Interfacial buffer layers such as electron and hole transporting layers are crucial in maximizing power conversion efficiencies of organic solar cell (OSC) devices. In this manuscript, we studied the role of solution processed ZnO as electron transport layer in devices with Aluminum (Al) cathode on opaque substrates in inverted device configuration where Al and Gold (Au) are used as cathode and anode, ZnO and MoO3 are used as electron and hole transport layers respectively whilst P3HT:PC(60)B3M is used as active layer (ActL). The results show that a thin layer of Aluminum Oxide (Al2O3) present on thermally evaporated Al cathode acts as an electron transport layer (ETL) when ZnO is absent, with surface states present on Al2O3 layer aiding the electron transport to cathode. The presence of ZnO as ETL between Al and active layer leads to a decrease in the efficiency of solar cell from 2.9% to 1.9%, attributed to the trapping of electrons at Al/ZnO interface due to unfavourable band alignment created by the presence of composite Al2O3/ZnO layer, implying that an additional ETL is not required when Al is used as a bottom electrode and could be an added advantage due to reduced processing steps.