Device degradation studies of CIGS solar cells using in-situ high temperature X-ray diffraction

In-situ X-ray diffraction (HT-XRD) was used to study the degradation mechanism of the CIGS device structure SS/Mo/CIGS/CdS/ITO. Temperature ramp HTXRD experiments carried out in both N₂ and forming gas ambient revealed formation of the solid solution γ-CuCd₂(Ga_xIn_1-x)Se₄ at ~400 °C. Time dependent XRD patterns were collected at constant temperature for four temperatures in the range 420 to 480 °C to extract the first order rate parameters for formation of this phase using the Avrami model. Activation energy of 233.5 (±45) kJ/mol and pre-exponential value 8.3×10¹³ s^-1 were estimated. Extrapolation of the reaction rate to a typical module operating temperature (50 °C) indicated the rate of formation of this complex compound is sufficiently low that the extent of reaction is negligible during the module lifetime of 30 years.