Enhancing Open-Circuit Voltage in Infrared PbS Quantum Dot Heterojunction Solar Cells Using ZnO Nanowires Passivated by Atomic Layer Deposition of Al2O3

Abstract

PbS colloidal quantum dot (CQD)-based solar cells hold promise for solution-processed solar cells with wideband spectral sensitivity from the visible to the infrared region. In particular, an approximately one micrometer thick nanocomposite structure composed of the densely and intricately mixed infrared-absorbing PbS QDs and ZnO nanowires (NWs) effectively enhances the external quantum efficiency of photocurrent from the visible to the infrared spectrum because of the formation of spatially separate carrier pathways. This enlarged heterointerface makes the nanocomposite structure a promising candidate for a solar cell structure for high-efficiency infrared photovoltaics. However, since the recombination reaction mainly occurs at the heterojunction, improving open-circuit voltage (V_oc) is a critical challenge to fully capitalize on the performance of a nanocomposite with the enlarged heterojunction interface. To address this, we utilized the atomic layer deposition (ALD) technique to passivate the surface defects of ZnO NWs with Al₂O₃. A detailed analysis using high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS) confirmed that a precisely controlled ALD process enables the deposition of the conformal Al₂O₃ layer with the target thickness (3 nm) uniformly across the surfaces of ZnO NWs within the nanocomposite. Moreover, incorporating infrared-absorbing PbS quantum dots into the nanocomposite structure led to an increase in open-circuit voltage without compromising the short-circuit current density.