Оптичні характеристики тонких плівок SnO2 як сонячних елементів, досліджених методом кутової спектроскопічної еліпсометрії

У даній роботі методом розпилювального піролізу отримано наноструктуру ТСО в тонкій плівці оксиду олова. Різні параметри осадження були встановлені та оптимізовані для приготування вихідного розчину з дигідрату хлориду олова, нанесеного на очищену скляну підкладку. Була досягнута характеристика тонкого шару чистого оксиду олова, нанесеного методом конденсації. Методом рентгенівської дифрактометрії показано, що плівка оксиду олова полікристалічна з тетрагональною структурою, яка складається в основному з орієнтацій (101), (211) та інших менш інтенсивних. Ультрафіолетова спектроскопія підтвердила високу прозорість тонкого шару чистого SnO2 з пропусканням 95 % при 600 нм. Величина оптичної щілини осадженого зразка дорівнює 3,95 еВ. Спектроскопічні еліпсометричні вимірювання параметрів Psi і delta були проведені під різними кутами падіння 65°, 70° і 75°. Оптичну постійну шару SnO2 було змодельовано за допомогою B-сплайнової моделі. Достовірність еліпсометричного підгонки визначена при куті падіння 75°, що вказує на мінімальний MSE, рівний 4,022. Результати SE характеристики тонкої плівки SnO2 на скляній підкладці показали, що товщина шару, показник заломлення та коефіцієнт екстинкції дорівнюють 219,78 нм, 1,41 та 0,123 відповідно. Отримані структурні та оптичні параметри підтвердили, що утворився тонкий шар SnO2. Цей шар продемонстрував широку смугу пропускання та високу прозорість типу напівпровідника TCO, який можна використовувати як антибліковий шар в сонячних елементах.In this work, we prepared a nanostructure of TCO in a thin film of tin oxide using the spray pyrolysis method. The different deposition parameters have been set and optimized to prepare the source solution from Tin chloride dihydrate deposited on a cleaned glass substrate. The characterization of the thin layer of pure tin oxide deposited by spraying was achieved. The X-ray diffractometer shows that the tin oxide film is polycrystalline with a tetragonal structure, which consists mainly of orientations (101), (211) and other less intense. Ultra-violet-spectroscopy approved an excellent transparency of the thin layer of pure SnO2 with a transmission of 95 % at 600 nm. The optical gap value of the deposited sample is equal to 3.95 eV. The spectroscopic ellipsometry measurements of the Psi and delta parameters were carried out at various angles of incidence 65°, 70° and 75°. The optical constant of the SnO2 layer was modeled using a B-spline model. The goodness of the ellipsometric fitting was found at an incidence angle of 75°, which indicates a minimum MSE equal to 4.022. The SE characterization results of SnO2 thin film on the glass substrate have shown that the layer thickness, the refractive index and the extinction coefficient are equal to 219.78 nm, 1.41 and 0.123, respectively. The resulting structural and optical parameters confirmed that a thin SnO2 layer was formed. This layer showed a wide bandwidth and high transparency of a type of TCO semiconductor, which can be used as an anti-reflective layer in solar cell devices

Improving Light Harvesting in Dye-Sensitized Solar Cells Using Hybrid Bimetallic Nanostructures

In this work we demonstrate improved light trapping in dye-sensitized solar cells (DSSCs) with hybrid bimetallic gold core/silver shell nanostructures. Silica-coated bimetallic nanostructures (Au/Ag/SiO₂ NSs) integrated in the active layer of DSSCs resulted in 7.51% power conversion efficiency relative to 5.97% for reference DSSCs, giving rise to 26% enhancement in device performance. DSSC efficiencies were governed by the particle density of Au/Ag/SiO₂ NSs with best performing devices utilizing only 0.44 wt % of nanostructures. We performed transient absorption spectroscopy of DSSCs with variable concentrations of Au/Ag/SiO₂ NSs and observed an increase in amplitude and decrease in lifetime with increasing particle density relative to reference. We attributed this trend to plasmon resonant energy transfer and population of the singlet excited states of the sensitizer molecules at the optimum concentration of NSs promoting enhanced exciton generation and rapid charge transfer into TiO₂

Digital Transfer Growth of Patterned 2D Metal Chalcogenides by Confined Nanoparticle Evaporation

