We introduce indium arsenide colloidal quantum dot films for photovoltaic devices, fabricated by two-step surface modification. Native ligands and unwanted oxides on the surface are peeled off ...followed by passivating with incoming atomic or short ligands. The near-infrared-absorbing n-type indium arsenide colloidal quantum dot films can be tuned in energy-level positions up to 0.4 eV depending on the surface chemistry, and consequently, they boost collection efficiency when used in various emerging solar cells. As an example, we demonstrate p-n junction between n-type indium arsenide and p-type lead sulfide colloidal quantum dot layers, which leads to a favorable electronic band alignment and charge extraction from both colloidal quantum dot layers. A certified power conversion efficiency of 7.92% is achieved without additionally supporting carrier transport layers. This study provides richer materials to explore for high-efficiency emerging photovoltaics and will broaden research interest for various optoelectronic applications using the n-type covalent nanocrystal arrays.
Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (D) down to 1.5 ...nm, and found an abrupt transition at D ≈ 4 nm in the air stability as the QD size was varied from 1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density functional theory calculations reveal that the stability transition is closely associated with the shape transition of oleate-capped QDs from octahedron to cuboctahedron, driven by steric hindrance and thus size-dependent surface energy of oleate-passivated Pb-rich QD facets. This microscopic understanding of the surface chemistry on ultrasmall QDs, up to a few nanometers, should be very useful for precisely and accurately controlling physicochemical properties of colloidal QDs such as doping polarity, carrier mobility, air stability, and hot-carrier dynamics for solar cell applications.
Abstract
Despite the technological importance of colloidal covalent III-V nanocrystals with unique optoelectronic properties, their synthetic process still has challenges originating from the complex ...energy landscape of the reaction. Here, we present InP tetrapod nanocrystals as a crystalline late intermediate in the synthetic pathway that warrants controlled growth. We isolate tetrapod intermediate species with well-defined surfaces of (110) and (
$$\bar{1}\bar{1}\bar{1}$$
1
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) via the suppression of further growth. An additional precursor supply at low temperature induces
$$\bar{1}\bar{1}\bar{1}$$
1
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-specific growth, whereas the 110-directional growth occurs over the activation barrier of 65.7 kJ/mol at a higher temperature, thus finalizes into the (111)-faceted tetrahedron nanocrystals. We address the use of late intermediates with well-defined facets at the sub-10 nm scale for the tailored growth of covalent III-V nanocrystals and highlight the potential for the directed approach of nanocrystal synthesis.
Power conversion efficiencies of colloidal quantum dot solar cells, which have focused mainly on lead chalcogenide systems until recently, have increased rapidly and currently exceed 12%. Among the ...many issues involved in commercialization of this technology as a consumer product, lead-based materials in these systems must be replaced. This requires the use of a low-cost, low-loss, and non-toxic chemical, along with the development of an eco-friendly manufacturing process. Herein, we review recent progress in ecofriendly colloidal quantum dot photovoltaics, with a focus on two aspects. First, we examine non-toxic or less-toxic quantum dot materials designed for efficient thin-film based solar cells by considering factors such as bandgap tunability, exciton binding energy, and more. We then present the performance of quantum dot solar cells using these green quantum dot materials, and discuss the scientific and technological issues facing them. Second, we review fabrication methods of quantum dot thin films with low-cost, lowwaste, and non-toxic chemicals, for use in eco-friendly manufacturing processes.
Colloidal quantum dots (CQDs) are promising light harvesting materials for realization of solution processible, highly efficient multipurpose photovoltaics (PVs). Here, PbS CQD solar cells are ...reported with improved certified power conversion efficiency performance of 10.4% by simply controlling protic solvents (alcohols) in ligand exchange process. With shorter chain alcohols, the mobility of charge carriers is an order‐of‐magnitude improved due to the enhanced interparticle coupling; on the other hand, excessive removal of passivating ligands by very protic solvent, methanol (MeOH) induced undesirable traps on CQD surface. Consequently, it has been found that high performance CQD PVs require a solvent engineering for balance between native leaving ligands with incoming ligands during ligand exchange process for well‐controlled surfaces of CQDs and enhanced carrier concentration of conductive CQD films.
