Naphthalene-diimide selenophene copolymers as efficient solution-processable electron-transporting material for perovskite solar cells

Three naphthalene-diimide semiconducting polymers (NDI-Se, NDI-BiSe, NDI-TriSe) were synthesized via copolymerization of naphthalene-diimide and selenophene building blocks (selenophene, biselenophene, and triselenophene). The physical, optical, electrochemical and electronic properties of these polymers are characterized by thermogravimetric analysis (TGA), UV–Vis, X-ray powder diffraction (XRD), cyclic voltammetry (CV) and space charge limited current (SCLC). These polymers show excellent thermal stability, high crystallinity, and well matched energy level to that of perovskites. Utilizing these polymers as single and solution processed electron transporting layer (ETL), p-i-n type perovskite solar cells with a configuration of ITO/NiO_x/MAPbCl_0.2I_2.8/ETL/Ag were successfully fabricated with the highest efficiency (14.0%) achieved from NDI-TriSe solar cell, which is comparable to the standard cell (14.6%) using thermally deposited C₆₀/BCP bilayer ETL. Our results demonstrate NDI-selenophene based copolymers are very promising electron transporting materials for highly efficient and solution processable perovskite solar cells.