Developing methods for the facile synthesis of two-dimensional (2D) metal chalcogenides and other layered materials is crucial for emerging applications in functional devices. Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor-phase synthesis. Here, we demonstrate a method to control these parameters in the growth of metal chalcogenide (GaSe) and dichalcogenide (MoSe₂) 2D crystals by precisely defining the mass and location of the source materials in a confined transfer growth system. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a substrate by pulsed laser deposition (PLD) at room temperature. This <i>source</i> substrate is then covered with a <i>receiver</i> substrate to form a confined vapor transport growth (VTG) system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate, the temperature of which is controlled by the background gas pressure. Large monolayer crystalline domains (∼100 μm lateral sizes) of GaSe and MoSe₂ are demonstrated, as well as continuous monolayer films through the deposition of additional precursor materials. This PLD–VTG synthesis and processing method offers a unique approach for the controlled growth of large-area metal chalcogenides with a controlled number of layers in patterned growth locations for optoelectronics and energy related applications

Ultrafast Charge Transfer and Hybrid Exciton Formation in 2D/0D Heterostructures

Photoinduced interfacial charge transfer is at the heart of many applications, including photovoltaics, photocatalysis, and photodetection. With the emergence of a new class of semiconductors, i.e., monolayer two-dimensional transition metal dichalcogenides (2D-TMDs), charge transfer at the 2D/2D heterojunctions has attracted several efforts due to the remarkable optical and electrical properties of 2D-TMDs. Unfortunately, in 2D/2D heterojunctions, for a given combination of two materials, the relative energy band alignment and the charge-transfer efficiency are locked. Due to their large variety and broad size tunability, semiconductor quantum dots (0D-QDs) interfaced with 2D-TMDs may become an attractive heterostructure for optoelectronic applications. Here, we incorporate femtosecond pump–probe spectroscopy to reveal the sub-45 fs charge transfer at a 2D/0D heterostructure composed of tungsten disulfide monolayers (2D-WS₂) and a single layer of cadmium selenide/zinc sulfide core/shell 0D-QDs. Furthermore, ultrafast dynamics and steady-state measurements suggested that, following electron transfer from the 2D to the 0D, hybrid excitons, wherein the electron resides in the 0D and the hole resides in the 2D-TMD monolayer, are formed with a binding energy on the order of ∼140 meV, which is several times lower than that of tightly bound excitons in 2D-TMDs

Ultrafast Dynamics of Metal Plasmons Induced by 2D Semiconductor Excitons in Hybrid Nanostructure Arrays

With the advanced progress achieved in the field of nanotechnology, localized surface plasmon resonances are actively considered to improve the efficiency of metal-based photocatalysis, photodetection, and photovoltaics. Here, we report on the exchange of energy and electric charges in a hybrid composed of a two-dimensional tungsten disulfide (2D-WS₂) monolayer and an array of aluminum (Al) nanodisks. Femtosecond pump–probe spectroscopy results indicate that within ∼830 fs after photoexcitation of the 2D-WS₂ semiconductor energy transfer from the 2D-WS₂ excitons excites the plasmons of the Al array. Then, upon the radiative and/or nonradiative damping of these excited plasmons, energy and/or electron transfer back to the 2D-WS₂ semiconductor takes place as indicated by an increase in the reflected probe at the 2D-exciton transition energies at later time delays. This simultaneous exchange of energy and charges between the metal and the 2D-WS₂ semiconductor resulted in an extension of the average lifetime of the 2D-excitons from ∼15 ps to ∼58 ps in the absence and presence of the Al array, respectively. Furthermore, the indirectly excited plasmons were found to live as long as the 2D-WS₂ excitons exist. The demonstrated ability to generate exciton–plasmon coupling in a hybrid nanostructure may open new opportunities for optoelectronic applications such as plasmonic-based photodetection and photocatalysis

Interlayer Coupling in Twisted WSe₂/WS₂ Bilayer Heterostructures Revealed by Optical Spectroscopy

van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe₂/WS₂ bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS₂ and WSe₂ grown by chemical vapor deposition. After annealing the WSe₂/WS₂ bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm^–1 that is sensitive to the number of WSe₂ layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe₂/WS₂ bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed broadening of the WSe₂ A exciton absorption band in the bilayers suggests fast charge separation between the layers, which was supported by direct femtosecond pump–probe spectroscopy. Density functional calculations of the band structures of the bilayers at different twist angles and interlayer distances found robust type II heterojunctions at all twist angles, and predicted variations in band gap for particular atomistic arrangements. Although interlayer excitons were indicated using femtosecond pump–probe spectroscopy, photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition is very weak. However, the interlayer coupling for the WSe₂/WS₂ bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics