We use SENTAURUS DEVICE simulation to investigate the effect of "passivated emitter and rear cell" (PERC) and "passivated emitter and rear, totally-diffused" (PERT) device architecture on the solar cells' weak light performances. Injection-dependent carrier lifetimes can also strongly influence the fill factor and weak light performance of solar cells. To focus on the effect of the device architecture alone, we present here simulations with essentially injection independent carrier lifetimes. In our simulations we find that at 1/10 of AM 1.5G ("tenth of one sun") the standard industrial cell architecture with full-area BSF loses about 1.7% efficiency and the similar PERT cell structure loses 1.4% to 1.6%, depending on wafer quality. The PERC cells suffer only 1.2% to 1.5% loss of efficiency at 1/10 sun. The lower losses for PERC solar cells result from the fact that at lower illumination intensity the relatively high resistance of PERC cells is less significant. We furthermore find that only for PERC solar cells the optimum wafer doping concentration depends on the illumination intensity.