Application of a substrate bias to control the droplet density on Cu(In,Ga)Se2 thin films grown by pulsed electron deposition

One of the main shortcomings in the fabrication of thin-film solar cells by pulsed high-energy deposition tech- niques (i.e. Pulsed Laser Deposition or Pulsed Electron Deposition -- PED), is the presence of a significant number of particulates on the film surface. This affects the morphological properties of the cell active layers and, ultimate- ly, the performance of the final device. To reduce the density of these defects, we deposited a Cu(In,Ga)Se2 (CIGS) thin film by PED and studied the effect on the film morphology when a DC bias was applied between the sub- strate and the target. Our results show that a negative substrate voltage, comprised between 0 and -300 V, can not only reduce the droplet density on the CIGS film surface of about one order of magnitude with respect to the standard unbiased case (from 6 × 105 to 5 × 104 cm- 2), but also lower the maximum particulate size and the surface smoothness. When a positive voltage is applied, we observed that a significant increase in the droplet surface density (up to 108 cm- 2) occurs. The abrupt change in the preferred crystal orientation (switching from (112) to (220)/(204) by applying negative and positive biases, respectively) is also a direct consequence of the applied DC voltage. These results confirm that the external DC bias could be used as an additional parameter to control the physical properties of thin films grown by PED.