TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Composition: The ZnS content of the buffer layer of the solar cell has a continuous concentration gradient and the ZnS content decreases in a direction from the first buffer layer to the second buffer layer. The ZnS content of the first buffer layer ZnS/Zn(S,O,OH) is about 0.25 to about 0.63 and a ZnS content of the second buffer layer ZnS/Zn(S,O,OH) is less than about 0.25, optionally the ZnS/Zn(S,O,OH) of the first buffer layer is from about 0.25 to about 0.61, optionally from about 0.27 to about 0.60, and optionally from about 0.29 to about 0.50. ZnS/Zn(S,O,OH) of the second buffer layer is from about 0.10 to about 0.24, optionally from about 0.15 to about 0.23, and optionally from about 0.19 to about 0.23. A surface of the light-absorption layer of the solar cell has a sulfur concentration of more than 0.5 atomic percentage (at.%). Concentration of zinc sulfate (ZnSO4) is in a range of about 0.01 M to about 0.1M, optionally, where the thiourea (SC(NH2)2) concentration is in a range of about 0.2M to about 1.3M and, optionally, where the ammonium hydroxide (NH4OH) concentration is in a range of about 1 M to about 5M. Preferred Parameters: The total buffer layer is between 1 nm to about 200 nm in thickness, optionally 1.5 nm to about 10 nm and optionally 1.7 nm to about 8 nm. A thickness ratio of the first buffer layer to the second buffer layer in the solar cell is in a range of 3:1 to 1:3, optionally, a thickness ratio of the first buffer layer to the second buffer layer in the solar cell is in a range of 1.1:0.9 to 0.9:1.1. Preferred Components: The light-absorption layer region is a composition represented by Cu(In1-xGax)(Se1-ySy)2, where 0 less than or equal to x not less than 1, 0 not less than y not less than 1. The solar cell also contains a third buffer layer disposed on the first buffer layer, optionally formed of an intrinsic zinc oxide (ZnO). Preferred Process: The solar cell is produced by a process which involves preparing an aqueous solution including ZnSO4, SC(NH2)2 and NH4OH; and immersing a light absorption layer including a p-type compound semiconductor in the aqueous solution for about 7 minutes to about 30 minutes. After the forming of the buffer layer, the process includes annealing at a temperature of about 100 to about 300 degrees C in an atmospheric condition. Preferred Conditions: The temperature of the aqueous solution is in a range of about 55 degrees C to about 70 degrees C. The pH of the aqueous solution is in a range of about 10 to about 13.