Energy level alignment of colloidal quantum dots: A comparative study on hole injection barrier with organic materials

Understanding energy level alignment (ELA) at interfaces is essential for optimizing charge injection in colloidal quantum dot (CQD)-based optoelectronic devices. This study compares the ELA behaviour of PbS CQD films with that of conventional organic materials. Ultraviolet photoelectron spectroscopy (UPS) and hole-only device (HOD) measurements reveal that PbS CQDs exhibit early Fermi level pinning within the bandgap, unlike organic materials, where pinning occurs near the HOMO level. This early pinning is attributed to high densities of surface defects and ligand-induced states. The strong downward band bending in PbS films indicates significant space charge accumulation and a negatively charged interface. Despite a higher hole injection barrier (∼1.08 eV), PbS CQD-based HODs exhibit low trap-filled limit voltages and reduced trap densities, implying effective defect passivation. Additionally, strong interfacial dipoles at the CQD/substrate interface influence the energy landscape. These findings highlight the unique interfacial physics of CQDs and offer important guidance for the design of high-performance CQD-based devices.