Characterization of the heavily doped emitter and junction regions of silicon solar cells using an electron beam

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Abstract
The electron-beam-induced-current (EBIC) technique is applied to the investigation of heavily doped emitter and junction regions of silicon solar cells via the beam specimen configuration in which the junction is parallel to the surface and the beam is perpendicular to it. It is demonstrated that this technique is very well suited to the investigation of the emitter and junction regions. Even though the experimental EBIC data are collected under three-dimensional conditions, it is shown analytically with the help of two numerical examples that the solutions obtained with one-dimensional numerical modeling are adequate. EBIC data for bare and oxide-covered emitter surfaces are measured and compared with theory. Good agreement is obtained when a cell quality factor of 0.89 is assumed, thus suggesting that ∼11% of the collected carriers recombined in the cell depletion region. This result is similar to Possin’s finding that there was evidence for ∼10% of recombination in the junctions of some of his silicon diodes with very shallow junctions [J. Appl. Phys. 50, 4033 (1979)]. It appears that further improvement of the solar cell conversion efficiency is possible if we can consistently make cells with an unity cell quality factor. The improvement in collection efficiency when an emitter surface is covered with an ∼100-A SiO2 film varies with beam energy. For a cell with a junction depth of 0.35 μm the improvement is ∼54% at 2 keV.

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