Back contact formation in thin cadmium telluride solar cells

We present a model describing the undesired roll-over which is a well-known phenomenon in the current–voltage characteristics of CdTe solar cells. Therein, the roll-over is ascribed to a Schottky barrier at the back contact which is effective as a reverse diode. The formation of this barrier is investigated depending on the CdTe absorber thickness as well as on the employed back contact metal. Computer simulations of the energy band diagram reveal that the back contact barrier can be reduced and even eliminated for sufficiently thin absorbers. The reason is the spatial overlap between the space-charge regions of the p–n heterojunction with the one of the back contact. This behaviour correlates with experimental current–voltage data of solar cells with a simple gold back contact. In the latter, the roll-over is considerable for absorbers with 3 to 5 μm thickness, diminishes when the absorber thickness is reduced and finally vanishes when the absorber thickness is approximately 1 μm. The investigations show that thickness reduction can be employed in order to suppress the roll-over phenomenon in CdTe solar cells.