E1/E2 defects are the typical negative-U centers in n-type 6H silicon carbide (SiC). They are the main contributors to non-radiative recombination, which limits the carrier lifetime. In this study, two fluorescent 6H silicon carbide (f-SiC) samples and one bulk substrate were characterized via time-resolved photoluminescence (TRPL) and static photoluminescence (PL) measurements, where all the samples were nitrogen-boron co-doped 6H n-type. The existence of E1/E2 defects, which caused the diminution of the internal quantum efficiency (IQE) and luminescence intensity of each sample, was confirmed by applying a carrier dynamics model based on negative-U centers. The carrier dynamics simulation reveals that the density of the E1/E2 defects in bulk 6H SiC is two orders of magnitude higher than that of the f-SiC sample, causing much lower PL intensity in the bulk substrate compared to the two f-SiC samples. The IQE of the two f-SiC samples was extracted from the corresponding TRPL results, where the contrast between their IQE was further confirmed by the related PL measurement results. The slight difference in IQE between the two f-SiC samples was attributed to slightly different E1/E2 defect concentrations. On the other hand, by implementing a steady-state donor-acceptor-pair (DAP) recombination calculation, it was found that the f-SiC sample with lower IQE had a higher DAP transition probability due to the higher doping level. This prompted further optimizations in the f-SiC crystal growth conditions in order to decrease the E1/E2 defects while maintaining the correct doping parameters.