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Based on density functional theory,first-principles calculation is applied to study the electronic properties of undoped and Ag-doped ZnO-Σ7 (1230) twin grain boundaries (GBs).The calculated result indicates that the twin GBs can facilitate the formation and aggregation of Ag substitution at Zn sites (Agzn) due to the strain release.Meanwhile,some twin GBs can also lower the ionization energy of Agzn.The density of state shows that the O-O bonds in GBs play a key role in the formation of a shallow acceptor energy level.When Agzn bonds with one O atom in the O-O bond,the antibonding state of the O-O bond becomes partially occupied.As a result,a weak spin splitting occurs in the antibonding state,which causes a shallow empty energy level above the valence band maximum.Further,the model can be applied to explain the origin of p-type conductivity in Ag-doped ZnO.