Microdefects and point defects optically detected in Cu(In,Ga)Se2 thin film solar cells exposed to the damp and heating

Optically detected changes have been studied in Cu(In,Ga)Se2 (CIGS) thin film solar cells, which were exposed to the damp and heat test by IEC 1215 international standard recommendations. High-resolution optical microscopic images at T=300 K and emission properties at T=20 K of ZnO/CdS/CIGS devices were characterized and compared to the tested non-incapsulated device. The near-gap photoluminescence peak at 1.191 eV for the baseline device drastically decreases after the test. Long wavelength emission bands at 1.13 and 1.07 eV, associated with optical transitions through defect levels in absorber, retain their intensity and spectral position. Microscopic surface morphology deteriorates after the test: appearance of micro-scale defects and reduction of optical reflectivity have been observed in blue-violet light and polarization with good contrast. A decrease of conversion efficiency of the exposed solar cell is caused by the degradation of upper wide-gap films and heteroboundary between CdS and CIGS.