Comparison of ZnO buffer layers prepared by spin coating or RF magnetron sputtering for application in inverted organic solar cells

We compared the electrical, optical, structural, and morphological properties of radio-frequency (RF) magnetron-sputtered ZnO and solution-processed ZnO nanoparticle (NP) buffer layers on ITO cathodes for use in inverted polymer solar cells (IPSCs). Continuous sputtering resulted in integration of the ZnO buffer layer in the ITO cathodes, which were then used as transparent cathodes for IPSCs. Although the electrical, optical, and morphological properties as well as work function of RF-sputtered ZnO film were similar to those of solution-processed ZnO NP film, the power conversion efficiency (PCE) of IPSCs with an RF-sputtered ZnO buffer layer was much lower than that of IPSCs with a solution-processed ZnO NP buffer layer due to vertical phase segregation of the organic active layer. However, intentional bias sweeping of IPSCs with an RF-sputtered ZnO buffer layer improved performance due to diffusion of PC₇₀BM through the PV-D4610 donor layer and formation of a suitable heterojunction structure. Based on transmission electron microscope examination and dark current-voltage curves, we suggest a possible mechanism to explain the difference in behavior of RF-sputtered ZnO and solution-processed ZnO NP buffer layers in IPSCs.