The effect of the side-chain length of poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT) on the performance of hybrid polymer-metal oxide photovoltaics (PVs) utilizing zinc oxide (ZnO) acceptor is investigated. The pATBT attached with a dodecyl side chain (pATBT-C12) in hybrid photovoltaics with ZnO was compared to pATBT with a hexadecyl side chain (pATBT-C16). Atomic force microscopic analysis reveals a smoother surface for the pATBT-C16 photoactive layer compared to the pATBT-C12. For hybrid PVs using pATBT-C16, the relative intensity of the external quantum efficiency (EQE) increased particularly in wavelength region associated with the ZnO. Furthermore, the EQE spectrum shows a red shift for pATBT-C16 indicating better structural ordering compared to hybrid PVs with pATBT-C12. As a result, the hybrid PV utilizing pATBT-C16:ZnO blend layer is observed to display a better performance with a power conversion efficiency of 1.02% compared to 0.672% of pATBT-C12:ZnO PV.