Methylammonium lead iodide perovskite solar cells with Zn1-xMgxO buffers (hole-blocking layers) were fabricated to optimize conduction band offset (CBO) of buffer/perovskite absorber interface. The Zn1-xMgxO films were prepared by sol-gel and sputtering methods. It is disclosed that the Zn1-xMgxO buffers with increasing [Mg]/( [Mg] + [Zn]) from 0 (pure ZnO) to 0.15 prepared by the sol-gel method lead to the enhancement of conversion efficiency (q) from 10.7 to 15.1%, attributed to the improvement of the CBO. However, the Zn1-xMgxO buffers with enhancing [Mg]/([Mg] + [Zn]) from 0 to 0.10 deposited by the sputtering method yield the decrease in eta from 6.6 to 5.6%. The lower eta in the case of the sputtering method is mainly caused by the lower short-circuit current density (J(SC)). According to micro-scale current path distributions of ZnO-coated SnO2:F (FTO) substrates, local current easily flows in case of ZnO-coated FTO sample by sol-gel method. On the other hand, the local current hardly flows especially at peak regions of sample surface in case of ZnO-coated FTO sample by sputtering method. This is because ZnO (or Zn1-xMgxO with [Mg]/([Mg] + [Zn]) of 0) by sputtering has higher resistance than that by sol-gel method and tends to be formed on peak regions of the FTO surface. Consequently, the low J(SC) of the perovskite solar cells with the buffer by sputtering method is observed.