Effect of impurities in the CdS buffer layer on the performance of the Cu(In,Ga)Se2 thin film solar cell

The highest efficiencies of Cu(In, Ga)Se-2 (CIGS) thin film solar cells have been achieved when incorporating a thin CdS buffer layer grown by chemical bath deposition (CBD). The reason for this success has recently been discussed in terms of a pure Cd-doping effect in the CIGS layer. Such a model suggests that the bulk properties of the CBD-CdS buffer layer would be of minor importance. In this work, CBD-CdS layers having different bulk properties (i. e., concentrations of incorporated impurities) were employed in a number of CIGS solar cells. To further explore the bulk versus interface properties, half of these CIGS devices were subjected to a pre-deposition of an additional intermediate ultrathin layer. Moreover, CIGS devices made with CBD-CdS layers of different thickness were fabricated. Both standard and temperature dependent current-voltage (I - V) measurements were performed. The results indicate that the bulk properties of the CBD- CdS buffer layer indeed play an important role in the formation of the CdS/CIGS heterojunction. By increasing the impurity concentration or the thickness of the CBD- CdS layer, the open-circuit voltage V-oc was observed to substantially increase. This favorable effect was counteracted by the appearance of a crossover effect in the I - V characteristics for devices with the highest impurity concentration or thickness of the CBD- CdS layer. The pre-deposition of an ultrathin layer did not affect these results. The presence of the crossover effect was strongly correlated to the appearance of trap-assisted tunneling in addition to the thermally assisted tunneling. The observed crossover effect was suggested to originate from too high a number of impurities (defect states) in the bulk of the CdS layer. (C) 1999 American Institute of Physics. [S0021-8979(99)00409-0].