Researchers from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, in collaboration with Jingdezhen Ceramic University and Aalto University, have developed a surface engineering technique that significantly enhances the performance of quantum dot solar cells. Their study, published in the Journal of Colloid and Interface Science, demonstrates how chloride and potassium co-passivated zinc oxide nanocrystals improve power conversion efficiency and device stability.

Colloidal zinc oxide (ZnO) nanocrystals serve as electron transport layers in lead sulfide (PbS) quantum dot solar cells. However, surface defects caused by oxygen vacancies and hydroxyl groups trap electrons and hinder energy conversion. Traditional single-ion passivation methods only partially addressed these issues. The research team hypothesized that simultaneously incorporating chloride anions (Cl^-) and potassium cations (K⁺) could more comprehensively suppress defects.

The scientists engineered an in-situ passivation strategy during nanocrystal synthesis. By adding zinc chloride or potassium chloride to the precursor solution, Cl^- and K⁺ ions were incorporated into growing ZnO nanocrystals without lattice doping. Advanced characterization confirmed successful surface modification: X-ray photoelectron spectroscopy detected Cl and K on nanocrystal surfaces, electron paramagnetic resonance revealed a 60% reduction in oxygen vacancy signals, and photoluminescence spectra showed suppressed defect-related emissions.

The dual-ion mechanism operates through complementary actions: K⁺ ions remove hydroxyl groups via deprotonation reactions, while Cl^- ions fill zinc vacancy sites. This synergy creates a smoother pathway for electron extraction at the critical interface between the electron transport layer and quantum dot absorber.

Quantum dot solar cells incorporating the dual-passivated ZnO nanocrystals achieved a champion power conversion efficiency of 11.17% - a significant improvement over devices using pristine ZnO (9.41%) or chloride-only passivated ZnO (10%). The optimized cells also demonstrated superior stability, retaining 83% of initial efficiency after 200 hours of continuous operation.

Performance gains stemmed from multiple factors: improved charge extraction confirmed by time-resolved photoluminescence, reduced interfacial recombination verified by light-intensity-dependent measurements, and favorable energy band alignment. The cross-linked morphology of passivated nanocrystals also enabled better coverage on rough conductive substrates, minimizing electrical shorts.

This work addresses a bottleneck in quantum dot photovoltaics: inefficient charge extraction at the electron transport interface. The solution-processed, room-temperature synthesis offers a scalable approach applicable to other solution-processed solar technologies. By suppressing defect-mediated degradation pathways, the strategy concurrently enhances efficiency and operational stability.

The chloride/potassium passivation approach could potentially benefit perovskite solar cells and other optoelectronic devices where metal oxide interfacial defects limit performance.