Defect Passivation of Low-Temperature Processed ZnO Electron Transport Layer with Polyethylenimine for PbS Quantum Dot Photovoltaics

Lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs) present the distinctive ability to utilize short-wave infrared light, good ambient stability, and convenient solution-based fabrication processes and thus attract much attention in the photovoltaic research field. The performance of CQDSCs has been improved by constructing the ZnO/PbS heterojunction, due to suitable band levels and electron mobility of ZnO electron transfer layer (ETL). However, the huge number of defects in low-temperature processed ZnO cause an unbalanced carrier-related processes, which restrict further performance enhancement and flexible production of CQDSCs. Here, we described a facile method to passivate defects in low-temperature sol–gel ZnO by introducing polyethylenimine (PEI) into the precursor solution. Versus the original ZnO film, the composite ZnO:PEI films exhibit better crystallization because of the Zn–N interaction. A series of electronic analyses have shown that the addition of PEI reduces the work function (WF) of ZnO and increases the built-in voltage (Vbi) at the heterojunction interface, suggesting that the carrier separation is improved in the depletion region of solar cells. The carrier transport in ZnO ETL is also optimized by PEI, since the electron mobility of ZnO is maximized when the mass faction of PEI is 5%. In addition, the carrier recombination is effectively suppressed in the ZnO:PEI based solar cells proved by the increased carrier lifetime. Consequently, a power conversion efficiency (PCE) of 7.30% was achieved with the ZnO:PEI 5% film versus 5.84% for the reference cell—this was attributed to the optimized carrier-related processes.