Non-injection Synthesis of Doped Zinc Oxide Plasmonic Nanocrystals

Plasmonic metal oxide nanocrystals bridge the optoelectronic gap between semiconductors and metals. In this study, we report a facile, non-injection synthesis of ZnO nanocrystals doped with Al, Ga, or In. The reaction readily permits dopant/zinc atomic ratios of over 15%, is amenable to high precursor concentrations (0.2 M and greater), and provides high reaction yields (>90%). The resulting colloidal dispersions exhibit high transparency in the visible spectrum and a wavelength-tunable infrared absorption, which arises from a dopant-induced surface plasmon resonance. Through a detailed investigation of reaction parameters, the reaction mechanism is fully characterized and correlated to the optical properties of the synthesized nanocrystals. The distinctive optical features of these doped nanocrystals are shown to be readily harnessed within thin films that are suitable for optoelectronic applications

Aqueous Synthesis of High-Quality Cu₂ZnSnS₄ Nanocrystals and Their Thermal Annealing Characteristics

Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by Sn₂S₆^4– and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes

Fabrication of Back-Contact Electrodes Using Modified Natural Lithography

The fabrication of back-contact electrodes with micron-sized features by microsphere lithography is implemented via a modified “natural lithography” approach. The solution-based assembly of microsphere beads on a substrate occurs via the electrostatic attraction between the molecular monolayer-functionalized substrate and the micron-sized polystyrene microbeads with carboxyl surface groups. Through a modification of the original “natural lithography” method, the density of the microbeads used as a lithographic mask can be increased 5-fold. The resulting back-contact electrodes are used for the fabrication of perovskite solar cell devices and the examination of their potential. Devices with electrodes fabricated using a modified “natural lithography” approach showed a 3.5-fold increase in performance compared to the devices with electrodes made using the original method

In Situ Formation of Reactive Sulfide Precursors in the One-Pot, Multigram Synthesis of Cu₂ZnSnS₄ Nanocrystals

Herein we outline a general one-pot method to produce large quantities of compositionally tunable, kesterite Cu₂ZnSnS₄ (CZTS) nanocrystals (NCs) through the decomposition of in situ generated metal sulfide precursors. This method uses air stable precursors and should be applicable to the synthesis of a range of metal sulfides. We examine the formation of the ligands, precursors, and particles in turn. Direct reaction of CS₂ with the aliphatic primary amines and thiols that already constitute the reaction mixture is used to produce ligands in situ. Through the use of ¹H and ¹³C nuclear magnetic resonance, Fourier transform infrared spectroscopy, and optical absorption spectroscopy, we elucidate the formation of the resulting oleyldithiocarbamate and dodecyltrithiocarbonate ligands. The decomposition of their corresponding metal complexes at temperatures of ∼100 °C yields nuclei with a size of 1–2 nm, with further growth facilitated by the decomposition of dodecanethiol. In this way the nucleation and growth stages of the reaction are decoupled, allowing for the generation of NCs at high concentrations. Using in situ X-ray diffraction, we monitor the evolution of our reactions, thus enabling a real-time glimpse into the formation of Cu₂ZnSnS₄ NCs. For completeness, the surface chemistry and the electronic structure of the resulting CZTS NCs are studied

Mimicry of Sputtered <i>i-</i>ZnO Thin Films Using Chemical Bath Deposition for Solution-Processed Solar Cells

Solution processing provides a versatile and inexpensive means to prepare functional materials with specifically designed properties. The current challenge is to mimic the structural, optical, and/or chemical properties of thin films fabricated by vacuum-based techniques using solution-based approaches. In this work we focus on ZnO to show that thin films grown using a simple, aqueous-based, chemical bath deposition (CBD) method can mimic the properties of sputtered coatings, provided that the kinetic and thermodynamic reaction parameters are carefully tuned. The role of these parameters toward growing highly oriented and dense ZnO thin films is fully elucidated through detailed microscopic and spectroscopic investigations. The prepared samples exhibit bulk-like optical properties, are intrinsic in their electronic characteristics, and possess negligible organic contaminants, especially when compared to ZnO layers deposited by sol–gel or from nanocrystal inks. The efficacy of our CBD-grown ZnO thin films is demonstrated through the effective replacement of sputtered ZnO buffer layers within high efficiency solution processed Cu₂ZnSnS_4<i>x</i>Se_{4(1–<i>x</i>)} solar cells

