Sodium Incorporation for Performance Improvement of Solution-Processed Submicron CuIn(S,Se)₂ Thin Film Solar Cells

Low-cost solution-processed CuIn(S,Se)₂ (CISSe) has great potential for large-scale production of photovoltaics (PV). However, low power conversion efficiency caused by poor crystallinity is one of the main drawbacks compared to vacuum-processed CISSe solar cells. In this work, three strategies for sodium (Na) incorporation into solution-processed CISSe by soaking in sodium chloride (NaCl) aqueous-ethanol solution [1 molarity (M) for 10 minutes (min)], either prior to absorber deposition (pre-deposition treatment, Pre-DT), before selenization (pre-selenization treatment, Pre-ST), or after selenization (post-selenization treatment, PST) are researched. The Pre-ST CISSe solar cells achieve a better PV performance than those from the other two strategies of Na incorporation. For optimization, soaking times (5, 10, and 15 min) and NaCl concentrations (from 0.2 to 1.2 m) of the Pre-ST are researched. The highest efficiency achieved is 9.6% with an open-circuit voltage (V_oc ) of 464.5 mV, a short-circuit current density (j_sc ) of 33.4 mA cm^-2 , and a fill factor (FF) of 62.0%. Compared to the reference CISSe solar cell, V_oc , j_sc , FF, and efficiency of the champion Pre-ST CISSe device are improved absolutely by 61.0 mV, 6.5 mA cm^-2 , 9%, and 3.8%, respectively. Simultaneously, the open-circuit voltage deficit, the back contact barrier, and the bulk recombination are found to be reduced for Pre-ST CISSe.