•HC-Se facilitates the Cu–Se and In–Se reaction at lower temperatures.•“Polygon grains” in HC-Se samples are necessary for selenium diffusion since they make the film incompact.•HC-Se atmosphere delays compact CIS formation at the first heating.•Ga is more uniform throughout CIGS films in HC-Se atmosphere.•Cell performance is improved in HC-Se samples due to shunt conductance reduction.Thermal-cracking system has been adopted to produce cracked selenium and the influence of cracked selenium flux on the structure and reaction pathway during the first-step selenization is investigated. High Cracked-Selenium (HC-Se) may facilitate the Cu–Se and In–Se reaction at lower temperatures. The “Polygon grains” observed in the HC-Se samples play a key role in further selenium diffusion into the film since they make the film more incompact. In addition, activation energy analysis indicates that Cu2−xSe and β-In2Se3 formed in the samples prepared in HC-Se atmosphere may result in different growth pathway of Cu(In1−xGax)Se2 (CIGS) thin film compared with that prepared in Low Cracked-Selenium (LC-Se) atmosphere during the first-step selenization, which restrains lamination in CIGS films effectively so that the distribution of Ga is more uniform throughout CIGS films and no small grains of CuGaSe2 (CGS) accumulate near the Mo back-contact. As a result, the shunt conductance in this CIGS thin film device prepared in HC-Se is reduced, and the fill factor, open-circuit voltage, as well as the cell efficiency are improved.

In this work the selenization reactions and reaction paths in CuInxGa1-xSe2 thin films prepared by sputtering and post-selenization process are investigated. The in-situ electrical resistance measurement technique is applied to monitor all the selenization reactions. The crystal structure is determined by X-ray diffraction (XRD) measurement. From the analysis of resistance-temperature curves and the XRD patterns, the phase evolutions of various crystalline and selenization reaction paths have been obtained. From these measurements, the reaction mechanisms and kinetics in the CuInGa–Se system are further understood.