Willem Walravens

Bio: Willem Walravens is an academic researcher from Ghent University. The author has contributed to research in topics: Quantum dot & Nanocrystal. The author has an hindex of 9, co-authored 13 publications receiving 1191 citations. Previous affiliations of Willem Walravens include Columbia University.

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

Abstract: 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. 1H 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 fi...

A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit

Abstract: We report a tunable library of N,N,N′-trisubstituted selenourea precursors and their reaction with lead oleate at 60–150 °C to form carboxylate-terminated PbSe nanocrystals in quantitative yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6–56.7 μM) and the size following complete precursor conversion (d = 1.7–6.6 nm) to be controlled. Narrow size distributions (σ = 0.5–2%) are obtained whose spectral line widths are dominated (73–83%) by the intrinsic single particle spectral broadening, as observed using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320–65 meV, d = 1.6–4.4 nm) that derives from exciton fine structure and exciton–phonon coupling rather than broadening caused by the size distribution.

Size and Concentration Determination of Colloidal Nanocrystals by Small-Angle X-ray Scattering

Abstract: The accurate determination of the dimensions of a nano-object is paramount to the development of nanoscience and technology. Here we provide procedures for sizing quasi-spherical colloidal nanocrystals (NCs) by means of small-angle X-ray scattering (SAXS). Using 2.5 to 10 nm PbS NCs as a model system, the protocols outline the extraction of the net NC SAXS pattern by background correction and address the calibration of scattered X-ray intensity to an absolute scale. The NC size distribution is retrieved by fitting the corrected SAXS pattern either to parametrized analytical distributions or to a distribution constructed through a Monte Carlo approach. We compare the two methods and show that they yield nearly identical estimates of the NC diameter in the case of an NC ensemble with a monodisperse and monomodal size distribution. Extending the analysis to PbSe, CdSe, and CdS NCs, we provide SAXS-calibrated sizing curves ranging from 2.5 to 7 nm, which relate the NC diameter and the NC band-gap energy as de...

Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

Abstract: 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 o...

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

Abstract: 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.

Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics.

Abstract: We show nanoscale phase stabilization of CsPbI 3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI 3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI 3 (α-CsPbI 3 )—the variant with desirable band gap—is only stable at high temperatures. We describe the formation of α-CsPbI 3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.

Properties and potential optoelectronic applications of lead halide perovskite nanocrystals

Abstract: Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber materials in photovoltaics but have also proven to be disruptive in the field of colloidal semiconductor nanocrystals (NCs). The most important feature of LHP NCs is their so-called defect-tolerance—the apparently benign nature of structural defects, highly abundant in these compounds, with respect to optical and electronic properties. Here, we review the important differences that exist in the chemistry and physics of LHP NCs as compared with more conventional, tetrahedrally bonded, elemental, and binary semiconductor NCs (such as silicon, germanium, cadmium selenide, gallium arsenide, and indium phosphide). We survey the prospects of LHP NCs for optoelectronic applications such as in television displays, light-emitting devices, and solar cells, emphasizing the practical hurdles that remain to be overcome.

Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals

Abstract: Lead halide perovskites (LHPs) in the form of nanometre-sized colloidal crystals, or nanocrystals (NCs), have attracted the attention of diverse materials scientists due to their unique optical versatility, high photoluminescence quantum yields and facile synthesis. LHP NCs have a 'soft' and predominantly ionic lattice, and their optical and electronic properties are highly tolerant to structural defects and surface states. Therefore, they cannot be approached with the same experimental mindset and theoretical framework as conventional semiconductor NCs. In this Review, we discuss LHP NCs historical and current research pursuits, challenges in applications, and the related present and future mitigation strategies explored.

Anion-exchange red perovskite quantum dots with ammonium iodine salts for highly efficient light-emitting devices

Abstract: Perovskite quantum dots have significant potential for light-emitting devices because of their high colour purity and colour tunability in the visible spectrum. Here, we report highly efficient red perovskite quantum dot-based light-emitting devices. The quantum dots were fabricated by anion exchange from pristine CsPbBr3 using halide-anion-containing alkyl ammonium and aryl ammonium salts. Anion-exchange quantum dots based on ammonium iodine salts exhibited a strong redshift from green emission to a deep-red emission at 649 nm as well as higher photoluminescence quantum yields. Furthermore, the quantum dot-based light-emitting device with the alkyl ammonium iodine salt exhibited an external quantum efficiency of 21.3% and high colour purity, with Commission Internationale de l’Eclairage coordinates of (0.72, 0.28), while the light-emitting device with the aryl ammonium iodine salt showed an external quantum efficiency of 14.1%. Finally, the operational stability of the latter was 36 times higher because the surface ligand density of the corresponding quantum dots was lower. Perovskite quantum dots (QDs) are synthesized via an anion-exchange process where CsPbBr3 is used to realize a highly efficient red light-emitting diode (LED). The perovskite QD-based LED exhibits the highest external quantum efficiency of more than 20% compared with perovskite LEDs.

50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control

Abstract: Solution-processed CsPbBr3 quantum-dot light-emitting diodes with a 50-fold external quantum efficiency improvement (up to 6.27%) are achieved through balancing surface passivation and carrier injection via ligand density control (treating with hexane/ethyl acetate mixed solvent), which induces the coexistence of high levels of ink stability, photoluminescence quantum yields, thin-film uniformity, and carrier-injection efficiency.