In the event that diffusion length is longer than the absorber layer thickness then one would expect that all the incident photons would reach the external circuit. In this event one would expect, since the incident radiation has a constant intensity for the same conditions of test, that the short-circuit current would be independent of the grain size and of the diffusion length. In fact, as shown in Figure 1.11, this is validated by the collected data. Hence, any systematic variation for this constraint on diffusion length/absorber layer thickness of the efficiency must be due to the open circuit voltage.

Figure 1.11: Showing independence of short-circuit current on grain size and diffusion length for solar cells in which diffusion length exceeds absorber layer thickness. Grain size, +; diffusion length, filled dots. (Data from Ref. [129].)

When the relation between diffusion length and absorber layer thickness is reversed we should expect that the short-circuit current would depend upon diffusion length and grain size. As shown in Figure 1.12 this expectation is obeyed by the data. (For both figures the data were produced by a variety of different sources and scatter in these results should thus be expected, as revealed. However, the conclusions drawn above are not dependent on this scatter.)

Now, we plot for...