The wet repair of sidewall damage in flexible inverted metamorphic five-junction solar cells

Abstract

The more III-V sub-cells are stacked in series, the higher the theoretical efficiency. However, the significant mismatch stress and the increased thickness of the epitaxial layers in the directly grown five-junction solar cells adversely affect the device fabrication. The lateral etching damage on the cell sidewalls is exacerbated during the chip isolation process. This work uses a mixed solution of citric acid and hydrogen peroxide to perfectly clean and repair the damage to the side walls of five-junction solar cells. The leakage current density is reduced from 2.51 mA/cm² to 1.45 mA/cm², which increases the shunt resistance from 0.70 kΩ·cm² to 10.37 kΩ·cm², and restores the fill factor (FF) of the cell from 65.08 % to 85.01 %. The scanning electron microscope (SEM) and auger electron spectroscopy (AES) measurements have demonstrated that the wet repair process can effectively remove contaminants from the sidewalls and therefore reduce additional current losses. Ultimately, the conversion efficiency (Eff) of the flexible five-junction (5 J) solar cell is 36.24 % (The size is 8.03 cm²) with the open-circuit voltage (V_oc) of 4.77 V under AM 1.5G spectrum. The wet repair of sidewall damage is expected to produce high-efficiency, large-size, flexible cells with five or even more junctions.