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.