A comparative study of ZnO i-layer deposited with ALD and PVD for CIGS solar cells

Abstract: Two identified setbacks for CIGS based devices in order to obtain higher efficiency are parasitic absorption in the window layer structure and losses in open-circuit voltage due to bad interfaces. This study investigated how the performance of the solar cell is affected by depositing intrinsic ZnO (i-ZnO) and ZnMgO with atomic layer deposition (ALD) instead of the conventional sputtering. No significant improvement in fill factor was obtained by the use of ALD compared to sputtering, leading to the conclusion that pinholes in the sputtered film are not a detrimental factor for the cell. As the thickness of the i-layer increased, an increase in FF was observed for the ALD-deposited i-layer, whereas a decrease was observed for the sputtered i-layer. The open-circuit voltage was considered constant between the two series with only small fluctuations, indicating that the defect chemistry of the i-ZnO/CdS interface was not improved with the use of ALD. In this study it is shown that a gain in short-circuit current can be obtained for CIGS solar cells in the high energy region of the spectrum by reducing the thickness of the i-ZnO, as well as alloying the ZnO with Mg. When compared with a baseline layer sample with a sputtered i-layer thickness of around 90 nm, the estimated gain in short-circuit current density without a loss in fill factor was 0.14 and 0.20 mA/cm2 for ALD and sputtering, respectively. For the series with a ZnMgO i-layer, the highest estimated gain was 0.17 mA/cm2. This was observed for the sample with a 4:1 (Zn:Mg) pulse ratio, whereas higher Mg contents yielded a too high band gap that resulted in an electron blocking barrier.