Polyethylenimine-based bifunctional interfacial layer for efficient quantum dot photovoltaics

Interface engineering, which efficiently optimizes the interfacial carrier collection and recombination, has been proven to be of importance for the emerging colloidal quantum dot solar cells (CQDSCs). Compared with the attractive modification efforts at the interface between quantum dots and the anode/electron transport layer (ETL), the interface between fluorine-doped tin-oxide (FTO) cathodes and the ETL, for which there exists a band alignment mismatch and high trap density in ZnO, has been investigated less. Herein, two kinds of ethylenimine-based polymers, branched by only ethylenimine groups (b-PEI) and by both ethylenimine/ethoxylated groups (e-PEI), respectively, are introduced as bifunctional interfacial layers (BILs) in lead sulfide (PbS) CQDSCs. PEI-based BILs were utilized to modulate the work function of an FTO cathode for optimizing the band alignment at the FTO/ZnO interface and to control the crystallinity of ZnO for reducing its traps. These BILs suppressed the interfacial carrier recombination and achieved a power conversion efficiency (PCE) of 11.28% in CQDSCs, which was much superior to the PCE of the reference device without BIL (10.29%). Also, the branched side chain of PEI-based BILs plays a crucial role in rationally modulating the Schottky barrier to gain different interface-optimization effects. Our work has laid a foundation for the commercial application of CQDSCs due to the advantage of low-temperature solution processability, low-cost, and scalable manufacturing.