The control of short‐chain alcohols in ligand exchange is proven to be very crucial for improving optoelectronic properties of PbS colloidal quantum dot (CQD) films. MeOH commonly used for ligand exchange of CQDs creates too many uncontrolled surface traps, but EtOH balances the ligand exchange and surface trap density, enabling a high certified power‐conversion‐efficiency of 10.4%.
Controlling the thickness of quantum dot (QD) films is difficult using existing film formation techniques, which employ pre-ligand-exchanged PbS QD inks, because of several issues: 1) poor colloidal ...stability, 2) use of high-boiling-point solvents for QD dispersion, and 3) limitations associated with one-step deposition. Herein, we suggest a new protocol for QD film deposition using electrical double-layered PbS QD inks, prepared by solution-phase ligand exchange using methyl ammonium lead iodide (MAPbI
). The films are deposited by the supersonic spraying technique, which facilitates the rapid evaporation of the solvent and the subsequent deposition of the PbS QD ink without requiring a post-deposition annealing treatment for solvent removal. The film thickness could be readily controlled by varying the number of spraying sweeps made across the substrate. This spray deposition process yields high-quality n-type QD films quickly (within 1 min) while minimizing the amount of the PbS QD ink used to less than 5 mg for one device (300-nm-thick absorbing layer, 2.5 × 2.5 cm
). Further, the formation of an additional p-layer by treatment with mercaptopropionic acid allows for facile hole extraction from the QD films, resulting in a power conversion efficiency of 3.7% under 1.5 AM illumination.
Colloidal quantum dots (QDs) are nanosized semiconductors whose electronic features are dictated by the quantum confinement effect. The optical, electrical, and chemical properties of QDs are ...influenced by their dimensions and surface landscape. The surface of II-VI and IV-VI QDs has been extensively explored; however, in-depth investigations on the surface of III-V QDs are still lagging behind. This Perspective discusses the current understanding of the surface of III-V QDs, outlines deep trap states presented by surface defects, and suggests strategies to overcome challenges associated with deep traps. Lastly, we discuss a route to create well-defined facets in III-V QDs by providing a platform for surface studies and a recently reported approach in atomistic understanding of covalent III-V QD surfaces using the electron counting model with fractional dangling bonds.
Wet chemical synthesis of covalent III‐V colloidal quantum dots (CQDs) has been challenging because of uncontrolled surfaces and a poor understanding of surface–ligand interactions. We report a ...simple acid‐free approach to synthesize highly crystalline indium phosphide CQDs in the unique tetrahedral shape by using tris(dimethylamino) phosphine and indium trichloride as the phosphorus and indium precursors, dissolved in oleylamine. Our chemical analyses indicate that both the oleylamine and chloride ligands participate in the stabilization of tetrahedral‐shaped InP CQDs covered with cation‐rich (111) facets. Based on density functional theory calculations, we propose that fractional dangling electrons of the In‐rich (111) surface could be completely passivated by three halide and one primary amine ligands per the (2×2) surface unit, satisfying the 8‐electron rule. This halide–amine co‐passivation strategy will benefit the synthesis of stable III‐V CQDs with controlled surfaces.
InP colloidal tetrahedral nanocrystals were synthesized through a simple acid‐free approach using tris(dimethylamino) phosphine and indium trichloride dissolved in oleylamine. Their formation was attributed to the unique stabilization of the In‐rich (111) facets by co‐passivation with halide and primary amine.
Thermoelectric Generators In article number 2301252, Hyekyoung Choi, Seung I. Cha and co‐workers develop a wearable thermoelectric generator (TEG) using a partially air‐filled deformable gasket as a ...leg support with a negative Poisson's ratio. The gasket tightly wraps around human skin to minimize heat loss and prevents thermal leakage from the thermoelectric material, resulting in an enhanced temperature gradient, thereby improving overall performance.
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