Analysis of the interaction of an electron beam with a solar cell

In continuation of previous work, the short circuit current Isub(SC) generated by a collimated electron beam impinging on an N-P junction (solar cell) is investigated in a configuration in which the beam scans the front surfaces of a solar cell crossing the ohmic contact strips. The analysis employs Fourier and Wiener-Hopf techniques and shows that even in the idealized case of uniform doping in both the N-material and the P-material the scanning electron beam gives little information about junction parameters (diffusion lengths, surface recombination velocities etc.). A recently proposed method for measuring the surface recombination velocity by means of changing the beam energy is inapplicable for shallow junctions (junction depth approximately 0.1 μm) due to the fact that the radius of the beam-semiconductor interaction volumes is larger or comparable to the characteristic lengths, junction depth, depletion layer width and diffusion length of minority carriers in the N-material. The uncertainties of the distribution in space of excess carriers generated by the electron beam prevent an accurate determination of junction parameters. If, however, the ohmic contact on the back surface of a solar cell is partially removed, scanning across the free surface toward the ohmic contact yields useful information about the bulk diffusion length. (author)