engleski

Tailoring Perovskite-Type Core-Shell Nanoarchitecture of Charge Transfer Layer in Solar Cells

A key task in multilayer solar cell (SC) is to ensure overall compatibility of the active and passive layers. 3rd generation SC ensure this task by allowing high-efficiency transport of charge carriers between the absorbing material and electrode, which is the duty of the charge transfer layer (CTO). Titania and zincite stand out among prominent electron transfer materials (ETO), however in addition to chemical composition modification approach, the use of various nanostructural arrays (nanotubes, nanorods, etc.) contributes more to overall efficiency upgrade. Namely, nano-arrays display substantial surface roughness and thereby addresses important problem of SC’s such as low lifetime of photogenerated carriers and optical gap decrease (transparency, efficiency, stability), consequently allowing the layers to remain thin. SC assembly starts with conductive transparent oxide substrate, commonly indium-doped tin oxide glass ; thereon titanium is deposited using magnetron sputtering or electron beam evaporation. Microstructural and structural results strongly correlate the energy level of Ti deposition to yield morphology and further on anodization susceptibility. Titanium anodization etching process was employed as a convenient way to produce the anatase nanotubes having various geometries (length, diameter and wall thickness). Two-step growth from solution was employed to prepare nanorods of zincite having various geometries (length, distance). Extensive structural, microstructural and optical characterisation was performed to identify (and optimise) conditions required for preparation of uniform layers at large scale, which is the limiting factor for 3rd generation SC scale-up. It was found that all SC problems basically start at the layers boundary. This investigation aims to interface the ETO layer with additional buffer layer in perovskite-type of structure. The ferroelectric properties of the buffer material allow specific electric environment and enable additional separation of the charge carriers. The buffer layer is introduced on behalf of sol-gel synthesis as a core-shell coating of the previously achieved ETO nano-arrays. The ordered CTO nanorchitecture remained unaffected after the addition of buffer layer, i.e. enough space remains available for the infiltration of the active material. These nanostructured layers were then infiltrated with photoactive materials, i.e. small molecules, methylammonium iodide perovskites and polymer bulk-heterojunction blends. Small molecules fully infiltrate the nanoarchitecture and ensure charge transfer effect, while for the material with bigger building units the infiltration level still has to be confirmed. Non-contacted layers were characterized using structural methods (X-ray diffraction (XRD), grazing-incidence XRD, small and wide angle scattering), spectroscopy methods (Raman, absorbance and photoluminescence spectroscopy) and microscopy (field-emission scanning electron microscopy, transmission electron microscope, energy dispersive spectroscopy). The assembly was finished using standard hole transmitting material and silver electrodes. Contacted solar cell were checked for electric properties (current-voltage measurements, impedance spectroscopy, quantum efficiency spectroscopy), offering information on relevant time scales for electronic transport and recombination. Overall information reveals interfacial phenomena with respect to layers geometry (good layer homogeneity and stability), but also indicates in complexity of mechanisms behind the composite in terms of distinguishing of the contributions of the layers and surface imperfections. Preparations of such advanced interfaces significantly attribute to SC behaviour and act as milestone in their future upgrade.

Solar cell ; Thin films ; Nanoarchitecture ; Anodization ; TiO2 nanotubes ; ZnO nanorods ; Perovskite

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