Lift-off process reducing crack formation and its Cu(In,Ga)Se2 thin film solar cell applications

Abstract

► We develop the technique to transfer a Cu(In,Ga)Se 2 layer without forming cracks. ► The thickness of the transferred films is important in the crack reduction. ► This technique is applied to Cu(In,Ga)Se 2 solar cell fabrication. ► The crack reduction is effective to improve the solar cell performance. To reduce cracks caused by the lift-off process in a Cu(In,Ga)Se 2 (CIGS) layer, we focused on increasing the transferred layer thickness. We investigated the relationship between crack formation and the transferred layer thickness which is controlled by a Mo back electrode thickness. We found that the cracks were reduced by increasing the back electrode thickness. We suggest that the dominant factor of the crack reduction is attributed to the increase of the film hardness by increasing the Mo back electrode thickness. Next, we applied this crack reduction method to the solar cell fabrication. From the comparison of the 0.2-μm-thick Au single and 0.2-μm-thick Au/1.6-μm-thick Mo stacked back electrode lift-off CIGS solar cells, we investigated advantages of our crack reduction method. The crack formation was reduced only for the stacked back electrode lift-off solar cell. From the spatial distribution evaluation of an external quantum efficiency (EQE), we found that the crack reduction leads to not only the increase of an average EQE but also the decrease of EQE dispersion. In the solar cell parameters, the stacked back electrode lift-off solar cell without cracks showed the short-circuit current density and fill factor higher than those of the single back electrode lift-off solar cell with cracks. As a result, the conversion efficiency improvement as high as approximately 1% (an absolute value) was obtained. Moreover, the stacked back electrode lift-off solar cell showed the diode parameters (the diode ideality factor, the saturation current density, and series resistance) better than those of the single back electrode lift-off solar cell in the dark current density–voltage characteristics. We concluded that this high fill factor was attributed to the better diode performance. We therefore found that the stacked back electrode structure was very effective for improving the solar cell performance using the lift-off process.