Polymer Interfacial Layer-Enabled Back-Contact Engineering for Quantum Dot Solar Cells

Modulation of the back-contact interface in PbS colloidal quantum dot solar cells (CQDSCs) is critical for regulating carrier transport and extraction, thereby profoundly influencing the overall device performance. Conventionally, the back-contact interface is built up by vacuum deposition of Au on the p-type PbS CQD layer; however, high-energy Au atoms may penetrate into the PbS CQD layer, causing the deterioration of the back-contact interface. To address the issue, we herein propose a polymer interfacial layer-enabled back-contact engineering strategy. The insertion of insulating poly(methyl methacrylate) (PMMA) into the Au/PbS interface increases the charge extraction efficiency from 70 to 85% and reduces the reverse saturation current by ca. 2 orders of magnitude, to 1.3 × 10^-5 mA cm^-2, resulting in an improved power conversion efficiency of 12.2%. X-ray photoelectron and photoluminescence spectroscopy verify that the polymer layer effectively blocks Au atom penetration. Replacing PMMA with hole transport polymers, such as PTAA and PM7, as well as PCBM with electron-transport property, also improves device performance; however, the enhancement is less pronounced than that achieved with the insulating PMMA. These observations suggest that Au penetration-induced interface deterioration is a non-negligible factor influencing the quality of the Au/PbS back-contact interface. The insulating PMMA interlayer also improves the device stability, offering a convenient, low-temperature approach for the back-contact interface engineering of CQD optoelectronic devices.