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• The low-electron-density defect is detected by positron annihilation spectra. • A quantitative correlation between defects and performance is demonstrated. • The correlation is pervasive in various materials and photochemical processes. • Decreasing the defect concentration would promote electron transfer rate. • The sample with lower defect concentration shows higher photochemical activity. Establishing definite correlations between specific defects and physicochemical processes is the prerequisite for applying defect-engineering to optimize material properties. However, systematic investigations are required to fulfill this task. In this work, for the first time, a kind of defect with low electron density is quantitatively demonstrated to be correlative with photocatalysis efficiency by using perovskite oxide Na 0.5 Bi 2.5 Ta 2 O 9 nanosheets as the model material, in which the defect was systematically introduced and engineered. More importantly, the correlation is found to be not only valid for photocatalysis in Na 0.5 Bi 2.5 Ta 2 O 9 , but also is pervasive in various materials and different photochemical processes, though it has been overlooked in previous reports. The defect with low electron density acts as the recombination center for photogenerated carriers, therefore it is detrimental to the photo(electro)chemical performance, and the elimination of this kind of defect will be critical for improving photochemical efficiency. [ABSTRACT FROM AUTHOR]