| Lead sulfide（PbS）colloidal quantum dots（CQDs）are widely utilized in optoelectronic devices such as field effect transistors,photodetectors and solar cells（SCs）.owing to their unique quantum confinement effect,tunable optoelectronic properties and solution processability.Since the length of the ligands on the surface of PbS CQDs determines the inter-dot distance and thus affects the inter-dot carrier transport rate.Aiming at promoting the carrier collection in PbS CQD photovoltaic devices（PVs）,long-chain insulating ligands passivated on the CQD surface that controls growth and inhibit aggregation during synthesis need to be replaced by inorganic atomic ligands/short-chain organic ligands in order to enhance the electronic coupling between adjacent CQD and facilitate charge transport.In recent years,solution-phase ligand exchange（SPLE）has been widely employed as an effective surface ligand exchange method.However,the SPLE process is easily affected by the internal factors and external environment（type of ligand and counterion and reaction temperature,etc.）,resulting in some issues such as long-chain insulating residues on the surface of CQD,insufficient passivation of trap states,and less coverage of short-chain ligands,which is detrimental to the preparation and development of high-performance devices.Therefore,in view of the above concerns,we optimized the PbS CQD surface ligands in this paper and regulated the type and quantity of CQD surface ligands during the SPLE and subsequent treatment to achieve the improvement of PbS CQD solar cell performance.The specific research results are as follows:1.The complementary treatment with europium acetylacetonate（Eu（acac）3）achieved surface passivation and energy level alignment of PbS CQDs.After the conventional lead halide-based SPLE process,Eu（acac）3with a concentration of 0.005M was introduced to supply the passivation of the CQD surface.X-ray photoelectron spectroscopy（XPS）analysis demonstrated that Eu（acac）3 could act on the surface of PbS CQD without affecting the original lead halide ligands.This complementary passivation could reduce the trap state density of PbS CQD films from 4.33×1016 cm-3to 3.65×1016 cm-3.Simultaneously,the ultraviolet photoelectron spectroscopy（UPS）test proved that the adsorption of Eu（acac）3 induced the conduction band and valence band of the CQD film shifted up by 0.2 e V relative to the vacuum energy level,as well as the Fermi level（EF）shifted up by 0.08 e V.After the treatment with Eu（acac）3,the band edge position of the CQD absorb layer was more appropriate for the existing electron and hole transport layers which contributed to improve the collection of carriers.The complementary passivation and band alignment effect of Eu（acac）3 on PbS CQD increased the open circuit voltage(Voc)of the device from 0.59 V to 0.61 V,the photoelectric conversion efficiency（PCE）from 9.1%to 10.1%and inhibited the attenuation of the device.Consequently,this effective and convenient method can compensate for the loss of performance and stability resulting from insufficient surface coverage.2.The solution post-annealing treatment promoted the reconfiguration of surface ligands and achieved improved performance of PbS CQD devices.After the SPLE process,the PbS CQD solutions were annealed at 30°C,50°C and 80°C,respectively.The heating process could promote the desorption of residual oleic acid（OA）and hydroxyl ligands on the surface of PbS CQD,and promote the approach and passivation of unreacted lead halide in the solution to the CQD surface,the best passivation effect was obtained on the surface of quantum dots when the annealing temperature was 50°C.As confirmed by 1H nuclear magnetic resonance spectroscopy（1H NMR）and XPS test,after annealing the average number of OA ligands on the CQD surface decreased from 6.9 to 3.7,and the oxygen elemental frontal ratio decreased from 0.25 to 0.2while the ratio of surface halogen atoms increased from 0.67 to 0.74 after annealing at 50°C,which suggested that the unreacted lead halide in solution may passivate on the vacancies after desorption of ligands such as OA and hydroxyl groups during annealing.Dynamic light scattering（DLS）and transmission electron microscopy（TEM）tests also demonstrated that this strategy reduced aggregation induced in conventional SPLE and optimized the mono-dispersity of CQDs.Moreover,compared to the control sample,this helped produce CQD films with double the carrier mobility from 1.09×10-3 cm2·V-1·s-1 to 2.40×10-3 cm2·V-1·s-1,and the trap state density declined from 1.41×1016 cm-3 to 0.83×1016 cm-3,the corresponding device exhibited a photoelectric conversion efficiency of nearly 11%,which was 17%higher than that of the control sample.This method further developed the completion degree of ligand exchange and optimized the composition of ligands on the CQD surface by introducing the solution post-annealing treatment.3.Acid-based additive-assisted ligand exchange for narrow-bandgap PbS CQD enabled high-performance near-infrared PVs.As the band gap of the quantum dot narrows to 4 nm（1.07e V）,the proportion of[100]crystal planes in the quantum dot increased.The[100]crystal planes exposed during ligand exchange would promote the quantum dot have the tendence of aggregation and make it difficult to achieve stable ligand exchange.To address this issue,we introduced guanidine hydriodate（GAI）with a higher acidity into the ligand exchange system of CQD with narrow band gaps.Higher acidity would promote the desorption of OA ligands on the CQD surface,while the sterically hindered guanidine cations played a role in blocking adjacent CQD in solution,inhibiting the exposed[100]facet-induced aggregation.DLS and TEM tests showed that the addition of GAI reduced CQD aggregations and narrowed the size distribution.According to XPS results,the SPLE process improved the proportion of halide on CQD surfaces by 5%,and the ratio of oxygen-related groups like Pb-OH and Pb-COO decreased from 3.6%to 1.8%.Additionally,a PCE of 9.28%was achieved in narrow-bandgap PbS CQD（1.07 e V）solar cells by introducing the solution post-annealing method,which was at a relatively high level among the reported narrow-bandgap CQD devices. |
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