Synthesis and Structure of New Lanthanoid Carbonate “Lanthaballs”

New insights into the synthesis of high-nuclearity polycarbonatolanthanoid complexes have been obtained from a detailed investigation of the preparative methods that initially yielded the so-called “lanthaballs” [Ln₁₃(ccnm)₆(CO₃)₁₄(H₂O)₆(phen)₁₈] Cl₃(CO₃)·25H₂O [<b>α-1Ln</b>; Ln = La, Ce, Pr; phen = 1,10-phenanthroline; ccnm = carbamoylcyanonitrosomethanide]. From this investigation, we have isolated a new pseudopolymorph of the cerium analogue of the lanthaball, [Ce₁₃(ccnm)₆(CO₃)₁₄(H₂O)₆(phen)₁₈]·Cl₃·CO₃ (<b>β-1Ce</b>). This new pseudopolymorph arose from a preparation in which fixation of atmospheric carbon dioxide generated the carbonate, and the ccnm ligand was formed in situ by the nucleophilic addition of water to dicyanonitrosomethanide. From a reaction of cerium­(III) nitrate, instead of the previously used chloride salt, with (Et₄N)­(ccnm), phen, and NaHCO₃ in aqueous methanol, the new complex Na­[Ce₁₃(ccnm)₆(CO₃)₁₄(H₂O)₆(phen)₁₈]­(NO₃)₆·20H₂O (<b>2Ce</b>) crystallized. A variant of this reaction in which sodium carbonate was initially added to Ce­(NO₃)₃, followed by phen and (Et₄N)­(ccnm), also gave <b>2Ce</b>. However, an analogous preparation with (Me₄N)­(ccnm) gave a mixture of crystals of <b>2Ce</b> and the coordination polymer [CeNa­(ccnm)₄(phen)₃]·MeOH (<b>3</b>), which were manually separated. The use of cerium­(III) acetate in place of cerium nitrate in the initial preparation did not give a high-nuclearity complex but a new coordination polymer, [Ce­(ccnm)­(OAc)₂(phen)] (<b>4</b>). The first lanthaball to incorporate neodymium, namely, [Nd₁₃(ccnm)₄(CO₃)₁₄(NO₃)₄(H₂O)₇(phen)₁₅]­(NO₃)₃·10H₂O (<b>5Nd</b>), was isolated from a preparation similar to that of the second method used for <b>2Ce</b>, and its magnetic properties showed an antiferromagnetic interaction. The identity of all products was established by X-ray crystallography

Cu₂ZnGeS₄ Nanocrystals from Air-Stable Precursors for Sintered Thin Film Alloys

The synthesis of an air and moisture stable germanium complex and its use in the synthesis of ternary and quaternary copper containing nanocrystals (NCs) is described. Through the use of ¹H-/¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopies, thermogravimetric analysis, and powder X-ray diffraction, the speciation and chemistry of this precursor is elucidated. This germanium source is employed in the gram scale, noninjection synthesis of Cu₂ZnGeS₄ (CZGeS) and Cu₂GeS₃ (CGeS) NCs using a binary sulfide precursor approach. To demonstrate the versatility of such NCs for fabricating thin films suitable for high-efficiency optoelectronic devices, they are blended with Cu₂ZnSnS₄ (CZTS) NCs and selenized to form homogeneously alloyed Cu₂ZnSn_<i>x</i>Ge_1–<i>x</i>S_<i>y</i>Se_4–<i>y</i> (CZTGeSSe) thin films. The structural, optical, and electronic properties of such thin films are studied using X-ray diffraction, scanning electron microscopy, UV−vis−NIR spectroscopy, and photoelectron spectroscopy in air. These measurements demonstrate collectively that incorporating Ge into micrometer-sized, tetragonal Cu₂ZnSnS_<i>x</i>Se_4–<i>x</i> (CZTSSe) provides a facile manner in which the conduction band energy can be readily tuned. The strategy developed herein provides a pathway to controlled levels of Ge incorporation in a single step process, thus avoiding the need for intra-alloyed Cu₂ZnSn_<i>x</i>Ge_1–<i>x</i>S₄ nanocrystals