Single junction and tandem (a-Si:H/a-Si:H) thin film PV modules were fabricated by a production line large batch process with different process parameters and light soaked in outdoor sunlight. The performances were analyzed and compared with module stability data obtained from a large-area, uniform indoor solar-radiation simulator. Tandem (double junction) modules are notably more stable with higher post-exposure output power compared to single junction modules. Optimizations in a-Si:H deposition have led to typical light-induced degradation of about 10% and 20%, respectively, for tandem and single junction PV modules. Grading scheme at the p-i interface and i-layer growth rate greatly influence the 'stabilized' output power of manufactured a-Si:H PV modules. The presence of carbon in the i-layer resulted in higher initial, but lower degraded, module power for both single junction and tandem modules.