Willem Walravens

Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals.

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. (1)H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.

Slow recombination in quantum dot solid solar cell using p–i–n architecture with organic p-type hole transport material

The interfaces between different materials in the heterojunction colloidal quantum dot (QD) solar cell play an important role for charge carrier separation, recombination and collection. Here, an organic–inorganic hybrid p–i–n architecture for the heterojunction PbS QD solid solar cell is constructed to increase the charge extraction and reduce charge recombination. Heavily doped poly(3-hexylthiophene-2,5-diyl) (P3HT) is applied as hole transport interlayer between the QD film and metal contact electrode. The results show that the P3HT interlayer diminishes the charge carrier recombination at the QD film/metal contact electrode interface leading to increased open-circuit voltage and increased electron life time. Furthermore, after incorporation of P3HT interlayer an additional p–i heterojunction might form at P3HT/QD film interface resulting in increased depletion region, which promotes charge carrier extraction under working conditions. Two other organic p-type interlayers are also investigated, however, the results indicate that a barrier for charge extraction is formed for these devices, which is explained by the difference in energy levels. The solar cell with the P3HT interlayer exhibits a power conversion efficiency of 5.1% at 1 sun of illumination and ambient atmosphere, which is ∼20% higher compared to the solar cell without any hole transport interlayer.

Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to restore nanocrystal stoichiometry can trigger the formation of epitaxial superlattices of PbSe and PbS quantum dots. More specifically, we show that both chalcogen-adding (sodium sulfide) and lead oleate displacing (amines) additives induce small area epitaxial superlattices of PbSe quantum dots. In the latter case, the amine basicity is a sensitive handle to tune the superlattice symmetry, with strong and weak bases yielding pseudohexagonal or quasi-square lattices, respectively. Through density functional theory calculations and in situ titrations monitored by nuclear magnetic resonance spectroscopy, we link this observation to the concomitantly different coordination enthalpy and ligand displacement potency of the amine. Next to that, an initial ∼10% reduction of the initial ligand density prior to monolayer formation and addition of a mild, lead oleate displacing chemical trigger such as aniline proved key to induce square superlattices with long-range, square micrometer order; an effect that is the more pronounced the larger the quantum dots. Because the approach applies to PbS quantum dots as well, we conclude that it offers a reproducible and rational method for the formation of highly ordered epitaxial quantum dot superlattices.