SnS₄^4– Metal Chalcogenide Ligand, S^2– Metal Free Ligand, and Organic Surface Ligand Toward Efficient CdSe Quantum Dot- Sensitized Solar Cells

Inorganic surface ligands such as metal chalcogenides ligand SnS₄^4– and metal free ligand S^2– were introduced for CdSe quantum dot sensitized solar cell (QDSSC) applications. The SnS₄^4– ligand QDs were successfully deposited onto TiO₂ photoanode through metal ion coordination. In solution, metal–ammonia complexes of Zn²⁺, Cd²⁺, and Cu²⁺ can be coordinated by the SnS₄^4– ligands and reverse the zeta potential. Similarly, the metal ions can be sandwiched by SnS₄^4– ligands and the photoanode. Using the metal ion bridged SnS₄^4– ligand QDs as the sensitizer, photovoltaic properties of the QDSSCs have been studied. Cd²⁺ mediated deposition case showed better photovoltaic performance than the cases of Zn²⁺ or Cu²⁺. To further investigate the surface ligand effect on QDSSC, organic/inorganic mixed surface CdSe QDs were introduced using partial ligand exchange after the deposition onto the TiO₂ photoanode. The postdeposition surface ligand exchange with inorganic ligands such as SnS₄^4– and S^2– is thought to retain the initial organic ligands between QDs and TiO₂ photoanode and selectively replace the QD ligands that would contact the electrolytes. Metal free S^2– ligand QDSSC showed the best photovoltaic performance recording 1.4 times enhanced photocurrent and 1.5 times enhanced photoconversion efficiency when compared with the initial organic surface ligand QDSSC. Comparison studies on the photovoltaic properties of QDSSCs with different surfaces suggest that (i) the Cd–SnS₄^4– complex and SnS₄^4– surface ligand act as efficient electron traps hurdling the photovoltaic performance severely, (ii) S^2– surface ligand works as an efficient hole trap only at the interface between the QD and TiO₂, and (iii) S^2– surface ligand blocks back electron transfers better than the initial organic ligand

Layer-by-Layer Quantum Dot Assemblies for the Enhanced Energy Transfers and Their Applications toward Efficient Solar Cells

Two different quantum dots (QDs) with an identical optical band gap were prepared: one without the inorganic shell and short surface ligands (BQD) and the other with thick inorganic shells and long surface ligands (OQD). They were surface-derivatized to be positively or negatively charged and were used for layer-by-layer assemblies on TiO₂. By sandwiching BQD between OQD and TiO₂, OQD photoluminescence showed seven times faster decay, which is attributed to the combined effect of the efficient energy transfer from OQD to BQD with the FRET efficiency of 86% and fast electron transfer from BQD to TiO₂ with the rate of 1.2 × 10⁹ s^–1. The QD bilayer configuration was further applied to solar cells, and showed 3.6 times larger photocurrent and 3.8 times larger photoconversion efficiency than those of the device with the OQD being sandwiched by BQD and TiO₂. This showcases the importance of sophisticated control of QD layer assembly for the design of efficient QD solar cells

Layer-by-Layer Assemblies of Semiconductor Quantum Dots for Nanostructured Photovoltaic Devices

A multilayer of quantum dots (QDs) is preferred for QD-sensitized solar cells over a monolayer counterpart to fully utilize the sunlight incident into a relatively thin-film-based photovoltaic device. A controlled assembly of QD multilayers such as layer-by-layer (LbL) assemblies can provide a model system to study the interactions between the QD layers and can offer an optimal device configuration for efficient solar power conversion. Recently, we have proposed a LbL QD assembly using electrostatic interactions of the surface charges and have successfully prepared a controlled multilayer of QD on the surface of mesoporous metal oxide films. The as-prepared tailor-made QD multilayers not only guaranteed the sufficient absorption of incident solar light but also provided a toolbox for the study and optimization of electron/energy transfers between QD layers

Exciton-to-Dopant Energy Transfer in Mn-Doped Cesium Lead Halide Perovskite Nanocrystals

We report the one-pot synthesis of colloidal Mn-doped cesium lead halide (CsPbX₃) perovskite nanocrystals and efficient intraparticle energy transfer between the exciton and dopant ions resulting in intense sensitized Mn luminescence. Mn-doped CsPbCl₃ and CsPb­(Cl/Br)₃ nanocrystals maintained the same lattice structure and crystallinity as their undoped counterparts with nearly identical lattice parameters at ∼0.2% doping concentrations and no signature of phase separation. The strong sensitized luminescence from d–d transition of Mn²⁺ ions upon band-edge excitation of the CsPbX₃ host is indicative of sufficiently strong exchange coupling between the charge carriers of the host and dopant d electrons mediating the energy transfer, essential for obtaining unique properties of magnetically doped quantum dots. Highly homogeneous spectral characteristics of Mn luminescence from an ensemble of Mn-doped CsPbX₃ nanocrystals and well-defined electron paramagnetic resonance spectra of Mn²⁺ in host CsPbX₃ nanocrystal lattices suggest relatively uniform doping sites, likely from substitutional doping at Pb²⁺. These observations indicate that CsPbX₃ nanocrystals, possessing many superior optical and electronic characteristics, can be utilized as a new platform for magnetically doped quantum dots expanding the range of optical, electronic, and magnetic functionality

Effects of Direct Solvent-Quantum Dot Interaction on the Optical Properties of Colloidal Monolayer WS₂ Quantum Dots

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS₂ QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS₂ QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS₂ QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments

Colloidal Single-Layer Quantum Dots with Lateral Confinement Effects on 2D Exciton

Controlled lateral quantum confinement in single-layer transition-metal chalcogenides (TMCs) can potentially combine the unique properties of two-dimensional (2D) exciton with the size-tunability of exciton energy, creating the single-layer quantum dots (SQDs) of 2D TMC materials. However, exploring such opportunities has been challenging due to the limited ability to produce well-defined SQDs with sufficiently high quality and size control, in conjunction with the commonly observed inconsistency in the optical properties. Here, we report an effective method to synthesize high-quality and size-controlled SQDs of WSe₂ via multilayer quantum dots (MQDs) precursors, which enables grasping a clear picture of the role of lateral confinement on the optical properties of the 2D exciton. From the single-particle optical spectra and polarization anisotropy of WSe₂ SQDs of varying sizes in addition to their ensemble data, we reveal how the properties of 2D exciton in single-layer TMCs evolve with increasing lateral quantum confinement

SnS₄^4–, SbS₄^3–, and AsS₃^3– Metal Chalcogenide Surface Ligands: Couplings to Quantum Dots, Electron Transfers, and All-Inorganic Multilayered Quantum Dot Sensitized Solar Cells

Three inorganic capping ligands (ICLs) for quantum dots (QDs), SnS₄^4–, SbS₄^3– and AsS₃^3–, were synthesized and the energy levels determined. Proximity between the ICL LUMO and QD conduction level governed the electronic couplings such as absorption shift upon ligand exchange, and electron transfer rate to TiO₂. QD-sensitized solar cells were fabricated, using the ICL-QDs and also using QD multilayers layer-by-layer assembled by bridging coordinations, and studied as a function of the ICL ligand and the number of QD layers