Zinc Oxide Coated Carbon Dot Nanoparticles as Electron Transport Layer for Inverted Polymer Solar Cells

Interface engineering is a powerful tool to improve the performance of polymer solar cells (PSCs), and zinc oxide (ZnO) is a significant interfacial material for PSCs. However, ZnO is easy to agglomerate, which leads to low charge conductivity and poor stability; moreover, the hydroxyl groups on its surface also lead to a large number of defects, which restricts the application of ZnO. In order to improve the dispersion stability of ZnO nanoparticles and inhibit its surface defects, ZnO coated carbon dot (CD@ZnO) nanoparticles are first synthesized by direct particle precipitation. The introduced CD induces and participates in the growth of ZnO crystal. As a result, CD@ZnO nanoparticles show better colloidal stability, wider energy band gap, and fewer surface defects, which enhances the exciton extraction and restrains the charge recombination at the interface of the active layer and electron transport layer (ETL) of PSCs. Therefore, the device based on poly[[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]-2,5-thiophenediyl]:3,9-bis(1-oxo-2-methylene-3-(1,1-dicyanomethylene)-5,6-difluoroindanone)-5,5,11,11-tetrakis(4-n-hexylphenyl)dithieno[2,3d:2′,3′d′]-s-indaceno[1,2-b:5,6-b′]dithiophene with CD@ZnO as ETL exhibits a greatly strengthened power conversion efficiency of 12.23% compared to 11.26% of the reference device. Meanwhile, the CD@ZnO ETL also achieved a big performance boost in fullerene-based solar cells. This work offers an available method using CDs to modify ZnO for highly efficient PSCs.