Improving the photovoltaic performance of CIGS solar cells with the modified 3-stage co-evaporation process

Abstract

The 3-stage co-evaporation process has been adopted as one of the standard deposition techniques for developing high efficiency CuIn1-xGaxSe2 (CIGS) thin film solar cells. This method often results in a double-graded [Ga]/([Ga] + [In]) or x value, leading to a double-graded bandgap in the CIGS films. The main challenge is to increase open-circuit voltage (Voc) without losing short-circuit current density (Jsc) of the devices. To achieve this goal without extra elemental sources or processes during fabrication, the standard 3-stage co-evaporation process could be modified by adjusting the deposition rate of Ga in the 1st and 3rd stages to enhance the double-graded profile of x. In the 1st stage, x1 is varied in the range of 0.37–0.60, whereas x3 is varied in the range of 0.15–0.37 in the 3rd stage. This is to be compared with our reference 3-stage process using constant x1 = x3 = 0.37. The effects of the x values on the photovoltaic properties of the CIGS devices and the elemental depth profiles of the CIGS absorber have been examined. In this work, the champion device has a maximum efficiency of 17.61 %, a Voc of 698.4 mV, a Jsc of 33.81 mA/cm2 and a fill factor (FF) of 76.12 %. This was achieved by using x1 = 0.55 and x3 = 0.25. These parameters demonstrate a significant beneficial improvement over reference 3-stage process and result in the notch position (the minimum of the x value) being approximately 500–800 nm below the surface of the CIGS films.