Organic polymer solar cells (PSCs) have lots of advantages for low-cost technology , light weight, easy fabrication and have drawn a great deal of attention. In recent years, many research groups are devoted to the study of processing, materials, and device structures. As a result, the PCE increases rapidly in a short time. In this study, we investigate the inverted structure solar cells with zinc oxide as the electron transport layer because of its stability. The ZnO film in our solar devices is deposited by sol-gel technique. Our study shows that more ZnO layers lead to flatter film. This would make the contact between ZnO and the active layer better. The short circuit current (Jsc) of our device is enhanced. The PCE of P3HT:PC61BM device with 3-layer ZnO is 3.78%. It should be noted that, the conductivity of ZnO thin film can be improved by addition of small amount of Al. The PCE of device with 3-layer of AZO is 3.91%. In addition, we use a low bangap material, PBDTTT-C-T, to increase short circuit current because of its broad light absorption enhancement. The surface morphology of these devices can be modified by additives such as DIH. The maximum PCE of PBDTTT-C-T:PC71BM devices is 6.51% with 4% DIH because the cluster size of organic material is suitable for carrier extraction in this mixture ratio. Finally, we study the lifetime of aforementioned devices. The PCE of the device without encapsulation under N2 environment is still above 97% after 900 hours. The PCE of the device without encapsulation under ambient environment of air is above 79% after 900 hours.