Insights into the nonideal behavior of CdS/CdTe solar cells

One-dimensional device modeling (AMPS-1D) is used to understand and explain the departures from ideal behavior seen in CdS/Cd-Te solar cells, such as rollover and crossover of the dark and light J-V curves and shifts in V_oc. Large variations of these departures occur from cell to cell and for the same cell after environmental stress. J-V curve rollover in the dark (and to some extent in the light in forward bias beyond V_oc) is explained by the inclusion of a blocking contact at the back, which limits majority-carrier injection. Crossover of light and dark J-V curves occurs because light can supply additional electron current through the cell in the rollover region, becoming the principal component of the total current at the back of the cell. Decreases in V_oc occur for light doping, and/or compensation of dopant accepters by donor recombination centers, and/or thinner cells. The authors show that the back-contact energy barrier to holes influences cell characteristics far more than the same contact energy barrier to electrons at the front. Finally, rollover of a different type that occurs in the power quadrant is explained by the inclusion of a thin n-layer under the back-contact metal