Surface chemistry plays a critical role in the performance of quantum dot solar cells (QDSCs) by influencing charge carrier dynamics, stability, and optoelectronic properties. The surface of quantum dots (QDs) is inherently reactive due to dangling bonds and unsaturated coordination sites, which can introduce trap states that hinder charge extraction and promote recombination. Effective surface engineering strategies, including ligand exchange, atomic passivation, and shell growth, are essential to mitigate these effects and enhance device efficiency.

Ligand engineering is a fundamental approach to modifying QD surfaces. Organic ligands, such as oleic acid and oleylamine, are commonly used during synthesis to stabilize colloidal QDs. However, these long-chain insulating ligands impede charge transport in solid-state films. Replacing them with shorter or conductive ligands improves interdot coupling and reduces series resistance. For example, thiol-based ligands like 1,2-ethanedithiol (EDT) or mercaptopropionic acid (MPA) enhance carrier mobility by facilitating closer packing of QDs. Inorganic ligands, such as halides (Cl⁻, Br⁻, I⁻) or pseudohalides (SCN⁻), further improve conductivity by eliminating insulating organic layers while passivating surface defects. Studies have demonstrated that iodide-treated PbS QDs exhibit higher photovoltaic performance due to improved charge transport and reduced trap density.

Atomic passivation involves the direct binding of atoms or small molecules to undercoordinated surface sites, neutralizing electronic traps. Chalcogenide (S, Se, Te) or halide passivation is particularly effective for II-VI and IV-VI QDs. For instance, cadmium selenide (CdSe) QDs treated with sulfide ions show suppressed surface recombination due to the formation of a Cd-S bond that fills selenium vacancies. Similarly, lead sulfide (PbS) QDs benefit from chlorine or bromine passivation, which not only reduces trap states but also tunes the band alignment for better charge extraction. Atomic hydrogen treatment has also been explored, effectively passivating defects in silicon QDs by saturating dangling bonds without introducing additional impurities.

Shell growth is another strategy to passivate surface states by epitaxially growing a wider-bandgap material around the QD core. This creates a core-shell structure that confines charge carriers within the core while shielding them from surface defects. For example, CdSe/CdS core-shell QDs exhibit enhanced photoluminescence quantum yield and stability compared to bare CdSe QDs due to the suppression of non-radiative recombination at the interface. Similarly, PbS QDs with a thin lead chloride (PbCl₂) shell demonstrate improved open-circuit voltage and fill factor in solar cells by reducing interfacial recombination. Graded shells, where the composition gradually changes from core to shell, further minimize lattice mismatch and strain-induced defects.

Surface states significantly impact charge transport and recombination in QDSCs. Unpassivated surfaces introduce mid-gap states that act as traps for electrons and holes, leading to Shockley-Read-Hall recombination. These traps also cause Fermi-level pinning, which limits the open-circuit voltage. Surface oxidation exacerbates these issues, particularly in sulfide or selenide QDs, by creating deep traps that are difficult to eliminate. For example, oxidized PbS surfaces form sulfates or oxides that degrade device performance over time. Effective passivation must therefore address both intrinsic defects and extrinsic degradation pathways.

Several passivation strategies have proven effective in mitigating surface-related losses. Hybrid passivation, combining organic and inorganic ligands, leverages the advantages of both approaches. A notable example is the use of zwitterionic ligands, which provide electrostatic stabilization while maintaining good charge transport. Another successful strategy is post-deposition treatments, such as exposure to molecular iodine or thiols, which chemically modify the QD surface after film formation. For instance, iodine-treated PbSe QD films show enhanced carrier mobility and reduced trap density due to the formation of a lead iodide layer that passivates selenium vacancies.

In conclusion, surface chemistry is pivotal in optimizing the performance of quantum dot solar cells. Ligand engineering, atomic passivation, and shell growth are essential tools for reducing trap states, improving charge transport, and minimizing recombination. The choice of passivation strategy depends on the QD material system and the desired balance between conductivity and stability. Continued advances in surface modification techniques will be crucial for achieving high-efficiency, stable QDSCs.